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An approach to generating pulse trains with user-defined pulse positions in a phase-modulated optical frequency-shifting loop (OFSL) is proposed and experimentally demonstrated. In this method, the OFSL operates in the integer Talbot state, i.e., the repetition frequency of the driving waveform is equal to the free spectral range (FSR) of the OFSL. The phase of the optical field in the OFSL is manipulated by using an electro-optic phase modulator (PM) in each round trip. Then, pulses will be generated in the positions that the additional phase introduced by the PM is equal to an integer multiple of 2π in each round trip. Hence, the positions of the generated pulses can be controlled by designing the driving waveform. In the experiment, we firstly demonstrate the ability of the proposed scheme to control the pulse positions by applying four types of signals, namely, a linearly chirped waveform, a dual-chirp waveform, a quadratically chirped waveform, and a sinusoidal frequency-modulated waveform, to the PM. In addition, the proposed scheme can also be used to generate pulse trains with coded pulse positions. To verify this ability, two groups of coding sequences, i.e., “0101101110” and “1010010001,” are used to encode the pulse positions in a round-trip time of the OFSL. The proposed scheme can generate pulse trains with user-defined pulse positions, which can be used as a pseudo-random sampling source in compressed sensing (CS) systems.
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