Research On The Influence Of Medium Temperature On Stimulated Brillouin Scattering In Liquid Water

There have been considerable excellent accounts of the theory and experiment ofBrillouin scattering in different media during the past decades of development. With thedevelopment of ocean lidar system based on Brillouin scattering technique, some basicproperties of the parameters of SBS have still need to be studied theoretically andexperimentally, such as the propagation characteristics of high energy pulse laser inwater, and the influence of medium properties changes on scattering process. In thispaper, we have studied the effect of medium temperature change on SBS threshold andpulse compression using pure water as SBS medium. Meanwhile, we have calculatedand analyzed the dependence of phonon lifetime and gain coefficient of SBS onmedium temperature. Finally, the effects of ambient temperature, humidity and pressureon attenuation coefficient of high energy pulse laser in liquid water have beeninvestigated.Firstly, we have analyzed the relationship between phonon lifetime, threshold value,gain coefficient of SBS and medium temperature theoretically, and deduced atheoretical equation. Experimentally, the used light source is an injection-seeded Nd:YAG laser (Continuum Powerlite Precision II8000) with pulse duration8ns andoperating at532nm, its repetition rate is10Hz. The line-width is adjustable. The pulseenergy of the laser is adjusted by changing the time delay between the oscillator and theamplifier. The water temperature is controlled by circulating the water temperaturecontroller. For each temperature, the attenuation coefficient of pulse laser has beenmeasured in wide-band and narrow-band respectively and the temporal waveforms ofpulse duration for temperature been collected, six repeat measurements were made,from these, an average attenuation coefficient was calculated with standard errorbars.The results show that the threshold value of SBS is inversely proportional tomedium temperature and decreases with the increase of temperature. The pulse durationis proportional to temperature, the higher of the temperature is, the wider pulse durationis, so that the compressing effect is weaker. Through calculation and analysis, thetemperature change will affect greatly the gain coefficient and phonon lifetime. Theincrease of the temperature will lead to the increase in the gain coefficient and phonon lifetime, both are in direct proportion to medium temperature.Based on the previous works, the attenuation coefficient of pulse laser has beenmeasured according to the change of ambient temperature, humidity and air pressure inone year. The results show that the attenuation coefficient is in direct proportion toambient temperature and humidity, isinversely to atmospheric pressure.