This is the second post about my experiments with laser diodes, you can read the first one here. In this post I cover adding a lens to focus the beam and protect the diode, building a suitable power supply to drive the laser without destroying it, and finally putting the whole assembly in a safe to use mounting frame.
Adding a Lens
The easiest, and safest way, to mount your diode is to use a pre-made laser diode assembly. These can be purchased on eBay for a reasonable price (just search for "laser diode housing") and include a heatsink, a mount for the TO-18 diode case and an adjustable lens to focus the beam.
I was a little unsure how to assemble everything at first, the image to the left shows how all the various parts fit together. For the most part there are no tools required to assemble everything, you do need to fit the laser diode into the mount provided which does require some force.
Make sure that you solder the power leads to the diode before mounting it. You will need a reasonable length on the cables (about 15cm to 20cm should be fine) so you have a bit of freedom of movement.
To avoid damaging the diode I used a file to enlarge the hole for it but I overdid it a bit and made the hole a bit too large. To compensate for that I used some wax to ensure the diode wouldn't move - it's probably not the best solution but it seems to be working fine.
Normally you would need to apply enough pressure to force the diode into place without damaging. Most advice I have seen on this is to use a small vice to do the task.
Building a Power Supply
If you have extracted a diode from a DVD/RW drive as described in the previous post you should have a 650nm RED laser with a power rating between 300mW and 400mW (a 16X DVD/RW drive should have a 300mW diode, a 32X will generally have a 400mW).
It can be difficult to determine the exact specifications of the laser diode you have (on all the devices I've pulled apart the diodes have no part numbers or any other markings on them) so getting the power supply right requires a little bit of trial and error.
The best place to start is with a 200mA constant current supply. These can be built around an LM317 voltage regulator.
In this case we need a power supply that regulates the current, rather than the voltage. The LM317 is perfect for this task - it can provide an output current of up to 1.5A and requires minimal external circuitry. The schematic to the right shows the design I used.
The output current is controlled by the resistor network between the output and adjust pins on the regulator chip using the formula:
``` I = 1.25 / R
The network I use has two 10 Ohm resistors in parallel (giving a 5 Ohm total resistance) and a trimpot in series for fine tuning. This allows for a minimum resistance of 5 Ohms and a maximum output current of 250mA. The laser diode has a forward voltage drop of 2.5V so the total power passing through the circuit will be over 500mW - standard 1/2W or 1/4W resistors are not good enough so I used 10W wire-wound resistors instead.
This is jumping ahead a little bit but I found that a 200mA supply is not to generate the full 300mW output the laser is capable of due to the diodes conversion efficiency. We need around 350mA output instead. I added another 10 Ohm resistor to the parallel network reducing the base resistance to 3.3 Ohm and allowing for over 600 mA if needed. An alternative solution would be to reduce the resistance of the two existing resistors - if you swap the 10 Ohm resistors with 5.6 Ohm ones you will get a similar boost in the output current range.
Tuning the Current
To start with we want the regulator to output a steady 200mA before we attach the laser diode so we need to tune the power supply first. You will need an input power of 9V to 12V, 3 1N4001 diodes and a multimeter capable of measuring up to 500mA of current.
Connect the 3 diodes in series (this will simulate the 2.5V forward voltage drop of the laser diode) and connect this to the output of the power supply with the multimeter in series. Apply the input voltage and adjust the trimpot until you get a steady 200mA reading on the multimeter.
Tuning the Power
I recommend starting with the 200mA current for your initial work. This is not driving the laser at it's full capacity though (the output will actually be about 180mW).
I haven't been able to find detailed datasheets for the laser diodes so ensuring maximum power output mostly done through guesswork and experimentation. The biggest issue is the conversion efficiency of the diode and this is effected by heat as well (you don't want the diode to get much above 60 degrees Celsius).
As a general rule of thumb I've found the output is about 86% of the input current - so at 200mA you are getting around 170mW, at 350mA you are getting around 300mW. If you have a 400mW laser (out of a 32X DVD writer for example) you will need to be driving it at 460mA. Please note that these are guesstimates and have worked with the lasers I have - pushing the current too high (or running the laser for extended periods at high current) will damage it.
Building a Frame
At this stage we have a powered laser with a focusing lens and protective casing and that's probably all that's needed for basic experimentation. I was a bit concerned about the cylindrical lens casing however - I didn't want to have it slip out of my hands and accidentally beam light directly into my (or any other hapless bystanders) eyes. I also wanted to a way to keep the laser a constant distance away from the target surface so I could replicate experiments with some consistency.
To help with this I designed a simple square frame that I could mount the lens assembly in. The design is pretty straight forward (see the image to the right) so I won't cover it in too much detail. You can download the OpenSCAD and STL files for it here.
Once the frame is put together it's a simple matter of inserting the lens assembly and positioning it so the focal point for the laser light is in the centre of the bottom part of the frame. Now I can simply place the frame on the material I want to test against and move it around to test the time it takes to burn or etch that surface.
Now that we have everything in place it's time to find out what a laser of this power can and can't burn. Ideally I would like to be able to burn a thin layer of black paint from a copper surface (and thus expose the copper for later acid etching) but there are other interesting applications as well (cutting stencils out of thin material or slicing up foam for simple constructions for example). In the next post I'll detail the results of those experiments.