Optical Chopper System
18488-D02 Rev G, September 24, 2012 Page 6 www.thorlabs.com
Part 2. Product Overview
The MC2000 Optical Chopper is a precision instrument utilizing advanced features to meet the most
demanding applications. The MC2000 uses a phased-locked loop (PLL) motor speed control design to
precisely maintain the chopping speed and phase to a reference signal. An internal, crystal-stabilized,
frequency synthesizer provides an accurate and stable reference frequency for long-term stable
performance.
Unlike conventional, open-loop speed control designs, the PLL speed control circuit also allows the
MC2000 chopper to be synchronized to external reference signals, including other MC2000 choppers
and reference sources such as DSP lock-in amplifiers.
For more advanced measurements, the MC2000 can lock to a harmonic, sub-harmonic, or fractional-
harmonic of an external reference frequency. The on board microprocessor is used to multiply the
external reference up to the 15th harmonic or to divide the reference down to the 15th sub-harmonic. By
combining both the frequency multiplication and division together, a fractional harmonic can be
obtained.
The MC2000 also supports 2-frequency chopping from a single chopper blade. A special blade is
available with 7 outer slots and 5 inner slots. This slot combination allows a single beam to be split and
individually modulated for ratio metric experiments. Another example application is a pump-probe
experiment where the pump beam is modulated at the outer frequency while modulating a probe beam at
the inner frequency. The MC2000 provides the sum and difference frequencies of the 2-frequency blade
for accurate lock-in detection of the frequency-mixed response.
A high quality, rare earth magnet, DC motor and a photo-etched optical chopper wheel are the top two
design elements responsible for this device’s high precision. The compact optical head has a wide base
for extra stability. The base is slotted for two 1/4"-20 mounting screws on 3" centers. The interface cable
uses circular snap-on connectors for easy setup.
The MC2000 controller includes a 240 x 128 pixel graphics display for
setting and monitoring chopper functionality. All of the functions are
accessible through a front panel control knob with turn and push control.
Multiple user setups can be easily saved and recalled from non-volatile
memory. A USB interface is included as a standard feature for remote
PC control of the MC2000.
Thorlabs offers a number of different blades to extend the range of the
chopping frequency. A standard 10-slot blade is included with the
MC2000. 2-slot, 15-slot, 30-slot, 60-slot, and 2-frequency blades are
also available.
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