How to transmit data with a laser beam?

Question

I am working on a project about data transmission with lasers. The project is very young so I can modify my current planned setup if necessary.

This is the actual planned setup:

Every raspberryPi can control a laser (probably via a dedicated driver) and a phototransistor. After a "syncing" phase the devices can communicate turning on and off their own laser to send a one or a zero respectively. To help receive the laser beam I am planning to add some kind of lens.

My goal is to archive enough bandwidth to make a voice call (see below for more detaild) between the two devices, about 5 KBps.

The expected working distance is at least 4 meters, up to 20 meters if possible.

I thought about infrared laser to avoid some disturbs from artificial ligth, sun, etc.

What kind of (possibly low cost) laser and photransistor can I use in this setup? I have searched online for some lasers but I am concerned about the speed at which the laser can turn on and off.

Clarification: I have not been very clear about what i wanted to archive with the project. Sorry. My goal is to be able to transmit arbitrary data (bytes) over this channel. With this first prototype the goal is transmitting fast enougth to carry a phone call. The voice is first received by a microphone connected to a sound card, converted to binary data, compressed, sent through the laser channel, received by the other end, decompressed and played through a speaker. In future prototypes with more bandwith i'd like to transmit other kind of data such as small files or video streaming.

Answer 1

My answer is about the use of laser diodes.

Using a laser to transmit optical information requires a little knowledge. The first bit of knowledge is the threshold current that is needed for the device to commence lasering. Below this current, the lasering is poor or zero and there is a time lag on returning to lasering when current rises again. This means your data can be interrupted. So, if you want a good system, stay above the lasering theshold. However, that threshold is temperature dependent: -

Picture Source

So where do you pitch the forward current to cover the likely (or possible) variations in temperature (bearing in mind the self heating of the device). What about this: -

If I choose a minimum current of 55 mA I can always produce laser light. See the blue line I've added to the picture above. But if the maximum output power is 10 mW, I can't modulate the light with a current greater than about 65 mA at 0 degC. This gives me a light output that varies between about 6 mW and 10 mW at 0 degC and between about 1 mW and 3 mW at 50 degC.

But you could go for more sophisticated approach. You could make the average laser current rise with temperature using a thermistor. So, at 0 degC the average current is about 52 mA and at 60 degC it's about 78 mA. This now positions the unmodulated light output at 5 mW across the range of temperatures.

This is important because now you can modulate the light output between 1 mW and 10 mW - this is called the extinction ratio (by the way) i.e. the ratio between lightest and darkest laser light output levels.

It's still a little tricky though because to get 1mW to 10 mW at 0 degC requires a change in current of about 25 mA whereas at 50 deg C you'll never quite get 10 mW even with 100 mA but you might get 1 mW to 9 mW with a change in current of 45 mA.

So the laser output gain reduces with temperature.

I'm labouring these two points (threshold and gain temperature sensitivity) because be under no illusion, to get a reliable free-space transmission system working across a gap of 20 metres requires every tweak possible to make it work successfully.

You'll need a small (but not parallel) "diversion angle" to make your system usable but, because of this, you will be receiving micro watts (at best) at 20 metres and you may be "in the noise" of your receiver.

Just think about that tiny little window in your photodiode (yes, use a photodiode for speed) - you might have an active area of 1 square mm. If your laser is illuminating a 20 m distant target with an area of of 100 mm x 100 mm you have spread your 5 mW average power thinly over 10,000 square mm and so your receiver window acquires just 500 nW and that's just on a good day!

Do not underestimate the difficulties and the head scratching. Do not under-estimate the length of time you might spend trying to understand the equivalent input noise of this or that photodiode. It can be done but it can also be very tricky. Choose a photodiode that is as big as you can but not so big that it is too slow for the data rate. If you can get 4 sq mm then you have quadrupled the receive power compared to 1 sq mm.

If you are going to use a lens (or you need a lens) mount it at the laser end and not the photodiode end. Mounting at the photodiode end is much less effective given that the lens can only receive and focus the power that hits the lens. Hence a small receiver lens is of moderate use compared to a lens at the laser used for focussing a slightly divergent beam.

Answer 2

The easiest type of laser to use will be a diode laser. Using pretty much any other type of laser will be a massive hassle, so stick with that. Diode lasers can switch extremely fast, to the point where the limiting factor is usually the drive circuitry. It's relatively unlikely that you'll need to switch at such a high speed.

For the receiver, the speed of the component is becomes more of a bottleneck. A phototransistor typically switches in the 100's of KHz, which may not be enough. If you need soething faster, you can go with a photodiode. This one switches at 5ns, certainly enough for your application.

Edit: Since you specify 5kBps, a phototransistor should be fine. Also, you're already using a laser, so you probably don't need a lens...

Answer 3

I recommend against using IR components, for no other reason than you cannot see them. For this type of experimental setup, a visible beam is much easier to align and diagnose. Plus, it makes for a much more effective demonstration.

The guts of a small laser pointer could work for the transmitter. Pro = more than enough beam power for a few meters. Con = requires pinpoint alignment with the receiver. As an alternative, some very-high-brightness red LEDs get their brightness spec by having a very narrow beam width. Not nearly as narrow a a laser pointer, so it should have enough beam spread st a few meters to make alignment less critical.

Start with something simple like a 100 Hz squarewave. At the receiver, pass the recovered waveform directly to the audio amp and speaker. When that works, increase the frequency until you reach a few kHz. If the recovered waveform is nice and square, this indicates that higher frequencies are worth trying.

5 kBps is 40 kHz, which you will not be able to hear directly. However, as indicated above, legible speech is possible with much lower data rates.

Note - be very careful about eye protection.