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Optical Chopper Frequently Asked Questions (FAQ)

 Please contact us with any questions not covered below.

1. What are optical choppers anyway?

2. Can the Scitec range of optical choppers be used in vacuum?

3. I want to be able to chop an X mm wide beam at Y Hz. Why can't I?

4. What can I do to get the maximum chopping rate with a large beam diameter?

5. What is the fastest speed that a mechanical rotating chopper can chop at?

6. I would like my beam chopped so that I get a sine wave (or other waveform) output. How do I achieve this?

 There are also answers available in our application notes section.

 1. What are optical choppers anyway?

Optical choppers are mechanical devices that physically block a light beam of some type.

Rotating optical choppers are perhaps the most common form and are the type produced by Scitec Instruments Ltd. A metal disc with slots etched into it is mounted on a DC motor and rotated. The disc is placed in the path of the light beam which will then cause the beam to be periodically interrupted by the blocking part of the disc.

Mechanical optical choppers are useful where it is not possible to control the light source directly or at the speeds required. For example a standard filament light bulb can be pulsed to a few 100 Hz though the depth of modulation is limited. If it is required to switch the light on an off completely at 20 kHz, then the use of a mechanical optical chopper is required.

 2. Can the Scitec range of optical choppers be used in vacuum?

The motors used are not designed to operate in a vacuum and it is therefore not recommended. However, we do know of some customers who have successfully operated the systems in a vacuum. The main issues are the loss of lubrication fluid causing the motor lifetime to be significantly reduced and outgassing of the motor materials.

One option for customers is to purchase the required discs from Scitec and then to mount them on a stepper motor / dc motor designed for vacuum use.

 3. I want to be able to chop an X mm wide beam at Y Hz. Why can't I?

Please calculate the speed the disc surface needs to travel using the following formula:

Speed = X · Y · 0.0072 km/hr (or alternatively Speed = X · Y · 0.00447 miles/hr)

If the resultant figure is greater than 1193 km/hr (or 741 miles/hr) then you will need to break the sound barrier to reach the required chopping rate. We can almost reach half this figure with our high speed chopper in the right conditions but it makes rather a fuss about it.

Please note that the formula does not include the diameter of the disc. This is due to the limiting factor for the speed of optical choppers being wind resistance. To a first approximation, the speed at the edge of the disc is directly related to the wind resistance and is independant of the diameter of the disc. As an example our 200mm diameter system will only spin at approximately half the speed of our 102mm diameter system with the same power motor. Hence the speed at the edge of the disc is the same.

Please also see the following answer if you want to break this rule.

 4. What can I do to get the maximum chopping rate with a large beam diameter?

If you have a large beam diameter but wish to chop at a fast rate then there are a couple of tricks that you can use. Unfortunately, both methods mean that you will loose at least 50% of your signal strength and you won't get exactly 100% blocking in the dark state.

Method 1: Place a stationary disk with the same number of slots in front (or behind) the rotating disc. As the discs go in and out of phase the beam is either blocked or 50% is allowed through. More info here.

Method 2: Place two chopper systems with the same blades in front of each other and rotating in opposite directions. The chopping rate is the sum of the individual chopping rates and the maximum beam size is the slot length rather than the slot width. Using this technique, a 10 mm beam can be chopped at 240 kHz using our high speed chopper system.

 5. What is the fastest speed that a mechanical rotating chopper can chop?

Our current record is 120 kHz with our high speed system. This is about the limit with 0.25 mm thick base material using normal photo etching processes. However, we have access to electro formed components which allow the number of slots to be increased considerably. Using this technology 1 to 2 MHz should be possible though the slot width will be rather small. If you have a pot of money and would like us to make a prototype then please get in touch.

 6. I would like my beam chopped so that I get a sine wave (or other waveform) output. How do I achieve this?

If your optical beam has a gausian profile then it is possible to get a fairly close approximation to a sine wave using our standard chopping discs. Please select a disc that has a slot width that is the same or wider than your beam. Rotate the chopper relative to the beam until both sides of the beam just touch the chopping disc.

A square wave output can be achieved by using a disc with slot widths that are many times wider than the beam diameter though this does restrict the chopping rate.

Triangular output waveforms are produced by the 300HF high frequency accessory.

Other waveforms can be achieved by either mounting multiple blades on the chopping head at one time or by producing a custom disc. Contact us with your requirements and we will feed them into a simulation we have to find the slot shape required.

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