Application Of Pseudo-Random Sequences To The Measurement Of Weak Light Signals

It is a crucial problem that how to detect weak light signals accurately in the measurement of optical fiber performance and its applications or other optical systems. The key of the solution to this problem is to find some new detection methods. In this paper, on a base of many references, pseudo random sequences (PNs) were used in the optical measurement, weak light signals under strong interferences and noise background could be detected effectively using the nice randomness of pseudo random sequences. This correlation detection method was investigated in detail, studied systematically. The research works are as follows:In this paper, we have analyzed the characteristics of various pseudo random sequences, discussed all kinds of generation methods and improved on searching algorithm for primitive polynomials of m sequences. All above works provided a theoretical foundation for the generation of pseudo random sequences.The unit impulse response of a linear system could be distinguished by pseudo random sequences using correlation identification method. We applied this method to the optical measurement system. Through theoretical analysis and simulation, the relation between the performance of measurement and the parameters of PNs was shown that the linearity of system, mean square error and improvement factor of signal-to-noise ratio all were determined by the period of PNs. In order to eliminate various interferences, weighted average method was used.For detecting the time domain waveform of input signals, the principle of spread spectrum communication found its applications for an optical weak signal measurement system. All the varied spectrum of signals, interferences and noise were shown in the process of spreading and despreading. A pseudo random sequence after a comb filter for despreading removed the spectrum of interference from the range of signals, and therefore avoided harmonic interference. A simulation experiment verified the measurement accuracy using this method.Finally, the experimental devices of weak light signal measurement system based on pseudo random sequence were discussed at the end of this paper. We have finished the design and debug of all circuits and software in the system. Experimental results testing with two types of signals showed that the measurement accuracy was accordant to the theoretical analysis and this method was a valid method for the detection of weak light signals.There are many research works which need to be explored on the application of PNs to the detection of weak optical signals either in theory or on experiment. In theory, it is necessary to study the inherent relation between correlation identification and the principle of spread spectrum and to find a uniform correlative detection theory on weak signals. For experiments, there is a need for the improvement on signal generators modulation and the processing by hardware.