Font Size: a A A

Investigation Of Photon Localization Using The Random Laser And Traditional Laser Theory

Posted on:2010-02-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:S Z FanFull Text:PDF
GTID:1118360278474489Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Random laser and photon localization are both newly discovered less than two decades ago. Random laser is a special laser without mirrors. Its spectral width is narrow just like traditional lasers. And many properties of the random laser are different with the traditional laser. The random laser is robust because of the mirrorless structure, the scale can be very small, and the emission is not as directional and coherent as the traditional laser. These properties make the random laser an excellent candidate in the potential applications in laser displaying, lighting technology, monitoring of the liquid flow, medical biometrics, geological study, encoded marking of documents or material and so on. And because that the feedback of the random laser is believed to be the photon localization, it is also important in the study of the photon localization. The photon localization comes from the interference effect of the light wave. When the light is strongly scattered in the medium, the interference may lead to the slow-down of the light diffusion. And the light is expressed as localized when the diffusion coefficient is approaching zero. The studying of the photon localization is important in exploring the underlying physics of the light and in studying the interaction of the light with the matter microscopically. And also it is important in the engineering of the random laser, the optical storage, the laser protection and so on.So in this thesis, we study the properties of the photon localization by using the random laser and the traditional laser theory. The rate equations combined with diffusion theory are employed in the studying of the random laser. As a powerful laser with good beam quality is needed in the studying of the random laser as the pumping source, we first study the precise determination of the thermal focal length in the end-pumped solid state laser in the second chapter, which is useful in designing the laser with high power and good beam quality. In the third chapter, we study the pre-localization: coherent backscattering. We show a simple method of measuring weak coherent backscattering signal. In chapter four, we compared the random laser and various emissions from the dye solution. In chapter five and six, we show the studies of the random laser on the spectral and the temporal properties. The studies in chapter four to six show the localization in certain samples.The main works in the thesis are:1. The thermal distribution in the end-pumped solid state laser is studied. Using the diffraction theory of aberrations, we give the more precise determination of the thermal focal length. We show that the thermal focal length is determined not only by the pumping beam radius, but also the laser beam radius. Two pumping distributions are considered theoretically. The simplified equations are also presented for convenience. Experimental results are also given. This study is useful in the optimization of the laser with high power and good beam quality.2. A simple method of measuring the weak coherent backscattering signal using CCD is studied. The CCD is used to record the weak signal of the coherent backscattering. And the obtained data image is integrated to increase the signal to noise ratio. The optimized integration interval is suggested. The equation for fitting is studied to obtain the useful information. Simulation is presented to validate the method and to make sure the results are exclusive. The dependence of the fitting error on the noise is presented. Experiment results are given. And we give two new experimental configurations based on this method. The new configuration uses a cylindrical lens instead of the normal lens, which make the numerical process much easier and the detail of the coherent backscattering peak is maintained.3. The fluorescence, the transverse amplified spontaneous emission, the longitudinal amplified spontaneous emission, the random laser and the emissions which we call the longitudinal amplified spontaneous emission with partial coherent feedback and with stronger coherent feedback from the Rhodamine 6G dye solution under picosecond pulse pumping are compared. We show the effect on the spectra and the pulse width of the degree of the coherent feedback. And we show that in certain dye concentration and scatterer density, the random laser show partial coherent feedback. This study changed the idea that in such random laser there is no coherent feedback, and may help to increase the knowledge on the laser physics.4. The spectral properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping are studied. The spectral peaks are shown to be at different wavelength for various dye solutions and scatterer densities. The inflection points of the spectral peak from blue shift to red shift are found for various dye concentration with increasing scatterer density. The inflection points are shown at different scatterer density for different dye concentration. Using the diffusion theory, the average optical path length can be estimated and the inflection points are believed to be the points when the system changing from the diffusive to weakly localized.5. The temporal properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping are studied. A special fiber is used to measure the temporal properties. The build-up time of the random laser from samples with various dye solutions and scatterer densities. We studied the dynamic of the random laser using the traditional laser theory and the diffusion theory. The results from the theory and the experiments are agreed in the trend.The main innovations in the thesis:1. For the first time, we give the more precise determination of the thermal focal length of the end-pumped solid state lasers. We show that the thermal focal length depends on the laser beam radius for the first time. We give the simple equations for convenience.2. For the first time we present a simple method for recording the coherent backscattering using CCD. We present the optimized selection of the integration interval. We calculated the dependence of the error of the results on the noise.3. For the first time we show a new experimental configuration based on the method we present. A cylindrical lens is used instead of normal lens to make the process of the CBS image much easier. The detail of the CBS peak is maintained using this configuration.4. By comparing the transverse amplified spontaneous emission, the longitudinal amplified spontaneous emission, the random laser and the emissions which we call the longitudinal amplified spontaneous emission with partial coherent feedback and with stronger coherent feedback, for the first time we show the effect on the spectra and the pulse width of the degree of the coherent feedback. And for the first time we show that in certain dye concentration and scatterer density, the random laser show partial coherent feedback. This study changed the idea that in such random laser there is no coherent feedback, and may help to increase the knowledge on the laser physics5. By studying the spectral properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping, for the first time we find the inflection points of the spectral peak from blue shift to red shift for various dye concentration with increasing scatterer density. The inflection points are shown at different scatterer density for different dye concentration.6. By studying the spectral properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping, for the first time we give the method of estimating the average optical path length of the random laser by using the spectral peak and the diffusion theory.7. By using the diffusion theory and the dependence of the obtained average optical path lengths on the scatterer density, for the first time we show that the inflection points of the spectral peak is showing that the system is changing from the diffusion system to a weakly localized system.8. For the first time, the build-up time of the random laser from the dye solutions with nanoscatterers under picosecond pulse pumping with various dye solutions and scatterer densities are measured. We studied the dynamic of the random laser using the traditional laser theory and the diffusion theory. The results from the theory and the experiments are agreed in the trend.
Keywords/Search Tags:Photon localization, Random laser, Coherent backscattering, Dye laser solution, Nanoscatterer, Thermal lens
PDF Full Text Request
Related items