Glossary entry

DFB laser

Distributed feedback laser — a semiconductor laser in which a Bragg grating integrated along the cavity selects a single longitudinal mode, producing narrow-linewidth single-frequency emission.

A distributed feedback (DFB) laser uses a Bragg grating integrated along the length of the active region to provide wavelength-selective optical feedback. Only wavelengths satisfying the Bragg condition

\lambda_B \;=\; 2 \, n_\text{eff} \, \Lambda

experience constructive feedback, where $n_\text{eff}$ is the effective index of the waveguide mode and $\Lambda$ is the grating period. All other longitudinal modes are suppressed, producing single-mode emission.

Standard performance metrics for telecom DFB devices:

Parameter	Typical value
Threshold current	5–20 mA
Side-mode suppression ratio (SMSR)	$>$ 35 dB
Spectral linewidth	1–10 MHz (sub-MHz with external feedback)
Wavelength temperature coefficient $d\lambda/dT$	0.08–0.10 nm/°C
Wavelength current coefficient $d\lambda/dI$	$\sim 0.01$ nm/mA
Operating wavelength range	1270–1620 nm (telecom O, E, S, C, L bands)

The wavelength temperature coefficient of DFB devices ( $\sim 0.09$ nm/^\circ C) is significantly smaller than that of Fabry–Pérot devices ( $\sim 0.4$ nm/^\circ C), because $\lambda_B$ is set by the grating geometry rather than by the gain peak. Temperature stability of the emission wavelength enables dense wavelength-division multiplexing (DWDM) on 50 GHz and 100 GHz channel grids.

DFB lasers are the dominant source for fiber-optic telecom links operating in the 1300–1550 nm band, used in coherent transmission systems, intra-datacenter optical interconnects, and gas sensing.