EP2144207A1

EP2144207A1 - Optical fiber anti-intrusion system

Info

Publication number: EP2144207A1
Application number: EP09008963A
Authority: EP
Inventors: Paolo Tagliabue; Natale Claudio Pistoni
Original assignee: GPS Standard SpA
Current assignee: GPS Standard SpA
Priority date: 2008-07-11
Filing date: 2009-07-09
Publication date: 2010-01-13
Also published as: ITTO20080535A1; IT1391026B1

Abstract

Optical fiber anti-intrusion system (10) having a luminous source (11); a luminous conductor means (12) coupled on a first end to the luminous source (11) and acting as a sensor on behalf of a pressure and/or a handling. The luminous conductor means (12) is capable of transferring a photon beam emitted from the luminous source (11) towards an end opposed to the first end of the luminous conductor means (12) in one or more propagating optical modes; a photodetector (15) is capable of detecting the presence of said photon beam coming out of the luminous conductor means (12); the optical fiber anti-intrusion system (10) possesses luminous filtering means (14) interposed between the opposed end of the luminous conductor means (12) and the photodetector (15); the luminous source (11) is supplied by an electrical signal of constant or non constant waveform; the luminous source (11) is provided with an optical isolator being capable of reducing the retroreflections in said source.

Description

The present invention relates to the field of anti-intrusion systems and, in particular, it refers to an optical fiber anti-intrusion system.
There are known anti-theft devices for the perimeter monitoring designed to detect the presence of people that are passing through areas outside or in proximity of buildings.
These anti-theft devices 1, as shown in Figure 1, are arranged on a perimeter 2 of a building 3 and are connected to a processing system 4 designed to typically activate an alarming signal when a sensor 5 is stimulated, for example by means of a pressure of a foot of an ill-intentioned person.
Anti-intrusion systems using optical fiber that are used as sensors 5 are known.
An optical fiber is typically composed of a vitreous or plastic material and comprises at least an inner layer called core (fiber core) and an outer covering layer (hereinafter called using its technical name, cladding); the core typically possesses an index of refraction higher than the cladding; in this way, an optical beam, emitted either in the visible spectrum or in the IR, is typically confined inside the core by means of successive total reflections. According to the number of optical modes that can pass inside the core of an optical fiber, there are single-mode optical fibers (SMF) and multi-mode optical fibers (MMF); these last ones present a core larger than the one of the single-mode fibers.
Over the last years, the optical fibers have been efficiently used also outside the field strictly related to data transmission (for example in optical fiber nets) as sensors designed to identify an intrusion inside the perimeter 2 of a building 3.
In particular, the document US42964513 describes an anti-intrusion system using optical fibers that uses the principle of the differential detection of the signal present in a core of an optical fiber with the one contained in the cladding of the fiber itself. The two signals are amplitude-modulated.
On the other hand, the document US 4297684 describes an intrusion detection system wherein a multi-mode optical fiber is used as sensor with detection of the changes of the speckle figure caused by the handling of the fiber itself due to the pressure of the ill-intentioned person weight. In fact, the optical fiber is positioned in the ground and the system possesses also a spatial filter substantially constituted by a pin-hole, that is to say a very small hole made on a screen opaque to the light, that permits to detect the variations together with a photodiode.
The document US 5712937 also describes the entry of a light beam deriving from a plurality of photoemitters inside a single optical fiber.
Finally, the document US 7092586 describes a system suitable for the detection of the tampering of an optical cable for telecommunication containing multi-mode fibers and wherein a laser-type optical source illuminates a single-mode type input optical fiber.
The optical fiber anti-intrusion systems of known type are characterized in that they have the disadvantage of possessing an high output optical noise, that therefore does not permit to correctly detect small variations or movements of the fiber itself and can cause missed alarms.
The purpose of the present invention is to realize an optical fiber anti-intrusion system, that is free from the above described drawbacks.
According to the present invention, an optical fiber anti-intrusion system is realized as claimed in claim 1.
The invention will be now described with reference to the attached drawings, that illustrate a nonlimiting example of embodiment, wherein:

Figure 1 shows a simplified diagram of a perimeter anti-intrusion system for buildings;
Figure 2 shows a simplified diagram of the optical fiber anti-intrusion system according to the present invention;
Figure 3 shows an optical image produced by the anti-intrusion system of Figure 2;
Figures 4a, 4b show two images obtainable through anti-intrusion devices according to the present invention.

With reference to Figure 2, the optical fiber anti-intrusion system is designated as a whole with the numeral 10.
The anti-intrusion system 10 comprises a luminous source 11 connected to a multi-mode optical fiber 12 having a core 12.1 and a cladding 12.2 that acts as sensor.
In detail, the luminous source 11 is preferably a single-mode Distributed FeedBack laser (also known as DFB laser) that comprises at least a totally reflecting mirror 11.1, a semi-reflecting mirror 11.2 directed toward the optical fiber 12, a layer of active semiconductor 11.3 and an optical grating 11.4 that realizes the distributed feedback.
In order to minimize the signal losses within the optical fiber 12, that can be also very long for covering the perimeter of a building, lasers with emission spectrum substantially centered on the third absorption window of the optical fibers, having a central wavelength of 1550nm are used. In fact, it is properly in the third window that the fibers present a lower absorption and, consequently, lower losses of optical power along their length.
The coupling between the luminous source 11 and the multi-mode optical fiber 12 is preferably made through a single-mode optical fiber interposed between them.
The coherent photon beam emitted by the luminous source 11 propagates in direction of the optical fiber 12 and within it in many modes, that is to say following different optical paths each having its own propagation characteristics; the number of the optical paths and the respective propagation characteristics of each single optical path depend on the geometrical characteristics of the fiber itself, on the emission wavelength of the photon beam and:

on the index of refraction n1 of the core 12.1 of the optical fiber 12;
on the index of refraction n2 of the cladding 12.2 of the optical fiber 12.

If the photon beam emitted by the luminous source 11 is of monochromatic type, as output of the optical fiber 12 it is visible an image 100 (Figure 3) that is not homogeneous, but characterized by the presence of a group of brilliant and dark points (Speckle image). The image 100 is the result of the constructive or destructive interference of the various optical modes that propagate within the optical fiber 12.
The deformation of the fiber 12, for example a squashing or a flexion, causes a variation of the disposal of the brilliant points and of the dark points of the image 100 and this variation is used for permitting the detection of an intrusion inside a perimeter of a building.
In fact, the optical fiber 12 is suitable for being positioned in the ground, appropriately covered by one or more cladding layers 13 designed to prevent the contamination from external agents such as humidity, or on loose links, electrically welded and in stone enclosures.
More in detail, in order to realize the detection of intrusion, a terminal part 12.3 of the optical fiber 12 is coupled to a spatial filter 14 that permits the passage of a restricted part of the luminous beam produced by the output of the optical fiber 12.
After the variation of the image 100 due to a compression and/or flexion of the optical fiber 12, a variation sensibly higher is reached in a second image 101 detected behind the spatial filter 14 for example through a photodiode 15.
The photodiode 15 substantially measures the integral of the optical energy present on the image 101 and, when the optical fiber 12 is disturbed, it measures a luminous variation that is transformed into an alarm signal by a visual and/or acoustic device.
This variation can present a threshold level both temporal and in terms of the measured variation, under which the alarm signal is not activated.
More in detail, as shown in Figures 4a and 4b, the image 101 and a corresponding image 101' obtained in a condition of perturbation have an extremely different dark and light areas ratio. The photodiode 15 substantially calculates the following integral: $I = \int_{x} \int_{t} Sc (x t) dxdt$

wherein Sc(x,t) represents the surface of the dark areas that is function of the position within the image 100 and also of the time.
In detail, the spatial filter 14 can comprise either a pin-hole filter or a section of single-mode optical fiber. By using a single-mode optical fiber of standard size as spatial filter 14, coupled to the optical fiber 12, we would obtain a size of the hole equal to the core of the single-mode fiber that is tipycally of 9 µm whereas the diameter of the core 12.1 of the optical fiber 12 (multi-mode) is of 50ö62, 5µm.
In order to minimize the losses between the single-mode optical fiber and the optical fiber 12, a fusion splice that realizes a stable and safe connection can be used.
The piloting current of the luminous source 11 is of constant or non-constant type, for reducing the noise associated to the speckle figure; in this case, an amplitude modulation with frequencies of 20-50 KhZ, for example sinusoidal, is superimposed at a constant mean value; this modulation permits to use also a second type of luminous source consisting of a Fabry-Perot laser, characterized by reduced performances in terms of spectral purity of the emitted photon beam for the presence of many longitudinal modes. This lack of purity is counterbalanced by the modulation of the piloting current.
In this case, the presence of a current modulator, that can be inside (or integrated to) the luminous source or outside it is necessary.
In fact, it has been demonstrated that the absence of this modulation causes a huge uncertainty of measurement of the change on the image 101 when the perturbation of the fiber has the frequence of 200-300 Hz up to a maximum of 1500 Hz, with the serious risk of running into false alarms or missed alarms.
The advantages of the present invention are evident from the preceding description. In detail, the optical fiber anti-intrusion system described up to this point permits to ensure an effective detection of the passage of ill-intentioned people within the perimeter of a building and a high rejection toward tipycal interferences of the systems of this type also thanks to a modulation of the current injected into the optical source.
Some variants can be applied to the device described up to this point. In detail, between the luminous source 11 and the optical fiber 12, optical isolators designed to drastically reduce the retroreflections directed toward the cavity of the optical source itself that are sources of further noises on the detected signal and therefore of uncertainty in the determination of the intrusion can be interposed.
Finally, the optical fibers 12 can be coupled in substantially parallel direction one with respect to the other in such a way as to ensure first of all a systemic redundancy (that is to say protection in case of fiber failures for example after the breaking of the photodetector) and for a higher safety of trampling or breaking in case of application on enclosures. In this case, the alarm would go off alternatively or in combination, if the first or second optical fiber are stimulated.

Claims

An optical fiber anti-intrusion system (10), having a luminous source (11); a luminous conductor (12) coupled on a first end to said luminous source (11) and acting as a sensor on behalf of a pressure and/or a handling; said luminous conductor (12) being capable of transferring a photon beam emitted from said luminous source (11) towards an end opposed to the first end of said luminous conductor (12) in one or more propagating optical modes; a photodetector (15) being capable of detecting the presence of said photon beam coming out of said luminous conductor (12);
said optical fiber anti-intrusion system (10) further having luminous filtering means (14) interposed between said opposed end of the luminous conductor (12) and said photodetector (15), characterized in that said luminous source (11) is supplied by an electrical signal of constant or non constant waveform.
The anti-intrusion system according to claim 1, wherein said luminous conductor (12) is an optical fiber having a core (12.1) and a cladding (12.2) covering said core (12.1).
The anti-intrusion system according to claim 1, wherein said luminous source (11) is supplied by an electrical current having a constant average value to which a signal of variable modulation is superimposed.
The anti-intrusion system according to claim 1, wherein said luminous source (11) is a semiconductor laser.
The anti-intrusion system according to claim 4, wherein said laser is a Distributed FeedBack laser.
The anti-intrusion system according to claim 4, wherein said laser is a Fabry-Perot cavity laser.
The anti-intrusion system according to claim 4, wherein said laser is provided of an optical isolator being capable of reducing the retroreflections in the cavity.
The anti-intrusion system according to claim 1, wherein said luminous filtering means (14) are constituted by an optical fiber; said optical fiber having a core and a cladding covering said core; said optical fiber being capable of helping the transit of substantially only one optical mode.