The following file has been taken from the October 1987 issue of Radio-Electronics.  It has been summarized to shorten the length of the file.  A hi-res schematic accompanies this file.


                             THE LASER LISTENER



                    Introduction and Theory of Operation


     Breaking and entering to plant a listening device is one way to "bug a room."  Unfortunately, it can earn someone a long jail term too.  A better and safer way is to use a laser beam to eavesdrop on the window across the street.
     The sound waves generated by nearby conversation will cause the glass in a window to vibrate very slightly.  If a laser beam is bounced off the window, its reflection will be modulated by the vibrations.  All that's needed to hear what is being said is a demodulating device that extracts the audio from the reflected laser beam.  That technique is used by sophisticated "survelliance experts," but you can easily duplicate that feat by using a hobbyist's laser and the inexpensive Laser Listener demodulator described in this article.  If you need something a little more sophisticated, it can be made part of the riflescope aimed laser-bug system that is shown in Fig. 2.


                                Laser Basics


     Although the details underlying the generation of laser light are beyond the scope of this text, an understanding of some of the characteristics of a laser beam as compared to ordinary light will be helpful in assembling a laser-listener system.
     Light is considered to be comprised of packages of energy particles called photons.  However, light is also electromagnetic radiation and behaves like radio waves, although at a much higher frequency.  The perceived color of visible light is determined by the radiation's wavelength, which is usually given in micrometers.  The shorter wavelengths are perceived as violet, the longer wavelengths as red.  The spectrum below the visible portion is called infrared; the spectrum above is called ultraviolet.
     The light emitted by a conventional incandescent or fluorescent source contains a wide range of frequencies, and the photons are emitted randomly and spontaneously in all directions.  On the other hand, in a laser light source the photons are released in one direction, at one frequency, making the laser light highly directional and pure in color.  (An analogy would be to liken ordinary light to white noise, while the laser is likened to a sinewave -- a single pure tone.)  Since all of the light emitted by a laser is coherent (has the same frequency), constructive or destructive interference occurs when two beams of laser light meet at the same place and time.
     The beams cancel each other when out of phase (destructive interference).  They are additive when in phase (constructive interference).  It is the interference between the beams that enables the movement of any reflecting surface to be sensed by a device called a interferometer.  An interferometer is a beam splitter -- usually a piece of partially-mirrored glass -- that deflects only a small part of a beam aimed through the glass.  It can be used to reflect both the source and the reflected laser beams so that their phasing or amplitude can be compared by a receiver.
     The major problems with using interferometry for eavesdropping is that only a part of the laser's energy is directed at the target, limiting the working range, and the interferometer is sensitive to the diffusion of the sounds target's reflections caused by tremors in the mountings of the interferometer, the laser, and the reflective target.  For super-snooping, a direct reflection from the target is preferred because the collimated nature (parallelism) of laser light also allows modulation of the beam.


                            The Prototype's Laser


     Regardless how we choose to eavesdrop, we must start out with a laser, so we'll cover the prototype laser bug's laser unit first.  It's a Heathkit model ETS-4200 Laser Trainer, a Helium Neon (HeNe) unit having a output power of 0.9 milliwatts.  It has a beam diversion of 1.64 milliradians, which produces a spot of light 1 1/2 inches in diameter at 200 feet.  Although 0.9 milliwatts doesn't appear to be much power, it can cause extreme eye damage if allowed to shine or be reflected directly into the eye, or if viewed directly through any optical device such as a telescope, binocular, etc.  The beam may be safely viewed only if projected onto a non-reflective surface such as a white sheet of paper.
     If you want to keep costs at rock-bottom, or just want the excitement of a complete home-brew project, another alternative is to assemble the helium-neon laser shown in the June 1986 issue of Radio-Electronics.  Also, if you want to build a laser from your own design, helium-neon tubes are often available from "surplus" distributors.


                                The Reciever


     The Laser Listener's receiver is relatively easy to build and adjust.  It is designed to drive a 4-20 ohm headphone or speaker, which permits just about an high-fidelity or Walkman-type headphone to be used for monitoring.  The circuit shown in the accompanying hi-res screen uses a photo transistor (Q1) for a sensor, and has a meter (M1) that indicates the relative signal strength of the reflected laser beam.  Because the meter only responds to the amplitude modulation of the reflected laser beam, it is unaffected by ambient light and the relative intensity of the laser beam.  An adjustable polarizing light filter can be installed in the front of Q1 to avoid swamping of the phototransistor by very high ambient light.
     Phototransistor Q1 is an inexpensive type usually called an IR detector, which means that it is specifically sensitive to infrared light.  Tests comparing the unit specified in the parts list with other less readily-available and more expensive devices show no measurable differences in performance in the prototype receiver.  No base connection is used for Q1 because the reflected laser light controls the collector current.  The audio signal developed across collector load-resistor R1 is coupled by C2 to voltage-controlled atenuator IC1, which has a greater than 30-dB gain variation; it serves as both a pre-amplifier and as an electronic volume control.
     Resistor R2 and capacitor C1 decouple (filter) the power supply voltage to Q1 and IC1.  Be sure to take extreme care not to eliminate or accidentally bypass the filter because that will cause unstable operation.  The gain of Q1 and IC1 is too great to permit non-decoupled operation from the power supply.
     The output from IC1 is fed through C4 to amplifier IC2.  Resistor R4, and capacitators C5 and C7, tailor IC2's frequency response and ensure stable operation with varying drive levels and output loads.
     The output of IC2 is split into two paths: One goes into output-jack J1 via C6; the other feeds voltage-follower IC3, which drives the meter circuit consisting of D1, D2, C11, R8, and M1.  The time constant created by the values of R8, C11, and M1's DC resistance was selected to provide a comfortable damping of the meter pointer's gyrations.  The value of C11 may be caried to change the pointer's response.  Increasing the value of C11 provides a smoother response; decreasing C11's value will cause the pointer to more closely track the variations in the laser beam's modulation.


                                Construction


     The prototype receiver was assembled on a modified Radio Shack 276-170 pre-drilled PC board, which has strips of copper foil on the underside that connect the component mounting holes.  (A board with a parts-placement template in place is available from the source given in the Parts List.)  Nothing about the layout is critical as long as you follow the usual precaution of keeping the input and output connections reasonably separated.
     Check your parts layout against the foil strips on the underside of the board.  If it appears that any will be too long, cut them to size before mounting any components.  Cut each foil strip exactly as long as needed so that a foil carrying the input signal doesn't end up running adjacent to an output connection.
     For best results when making connection to the foils, use a small pencil-tipped soldering iron and .040 diameter rosin core solder.  If your layout requires jumpers between component mounting holes, use #22 solid, bare wire.  Insulated jumpers are #22 solid, insulated wire.  Connections between the copper foils should be #18 insulated wire because it's a precise push-fit for the holes in the special prototyping board.
     The enclosure is a 6 1/2 X 2 1/8 X 1 5/8 inch aluminium cabinet.  Phototransistor Q1 protrudes from one end of that enclosure and is mounted with a dab of household cement.  Position Q1 correctly before gluing it in place and be very careful to not get glue on the surface of the lens.  Do not use cyanoacrylate-based instant glue because it might cloud the transistor's plastic lens.  Output-jack J1, gain-control potentiometer R5, and the meter are mounted on the side of the cabinet so as to encourage the user to face at a right angle to the source of laser light, thereby lessening the chance of looking directly into the reflected beam.
     The board is mounted in the enclosure with four 3/4 inch 6-32 machine screws.  Use 1/8 inch insulated spacers between the board and the enclosure to insure adequate clearance between the enclosure an the board's foil side.  A ground lug located at one mounting screw in soldered to the circuit-board's ground foil to provide the ground connection between the board and the cabinet.  The connections between the board and the panel-mounted components can be #18-22 stranded, insulated wire.


                             Optical Attenuator


     The optical attenuator assembly, for which construction details are shown in the second accompanying hi-res picture, mounts over phototransistor Q1.  The hi-res picture shows how it is installed over Q1 and the individual details for each component in the assembly.  The front of the assembly is painted flat white so that the reflected laser beam can be easily seen.  The attenuator is built in such a way that the photo transistor can see the laser beam directly, or through a combination of one or two polarizing filters.  When both filters are in place, rotation of the large-diameter filter mount will cause a gradual decrease in light transmission (to almost total blockage within 90 degrees of rotation), which allows the receiver to be used over a wide range of light intensities without swamping the photo detector.
     The attenuator has an inner filter and an outer filter made from brass telecopic tubing.  Each filter consists of two sections: a filter base that is soldered to a small mounting plate made from brass sheet (the painted target), and a filter mount that slips over the base.  Polaroid filters cut from neutral-tint polarized sunglasses are cemented to one end of each filter mount to complete the attenuator.  When complete, the entire optical attenuator's mounting plate is secured on the enclosure over phototransistor Q1.


                              Types of Lasers:

WARNING!

     Extra precautions must be taken because of a laser's intense concentrated energy.  Among other factors, the hazards presented depend on the power density, the frequency of the beam, and the time of exposure.  Guidelines have established the classifications of lasers.  A brief description of the classification is as follows:

     Class I:  Low power beam.  Not known to produce any biological injuries to the eye or skin.
     Class II:  Reserved for visible-light lasers only.  They are limited to less than 1-milliwatt output.  Eye damage will result if stared into for longer than 1 second.  The normal blink response of the human eye will provide protection.  Eye damage will occur if the beam is viewed directly by optical instruments.  Direct (specular) reflection, as from a mirror, should be considered to be the direct beam.  Diffuse reflection of the light may be viewed.
     Class III:  Instantaneous eye damage will occur if exposed to the direct beam.
     Class IV:  Both direct exposure or direct and diffuse reflections will produce eye damage.  Exposure of the skin to the beam is hazardous.  The beam is considered to be a fire hazard.