Compensation of Strong Thermal Lensing in High-Optical-Power Cavities

Abstract

In an experiment to simulate the conditions in high optical power advanced gravitational wave detectors, we show for the first time that the time evolution of strong thermal lenses follows the predicted infinite sum of exponentials (approximated by a double exponential), and that such lenses can be compensated using an intracavity compensation plate heated on its cylindrical surface. We show that high finesse 1400 can be achieved in cavities with internal compensation plates, and that mode matching can be maintained. The experiment achieves a wave front distortion similar to that expected for the input test mass substrate in the Advanced Laser Interferometer Gravitational Wave Observatory, and shows that thermal compensation schemes are viable. It is also shown that the measurements allow a direct measurement of substrate optical absorption in the test mass and the compensation plate.

Discriminating the role of sample length in thermal lensing of solids

“…Much effort has been made on the study of diode pumped solid state lasers (DPSSL), especially when operating in an end-pumping configuration due to the highly localized heat deposition achieved in this configuration [1]. For instance, compensation of TL effects is a key issue for the performance of interferometric gravitational-wave detectors [2]. Monitoring the dynamic process of optical path difference (OPD) with photothermal methods enables direct quantitative access to many physical properties of a large class of materials.…”

“…The description of wavefront distortion induced by laser absorption in optical elements is fundamental in the design and evaluation of solid state lasers, optical windows, and other passive optical components for high power laser systems [1][2][3][4][5]. Thermal lensing is the dominant effect for beam quality and power scaling of solid state lasers.…”

Observation of optical torsional stiffness in a high optical power cavity

“…The charge-coupled device ͑CCD͒ camera located behind the ETM measures the transmitted beam profile. 15 We measured the resonant frequency 1yaw of the ITM yaw degree of freedom as a function of the total cavity g-factor, which was varied by thermal tuning the focal length of the CP. The spectrum analyzer, shown in Fig.…”

Direct measurement of absorption-induced wavefront distortion in high optical power systems

“…1, consists of a flat sapphire input-coupling mirror, or input test mass (ITM), that is reversed so that the substrate is within the cavity (to increase the power absorbed in the substrate) and a highly reflecting concave mirror, or end test mass (ETM). The cavity also contains a fused-silica compensation plate (CP) that was used previously to demonstrate wavefront correction by conductive heating of the plate [18]. The optical and physical properties of the mirrors and CP are listed in Table 1.…”

“…Because of the vastly different thermal diffusivities of sapphire and fused silica, the thermal lenses will form at significantly different rates. This effect has previously been observed indirectly by monitoring the behavior of the cavity mode [18]. The induced wavefront distortion is measured using a Hartmann wavefront sensor (HWS) [16] in an off-axis configuration to enable the distortion induced by each optic to be measured separately.…”