Properties Investigation On Linearly Chirped Pluses Based On Cascaded Gratings

Microwave radar technology has been mature, and been widely used in the ranging, velocity measurement, imaging and other fields. At present, the science and technology has been improved continually, requirements of microwave radars are also increasing, prompting the microwave radar ranging accuracy to be further improved to meet the new demand. Pulse compression technique is one of the primary means of enhancing ranging distance and ranging accuracy. Linearly chirped pulse signals, which are used in pulse compression microwave radar systems, are generally generated electronically, however the frequency of the microwave signals is restricted to a low microwave frequency of 10 GHz by the electronic bottleneck of the traditional electronics. Using the means of microwave photonics, the linearly chirped pulse signal with a high frequency can be generated by steering clear of the electronic bottleneck, and get a large pulse compression ratio.In this dissertation, the generation and the properties of the linearly chirped pulse signals, which are generated based on cascade chirped fiber Bragg gratings, are researched. An original system is improved by changing the reflectivity of chirped fiber Bragg gratings, and the generation of the linearly chirped pulse signals is proposed in the extreme cases of low reflectivity and high reflectivity. In the case of low reflectivity, the system will generate a sinusoidal linearly chirped pulse signal with the contrast ratio of 1. In the simulated analysis using MATLAB software, a linearly chirped pulse signal with an instantaneous frequency range from 27.63 GHz to 11.84 GHz and a pulse width of 1.24 ns is generated, and the pulse compression ratio is 98.04. Aiming at the shortage of a short pulse width, a method of pulse stitching is proposed. In the simulated analysis, a wide pulse stitched by two short pulses is generated, which has a pulse compression ratio of 78.96. In the case of high reflectivity, the system will generate a comb linearly chirped pulse signal. In the simulated analysis using MATLAB software, a comb linearly chirped pulse signal with a pulse width of 12.4 ns is generated, and the pulse compression ratio is 517.05. The comb linearly chirped pulse signal is better than the sinusoidal linearly chirped pulse signal in respects of pulse compression and anti-interference ability.The generation system for linearly chirped pulse signals only uses chirped fiber Bragg gratings and a length of fiber, expect necessities, laser and photoelectric detector, so the cost of the system is very low. And the generated signals have a large pulse compression ratio and strong anti-interference ability. It is suitable for modern microwave systems.