ƒ-Theta objectives, also known as flat-field objectives or scan objectives, are used in many industries. While standard lenses display the laser beam on a spherical shell, but not on an even field, ƒ-Theta objectives have one major advantage: You can use them to position the laser focus on a level image field, while the focus size remains almost constant. The position of the focus point (image height) is proportional to the scanning angle.
Lenses used in combination with XY galvanometer scanners are called ƒ-theta lenses, plane field objectives or simply scan lenses. ƒ-theta lenses are used in various applications from industrial material processing, drilling, welding of synthetic material and cutting in addition to medical and biotechnology (confocal microscopy, ophthalmology) to science and research. The design and the quality of the optical components are playing a decisive role.
Short pulse lasers and ultra-short pulse lasers, but also lasers with high average power issue a exceptional challenge for lenses. Processed are strongly influenced by the characteristics of regular optical glasses. For instance thermal effects change both the beam shape and the working distance. At this point fused silica presents its crucial advantage by lower sensitivity to thermal effects in comparison to optical glasses and is therefore highly recommended for the use with the named laser sources above.
Telecentric ƒ-theta lenses provide a beam with perpendicular incidence onto the image surface. This is necessary for drilling holes and structuring surfaces in the third dimension (depth). The downside is generally a smaller focal plane compared to a non-telecentric lens, because the size of the frontal lens elements has to be larger than the scan area. Sill offers for both kinds of ƒ-theta lenses a large variety of options and focal distances.
While the dispersion in the material can neglected for fs-pulses, the color error caused by the spectral width of the pulse plays a decisive role. The spectral width of the pulse is increasing rapidly by shorter pulse durations and for longer wavelengths. This results in a color error where the axial and lateral position of a focused beam varies for different colors. The amount of deviation of the resulting spot depends on the focal length and the wavelength. Sill offers ƒ-theta lenses which are especially designed for the use with femtosecond lasers and correct this color error leading to a consistent beam shape over the complete scan area.
For online inspection systems we successfully introduced color corrected ƒ-theta lenses for 1064 nm and 532 nm to the market. Color correction offers usage of one lens for several manufacturing steps at the same time.
In addition, we offer color corrected ƒ-theta lenses designed for confocal microscopy applications optimized for a wavelength range from 450 nm to 650 nm and a lens optimized for 355 nm and 1064 nm. The focal lengths and working distances are identical for several wavelengths, resulting in identical image fields for the laser and inspection wavelength respectively for a whole wavelength range.