Quantitative measurement of thermal lenses using wave front sensor

Radiation-induced thermal lenses in optical components are known to cause considerable problems in many laser radiation applications. For example, in semi-conductor lithography, the so-called 'lens heating' resulting from absorbed UV laser radiation can lead to a reduction in the level of achievable resolution. Another example in which thermal effects play a key role is the structure of powerful solid state lasers. If designers want to achieve good stability and protect the lenses from destruction, they cannot avoid taking into account the thermal lens produced by the laser rod.

Laser-Laboratorium Göttingen has now developed a measurement system for quantitatively recording this effect. It is based on an enhanced Hartmann-Shack wave front sensor, characterised by extreme sensitivity and which can now detect wave front changes in the range of lambda/10 000. This represents deviations of less than 0.1 nm over a sensed area of several square centimetres. The benefit of this over interferometric procedures is that despite the high level of sensitivity, the measurement system is very compact and can be used flexibly. The wave front is easily recorded in real time.

The figure shows the photothermal measurement of a quartz plate that has been radiated by an excimer laser at a wave length of 193 nm. Within a few seconds of the excimer laser being switched on, the flat wave front of a collimated laser beam is 'hidden’ by the change in refraction index and thermal expansion of the quartz plate into a rotationally symmetrical cavity corresponding to the moulding of a convex lens. In the example shown, its focal width (defocus) is around 1 km (!) and the total wave front deflection (tip to tip) is around 2 nm.

This new procedure can firstly be used to accurately measure wave front changes induced by the Zernike analysis such that actions to compensate (e.g. using adaptive lenses) can be taken. Secondly, the photothermal measurement can also be used to quickly evaluate the quality of optical components because the degree of wave front change is directly proportional to the thermal absorption losses. This opens up another area in which the Hartmann-Shack sensor can be used for the real-time characterisation of laser radiation (beam profile, propagation, M² measurement).

Laser-Laboratorium Göttingen is a partner to PhotonicNet, one of nine regional networks of expertise organised for optical technologies (OT) in Germany. Key tasks include offers such as forums and working parties organised throughout the country on current technology issues, information management, monitoring of research applications, advice for business start-ups and the sharing of know-how. PhotonicNet will be sponsored by the German Federal Ministry of Education and Research (BMBF) until 2008. The state of Lower Saxony is also sponsoring the network.