Formula used
This calculator estimates the fundamental frequency linewidth using a modified Schawlow–Townes style expression (FWHM):
Δν = π · h · ν · (Δνc)² / Pout
Where ν = c/λ, Pout is output power, and Δνc is the passive cavity linewidth (FWHM). The tool also applies optional broadening factors:
Δνtotal = Δν · nsp · K · (1 + α²)
Notes: Δνc can be entered directly, or derived from photon decay time via Δνc = 1/(2π τc), or from cavity Q via Δνc = ν/Q.
How to use this calculator
- Enter the laser wavelength and output power.
- Select how you want to specify the cavity linewidth: Δνc, photon decay time τc, or cavity Q.
- Fill the corresponding field, then optionally add nsp, α, and K.
- Click Estimate Linewidth to see results above the form.
- Use the CSV/PDF buttons to export the computed report.
Example data table
| λ (nm) | Pout (mW) | Input option | Value | nsp | α | K | Estimated Δν (MHz) |
|---|---|---|---|---|---|---|---|
| 1550 | 10 | Δνc | 1 MHz | 1.0 | 0 | 1.0 | Varies by assumptions |
| 1064 | 50 | τc | 10 ns | 1.0 | 0 | 1.0 | Varies by assumptions |
| 780 | 5 | Q | 2×108 | 1.2 | 3 | 1.3 | Varies by assumptions |
Use the calculator to compute the exact values for each row.
Engineering notes
- This estimate targets the quantum-limited linewidth; technical noise (vibration, current noise, thermal drift) can dominate in practice.
- Different references use slightly different conventions and factors; treat results as an order-of-magnitude design guide.
- For very narrow linewidth systems, measurement bandwidth and definition (instantaneous vs integrated) also matter.