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Fibre Gratings and Devices for Sensors and Lasers
Abstract
Although mainstream grating writing, more often than not using single photon excitation of germanosilicate based defects with CW 244 nm light, remains the key technology for complex devices it is now being complemented by a whole host of processes which can enhance and tailor the properties of both conventional and not-so-conventional fibre Bragg gratings. Further, processes for writing of gratings in non-germanosilicate fibres have also continued to develop and include multi-photon excitation directly into the band edge of the glass. It is now possible to custom tailor a gratings property based on the application and the nature of production as well as custom tailor the grating writing process to suit the type of fibre and application. Examples and suggestions where these can benefit sensors and lasers are outlined.
... Bragg gratings in fibers are typically classified into eight different categories [7]; these are defined by glass composition (doping and whether hydrogen loaded), laser exposure, and thermal properties [7]. The underlying mechanisms of laser-induced refractive index changes are debated [8], but it is commonly accepted that defects in the material cause the change in refractive index upon UV exposure [7]. ...
... Bragg gratings in fibers are typically classified into eight different categories [7]; these are defined by glass composition (doping and whether hydrogen loaded), laser exposure, and thermal properties [7]. The underlying mechanisms of laser-induced refractive index changes are debated [8], but it is commonly accepted that defects in the material cause the change in refractive index upon UV exposure [7]. ...
... Bragg gratings in fibers are typically classified into eight different categories [7]; these are defined by glass composition (doping and whether hydrogen loaded), laser exposure, and thermal properties [7]. The underlying mechanisms of laser-induced refractive index changes are debated [8], but it is commonly accepted that defects in the material cause the change in refractive index upon UV exposure [7]. The color centre model describes the photo-induced change in refractive index according to the Kramers-Kronig relation [9]. ...
We present the first substantive investigation into the photosensitivity response of planar-doped silica to pulsed 213 nm light. We look at the response over a broad range of fluences and average powers to identify suitable regimes for simultaneous waveguide and Bragg grating writing. Unlike previously reported work, we do not observe any clear evidence of a similar non-linear photosensitivity response in B/Ge doped silica. We discuss laser-induced damage, saturation of photosensitivity, and grating response. This paper presents writing regimes for small spot direct UV writing where the photosensitivity and grating response are optimum, thereby confirming the suitability of the fabrication approach for complex devices.
... Type I gratings [4] are inscribed mostly in photosensitive fibers and they are based on defects creation inside the silica fiber core [5]. However, these gratings erased at temperature higher than 400°C [4]. ...
... Type I gratings [4] are inscribed mostly in photosensitive fibers and they are based on defects creation inside the silica fiber core [5]. However, these gratings erased at temperature higher than 400°C [4]. Whereas type II FBGs [4] are inscribed in all fibers types, where the energy at the focal point is above the silica damaged threshold resulting of the creation of nanostructure such as nano-gratings [6]. ...
... However, these gratings erased at temperature higher than 400°C [4]. Whereas type II FBGs [4] are inscribed in all fibers types, where the energy at the focal point is above the silica damaged threshold resulting of the creation of nanostructure such as nano-gratings [6]. These FBGs can withstand temperature higher than 900°C [4]. ...
... The fluence curve is generally expected to be different for different fibres, as absorption varies for different glass composition. In Fig. 2 [2] Grating Type Description Type I Gratings show a red shift during inscription. Gratings are found in photosensitive fibres. ...
... there are two mechanisms involved in changing the refractive index of a material with UV radiation: colour centres and densification. The type of mechanism has an impact on the properties of the Bragg gratings, especially regarding their resilience to temperature [2]. Table 2.1 shows an overview of the different gratings types, which were identified in the following experiments. ...
... To analyse the grating types for SSDUW, 11 gratings were written with different fluences into GF4A in a range from 10-40 kJ/cm 2 and 14 gratings in SMF28 with fluences The hydrogen-loaded fibre SMF28, does not show a dip in Fig. 2.8. Fibres with the presence of hydrogen are classified into different types, known as Type IH and Type IHp gratings and they do not show the characteristic roll over [2]. ...
This thesis investigates the capability of using an optical platform as data storage for calibration data, which is used for harsh environments. The optical device is based on fibre Bragg gratings, which are fabricated with small spot direct ultraviolet writing (SSDUW). Specifically, this work investigated different writing methods for co-locating gratings, aimed to achieve a high grating density to meet the required storage capacity. Colocating more than 40 gratings have been achieved with two different techniques. For read out, a system was chosen that allowed the read out of both the spectral and the spatial domain and was able to differentiate between different grating types, and hence contributing to a larger data storage capacity. The three parameters contributing to the storage capacity per unit length are: number of co-located gratings, the ratio between spectral grating bandwidth and the bandwidth of the light source and the amount of grating types that can be distinguished between each other. In this thesis not only the optical technology behind the storage device was investigated, but also the information technology required to translate calibration data into a spectral pattern and the correction methods to manage errors. Through this work, numerous findings have been made; the evaluation of different inscription methods for co-located fibre Bragg ratings; achieving more than 40 co-located Bragg gratings using SSDUW; the inscription of individual cores of a multicore fibre using SSDUW; the use of machine learning algorithms to identify mechanical vibrational modes of a cantilever; the classification of gratings inscribed with SSDUW into the widely used grating framework.
... These thresholds depend on the fiber composition, the possible fiber treatments undertaken to improve its photo-sensitivity (such as the H2 loading), the inscription technique, and the laser wavelength. Table 2 reports the most common grating types defined that have been defined, with the most important inscription conditions, the cause of the refractive index modification, and the maximum temperature each type can withstand, together with some of the most important references (for a more complete classification, see reference [54]). Color centers 1 . ...
... Micro-voids surrounded by a shell of densified silica. T > 1000 °C [59,60] 1 It is worth noticing that, according some authors, type I FBGs can be due, along with color centers, to structural changes and densification [54]; however, the latter should not be erased by thermal treatments at temperatures lower than 500 °C. So, the subject is still controversial. ...
Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments.
... -Tin-doped FBG performed up to 826.7 • C [10]. -Boron-doped highly silica fiber worked until 1295 • C [11]. -Fluorine-doped silica fiber with a thermal operation ranging from 1000 to 1200 • C [12]. ...
... [ [10][11][12] Tetrahedral FBG (TFBG) ...
FBG sensors are used in many scientific and industrial fields for assessing the structural integrity of mechanical components and in very high (above 600 °C) or very low (cryogenic) temperature applications. The main concerns with the use of such sensors in applications involving extreme temperatures are related partly to the instability of the reflected spectrum, which tends to dissolve into the noise floor, and partly to the degradation of the mechanical properties of the optical fiber, which tends to worsen the inherent brittleness. All of this raises the need for a robust nickel protective coating to ensure the grating’s integrity in high-temperature environments. In addition, the inherent brittleness of fiber-optic gratings leaves one to wonder whether it is possible to recover a broken, seemingly unusable sensor. In this way, a single-peak commercial FBG was intentionally broken in the middle of the grating length and re-spliced, inducing a strongly asymmetric chirped-like spectrum; then, a nickel coating was electrodeposited on its surface. The most important outcome achieved by this work is the regeneration of a highly distorted reflected spectrum through three thermal cycles performed from room temperature up to 500, 750, and 800 °C, respectively. After reaching a temperature of at least 700 °C, the spectrum, which has been drastically altered by splicing, becomes stable and restores its single peak shape. A further stabilization cycle carried out at 800 °C for 80 min led to an estimation of the stabilizing time of the new single-peak reflected spectrum.
... The main advantage of the interferometric technique [51] [52] is that the Bragg wavelength can be changed. In addition, few components are necessary. ...
... This method takes a relatively long process time because the step-by-step process. Several types of Fiber Bragg grating exist such as the uniform Bragg reflector, the chirped Bragg grating and the blazed Bragg grating [52]. They are distinguished by their grating pitch, tilt angle or spacing between grating planes and fiber axis. ...
In the transportation system domain, heating problems appear with the temperature increase in different types of electrical machines. In the classical design of electrical machines, thermal analysis should be considered in the initial design, control and monitoring of electrical machines. The measurement of local temperature especially in the rotor is important for several reasons such as extending the lifetime of the electrical machine components, and localizing the hot spots inside the machine which allows the development of appropriate cooling systems and protects the machine. Numerous approaches for temperature measurement can be used such as thermocouples, thermistors, infrared sensors or infra-red cameras. This thesis presents a non-contact technique that measures the temperature of the rotor of a small machine using Fiber Bragg Gratings (FBGs) sensor. Monitoring local temperature especially inside the rotor is important in order to detect early thermal aging of the machine. Hot spot in the rotating parts can be localized by using this technique. The main originality of the proposed work is measuring high temperatures (70°C) with high speed of rotation (860 RPM) of rotating machines and most importantly integrating the FBG sensor into a geometrically small scale electrical rotor of vehicles. The FBG sensor response has been simulated using Transfer matrix method (TMM). After that, the FBG has been calibrated from 20 °C to 70 °C using a heating furnace fabricated at our laboratory. A small rotating machine with embedded FBG has then been designed and fabricated. The temperature of the rotor has been changed while rotating the machine and wavelength shifts due to temperature variations have been experimentally measured up to 860 RPM. A temperature sensitivity of 4.7 pm/°C have been experimentally reached. The ability of this sensor to monitor real time temperature variations of the rotor has been experimentally validated.
... Display fiber Bragg gratings (FBGs) are optical devices formed by introducing periodic refractive index modulation (RIM) into the core of an optical fiber. They can be widely applied in fields, such as optical fiber sensors [1], fiber lasers [2], and optical filters [3], and play an important role in the development of modern optics [4]. Multi-mode waveguides, particularly multimode fibers (MMFs), have been deployed in numerous scientific and industrial applications, such as astronomical signal filtering [5], space-division multiplexing (SDM) transmission [6], singlefiber imaging [7], 3D holographic optical manipulation [8], and high-temperature sensing (such as multi-mode sapphire FBGs) [9]. ...
In this Letter, we propose the use of a femtosecond laser (FL) plane-by-plane (Pl-b-Pl) method to achieve narrowband single-mode reflection peak excitation directly in multi-mode no-core fibers (NCFs). The effects of different grating offsets on grating reflectivity were verified. We inscribed orthogonal fiber Bragg gratings with an offset distance of 15 m in multi-mode no-core fibers, and the bending sensitivities of the two gratings can reach −2.79 and −2.93 dB/m⁻¹. This structure can be used as a two-dimensional vector bending sensor, especially for detecting very small bends.
... Our approach utilizes the dispersive Fourier transform concept [11][12][13] to map the frequency of a temporally-short optical pulse onto the temporal envelope of the output pulse. We use advances in chirped fiber Bragg grating (CFBG) technology [14,15] to achieve large linear group-delay dispersion (GDD) across a broad frequency range outside the telecommunications spectral region. This technique allows rapid data acquisition rates of high-resolution spectral measurements. ...
A fiber-integrated spectrometer for single-photon pulses outside the telecommunications wavelength range based upon frequency-to-time mapping, implemented by chromatic group delay dispersion (GDD), and precise temporally-resolved single-photon counting, is presented. A chirped fiber Bragg grating provides low-loss GDD, mapping the frequency distribution of an input pulse onto the temporal envelope of the output pulse. Time-resolved detection with fast single-photon-counting modules enables monitoring of a wavelength range from 825 nm to 835 nm with nearly uniform efficiency at 55 pm resolution (24 GHz at 830 nm). To demonstrate the versatility of this technique, spectral interference of heralded single photons and the joint spectral intensity distribution of a photon-pair source are measured. This approach to single-photon-level spectral measurements provides a route to realize applications of time-frequency quantum optics at visible and near-infrared wavelengths, where multiple spectral channels must be simultaneously monitored.
... The use of femtosecond lasers allows to inscribe FBGs in virtually all kinds of optical fibers, sometimes even through their coatings. Four main types of FBGs are distinguished: Type I, II, III, or R FBGs [15], whose different characteristics are detailed in [16]. The grating typically has a length , from a few mm to a few cm, with the sensitivity confined to this functionalized zone. ...
This review focuses on silica-based optical fibers, guiding light through the total internal reflection mechanism, that are currently used in telecommunications and sensor networks. Section I briefly introduces the fiber and fiber sensor technologies. Section II presents their various applications in radiation environments. In section III, the radiation effects on optical fibers are explained, first at a microscopic scale, with the description of point defects generated by irradiation. Then, the macroscopic consequences of the appearance of these point defects are discussed, in particular the phenomena of radiation-induced attenuation (RIA), radiation-induced emission (RIE) and radiation-induced refractive index change (RIRIC). RIA impacts nearly all applications under irradiation. We then detail the various parameters, intrinsic or extrinsic to the optical fibers that influence their amplitudes and kinetics. This improved knowledge paved the way for the radiation hardening of optical fibers through various approaches, either at material, component or system levels, that are described in section IV. Recent advances are presented for passive and for active rare-earth doped optical fibers and systems. Then, the use of fiber optic sensors is introduced, starting in section V with the latest advances in radiation-hardened point or distributed sensors. Then, the potential of this technology for radiation detection, beam instrumentation and dosimetry applications is assessed in section VI. Finally, the future prospects and the main challenges to be met in order to accelerate the implementation of this technology in the harshest environments are discussed.
... [16][17][18] In addition, their immunity to electromagnetic interference and high sensitivity further enhance their suitability for harsh environments. 19 FBGs also play a critical role in telecommunications, where they are used as wavelength division multiplexing (WDM) filters and dispersion compensators. In WDM systems, FBGs enable the separation of different communication channels, enhancing the capacity of optical networks. ...
Fiber optic sensors based on fiber Bragg grating (FBG) technology have the potential to revolutionize the way vital signs of the human body are measured and monitored. By leveraging their unique properties, these sensors can provide accurate and reliable data, thus enhancing the effectiveness of wearable devices. The integration of FBG sensors into different materials not only broadens their application scope but also improves user comfort and device practicality. However, some challenges remain in optimizing the embedding process to ensure sensor performance and durability. This review provides an overview of FBG technology employed for measuring vital signs of the human body reported in the past decade. The focus of the review is on the FBG embedding strategies into different materials, categorized into these three main groups (i.e., 3D printed, textiles, and polymers) and explores the implications of embedding fiber optic sensors in each category. Furthermore, it discusses the potential impact of these embedded sensors on the accuracy, comfort, and practicality of wearable devices designed for monitoring vital signs, highlighting the potential of these sensors to transform the field of health monitoring. Future research directions may include exploring new materials for embedding and refining sensor design further to improve the accuracy and comfort of these wearable devices. Ultimately, the evolution of fiber optic sensors could significantly advance the field of human vital sign monitoring, paving the way for more sophisticated and user-friendly health monitoring systems.
... И наиболее перспективными, с точки зрения устойчивости к температуре, были признаны два типа ВБР. К первому типу относятся решетки Type II [19][20][21]. ...
The relevance of studies on the thermal stability of fiber Bragg gratings arises from the prospects of their applications in difficult operating conditions at elevated temperatures. One of the highest-temperature types of such structures formed in silica glass fibers are regenerated Bragg gratings. Generally, the regeneration effect is observed in gratings inscribed in molecular hydrogen-loaded fibers and/or in highly-doped germanosilicate fibers. This paper describes the first reported regeneration of gratings created via point-by-point inscription with femtosecond laser radiation in standard telecommunication fiber Corning SMF-28 without the presence of hydrogen in technological processes. The paper proposes an explanation of the possible mechanism for the formation of such regenerated structures.
... According to the nature of the refractive index modification and the structural changes induced by fs-lasers, different types of gratings are identified [9][10][11]. Type I modifications, corresponding to an isotropic index change in the glass matrix, and mostly related to defect formation, densification, and fictive temperature change, present relatively low thermal stability (<600 • C for few hours in silica) [12]. ...
Fiber Bragg gratings are key components for optical fiber sensing applications in harsh environments. This paper investigates the structural and chemical characteristics of femtosecond laser photo-inscribed microvoids. These voids are at the base of type III fs-gratings consisting of a periodic array of microvoids inscribed at the core of an optical fiber. Using high-resolution techniques such as quantitative phase microscopy, electron transmission microscopy, and scattering-type scanning near-field IR optical microscopy, we examined the structure of the microvoids and the densified shells around them. We also investigated the high-temperature behavior of the voids, revealing their evolution in size and shape under step isochronal annealing conditions up to 1250 °C.
... Porous NGs lamellar structuring induces a strong birefringence optical response from negative index variation [6], and can even lead to optical chirality [7]. Consequently, they have drawn attention for applications/fields including birefringent optical devices [8], 3D geometric phase optics [9], optical data storage [10,11], microfluidic [12], structural health monitoring and high-temperature sensing [13][14][15]. ...
... The high-temperature sensing performance of different material fibers and different types of FBGs is summarized in Table 1. The term "Type" in Table 1 and in the rest of this paper refers to the underlying photosensitivity mechanism by which grating fringes are produced in the optical fiber [60] . ...
... Let us consider a filter of width Δλ which suppresses the main carrier frequency by α T ≪ 1 while ensuring an Oð1Þ transmission at the location of the sideband. In Fig. 1 we consider filter efficiencies of α T ¼ 10 −10 ::10 −20 and a bandwidth of Δλ ≃ 100 kHz, which may e.g., be achieved by employing optical cavities tuned to the sideband frequency [17], or potentially also by stacking multiple fiber Bragg gratings [18,19]. We will neglect propagation effects induced by the GW in this filtering system, noting that they can be suppressed by choosing a suitable geometry (e.g., parallel to the incoming GW). ...
High frequency gravitational waves can be detected by observing the frequency modulation they impart on photons. We discuss fundamental limitations to this method related to the fact that it is impossible to construct a perfectly rigid detector. We then propose several novel methods to search for O(MHz−GHz) gravitational waves based on the frequency modulation induced in the spectrum of an intense laser beam, by applying optical frequency demodulation techniques, or by using optical atomic clock technology. We find promising sensitivities across a broad frequency range.
... They are created using a specific high temperature annealing regime of UV-laser-induced gratings that erases the laser-induced index change but relaxes internal fiber stresses that create a weak index modulation that has similar resilience to the temperature of thermally stable femtosecond laser-induced gratings. Their discussion here is beyond the scope of this Tutorial; however, interested readers are referred to the following review articles for more information 14,15 In this Tutorial, recent developments in harsh environment applications of fs-laser-induced FBGs in silica-based fibers will be presented. This will include a discussion of the basic sensing principle of the FBG, a summary of mechanisms for both the traditional single UV-photon and the femtosecond laser-material interaction and laser-induced index change, and an overview of popular femtosecond laser-induced FBG inscription techniques and their unique attributes, followed by some examples of extreme environment applications. ...
The technique of femtosecond laser-induced inscription of fiber Bragg gratings creates a structure in the optical fiber that can be used effectively as a sensor especially when deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices can be made that are suitable for sensing applications involving high temperature, pressure, ionizing radiation, and strain. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation, and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.
... In itself, the fabrication of FBGs using a femtosecond laser has attracted considerable attention since the procedure was initially proposed [31]. When compared with the traditional ultraviolet (UV) phasemask-based technique [32][33][34], femtosecond laser direct writing of FBGs is more flexible in terms of its ability to tune the reflection wavelength, and the fiber itself is not required to be photosensitive. There is also no need to peel off the fiber coating during the femtosecond laser writing process, which provides the fiber device with better mechanical strength. ...
Optical fibers are typically used in telecommunications services for transmission of data, fiber tags are essential to distinguish between different transmission fibers or channels to ensure the working functions of the communication system. Traditional physical entity marking method for fiber labelling is bulk in size, easy confusion and most importantly the label information can be easilly accessed by all potential users. In this work, we propose an encrypted optical fiber tag based on encoded fiber Bragg grating array fabricated by a pointby-point femtosecond laser pulse chain inscription method. Gratings with different resonant wavelengths and reflectivities can be achieved by adjusting the grating period and refractive index modulations. It is demonstrated a binary data sequence carried by a fiber tag can be inscribed into the fiber core in the form of FBG array, and the data can be encryted by proper design of the spatial distributions of FBGs with varing reflection wavelengths and reflectivities.The proposed fiber tag technology could be used for port identification, encrypted data storage and transmission in fiber networks.
... At present, equipping nuclear installations with optical fibre light guides (hereinafter referred to as "optical fibre" or OF) and fibre-optic sensors based on fibre Bragg gratings (FBGs) [1][2][3][4][5] is an actively growing trend in the nuclear industry [3,6] and science [7]. ...
A method for in-situ studies of the optical fibres (OFs) and the fibre Bragg grating (FBG) sensors radiation
resistance under irradiation at the IVG.1 M research reactor (Kurchatov, Kazakhstan) was developed. For this
purpose, an irradiation ampoule device (AD) was developed that makes it possible to carry out joint irradiation of
investigated samples with a fast neutron flux of up to 2.4 1013 n/(cm2⋅s) and a dose rate of up to 1.57 kGy/s in
the temperature range from 200 to 700 ◦C and pressures of 10–80 Pa.
Studies of radiation-induced attenuation (RIA) of single-mode OF in various protective coatings (copper,
aluminium and polyimide at a wavelength of 1.55 μm and in the spectral range from 1.1 to 1.7 μm were carried
out. Fast neutron fluence and absorbed dose were ФE>0.1MeV=1.72⋅1017 n/cm2 and Dγ=11.3 MGy, respectively. It
has been established that the method of fibre winding in AD has a critical influence and leads to a parasitic
contribution of temperature-induced stress-related losses to optical losses arising during reactor irradiation.
The operability of FBG sensors up to a temperature of 500 ◦C has been demonstrated.
The developed methods make it possible to conduct in-situ studies of the radiation resistance of fibre-optic
components under conditions close to their applications in thermonuclear and nuclear facilities.
... In fig. 1 we consider filter efficiencies of α T = 10 −10 ..10 −20 and a bandwidth of ∆λ 100 kHz, which may e.g. be achieved by employing optical cavities tuned to the sideband frequency [16], or potentially also by stacking multiple fiber Bragg gratings [17,18]. We In all cases we have set τ = 1 s, L = 1 m, ω S γ /2π = 2 × 10 14 Hz and P = mW. ...
High-frequency gravitational waves can be detected by observing the frequency modulation they impart on photons. We discuss fundamental limitations to this method related to the fact that it is impossible to construct a perfectly rigid detector. We then propose several novel methods to search for O(MHz-GHz) gravitational waves based on the frequency modulation induced in the spectrum of an intense laser beam, by applying optical frequency demodulation techniques, or by using optical atomic clock technology. We find promising sensitivities across a broad frequency range.
... Conventional Type I FBGs suffer from reflectivity decay when the temperature is over 200 • C [4], which largely influences their sensing performance at high temperatures. High temperature-resistant FBGs like femtosecond laser-inscribed FBGs (fs-FBGs) have been reported to survive at temperature over 1000 • C [5,6]. Besides, decoupling temperature and strain in a FBG is another challenge due to the intrinsic cross sensitivity between each other, which limits their ability to provide accurate results when both temperature and strain vary simultaneously. ...
Metal coatings can protect the fragile optical fiber sensors and extend their life in harsh environments. However, simultaneous high-temperature strain sensing in a metal-coated optical fiber remains relatively unexplored. In this study, a nickel-coated fiber Bragg grating (FBG) cascaded with an air bubble cavity Fabry-Perot interferometer (FPI) fiber optic sensor was developed for simultaneous high temperature and strain sensing. The sensor was successfully tested at 545 °C for 0-1000 µɛ, and the characteristic matrix was used to decouple temperature and strain. The metal layer allows easy attachment to metal surfaces that operate at high temperatures, enabling sensor-object integration. As a result, the metal-coated cascaded optical fiber sensor has the potential to be used in real-world structural health monitoring.
... f diffe e e f FBG Finally, to help the reader to understand better the different type of Bragg gratings and find the information more convenient, we summarize several key informations in Table 1-2, such as the main characteristics, fabrication techniques, maximum temperature of operation and wavelength shift as a function of temperature ( for the various types of fiber Bragg gratings we have discussed above. [194,224] fs-IR-Type I Type I IR-fs modifications is featured by an isotropic growth of the refractive index with respect to the non-irradiated regime. They mostly are based on a glass fictive temperature Tf increase when the duration of the heat pulse corresponding to the light energy is greater than the glass relaxation ime T G T hea i c i hea m d l with a partial contribution (typically 20%) of point defects. ...
Harsh environmental sensing using optical fiber technologies is a rapidly growing field of research. A new generation of devices that operate in extreme environments (particularly those operating above 700°C and up to 1500°C), including environments with additional challenges such as radiation and intense optical fields (e.g., high power lasers), must be capable of withstanding gradual annealing and degradation, or aging, over time. Of particular relevance, these devices include live diagnostics with embedded optical sensor composites for aircraft, helicopters, and space infrastructure, temperature/pressure sensing in increasingly deeper and hotter oil bores, or again monitoring and mapping of temperature distribution in power plants, furnaces, and chemical reactors. The key enablers to meet these requirements are related to the intrinsic nature of the glass material that composes the devices, and more specifically its glass structure and associated properties such as viscous flow, chemical migration, and stress relaxation. Thermal stability of these devices may therefore be achieved by judicious application of glass preparation, glass annealing and glass laser irradiations.From a fundamental point of view, femtosecond laser direct writing allows to take profit of light forces that act on the plasma and its nanostructuring leading to the nucleation - growth of nanocavities or even nanocrystals depending on the chosen compositions. These structural modifications could be an ideal support to imprint various optical functions that could withstand high temperature operation.In this context, the objectives of this Ph.D. thesis is to figure out the relationship between different type of modifications induced by fs laser in glasses and fibers (particularly silica and aluminosilicate) that are fabricated by various methods, and make the link between chemistry of materials and femtosecond laser-induced transformations. We seek to analyze the effect of the glass chemical composition and the laser writing parameters on the thermal stability of the imprinted optical properties.On the one hand, we tentatively reveal the underlying mechanisms at the origin of the observed birefringence and their entanglement in bulk silicate glasses: oriented point defects, stress fields from several contributions, nanostructures and texturation, etc. On the other hand, non-conventional fiber fabrication methods such as the molten core technique or 3D printing preform are also investigated to tentatively improve the thermal stability of fs-laser imprinted modifications through the aspect of glass preparation. These discoveries pave the way towards a new generation of glass based laser written devices, both in bulk and fibers forms, for either high temperature operation, high laser power propagation or long lifetime applications like optical data storage, sensors for structural heath monitoring or optics in harsh environment.
... Their ability to support and transmit tremendous volumes of data through low loss glass is unmatched by any other technology. And with novel material processing, it is supported by a range of extended fibre and integrated silicon technologies that will soon bring dense wavelength division multiplexing (DWDM) from the laboratory to the field [25][26][27]. Processing existing silica fibre technologies that can also support dopants and other materials such as aluminium oxide, has led to the demonstration of waveguides that can operate above 1200 • C [28], making optical technologies ideal for extreme conditions including off world. With few exceptions, more varied fibreless technologies are presently confined to the fibre ends or edges of the internet where mobile transport has become a necessity. ...
To address climate change, environmental monitoring and wellness more generally on a global scale, a new concept is presented - the bionic internet-of-things, or b-IoT. We propose the utilization of existing organic “sensor” technology that nature has provided and discuss a future adapting these to an existing inorganic internet to truly open up a global IoT. The use of organisms, in the first instance plants, bring an additional physical and psychological factor, connecting up living in things in a way that is consistent with natural symbiosis but extended over a global and potentially galactic scale. These plants not only monitor the environment, they interact to enable it to thrive, producing an ecosystem that consumes CO2, generates oxygen, recycling land and providing an environment for other organic species to develop. In contemporary real estate development, the need for a more whole ecosystem approach is recognized and that technology plays a vital role towards that. Thus we identify wellness and wellbeing as an integral part of all future technology development. A fundamental challenge is connecting such sensors to the IoT. We briefly review technologies of relevance in the context of material, health and environmental considerations, and discuss novel transducer mechanisms. To assess sensor capability we review our recent work on measuring leaf material properties using contact angle mapping, demonstrating a diversity of potential for environmental monitoring from this method alone. We also review some examples of common botanical properties that already exist which can in principal be readily coupled to existing transducers to create the hybrid b-IoT. We briefly speculate into the future of materials at the sensor end and into reaching space that can meet low cost and provide advanced functionality to help connectivity and integrate fibre and fibreless technologies.
... Note that temperature only causes the wavelength shift of Hi-Bi FBGs. However, these UV-induced Hi-Bi FBGs cannot withstand a high temperature above 450°C [15]. ...
We demonstrate the fabrication of a new highly birefringent cladding fiber Bragg grating (Hi-Bi CFBG) consisting of a pair of sawtooth stressors near the fiber core by using a femtosecond laser direct writing technology. The unique sawtooth structure serves as in-fiber stressor and also generates Bragg resonance due to its periodicity. After optimization of laser pulse energy, the Hi-Bi CFBG with a high birefringence of 2.2 × 10⁻⁴ and a low peak reflectivity of ∼ -24.5 dB (corresponding to ∼ 0.3%) was successfully fabricated in a conventional single-mode fiber (SMF). And then, a wavelength-division-multiplexed Hi-Bi CFBGs array and an identical Hi-Bi CFBGs array were successfully constructed. Moreover, a simultaneous measurement of torsion and strain at high temperature of 700 °C was realized by using the fabricated Hi-Bi CFBG, in which the torsion can be deduced by monitoring the reflection difference between the two polarization peaks and strain can be detected by measuring polarization peak wavelength. A high torsion sensitivity of up to 80.02 dB/(deg/mm) and a strain sensitivity of 1.06 pm/µɛ were achieved. As such, the proposed Hi-Bi CFBG can be used as a mechanical sensor in many areas, especially in structural health monitoring at extreme conditions.
... Such UV-illuminated devices exhibit great improvement in the SNR of DTS. However, the UV-induced type I refractive index modulation could not survive at high temperature above 450°C [20]. ...
High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼ -55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000°C, showing a temperature sensitivity of -2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.
The study of the thermal stability of fiber Bragg gratings (FBGs) is an important task, the relevance of which is due to the need to expand the operating temperature range of FBG-assisted sensors. Regenerated Bragg gratings are among the most promising candidates for this purpose. In this article, we for the first time demonstrate the regeneration of FBGs inscribed in a standard telecommunication optical fiber Corning SMF-28 via point-by-point method with femtosecond laser radiation. The inscription was performed without preliminary molecular hydrogen loading of the target fiber. Regeneration was observed during thermal annealing at a temperature of
C. A characteristic comparison was conducted with gratings inscribed with standard UV phase mask fabrication technology. In this article, we suggest an explanation of the formation mechanisms of such gratings, which imply clustering of irradiated core areas coupled with the effect of relaxation and redistribution of mechanical stresses in the optical fiber.
Single-frequency distributed Bragg reflector fiber lasers (DBR FLs) are attractive as sensing elements for detecting weak vibration or acoustic signals in extreme environments. However, conventional UV-written DBR FLs operate with two orthogonal polarization modes and can hardly operate in high-temperature environments. Herein, we propose the fabrication of polarization-controllable DBR FLs by using a slit beam shaping femtosecond (fs) laser point-by-point technology. High-quality fiber Bragg grating Fabry-Perot (FBG-FP) cavities with insertion loss as low as 0.2 dB are directly inscribed in Er-doped fibers to create DBR FLs. Both single-polarization and dual-polarization DBR FLs are created by changing the fs laser-induced birefringence using a mechanical slit. In addition, a DBR FL array consisting of eight DBR FLs is also successfully created. Experimental results show that the fabricated DBR FL can withstand a high temperature up to 800 °C and the laser linewidth increases from 1.55 kHz to 10.8 kHz as temperature raising from 25 °C to 800 °C. Furthermore, high-temperature vibration sensing at 800 °C is realized by using a single-polarization DBR FL, achieving an acceleration sensitivity of 0.319 rad/(m/s
2
). Moreover, a dual-polarization DBR FL is served as an ultrasonic sensor, realizing the ultrasonic non-destructive evaluation (NDE) in a 7075-aluminum plate.
High-temperature resistant fiber Bragg grating (FBG) has a wide application in aerospace, energy, smelting, and other high-temperature sensing fields. However, the general FBG will experience grating thermal decay and fiber thermal embrittlement when working for a long time in high-temperature environments, resulting in the sensors being unable to be applied in stress–strain-related sensing. Therefore, the thermal stability and mechanical properties of FBG in high-temperature environments are crucial for health monitoring of high-temperature or high-pressure equipment. Here, we developed an effective approach of thermal regeneration and annealing for FBG by studying the influence of different annealing conditions on the axial stress and mechanical properties of FBG and then obtained a thermally regenerated fiber Bragg grating (RFBG) with high mechanical properties and high-temperature resistance. Compared with ordinary RFBG, the mechanical strength of RFBG obtained under optimized regeneration and annealing conditions is increased by about four times. Furthermore, six optimized RFBG sensors are utilized to simultaneously and stably monitor the temperature field distribution and local stress–strain state of a high-temperature autoclave. The corresponding mean temperature sensitivity and stress–strain sensitivity are 16.50 pm/°C and 1.25 pm/
, respectively. This work proposes an effective high-temperature and stress–strain sensing technology, which is expected to be used for structural health monitoring in high-temperature environment.
High Order Fiber Bragg Gratings inscribed with fs-lasers by using the point-by-point technique are promising components for metrological purposes. In this study, we compare the radiation-induced Bragg wavelength shifts of low-diffraction-order (2) and high-diffraction-order (745) FBGs photo-written around 1550 nm at the FemtoBragg platform (CEA List) in uncoated Ge-doped and P-doped core fibers. Our interrogation setup allows us observing simultaneously 80 resonance peaks for the high order grating. We irradiated all the gratings up to 720 kGy(SiO 2 ) dose under X-rays. We show that low- and high-order FBGs have the same radiation response. Moreover, the RI-BWS – and by extension, the core index variation – are wavelength-independent over the studied wavelength range, despite the growth of a strong radiation-induced attenuation.
Fiber optic temperature sensors for small-scale heat sources have an urgent need in many industrial fields, for example, the temperature field of the laser heat source, mini LED, microchip, and so on. To the best our knowledge, we first propose and demonstrate an ultrashort fiber optic temperature sensor based on a Fabry–Perot (FP) interferometer which combines a low reflection fiber Bragg grating (FBG) with Fresnel reflection (i.e., ~4%) on the fiber end face. This unique FBG exhibits a broad Gaussian spectrum with a full-width at half-maximum (FWHM) bandwidth of 12.4 nm. This FBG-FP cavity structure was fabricated by a femtosecond laser line-by-line (LbL) scanning technology, and it has stronger mechanical strength due to no damage to the fiber material compared to other fiber optic FP interferometers. The free spectrum range (FSR) of the FBG-FP cavity is about 1.56 nm, corresponding to the cavity length can be calculated to be about
. The selected dip wavelength of the FBG-FP cavity sensor changes linearly with the temperature increase, corresponding to the temperature sensitivity of 10.8 pm/°C. Moreover, the FBG-FP cavity sensor exhibits a high spatial resolution of
and it can be used to measure the temperature of the small-scale laser spot due to its compact structure. The experimental results show the temperatures at different positions of 1064 nm laser exposure area are 138.4 °C, 111.5 °C, and 74.4 °C, respectively.
Copper Zinc Ferrite nanoparticles are dispersed in distilled water as their carrier liquid for preparing the different concentrations for magneto-optical characterization, and are subjected to uniform field where the Magneto-Optic rotation (MOR) is studied using green He-Ne laser of wavelength 543.5 nm to propagate through CZF. These ferrites are generally isotropic in nature, but under magnetization, the magnetic nanoparticles aligned themselves along the direction of an easy axis of the fluid, resulting to an increasing magnetic moment along an easy axis and decreasing magnetic moment along a hard axis of the ferrofluid, thus, making their isotropic behavior anisotropic.
The power spectral density (PSD) of a surface is a mathematical tool that
decomposes a surface across a range of wave-vectors i.e., as a function
of the inverse of the wavelength. PSD measurement provides concrete
topographical description of roughness, growth regime, fractal
dimensions and correlation length. This frequency-domain
characterization provides pragmatic and statistical information on
characteristic features which compose the microstructure of the
substrates and thus mechanisms governing the surface morphology.
In this work, two dimensional average PSD functions have been
derived from the Fourier Transform (FT) of the topographic micrograph
by angle averaging the Fourier Transform in all directions. These spectra
give a guide to study the frequency-domain characteristics of oblique Ar+
sputtered Si(111) surfaces across spatial frequency ranging from 0.1μm-
1 to 25.7μm-1 in the reciprocal space. The power law dependence of the
derived PSD curves on the spatial frequency has been deduced and
discussed thoroughly. Moreover, spatial frequency analysis of PSD is
used to determine characteristic dimensions of topographical features in the surface plane. Surface scaling analysis via PSD demonstrates that
surface morphology of the sputtered samples relies on ion beam
stimulated mass rearrangement inside the amorphous layers.
In this work the mechanical properties of a Photonic Crystal Fiber is achieved using concepts of the rules of mixture of composite materials in order to find the effective area, effective Young’s modulus and the effective density. Results achieved by analytical method are compared to results achieved through a commercial software. Simulation results of acoustic-induced modulation of a Photonic Crystal Fiber Bragg grating obtained using adapted Finite Element and Transfer Matrix Methods is showed. The methodology applied to study the device consists of the achieving of Photonic Crystal Fiber mechanical properties; the strain field in the whole structure is obtained by using the Finite Element Method. Further, the calculated strain field is used on the Transfer Matrix algorithm to obtain the Fiber Bragg Gratings reflected spectrum. The results are compared with conventional Fiber Bragg Gratings Acousto-Optic modulator. Numerical results show that, due to its smaller stiffness, compared to conventional Fiber Bragg Gratings the presence of Photonic Crystal Fiber Bragg Gratings causes an increasing on strain field along the grating when the modulator is excited by particular frequencies. This way, Photonic Crystal Fiber Bragg Gratings can be used in acousto-optic modulators increasing the acousto-optic efficiency and avoiding the need of tapers.
In this article, a cost-effective and fast interrogating system for wide temperature measurement with Fiber Bragg Gratings is presented. The system consists of a Vertical Cavity Surface Emitting Laser (VCSEL) with a High Contrast Grating (HCG)-based cavity that allows for the fast tuning of the output wavelength. The work focuses on methods of bypassing the limitations of the used VCSEL laser, especially its relatively narrow tuning range. Moreover, an error analysis is provided by means of the VCSEL temperature instability and its influence on the system performance. A simple proof of concept of the measurement system is shown, where two femtosecond Bragg gratings were used to measure temperature in the range of 25 to 800 °C. In addition, an exemplary simulation of a system with sapphire Bragg gratings is provided, where we propose multiplexation in the wavelength and reflectance domains. The presented concept can be further used to measure a wide range of temperatures with scanning frequencies up to hundreds of kHz.
We have designed, fabricated and characterized a low-frequency vibration acceleration sensor. The sensor consists of a double-clamped beam and a long-period fiber grating (LPFG) that inscribed in a single mode fiber (SMF) by femtosecond laser. The LPFG was glued tightly to the beam top surface. Its transmission spectrum changes with beam bending, enabling vibration acceleration measurements. The experimental results display that the sensor has an accurate and steady frequency response over the frequency range of 1.5 ∼ 10 Hz with a maximum frequency response error of 4.4 %. The maximum measurable acceleration is 0.3 g, and the maximum acceleration sensitivity is 2.8 mW/g with a linearity greater than 99.6 %. The temperature drift of the sensor is less than 0.001 g/℃. The sensor has good immunity to electromagnetic interference and reliability.
Thermal cycling tests of a Fiber Bragg Grating inscribed in a sapphire fiber with a phase mask and a femtosecond laser are performed up to 1150°C in order to assess the repeatability of high temperature measurements.
Modern optoelectronic devices use the advantage of digital systems for data processing aimed at delivering reliable information. However, since commonly used DACs have limited accuracy, some artefacts can be observed in data streams, especially in systems designed for continuous, long-term process monitoring. In this paper, the authors’ experience with data enhancement using a fibre-optic rotational seismograph (FORS) operating in a closed-loop mode is presented and discussed. Generally, two kinds of enhancement are described. The first one uses suitable filtering techniques adequate for FORS noise investigation, as well as a suitable data resampling method for transmitted data file size reduction. The second one relates to the artefacts observed during data recording in real time. The recording starting point is triggered when the detected signal exceeds a middle signal level and, therefore, the existence of artefacts generally disturbs the recording process. Although the artefacts are easily recognised by human eyes even at first sight, their automatic elimination is not so easy. In this paper, the authors propose a new concept of signal filtering to solve the above problem.
Type III (void) femtosecond Fiber Bragg Gratings inscribed with the point-by-point technique in the SMF-28 fiber and thermally annealed were irradiated under both X-rays and temperature ranging from -75°C up to 150°C.
We propose a non-destructive, all-optical technique to imprint embedded lateral superlattices near semiconductor heterostructures by illuminating the samples with a stable interference pattern generated by a phase diffraction grating. We demonstrate the technique on an ultrahigh mobility GaAs/AlGaAs sample with a Si δ-doping by inducing a persistent charge redistribution at cryogenic temperatures in the doping layer containing DX-centers. Weiss commensurability oscillations in the magnetoresistance of the light-induced superlattice are observed and analyzed to obtain its characteristics.
Fiber‐optic technology emerged originally for applications in data transmission and telecommunications. However, sensors based on fiber‐optics have been developed rapidly because of their excellent sensing performances and capability to function in remote and harsh environments. The usage of fiber‐optic sensors has flourished in many fields over the past 30 years due to the fiber‐optic's inherent advantages: cost‐effectiveness, miniaturized size, light weight, and immunity to electromagnetic interference. This work reviews the fiber‐optic sensors based on Bragg gratings, long period gratings, interferometers, surface plasmon resonance, fluorescence, and light diffusion. Brief theory of sensing principle, fabrication method, applications, advantages and disadvantages of the different fiber‐optic sensors, are addressed. Recent progress in numerous sensing fields, including environmental, industrial, and biomedical are discussed for each class of fiber‐optic sensors. The review highlights the methods and techniques used to overcome the sensing challenges. Finally, prospect of future developments of fiber‐optic sensors is summarized. Fiber‐optic sensors based on Bragg gratings, long‐period gratings, interferometry, surface plasmon resonance (SPR), fluorescence, and light diffusion are analyzed. The foundations of chemical and biosensing are addressed and the recent progress in numerous sensing fields, including environmental, industrial, and biomedical applications, are discussed. The review highlights the strategies and techniques used to overcome the sensing challenges.
This paper presents a comprehensive review of optical fiber sensors (OFSs), including FBG, distributed optical fiber sensor and Fabry-Perot interferometer, and their applications within harsh environments, which include extremely high temperatures (from 275 °C to 1750 °C) and low temperatures (from −271.15 °C to −40 °C, namely cryogenic conditions: from 2 K to 233.15 K), and high levels of ionizing radiation (with a maximum gamma dose up to 2 GGy, and a maximum neutron fluence of approximately
n/cm
2
). After a brief introduction of the principles of OFSs and mechanisms of interrogation, this paper focuses on the existing works for the above three operating environments. Attention have been paid to material selection for fabricating fibers, effects of doping with rare earth elements, femtosecond laser engraving, pre-processing and post-processing (i.e., annealing) that are employed to overcome issues faced byOFSs in extreme temperatures and radiation environments. Application examples and practical test cases are also presented. Through these examples, the limitations in the current state-of-the-art are acknowledged and the key problems are identified. Potential solutions to some of these problems are also elucidated. A feature of this paper is the amalgamation of many research methodologies and outcomes in three seemingly distinct environmental conditions in one place so that different solution techniques can be integrated to advance OFS technologies, especially for extreme environment applications.
We propose and experimentally demonstrate a slit beam shaping method for femtosecond laser Point-by-Point (PbP) inscription of highly localized Fiber Bragg Gratings (FBGs). The influence of slit width on the shape and area of Refractive Index Modulation Region (RIMR) induced by a single femtosecond laser pulse was investigated. The shape of RIMR can be changed from a spot to a line with an enlarged RIMR, and hence enhances the coupling strength of core mode and cladding modes. The RIMRs were precisely assembled along the fiber core, producing highly localized FBGs with a spectral comb of pronounced Cladding Mode Resonances (CMRs) intensity of more than 30 dB, a wide wavelength span of 240 nm and a low insertion loss of 0.3 dB. Note that the total processing time for fabricating such a highly localized FBG only requires ∼3.7 s. Subsequently, by including tilted angle on the slit, highly localized tilted FBGs were fabricated and these FBGs show adjustable envelope on the CMRs. Moreover, we investigated the surrounding Refractive Index (RI) response and thermal characteristics of the fabricated highly localized FBGs, which exhibit a sensitivity of 510.87 nm/RIU in a wide RI measurement range and excellent high temperature resistance at 1000°C. Therefore, such highly localized FBGs could potentially be used for multi-parameter sensing in many extreme environments.
Monolithic integrated spectrometers are ideal for use in portable testing equipment, however current research tends to focus on high-resolution platforms at near-infrared wavelengths. This thesis investigates a dispersive spectrometer platform based on blazed chirped Bragg gratings which has high resolution, large bandwidth, and low cost. As such it is suitable for portable Raman spectroscopy and optical coherence tomography.
Blazed chirped Bragg gratings diffract and focus light, with the angle of diffraction dependent on the wavelength of the input light. As such, the detected intensity distribution along a detector array can be used to measure the light spectrum. This thesis shows flexible fabrication and characterisation of these devices using small-spot direct UV writing, as well as methods of efficiently modelling their output intensity distribution.
Focussing aberrations of blazed chirped Bragg gratings are investigated and an ideal chirp function is derived to eliminate such aberrations. This is experimentally verified, enabling a device which operates over the 1440 nm to 1640 nm wavelength range. The device exhibited a peak resolution of 1.8 nm at 1560 nm and a typical resolution of 2.6 nm across a 100 nm range.
Scalar diffraction modelling was used to investigate the bandwidth of devices, showing 3 dB bandwidths of greater than 210 nm for devices operating at 1550 nm using a novel 45° detector mounting scheme. This mounting scheme also flattens the resolution response with wavelength, resulting in greater spectral resolution further from the design wavelength.
Finally a blazed chirped Bragg grating with variable blaze angle was demonstrated to offset fabrication effects and further increase device resolution and sensitivity. To our knowledge this is the first example of Bragg gratings with varying blaze angle along their length. Devices operating at wavelengths close to 780 nm achieved measured resolutions of 0.4 nm to 0.5 nm, though it is suspected that this is limited by aberrations inside the characterisation system. Modelling shows that fully optimised devices operating at 780 nm should achieve resolutions of 0.3 nm, as well as bandwidths exceeding 100 nm.<br/
By use of high-intensity (≈200 GW/cm²) femtosecond 264-nm laser light and a phase mask technique, Bragg grating inscription in a range of different photosensitive and standard telecom fibers (both H 2 - free and H 2 - loaded ) was studied. The dependences of the induced refractive index modulation versus the incident fluence as well as the thermal decay curves were compared with similar dependences for gratings fabricated by other existing methods. It was shown that with high-intensity UV laser irradiation, two-quantum photoreactions occur in the irradiated fiber core, that result in a significant photosensitivity enhancement of the investigated fibers in comparison with conventional low-intensity 248-nm exposure (by 6–128 times, depending on fiber type and irradiation intensity).
Nitrogen-doped silica-core fibre is discussed in the context of a technological basis for the fabrication of Bragg gratings for sensors with enhanced temperature resistance up to 900 °C. To estimate the applicability of this fibre type for the manufacture of high-temperature Bragg grating sensors, the following features are analysed: technology for fabricating fibre preforms, fibre characteristics and particularities of Bragg gratings written therein. Experimental data on the degradation of the gratings' characteristics resulting from a long-term (up to 4 months) annealing at elevated temperatures are presented and discussed. The practical application of this type of sensor in thermometry is given as an example.
The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E′-centers are discussed. The effects of glassy disorder have a profound effect on defect properties and interconversion processes in silica.
Type II phase gratings were produced by the use of holographic side writing in high-birefringence optical fibers with UV fluences of 0.06 J/cm² over 10 times lower than that previously reported for standard fibers. The grating growth, transmission and reflection spectra, temperature response, short-wavelength light ejection, and high-resolution confocal microscopy images are reported. Diffraction theory is used to interpret the grating microstructure revealed by confocal microscopy. Each period of the grating is shown to consist of a plate of oriented cracks, and arguments relating to the arrangement of the cracks and crack growth are linked to the observed grating-growth dynamics.
We have demonstrated fast formation [~1500 pulses at ~1 (J/cm²)/pulse] of fiber gratings with highly negative index modulations (~−3 × 10⁻⁴). We have found that the maximum negative index modulations that are achieved do not depend on the pulse intensities, although the inverse of the time taken to reach the negative index-modulation maximum varies linearly with the pulse intensities. This prompts us to use a three-energy-level system to model the photosensitivity in boron-doped germanosilicate fiber. All the necessary parameters of the model can be determined from a single growth measurement of the average index change, and the model’s prediction fits well the measured index-modulation growth.
We present a novel method for three-dimensional optical data storage that has submicrometer size resolution, provides a large contrast in index of refraction, and is applicable to a wide range of transparent materials. Bits are recorded by use of a 0.65-N.A. objective to focus 100-fs laser pulses inside the material. The laser pulse produces a submicrometer-diameter structurally altered region with high contrast in index of refraction. We record binary information by writing such bits in multiple planes and read it out with a microscope objective with a short depth of field. We demonstrate data storage and retrieval with 2-μm in-plane bit spacing and 15-μm interplane spacing (17 Gbits/cm³). Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.
With the goal of being able to create optical devices for the telecommunications industry, we investigated the effects of 810-nm, femtosecond laser radiation on various glasses. By focusing the laser beam through a microscope objective, we successfully wrote transparent, but visible, round-elliptical damage lines inside high-silica, borate, soda lime silicate, and fluorozirconate (ZBLAN) bulk glasses. Microellipsometer measurements of the damaged region in the pure and Ge-doped silica glasses showed a 0.01–0.035 refractive-index increase, depending on the radiation dose. The formation of several defects, including Si E′ or Ge E′ centers, nonbridging oxygen hole centers, and peroxy radicals, was also detected. These results suggest that multiphoton interactions occur in the glasses and that it may be possible to write three-dimensional optical circuits in bulk glasses with such a focused laser beam technique.
Narrow-line, permanent Bragg reflection gratings have been created in Ge-doped silica-core optical fibers by interfering beams of a single 20-ns pulse of KrF excimer laser light. Of the fibers studied, the highest reflectance value of ~2% was observed with a linewidth (FWHM) of 0.1 nm, which corresponds to a 2-mm grating length with an index modulation of ~3 × 10⁻⁵.
A new class of refractive index sensors using solid core photonic crystal fibres isdemonstrated. Coherent scattering at the cladding lattice is used to optically characterizematerials inserted into the fibre holes. The liquid to solid phase transition of water uponfreezing to ice 1h is characterized by determining the refractive index.
The effect of the microstructure on transversely coupled laser light into the core of a photonic crystal fiber is investigated. Computational two-dimensional modeling and direct experimental measurements indicate that there exist angles and positions of the fiber microstructure, relative to a transversely launched laser beam, that preferentially couple laser light into the fiber core. The implications of these observations on long period and fiber-Bragg grating fabrication in photonic crystal fibers are discussed.
By controlling the fibre geometry, the fraction of optical field within the holes and the inserted material of a photonic crystal fibre, we demonstrate that it is possible to engineer any arbitrary wavelength-dependent thermo-optic coefficient. The possibility of making a fibre with a zero temperature dependent thermo-optic coefficient, ideal for packaging of structured fibre gratings, is proposed and explored.
The formation of two grating types in SMF-28 fiber by focusing 125 fs, 0.5–2 mJ pulses through a phase mask onto a fiber sample is studied. The first type, specified as type I-IR, occurs below the damage threshold of the medium. The scaling behavior of the type I-IR gratings with field intensity and annealing properties suggests that their formation is related to nonlinear absorption processes, possibly resulting in color center formation. The second type, denoted as type II-IR, occurs coincidentally with white light generation within the fiber. These type II-IR gratings are stable at temperatures in excess of 1000 °C and are most likely a consequence of damage to the medium following ionization.
A distributed feedback laser is fabricated in Er³⁺-doped photonic crystal fibre. Preferential single-mode lasing is obtained with no special consideration of polarisation issues. The results demonstrate practical implementation of a multi-photon writing process for complex structures in these optical fibres. No hydrogen loading and no germanium are involved.
Low temperature (sub 1000°C) thermal hypersensitisation is reported in germanosilicate optical waveguides. Gratings are written using a CW 266nm laser source. In contrast to laser hypersensitisation, thermal excitation is generally dispersive involving a range of specific glass sites. More complex grating profiles presenting evidence of solid-state autocatalysis and bistability at increasingly high sensitisation temperatures are observed. More specifically, at 500°C, a behaviour resembling type IIA grating response is observed.
We report on waveguide writing in fused silica with a novel commercial femtosecond fiber laser system (IMRA America, FCPA µJewel). The influence of a range of laser parameters were investigated in these initial experiments, including repetition rate, focal area, pulse energy, scan speed, and wavelength. Notably, it was not possible to produce low-loss waveguides when writing with the fundamental wavelength of 1045 nm. However, it was possible to fabricate telecom-compatible waveguides at the second harmonic wavelength of 522 nm. High quality waveguides with propagation losses below 1 dB/cm at 1550 nm were produced with 115 nJ/pulse at 1 MHz and 522 nm.
We describe an Er³⁺-doped aluminosilicate core photonic crystal fibre laser incorporating distributed Bragg reflectors written by two-photon 193nm irradiation through an optical phase mask as the feedback elements. The laser is diode pumped at 980nm and evidence of dual linewidth laser operation close to threshold is observed. However, at higher pumping levels gain competition preferentially selects one laser line.
Strong Bragg grating fabrication in cheap, low power (rated 15W - intensity ~0.054Jcm⁻²) UV-lamp hypersensitised optical fibres is demonstrated. No optics has been used. Comparable results with recent 355nm laser hypersensitisation are obtained.
Using ultraviolet femtosecond pulses with high irradiance stability, we measured the two-photon absorption (TPA) coefficients in a number of substances with a total accuracy of ∼10%. Six commercial fused-silica samples (KU-1, Corning 7940, SQ, Suprasil, Herasil, and Infrasil) possess TPA coefficients (β values) of ∼2 × 10⁻¹¹ cm/W. For crystalline quartz and sapphire, the following β values were obtained: (1.2 ± 0.2) × 10⁻¹¹ and (9.4 ± 1.2) × 10⁻¹¹ cm/W, respectively. In β-barium borate crystal the TPA coefficient depends on crystal cut, beam polarization, or both and varies from (47 ± 5) × 10⁻¹¹ to (68 ± 6) × 10⁻¹¹ cm/W. For eight liquids that were studied (water, heavy water, ethanol, methanol, hexane, cyclohexane, 1,2-dichloroethane, and chloroform) the β value lies from (34 ± 3) × 10⁻¹¹ to (95 ± 11) × 10⁻¹¹ cm/W.
We report on grating writing in air–silica structured optical fibers with pure silica cores by use of two-photon absorption at 193 nm. A decrease in propagation loss with irradiation was observed. The characteristic growth curves were obtained.
We report on what is to our knowledge the first fabrication of fiber Bragg gratings by UV femtosecond radiation. The Bragg gratings, with photoinduced refractive-index modulation up to 1.92 × 10 - 3 in H 2 -loaded SMF-28 and up to 1.05 × 10 - 3 in Nufern GF1 fibers, were written by high-intensity ( 31 – 77 ‐ GW / cm 2 ) femtosecond pulses at 264 nm. The dependence of the refractive-index modulation on intensity at equal fluences points to a two-photon absorption mechanism for grating inscription.
We compare the cladding patterns present in grating structures fabricated with an ultrafast laser and a phase mask with a cw beam interference model. We find that the observed patterns agree well with the model results for picosecond pulses; however, for femtosecond pulses, we show that the full bandwidth and the pulsed nature of the sources must be considered because the pattern can be affected by group-velocity walk-off. An interesting consequence of order walk-off is the possibility of pure two-beam interference generation with a phase mask in the femtosecond pulse regime.
The threshold for the fabrication of fiber Bragg gratings with ultrafast 800-nm radiation and a phase mask was studied in SMF-28 and all-silica core fiber by use of 125-fs pulses. High-pressure molecular hydrogen loading ( H 2 loading) was observed to significantly lower the grating writing threshold in standard Ge-doped telecommunication fiber. No reduction was observed with all-silica core fiber. The index change appeared to be confined to the Ge-doped core region of the fiber. Gratings in H 2 -loaded SMF-28 had thermal annealing behavior similar to UV-induced gratings. Unlike UV-induced H 2 -loaded gratings, no absorption associated with Ge–OH defect formation was observed.
Silicon dioxide plays a central role in most contemporary electronic and photonic technologies, from fiber optics for communications and medical applications to metal-oxide-semiconductor devices. Many of these applications directly involve point defects, which can either be introduced during the manufacturing process or by exposure to ionizing radiation. They can also be deliberately created to exploit new technologies.
This book provides a general description of the influence that point defects have on the global properties of the bulk material and their spectroscopic characterization through ESR and optical spectroscopy.
Optical fiber sensors present interesting properties like large dynamic range of measures, great sensitivity and flexibility, insensitivity to electromagnetic fields, no local electronics allowing measurement in hazardous medium. Different applications concerning measurement of thermal, mechanical, electrical current, variables are described in this paper.
In the development of a new research field, it is often very fruitful to have a close interaction between theory and experimental work, and the area of photonic crystal fibres is in no way different on this issue. It is, therefore, an integrated part of the research development to study the theoretical treatment of the problems and challenges leading to the understanding of these new fibres, and as a key element of this work, the numerical modelling of the complex fibre structures become important.
In the world of scientific investigations, we are often facing the situation that a complex problem or the fascination of a given idea forces us to focus so much that we have a tendency to overlook the things, which should have been the true inspiration of our work. Many times, it is first at the point, when we have determined a solution to a simple sub-problem that we might generalize the finding and obtain a wider perspective. Then the situation becomes even more rewarding, when we from the more general theory become able to solve new and unforeseen problems, which at first glance had very little to do with the first sub-problem.
The course of the changes produced by exposure in nuclear reactors and the subsequent alteration of the radiation-induced property changes caused by heating were followed by measuring the dilatation, refractive index, rotatory power, and birefringence. The fast-neutron dosages determined by a comparison of radiation damage in several substances are used to give a consistent scale for presenting the effects seen from the initial changes to nearly the saturation state, despite the many irradiation facilities used for the work. The property changes resulting from heating specimens irradiated to various extents are given. The damaging phenomena are explained in terms of the interaction of the scattered atoms with the silica structure, and the annealing phenomena in terms of the behavior of the anticipated structural changes. On the basis of the effect of displaced atoms and thermal spikes on solid structures there are delineated a number of classes of radiation damage among insulators. For one of these, silica is taken to be the prototype. Thermal spikes cause the displaced atoms to be accommodated by a local deformation, which is depicted as analogous to the rapid inversions in which a minor reorientation of the Si-O tetrahedra occurs without extensive disruption of the structure. The resulting gradually accumulating disorder is considered the basic formulation of a metamictization for which the damaging of quartz thus becomes the simplest example. The same basic processes are considered to take place in vitreous silica; but since the substance is already disordered, the changes which take place are minor, the slight compacting of the structure evidently representing mainly the partial freezing-in of a high-temperature state produced by the thermal spike.
DOI:https://doi.org/10.1103/PhysRev.92.1064
Ge-doped preform plates have been OH-flooded and subsequently exposed to UV pulses from ArF laser (λp=193nm) under typical conditions of Bragg grating (BG) inscription. Both Fourier Transform Infrared (FTIR) and Vacuum Ultraviolet (VUV) absorption spectroscopy have been carried out after each step of the sensitization and inscription processes. It is shown that the sensitization process not only induces the formation of a large concentration of hydroxyl species but also that of hydride species and GODC defects. The UV exposure leads to strong changes in both the VUV and FTIR absorption spectra of the sensitized plates. These changes are analyzed with a view to getting a better understanding of the OH-flooding-induced sensitization at 193nm.
Using classical dynamics simulations, we model the long-lived structural effect of ultraviolet irradiation on amorphous silica. We find a significant increase in density of a model of amorphous silica following localized energy deposition, in agreement with experimental observation. We present evidence that this densification arises as a result of the rapid local cooling that follows irradiation. Similar high density forms of amorphous silica are found following fast quenches of bulk samples. In support of this proposal we demonstrate that very rapidly quenched silica undergoes dilation, rather than compaction, on irradiation.
We report the first demonstration of autocatalysis and oscillatory behaviour in the solid-state where no decomposition process is involved. Our material system is solid-state silica glass impregnated with hydrogen. It is at the heart of photosensitivity in glass-based optical waveguides and devices, which have many applications including telecommunication devices, fibre lasers, and optical sensors. Consequently, the results reported here extend the engineering of chemical complexity to a previously uncharted area in materials science of particular relevance to photonic applications. Diagnosis is carried out optically, opening up a new approach to characterisation of such catalytic processes generally.
The irradiations of vitreous amorphous silica with subband-gap laser radiation at 248 nm induces reversible-irreversible compaction. The former is consistent with the creation of a wide continuum of densified amorphous silica (a-SiO2) polymorphs whilst the second stage corresponds to the ablation through laser radiation of the damaged a-SiO2. These observations are discussed in terms of conformational structure modifications (i.e., changes in the distribution of planar rings) and stable superficial defect creation.
Semiconductor surface damage, produced by a focused beam of ruby laser light, consisted of regular patterns of cracks and a system of parallel straight lines. The system of grooves (spacing of about 0.0001 cm) can be explained on the basis of diffraction effects at the focus of a lens. The regular crack patterns were related to the cleavage habits of the semiconductors by x-ray analysis. (Author)
For centuries, it had been the dream of alchemists to turn inexpensive metals into gold. Certainly, it is not enough from an alchemist’s point of view to transfer only the appearance of a metal to gold. However, the possibility of rendering a certain metal to a completely different color without coating can be very interesting in its own right. In this work, we demonstrate a femtosecond laser processing technique that allows us to create a variety of colors on a metal that ultimately leads us to control its optical properties from UV to terahertz.
A precision phase-shifting approach to fabricate various phase-shifted gratings using different combinations of polarized ultraviolet (UV) light is demonstrated. In doing so, the difference between s- and p-polarized light reported by others is confirmed. However, we reveal added complexity for the role of hydrogen and deuterium in the UV-induced process. Previous arguments for the origins are systematically ruled out by reviewing existing literature. We note that the birefringence is made up of at least two components with different thermal stabilities, one consistent simply with molecular hydrogen being present in the system. Overall the birefringence, by deduction, is associated with anisotropy in hydrogen reactions within the fiber. As a result they lead, through known mechanisms of dilation in glass, to anisotropic stress relaxation that can be annealed out, with or without hydrogen remaining, at low temperatures close to 125 °C.
An approach to accessing air holes in a structured optical fiber with a distributed-feedback (DFB) laser based on higher order mode lasing is proposed and demonstrated. A narrow linewidth DFB fiber laser is fabricated in rare-earth-doped structured optical fiber. A higher order mode is shown to lase. Dual laser operation in both fundamental and higher order modes is also achieved. Numerical simulation of the mode profiles within the fiber using the adjustable boundary conditions-Fourier decomposition method supports the experimental results. Laser performance for each mode is characterized including imaging the emission of pump and lasing mode intensity profiles.
Phosphosilicate fibre gratings can be stabilized at temperatures in excess of 500 °C for sensor applications by optimizing thermal and UV presensitization recipes. Furthermore, the use of 193 nm presensitization prevents the formation of OH absorption bands, extending the use of fibre gratings across the entire wavelength spectrum. Gratings for operation at 700 °C retaining up to 70% reflectivity after 30 min are demonstrated.
Liquid filling of photonic crystal fibres reduces the scattering from air–glass interfaces during Bragg grating writing in many layered photonic crystal fibres. Within experimental uncertainty, the grating index modulation of a grating written in germanium-doped photonic crystal fibre with 10 rings of holes was more than doubled for a fixed fluence.
An insight into the present understanding of point defects in the simplest and the most radiation-resistant oxide glass, glassy silicon dioxide (silica) is presented. The defects and their generation processes in glassy and α-quartz forms of silicon dioxide are significantly different. The only defect, confirmed to be similar in both materials, is oxygen vacancy. In silica, additional defects of dangling bond type are generated from precursor sites formed by strained Si-O bonds, and by modifier ions. The optical absorption spectra of silica are dominated by paramagnetic dangling bond type defects: silicon dangling bond (“E′-center”) and oxygen dangling bond (“non-bridging oxygen hole center, NBOHC”). Radiation-induced interstitial oxygen atoms exist in peroxy linkage (Si-O-O-Si) form, they can react with oxygen dangling bonds to create peroxy radicals or dimerize into interstitial O2 molecules. Hydrogen doping helps to reduce the defect concentration, however, creates new precursors in the form of hydroxyl groups and may stimulate O vacancy generation. Doping by fluorine reduces the number of strained Si-O bonds and results in glass, which has higher vacuum ultraviolet transparency and higher resistance to excimer laser light than pure silica. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Three-dimensional (3D) micromachining of photosensitive glass is demonstrated by photochemical reaction using femtosecond (fs) laser for lab-on-a-chip application. True 3D hollow microstructures embedded in the glass are fabricated by fs laser direct writing followed by heat treatment and successive wet etching. The modification mechanism of the photosensitive glass by the fs laser and advantage of this process are discussed. Various microcomponents for the lab-on-a-chip devices such as microfluidics, microvalves, microoptics, microlasers, etc. are fabricated by using this technique and their performance is examined .
Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused
800-nm, 130-fs laser pulses at fluences between 5 and 200J cm-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide
evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass
shows a broad fluorescence band at 630nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses.
Waveguides that support the fundamental mode at 633nm have been fabricated inside fused silica by scanning the glass along
the fs laser beam axis. The index changes are estimated to be approximately 0.07×10-3.
This paper demonstrates the first air pure-silica photonic crystal Fresnel fibre with a water core and a Bragg grating, which opens up plenty of possibilities for liquid and gas sensing, and low cost in-fibre nonlinear devices fabrication.
The dilatations (negative) caused by neutrons, 40–600 keV electrons, 140 keV H+, D+, He+, and gamma rays are given as a function of dose and are compared. The presaturation dependence is linear for neutrons, He+, and D+, and depends on the 0.5–0.7 power of dose for H+, electrons, and gamma rays. The dilatations are not temperature dependent from about 0°–100°C, hence are not thermally activated. The dilatations are explained as compaction of the silica structure resulting from oxygens moving into some of the ``free volume,'' the structural change being similar to that occurring on pressure, shock, or thermal compaction of vitreous silica. The mechanism given for the neutron‐induced compaction involves the state of high vibrational excitation developed in the slowing down of scattered atoms. The compaction by gamma rays, electrons, protons, or deuterons is explained as caused by transient ionization relaxing SiO binding to permit oxygens to move into the ``free volume'' where they may be locked‐in on recombination by similar incidents occurring nearby. The reduced power dependence is explained as a ``hardening'' effect; succeeding events are more difficult to perform or are performed with lower probability. The compaction caused by He+ (140 keV) seems to possess an equal contribution from ionization compaction and that caused by energy transfer to atomic vibrations.
We analyzed the action of fire, causing degradation in a concrete cantilever beam using dynamic testing. The structure was fitted with two fibre Bragg gratings (FBG) sensors. One of them measured vibration and the other measured the temperature inside of the cantilever beam, while the beam was exposed to fire. A high-temperature probe based on a simple packaging system, which isolates the sensing FBG from any mechanical action, was developed. A low-cost fibre Bragg grating interrogation system, including easy assembly and maintenance, was used for the measurements. The temperature in the cantilever beam increased until 150 °C and a reduction in the strength of concrete was observed through modal analysis. Results reveal a considerable reduction in strength occurs even with exposures to moderate temperatures (less than 90 °C).
In an attempt to write optical waveguides in bulk glasses, photo-induced refractive index changes were continuously produced by focusing an ultra-short laser pulse through a microscope objective and translating the sample parallel to the axis of the laser beam. The resulting linear refractive index changes were written inside the bulk glasses along the path traversed by the focal point of the laser. From field intensity distributions in the output of guided light for these waveguides, we demonstrated that permanent optical waveguides could be successfully formed in various types of glass. In addition, through analyzing the near-field pattern with a CCD camera, we confirmed that single mode waveguides of the graded index type could be formed by a writing technique using an ultra-short laser pulse.
Improved understanding of the underlying fundamental mechanisms of the photosensitivity used to create index change within optical fibers can lead to substantial improvements in optimizing these changes for fiber grating writing. Recent work in unravelling and understanding photosensitization has lead to results which suggest that with an appropriate UV writing recipe one can achieve photostabilization of the gratings equivalent to sufficient thermal annealing of Bragg gratings to ensure stable operation over 25 years. In this paper those results and the mechanism of sensitization are reviewed.
Light scattering which peaks in the plane of light polarization is observed in glass pumped by intense laser radiation. The phenomenon is interpreted in terms of the angular distribution of photoelectrons in isotropic solid state materials.
The so-called characteristic curve describing photosensitivity change is elaborated and shown to be a powerful tool for understanding and characterizing photosensitive growth both at a fundamental and practical level. It has been used successfully to diagnose when optimal hypersensitization has been achieved and the physical basis for this is explained. By way of example, previous results using 355 nm hypersensitization are re-examined. Evidence of single site-selective glass relaxation through direct laser excitation offers a new approach to accessing and studying induced relaxations.