Inexpensive Diode based Red LASER
Collimated Beam From my Laser
Before you consider
building such a device, you should be aware of the risks of Class IIIB lasers
(5mW -500 mW). Laser diodes in newer DVD burners can produce continuos (CW)
optical outputs of 150 mw. If overdriven, the power may exceed 200mW which is
hundreds of times more powerful than inexpensive laser pointers. A highly focused
beam of this intensity can burn objects that absorb the red light emissions
and this includes your eyes. To ensure safety, you should wear laser goggles
designed to absorb the wavelength of the laser emissions. Follow this
link and scroll to the bottom of that page for a real-life account of an
experimenter who has suffered permanent eye damage from a less powerful laser.
The laser diode
I used was extracted from an LG Electronics GSA H54N DVD drive (cost approx
$32). This drive can burn multilayer DVDs at 18X. This spec indicates that the
diode is of a high output. Using older drives will usually result in a lower
output and/or shorter diode life. In this DVD burner, the laser diodes are not
covered with a glass window so the actual semiconductor crystal is exposed to
the environment, therefore an airtight enclosure is required.
I cobbled together a number of parts some of which were left over from older projects etc namely a once-used die cast aluminum enclosure (better for maintaining optical alignment). I also decided to build the device as a "console" that runs off of a 6v adapter.
Laser Console with cover removed.
Top cover showing DIP switches for setting current.
I won't go into the details of identifying and removing the diodes since other sites cover this already (see links below). Instead I will mention the things that are specific to this project.
The Laser diode is being driven by an LM317T voltage regulator mounted on a small piece of scrap aluminum. The regulator however, is wired as a current regulator instead. This limits the diode current consumption. Without a current limiter, the diode will naturally start to draw more current as it heats up, possibly shortening its life. The source power supply is a 6 volt DC, 500ma power adapter. A 1000uf 16v electrolytic capacitor is shunted across the input to the regulator where the power adapter feeds in. Connected across the output of the LM317T, is a 10nf ceramic capacitor which will absorb any transients that may be produced during power up or down. The wire going to the diode should be short in order to reduce any di/dt transient voltages. The current is set by the 25 ohm 5% tol 2 watt resistors. 2 watt resistors are not necessary but they were the only 25 ohm resistors I had available. The resistors are switched into the circuit in parallel via a DIP switch which I surface mounted on the back of the PC board (shown on second photo). As each resistor gets switched into the circuit, about 50mA of drive current are added in. The red tape around the switch is temporary (pending a new gasket) and is used to keep dust from entering the enclosure. The total drive current can be set to 50 x 5 =250 mA. It is possible that a higher drive current could be applied although there is a strong possibility that the diode life will be shortened significantly. Normally, I operate the device at 200ma or less (4 switches in the "on" position)
The actual laser diode had an aluminum housing surrounding it when mounted in the drive. I did remove this however. I then mounted the diode on a heatsink suitable for the enclosure. It was mounted with an industrial epoxy mixed with fine silicon carbide powder (a home brew). This paste is a much better conductor than silicone/ZnO and more economical than silver. The Diode now barely gets warm to the touch even when run at 200ma continuous current. Some other designs I have seen place the diode in a small enclosure. While this is simpler and more convenient, it does not provide sufficient cooling to the diode resulting in a potentially shorter diode life.
Optics and Optical Housing:
Laser Diodes need a collimating lens because the light will naturally fan out from the end of the diode mirror. I first tried a lens from a dollar store laser pointer but the beam focus was poorer and the lens was not coated. I had the best luck with the lens which is found on the drive itself. This is the lens that is last in the series and is the disc focusing lens. Unfortunately this lens has a very short focal point and therefore must be mounted very close to the diode surface (1-2mm approx) making alignment a real headache. Nonetheless I mounted the lens on the leftover laser pointer cap. The cap then threaded by luck onto the lamp thread that passes through the housing where an outer threaded nut maintains it in position. Plumbers Teflon tape was wrapped onto the threads to reduce the "play" and allow a more precise focal adjustment. This arrangement allows me to focus the beam to any focal point required. Alignment of the diode and heatsink was done with the mounting screws and some springy lock washers.
The photo at the top of the page was taken in darkness. In the photo, the beam is visible as it is to the naked eye. The beam is much more speckled than what the camera reveals -due to the approx. one second exposure time. The beam is actually being scattered by water vapor and possibly smog. The brighter dots along the beam are larger dust particles crossing the beam path.
Questions? You can email me at:
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Sam's Laser FAQ
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