Abstract:
A secure fiber optic data transmission system includes a transmitter having a light source, a phase modulator for phase modulating the light source, and a controller for controlling the phase modulator as a function of an input electronic data stream and a second electronic data stream having a delay, the phase modulator creating a phase-modulated optical signal. An optical fiber receives the optical signal and a receiver receives the optical signal from the optical fiber. The receiver has a splitter for splitting the optical signal into a first path and a second path. The second path has a second path length longer than the first path length, the second path length being a function of the delay in the second electronic data stream.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to telecommunications and more particularly to improving security and data transmission over fiber optic networks. 
     2. Background Information 
     In current fiber optic networks, an electronic data stream is fed to a laser amplitude modulator. The laser amplitude modulator typically pulses or alters the laser output to create an amplitude-modulated optical signal representative of the electronic data stream. The laser amplitude modulator and laser thus define a transmitter for transmitting the optical signal over an optical fiber, which is then received by a receiver. The receiver for the amplitude-modulated optical signals of the optical data typically includes a photodiode to convert the optical signals back into the electronic data stream. 
     The reading of the amplitude-modulated optical data signals using a photodiode is straightforward: the optical signals either produce an electric output at the photodiode or they do not. As a result, an output electronic data stream of zeros and ones is generated. 
     However, optical fiber may be tapped. The optical fibers can be spliced or even merely clamped so as to obtain optical signals from the fiber. It also may be possible to tap fibers without physically touching the optical fiber, for example by reading energy emanating or dissipating along the fiber. Amplitude-modulated optical signals, with their ease of detection from a photodiode, require that only a small amount of energy be tapped and passed through the photodiode in order to be converted into a tapped electronic data stream. 
     To confront non-secure optical and non-optical data lines, it has been known to use public key/private key encryption so that the data stream being transmitted is encoded in a format that makes it difficult to decode. Encryption however has several drawbacks, including the need for extra processing steps and time. Moreover, public key/private key encrypted data can be cracked, and the devices and algorithms for doing so are constantly improving. 
     U.S. Pat. No. 5,455,698 purports to disclose a secure fiber optic communications system based on the principles of a Sagnac interferometer. A data transmitter is a phase modulator for modulating counter-propagating light beams sent by a receiver round a loop. The receiver includes a light source, a beamsplitter for splitting light from the light source into counter-propagating light beams and for receiving the phase-modulated light beams, and an output detector. U.S. Pat. No. 5,223,967 describes a similar Sagnac-interferometer-based system operating over a single optical fiber. 
     The Sagnac-interferometer-based systems described in these patents have the disadvantage that they require the light to travel over a loop, whether back and forth in a single fiber or over a long length looped fiber. As a result, either the link budget for the single fiber must be doubled, reducing the data carrying capacity for a single fiber, or else a looped fiber with significant and expensive extra length of at least twice that of a single fiber must be laid between the transmitter and the receiver. Moreover, the receiver contains the light source, as opposed to the current installed base where the transmitter has the light source. 
     The Sagnac-interferometer-based systems thus are expensive to build and operate, and do not work particularly well with existing systems. 
     U.S. Pat. No. 6,072,615 purports to describe a method for generating a return-to-zero optical pulses using a phase modulator and optical filter. The RZ-pulse optical signal transmitted over the fiber is easily readable by a detector. 
     U.S. Pat. No. 5,606,446 purports to describe an optical telecommunications system employing multiple phase-compensated optical signals. Multiple interferometric systems are combined for the purpose of multiplexing various payloads on the same optical transmission path. The patent attempts to describe a method for providing fiber usage diversity using optical coherence length properties and a complex transmit/receive system. Each transmitter has a splitter, a plurality of fibers and a plurality of phase modulators to create the multiplexed signal, which is then demultiplexed at the receiver. This system is complex and expensive. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved security optical fiber transmission system and device. An alternate or additional object of the present invention is to provide high bandwidth optical data transport via transmission and recovery of phase-modulated optical signals. Yet another alternate or additional object of the present invention is to provide a simple yet secure phase-modulated optical data transmission system. 
     The present invention provides a fiber optic data transmission system comprising a transmitter having a light source, a phase modulator for phase modulating the light source and a controller for the phase modulator. The controller controls the phase modulator as a function of an input electronic data stream and a second electronic data stream employing a delay, so as to create an encoded phase-modulated optical signal in the light passing through the phase modulator. The system also includes an optical fiber receiving the optical signal and a receiver receiving the optical signal from the optical fiber. The receiver includes a splitter for splitting the optical signal into a first path and a second path. The second path has a second path length longer than the first path length, the second path length being a function of the delay in the second electronic data stream. The receiver also includes a coupler for coupling the first path and the second path together so as to create an optical output signal. 
     With the system of the present invention, the receiver functions as an interferometer. An attempt to read the optical signal in the fiber, for example from a tap, requires knowledge of the delay and the creation of a precise physical delay path in the interferometer. Optical detectors with photodiodes do not have the bandwidth to measure the phase-modulated optical signal directly, since photodiodes are only capable of determining whether or not light is present. 
     The interferometer of the receiver of the present invention requires a significant amount of light to pass through the splitter and coupler, so that a tap would have to remove a significant amount of energy from the optical fiber in order to resolve the optical signal without a significant bit error rate. Detection of a tap on the system of the present invention, for example through a detection device reading a level of light energy in the fiber, becomes almost certain. 
     Moreover, the tap would have to match the interferometer delay in the second path to the electronic delay imposed by the controller, which is not always known. 
     The controller preferably includes an exclusive-or gate function, the input data stream being fed to an input of the exclusive-or gate and the second data stream being an input of the exclusive-or gate and a function of the output of the exclusive-or gate. The second data stream thus runs in a feedback circuit, which preferably includes a delay circuit delaying the second data stream by an amount of time directly proportional to the bit rate. As such, the controller may comprise a delayed-feedback exclusive-or gate. 
     The delay circuit may delay the second data stream by an amount of time directly proportional to a predetermined number of bits. When the data rates are at 155 Mb/sec (OC-3) or higher, the amount of delay preferably is greater than one bit. However, for data rates below OC-3, the electronic delay can be a fraction of the bit time, as long as the fraction is a power of two, for example one-half, one-quarter, one-eighth, etc. The fractional delay permits the present system to run relatively low data rates, such as T1, without requiring a long coherence length source. 
     The light source preferably is a continuous wave laser, for example a semiconductor laser operating at approximately 1550 nm or other wavelengths. The transmitter of the present invention requires only one phase modulator, and can operate at speeds of up to 10 Gb/s and even faster. 
     The receiver may include a detector for converting the output optical signal into an electronic output data stream. Preferably, the path length difference between the first path length and second path length is a function of the delay and the speed of the light in the fiber. The distance delays the light traveling in the second path with respect to light in the first path by a second delay, the second delay preferably being equal to the delay imposed at the controller. The second delay may vary slighty from the first delay, as long as the detector at the detector can read the output signal. The actual permissible difference will depend on the light source and any electronic filtering of the output signal. 
     The system preferably includes a detector for detecting a tap or loss of energy in the optical fiber. Most preferably, the detector is an energy sensor, which may or may not include programmable “trip” levels, which can monitor the amplitude of the light in the fiber. If a tap occurs, it must couple off a significant amount of energy to pass through an interferometer with a low bit error rate, thus making detection of the tap by the detector highly likely. 
     Depolarizers preferably are located between the light source and the phase modulator, and in the first path of the receiver. The depolarizer in the receiver alternatively may be in the second path. 
     The present invention also provides a transmitter comprising a light source, a phase modulator for phase modulating the light source, and a controller controlling the phase modulator, the controller including a delayed-feedback exclusive-or gate. 
     In addition, the present invention also provides a receiver comprising an interferometer, the interferometer having a first path and a second path propogating light at a delay with respect to the first path, the delay being a function of a delay imposed by a phase-modulator controller in a light-emitting transmitter. 
     A method for transmitting secure data is also provided comprising the steps of: 
     transmitting light from a light source in a transmitter; 
     electronically imposing a delay on an electronic data stream; and 
     phase modulating the light in the transmitter as a function of the selected delay and an electronic data input stream. 
     Preferably, the phase modulated data is a function of an output of a delayed-feedback exclusive-or gate. 
     The method further may include receiving optical signals in a receiver, splitting the optical signals into a first and second path and imposing a second delay on light in the second path with respect to light in the first path. The second delay is a function of the electronically-imposed delay and most preferably is equal to the electronically-imposed delay. The first and second paths then are recombined so as to generate an output optical signal, which may be read by an optoelectronic detector. 
     The method preferably includes monitoring a fiber for intrusion. The monitoring preferably includes monitoring an energy level in the fiber with programmable trip levels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred continuous-wave laser embodiment of the present invention is described below by reference to the following drawings, in which: 
     FIG. 1 shows a schematic of the system of the present invention; 
     FIG. 2 shows details of the circuit of the controller of FIG. 1; 
     FIG. 3 shows in a larger view the interferometer FIG. 1; 
     FIG. 4 shows details of an electronic data stream and the respective phase-modulated optical signals of the present invention, in representative binary form; and 
     FIG. 5 shows details of other electronic data streams and phase-modulated optical signals of the present invention, in representative binary form. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a preferred embodiment of a secure telecommunications system  1  according to the present invention. The system  1  includes a transmitter  10 , an optical fiber  20 , and a receiver  30 . Transmitter  10  includes a continuous wave coherent laser  12 , for example a semiconductor laser emitting a narrow band of light at approximately 1550 nm, or at other wavelengths. Light emitted from laser  12  is depolarized by a depolarizer  14  and passes through a phase modulator  16 , for example a Mach-Zender phase modulator. An electronic controller  18 , for example a PLC, controls phase modulator  16 . Controller  18  is also programmable to control the optical power output of light emitted by laser  12 . Preferably, the power output is set as low as possible for a given optical span, while maintaining a low bit error rate. This reduces the light available for any tap. 
     Depending on the controller output, phase modulator  16  either imparts no phase shift to the light or a 180-degree phase shift on the light passing through phase modulator  16 , thus creating an optical signal  22 , which represents a stream of binary bits. Optical signal  22  is transmitted over fiber  20  to receiver  30 . Receiver  30  includes a coupler/splitter  31 , functioning as a splitter, a light monitoring detector  32 , a coupler/splitter  34 , functioning as a splitter, and a coupler/splitter  36 , functioning as a coupler. The coupler  34  and splitter  36  together define part of an interferometer  40 , as will be described with reference to FIG.  3 . 
     FIG. 2 shows a schematic of part of the circuitry of controller  18  of FIG.  1 . Input data identified as DSI forms an input B of an exclusive-or gate  118 . The other input A of the exclusive-or gate  118  is a feedback loop  119 , which feeds back the output of exclusive-or gate  118 , and provides an electronic delay circuit  120 , which causes output OP to arrive at input A with a delay, for example, a certain number of bits later. Exclusive-or gate  118  thus is a delayed-feedback exclusive-or gate, which outputs an output electronic data stream OP for controlling phase modulator  16 . Phase modulator  16  phase modulates the light output from the laser  12  based on the electronic data stream OP. Optical signal  22  in FIG. 1 thus corresponds to the data in electronic data stream OP. 
     Optical signal  22  of FIG. 1, which preferably has a constant maximum amplitude, then passes to receiver  30 . Splitter  31  splits off a portion of the light, directing part of the optical energy to the light monitoring detector  32  and passing the remaining light to the interferometer  40 . A detector  32 , for example a light energy detector, monitors the light energy in the fiber  20  via the light energy coupled to the detector by splitter  31 , the light energy being a function of the amplitude. If the amplitude drops, most likely from a tap, the detector alerts the receiver and can, for example, sound an alarm or alert network maintenance personnel. Additionally, since the receiver is generally part of a component box, which also includes a transmitter, the component box transmitter can send a signal back to the component box containing transmitter  10  so as to instruct transmitter  10  to stop sending data, or to send data over a standby fiber. Detector  32 , while preferably part of receiver  30 , also could be located separately from receiver  30 , for example where fiber  20  enters a building or other secure environment. 
     Optical signal  22  after passing splitter  31  then enters interferometer  40  at an input  41  of splitter  34 . Splitter  34  splits the light entering input  41 , so that the signal OP travels over both a first fiber  43  and a second fiber  45 . A depolarizer  48  may depolarize light passing through fiber  43 , preferably, or fiber  45  as an alternative. Second fiber  45  includes a delay fiber  46  which may include a fiber loop of a desired length. Delay fiber  46  then provides an input to coupler  36  which recombines the delayed signal with the non-delayed signal propagating through fiber  43  and depolarizer  48  at output  42 . The physical delay imposed by the interferometer  40  in the second light path through fiber  45 , with its delay loop  46 , with respect to light passing through the first light path through fiber  43  and depolarizer  48  is selected to match as closely as possible an electronic delay time ED imposed by electronic delay circuit  120  of the controller  18 . If the first path in the interferometer  40  has a length L 1  and the second path a length L 2 , the length L 2  is selected, preferably by sizing loop  46 , as a function of L 1 , the speed of light v in fibers  43  and  45 , the light propagation delay through the depolarizer  48 , DPD, and the electronic delay time ED. The speed of light in the fibers may be estimated as a function of the wavelength and the type of fiber used. The length L 1  is known. When depolarizer  48  is in path  43 , L 2  is then chosen to approximate, and preferably equal, the amount (ED+DPD)*v+L 1 . The actual permissible difference between the two amounts depends on the light source and the accuracy of any electronic filtering of the output signal. 
     The light recombining at output  42  thus recombines the signal OP with a delayed signal OPD, delayed by an amount of time equivalent to the electronic delay time ED. If the data in the OP and OPD signals each represents a zero, or each represents a one, at the inputs  44  and  47  to coupler  36 , the signals will destructively interfere when recombined at output  42  of coupler  36 . Output detector  38  then detects no light and a produces a zero signal. If one of the data bits in the OP and OPD signals represents a zero and the other one represents a one, at the inputs  44  and  47  to coupler  36 , the signals will constructively interfere when recombined at coupler output  42 . Output detector  38  then detects light and produces an electronic signal representative of a one. 
     The interferometer  40  comprising coupler/splitter  34  and  36 , fibers  43  and  45 , delay fiber  46 , and depolarizer  48  thus functions as an optical exclusive-or gate with one input leg delayed for signals arriving at input  41  of coupler  34 . Interferometer  40  as a whole thus optically and physically “decodes” the signal OP produced by the delayed-feedback exclusive-or gate  118  of FIG.  2 . 
     FIG. 4 shows a schematic example of the functioning of the system  1  with a two-bit delay imposed by delay circuit  120 . The electronic data stream input DSI is also the input B for exclusive-or gate  118 . The first two delayed bits from input A are determined by the previous two bits in stream B, and as will be demonstrated with respect to FIG. 5, do not affect the functioning of the system  1 . Assuming for purposes of FIG. 4 that the delayed bits  64  entered input A as zero and zero, the output OP is as shown. Phase modulator  16  then converts this electronic data stream OP into optical signal  22  representative of OP. The interferometer  40  then creates delayed optical signal OPD, also delayed two bits from the optical signal representative of OP. At combiner  36 , the two signals OP and OPD produce, at output  42  and photodiode detector  38 , the data stream output DSO. As shown, input data stream DSI and output data stream DSO are the same after accounting for delay and initialization. 
     FIG. 5 shows the effect of having a different first two delayed bits  65  from input A on the same data stream input DSI of FIG.  4 . While the data stream OP and OPD thus differ from those in FIG. 4, the resulting data stream output DSO is the same as in FIG.  4 . 
     System  1  provides a secure method for transmitting data over a single optical fiber, which is difficult to decode if tapped, and also permits excellent detection of the existence of a tap.