Locally Nulled Sine-Wave Total Power Alarm for Intrusion Detecting Optical Communications Systems

The invention relates to a system for detecting an unauthorized attempt to tap the optical fiber of an optical communications system. Tapping an optical fiber line usually requires extracting some power from it. Conventional tap detectors measure the received power and produce an alarm if the received power is less than expected. The present invention improves upon prior optical fiber tap detectors by modifying the original data signal such that the transmitted signal includes synchronizing periodic waveform information as well as a fixed total power. The synchronizing information is detected in the receiver and used to produce a local signal which is combined with the transmitted signal. When the combined signal is out-of-balance as a result of extraction of power due to tapping of the optical fiber, an alarm is activated.

As set forth above, intrusion detection optical systems are known in the prior art as evidenced by the U.S. Pat. No. 4,435,850 to Bowen et al and Steensma U.S. Pat. No. 4,207,561. The Bowen et al patent, for example, discloses a secure fiber optic data transmission system wherein a data signal is modulated onto a constant power reference signal. Detection and alarm circuitry is provided in the receiver. The alarm is triggered when the detected reference signal power is above or below a predetermined threshold. The Steensma patent discloses an optical communication system including means in the transmitter for detecting intrusion. The transmitted signal is reflected from the receiver back through the optical fiber to the transmitter where it is correlated with the original. An alarm is triggered when the correlator output falls below a given value. It is also known in the art to optically transmit a data signal with a reference signal produced by a voltage-controlled oscillator as shown in the U.S. patent to Crochet et al U.S. Pat. No. 4,249,764.

While the prior devices normally operate satisfactorily, they are generally characterized by an alarm "window", whereby changes in the transmitted signal as a result of instrusion must be significant in order to be detected. Moreover, the size of the window depends on the stabilities of the transmitter, fiber, and avalanche photodiode (APD) receiver. Since the APD is the least stable component, expensive steps must be taken to stabilize the APD, thereby increasing the cost and complexity of the prior devices.

The present invention was developed in order to overcome these and other drawbacks of the prior devices by providing a locally nulled periodic subcarrier power alarm for optical communication systems.

Accordingly, it is a primary object of the present invention to provide an improved intrusion detection optical system including a transmitter for generating a combined multiplexed data and periodic subcarrier alarm waveform optical signal, an optical fiber for transmitting the combined signal, and a receiver connected with the fiber for receiving the combined signal. The receiver includes a local signal generator which generates a local optical signal including a periodic wave component matched with the periodic subcarrier alarm waveform of the transmitted combined signal. An avalance photodiode combines the transmitted and local optical signals to produce a composite output signal. A bandpass filter is connected with the output of the avalanche photodiode and passes only non-zero periodic wave components from the composite output signal. Any non-zero wave components above a threshold value are detected by a synchronous detector connected with the bandpass filter to indicate an intrusion into the optical fiber.

According to a more specific object of the invention, the periodic waveform comprises a sine wave. More particularly, the transmitter includes a first oscillator for producing sine wave sync signal multiplexed with the data to be transmitted via the optical fiber and a sine wave signal combined with the multiplexed data signal for transmission therewith. The receiver also includes an oscillator which receives the transmitted sine wave sync signal and generates a sine wave for addition to the local optical signal which is matched in frequency and phase to the sine wave signal from the transmitter.

According to yet another object of the invention, power sources are provided in both the transmitter and receiver to provide power to the transmitted and local signals.

It is yet another object of the invention to provide an intrusion detection of optical communication system whcih is capable of duplex operation with alarm activated data shutoff.

Other objects and advantages of the invention will become apparent from a study of the following specification, when viewed in the light of the accompany drawing, in which:

FIG. 1 is a block diagram of the intrusion detection optical, communication system according to the invention; and

FIG. 2 is a block diagram of the invention for use in a duplex mode of operation.

Referring first to FIG. 1, the improved intrusion detection optical communication system comprises a transmitter 2, a receiver 4, and an optical fiber 6 connected therebetween. Data to be transmitted, which may be in either analog or digital form, is delivered to a mutiplexer 8 within the transmitter where the data is multiplexed with a sync signal generated by an oscillator 10. The multiplexer 8 may operate in either the time or frequency domain.

The oscillator 10 also generates a periodic waveform, and more particularly a sine wave, which is delivered to an adder 12. As shown in the drawing, the output from the multiplexer is connected with the adder, as is the power output from a DC source 14. The adder combines the multiplexed sync and data signal form the multiplexer 8, the DC source 14 and the sine wave signal from the oscillator 10 to produce a data and sine wave output signal. This output signal, which is in electrical form, is delivered to an optical source 16 where it is converted to an optical format. The combined optical output signal is transmitted to the receiver 4 via the optical fiber 6 connected with the transmitter optical source 16.

The sine wave generated by the oscillator 10 has a frequency f.sub.p measured by Hertz. The output of the oscillator is thus expressed as A. Sin (2.pi.f.sub.p t) The multiplexed data signal from the multiplexer 8 may be expressed as s(t). Accordingly, the optical intensity of the transmitter output signal may be expressed mathematically as

where

C and S are non-negative constants;

m.sub.p (0<m.sub.p .ltoreq.1) is the alarm modulation index;

and

w.sub.p =2.pi.f.sub.p.

Within the receiver 4, the other end of the optical fiber is connected with an avalanche photodetector (APD) 18 which passes the received optical signal from the transmitter to a demultiplexer 20 to demultiplex the data portion of the receive signal from the sine wave sync signals. The demultiplexer may also operate in the time or frequency domain in accordance with the domain of operation of the transmitter multiplexer 8. One output of the demultiplexer delivers the received data, while the other output of the demultiplexer is arranged in a feedback circuit for detecting a tap or intrusion into the optical fiber as will be set forth in greater detail below.

The sine wave sync portions of the received optical signal are delivered to a phase locked oscillator 22 via a delay circuit 24. The oscillator 22 generates a sine wave that is matched in frequency and phase to the sin (w.sub.p t) component of the transmitted signal I.sub.f (t). The sine wave output of the phased locked oscillator 22 is coupled to the reference input of a synchronous detector 26 as will be discussed in greater detail below. The output of the oscillator 22 is also coupled to the input of an inverting amplifier 28 having unity gain, whereby the output of the amplifier is equal in magnitude but opposite in sign to its input. The amplifier output is connected with a receiver adder 30 as is the power output from a DC source 34. The electrical output from the adder 30 drives an optical source 34 which converts the intensity modulated adder output to optical form, whereby a local or feedback intensity modulated optical signal is delivered to the avalanche photodiode 18 via an optical fiber 35. This local optical intensity signal can be written mathematically as

where

C.sub.n is a non-negative constant; and

m.sub.n (0<m.sub.n .ltoreq.1) is its modulation index.

The avalanche photodetector 18 combines the transmitted and local signals I.sub.f (t) and I.sub.n (t) to produce an electrical output I.sub.r (t) as follows: ##EQU1##

In normal operation where there is no intrusion or tapping of the optical fiber 6, the parameters C, m.sub.p, C.sub.n, and m.sub.n are adjusted to make

which nulls the Sin (w.sub.p t) component of I.sub.r (t).

The output of the avalanche photodiode is delivered to a narrow bandpass filter 36 having a passband centered at frequency f.sub.p. The filter 36 is operable to block all signals except any non-zero sin (w.sub.p t) component which is passed to an input of the synchronous detector 26.

Under normal (i.e. non-intrusive) operation where the Sin (w.sub.p t) component of I.sub.r (t) is nulled, the output of the bandpass filter is zero. This is sensed by the synchronous detector whose output thus is also zero.

On the other hand, if a tap is present on the fiber 6, some of the power propagating within the fiber is extracted, and the parameter C is reduced which disrupts the null condition set forth above. The resultant non-zero Sin (w.sub.p t) component of I.sub.r (t) from the avalanche photodiode 18 passed through the bandpass filter 36 to the synchronous detector 26. Since the reference input of the synchronous detector is always non-zero (i.e. the output of the phase locked oscillator 22), the DC ouput of the detector is non-zero. When the magnitude of this output exceeds a given threshold value, an alarm circuit 38 is activated to indicate the intrusion into the fiber of the optical communication system.

Duplex operation with alarm activated data shutoff can be implemented as shown in FIG. 2. The activation of the alarm 40 in terminal B terminates the data transmission from terminal B to terminal A. This activates the alarm 42 in terminal A which terminates data transmission from terminal A to terminal B. Similar results occur if the alarm in terminal A is activated first.

In lieu of the sine wave oscillators 10, 22 in the intrusion detection system shown in FIG. 1, any type of signal generators may be used which produce a periodic waveform having a repetition rate f.sub.p Hz. The frequency band of this waveform, however, must not overlap the frequency band of the transmitted data.

Moreover, in addition to the avalanche photodetector, the receiver photodetector may comprise any device which produces an electrical output proportional to the total intensity of all incident optical signals.

While in accordance with the provisions of the patent statute the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those skilled in the art that various changes or modifications may be made without deviating from the inventive concepts set forth below.