Correcting polygon scanner cross-scan (wobble) error … if needed

Polygon scanners have been used in laser printers, copy machines and other lithographic applications for more than 40 years.  It is well proven that polygon scanners are capable of high speed scanning with “photographic quality” accuracy.  I like the term “photographic quality” because it requires the highest accuracy and the eye is very good at spotting misplaced pixels in a photograph.  

In the world of material processing, galvanometer (galvo) scanners dominate.  However, with high power and ultra-fast (picosecond & femtosecond) lasers, galvos may not be fast enough.  With a need for speed, more and more people are looking at implementing polygon scanners which can be 10 to 100 times faster than galvo scanners.  POLYGON SCANNERS

Every tool has its strengths and weaknesses.  Galvo scanners have been around so long, the weaknesses are well understood by designers and often unknown to the user because there are corrections going on in the background.  One of the characteristics of a polygon scanner is cross-scan error which is a deviation perpendicular to the scan line.  This is also referred to as wobble or dynamic track error.  There is non-repeatable wobble error from the motor bearings.  There is also repeatable error from polygon facet to datum error.  There is no such thing as a perfect polygon scanner.  There is always some wobble error.  Specs typically range from 15 to 60 arc seconds of mechanical error.  This is insignificant in many applications but not good enough in very high accuracy situations.  This article is to assist designers who need to reduce wobble errors.  Here are four methods:

Passive Optical Cross-Scan Error Reduction
This technique reduces both repeatable and and-non-repeatable errors.  
The beam is shaped to have an elongated spot on the polygon mirror facet.  After the polygon, it is re-imaged with a cylindrical lens.  It is illustrated here:

Here is an expired Xerox patent covering the technique.  It describes using some adjustable elements to align the optics and achieve submicron wobble correction.  POLYGON SCANNER WOBBLE CORRECTION PATENT

Galvanometer Scanner Correction
This can reduce the repeatable polygon mirror error.  It cannot reduce the non-repeatable motor bearing errors.  However, motor bearing error may be very small by the use of high precision bearings.
If you have a polygon scanning the fast X-axis and a galvo doing the slower Y-axis scan (for an X-Y raster scan) the galvo can be used to implement the repeatable cross-scan error correction.  The polygon facet errors can be stored in a lookup table and the polygon scanner can generate a 1/rev pulse indicating Facet #1.  Software can determine which facet is scanning and use the correction angle in the lookup table to have the galvo shift position very slightly in the cross-scan (Y-axis) correcting for each facet.

Stage Correction (similar to galvo correction)
This can reduce the repeatable polygon mirror error.  It cannot reduce the non-repeatable motor bearing errors.  However, motor bearing error may be very small by the use of high precision bearings.  
If you have a polygon scanning the fast X-axis and the target is on a moving stage (or reel to reel) the stage movement can be controlled to implement the repeatable cross-scan error correction.  The polygon facet errors can be stored in a lookup table and the polygon can generate a 1/rev pulse indicating Facet #1.  Software can determine which facet is scanning and use the correction angle in the lookup table to have the stage offset the normal step very slightly correcting for each facet.

Software Correction
This can reduce the repeatable polygon mirror error.  It cannot reduce the non-repeatable motor bearing errors.  However, motor bearing error may be very small by the use of high precision bearings.  
Cross-scan error correction is usually not needed for inspection and LIDAR applications.  However, image data may be improved by knowing more accurately where the beam is pointing on each scan.  The polygon facet errors can be stored in a lookup table and the polygon can generate a 1/rev pulse indicating Facet #1.  Software can determine which facet is scanning and use the correction angle in the lookup table to slightly change the positional data in the image or the point cloud.

Designers who are very competent designing galvo scan heads may feel intimidated when designing their first Polygon Scan Head.  While polygon scanners work quite differently from galvanometer scanners, there are proven methods to implement accurate high speed scanning.  Feel free to ask us if you have any questions.