Power spectra of FSK modulated laser fields

Abstract

In a continuous phase frequency shift keying (CPFSK) coherent optical communication system, the optical signal is usually generated by modulating the bias current of a singlemode laser diode. This generation process is always accompanied by a simultaneous intensity modulation (IM) of the emitted light. The authors derive an exact expression for the power spectra of the optical CPFSK signal, taking this simultaneous IM effect into account. From the derived formula, the asymmetrical shape of the power spectra observed earlier by other investigators is shown and explained.