Temporarely Reducing a 200 mW Laser to 1 mW for safety

Hi,
i hope this is the correct forum, it might also fit into the safety section.

I'm currently working on a concept for a CNC Laser rig. The main target is to expose photo positive pcb material to create circuit boards.
As a bonus i might want to brand some soft materials and maybe vaporize a layer of black coating from a copper clad board.

I'm currently planing to use this 405 nm Laser:
DealExtreme: $29.10 Focusable 200mW 405nm Burns Match + Money Detector Blue-Violet/Royal Purple Laser Pen (2*CR2)

The whole rig will of course be covered with an appropriate safety hood with an automatic off switch.

However i figure that i will need to focus the laser from time to time. For that purpose i would like to include a secondary circuit that physically limits the power output to safe levels. While i will be using protective goggles, i want to make sure that there is no way the full 200mW beam can be output with an open cover.

From what i have read so far, there are 3 ways to limit the power:
- Adding some opaque material in the beam
-- This is not usefull since it might change the focus, and the full power is still output, just absorbed early on

- Limiting the current
-- This is theoretically the best option, but I probably won't be able to limit the current to get it down to 1mW, because of the turn on threshhold

- PWM (As in switching the beam on and off)
-- This is the method i'm most used to from my current electronics experience, however this will still output the full power beam for every pulse, so i will need a frequency > 5kHz and i will need to be carefully to select a power supply that works with PWM. Also the circuit has to be very robust to make sure it doesn't get stuck in the on position.


My approach so far would be to combine the last two options. Limit the current to a bit over the threshhold and then use a 555 timer to generate a frequency of ~10kHz with a duty cycle of maybe 1% or less, depending on the current.
The whole laser could than be switched from the normal full power supply to the lower power supply to focus it.

Would this provide a somewhat safe environment to operate the laser? (The laser would still be pointed downwards, but there might be accidental reflections, i might cross the beam with my hand, it would be focused on the same area for a couple of minutes, etc...

Why don't you just have a calibration area on your boards, run it at a moderate power (~30mW or something), and then perform the focusing on that section? You could also fix it so it'll focus at a certain height and use a camera on the side to determine what height the surface is at.

Basically this is what i want to do, just that instead of a moderate power of ~30mW i want to do this at a minimal power of ~1mW. I doubt that i can just reduce the current so drastically, that's why i want to try to limit the current and then apply the PWM.

By the way, are there any good ways to automatically focus a laser?
I was thinking of using a light dependant resistor below a pinhole and varying the height of the pointer until i get minimum resistance. I'm just not sure if a resistor with a pinhole opening is the best way to go.

NsN said:

Hi,
- PWM (As in switching the beam on and off)
-- This is the method i'm most used to from my current electronics experience, however this will still output the full power beam for every pulse, so i will need a frequency > 5kHz and i will need to be carefully to select a power supply that works with PWM. Also the circuit has to be very robust to make sure it doesn't get stuck in the on position.


My approach so far would be to combine the last two options. Limit the current to a bit over the threshhold and then use a 555 timer to generate a frequency of ~10kHz with a duty cycle of maybe 1% or less, depending on the current.
The whole laser could than be switched from the normal full power supply to the lower power supply to focus it.

Click to expand...

I have used PWM on current sources, mainly to do some research into average vs peak current performance of laser diodes. Its not hard to pull off at all, as long as you use a proper driver (opamp based, or even the 2-transistor circuit).

Modulating at around 10 kHz should be fine, if you go much higher capacitances in the laser diode and its wiring may start to interfere with the current source. You can reduce the output power to whatever you like using this method, and go to full power by adjusting the duty cycle. A nice thing about PWM is that you can get a linear control using a potmeter, so you can set the exact average power you want.