View Laser Bounce Listening Device for additional notes and technical information of older devices.

Internal view of a stock Polytec LSV-065 Laser Surface Velocimeter showing the laser beam paths.

The Peltier-cooled laser diode (670 nm) is on the left.  Next is the Bragg modulator (+15 VDC) with an internal 40 MHz oscillator and divided-down 10 MHz oscillator reference output.

The Bragg modulator feeds a prism/beam splitter combination which splits the beam into an unmodulated (RED path) and modulated (BROWN path) beam.

The beam then leaves through steering optics.  On receive, a centrally-mounted plano-convex lens focuses the reflected (GREEN) beam onto an avalanche PIN photodiode.

The target modulation is then imposed on this 40 MHz carrier and the output (50Ω) of the photodiode module is this signal which we'll need to demodulate:

• Doppler (velocity) = Fairly easy, standard FM discriminator.
• Phase (displacement) = Much more complicated, but the sensitivity is insane.

Here is a helpful Polytec PDF file titled Basic Principles of Velocimetry,

We'll need to remove the beam-steering optics, the receive optics, and the avalanche PIN photodiode module.

Alternate view with the front-end modules removed, but with proper power now applied.  Go here for the DBM-17W2S connector pin-out.

Note the two little red dots on the right.

The top dot is the unmodulated reference beam.

The bottom dot is the 40 MHz modulated signal beam.

Alternate view with the front-end modules removed, but with proper power now applied.

The power (+15 VDC) to the Bragg modulator was removed, so only the unmodulated reference beam is output.

Close-up view of the output while the Bragg modulator was powered off.

The top dot is the unmodulated reference beam.

The bottom small dot is leakage from the Bragg modulator.  This is called the "zeroth-order" modulation and is normal, but we should make note to applied stray beam capture material internally.

Close-up view with everything properly powered.

The top dot is the unmodulated reference beam.

The middle dot is the modulated signal beam.

The bottom dot is the stray "zeroth-order" modulation beam (near the screwdriver tip).

Referring to Figure 1 in the U.S. Patent 6.714,306, we'll need to add a beam bender and a beam splitter.

Shown here are a Hewlett-Packard 10707A Beam Bender and a Hewlett-Packard 10701A 50% Beam Splitter (just for fitting).  The internals of the LSV-065 had to be milled out slightly to allow these optics to fit.

Other less-expensive optic modules will work, but these HP ones are much more convenient.

It seemed to work out well.  I was going to just drill a hole for the receive laser output, but milled the side of the case instead.

I'm not sure what they mean with "an optical delay 21 equal to the delay encountered in the sampling process" in the patent.  I know lasers can change their phase as the beam travels, so you technically need "both" beams the same length for proper phase demodulation.

I have no idea how to do that, so we'll just skip that step for now.  LOL.  (Optical Delay Notes)