What they are. How they work. By Insight Technologies

Laser eavesdropping systems differ significantly from traditional audio surveillance and monitoring systems in that they are completely removed from the room being monitored. No microphone, radio transmitter, recorder, or electronic device of any kind is required in the room. The only indication that an eavesdropper is present is an invisible laser beam reflecting off the exterior surface of a window.

When several years ago, the national news media revealed that there was a serious health risk for employees of the U.S. embassy Moscow posed by the continuous bombardment of the embassy by microwaves, it was mentioned that the exposure resulted from the microwave beams of microwave eavesdropping devices operated by Soviet intelligence agencies. These devices were Maser-Bounce Listening Devices, older, lower frequency versions of Laser-Bounce
Listening Devices, the basis of laser eavesdropping systems.

Laser eavesdropping systems are more effective than maser eavesdropping systems because laser light beams are much harder to detect than maser microwave beams.

Like the lasers on which they are based, laser eavesdropping systems were once very exotic, very sophisticated, and very expensive - the exclusive province of university laboratories,intelligence agencies, and the military. Today both lasers and laser eavesdropping systems are cheap, plentiful, easy to obtain, and easy to use.

When T. H. Maiman demonstrated the first laser in 1960, the lasing element was a rod of specially grown ruby crystal optically "pumped" by a xenon flash lamp. Needless to say, the laser was expensive and temperamental. Today, there are many types of lasers, optically-pumped solid lasers, gas lasers, gas dynamic lasers, liquid lasers, tunable die lasers, organic lasers, chemical lasers, and most importantly, semiconductor diode lasers.

Based on small semiconductor chips, the semiconductor diode laser (laser diode) has revolutionized our world and found its way into almost every aspect of our lives. Semiconductor laser diodes can be made which, at room temperature, radiate light with wavelengths (colors) from 420 nanometers (violet) to 1550 nanometers (infrared). A nanometer (nm) is a billionth (10-9) of a meter, about 39 billionths of an inch. It is semiconductor laser diodes operating in the near infrared region (700 nm - 1550 nm) that are most often used in laser eavesdropping systems.

In 1992, almost 33,000,000 infrared laser diodes were sold for use in fiber optic communications, optical memories, bar code scanners, laser printers, and other applications. The economies of scale are evident. The price of lasers has dropped to the point that anyone can afford one

Anyone can buy a laser pointer to enhance their presentations for $100 or a laser gun sight toimprove their aim for $200. Laser diodes (either red or infrared) can be bought for $20 to $30 apiece from almost any electronics parts distributor.

Cheap laser diodes make laser eavesdropping systems, once prohibitively expensive and used only in intelligence operations and very high stakes industrial espionage, affordable and usable by anyone with an urge to listen.

For $200, anyone can buy a very serviceable laser eavesdropping system by mail from ads in the back of magazines like Popular Science and Popular Electronics. Naturally, more expensive and sophisticated systems are available for professionals.

Laser eavesdropping systems require either a continuous wave (CW) or a very high duty cycle laser in the transmitter and a photodetector in the receiver. While any combination of CW laser and photodetector can be used to listen, the operational advantages of using an uncooled detector and the added covertness of an invisible infrared laser beam coupled with the low cost and easy
availability of such laser/detector combinations are so compelling that almost all laser eavesdropping systems use a near-infrared laser diode and either a silicon or indium gallium arsenide photodetector.

Laser eavesdropping systems operating in the near-infrared region use lasers which radiate an invisible beam with a wavelength somewhere between 700 and 1550 nanometers (nm).The reflected beam is received by either a silicon photodetector operating between 400 nm and 1100 nm or an indium gallium arsenide photodetector operating between 800 nm and 1700 nm.

To use a laser eavesdropping system to monitor a conversation, the eavesdropper simply points the laser eavesdropping system laser at a window of the room in which the target conversation is occurring and illuminates the surface. A window is preferable to other surfaces because it is generally flatter and more reflective than other building material. The eavesdropper then centers the reflected laser beam on the laser eavesdropping receiver. The basic rule of optics (the angle of incidence equals the angle of reflection) governs the
placement of the laser and the receiver. Normally, it is best to position both the laser and the receiver at a right angle to the window.

The window behaves like a microphone. In a microphone, sound vibrates the plates of a capacitor and causes output voltage fluctuations which can be manipulated electronically and later reconstituted as sound. Similarly, sound vibrates the surface of the window and produces interference patterns in the reflected laser beam. The photodetector in the receiver converts these interference patterns to voltage fluctuations which are electronically manipulated and reconstituted as sound.

Once the sound has been converted to an electrical signal, it is irrelevant whether the signal was generated by a microphone or a laser receiver.

The effective range of a laser eavesdropping system is determined by the characteristics of its laser, its system optics, its photodetector, and the weather. Most cheap systems work at ranges out to 100 meters.Better systems can work at longer ranges.

It is not easy to protect your conversation from the laser eavesdropping system. During the 1970's and 1980's, security personnel developed standard techniques to attempt to counter laser-bounce and maser-bounce listening systems. Most techniques involved trying to mask conversations by beaming noise of various types at the windows of a conference room.

Recent improvements in electronic and computer enhancement / signal recovery
techniques have rendered "window shakers", beaming noise at the windows, and other similar jamming techniques obsolete.

Modern electronics routinely allow the recovery of conversations 30 dB below (1000 times weaker than) the background noise level. With specialized commercial equipment, it is possible to recover information that is 60 dB below (1,000,000 times weaker than) the background noise.

Modern sophisticated computer enhancement techniques make it possible to recover a conversation even if the background noise is billions of times louder than the conversation. In fact, military and intelligence communications systems typically require signal isolation levels significantly greater than 110 dB (100,000,000,000) to prevent the enemy from recovering voice or data which might be unintentionally transmitted.

If you can't protect your conversations against laser eavesdropping systems by electronic means, how can you maintain confidential information without abandoning all rooms with exterior walls and windows?

The answer is to detect the presence of the laser eavesdropping system and then take whatever action is required - moving the conversation to a room with only interior walls, dispatching a security team to find and remove the surveillance equipment, etc.

Normal counter-surveillance equipment, sweepers, bug detectors, etc., are designed to detect electronic systems within the room and will not detect the presence of the infrared laser beam.

Insight Technologies offers an inexpensive Laser Detector which can detect the laser beam of a laser eavesdropping system and notify security of the attempted

What They Are

Laser eavesdropping systems are covert listening devices.

Laser eavesdropping systems are covert because they work from a distance and provide no electronic clues to security personnel that the conversation is being monitored. There are no radio transmissions from the building, no electronic variations in phone transmissions, no electronic equipment or bugs of any kind in the room being monitored. There is only an invisible beam of laser light reflecting off the outer surface of a window.

The laser eavesdropping system contains two basic elements, a laser and a laser receiver. Most laser eavesdropping system lasers radiate in the near infrared spectrum between 700 nm and 1550 nm. The reasons for this are straightforward.
A laser which radiates in that spectrum has an invisible beam with good atmospheric
transmission characteristics. Operation in that spectral region is compatible with the use of
inexpensive, uncooled silicon or indium gallium arsenide detectors in the receiver.

The rationale behind selection of a near infrared laser is equally straightforward. Covert operation requires an invisible beam. This eliminates lasers which operate in the visible light
spectrum. Atmospheric absorption factors eliminate ultraviolet lasers. Therequirement that the laser be portable eliminates many of the more exotic lasers and further limits the choice to the infrared spectrum.

Infrared radiation is often associated with heat. This is somewhat ironic because the longer the wavelength of light, the cooler the "blackbody" radiator which produced it. Infrared light is significantly "cooler" than visible or ultraviolet light. For this reason, night vision devices which
are intended to detect the "heat" from living beings operate in the mid-IR(10,000 to 14,000 nm)
region and air to air missile seekers which home in on the hot metal parts ofjet engines operate in the near-IR (1,000 to 4,000 nm). This also means that the longer the wavelength of the laser light, the more potential sources of interference ("noise") exist.

For a laser eavesdropping device, it is important to design the system to avoid as much interference or "noise" as possible. Consequently the shorter wavelengths of lasers in the near IR region, make them preferable to lasers in the mid and far IR regions. By process of elimination, the laser of choice for laser eavesdropping devices is a semiconductor laser diode operating in the near IR (between 700 nm - 1550 nm).

The selection of a near-IR laser is reinforced by the choice of detectors for the receiver. The longer wavelengths typically require cryogenically cooled detectors for optimum sensitivity, while the near infrared spectrum can be received by either silicon or indium gallium arsenide
uncooled detectors. Both silicon and indium gallium arsenide detectors are readily available, relatively inexpensive, reliable and effective in the near IR spectrum. Silicon detectors typically operate in the spectrum of 400 nm to 1100 nm. Indium gallium arsenide detectors operate in the spectral range of 800 nm to 1700 nm.

The physical arrangement of a typical laser eavesdropping system is as follows:

Laser: The laser optical package will be mounted on a stable platform, similar to a camera tripod or a surveyor's transit. The laser optical package will consist of the laser, its power supply (usually batteries), focusing optics, and aiming optics. The complexity of this element is a function of the sophistication and seriousness of the eavesdropper. Some mail order systems use a rifle scope sight arrangement to point the laser.

Receiver: The receiver will also be mounted on a stable platform. It could be collocated with the laser transmitter or at a separate location. The location is dictated by the geometry of the target area. The receiver contains collection and focusing optics, filters, the detector, and supporting electronics. The complexity and function of the electronics package is dependent on the sophistication and seriousness of the eavesdropper. In the simplest case, the detector package simply converts the reflected laser interference patterns to sound and feeds it to earphones and a
tape recorder. More sophisticated devices contain signal conditioning electronics.

How They Work

Laser Eavesdropping Devices work because of the two unique characteristics of laser light - laser light is monochromatic and coherent.

Monochromatic means that all the light waves in a laser beam are exactly the same color and hence, have exactly the same wavelength. This is important because it allows the use of a monochromatic filter on the receiver to filter out all light with color different than the laser beam. This greatly simplifies signal reception. It is the equivalent of being in a crowded football stadium and having a device which filters out all the sound except the person to whom
you wish to talk.

From a system design perspective, the ability to use a monochromatic filter greatly improves the signal to noise ratio at the detector and allows the use of lower power lasers. This makes the system smaller, cheaper, and more practical.

It is the added element of coherence which makes laser light so special. What coherency means is that all the light waves radiated by the laser are in phase. A beam of light can be represented as an electromagnetic wave.

An electromagnetic wave is comprised of two varying fields, an electric (E) field and a magnetic (H) field sinusoidally varying in amplitude at right angles to the direction of propagation of the light wave.

For most purposes, the amplitude of the light wave is given by the amplitude of the electric (E) field. The distance between two successive points of maximum amplitude of the E field is called the wavelength l.

At a given point along the wave the amplitude y is normally represented by the equation y = a sin(wt - kx) where w is the angular frequency (w= 2pl/c), c is the speed of light, t is time and k is the propagation number or wave number (k= 2p/l). The phase at any given point along the wave at any given time is defined by the quantity (wt - kx).

The absolute phase is not particularly important. What is important is the relative phase between two light waves. This is because when light waves are superimposed they interfere with one another and add either constructively or destructively depending on the relative phase.

That is, when the waves are "in phase" they reinforce each other and create a greater amplitude and when they are completely "out of phase" the two waves cancel each other out. The patterns of light and dark which result from the adding of light waves are called interference patterns and are the basis on which laser eavesdropping devices work.

A laser emits coherent light. Laser light is in phase. There is no destructive interference in the beam. Although laser beams can be collimated, the beam in a laser eavesdropping system is typically focused so that the beam has a diameter of about a foot when it hits the target window.

The beam is reflected from the window and the individual light waves add at the receiver to create an interference pattern. The reason that an interference pattern exists is because as the beam diverges on the way to the window and back to the receiver, the light waves in different parts of the beam travel different distances.

The round trip distance of a light wave at the edge of the beam from the laser to the window to the receiver could easily be several feet longer than the round trip distance of a light wave at the center of the beam. This means that the time it takes to reach the receiver collection optics is different for light waves in different parts of the laser beam.

Since phase is time dependent, waves with different reflection paths may have different phases when they reach the receiver focusing optics and an interference pattern detectable by the receiver is created.

The laser beam is focused on the window. Sounds are vibrations in the atmosphere. Sound vibrations in the room hit against the window and cause it to vibrate. As the window vibrates, the relative phase of the light waves being reflected change depending on the changes in the
distance between the receiver and the individual reflecting element and the angle of reflecting surface. The result is a change in the interference pattern at the receiver which corresponds
to the vibration of the reflecting surface.

The detector converts these fluctuations in light intensity into voltage fluctuations which can be reconstituted as sound.

Some points should be noted at this point. The vibration of the window required to affect the interference pattern at the receiver is small, less than half a wavelength. The wavelength l of the laser light used in laser eavesdropping systems is somewhere between 700 nanometers and 1550

That means that part of the window surface need only move a little over a hundred thousandth of an inch to have a significant impact on the interference pattern at the receiver.

How to Protect Yourself From Them

The only way to protect yourself from a laser eavesdropper is to move the conversation which you wish to protect to an interior room. However, if you wish to use rooms with windows for meetings and normal conversations, you need to be able to detect the presence of the laser eavesdropping system laser beam.

Insight Technologies has developed a laser eavesdropping system detector which will detect the presence of laser beams in the near infrared and sound an alarm.

The Insight Technologies laser detectors are compact devices (approximately 2 inches by 4 inches by 1 inch) which hang in the center of a vulnerable window. The device runs on a 9 volt battery. The laser detector has three modes - On, Off, and Test. In the Test mode, a standard
television remote control can be used to verify device operation.

To use the device, look out each of the windows in the room which you wish to protect and determine whether or not there are potential locations from which an eavesdropper could aim a laser at the window. If there are, then hang an Insight Technologies laser detector in the center of the window.

When eavesdroppers set up a laser eavesdropping system, they first mount the laser on a stabilized platform and aim it optically at the target window. At this point the beam is fairly wide. The receiver is then aimed at the window to intercept the reflected beam. After the receiver is centered on the reflected beam, the eavesdropper will then tighten the focus of the
laser beam for optimum sensitivity.

The initial laser beam will cover most of the window. The final beam will be about a foot in diameter and will be aimed at the center of the window formaximum sensitivity. Both beams are easily detectable by the Insight Technologies laser detector.

As soon as the eavesdropper starts to focus the laser beam on the window, the Insight Technologies laser detector will detect the presence of the beam and sound an alarm. The system will also light a red light which will stay lit after the alarm cycles off to notify you that the eavesdropper focused the laser eavesdropping device on the window. When the alarm
sounds, either move the conversation to a safe room or send a security team to remove the laser eavesdropper.


Any room with windows is vulnerable to an eavesdropper with a laser eavesdropping device. The eavesdropper, from a remote location, can listen to and record any conversation in the room.

The operation of a laser eavesdropping system is completely undetectable by electronic sweepers and standard countersurveillance equipment. The only indication of the operation of a laser eavesdropping system is an invisible laser beam reflecting off the exterior surface of a room window.

Laser eavesdropping systems are cheap and readily available. A basic laser
eavesdropping system can be bought by mail for $200 by anyone with an urge to listen.. Systems with custom lasers, optimized receivers, more complex optics and filters, and more sophisticated signal processing electronics are readily available to professionals willing to pay for performance.Security people understand the electronic eavesdropping (bugging) threat and have taken steps
to effectively counter it. The familiarity of the battle against the electronic eavesdropping threat provides a false sense of security.

Laser eavesdropping systems are covert, readily available, effective, and completely undetectable by standard anti-bugging devices and procedures and as such represent a new dimension in the assault on privacy.

Advances in signal processing technology and the ready availability of powerful personal computers make traditional "window-shaker" countermeasures obsolete and ineffective.

The only effective method of countering a laser eavesdropping system is to detect the presence of the invisible laser beam on the surface of the window and move the conversation to
be protected to an interior room while other measures are being taken.

Insight Technologies has developed a laser detector system which can detect and warn of the operation of laser eavesdropping systems and can let you protect your privacy.

For additional information, please write to Insight Technologies, 10235P Spartan Drive, Cincinnati, Ohio 45241.