Showing papers on "Brillouin scattering published in 1999"

Characterization of the Brillouin-loss spectrum of single-mode fibers by use of very short (<10-ns) pulses.

Abstract: The characterization of the Brillouin-loss spectrum of single-mode fibers with very short (<10-ns) pulses has been studied. It was found that the Brillouin-loss signal intensity is linearly related to the duration of the pump pulse used to obtain the spectrum. In contrast with the uniform trend of the signal, three distinct behaviors were observed in the spectral linewidth. At long pulse durations the linewidth was constant at approximately 40 MHz. Pulse durations of the order of the phonon lifetime resulted in a broader spectrum, reaching a maximum width of ~100 MHz at 5 ns. Reducing the pulse duration further resulted in a sudden narrowing of the Brillouin line.

Elastic constants of tetrahedral amorphous carbon films by surface Brillouin scattering

Abstract: The elastic constants of tetrahedral amorphous carbon (ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H) thin films were determined nondestructively by surface Brillouin scattering. Besides the usual Rayleigh surface mode, we also observe a new pseudosurface acoustic mode of longitudinal polarization, which is a modified version of the longitudinal guided mode usually found in slow-on-fast supported films. The Young’s modulus E of a ta-C with 88% sp3 is 757 GPa, and the shear modulus G is 337 GPa. The moduli of ta-C:H with 70% sp3 and 30 at. % H are lower, E=300 GPa and G=115 GPa.

Elastic constants of tetrahedral amorphous carbon films by surface Brillouin scattering carbon thin films

Abstract: The elastic constants of tetrahedral amorphous carbon (ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H) thin films were determined nondestructively by surface Brillouin scattering. Besides the usual Rayleigh surface mode, we also observe a new pseudosurface acoustic mode of longitudinal polarization, which is a modified version of the longitudinal guided mode usually found in slow-on-fast supported films. The Young’s modulus E of a ta-C with 88% sp3 is 757 GPa, and the shear modulus G is 337 GPa. The moduli of ta-C:H with 70% sp3 and 30 at. % H are lower, E=300 GPa and G=115 GPa.

Brillouin light scattering from quantized spin waves in micron-size magnetic wires

Abstract: An experimental study of spin-wave quantization in arrays of micron-size magnetic ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ wires by means of Brillouin light-scattering spectroscopy is reported Dipolar-dominated Damon-Eshbach spin-wave modes laterally quantized in a single wire with quantized wave vector values determined by the width of the wire are studied The frequency splitting between quantized modes, which decreases with increasing mode number, depends on the wire sizes and is up to 15 GHz The transferred wave vector interval, where each mode is observed, is calculated using a light-scattering theory for confined geometries The frequencies of the modes are calculated, taking into account finite-size effects The results of the calculations are in a good agreement with the experimental data

Laser beam combining and cleanup by stimulated Brillouin scattering in a multimode optical fiber

Abstract: A new technique for combining low-power laser beams has been demonstrated by use of semiconductor diode lasers. The technique, which is appropriate for any single-longitudinal-mode laser, is based on stimulated Brillouin scattering (SBS) in long multimode optical fibers. It produces a clean Gaussian-like beam that corresponds to the fundamental fiber mode, irrespective of the profile of the pump. Coherent as well as incoherent combining was demonstrated, and conversion slope efficiencies as high as 67% and 83% were shown to be achievable for the single-pass and ring-cavity SBS geometries, respectively.

Degradation of modulation and noise characteristics of semiconductor lasers after propagation in optical fiber due to a phase shift induced by stimulated Brillouin scattering

Abstract: Here we demonstrate theoretically that stimulated Brillouin scattering (SBS) can induce a phase shift of the optical carrier relative to its sidebands due to the waveguiding effect of the optical fiber on the acoustic wave. This causes conversion of frequency modulation to intensity modulation, which results in an increase in the relative intensity noise and degradation of the modulation response of directly modulated lasers after propagation in an optical fiber, in agreement with our experimental observations. Suppression of SBS can be achieved at low frequencies and high modulation powers due to the laser adiabatic chirp.

Brillouin scattering study of bulk gan

Abstract: High-resolution Brillouin scattering experiments have been performed for a high-quality free-standing gallium nitride (GaN) substrate. Elastic stiffness constants are reported. A comparison is made with the results of an earlier study for a GaN thin film on sapphire substrate.

Trade-off between the spatial and the frequency resolutions in measuring the power spectrum of the Brillouin backscattered light in an optical fiber

Abstract: We theoretically analyze the relation between the pulse width of light launched into an optical fiber and the resultant power spectrum of spontaneous Brillouin backscattered light. Through this analysis, we determine numerically that the bandwidth of the Brillouin backscattered light becomes wider, and thus the measurement accuracy in determining the peak-power frequency degrades in approximately inverse proportion to the launched pulse width. Experimental results with various pulse widths are in good agreement with the derived theoretical results.

Amplitude squeezing of light by means of a phase-sensitive fiber parametric amplifier

Abstract: We experimentally demonstrate generation of bright sub-Poissonian light by means of parametric deamplification in a phase-sensitive fiber amplifier that is based on a balanced nonlinear Sagnac interferometer. On direct detection, the photocurrent noise falls below the shot-noise limit by 0.6±0.2 dB (1.4 dB when corrected for detection losses). To observe the noise reduction we employed a scheme that used two orthogonally polarized pulses to cancel the noise that arises from the predominantly polarized guided-acoustic-wave Brillouin scattering in the fiber. We also present a simplified semiclassical theory of quantum-noise suppression by this amplifier, which is found to be in good agreement with the experimental results.

Structural monitoring by use of a Brillouin distributed sensor

Abstract: The testing of a fiber-optic distributed-strain sensor attached to a simple structural member is reported. A Brillouin scattering-based sensor system was used to measure both tensile and compressive strains along the length of a cantilever beam subjected to various loads. The sensing fiber was attached to the beam in such a way that some sections experienced uniform strain, whereas others were subjected to a nonuniform strain distribution. A spatial resolution of 0.4 m was used, and a measurement precision of approximately ±50 µe was achieved.

Pulse compression to the subphonon lifetime region by half-cycle gain in transient stimulated Brillouin scattering

Abstract: A new approach to the transient pulse compression by stimulated Brillouin scattering (SBS) is presented. The theoretical analysis involving the time-dependent SBS gain in explicit form leads to a nonlinear system of partial differential equations, solved numerically by a generalization of the split-step method. It is shown theoretically and confirmed experimentally that the phonon lifetime is not always an appropriate parameter that determines the lower limit to the pulse duration in SBS compressors. A half-cycle gain regime is found for pulses shorter than the phonon lifetime. Hence, under proper conditions, pulses as short as half the acoustic period can be produced.

PbMg1/3Nb2/3O3 as a model object for light scattering experiments

Abstract: A classical relaxor material, PbMg1/3Nb2/3O3 (PMN), has a long history of investigations by Raman and Brillouin spectroscopies. Earlier results on Raman scattering have emphasized a predominant role of disorder and, on the whole, confirmed a model of compositional fluctuations, first suggested by Smolensky and his colleagues. The extensive microstructural studies of PMN in recent years require re-consideration of many conclusions. In the present paper, we initiated analysis of rather complex Raman scattering in order to reach a definite agreement with the recent direct microstructural studies. Different unambiguous features were analyzed in light of polarization measurements, a breakdown in the Raman selection rules, assignment of modes consistent with the Fm3m space group, the mode behavior for some ion substitutions, the key role of the high temperature transformation of the spectra in explaining the origin of Raman active modes, and some other aspects. The behavior of optical phonons is predom...

Self-pulsing and dynamic bistability in cw-pumped Brillouin fiber ring lasers

Abstract: The nonlinear dynamics of cw-pumped Brillouin long-fiber ring lasers that contain a large number of longitudinal modes N beneath the Brillouin gain curve is controlled by a single parameter, namely, the Stokes feedback R. Below Rcrit, a stable train of dissipative solitonic pulses is spontaneously structured at the round-trip frequency fr without any additional intracavity mode locking. Experimental observations in cw-pumped fiber ring cavities, supported by numerical simulation in a coherent space–time three-wave model that includes the optical Kerr effect, prove the universality of the self-pulsing mechanism. Stability analysis shows that below Rcrit the steady Brillouin mirror regime is destabilized through a Hopf bifurcation. For R

Tensile-strain coefficient of resonance frequency of depolarized guided acoustic-wave Brillouin scattering

Abstract: With the aim of making a tensile-strain sensor or a temperature sensor based on depolarized guided acoustic-wave Brillouin scattering (GAWBS), we experimentally investigate the effect of tensile strain applied to a fiber. We pay our attention to the resonance frequency of depolarized GAWBS by the TR/sub 25/ mode of the acoustic wave. By unjacketing the fiber, the peak level of the resonance spectrum increases and the full-width at half-maximum narrows to 500 kHz, which increases the accuracy of measurement. The experimentally obtained tensile-strain coefficient is 1.8. On the basis of our experimental results, we discuss the way to apply depolarized GAWBS to a tensile-strain sensor or a temperature sensor.

Multiwavelength all-optical clock recovery

Abstract: Multiwavelength clock recovery is especially desirable in systems that use wavelength-division-multiplexed technology. A multiwavelength clock-recovery device can greatly simplify costs by eliminating the need to have a separate regenerator for each wavelength. This letter discusses multiwavelength all-optical clock recovery using stimulated Brillouin scattering.

Characterization of plasma and laser conditions for single hot spot experiments

Abstract: The TRIDENT laser system at the Los Alamos National Laboratory is being used for fundamental experiments which study the interaction of self-focusing, stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in a near-diffraction-limited (single) laser hot spot in order to better understand the coupling between these plasma instabilities The diffraction limited beam mimics a single hot spot found in speckle distributions that are typical of random or kinoform phase plates (RPP or KPP) used for spatial smoothing of laser beams A long scale length, hot plasma (∼1 mm, ∼06 keV) is created by a separate heater beam, and the single hot spot beam is used to drive parametric instabilities The focal plane distribution and wave-front of the interaction beam are characterized, and its intensity can be varied between 1014–1016 W/cm2 The plasma density, temperature, and flow profiles are measured using a gated imaging spectroscopy of collective Thomson scattering from the heater beam Results of the laser and plasma characterization, and initial results of backscattered SRS, SBS, and beam steering in a flowing plasma are presented

Dynamic Behavior of Stimulated Brillouin Scattering in a Single-Mode Optical Fiber

Abstract: We theoretically and experimentally investigate the transient stimulated Brillouin scattering (SBS) in a single-mode fiber. The experiment was conducted using a single-mode pulsed YAG laser and demonstrates that SBS can occur even in the nanosecond region. Due to the transient nature of SBS, pulse narrowing of the transmitted pulse is observed. The experimental results agree with the theory based on the coupled-amplitude equations for the pump, Stokes, and acoustic waves. The steady-state solution of SBS is obtained when the pulse width of the incident pulse is one 100-fold greater than the round-trip time within the fiber. We show that the instability of SBS depends on the magnitude of the nonlinear refractive index of the fiber. SBS instability behavior appears at different pump power levels when the nonlinear refractive index is an order of magnitude larger than that of the fused-silica fiber.

Radiative properties of neutral germanium obtained from excited-state lifetime and branching-ratio measurements and comparison with theoretical calculations

Abstract: Using time-resolved UV laser-induced fluorescence from a laser-produced plasma, natural radiative lifetimes have been measured for 21 levels of Ge I belonging to the odd-parity $4p4d,4p5d,$ and $4p6s$ configurations. Stimulated Brillouin scattering in water has allowed us to compress Nd:YAG laser pulses pumping a dye laser (where YAG denotes yttrium aluminum garnet), thus yielding 1-ns tunable laser pulses to enable accurate measurements of short lifetimes. Branching ratios of Ge I have been measured by inductively coupled plasma emission spectrometry, the intensity calibration being performed by means of Ar lines emitted by a hollow-cathode lamp. The experimental lifetimes and branching ratios have been combined in order to provide a set of accurate transition probabilities for the $4p\ensuremath{-}5s$ and $4p\ensuremath{-}4d$ transitions. A relativistic Hartree-Fock calculation, taking configuration-interaction and polarization effects into account, has been combined with a least-squares optimization procedure of the Slater and spin-orbit integrals in order to test the ability of this approach to correctly predict radiative properties of the group-IV elements. Good agreement between experimental and theoretical transition probability values has been achieved for most of the transitions considered.

Monitoring of Large Structure Using Distributed Brillouin Fibre Sensing

Abstract: On-site distributed measurements using a sensor based on stimulated Brillouin scattering are presented. Long fibre length can be used, so that a dense 2D or 3D measurement of strain or temperature can be obtained in large structure.

Highly sensitive self-mixing measurement of Brillouin scattering with a laser-diode-pumped microchip LiNdP(4)O(12) laser.

Abstract: Brillouin scattering has been measured with a high degree of sensitivity by a system built around a laser-diode-pumped microchip LiNdP(4)O(12) laser. An efficient self-mixing modulation effect that is due to the interference between a lasing field and a weak field fed back from an acousto-optic modulator by means of a phase-conjugate reflection was used. Laser-Doppler velocimetry that can discriminate the direction of motion has been demonstrated.

Dynamic Behavior of Stimulated Brillouin Scattering in a Single-Mode Optical Fiber

Abstract: We theoretically and experimentally investigate the transient stimulated Brillouin scattering (SBS) in a single-mode fiber. The experiment was conducted using a single-mode pulsed YAG laser and demonstrates that SBS can occur even in the nanosecond region. Due to the transient nature of SBS, pulse narrowing of the transmitted pulse is observed. The experimental results agree with the theory based on the coupled-amplitude equations for the pump, Stokes, and acoustic waves. The steady-state solution of SBS is obtained when the pulse width of the incident pulse is one 100-fold greater than the round-trip time within the fiber. We show that the instability of SBS depends on the magnitude of the nonlinear refractive index of the fiber. SBS instability behavior appears at different pump power levels when the nonlinear refractive index is an order of magnitude larger than that of the fused-silica fiber.

Noise characteristics of fiber-based optical phase conjugators

Abstract: This paper presents a theoretical and experimental analysis of the noise characteristics of a fiber based optical phase conjugator. The results are used to derive an expression for the noise figure (NF) of the device. The lowest measured NF is 18 dB, at -15 dB net conversion efficiency. This NF was limited by stimulated Brillouin scattering (SBS) and amplified spontaneous emission from an optical amplifier. Using the experimental parameters, the theoretical values agree well with the measurements, and predicts in our particular case a minimum NF of 10 dB at high conversion efficiencies. Furthermore, we investigate the influence of optical amplifiers before and after the conjugator, and, with all excess noise eliminated, we predict the quantum limit of 3 dB at efficiencies much greater than 100%.

Brillouin Scattering Based Distributed Sensors for Structural Applications

Abstract: Over the past four years the Fibre Optics Group at the University of New Brunswick has been developing a distributed sensor for use in smart civil structures. By using Brillouin loss, the sensor is capable of measuring strain or temperature at any region on a sensing fibre, even those that are kilometers in length. This paper outlines the development of the sensor system, and discusses some of the experiments that have been performed with it. New results are presented that demonstrate 100 mm spatial resolution under laboratory conditions. This represents a four-fold improvement in the spatial resolution over previously reported results attained with a Brillouin scattering based distributed sensor.

Monitoring of large structure using distributed Brillouin fibre sensing

Abstract: On-site distributed measurements using a sensor based on stimulated Brillouin scattering are presented. Long fibre length can be used, so that a dense 2D or 3D measurement of strain or temperature can be obtained in large structure.