An essential requirement when keeping lasers positioned correctly is the use of a sensor that can record the beam position to sub-micron accuracy without interfering with the direction of the beam.
A PSD measures the so-called photometric centroid of a ‘splodge’ of light incident on the sensor. Electrons flow to X- and Y- electrodes and by measuring the voltage the position of the spot of light can be measured. Both PCB for OEM applications or bench top boxes for laboratory applications are available.
The PSD module contains a pellicle beamsplitter to take a small amount of light for position measurement without causing lateral displacement of the laser beam. If environmental temperature, vibration and ambient light effects are controlled sufficiently, the 0.02mm over 100m measurement accuracy that is possible with the PSD module equates to a pointing accuracy of 0.04 arc seconds.