Things you need to know about specifying polygon mirrors for laser scanning

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 higher speed, people are looking at implementing polygon scanners which can be 10 to 100 times faster than galvo scanners.  POLYGON SCANNERS

Someone who is very experienced designing galvanometer scan heads, may find it difficult to specify optics for a Polygon Scan Head.  Even if the optics are well understood, the mechanics of a polygon scanner are so different from a galvo scanner that some guidance may be helpful.  This article covers a few important things unique to polygon mirrors including some topics which you may not find elsewhere.

FACET TO DATUM ERROR CAN BE MITIGATED
You may not need to pay a fortune for a polygon mirror or polygon scanner which has very tight facet to datum accuracy.  This error may be mitigated.  It is described here:  ERROR CORRECTION

FACET TO FACET ERROR CAN BE TOTALLY ELIMINATED
Every polygon mirror has some facet to facet angle error.  It may be as small as a few arc seconds or much greater.  Uncorrected, each scan line will be slightly off in the scan axis direction.  Using a Start Of Scan detector can totally eliminate this error.  The SOS detector is placed just ahead of the working area.  When the beam triggers the detector, you know exactly where the beam is irrespective of facet to facet error.  The detector pulse starts a timer which is used to know where the beam is across the scan.

SILVER COATINGS ARE NOT COMMONLY AVAILABLE FOR POLYGON MIRRORS
A lot of galvo mirror manufacturers offer silver coatings.  Silver is very desirable for its broad band reflectivity in the visible region.  The problem with silver is it will oxidize (turn black) when exposed to oxygen and corrosive things in the air (especially salts).  Silver coated galvo mirrors may last a long time but coating failure is not uncommon.  Glass galvo mirrors are very smooth so chances of applying a well sealed and highly protective overcoat over silver is possible.
Galvo users expecting to buy polygon scanners with a silver coating may be surprised to know that silver is not available.  Most polygon mirror manufacturers will not offer silver because experience is the coating will not last.  Polygon mirrors are made in ways completely different from galvo mirrors.  Diamond Turned polygon mirrors are rough around the facet edges making it difficult to reliably seal a silver coating.

Many companies have tried to produce a long lasting silver coating on polygon mirrors but the coatings fail as oxidants penetrate through the rough edges resulting in black facets.  To my knowledge, no one has ever achieved a silver coated polygon that met industry expectations.

Typically, in the UV to visible region, polygon mirror manufacturers will offer dielectric coatings designed to peak at a specific wavelength.  For a broadband visible coating, Precision Laser Scanning’s alternative to silver coating is an enhanced aluminum dielectric coating that gives broadband reflection.  It is not as good a reflector as silver but it is very durable and more suitable for multi-spectral systems than a coating that is peaked for a single wavelength.

99.9% REFLECTIVITY WITH POLYGON MIRRORS IS NOT REQUIRED
Galvo mirror manufacturers will claim 99.9% reflectivity for some wavelengths.  That may be possible with an optimum substrate with extremely low surface roughness.  That is typically NOT possible with polygon mirrors.  Nor is it essential!
Galvo mirrors have little capacity to dissipate heat that is not reflected by the mirror.  In a high power system, reduced reflectivity may lead to mirror failure from heat absorption.

Polygon scanners have a much higher laser damage threshold compared to galvos.  So, although a polygon mirror does not reflect as well as a galvo mirror, the ability of a polygon mirror to dissipate heat is so far superior to a galvo mirror, polygon mirrors have a much higher laser damage threshold.  LASER DAMAGE THRESHOLD

WHAT SURFACE ROUGHNESS DO YOU REALLY NEED?
Inexpensive galvo mirrors may claim better than 20Å RMS surface roughness.  It is not difficult to make galvo mirrors with very low surface roughness, but is it really needed?
Polygon mirrors are most often diamond machined.  Machining leaves tool marks that are invisible to the naked eye but are measurable and will cause scatter. Older diamond machining equipment can reliably produce polygon mirrors with better than 70Å RMS surface roughness.  Newer diamond turning machines and techniques can produce polygon mirror facets with 50Å to 25Å RMS surface roughness.  It is possible to achieve <10Å RMS surface roughness on polygon mirrors using polishing.  Polishing is done one facet at a time and it may take days or weeks to polish one polygon.  Polishing polygon mirrors is very expensive.  Polishing may not be needed unless you are working in UV and need very low scatter.
As a general guide:
70Å RMS is fine for IR but scatter increases as the wavelength gets shorter.  It could be unacceptable in some visible and most UV applications.
50Å RMS surface roughness is fine for IR to visible and for UV applications that are not scatter sensitive.
25Å RMS surface roughness is fine for IR to UV applications provided the UV application is not extremely scatter sensitive.
10Å RMS surface roughness is good for any laser scanning application from IR to UV.  However, it is a very expensive option for polygon mirrors.

If you have any questions about polygon mirrors, feel free to contact Precision Laser Scanning at:
1-480-515-1643
info@precisionlaserscanning.com