---
title: "Raman & Brillouin Lasers Interactive Lab | Sandbox Physics"
description: "Can transparent glass become a laser gain medium? Yes."
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language: "en"
page_type: "lab"
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  zh-CN: "https://sandboxphysics.com/zh/labs/raman-brillouin-lasers/"
last_modified: "2026-10-27"
---

L27 · Lasers powered by scattering

# Raman & Brillouin Lasers

**Can transparent glass become a laser gain medium?** Yes. A strong pump scatters off vibrations in the glass, molecular vibrations for Raman and sound waves for Brillouin, and amplifies light shifted to a lower frequency, called Stokes light. No population inversion is needed. Above threshold, the next Stokes order can grow, drain the previous one and hold it at a fixed level.

Pump a silica fiber past a Stokes threshold and watch the next order clamp the one before it. Capture a transient and calculate a steady pump sweep.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

The model uses spatially uniform, mean-field power equations. The pump is continually replenished and is not resonant, and there are up to three set Stokes modes. Raman light travels with the pump; bulk Brillouin orders alternate direction. This teaching cavity is defined by assumed silica gains, fast material relaxation and logarithmic round-trip losses. Photon-energy ratios keep track of the heat left behind as phonons. The set pump spectrum broadens the Brillouin gain; that width is not the emitted linewidth. A separate model of small phase fluctuations takes the same calculated steady powers, thermal phonon occupation at 300 kelvin, vacuum noise and the anti-Stokes noise from the next order, and calculates the fundamental Lorentzian linewidth of each lasing order. You can record a Gaussian phase walk for the first order and compare its measured diffusion and field coherence with that reference. This low-offset noise floor leaves out the transfer of technical pump noise, amplitude fluctuations, material dispersion, polarization and non-Gaussian behavior near threshold. The phase record is not the power transient. Transients are frozen copies, and the steady-state sweep and phase-noise reference are calculated diagnostics, not hardware measurements. Fiber routes, colors and dimensions are schematic.

Laser foundations · Light, gain and feedback

[L01 · Compare sources](https://sandboxphysics.com/labs/laser-source-comparison/) [L02 · Count transitions](https://sandboxphysics.com/labs/light-matter-transitions/) [L03 · Build inversion](https://sandboxphysics.com/labs/population-inversion/) [L04 · Amplify once](https://sandboxphysics.com/labs/single-pass-amplifier/) [L05 · Add feedback](https://sandboxphysics.com/labs/passive-optical-resonator/) [L06 · Start from noise](https://sandboxphysics.com/labs/laser-noise-startup/) [L07 · Build and debug](https://sandboxphysics.com/labs/build-debug-laser/) [L08 · Measure and diagnose](https://sandboxphysics.com/labs/laser-diagnostic-bench/) [L09 · Select axial modes](https://sandboxphysics.com/labs/gain-longitudinal-modes/) [L10 · Confine the beam](https://sandboxphysics.com/labs/stable-resonator-geometry/) [L11 · Select spatial modes](https://sandboxphysics.com/labs/transverse-laser-modes/) [L12 · Propagate and measure](https://sandboxphysics.com/labs/gaussian-beam-quality/) [L13 · Align and match](https://sandboxphysics.com/labs/alignment-mode-matching/) [L14 · Optimize extraction](https://sandboxphysics.com/labs/output-coupler-optimization/) [L15 · Drive the dynamics](https://sandboxphysics.com/labs/laser-dynamics-relaxation/) [L16 · Store and release](https://sandboxphysics.com/labs/q-switched-giant-pulse/) [L17 · Lock phases into pulses](https://sandboxphysics.com/labs/mode-locking-frequency-comb/) [L18 · Calibrate the comb](https://sandboxphysics.com/labs/optical-frequency-comb/) [L19 · Inject carriers](https://sandboxphysics.com/labs/semiconductor-laser-diode/) [L20 · Compare gain media](https://sandboxphysics.com/labs/laser-medium-zoo/) [L21 · Distributed gain](https://sandboxphysics.com/labs/fiber-fbg-laser/) [L22 · Choose feedback](https://sandboxphysics.com/labs/laser-feedback-architectures/) [L23 · Balance ultrafast pulses](https://sandboxphysics.com/labs/ti-sapphire-oscillator/) [L24 · Stretch and compress](https://sandboxphysics.com/labs/chirped-pulse-amplification/) [L25 · Trap and extract](https://sandboxphysics.com/labs/regenerative-multipass-amplifier/) [L26 · Convert frequencies](https://sandboxphysics.com/labs/nonlinear-frequency-conversion/) [L27 · Scatter and cascade](https://sandboxphysics.com/labs/raman-brillouin-lasers/) [L28 · Synchronize a slave](https://sandboxphysics.com/labs/injection-locking/) [L29 · Lock to a reference](https://sandboxphysics.com/labs/pdh-frequency-lock/) [L30 · Measure phase noise](https://sandboxphysics.com/labs/laser-linewidth-phase-noise/) [L31 · Scatter for feedback](https://sandboxphysics.com/labs/random-laser/) [L32 · Diagnose thermal scaling](https://sandboxphysics.com/labs/thermal-lensing/) [L33 · Combine coherent fields](https://sandboxphysics.com/labs/coherent-beam-combining/) [L34 · Sustain a Kerr pulse](https://sandboxphysics.com/labs/microresonator-kerr-comb/) [L35 · Return a delayed field](https://sandboxphysics.com/labs/external-feedback-chaos/) [L36 · Scan without mode hops](https://sandboxphysics.com/labs/tunable-laser-scan/)

Physical basis: [MIT: coherence and spectrum](https://ocw.mit.edu/courses/res-6-006-video-demonstrations-in-lasers-and-optics-spring-2008/resources/coherence-length-and-source-spectrum/) · [Rate equations](https://ocw.mit.edu/courses/6-977-ultrafast-optics-spring-2005/resources/chapter2/) · [Pumping and gain media (§7.2)](https://ocw.mit.edu/courses/6-974-fundamentals-of-photonics-quantum-electronics-spring-2006/bf3bdeb971819ddc9c68719d28bb2745_classnotes.pdf) · [Laser dynamics](https://ocw.mit.edu/courses/6-977-ultrafast-optics-spring-2005/resources/chapter4/)

Physics tutorial

## Scattered photons can sustain an oscillator

**Background** The pump light supplies the energy for both the Stokes light and the vibration it leaves in the material.

**Why it matters** Transparent glass can still amplify light by scattering. Pump a silica fiber past a Stokes threshold, watch the next order clamp the one before it, then capture a transient and calculate a steady pump sweep.

Start with the essentials

- **Focus question**: Can transparent glass become a laser gain medium?
- **One-sentence intuition**: When the next Stokes order starts oscillating, it drains its parent and clamps it; it does not simply add more output.

### Core mathematical model

#### One photon becomes light and vibration

$$
\begin{aligned}\nu_{j-1}&=\nu_j+\Omega_j\\h\nu_{j-1}&=h\nu_j+h\Omega_j\end{aligned}
$$

Each scattering event moves one photon from an order to the next, but the photons in the two orders carry different energies.

#### Average gain over the fiber

$$
\begin{aligned}C_j&=g_jL_{\rm eff}/A_{\rm eff}\\L_{\rm eff}&=(1-e^{-\alpha L})/\alpha\end{aligned}
$$

The gain overlap includes the set error in mode spacing. A spectrally broad pump lowers the Brillouin gain.

#### Stokes growth and depletion

$$
\begin{aligned}\dot P_j&=G_j-D_j\\G_j&=C_jP_{j-1}P_j-\Lambda P_j\\D_j&=r_jC_{j+1}P_jP_{j+1}\end{aligned}
$$

Time here is counted in round trips. The frequency ratio in the depletion term accounts for the energy carried off by the emitted phonon.

#### First oscillation threshold

$$
\begin{aligned}P_{0,\rm th}&=\Lambda/C_1\\P_{\rm in,th}&=a\Lambda/C_1\end{aligned}
$$

The pump in the fiber decays with coefficient a. Each further Stokes order needs more power from its parent.

#### Heat left behind as phonons

$$
\begin{aligned}Q_{\rm ph}&=\sum_j(r_{j-1}-1)S_j\\S_j&=C_jP_{j-1}P_j\\r_{j-1}&=\nu_{j-1}/\nu_j\end{aligned}
$$

The fraction of optical energy lost is small for Brillouin scattering and much larger for Raman scattering.

#### In steady state the parent is clamped

$$
\begin{aligned}C_jP_{j-1}&=\Lambda+r_jC_{j+1}P_{j+1}\\P_j&>0\end{aligned}
$$

This relation gives every order that oscillates in a steady-state branch. The last order has no higher order to drain it.

#### Fundamental linewidth of each output

$$
\begin{aligned}N_m&=P_m t_{\rm rt}/(h\nu_m)\\A_m&=\gamma(\bar n+1)\\B_m&=2\mu_mN_{m+1}(2\bar n+1)\\\Delta\nu_m&=(A_m+B_m)/(4\pi N_m)\end{aligned}
$$

A separate model of small phase fluctuations at low offset frequency uses the same steady powers. When the next order lases, it adds anti-Stokes noise. Technical noise from the pump is left out.

#### Record phase, then test coherence

$$
\begin{aligned}d\varphi&=\sqrt{2\pi\Delta\nu_m}\,dW\\g^{(1)}(t)&=e^{-\pi\Delta\nu_m|t|}\\\widehat{\Delta\nu}&=\frac{\operatorname{Var}(\delta\varphi)}{2\pi\delta t}\end{aligned}
$$

The simulated phase record lets you check the diffusion and the coherence. The Fourier transform of the exponential coherence is a Lorentzian with this full width.

### Common difficulties

#### The gain width is the output linewidth

**Typical misconception** The gain bandwidth shown tells you the linewidth of the emitted laser light.

**Better mental model** The gain bandwidth belongs to the power model. The separate phase model calculates the steady, phase-matched noise floor and records how the phase diffuses. Technical pump noise and fluctuations near threshold are outside its scope.

#### More pump always grows the first order

**Typical misconception** Every Stokes order keeps growing as the pump increases.

**Better mental model** Once the next order oscillates, it drains its parent, and the parent stays clamped on that branch.

### Run the experiment

1. 01

   #### Cross the threshold

   Compare Below threshold with Raman, then drag the pump dial. Each setting records six hundred round trips automatically.

   **What to observe:** The fixed seed light dies away below threshold and grows above it, until pump depletion limits the growth.

2. 02

   #### Follow the energy into another order

   Choose Cascaded orders and follow the three colored outputs and the pump sweep below. Open Advanced controls to compare One allowed order.

   **What to observe:** As a new order grows, its parent is clamped. The separately calculated sweep shows the thresholds one after another.

3. 03

   #### Broaden the pump

   Compare narrow and broad Brillouin pumping at the same power, and look at the backward and forward ports.

   **What to observe:** Less overlap raises the threshold. Odd and even Brillouin orders leave through opposite ports.

4. 04

   #### Measure the output phase

   Open Advanced controls, choose One allowed order and click Record output phase. Expand the separate phase-noise record below, then compare it with a cascade.

   **What to observe:** The emitted linewidth is not the gain bandwidth, and in a cascade the next order adds its own noise channel.

Physical conclusion

Look at the threshold, the power in each order and the phonon heat together, and remember that a narrow gain window does not measure the linewidth.

## Related experiments and articles

Probes and measurement · Reading path

### Molecules, forces and ultrafast dynamics

Articles on this path that use this experiment, followed by the other experiments they suggest.

1. [Article **Raman: the shifted lines moved with the source**](https://sandboxphysics.com/stories/probe-evidence/raman-shift-and-molecular-evidence/)
2. [Lab **Spontaneous Raman Spectrometer** Measure a spontaneous Raman spectrum](https://sandboxphysics.com/labs/spontaneous-raman-spectrometer/)
3. [Lab **Vibrational EELS: Resolution & Aloof Probes** Compare electron energy-loss spectroscopy](https://sandboxphysics.com/labs/vibrational-eels/)

Waves & optics · Starter path

## From one ray to interference measurement

Start with refraction, find where the ray picture stops working, then use interference to measure tiny distances.

[Start here → **Optical Boundary Explorer** Turn the incident angle and watch reflected and transmitted power trade off. Then switch to the critical-angle scene. Observe · 11 min](https://sandboxphysics.com/labs/optical-boundary-explorer/)

1. [1 Optical Boundary Explorer](https://sandboxphysics.com/labs/optical-boundary-explorer/)
2. [2 Rainbow Caustic Observatory](https://sandboxphysics.com/labs/rainbow-laboratory/)
3. [3 Camera Obscura](https://sandboxphysics.com/labs/camera-obscura/)
4. [4 Coherent Wave Composer](https://sandboxphysics.com/labs/coherent-wave-composer/)
5. [5 Young Double-Slit Bench](https://sandboxphysics.com/labs/young-double-slit-bench/)
6. [6 Michelson Precision Lab](https://sandboxphysics.com/labs/michelson-precision-lab/)

[View the full path →](https://sandboxphysics.com/topics/waves-and-optics/#starter-path)