MC2000B Optical Chopper Chapter 3: Description
Rev A, Jul 5, 2016 Page 3
Chapter 3 Description
3.1. Product Overview
The MC2000B Optical Chopper is a precision instrument that utilizes advanced features to create pulses of light
from a continuous beam. This beam can be from a coherent or incoherent light source. The MC2000B is the
enhanced version of the MC2000. It offers faster locking times, better graphics, and a larger selection of blades.
The MC2000B uses a phased-locked loop (PLL) motor speed control design to accurately maintain the chopping
speed as well as the phase of a reference signal. An internal, crystal-stabilized, frequency synthesizer provides
an accurate and stable reference frequency for reliable, long-term performance.
Unlike conventional, open-loop speed control designs, the PLL speed control circuit also allows the MC2000B
chopper to be synchronized to external reference signals. This includes other MC2000B choppers, and reference
sources such as DSP lock-in amplifiers.
For more advanced measurements, the MC2000B locks to a harmonic, sub-harmonic, or fractional-harmonic of
an external reference frequency. The onboard microprocessor multiplies the external reference up to the 15th
harmonic, or divides the reference down to the 15th sub-harmonic. A fractional harmonic can be obtained by
combining both the frequency multiplication and division together.
Additionally, the MC2000B supports two-frequency chopping from a single blade. Special (Inner/Outer) 4/7, 5/7,
3/30, 8/60, and 53/60 harmonic blades are available. These slot combinations split single beams for individual
modulation in ratio metric experiments. Another example application is a pump-probe experiment where the pump
beam is modulated at the outer frequency, and a probe beam is modulated at the inner frequency. The MC2000B
provides the sum and difference frequencies of the two-frequency blades for accurate lock-in detection of the
frequency-mixed response.
Note, on two-frequency blades, the chopper wheel reference derives from two photo-interrupters on the optical
head that sense the motion of the chopper blade. A third set of slots on the rim of the blade are added, because
the second photo-interrupter is unable to physically measure the inner slots. These slots match the inner slots in
exact orientation and frequency.
The top two design elements responsible for this device’s high precision are a high-quality DC motor with a rare
earth magnet, and a photo-etched optical chopper wheel. 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 MC2000B controller includes a 240 x 128 pixel graphics display for setting and monitoring the chopper’s
functions. All of the functions are accessible through a front panel knob with turn and push control. Multiple user
setups can be easily saved and recalled from non-volatile memory. A USB interface is a standard feature for
remote PC control of the MC2000B.
