Enter Design Inputs
Example Data Table
| Example | Wavelength | Power | Cavity Length | Estimated Linewidth | Coherence Length |
|---|---|---|---|---|---|
| Ultra-stable fiber cavity | 1550 nm | 20 mW | 10 cm | 0.006757 Hz | 14,122,040.545340 km |
| Short diode cavity | 980 nm | 8 mW | 0.05 cm | 3.1032 MHz | 30.751069 m |
| Gas resonator style | 632.8 nm | 1.2 mW | 25 cm | 0.014027 Hz | 6,803,185.727587 km |
Formula Used
ν0 = c / λ
ρ = R1 × R2 × (1 − Lrt)
αtot = ln(1 / ρ) / (2L)
τp = 1 / (vg αtot), where vg ≈ c / n
Δνcav = 1 / (2π τp)
Δνest = F × nsp × (1 + αH2) × [hν0 / (4π P τp2)]
τc ≈ 1 / (π Δνest)
Lc ≈ c τc
This model is a practical engineering estimate. It is most useful for comparing design changes, seeing scaling trends, and understanding how power, cavity loss, reflectivity, and phase-amplitude coupling influence linewidth.
How to Use This Calculator
- Choose a preset if you want sensible starting values for a common laser family.
- Enter wavelength, output power, cavity length, and refractive index.
- Add both mirror reflectivities and the estimated internal round-trip loss.
- Set the spontaneous emission factor, Henry alpha factor, and excess noise multiplier.
- Press the calculate button. The result appears above the form under the header.
- Review the linewidth, coherence, cavity linewidth, finesse, and the graph showing linewidth sensitivity to output power.
Frequently Asked Questions
1) What does laser linewidth represent?
Laser linewidth is the spectral width of the optical output around its center frequency. Narrower linewidth usually means better coherence, cleaner interference performance, and improved frequency stability.
2) Why does higher output power often reduce linewidth?
In Schawlow-Townes style models, linewidth scales roughly inversely with output power. More photons in the lasing mode reduce the relative impact of spontaneous emission noise.
3) Why are mirror reflectivities important?
Mirror reflectivities help determine cavity loss and photon lifetime. Higher reflectivity generally keeps photons in the resonator longer, which can reduce passive cavity linewidth and narrow the estimated laser linewidth.
4) What is the Henry alpha factor?
The Henry alpha factor links amplitude changes to phase changes. In semiconductor lasers, it can strongly broaden linewidth, which is why the calculator includes the term (1 + αH2).
5) Is this result exact for every laser?
No. This is an engineering estimate. Real linewidth can be wider because of thermal drift, current noise, acoustics, vibration, multimode behavior, pump fluctuations, and measurement setup limitations.
6) Does a longer cavity always mean a narrower linewidth?
Not always. A longer cavity often improves photon lifetime and reduces free spectral range, but the result also depends on distributed loss, mirror losses, output coupling, and technical noise sources.
7) Why can measured linewidth be much broader than predicted?
Measured linewidth may include low-frequency drift, environmental disturbances, current modulation, thermal wander, and flicker noise. These effects can dominate over the intrinsic linewidth floor predicted by a quiet cavity model.
8) Which inputs should I refine first for better accuracy?
Start with output power, cavity length, reflectivities, and internal loss. Then improve spontaneous emission factor, alpha factor, and excess noise assumptions using device data, measurements, or vendor specifications.