Simple methods for controlling laser beam

Here is some general information about techniques used for controlling laser. I tell you some basic information and results of experiments I have made with my small 1 mW HeNe laser.

Laser scanning can be done in many ways. The most common ways to control laser beam are rotationg mirrors and galvanometers.

Rotating mirror is simply a mirror connected to electric motor so that it can be rotated. One rotating mirror can be easily used for generating circles. When you use two rotating mirrors, you can generate interresting lissajous-figure like figures when changing the rotation speeds of those motors. Very easy and quite nice looking results. I tried this method with two small mirrors and two old 12V fans. I connected the mirrors to the centers of the fans with double-side adhesive tape (two pieces of tape to other and and one piece to other end). The I connected the fans to two variable power supplies to control the speed of mirror rotation. Then I located the mirrors so that the laser first hit one mirror end the reflects the other mirror.

When the mirror is mounted to the motor in the other way, you get linear scan to one direction. This system is used in some laser printers and bar code scanners. You can also use a small prism instead of mirror to get nice results also.

Galvanometers are mirrors which are controlled by magnetic field produced by electric coil. Those mirrors turn few degrees when current in the controlling coil is changed. When you have two galvanometers you can use one to control x direction and another to control y direction so you get quite nice 2d scanning system. You can get nice lissajous figures when you feed those galvanometers with sine waves which have different frequency (circuit or ellipses when same fequency) You can also use computer and two D/A converters to make more complicated figures. Galvanometers can usually follow to frequencies of few hundred herz and cheap galvanometers cost around $100.

A cheap hack to replace galvanometers is to glue mirros to a loudspeaker and use those mirrors like galvanometers. This is far from ideal, but can work nicely if done properly.

It is also possible to connect a mirror to a stepper motor. In this way it is easy to control laser beam quite accurately. For example 400 steps per rotation stepper motors give accuracy better than one degree and with microstepping this resolution can be increased form that. I experimeted with two stepper motors and control circuit taken from two broken disk drives. I made some interresting test with this system but I found that stepper motors have some disadvantages. When you try to stop a motor, the rotor will oscillate some time around the step place it should stop. This does not look nice.

When doing more complicated system, there is need for starting and shutting down laser beam quite often. When using semiconductor lasers this is very easy and fast, but with HeNe laser this is more complicated. I have read that HeNe lasers can be modulated to some degree with some hacks, but completely shutting down and starting quicly is a problem. The laser I have has a TTL level input to control laser output, but the laser can not start and stop faster that about five times a second. Mechanical shutter is a good way to stop laser beam, but it has also limited speed. I have also head that liquid crystal shutters are also used in some applications.