SR542 Optical Chopper

An optical chopper provides a direct way to modulate laser light. The SR542 Precision Optical Chopper offers enhanced features and configurability as well as optimized chopping performance. Whether your application requires best-in-class phase jitter performance, tracking an external reference, a wide frequency range, two-wave photo mixing, or synchronization of multiple choppers, the SR542 will get the job done.

Flexible Configuration

The chopper can be synchronized to a variety of input sources including an external frequency reference (sine or TTL/CMOS), the internal clock (crystal oscillator), the VCO input (0 to +10 VDC), or the AC Line (50 Hz to 60 Hz). Long-term frequency drift is virtually eliminated. Chop rates as low as 0.4 Hz or as high as 20 kHz can be achieved (blade-dependent).

A user-programmable multiplier and divider enable chopping rates at a harmonic, sub-harmonic, or fractional harmonic of the primary frequency.

The user can select the inner slots, outer slots, or the shaft (one-pulse-per-revolution) to be phase-locked to the scaled primary reference. A phase offset is easily adjusted with 0.01° resolution, and a relative phase can be set as a zero-phase reference.

Chopper Head

Chopper Blades           X Chopper Blades

The chopper head employs a slotless, brushless DC motor (BLDC), which minimizes mechanical vibrations and helps keep your optical setup quiet. Furthermore, without the wear-and-tear experienced by the commutator brushes of a DC motor, the operating life of the chopper head is extended.

Precision photo-etched blackened stainless steel chopper blades come in a variety of single and dual frequency designs, plus a variable duty factor design. Tight tolerances for the blade etching ensures low phase jitter of the optically chopped signals.

The chopper head can be mounted to a standard one inch optical breadboard using the slotted base plate (two inch slot spacing) or to a half inch optical post with the included clamping knob.

Chopper Head with Shroud           X Chopper Head with Shroud

A removable shroud prevents accidental collisions with the operational chopper head.

While BLDC motors are typically controlled with high frequency PWM signals, the SR542 uses an analog current amplifier with 3-phase unchopped sinusoidal drive currents to eliminate the high frequency EMI challenges of traditional PWM drivers.

Reference Outputs

Six always-available rear-panel reference outputs provide flexibility for synchronization of multiple instruments, including the ability to cascade choppers or trigger other instruments (lock-in amplifier, oscilloscope, etc.). Available outputs include Source (the primary internal clock, which is either free running, or phase-locked to the AC Line, Ext Sync, or VCO Input), Inner Slots, Outer Slots, Shaft (once-per-mechanical-revolution), Sum and Difference (f_outer ± f_inner). All outputs are 0 to +5 V square waves through 50 Ω.

Intuitive Front Panel

The front panel of the SR542 controller displays a block diagram of the chopper control loop, which makes it easy and intuitive to see and adjust the instrument configuration. Numeric settings can be entered via either the number keypad or rotary knob.

Shutter Mode

The SR542 can also be operated as a simple optical shutter. With an Internal frequency set to 0 Hz, the static rotational orientation of the chopper blade can be controlled by the phase setting. By proper choice of phase, the user can alternately pass or block the optical beam.