In co-packaged optics the laser is kept off the switch/compute ASIC and supplied as a separate external module (DFB/DBR laser arrays, sometimes plus a comb source and a star coupler) feeding light into the photonic engine over fibre. The reason is structural: laser junctions are temperature-sensitive and wavelength-critical, and the ASIC runs extremely hot, so isolating the laser preserves wavelength stability and reliability/serviceability (ref).
This separation is what makes the light source a merchant, separately-sourced supply layer — the durable private wedge in Photonic Light Source Supply (ELS forecast ~10% of a >$20B CPO market by 2036). It is the deliberate contrast to the Photodetector & Optical Receiver Front-End / Photonic Photodetection Layer case: detectors are CMOS-native and grow monolithically on the silicon-photonics die, so they do not de-integrate into a sourced layer the way lasers do. Lasers leave the die; detectors stay on it — which is why one is a supply thesis and the other a no-wedge market.