All-fiber radio frequency-modulated pulsed laser based on frequency-shift feedback loop

Abstract

Lidar-radar by using an radio frequency modulated (RF-modulated) laser transmitter is a powerful technique for applications involving remote sensing. The method is based on the use of an optically carried RF signal in order to acquire the merits of both the directivity of the optical beam (lidar) and the accuracy of RF signal processing (radar). Compared with single-frequency coherent lidars, lidar-radars are less sensitive to atmospheric turbulence and the speckle noise induced by target roughness. For long range detection, pulsed operation is usually required because of the high peak power. In order to meet the requirement for long range detection, an RF-modulated pulse train based on an all-fiber frequency shifted feedback loop is proposed in this paper. A continuous-wave single-frequency fiber laser (seed laser) is coupled into a fiber link and an acousto-optic chopper is used as a frequency shifter and beam chopper. A Yb3+-doped fiber amplifier is used to compensate for the loss of the signal in the fiber loop. The pulse duration is determined by the open time of acousto-optic chopper, which is fixed at 110 ns. A square wave generated by an arbitrary waveform generator is used as a trigger signal of the acousto-optic chopper. The RF within the pulse results from the interference of frequency shifed pulse with the seed laser. By inserting a 10 km fiber in the loop and accurately controlling the trigger cycle of the acousto-optic chopper equal to the roundtrip time of the loop, the pulse train generated by acousto-optic chopper can circulate in the loop, leading to the generation of RF-modulated pulse with about 20 kHz repetition rate and 110 ns width. The gain provided by fiber amplifier in the loop partially compensates for the loss. By adjusting the gain of fiber amplifier, the modulation depth of RF within the pulse can be continuously adjusted and the maximum modulation depth is 0.67. We also present an time-delayed scalar interference model which includes the loop length, trigger cycle, frequency-shift, and the gain. According to the theoretical model, the RF-modulated pulse affected by trigger cycle and fiber amplifier is numerically simulated. The experimental results accord well with theoritical predictions. The RF-modulated pulse has the advantage of high pulse-to-pulse coherence, which provides potential applications in lidar-radar detection. Besides, with an additional frequency doubling stage one can obtain a source for underwater detections and communications. Extension of the scheme to the 1.5 μm telecommunication window is straightforwardfor various radio-over-fiber applications.