A cheap, low-power 850 nm laser that emits from the wafer surface — the source behind every multimode SR module.
What a VCSEL Is
A vertical-cavity surface-emitting laser builds its cavity vertically, between two distributed Bragg reflector mirror stacks grown above and below a very thin active region, and emits perpendicular to the wafer surface. An edge-emitting laser such as a DFB emits sideways from a cleaved facet and cannot be finished until the wafer has been cut.
That geometric difference rewrites the economics. VCSELs can be tested on the wafer before dicing, so bad die are discarded before any packaging cost is spent; the beam is circular rather than elliptical, so coupling into a fibre is straightforward; and the threshold current is a few milliamps rather than tens. The result is the cheapest laser in volume manufacture by a wide margin.
Why It Is Multimode Only
The vertical cavity is short, which means it supports several transverse modes and its output is comparatively broad — up to 0.6 nm RMS in 100GBASE-SR4, against roughly a tenth of a nanometre for a DFB. A source that wide launched into a 9 micron single-mode core would suffer badly from chromatic dispersion, and the multi-transverse-mode beam would not couple well in any case.
Paired with multimode fibre it is a good match, but the fibre then imposes its own limit. Modal dispersion, quantified as effective modal bandwidth, sets a bandwidth-distance product that no amount of extra launch power will improve. This is why SR reaches are stated per fibre grade and per lane rate rather than as one number.
Reading the Reach Table Correctly
10GBASE-SR runs 300 m on OM3 and 400 m on OM4. Move to 25G per lane and the same 850 nm VCSEL gets you 70 m and 100 m; 100GBASE-SR4 is four of those lanes and inherits exactly the same 70 and 100 metres. 400GBASE-SR8 at 50G per lane does not do better.
The pattern is not laser degradation, it is the bandwidth-distance budget being spent on symbol rate instead of distance. OM5 does not change it for single-wavelength SR either; OM5 exists to support SWDM, which puts four VCSELs at different wavelengths down one fibre pair. The multimode reach calculator has the full table by standard and fibre grade.
Where You Meet It
Every SR-class multimode module you have ever installed, plus SWDM variants and most multimode AOCs. VCSELs never appear in single-mode products, so the moment a link needs to cross more than about 100 metres at modern lane rates the source technology changes and so does the cost structure — which is the real content of the single-mode versus multimode decision. Our multimode fibre guide covers how to choose the glass.
VCSEL reference
| Wavelength | 850 nm; SWDM adds roughly 880, 910 and 940 nm |
|---|---|
| Emission | perpendicular to the wafer surface, circular beam |
| RMS spectral width | 0.6 nm maximum in 100GBASE-SR4 — several times a DFB's line |
| Threshold current | a few milliamps |
| Modulation | direct — there is no external modulator |
| Wafer-level test | yes, before dicing — the root of the cost advantage |
| OM4 reach | 400 m at 10G per lane, 100 m at 25G and 50G per lane |
| OM3 reach | 300 m at 10G per lane, 70 m at 25G and 50G per lane |
| Not used for | single-mode — the beam and the linewidth are both wrong for a 9 micron core |
Related Terms
Effective modal bandwidth is what actually caps the reach, SWDM is how several VCSELs share a fibre, and EML is the single-mode alternative.