Lasers by form factor & data rate

Which laser sits inside a module is decided by two things — the rate per lane the laser must modulate and the reach it must cover — and both have marched upward with every form factor generation. This page maps the laser and receiver technology used at each step, from 1G GBICs to 800G OSFPs.

The rule of thumb

Rate pushes the same way: up to 10G a directly modulated laser is fine; at 25G DML is marginal and EML/VCSEL-25G take over; from 50G PAM4 per lane, EML or silicon-photonics modulators dominate; at 100G+ per lane VCSELs reach their ceiling.

Generation by generation

1G — GBIC and SFP (1990s → )

The FP laser's multi-mode spectrum was acceptable at 1.25 Gb/s; VCSELs made 1G multi-mode optics almost free. See GBIC, SFP.

10G — XFP, SFP+ (2002 → )

10G is where the EML and the APD became standard for anything beyond 10 km, and where XFP's in-module CDR gave way to SFP+'s host-side design (XFP, SFP+).

25G — SFP28 (2015 → )

RS-FEC arrives with 25G, letting cheaper lasers meet the spec at the pre-FEC BER point.

40G — QSFP+ (2010 → )

The first mass-market parallel optics; VCSEL arrays on a board with a lens array and MPO ferrule set the pattern for every SR4/SR8 since (QSFP+, Chip-on-board).

100G — QSFP28 (2015 → )

100G is the generation where silicon photonics entered volume production (PSM4, DR) and where the split between uncooled DFB (CWDM4, 2 km) and cooled EML (LR4, 10 km) defined the price ladder (QSFP28).

200G / 400G — QSFP56, QSFP-DD, OSFP (2019 → )

PAM4 makes the DSP part of every module and pushes lasers to 100G per lane — EML and SiPh territory (QSFP-DD, OSFP).

800G and 1.6T — OSFP, QSFP-DD800, OSFP-XD (2022 → )

Cross-cutting trends

1. From direct to external modulation — DML for ≤ 25G, EML/MZM above.
2. From one laser per lane to one laser per module — SiPh splits a CW source across lanes.
3. From hermetic TO-cans to bare dies — chip-on-board for parallel optics, SiPh integration for single-mode.
4. VCSEL reach shrinks as rate grows — 550 m at 1G, 100 m at 25G, 50–100 m at 100G; single-mode DR is taking over even short data-centre links.
5. Coherent moves into pluggables — the transport receiver now fits in a QSFP-DD.
6. Power is the new limit — cooled EMLs, DSPs and ITLAs push 400G/800G modules to 15–20 W, driving LPO and co-packaged optics.

Why it matters for coding and diagnostics

The identity bytes a switch reads — wavelength, reach, compliance code — are a proxy for the laser inside. Coding a VCSEL SR4 module as an LR4 does not change its 850 nm lasers; the host will expect single-mode reach it cannot deliver. And the DDM signature differs by laser type — a VCSEL's few-mA bias versus an EML's tens of mA plus a TEC — so a bias value that is "normal" for one is an alarm for the other (Laser types, Receivers).