---
title: "Laser Diagnostic Bench Interactive Lab | Sandbox Physics"
description: "Which measurements tell laser light apart from fluorescence and amplified spontaneous emission? It takes several; no single one settles it."
canonical_url: "https://sandboxphysics.com/labs/laser-diagnostic-bench/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/laser-diagnostic-bench/"
last_modified: "2026-10-27"
---

L08 · Synthesis and measurement

# Laser Diagnostic Bench

**Which measurements tell laser light apart from fluorescence and amplified spontaneous emission?** It takes several; no single one settles it. Output that suddenly climbs faster once the pump passes a threshold, separate cavity modes in the spectrum, Michelson fringe contrast that fades and then returns as the path difference grows, and a beam that propagates well together make the case. A linewidth too narrow for the spectrometer to resolve is only an upper limit.

Connect an unknown source to six instruments. Record a pump sweep, a spectrum at finite resolution, the beam caustic, a polarization scan, Michelson visibility and a fast photodiode trace, then decide from the evidence what the source is.

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

The four source families at 1064 nm are phenomenological models, not devices calibrated to real materials. One source description feeds every instrument: normalized Lorentzian lines, Gaussian-Schell beam propagation and a mix of linearly polarized and unpolarized light. The spectrometer convolves the spectrum with a Lorentzian instrument response, so a measured peak width may still be unresolved. Beam quality comes from fitting the caustic across several planes, not from one image, and it is a teaching proxy, not an ISO measurement. The Michelson interferometer assumes balanced arms and perfect spatial and polarization overlap; the optical path difference is twice the mirror travel. The photodiode samples classical complex fields with a finite bandwidth, and its RF bins are not a calibrated noise power spectral density or quantum correlations. No finite set of these measurements can identify every real source uniquely.

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

## Identify a source from converging evidence

**Background** Fluorescence, amplified spontaneous emission (ASE) and laser output can look alike in brightness, directionality or polarization. To tell them apart, combine the response to the pump with spectral, spatial and time measurements.

**Why it matters** Real instruments have limited resolution and bandwidth, so what a detector records is not the ideal property of the source. Several kinds of source can also give similar results in any single measurement.

Start with the essentials

- **Focus question**: What evidence tells a narrow ASE source from a laser oscillator, or a multimode laser from a single-mode one?
- **One-sentence intuition**: A kink in the pump response, resolved mode structure, revivals in coherence and beam quality each add a different piece of evidence. A linewidth the instrument cannot resolve is only a bound.

### Core mathematical model

#### Finite spectral resolution

$$
\begin{aligned}S_{\mathrm{rec}}(\nu)&=(S*K)(\nu)\\\Delta\nu_{\mathrm{rec}}&=\Delta\nu_s+\Delta\nu_{\mathrm{inst}}\end{aligned}
$$

Adding the widths works only for a single Lorentzian line measured with a Lorentzian instrument response. Overlapping multimode peaks need not follow this rule.

#### The spectrum predicts coherence

$$
\begin{aligned}g^{(1)}(t)&=\sum_j p_j e^{-\pi\Delta\nu_j|t|}\\&\qquad\cdot e^{2\pi i\nu_jt}\end{aligned}
$$

The same normalized mix of Lorentzian lines drives both the interferometer and the sampled complex fields. Each spectral component accumulates its own phase as the delay increases. The components can cancel and later align again, so a multimode comb makes visibility dip and revive.

#### Michelson interferometer with balanced arms

$$
\begin{aligned}V(\delta)&=|g^{(1)}(\delta/c)|\\P_d&=\frac{P}{2}[1+V\cos\phi]\end{aligned}
$$

The optical path difference is twice the mirror travel. The phase scan records one output port; the two ports together carry all the input power.

#### Beam quality from a caustic, not one image

$$
\begin{aligned}w^2(z)&=w_0^2+\theta^2z^2\\M^2&=\frac{\pi w_0\theta}{\lambda}\end{aligned}
$$

The model uses a circular Gaussian-Schell source with a second-moment radius. A wider beam in one plane can be either a large waist or rapid divergence. A fit along the propagation direction separates these and gives the beam-quality proxy; the image shows only the chosen camera plane.

#### Analyzer transmission

$$
P_\theta=P[(1-p)/2+p\cos^2\theta]
$$

The light is part linearly polarized and part unpolarized. Highly polarized ASE is included on purpose, because polarization alone cannot prove that a laser is oscillating.

### Common difficulties

#### The first dip is not always the coherence length

**Typical misconception** Every dip in visibility marks a unique coherence length.

**Better mental model** The readout reports where the visibility first drops to one over e within the finite scan. Multimode revivals and long coherence that the scan cannot resolve mean no single number describes every case.

#### Detector noise depends on how you measure

**Typical misconception** The recorded relative RMS is an intrinsic noise specification of the laser.

**Better mental model** It depends on the bandwidth, the time window and the random realization. The RF bins from the finite window are a transform of the same trace, not calibrated RIN or a quantum photon-statistics result.

### Run the experiment

1. 01

   #### Learn the four known sources

   Predict which source will show a threshold kink, then compare the four known-source presets. All six instruments are acquired automatically; switch between Pump sweep, Spectrum and Coherence.

   **What to observe:** Fluorescence and ASE respond smoothly to the pump. The lasers show a threshold kink in this teaching model, and multimode coherence revives instead of simply decaying.

2. 02

   #### Find the instrument limits

   Choose Known single-mode laser, select Spectrum and widen the response. It updates as you drag; narrow the response again.

   **What to observe:** A peak limited by resolution does not give the true linewidth. The demonstration reacquires all instruments at the new setting. In an unknown challenge, only the selected instrument is reacquired; the other records are hidden until measured again.

3. 03

   #### Add independent measurements

   Acquire the beam caustic and polarization. For a multimode source, compare fast traces at broad and narrow detector bandwidth.

   **What to observe:** The beam-quality proxy uses several planes. A narrow electrical bandwidth hides the resolved beating between modes. ASE can also be strongly polarized.

4. 04

   #### Classify an unknown source

   In Advanced controls, load an unknown source. One instrument is acquired at entry; use the instrument buttons to collect the pump sweep, spectrum and coherence, plus either the beam or the fast trace, then submit a class.

   **What to observe:** The challenge needs several pieces of evidence that agree. A correct answer matches this teaching family; it is not a proof that would hold for every possible real source.

Physical conclusion

All measurements must come from the same setup and agree. Combine independent instruments, and keep what you infer separate from the recorded data and the instrument response.

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)