Laser Range Finder - optics help

Hello,

I am currently designing a laser range finder. I am using a plate beam splitter to split a 650nm 10mW laser light into two parts. I then want one part to go into a photodiode and preamp, and the other to be send out a telescope and recieved into another photodiode. How can I accomplish this? I cannot see how I can do this. What telescope assembly should I use for the 100meters ?

Help is very appreciated!!!!!!

Steve

 

[MichaelBrown]

How do you plan to capture the length or range with this?

Are you capturing the time required to reach the 100m target with respect to a known distance (other half of beam)?

I am not an expert. Just interested, I am working on a length calibration device. michael_brown@albint.com

 

[DrDoug]

I also wonder how you will obtain the range to target, and why you want to split the beam at all. You obviously need to time the laser pulse very accurately. Some quick points, some no doubt obvious:
1) The laser must be pulsed - not a continuous beam.
2) For a range of only 100 meters, you will probably encounter backscatter problems - the pulse reflecting off aerosols in the atmosphere inbetween the laser and the target.
3) For 100 meter range, you may not need to send out the pulse through a telescope at all - after 100 meters, it will only have spread out laterally by a relatively small amount. You will, however, need a telescope to capture the diffusely reflected light from the target.
4) Range finders are not the easiest things in the world to build. The timing accuracy required is formidable, placing stringent demands on the electronics design.

 

[nbucska]

What is the MINIMUM distance and what kind of accuracy do you need? <nbucska@pcperipherals.com>

 

[gamarilla]

I know of a device that modulates a laser source and uses the reflected light as an input to an oscillator. It works between 4Mhz to 50 Mhz for distance between 0 and 100m.
It is very sensitive to temperature changes, and has to be compensated. It is not affected by ambient light, due to it´s high frequency modulation.
I don´t know other details.

 

[IRstuff]

Depending on what your exact requirements are, the t0 pulse information does not usually require beamsplitting. Instead, a simple PIN diode can be placed anywhere in the optical path that provides some amount of backscattered laser energy into the PIN diode.

The beamsplitter may or may not be required, again, depending on your specific design requirements. Other possibility is to share the aperture; I've seen designs that make the beam come through the center 50% of the aperture and a annulus is used to collect the return energy. Ditto with a slit for the transmitter and the main mirror for the return beam. A beamsplitter design usually requires that the transmitter beam be highly polarized and that the beam splitter be polarized. The return beam will tend to be randomly or at least, less polarized, allowing the energy to be directed into the receiver path.

Alternately, you can simply leave the design as a bistatic design, e.g., a separate optics for the receiver altogether.

In either approach, you'll need to be concerned about boresighting the laser with the receiver and how to minimize the field of view of the receiver to minimize the amount of spurious background noise. While the output beam certainly does not have to be collimated, magnification will reduce the divergence, thus allowing some potential improvement in reducing receiver field of view. The downside is that magnification increases the beam diameter at the transmitter, thus, at close ranges, there may not be sufficient gain to make that worthwhile.

Timing can be done a couple of different ways. The most common is to use a high speed counter that starts with the receipt of the t0 pulse and stops when the return pulse is received. we're talking 0.15m/ns after accounting for round trip time, so you'll need something that can count in the hundreds of MHz to get reasonable resolution of range. Another approach is to use the t0 pulse to start a precision current source charging a high quality capacitor. The charging stops with the return pulse and the voltage of the capacitor is proportional to time and hence distance.

You'll also need to either have truckloads of excess energy so that you can set a fixed threshold and be done with that. Otherwise, you need to have some sort of thresholding than can adapt to the different lighting backgrounds that you might encounter. TTFN

 

[IRstuff]

Alternately, you might simply consider buying a Leica rangefinder; the most expensive model, the DistoPro4 is under $800 US, so the lower performance ones should be evan cheaper. TTFN