External laser source (ELS)

Co-packaged optics moves modulators and photodetectors next to the switch ASIC, and in nearly every shipping design the lasers conspicuously do not make the trip. They are pulled out into external laser sources: pluggable modules holding the DFB or comb lasers, delivering continuous-wave light to the optical engines over polarization-maintaining fiber. The engine modulates light it did not generate.

Three separate arguments push the laser out, and they reinforce rather than compete. Thermally, the lasers sit in the worst location a package designer could pick: a switch ASIC lid runs tens of degrees hotter than a pluggable cage, and laser threshold, efficiency, and wavelength all degrade with temperature while reliability wear-out accelerates. Statistically, the laser dominates the failure budget of an optical engine, and a failed integrated laser inside a co-packaged switch would mean scrapping or reworking the most expensive board in the rack; a failed ELS is a hot-swap. Economically, separating the parts decouples their yields and their supply chains, and lets the same engine design take light from whichever source technology wins.

The interfaces are standardized. OIF published the External Laser Small Form-Factor Pluggable (ELSFP) implementation agreement in 2023, defining a blind-mate pluggable laser module with PM-fiber output for exactly this role, and the CW-WDM MSA defines the wavelength grids, 8, 16, and 32 lines in the O-band, that multi-wavelength sources and engines agree on. An ELS then ranges from a single high-power DFB feeding a bank of microring modulators through a fiber splitter, to a full comb source supplying every WDM lane at once; Nvidia's co-packaged switches use the split-and-modulate arrangement, which is how a port comes to need several times fewer lasers than a pluggable module of the same capacity.

The accounting that the architecture must survive is optical rather than rhetorical. Light leaving the ELS pays a connector, a fiber-to-chip coupler, and an on-chip distribution tree before it reaches any modulator: a 1 by 8 split alone is 9 dB, and a representative full path from ELS output to ring is 11 to 12 dB, so delivering 1 mW per wavelength at the modulator can demand some 15 mW per wavelength at the source. At a diode wall-plug efficiency of 15%, the light for an eight-wavelength, 1.6 Tb/s engine costs roughly 0.8 W of electrical power, about 0.5 pJ/bit before any electronics, which is why source wall-plug efficiency and coupling loss appear together in every serious CPO power analysis. Redundancy is bought the same way: an N+1 spare laser in the shelf costs one more line of the same arithmetic and removes the most likely single point of failure.

References: OIF, External Laser Small Form-Factor Pluggable (ELSFP) Implementation Agreement (2023); CW-WDM MSA specifications, cw-wdm.org. The design pressures and the comb-laser contenders are covered in Light sources for co-packaged optics, and the co-packaged optics entry gives the surrounding architecture.