We investigated the loss modulation effect on the second-harmonically mode-locked erbium-doped fiber laser based on the Sagnac loop reflector with a Y-branch LiNbO 3 phase modulator. We found that even a small loss modulation originating from the phase modulator could cause severe peak power difference between two output pulses in a modulation period. It was also found that the peak power difference was sensitively dependent on the modulation frequency. By numerical simulation, we quantitatively analyzed the relation between the peak power difference and the amount of loss modulation, which agreed well with the experimental results.

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An all-digital closed-loop signal processor for the open-loop FOG was built. It is designed to have a potential of 36-bits resolution over the 27~ range and its measurement range is not limited within the fringe. Test results showed that the random walk of the processor is about 3 prad. Introduction:FOG signal processor can be grouped into two categories; the open-loop and the closed-loop . The first is simple and cheap, but suffers from limited dynamic range, low in sensitivity, and poor stability. Closed-loop approach is a good solution to the problems of the open-loop, but is optically complex and expensive. In order to avoid the problems with open-loop approaches and obtain the performance of the closed-loop approach with reasonable cost, many researchers have tried to develop a practical closed-loop signal processor for an open-loop FOG for the last 20 years['-51. Some of the techniques that have practical importance are the synthetic heterodyne13' and the digital integrating processor (DIP)'4'. The DIP processor, which was demonstrated by a Stanford group, adopted an electronic closed-loop that is amenable to digital implementation. This signal processor utilizes a square pulse train with adjustable spacing in response to the rotation-induced Sagnac phase shift , as shown in Figure 1. The rotation rate is measured by multiplying the detector output from the FOG with the above-mentioned pulse train and adjusting the pulse spacing such that the dc component of the multiplied signal is maintained to be zero.However, in order for the new processor to get the expected performance, it is required that the processor must have a square pulse train, a multiplier, a phase modulator and a sinusoidal waveform for driving the phase modulator, all in its ideal form. Otherwise, its output will be deteriorated by offset and its drift.