In a scanning system, a laser beam moves linearly in one or two dimensions by usually using a galvanometer or scan mirrors. The focused laser can then be used for engraving images in different materials, and drilling, cutting or other applications. However, when using traditional lenses in this application, the profile of the focused beam changes as the beam moves away from the optical axis. The reason for this is those lenses focus light on a curved surface (this is one of the origins of some aberrations such as field curvature). The change in the focused beam can cause changes in the optical power on the material’s surface causing errors in manufacturing.
The focal point of a regular lens is a curved plane.
A way to solve this problem is by using what is called “flat-field” scanning lenses that focus light in a flat plane at the focal point. However, traditional flat-field lenses create an image that is proportional to the lens focal point and to the scanning angle tangent. That means, as the scanning angle changes linearly, the image will not move linearly. This can also cause manufacturing errors.
F-theta lenses can create this flat field and they are used in a wide variety of applications including laser welding, laser marking/engraving, laser drilling, laser displays, LIDAR, scanning laser microscopy, and optical coherent tomography.
They are called f-theta lenses because the image height created by these lenses is linearly proportional to the focal length and the scanning angle theta .
Difference in the focal plane for a convention lens, flat field scanning lens, and f-theta lens (image source Thor Labs)
There are different ways to decide what kind of f-theta lens is the right one for you. Things to keep in mind are:
Wavelength of operation. Are you working at a single wavelength? If so, are you in the visible, IR, or UV ranges?
Spot size. This may be critical if you are in manufacturing.
Scanning area. Sometimes referred to as Scan Field Diameter (SFD). It is a diagonal on the image plane where the beam can be focused.
Another parameter to consider is if you need a telecentric or non-telecentric system. In an telecentric lens, rays on the image plane are parallel to the optical axis. Therefore, the beam won’t shift laterally if the object is displaced from the focal plane. For example, if you are working with an engraver but cannot guarantee the distance between the plate that needs to be engraved and the f-theta lens you may need a telecentric lens to guarantee that your image is not distorted when engraved.
Example of a telecentric f-theta lens
(Please see this article for a detailed explanation of wide angle lens types including about f-theta lenses)
