Silicon photonics
In CPO, an external laser source—placing the laser outside the switch IC package and supplying only light via fiber—is often considered. What is the main reason?
- ALasers tend to lose efficiency and lifetime at high temperatures, so they are kept away from the hot switch IC and made easier to replace
- BPlacing the laser outside the package completely eliminates coupling loss when carrying the light through fiber
- CSilicon modulators can generate light from electrical signals alone, without laser light
Show answer and explanation
Answer: A. Lasers tend to lose efficiency and lifetime at high temperatures, so they are kept away from the hot switch IC and made easier to replace
As temperature rises, a semiconductor laser's threshold current increases, its efficiency drops and degradation accelerates. In CPO, the optical engines are placed right next to a switch IC consuming several hundred W, so a laser inside the package would have to operate at harsh temperatures. An external laser source can run in a relatively cool location and can be replaced by plugging and unplugging, for example from the front panel. In exchange, the coupling loss of guiding the laser light to the optical engine through fiber increases, and the laser output must be raised accordingly. Modulators do not generate light.