Naked Science Forum

Non Life Sciences => Technology => Topic started by: Intellicide on 19/11/2014 06:08:33

Title: Can a Laser Diode work in reverse?
Post by: Intellicide on 19/11/2014 06:08:33
Can a laser diode/ laser emitter work in reverse? Could it receive a laser, while still being able to emit a laser? If for example you get a laser pointer, and you shine it at a mirror for enough time to emit a laser but not enough time for it to bounce back (which I guess is about 1000 billionths of a second or something), then when the laser returns back to the laser pointer, could it go back inside? Can laser diodes/emitters work in both directions?
Title: Re: Can a Laser Diode work in reverse?
Post by: CliffordK on 19/11/2014 07:00:56
It would be much easier to do your experiment using two separate laser devices. 

What you need is a photo receptor or solar cell.  I don't think a diode can generate power, although I'm not quite sure why not.
Title: Re: Can a Laser Diode work in reverse?
Post by: alancalverd on 19/11/2014 09:18:10
In principle you might get a photoelectric signal from any semiconductor diode but in practice this experiment would be very difficult to do. The problem is that laser diodes are not particularly efficient: the junction will get hot during the transmission pulse and any received pulse will get lost in the residual thermal noise.

You could however measure the speed of light by using two fast photodiodes, one receiving scattered light from close to the source and the other seeing only the reflected beam. Measure the time between the leading edges of the photodiode pulses.   

I think the corner cube reflector left on the moon by the Apollo mission is still accessible, and this gives you a decent timebase of a couple of seconds, so all you need is a laser, a telescope, and a stopwatch! shows you how to do it, and have fun and drunken sex at the same time. It's one of the best adverts for a career in experimental physics. 
Title: Re: Can a Laser Diode work in reverse?
Post by: evan_au on 19/11/2014 09:46:32
I don't think a diode can generate power

A semiconductor diode is constructed as a boundary between some "P-Type" and "N-Type" material. It builds up an internal electric field within the device.
If the semiconductor material is transparent, and in a transparent housing..
      ..and light falls on the diode with energy greater than the bandgap
             Then an electron will be kicked across this internal electric barrier
                    Which provides a small current derived from the light.

A semiconductor LED or laser or silicon solar cell are all diodes (the "D" in LED stands for Diode). All can work as photodiodes  ( I can remember filing the paint off a regular diode to make a light detector.

However, the reverse is not true - not all diodes can emit light. The indirect bandgap ( of silicon means that it does not emit light very well.

The large area of silicon solar cells means that they have a large capacitance, and don't respond rapidly to changes in illumination, so they don't make good pulse detectors.

Can a laser diode/ laser emitter work in reverse?
Yes, laser diodes have the right bandgap structure to both emit and respond to light.

The frequency emitted by the laser diode will be defined by the bandgap. Best detection is achieved by frequencies higher than the bandgap. So it won't be a very efficient detector of its own emissions.

The returning light pulse must approach the laser pointer at the same angle it is emitted, or it will miss the angles captured by the lens.
The laser emits light when it is forward biased, but detects light best when it has no bias voltage (or slightly reverse biased). Rapid switching between the two would require some tricky circuit design.

In CD/DVD players and telecommunications applications, separate lasers and photodiodes are used, so each can be optimised for each task and more easily coupled into the relevant circuitry. Both can then operate simultaneously.