External cavity laser (ECL)
A laser built from a semiconductor gain chip and a wavelength-selective reflector outside the chip. The long cavity and sharp filtering buy narrow linewidth and wide tunability at the cost of size.
An external cavity laser removes one of the semiconductor laser's mirrors and replaces it with external optics: the gain chip (one facet AR-coated to kill its own cavity) supplies amplification, while an external element (diffraction grating, interference filter, or increasingly a photonic-integrated ring-resonator mirror) selects wavelength and closes the cavity.
The architecture attacks linewidth through both available levers. The long cavity stretches photon lifetime, shrinking the cold-cavity linewidth that enters the Schawlow–Townes formula squared; and the external filter decouples frequency selection from the carrier-laden gain medium, suppressing the α-mediated noise conversion. Bench ECLs reach 10–100 kHz routinely; hybrid-integrated designs pairing a III-V gain chip with ultra-low-loss Si₃N₄ ring mirrors have pushed intrinsic linewidths to the kHz and even sub-kHz class: laboratory coherence in a butterfly package.
Classic free-space geometries carry the names Littrow (grating retro-reflects its first order back into the chip; tuning rotates the grating; the output beam angle moves unless compensated) and Littman–Metcalf (grazing-incidence grating plus tuning mirror; double-pass filtering narrows further, fixed output direction, at some output-power cost). Both target mode-hop-free tuning: rotating about a designed pivot scans grating selection and cavity length synchronously, so one longitudinal mode is stretched continuously across tens of GHz rather than hopping.
Tunability is the second selling point: gain chips are spectrally broad, so a C-band ECL sweeps ~40 nm or more, the basis of the tunable sources in every WDM lab and of swept-source instrumentation. Integrated ECLs tune via ring-heater Vernier action across similar ranges.
Costs relative to a DFB: mechanical/thermal sensitivity (the cavity is only as stable as its mount), acoustic pickup in free-space versions, slower tuning, larger package, higher price. The DFB remains the deployed-transmitter answer; the ECL is the instrument-grade and coherent-source answer.