GB2108743A - Laser security system - Google Patents

Abstract

A weather proof laser security system for protecting property whether outdoors or indoors, underwater or in space, in which a laser beam is coded and split to form a laser fence having multiple strands, each strand beam propagating a distance consistent with the prevailing weather conditions such that the beams irradiate a detector array. An alarm is activated when two or more beams are blanked. Changing weather conditions are compensated by increasing or decreasing the number of posts in the laser fence. <IMAGE>

Description

SPECIFICATION Coded, weather proof laser security fence This invention relates to a weather proof laser security system, with coded laser beams, for the protection of valuable property situated outdoors, indoors, in vauits, underwater and in space, consisting of laser beam generators, arrays of laser beam reflectors, arrays of laser beam detectors, random code generators and detection systems, alarm control units, interconnecting cables, standby power units, fixed posts containing said components and adjustable posts containing said components, weather monitors and light meters. When used outdoors, my invention adjusts automatically to bad weather conditions by increasing the number of posts in said laser security system so as to compensate for the increased attenuation of said laser beams by the deteriorating weather.When used indoors, my invention is protected from detrimental weather conditions either by mounting within individual walls of said buildings or with the cavities formed by the outer and inner walls, double layer ceilings, double glazed windows and appropriately drilled doors and their associated frameworks of said buildings. When used underwater my invention responds to the transmissivity of the water whilst space deployment is equivalent to indoor operation provided adequate precautions are taken to ensure that direct sunlight does not affect the detector arrays.

Any intruder, or intruding object, intercepting one or more of the said laser beams forming said laser fence, sets off the alarm by reducing the intensity of said laser beam which then causes a change in the current flow from the optical detector arrays. The said current flow is then conducted via cables to the control and alarm units. However, in order to operate with a realistic false alarm threshold in terms of birds, moths, dogs, and cats it is arranged that two or more laser beams within the fence must be intercepted in order to set off the alarm although once a single beam is intercepted the situation is monitored and the alarm will be activated if the situation persists. If the intrusion is in the form of smoke then my invention also acts as a fire alarm system.

My invention can also generate laser beams that bypass the next post in the sequence to be detected by the following or other posts. In this way it is possible to cover any intrusion of the main fence, for example by parachutists or exotic tunnulers.

Background of the invention This invention has benefited from experience gained with outdoor range testing of early laser ranging devices and the emphasis that had to be placed on the provision of safety boundaries between which the laser could only be fired. The problem of providing such a security boundary naturally lead to consideration that this field was yet another application of lasers generated by the laser itself. However, it was soon realised that using the laser as a safety fence was a relatively simplistic application compared to using the laser as a security fence because in the former the intrusion would be accidental whereas in the latter case the intrusion could be directed by a superior laser specialist than the one designing the fence in the first place.The crucial analogy is the penetration of an electrified submarine net by clipping on various jump leads then snipping off a given section. The optical equivalent is to insert thin laser beam reflectors in to the laser security fence beams, analyse the faintly reflected laser beam then gradually inset an optically bypass system so that a doorway can be made in the said laser security fence without affecting the intensity distribution and levels on the detector arrays in the process. If follows that a fool-proof laser security fence is relatively easy to construct and all prior art laser security fences can be considered foolproof, at least in good weather, in the sense that a fool would simply walk straight through the fence without realising it existed.

Unfortunately, what is required is a genius proof laser security fence in the sense that to penetrate such a security fence would cost much more than the value of the object being protected. For example, if a laser security fence was used in a bank to secure a million dollars, then it should cost at least one million dollars for any potential thief to penetrate the fence. To do so would require a high technical understanding of laser, optical and electronic equipment and also a very high level of understanding of physics. In short the fundamental difference between my invention and prior art laser based security systems is simply the very wide gap in scientific expertise and hence costs required even to attempt to penetrate said fence.

It was very clear from early laser ranging tests that bad weather and lasers simply did not go together with the absorption and scattering coefficients making the exponential diminution of laser beam intensity with distance a reality. In other words the only way to beat the bad weather exponential is to move to laser beam generators and detectors closer together. Unfortunately it is not possible to do this with a security fence which may extend outdoors for many kilometers. Either one has to build a close pole fence which would look rather odd in clear weather when the pole to pole operating range was several kilometers.

Under the worse operating conditions the line of sight visibility is down to about 100 meters. If visibility is less than this distance then either more laser fence poles would have to be deployed or valuable merchandise should never be stored in such a locality unless special foot patrols can be guaranteed during extreme weather conditions. It is often imagined that the intruder should not be aware of the laser fence and therefore atmospheric scattering during bad weather would be very detrimental. This is not the situation that is first imagined because the scattered light itself is also affected and is more difficult to detect than a similar level of such light being transmitted through a clear atmosphere.On the other hand, in a genius proof laser security fence there is simply no point in trying to hide the beams from the potential intruder, in fact the opposite should be the case because it would be a challenge that could not be met. On the other hand, on a clear day, a dense row of laser fence posts through the countryside would be an eyesore equivalent to a complex wire fence and both should be avoided.

Therefore pop-up poles which only appear in bad weather and therefore cannot ever be an eyesore, have an advantage both from the asthetic and security viewpoints.

With indoor uses of the invention weather problems can be eliminated and it must be made as difficult as possible for the potential intruder to know whether or not a laser security fence is installed. Ideally the laser fence should be installed so as to propogate along the wall cavities, through a ceiling cavity, underfloor cavity, between the panes of double glazed windows, through doorframes and the doors themselves so as to enclose the whole building. It has often been the case the the windows and doors of banks have been ouffitted with extensive security devices and the thieves simply dig a hole through the wall from next-door, steal the valuables and are away a long tinie before the theft is discovered.

In underwater applications of the invention the mean visibility is determined and the laser fence set accordingly. On the other hand the use of the invention in space provides for operation in an ideal environment provided precauations are taken to ensure that direct sunlight is not influencing the detector arrays.

Description of the prior art Laser based prior art systems come in several configurations, all of which have the advantage over microwave security systems in that they can be used near airports where the microwave background noise has to be minimised.

Furthermore, noise threshold levels can be high making laser based systems superior to acoustically based systems. In banks, infra-red surveyance systems are popular and are sensitive to one degree temperature difference or less. A sophisticated bank robber should outfit himself with a suit of aluminium foil to even his body temperature relative to the environment in order to overcome the distinguishable one degree difference in temperature required by infra-red detectors.

Prior art laser based security systems came in two forms, either a fixed distance was maintained between transmitter and detector or the transmitter was traversed like a narrow beam searchlight from one detector to the other. All of these prior art systems are weather affected and become inoperable in bad weather. Furthermore, they cannot distinguish between a large and small intruder. Yet another defect with prior art laser based security systems is the ease with which they can be penetrated. In the case of the moving beam system, the intruder simply uses a laser viewer and sidesteps the beam. In the case of the fixed beam systems one again uses a laser viewer, locates the position of the beam, locates the position of the detectors, shines an equivalent power onto the detectors as they shield the laser beam.

Summary of the invention The subject invention is an apparatus for generating and directing a series of coded laser beams of low divergence onto an array of optical detectors at a distance determined by the weather conditions prevailing at the time. The apparatus is set such that if a single beam of the laser fence is blocked, then the alarm does not go off until the nature of the blockage can be determined, if more than one beam is blocked then the alarm goes off instantly. The propagating beams can be made to follow a given path by appropriate reflections off an array of passive mirrors.

One object of this invention is to provide a weather-proof laser security fence which has a minimum of visible components at all times. A particular example would be to transmitterdetector posts separated by five kilometers on a clear day whilst the post separation automatically decreased to as low as one hundred meters in bad weather.

Another object of the invention is to enclose the inside of a building or bank vault with a network of laser beams, which if any one is blocked the alarm is put on standby and the situation assessed whilst if any two are blocked the alarm goes off instantaneously.

Another object of the invention is to provide a series of randomly coded laser beams via the rotation of the beam polarisation vector.

Another object of the invention is to provide a randomly coded intensity modulation of the laser beams forming the security fence.

Another object of the invention is to provide means of directing one or more of the generated laser beams in a non parallel direction to the other laser beams either within the plane of the laser fence or at any angle to it.

Another object of the invention is to provide a laser security fence with alternate beams, or a randomly selected number of beams, counter propogating between posts, emitting the same or different wavelengths.

Another object of the invention is to provide self-contained operation which is independent of external sources for 24 hours, and is automatically switched to such internal power source in the event of external mains failure, intentional or otherwise.

Another object of the invention is to provide backup components in case of failure leading to fence breakdown.

Broadly this invention provides apparatus for generating, selecting, directing, coding, detecting and maintaining a series of laser beams to form a laser security fence comprising: means of generating a laser beam via a gas laser tube, semiconductor laser solid state laser or other suitable lasing medium; means of selecting sub beams from the primary laser beam and directing them onto the detector arrays; means of coding said generated laser beams by rotation of polarisation and intensity modulation in a predetermined and random manner; means of collimating the said laser beams so that maximum intensity and minimum field of view is maintained at the detector end; means of directing laser fence to follow given path using an array of passive mirrors.

In a preferred embodiment this invention provides an apparatus for generating, monitoring and maintaining a laser security fence which can completely enclose a building, surround an outdoor site to a height of three meters, underwater sites and locations in outer space. The invention is considered to be both fool and genius proof in the sense than an ignorant thief would be caught immediately he intercepted the laser fence whilst the genius would have to utilise very expensive equipment and expertise to intercept the fence with only a remote chance of not activating it.

In its simplest configuration the invention consists of a Helium-Neon laser tube run off a 12 volt battery, with its output beam being expanded to five cms and therefore collimated to better than one ten thousandth of a radian before which it has passed through a rotatable polariser, and after which it passes through a series of beam splitters so that eight laser beams of approximately equal intensities are generated as a parallel fence and propagated towards the detector post where each of the said laser beams irradiates its particular detector and only that detector. The output of the detector array is connected to the control unit in such a manner that if one of the said laser beams is blocked the alarm does not go off.In other words one can wave an arm up and down through the laser fence intercepting all beams in turn but nothing happens but the alarm goes off if one cuts two or more beams with the body.

Used as a bank security and fire alarm system the laser fence is generated and detected at as few sites as possible. For the wall, windows, doors laser fence the beams are generated and detected at one site within the bank and guided along the wall cavities via a series of passive mirror arrays.

Brief descriptions of the several views of the drawings A better understanding of the invention will be gained from the following description taken in conjunction with the drawings.

In the drawings: Figure 1 shows a preferred arrangement set up for weather proof operation; Figure 2 shows another arrangement with counter propagating laser beams; Figure 3 shows another arrangement with offset laser beams; Figure 4 shows another arrangement with the laser fence propogating around the wall cavities of a building; Figure 5 shows the laser fence traversing a double glazed window.

Detailed description Now having particular regard to the numerals on the drawings, 1 indicates the post which acts as housing for laser beam generating components, numeral 2 indicates the generator of the laser beam indicated by numeral 3. Numeral 4 indicates the laser beam intensity modulator, numeral 5 indicates the laser beam frequency modulator, numeral 6 indicates the laser beam polariser, numeral 7 the laser beam expanding telescope and numeral 8 the laser beam reflector array which produces multiple beams from the primary beam which are each of comparable intensity.

Numeral 9 indicates a standby power supply in case of mains failure. A prior art sensing element to avoid the possibility of an intruder using the post 1 as a means of penetrating the fence is mounted on each post. Alternatively the end reflector in array 8 could be made partially transmitting to allow a portion of laser beam 3 to be transmitted out of the top of post 1 and used as part of a detection means.

Numeral 10 indicates the laser security fence the spacing depending on the user requirements whilst numeral 11 indicates the detector post containing detector array indicated by numeral 12, with associated optics indicated by numeral 13.

Numeral 14 indicates a hole in the ground containing the pop-up post indicated by numeral 15, which has to contain both transmission and detection components as well as means of activating pole to its operational position via light level data supplied by both light monitors (not shown) and fence control indicated by numeral 16.

Numeral 1 7 indicates a counter propogating laser fence generated and detected using posts indicated by numeral 18. In an operational fence posts 18 would require two reflector and detector arrays.

Numeral 19 indicates the layout of the main fence posts indicated by numeral 20 while numerals 21, 22 and 23 indicate secondary fences.

Numeral 24 indicates a bank vault bounded by cavity walls indicated by numeral 25. Numeral 26 indicates double glazed windows and numeral 27 indicates doorways into the vault.

The laser fence generated in post 1 within enclosure indicated by numeral 28 is redirected by mirror array indicated by numeral 29 along cavity indicated by numeral 30. Numeral 31 indicates holes in door framework indicated by numeral 32.

It is emphasised that the above teachings are exemplary and not limitative of the scope and applicability of this invention.

A particular use of this invention is to protect valuable equipment stored or being repaired and serviced outdoors or within a large complex of buildings which have to be secured collectively. If the equipment is valued at many millions of dollars, for example large commercial aircraft, then it is necessary for the laser security fence to be such that it would be prohibitively expensive for anyone to even attempt its penetration. Once the fence is set up it has to be coded in such a manner that it will be impossible for anyone to insert an optical by-pass loop, even with inside information. This is achieved by using a computer based random number generator which controls all the beam codes based on polarisation, amplitude and frequency modulation. The more expensive the items to be secured then the more complex must be the coding.

An intruder trying to penetrate the laser fence of this invention cannot interfere in such a manner as to set off the alarm. The only thing that can be done is to break one of the beams only. This is to allow small objects such as birds to pass through without setting off the alarm. However, if any one beam in the fence is broken for a period in excess of that required to allow for the passage of a moving animai then the alarm has to be activated.

The reason for this is that an intruder could otherwise tamper with each of the fence beams in turn and take his time to try and set up an optical doorway. One way such a doorway could be attempted is to simply match each beam onto its respective detector so that a doorway is set up between the intuder and the detector post. The problem is made much more difficult with counter propogating beams.

To set up an optical doorway, the intruder would have to set a very thin beamsplitter into each of the fence beams in turn so as to determine its exact intensity, polarisation and frequency distribution. The need to insert a thin beamsplitter into the laser fence beams arises due to the fact that a few percent intensity variation in one beam relative to the others will be easily detectable in the fence control centre. With a thick beam splitter inserted into any of the beams such an intensity variation would be easily detected and the alarm sounded.If the intruder succeeds in inserting the thin beam splitter into one of the fence laser beams without setting off the alarm, then he would be able to extract a portion of one of the fence beams for analyses in what must be a very sophisticated and hence very expensive optical laboratory staffed by highly qualified and hence expensive optical physicists.

The reflected laser beam can then be analysed at will and all of its parameters at a particular instant determined. However, this information in itself would be worthless to any potential intruder because they would immediately know that the information varied randomly in a period short compared to that required to modify any optical circuit required to keep pace with such a modification such as not to set off the alarm. For example, if the fence was tampered with in such a way as to insert an optical dogleg doorway, then the reflectivity on the mirrors involved would be polarisation dependent. By randomly varying the beam polarisation and also the detector polarisation, it would be impossible to adjust the reflector orientation in the time required to keep up with the changes.Also, using a thin beam splitter leads to problems with keeping the reflectivity constant due to mode beating. Thus by changing the frequency components in the laser beam the reflectivity would again be randomly affected. Intensity modulation is of advantage when the intruder attempts to insert another laser beam into the fence in place of the existing beams. If the real laser beams are randomly varied in intensity it would be extremely difficult to duplicate such intensity variation using other laser beam generators. It should now be clear that prior art laser security beams are relatively simple to penetrate since there is no major problem with the insertion of a dog leg doorway through which the intruder can pass.

The coded beams of the present invention can be of such a complexity that it becomes uneconomic to penetrate the laser fence. In fact the mobile optical laboratory required to duplicate such a fence would be far more expensive than all the valuables in the protected sites.

Means as provided in each post of this invention to keep the surfaces of the optical elements at a temperature such that atmospheric moisture will not condense and distort the propogation of said laser beams.

This invention can be used to protect valuable equipment parked or stored outdoors or in a group of buildings, the secured perimeter being up to one hundred kilometers or more in length with one or more control units. Examples of such applications are airline maintenance yards, defence bases, airports, naval bases and national boundaries. Indoors, the laser fence can secure small valuable objects such as statues, paintings, jeweliry and artifacts. Also bank vaults, prisons and depositories of national estate. Underwater applications include harbour entrances whilst space applications include the security of space vehicles, orbiting space stations and communication stations.

Although the preferred embodiments of my invention have been shown and described herein, it should be clear that modification and variation may be made without departing from what is considered to be the invention.

Claims (10)

Claims

1. System for generating, coding, expanding and splitting a laser beam to form a laser fence consisting of more than two strands, each strand beam propogating a distance consistent with the prevailing weather conditions such that the said beams irradiates a detector which is part of a detector array with each detector matching a particular laser beam of said fence, such that when two or more of said beams are blocked the alarm at the laser fence control unit is activated as a result of a varying current flow in said detectors, the systems being capable of reacting to changes in weather conditions by increasing or decreasing the number of posts in said laser fence.

2. System as claimed in claim 1 where the said fence posts contain both transmitting and detection optics and electronics such that the said laser fence is formed by alternately counter propogating laser beams.

3. Systems as claimed in claims 1 and 2 where the said laser beams are coded by changing their polarisation.

4. Systems as claimed in claims 1 and 2 where the said laser beams are coded by modulating their intensity.

5. Systems as claimed in claim 1 and 2 where the main laser beams are coded by changing their frequency.

6. System as claimed in claims 1 and 2 where additional laser beams can propogate both in the plane of said laser fence and at an angle to said plane such they can be detected and activate the alarm if interfered with.

7. System as claimed in claim 1 with the laser fence propogating along the cavity between the outer and inner walls of banks.

8. System as claimed in claim 1 where the laser fence propogates between the panes of a double glazed window.

9. System as claimed in claim 1 where the laser fence propogates through holes drilled in the plane of a door.

10. System as claimed in claims 1 and 2 where laser fence can be maintained by a standby power supply in the event of mains blackout.