Fundamental Limits for Linearity of CATV Lasers

We have considered nonlinear distortions in a subcarrier multiplexed optical transmission system arising from carrier heating effects in the source laser. We included two sources of carrier heating: free-carrier absorption of the coherent radiation in the cavity, and the power flux into the active layer, associated with the input current. Resulting from the very existence of modulated optical and electrical signals, these effects cannot be avoided in any laser structure. Simple analytic expressions have been derived for the intermodulation distortion in multichannel single-mode lasers. For an 80-channel 60–540 MHz system with 3% optical modulation depth per channel, the composite second-order (CSO) distortion brought about by the hot-carrier effects is near the tolerance limit for CATV systems.