Laser projector: introduction

First of all, some warnings. This page is (duh!) about lasers. Lasers are potentially dangerous, and may damage your vision, especially focussed (collimated) laser beams. But let's not forget the very high voltages used to power gas laser tubes, which can cause severe burns or even death. If you decide to go experimenting with lasers please inform yourself about the needed precautions, the risks involved and grab some theory behind the subject. It could avoid dangerous or even life-threatening situations.

I can and will not accept responsibility for damages that spring from the use of the information in this document.

This project is on hold, probably indefinitely... at the bottom of the page you can read why. This page is maintained as an archive.

You made it to the laser page! Now things are getting really interesting. I've been working on a laser scanner for some time now (the idea has been wandering around for years) but lately I finally have the resources to actually put something together. The idea is: project line figures on a wall by moving a laser beam with a mirror, preferrably without the help of a PC. The latter because I wanted to be able to put it in a light grid in a venue I used to work at (PARA), and I did't want to have to bring my PC every week to do the light show. Since 2001 I have been toying around with embedded microcontrollers. I good example of this is the FC-50 foot controller on the main page. I should be able to use a similar microcontroller to control the laser beam.

If you're an electronics enthousiast, like myself, you probably have tinkered with little lights and LEDs and stuff like that, when you were a kid. And maybe still today, like me. If so, you must have dreamed of ever owning a laser and experimenting with it. For some years, semiconductor (diode) lasers have been on the market and are even cheap enough to buy a few of them. They are widely available as laser pointers, though most of them use a limited-visibiity (670 or 650 nm) low power (1 mW) laser, because they only cost a few bucks. A high visibility (632 nm) more powerful (3-5 mW) one is much more expensive, but, of course, has much much better visibility. Below is a table of laser classes. Most lasers are class 1 thru 3A. You'll find them in CD and DVD drives, laser pointers, simple laser effects equipment, security systems, etc. Low power class 3B lasers are used mainly in DVD (re)writers and medical equipment. High power class 3B and class 4 are for industrial uses, such as welding, cutting and for scientific purposes such as nuclear fusion experiments.

Class 1 Visible laser radiation with a light output power of less than .25 mW. Completely harmless.
Class 2 Visible laser radiation with a light output power of less than 1 mW. Harmless. Eye blink reflex is sufficient to prevent eye damage.
Class 3A Visible laser radiation with a light output power of 1 to 5 mW. Potentially dangerous. Looking into a focussed (collimated) beam can cause eye damage.
Class 3B Visible laser radiation with a light output power of 5 to 500 mW. Dangerous. Precautions to avoid directly looking into the beam are strongly recommended.
Class 4 Visible laser radiation with a light output power of more than 500 mW. A laser rig of this class is extremely dangerous, even when you are not touching the beam and even slight refractions on matte surfaces can cause eye damage or burns. When the beam is touched, severe, deep burns will occur. A 1 watt collimated laser beam has a brightness comparable to a 10,000,000 watts filament lamp at very close distance!

Laser diodes

For my project, I've chosen to walk the easiest and safest route: a laser diode (right picture). A laser diode is not a funked up LED. It's a real laser. The only things these two have in common are: they're both semiconductors (diodes to be more precise), and they both emit almost monochromatic (one frequency) light. Do not confuse a laser diode with a diode laser module (left picture). A plain laser diode is cheaper, but much harder to use, because they are EXTREMELY sensitive parts. If you pump too much current through a laser diode, it will die instantly because of the sheer light intensity. A specific laser diode is built to emit a certain maximum output power and the laser crystal will be damaged when this maximum is exceeded. It will not melt or burn out or anything, it will just stop funtioning, turning into a poor excuse for an LED. Furthermore, a laser diode doesn't put out a narrow beam. It needs some kind of optic to focus the light into a beam. That's why you want a module. A module is more expensive, but has a very precise current regulator on board and it monitors the actual light output of the laser diode unit by sensing the laser diodes built-in photodiode - most laser diodes have one. Power glitches and noise are filtered out in order to limit the current through the laser diode within microseconds. Even with a module it is recommendable to use a regulated power supply to absolutely minimize the chance of ruining the expensive laser diode. A module also has a built-in optic that focuses the light into a narrow beam.

Laser diodes come in different colours and classes. The classes are explained above. The colour of light is expressed in nanometers (nm, one billionth of a meter), which really is the length of one light wave. Visible light is in the approximate range of 680 (deep red) to 430 nm (violet). IR (infra red) laser diodes, like the ones used in regular CD players are very easy to come by and cost only a few quarters to a few dollars. Unfortunately, they only emit invisible light (780 nm). Common visible laser diodes have limited visibility, because they emit light of a colour our eyes are quite insensitive to (670 nm). The best visibility in the red range comes from 630 nm diodes, but they require a very expensive production process, so they're much more costly. Commercially available laser diodes range from .1 to about 10 mW. A 1 mW 670 nm diode may sell for USD30, while a 10 mW 630 nm diode may cost USD300 (as of this writing in 2001), but its apparent brightness is 50 times as high.

Projector

In order to project a line figure on a wall, one needs to control the direction of the laser beam. Preferrably a few hundred movements per second, to allow for complex figures without flickering. If you want to control a laser beam, you have to either move the laser head or use a controllable mirror. The first option is a no-go, because a laser head is usually made of almost solid brass (see module picture above) and therefor too heavy to control very fast. A small 1x1 cm mirror, here next to a blue LED, is much lighter and will allow very fast movement.

Now, how are you going to control a mirror with a few hundred movements per second? What we need is 2 linear motors, one for X motion and one for Y motion, so we can just put varying voltages on these things and the laser beam moves according to these voltages. Linear is the keyword here. It means we don't have to track and correct the movement of the motors. Professional designs use galvanometers: high power linear rotary motors. They cost hundreds of dollars. Way too expensive for an amateur like me. The most common type of linear motor is in something everyone has: loudspeakers! Unfortunately they do not provide a rotary motion. A bearing of some kind has to be constructed to allow the mirror to swivel.

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