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Français 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
| Reach | Fibre | Laser | Receiver |
|---|---|---|---|
| ≤ 100–500 m | multi-mode | VCSEL 850 nm, direct modulation | PIN |
| 500 m – 2 km | single-mode | DFB (DML) or SiPh 1310 nm | PIN |
| 10 km | single-mode | DFB (DML) up to 10–25G; EML at 50–100G per lane | PIN |
| 40 km | single-mode | EML 1550 or LAN-WDM, cooled | PIN or APD |
| 80 km+ | single-mode | EML + APD, or coherent tunable | APD / coherent |
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 → )
| PMD | Laser | Detector |
|---|---|---|
| 1000BASE-SX | 850 nm VCSEL | GaAs PIN |
| 1000BASE-LX | 1310 nm FP (later DFB) | InGaAs PIN |
| 1000BASE-EX / ZX | 1310 / 1550 nm DFB | PIN; APD on 80–120 km ZX |
| CWDM SFP | uncooled DFB on the 20 nm grid | PIN, APD on 80 km+ |
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 → )
| PMD | Laser | Detector |
|---|---|---|
| 10GBASE-SR | 850 nm 10G VCSEL | PIN |
| 10GBASE-LRM | 1310 nm FP/DFB + host EDC | PIN |
| 10GBASE-LR | 1310 nm DFB, direct modulation | PIN |
| 10GBASE-ER | 1550 nm EML, cooled | PIN / APD |
| 10GBASE-ZR / 80 km | 1550 nm EML, cooled | APD |
| DWDM 10G, tunable | cooled DFB or tunable (DBR/ITLA) + EML | APD |
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 → )
| PMD | Laser | Detector |
|---|---|---|
| 25GBASE-SR | 850 nm 25G VCSEL | PIN |
| 25GBASE-LR | 1310 nm 25G DFB (DML) or EML | PIN |
| 25GBASE-ER | 1310 nm EML | APD |
RS-FEC arrives with 25G, letting cheaper lasers meet the spec at the pre-FEC BER point.
40G — QSFP+ (2010 → )
| PMD | Laser | Detector |
|---|---|---|
| 40GBASE-SR4 | 4 × 10G VCSEL array, chip-on-board | 4 × PIN array |
| 40GBASE-LR4 | 4 × DFB on CWDM 1271–1331 nm, uncooled | 4 × PIN + demux |
| 40G PSM4 | 4 × 1310 nm DFB | 4 × PIN |
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 → )
| PMD | Laser | Detector |
|---|---|---|
| 100GBASE-SR4 | 4 × 25G VCSEL (COB) | 4 × PIN |
| 100G CWDM4 / CLR4 | 4 × 25G DFB (DML), uncooled CWDM | 4 × PIN + demux |
| 100G PSM4 | 4 × 25G DFB or SiPh MZM from one CW laser | 4 × PIN (Ge on SiPh) |
| 100GBASE-LR4 / ER4 | 4 × 25G EML on LAN-WDM, cooled | PIN / APD |
| 100GBASE-DR / FR / LR | 1 × 100G PAM4 EML or SiPh | PIN, linear TIA + DSP |
| 100G DWDM / ZR-class | tunable + EML or coherent | APD / coherent |
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 → )
| PMD | Laser | Detector |
|---|---|---|
| 200G/400G SR4 / SR8 | 50G PAM4 VCSEL arrays (COB) | PIN arrays + DSP |
| 400GBASE-DR4 | 4 × 100G PAM4 EML or SiPh MZM, 1310 nm | PIN + DSP |
| 400GBASE-FR4 / LR4 | 4 × 100G EML on CWDM, cooled | PIN |
| 400GBASE-FR8 / LR8 | 8 × 50G EML on LAN-WDM | PIN |
| 400ZR / ZR+ | tunable nano-ITLA + SiPh coherent modulator | coherent receiver |
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 → )
| PMD | Laser | Detector |
|---|---|---|
| 800G SR8 | 100G PAM4 VCSEL (the VCSEL ceiling) | PIN + DSP |
| 800G DR8 / 2×DR4 | 8 × 100G EML or SiPh | PIN + DSP |
| 800G 2×FR4 | 8 × 100G EML on CWDM | PIN |
| 1.6T DR8 (200G/lane) | 200G PAM4 EML, SiPh, thin-film lithium niobate modulators | PIN + DSP |
| LPO variants | same lasers, no module DSP — host equalises | linear TIA only |
Cross-cutting trends
- From direct to external modulation — DML for ≤ 25G, EML/MZM above.
- From one laser per lane to one laser per module — SiPh splits a CW source across lanes.
- From hermetic TO-cans to bare dies — chip-on-board for parallel optics, SiPh integration for single-mode.
- 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.
- Coherent moves into pluggables — the transport receiver now fits in a QSFP-DD.
- 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).