Abstract
In this work, a novel designed fiber ring laser by self-feedback mode-locked weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) is established. By directly-modulating, the WRC-FPLD is used simultaneously as the gain medium and the optical pulsed carrier source in the fiber ring. The characteristic of the 10 GHz mode-locked optical pulse-train beyond modulation bandwidth operation of the WRC-FPLD has been demonstrated. First, with 90% self-feedback ratio and 28 dBm modulation power via circulated fiber ring, the enhancement of mode-locked force can be obtained, and the modification on the self-injection structure is discussed with the mode-locking theory. Besides, when optimizing the operation, the broadening longitudinal mode of the WRC-FPLD caused by the transformation from the gain-switching to mode-locking is demonstrated because of the degradation of the longitudinal mode suppression in 30 dB. The optimized mode-locking output of the self-feedback WRC-FPLD fiber ring with pulsewidth, timing jitter, pulse extinction ratio, and spectral linewidth of 22 ps, 153 fs, 13.65 dB, and 7.6 nm are obtained by setting the WRC-FPLD bias and self-feedback ratio at 60 mA and 90%, respectively. Then, we also discuss the performance of output pulse on the transformation from gain-switching to mode-locking by changing the feedback ratio and reflectivity of the WRC-FPLD with the theoretic model of optical injection locking. By studying the conversion of pulse sharp, longitudinal mode linewidth, and frequency chirping, some trends of the competition between the intra-cavity of WRC-FPLD and external fiber ring can be explained by the enhancement of mode-locked force. In order to retain the better performance of optical pulsation and selective longitudinal mode frequency for DWDM channel spacing at the same time, the dual cavity built of an extra fiber ring is demonstrated. Under the 90% self-feedback mode-locked fiber ring laser, additional oscillation condition causes the variation on the modulation response of the system is proposed, which induces the change on the mode-locked force. When the different forces of oscillation are described by changing dual fiber ring ratio from 10% to 90%, the performance of optical pulsation can be observed. The self-feedback mode-locked WRC-FPLD in dual cavity which cannot implement the channelized transmission because of the degradation on the performance of optical pulse-train is also investigated. Moreover, the extra oscillation condition which causes the degeneracy on mode-locked force regardless of the dual cavity ratio of 10% or 90% would obtain the pulsewidth of 24 ps and 30 ps, respectively. Finally, 90% self-feedback mode-locked WRC-FPLD with an external fiber ring has a competition with the intra-cavity of directly-modulated laser diode. Optical pulsation transferring from harmonic mode-locking to high-speed repetition rate is observed by detuning RF frequency under direct modulation. By means of changing the parameters of operation, the transformation from the competition can be demonstrated on another force, which means the domination of gain-switched mode-locking. The result of repetition rate enhancement up to 67 GHz is investigated and obtained by the adjustment on the biased current and frequency.
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