Thermo-optically driven adaptive mirror for laser applications

Abstract

An adaptive mirror is investigated that is based on the deformation of the reflective surface due to thermal expansion of an underlying material that can be locally heated with an external light source. The mirror is made from a glass plate coated with a 1-mm-thick layer of a material with a large thermal expansion coefficient that was finally sputter coated with a thin gold film. The adaptive mirror is thermo-optically heated with an incandescent lamp. To generate the desired temperature pattern an aperture mask is used. The deformation of the adaptive mirror is measured with a Michelson interferometer. It is shown that the spatial resolution and amplitude of the deformation are sufficient for the application as an adaptive mirror. As a possible application the suitability of this mirror type as part of a laser resonator to generate a super-Gaussian mode is discussed. For many laser applications intensity distributions other than the TEM 00 fundamental mode are desired [1]. Among different custom modes the super-Gaussian mode (also referred to as the top-hat mode) features advantages which are important for numerous purposes, such as higher extraction efficiency of laser radiation out of the laser rod due to a higher spatial overlap with the pump distribution and the generation of higher harmonics in the field of non-linear optics [2]. The intra-cavity generation of custom modes requires a resonator mirror with a surface that deviates from a conventional flat or spherical surface. The intensity distribution emitted from a cavity can be customised by replacing one of the mirrors with a graded-phase mirror. With such an optical element, it has been shown that the generation of a super-Gaussian mode can easily be achieved with good efficiency [3,4]. The main goal is now to replace the currently fixed graded-phase mirror with an adaptive mirror to allow for continuous and controlled variations of the generated laserbeam properties during laser operation.However, currently known adaptive optical systems such as deformable mirrors with piezo-actuators [5,6] and electrou Fax: +41-31/631-3765, E-mail: hoeudae@yahoo.com statically deformable membrane mirrors [7,8] are not suited for intra-cavity beam control of lasers in the visible and nearinfrared spectral region. Either they cannot be modulated with sufficient spatial resolution or they cannot withstand the power levels reached in modern laser systems. A solution can be found with the use of an adaptive mirror that is thermooptically driven with an external light source. The pattern of the light source irradiated onto the mirror may be temporally constant, as with an incandescent lamp in combination with a mask, or it can be dynamic with the projection of a temporally varying pattern. For this purpose two mechanisms are possible: thermal expansion and thermally induced change of the refractive index.In this letter we concentrate on an adaptive mirror generated by thermal expansion of the underlying material. The mirror consists of a glass substrate carrying ...

Generation of custom modes in a Nd:YAG laser with a semipassive bimorph adaptive mirror

“…The development of novel thermo-optical and thermomechanical adaptive devices is in progress [31][32][33]. We expect a high lateral resolution and large phase shifts in the range of 20 diopters.…”

Thermal lensing: outside of the lasing medium

“…This heating light was provided and formed using a video projector and the mirror placed between the reflective layer and the substrate, was covered with an absorptive layer that was heated by the light provided by the projector. Note however, that the authors did relate the operation of their thermal mirror to thermo-elastic deformations only; thus, this type of device cannot be called a thermal lens, similarly as the device described by Michel et al [160] working on the principle very similar to that of Saathof et al…”

Thermo-optically driven adaptive mirror

“…Mirrors based on thermo-optically induced modifications of the surface itself or on modifications of a medium in front of the surface are also described 4,5 . They offer a promising alternative to conventional adaptive optics.…”

“…A rather elegant way is the irradiation of light for heating an adaptive mirror based on thermal expansion. Such a system using a layer of Sylgard 184 with a reflective coating has been investigated 4 . A similar solution is found with a mirror based on thermal dispersion 5 .…”