Broadband SLDs and narrow-linewidth lasers sit at opposite ends of the coherence spectrum — one engineered short, the other long.
Coherence length is the maximum path difference over which two beams still form clear interference fringes. It is set entirely by the source bandwidth: the wider the spectrum, the faster fringes from different wavelengths walk off each other. The two coefficients come from lineshape — Gaussian spectra (SLDs and most broadband sources) use (2ln2/π)·λ²/Δλ, while phase-noise-dominated single-frequency lasers use the Lorentzian c/(π·Δν).
SLDs are engineered wide on purpose: pushing coherence down to tens of microns suppresses coherent noise from Rayleigh backscatter and polarization cross-coupling in fiber gyros, and directly sets OCT axial resolution. DFB / ECL lasers are engineered narrow: stretching coherence to tens of meters or kilometers enables coherent detection, distributed sensing and heterodyne interferometry. When selecting, ask: how much path difference must your application "remember"?
Related tools: OCT axial resolution · Spectral ripple & FSR · Δλ↔Δν converter
※ Formulas on this page assume ideal models; all device parameters shown are typical values — refer to the datasheet and the serialized factory test report shipped with each unit. For selection support, contact sales@lncetek.com.