Spectral Width Converter Δλ ↔ Δν

Converter

Center wavelength λ (nm) Width Δλ

⇄

Width Δν

Δν = c · Δλλ² Δλ = λ² · Δνc First-order approximation for Δλ ≪ λ; valid for both bandwidth and linewidth

Typical device widths (@1550 nm, typical values)
Device	Usually quoted as	Converted	Note
External-cavity laser (ECL)	10 kHz	0.00008 pm	Ultra-narrow, coherent sensing
DFB narrow-linewidth	2 MHz	0.016 pm	Single mode, TDLAS / coherent
Memory anchor	100 MHz	0.8 pm	≈125 MHz per pm at 1550 nm
OSA resolution	0.02 nm	2.5 GHz	Why an OSA cannot measure MHz linewidths
SLD broadband	50 nm	6.24 THz	Low-coherence source, FOG / OCT

Why vendors mix two unit systems

Narrow-linewidth lasers (DFB / ECL) are specified in MHz / kHz because linewidth is set by phase noise and measured in the frequency domain (self-heterodyne). Broadband devices (SLD) and spectrometers use nm / pm — a grating OSA reads wavelength directly. The two convert via Δν = c·Δλ/λ², but the conversion depends on center wavelength: the same 0.1 nm is ≈17.5 GHz at 1310 nm yet ≈12.5 GHz at 1550 nm.

A common pitfall: "measuring" a DFB's MHz-class linewidth with an OSA (0.02 nm ≈ 2.5 GHz resolution) — you only read the instrument's resolution. MHz linewidths require the self-heterodyne method, which is what our factory reports use for narrow-linewidth grades.

Spectral Width Δλ ↔ ΔνBandwidth Unit Conversion

Why vendors mix two unit systems