Laser emitter for serial communications?

Question

I want to build a 2-way serial communications rig using low cost LED lasers. 9600 baud would be ok, but faster speeds would be more useful. (This is purely a hobby project. The further I can establish a communications link the cooler it will be, but there is no hard requirement.)

Initially I'll probably use red LED lasers because they're easy to aim, but will eventually switch to IR. My thinking is that I will put an iR phototransistor in a parabolic reflector (like an old car headlight assembly) and put a cheap visible light blocking/IR pass filter in front (An "IR notch filter" to use signal processing terms for an optical filter) and put the whole assembly in a cylindrical tube. That way ambient light will be greatly attenuated and with the parabolic reflector pointed roughly in the direction of the sending laser even a "near miss" should be enough to generate a decent signal in the receiver, and if the beam is bigger due to being scattered over distance, the parabolic reflector will serve to focus it on my receiving phototransistor.

The dirt-simplest way to do this would be to power an off-the-shelf laser pointer through a transistor, and apply a CMOS level control signal to the gate of the transistor.

However I suspect that the voltage/current regulator on the laser pointer would tend to flatten out the high frequency switching of the power and prevent clean switching of the signal. (Most voltage regulators I've seen include one or more capacitors to flatten out the signal) Am I right in my suspicions?

I could buy a laser diode, regulator, and collimating lens and attempt to switch the output of the regulator into the laser diode, but I'm a software developer who likes to tinker with electronics, not an EE.

Are there low-cost solutions to sending digital signals with LED lasers? I'm willing to spend money on it, but would like to keep the costs down.

Answer 1

First, just to pick nits, an LED is not a laser, and vice-versa.

That said, you might try eBay, searching for "laser module TTL", and you can get quite powerful lasers which can be digitally modulated, usually in excess of 10 kHz - that's why you need to specify TTL. Beware of the small cylindrical units - they typically cannot run for more than 60 seconds without overheating unless you provide extra heatsinking. And the bigger ones will have problems with eye safety, but you can get around that by defocussing them. However, you really need to start learning about the ins and outs of eye safety, since more or less by definition you're going to be pointing lasers at people's eyes. Nowadays folks tend to really freak out if they find themselves looking down the beam of a laser (the wussies).

While you're on eBay, consider finding very narrowband optical filters at the wavelength you're planning to use. When you're operating in the great outdoors, Mr. Sun is not your friend.

Putting a detector in an old headlight is a decent idea in principle, but the details of actually making a robust mount will be more of a challenge than you think. Plus, the better your system gets at concentrating your incoming light, the narrower its field of view and the more difficult aiming it gets. Maintaining alignment also gets iffier, too.

As an aiming guide, you might consider sending a constant stream of null data, as close to alternating ones and zeros as you can, and producing an audio output from the receiver. This will let you aim your system by listening to the tone, and such fast audio feedback will be enormously useful.

I'm not bringing this up to discourage you, just to let you know that you're in for a tussle.

Answer 2

I think the hardest part will be keeping the beam focussed tight so I'd strongly consider using a light pen as the starting point and stripping the electronics back to be able to drive the laser directly. It's quite easy to drive lasers - you just have to bias them so that a digital low doesn't take it past its extinction point or things get slow - in other words keep it biased at say 10mA (or whatever the diode is rated for) and wobble the bias with an AC 5mAp-p that represents your data. Be aware that you'll probably need NRZ type data to keep this working so be either prepared to manchester encode the data or use a scrambler and descrambler at the receiver.

Answer 3

You're right about the regulator chips. They often take optical feedback from the laser to ensure flatness over time (anti-aging - wear out). Most lasers come in packages with three leads, power in, ground and PD (Photodiode) out for his exact reason. However most of these driver chips have an enable input and the fact is you're driving this so slowly that you might just be able to modulate the laser fast enough even with the regulator in place.

I would recommend spreading the output beam out as much as possible to ensure eye safety. So take the asphere off the front of the laser and put it at the focus of an ellipsoid. Be safe!