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Experimental Investigation of Supercontinuum Generation in Photonic Crystal Fibers Pumped With Sub-ns Pulses
Abstract
We experimentally investigate a supercontinuum generation in a series of photonic crystal fibers pumped with sub-ns pulses. The fibers are designed to have zero dispersion wavelengths close to 1064 nm, but they differ with respect to the lattice constant and air hole size. We focus on the supercontinuum generation mechanism and on the shape of spectra generated with the use of these photonic crystal fibers. A comprehensive study on the influence of the fiber structural parameters on the supercontinuum outcome indicates how to tailor these parameters for specific application where particular spectral characteristics are desired.
... Moreover, employing PCFs for supercontinuum generation allowed the use of relatively low peak power pump sources since low peak power of pump can be compensated by long length of the nonlinear medium. Consequently, supercontinuum generation in PCF has been demonstrated using subnanosecond, nanosecond and even continuous-wave laser radiation [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] which opened new application possibilities and brought additional insight on the physics of supercontinuum generation. Rapid progress in supercontinuum generation research lead to various applications such as spectroscopy [32], telecommunications [33], frequency metrology [34,35], optical coherence tomography [36,37], etc. and evolved into commercial broadband supercontinuum sources based on mode-locked fiber oscillators delivering ultrabroadband radiation from UV to IR spectrum range. ...
We present an experimental and numerical investigation of supercontinuum generation in polarization-maintaining highly nonlinear photonic crystal fiber using subnanosecond pulses of a passively Q-switched neodymium-doped yttrium aluminum garnet microlaser. For analysis of temporal and spectral characteristics, we performed spectrogram measurements using a streak camera. Supercontinuum spectrograms revealed the presence of cladding modes in our photonic crystal fiber and showed that part of the supercontinuum light was actually generated in the cladding modes. Moreover, numerical simulations indicated that distinct cladding modes can exhibit different dispersion, which adds more complexity to supercontinuum generation processes.
... The unique properties of IG MOFs allow the possibility of controlling their chromatic dispersion in a wide wavelength range by varying a hole diameter d and a hole-to-hole spacing, pitch Λ. To date, IG MOFs with zero dispersion wavelengths in the visible and near-infrared wavelengths [3]- [6], with two zero dispersions [7], [8], with varying dispersion [8]- [10], with flattened and near zero dispersion [11]- [13], and even with ultra-flattened and near zero chromatic dispersion for wavelengths of about 1.0 to 1.8 μm [14]- [19] have been reported. IG MOFs with ultra-flattened dispersion can be used in wide-band supercontinuum generation, dispersion compensation, ultra-short soliton pulse transmission, optical parametric amplification, wavelength-division multiplexing or pulse reshaping [15]- [20]. ...
We fabricated two types of special nonlinear, index-guiding air-silica microstructured fibers for supercontinuum (SC) generation. One fiber has an irregular structure of the holes in the cladding, and the other one has a regular structure. In both fibers, SC was generated by pumping the fibers with femtosecond pulses. The spectral features of the SC and the optimum pumping conditions were identified through mode dispersion analysis. It was found that SC in the fiber with irregular cladding can be generated by pumping the fiber with a broad wavelength range.
We investigate the influence of air holes on phase sensitivity in microstructured optical fibers to longitudinal strain. According to the numerical simulations performed, large air holes in close proximity to a fiber core introduce significant compression stress to the core, which results in an increase in the effective refractive index sensitivity to longitudinal strain. The theoretical investigation is verified by an experiment performed on four fibers drawn from the same preform and differentiated by air hole diameter. We show that introducing properly designed air holes can lead to a considerable increase in normalized effective refractive index sensitivity to axial strain from -0.21 e-1 (for traditional single mode fiber) to -0.14 e-1.
In this paper we present possibilities of tuning spectrum of supercontinuum with the use of temperature change. Our study is based on the information about the role of dispersion characteristics in the process of nonlinear effects generation in nanosecond pulse regime. We obtain tunable spectrum effects in microstructured fiber and we show how to optimize its properties. Our experimental results showing nonlinear effects generation in fiber pumped in normal and anomalous dispersion regime enables to determine how the nonlinear effects depend on temperature changes. We show that even small changes of dispersion characteristic of microstructured fibers enable to obtain significant modification of generated spectra when four wave mixing is dominant effect. Controllable generation of tunable supercontinuum can be used in numbers of potential applications such as diagnostics and measurement systems. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
We present a series of three fold symmetric silica microstructured fibers for nonlinear application. The fibers are designed to have zero dispersion wavelength far from the pump wavelength. In our fibers a domination of nonlinear effects is precisely controlled by selectively increased three air holes in the vicinity of the core. Our fibers allow for the broadband single mode spectrum generation and the achievement of flat spectra even after changes of fiber dispersion caused by fabrication inaccuracies.
Based on the generalized nonlinear Schrödinger equation, nonlinear chirped-pulse propagation and supercontinuan (SC) generation in dispersion-flattened dispersion-decreasing fibers (DFDFs) were investigated numerically. The simulation results indicate that the effect of initial chirp parameter on pulse propagation and SC generation is highly related to the choices of pumping conditions and fiber parameters. When DFDF has small normalized quadratic dispersion coefficient, proper positive chirps can significantly enhance the SC bandwidth, while negative chirps or too large positive chirps suppresses the SC bandwidth. There is a wide range of positive chirps that can enhance the SC bandwidth, but the range of proper positive chirps become narrower as input soliton order decreases; When DFDF has large normalized quadratic dispersion coefficient and the pump pulse is a lower-order soliton, the enhancement of SC bandwidth by initial chirp parameter is insignificant, and the widest SC spectrum is generated when the initial chirp is close to zero.
In presented work, we examined the structures of dual-core fibers paying special attention to the possibility of using them for sensing. In the hole-assisted fiber structure, the character of propagation in the cores was changed fluently, by post-processing the fiber, i.e. tapering with collapsing the holes. Fiber post-processing changed the conditions for supermodes interference and thus the different scale of power transfer between cores was observed. In the paper we investigated the influence of the taper parameters (taper waist, length and ratio) on the properties of the fiber. We have also studied the behaviour of the transmitted signal, while putting post-processed segment of fiber into different external conditions. Presented research shows a great potential of using modified hole-assisted fibers as sensing elements.
We study the generation of supercontinua in air-silica microstructured fibers by both nanosecond and femtosecond pulse excitation. In the nanosecond experiments, a 300-nm broadband visible continuum was generated in a 1.8-m length of fiber pumped at 532 nm by 0.8-ns pulses from a frequency-doubled passively Q-switched Nd:YAG microchip laser. At this wavelength, the dominant mode excited under the conditions of continuum generation is the LP11 mode, and, with nanosecond pumping, self-phase modulation is negligible and the continuum generation is dominated by the interplay of Raman and parametric effects. The spectral extent of the continuum is well explained by calculations of the parametric gain curves for four-wave mixing about the zero-dispersion wavelength of the LP11 mode. In the femtosecond experiments, an 800-nm broadband visible and near-infrared continuum has been generated in a 1-m length of fiber pumped at 780 nm by 100-fs pulses from a Kerr-lens model-locked Ti:sapphire laser. At this wavelength, excitation and continuum generation occur in the LP01 mode, and the spectral width of the observed continuum is shown to be consistent with the phase-matching bandwidth for parametric processes calculated for this fiber mode. In addition, numerical simulations based on an extended nonlinear Schrödinger equation were used to model supercontinuum generation in the femtosecond regime, with the simulation results reproducing the major features of the experimentally observed spectrum.
We review supercontinuum generation in optical fibers for particular cases where the nonlinear spectral broadening is induced by pump radiation from fiber-format sources. Based on numerical simulations, our paper is intended to provide experimental design guidelines tailored ytterbium and erbium-based pumps around 1060 and
1550
nm
, respectively. In particular, at
1060
nm
, we consider conditions under which the generated spectra are phase and intensity stable, and we address the dependence of the supercontinuum coherence on the input pulse parameters and the fiber length. At
1550
nm
, special attention is paid to the case of dispersion-flattened dispersion-decreasing fiber, where we revisit the underlying physics in detail and explicitly examine the use of such fiber for supercontinuum generation with pumps of peak power in the range
200
–
1200
W
and sub-10m fiber lengths. We show that supercontinuum generation under such conditions can be highly coherent and can be applied to nonlinear pulse compression.
We report, to the best of our knowledge, the first turn-key and compact dual-output sub-nanosecond supercontinuum source applicable to ultrabroadband multiplex coherent anti-Stokes Raman scattering microspectroscopy of biological samples. The light source design and the microspectroscopy setup are described in details. Easy multicolour imaging of Caenorhabditis elegans nematode is operated successfully and discussed. Copyright © 2011 John Wiley & Sons, Ltd.
A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.
Long-pulse supercontinuum sources are initiated by modulation instability and consequently suffer from stochastic shot-to-shot variations of their spectral power density. In this paper, we provide a measurement of pulse-to-pulse fluctuations over the whole supercontinuum spectrum, and we show that their spectral dependence follows the group index curve of the fiber. Then, we demonstrate a significant reduction of supercontinuum pulse-to-pulse fluctuations in the visible by using a photonic crystal fiber with longitudinally tailored guidance properties. We finally show numerically that this new source would allow a significant improvement of the signal-to-noise ratio in fluorescence microscopy.
Enhancement of the bandwidth of supercontinuum generated in microstructured fibers with a tailored dispersion profile is demonstrated experimentally. The fibers are designed to have two zero-dispersion wavelengths separated by more than 700 nm, which results in an amplification of two dispersive waves at visible and infrared wavelengths. The underlying physics behind the broad continuum formation is discussed and analyzed in detail. The experimental observations are confirmed through numerical simulations.
Photonic crystal fibres exhibiting endlessly single-mode operation and dispersion zero in the range 1040 to 1100 nm are demonstrated. A sub-ns pump source at 1064 nm generates a parametric output at 732 nm with an efficiency of 35%, or parametric gain of 55 dB at 1315 nm. A broad, flat supercontinuum extending from 500 nm to beyond 1750 nm is also demonstrated using the same pump source.
Numerical simulations are used to study the temporal and spectral characteristics of broadband supercontinua generated in photonic crystal fiber. In particular, the simulations are used to follow the evolution with propagation distance of the temporal intensity, the spectrum, and the cross-correlation frequency resolved optical gating (XFROG) trace. The simulations allow several important physical processes responsible for supercontinuum generation to be identified and, moreover, illustrate how the XFROG trace provides an intuitive means of interpreting correlated temporal and spectral features of the supercontinuum. Good qualitative agreement with preliminary XFROG measurements is observed.
We report on the influence of the choice of the pump wavelength relative to the zero-dispersion wavelength for continuum generation in microstructured fibers. Different nonlinear mechanisms are observed depending on whether the pump is located in the normal or anomalous dispersion region. Raman scattering and the wavelength dependence of the group delay of the fiber are found to play an important role in the process. We give an experimental and numerical analysis of the observed phenomena and find a good agreement between the two.
We demonstrate, for the first time to our knowledge, optical parametric oscillation based on four-wave mixing in microstructure fiber. The measured wavelength-tunability range of the device (40 nm) and the threshold-pump peak power (34.4 W) are in good agreement with the theory of four-wave mixing in optical fibers. The ellipticity of the fiber's polarization modes allows the device to be implemented in a relatively simple Fabry-Perot configuration. Spectral peaks that are due to cascaded-mixing processes are easily observed in our setup, which may provide a way to extend the tunability range of existing high-power lasers.
We describe an experiment in which a train of femtosecond pulses is coupled into a photonic crystal fiber (PCF) by means of an offset pumping technique that can selectively excite either the mode LP(01) or LP(11) or LP(21). The PCF presents a wide range of wavelengths in which the fundamental mode experiences normal dispersion, whereas LP(11) and LP(21) propagate in the anomalous dispersion regime, generating a supercontinuum based on the soliton fission mechanism. We find that the existence of a cut-off wavelength for the higher-order modes makes the spectral broadening asymmetrical. This latter effect is particularly dramatic in the case of the LP(21) mode, in which, by using a pump wavelength slightly below cut-off, the spectral broadening occurs only on the blue side of the pump wavelength. Our experimental results are successfully compared to numerical solutions of the nonlinear Schrödinger equation.
We report UV four-wave mixing in the LP<sub>02</sub> mode of a photonic crystal fiber when pumped by a frequency-doubled 532 nm microchip laser in the normal dispersion regime. A pure LP<sub>02</sub> mode was generated for the pump light by a broadband all-fiber mode converter. Ultraviolet signal wavelengths as short as 342 nm were generated.
We demonstrate an experimental study of the chromatic dispersion properties for a series of microstructured fibers (MSFs) dedicated for a supercontinuum generation. With white-light interferometry application we analyze experimentally how the small variations of structural parameters, i.e. an air-hole diameter and a lattice constant, influence dispersion characteristics in different groups of MSFs. Our study provides useful information on how to design the fiber which is less sensitive to the fabrication imperfections. Moreover those investigations are the initial step to the development of the customized or tunable supercontinuum light sources based on MSFs with slightly changed structural parameters which can generate light with a different spectrum range, adapted to a proper application.
Ultrawideband supercontinua have been generated using ultrashort pulses and zero-dispersion highly nonlinear fiber. However, they have inherent large noise and spectral fine structure. We generated a widely and flatly broadened, low-noise, highly coherent, high-quality supercontinuum and used it to demonstrate ultrahigh-resolution optical coherence tomography in several wavelength regions.
Three centuries after Newton's experiments on the decomposition of white light into its spectral components and the synthesis of white light from various colors, nonlinear-optical transformations of ultrashort laser pulses have made it possible to produce an artificial white light with unique spectral properties, controlled time duration, and a high spectral brightness. Owing to its broad and continuous spectrum, such radiation is called supercontinuum. The laser generation of white light is an interesting physical phenomenon and the relevant technology is gaining in practical implications — it offers novel solutions for optical communications and control of ultrashort laser pulses, helps to achieve an unprecedented precision in optical metrology, serves to probe the atmosphere of the Earth, and suggests new strategies for the creation of compact multiplex light sources for nonlinear spectroscopy, microscopy, and laser biomedicine. Here, we provide a review of physical mechanisms behind the laser generation of white light, examine its applications, and discuss the methods of generation of broadband radiation with controlled spectral, temporal, and phase parameters.
Lasers are increasingly important components within biological imaging and analysis systems. Applications range from flow cytometry to confocal and multi-photon microscopy. However, many commercial imaging systems are still limited in performance by the availability of suitable laser sources-limited in the availability of wavelengths and in the size, cost, power and reliability of conventional laser technologies. Ultrafast fibre lasers offer turnkey, compact and reliable solutions for next generation biomedical imaging systems.
In recent years optical fibers having a complex microstructure in the transverse plane have attracted much attention from both researchers and industry. Such fibers can either guide light through total internal reflection or the photonic bandgap effect. Among the many unique applications offered by these fibers are mode guidance in air, highly flexible dispersion engineering, and the use of very heterogeneous material combinations. In this paper, we review the different types and applications of microstructured optical fibers, with particular emphasis on recent advances in the field.
We have developed a new multimodal molecular imaging system that combines CARS (coherent anti-Stokes Raman scattering), SHG (second harmonic generation), THG (third harmonic generation) and multiplex TSFG (third-order sum frequency generation) using a subnanosecond white-light laser source. Molecular composition and their distribution in living cells are clearly visualized with different contrast enhancements through different mechanisms of CARS, SHG, THG and TSFG. A correlation image of CARS and TSF reveals that the TSF signal is generated predominantly from lipid droplets inside a cell as well as the peripheral cell wall.
We demonstrate how the gradient of the tapering in a tapered fiber can significantly affect the trapping and blueshift of dispersive waves (DWs) by a soliton. By modeling the propagation of a fundamental 10 fs soliton through tapered fibers with varying gradients, it is shown that the soliton traps and blueshifts an increased fraction of the energy in its DW when the gradient is decreased. This is quantified by the group-acceleration mismatch between the soliton and DW at the entrance of the taper. These findings have direct implications for the achievable power in the blue edge of a supercontinuum generated in a tapered fiber and explain observations of a lack of power in the blue edge.
We demonstrate quasi-supercontinuum (quasi-SC) generation around the 1.3 μm wavelength region using ultrahigh-speed, wavelength-tunable, femtosecond soliton pulses based on an ultrashort-pulse laser system operating at a wavelength of 1.0 μm. The wavelength tuning range was from 1.0 to 1.9 μm, and the scanning speed was up to 1.3 MHz. A Gaussian-like quasi-SC with a bandwidth of 220 nm was generated at 1220 nm. The generated quasi-SC was used in an optical-coherence tomography system. High axial resolutions of 5.1 μm in air and 3.7 μm in tissue were obtained. A maximum sensitivity of 100 dB was achieved, and ultrahigh-resolution images of a hamster cheek pouch were observed.
A new model of pump noise in supercontinuum and rogue wave generation is presented. Simulations are compared with experiments and show that the new model provides significantly better agreement than the currently ubiquitously used one-photon-per-mode model. The new model also allows for a study of the influence of the pump spectral line width on the spectral broadening mechanisms. Specifically, it is found that for four-wave mixing (FWM) a narrow spectral line width ( similar 0.1 nm) initially leads to a build-up of FWM from quantum noise, whereas a broad spectral line width (succeeds, similar 1 nm) initially leads to a gradual broadening of the pump spectrum. Since the new model provides better agreement with experiments and is still simple to implement, it is particularly important that it is used for future studies of the statistical properties of nonlinear spectral broadening, such as the formation of rogue waves.
Focus Serial: Frontiers of Nonlinear Optics
Optical fibers and waveguides provide unique and distinct environments for nonlinear optics, because of the combination of high intensities, long interaction lengths, and control of the propagation constants. They are also becoming of technological importance. The topic has a long history but continues to generate rapid development, most recently through the invention of the new forms of optical fiber collectively known as photonic crystal fibers. Some of the discoveries and ideas from the new fibers look set to have lasting influence in the broader field of guided-wave nonlinear optics. In this paper we introduce some of these ideas.
We report a 1 mum cw-pumped supercontinuum that extends short of the pump wavelength to 0.65 mum. This is achieved by using a 50 W Yb fiber laser in combination with a photonic crystal fiber with a carefully engineered zero-dispersion wavelength. We show that the short-wavelength generation is due to a combination of four-wave mixing and dispersive wave trapping by solitons. The evolution and limiting factors of the continuum are discussed.