Laser damage threshold – Galvo Mirror vs Polygon mirror

Laser damage to galvanometer (galvo) mirrors is a major issue in some high power applications.  The galvo mirror(s) may be the weak link preventing use of higher power lasers which would allow processing material at a faster rate.  This is one reason to consider a high speed polygon scanner but how do polygon mirrors hold up in high power systems?

Galvo Mirror
The hot beam dwells on the same mirror area.
The mirror is thin & light so it can reciprocate fast.  This means there is minimal material backing the reflective thin film coating to pull the heat away.

Polygon Mirror
The beam moves all around the circumference of the mirror, spreading the heat over a large area.
The polygon mirror facets are backed by a substantial metal mass (usually aluminum or copper) that acts as a highly effective heat sink.
The polygon mirror spins rapidly which gives excellent air cooling.

SUMMARY
Galvo mirrors have a hot spot where the beam is reflected.  Even though the mirror coating is highly reflective, there is heating.  The heating increases with age as dust and organics contaminate the mirror coating and reduce reflectivity.  The thin galvo mirror substrate has poor heat conductivity so the coating is susceptible to overheating and sudden failure.  

Polygon mirrors rotate at a constant speed so there is no need to lighten them.  The high mass of a metal polygon is actually a benefit for speed stability (accuracy).  An aluminum or copper polygon conducts heat much better than a silicon galvo mirror.  The substantial metal polygon pulls heat rapidly from the facet coatings to keep them cool.  

Polygon scanners are inherently far superior to galvo scanners for laser damage threshold.  Therefore, polygon scanners are more durable with high power densities.

Polygon Scanners