Font Size: a A A

Measurement Of Underwater Parameters Based On Stimulated Brillouin Scattering And Related Basic Physics

Posted on:2014-02-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:J L ShiFull Text:PDF
GTID:1220330398485706Subject:Physical Electronics
Abstract/Summary:PDF Full Text Request
Stimulated Brillouin scattering (SBS) is an important subject in nonlinear optics, the research of SBS has a long history. SBS has been widely used in several applications, especially in phase conjugation, pulse duration compression, amplification, and slow ligh, even in Brillouin lidar for remote sensing of the ocean. However, some basic properties of the parameters of SBS have still been seldom studied experimentally, such as the temperature dependence, In this thesis, the influence of temperature on the properties of SBS (pulse duration, linewidth, threshold value and gain coefficient) has been investigated. Then, some parameters include sound velocity, bulk viscosity and thermocline were measured based on the temperature dependence of SBS. Moreover, the influence of atmosphere environment (air pressure, air temperature and air humidity) on the attenuation coefficient of light in water has been analyzed.Brillouin lidar system has been developed for a long period, there are some mature systems, such as a Brillouin lidar based on Fabry-Perot scanning interferometer, and edge detection technique, etc.. However, the lidar system based on spontaneous Brillouin scattering has some disadvantages in applications. The major problem is that the spontaneous signal is uncontrollable, and the crosstalk of signal from different depths makes a relatively low depth resolution in detecting. Compared with spontaneous scattering, SBS is easier to detect due to its phase conjugation property and strong scattering signal. So, the technique based on stimulated Brillouin scattering can improve the performance of lidar system greatly, In this paper, a SBS system which composed of cylinder lens, Fabry-Perot etalon and ICCD is made to boost the performance of lidar. Using this system, the interference ring shaped by F-P etalon can be concentrated to two spots, therefore, the detecting intensity and the signal-to-noise ratio (SNR) can be improved significantly.As the several important parameters, the pulse duration, line-width, threshold value and gain coefficient of SBS, only the properties at room temperature were given for some materials. It must be addressed that this temperature dependence is actually important, and has great significance in applications. However, the temperature dependence of these parameters is not clear now, even little work was reported. In this paper, the influence of temperature on the properties of SBS has been investigated theoretically and experimentally. The results indicate that, the measured pulse duration of SBS decreases with the decrease of temperature; the lower of the temperature is, the more obvious of the pulse compression effect is. The line-width of SBS increases with the decrease of temperature. Moreover, the results also show that the threshold value of SBS decreases exponentially with an increase of temperature, whereas the gain coefficient increases with the increase of temperature.The ultimate aim for developing the SBS lidar is used for remote sensing in ocean. In this paper, the measurement of underwater parameters (sound velocity, bulk viscosity and thermocline) based on SBS was conducted. In the study of sound velocity, the theoretical values were given respectively in the pure water and seawater with different salinities. Based on the theoretical analysis, the sound velocities were measured respectively in the pure water and seawater with the salinity of35‰experimentally. Based on the temperature dependence of pulse duration and line-width of SBS, this paper details a new method for measuring the bulk viscosity of water which using the Fourier transform of pulse duration. Meanwhile, the measurement of thermocline in water has been done using SBS method. The distribution and features of the thermocline have been analyzed. On the basis of experiment, the time response of thermocline has been presented by simulating the thermal equation with some special conditions.In order to improve the detecting performance of SBS lidar, the attenuation coefficient of532nm light in water under different atmospheric conditions was investigated. Measurements were made over more one year period at the same location and show that the attenuation coefficient is significantly influenced by the atmospheric environment. It is lowest when the atmospheric pressure is high and temperature is low, and is highest when the atmospheric pressure is low and temperature is high. The maximum attenuation coefficient of pure water in these studies was about three times the minimum value. The mechanism of the phenomena is discussed. These results are also important in underwater acoustics.
Keywords/Search Tags:Stimulated Brillouin Scattering, Pulse duration, Linewidth, Threshold value, Gain coefficient, Thermocline, Atmosphere environment, Attenuation coefficient
PDF Full Text Request
Related items