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Controller Interface Screenshot

Features

• Crystal-Stabilized, Phase-Locked Feedback Loop Suppresses Low Frequency Drift and Phase Jitter
• Harmonic, Subharmonic, and Fractional Harmonic Chopping with Sum and Difference Reference Outputs
• Microprocessor Controlled
• Single- and Dual-Frequency Blades Available
• Harmonic Frequency Blades Available for Pump-Probe and Other Nonlinear Experiments
• Save and Recall User Setups in Non-Volatile RAM
• USB Interface
• Control Software Package Included

The MC2000B Optical Chopper is a precision instrument that utilizes advanced features to modulate light from a continuous beam. The controller uses a phase-locked loop (PLL) motor speed control design to precisely maintain the chopping speed and phase relative to a reference signal. An internal, crystal-stabilized, frequency synthesizer provides an accurate and stable reference frequency for stable long-term performance. The MC2000B Chopper offers faster locking times, interface improvements, and a larger selection of blades than our previous-generation MC2000 Optical Chopper.

An optical switch is used to get the reference signal when the chopper blades pass through and interrupt the beam path. The switch incorporates an LED with a wavelength range of 850 nm - 940 nm that is operated at the 15 mA threshold current to minimize light output from the LED.

Unlike conventional, open-loop speed control designs, the PLL speed control circuit also allows the MC2000B chopper to be synchronized to external reference signals, including other MC2000B choppers and reference sources such as DSP lock-in amplifiers. Please see the Operations tab for more details on the MC2000B chopper and its applications.

The MC2000B design features a high quality, rare earth magnet, DC motor. The photo-etched optical chopper wheel blade is made of 0.01" thick blued clock spring steel for high precision. The compact optical head has a wide base for extra stability, and the base is slotted for two 1/4"-20 (M6) mounting screws on 3" (76.2 mm) centers. The mounting slots have a 1.5" (38.1 mm) radius. The interface cable uses circular snap-on Hirose connectors for easy setup.

The MC2000B 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 MC2000B. A software package with a GUI to control the MC2000B is included with each controller and can also be downloaded from the Software tab. The package also supports controlling the MC2000B via programs written in C++ or LabVIEW.

Please Note: The MC2000B should be shielded from stray light sources because light contamination in the optical sensor will cause jitter.

Operation

An optical chopper is an electromechanical instrument that periodically interrupts a beam of light. The MC2000B Optical Chopper is a rotary type composed of a benchtop controller, motor head assembly, and slotted chopper blade/wheel.

Applications

MC2F57B Dual Frequency Blade

Unlike conventional, open-loop speed control designs, the PLL speed control circuit also allows the MC2000B chopper to be synchronized to external reference signals, including other MC2000B choppers and reference sources such as DSP lock-in amplifiers.

For more advanced measurements, the MC2000B 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.

In order to maximize the slope of the signal as the blade rotates (i.e., minimize the time to fully open or closed), the beam should be focused onto the widest portion of the slot in the chopper blade.

The MC2000B also supports 2-frequency chopping from a single chopper blade (example MC2F57B blade shown at right). We offer five different dual frequency blades (see below or refer to the Specs tab for details). The combination of dual frequency slots on one blade allows a single beam to be split and individually modulated for ratiometric 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 MC2000B provides the sum and difference frequencies of each dual frequency blade for accurate lock-in detection of the frequency-mixed response.

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MC2000B Software Screenshot

Software with GUI for Remote Operation of the MC2000B Chopper Controller

Software

Version 2.1.0

Software package to control the MC2000B. The installer includes the necessary drivers. This software package is compatible with the lastest firmware for the system and supports writing programs to control the MC2000B using C++ or LabVIEW.

Insights into Beam Characterization

Scroll down to read about:

• Beam Size Measurement Using a Chopper Wheel

Click here for more insights into lab practices and equipment.

Beam Size Measurement Using a Chopper Wheel

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Figure 2: The blade traces an arc length of Rθ through the center of the beam and has an angular rotation rate of f. The chopper wheel shown is MC1F2.

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Figure 1: An approximate measurement of beam size can be found using the illustrated setup. As the blade of the chopper wheel passes through the beam, an S-curve is traced out on the oscilloscope.

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Figure 4: The diameter of a Gaussian beam is often given in terms of the 1/e² full width.

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Figure 3: Rise time (t_r ) of the intensity signal is typically measured between the 10% and 90% points on the curve. The rise time depends on the wheel's rotation rate and the beam diameter.

Camera and scanning-slit beam profilers are tools for characterizing beam size and shape, but these instruments cannot provide an accurate measurement if the beam size is too small or the wavelength is outside of the operating range.

A chopper wheel, photodetector, and oscilloscope can provide an approximate measurement of the beam size (Figure 1). As the rotating chopper wheel's blade passes through the beam, an S-shaped trace is displayed on the oscilloscope.

When the blade sweeps through the angle θ , the rise or fall time of the S-curve is proportional to the size of the beam along the direction of the blade's travel (Figure 2). A point on the blade located a distance R  from the center of the wheel sweeps through an arc length (Rθ ) that is approximately equal to the size of the beam along this direction.

To make this beam size measurement, the combined response of the detector and oscilloscope should be much faster than the signal's rate of change. 

Example: S-Curve with Rising Edge
The angle (θ = ft_r ) subtended by the beam depends on the signal's rise time (Figure 3) and the wheel's rotation rate (f ), whose units are Hz or revolutions/s. The arc length (Rθ = R ⋅ ft_r ) through the beam can be calculated using this angle. For a small Gaussian-shaped beam (Figure 4), a first approximation of the 1/e² beam diameter (D ),

includes a factor of 1.56, which accounts for the portion of the beam measured between the 10% and 90% intensity points being smaller than the 1/e² beam diameter.

Date of Last Edit: June 22, 2021
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