Technicon: F-theta lenses

Overview

General Information

Lenses used in combination with XY galvanometer scanners or polygon scanners are so-called f-theta lenses, plane field objectives or scan lenses.

 

Our f-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 f-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. The calculated values take a maximum vignetting of 1 % into account.

 

Lenses which are marked with an ! are manufactured on request only.

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.

Internal ghosts

Ghosts or back reflections occur as a portion of laser light is reflected from a lens or protective window surface to a previous lens element.

 

Laser lenses are coated with anti-reflective coatings which transitions the light from the index of refraction of air to the refractive index of the bulk material of the lens. This reduces the back reflections of each surface from 4 % to 0.2 %. In spite of low-absorption losses a usage of lenses with internal ghosts and SP and USP lasers often results in exceeding the damage threshold of the coating or bulk material.

 

Most scan lenses have anywhere from two to six lens elements. The solution is a special design which prohibits internal ghosts nearby any lens element. We strongly recommend the use of such “ghost free” lenses in combination with high and mid power lasers (up in the kilowatt-range) as well as with short pulsed lasers. USP usable and ghost free lenses are marked with a . They consist of glasses with a low temperature coefficient (e.g. fused silica) without any kitted surfaces.

External ghosts

Ghosts are back reflections from lens surfaces or from the protective window. High power lasers are able to damage optical elements which are positioned nearby the back reflection.

 

On the one hand the focus of an internal ghost (see page 31) is positioned on top of a glass surface inside the lens. Lenses with internal ghosts are generally not suitable for high power lasers.

 

On the other hand there are external ghosts whose foci are positioned outside of the housing. High power lasers can be used safely in this case. But it is important to choice the distance between lens and the rest of the optical setup advisedly. If the external ghost is on top of an optical element (normally scanner mirror) a damage be generated there.

 

On the datasheet there is a field called “back reflection position” which specifies all external ghost positions. The distance is measured between the focal point on the optical axis and the frame border. The middle chief ray is the basic for the simulation. Tilting the beam results in a position change of the external ghost. Because of that optical elements in front of the lens should not be positioned nearby the ghost position (minimum ± half laser beam diameter).

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 Optics 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.

Polychromatic lenses

There are two different polychromatic lens types.

 

On the one hand there are color corrected lenses, which become more important for modern high power lasers nowadays. Color corrected f-theta lenses ensure a high process quality for applications with ultrashort pulsed lasers (some fs). According to the uncertainly principle of Küpfmüller (analogous to the uncertainly principle of Heisenberg), the product of bandwidth and pulse width is constant. So the spectral bandwidth increases with decreasing pulse width.

 

Different spectral waves of USP lasers focus with an off set in the direction and vertically to the propagation direction. This effect increases the spot diameter, reduces the energy density and compensates the advantage of the short pulse for standard lenses.

 

Color corrected lenses enable a low spot size and high energy density even for very short pulses.

On the other hand there are multispectral lenses, which are usually used for online process monitoring. In contrast to color corrected lenses they are corrected for different narrowband wavelengths – typically the working and the observation wavelength. Sometimes multispectral lenses are utilized for processes with diverse lasers. So it is possible to switch the lasers without exchanging the lens.

Scan length

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

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.

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 diameter diagram

The spot diameter diagram is a color diagram which indicates the spot diameter variation depending on its field position. The color gradient ranges from the smallest value in white to the largest value in blue. Both axis cover the max. scan field. The scales on the axes show the position in the working area [mm] with the middle placed point of origin. At the lower and upper end of the axes you can see the minimum / maximum field position and the mechanical mirror tilt [°].

 

The size of the beam diameter depends on the laser beam quality factor M² and the entrance beam diameter. Sill assumes M² to be equal to one, thus a rough estimation is done by multiplication of the actual M² of the laser. The spot diameter is the diameter of the circle which includes 86.5 % (1/e²) of the impacting laser power.

 

The spot diameter diagram is 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 input beam diameter used in simulation are given in the text below the diagram.

 

Because of aberrations there is a dependence between spot diameter and field position.

 

Most designs are diffraction limited on the entire scan field. But even these lenses show a varying spot size, because the diffraction limit changes over the scan field. The percentage value on top of the color scale specifies the intensity of this variation.

  

 Example: Spot diameter diagram of S4LFT4010/292 f-theta lens


Telecentricity error

The telecentricity error specifies the deviation of the laser beam from perpendicular. Usually it is maximum in the corner of the XY scan field. Note, that the telecentricity error is always zero on the optical axis. The telecentricity error or maximum incidence angle is a sign of quality for telecentric or entocentric lenses. 

 

Perfect telecentricity is only possible if all light comes from the object sided focus.

 

In principle you can say: the larger the space between the mirrors, the larger the distance from one mirror to the object sided focus the larger the telecentricity error.

Black Box files for Zemax OpticStudio®

Standard Sill lenses are calculated for a certain scan head, but it is also possible to use them in combination with other scanners. A variation of the mirror distances or of the input beam diameter influences specifications like e.g. scan field size, spot diameter or telecentricity error. Black Box fi les can be helpful for simulating specifications of a complete system within a custom specific environment. This applies not only to f-theta lenses but also to lens systems which should be integrated into an optical setup. These fi les show the performance of a lens without disclosing its design.

 

For opening you have to save the fi le in the folder “Zemax”→”Black Box” so that the program has access to the data. After that you can insert a new surface into your design fi le and set “Surface Type” to “Black Box Lens”. In order to insert the Black Box put the full name of the Black Box fi le into the field “comment” (e.g. “f-theta-lens.ZBB”). In principle it is possible to get a Black Box fi le of any Sill lens on request.