Intermodulation distortion in multi-carrier operation of semiconductor lasers

Direct analogue modulation of a semiconductor laser introduces the possibility of distortion being generated as a result of nonlinearities in the conversion process. Second-order and third-order terms are likely to dominate, and, in the particular case where a multi-carrier FDM signal is used, their relative importance will depend on the system design. For narrowband operation, only third-order intermodulation products will fall in-band and contribute to the intensity fluctuations, but if the FDM signal occupies a large fractional bandwidth, then both types will be important. To predict the significance of this nonlinear distortion, laser operation can be modelled by the rate equations, which must then be solved for situations where the drive current consists of a bias term together with sinusoidal components representing the wanted multi-carrier modulation. To investigate how the carrier-to-intermodulation ratio varies as a function of multi-carrier drive current, experiments were also carried out using a number of different modulation depths. These ranged from cases where peak current excursions did not drive the laser below threshold, to those where the mean square modulating current exceeded the bias current