Research On Sub Nanosecond Pulse Laser And Its Coherent Detection

The laser was born in 1960 s, comparing to the traditional light,it has series of merits such as good monochromatic performance, good coherence, small divergence angle and high brightness, it provides high performance light source for the development of optical coherence detection. The laser detection has the advantages of higher transmission efficiency, stronger anti-interference performance, smaller sizes and so on. It is gradually replacing the traditional detection in some fields. The laser acts as probes in the rapid access to information process, when outputting the same light energy, the processing of laser detection such as the transmission distance, signal to noise radio(SNR), receiving sensitivity and range resolution are increasing when the width of the laser pulse become shorter. The acquisition, transmission, storage and processing of information are important applications of the nanosecond laser pulse. The research work of short pulse coherence detection provide a new study direction in the detection fields.In this thesis, we mainly study the nanosecond pulse laser and its coherent detection. The single-frequency, narrow-line-width and low-noise non-planar ring oscillator laser(NPRO) is designed and developed as the local light. The singlefrequency, narrow-pulse-width and high-peak-power passively Q-switched micro-chip laser is designed and developed as the pulse emission light source. We should ensure the frequency of the two light are matching. Pulse laser has a wider spectrum, theoretically speaking, only when the frequency of the two light are completely consistent, can we obtain the best sensitivity. The two beams of the same wavelength laser access to the 98/2 fiber couple, and coherent phenomenon occurs in it. Then access the coherent light to the photoelectric detector(10GHz), we can get the obvious optical beat phenomenon in the oscilloscope. By controlling the power value of the local oscillator,we can see the amplitude change of the pulse signal in the oscilloscope, thus verifying the detection of small signal. To avoid the power of the signal become unstable when the frequency of the two laser become identical, we input the local light and the signal light to the 90 o hybrid in the detection processing. After the two lights occurring coherence, the hybrid output 4 way signal which the phase difference is 90 o. We select the 0o and 180 o as the I channel, and the 90 o and 270 o as the Q channel, then input the two channels signal to the balanced detection amplifier which fix PINs. Ensure that the length of the optical fiber from the 90 o hybrid to the balanced detection amplifier are completely consistent, in this way the phase difference between the I and Q channel is 90 o. Then select the two signal to do sum of squares by the oscilloscope, in the final we get the stable pulse signal which consistent with the calculated results.The coherent detection can improve the SNR and detection sensitivity, according to the experiments, the signal to noise ratio in the coherent detection is 45.6dB, and the minimum acceptable optical power is-55.3dbm; nanosecond pulse laser has wider spectrum, so its characteristic will relax the requirement for frequency stability of the local oscillator laser in the detection processing.