Brillouin Lines Research Articles

Brillouin light scattering and second harmonic generation in barium strontium niobate films

Temperature dependences of Brillouin light scattering spectra from Ba0.5Sr0.5 Nb2O6 (SBN 50) film were obtained. The deviations of the Brillouin shift dependence from linear near ferroelectric phase transition are due to electric fields in the local regions with symmetry breaking. A maximum of Brillouin line width is more than twice of the corresponding width in a bulk crystal of the same composition. This broadening is attributed to the confinement of the acoustic phonons. Optical second harmonic generation measurements (SHG) showed that the transition from the polar to the nonpolar phase occurred at a temperature characteristic of SBN-50.

The Brillouin linewidth in water as a function of temperature and salinity: the missing empirical relationship

Brillouin scattering in water has been proposed as a method to measure temperature profiles in the ocean. Spontaneous Brillouin scattering can be understood as light scattering on phonons in the water originating from thermal fluctuations. The scattering signal usually features three peaks in its spectrum, of which one is attributed mainly to scattering from suspended particles and shows no frequency shift from the incident light, while the other two are shifted by the phonon frequency leading to a linear dependency of this Brillouin shift to the speed of sound. The speed of sound is dependent on temperature and salinity. To measure temperature and salinity independently, a second measurement parameter is necessary. The spectral width of the Brillouin lines constitutes a measure for the phonon lifetime and depends on various macroscopic water properties. This linewidth was investigated in a laboratory using a scanning FPI (Fabry-Perot interferometer) over a temperature range from 1.8 °C to 35 °C and 13 salinities from 0 ppt to 36 ppt. A strong temperature dependence was confirmed and a weaker, nonlinear dependency on salinity was found. An empirical model was created to describe the data for the purpose of enabling remote sensing of water temperature profile and salinity simultaneously in the future.

Modified Landau–Placzek ratio of the liquid metal rubidium beyond hydrodynamics

The intensity ratio of the Rayleigh line and the Brillouin lines can be derived within hydrodynamics and is known as the Landau–Placzek (LP) ratio. This ratio is directly related to the ratio of specific heats of the fluid. Within the microscopic wave vector range, which can be probed by inelastic neutron scattering, the intensity ratio for simple liquid metals deviates distinctly from the hydrodynamic prediction of the LP-ratio. We derive the intensity ratio from experimental data of liquid rubidium, which shows an enhanced LP-ratio by a factor 8 compared to the hydrodynamic prediction. This strong deviation indicates a further relaxation process in the microscopic wave vector range beyond hydrodynamics. That relaxation process is the viscoelastic reaction of the simple liquid to density fluctuations. Taking this process into account a modified LP-ratio is able to describe the data quite well.

Multi-wavelength Brillouin Raman random fiber laser with double Brillouin frequency shift

In this paper, a linear full-open-cavity Raman random fiber laser generating multi-wavelength Brillouin lines with double Brillouin frequency shift is proposed. The odd-order and even-order Stokes lights are respectively output from different end. Our contrastive experiments with forward and backward Brillouin pumping schemes reveal that the signal-to-noise ratio (SNR) of the laser output by backward pump is 24.5 dB, and the SNR of the laser output by forward pump is 19.5 dB under same Raman pump power of 831.8 mW. The SNR of backward-pumped laser output is higher and the wavelength number is more than that of forward-pumped case. But the Raman pump threshold for Brillouin Stokes light generation is higher for backward-pumped laser, which results in no stable Brillouin light generation at low Raman pump power. It is believed that the results not only can provide different ideas for the further optimization of random fiber laser structure, but also can help to further expand the application range of multi-wavelength Brillouin Raman lasers in optical communication systems and optical sensing fields.

Multi-wavelength Brillouin-erbium fiber laser with more than 95 lines based on a dual-ring structure

We demonstrate a multi-wavelength Brillouin fiber laser based on a dual-ring structure to take advantage of the forward and backward Brillouin gain simultaneously with an injected tunable scanning lasing. We used two erbium-doped fiber amplifiers for the amplification of the seed lasing and the operated Brillouin laser. The cascaded stimulated Brillouin effect in the section of dispersion-shifted fiber contributes to the free oscillation suppression in cavity. When the laser operates at the center wavelength of 1565 nm, the output spectra shows the maximum Brillouin lines of more than 95 with a wavelength spacing of about 0.08 nm under the maximum pump. Through adjusting of the seed wavelength and the tunable filter in cavity, the output spectra can be shifted from 1540 to 1570 nm. This fiber laser will find potential applications in optical measurement and sensing because of its large line number, stability, tunability and fixed wavelength spacing.

Brillouin spectroscopy of biorelevant fluids in relation to viscosity and solute concentration.

The measurement of intracellular viscoelastic properties by Brillouin scattering is a rapidly developing field in biophysics and medicine. Here, the Brillouin spectroscopy is applied for a number of aqueous solutions of biorelevant molecules to reveal relations between the Brillouin line parameters (frequency and width) and viscosity or solute concentration. It is found that for the majority of the studied biorelevant molecules the solute concentration governs the Brillouin frequency in a universal manner. On the other hand, the relations between the macroscopic viscosity and Brillouin peak parameters are different for different solutes. We conclude that for biological fluids the viscosity evaluation from Brillouin data needs prior knowledge about the chemical composition. This result challenges the fidelity of the indirect experimental determinations of the cellular viscosity, when small molecule solutions are used for the calibration.

Dependence of acoustic anomalies on chemical composition in strontium barium niobate crystals (from conventional ferroelectric to relaxor) probed by Brillouin light scattering

Brillouin spectra in a series of strontium barium niobate crystals SrxBa1-xNb2O6 of different chemical composition (x = 0.3, 0.5, 0.6, 0.75) are investigated in various spectral geometries and in a wide temperature ranges including ferroelectric and paraelectric phases. It is shown that for all crystals studied, a pronounced temperature dependences of position and width of the Brillouin lines are clearly manifested only when the acoustic wave vector is parallel to the polar axis Z. Exponential temperature dependence of anomalies of elastic modules was obtained. The exponent rate decreases with increasing of disordering in crystals under study.

Brillouin study of elastic properties of nanometric phospholipid layers in aqueous suspensions of vesicles

This work is devoted to the development of an experimental technique based on Brillouin spectroscopy for the study of aqueous suspensions of multilayer vesicles of phospholipids at different temperatures using the example of DPPC phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine). The temperature dependence of the Brillouin line position is studied. A comparison of the behavior of Brillouin lines of polycrystalline ice, dry vesicles, and vesicles suspension reveals the contribution of phospholipid layers to the Brillouin spectrum. Analysis of the data makes it possible to estimate the value of sound velocity of the vesicles membranes and its temperature dependence. [AQ1] [AQ2]

Image analysis applied to Brillouin images of tissue-mimicking collagen gelatins.

Brillouin spectroscopy is an emerging analytical tool in biomedical and biophysical sciences. It probes viscoelasticity through the propagation of thermally induced acoustic waves at gigahertz frequencies. Brillouin light scattering (BLS) measurements have traditionally been performed using multipass Fabry-Pérot interferometers, which have high contrast and resolution, however, as they are scanning spectrometers they often require long acquisition times in poorly scattering media. In the last decade, a new concept of Brillouin spectrometer has emerged, making use of highly angle-dispersive virtually imaged phase array (VIPA) etalons, which enable fast acquisition times for minimally turbid materials, when high contrast is not imperative. The ability to acquire Brillouin spectra rapidly, together with long term system stability, make this system a viable candidate for use in biomedical applications, especially to probe live cells and tissues. While various methods are being developed to improve system contrast and speed, little work has been published discussing the details of imaging data analysis and spectral processing. Here we present a method that we developed for the automated retrieval of Brillouin line shape parameters from imaging data sets acquired with a dual-stage VIPA Brillouin microscope. We applied this method for the first time to BLS measurements of collagen gelatin hydrogels at different hydration levels and cross-linker concentrations. This work demonstrates that it is possible to obtain the relevant information from Brillouin spectra using software for real-time high-accuracy analysis.

Ultra-narrow linewidth Brillouin laser with nanokelvin temperature self-referencing

Ultrastable lasers serve as the backbone for some of the most advanced scientific experiments today and enable the ability to perform atomic spectroscopy and laser interferometry at the highest levels of precision possible. With the recent and increasing interest in applying these systems outside of the laboratory, it remains an open question as to how to realize a laser source that can reach the extraordinary levels of narrow linewidth required and still remain sufficiently compact and portable for field use. Critical to the development of this ideal laser source is the necessity for the laser to be insensitive to both short- and long-term fluctuations in temperature, which ultimately broaden the laser linewidth and cause drift in the laser’s center frequency. We show here that the use of a large mode-volume optical resonator with 2 m of optical fiber, which acts to suppress the resonator’s fast thermal fluctuations, together with stimulated Brillouin scattering optical nonlinearity presents a powerful combination that enables lasing with an ultra-narrow linewidth of 20 Hz. To address the laser’s long-term temperature drift, we apply two orthogonal polarizations of the narrow Brillouin line as a metrological tool that precisely senses a minute change in the resonator’s temperature at the level of 85 nK. The precision afforded by this temperature measurement enables new possibilities for the stabilization of resonators against environmental perturbation.

Non-contact mechanical and chemical analysis of single living cells by microspectroscopic techniques

Innovative label-free microspectroscopy, which can simultaneously collect Brillouin and Raman signals, is used to characterize the viscoelastic properties and chemical composition of living cells with sub-micrometric resolution. The unprecedented statistical accuracy of the data combined with the high-frequency resolution and the high contrast of the recently built experimental setup permits the study of single living cells immersed in their buffer solution by contactless measurements. The Brillouin signal is deconvoluted in the buffer and the cell components, thereby revealing the mechanical heterogeneity inside the cell. In particular, a 20% increase is observed in the elastic modulus passing from the plasmatic membrane to the nucleus as distinguished by comparison with the Raman spectroscopic marker. Brillouin line shape analysis is even more relevant for the comparison of cells under physiological and pathological conditions. Following oncogene expression, cells show an overall reduction in the elastic modulus (15%) and apparent viscosity (50%). In a proof-of-principle experiment, the ability of this spectroscopic technique to characterize subcellular compartments and distinguish cell status was successfully tested. The results strongly support the future application of this technique for fundamental issues in the biomedical field.

Characterization of Brillouin scattering in a few-mode fiber

The few-mode fiber can be used to transmit limited orthogonal modes, which has the advantages of small modular interference and easily controlled modes. The Brillouin scattering sensor based on the few-mode fiber can effectively reduce the cross sensitivity of multi parameter measurement, and realize the measurement of multi physical quantity. In this paper, based on the wave optics theory, the Brillouin scattering spectrum parameters of the step-index few-mode fiber are analyzed, such as frequency shift, line width and peak gain and so on. Firstly, the transmission modes of the few-mode fiber are analyzed. The finite element analysis result shows that there are 5 kinds of transmission modes:LP01, LP11, LP21, LP02 and LP31, and their effective refractive indexes are 1.4664, 1.4652, 1.4637, 1.4630 and 1.4616, respectively. Secondly, the mathematical models of the Brillouin frequency shift, line width and peak gain of different modes in the few-mode fiber are analyzed. Finally, the parameters of Brillouin scattering spectrum with different modes' superposition are also discussed. In the few-mode fiber, due to the different effective refractive index, the light of each mode is propagated along its respective path and interacts with the particles in the fiber, thus producing different Brillouin scattering spectrum. The simulation results show that the frequency shift of the Brillouin scattering spectrum of each mode is in a range of 10.19-10.23 GHz, and the frequency shift increases with the decrease of the mode order. The Brillouin line width of each mode is in a range of 32-34 MHz, and the line width also increaseswith the decrease of the mode order. Moreover, the relative amplitude of the Brillouin scattering gain spectrum increases with the decrease of the mode order. The mathematical models of this paper are used respectively to analyze the Brillouin scattering spectra of other types of step-index few-mode fibers. It is shown that the Brillouin frequency shift, Brillouin line width and peak gain of other types of step-index few-mode fibers also increase with the decrease of the mode order. In a step-index few-mode fiber, intramodal Brillouin scattering spectrum and the intermodal Brillouin scattering spectrum are both in line with the distribution of Lorenz curve. However, the intermodal Brillouin scattering spectrum of modes' superposition leads to the line width broadening of the Brillouin scattering spectrum, and the relative amplitude of the intermodal Brillouin scattering spectrum of modes' superposition being generally smaller than that of intramodal.

Precise Determination of Brillouin Scattering Spectrum Using a Virtually Imaged Phase Array (VIPA) Spectrometer and Charge-Coupled Device (CCD) Camera.

Brillouin spectroscopy is an emerging tool for microscopic optical imaging as it allows noninvasive assessment of viscoelastic properties of materials. The use of atomic-molecular absorption cells as ultra-narrow notch filters allows acquisition of Brillouin spectra from turbid samples despite their strong elastic scattering. However, such systems alter the shapes of the Brillouin lines, making the precise determination of the Brillouin shift difficult. In this report, we propose a simple method for analyzing the Brillouin spectrum using a customized least-square fitting algorithm. The absorption spectrum induced by the atomic-molecular cell was taken into consideration. The capability of the method is confirmed by processing experimental spectroscopic data from the pure water at different temperatures. The accuracy of the measurements of ±1 MHz spectral line shift is experimentally demonstrated.

Theoretical investigation of tunable pulse broadening cancellation via doublet Brillouin gain lines in an optical fiber

Tunable pulse broadening cancellation via doublet Brillouin gain lines in a silica fiber was theoretically investigated. Minimum broadening factor of 0.96 with time delay of 134.5ps is achieved by optimizing doublet Brillouin lines for 200ps Gaussian pulse with pulse distortion. Frequency separation between two gain lines and spectrum bandwidth of single gain line and peak gain values are optimized and considered in the present paper.

Tunable Brillouin-Erbium Comb Fiber Laser in a Linear Cavity With a Single-Mode Tellurite Fiber

We demonstrate a Brillouin-erbium fiber laser in a linear cavity with a single-mode tellurite fiber. Only 132-m tellurite fiber is used in the cavity because of the high Brillouin gain coefficient. Both the backward and forward Brillouin lines are reflected into the cavity, which experience dual-pass Brillouin gain. With a tunable filter, the gain is equalized in the erbium-doped fiber bandwidth. About 40 comb lines spaced by 7.91 GHz are obtained and the line number is kept in the tunable range from 1528 to 1565 nm with 37 nm. The average output power fluctuates from 44 to 48 mW in the tunable range.

Tunable hybrid Brillouin-erbium comb fiber laser in a composite cavity with a single-mode tellurite fiber

We demonstrate a hybrid Brillouin-erbium fiber laser in a composite cavity with a single-mode tellurite fiber. Only a 200 m long tellurite fiber is used in the cavity due to the high effective nonlinearity. The double pass of the tellurite fiber in the linear part is advantageous for improving the Brillouin gain and increasing the comb lines. With a tunable filter, more than 70 comb lines spaced by 7.888 GHz are obtained at 1565 nm. The line number is kept around 60 in the tunable range of 30 nm from 1535 to 1565 nm. Compared with silica fibers, the tellurite fiber can generate a larger number of Brillouin lines in a wider tunable range.

An analytical model for Rayleigh–Brillouin scattering spectra in gases

An analytical model for Rayleigh–Brillouin scattering (RBS) spectra of gases is developed and discussed in this paper. The well-recognized S6 and S7 models only have a numerical form and are complex to process in real applications; simple Gaussian models are imprecise and suffer from greater error. A new model (V3) based on the spectral line broadening theory and the Voigt profile is established. This model is easy to process as it has an analytical form, and it is proved to have good accordance with the S7 model. In addition, this model can separate the Rayleigh and Brillouin lines, and the measured Brillouin shift is proved to match theoretical values.

Spacing-Switchable Multiwavelength Fiber Laser Based on Nonlinear Polarization Rotation and Brillouin Scattering in Photonic Crystal Fiber

A new spacing-switchable multiwavelength erbium-doped fiber laser (EDFL) is demonstrated using switchable nonlinear polarization rotation (NPR) and stimulated Brillouin scattering (SBS) effects. The laser employs a 100-m-long photonic crystal fiber in conjunction with a four-port circulator in a figure-of-eight arrangement to provide the intensity-dependent transmission effect, as well as to discriminate the even-order and odd-order Brillouin Stokes to have a double-frequency Brillouin Stokes output. Without the Brillouin pump (BP), the laser operates in an NPR mode to produce at least 22 lasing wavelengths with a side mode suppression ratio of more than 10 dB and a wavelength spacing of 0.27 nm. In the Brillouin erbium fiber laser mode, at least 30 Brillouin lines with a spacing of 0.16 nm are obtained at BP power of 3 dBm and 980-nm pump power of 120 mW.

Note: Higher resolution Brillouin spectroscopy by offset stabilization of a tandem Fabry-Pérot interferometer

A simple modification to a Sandercock-type tandem Fabry-Pérot interferometer is demonstrated. By adding an independent reference laser with temperature tunability, narrow Brillouin lines that are tens GHz shifted from the Rayleigh line can be recorded with much higher frequency resolution than in the original system.

Analytical model for Rayleigh–Brillouin line shapes in air: errata

In the recent publication by B. Witschas [Appl. Opt.50, 267 (2011)]APOPAI0003-693510.1364/AO.50.000267 three typing errors were found and are corrected in the following.