Stimulated Brillouin Gain Spectra Research Articles

3.05 kW, 13.7 GHz linewidth fiber amplifier based on PRBS phase modulation for SBS suppression

In this paper, we establish a multi-stage fiber amplifier with pseudo-random binary sequence (PRBS) phase modulation. The stimulated Brillouin gain spectra of the main amplifier with both the unmodulated and pseudo-random binary sequence phase modulated configuration are measured (with corresponding output power), and the stimulated Brillouin scattering (SBS) threshold is investigated experimentally and theoretically. The pseudo-random binary sequence phase modulation parameters are optimized by theoretical simulation. With a two-stage preamplifier chain and a counter-pumping main amplifier stage, a maximum 3.05 kW output power with a slope efficiency of 85.9% is obtained experimentally. The central wavelength of the fiber amplifier is 1050 nm, associated with a full-width at half-maximum linewidth of 13.7 GHz. The stimulated Brillouin scattering reflectivity is below 0.01% at 3.05 kW at 13.7 GHz, which indicates that stimulated Brillouin scattering can be suppressed efficiently at this power and linewidth level.

Stimulated Brillouin Gain Spectroscopy of Superfluid Helium-4

We report stimulated Brillouin gain spectra of bulk superfluid helium-4. Stimulated Brillouin gain spectroscopy is a powerful optical method to probe the acoustic properties of liquid helium in a narrow spatio-temporal domain with moderate optical power and fast acquisition rates. We determine the velocity of first sound of superfluid helium-4 from the stimulated Brillouin gain spectra between 0.90 and 2.17 K and compare it to well-known available data, which validate our experimental method.

High-speed Continuous-wave Stimulated Brillouin Scattering Spectrometer for Material Analysis.

Recent years have witnessed a significant increase in the use of spontaneous Brillouin spectrometers for non-contact analysis of soft matter, such as aqueous solutions and biomaterials, with fast acquisition times. Here, we discuss the assembly and operation of a Brillouin spectrometer that uses stimulated Brillouin scattering (SBS) to measure stimulated Brillouin gain (SBG) spectra of water and lipid emulsion-based tissue-like samples in transmission mode with <10 MHz spectral-resolution and <35 MHz Brillouin-shift measurement precision at <100 ms. The spectrometer consists of two nearly counter-propagating continuous-wave (CW) narrow-linewidth lasers at 780 nm whose frequency detuning is scanned through the material Brillouin shift. By using an ultra-narrowband hot rubidium-85 vapor notch filter and a phase-sensitive detector, the signal-to-noise-ratio of the SBG signal is significantly enhanced compared to that obtained with existing CW-SBS spectrometers. This improvement enables measurement of SBG spectra with up to 100-fold faster acquisition times, thereby facilitating high spectral-resolution and high-precision Brillouin analysis of soft materials at high speed.

High-speed stimulated Brillouin scattering spectroscopy at 780 nm

We demonstrate a high-speed stimulated Brillouin scattering (SBS) spectroscopy system that is able to acquire stimulated Brillouin gain point-spectra in water samples and Intralipid tissue phantoms over 2 GHz within 10 ms and 100 ms, respectively, showing a 10-100 fold increase in acquisition rates over current frequency-domain SBS spectrometers. This improvement was accomplished by integrating an ultra-narrowband hot rubidium-85 vapor notch filter in a simplified frequency-domain SBS spectrometer comprising nearly counter-propagating continuous-wave pump-probe light at 780 nm and conventional single-modulation lock-in detection. The optical notch filter significantly suppressed stray pump light, enabling detection of stimulated Brillouin gain spectra with substantially improved acquisition times at adequate signal-to-noise ratios (∼25 dB in water samples and ∼15 dB in tissue phantoms). These results represent an important step towards the use of SBS spectroscopy for high-speed measurements of Brillouin gain resonances in scattering and non-scattering samples.

High resolution stimulated Brillouin gain spectrometer

We describe a stimulated Brillouin gain (SBG) spectrometer based on low power continuous-wave frequency-stabilized lasers. The high resolution and broad tuning range of this spectrometer are demonstrated through a SBG spectrum of glycerol in the glassy state (T=146 K). The narrow Brillouin linewidth (17 MHz half-width at half-maximum) and large Brillouin shift (17.31 GHz) in this spectrum illustrate the extremely high equivalent ‘‘finesse’’ of the instrument, a finesse that would be difficult to achieve through interferometric techniques. The accuracy and precision with which the Brillouin shift can be measured are demonstrated using SBG spectra of liquid methylene chloride as an example. Our Brillouin shift values obtained over the temperature range 178–300 K are in excellent agreement with values measured previously by others using spontaneous and stimulated Brillouin scattering.

Rotational–translational coupling and asymmetric line shapes in the high resolution stimulated Brillouin gain spectra of liquid carbon disulfide

We have measured high resolution stimulated Brillouin gain spectra of liquid carbon disulfide at a number of temperatures between 158 and 301 K. As the temperature decreases the shape of the Brillouin peak becomes increasingly asymmetric, with the intensity skewed away from the center of the spectrum. We show that this unusual asymmetry results from the coupling between rotational and translational motions. Rotational–translational coupling is well known to give rise to line shape distortions in the depolarized light scattering spectra of nonspherical molecules (e.g., the ‘‘Rytov dip’’), but the observation of line shape distortions (the asymmetry) in the polarized Brillouin lines has not been noted before. In the latter case, the asymmetry in the Brillouin peak can be traced to a cross correlation between the collective molecular orientation variable and the number density fluctuation and can be viewed as an acoustically induced birefringence. A calculation using the theory of rotational–translational coupling and experimental values for the relevant parameters reproduces quantitatively the observed asymmetry in the Brillouin lines over the entire temperature range.

High-resolution stimulated Brillouin gain spectroscopy of liquid carbon disulfide: Dependence of the linewidths and peak gain on scattering angle

We have used narrow linewidth continuous-wave lasers to obtain high-resolution stimulated Brillouin gain (SBG) spectra of liquid carbon disulfide at various scattering angles. For back-scattering angles, the linewidths of the SBG peaks reflect the natural lifetimes of the stimulated acoustic waves, whereas for forward-scattering angles the linewidths are dominated by laser jitter and by the uncertainty in the scattering wave vector that results from focusing the laser beams in the interaction region. The peak gain for the forward-scattering SBG spectra is an order of magnitude smaller than that observed at a back-scattering angle of 178°. The dependence of the peak gain on scattering angle can be accounted for quantitatively by considering the angle dependencies of the Brillouin frequency, linewidth, and laser beam crossing efficiency.