Intensity noise characteristics of mode-locked lasers

Abstract

Recently, many theoretical models for modulation response and mode-locking performance of strong external cavity lasers have been described in the literature. However, no studies have been made showing noise characteristics of these lasers at the mode-locked condition. In this paper, effect of spontaneous noise on mode-locked hybrid soliton pulse source (HSPS) utilizing linearly chirped raised-cosine fiber Bragg grating (FBG) and relative intensity noise (RIN) is investigated. The model is based on a time domain solution of the coupled-wave equations including spontaneous noise. The laser cavity is divided into sections with equal effective length and each laser section the carrier density is calculated from the rate equations. The realization of long distance soliton based transmission systems requires a reliable, stable source of transform limited pulses of the correct pulsewidth giving a time-bandwidth products (TBP) of 0.5 or less. In this work, the results show that without noise near transform-limited pulses from mode-locked HSPS are generated over a tuning range of 1 GHz (2-3 GHz) around fundamental mode-locking frequency of 2.5 GHz, with a pulsewidths of 45 ps, and TBP is 0.394, as required for a practical soliton transmission system if linewidth enhancement factor (.) is taken 2. This range was found approximately 1 GHz for linearly chirped FBGs [1-2]. If spontaneous noise is considered, in this case, RIN has a peak at 2.4 GHz giving a RIN value of –94.16 dB as seen in Fig. 1. At this frequency pulsewidth and TBP decreases and TBP becomes a meaningless data for that signal giving a TBP of 0.006. However, mode-locking is difficult to achieve for large value of . because of increasing noise. Therefore, transform-limited pulses are not generated over a wide tuning range and proper mode-locking range where transformlimited pulses are generated reduces. . determines spectral linewidth and frequency chirp. Although zero dependence is impossible to obtain in practice, in our simulation . is varied 0 to 5. As seen in Fig. 1 RIN increases for most of the mode-locking frequencies for 5 value of its and transform-limited pulses are generated only at the mode-locking frequency range of 2.7-3 GHz. RIN is very low for its zero value but for this case, TBP is greater than 0.5 with and without noise and so again soliton type pulse are not generated for most of the mode-locking frequencies.