Showing papers on "Brillouin scattering published in 2007"
TL;DR: In this article, the authors describe a method for storing sequences of optical data pulses by converting them into long-lived acoustic excitations in an optical fiber through the process of stimulated Brillouin scattering.
Abstract: We describe a method for storing sequences of optical data pulses by converting them into long-lived acoustic excitations in an optical fiber through the process of stimulated Brillouin scattering. These stored pulses can be retrieved later, after a time interval limited by the lifetime of the acoustic excitation. In the experiment reported here, smooth 2-nanosecond-long pulses are stored for up to 12 nanoseconds with good readout efficiency: 29% at 4-nanosecond storage time and 2% at 12 nanoseconds. This method thus can potentially store data packets that are many bits long. It can be implemented at any wavelength where the fiber is transparent and can be incorporated into existing telecommunication networks because it operates using only commercially available components at room temperature.
333 citations
TL;DR: In this article, continuous-wave single-frequency master- oscillator power-amplifier (MOPA) sources based on ytterbium-doped fibers (YDFs) with particular attention to their recent advances and their experimental results were discussed.
Abstract: We discuss continuous-wave single-frequency master- oscillator power-amplifier (MOPA) sources based on ytterbium- doped fibers (YDFs) with particular attention to their recent advances and our experimental results. This includes a 402-W plane-polarized MOPA source and a 511-W random-polarized MOPA source. In these MOPAs, the final-stage high-power amplifier operates with high efficiency of 70%-80%, and a high gain of over 20 dB in a near diffraction-limited beam. We report at least 7-dB enhancement of the stimulated Brillouin scattering (SBS) threshold for the 402-W polarization-maintaining (PM) YDF. In fact, we did not see any sign of SBS even at the highest output power. We eventually observed SBS appearance at around 500 W for the non-PM YDF. The observed SBS strengths were far weaker than expected in theory, unless we allowed for the Brillouin gain reduction from thermal Brillouin gain broadening induced by the quantum-defect heating.
243 citations
01 Jan 2007
TL;DR: In this article, the authors investigate slow light via stimulated Brillouin scattering (SBS) in a room temperature optical fiber that is pumped by a spectrally broadened laser and find that partial overlap of the Stokes and anti-Stokes resonances can actually lead to an enhancement of the slow light delay-bandwidth product when Deltaomegapsime 1.3OmegaB.
Abstract: In this paper, we investigate slow light via stimulated Brillouin scattering (SBS) in a room temperature optical fiber that is pumped by a spectrally broadened laser. Broadening the spectrum of the pump field increases the linewidth Deltaomegap of the Stokes amplifying resonance, thereby increasing the slow-light bandwidth. One physical bandwidth limitation occurs when the linewidth becomes several times larger than the Brillouin frequency shift OmegaB so that the anti-Stokes absorbing resonance substantially cancels out the Stokes amplifying resonance and, hence, the slow-light effect. We find that partial overlap of the Stokes and anti-Stokes resonances can actually lead to an enhancement of the slow-light delay-bandwidth product when Deltaomegapsime1.3OmegaB. Using this general approach, we increase the Brillouin slow-light bandwidth to over 12 GHz from its nominal linewidth of ~30 MHz obtained for monochromatic pumping. We controllably delay 75-ps-long pulses by up to 47 ps and study the data-pattern dependence of the broadband SBS slow-light system
183 citations
TL;DR: A high Q-factor photonic microwave filter showing tuning and reshaping capabilities and based on stimulated Brillouin scattering is demonstrated, demonstrating the wide tuning range of the filter, its reshaping capability, and Q factor of 670.
Abstract: A high Q-factor photonic microwave filter showing tuning and reshaping capabilities and based on stimulated Brillouin scattering is demonstrated. The filter bandpass can be continuously tuned, changing the microwave oscillator used to generate the pump power, and the filter shape can be modified by modulating the microwave tone. A single bandpass over the microwave spectrum can be obtained by using single-sideband suppressed carrier modulation. Experimental results demonstrate the wide tuning range of the filter, its reshaping capability, and Q factor of 670.
145 citations
TL;DR: In this paper, the authors presented a BOTDA system based on dark-pulse scattering that provides improved resolution, accuracy, and acquisition time over conventional BOTSDA systems, without the severe limitations on sensing length often imposed by other high-resolution techniques.
Abstract: Brillouin scattering-based distributed fiber-optic sensing is a powerful measurement tool that uses the inelastic scattering of incident light by an acoustic wave (phonon) to determine strain and/or temperature conditions of the fiber. Since the original Brillouin-time-domain-analysis (BOTDA) technique was proposed, several other analysis methods have been introduced to improve sensing performance in four key areas: spatial resolution; measurement accuracy; total sensing length; and measurement-acquisition time. The four factors are generally interrelated and improvements to one factor often come at the cost of one or more of the others. For example, one system might sacrifice spatial resolution for total sensing length, while another might sacrifice accuracy to gain acquisition speed. We present a BOTDA system based on dark-pulse scattering that provides improved resolution, accuracy, and acquisition time over conventional BOTDA systems, without the severe limitations on sensing length often imposed by other high-resolution techniques. Theoretical validation of the method is given, and experimental results are presented that demonstrate 20-mm resolution strain measurements with an accuracy of plusmn20 muepsiv, which is the highest spatial resolution yet reported for a BOTDA system
136 citations
TL;DR: Broadband slow light is demonstrated by using stimulated Brillouin scattering in optical fibers based on a double BrillouIn pump, the peaks of which are spectrally separated by twice the Brillou in frequency.
Abstract: Broadband slow light is demonstrated by using stimulated Brillouin scattering in optical fibers based on a double Brillouin pump, the peaks of which are spectrally separated by twice the Brillouin frequency The loss spectrum generated by one of the pump waves is fully compensated by the gain spectrum of the other one, which permits the enlargement of the bandwidth to 25 GHz and a variable time delay of up to 109 ps with 37 ps pulses
132 citations
TL;DR: Simulations demonstrate the potential for a fiber designed to suppress stimulated Brillouin scattering through a reduction in the overlap of the optical and acoustic fields to achieve greater than 1000 Watts of output power.
Abstract: High power operation of narrow linewidth optical fiber amplifiers is usually limited by the onset of stimulated Brillouin scattering. In this paper, we present results demonstrating over 500 Watts of power in a single mode beam from a fiber designed to suppress stimulated Brillouin scattering through a reduction in the overlap of the optical and acoustic fields. Simulations demonstrate the potential for this fiber to achieve greater than 1000 Watts of output power.
126 citations
TL;DR: In this article, the authors showed that the elastic wave velocities for a nearadiabatic lower mantle with a bulk composition dominated by magnesium silicate perovskite are consistent with the average lower mantle seismic velocity structure.
101 citations
TL;DR: This work provides a complete experimental characterization of stimulated Brillouin scattering in a 160 m long solid-core photonic crystal fiber, including threshold and spectrum measurements as well as position-resolved mapping of the BrillouIn frequency shift.
Abstract: We provide a complete experimental characterization of stimulated Brillouin scattering in a 160 m long solid-core photonic crystal fiber, including threshold and spectrum measurements as well as position-resolved mapping of the Brillouin frequency shift. In particular, a three-fold increase of the Brillouin threshold power is observed, in excellent agreement with the spectrally-broadened Brillouin gain spectrum. Distributed measurements additionally reveal that the rise of the Brillouin threshold results from the broadband nature of the gain spectrum all along the fiber and is strongly influenced by strain. Our experiments confirm that these unique fibers can be exploited for the passive control or the suppression of Brillouin scattering.
95 citations
TL;DR: In this article, the authors reported the highest speed distributed sensing of dynamic strain based on stimulated Brillouin scattering in optical fibers, achieving a sampling rate of 1 kHz, more than an order of magnitude higher than the former best result.
Abstract: We report the highest speed distributed sensing of dynamic strain based on stimulated Brillouin scattering in optical fibers. A sampling rate of 1 kHz, more than an order of magnitude higher than the former best result, is achieved by applying a simplified Brillouin optical correlation domain analysis with optimized time gates and an unbalanced Mach-Zehnder delay line. In experiments, we present the measurement of various dynamic strains at the maximum frequency of 200 Hz with 10-cm spatial resolution and 20-m measurement range.
94 citations
TL;DR: In this paper, the authors used the 246 Landau potential with saturation terms to describe the elastic anomalies associated with octahedral tilting transitions in perovskites that are associated with the M and R points of the Brillouin zone.
Abstract: Landau theory has been used to develop expressions for the elastic anomalies that accompany octahedral tilting transitions in perovskites that are associated with the M and R points of the Brillouin zone. The master equation is a 246 Landau potential with saturation terms that provides phenomenological descriptions of transition sequences from a parent cubic structure through tetragonal or rhombohedral intermediates to orthorhombic or monoclinic product structures. Data from the literature have been used to determine values for all the coefficients required to generate a quantitative description of the Pm 3 m ↔ I 4/ mcm transition in SrTiO 3 , which is taken as a model system. Solutions to the Landau expansion have been adapted to include the general influence of hydrostatic pressure and non-hydrostatic stress on transition temperature and the evolution of the order parameter. Critical examination of elastic constant data from the literature reveals inconsistencies between the results of measurements on tetragonal samples using ultrasonic rather than Brillouin scattering methods. An internally consistent data set has, nevertheless, been assembled. Good qualitative agreement was obtained between the general pattern of calculated and observed variations of all the single crystal elastic constants, and semi-quantitative agreement was obtained for C 11 , C 33 , C 12 , and C 13 . Some inconsistencies remain in relation to the temperature dependence of the square of the soft mode frequencies in the tetragonal phase, which follow the square of the order parameter rather than its inverse susceptibility, but the 246 potential seems to provide a good description of the structural evolution of SrTiO 3 over a wide temperature interval up to the cubic-tetragonal transition point.
TL;DR: The results of the numerical simulations reveal that this high-frequency phonon is indeed trapped within the fiber core by the air-hole microstructure, in good agreement with experimental measurements.
Abstract: We experimentally investigate guided acoustic wave Brillouin scattering in several photonic crystal fibers by use of the so-called fiber loop mirror technique and show a completely different dynamics with respect to standard all-silica fibers. In addition to the suppression of most acoustic phonons, we show that forward Brillouin scattering in photonic crystal fibers is substantially enhanced only for the fundamental acoustic phonon because of efficient transverse acousto-optic field overlap. The results of our numerical simulations reveal that this high-frequency phonon is indeed trapped within the fiber core by the air-hole microstructure, in good agreement with experimental measurements.
TL;DR: In this paper, the authors describe stimulated Brillouin scattering and stimulated Raman scattering processes in optical fiber and their thresholds, reduction in power penalty and applications along with comparative study of these effects are also presented.
Abstract: The nonlinear scattering effects in optical fiber occur due to inelastic-scattering of a photon to a lower energy photon. This paper describes stimulated Brillouin scattering and stimulated Raman scattering processes. Their thresholds, reduction in power penalty and applications along with comparative study of these effects are also presented.
TL;DR: In this paper, a double-pulse optical time-domain reflectometry (DP-BOTDR) system is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution.
Abstract: A novel Brillouin optical time-domain reflectometry (BOTDR) system, called a double-pulse BOTDR (DP-BOTDR) system, is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution. Our experiment confirmed that the DP-BOTDR system enables us to measure the distributed Brillouin frequency shift, i.e., the distributed strain and temperature, with a spatial resolution of 20 cm. This spatial resolution is five times better than that provided by the conventional single-pulse BOTDR system.
TL;DR: This work investigates the amplitude flatness of Rayleigh-assisted Brillouin-Raman comb laser in a linear cavity in which feedbacks are formed by high-reflectivity mirror and obtains an average signal-to-noise ratio of 17 dB.
Abstract: We investigate the amplitude flatness of Rayleigh-assisted Brillouin-Raman comb laser in a linear cavity in which feedbacks are formed by high-reflectivity mirror. The optimization of Brillouin pump power and wavelength is very crucial in order to obtain a uniform power level between Stokes lines. The Brillouin pump must have a relatively large power and its wavelength must be located closer to the Raman peak gain region. The flat-amplitude bandwidth is also determined by the choice of Raman pump wavelengths. A flat-amplitude bandwidth of 30.7 nm from 1527.32 to 1558.02 nm is measured when Raman pump wavelengths are set to 1435 and 1450 nm. 357 uniform Brillouin Stokes lines with 0.086 nm spacing are generated across the wavelength range. The average signal-to-noise ratio of 17 dB is obtained for all the Brillouin Stokes lines.
TL;DR: Wave front bowing of electron-plasma waves (ion-acoustic waves) from trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time and self-focusing from trapped particles modulational instability (TPMI) is shown to occur in 2D and 3D SRS simulations.
Abstract: Stimulated Raman (SRS) and Brillouin scattering (SBS) are examined in the kinetic regime using particle-in-cell simulations. Wave front bowing of electron-plasma waves (ion-acoustic waves) from trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time. Self-focusing from trapped particle modulational instability (TPMI) is shown to occur in 2D and 3D SRS simulations. The key physics of SRS saturation is identified as a combination of wave front bowing, TPMI, and self-focusing: Bowing marks the beginning of SRS saturation and self-focusing terminates SRS. Ion-acoustic wave bowing also contributes to SBS saturation. Velocity diffusion by transverse modes and rapid loss of hot electrons in regions of small transverse extent formed from self-focusing dissipate wave energy and increase Landau damping, despite trapping that reduces Landau damping initially.
TL;DR: In this article, flat-top sharp optical filters of gigahertz bandwidth are realized using stimulated Brillouin scattering (SBS) to convert double sideband to single sideband (SSB) modulations for 1-GHz wide linear-frequency-modulated (LFM) signals of arbitrary radio-frequency carrier.
Abstract: Flat-top sharp optical filters of gigahertz bandwidth are realized using stimulated Brillouin scattering (SBS). Pump chirp control of the SBS process enables versatile programming of the filter shape and bandwidth. The operating wavelength of the filters is tunable, and their frequency response is inherently aperiodic. Full widths at half maximum of 1.3-2.5 GHz are demonstrated, with a filtering selectivity up to 30 dB and an rms ripple of 0.5-1 dB. The filters are used to convert double sideband to single sideband (SSB) modulations for 1-GHz-wide linear-frequency-modulated (LFM) signals of arbitrary radio-frequency carrier. Such SSB modulation is highly instrumental for photonic implementations of true time delay, for example, in antenna beam-forming. The peak-side-lobe ratio of the processed LFM signal was -32 dB; its main lobe was broadened by only 4%. The integrated side-lobe ratio, which is limited by noise from spontaneous Brillouin scattering, was better than 21 dB, which is a sufficient value for most systems. The technique results in a large modulation index and does not lead to harmonic distortions.
TL;DR: In this article, a multiple-wavelength Brillouin fiber laser (BFL) was demonstrated using an injection of intense light at 1480 nm, where the light generated Raman gain at around 1560 nm to amplify the backscattered light in the medium and allowed the generation of multiple lines by selffeedback-seeding effects.
Abstract: A multiple wavelength Brillouin fiber laser (BFL) is demonstrated using an injection of intense light at 1480 nm. The light generates Raman gain at around 1560 nm to amplify the Brillouin-Rayleigh backscattered light in the medium and allows the generation of multiple lines by self-feedback-seeding effects. Three Stokes are obtained in the BFL with the maximum power of the first Stokes at approximately -6 dBm from the injection of 1480 nm light at 150 mW. The line spacing is measured to be approximately 0.08 nm. There is no significant change in the power of the first Stokes with the injection of 1480 nm light; however the powers of additional Stokes are highest at Brillouin pump of 1560 nm.
TL;DR: In this paper, a method to achieve flat-top gain spectrum through overlapping multiple gain lines, which can be used to increase slow light bandwidth and relative pulse delay, is presented, where a phase modulator is used to modulate the phase of the pump wave.
Abstract: We present a method to achieve flat-top gain spectrum through overlapping multiple gain lines, which can be used to increase slow light bandwidth and relative pulse delay. A tunable gain bandwidth can be realized by changing the number of spectral lines and frequency separation between adjacent spectral lines. We demonstrate the method in a SBS-based slow light system. A phase modulator is used to modulate the phase of the pump wave, generating a pump wave with multi-line spectrum and achieving a Brillouin gain bandwidth of ~ 330 MHz.
TL;DR: In this paper, a model connecting the observed phenomenon with an interference of different spin-wave modes existing in the stripe due to the finite-size effect is proposed, and the spatial profile of the spinwave beam demonstrates a focusing at a certain distance from the excitation source depending on the stripe width.
Abstract: Excitation and propagation of spin waves in Permalloy microstripes magnetized in their plane perpendicularly to the axis have been investigated by means of microfocus Brillouin light scattering spectroscopy with high spatial resolution. We show that the spatial profile of the spin-wave beam demonstrates a focusing at a certain distance from the excitation source depending on the stripe width. A model connecting the observed phenomenon with an interference of different spin-wave modes existing in the stripe due to the finite-size effect is proposed.
TL;DR: A novel technique that enables coherent detection of spontaneous Brillouin scattering in radio-frequency region has been demonstrated for distributed measurements of temperature and strain in long fiber by using a CW single-frequency BrillouIn fiber laser.
Abstract: A novel technique that enables coherent detection of spontaneous Brillouin scattering in the radio-frequency (<500 MHz) region with excellent long-term stability has been demonstrated for distributed measurements of temperature and strain in long fiber. An actively stabilized single-frequency Brillouin fiber laser with extremely low phase noise and intensity noise is used as a well-defined, frequency-shifted local oscillator for the heterodyne detection, yielding measurements of spontaneous Brillouin scattering with high frequency stability. Based on this approach, a highly stable real-time fiber sensor for distributed measurements of both temperature and strain over long fiber has been developed utilizing advanced digital signal processing techniques.
TL;DR: Results of the calculations are in reasonably good correspondence with proper measurements in silica fibres, which have found in the literature and have done ourselves in fibre-samples without and with cavity feedback.
Abstract: We show that in optical fiber the threshold exponential gain, Gth, for stimulated Brillouin scattering initiated by spontaneous Brillouin scattering is functionally and strongly dependent on the material, length and numerical aperture of the fiber and the pump wavelength. For silica fiber we show that the value of Gth at λ≅1 µm ranges from as low as ~5 in long fiber (≥ few kms) to ~10–12 in fibers of ~100 m length and ~20–23 for very short fibers (<10 cm).
TL;DR: It is concluded that optical storage times in excess of one second could be achieved in millimeter sized cavities and theoretically derive Raman gain of CaF(2).
Abstract: The temperature dependence of the processes which fundamentally limit optical quality factor of ideal crystalline whispering gallery mode resonators is investigated. The example of CaF(2) is used to show that spontaneous Raman scattering is the main limitation of the quality factor at low temperatures. Stimulated Raman scattering is also shown to be important at any temperature. We experimentally demonstrate nonlinear absorption due to stimulated Raman scattering in a real cavity at room temperature and theoretically derive Raman gain of CaF(2). We conclude that optical storage times in excess of one second could be achieved in millimeter sized cavities.
TL;DR: In this paper, the effects of the intensity modulation (IM) of light source on fiber-distributed sensors based on Brillouin optical correlation domain analysis (BOCDA) are analyzed by numerical simulation, and the results are experimentally confirmed.
Abstract: The effects of the intensity modulation (IM) of light source on fiber-distributed sensors based on Brillouin optical correlation domain analysis (BOCDA) are analyzed by numerical simulation, and the results are experimentally confirmed. We show that the shape of the Brillouin gain spectrum in the BOCDA system has a particular dependence on the optical spectrum of light source and that it can be controlled and tailored by proper modification of the optical spectrum using an additional IM. In the experiments, we applied several IM waveforms based on the simulation results for confirmation. Additionally, a distributed strain measurement along a 1-km optical fiber with a 30-cm spatial resolution is demonstrated by applying a proper IM scheme, which is the longest range reported using the BOCDA system
TL;DR: The acoustic properties of three polymer elastomers have been measured from ambient pressure to approximately 12 GPa by using Brillouin scattering in high-pressure diamond anvil cells using P-V isotherms to extract the isothermal bulk modulus and its pressure derivative for each material.
Abstract: The acoustic properties of three polymer elastomers, a cross-linked poly(dimethylsiloxane) (Sylgard® 184), a cross-linked terpolymer poly(ethylene-vinyl acetate-vinyl alcohol), and a segmented thermoplastic poly(ester urethane) copolymer (Estane® 5703), have been measured from ambient pressure to approximately 12GPa by using Brillouin scattering in high-pressure diamond anvil cells. The Brillouin-scattering technique is a powerful tool for aiding in the determination of equations of state for a variety of materials, but to date has not been applied to polymers at pressures exceeding a few kilobars. For the three elastomers, both transverse and longitudinal acoustic modes were observed, though the transverse modes were observed only at elevated pressures (>0.7GPa) in all cases. From the Brillouin frequency shifts, longitudinal and transverse sound speeds were calculated, as were the C11 and C12 elastic constants, bulk, shear, and Young’s moduli, and Poisson’s ratios, and their respective pressure dependenc...
TL;DR: In this article, a comprehensive observation of the stimulated Brillouin scattering (SBS) phenomena in a high power nanosecond linearly polarized Yb-doped double-clad fiber amplifier is presented.
Abstract: A comprehensive observation of the stimulated Brillouin scattering (SBS) phenomena in a high power nanosecond linearly polarized Yb-doped double-clad fiber amplifier is presented. According to the behavior of SBS, the amplification process is divided into four regions. All the temporal, spectral and power characteristics of SBS in both forward and backward directions in each region are described in detail. Cascaded SBS with up to 30 orders of Stokes waves and 10 orders of anti-Stokes waves is observed. The fiber damage caused by the giant SBS pulses is reported. And the influence of the linewidth, pulse duration, feedback and polarization on SBS is discussed.
TL;DR: In this paper, a novel scheme to implement multitap tunable incoherent microwave photonic filters with all-optical complex coefficients based on Brillouin signal processing of optical single-sideband signals was proposed.
Abstract: We propose a novel scheme to implement multitap tunable incoherent microwave photonic filters with all-optical complex coefficients based on Brillouin signal processing of optical single-sideband signals. A six-tap filter is experimentally demonstrated to highlight the enhanced flexibility in the synthesis of frequency responses provided by complex coefficients, particularly in terms of tuning.
TL;DR: In this paper, the laser-induced damage threshold (LIDT) in different multimode glass fibers with a core diameter of 200 μm was determined, and an antireflective coatings for fiber end-faces with high LIDT was developed to enhance the SBS reflectivity and reduce unwanted front-face reflections.
Abstract: Stimulated Brillouin scattering (SBS) and the laser-induced damage threshold (LIDT) in different multimode glass fibers with a core diameter of 200 μm are determined Antireflective coatings for fiber end-faces with high LIDT are developed, which enhance the SBS reflectivity and reduce unwanted front-face reflections The morphology of pump- and Stokes-radiation induced damages is investigated using atomic force microscopy
TL;DR: In this paper, a multiwavelength linear-cavity Brillouin-Erbium fiber laser pumped by a 1480 nm pump laser is demonstrated to operate in the L-band region.
Abstract: In this paper, a multiwavelength linear-cavity Brillouin-Erbium fiber laser pumped by a 1480 nm pump laser is demonstrated to operate in the L-band region. The issue of low gain efficiency of L-band in Erbium-doped fiber pumped with 980 nm pump lasers is resolved with this pumping scheme. The amount of 1480 nm pump power to produce the first Brillouin Stokes line oscillating in the cavity is 23.4 mW only. The proposed laser configuration has a wide tuning range of 16 nm from 1591 to 1607 nm with six Brillouin Stokes lines within this tuning range. The maximum number of 23 Brillouin Stokes lines with a spacing of 0.089 nm is achieved by setting the Brillouin pump wavelength at 1595.5 nm and its power is set at 3.2 mW.
26 Mar 2007
TL;DR: In this paper, the authors report the generation of slow light using Brillouin amplification in a short length of highly nonlinear bismuth-oxide fiber, and demonstrate a five-fold reduction in group velocity for ~200-ns pulses, which they believe to be a record for a slow light propagation in an optical fiber.
Abstract: We report the generation of slow light using Brillouin amplification in a short length of highly nonlinear bismuth-oxide fiber. By using just 2 m of fiber, we demonstrate a five-fold reduction in group velocity for ~200-ns pulses, which we believe to be a record for a slow-light propagation in an optical fiber. Moreover, by virtue of the high nonlinearity per unit length of this fiber, we achieve this at a very modest pump power level of just ~400 mW and with a low inherent device latency of 14 ns. These results highlight both the merits and practicality of using high nonlinearity nonsilica fibers for slow-light devices