How to build an ultra high speed photo detector? (for detecting red laser pulse)

Some hints:

1. Capacitance is the usual problem with photodiode speed. Smaller photodiodes, PIN type, reverse bias.
2. Reduce load impedance, since T = RC
3. Avalanche photodiodes are yet faster.
4. If you compare differences in timing between 2 optical paths of different lengths, you can cancel out any delays common between the 2, which makes it possible to measure c with fairly slow equipment.

Paul Mathews

Yes but it is difficult to do this over a reasonable distance without high power lasers and avalanche detectors and highly selective optical filter wich are all quite expensive, however a large lense helps and is cheap.

The amplification needed to recover your very fast pulse edge from the loss to/from target and the capacitance of the detector will also amplify a lot of noise.

However there are a couple of other ways of doing this, using a sweep modulated laser beam and using a synchronous detector to detect the phase change over a frequency range gives very good results from a low power laser, using a heterodyne optical receiver improves this method enourmously, Ive experimented with this with modulation frequencies upto 1ghz.

Another way is to use a tuneable laser and compare the interference from the target with the interference from a tunable cavity, to give absolute distance, the resolution is quite remarkable considering the distance it works over.

Somone was going to look inside one of those Lieca range finders but not heard anything since.

Colin =^.^=

Photodiodes have substantial shunt capacitance which limits their response time, especially with weak signals. That's why you don't see any infrared LAN's-- the signals are too weak and low bandwidth to get very far.

The usual trick is to use FM instead of AM -- this allows you to extract the echo difference frequency, which is at a manageable rate-- low megahertz instead of several GHz.

Why not just use a spinning mirror, like Michelson and Morley did in 1879? ;-)

Good Luck! Rich

You'll need a fast silicon PIN detector, or a silicon or germanium avalanche photodiode. The APD would give a lot more signal, but needs a high-voltage adjustable supply and protection circuits. A pin detector will need a lot of gain, a fast TIA or a MMIC or something.

You can buy integrated 150-600 MHz pin-tia widgets, usually in a small TO-can with a flat or a ball lens, for $20 or some such. Google "pin-tia". Try Lasermate and Appointech too. Most pin-tia's have AGC, which trashes prop delay when it kicks in.

The really fast stuff is GaAs, but it's blind at visible wavelengths.

I may have a couple of 850 nm pin-tia gadgets around here that I can spare... I'll have to look in my box of samples.

Oh, Newark stocks the Optek parts, and their silicon pins are OK. Apply lots of bias voltage, 24 maybe, to speed them up. Their vcsel lasers suck.

John

Seems to me what I read in the HP Journal about an HP distance-measuring transit, back when they were in that business, is that they used sinusoidal AM modulation at discrete frequencies. You can then take a sample of the transmitted light locally and compare the phase of the modulation with the return signal's modulation phase. There's an ambiguity of the number of cycles, but if you do that at just a few carefully chosen frequencies, the ambiguity can be resolved. Then for accurate distance measurement, you must compensate for the medium through which the light is propagating: humidity and temperature of the air are important, if it's air, for example. There are various ways to compare phase accurately to effective resolution of a few picoseconds, I'm sure, given some time to do averaging, and I wouldn't be surprised to learn that it can be done quite a bit better than that. In fact, the averaging gives a pretty big advantage to the modulated continuous signals, over a pulsed system that can only handle relatively few pulses per unit time.

Cheers, Tom

Nice project. Why don't you use the PIN detector used for laser beam monitor inside a laser diode from a CD reader/writer ? You want to keep costs low, but what tool do you have for measuring 100 pS range ?

success, Vasile