Abstract:
An intrusion detector and method of operation which include a light source having a certain correlation length, and first and second light paths, wherein at least the first path extends through a secured area. A modulator, which is advantageously either a phase rotation modulator or path length modulator, is provided in the second path. At least one optical detector is placed to detect light traveling along the two paths. A comparator is electrically coupled to the detector to compare the detected signal with a signal applied to the modulator.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to intrusion detectors and, more particularly, to a low cost photon intrusion detector. 
   BACKGROUND OF THE INVENTION 
   Laser and light beam intrusion detectors are now employed extensively to protect against unauthorized intrusion into secured areas. Such detectors usually take the form of interferometers employing interference effects to detect disturbances. For example, it has been proposed to utilize a fiber loop with counter-propagating beams, and detect phase changes between the two beams and between one of the beams and a reference beam in order to detect a disturbance on the fiber loop. It was also proposed that a frequency or phase modulator could be inserted in the apparatus to improve sensitivity. (See U.S. Pat. No. 4,885,462 issued to Dakin, and U.S. Pat. No. 5,012,088 issued to Cole, et al.) It has also been suggested to propagate two light beams in the same direction in two fibers, and utilize one beam to measure a physical variable and the other beam as a reference. (See U.S. Pat. No. 5,004,913 issued to Kleinerman.) Use of two overlapping fiber loops has also been proposed to determine the position of any disturbance. (See U.S. Pat. No. 5,355,208 issued to Crawford et al.) Backscattered light in combination with a Mach-Zehnder interferometer may also be used for intrusion detection. (See U.S. Pat. No. 5,194,847 issued to Taylor et al.) Fiber optic vibration detectors have been suggested employing Michelson type interferometers with one fiber used to detect the vibration and another fiber used as a reference. (See U.S. Pat. No. 5,381,492 issued to Dooley et al.) 
   Secure information transmission systems have also been proposed employing a Mach-Zehnder or Sagnac interferometer where at least one of the light paths has a phase or path length modulator. (See U.S. Pat. No. 5,140,636 issued to Albares.) A random path length modulator may be inserted in the loop. (See U.S. Pat. No. 5,694,114 issued to Udd.) 
   For systems detecting physical intrusion in a secured place, it may be possible for an intruder to defeat the system by diverting the optical beam and injecting a substitute interference pattern. For high security applications, systems are often supplemented by using multiple beams and other detection schemes, which can be fairly costly. 
   It is desirable, therefore, to provide a low cost intrusion detector which is not vulnerable to beam diversion. 
   SUMMARY OF THE INVENTION 
   The present invention in one aspect is an intrusion detector which includes a light source having a certain correlation length, and first and second light paths, at least the first path extending through a secured area. A modulator selected from phase rotation and path length modulators is provided in the second path. At least one optical detector is placed to detect light generated by the two paths. A comparator is electrically coupled to the detector to compare the detected signal with a signal applied to the modulator. 
   In accordance with another aspect, the invention is a method for detecting an intruder including the steps of splitting light from a light source having a certain correlation length into a first and second optical path, at least the first path extending through a secured area. The light in the second path is modulated applying a signal to a phase rotation or path length modulator. Light produced by the two paths is detected and the detected signal is compared with the signal applied to the modulator. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice in the industry, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. 
       FIG. 1  is a schematic illustration of an intrusion detector in accordance with a first embodiment of the invention; 
       FIG. 2  is a schematic illustration of various waveforms utilized in the detector of  FIG. 1 ; 
       FIG. 3  is a flow diagram illustrating method aspects of the invention in accordance with an embodiment of the invention; and 
       FIG. 4  is a schematic illustration of an intrusion detector in accordance with a second embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings, wherein like reference numerals refer to like elements throughout,  FIG. 1  is a schematic illustration of intrusion detector  100 , in accordance with an embodiment of the invention. The detector utilizes light source  101  having a known correlation length. As known in the art, the term “correlation length” is the length of a photon, and determines the amount of overlap (between two beams) that will produce interference. The correlation length will be a function of the light source and will generally be in the range of 1 millimeter (mm) to 30 centimeters (cm). In some embodiments, the light source is a light emitting diode having a wavelength in the infra-red or visible spectrum. Other light sources such as incandescent or fluorescent light can also be used. 
   The light from source  101  is incident on optical splitter  102 , which splits the light, usually equally, into two beams that travel along light paths  103  and  105 . One light path (e.g., light path  103 ) is directed through secured area  104 . The secured area can be a room, a building, or any other structure that is to be protected from unauthorized physical intrusion. 
   Light path  103  is steered, either inside or outside the secured area, so that the beam is made incident on photodetector  109 . In this example, the beam is steered by a combination of mirror  107  and optical combiner  108 , but in other embodiments, other standard optical components can suitably be used. 
   The other light path, light path  105 , is directed to modulator  106 . The modulator can be any standard optical component that changes either the optical path length or the phase rotation of the incident beam in response to an electrical signal from modulation source  110 . For example, in some embodiments, modulator  106  is a crystal whose index of refraction is altered by the electrical signal. In some other embodiments, modulator  106  is one or more mirrors whose position or shape is modified by the source signal. Modulation source  110  is adapted to produce a random series of pulses, as illustrated by waveform  201  of FIG.  2 B. Modulation source  110  is electrically coupled to low pass filter  114 . 
   After passing through modulator  106 , the beam in light path  105  is diverted by mirror  111  to combiner  108 . The combiner combines the beam in light path  103  with the beam in light path  105 . The combined beams are made incident on detector  109  (e.g., photodetector), which produces an electrical signal in response thereto. 
   The electrical output of the detector  109  is coupled to demodulator  115 , which is, in turn, electrically coupled to one input of comparator  112 . The other input of comparator  112  is electrically coupled to low pass filter  114 . Alarm circuit  113  is advantageously electrically coupled to the output of comparator  112 . 
   Although in this embodiment, light path  105  is wholly outside secured area  104 , in some other embodiments, it is wholly or partially inside the secured area. 
   With further reference to the waveforms of FIG.  2  and the flow diagram of  FIG. 3 , in operation, light source  101  produces light having essentially constant intensity and a certain correlation length. This is illustrated by waveform  200  of FIG.  2 A and task  300  of FIG.  3 . The beam from light source  101  is split into two beams traveling along the two paths  103  and  105 , as per task  301 . The beam in one path (e.g., path  103 ) is directed through secured area  104 , as illustrated in task  302 . At the same time, the beam in path  105  is modulated by modulator  106  in response to the random pulses generated by modulation source  110 , as illustrated by waveform  201  in FIG.  2 B and task  303  in FIG.  3 . 
   It will be noted that, in this embodiment, the signal from modulation source  110  comprises pseudo-random pulse width component  202 , and, superimposed thereon, is band-limited signal  203 , with a high rate of change of phase. It is desirable that the pseudo-random component has intensity I 1 , which is much greater than the correlation length. For example, in some embodiments, the intensity is in a range of about 2 times to 10 times the correlation length. The band-limited component preferably has intensity I 2 , which is approximately one-half the correlation length. 
   Assuming that there is no disturbance in path  103 , the beams from paths  103  and  105  are combined by combiner  108 , as illustrated in task  304 , to produce the interference signal illustrated by waveform  204  in FIG.  2 C. The interference signal is detected (per task  305 ) by detector  109 . The detector generates an electrical signal in response thereto. After passing through demodulator  115 , the signal is illustrated by waveform  205  of FIG.  2 D. As illustrated by task  306 , the detected signal is compared with the signal from modulation source  110  after the latter has passed through the low pass filter  114  (waveform  206  in FIG.  2 E). 
   If the detected signal matches the modulating signal, as indicated by decision task  307 , the detector continues to operate normally. In this example, the two signals are 180 degrees out of phase, and the resulting signal from the comparator is zero. Consequently, no alarm is generated. It will be noted that the signals “match” if the detected signal has some predetermined relationship with the signal from the modulation source. 
   If, however, the signals do not match, an alarm is generated, as per task  308 , by alarm circuit  113 . This mismatch can occur in a number of ways. If the beam in path  103  is blocked by an intruder, no interference pattern will be generated at detector  109 . If an intruder attempts to divert the beam in path  103 , the path length of path  103  will change. No interference pattern will be generated if the change in path length is greater than the correlation length of the photons. 
   These examples produce a detected signal after passing through demodulator  115 , as illustrated by waveform  207  of FIG.  2 F. It will be noted that, since no interference pattern is generated, the signal has an essentially constant intensity indicative of the light source  101 . Since the detected signal no longer matches the signal from the modulation source (waveform  206 ), comparator  112  generates a signal, such as illustrated by waveform  208  of  FIG. 2G , and an alarm results. 
   If the intruder is somehow able to change the path length by an amount less than the correlation length, or attempts to inject a bogus interference pattern, the pattern detected by detector  109  will not match the signal from modulation source  110 , and, again, an alarm will be generated. In other words, even if a series of random pulses is generated by detector  109 , these pulses will not cancel out the signal ( 206  of  FIG. 2E ) from modulation source  110  and an alarm signal will be generated. 
   Thus, it can be seen that the intrusion detector according to the invention provides enhanced security in a cost-effective manner. 
     FIG. 4  is a schematic illustration of a detector in accordance with a further embodiment of the invention. It will be noted that this embodiment utilizes essentially the same components, which are numbered as in FIG.  1 . The main difference is that the light beams are carried by optical fibers rather than free space. In particular, light from source  101  is carried by optical fiber  401  to optical coupler/splitter  402 , where a portion of the light is coupled to optical fiber  403  and the remaining portion continues along fiber  401 . About 50 percent of the initial light is “split off” to fiber  403 . Fiber  401 , which forms one of the light paths, is sent through secured area  404 , while fiber  403 , which forms the other light path, is sent through modulator  106 . 
   After passing through modulator  106 , the light beam in fiber  403  is recombined with the light in fiber  401  by optical splitter/combiner  408 . The recombined beam in fiber  401  is coupled to detector  109  for detection of the resulting interference pattern, as before. The operation is otherwise as previously described. 
   One of the advantages of this embodiment is that secured area  404  can be fairly small, such as a container. The container would have a fiber attached to it. An intruder attempting to open the container would break the fiber and, as a result, no interference pattern would be formed at detector  109  and an alarm would be generated. Attempts to by-pass the fiber or inject a false pattern would also trigger an alarm for the reasons previously described. 
   Although the invention has been described with reference to exemplary embodiments, it is not limited to those embodiments. For example, although the embodiments described involve co-propagating beams, in some other embodiments, counter-propagating beams can be used. While in the illustrative embodiments, the comparator, demodulator, low pass filter, and alarm are illustrated as separate circuits, in some other embodiments, they are part of a single integrated circuit or several integrated circuits. Furthermore, in some other embodiments, the demodulator and low pass filter are replaced by other components that enable the signals in the two paths to be compared. Additionally, although the signals from the two paths are designed for total destructive interference in the illustrative embodiments when no intruder is present, other arrangements can be used. For example, if the light is not split equally, a dc component could be present even if the signals match. Rather, the appended claims should be construed to include other variants and embodiments of the invention which can be made by those skilled in the art without departing from the true spirit and scope of the present invention.