Analysis of direct-discrimination of FSK modulated optical signal using injection-locked DFB lasers

Abstract

Direct discrimination of FM and FSK optical signals using injection-locked DFB semiconductor lasers is analysed. The demodulation eficiency is found to be independent of the optical injection level in the steady state. However, the optical injection level is important in determining the dynamic response. There exists an optimum optical injection rate to achieve the highest demodulation speed. The noise analysis revealed that the signal to noise ratio of this method is of the same order as that of the passive interferometer demodulation method. IntroductionRecently, it was confirmed, both experimentally and theoretically, that the stable locking band of a distributed feedback (DFB) semiconductor laser, in the configuration of external optical injection locking, is symmetrically centred around the frequency of the free-running slave laser (SL) in the low optical injection level. The variation of the junction voltage was measured to be almost linearly related to the frequency detuning throughout the locking band within the unconditionally stable locking regime [l, 2, 31. This special property of DFB semiconductor lasers permits a simple method to directly demodulate the frequency-modulated (FM) optical signal. The first detailed experimental demonstration of this method was recently performed by H. Nakajima [ 3 ] . The reported results are promising because the method allows the demodulation of optical FM signals directly and without the interferometer. The purpose of this paper is to present a theoretical analysis of injection-locked DFB semiconductor lasers used as discriminators for FM and FSK optical signals. The static solution of the rate equation reveals that the FM demodulation efficiency is independent of the optical injection level in the steady state. However, the optical injection level is important in determining the dynamic response. Both a small-signal analytical study on FM optical signal demodulation and a large-signal numerical simulation for FSK optical signal demodulation are described. The noise analysis indicates that the signal-tonoise ratio of the present method is in the same order as that of the passive interferometer demodulation method. Some limitations are pointed out and a parameter opti- Small-signal analysisIn DFB semiconductor lasers, the side longitudinalmodes are highly suppressed by the distributed grating, so that the theoretical analysis can be based on the following well known single-mode Van and N , are the steady state resonance angular frequency and carrier number of SL, respectively, without optical injection. As the gain nonlinearity is included in this paper, the carrier number is not clamped at its threshold value when the laser operates above threshold. Therefore, we will use the steady state values such as N , and I, for the expansion in the calculation. o, and o are the angular frequencies of ML and SL, C is the carrier injection rate, @ is the phase of the injection locked laser field, a = -2(Sw/SN)/(SG/SN) is the linewidth enhancement factor. T , is t...