GB2361058A

GB2361058A - Optical intruder detection system

Info

Publication number: GB2361058A
Application number: GB0029099A
Authority: GB
Inventors: Paul David Townsend
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1999-03-17
Filing date: 2000-03-15
Publication date: 2001-10-10
Anticipated expiration: 2020-03-15
Also published as: GB2361308A; FR2801401A1; GB0029099D0; FR2801400A1; FR2801400B1; GB0029101D0; GB2361308B; GB2361058B

Abstract

The intruder detection system, suitable for securing a room, has an optical signal source 1 and a number of spatially separated collection points 3 which act as inputs to a sensor system. The optical signal is scattered before it is collected and sensed. A processor triggers an alarm in response to a change in the sensed optical signal. The optical signal may be pules of laser light which may be sent to a scattering target to produce scattered optical signals. The collection points 3 may be connected to one sensor via an optical network (fig 2,3) which may include different optical delay lines (fig 2,3). The sensor may be a single photon detector such as an avalanche photo diode. The optical signals may comprise a plurality of different wavelength signals (fig 4), and the system may include wavelength multiplexers.

Description

2361058 1 Detection System The present invention relates to an intruder

detection system, and in particular to a system that uses an optical signal to detect the presence of an 5 intruder.

It is well known to construct an intruder detection system by aligning an optical source and an optical detector, and then registering any interruption in the optical signal when an intruder passes between the source and the detector. A significant weakness in conventional systems is that it is possible for a would-be intruder to observe the path of the optical signal, for example as a result of scattering of the signal by particles in the atmosphere. This makes it possible for the intruder to avoid the path of the optical signal, or to employ countermeasures to avoid detection. Even if the optical signal is in the infra-red domain, and so is not directly visible, it can still be observed if the intruder uses appropriate aids such as night vision binoculars.

According to a first aspect of the present invention, there is provided a method of intruder detection comprising:

a) outputting an optical signal b) detecting the optical signal; and c) triggering an intrusion indication in response to a change in the detected optical signal; characterised in that step (b) includes receiving scattered optical signals at a plurality of spatially distributed optical inputs.

The present invention makes it far harder for an intruder to observe and avoid the path of the optical signal, by detecting scattered signals at a number of different locations, instead of merely detecting a localised beam.

Preferably the method includes transmitting optical signals from the plurality of spatially distributed optical inputs to a common detector. Preferably the method includes applying different respective delays to signals received at different optical inputs.

These preferred features of the invention make it possible to realise the advantages of multiple detection points without significantly increasing the cost of the system. This is achieved by using a common detector for the different inputs.

1 2 Preferably the method includes detecting the optical signal using a singlephoton detector.

Use of a single-photon detector allows detection even of very faint signals resulting from scattering, making it difficult for the scattered signal to be observed.

Preferably the optical signal is a narrow-band signal, and the step of detecting the optical signal includes passing the optical signal through a narrow-band filter matched to the narrow-band source. This further aids discrimination of a faint optical signal in the presence of ambient light.

According to a second aspect of the present invention, there is provided an intruder detection system comprising:

a) an optical signal source b) an optical detection system; and c) a processor connected to the optical detection system and arranged to trigger an intrusion indication in response to a change in the detected optical signal; characterised in that the optical detection system (b) includes a plurality of spatially distributed optical inputs.

Systems embodying the present invention will now be described in further detail, by way of example only, with reference to the accompanying drawings, in which:

system; system.

Figure 1 is a diagram showing a first example of an intruder detection Figure 2 is a diagram showing a second example of an intruder detection Figure 3 is a diagram showing a third example of an intruder detection system.

Figure 4 is a diagram showing an intruder detection system employing wavelength multiplexing.

Figure 5 is a diagram showing an optical detection system employing a 30 single-photon optical source in combination with wavelength multiplexing.

As shown in Figure 1, an intruder detection system comprises a source 1 of short multi-photon pulses. The source 1 outputs a beam B that is directed at a scattering target 2. In this example, the source 1 is mounted close to floor level on a 3 wall and the scattering target 2 is mounted on a ceiling of the room in which the detection system is deployed. The scattering target scatters the photons generally through the room in three dimensions. A number of optical input ports 3 are fixed to the walls of the room in different locations. The optical input ports comprise beam collection optics connected to a respective optical fibre. Each optical fibre forms one branch of a passive optical network (PON). The different branches of the passive optical network includes different respective optical delays, so that optical signals from different optical input are separated in the time domain. The optical delays may be implemented by loops of optical fibre of differing lengths.

The passive optical network is connected at its head end to an optical detection system. The optical detection system comprises a narrowband filter and a photo detector. In the present example, the photo detector is a single-photon detector. The narrowband filter is matched to the wavelength of the optical source. The output of the single-photon detector is processed by a personal computer 6. The personal computer 6 has an electrical connection to the optical source 1 and receives electrical synchronisation pulses from the source 1. In operation, the count rate of detection events in the single-photon detector that are synchronised with the production of the pulses by the source 1 is measured. In the computer 6, the count rate is compared with pre- determined upper and lower threshold values. If these threshold values are exceeded, for example because an intruder has entered the room and in so doing has reduced the level of light reaching one or more of the detectors, then an intruder alarm is triggered. The threshold levels for intruder detection may be set by running the detection system in a training phase during which the computer measures the count rate and the variation in the count rate in the absence of any intruder. This then establishes the bounds for possible variations in the count rate and any departure from those bounds is taken to indicate the presence of an intruder. Optionally, in order to further enhance the sensitivity of the system while maintaining an acceptably low rate of false alarms, the computer may use a heuristic algorithm to learn the expected profile of the count events in the absence of an intruder. The heuristic algorithm may use a neural-network technology.

The components used in the system and the operation of the system will now be described in further details.

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4 The optical source 1 in this example is a pulsed semiconductor laser. An appropriate device is available commercially as the PIDL 800 (PicoQuant GmbH). The semiconductor laser is driven by a pulse generator.

The single-photon detector may be, for example, an avalanche photo-diode (APD) biased beyond reverse breakdown and operating in Geiger mode in order to achieve single-photon sensitivity. Silicon APIDs such as the SPCM-100-AQ (EG and G Opto-Electronics Canada) can be used in the 400106Onm wavelength range, while germanium, or InGaAs devices such as the NDL5131P1 (NEC Corporation) or C30644JT (EG and G Opto-Electronics Canada) can be used in the 1000-1550nm wavelength range with appropriate avalanche quenching circuitry. The narrowband filters used at the detector may be fabricated using photosensitive Bragg grating technology, or micro-optic thin film dielectric filter technology (e.g. TB1300A manufactured by MS-Fitel).

The personal computer (PC) is a commercially available system using, for example, a Pentium Ill (trade mark) microprocessor. The computer includes a photon counting card such as the TimeHarp 100 (PicoQuant GmbH) that processes the output of the APD. The card returns a value for the photon count rate to an intruder detection application running on the PC.

Figure 2 shows a second example of a system embodying the present invention. In this case, the multiple optical input ports 3 of the system of Figure 1, are replaced by multiple combined inputloutput ports 3. As in the first example, the optical ports are connected to a passive optical network (PON) having a tree topology. The passive optical network is connected at its head end to a pulsed optical source 1. Each input/output port directs a beam of light towards a scattering target 2. Scattering from the target and possible further backscatter from the walls of the room andlor objects in the room creates a complex web of sensor paths. The scattered light is received at the multiple inputloutput ports and is transmitted via the passive optical network and via an optical circulator to an optical detection system 5. The detection system 5, as in the first example, includes a single-photon detector and a narrowband filter. The output of the detection system is processed by a personal computer running the intruder detection application, as described previously with regard to Figure 1. An appropriate circulator is available commercially as model 0803-A (New Focus Inc.).

Figure 3 shows a third example of the system embodying the present invention. This example is generally similar to the system of Figure 2 except that the combined input/output ports of Figure 2 are replaced by separate ports for the optical output and the optical inputs respectively. The optical output ports are distributed at different spatial locations and are connected via a first passive optical network to a classical source. The optical inputs are connected via a second passive optical network to the optical detection system described previously with respect to Figure 2.

In the examples so far considered, the optical signals suffer loss on their way through the return PON. That is to say, if we have e.g. 10 sensors (11:10 splitter) there will be a 10d13 loss for each photon entering the receiving PON. With large numbers of sensors this would limit the received count rate and therefore reduce the efficiency of the system. However, if the collection of ambient light is not a limiting factor in the system, a broader bandwidth source (and matched filter) may be used together with WIDIV1 (wavelength division multiplexing) techniques to reduce the path loss for the individual sensors. For example, if we use a source with central wavelength L and a wide bandwidth, dL, the splitter may be replaced by an arrayed waveguide (AWG) multiplexer that divides the light on the single input fibre onto e.g. 10 output fibres each carrying a distinct wavelength of bandwidth -dUlO (i.e. each sensor now operates at a different wavelength spread over the range L-dU2 to L+dU2). The receiver PON will now contain another AWG instead of a splitter that recombines the 10 wavelengths onto a single output fibre that connects to the detector. In principle the loss in each return path can be reduced to zero using this technique, although in practice the improvement will depend upon the loss of the AWG (these are widely available components commonly used for multiplexing and demultiplexing in dense WIDIV1 communication systems). Short pulses are still used in this implementation to preserve the time resolution, but the pulses are now strongly chirped to provide the larger bandwidth required by this implementation. An appropriate AWG device is available commercially as AWG-MD-1x32 from Photonic Integration Research Inc.

Figure 4 shows a detection system employing wavelengths multiplexing as described above. The detection system is generally similar to that of Figure 2, in that it uses combined optical outputlinput ports. However, in place of the splitter used to combine signals from the branches of the optical network in the system of Figure 2, an arrayed waveguide multiplexer (AWG) is used. For ease of illustration, the Figure shows 6 only four of the branches of the optical network and a corresponding four wavelegth channels. In practice, more branches and more channels may be employed. For example, ten channels may be used as described above. Note that in this implementation sensors 1 -10 each have distinct output wavelengths L1 -1-110. Any given sensor (number 1 for example) may receivelcollect scattered photons with wavelengths in the range L1.110, since these photons could originate from any of the sensor outputs. However, because of the wavelength filtering effect of the AWG only photons of wavelength Ll received by sensor 1 will reach the detector. That is to say, each sensor will only 'see' its own returning photons. So, unlike the previous embodiment, no cross-talk between sensors is 10 possible and hence the number of possible detection paths is reduced.

The AWG device described above is also suitable for use in the systems of our co-pending application G139906208.5 filed 17 March 1999 the contents of which are incorporated herein by reference. Figure 5 shows a system of this sort in which a chirped single-photon pulse source is coupled via a first AWG and a corresponding first passive optical network 8a to produce multiple free-space beams. The beams are received at corresponding optical input ports, are transmitted via a second passive optical network 8b and coupled via a second AWG to a single optical fibre 4 that passes to the optical detection system 5.

7

Claims

2.

6.

7.

8.

1 A method of intruder detection comprising:

(a) outputting an optical signal; (b) detecting the optical signal; and (c) triggering an intrusion indication in response to a change in the detected optical signal; characterised in that step (b) includes receiving scattered optical signals at a plurality of spatially distributed optical inputs. A method according to Claim 1, including transmitting optical signals from the plurality of spatially distributed optical inputs to a common detector. A method according to Claim 2, including applying different respective delays to signals received at different optical inputs. A method according to any one of the preceding claims including detecting the optical signal using a single-photon detector. A method according to any one of the preceding claims, in which the optical signal is a narrowband signal, and the step of detecting the optical signal includes passing the optical signal through a narrowband filter matched to the narrowband source. An intruder detection system comprising: (a) an optical signal source; (b) an optical detection system; and (c) a processor connected to the optical detection system and arranged to trigger an intrusion indication in response to a change in the detected optical signal; characterised in that the optical detection system (b) includes a plurality of spatially distributed optical inputs. A system according to Claim 6, including a plurality of spatially distributed optical outputs. A system according to Claim 7, comprising a plurality of combined optical inputfoutput ports.

8 9. A system according to any one of Claims 6 to 8, in which the plurality of spatially distributed optical inputs are connected in common to a single optical detection system., 10. A system according to any one of Claims 6 to 9, in which the optical detection system includes a single photon detector.

11. A method of intruder detection comprising:

(a) outputting an optical signal; (b) detecting the optical signal; and (c) triggering an intrusion indication in response to a change in the detected optical signal; characterised in that in step (b) the signal is detected by a single-photon detector.

12. A method of intruder detection comprising:

(a) outputting an optical signal; (b) detecting the optical signal; and (c) triggering an intrusion indication in response to a change in the detected optical signal; characterised in that step (a) includes outputting optical signals at a plurality of different wavelengths and in step (c) signals from a plurality of spatially distributed input locations are coupled to a common detector via a wavelength multiplexer.

13. A method according to any one of claims 1 to 5 in which:

step (a) includes outputting optical signals at a plurality of different wavelengths and in step (c) signals from a plurality of spatially distributed input locations are coupled to a common detector via a wavelength multiplexer.