Reflection Isolator 2018-05-16T14:33:39+00:00

Reflection Isolator C-RI

Principle of operation

Optically the reflection isolator is identical to the manual attenuator, but is not intended to allow the out-going
beam power to be varied. It is installed in the system to allow the out-going beam to pass without loss.
However, any beam travelling back towards the laser and polarised in the orthogonal direction will be
attenuated. Using two uncoated Brewster plates the transmittance will be 6.4% and two coated plates, about
0.04%.
In some laser systems this function is useful to prevent a beam reflection from the work piece travelling all
the way back into the laser cavity where it can cause instability to the power or damage in a high power
laser. The laser user will need to consult the laser manufacturer to estimate what power level can be
tolerated by the cavity and hence whether coated or uncoated plates are necessary (or even more than one
isolator in series).

Backreflection

Fig 1: Set up required for the reflection isolator

As mentioned above, the backward reflection must be polarised orthogonal to the out-going beam, so
somewhere in the system between the isolator and workpiece, the polarisation must be rotated through 90
in order for the isolator to work. This will happen in a system where a /4 (1/4-wave or 90) phase retarder
mirror is used to convert the linearly polarised beam into a circularly polarised one. Such an arrangement is
common and is widely used for metal processing. The metals Aluminium and Copper are particularly noted
for sending back an unwanted reflection towards the laser. Fig 1 shows the arrangement that is required.
The out-going beam is polarised for maximum transmission through the isolator and 45 to the S and P
directions for the phase retarder mirror. After reflection the beam is circularly polarised and is used in this
state to process the material. If a backward reflection occurs, it is reflected off the phase retarder a second
time and undergoes another /4 phase retardation. This second reflection does not ‘undo’ the first to give a
zero retardation but adds to it to give a /2 (or 180) retardation. The result is to re-create linear polarisation
but orthogonal to the out-going beam. Therefore the isolator will dump the beam.
Note that inaccuracies in the phase retarder coating and alignment of the polarisation to the mirror will result
in elliptical polarisation in the reflected beam, with a less efficient dumping of the power in the isolator.
Reflection isolator use
The isolator is installed in the same way as the attenuator, except there is no ring to adjust the power. The
arrow on the barrel defines the P plane of the Brewster plates and the preferred direction of propagation
(more relevant for the coated plate version). Rotate the isolator in the beam until this arrow is aligned to the
direction of polarisation or using a power meter, gives the maximum transmittance. Then lock in place.