Guided acoustic wave Brillouin scattering in photonic crystal fibers

Intramode Brillouin Scattering Properties of Single-Crystal Lithium Niobate Optical Fiber

Ordinary step-type fiber usually has only one obvious Brillouin scattering gain peak with a low gain coefficient, resulting in a poor sensing performance. As a promising material for nonlinear photonics, lithium niobate can significantly improve the Brillouin gain due to its higher refractive index when replaced with the core material. Furthermore, the higher-order acoustic modes make the Brillouin gain spectrum exhibit multiple scattering peaks, which could improve the performance of sensors. In this study, we simulated the Brillouin scattering properties of different modes of intramode in step-index lithium niobate core fibers. We analyzed the intramode-stimulated Brillouin scattering properties of different pump–Stokes pairs for nine LP modes (LP01, LP11, LP21, LP02, LP31, LP12, LP41, LP22, and LP03) guided in fiber. The results show that both the effective refractive index and Brillouin scattering frequency shift are decreased with the increase in the nine mode orders, and the values of which are 2.2413 to 2.1963, and 21.17 to 20.73 GHz, respectively. The typical back-stimulated Brillouin scattering gain is obtained at 1.7525 m−1·W−1. These simulation results prove that the Brillouin gain of the LiNbO3 optical fiber structure can be significantly improved, which will pave the way for better distributed Brillouin sensing and for improving the transmission capacity of communication systems.

Guided acoustic Brillouin scattering measurements in optical communication fibers

Guided acoustic Brillouin (GAWBS) noise is measured using a novel, homodyne measurement technique for four commonly used fibers in long-distance optical transmission systems. The measurements are made with single spans and then shown to be consistent with separate multi-span long-distance measurements. The inverse dependence of the GAWBS noise on the fiber effective area is confirmed by comparing different fibers with the effective area varying between 80 µm² and 150 µm². The line broadening effect of the coating is observed, and the correlation between the width of the GAWBS peaks to the acoustic mode profile is confirmed. An extensive model of the GAWBS noise in long-distance fibers is presented, including corrections to some commonly repeated mistakes in previous reports. It is established through the model and verified with the measurements that the depolarized scattering caused by TR_2m modes contributes twice as much to the optical noise in the orthogonal polarization to the original source, as it does to the noise in parallel polarization. Using this relationship, the polarized and depolarized contributions to the measured GAWBS noise is separated for the first time. As a result, a direct comparison between the theory and the measured GAWBS noise spectrum is shown for the first time with excellent agreement. It is confirmed that the total GAWBS noise can be calculated from fiber parameters under certain assumptions. It is predicted that the level of depolarized GAWBS noise created by the fiber may depend on the polarization diffusion length, and consequently, possible ways to reduce GAWBS noise are proposed. Using the developed theory, dependence of GAWBS noise on the location of the core is calculated to show that multi-core fibers would have a similar level of GAWBS noise no matter where their cores are positioned.

Chromatic dispersion dependence of GAWBS phase noise compensation with pilot tone

We describe experimental and numerical results regarding the influence of chromatic dispersion in optical fibers on guided acoustic-wave Brillouin scattering (GAWBS) phase noise compensation with a pilot tone (PT). We compared the compensation performance for GAWBS phase noise generated in an ultra-large-area fiber (ULAF) where DULAF = 21 ps/nm/km with that in a dispersion-shifted fiber (DSF) where DDSF = -1.3 ps/nm/km and found that the performance depends strongly on chromatic dispersion. The numerical analysis shows that the group delay between the signal and PT caused by chromatic dispersion reduces the GAWBS noise correlation between them, which degrades the compensation performance for GAWBS phase noise. It is clarified that a condition for effective GAWBS compensation is that the group delay between the signal and PT should be less than half the period of the GAWBS phase noise component.

Efficient interrogation method of forward Brillouin scattering in optical fibers using a narrow bandwidth long-period grating

A new technique, to the best of our knowledge, for the characterization of the effective refractive index modulation in optical fibers due to transverse acoustic mode resonances excited by electrostriction is reported. The resonances excited by an optical pulse are probed by a narrow bandwidth long-period grating (LPG) inscribed in the fiber, which is interrogated by a continuous wave (CW) beam. The LPG used in this experiment has a narrow bandwidth and high sensitivity to small mode index perturbations, allowing the measurement of index variations from below 10^-9 to 10^-6. Radial and torsional-radial acoustic modes were characterized up to 1.1 GHz. The linewidth of resonances was found to be much shorter than in previous reports in which long fiber lengths are typically required, obtaining Q factors as high as 5000.

Theoretical and experimental analyses of GAWBS phase noise in various optical fibers for digital coherent transmission

We describe the fiber structural dependence of guided acoustic-wave Brillouin scattering (GAWBS) phase noise in a digital coherent optical fiber transmission. We present theoretical and experimental analyses of GAWBS phase noise spectra in three types of optical fibers and show that the GAWBS resonant modes are distributed over a wider bandwidth as the effective core area of the fiber becomes smaller. We also use a vector signal analysis to show phase fluctuations caused by GAWBS. On the basis of these analyses, we show that the GAWBS phase noise fluctuation has a Gaussian distribution, which is used to evaluate its influence on the BER characteristics in a coherent QAM transmission. As a result, we found that the error-free transmission distance in SSMF is limited to 4600, 1200, and 340 km with 64, 256, and 1024 QAM, respectively, assuming a hard-decision FEC with a 7% overhead. These results provide useful insights into the influence of GAWBS on digital coherent transmission.

High Resolution Brillouin Sensing of Micro-Scale Structures

Brillouin distributed measurement techniques have been extensively developed for structural health monitoring using fibre optic nerve systems. The recent advancement in the spatial resolution capabilities of correlation-based Brillouin distributed technique have reached the sub-mm regime, making this approach a suitable candidate for monitoring and characterizing integrated photonic devices. The small dimension associated with the short length of these devices—on the order of the cm- and mm-scale—requires high sensitivity detection techniques and sub-mm spatial resolution. In this paper, we provide an overview of the different Brillouin sensing techniques in various micro-scale structures such as photonic crystal fibres, microfibres, and on-chip waveguides. We show how Brillouin sensing is capable of detecting fine transverse geometrical features with the sensitivity of a few nm and also extremely small longitudinal features on the order of a few hundreds of μ m . We focus on the technique of Brillouin optical correlation domain analysis (BOCDA), which enables such high spatial resolution for mapping the opto-acoustic responses of micro-scale waveguides.

Brillouin spectroscopy of a hybrid silicon-chalcogenide waveguide with geometrical variations

Recent advances in design and fabrication of photonic-phononic waveguides have enabled stimulated Brillouin scattering in silicon-based platforms such as underetched silicon waveguides and hybrid waveguides. Due to the sophisticated design and, more importantly, high sensitivity of the Brillouin resonances to geometrical variations in micro- and nano-scale structures, it is necessary to have access to the localized opto-acoustic response along those waveguides to monitor their uniformity and maximize their interaction strength. In this Letter, we design and fabricate photonic-phononic waveguides with a deliberate width variation on a hybrid silicon-chalcogenide photonic chip and confirm the effect of the geometrical variation on the localized Brillouin response using a distributed Brillouin measurement.

Optomechanical time-domain reflectometry

Optical fibres constitute an exceptional sensing platform. However, standard fibres present an inherent sensing challenge: they confine light to an inner core. Consequently, distributed fibre sensors are restricted to the measurement of conditions that prevail within the core. This work presents distributed analysis of media outside unmodified, standard fibre. Measurements are based on stimulated scattering by guided acoustic modes, which allow us to listen where we cannot look. The protocol overcomes a major difficulty: guided acoustic waves induce forward scattering, which cannot be mapped using time-of-flight. The solution relies on mapping the Rayleigh backscatter contributions of two optical tones, which are coupled by the acoustic wave. Analysis is demonstrated over 3 km of fibre with 100 m resolution. Measurements distinguish between air, ethanol and water outside the cladding, and between air and water outside polyimide-coated fibres. The results establish a new sensor configuration: optomechanical time-domain reflectometry, with several potential applications.

Surface Brillouin scattering in photonic crystal fibers

We report, to the best of our knowledge, the first experimental observation of surface Brillouin scattering in silica-based photonic crystal fibers, arising from the interaction between guided light and surface acoustic waves. This was achieved using small-core and high air-filling fraction microstructured fibers that enable a strong opto-acoustic coupling near the air holes while mitigating the acoustic leakages in the microstructured cladding. It is further shown that this new type of light scattering is highly sensitive to the fiber air-hole microstructure, thus providing a passive and efficient way to control it. Our observations are confirmed through numerical simulations of the elastodynamics equation.

Analysis of acousto-optic interaction based on forward stimulated Brillouin scattering in hybrid phononic-photonic waveguides

We present the generation of forward stimulated Brillouin scattering (FSBS) in hybrid phononic-photonic waveguides. To confine the optical and acoustic waves simultaneously, a hybrid waveguide is designed by embedding the silicon line defect in the silicon nitride phononic crystal slab. By taking into account three kinds hybrid waveguide, the appropriate structural parameters are obtained to enhance the acousto-optic interaction. We fabricate the honeycomb hybrid waveguide with a CMOS compatible technology. The forward Brillouin frequency shift is measured up to 2.425 GHz and the acoustic Q-factor of the corresponding acoustic mode is 1100. The influences of pump power, acoustic loss, nonlinear optical loss and lattice constant on the acousto-optic interaction in FSBS are analyzed and discussed. The proposed approach has important potential applications in on-chip all-optical signal processing.

Depolarized guided acoustic wave Brillouin scattering in hollow-core photonic crystal fibers

By performing quantum-noise-limited optical heterodyne detection, we observe polarization noise in light after propagation through a hollow-core photonic crystal fiber (PCF). We compare the noise spectrum to the one of a standard fiber and find an increase of noise even though the light is mainly transmitted in air in a hollow-core PCF. Combined with our simulation of the acoustic vibrational modes in the hollow-core PCF, we are offering an explanation for the polarization noise with a variation of guided acoustic wave Brillouin scattering (GAWBS). Here, instead of modulating the strain in the fiber core as in a solid core fiber, the acoustic vibrations in hollow-core PCF influence the effective refractive index by modulating the geometry of the photonic crystal structure. This induces polarization noise in the light guided by the photonic crystal structure.

Brillouin light scattering from surface acoustic waves in a subwavelength-diameter optical fibre

Brillouin scattering in optical fibres is a fundamental interaction between light and sound with important implications ranging from optical sensors to slow and fast light. In usual optical fibres, light both excites and feels shear and longitudinal bulk elastic waves, giving rise to forward-guided acoustic wave Brillouin scattering and backward-stimulated Brillouin scattering. In a subwavelength-diameter optical fibre, the situation changes dramatically, as we here report with the first experimental observation of Brillouin light scattering from surface acoustic waves. These Rayleigh-type surface waves travel the wire surface at a specific velocity of 3,400 m s⁻¹ and backscatter the light with a Doppler shift of about 6 GHz. As these acoustic resonances are sensitive to surface defects or features, surface acoustic wave Brillouin scattering opens new opportunities for various sensing applications, but also in other domains such as microwave photonics and nonlinear plasmonics.

In optical fibres, stimulated Brillouin scattering is a fundamental interaction where light generates bulk elastic waves and it is backward scattered by them. Here, Beugnot et al. demonstrate the generation of backward-propagating surface acoustic wave Brillouin scattering in subwavelength-diameter optical fibres.

Analysis of enhanced stimulated Brillouin scattering in silicon slot waveguides

Stimulated Brillouin scattering has attracted renewed interest with the promise of highly tailorable integration into the silicon photonics platform. However, significant Brillouin amplification in silicon waveguides has yet to be shown. In an effort to engineer a structure with large photon-phonon coupling, we analyzed both forward and backward Brillouin scattering in high-index-contrast silicon slot waveguides. The calculations predict that gradient forces enhance the Brillouin gain in narrow slots. We estimate a currently feasible gain of about 10(5) W(-1) m(-1) in horizontal slot waveguides, which is an order of magnitude larger than in a stand-alone silicon wire. Such efficient coupling could enable a host of Brillouin technologies on a mass-producible silicon chip.

Observation of acoustically induced modulation instability in a Brillouin photonic crystal fiber laser

We report the experimental observation of self-induced modulation instability (MI) in a Brillouin fiber laser made with a solid-core photonic crystal fiber (PCF) with strong anomalous dispersion. We identify this MI as the result of parametric amplification of optical sidebands generated by guided acoustic modes within the core of the PCF. It is further shown that MI leads to passive harmonic mode locking and to the generation of a picosecond pulse train at a repetition rate of 1.15 GHz which matches the acoustic frequency of the fundamental acoustic mode of the PCF.

Temperature coefficient of the high-frequency guided acoustic mode in a photonic crystal fiber

High-frequency guided acoustic Brillouin modes have recently been observed in small-core silica photonic crystal fibers. In this paper, we investigate the temperature dependence of the optical sideband frequency generated by one of these guided acoustic waves. The experimental results show a temperature coefficient of 100 kHz/°C at an acoustic resonance frequency of 1.15 GHz and are in very good agreement with the theoretical predictions. This coefficient demonstrates a temperature sensitivity 10 times larger than that previously reported in conventional single-mode fibers, which is promising in view of potential applications to optical fiber sensors.

Frequency-selective excitation of guided acoustic modes in a photonic crystal fiber

We present experimental and numerical results demonstrating the simultaneous frequency-selective excitation of several guided acoustic Brillouin modes in a photonic crystal fiber with a multi-scale structure design. These guided acoustic modes are identified by using a full vector finite-element model to result from elastic radial vibrations confined by the wavelength-scale air-silica microstructure. We further show the strong impact of structural irregularities of the fiber on the frequency and modal shape of these acoustic resonances.

FSBS resonances observed in a standard highly nonlinear fiber

Forward stimulated Brillouin scattering (FSBS) is observed in a standard 2-km-long highly nonlinear fiber. The frequency of FSBS arising from multiple radially guided acoustic resonances is observed up to gigahertz frequencies. The tight confinement of the light and acoustic field enhances the interaction and results in a large gain coefficient of 34.7 W(-1) at a frequency of 933.8 MHz. We also find that the profile on the anti-Stokes side of the pump beam have lineshapes that are asymmetric, which we show is due to the interference between FSBS and the optical Kerr effect. The measured FSBS resonance linewidths are found to increase linearly with the acoustic frequency. Based on this scaling, we conclude that dominant contribution to the linewidth is from surface damping due to the fiber jacket and structural nonuniformities along the fiber.

Vector Brillouin optical time-domain analyzer for high-order acoustic modes

Thanks to a double-frequency phase modulation scheme, we report a vector Brillouin optical time-domain analyzer (BOTDA). This BOTDA has a high immunity level to noise, and it features a phase spectrogram capability. It is well suited for complex situations involving several acoustic resonances, such as high-order longitudinal modes. It has notably been used to characterize a dispersion-shifted fiber, allowing us to report spectrograms with multiple acoustic resonances. A very high 57 dB dynamic range is also reported for 100-ns-long pulses simultaneously with a 16 cm numerical resolution.

An efficient method of all-optical buffering with ultra-small core photonic crystal fibers

We propose a new method of all-optical buffering with ultra-small core photonic crystal fibers (PCFs) based on stimulated Brillouin scattering. The large refractive index contrast between the near-hollow cladding and the pure-silica core contributes to high nonlinearity of the PCF. Considering the unusual gain spectrum of the ultra-small core PCF, we numerically investigate the influence of these factors to the buffering efficiency. A PCF with a length of 1 meter and a core diameter of 1.06 micrometer is simulated as the storage medium in this paper. It is shown that we can obtain a good buffering efficiency under a very low control power of 2 watt, which promises a significant improvement for the all-optical communication system.

Complete experimental characterization of stimulated Brillouin scattering in photonic crystal fiber

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.

Structural dependence of guided acoustic-wave Brillouin scattering spectra in hole-assisted fiber and its temperature dependence

We describe the guided acoustic-wave Brillouin scattering (GAWBS) characteristics of hole-assisted fiber (HAF). We clarify numerically and experimentally that the GAWBS spectrum corresponding to a particular acoustic mode is observed for HAF and that the efficiency for that mode can be controlled simply by designing the air-hole size and position. We also reveal the temperature dependence of the GAWBS characteristics in HAF.