Nonlinearity of the chairacteristic of transformation thinwire bolometer of focused laser radiation

Abstract

r;liameters of focused laser radiation allow to estimate its radiatence, and by use in technology define quali:j of carried out processes, for example such, as cutting speed of metal, melt depth and width of a cut of a sket material. The means of measurements of a focal stain laser parameters should have high optical durability and large spatial accuracy of determination of intensity distribution in section of a beam. In a visible range of a spectrum use photographic ways [l], moving of an opaque plate with a small aperture in CTOSS section of a beam and method of a replaceable stops with receivers of radiation, and also way of an evaporating film on a transparent plate [2]. The listed ways require essential easing of radiation, are rather labour-consuming and result in significant errors of measurements results. Complexities of use of known ways of i! focal stain parameters measurements are increased with transition in infrared range of spectrum. One of possible ways of perfection of measurement methods of a focal stain parameters of continuous laser radiation is use of thinwire bolometers with diameters in units of micron, having high optical durability and large spatial accuracy at their moving in section of a beam. The large density of radiation capacity result in significant temperatures of heating of irradiated bolometer areas and under these conditions its basic physical parameters (temperature factor of resistance a, factor of efficiency of absorption q, factor of heat exchange with external environment y, the specific heat capacity c and factor of heat conduction h) do not remain constant, that results in occurrence of nonlinearity of the transformation characteristic, amplifying non-uniformity of falling capacity distribution, and systematic errors of registered parameters of radiation. In work [3] researches of a thinwire bolometer of a focal stain of the laser on the basis of the decision of the stationary equation of heat conduction with variable factors are begun. The given work is continuation of these researches by the decision of the non-stationary equation of heat conduction with variable factors. In a rather large interval of temperatures the basic physical parameters of a bolometer have linear dependences: a(T)=ao+alT; q(T)=qo+qlT; y(T)=yo+ylT; c(T)=co+c~T and h(T)=ho+hlT, ao, qo, yo, CO and Lo meanings of parameters at temperature of environment, and a~, 41, yl, CI and XI - factors of temperature dependence of these parameters. Under these conditions quasi-lineal non-stationary equation of heat conduction for bolometrical element, irradiated with Gaussian beam of radiation, will be: where 2.L - complete length of a bolometer; z=x/L - relative coordinate of a bolometer, varied from -1 up to 1 ; t - time, and @=UT - normalized time, where T=m'cD/yo -thermal time constant of a bolometer with linear weight m; SH=x d'14.L' - normalized area of bolometer section with diameter d; DH=D/L - normalized rootmean-square radius of an optical beam with the valid meaning 0; T=T(z,O) - temperature of a bolometer in a point with coordinate z at the moment of time 0; Pk - effective falling capacity on a bolometer; P complete capacity of radiation falling on a bolometer; k=cos2$+KDsin2$ - polarizable factor of interaction of radiation with a cylindrical bolometer, showing dependence of the factor of absorption efficiency on a corner between direction of linear polarization and axis of bolometer; KD - dichroizm factor of a bolometer equal to the relation of the factors of absorption efficiency for perpendicular and parallel axis of a bolometer polarizations of radiation. The equation was solved with the entry condition T (z, 0) =O and boundary conditions T (&I,@) =O. The decision of the equation is received numerically through a method of final differences under the