Lenses for femtosecond lasers

24. April 2015

The pace in the development of short pulse and ultrashort pulse lasers has been amazingly rapid. Laser systems utilizing picosecond lasers are efficient tools in many industrial and scientific applications. 

Besides “cold ablation”, another key feature of short pulse lasers are nonlinear effects like multi photon absorption and self-focusing in transparent materials. In glass cutting applications, these effects are utilized for modifying the index of refraction and to distribute the laser energy along the beam axis, i. e. maintaining a near-constant beam radius over many Rayleigh lengths, thus providing a very large depth of field. These effects are well suited in contour cutting (filament cutting) of uncured, chemically hardened glass (cover glasses of smart phones) and sapphire, resulting is very high quality edges and very little material removal. The glass is cut by plasma dissociation leading to cutting kerfs smaller than 1 µm. This results in a kerf  which is much smaller than the diffraction limited laser spot focus diameter. Sill Optics now offers a range of specially designed lens for these short pulse lasers.

For pulse lengths shorter than 1 ps, the laser creates a noticeable spectral bandwidth, which can have an impact on the spot performance. Thus, an 800 fs Gaussian shaped pulse has a spectral width of approx. 2 nm and a 250 fs pulse a width of almost 7 nm. Usually, laser lenses are corrected for monochromatic light. The spectral bandwidth of short pulse lasers results in so-called color errors both in and transverse to the propagation direction. The resulting spot will be distorted and blurred, as the laser pulse intensity is spread onto a larger area inhibiting multi photon processes.

Sill Optics has introduced a telecentric f-Theta lens with a focal length of 100 mm and a maximum field size of 35 mm x 35 mm. The unique feature of this lens is the color correction from 1.0 µm to 1.1 µm, i.e. for a 100 nm wide spectrum. For a 10 mm beam the lens is diffraction limited. This f-Theta lens is available for purchase under part number S4LFT7010/450. A version for 1.500 nm – 1.600 nm will follow soon.