Optothermal Signal Simulation And Parameter Reconstruction For Semi-transparent Media Under The Laser Irradiation

Since the1960s, laser technology has been developed constantly. It has beenwidely applied in defense, biomedicine, communications, information processing,industrial processing, inspection diagnosis and many other fields. The essence ofthese applications is the interaction of the various types of laser with differentsubstances. Studying the interaction mechanism of laser in the semi-transparentmedium is of great importance in many fields, such as atmospheric remote sensing,laser propulsion, optical tomography, infrared nondestructive testing, the physicalproperty measurement and so on. In many cases, the interaction of laser andsemi-transparent body is accompanied by photothermal effects, i.e., there will beradiation signals (optical signals) or temperature responses (thermal signals) in thesurface of media. Systematically analysing the photothermal signals which are richof the inner information of the medium, and summarizing the influencing factorsand variation of these signals are the theoretical premise to reconstruct accuratelythe inner information of the medium, which includes the inversion of internal opticalparameters of the medium, geomitic features of inclusions, shapes, sizes and otherparameters of defects and so on.This thesis takes the application of photothermal signals obatained fromparticipating media exposed to the laser irradiation as research background. Thenthe time or frequecy-domain singnals have been simulated. Meanwile, thereconstruction for the iner parameters of media, and radiation propertiesmeasurement for semi-transparent media have been studied systematically. Themajor works could be summarized as the following four aspects:1. The finite volume method is used to solve the transient andfrequency-domain radiative transfer model describing the semi-transparent mediaexposed the square or Gauss type short pulse. Thus the optical radiation signals(transmitted and reflected signals) are simulated correctely. After that, the influencesof laser parameters and the medium optical properties on optical radiative signals.Futhermore, investigate the similarity research of transient radiative transferproblem, and propose the concept of optimal detecting distance for reflected signalsand critical optical thickness. Moreover, sensitivities for laser parameters andoptical properties of the medium with respect to time-domain optical radiationsignals were discussed, and the time domain signal selection criteria in the study ofparameters reconstruction inverse problem was determined. In addition, the relationbetween time-domain signal peak value and pulse width is studied with refractiveindex matching and mismatching, and put forward the concept of dimensionless peak reflection signals. This paper also analyzes the frequency domain signalamplitude and phase angle’s change laws with the pulse width and the attenuationcoefficient, the albedo of the medium.2. The particle swarm optimization (PSO) and its improved algorithms areintroduced into the study of inverse problem of transient radiative transfer andcoupled thermal radiation heat transfer. Use the time or frequency domain signalsand PSO-based algorithms to inverse the optical parameters and geometricalcharacters within one dimensional homogenous media, multi-layer media and twodimensional media. Based on the stochastic particle swarm optimization (SPSO), byintroducing the random direction searching items into the speed evolutionformulations of the particle swarm, a fast particle swarm optimization algorithm(RD-PSO) was obtained. In addition, this paper established the PSO inversion modelusing peak time-domain signals. Studies show that only using peak reflection signalcan reconstruct the optical parameters of the homogeneous and heterogeneousplanar medium accurately.3. The coupled conduction and radiation heat transfer modle based on theparticipating midium exposed laser is established, and the temperature response issimulated by the finite volume method. The PSO-based algorihtm is used to estimatethe conduction-radiation parameter, scttering albedo, and the emissivity of ahomogenous slab. Futhermore, the temperature response on surfaces of thetwo-dimensional ceramic medium containing defects in a futher step is investigated.The sensitivities for defect size and convective heat transfer coefficient with respectto transient temperature on medium boundary are analyzed, and the PSO-basedalgorithm was used to reconstruct the internal defects in the ceramic medium.4. The common measured methods for radiative properties (reflectance andtransmittance properties, absorption and scattering properties, and emissionproperties) are summarized. A experimental table to measure the emissivity ofsemitransparent medium is developed. Meanwhile, the calculating formula withdeduction of background radiation for spectral emissivity is given. The experimentalstudy of the spectral emissivity characteristics of semitransparent material wascarried out, and one sapphire sample and other materials is tested. In addition, thephotothermal properties of the glass sample with black coating were measured, andthe FVM modle of the coupling conductive-radiative heat transfer was validatedbased on the experiment study.