Showing papers on "Brillouin scattering published in 2016"

Cavity Optomagnonics with Spin-Orbit Coupled Photons.

Abstract: We experimentally implement a system of cavity optomagnonics, where a sphere of ferromagnetic material supports whispering gallery modes (WGMs) for photons and the magnetostatic mode for magnons. We observe pronounced nonreciprocity and asymmetry in the sideband signals generated by the magnon-induced Brillouin scattering of light. The spin-orbit coupled nature of the WGM photons, their geometrical birefringence, and the time-reversal symmetry breaking in the magnon dynamics impose the angular-momentum selection rules in the scattering process and account for the observed phenomena. The unique features of the system may find interesting applications at the crossroad between quantum optics and spintronics.

Optomagnonic Whispering Gallery Microresonators.

Abstract: Magnons in ferrimagnetic insulators such as yttrium iron garnet (YIG) have recently emerged as promising candidates for coherent information processing in microwave circuits. Here we demonstrate optical whispering gallery modes of a YIG sphere interrogated by a silicon nitride photonic waveguide, with quality factors approaching 10^{6} in the telecom c band after surface treatments. Moreover, in contrast to conventional Faraday setups, this implement allows an input photon polarized colinearly to the magnetization to be scattered to a sideband mode of orthogonal polarization. This Brillouin scattering process is enhanced through triply resonant magnon, pump, and signal photon modes within an "optomagnonic cavity." Our results show the potential use of magnons for mediating microwave-to-optical carrier conversion.

[INVITED] State of the art of Brillouin fiber-optic distributed sensing

Abstract: Fiber-optic distributed sensing, employing the Brillouin effect, is already a commercially available measurement technique for the accurate estimation of the static strain/temperature fields along tens of kilometers with a spatial resolution of the order of a meter. Furthermore, relentless research efforts are paving the way to even much wider usability of the technique through recently achieved enhanced performance in each of its critical dimensions: measurement range has been extended to hundreds of kilometers; spatial resolution is of the order of a centimeter or less, signal to noise ratio has been significantly improved; fast dynamic events can be captured at kHz’s sampling rates; and a much better understanding of the underlying physics has been obtained, along with the formulation of figures of merit, and the preparation and early adoption of appropriate standards and guidelines. This paper describes the basics, as well as the state of the art, of the leading Brillouin interrogation methods, with emphasis on the significant progress made in the last 3 years. It also includes a short introduction to coding, which has proven instrumental in many of the recently obtained performance records.

Contributed Review: Distributed optical fibre dynamic strain sensing.

Abstract: Extensive research on Brillouin- and Raman-based distributed optical fibre sensors over the past two decades has resulted in the commercialization of distributed sensors capable of measuring static and quasi-static phenomena such as temperature and strain. Recently, the focus has been shifted towards developing distributed sensors for measurement of dynamic phenomena such as dynamic strain and sound waves. This article reviews the current state of the art distributed optical fibre sensors capable of quantifying dynamic vibrations. The most important aspect of Rayleigh and Brillouin scattering processes which have been used for distributed dynamic measurement are studied. The principle of the sensing techniques used to measure dynamic perturbations are analyzed followed by a case study of the most recent advances in this field. It is shown that the Rayleigh-based sensors have longer sensing range and higher frequency range, but their spatial resolution is limited to 1 m. On the other hand, the Brillouin-based sensors have shown a higher spatial resolution, but relatively lower frequency and sensing ranges.

Ultrahigh-speed distributed Brillouin reflectometry

Abstract: Optical fibre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on account of their capacity for distributed strain and temperature measurements. However, real-time distributed strain measurement has been achieved only for two-end-access systems; such systems reduce the degree of freedom in embedding the sensors into structures, and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at a point along the sensing fibre. Here, we demonstrate real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique. In this method, the Brillouin gain spectrum is obtained at high speed using a voltage-controlled oscillator, and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform; the phase delay is subsequently converted into a voltage, which can be directly measured. When a single-point measurement is performed at an arbitrary position, a strain sampling rate of up to 100 kHz is experimentally verified by detecting locally applied dynamic strain at 1 kHz. When distributed measurements are performed at 100 points with 10 times averaging, a repetition rate of 100 Hz is verified by tracking a mechanical wave propagating along the fibre. Some drawbacks of this ultrahigh-speed configuration, including the reduced measurement accuracy, lowered spatial resolution and limited strain dynamic range, are also discussed. An optical fibre sensing scheme that measures strain with a high spatial resolution and a very high sampling rate has been developed. Optical fibre sensors based on Brillouin scattering are promising for monitoring structural health. The system built by Yosuke Mizuno of Tokyo Institute of Technology and colleagues measures the frequency shift induced in the fibre’s Brillouin gain spectrum on stretching the fibre. This frequency shift is converted into a phase delay of a sinusoidal waveform, which enables the direct detection of the frequency shift. The approach allows single-point strain measurements to be performed at a rate of up to 100 kilohertz at any point along the fibre. Distributed measurements at multiple points along the fibre are also possible, although at lower repetition rates. Importantly, the scheme only requires access from one end of the fibre.

Optomechanical sensing of liquids outside standard fibers using forward stimulated Brillouin scattering

Abstract: The analysis of chemical species is one of the most fundamental and long-standing challenges in fiber-optic sensors research. Existing sensor architectures require a spatial overlap between light and the substance being tested and rely either on structural modifications of standard fibers or on specialty photonic crystal fibers. In this work, we report an optomechanical fiber sensor that addresses liquids outside the cladding of standard, 8/125 μm single-mode fibers with no structural intervention. Measurements are based on forward stimulated Brillouin scattering by radial, guided acoustic modes of the fiber structure. The acoustic modes are stimulated by an optical pump pulse and probed by an optical signal wave, both confined to the core. The acoustic vibrations induce a nonreciprocal phase delay to the signal wave, which is monitored in a Sagnac interferometer loop configuration. The measured resonance frequencies and excitation strengths of individual modes agree with the predictions of a corresponding quantitative analysis. The acoustic reflectivity at the outer cladding boundary and the acoustic impedance of the surrounding medium are extracted from cavity lifetime measurements of multiple modes. The acoustic impedances of deionized water and ethanol are measured with better than 1% accuracy. The measurements successfully distinguish between aqueous solutions with 0, 4%, 8%, and 12% concentrations of dissolved salt. The new fiber-sensing paradigm might be used in the monitoring of industrial processes involving ionic solutions.

Distributed shape sensing using Brillouin scattering in multi-core fibers

Abstract: A theoretical and experimental study on the response of Brillouin scattering in multi-core optical fibers (MCF) under different curving conditions is presented. Results demonstrate that the Brillouin frequency shift of the off-center cores in MCF is highly bending-dependent, showing a linear dependence on the fiber curvature. This feature is here exploited to develop a new kind of distributed optical fiber sensor, which provides measurements of a distributed profile mapping the longitudinal fiber shape. Using conventional Brillouin optical time-domain analysis with differential pulse-width pairs, fully distributed shape sensing along a 1 km-long MCF is practically demonstrated. Experimental results show a very good agreement with the theoretically expected behavior deduced from the dependence of the Brillouin frequency on the strain induced by the fiber bending over a given core. The analysis and results presented in this paper constitute the first demonstration of distributed bending sensing, providing the cornerstone to further develop it into a fully distributed three-dimensional shape sensor.

Tailoring of the Brillouin gain for on-chip widely tunable and reconfigurable broadband microwave photonic filters

Abstract: An unprecedented Brillouin gain of 44 dB in a photonic chip enables the realization of broadly tunable and reconfigurable integrated microwave photonic filters. More than a decade bandwidth reconfigurability from 30 up to 440 MHz, with a passband ripple <1.9 dB is achieved by tailoring the Brillouin pump. The filter central frequency is continuously tuned up to 30 GHz with no degradation of the passband response, which is a major improvement over electronic filters. Furthermore, we demonstrate pump tailoring to realize multiple bandpass filters with different bandwidths and central frequencies, paving the way for multiple on-chip microwave filters and channelizers.

1.89 kW all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality.

Abstract: In this manuscript, we demonstrate high power, all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality by simultaneously suppressing detrimental stimulated Brillouin scattering (SBS) and mode instability (MI) effects. Compared with strictly single frequency amplification, the SBS threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB by subsequently using three-stage cascaded phase modulation systems. Output powers of 477 W, 1040 W, and 1890 W are achieved with full widths at half maximums (FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. The MI threshold is increased from ~738 W to 1890 W by coiling the active fiber in the main amplifier. Both the polarization extinction ratio (PER) and beam quality (M2 factor) are maintained well during the power scaling process. To the best of our knowledge, this is the first demonstration of all-fiberized amplifiers with narrow linewidth, near linear polarization, and near-diffraction-limited beam quality at 2 kW power-level.

Seeing cells in a new light: a renaissance of Brillouin spectroscopy

Abstract: Over the years, light scattering from acoustic waves has grown to be increasingly important in the fields of biology and medicine. This type of scattering, known as Brillouin scattering, has already seen a plethora of applications in fields such as physics. However, the potential for Brillouin scattering for medical imaging and diagnostics has only recently been considered. In this work, we summarize most of the applications of Brillouin scattering in biology to date, and some current work in our lab showing how Brillouin scattering is a worthy prospect for many emerging problems in biology and medical diagnostics.

Brillouin scattering self-cancellation

Abstract: The interaction between light and acoustic phonons is strongly modified in sub-wavelength confinement, and has led to the demonstration and control of Brillouin scattering in photonic structures such as nano-scale optical waveguides and cavities. Besides the small optical mode volume, two physical mechanisms come into play simultaneously: a volume effect caused by the strain-induced refractive index perturbation (known as photo-elasticity), and a surface effect caused by the shift of the optical boundaries due to mechanical vibrations. As a result, proper material and structure engineering allows one to control each contribution individually. Here, we experimentally demonstrate the perfect cancellation of Brillouin scattering arising from Rayleigh acoustic waves by engineering a silica nanowire with exactly opposing photo-elastic and moving-boundary effects. This demonstration provides clear experimental evidence that the interplay between the two mechanisms is a promising tool to precisely control the photon-phonon interaction, enhancing or suppressing it.

Wide-range, high-precision multiple microwave frequency measurement using a chip-based photonic Brillouin filter

Abstract: Spectrum analysis is a key functionality in modern radio frequency (RF) systems. In particular, fast and accurate estimation of multiple unknown RF signal frequencies over a wide measurement range is crucial in defense applications. Although photonic techniques benefit from an enhanced frequency estimation range along with reduced size and weight relative to their RF counterparts, they have been limited by a fundamental trade-off between measurement range and accuracy. Here, we circumvent this trade-off by harnessing the photon and phonon interactions in a photonic chip through stimulated Brillouin scattering, resulting in an accurate estimation of multiple RFs of up to 38 GHz with a record-low error of 1 MHz.

Elastic Constants, Optical Phonons, and Molecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3 Hybrid Perovskite

Abstract: Low frequency dynamics has been studied in a CH3NH3PbBr3 hybrid perovskite single crystal by using four different spectroscopy techniques: coherent inelastic neutron, Raman and Brillouin scatterings, and ultrasound measurements. Sound velocities were measured over five decades in energy to yield the complete set of elastic constants in a hybrid halide perovskite crystal in the pseudocubic plastic phase. The C44 shear elastic constant is very small, leading to a particularly low resistance to shear stress. Brillouin scattering has been used to study the relaxation dynamics of methylammonium cations and to evidence translation-rotation coupling associated with the cubic to tetragonal phase transition at Tc ≈ 230 K. Low frequency and highly damped optical phonons observed using both Raman and inelastic neutron below 18 meV, do not present softening close to Tc. The critical dynamics at Tc ≈ 230 K is compatible with an order-disorder character, dominated by relaxational motions of the molecules.

Stimulated Brillouin Scattering Microscopic Imaging.

Abstract: Two-dimensional stimulated Brillouin scattering microscopy is demonstrated for the first time using low power continuous-wave lasers tunable around 780 nm. Spontaneous Brillouin spectroscopy has much potential for probing viscoelastic properties remotely and non-invasively on a microscopic scale. Nonlinear Brillouin scattering spectroscopy and microscopy may provide a way to tremendously accelerate the data aquisition and improve spatial resolution. This general imaging setup can be easily adapted for specific applications in biology and material science. The low power and optical wavelengths in the water transparency window used in this setup provide a powerful bioimaging technique for probing the mechanical properties of hard and soft tissue.

Unifying Brillouin scattering and cavity optomechanics

Abstract: So far, Brillouin scattering and cavity optomechanics have been mostly disconnected branches of research, although both deal with photon-phonon coupling. This begs for the development of a broader theory that contains both fields. Here, we derive the dynamics of optomechanical cavities from that of Brillouin-active waveguides. This explicit transition elucidates the link between phenomena such as Brillouin amplification and electromagnetically induced transparency. It proves that effects familiar from cavity optomechanics all have traveling-wave partners, but not vice versa. We reveal a close connection between two parameters of central importance in these fields: the Brillouin gain coefficient and the zero-point optomechanical coupling rate. This enables comparisons between systems as diverse as ultracold atom clouds, plasmonic Raman cavities, and nanoscale silicon waveguides. In addition, back-of-the-envelope calculations show that unobserved effects, such as photon-assisted amplification of traveling phonons, are now accessible in existing systems. Finally, we formulate both circuit- and cavity-oriented optomechanics in terms of vacuum coupling rates, cooperativities, and gain coefficients, thus reflecting the similarities in the underlying physics.

Signatures of the Self-Similar Regime of Strongly Coupled Stimulated Brillouin Scattering for Efficient Short Laser Pulse Amplification.

Abstract: Plasma-based laser amplification is considered as a possible way to overcome the technological limits of present day laser systems and achieve exawatt laser pulses. Efficient amplification of a picosecond laser pulse by stimulated Brillouin scattering (SBS) of a pump pulse in a plasma requires to reach the self-similar regime of the strongly coupled (SC) SBS. In this Letter, we report on the first observation of the signatures of the transition from linear to self-similar regimes of SC-SBS, so far only predicted by theory and simulations. With a new fully head-on collision geometry, subpicosecond pulses are amplified by a factor of 5 with energy transfers of few tens of mJ. We observe pulse shortening, frequency spectrum broadening, and down-shifting for increasing gain, signatures of SC-SBS amplification entering the self-similar regime. This is also confirmed by the power law dependence of the gain on the amplification length: doubling the interaction length increases the gain by a factor 1.4. Pump backward Raman scattering (BRS) on SC-SBS amplification has been measured for the first time, showing a strong decrease of the BRS amplitude and frequency bandwidth when SBS seed amplification occurs.

Proposal of Brillouin optical frequency-domain reflectometry (BOFDR).

Abstract: We demonstrate a Brillouin optical frequency-domain reflectometry (BOFDR) technique, which can measure the strain and/or temperature along an optical fiber with one-end access, by detecting the spontaneous Brillouin scattering from a sinusoidally modulated pump light. Compared to the Brillouin optical frequency-domain analysis (BOFDA), we show that BOFDR measurements are free from the distorting components related to acoustic wave modulation, thus simplifying data processing.

Universal nonlinear scattering in ultra-high Q whispering gallery-mode resonators.

Abstract: Universal nonlinear scattering processes such as Brillouin, Raman, and Kerr effects are fundamental light-matter interactions of particular theoretical and experimental importance. They originate from the interaction of a laser field with an optical medium at the lattice, molecular, and electronic scale, respectively. These nonlinear effects are generally observed and analyzed separately, because they do not often occur concomitantly. In this article, we report the simultaneous excitation of these three fundamental interactions in mm-size ultra-high Q whispering gallery mode resonators under continuous wave pumping. Universal nonlinear scattering is demonstrated in barium fluoride and strontium fluoride, separately. We further propose a unified theory based on a spatiotemporal formalism for the understanding of this phenomenology.

Effect of Sodium Oxide Modifier on Structural and Elastic Properties of Silicate Glass

Abstract: Molecular dynamics (MD) simulations and Brillouin light scattering (BLS) spectroscopy experiments have been carried to study the structure of sodium silicate glasses (SiO2)(100–X)(Na2O)X, where X ranges from 0 to 45 at room temperature. The MD-obtained glass structures have been subjected to energy minimization at zero temperature to extract the elastic constants also obtained by BLS spectroscopy. The structures obtained are in good agreement with the structural experimental data realized by different techniques. The simulations show that the values of the elastic constants as a function of X (i.e., Na2O mol %) agree well with those measured by BLS spectroscopy. The variations of elastic constants C11 and C44 as a function of Na2O mol % are discussed and correlated to structural results and potential energies of oxygen atoms.

In Vivo Brillouin Analysis of the Aging Crystalline Lens.

Abstract: Purpose To analyze the age dependence of the longitudinal modulus of the crystalline lens in vivo using Brillouin scattering data in healthy subjects. Methods Brillouin scans were performed along the crystalline lens in 56 eyes from 30 healthy subjects aged from 19 to 63 years. Longitudinal elastic modulus was acquired along the sagittal axis of the lens with a transverse and axial resolution of 4 and 60 μm, respectively. The relative lens stiffness was computed, and correlations with age were analyzed. Results Brillouin axial profiles revealed nonuniform longitudinal modulus within the lens, increasing from a softer periphery toward a stiffer central plateau at all ages. The longitudinal modulus at the central plateau showed no age dependence in a range of 19 to 45 years and a slight decrease with age from 45 to 63 years. A significant intersubject variability was observed in an age-matched analysis. Importantly, the extent of the central stiff plateau region increased steadily over age from 19 to 63 years. The slope of change in Brillouin modulus in the peripheral regions were nearly age-invariant. Conclusions The adult human lens showed no measurable age-related increase in the peak longitudinal modulus. The expansion of the stiff central region of the lens is likely to be the major contributing factor to age-related lens stiffening. Brillouin microscopy may be useful in characterizing the crystalline lens for the optimization of surgical or pharmacological treatments aimed at restoring accommodative power.

Fano Scattering: Manifestation of Acoustic Phonons at the Nanoscale.

Abstract: Size-dependent asymmetric low-frequency Raman line shapes have been observed from silicon (Si) nanostructures (NSs) due to a quantum confinement effect. The acoustic phonons in Si NSs interact with an intraband quasi-continuum to give rise to Fano interaction in the low-frequency range. The experimental asymmetric Raman line shape has been explained by developing a theoretical model that incorporates the quantum-confined phonons interacting with an intraband quasi-continuum available in Si NSs as a result of discretization of energy levels with unequal separation. We discover that a phenomenon similar to Brillouin scattering is possible at the nanoscale in the low-frequency regime and thus may be called “Fano scattering” in general. A method has been proposed to extract information about nonradiative transitions from the Fano scattering data where these nonradiative transitions are involved as an intraband quasi-continuum in modulation with discrete acoustic phonons.

Stimulated Brillouin laser and frequency comb generation in high-Q microbubble resonators.

Abstract: We report on the stimulated Brillouin laser (SBL) and over-dense frequency comb generation in high-Q microbubble resonators (MBRs). Both first-order and cascaded SBL are achieved due to the rich high-order axial modes in the MBRs, although the free spectral range (FSR) of azimuthal mode of the MBR is severely mismatched with the Brillouin shift. The SBL is also generated by varying the internal pressure of MBR at fixed initially non-resonant pump light wavelength. In addition, over-dense frequency combs are realized with comb spacings that are one and two FSRs of aixal mode.

A Hamiltonian treatment of stimulated Brillouin scattering in nanoscale integrated waveguides

Abstract: We present a multimode Hamiltonian formulation for the problem of opto-acoustic interactions in optical waveguides. We develop a quantised Hamiltonian representation of the acoustic field and then introduce a full system with a simple opto-acoustic coupling that includes both photoelastic/electrostrictive and radiation pressure/moving boundary effects in a particularly transparent manner. The interaction is applied to a Fermi's golden rule calculation of spontaneous Brillouin scattering in uniform waveguides. The Heisenberg equations of motion are then used to obtain coupled mode equations for quantised envelope operators for the optical and acoustic fields. We show that the coupling coefficients obtained coincide with those established earlier. Our formalism provides a new basis for future work involving quantum photon and phonon noise in the low intensity limit, phonon–phonon scattering and anharmonicity effects.

Noise and dynamics in forward Brillouin interactions

Abstract: In this paper, we explore the spatio-temporal dynamics of spontaneous and stimulated forward Brillouin scattering. This general treatment incorporates the optomechanical coupling produced by boundary-induced radiation pressures (boundary motion) and material-induced electrostrictive forces (photo-elastic effects), permitting straightforward application to a range of emerging micro- and nano-scale optomechanical systems. Through a self-consistent fully coupled nonlinear treatment, developed within a general Hamiltonian framework, we establish the connection between the power spectral density of spontaneously scattered light in forward Brillouin interactions and the nonlinear coupling strength. We show that, in sharp contrast to backward Brillouin scattering, noise-initiated stimulated forward Brillouin scattering is forbidden in the majority of experimental systems. In fact, the single-pass gain, which characterizes the threshold for energy transfer in back-scattering processes, is negative for a large class of forward Brillouin devices. Beyond this frequent experimental case, we explore mechanisms for dispersive symmetry breaking that lead to amplification and dynamics reminiscent of backward Brillouin scattering.

Guided acoustic and optical waves in silicon-on-insulator for Brillouin scattering and optomechanics

Abstract: We numerically study silicon waveguides on silica showing that it is possible to simultaneously guide optical and acoustic waves in the technologically important silicon on insulator (SOI) material system. Thin waveguides, or fins, exhibit geometrically softened mechanical modes at gigahertz frequencies with phase velocities below the Rayleigh velocity in glass, eliminating acoustic radiation losses. We propose slot waveguides on glass with telecom optical frequencies and strong radiation pressure forces resulting in Brillouin gains on the order of 500 and 50 000 W−1m−1 for backward and forward Brillouin scattering, respectively.

Stimulated Brillouin Scattering in Photonic Integrated Circuits: Novel Applications and Devices

Abstract: The last few years have seen major progress in harnessing on-chip photon–phonon interactions, leading to a wide range of demonstrations of new functionalities. Utilizing not only the optical response of a nonlinear waveguide—but also acoustic resonances—enables the realization of microwave devices with unprecedented performance, otherwise hard to achieve in all-optical processing schemes or electronically. Here, we overview on-chip stimulated Brillouin scattering (SBS) with special emphasis on microwave sources and microwave signal processing schemes. We review the different material platforms and structures for on-chip SBS, ranging from chalcogenide rib waveguides to hybrid silicon/silicon-nitride structures, high-Q photonic–phononic silica microresonators, and suspended silicon nanowires. We show that the paradigm shift in SBS research—from long length of fibers to chip-scale devices—is now moving toward fully integrated photonic–phononic CMOS chips.

Background-free Brillouin spectroscopy in scattering media at 780 nm via stimulated Brillouin scattering.

Abstract: We demonstrate the effectiveness of stimulated Brillouin scattering for background-free Brillouin spectroscopy in scattering media within the biological spectral window. Using two nearly counter-propagating continuous-wave diode laser beams at 780 nm, we acquired transmission stimulated Brillouin point spectra in 10 mm and 500 μm thick Intralipid tissue phantoms with ∼100 μm and ∼16 μm diameter focal points, respectively. Stimulated gain spectra with high signal-to-noise ratio (8.7-30.7 dB) and frequency accuracy (6-72 MHz) were obtained at 20 MHz/10 ms and 20 MHz/100 ms through 0.24-3.36 mean-free paths of tissue phantoms. Our results suggest that stimulated Brillouin gain can be useful for imaging of Brillouin resonances in submillimeter-thick scattering samples.

Slope-Assisted Brillouin Optical Correlation-Domain Reflectometry: Proof of Concept

Abstract: Exploiting the slope of the Brillouin gain spectrum, we develop a new configuration of Brillouin optical correlation-domain reflectometry, which can measure strain (or temperature) and optical loss distributions simultaneously with a high sampling rate. The strain, temperature, and loss dependence coefficients of the output signal are measured to be $1.95\times 10^{-4}\ \text{dB}/\mu\varepsilon$ , $4.42\times 10^{-3}\ \text{dB/K}$ , and 0.191, respectively, which are consistent with the theoretical predictions. We also verify the basic operation of simultaneous measurement of the three parameters.

Novel scanning method for distortion-free BOTDA measurements

Abstract: Systematic errors induced by distortions in the pump pulse of conventional Brillouin distributed fiber sensors are thoroughly investigated. Experimental results, supported by a theoretical analysis, demonstrate that the two probe sidebands in standard Brillouin optical time-domain analyzers provide a non-zero net gain on the pump pulse, inducing severe distortions of the pump when scanning the pump-probe frequency offset, especially at high probe power levels. Compared to the impact of non-local effects reported in the state-of-the-art, measurements here indicate that for probe powers in the mW range (below the onset of amplified spontaneous Brillouin scattering), the obtained gain and loss spectra show two strong side-lobes that lead to significant strain/temperature errors. This phenomenon is not related to the well-known spectral hole burning resulting from pump depletion, but it is strictly related to the temporal and spectral distortions that the pump pulse experiences when scanning the Brillouin gain/loss spectrum. As a solution to this problem, a novel scanning scheme for Brillouin sensing is proposed. The method relies on a fixed frequency separation between the two probe sidebands, so that a flat zero net gain is achieved on the pump pulse when scanning the pump-probe frequency offset. The proposed technique is experimentally validated, demonstrating its ability to completely cancel out non-local effects up to a probe power ultimately limited by the onset of amplified spontaneous Brillouin scattering. The method allows for one order of magnitude improvement in the figure-of-merit of optimized long-range Brillouin distributed fiber sensors, enabling measurements along a 100 km-long sensing fiber with 2 m spatial resolution and with no need of added features for performance enhancement.

High-extinction virtually imaged phased array-based Brillouin spectroscopy of turbid biological media

Abstract: Brillouin microscopy has recently emerged as a powerful technique to characterize the mechanical properties of biological tissue, cell, and biomaterials. However, the potential of Brillouin microscopy is currently limited to transparent samples, because Brillouin spectrometers do not have sufficient spectral extinction to reject the predominant non-Brillouin scattered light of turbid media. To overcome this issue, we combined a multi-pass Fabry-Perot interferometer with a two-stage virtually imaged phased array spectrometer. The Fabry-Perot etalon acts as an ultra-narrow band-pass filter for Brillouin light with high spectral extinction and low loss. We report background-free Brillouin spectra from Intralipid solutions and up to 100 μm deep within chicken muscle tissue.