PEM200 User Manual
44
9
Maintenance
Calibration
PEM200 modulators are calibrated at the factory before shipment. For most
applications, recalibration is neither necessary nor appropriate. For a few applications,
however, an
on-site
calibration procedure will be necessary if optimum performance is
to be achieved. Hinds’ engineers will be glad to advise users on whether on site
calibration is appropriate in a particular case.
The Calibration appendix contains an extensive discussion of the theory of retardation
calibration and a variety of methods for establishing known calibration points.
Fortunately, photoelastic modulators are to a large extent self-calibrating, and most of
the equipment needed for calibration will be required for the user’s experiment.
Adjustment of Peak Retardation Calibration
The most common reference point used for calibration is half-wave peak retardation or
an integral multiple of half-wave. This condition may be easily observed using an
oscilloscope, as described in the Calibration appendix. Even when other reference
points are used, this method should be regarded as the “coarse adjustment” on the
modulator calibration.
The peak retardation calibration of the modulator is adjusted by adjusting modulation
drive (:MOD:DRV #) and fitting appropriate firmware points. Please contact Hinds
Instruments for assistance with these commands. With the appropriate optical bench
setup and detection system, the modulation amplitude can be adjusted until the
calibration condition is obtained. For example, in the case of the half-wave calibration,
the appropriate flat toped (or flat-bottomed) characteristic waveform is observed on the
oscilloscope.
Maintenance
The optical head contains an optical element and requires the same care in use as
other components such as lenses and mirrors. The unit should not be operated in a
dusty, corrosive, or otherwise contaminating environment. Small amounts of dust may
be removed from the optical element with a soft brush such as an artist’s camel-hair
brush. If more serious contamination occurs, contact Hind’s engineers.
