Elena Lopez Aviles, a Senior Optical Engineer at Coherent, explains the phenomenon of Stimulated Brillouin Scattering (SBS) in optical fibers. She details the underlying physics, where incident light interacts with thermally generated acoustic phonons, creating a moving refractive index grating that backscatters light with a Doppler-shifted frequency. While SBS limits power transmission in communication systems, its sensitivity to medium density makes it valuable for sensing applications (temperature, strain) and enables Brillouin lasers/amplifiers. The presentation outlines a numerical method using COMSOL Multiphysics (Wave Optics, Acoustics modules) integrated with MATLAB via LiveLink to calculate the SBS gain spectrum. This involves defining material properties, geometry, solving for optical and acoustic modes, calculating acousto-optic overlap integrals, and summing modal contributions to predict the fiber's SBS characteristics. Coherent utilizes this capability to design specialty optical fibers with tailored SBS gain levels for specific applications.
Stimulated Brillouin Scattering (SBS) in Optical Fibers: Physics and Numerical Modeling
[00:00:00] Introduction: Elena Lopez Aviles from Coherent introduces the topic of Stimulated Brillouin Scattering (SBS) in specialty optical fibers fabricated at their Norsal facility.
[00:00:32] SBS Phenomenon Explained: SBS is a non-linear process where light propagating through a fiber interacts with vibrating atoms (acoustic phonons). This interaction creates a moving periodic modulation of the refractive index (a grating) due to interference between incident and backscattered light.
[00:01:34] Frequency Shift and Backscattering: The moving grating strongly reflects light backward. Due to the Doppler effect associated with the grating moving at the acoustic velocity, the backscattered (Stokes) light is downshifted in frequency compared to the incident (pump) light.
[00:01:51] Power Limitation and Threshold: As input power increases, transmitted power eventually saturates because significant power is reflected backward via SBS. The SBS threshold (power level where this becomes significant) can be low (~1 mW for CW/long pulses) and depends on the pump spectrum.
[00:02:33] SBS Applications: While limiting in communications, SBS is useful for sensors (exploiting sensitivity to temperature/strain), fiber lasers, and amplifiers.
[00:02:49] Classical SBS Description: SBS involves interaction between the pump light, the Stokes light (backscattered), and an acoustic wave. The pump induces an index grating, which scatters the pump light (Bragg diffraction).
[00:03:32] Conservation Laws and Phase Matching: Energy and momentum must be conserved. The frequency shift corresponds to the acoustic wave frequency (Ω), and the wave vector difference matches the acoustic wave vector (q). A phase-matching condition relates Ω, q, pump wavelength, and effective index.
[00:04:06] Acoustic Velocity and Modes: The acoustic wave frequency and wave vector are related by the acoustic velocity (Vm), which differs for various acoustic modes (m) in the fiber.
[00:04:15] Directionality of Scattering: SBS is predominantly a backward process because the phase-matching condition is best met for backward scattering (angle = π). Weak spontaneous forward scattering (GAWBS) can occur due to guided acoustic waves.
[00:05:39] Brillouin Gain Spectrum: The growth of the Stokes wave is described by the Brillouin gain spectrum, G(Ω), which has a Lorentzian shape. Key parameters include the peak gain (G₀, depends on material properties), Brillouin linewidth (ΓB, related to acoustic mode lifetime/loss), resonant frequency (Ωm), and the Acousto-Optic effective Area (Am, dependent on the overlap between optical and acoustic modes).
[00:07:33] Numerical Calculation Approach: Coherent uses COMSOL Multiphysics (Wave Optics, Acoustics modules) and the LiveLink for MATLAB to numerically calculate the SBS gain spectrum.
[00:07:51] Simulation Workflow:
MATLAB sets optical/acoustic parameters in COMSOL and sweeps frequencies.
COMSOL model defines parameters (material properties like Pockels coefficient), geometry (e.g., step-index or custom profiles using quarter symmetry), materials (assigning refractive index and acoustic velocity profiles), damping (related to Brillouin linewidth), and meshes (finer for acoustics).
COMSOL solves for the fundamental optical mode.
For each frequency in the sweep, COMSOL solves for multiple acoustic modes near the phase-matching condition.
Overlap integrals between the optical mode and each acoustic mode are calculated to determine the acousto-optic area (Am).
MATLAB sums the gain contributions from all relevant modes at each frequency to construct the final SBS gain spectrum.
[00:15:08] Example and Validation: An example SBS gain spectrum for a step-index fiber is shown. Calculations for custom profiles show good agreement with literature and experimental results.
[00:15:41] SBS Advantages vs. Disadvantages: Undesired in high-power transmission due to power limits. Advantageous for sensing, as the acoustic velocity (and thus Brillouin frequency shift) is sensitive to density changes caused by temperature or strain.
[00:16:27] Conclusion and Capability: The numerical method allows prediction of SBS gain spectra. Coherent has the capability to design and fabricate optical fibers with specific, tailored SBS gain levels depending on the target application.