Computer modeling of the nonuniform FM transfer function of semiconductor lasers for the study of coherent optical systems

This paper proposes an accurate computer model of the nonuniform FM response of semiconductor lasers, to be used in the computer-aided design of coherent optical communication systems. The model is communications engineer oriented and does not involve the physical insight of the device. The main idea of this approach is that the FM response of the laser can be approximated by a recursive digital filter based directly on measurements of the FM response. The procedure is divided into two steps : First, measurements of the FM response are fitted by a rational interpolant using the theory of multi-point Padé approximants. Then, the impulse invariant transformation is used to calculate digital filter coefficients. The procedure is applied in the case of a conventional single-electrode distributed-feedback (DFB) laser. The calculated digital filter is used to study the influence of the nonuniform FM response on the performance of a coherent heterodyne CPFSK system with differential receiver operating at 1 Gb/s. The sensitivity penalty is given as a function of signal-to-noise ratio, phase noise and sequence length by a semi-analytical technique. Theoretical and experimental results are in excellent agreement