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The SR540 chopper will handle all your optical chopping requirements—from
simple measurements to dual-beam and intermodulation experiments.
The SR540 has a 4-digit frequency display, front-panel frequency
control, analog voltage frequency control and two reference outputs
with selectable operating modes. Two anodized aluminum blades are
provided: a 5/6 slot blade for frequencies up to 400 Hz, and a 25/30
slot blade for frequencies up to 3.7 kHz. Reference outputs are
provided for frequencies corresponding to each row of slots, as
well as the sum and difference frequencies.
Optical Chopper Applications
Single-Beam Experiment
In this application, a single optical beam is chopped by the outer
row of slots, and the reference output from the right BNC is used
to lock the lock-in amplifier to the chop frequency. The inner row
of slots could also be used, in which case the left BNC would be
the reference output. In either case, the REFERENCE MODE switch
is in the “up” position.
Dual-Beam Experiment
In this arrangement, the output from a single source is split and
chopped at two different frequencies by the two rows of chopper
slots. One beam passes through the experiment while the other is
used as a reference beam. The beams are recombined and sent to the
same detector. Two lock-ins are used to detect the signals at ƒinner,
corresponding to the experimental signal, and ƒouter,
corresponding to the reference beam. If the detected signal in the
experimental arm is ratioed to the detected signal in the control
arm, then effects due to changing source intensity and detector
efficiency are removed.
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