How to choose an Optical Chopper or Shutter

In general, we have three types of devices to modulate an optical beam: rotating disk choppers, tuning fork (resonant vane) choppers, and shutters which open or close on command.

Rotating disk choppers are laboratory workhorses.  The chopping frequency is set by the user and the aperture is large compared to other devices.  However, motors limit use to near-room-temperature and atmospheric-pressure environments and motors wear out if used continuously.

Tuning fork choppers are made to operate at one frequency and cannot be significantly changed thereafter.  Apertures are small at high frequencies.  But they have no wearing parts so will last indefinitely, they are small, and they can be constructed for use at high or low temperatures or in a vacuum.

Shutters are attractive when the ability to control the aperture at an instant is important.  Our shutters can respond to arbitrary sequences of openings and closings under TTL control.  However, they respond in 10 milliseconds or slower, so the maximum frequency attainable is not very fast.

The following sequence of questions are factors to keep in mind when selecting an optical chopper or shutter.

• Does your application require chopping at a single known frequency?
• What range of frequencies does your chopping need fall in?
• Do you need to chop two light beams at different frequencies at the same time on the same chopper?
• Do you need to operate on a non-periodic basis?
• What size (aperture) beam do you want to chop?
• Do you need to stop the blade in a closed or open position?
• Does convenience matter in adjusting frequency?
• Is a digital display of the frequency readout useful?
• Is there a particular duty cycle you are looking for?
• Do you need variable aperture width?
• Do you need extremely accurate operation?
• Is the space available for the chopper or shutter limited?
• Are there environmental factors which should be taken into consideration?
• Do you need a long-life chopper or shutter?
• Is there any need to synchronize the chopper or shutter to an external clock or another chopper?

Does your application require chopping at a single known frequency? 

If you know the single frequency you want, a tuning fork chopper can be a simple, cost-effective solution. If, however, you want to vary the chopping frequency over a range, the tuning fork choppers (CH-10 and CH-20) cannot be used.  Try a rotating chopper in this case.

What range of frequencies does your chopping need fall in?

The following table shows chopping options.

Chopper or Shutter	Possible Operation Frequencies
300 Series (normal, which includes the 2-slot, 5-slot, 10-slot, 30-slot, 200-slot, and 445-slot blades, whose ranges are added for this total)	2-44,500Hz
300 Series, 200mm disk, 2-slot	5-110Hz
300 Series, 200mm disk, 4-slot	10-220Hz
300 Series, 250mm disk, 2-slot	5-75Hz
320 Series (normal, which includes the 3/30-slot, 4/7-slot, 8-60-slot, and 53/60-slot blades, whose ranges are added for this total)	7.5-5,300Hz inner, 75-6,000Hz outer
350 Series, 2-slot	5-400Hz
350 Series, 4-slot	10-800Hz
360 Series, 2 and 4-slot	same as 350 Series
360 Series, 40-slot	100-8,000Hz
CH-10	10Hz to 1kHz
CH-20	200Hz to 6kHz
CH-50	DC to 12Hz
CH-60	DC to 50Hz
CH-61	DC to 50Hz
SH-10	DC to 30Hz
SH-20	DC to 30Hz

 A couple options extend ranges on the 300 and 320 systems. The 300 and 320 series can have reduction gearing mounted on the motor, which adjusts downward the chopping frequency (although reduction gearing is not suitable for the 200 slot disk). Resulting ranges for the 300 systems are, at 7.2:1 reduction, 0.3 to 417Hz; 20:1, 0.1 to 150Hz; 32:1, 0.06 to 93.8Hz; 64:1, 0.03 to 46.88Hz; 131:1, 0.015-22.9Hz; 199:1, 0.010-15.1Hz; and 325:1, 0.006-9.23Hz. The range of the 300 systems can be extended upward with the use of the 445-slot blade (to 2-44500Hz with all six blades).

Do you need to chop two light beams at different frequencies at the same time on the same chopper? 

The 320C and 320CD use blades with two series of slots and two output references are produced.

Do you need to operate on a non-periodic basis?

The low frequency optical shutters and safety shutters can be open or shut on demand, with the following response time.

Shutter	Response Time
CH-50	30mSec
CH-60	10mSec
CH-61	10mSec
SH-10	15mSec
SH-20	15mSec

What size (aperture) beam do you want to chop?

The size beam on the rotating choppers is measured as the maximum diameter possible per blade.  Values listed are approximate.

Unit / Disk	Aperture
300C, 300CD
300-2	32mm
300-5	23mm
300-10	13mm
300-30	4.8mm
300-200 (200-slot blade)	0.75mm (22mm with 300HF-20K)
300-445 (445-slot blade)	0.35mm (10mm with 300HF-44K)
200mm 2-slot (for 300C/CD)	78mm
200mm 4-slot (for 300C/CD)	54mm
250mm 2-slot (for 300C/CD)	101.9mm
320C, 320CD
320-3/30	12.5mm and 4mm
320-4/7	12.5mm and 12.5mm
320-8/60	8mm and 2mm
320-53/60	1.4mm and 2.2mm
350C, 350CD, 360C
350-2 (2-slot miniature blade)	9mm
350-4 (4-slot miniature blade)	8mm
350-40 (40-slot miniature blade)	0.9mm

• The apertures on the boxing on the head of the 350 system are 9mm in diameter.
• The 300HF-20K increases the beam diameter of the 200-slot disk to 22mm, though with a maximum 50% transmittance.
• The 300HF-44K increases the beam diameter of the 445-slot disk to 10mm, though with a maximum 50% transmittance.
• With the 250mm blade, the optoswitch is removed and hence there is not reference signal.  Also it is not possible to rest the head directly on a bench; the head must be mounted on an optical stalk.
• The 300P and 320 blade protectors apertures are 33mm in diameter.

For the tuning fork choppers, maximum beam diameter depends upon the frequency for which the chopper is manufactured, as well as the duty cycle selected. Please see the Tuning Fork Tutorial for an explanation of duty cycles on tuning fork choppers.

Each optical shutter has a fixed size aperture regardless of operating frequency.

Do you need to stop the blade in a closed or open position? 

The low frequency optical shutters (CH-50, CH-60, and CH-61) and laser safety shutters (SH-10 and SH-20) can be set to stop in the open or closed position.

Does convenience matter in adjusting frequency? 

The 300C, 300CD, 320C, 320CD, 350C, and 350CD have a 10-turn potentiometer for frequency adjustment. The 300C OEM and 350C OEM (versions A and B), as well as the 360C OEM, have a trim pot on the circuit board which can be used to adjust the motor speed. The CH-50, CH-60, and CH-61 have a trim pot accessible on the printed circuit board of the appropriate driver. All of the previous can also be controlled via an input signal from an external clock. The laser safety shutters (SH-10 and SH-20) can only be controlled via signal from an external clock.

Is a digital display of the frequency readout useful?

The 300CD, 320CD, and 350CD provide a 5-digit digital readout of the frequency. Display resolution is reported in Hz with a 1 second update time, or in 1/10th Hz with a 10 second update time. With the 320CD, only the outermost frequency is reported.  The 300C OEM systems do not come with the digital display, but it can be added to either version A (with the addition of several wires and a DPDT switch) or version B (no modifications or additional parts necessary).  The part number for the display is 300F.

Is there a particular duty cycle you are looking for?  Do you need variable aperture width?

If you are interested in tuning fork choppers and you are not working with a very small diameter beam, please read the Tuning Fork Tutorial for a discussion on how 50% and 90% duty cycles are manifested in tuning fork choppers.  The rotating choppers (300 systems and 320 systems) are available with blades of a 1:1 mark:space ratio.  To obtain lower duty cycles (shorter periods of transmission) or smaller aperture widths, two blades can be mounted together such that their alignment covers the appropriate amount of the aperture (up to blocking it entirely).  The optical shutters (SH-10 and SH-20, as well as CH-50, CH-60, and CH-61) can function at an arbitrary duty cycle if they are given appropriate input.

Do you need extremely accurate operation?

Resonating choppers and shutters have extremely high stability, which can be increased with the use of appropriate drivers.  The tuning fork choppers (CH-10 and CH-20) have an stability of 0.05%, which can be improved by a factor of 10 with the use of the AGC driver, yielding an amplitude regulation of 0.01% or better typical (depending on temperature range), with an amplitude adjustment range of 20% to 100% of full aperture.

The CH-50, CH-60, and CH-61 optical shutters, when used with the appropriate drivers (DCH-50 or DCH-60), and the SH-10 and SH-20, have a stability limited only by the clock they follow.

Rotating optical choppers show good stability.  Stability on the 300 systems, 320 systems, 350 systems, and the 360 system is better than 0.01%/C.  Long-term frequency stability on these systems is better than +/-0.1% of reading within 10 to 100% maximum frequency.  However, short term frequency instability is phase jitter, which increases with the number of slots on the disks. Also, gear reduction increases general jitter on the chopper. 

Is the space available for the chopper or shutter limited?

Dimensions are available for all our optical choppers and shutters.  The 350 chopper has a miniature chopping head; the 360 chopper has an ultra-miniature head (52mm from base of mount to top of blade).  The size of the CH-10 and CH-20 depend on the frequency (with lower frequencies requiring larger blades).  The largest of the CH-10 choppers is 72mm long; all of the CH-20 choppers are smaller than the 360 chopper head.  Optical shutters are moderately small.

Are there environmental factors which should be taken into consideration?

If you will be using the optical choppers or shutters in harsh environmental conditions, such as vacuum operation, cryogenic or high-temperature operation, or in a high-vibration environment, tuning fork choppers offer the best durability.  The standard CH-10 and CH-20 choppers operate in vacuum up to 10-6 torr, beyond which they continue to function but outgas at an undesirable rate.  Modifications extend the range of their operation to either 10-8 or 10-10 torr.  Standard operating temperatures are -40C to +65C.  High temperature options to 200C and low temperature options to cryogenic are also feasible.  For high-vibration environments, steady operation can be maintained with the CH-10 or CH-20 with an AGC driver (see accurate operation).

Do you need a long-life chopper or shutter?

Tuning fork choppers and long-life versions of the optical shutters offer extremely long lifetimes.

SH-10 and SH-20   2 million cycles minimum

SH-10-L and SH-20-L (long life version)   50 million cycles minimum

Is there any need to synchronize the chopper or shutter to an external clock or another chopper?

The 300Synch allows the 300C and 300CD optical choppers to be synchronized as "slave" to any other chopper or arbitrary periodic external "master" reference signal.  It will accept as signal all types of TTL signals, all types of CMOS signals, square, sine, triangular, sawtooth, or similar waveforms from 100 mV peak to peak to 10V peak to peak.  Square waves are preferred as they produce least phase jitter.  The slave reference signal must be TTL, and some choppers having TTL reference output have been successfully incorporated as "slave" into a synchronization system using the 300Synch.  Note that phase jitter can become a significant problem with the 300Synch, particularly when both master and slave are rotating choppers.  As a result of difficulties in aligning jittery or infrequent signals, operation of the synchronizer is limited to 10 to 3000Hz.

The PLD-1C driver for the CH-10 and CH-20 locks a tuning fork chopper to an external clock signal (at the chopper's operation frequency), with a phase relationship set at the factory to customer requirements and user adjustable in a range of +/-45 degrees.

The PLD-2C driver for the CH-10 and CH-20 two tuning fork choppers in a "master"/"slave" mode.  This enables ultra stable modulating beam motions.  The two choppers need not be the same frequency.  The phase relationship of the slave to the master is set at the factory to the customer's requirements and is user adjustable +/-45 degrees.

The CH-50, CH-60, CH-61, SH-10, and SH-20 optical shutters can receive their input from an external clock signal, and are locked in phase with the clock input.

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