In general, the laser hazard depends on the laser power, the output beam diameter, and the laser wavelength. For a class IIIa or 3R laser (the "IIIa" designation is basically obsolete, although it remains in use for products certified before the new classes were defined), you're at low risk if you don't force yourself to stare into the beam. If the beam just happens to stray into your eye, you'll generally have a reflex response to look away from the painfully bright light. (Don't fight this reflex -- keep yourself safe). Note that the high end of the class 3R power limits is defined by the power where 50% of people will have an "aversion response" sufficient to avoid injury --- and the other 50% won't.
Reducing the current to a diode laser will reduce the output power, and thus make the output safer. However, most laser accidents don't happen when the laser is operated as intended or as planned. You also need to consider all the possible "fault conditions" or ways that things can go wrong.
Say you design a control circuit that regulates the laser output to always be less than 1 mW (in most cases, a "safe" level) using a feedback photodiode. For real safety, you should also consider things like
- What if the optical feedback path to the photodiode is blocked (by dirt getting in your box)?
- What if the electrical path from the feedback diode to its amplifier is broken?
- What if there is a spike or drift in the power supply to the laser or laser drive circuit?
- What if some bit of metal junk gets in your box and short-circuits the laser to the power supply, bypassing the power controller?
- How are you calibrating the limit value for the photodiode current? If you're setting it with a pot, could the pot drift or be mis-adjusted after calibration? If you're using a digital circuit, could the EEPROM with the cal data be erased or damaged?
- etc.
These are the kind of conditions that marketable laser products need to consider before they can pass regulatory requirements. Before you risk your eyesight with your laser system, you should at least consider doing this kind of analysis for yourself.
If you used a laser with less power capability, you would know that before any of these kind of hazards could lead to the laser producing a dangerous beam, the laser itself would burn itself up. If you use a laser capable of producing 5 mW before burning out, you'd be wise to treat it with a proportional level of respect.
I've used the laser from a DVD burner and let me say that it doesn't burn much of anything!
I got several lasers from a couple of DVD burners and combined them with an adjustable focus lens that was specifically made for laser diodes. I then shot it at anything and everything. Here's what I can tell you about what burns and what doesn't:
Black Electrical Tape: You can burn a hole in this. It takes some time to get the laser focused right, but when it does you can burn very small holes. It's hard to actually "cut" the tape with the laser. It's more like making a series of very small holes than slicing a line through it.
White paper: Forget about it. The laser, even when well focused, just reflects off of this and does nothing.
Black paper: Will put small holes in it. By small, I mean something around 0.5 mm. Larger holes are difficult and needs to be made from a bunch of smaller holes.
Brown Paper (a.k.a. Cardboard): With time and effort it can make small holes. The holes only go through the first layer and not completely though the cardboard.
Plastic: Will make very small marks in black plastic. No holes. Does nothing to light colored plastic.
Balloons: Might pop a dark colored balloon. Nothing to a light colored balloon.
Matches: It can work, but it's hard. It doesn't work on white or red tipped matches. Darker colors, or matches marked with a black marker, will ignite.
So... It won't work for ping-pong balls. But it's still cool and worth making one to just play with.
Warning: You MUST use eye protection with this! Seriously. I speak from hard-earned experience. The laser, when reflecting off of brown cardboard, has enough energy to make your eyes hurt after 30 minutes of playing around. If you accidentally got hit directly in the eyes or got a reflection off of something shiny then you'd be in a world of hurt. At this wavelength and optical power, a simple pair of dark sunglasses is good enough in a pinch. Of course a real pair of laser goggles is preferred.
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Oh, @tomnexus gave me a clue... I think there is only one photodiode (PD) and as he said the rings scatter light that is not "on axis". So you find a signal maximum when the laser light is perfectly aligned, (cute).
And to answer the question: Yes I think you can use a commercial PD. You'll want the PD area to fill the end area of the rod... there might be some diffusor in the middle there between the PD and rod.