---
title: "Build & Debug Your First Laser Interactive Lab | Sandbox Physics"
description: "When a laser you built won't lase, how do you find the fault?"
canonical_url: "https://sandboxphysics.com/labs/build-debug-laser/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/build-debug-laser/"
last_modified: "2026-10-28"
---

L07 · Synthesis and measurement

# Build & Debug Your First Laser

**When a laser you built won't lase, how do you find the fault?** Check the excited population, the return path through the mirrors and the losses on each round trip. Extra pump cannot replace missing feedback. A narrow aperture can raise the threshold beyond the available pump. Once the laser oscillates, compare useful output, absorbed-pump efficiency, mode purity and heat.

Assemble a pump, a three- or four-level medium, curved mirrors, an aperture and a detector. Find hidden faults from the population, modal gain and output, then balance efficiency, mode purity and heat.

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

This is an illustrative classical rate model with three modes: ten billion emitters, emission at 1064 nm, an absorbed pump at 808 nm, an upper-level lifetime of 2 μs and a peak cross section of 0.000000000000000008 square metres. Concave mirrors have positive radii. Paraxial Gaussian geometry sets the mode area and the clipping at the aperture, which the beam passes twice per round trip. Marginal and unstable geometries have no supported mode in this model; real unstable-resonator lasers are outside its scope. The thin gain medium and aperture share the cavity midpoint in the model; their separation in the drawing is schematic. Axial length, transverse size and mirror curvature use separate display scales. The Gaussian envelope is capped at the viewport display limit. The three chosen longitudinal modes share one homogeneous population. Spontaneous emission seeds the modes as a continuous average source. Pump depletion, spatial hole burning, thermal lensing and optical phase are left out. Heat is estimated by its minimum, the quantum defect, and the score reflects teaching goals.

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

## A working laser is a balanced system

**Background** Inversion alone is not enough. The cavity must support a mode that overlaps the gain, and enough light must stay in that mode for the gain to replace all round-trip losses.

**Why it matters** This capstone brings together the mechanisms of L02–L06. A part can be missing, a setting can be wrong, and even a laser that oscillates can be inefficient.

Start with the essentials

- **Focus question**: Can you find a hidden fault from the measured populations, modal gain and output, and then fix it?
- **One-sentence intuition**: Check stored excitation, the return path and round-trip losses separately to find what prevents growth before increasing the pump.

### Core mathematical model

#### Three- versus four-level inversion

$$
d_4=n_u,\qquad d_3=2n_u-1
$$

In a four-level system the lower laser level empties instantly; in a three-level system the lower level is the ground state. So the same upper-level fraction can amplify in one scheme and absorb in the other.

#### Three modes compete for one population

$$
\begin{aligned}\dot n_u&=R(1-n_u)-\frac{n_u}{\tau_u}\\&\quad-\frac{d}{N_a}\sum_jG_jS_j\end{aligned}
$$

Stimulated transitions drain the same population that sets the gain of every mode. Photon growth includes stimulated gain, logarithmic cavity loss and a small, continuous spontaneous source.

#### Growth of each mode and the output

$$
\begin{aligned}\dot S_j&=(G_jd-\kappa)S_j\\&\quad+\frac{\beta N_an_u}{3\tau_u}\\P_o&=h\nu\kappa_o\sum_jS_j\end{aligned}
$$

The output coupler removes photons continuously; multiplying its escape rate by the stored photon energy gives power. Only three chosen longitudinal modes are included. Changing the detuning can shift the gain toward a neighboring chosen mode. Distant modes and spatial hole burning are not modeled.

#### Stable cavity geometry and aperture loss

$$
\begin{aligned}g_i&=1-\frac{L}{\mathcal R_i}\\0&<g_bg_o<1\\Q_a&=\left[1-e^{-2a^2/w_m^2}\right]^2\end{aligned}
$$

The Gaussian beam that reproduces itself after one round trip of the ray matrix sets the mode radius at the medium. The beam passes the central aperture twice per round trip. Degenerate geometries on the stability boundary are excluded from this model.

#### Pump energy and minimum heat

$$
\begin{aligned}P_p&=h\nu_pN_aR(1-n_u)\\P_{h,\min}&=P_p\left(1-\frac{\nu}{\nu_p}\right)\end{aligned}
$$

Pump power here means the power absorbed in the effective one-way pump model. An absorbed pump photon has more energy than an emitted laser photon. The minimum heat comes only from that energy difference, the quantum defect: it leaves out nonradiative processes and does not calculate temperature or thermal lensing.

### Common difficulties

#### Unstable cavities are outside this model

**Typical misconception** An unstable cavity can never lase.

**Better mental model** This Lab handles only stable cavities with paraxial Gaussian modes. Real unstable-resonator lasers need a different diffraction model. The degenerate confocal boundary can support Gaussian modes, but this implementation excludes that boundary from its general matrix solution.

#### The score is for teaching

**Typical misconception** A high score proves the laser could be manufactured.

**Better mental model** The material constants are illustrative, only three modes are kept and heat is a minimum estimate, so engineering limits are missing. The score judges the design within this model only.

### Run the experiment

1. 01

   #### Build it and let it settle

   Choose Working four-level design and inspect the two startup curves. The twenty-microsecond response is already recorded; open All numerical results to check the last-microsecond stability flag.

   **What to observe:** The population builds up first, then stimulated emission clamps the inversion. The output can overshoot before it settles.

2. 02

   #### Remove an essential part

   In Advanced controls, remove the rear mirror, then put it back. Each edit records the next twenty microseconds from the existing population. Separately, disconnect the detector.

   **What to observe:** Without feedback the oscillator cannot run. Disconnecting the detector only takes away the measurement; the population dynamics carry on unchanged.

3. 03

   #### Diagnose, then repair

   Load a hidden fault in Advanced controls and pick a diagnosis from the recorded response. Repair it with the controls, without loading a preset, then evaluate.

   **What to observe:** The fault may be too little pump, no inversion, a spectral mismatch, clipping, the output coupling, the geometry or missing feedback.

4. 04

   #### Trade power against quality

   Compare the Too little output coupling and Too much output coupling presets, then Broad gain competition. Try to improve the working design with less pump.

   **What to observe:** Too little extraction can waste useful output; too much raises the threshold. Broad gain lets more modes compete. The score rewards several goals at once.

Physical conclusion

A good design weighs extracted power, efficiency relative to the absorbed pump, the share of the dominant mode, settling and quantum-defect heat. The brightest output is only one goal.

## Related experiments and articles

Probes and measurement · Reading path

### Controlled light and precision measurement

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

1. [Article **How an ammonia beam became a microwave oscillator**](https://sandboxphysics.com/stories/probe-evidence/maser-inversion-and-feedback/)
2. [Lab **Absorption, Spontaneous & Stimulated Emission** Watch a probe beam grow, then fade, in a prepared gas](https://sandboxphysics.com/labs/absorption-spontaneous-stimulated-emission/)
3. [Lab **Pumping & Population Inversion** Pump three gases with one lamp and see which one amplifies](https://sandboxphysics.com/labs/pumping-population-inversion/)
4. [Lab **Passive Optical Resonator** Compare stored light with self-sustained oscillation](https://sandboxphysics.com/labs/passive-optical-resonator/)
5. [Lab **Stable Resonator & Transverse Modes** Curve two mirrors and size an iris until one beam shape is left](https://sandboxphysics.com/labs/stable-resonator-transverse-modes/)
6. [Lab **Laser Noise Startup Workbench** Follow a cavity mode from its first seed](https://sandboxphysics.com/labs/laser-noise-startup/)
7. [Lab **Laser Startup from Noise** Watch many cavity frequencies compete for one gain](https://sandboxphysics.com/labs/laser-startup-from-noise/)

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)