Abstract
Laser-induced thermal lens in optical components causes wavefront distortion of the laser beam and may affect performance and stability of optical systems such as high-power lasers. The bulging of the heated area, the temperature dependence of the refractive index, and the photoelastic effects are responsible for phase shifts damaging beam quality. The theoretical background for laser-induced beam distortion is well understood and applies only for axially symmetric thermal loadings, with the assumptions that the stresses follow thin-disk or long-rod approximations. This, in fact, limits the overall applications of this model. In this work, we developed an unified theoretical model for the optical path change in optical materials regardless of its thickness. The modeling is based on the solution of the thermoelastic equation and has a real description of the surface deformation caused in the optical element. In the appropriated limits, as expected, the model retrieves the thin-disk and the long-rod type distributions. Furthermore, we provided time-dependent radial expressions for the temperature, surface displacement, and stresses. The theory presented in this paper provides simple analytical tools for designing laser systems, and complements previous work allowing one to access optical distortions of materials ranging from thin-disk to long-rod-like distributions.
©2012 Optical Society of America
Wavefront distortion arising from thermal lens in optical materials poses a major problem in high-power laser systems and has been subject to exhaustive investigation over the past few decades [1–9]. In such optical components, nonuniform heating caused by the laser beam absorption compromises the system’s performance affecting the optical quality of the beam [1,2]. This effect has been mostly investigated regarding phase shift induced in optical windows. Particularly for windows, the wavefront distortion originates from the refractive index gradient thermally induced in addition to the thermoelastic deformation of the windows surface.
The theoretical treatment for this effect has been investigated in great detail [3–6] and its bases relies on the fact that windows, or optical elements, are subject to axially symmetric thermal loading and thermoelastic properties are isotropic in the plane of the windows. In addition, the stresses are constrained to follow either the plane-stress or plane-strain approximations. The plane-stress approximation applies to thin-disk geometries [4], where the windows thickness is much smaller than the windows radius. The plane-strain model, on the other hand, uses the long-rod approximation that satisfies the condition of much larger thickness compared to radius. In the absence of mechanical loading, the optical windows are subjected to cylindrical symmetric radial and azimuthal stresses that are related in a simple manner to the temperature distribution within the material. The phase shifts responsible for the distortion of the beam arise from the change in path length as the heated area deforms, from the temperature dependence of the refractive index, and from thermoelastic effects associated with nonuniform heating pattern. We show here that those assumptions are, in fact, too restrictive and develop a unified analytical theory for the optical path change in optical materials regardless of its thickness. The modeling is based on solution of the thermoelastic equation and has a real description of the surface deformation by the optical element. In the appropriated limits, the model retrieves the thin-disk (plane-stress) and long-rod (plane-strain) type distributions. This generalized model complements previous work [3–6] in the sense that it allows us to access optical distortions of materials ranging from thin-disk to long-rod like distributions, and could have significant impact on designing laser systems and predicting windows degradation of optical materials. Moreover, this effect is the foundation of many photothermal methods for material characterization, and particularly important for the thermal lens spectrometry, where the optical path change is directly linked to the thermo-optical properties of solid and liquid materials [10–12].
For an axial symmetric beam propagating along the direction, the optical path is defined as
We consider here a low optical absorption material of thickness (path length) and refractive index centered in , as shown in Fig. 1. The material is surrounded by a nonabsorbing fluid medium of refractive index . The optical path length can be then written as where and are the laser-induced thermoelastic displacement components [13,14]. We define , and with regard to the change in index, , we must consider not only the temperature dependence () but also the stress induced thermoelastic effect () as [4] Note that . Using the linear transformation , the optical path length results in five contributions: where is the optical path before heating; are the contributions from temperature index variations caused in the sample () and in the fluid (); is the contribution from thermal stress; and is the contribution from thermoelastic expansion.In a first approximation, the thermal effects on the refractive index can be easily formulated as linearly dependent over the temperature range of interest as
is the thermo-optic coefficient as measured at the reference temperature for the sample () or for the fluid (). is the laser-induced temperature rise distribution. In this approximation, we may write andThe thermal stress index variation involves considerations relating to the thermoelastic effect and applies only to the sample as
for plane waves polarized along the radial direction, and for azimuthal polarizations. and refer to the stress-optic coefficients for stresses applied parallel and perpendicular to the polarization axis, respectively. are the stress tensor components. From Eqs. (9) and (10), we can introduce the symmetric and antisymmetric combinations of the radial and azimuthal aberrations [4], as with represents the averaged medium’s sensitivity to thermal lens. It combines the laser-induced change in index, and the contribution from bulging and photoelastic effect for the two polarizations. contributes only if the medium is stress-birefringent.Finally, the contribution originated from the surface deformation yields
Terms of second order proportional to the product are neglected here. The above expressions show us that the optical path change induced by the laser beam is a result of thermal, stress, and strain effects. This complex problem could be simplified under approximative assumptions. For instance, if the beam radius is smaller than the radial dimension of the sample, for a relative short exposure time , the temperature at the edges of the sample can be assumed constant. Moreover, if air is used as fluid, which is basically the case for optical windows in laser systems, , and the approximation of no heat flow across the interface material/fluid is valid [15]. Thus, the fluid contribution can be safely neglected—and will be omitted from this point.
We consider the case of a continuous wave incident beam with a Gaussian intensity profile. It is assumed that the attenuation of light intensity along the material thickness can be neglected—low optical absorption approximation. Thus, the temperature rise distribution with no axial heat flow to the surroundings is given by the solution of the following heat conduction differential equation [16]:
with the initial condition and boundary conditions and . , , and are specific heat, mass density, and thermal conductivity of the material, respectively. , is the optical absorption coefficient at the laser beam wavelength, , , and are the material’s surface reflectivity, the laser power, and its radius at the sample surface, respectively.The thermoelastic equation for the stress and surface displacement caused by a laser-induced nonuniform temperature distribution, in the quasi-static approximation, can be expressed in cylindrical coordinates by introducing the scalar displacement potential and the Love function following by the Poisson’s equation [17],
and the Biharmonic equation, is the linear thermal expansion coefficient, and the Poisson’s ratio.The components of the displacement vector () and the stress () are obtained from and by the relations
and where is the Young’s modulus and the Laplacian . Eqs. (16) and (17) are solved using free stress boundary conditions, , at the surfaces.Recently [13,15,18], we have obtained semianalytical expressions for the temperature and thermoelastic potentials. In those solutions, we were assuming that the radial dimension of the sample was large enough for the temperature to be considered constant at the edges of the sample, that is, . In fact, the solutions were validated by all numerical finite elemental analysis modeling with real boundary conditions [15,18]. Using this approximation, and assuming no azimuthal dependence for the temperature distribution, we can write the temperature rise as
is the Bessel function of the first kind, and is the Hankel transform of the -dependent temperature ( space). represents the thermal diffusivity of the sample. The scalar displacement potential and the Love function are and withThe above expressions allow us to write the displacement and stresses in a simple manner as
and where andThe symmetric and antisymmetric combinations of the radial and azimuthal aberrations to the optical path length lead to
and with . The stress contributions to the refractive index variation can be neglected for some optical materials because it is substantially smaller than the thermal contribution [4]. However, for materials which , the thermal expansion contribution can be suppressed and the stress contribution becomes relevant. As for , it contributes only if the material is stress-birefringent.Using Eqs. (14) and (27), the thermal expansion contribution can be written as
Finally, with Eqs. (8), (22), (33), and (35), we can write for free stress-birefringent material, that is , as
The general solutions presented here can be simplified if one take the following limits. For , the expression for recovers the plane-stress approximation, and, for , it recovers the plane-strain approximation, where and Eqs. (37) and (38) describe the well-known results for thin-disk and long-rod geometries [3,4].To illustrate and support the analytical solutions, Eqs. (36) to (40), we perform simulations considering physical parameters of a calcium fluoride windows () [6]. Figure 2 shows the normalized temperature, , and surface displacement, , profiles for windows with different thicknesses at an exposure time of and . The temperature profile has a sharp shape compared with the surface displacement for thick windows, which is caused by the mechanical inertia of the surface to expand upon temperature change. Both the temperature and the surface deformation profiles become similar only for very small thickness. This, in fact, validates the thin-disk, or the plane-stress, approximation. However, it is important to note that approximating the surface deformation by the temperature profile is reasonably acceptable only for very thin samples.

Fig. 2. Radial temperature (open circles) and surface displacement (continuous lines) profiles for the windows glass.
The limit cases, plane-stress and plane-strain, are better explored by analyzing the optical path change. Figure 3 shows the optical path scaled by a factor for and different thicknesses. was evaluated by Eq. (36) and the limit cases by Eqs. (37) and (38), respectively. The results illustrate well the total phase shift moving from a plane-stress approximation, for small , to the plane-strain as increases.

Fig. 3. Time dependence of the optical path for using the physical parameters of glass windows [6]. Solid lines were calculated using Eq. (36) and Eqs. (37) and (38) for plane-stress (open squares) and plane-strain (open circles) approximations, respectively.
Figure 4 shows the time and radial evolution of the optical path for a windows of thickness . The radial dependence of the optical path length induces aberration in the wavefront propagation. The corresponding phase shift is given by
where is the laser wavelength. In fact, Eq. (41) provides an expression for analyzing the thermal lens effect caused by a laser-induced optical element. This effect is important in high-power laser systems and has numerous applications in photothermal effect based techniques [10–12,15] for material characterization, such as in the thermal lens spectrometry. As for high-power laser systems, a criterion normally used to characterize the laser beam distortion is the Strehl ratio—the ratio of the intensity at the point of maximum intensity in the observation plane with and without phase aberration. For instance, in an untruncated Gaussian illumination with beam radius , it is given by [19] In a similar way, in the thermal lens spectrometry, the intensity of the center beam at the far field detector plane is given by [10] in which is a geometric parameter from the experimental setup, , and the probe beam radius in the mode mismatched configuration [10–12].In conclusion, we have developed a unified analytical theory for the wavefront distortion caused by laser-induced thermal lens. The theory applies not only to plane-stress and plane-strain type distributions, but also for stress and strain contributions within these limit cases. Radial and time-dependent expressions for the temperature, surface displacement, and stresses have been obtained. This generalized model could have significant impact on designing laser systems and predicting laser-induced windows degradation in optical materials. Furthermore, the optical path description presented in this work has direct application in thermal lens spectrometry, correlating optical path change to thermo-optical properties of solid materials.
ACKNOWLEDGMENTS
The authors are thankful to the Brazilian agencies CAPES, CNPq, and Fundação Araucária for the financial support of this work.
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