3502
Optical Chopper System
14
3
Using the Chopper
3.1
How the Model 3502 Optical Chopper Works
The Model 3502 Optical Chopper is designed to interrupt light paths in optical
experiments at frequencies from 4 Hz to 10.65 kHz. Both single- and dual-beam
experiments can be performed across a broad range of chopping frequencies. The
Chopper has a crystal-controlled frequency synthesizer that serves as an internal
reference frequency for locking the Chopper to a particular chopping frequency.
Reference frequencies can also be provided through the
Sync In
BNC connection
to allow the Chopper to lock to an external source.
Several measures ensure that jitter and drift of the chopping frequency is reduced
to a minimum. Precision photo-etched wheels are mounted on a high quality DC
motor. The Model 3502 Optical Chopper Head has a photo-sensor for monitoring
the chopping frequency of the outer part of the wheel. The Chopper Controller
then actively stabilizes the motor speed to match the desired chopping frequency.
This technique minimizes phase noise at the chopping frequency and provides for
long-term stable chopping with minimal frequency drift.
Figure 8 shows a block diagram of the Model 3502 Optical Chopper system.
Programmable divide/multiply circuitry allows for harmonic or subharmonic
locking of the Chopper to the reference frequency. In addition, the phase of the
chopping frequency may be varied over a -180 to +179 degree range with respect
to the reference frequency. A variety of TTL-level outputs are available for use in
triggering lock-in amplifiers, oscilloscopes, photon counters, or boxcar averagers.
The chopping frequency, as well as a number of other operating parameters, can
be viewed on the front panel display. Easy-to-use cursor keys provide easy
adjustment of operating parameters. From the front panel the user can store and
recall up to nine instrument setups. A USB 2.0 interface provides remote
operation of all instrument functions.
The Chopper Head can be mounted on a 1/2”-diameter post or bolted directly to
a standard optical bench. The Model 3502 Optical Chopper is supplied with six
Chopper wheels and a wheel cover. The Model 3502-BASE Optical Chopper does
not include any chopper wheels, and must be ordered separately.