Optical fiber security system

An optical fiber security system having two optical fiber strands each with a light transmission and detection capability such that a deformation as is caused by unwanted intrusion of one of the strands with respect to the other is detected.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to physical security systems utilizing a pressure 
sensitive optical fiber transducer system wherein the arrangement of 
optical fibers is combined with signal processing capabilities. 
2. Description of the Prior Art 
Physical security systems utilizing fiber optics are well known in the 
prior art and can be broken down into two designations, namely, point 
sensors and line sensors. A point sensor system can be described as one 
having a device which measures a disturbance at one point or several 
discrete points and transmits signals via an optical fiber to a control 
room or alarm system. In such a system a disturbance at certain discrete 
points is sensed by means of electrical or optical switches and the like. 
The optical fiber is used merely to transmit evidence of the disturbance 
to a detector device. A line sensor system is one in which a disturbance 
can be detected anywhere along the whole length of the fiber wherein the 
fiber itself is the sensing element. 
U.S. Pat. No. 4,275,296 to Adolfsson, U.S. Pat. No. 4,379,289 to Peek, U.S. 
Pat. No. 4,281,245 to Brogardh et al. and U.S. Pat. No. 4,367,460 to 
Hodard are examples of point sensor systems employing optical switches. 
These types of security systems are not suitable for perimeter or large 
area protection because of the large number of individual transducers 
and/or switches that are necessarily required therefor. 
The line sensor systems used in above ground applications are exemplified 
by U.S. Pat. No. 4,275,294 to Davidson and U.S. Pat. No. 4,370,020 to 
Davey wherein the optical fibers are strung out so that breakage or severe 
distortion anywhere along the length of the fiber by an intruder will be 
detected. 
Obvious disadvantages of these type systems include visibility of the 
fibers to an intruder, destruction of the fiber when an intrusion results 
and exposure to extraneous elements such as weather and the like. Line 
sensor systems utilizing underground installation of a fiber optic cable 
as evidenced by U.S. Pat. No. 4,321,463 to Stecher and U.S. Pat. No. 
4,297,684 to Butter overcome the above noted disadvantages of the above 
the ground type. Such systems are generally constructed with the sensing 
changes in the phase measurements of the light transmitted in the optical 
fiber and as a result are extremely costly, inflexible in terms of actual 
installation and subject to a high likelihood of a false alarm. 
No known prior art physical security system measures changes in the 
intensity of light transmitted through fiber optic strands. The only 
technology known to applicants related to measurement of change in 
intensity of light in fiber optic strands is embodied in the teachings of 
U.S. Pat. No. 4,342,907 to Macedo et al. In that patent it is noted that a 
cladded optical fiber subjected to pressure has inherent therein a change 
of the refractive index in the cladding material. The change of the 
refractive index results in a change in the light being transmitted in the 
optical fiber strand because light is allowed to escape from the fiber 
core. 
Heretofore no one has been able to utilize the principle taught in the 
Macedo et al patent in a physical security system. 
SUMMARY OF THE INVENTION 
The present invention is directed to a line sensor type optical fiber 
physical security system that overcomes the above noted disadvantages of 
the prior art. The inventive system utilizes optical fiber strands as 
pressure sensitive transducers, a photo optic transmitter, a photo optic 
receiver, a signal processing unit and an external response system in a 
unique combination that results in a relatively simple, trouble free 
physical security system. 
In the preferred embodiment two fiber optic loops are provided each loop 
having a light emitting diode (LED) transmitter connected to one end and a 
photodiode receiver on the other end. 
The LED transmitters are activated by a current stabilized pulse generator. 
The radiation received by each photodiode receiver is directed to a 
differential amplifier through a transimpedance amplifier. The two fiber 
waveguides are optically and electrically arranged so that only changes in 
one fiber strand with respect to the other are detected. The differential 
measurement capability results in an extremely high sensitivity concurrent 
with a high common mode rejection against effects or changes in both 
fibers. System noise is reduced by the provision of a bandpass filter that 
is connected to the output of the differential amplifier. If the 
differential optical signal exceeds a predetermined threshold a signal 
will be generated by an adjustable threshold detector to trigger a 
response device such as an alarm or the like. 
An automatic signal processing or servo loop operates to maintain the 
optical fiber loops balanced with respect to each other so as to 
compensate for long term drifts in conditions such as temperature and the 
like. Without such a balancing control environmental changes could effect 
the differential measurement referred to above. As change in temperature 
occurs, for example, the output of one of the LEDs is adjusted to maintain 
a balanced system. Transient signal changes are not sensed by the servo 
control circuit so that a change of condition of one fiber with respect to 
the other, as occurs when there is an intrusion, can be detected. 
In use the optical fiber strands are placed far enough apart from one 
another so that an intruder will cause a change in one with respect to the 
other. Either optical fiber strand loop can serve as the pressure 
sensitive transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 the intrusion alarm system is depicted in schematic form and is 
shown to consist of transmitter subsystem 1, pair of optical fiber 
transducers referred to generally as 2, receiver section 3, servo control 
subsystem 4 and various alarm devices generally referred to as 24. As will 
be explained in greater detail below transmitter subsystem 1 consists of 
current regulator 5, timer circuit shown as 6 and switch 7 and light 
emitting diodes (LED) 8 and 9. Current regulator 5, timer circuit 6 and 
switch 7 are generally referred to as the current pulse generator. 
Receiver section 3 is seen to consist of photodiodes 16 and 17, 
preamplifiers 18 and 19, differential amplifier 20, bandpass filter 21, 
threshold detector 22 and output signal timer 23. Servo control subsystem 
4 includes switch 26, integrator circuit generally designated as 28, 
driver circuit 29 and variable resistor 30. 
Optical fibers 14 and 15 may be a low-loss silicone polymer clad glass core 
fiber, such as Fiber Industries type Superguide B. However, the invention 
is not dependent on a specific type of fiber. Optical fibers ranging from 
200 .mu.m to 1000 .mu.m core diameter and polymer clad glass fibers as 
well as all plastic fibers have been tested and found equally well suited 
for this application. In the preferred embodiment fibers 14 and 15 are 
either buried underground or located underneath floor coverings. 
Referring to FIG. 2 transmitter subsystem 1 is seen in greater detail. 
Current regulator 5 consists of integrated circuit chip 40 and resistor 
R17. In the preferred embodiment integrated circuit chip 40 is a three 
terminal integrated circuit regulator commercially available from Motorola 
under the designation MC7812. Timer circuit 6 includes integrated circuit 
timer chip 42 for generating accurate time delays or oscillations. In the 
preferred embodiment an eight terminal chip is used that is commercially 
available from Signetics under the designation NE555. Switch 7 is a 
general purpose npn transistor which in the preferred embodiment is 
commercially available under the designation 2N222. 
Timing circuit 6 generates voltage pulses which are connected to the base 
of the on-off transistor switch 7. When there is no pulse from timing 
circuit 6 transistor 7 is in an open or "off" condition. Upon the 
occurrance of a 2 millisecond pulse from timing circuit 6 transistor 7 is 
closed or in an "on" condition so that current is passed from current 
regulator 5 to both LEDs 8 and 9. The current pulse through LEDs 8 and 9 
results in optical radiation being generated for transmission through 
optical fiber strands 14 and 15 via connectors 11 and 12 respectively. The 
pulse duration and pulse interval is determined by the values of the 
resistors R20 and R21 and capacitor C10 connected to terminals of the 
timer chip as shown in FIG. 2. In the preferred embodiment a pulse of 2 
millisecond duration with a pulse interval of 2 millisecond duration is 
achieved (using values designated in the drawings). As will be explained 
in more detail below potentiometer R22 is connected in parallel with LED 8 
to provide a coarse adjustment in the intensity of the light transmitted 
in fiber optic strand 14. The light emitting diodes described above may be 
Honeywell Fiber Optic LED type SE4352-003. 
Receiver section 3 is shown in schematic detail in FIG. 3 wherein it is 
seen that the ends of fiber optic strands 14 and 15 are connected to 
photodiodes 16 and 17 via the connectors 31 and 32 respectively. The 
photodiodes may be Honeywell fiber optic detectors, PIN diodes type SD 
3478-002. The anodes of the photodiodes operated in the photovoltaic mode 
are connected to the input leads of two operational amplifiers 18 and 19. 
The two operational amplifiers are actually contained in a single package 
to minimize temperature drift. The device including the two operational 
amplifiers is commercially available as integrated circuit "Operational 
Amplifier Model LF 353" from National Semiconductors. This particular 
device uses field effect transistor technology to provide a high impedance 
input combined with low noise. The current generated by the photodiode as 
a result of incident light on the fiber strands creates a voltage across 
resistors R1 and R2, respectively. Both preamplifiers are identical with 
regard to the arrangement and value of the components. The small 
adjustable capacitances in parallel to the resistors R1 and R2 is to 
reduce the bandwidth of the amplifiers and thus eliminate high frequency 
noise. 
The output from the two preamplifiers 18 and 19 is connected to a 
differential amplifier via resistors R3 and R4. The inverse polarity input 
is connected to the output by means of resistor R5, whereas the normal 
polarity input is connected to ground with a resistor R6. This is the 
circuit of a conventional differential amplifier. The gain of this 
amplifier is controlled by the ratio of resistors R5, R6 to resistors R3, 
R4. The integrated circuit selected may be an operational amplifier type 
MC1741 made by Motorola. Off-set voltage of the differential amplifier is 
reduced to zero by the potentiometer R7 connected to the off-set 
adjustment leads of the device. The MC1741 device is operated from a 
.+-.15 V powersupply. Capacitor C3 will filter noise from the powersupply. 
The purpose of the differential amplifier 20 is to amplify any voltage 
difference between the output from preamplifiers 18 and 19. 
The output from the preamplifier is connected to a bandpass filter 21. The 
appropriate combination of three resistors R8, R9, R10, and two capacitors 
C4, C5 will provide a narrow bandpass characteristic. For example, in the 
preferred embodiment the active filter has a center frequency of 250 Hz 
and a bandwidth of .+-.25 Hz at the 3 dB points (Q=10). Only the 
sinosoidal first harmonic of the signal generated by the transmitter 
subsystem 1 is passed by this filter. In this way any extraneous noise 
introduced by the photodiodes 16, 17, preamplifiers 18, 19, and 
differential amplifier 20 is eliminated. 
The output from the bandpass filter is connected to a threshold detector 
22. The combination of capacitor C6 and resistor R11 at the input provides 
for ac coupling. The resistive network comprised of potentiometer R13 and 
resistor R12 allows adjustment of the threshold at which the device is 
triggered. Typically the potentiometer is adjusted for a threshold level 
of 50 millivolts. If the input exceeds this level a large 10 V signal 
appears at the output of the threshold detector. Either a positive or 
negative signal exceeding the 50 millivolt level will cause a 10 V signal 
output. The active bandpass filter may use operational amplifier MC1741 
from Motorola. 
The output of the threshold detector 22 is connected to an output signal 
timer 23. This device provides an output signal of fixed duration every 
time the threshold device generates an output signal. The combination of 
circuit components and timer chip NE555 made by Signetics comprises a 
monostable multivibrator. The two resistors R13, R14 and capacitor C7 
provide ac coupling of the signal into the timer circuit and proper bias 
voltage for operation of the NE555. 
The duration of the output signal is determined by resistor R17 and 
capacitor C8. These values may be chosen to provide an output signal of 10 
seconds. Small resistor and capacitor values will provide shorter output 
signals; conversely large values can increase output signals to minutes if 
desired. The output signal lasting for 10 seconds activates a relay for 
this duration. Any external alarm indicator such as a loudspeaker, buzzer, 
bell, light, tape-recorder, etc., can be turned on by the closure of the 
relay contacts. As an example, FIG. 3 shows the activation of a buzzer 44 
by relay 43. Upon closure of relay contacts the buzzer 44 is connected to 
a 15 V powersource. The two diodes IN914 protect the integrated circuit 
chip NE555 from voltage spikes which may be generated by the coil of relay 
43. 
FIG. 4 shows details of the servo control subsystem 4. Its purpose as 
stated earlier is to maintain the light input to the preamplifiers 
balanced without affecting the transient response obtained from an 
intrusion signal. 
The first element of the servo control system 4 is switch 26. The switch 
may be a CM05 gate Model CD4066 made by RCA. The switch 26 connects the 
output of bandpass filter 21 via 25 with the input of the integrator 
circuit 28. The switch 26 is normally open and closes only after a signal 
is received via line 27 from timer transmitter circuit 42 shown in FIG. 2. 
If the switch is closed any voltage at input 25 which stems from a signal 
at the output of the bandpass filter 21 charges capacitor C11 of the 
integrator 28. 
The integrator 28 is comprised of an operational amplifier MC1741, an 
integrating capacitor C1, reset resistor R25 and resistors R23, R24. A 
voltage at the input of the integrator 28 will charge capacitor C11. The 
reset resistor R25 will slowly discharge capacitor C11. Resistors R23 and 
R24 provide the appropriate input levels. 
A small voltage present at input 25 of the switch will be sensed each time 
the transmitter is pulsed and will cause a slowly rising voltage on the 
capacitor. The build-up time is determined by resistor R23 and capacitor 
C11. The risetime may be 10 seconds. Small voltage differences occuring at 
each pulse are integrated and provide a constant voltage at the capacitor. 
The driver circuit 29 is an amplifier which brings the voltage to the 
appropriate level to drive transistor 30. The gain of this amplifier is 
determined by resistors R27 and R28. The output dc level is controlled by 
potentiometer R26. The capacitor C12 removes noise from the line. The dc 
output of amplifier 29 is set such that the transistor has an 
emitter-collector current of about 5 mA. The collector of transistor 
2N2222 is connected to LED 9 via line 13 shown in FIGS. 1 and 2. The 
transistor circuit 30 is actually performing as a variable resistor 
connected in parallel to LED 9. As mentioned before, a current of about 5 
mA is bypassed from the LED by this transistor. Depending on the voltage 
received from the integrator 28 and driver 29 the voltage of the base of 
transistor 30 changes up or down, thus increasing or decreasing the 
current flow through resistor R30. If the current through the transistor 
increases the current through the LED 9 will decrease and the radiation 
output from this light emitting diode will decrease, and vice versa. 
In operation, light pulses are generated by the light emitting diodes 8 and 
9 as a result of electric current pulses generated by timer 6 and switch 
7, and constant current source 5. The light pulses from diodes 8 and 9 are 
propagated through optical fibers 14 and 15. Radiation generated by diodes 
8 and 9, transmitted via optical fibers 14 and 15 are received by 
photodiodes 16 and 17. The output from each channel comprised of light 
emitting diodes 8 or 9, fiber cable 14 or 15, photodetector 17 or 16, and 
preamplifier 19 or 18, is directed into a common differential amplifier 
20. Provided that the light output from LED's 8 and 9 is adjusted 
properly, both channels can be balanced such that the output of 
photodetectors 16 and 17 is exactly equal. Therefore, the output of 
differential amplifier 20 will be zero and the subsequent circuits 21, 22, 
23 and 24 will be inoperative. 
If a small pressure is exerted anywhere along the length of either fiber 14 
or 15 an optical loss will occur at that point and less light will be 
received at detector 16 or 17. Such pressure can be the result of an 
intruder stepping on optical fiber 14 or 15 buried in the ground in an 
outdoor application, or located below a carpet in an indoor application of 
this invention. The radiation received at detectors 16 and 17 is no longer 
equal as a result of an optical loss introduced in one of the fibers by an 
intruder. This signal difference will be greatly amplified by the 
differential amplifier 20. After passing through the bandfilter 21 the 
differential voltage is incident on a threshold detector 22. If the signal 
exceeds a preset threshold the threshold device 22 issues a large voltage 
signal which triggers a timer circuit which in turn activates alarm 
devices 44 for a preset amount of time. 
By comparing the voltage levels at diodes 16 and 17 and amplifying the 
differences in amplifier 20 rather than measuring absolute values, a very 
highly sensitive system can be built. In order to prevent high false alarm 
rates any noise spikes which may be amplified by the operational amplifier 
20 and may trigger the threshold device 22 have to be suppressed. This is 
achieved with a narrow bandpass filter 21 which transmits only signals 
generated by the transmitter 1 and thus rejects noise spikes generated by 
the photodiodes 16, 17, preamplifiers 18, 19 or differential amplifier 20. 
Besides noise pulses which might trigger the threshold device, long term 
drift of one fiber output with respect to another fiber has to be 
controlled. In practice, temperature differences in the environment where 
fibers 14 and 15 are placed or temperature differences in the electronics 
will cause a slow drift of the outputs at 16 and 17. Small differences 
will be amplified by differential amplifier 20 and these signals will be 
transmitted by bandfilter 21 and will eventually trigger the threshold 
device 22. 
Slow voltage changes are sampled by the servo control loop 4. Small voltage 
differences which appear at the output of bandfilter 21 will charge 
capacitor C11 in the integrator circuit 28. The voltage at this capacitor 
which appears at the output of integrator 28 is amplified and used to 
control the current of transistor 2N2222. CED 9 and transistor 2N2222 are 
electrically connected in parallel. The current in transistor 2N2222 is 
controlled by the output from the integrator 28. Depending on the polarity 
of the voltage the current in transistor 2N2222 will either be decreased 
or increased. since transistor 2N2222 and LED 9 are fed from a constant 
current source 5, the sum of the current in both devices is constant. 
Therefore, controlling the current in transistor 2N2222 will adjust the 
current in LED 9 and thereby adjust the light output in this LED. The 
amount of control is dependent on the gain of the feedback loop which is 
determined by integrator 28 and amplifier 29.