Technicon: F-theta lenses

Overview

General Information

Lenses used in combination with XY galvanometer scanners or polygon scanners are so-called ƒ-theta lenses, plane field objectives or scan lenses. Our ƒ-theta lenses are used in various applications from industrial material processing (e.g. drilling, welding of synthetic materials or cutting) in addition to medical and biotechnological applications (confocal microscopy, ophthalmology) and science and research.

The design and the quality of the optical components play a key role in the lens performance. Standard lenses focus the laser beam on a spherical surface in contrast to an ideal flat or plane field. The use of ƒ-theta lenses provides a plane focusing surface and almost constant spot size over the entire XY image plane or scan field. The position of the spot on the image plane is directly proportional to the scan angle.

The scan length or scan area specifications in this catalog are based on mirror spacing of typical scan heads. For other scan systems the parameter “aperture stop” defines the distance of the geometrical center between the mirrors to the mechanical edge of the lens housing.

Short pulse laser (SP laser) and ultrashort pulse laser (USP laser) have special demands on optical elements:

SP lasers in picosecond range emit narrowband light with a bandwidth of approx. 1 nm. As peak power can be very high, non-linear effects (color centers, self-focusing, multi-photon absorption) can occur in certain glass materials.

In USP (ultra-short pulse) lasers in the femtosecond regime non-linear effects are even more of an issue. Additionally these lasers emit with a certain spectral bandwidth depending on the pulse length. That leads to color aberration because parts of waveband will be shifted both in and transvers to the direction of propagation. The result is a larger spot decreasing the energy density and limiting the advantages of ultrashort pulses.

A general statement about usability of a certain lens is not possible because of difference between available SP- and USP-laser sources and applications. Please contact us to discuss your requirements.

Optical and mechanical scan angle

The optical scan angle describes the maximum angle of the beam into the entrance aperture of the scan lens to avoid vignetting. Be aware that the max. mechanical scan angle, which describes the angle of the scan mirrors, is half the value of the optical scan angle.

Scan length

The scan length is the diagonal of the maximum scan field. It is depends on the scan angle and working distance of the lens. Note that for a telecentric f-theta, the max. output aperture has to be equal or larger than the required scan length(diagonal of the scan field). This provides a rough guideline for lens selection.

Aperture stop and scan-mirror distances

F-theta lenses are designed to focus a laser beam onto a planar image plane. They are often used in a scanning system with two galvanometer mirrors. One mirror is responsible for beam deflection in one direction and the second one for the perpendicular direction. For simulation purposes an aperture stop is placed exactly in the middle between both mirrors. In real applications, there is no mechanical border to create any kind of aperture stop there. The following sketch shows an illustration of the optical elements involved on the optical axis.

Spot-radius-diagram

The spot radius diagram indicates the spot radius variation depending on its position in the X-Y-scan field. The spot size is marked by the color gradient, ranging from the smallest value in blue (unit is always in microns [μm]) to the largest value in red. Both axis cover the max. scan field length and width, which is 35 x 35mm for the S4LFT4010/292. Note, that the PRAM parameters are the mechanical (not optical) scan angles in degrees of the two mirrors.

The radius values are dependent on the diffraction value and the entrance beam size. Sill assumes M² to be equal to one, thus a rough estimation is done by multiplication of the actual M² of the laser. The beam diameters are not always referred to the maximum clear aperture. In some applications beam intensities are so high, that vignetting at the 1/e² value would be unacceptable. Details about the laser parameters used in simulation are given in the text below the diagram.

As shown in the illustration below, the spot size changes depending on its position on the scan field. The reason is, that no lens can be designed to perfectly eliminate every aberration. Most designs compensate so all aberrations to lie below the diffraction limit, so they can be neglected. But even diffraction limited lenses show a varying spot size, because the diffraction conditions differ for every scan field position.

Maximum telecentricity error

The maximum. telecentricity error is a specification of telecentric lenses which indicates the deviation of the laser beam from perpendicular in the corner of the XY scan field or the specification of the incident angle for entocentric ones. Note, that the telecentricity error is always zero on the optical axis and in which is the center of the scan field.

Lens type recommendation

As previous explanations about LIDT demonstrated, the correct selection of a lens to the laser used and process requirements can be difficult and a general statement about usability is not possible. Therefore some basic lens properties are explained here, which are typically needed for specific laser types and give a rough guideline for any selection process.

Ghost free

Ghost (or back) reflections occur as a portion of the laser light is reflected back from a lens surface to a previous lens element.

Lens surfaces will typically reflect back about 4% of the light energy on each surface. Laser lenses are therefore coated with an anti-reflective coating which transitions the light from the index of refraction of the air to the refractive index of the bulk material of the lens. This reduces the back reflection from each surface to about 0.2%. Although 0.2% seems like a small amount, in a pulsed laser the peak power of the ghost spot can exceed the damage threshold of the coating or the bulk material.

Most scan lens have anywhere from 2-6 lens elements. The solution is to design the lens so no back reflections occur on or in any of the lens elements or on scan mirrors. Normally this is not a problem if the mirrors are placed as per the recommended design. This is accomplished by using an appropriate adapter ring.

We strongly recommend the use of such ghost free lenses with high power laser (up in the kilowatt-range) and as well as for short-pulse lasers.

Fused silica glass with low-absorption coating

Fused silica is a very resistive glass type which has also a very low thermal expansion coefficient compared to other optical glasses. Therefore it is commonly used to minimize thermal effects. Sill also uses a special low-absorption coatings with all fused silica objectives to minimize thermal effects further and increase typical damage thresholds. Fused silica combined with low-absorption coatings are recommended for the use with all high-power or short-pulse lasers.

Color correction

Color correction can be understood in two ways: First, some lenses are color corrected to be used with two wavelengths (e. g. 1064nm and 532nm) or a broad wavelength range (e.g. 405nm – 650nm). The second type of color correction relates to ultra-short pulse lasers. Their pulse width is so short, that due to the uncertainty principle, the wavelength range of the pulses is broad, that color errors play a significant role and can defocus the laser spot. Sill has developed and offers a new selection of special lenses to counteract those effects via color correction. These lens are specifically designed to be used with ultra-short pulse lasers.