A single-wavelength semiconductor laser with a grating built into the cavity — the workhorse source for single-mode optics.
What a DFB Laser Is
A distributed feedback laser has a diffraction grating etched along the length of its active region. The grating provides strong optical feedback at one wavelength and almost none at any other, so of all the longitudinal modes the cavity could support, one wins decisively. The output is a single narrow line rather than the comb a Fabry-Perot laser produces.
That single-mode behaviour is what makes long single-mode links possible. A broad source spreads in time as it travels, because different wavelengths move at different speeds in glass; a source with a very narrow line hardly spreads at all. See chromatic dispersion for the mechanism.
Temperature Sets the Wavelength
The grating period and the refractive index of the semiconductor together fix the emitted wavelength, and the index changes with temperature. A DFB therefore drifts by roughly 0.1 nanometres per degree Celsius — over a 0 to 70 degree case-temperature range, seven or eight nanometres of movement.
Whether that matters depends entirely on the grid. A CWDM channel is 20 nm from its neighbours with a passband of roughly plus or minus 6.5 nm, so eight nanometres of drift stays inside the filter and the laser can run uncooled — which is most of the reason CWDM optics are cheap. LAN-WDM packs four channels about 4.5 nm apart and DWDM puts them 0.8 nm apart on the 100 GHz grid; both need a thermoelectric cooler holding the die to a fraction of a degree.
Direct Modulation and Its Limit
The simplest way to send data is to switch the drive current. That works well and is what almost every 10G and 25G single-mode transmitter does, but every current change momentarily alters the carrier density, the refractive index and therefore the wavelength. The emitted pulse is smeared across a range of wavelengths before it enters the fibre, an effect called chirp.
Dispersion then turns that wavelength spread into time spread, and the eye closes with distance. Chirp, not launch power, is what caps how far a directly modulated source can usefully go — and the fix is to stop modulating the laser at all, which gives an EML.
Where You Meet It
Essentially every single-mode transceiver, in one of two roles. Directly modulated, it is the source in 10G LR and ER, 25G LR, 100G CWDM4 and PSM4, and 400G DR4. As the continuous-wave source inside an EML, it also sits behind 100G LR4 and ER4 and 400G FR4 and LR4 — along with every fixed-channel or tunable CWDM and DWDM optic. Uncooled DFBs dominate the cheap end of the range and cooled ones the dense-wavelength end.
Practically, the DFB is why a CWDM optic and a DWDM optic at nominally similar reach have such different prices and power figures. The grid calculator and the wavelength converter will map a channel number to the wavelength the laser has to hold.
DFB reference
| Wavelength selection | a Bragg grating written along the active region |
|---|---|
| Side-mode suppression ratio | 30 dB minimum in IEEE PMDs, 40 dB and better in practice |
| Wavelength drift with temperature | about 0.1 nm per degree C |
| CWDM grid | 20 nm spacing, roughly ±6.5 nm passband — uncooled is fine |
| LAN-WDM grid | about 4.5 nm spacing (800 GHz) — cooled |
| DWDM grid | 0.8 nm at 100 GHz spacing — thermoelectric cooler mandatory |
| Typical wavelengths | 1270 to 1610 nm |
| Modulation | direct for shorter reaches; external modulation gives an EML |