Pulsed laser systems working at relatively low frequencies (below say 1 kHz) often have a requirement to output every 2nd, 4th, 8th, etc pulse. This application note gives details of how a mechancial rotating optical chopper can be used to achieve this.

There are two main considerations, the shape of the optical chopper disk required and the synchronisation of the optical chopper disk with the laser pulses. Each of these problems will be considered in turn.

The shape required for the optical chopper disk can be very simple as shown below.

The outer set of 16 slots is used for synchronisation with the laser pulses. The inner set of slots has the laser shone through it and in this case will allow through every 4th pulse as shown below.

To allow through different numbers of pulses, different disks are required. This can get expensive due to the cost of having custom disks made. Fortunately, by mounting two disks simultaneously it is possible to produce a number of different options as shown in the following picture.

Disk 1 used by itself will allow every 2nd pulse to be allowed through. Mounting disk 1 and disk 2 together on the same chopper head will allow different numbers of pulses through depending on the mutual relationship. Position 1 allows 1 in every 4 pulses through, position 2 allows through 1 in every 8 and position 3 allows through 1 in every 16.

Care has been taken with design of the disks to ensure that they are balanced. Having two disks has the advantage that it is possible to have a visually unbalanced disk, such as that shown in position 3 above, that is actually balanced.

Please note that due to manufacturing tolerances, the combination of two disks in this way may not completely block light as shown. Small gaps where the disks overlap may allow a small amount of light through though this will not be a problem in a pulse picking application as these small gaps occur in between the pulses.

Synchronisation of the laser pulses with the rotating chopper disk is obviously critical. There are two ways of achieving this, the laser can be synchronised to the chopper or the chopper can be synchronised to the laser.

Method 1: Laser Synchronised to the Chopper

The optical chopper will generate a series of pulses from an opto-switch on the outside set of slots. This can be used to trigger the laser and is by far the simplest method of synchronisation since the laser will automatically track the optical chopper as it speeds up and slows down.

Method 2: Chopper Synchronised to the Laser

Synchronising the chopper to the laser is a lot harder than the other way around as the optical chopper is a mechanical device and can't react quickly to changes in operating frequencies. A system is required that monitors the laser pulses and the optical chopper reference and speeds up or slows down the optical chopper as required. The Scitec Instruments optical chopper synchroniser is suitable for this application.

Unfortunately, this system is not perfect as it can take the system up to 10 minutes to stabilise. Jitter is also a problem being approx ±15° for the 16 slot disk shown. Method 1 is therefore recommended wherever possible.