Applications

►Optical material testing

►Lens heating

►Transient image distortion of complex optics (e.g. F-Theta)

►Industrial quality assurance

►Suitable for all high power lasers
(solid state, excimer, CO2)

 

Any beam guiding optics that is employed in high power laser operation is subject to thermal stress inside the optical material due to radiation-induced absorption, that gives rise to a transient wavefront distortion of the transmitted laser radiation. This effect is known as thermal lensing that yields to unwanted, power dependent instabilities during process control („Focus shift“ at optics of material- processing laser)

The ProOpto GmbH is a distributor of a photothermal measurement system developped at Laser- Laboratorium Göttingen that allows a quantitative acquisition of this effect. It is based on an extremely sensitive Hartmann-Shack wavefront sensor, that is able to detect wavefront distorsions in the range of λ/10.000 (< 100pm) within a scanned testing area of a few square centimetres. The advantage compared to interferometric procedures is the compactness and flexibility next to the extremely high sensitivity. The corresponding wavefront distortion acquisition is carried out in real time.

Thermal lens in fused silica @193nm

 

Shown is the photothermal measurement of a Quartz plate that is irradiated by a fiber laser (1070 nm) of medium power. The originally planar wavefront of a collimated test laser is distorted to a rotationally symmetric depression within seconds after switch on of the laser due to change of refractive index as well as thermal expansion which corresponds to the formation of a convex lens.For this example, its focal length (Defocus) is around 1 km (!) at a total wavefront distortion (peak-to-peak) of around 2 nm.

Scheme of laser power absorption and heat transfer in bulk material

 

 

With this novel method it is on one hand possible to measure the induced wavefront distortions precisely via Zernicke analysis, so that action can be taken in order to compensate. On the other hand, the photothermal technique can be employed for fast absorption measurement of optical components.