designing a laser rangefinder

Give it a try.

Most diode lasers do have a monitor diode built in. Use this as reference and omit the splitter.

What kind of phase range would you wish to measure ?

Build the receiver first, then decide on the optics to achieve the range you have in mind.

Rene

I hope to achieve range of tens of metres. Most probably less than 30 metres. I'm using a normal laser pointer, trying to cut cost here.

your only hope is an impulsed laser pulse and apd and counter,,,,,

There is 0 chance for you to fabricate a hand held interferometer for laser range finding.. a pulse timer perhaps..

Good luck

Marco

Depends on the distance - how far do you want to go? A mile or so and you need CO2 - to teh street then maybe HeNe - in the lab then a pointer.

You really need to understand Interferometers if you are doing this work. The better coherence a laser has the more chance of success. You will need a MM Interfeometer. Also a Bragg Cell probably an RF down-convertor (and amplifier) and a PLL.

Ing

For the time being, i'm not targetting on building a hand-held unit. I just want to build a system that actually works based on what i have. gosh, any help anyone?

You didn't mention accuracy goal. You might be able to do this with triangulation.

Triangulation includes use of things like camera autofocus chips.

At 30 meters, round trip distance is 60 meters so max modulation frequency without phase redundancy in the range of interest is 5 MHz. One degree of phase resolution is then 2 inches total distance, or 1 inch of range. It's also about half a nanosecond at carrier frequency

-- but phase is preserved thru hetrodyning so you could down-convert before you do the phase measurement. You'll need a pretty stable master oscillator with low phase noise. This need not be costly if you have the time to fool with it.

I don't quite understand though. Currently i'm already designing a receiver circuit consisting of transimpedance, differentiator and buffer circuits. The output is then connected to an oscilloscope to obtain the phase. I'm building 2 receiver circuits: 1 for the initial signal (gotten from the spitter) and the other one is the reflected signals. So all i have to do is just compare the phases (theoretically) and obtain the distance. Correct?

Unless you just want the joy of designing and building, it would be easier to just buy one of these off the shelf - about $30 or so these days at your local hardware store

David

Yes, theoretically. Your received signal is likely to vanish in the noise. That is where the optics comes in.

Rene

Ditto. Finding the appropriate optics wouldn't be much of a problem. As for noise, i just need to design a good receiver circuit. I wouldn't have to worry to much about noise if i were to use a reflector as a target? at a range around 30 metres, I guess that would probably minimize the noise. I'll let u guys know the outcome :)

There was an article in Nuts-n-Volts a little while ago that described a TOF range finder setup. The author used a laser diode modulated with a square wave, ran the colimated beam through a hole in a mirror to the target. the return light was reflected in the mirror with hole and focused onto a photo diode.

The received signal was then compared to the transmit signal with an XOR gate, the output being a pulse, where the width represents the time of flight.

This is from memory, since I don't have the article in front of me. You'll need a fast scope (since 1ns aprox. 1ft), also I suspect the cleaner you can get the rise time for the transmit and receive, the more accurate measurements you'll get. One of my co-workers is trying to recreate this setup.

Of course, there other schemes you can use. Optical interferometry is very cool, as someone else mentioned, you'll need lasers with long coherence length (like HeNe), although some of the communication solid state lasers aren't too bad. You'll also need a way to frequency shift the transmitter in order to do absolute ranging to targets.

Typically, laser rangefinder use pulsed time of flight, some sort of modulation, or optical triangulation.

Later, -ingo