Laser types: VCSEL, FP, DFB, EML, tunable, SiPh

Six kinds of light source cover essentially every optical transceiver made. They differ in how the light is generated, how it is modulated, how pure the wavelength is — and therefore in cost, reach and the fibre they suit.

At a glance

VCSEL — vertical-cavity surface-emitting laser

The light exits perpendicular to the wafer, so VCSELs are tested on the wafer, made in arrays and coupled to multi-mode fibre with simple lenses. They are the cheapest and lowest-power lasers and the reason short-reach optics (SX, SR, SR4, SR8, AOC) cost what they do. Limits: 850 nm only on multi-mode fibre, so reach is set by modal bandwidth — roughly 550 m at 1G, 300–400 m at 10G on OM3/OM4, 100 m at 25G, 70–100 m at 50G PAM4. Per-lane rate is capped near 100G PAM4, which is why 800G SR8 was the last VCSEL generation planned at scale. See chip-on-board.

FP — Fabry–Pérot

The simplest edge-emitting laser: two cleaved facets form the cavity and it lases on several longitudinal modes at once. That broad spectrum spreads in the fibre (chromatic dispersion), so FP lasers are limited to low rates and short single-mode reaches — 1000BASE-LX at 10 km, 2 km at 2.5G. Cheap, uncooled, largely legacy today.

DFB — distributed feedback

A grating etched along the active region forces a single longitudinal mode: narrow linewidth, stable wavelength, and enough power for 10–40 km. Directly modulated (DML) DFBs are the workhorse of 10G LR, 25G LR, CWDM4 and coloured CWDM SFPs; drift of ~0.1 nm/°C is fine for a single wavelength or 20 nm CWDM spacing, so they run uncooled. For DWDM the same laser is put on a TEC to hold a 50 GHz slot. Direct modulation adds chirp — the wavelength shifts as the current switches — which through dispersion limits DML reach at 10G to about 40 km and at 25G to about 10 km.

EML — externally modulated laser

An EML keeps the DFB in continuous-wave operation and modulates the light with an electro-absorption modulator (EAM) integrated on the same chip. No chirp from the laser, high extinction ratio and 25G/50G/100G per-lane speeds make EMLs the choice for 10G ER/ZR, 100G LR4/ER4 (four LAN-WDM EMLs), 400G FR4/LR4/DR4 and 800G. Usually cooled for wavelength stability; higher cost and power than DFB.

Tunable lasers

For DWDM without a stock of 96 coloured part numbers, a tunable source: a DBR laser (tuned by current into grating sections), an external-cavity laser, or an ITLA — integrable tunable laser assembly — with micro-ITLA and nano-ITLA variants small enough for SFP+ and QSFP-DD. Tuning is thermal or electrical over the C-band; a wavelength locker holds the channel. See Tunable DWDM transceivers.

Silicon photonics (SiPh)

Silicon cannot lase, but it can modulate and route light extremely well. A SiPh transceiver uses an external InP CW laser (one laser can feed several lanes) and a silicon chip carrying Mach-Zehnder or ring modulators, waveguides, WDM mux and often the germanium photodiodes. Volume CMOS manufacturing, small size and easy integration make SiPh dominant in 100G PSM4/DR, 400G DR4, 800G DR8 and in the coherent modulators of 400ZR/ZR+. The laser remains the InP part that ages.

Direct vs external modulation

How to tell what a module has

The identity bytes say little about the laser directly, but three fields imply it: wavelength (850 → VCSEL; 1310 → FP/DFB/EML/SiPh; 1550/DWDM → DFB/EML/tunable), reach code and rate. DDM confirms the picture: Tx bias for a VCSEL sits at a few mA, for a DFB at 20–60 mA, for a cooled EML higher still, and a rising bias over months is the laser ageing (Failures). Which laser goes with which form factor and rate is mapped in Lasers by form factor & data rate.