Nonlinear spectral compression in fibre as a power-limiting device

Abstract

Spectral compression (SC) by self-phase modulation in optical fibre has been known for a long time and has since been reported for various parameters [1]. In this paper, we analyse the impact of amplitude fluctuations and a degraded optical signal-to-noise ratio (OSNR) of the seed pulses on the SC process by means of extensive nonlinear Schrodinger equation numerical simulation. Remarkably, our results show that the SC is rather stable against these pulse degradation factors, thereby revealing its potential for use in the context of optical regeneration of intensity-modulated signals. We therefore propose an optical scheme combining SC with an optical bandpass filter (OBPF) centred at the carrier frequency of the signal, which isolates the central part of the compressed spectrum, thereby converting the resulting change in central spectral brilliance into a change of the peak power of the temporal intensity profile. The computed intensity transfer function (TF) of the device [Fig. 1(a)] is shown to achieve both reduction of the amplitude fluctuations and improvement of the extinction ratio of a return-to-zero amplitude-shift keying pulse train [Fig. 1(b2), (c2)]. The statistical distribution of the peak power of the one-bit pulses highlights the reshaping of the amplitude fluctuations: at the regenerator output the distribution has no longer a Gaussian shape and is highly skewed to the left [2]. However, the amplitude noise rejection ability of the device diminishes for operation at degraded OSNR [Fig. 1(b3), (c3)]. Even though improvement of the extinction ratio is still observed, the statistical distribution of the one-bit level at the regenerator output maintains a Gaussian shape but is narrower than the input distribution. The improvement in signal quality is therefore marginal. We attribute this reduction of the device's performance to the influence of the jitter of the temporal width incurred by the input pulses because of the added amplified spontaneous emission noise. Indeed, as already demonstrated in the Mamyshev's regenerator [3], contrary to other nonlinear regenerators such as nonlinear optical loop mirrors, whose TFs only depend on the instantaneous pulse power, a variation in the input pulse duration affects the TF of our device by shifting the optimum operational power, thus impacting its beneficial effect.