Telecentric fused silica scan lenses for short pulse lasers

05. February 2019

Beam focusing has an important role to play in many spheres of technical optics.

A single lens with a suitable focal length is the simplest solution. For even machining surfaces, a single lens is not sufficient, because the curved image plane requires repositioning of the focus. To circumvent complex movements, so called f-theta lenses are designed to compensate this position shift and provide an even image area.

Additionally they feature a consistent spot size over the whole scanning plane. Telecentricity is an option which provides a perpendicular incidence on the working plane. Furthermore, it is desirable to enable large scan areas, as they allow treatment of larger work pieces without moving the optical unit or target.

Since several years, Sill offers series S4LFT4010 with 100 mm focal length. This group of f-theta lenses for different wavelength regions from UV to IR offer comparably small spot sizes for increased fluence. The S4LFT4125/075 is an enhanced version for lasers with ultraviolet radiation and consists of fused silica    equally. The scan area has been improved from 35 mm x 35 mm to 50 mm x 50 mm. Despite the higher focal length, a focal diameter of 8 µm (1/e²) can be reached inside the whole scan area with a 10 mm input beam diameter. S4LFT4126/292 (515 nm – 545 nm) and S4LFT4127/328 (1030 nm – 1090 nm) are two further options for scan lenses with a focal length of 125mm but designed for different wavelength regions.

Furthermore Sill introduced a series with 65 mm focal length for UV (S4LFT4067/075), green (S4LFT4066/292) and IR (S4LFT4065/328) providing field sizes of 15 mm x 15 mm to reach spot sizes in the range of 3 µm (UV) to 10 µm (IR).

All series incorporate fused silica lenses and our industry proven low absorption coating. Of course, there are not internal ghosts in lens elements or on scan mirrors and therefore compatible with short pulse lasers. The new lens series are diffraction limited for the specified input beam diameter. Furthermore there are no internal and external ghosts and therefore no back reflections nearby the scanner.