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Absorption and scattering of seawater for light waves seriously restrict the performance of laser radar for underwater target detection. Quantitative analysis of laser backscattering generated by seawater shows that the laser echo signal is seriously affected by seawater backscattering. The ability of range-gated technology and intensity modulation technology to suppress backscattering of seawater was analyzed and compared. A chaotic pulse laser with high frequency intensity modulation was proposed for underwater target detection and the scheme was designed. Based on the study of the characteristics of backscattering light and echo signal with different backscattering intensity in time and frequency domain, the ability of chaotic pulse lidar to suppress backscattering from seawater was determined by using cross-correlation noise level algorithm. The theoretical analysis shows that the target signal can still be extracted when the backscattered light intensity is 36 times that of the chaotic pulse laser.

Distributed fiber optic sensors are studied extensively, for monitoring abnormal events in continuous space, due to the advantages of immunity to electrical interference, non-conductivity and light weight. Moreover, the position of abnormal events, such as intrusions, could be determined directly without additional measurements. Among the various techniques, Sagnac interferometers prove to be promising for providing high sensitivity and large dynamic range in detecting intrusions. Two interference light beams are used which are naturally of equal optical path length in static status. When an intrusion occurs along the sensing fiber, the two light beams arrive at the intrusion position in different time and thus cause different phase changes induced by the intrusion. Analysis of the phase difference signal can predict the intrusion position, as well as the existence of the intrusion. As a Faraday rotator mirror (FRM) connected in the far-end of sensing fiber, both beams travel twice to the intrusion position after being reflected by the FRM. The propagation time interval T between the two interactions corresponds to the distance between the intrusion position and the far-end of sensing fiber Lx, which is further extracted as the localization of intrusion. Previously, the auto-correlation algorithm deals with the phase difference signal in the time domain and the null-frequency algorithm is used in frequency domain to calculate the distance. However the poor localization performance usually can not meet the requirement in high-quality monitoring applications. To determine the position of an intrusion effectively and accurately, the localization algorithm which deals with the phase difference signal in cepstrum domain is proposed in this article. Inspired by the research on the pitch examination we first introduce the algorithm for intrusion localization. Through theoretical analysis, the phase difference signal can be regarded as the convolution of the original waveform of intrusion and the T-related transform function. By applying the fast Fourier transform to the logarithmic spectrum, the phase difference signal is changed into the cepstrum domain, where the original waveform of intrusion and the transform function behave differently and are separated. The propagation time interval T, as well as the distance Lx, can be directly acquired from the peak produced by the transform function. In addition, to overcome the roughness in localization resolution brought by down-sampling of the phase difference signal, the decimator factor is scanned from 30 to 50 for multi-resolution localization at an original sampling rate 4 million/s-1. Besides the basic peak, high order peaks also emerge in the cepstrum in high signal-noise-ratio condition, which can also be used for localization. Since the localizations from different decimator factor and different peaks spread around the actual distance, an average of all reasonable localizations is calculated as the ultimate localization result for the intrusion. Firstly in experiments, intrusions occurring at a position 40.498 km are produced for the verification of the algorithm. The localizations are 40.489, 40.515 and 40.487 km, with localization errors as small as 9, 17 and 11 m respectively. Intrusions at different positions are tested and also correctly localized. For comparison, the standard deviations of localization error are respectively 695 m and 118 m for the auto-correlation algorithm and the null frequency algorithm, which are 58 times and 10 times of the result 12 m, which is obtained by the proposed cepstrum algorithm. The performance suggests promise to achieve better localization in practical applications.

The chaotic carrier with smooth spectrum and broad bandwidth is generated by a distributed feedback （DFB） semiconductor laser subject to unidirectional fiber ring optical feedback. The smooth spectrum of the obtained chaotic carrier hides the external cavity resonance frequency peak，therefore the security of the system is enhanced. By unidirectional coupling，we implement the stable chaos synchronization when the chaotic light is injected into a solitary DFB laser diode. The correlation coefficient of synchronization is 0.84. The effects of injection strength and optical frequency detuning on synchronization are investigated experimentally. The results show that in conditions of strong injected locking the synchronization can be achieved in a large range of frequency detuning，and the stronger injection corresponds to the larger range of frequency detuning in which chaos synchronization maintains. The synchronization quality improves with the injection strength increasing.

A semiconductor laser that generates a chaotic signal with optical feedback can be treated as a chaotic transmitter. Based on single mode rate equations of semiconductor laser with optical feedback and external injection, the numerical simulation shows that the bandwidth of the transmitter is expanded by external optical injection. When the injection index kinj is 0.39, the bandwidth is increased from the value of 2.7 GHz without optical injection to 14.5GHz. The results also reveal that, under the same injection strength, the enhancement of bandwidth depends evidently on the frequency detuning between the external injection laser diode and the chaotic carrier transmitter. The maximum bandwidth of the chaotic transmitter can be obtained when the frequency detuning is in the range from 2 to 4GHz. The bandwidth of the chaotic transmitter can also be enhanced by increasing the bias current.

The high frequency message filtering characteristics of the semiconductor laser which act as a receiver are numerically simulated in a scheme of closed-loop optical chaos communications system. Both transmitter and receiver are realized bysemiconductor lasers subject to strong optical injection. We examined the filtering properties of semiconductor laser with modulation message with different frequencies and amplitudes. The results demonstrate that the filtering effectiveness of the semiconductor laser receiver is higher for low frequency message and decreases as the message frequency approaches the relaxation oscillation frequency of the semiconductor laser. The quality of message recovery in such systems is shown to depend not only on the synchronization quality but also on the difference in amplitude of the message in the transmitter and receiver outputs.