Every metal cutting laser will contain a L/4 phase retarder, sometimes called a 90 degree phase shift or polarising mirror. This mirror circularises the laser beam polarisation to give even cut quality across all axes of motion.
In the early 1980s coating technology could only offer a polarising mirror with 45 degrees (or L/8) of phase shift, so two mirrors were needed to give the 90 degrees of phase shift used to circularly polarise the beam. Anyone remember the Ferranti MFK or Coherent EFA lasers?
The first 90 degree phase shift coatings were offered with 90 degrees +/- 6 degrees phase shift, then the market demanded +/- 3 degrees, and now +/- 1 degree is becoming widespread. This was in response to ever more demanding edge cut quality. LBP's current phase shift coating meets the most demanding specification of 90 Degree +/- 1 degree and each mirror has it's exact phase shift measured.