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Narrow-band optical band-pass filter using dual cascaded chirped FBGs
Abstract
In this paper, cascaded chirped fiber Bragg grating or CFBG based narrow-band bandpass filter capable of operating in C-band is proposed and analyzed. Cascading one linearly increasing chirp (LIC) FBG with another linearly decreasing chirp (LDC) FBG reduces the FWHM almost by five folds with a change in the normalized reflectivity. Analytical formulation based on a piecewise uniform approach (PUA) for the proposed structure(s) is also discussed. Extension of PUA approach for tilted FBGs is also presented in the paper. For the proposed structures, we have considered each CFBGs may have equal or different chirp rates keeping all other design parameters the same. As per the simulation results, the narrowest and widest FWHM achieved by the proposed structures are 1.56 nm and 1.77 nm, respectively. In such designs, the highest and lowest normalized reflectivity offered by the cascaded CFBG structures is approximately 0.81 and 0.39, respectively. The effect of temperature on the spectral characteristics for all the proposed structures is also studied. Simulation results reveal that the Bragg wavelength of the proposed cascaded structures will shift +1.701 nm for 100 °C increment in operating temperature in the absence of strain. The thermal sensitivity of all the proposed structures is estimated to be 17.01 pm/∘C.
... Such r.i. modifications inside the core of the fiber have led to the formation of many optical devices for sensing and communications purposes [3][4][5][6][7][8][9][10][11]. Although FBGs have been extensively used for different purposes, utilizing them to improve the optical transmission link performance has revolutionized the telecommunication industry [12][13][14][15][16][17]. ...
... Here, we have assumed that the considered FBG is operated in reflective mode and is produced in single mode fiber (SMF). The incident and reflected fields in transfer matrix form can be expressed as [5,7,23,24], ...
... Particularly, a chirped FBG or CFBG can be used for dispersion compensation purposes [32,33]. Generally, the grating pitch of a CFBG is not constant rather it is linearly varied as per the relation Λ N = Λ 1 + bz N , where b in terms of (nm/cm) represents the linear chirp parameter while N indicates the number of uniform FBG sections used to piecewise approximate the CFBG [7,22,34]. Λ 1 , Λ N are the grating pitch of the first and N th uniform FBG section, respectively. In a linearly increased chirp (LIC) FBG, the low-frequency components of a pulse through the CFBG get delayed more due to the increasing optical pitch in the grating region. ...
In this paper, an optical communication system constructed with cascaded fiber Bragg gratings or fiber Bragg gratings (FBGs) of varying lengths capable of operating in the C-band is presented. Here, initially, a passive optical device formed with four cascaded FBGs of varying lengths is proposed and analyzed. Subsequently, the proposed device is kept in the optical communication system to reduce the spectral width of the source, thereby enhancing the system’s performance. Analytical formulation based on the transfer matrix method of an optical device is incorporated. The effect of such a device on the system performance at various operating distances is discussed. Further, the system performance is studied with the apodized FBGs in the passive optical device. At the end, we investigated the effect of incorporating the cascaded FBG structure in the system utilizing four-level pulse amplitude modulation or PAM-4 modulation . As per the simulation results, the proposed device has given a maximum reflectivity of 98.39 % and a minimum FWHM of 0.175 nm for uniform FBGs. But with the apodized FBGs, an FWHM of 0.07 nm with a reflectivity of 59.81 % has been achieved. Simulation results reveal that the system formed with the proposed device has given better performance up to a distance of 105 km compared to the absence of the device. At the maximum operating distance of the system with apodized FBGs, the Q-factor and bit error rate (BER) are recorded as 6.712 and 9.5321×10⁻¹², respectively. Apart from the Q-factor and BER estimation, eye height is also used to estimate the system performance at various operating distances.
... In recent years, multiwavelength fiber lasers have been extensively used in optical sensing [1,2], fiber optic communication [3,4], the biomedical field [5,6], aerospace [7,8], and chemical engineering [9,10] due to their stable performance, high signalto-noise ratio, compact design, easy fabrication, and low cost. Stability is a crucial research focus, often enhanced by using fiber filters like fiber Bragg gratings (FBGs) [11][12][13], Michelson filters [14][15][16], Mach-Zehnder interferometers (MZIs) [17,18], Fabry-Perot filters (FPIs) [19,20], and Sagnac loop filters [21][22][23]. Stability is crucial in multiwavelength fiber laser research. ...
In this paper, a proposed filtering device using a dual Sagnac loop in parallel can switch channel spacing by adjusting the polarization controller angle, achieving outputs with 0.88 and 0.42 nm spacing. Theoretical simulations and experiments prove that it can be applied in the design of fiber lasers, and the impact of various parameters on the wavelength interval was analyzed. Adjusting the polarization-maintaining fiber parameters such as length and refractive index in the filter allows for new channel interval combinations. A multiwavelength fiber laser was constructed using this filter, achieving channel intervals of 0.88 and 0.42 nm in tests. The stable output laser signal is suitable for high-speed optical communication systems.
In open Radio access network (oRAN) 5G networks, a fronthaul segment between virtual Distribution unit (vDU) edge cloud and other external Remote radio units (RRUs) sites is very important to transmit high data rate to achieve 5G requirements. Performance parameters such as Quality factor (Q-factor) and Bit error rate (BER) of an optical fiber fronthaul segment are the main important parameters to enhance latency and bit rate. An Apodized fiber bragg grating (AFBG) is utilized with different apodization profiles to optimize linewidth of an optical signal leading to enhance performance. The designed AFBG is used with a data rate of 25 Gb/s optical Ethernet module at the vDU edge cloud to optimize the linewidth of the optical signal between the vDU and the RRU in the fronthaul optical fiber segment. The AFBG is investigated at different connections pre, post, and symmetrical connections, where different apodization functions are used in the AFBG design. It is found that the best results for the maximum Q-factor and minimum BER are 9.2 and 2.72e− 21, respectively, obtained from uniform function in the symmetrical connection. Finally, the proposed model linewidth is 0.408 nm, which is better than 1.56 nm that is evaluated by related work.
This paper proposes a system that aims to reduce the spectral width, Δλ, of the optical signal at transmitter for WDM system over distance 100 km. Also, a chirped fiber Bragg grating (CFBG) at the receiver is used to compensate dispersion. The proposed system consists of four cascaded FBGs connected between light source and optical fiber. Many apodization functions are investigated to enhance the performance of the FBG in the proposed system, and Δλ is obtained at every stage and apodization function. The Q-factor and bit error rate (BER) are obtained at distances 30, 40 and 50 km. It is found that Cauchy apodization function is the best one that reduces the reflective spectral width, Δλ, and achieves a maximum Q-factor and minimum BER at distances 30, 40 and 50 km at the last stage.
Smartphone-based interrogation of a fiber Bragg grating sensor is, to the best of our knowledge, reported for the first time. The smartphone flashlight LED was used as a light source, and a transmissive diffraction grating projected the CFBG spectra on the smartphone camera. In order to efficiently couple light from the smartphone LED to the fiber with CFBG, multimode fiber was used for inscription. Interrogation setup consists of a smartphone and low-cost off-the-shelf available components. Measurement principle was illustrated through the fiber strain caused by applied longitudinal force. Attained measurement sensitivity and resolution were validated via comparison with commercial spectrometer and theoretical results based on Cramer-Rao approach. Also, to the best of our knowledge, the influence of modal noise on the smartphone-based fiber optic sensor interrogation system performance is considered for the first time.
In this paper, a postdispersion compensation unit is proposed leading to a better performance for the optical communication systems. This unit utilizes a
chirped fiber Bragg grating (CFBG). For enhanced performance of the CFBG,
a proper apodization function is chosen to improve the quality factor (Q-factor) and the bit error rate (BER) of the system. A 110-km wavelength division multiplexing (WDM) optical link is investigated. The system performance is evaluated through its Q-factor, eye diagram, and BER showing best performance when using the Hamming apodization function.
https://doi.org/10.1002/dac.4551
In this paper, a cascaded fber Bragg grating (FBG) system is proposed to reduce the dispersion in the optical signal in single mode optical fbers. This consequently enhances
the system performance, which is evaluated by the bit error rate (BER) and quality factor (Q-factor). The proposed model consists of four uniform cascaded FBGs connected at
the transmitter to get narrow linewidth, Δλ, of the optical signal, which is a major cause
of the delay. The Optisystem7 is used to simulate the proposed model in a WDM system
with and without the model for distance 200 km. The system parameters are investigated
showing an enhanced performance with 12%, including eye diagram, Q-factor and BER. A
10−6–10−10 BER is achieved with a quality factor in the range 7–14, including the efects of
fber length, input power and FBG length.
Parabolic pulses with linear self-phase-modulation-induced frequency chirp are attractive in ultrafast laser fiber amplification system for the functionality of nonlinearities suppression. In this paper, we present an effective way of parabolic pulse evolution by passive spectral amplitude shaping with a pair of chirped fiber Bragg gratings (CFBG). By this approach, a high-energy high-peak-power Yb-doped fiber chirped pulse amplification (CPA) system is demonstrated. The oscillator is a dispersion-managed passively mode-locked Yb-doped fiber laser with a broadband Gaussian-shaped spectrum which is evolved to parabola in a following preamplifier with pre-chirping management by a CFBG compressor. The pulses are then stretched with a CFBG stretcher, based on frequency-to-time mapping, the temporal profiles of the pulses show an identical parabolic envelope to the spectrum. The shaped pulses are further amplified with three stages of all-fiber amplifiers and compressed by a grating-pair compressor. The pulse duration is compressed to 172 fs with a pulse energy of 27 µJ. The central pulse encompasses 72% of total pulse energy, corresponding to a pulse peak power of 113 MW. No obvious pulse degeneration is noticed at nonlinearity accumulation B-integral as high as 12 rad. This configuration shows a significant potential for nonlinearity tolerance in high-energy operation compared with conventional CPA system.
The purpose of this paper is to simulate and analyze the spectral characteristics of the fiber Bragg grating (FBG) to obtain narrow bandwidth and minimization side lobes in reflectivity. Fiber Bragg grating has made a big revolution in telecommunication systems. The existence of fiber Bragg grating is needed when an optical fiber amplifier and filter are used. They can be used as band reject filter or band pass filter for optical devices. The model equations of the cascaded uniform fiber Bragg grating and different cascaded apodization functions such as, Hamming apodized fiber Bragg grating, Barthan apodized fiber Bragg grating, Nuttal apodized fiber Bragg grating, Sinc apodized fiber Bragg grating and Proposed apodized fiber Bragg grating are numerically handled and processed via specially cast software to achieve maximum Reflectivity, narrow bandwidth without side lobes.
Fiber Bragg Gratings (FBGs) are one of the most popular technology within fiber-optic sensors, and they allow the measurement of mechanical, thermal, and physical parameters. In recent years, a strong emphasis has been placed on the fabrication and application of chirped FBGs (CFBGs), which are characterized by a non-uniform modulation of the refractive index within the core of an optical fiber. A CFBG behaves as a cascade of FBGs, each one reflecting a narrow spectrum that depends on temperature and/or strain. The key characteristic of CFBGs is that their reflection spectrum depends on the strain/temperature observed in each section of the grating; thus, they enable a short-length distributed sensing, whereas it is possible to detect spatially resolved variations of temperature or strain with resolution on the order of a millimeter over the grating length. Based on this premise, CFBGs have found important applications in healthcare, mechanical engineering, and shock waves analysis, among others. This work reviews the present and emerging trends in CFBG sensors, focusing on all aspects of the sensing element and outlining the application case scenarios for which CFBG sensors have been demonstrated.
A film sensor based on a tilted long-period fiber grating (TLPFG) inserted before a tilted fiber Bragg grating (TFBG) is proposed. The sensor is described theoretically using the transfer matrix method. This structure has two reflected peaks in the reflection spectrum. One peak is for the selected recoupled cladding mode of azimuthal order l = 2 and the other one is for the coupled core mode. The tilt angles of the TLPFG and TFBG and the mode order of the l = 2 cladding mode mainly determine the reflected power of the recoupled-(l = 2) cladding-mode peak in the reflection spectrum. By analyzing the relation between the film parameters (film refractive index and film thickness) and reflection spectrum, the characteristics of the film sensor are studied. The results show that this film sensor has a high sensitivity to the film parameters and increases the sensitivity of the film refractive index by two orders of magnitude in comparison with the normal cascaded long-period fiber grating (LPFG) and the fiber Bragg grating (FBG). The resolutions of the refractive index and the thickness of the sensing film are predicted to be 10⁻⁶ and 10⁻³ nm.
In this paper, we present the design and analysis of chirped fiber Bragg grating sensors (CFBG), optimized for temperature measurements. The transfer matrix method has been used to simulate the CFBGs and to study their thermal response. The simulations have been carried out with different temperature profiles in order to understand how the CFBG thermal response varies as function of the grating design parameters and, thus, to optimize the design for the specific application. Finally, to assess the numerical simulations, a series of experimental tests was performed showing very good agreement.
In this paper, we describe a novel method for spatially distributed temperature measurement with Chirped Fiber Bragg Grating (CFBG) fiber-optic sensors. The proposed method determines the thermal profile in the CFBG region from demodulation of the CFBG optical spectrum. The method is based on an iterative optimization that aims at minimizing the mismatch between the measured CFBG spectrum and a CFBG model based on coupled-mode theory (CMT), perturbed by a temperature gradient. In the demodulation part, we simulate different temperature distribution patterns with Monte-Carlo approach on simulated CFBG spectra. Afterwards, we obtain cost function that minimizes difference between measured and simulated spectra, and results in final temperature profile. Experiments and simulations have been carried out first with a linear gradient, demonstrating a correct operation (error 2.9 °C); then, a setup has been arranged to measure the temperature pattern on a 5-cm long section exposed to medical laser thermal ablation. Overall, the proposed method can operate as a real-time detection technique for thermal gradients over 1.5–5 cm regions, and turns as a key asset for the estimation of thermal gradients at the micro-scale in biomedical applications.
In this paper, the coupled mode theory is used to analyze apodized fiber Bragg gratings (FBGs). Since the profile of gratings varies with the propagation distance, the coupled mode equations (CMEs) of apodized FBGs are solved by the fourth-order Runge-Kutta method (RKM) and piecewise-uniform approach (PUA). We present two discretization techniques of PUA to analyze the apodization profile of gratings. A uniform profile FBG can be expressed as a system Corresponding author: H.-W. Chang (hchang@faculty.nsysu.edu.tw).
A method of measuring the complex modulation of a Bragg grating is derived from a one-dimensional model of light propagating in an optical fiber. Interference fringes between the Bragg grating and a reference air-gap reflector are measured, and a Fourier transform of the interference fringes generated as a laser is swept through the wavelength is used to compute the complex modulation function of the Bragg grating over a restricted domain. Supporting data, taken by temperature tuning a distributed feedback diode laser, are shown.
A full vector complex coupled mode theory (CMT) for the analysis of tilted fiber gratings is presented. With the combination of the perfectly matched layer (PML) and the perfectly reflecting boundary (PRB), the continuous radiation modes are well represented by a set of discrete complex modes. Simulation of coupling to radiation modes is greatly simplified and may be treated in the same fashion as guided modes. Numerical results of the tilted fiber Bragg gratings (TFBGs) with outer-cladding index equal, lower and higher than that of the inner-cladding indicate that the complex coupled mode approach is highly effective in the simulation of couplings to cladding and radiation modes in tilted fiber gratings. The reflective TFBGs are investigated by the proposed approach in detail.
A method of measuring strain over 30-cm intervals to an accuracy of 10 microstrain in unaltered low-loss communications-grade single-mode optical fiber is presented. The method uses a tunable external cavity diode laser to measure the reflected intensity of a reflector–fiber system as a function of wavelength. This measurement is performed with no strain applied to the fiber to produce a reference and then again after a strain has been induced. Cross correlation of the Rayleigh scatter spectra from a selected section of fiber in the strained and unstrained states determines the spectral shift resulting from the applied strain.
Whereas core-mode reflection and core-mode-to-radiation-mode coupling in tilted fiber Bragg gratings is well understood, as is coupling between a core mode and higher-order core and cladding modes in untilted gratings, here we analyze in detail the coupling among core modes and cladding modes in reflective and transmissive tilted fiber gratings. We show that strong coupling between an LP01 core mode and the exact (1m) cladding modes occurs in a transmissive tilted grating for nearly any tilt angle except angles close to 90°, whereas the LP01-to-(lm) cladding mode coupling (l ≠ 1) is appreciable only for tilt angles just below 90° (∼88°). In a reflective grating, strong coupling between the LP01 core mode and the exact (1m) cladding modes occurs only for angles less than ∼5°, whereas coupling to (1m) cladding modes for m > 1 occurs only for angles greater than ∼5°. Coupling among bound core modes exhibits a similar behavior, except that in general the coupling is stronger. Experimentally we show coupling to both higher-order bound core modes and cladding modes in a transmissive tilted grating at visible and near-infrared wavelengths.
In this letter, we analyze and develop the required basis for a precise grating design in a scheme based on two oppositely chirped fiber Bragg gratings, and apply it in several examples which are numerically simulated. We obtain the interesting result that the broader bandwidth of the reshaped pulse, the shorter gratings required
In this paper, the thermo-optic coefficient of silica fiber is analyzed and measured within a wide temperature range of 0-1200 °C for the first time. To obtain the thermo-optic coefficient, experiments were carried out by using fiber optic extrinsic Fabry-Perot interferometers (EFPIs) embedded in silica fiber. The optical cavity lengths of the EFPIs were measured with white-light interferometry. The experimental results show that the first and second orders of the thermo-optic coefficient are 1.090 × 10
-5
/°C and 1.611 × 10
-9
/°C
2
, respectively. The quadratic model for the temperature dependence of the basic optical parameter, the refractive index, is rigorous within a wide temperature range. Furthermore, this paper also provides a temperature calibration method for extrinsic Fabry-Perot interferometric temperature sensors. The temperatures measured by the sensors are in excellent agreement with those measured by the reference thermocouple. Hence, this calibration method is promising for temperature sensors with a wide working temperature range.
A chirped fiber Bragg grating (FBG) is fabricated using a non-uniform single-core As
Se
-PMMA tapered fiber that is pre-stretched by 0.52 mm during the inscription process. The inscription is based on a standing wave formed by two counter-propagating continuous-wave lights in the stretched non-uniform single-core As
Se
-PMMA taper. A periodical refractive index change by the standing wave in the high photosensitivity of the As
Se
core is induced, which creates a FBG along the taper. A strain gradient is formed along the grating due to the non-uniform structure when the elongation is changed, which produces a varying shift in the Bragg spacing at different position along the taper leading to a tunable chirped grating. The central wavelength and bandwidth are linearly dependent on the strain change added on the chirped grating. The wide tuning range is attributed to the low stiffness of the chalcogenide core and PMMA cladding. An exponential dependence of the amplitude of the chirping spectrum on the wave number is derived, which shows the potential for applications such as weak forces detections. Furthermore, long chirped FBGs can be easily inscribed using standing waves in As
Se
-PMMA tapered fibers, which is far beyond the practical length of phase masks and is desirable for chromatic dispersion compensation in high-bit-rate optical fiber transmission systems and distributed sensors. A 50 cm long chirped FBG with chirped wavelength range of ∼15 nm is inscribed and characterized. Fabrication of chirped FBGs in non-uniform single-core As
Se
-PMMA tapers opens the path towards the realization of novel sensors and devices.
Fibre Bragg gratings (FBGs) are obtained through a permanent and periodic refractive index modulation in the core of the single-mode optical fibre. For many years, they have been employed in telecommunication industry as a passive device for wavelength division multiplexing and dispersion compensation components, or in laser apparatus for laser fibre stabilization, Erbium amplifier gain flattening device and amplifier pump reflectors. In aerospace structures, FBGs are used as sensors for structural health monitoring of composite materials as they are able to perform measurements of several parameters inside the material in an elegant and low intrusiveness way. Based on the Bragg and optical fibre structure many kind of customizations can be applied on FBG sensors during the manufacturing process. Each of them gives to the FBG sensor different proprieties and sensing abilities. In this work, we address the numerical simulation of the reflected spectrum by a special FBG sensor called tilted FBG (TFBG), in which the core refractive index modulation is performed in way to obtain a tilted Bragg superstructure. By considering the classic Coupled-Mode theory for weakly-guided waveguides, we solved the mode propagation equations with the Transfer Matrix method (TMM) obtaining the TFBG reflectivity for different tilt angles.
In this paper, we proposed an eight channels dense-wavelength-division-multiplexing (DWDM)optical transmission system using two different dispersion compensation techniques. Dispersion compensation mechanism limits the pulse broadening effects of transmitted signal in optical communication systems. To overcome pulse broadening effects, linear Chirped Fiber Bragg Grating (CFBG)and dispersion compensation fiber (DCF)schemes are modeled, analyzed and compared for investigate the performance of DWDM system. The proposed system is designed for 10Gbps using return-to-zero (RZ)modulation format with an erbium doped fiber amplifier (EDFA)over a channel length of 150 km single mode fiber (SMF)and 45 mm CFBG/30 km dispersion compensation fiber using Opti-System7.0 simulator. Performance of designed system is explore and compared in terms of eye-diagram, gain (dB), noise figure (dB), output power (dBm), output optical SNR (dB), bit error rate (BER)and Q-Factor by varying three different parameters which are channel length (km), input power (dBm)and attenuation coefficient (dB/km). It is observed that the CFBG gives the best performance as dispersion compensator for proposed DWDM long-haul transmission system.
A novel half-bonded chirped fibre Bragg grating (CFBG) on a natural rubber diaphragm has been proposed to enhance the sensitivity and to compensate the temperature effects of a hydrostatic liquid level sensor. This innovative fabrication method resulted in the narrowing of the bandwidth with the hydrostatic pressure sensitivity recorded at -253 pm/kPa and water column sensitivity at -0.0253 nm/cm. The bandwidth modulation measurement was insensitive to temperature variations, when compared to centre wavelength modulation measurement. Furthermore, the proposed hydrostatic liquid level sensor is compatible with low cost photodetector.
A dispersion compensation scheme using the minimum number of cascaded fibre Bragg gratings (FBGs) is proposed for the transmission of 20 Gbps signal in 24 Wavelength Division Multiplexed (WDM) channels with a 0.4 nm channel spacing. The present work shows that four chirped FBGs (CFBGs) in cascade are sufficient to compensate the dispersion accumulated over 100 km of conventional fibre. The parameters of the CFBGs are optimized to obtain maximum quality factor for all the channels. The proposed setup is effective for simultaneous dispersion compensation in a WDM transmission system.
The Karhunen-Loeve transform (KLT) is used to retrieve the wavelength information of several fiber Bragg gratings (FBGs) that are acting as a multiplexed sensor. The modulated light of a broadband source is launched to the FBG cascade in order to capture the electrical frequency response of the system. Thanks to a dispersive media, the wavelengths of the FBGs are mapped in radiofrequency (RF) delays. Wavelength changes are determined by the amplitude change of the samples in the impulse response, a change which is followed by the eigenvalue calculated by the KLT routine. The use of the KLT routine reduces by three orders of magnitude the amount of points needed to have a sub-degree resolution in temperature sensing, while keeping the accuracy almost intact.
This paper presents the simulation results of coated Fiber Bragg Grating (FBG) as temperature sensor. As we know that bare FBG cannot be used as temperature sensor because they are made of silica and the thermal expansion coefficient of silica is very less. Hence bare FBG is less sensitive to temperature variation. To enhance the temperature sensitivity of the FBG sensor different types of materials are coated on FBGs whose thermal expansion coefficients are greater than that of silica. In this simulation Lead, Indium, Copper, Aluminium, and PMMA are coated on FBG and the simulation results are compared with the previous experimental work for the temperature range 50 K - 300 K. PMMA coated FBG sensor showed best sensitivity value among above coating materials.
Though people have experimentally studied the temperature dependence of the refractive index (RI) of optical fibers made of amorphous silica (a-SiO2), the temperature ranges in the previous experiments are usually below 800 K, and a rigorous theoretical model for the experimental results has still been absent. In this paper, the temperature dependence of the RI of a-SiO2 is studied theoretically and experimentally, where by means of an optical fiber delay line, the experiment is carried out in the temperature range of 301-1275 K for the first time, and the theoretical result agrees well with the experimental one. Our work has potential application in temperature sensors by means of optical fiber delay lines.
The delay of optical signal is determined by the refractive index and length of optical fiber, and temperature would have an intense influence on the index. To establish the relationship between refractive index and temperature, the temperature characteristics of refractive index was analyzed and the thermo-optical coefficient equation was derived according to the polarization of the induced electric dipole moment in SiO2 optical fiber. A measuring system based on optical fiber delay was carried out to measure the index within the temperature range of -30 degrees C to 70 degrees C and the experimental result was compared with the theoretical result. The final result shows that the relationship between refractive index and temperature is linear in the temperature range of discussion.
A design of chirped fiber Bragg gratings (CFBGs) for the compensation of higher order dispersion in dispersive delay lines of microwave photonic (MWP) filters driven by broadband sources is presented. It is shown that the modification of the dispersion slope of a nonlinearly CFBG can compensate for third-order dispersion in relatively broad optical bands, resulting in a linear correspondence between group delay and optical frequency. The concept is demonstrated by use of a custom modification of a commercial, low-ripple CFBG for dispersion compensation in 20 nm of the C-band. This broadband linear dispersive delay line is used to provide shape invariance in both the resonance’s amplitude and phase responses of single-passband MWP filters based on double-sideband modulation. The dispersion of the delay line is shown to be tunable in the range −170.0 to −143.3 ps/nm by the addition to the CFBG of different lengths of single-mode fiber, without compromising shape invariance.
The feasibility of thermal regeneration of chirped fiber Bragg gratings inscribed in commercial single-mode fibers using a 248-nm excimer laser has been experimentally demonstrated. Regenerated chirped gratings (RCGs) with a strong and symmetrical reflection profile can be achieved by 80 min of thermal treatment at a temperature of 800 °C. The thermal cycling test results show that the RCGs provide a stable and reproducible spectral response (both in bandwidth and in reflective intensity) over the temperature range from 25 °C to 1000 °C, indicating their good potentials for fiber laser and sensing applications under high-temperature conditions.
Direct experimental measurements of the thermo-optic and the thermal expansion coefficients for fixed temperature intervals (20-200°C, 200-500°C, 500-660°C, 660-780°C) in fused silica fiber containing fiber Bragg gratings (FBGs) were conducted. The diffraction efficiency of a FBG fluctuated with temperature between 2.01×10-4 and 0.17×10-4 while the temperature shift of the Bragg's peak was monitored between 1300 and 1311nm with sub-Angstrom precision. Numerical simulations were focused on the FBG's diffraction efficiency calculations accounting for the temperature drift of the gratings, and found to be in excellent agreement with obtained experimental data.It was found that the first–order thermo-optic coefficient changes between 1.29 and 1.85×10-5K-1 for the linear fit and at T=0°C its value was found to be close to 2.37×10-5K-1 for the polynomial fit of experimental data. The average thermo-optic coefficient undergoes a minimum in the vicinity of 440°C. Additional observation indicates a negative sign of the second-order thermo-optic coefficient. The value of thermal expansion coefficient was much less (0.5×10-5K-1) than that for the average thermo-optic coefficient. Based on the energy dispersive spectroscopy it was determined that thermal erasing of the FBGs at a temperature around 780°C corresponds to germanium sublimation in silica-based fibers.
Since the discovery of photosensitivity in optical fibers there has been great interest in the fabrication of Bragg gratings within the core of a fiber. The ability to inscribe intracore Bragg gratings in these photosensitive fibers has revolutionized the field of telecommunications and optical fiber based sensor technology. Over the last few years, the number of researchers investigating fundamental, as well as application aspects of these gratings has increased dramatically. This article reviews the technology of Bragg gratings in optical fibers. It introduces the phenomenon of photosensitivity in optical fibers, examines the properties of Bragg gratings, and presents some of the important developments in devices and applications. The most common fabrication techniques interferometric, phase mask, and point by point are examined in detail with reference to the advantages and the disadvantages in utilizing them for inscribing Bragg gratings. Reflectivity, bandwidth, temperature, and strain sensitivity of the Bragg reflectors are examined and novel and special Bragg grating structures such as chirped gratings, blazed gratings, phase-shifted gratings, and superimposed multiple gratings are discussed. A formalism for calculating the spectral response of Bragg grating structures is described. Finally, devices and applications for telecommunication and fiber-optic sensors are described, and the impact of this technology on the future of the above areas is discussed. © 1997 American Institute of Physics. S0034-67489701312-9
This study proposes a new approach to monitoring the damage process in holed CFRP laminates using an embedded chirped fiber Bragg grating (FBG) sensor. To this end, we experimentally and numerically investigated the damage process and the damage-induced changes in the spectrum shape. It was experimentally confirmed that multiple types of damage (e.g., splits, transverse cracks and delamination) appeared near a hole, and that the spectrum shape of the embedded chirped FBG sensor changed as the damage extended. Our proposed simulation for the reflection spectrum considering the damage agreed with the experiments. Furthermore, this study investigated the effect of each damage pattern on the changes in the spectrum shape. Finally, based on these discussions, we present simple damage identifications with the embedded chirped FBG for the holed CFRP laminates under completely unloaded conditions.
The ability to determine the centre position of a localized temperature change within a chirped fibre Bragg grating (CFBG) has been investigated as a function of grating strength. The intragrating sensor is based on the analysis of reflected power spectra arising from a CFBG. The technique uses a discrete Fourier transform (FFT) in which the measured spectrum of the CFBG due to a localized temperature change (heat source) was simulated using a FFT grating design model. The model operated on the reference spectrum and hypothesis temperature distributions, T (z), to generate a spectrum of a CFBG subjected to a hypothesis temperature disturbance. The simulated spectrum was fitted to the measured spectrum using a three-parameter automatic disturbance function fitting algorithm operating on position, width and amplitude of temperature change. RMS deviations to within 0.03 mm of applied values of position have been obtained.
In this paper, we describe the spectral characteristics that can
be achieved in fiber reflection (Bragg) and transmission gratings. Both
principles for understanding and tools for designing fiber gratings are
emphasized. Examples are given to illustrate the wide variety of optical
properties that are possible in fiber gratings. The types of gratings
considered include uniform, apodized, chirped, discrete phase-shifted,
and superstructure gratings; short-period and long-period gratings;
symmetric and tilted gratings; and cladding-mode and radiation-mode
coupling gratings
The historical beginnings of photosensitivity and fiber Bragg
grating (FBG) technology are recounted. The basic techniques for fiber
grating fabrication, their characteristics, and the fundamental
properties of fiber gratings are described. The many applications of
fiber grating technology are tabulated, and some selected applications
are briefly described