Abstract:
The present invention is an apparatus and method that identifies and localizes fiber breaks or faults automatically, utilizing a fiber optic data transceiver that has μOTDR functionality. The transceiver of the present invention is a single wavelength bi-directional transceiver that during normal operation sends and receives optical data streams in the same wavelength window using any protocol and reports the distance to the fault or multiple faults nearly instantaneously when the transfer of data is disrupted, without the need to have fiber lines dedicated for this purpose or physically connect and reconnect each fiber line to check for faults and eliminates the need to map out the distance to the remote transceiver.

Description:
This patent application claims priority to Provisional Application Ser. No. 61/521,311 filed on Aug. 8, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the detection of faults within a fiber optic network and the determination of the location of the detected fault and more specifically to the method and apparatus to determine the presence of, and location of, a fiber network fault, within a single optical fiber strand, utilizing, in particular, a transceiver that operates as both a digital bi-directional data transport on the same fiber, and a micro optical time-domain reflectometer (μOTDR). 
     2. Description of the Prior Art 
     In general terms, the detection of communication line faults has been a concern in the art for a long time. The advent of the communication age was accompanied by the proliferation of the necessary wires and cables to support that communication. As the communication networks grew larger, so too did the lengths of wires, cables, and now fiber optic lines, which now comprise networks thousands of kilometers in length. These wires, cables and optical fibers may be damaged for a number of reasons and need repair. Pinpointing the location of a fault is valuable information, regardless of whether the network is just a few meters or a few thousand meters in length. 
     The following are some examples of prior art fault detection devices: 
     U.S. Pat. No. 4,449,247 issued to Waschka (hereafter the “Waschka Patent”) depicts a system that transports additional data regarding the local status of each terminal, between the different terminals of a communication system. Unlike the present invention, the Waschka Patent does not check the status of the optical fiber or the location of a fault in the fiber, but merely has each station send out its own status to its neighboring terminal, receive status from a neighboring terminal and create separate alarms based on the various status notices. Additionally, the Waschka Patent recognizes only that it lost data and cannot determine if there is an electronic failure, a fiber failure, or a failure with the detector. Further, the Waschka Patent does not measure the distance to any fiber that is broken or determine if a fault is a fiber failure or an electronic failure. The Waschka Patent is merely a generic structure that sends and reads status reports to and from the various terminals without doing any type of analysis. 
     U.S. Pat. No. 5,586,251 issued to James A. Coleman et al. and assigned to the United States of America as represented by the Secretary of the Army on Dec. 17, 1996 for “Continuous On-Local Area Network Monitor” (hereafter the “Coleman Patent”), discloses a high level monitoring system of interconnected instruments having a plurality of connections to transmit and receive data regarding the status of the various segments of the system, including a preamble to notify the system that the following is not data. The Coleman Patent transmits information, doesn&#39;t look at the reflection of light and requires a plurality of transceivers and is incapable of detection by the transmission and receiving of light and using same to determine the location of a fault within the optic fiber itself. 
     U.S. Pat. No. 6,385,561 issued to John James Soraghan (hereafter the Soraghan Patent”) discloses detecting faults in electrical cables by coupling signals and comparing them and having separate input and output leads. The Soraghan Patent applies this same detection technique of coupling and comparing signals to fiber optic cables and storing sample signals. Soraghan is incapable of detecting a fault utilizing only one fiber and without any sampling, storage or comparisons. 
     U.S. Pat. No. 6,714,021 issued to Emrys J. Williams (hereafter the “Williams Patent”) discloses detecting faults within an electrical system by using electrical impedance and further requires a center plane, a back plane and a second back plane. The Williams Patent cannot be used for an optical circuit. 
     U.S. Pat. No. 7,139,668 issued to Eric Robert Bechhoefer (hereafter the “Bechhoefer Patent”) discloses the utilization of storing digitized electrical reflections and scoring them to determine the presence of an event. The Bechhoefer Patent is designed for a cable system, not a fiber optic system, and utilizes different components to transmit and receive than it does to determine an event in the system. 
     U.S. Pat. No. 7,218,388 issued to Gordon A. Keeler et al. and assigned to Sandia Corporation on May 15, 2007 for “VSCEL Fault Location Apparatus And Method” (hereafter the “Keeler Patent”) discloses the use of the same light source that transmits the light to also detect a reflection. Additionally, the Keeler Patent discloses the addition of an electronic circuit that changes the bias, and hence performance and characteristics, of the device, in order to perform a fault detection. Additionally, the transmitter in the Keeler Patent cannot, in normal operation, transmit and receive data, it can only transmit. The Keeler transmitter can receive data after a first and a second bias has been applied, after which it cannot transmit until the biases have been removed. The Keeler Patent does not have a data receiver in its system. 
     U.S. Pat. No. 7,239,680 issued to Sang T. Bui (hereafter the “Bui Patent”) discloses a method to diagnose an electrical system utilizing adaptive coefficients as filters which is used to determine the status of the cables as either open or short. An optic fiber doesn&#39;t have shorts, thus the Bui Patent cannot be used for fiber optic systems. 
     U.S. Pat. No. 7,558,212 issued to Jerome Edwin Olinski (hereafter the “Olinski Patent”), discloses a method for performing diagnostics by comparing the signal to stored data. Additionally, the Olinski Patent has mapped out and stored the physical locations of points along its pathway and it is to one of these points that the Olinski Patent directs a user as the location of the fault. In other words, the Olinski Patent requires prior knowledge of everything on its line, to which it then matches the response signal with the cable information and thus gives, as the location of the fault, the location of the nearest physical object on its line. 
     An Article dated February 1995 from the Hewlett-Packard Journal by Frank A. Maier and Harald Seeger entitled “Automation of Optical Time-Domain Reflectometry Measurements” (hereafter the “Hewlett Packard Article”) discloses the use of an OTDR which must be physically attached to each cable one at a time for testing for faults, whether by switching the lines internally or physically reconnecting the OTDR to the fiber. The Hewlett Packard Article does not teach that the same unit that is used to detect the fault is also used to transmit data. 
     SUMMARY OF THE INVENTION 
     Fiber Optic networks carry high priority data for multiple services. Any disruption of the network due to fiber break must be identified and repaired as quickly as possible. Current methodology to identify the location of a fiber break has multiple steps: 
     Recognizing that the link is down, meaning that there is no data transfer and/or high Bit Error Rate (BER) exists; 
     Identifying the particular fiber, or fiber pair, that constitutes the link that is down; 
     Sending a technician with specialized equipment, including an Optical Time Domain Reflectometer (OTDR) Tester to the patch panel where the fibers identified above are down or terminated; 
     Measuring the distance to the fault; 
     Consulting a detailed network map and/or database that translates the distance from the point of testing to physical location; and 
     Reporting the distance to a technical team that is dispatched to the location of the fault. 
     The present invention resolves all the problems of the prior art. The present invention is an apparatus and method that identifies and localizes fiber breaks (or faults) automatically. The present invention can transfer data during normal operation and report the distance to the fault or multiple faults nearly instantaneously when the transfer of data is disrupted. Unlike the prior art, there is no need in the present invention to have fiber lines dedicated for this purpose or physically connect and reconnect each fiber line to check for faults. The present invention eliminates the need to map out the distance to the remote transceiver—it is measured automatically during installation, or to transmit status messages. 
     The principal idea behind the current invention is to use a single wavelength bi-directional transceiver that during normal operation sends and receives optical data streams in the same wavelength window using any protocol (e.g. Ethernet per 802.3, SONET, or any other). When normal data transfer fails, the transceiver initiates a special mode in which its transmitter is sending high energy light pulses and the received signal, an echo from a fault location, is timed in reference to the transmitted signal. Since fiber break generates reflection of light at the break point, the time the reflected signal is received is the time it takes light to propagate from the laser to the fault (which is the location of the reflection) and back. Since the speed of light in the fiber is well known, the distance to the fault can be measured. Multiple faults can be identified if the receiver circuitry is capable to discern the timing of multiple reflections. 
     The inventors hereto have invented a new type of optical fiber fault detector. The present invention automates the first four steps above. This is a unique feature and ability in the art. Additionally, the present invention provides for automatic integration with mapping database(s) to locate physical location of the faults within seconds of the break/fault event. With such integration, all six of the above steps are automatically performed. The ability to have all six steps automatically performed is unheard of and is unique and innovative in the field and will provide a significant advantage to the network system which utilizes the present invention. 
     It is therefore a principal object of the present invention to describe the implementation of μOTDR, capable to detect and report fiber fault within a second, within a single fiber single wavelength data transceiver. 
     It is another object of the present invention to describe a single fiber single wavelength transceiver that is immune to reflections. 
     It is still another object of the present invention to provide OTDR functionality and capability within the transceivers of the optical fiber network itself. 
     It is still another object of the present invention to provide for fiber fault detection without requiring the dedication of fiber lines for fault detection purposes only. 
     It is still another object of the present invention to provide for an automated detection of faults by and within the same unit that is used to receive and transmit data. 
     It is still another object of the present invention to provide for an optical fiber fault detector that measures the exact link distance during installation without having to previously map out the entire fiber optic network. 
     It is still another object of the present invention to provide for an optical fiber fault detector that integrates with mapping data bases that depicts the fiber plant routes and enables a graphic presentation on the maps of the location of the fault. 
     It is still another object of the present invention to provide for an optical fiber fault detector that is used to determine a break or a cut in other systems, such as utility cables, power cables, network cables, and also to determine a break or cut other physical structures, such as fences, conduits, and pipelines. 
     It is still another object of the present invention to provide for a fiber fault detector for fiber optic systems having a transceiver that does not change its performance, characteristics and qualities in order to perform fault detection. 
     It is still another object of the present invention to provide for a fiber fault detector that detects a fault utilizing only one fiber and without any sampling, storage or comparisons. 
     It is still another object of the present invention to provide for a fiber fault detector that determines the location of a fault within an optic fiber by using the transmission and receiving of light within the optic fiber. 
     In accordance with the present invention, a fiber fault detector within a data transceiver having micro OTDR, μOTDR, capabilities and functions systems, methods, and apparatus are provided that substantially eliminate or reduce the disadvantages of traditional fiber optic fault detection systems and apparatus. 
     Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings, which are for the purpose of illustration only and not limitation, and in which: 
         FIG. 1  is a block diagram of a prior art bi-directional transceiver; 
         FIG. 2   a  is a block diagram of a bi-directional transceiver further illustrating an integrated micro optical time-domain reflectometer (μOTDR) of the present invention; 
         FIG. 2   b  is a block diagram of a bi-directional transceiver further illustrating an integrated micro optical time-domain reflectometer (μOTDR) and an including reflection amplitude measurement functionality of the present invention; 
         FIG. 3   a  is a schematic representation of the present invention of a single fiber optic link between two sites, incorporating CWDM MUX, under normal operation; 
         FIG. 3   b  is a schematic representation of the present invention of a single fiber optic link between two sites, incorporating CWDM MUX, after a fiber is broken; 
         FIG. 4   a  is the first part of an operation flow diagram of the μOTDR of the present invention; 
         FIG. 4   b  is the second part of an operation flow diagram of the μOTDR of the present invention; 
         FIG. 5  is operation flow diagram for the Reflection Immune Operation (RIO) of the present invention; 
         FIG. 6   a  is the first part of an operation flow diagram of an alternate embodiment of the μOTDR operation depicting the control of the conditions that will initiate the μOTDR measurement of the present invention; 
         FIG. 6   b  is the second part of an operation flow diagram of an alternate embodiment of the μOTDR operation depicting the distance measurement protocol of the present invention; and 
         FIG. 6   c  is the third part of an operation flow diagram of an alternate embodiment of the μOTDR operation depicting the integration of Delta Power Measurement into the μOTDR operation of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although specific embodiments of the present invention will now be described with reference to the drawings, it should be understood that such embodiments are by way of example only and merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the present invention. Various changes and modifications obvious to one skilled in the art to which the present invention pertains are deemed to be within the spirit, scope and contemplation of the present invention as further defined in the appended claims. 
     Referring now to  FIG. 1 , there is shown a block diagram of a B I -D IRECTIONAL  T RANSCEIVER    100 . The  TRANSCEIVER    100  is generally divided in two, having a FIBER O PTIC  R ECEIVER  S ECTION    130  and a F IBER  O PTIC  T RANSMITTER  S ECTION    140 . 
     In general, a standard, single fiber, single wavelength fiber optic bi-directional  TRANSCEIVER    100  has the following main functional elements: 
     A Digital Laser Diode ( LASER )  104  that transmits optical power pulses in response to input current. The  LASER    104  is a device that efficiently converts electrical power to optical power via a process called stimulated emission. L ASERS    104  work as a result of resonant effects such that the output of a laser is a coherent electromagnetic field. In a coherent beam of electromagnetic energy, all the waves have the same frequency and phase. Using supporting electronics the L ASER&#39;S    104  optical power can be modulated at high speed enabling transmissions of optical pulses at high rate. An example for a Laser is MITSUBISHI LASER DIODE ML9XX43 family. The laser package usually include a B ACK  F ACET  (BF) detector  103  that is a photodetector that detects the light from the back side of the  LASER    104  which is proportional to the  LASER&#39;S    104  output power. The measurement that is proportional to the output power from the laser enables implementation of automatic power control circuitry to maintain the stability of the laser over temperature. The signal from the BF  103  is processed in the  BACK FACET ELECTRONICS    114  and provided to the  MICROCONTROLLER    102 . 
     L ASER DRIVER  electronics  101  that controls the  LASER    104  and translates the input data stream to current pulses for laser modulation. The primary function of  LASER DRIVER    101  is to provide appropriate currents for bias and  MODULATION    118  of the  LASER DIODE    104 . The  BIAS    117  is a control line that controls the L ASER DRIVER    101  to provide constant current that pushes the  LASER DIODE    104  via line  116  to operate above its threshold value and into the linear region. M ODULATION CONTROL LINE    118  is a control line from the  MICROCONTROLLER    102  to the L ASER DRIVER    101  that sets the amplitude of the alternating current of the modulation output that is controlling the light output power from the LD  104  based on the input voltage waveform. To ensure proper performance over temperature the  BIAS    117  current tracks the changes in the  LASER    104  threshold current and the  MODULATION    118  current amplitude tracks the changes in the  LASER    104  slope efficiency. The  LASER DRIVER    101  also supplies the DC bias current via the  BIAS SUPPLY LINE    116  to the  LASER    104 . The  BIAS SUPPLY    116  is designed to decouple the DC current from the modulated current that generates the data of the optical signal. An example of a laser driver is SY88212L from Micrel a 3.3V 2.5GBPS with Integrated Bias and APC. 
     I MPEDANCE  M ATCH AND  E QUALIZATION    111 . It is important to maintain constant impedance at the cathode of the  LASER DIODE    104  such that the load on the high-speed output circuit versus frequency will remain stable, otherwise an unstable load on the output circuit will cause reflections, ringing, and other art effects, that will degrade the quality of the optical waveform. Proper characterization of the  LASER    104  and  LASER DRIVER    101  output impedance along with appropriate passive circuits minimize the effect of the  LASER    104  cathode impedance variation. 
     P HOTO DETECTOR  (PD)  108  that receives the light and translates it to electric current. P HOTO DETECTOR    108 , typically a PIN photodiode, functions exactly the opposite of a  LASER DIODE    104  where it absorbs photons and converts them into electrical current. P HOTODIODE DETECTORS    108  are current sources, with the current produced being proportional to the light intensity illuminating them. The  PHOTO DIODE    108  must be able to detect small digital signals over the top of any electromagnetic noise signals that are inherent in  PHOTO DIODE    108  itself and additionally in the light source and supporting electronics. It is enabling the detection of incident photons, and responds to the fastest changes in that incoming signal at a high rate. 
     T RANSIMPEDANCE  A MPLIFIER  (TIA)  109  that amplifies the current generated by the PD  108 . T RANSIMPEDANCE  A MPLIFIERS    109  translate the output from a very high impedance current source such as  PHOTO DIODES    108  current to a low impedance amplifier output in the voltage form. 
     An example for a Fiber Optic Receiver Section  130  assembly that contains the PD  108  and TIA  109  is inside AODR-P33N0-LM1-XI from Advanced Optronice Devices, al 0.25GBPS PIN-TIA assembly. The invention is applicable to any data rate and any communication protocol. The C ONNECTION    112  between the PD  108  and the TIA  112  is usually all packaged together with the PD  108 . The PD assembly includes an output that is proportional to the total incoming optical input. That output is delivered to the  MICROCONTROLLER    102 . This connection and the scaling electronics are not shown in the figures. The input optical power is stored in the  MICROCONTROLLER    102  and/or in its  MEMORY    115  and is available for reading by the host equipment through the IIC  BUS    119 . The IIC  BUS    119  is an example of the means to communicate with the  HOST EQUIPMENT    302 . 
     The output of the TIA  112  is connected to the next stage via L INE    113  that can be either AC or DC coupled. This next stage is the Q UANTIZER  E LECTRONICS    110 , for example MAX3747 from Maxim with signal detect and squelch/enable function built in, that translates the received signal by making decisions regarding the existence of a valid signal, i.e. if there is not a valid data signal stream, then L OSS  O F  S IGNAL  (LOS)  120  is high; if there is a valid data stream, then LOS  120  is low. The Q UANTIZER  E LECTRONICS    110  also makes decisions regarding the determination of 1&#39;s and 0&#39;s for the output data stream. The LOS  120  signal can be provided as an output of the transceiver LOS out  120   a  that is a separate I/O (input/output) pin, and/or provided to the M ICRO  C ONTROLLER    102  via control line LOS  120  to be available for monitoring through the IIC  BUS    119 . Known alternatively as P OST  A MPLIFIER  (PA)  110 , the Q UANTIZER  may simply amplify the weak signals and convert it in the digital stream. Some PA  110  have an inhibit input that forces no output data, regardless of the input from the TIA  109 . A  CONTROL LINE    121  from the  MICROCONTROLLER    102  can shut down the PA  110  output. Sometimes the LOS  120 ,  120   a  output is connected directly to the inhibit input of  CONTROL LINE    121  to allow effective squelching ensuring that when the signal is poor, meaning that the LOS  120 ,  120   a  are high, the output data is inhibited thus eliminating the noise from the data output. 
     M ICROCONTROLLER    102  monitors all the functions of the  TRANSCEIVER    100 , can function as the automatic power control, and provide communication means through a  BUS    119  (e.g. IIC bus) to the host equipment. I NTERNAL MEMORY    115  can be used to store values and events for external reporting through the  BUS    119 . The  MICROCONTROLLER  can supply the control signals to the  LASER DRIVER    101  both for modulation control via  MODULATION CONTROL LINE    118  and for bias control via  BIAS CONTROL LINE    117 . 
     The communication media, for both transmitted and received optical signals, is the  FIBER    107 . An optional  LENS    1056  close to the fiber tip focuses the light in both directions. O PTICAL BEAM SPLITTER    106  is the means that directs the portion of the received signal to the PD  108  and enables coupling the LD  104  optical output into the fiber through a  LENS    105 . The implementation used here  BY WAY OF AN EXA mple is not unique. It is possible to connect the power splitter outside the transceiver body and have fiber connections to the active parts—one to the Laser  104  and one to the Photo Detector  108 . A simple, fiber based, optical splitter can be used in-lieu of the Beam Splitter  106 . A LTERNATIVEly  an Optical Circulator (like SUN-OC-3-xx-P-X-X-X from GLsun Science and Tech Co.) can be used as a means to combine or separate the optical traffic in both directions into a single fiber. These are two of the several means to transport the data stream bi-directionally in a single fiber. In the application we are using the compact integrated solution as an example—without limitation. A  BEAM SPLITTER    106  is an optical device, positioned at about 45 degrees to the beam path, which can partially reflect and partially transmit an incident light beam such as laser beam, or receiving light beam from the external world through the  FIBER CABLE    107  into two optical light beams of similar or different optical power. It can reflect 60% of the light and transmit 40% (assuming negligible absorption losses), split it even 50/50 or any other ratio. To optimize operation, a  50 / 50  splitter is used in the present invention. The light coming from the fiber  107 , i.e. the signal transmitted from the other end of the link, is reflected by the  BEAM SPLITTER    106  and focused by the  LENS    105   a  into a P HOTODETECTOR  (PD)  108 . 
     At least one focusing  LENS    105  for each of the optical beams that couples optical light to and from the  FIBER    107  in/out of an active device LD  104  or PD  108 . 
     It is an industry practice to package the L ASER    104  with the BF  DETECTOR    103 , the  BEAM SPLITTER    106  and the PD  108  in a single optical device called BOSA. An example to advanced BOSA is in U.S. Pat. No. 6,652,158. Any person familiar with the art of bi-directional optical packaging for fiber optic communication knows how to optimize the design for improved coupling and reduced crosstalk. Crosstalk is the stray light from the L ASER  104  reflected by the    BEAM SPLITTER    106  inside the optical assembly and eventually trough multiple reflection finds its way into the  DETECTOR    108 . 
     Referring now to  FIG. 2   a , there is shown B I -D IRECTIONAL TRANSCEIVER    100  with the μOTDR  200  of the present invention. The fiber optic μOTDR  TRANSCEIVER    200  of the present invention, as depicted in  FIG. 2   a , has the following additional and novel functional elements: 
     The  FIBER  O PTIC  T RANSMITTER  S ECTION    240  is a modified version of the prior art 140. The additional elements and modifications are: 
     A  STATE MACHINE FIRMWARE    214  implemented in the processor logic  214  that recognizes when the data link is down and controls the μOTDR  200  operation. The  STATE MACHINE    214  is a means to transmit a high power pulse. The  STATE MACHINE&#39;S    214  function is to command the high power  PULSE ELECTRONICS CIRCUIT  to  215   a  to pulse the  LASER    104  at high current levels, producing a large optical signal with a fixed duration. The  STATE MACHINE    214   FIRMWARE LOGIC    400  as depicted in  FIGS. 4   a  and  4   b , in detail, recognizes if the data stream is interrupted, using the LOS  120  signal and/or the monitored value of the input optical power delivered by the PD  108  to the  MICROCONTROLLER    102 , and initiates the μOTDR  200  pulse. 
     A H IGH  P OWER  P ULSE DRIVER  circuitry  215   a  that is capable to drive a high current narrow pulse into the LD  104  to produce optical power in accordance to the amount of current supplied to the  LASER    104 . To pulse the  LASER    104  at very high current, the electronic circuit is designed to provide low electrical insertion losses but at the same time capable of delivering high current to the  LASER    104 . The DRIVER  215   a  can be a transistor like NE97833 a PNP transistor from CEL. The signal pass chosen in the present invention is via the bias control of the  LASER    104  and special I MPEDANCE  M ATCH    215   b  is used to decouple the  PULSE DRIVER    215   a  from the modulating signal path used for normal operation. The quality of the modulating current as measured by the open eye diagram of the  OPTICAL TRANSMITTER    140  should not be affected by the existence of the  PULSE DRIVER    215   a  during normal operation. The total  IMPEDANCE    215   b  seen from the  LASER DIODE    104  to the power supply should be optimized and a storage capacitor introduced if necessary to enable the high current pulse. Using the  DRIVER    215 A as described above results in optical power of more than 15 dBm pulsed for example for 400 nSec. 
     The  FIBER  O PTIC  R ECEIVER  S ECTION    230  of the present invention further contains (in addition to the 130 section of the prior art) a  DIFFERENTIAL PULSE SHAPER    216 , an  AMPLIFIER    217 ,  A NOISE FILTER    218  and a  COMPARATOR    219  that together are a means designed to detect the reflected light pulses. In order to achieve a higher sensitivity to detect the returning echo of the transmitted pulse a different optimized circuitry may be used. It is possible to use the LOS  120  output of the PA  110  as an echo detector for the μOTDR  200  operation, however the PA  110  is not optimized for single pulse detection since it is designed for continuous data stream. When LOS  120  is high, meaning there is no reliable data stream at the input, the μOTDR  200  is operational and the  RECEIVER CHAIN    230  that is producing the S IGNAL  D ETECTED  (SD) output  220  provided for echo detection and timing measurement. A matching circuitry depicted as differential P ULSE  S HAPER    216  that splits the signal at the input of the Q UANTIZER    110  and direct some signal to the special portion of  RECEIVER    230 . Broadband  AMPLIFIER    217 , for example an EL8101 a linear Op Amp from Intersil, a  NOISE FILTER    218  and a fast  COMPARATOR  (which in the present invention is a decision circuit that detects the existence of a pulse above a certain reference that is according to the noise floor)  219  complete the building block chain for the echo detector of the special  RECEIVER    230  that produces the output signal SD  220 . An overall sensitivity that exceeds −40 dBm can be achieved by standard design of electronic circuits. Using the special receiver as described above results in sensitivity to detect echo signals at a level of −40 dBm or better. 
     M ICROCONTROLLER    102  that executes the  STATE MACHINE    214   ALGORITHM    400  monitors both the LOS  120  and the SD  220 , controls the timing of the pulse into the LD  104 , is the means to measure the time until a reflection is detected, and calculates the distance to the fault. Among other things it also will calculate the timing between when the pulse was lunch at the  TRANSMITTER    240  and the time the optical pulse was received at  RECEIVER    230 , consequently calculating the fault distance location, based on the speed of light in a  FIBER    107 . Additionally, the duration of the pulse and its rate are also controlled by the  MICROCONTROLLER    102 . 
     The  MICROCONTROLLER    102  measures the time it take for a light pulse transmitted by the  LASER    104  to travel to a fault location, reflect due to fiber imperfection and travel back to the  TRANSCEIVER    100 . The stability of the  MICROCONTROLLER&#39;S    102  internal clock can be enhanced with an external quartz oscillator (e.g. C-002RX-32.768K-A from EPSON) and the resolution will depend on the clock frequency. For example  MICROCONTROLLER  MC68HC908SR12CFA from Freescale 102  operate at 8 MHz bus frequency hence enable time measurement resolution of 125 nSec which is the round trip time it takes to light travel inside 12.5 m of optic fiber. Faster microcontrollers can be used when higher resolution is required. 
     A  MEMORY ELEMENT    115 , which can be built within the  MICROCONTROLLER    102  or an external one retains the timing information and enables external device readout. 
     The implementation depicted above enable the  TRANSCEIVER    240  to measure the distance to any source of reflection when the echo signal attenuation is less than 55 dB—including the transmission loss and the fault reflection, such as a bad splice, cut, or break, with accuracy that enables quick and efficient discharge of repair technician based solely on the information provided by the  TRANSCEIVER    240  and the knowledge of the network topography (mapping). The functionality of the present invention to integrate with a service provider mapping that locates all fiber routes on a real world location presentation, i.e. a map, provides an immediate graphical representation of the location of the fiber break. 
     The present invention accomplishes this by measuring the distance to the fault, if transmission fails, or the receiving end, if transmission is complete. The present invention does not rely on determining the length of each  FIBER  prior to installation. When bringing up the link the transceiver of the present invention will detect any faults or breaks in the fiber, such as construction problems like a bad splice, cuts or any defects, and if no faults exist in the fiber, the present invention will detect the actual length of the fiber cable, because the receiver at the other end reflects some of the light, which light is then detected by the present invention and the time is measured from the time the high power pulse was sent to the time a reflection was detected is used to determine the length from end to end. 
     It can be seen that this functionality is useful for other applications. For example, the present invention can be used with a single fiber that is wrapped around or placed alongside a power line, telephone cable, pipeline, fence and the like. If vandals cut the copper line or cable, or if the pipeline breaks, the present invention immediately knows (1) that a break occurred, and (2) the exact location of the break. This provides first responders with a significant advantage, in that they can respond more quickly, so much so that they may still catch the vandals before they have completed removing the wire, and they can respond with accurate knowledge of the terrain and pinpoint location of the break. The same can be done for fences and other structures that are long and are otherwise difficult to maintain constant observation and monitoring. It can also be used in defense applications, for example around the perimeter of a base or for national border protection, wherein the fiber is routed on top of or within the protected infrastructure or fence, such that a break of the fence will break the fiber and the event will be detected by the present invention. 
     The invention is applicable to any single fiber transceiver that couples to a single fiber and uses at least one channel for single wavelength transport. 
     Referring now to  FIG. 2   b , there is shown a block diagram of the B I -D IRECTIONAL  T RANSCEIVER    100  with a μOTDR  200  and O PTICAL  R ETURN  L OSS  ORL  250  measurement of the present invention. 
     The  FIBER OPTIC  μOTDR  TRANSCEIVER    200  of the present invention as depicted in  FIG. 2   b  further has the following additional functional elements: 
     P EAK  D ETECTOR CIRCUIT    252  is an electronic circuit that detects peak RF voltage delivered to known load resistance, and by doing so, the reflected pulse power will also be measured, and is also the means to measure the time to receive the echo of the reflected pulse. The  PEAK DETECTOR    252  used for the present invention is fast enough to detect the pulse and discharge time and fast enough to be able detect the next μOTDR pulse. It also needs to be highly sensitive to measure small power levels. The output of the  PEAK DETECTOR    252  can have a built-in logarithmic amplifier, such as ADL5513 Logarithmic detector from Analog devices, and is connected to the  MICROCONTROLLER    102  via  LINE    220  and is reported together with the timing of the reflection. Measuring the reflected power enabling combined with a known transmitted High Power Pulse enable immediate evaluation of the Optical Return Loss  250  from the particular fault point being measured, which is the point that is the source for the echo signal being evaluated. These measurements can quantify minor reflections that may not impede a data link, may be used to fully characterize the fiber at the time of installation and/or first time link establishment. 
     The additional ORL  250  measurement of particular point of reflection can be logged by the reading instrument (e.g. network switch) to monitor stability over time. For example for a good link, the transceiver at the other end reflects power. The time information for the round trip pulse transmission (coming back from the farthest reflection) is the actual link length and the ORL  250  is a direct measurement of the total losses in the fiber pulse the reflection of the remote transceiver. This information is valuable for network monitoring. 
     The  FIBER OPTIC  μOTDR  TRANSCEIVER    250  as depicted above can have additional functional elements: 
     A  SQUELCH CONTROL    121  of the P OST  A MPLIFIER    110  that comes from the M ICROCONTROLLER    102 . As long as the LOS  OUT    215  from the  TRANSCEIVER    200  that is control M ICROCONTROLLER    102  is high the output of the PA  110  is inhibited. This inhibition reduces false and or reflected data from reaching the host equipment. 
     The output disable controls the data the host equipment sees and in conjunction with the link discovery process depicted in the flow diagrams (as shown in  FIGS. 4   a  and  4   b ) ensures that reflected data will not be seen by the hosting equipment. 
     Referring now to  FIGS. 3   a  and  3   b , there is shown a Functional diagram of a fiber optic network utilizing transceiver(s) with the present invention μOTDR Functionality. 
     The drawing depicts the fiber optic link between Site A and Site B in two conditions: 
     The first condition is shown in  FIG. 3   a  wherein there is illustrated the condition where dataflow exists, the fiber network is intact and no fiber fault is present. The second condition is illustrated in  FIG. 3   b , wherein there is illustrated the disruption of data flow, a fault in the fiber exists and one of the innovative elements of the present invention, the μOTDR  200  Optical Fiber Network with at least one  FIBER LINK    306  connecting Site A with site B. If there are more cites, as is usually the case, many more interconnections are possible BUT each link contains at least two  TRANSCEIVERS    303   a    305   a , one at each site, and optionally a CWDM MUX/D MUX    304  at each side as depicted in the  FIG. 3 . 
     One or more μOTDR  200  capable  TRANSCEIVERS    303   a ,  305   a  at Site A and one or more  TRANSCEIVERS    303   b ,  305   b  at Site B. Those can be operating on the same or on different wavelengths, and an μOTDR  TRANSCEIVER    303   a ,  305   a  of the present invention can be connected to a non-OTDR  TRANSCEIVER , and work within the scope of the present invention, but with a limiting functionality, single ended, of the network fault detection capabilities. 
     A S WITCH    302  or Media Converter or Router or any other networking apparatus  302 , is designated as host equipment that, can interface with fiber optic  TRANSCEIVER    303   a ,  305   a  and process data stream in one or more protocols, with at least one in Site A and at least one (similar or different) in Site B. Both  SWITCHES    302  on the same link must be capable to communicate in a common protocol (with Auto negotiation or with manual setup). 
     An optional CWDM M UX /D MUX    304  that enables combining few data streams of different wavelength (depicted as λ 1  to λ n ) on the same  FIBER LINK    306 , with one at each end of the network. 
       FIGS. 3   a  and  3   b  also show other  TRANSCEIVERS    303   b ,  305   b  (with or without the μOTDR  200  capability) connecting to the  SWITCH    302  and then connecting to other (or same) CWDM M UX /D MUX    304 . 
     Preferably each host equipment  SWITCH    302  has a capability to report in a generalized way the data it retrieves from the  TRANSCEIVER(S)    303   a ,  303   b ,  305   a ,  305   b  with μOTDR  200 . Such means for reporting can be SNMP (Simple Network Management Protocol). SNMP is an “Internet-standard protocol for managing devices on IP networks.” Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more. “It is used mostly in-network management systems to monitor network-attached devices for conditions that warrant administrative attention. SNMP is a component of the Internet Protocol Suite as defined by the Internet Engineering Task Force (IETF). It consists of a set of standards for network management, including an application layer protocol, a database schema, and a set of data objects. 
     The  SWITCHES    302  or any other host equipment that utilizes  TRANSCEIVERS    303   a ,  303   b ,  305   a ,  305   b  for fiber optic communication can be connected to a  NETWORK    301  enabling local and/or remote interrogation of the μOTDR  200  operating within utilizes  TRANSCEIVERS    303   a ,  303   b ,  305   a ,  305   b , as well as many other parameters customarily monitored for  NETWORK    301  elements. Some  SWITCHES    302  allow direct connection to a computer via a T ERMINAL INTERFACE . Such readout can be used locally at the Site. 
     State Machine 
     The  STATE MACHINE FIRMWARE    214  controls the functional flow of the μOTDR  200  operation. The functionality of a regular transceiver is not described here. It is important to note that in this description the validity of incoming optical signal into the “standard” elements of the  TRANSCEIVER    100  is designated by lack of LOS  120 ,  120   a  (L OSS  O F  S IGNAL ), or in other words LOS is Low or LOS=0, where is a presence of reflection during μOTDR  200  operation is designated here as S IGNAL  D ETECT  (SD)  220 . Reporting to the link status to the host equipment  SWITCH    302  is called LOS OUT    215 . 
     The sequence of operation is depicted in the flow diagram shown in  FIGS. 4   a  and  4   b  illustrating the  STATE MACHINE FLOW ALGORITHM    400 . Referring now to  FIGS. 4   a  and  4   b , there is shown the firmware flow diagram for the  TRANSCEIVER    303   a ,  303   b ,  305   a ,  305   b , with μOTDR  200  hardware capabilities. 
     There is shown in  FIG. 4   a , that when the  TRANSCEIVER    303   a ,  303   b ,  305   a ,  305   b  is powered at  STATE    410 , the  INITIAL STATE    420  is Tx—Disable. Tx  LASER DRIVER    101  can be enabled or disabled by the  MICROCONTROLLER    102 . For example the  MICROCONTROLLER  can suppress both the B IAS    117  and M ODULATION    118  and the  LASER TRANSMITTER    104  will not send any optical output. There are other ways to ensure that the  LASER    104  will not emit power. Most  TRANSCEIVERS    303   a ,  303   b ,  305   a ,  305   b  also have an external control through which the  HOST EQUIPMENT SWITCHES    302  can force the  TRANSCEIVERS    303   a ,  303   b ,  305   a ,  305   b  NOT to emit any light. Also the LOS OUT    215  (remember L OSS OF  S IGNAL  OUTPUT is a binary state that reports to the  HOST EQUIPMENT    302 , it is high when the received signal is missing or inappropriate) is kept high. The flow proceeds to the  DECISION POINT    440  that is looking for Low LOS  120  (remember LOS  120  is an internal variable that reports the received signal state at the P OST  A MP /Q UANTIZER    110  wherein low means good signal for data communication). If LOS  120  is low for at least 10 mSec, indicating input of steady source of light from the  FIBER    107 , the Yes output  440   b  goes to check whether there is an external control (e.g. from the  HOST EQUIPMENT    302 ) that Disables the Transmitter at  DECISION POINT    442 . If not, then the  TRANSMITTER    104  is enabled. The LOS OUT  215 state is now identical to the LOS  120  (normal operation) and the unit returns to  STATE    440 . For normal operation when the  FIBER  transports data the LOS is always low and the state machine is continuously looping through  440  to  442 . When LOS OUT    215  is low, the  MICROCONTROLLER    102  controls the output of the PA  110  (i.e. the Quantizer circuitry) via line  121  such that a PA  110  that employs squelch operation releases the data stream to the output of the transceiver. Without the PA  110  squelch the data at the output is noisy and unpredictable when the signal is at very low levels (below the level sufficient to trigger Low LOS  120 ). At this condition the  STATE MACHINE    214  is checking through the loop described above and data is presented at the output of the  TRANSCEIVERS    303   a ,  303   b ,  305   a ,  305   b.    
     If at  STATE    440  the LOS  120  is not Low for 10 mSec and LOS  120  is high for more than 100 mSec (basically each test can wait up to 100 mSec to wait for Low LOS  120 ) the flow branches to the No  BRANCH    440   a , and the  STATE MACHINE    214  enables the Tx-data (or idle signal) from the  HOST EQUIPMENT    302  is transmitted to the  FIBER    107  when Tx Enable is High and the initial LOS OUT  215 state is kept High—the  HOST EQUIPMENT    302  is signaled that no available data is to be expected. That was the condition leading into decision branch  440 . 
     If the present invention detects at  STATE    460  a valid signal LOS  120  Low on upon turning the Tx ON at  STATE    450  it can be due to reflection in the  FIBER NETWORK    306  (e.g. open SC-UPC connector will reflect partial power back to the source due to Fresnel glass to air interference). The present invention will continue to transmit data for a random time of 1-3 see, at  STATE    462 . The random time transmit at  STATE    462  is done to enable detection of the light at the other end of the link. The reason for the random timing at  STATE    462  is to ensure that the two units, that operates at the two ends of the link, are not synchronized—ensuring that there will be a time slot when one Tx is enabled while the other can detect it as an external source at  DECISION POINT    440 . Following the random Tx ON time, the  TRANSMITTER    104  is turned off (Tx Disable) at  STATE    463 . Now the LOS  120  condition is checked at  DECISION POINT    441  and if LOS  120  is Low, meaning that there is an external source of optical energy, then present invention will commence operation normally, through the Yes  BRANCH    441   b  to  442  as depicted above. 
     If the unit does not detect valid signal LOS  120  Low for &gt;10 mSec immediately upon turning the Tx ON, at  DECISION POINT    460 , then at  STATE    469  followed by  STATE    465  the present invention will transmit data for a random time of 1-3 sec, similarly to the above description. Afterwards the LOS  120  condition is checked at  DECISION POINT    441  and if it&#39;s Yes (meaning that there is an external source of optical energy) the unit will commence operation normally as depicted above, from 441 to 442 and then continuous looping  440  to  442  and back. This  STATE MACHINE    214  operation is one of the means that distinguishes between the reflection of the  TRANSCEIVER  and the incoming optical power. 
     In the case that LOS  120  (for 10 mSec) is not Low at decision point  441 , external signal is not detected at  441  the  STATE MACHINE  branches, through the No branch  441   a , to μOTDR operation. The No branch  441   a  continues on  FIG. 4   b . The points  1 ,  2 , and  3  at  FIG. 4   a  connect directly to points  1 ,  2 , and  3  at  FIG. 4   b.    
       FIG. 4   b  depicts the reflection measurement flow of the μOTDR operation. In state  443 , coming from point  2  the TFR (an internal variable that represents the round trip time of last reflection) is set to 0 and the reflection counter j is also set to 0. Next step, at  444 , the HPP (High Power Pulse) counter (n) is set to 0 since no pulse was transmitted yet for the current measurement cycle. High Power Pulse (for example 489 nSec wide) is sent via the  DRIVER CIRCUITRY    215   a  and  215   b  to the  LASER    104 . An internal timer starts at the end of the HPP, waiting for TFR before looking for reflection echo. At the first time TFR is 0 and the  DECISION POINT    445  will be entered immediately. However, if there were already measured reflections and the TFR is NOT 0 those reflections already detected will be ignored and the decision  POINT    445  will be entered only after waiting for the time it took to detect the previous reflection(s) looking for reflections farther away from those already accounted for. At  DECISION POINT    445  the status of SD  220  from the  COMPARATOR    219  is checked. 
     If no echo is detected SD  220  will not turn high and Tn will be set to 0. 
     If SD  220  is Detected the timer records the time from end of HPP to the time a signal is received as Tn. 
     After the Tn is recorded the  STATE MACHINE    214  verifies if 3 HPP were sent in the current condition (same TFR) at decision point  446 . The n counter is recording how many times the measurement with the current TFR is repeated. If the amount of tests has not reached the prescribed goal, 3 in this example (N=the number of repeated tests, can be any number chosen by the designer) n is incremented (n=n+1) and after total time delay of 651 μSec measured from the time the HPP was transmitted (this is the time we expect any reflection from a distance up to about 65 km) another HPP is transmitted. After transmitting the HPP n times (in the figures N is 3 as an example) the N records of the time to reflection (Tn) are compared at  DECISION POINT    447  (within 1 μSec representing reflection from a region of about 100 m length). If they are the same AND different from 0, then there is a valid fault detection. The average Tn from the 3 measurements is recorded as TRj—time to jth reflection. TFR is set to be TRj+1 uSec (ready to detect reflection 100 m farther away), j counter is incremented, and HPP loop restarts for the next reflection by going back to  BOX    444 . The j counter is the counter of valid reflections. Going back to  444  continues the measurement, looking for farther reflections. 
     If the N (e.g. 3) Tn are different or are equal to 0 (if they are all 0, then no reflection was detected, or if they are different indicating noisy measurement which is inconclusive and is ignored) the j counter is checked at  DECISION POINT    448 . If it is 0—no reflection was detected LOSout  120  is set to normal operation (looking at the output of the PA Quantizer and providing the signal to the host equipment) looping back to  DECISION POINT    440 . If j is not 0, indicating the last measurement was not valid, but previously valid measurements do exist, TRj of all prior j measurements is converted to distance by the  MICROCONTROLLER    102  and recorded in the  TRANSCEIVER MEMORY    115  (e.g. in page A2 of the EEPROM) the status of reflection, and the number of reflections are also recorded in the unit M EMORY    115  and the flow of the  STATE MACHINE  continues to go back to  440 . 
     Referring now to Table 1 below, there is shown a Data Presentation structure—enabling extracting fault information from the transceiver(s) 
     The way data that can be presented to the  HOST EQUIPMENT    302  by the transceiver memory according to present invention is depicted in Table 1. The data structure enables interrogation of the OTDR transceiver for multiple reflections. The distance to the farthest location is the default data presented in the example. The user (via SNMP or other means through the host equipment) can write the index of a closer reflection and get the actual distance to it. If power measurement capability, as depicted in  FIG. 2   b , is implemented in the  TRANSCEIVER  the ORL for each fault is reported as well index as above in Byte  127 —for example. 
     It is customary to include information within the transceiver memory that is accessible to the user. In the example below we use memory addresses that are available to use in the broadly accepted industry specification, but any address bytes or organizations of the address byte is clearly depicted here. 
     This data readout structure is a specific example of a general methodology that enables to save multiple reflections and read the reflections one at a time. It is designed to minimize memory usage while giving the user ultimate flexibility. The default presented values are for the farthest detected reflection. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 μOTDR Specific Codes in MICROCONTROLLER memory 
               
             
          
           
               
                 Byte 
                 No 
                 Name 
                 Description 
               
               
                   
               
               
                 123 
                 1 
                 OTDR_cntl 
                 This Byte is user Read/Write 
               
             
          
           
               
                   
                   
                   
                 Bit 
                 Description 
               
               
                   
                   
                   
                 7 
                 1 request update for information in bytes 124-127 based on index in bits 
               
               
                   
                   
                   
                   
                 0-4. 
               
               
                   
                   
                   
                   
                 OTDR software automatically resets bit 7 of byte 123 when the new index 
               
               
                   
                   
                   
                   
                 is processed. 
               
               
                   
                   
                   
                 6 
                 1 enables the μOTDR 0 disables the μOTDR (take up to 1 sec for 
               
               
                   
                   
                   
                   
                 update to propagate to Byte 124) 
               
               
                   
                   
                   
                 5 
                 1 enables the Delta Power Sensor (DPS); 0 disables the DPS 
               
               
                   
                   
                   
                 4 
                 1 enables (forces) single μOTDR measurement after DPS event (power 
               
               
                   
                   
                   
                   
                 delta detected) or after external Tx Disable. This state is named “Sense 
               
               
                   
                   
                   
                   
                 Priority”; 0 will cause DPS detection, to be followed by a link test - hence 
               
               
                   
                   
                   
                   
                 named “Link Priority) and Tx Disable will not initiate a μOTDR 
               
               
                   
                   
                   
                   
                 measurement. The bit is designated Run OTDR_once. 
               
               
                   
                   
                   
                 0- 
                 Numeric integer (4 bits representing an integer 0 to 15, bit 0 is LSB) that 
               
               
                   
                   
                   
                 3 
                 represents the index for the requested data. The largest index - [the 
               
               
                   
                   
                   
                   
                 Number of Reflections - 1] - is for the farthest reflection. Index 0 is for the 
               
               
                   
                   
                   
                   
                 first (closest to the unit) reflection. If the value is higher than the highest 
               
               
                   
                   
                   
                   
                 fault index [the Number of Reflections - 1] the highest index (farthest 
               
               
                   
                   
                   
                   
                 reflection) and its data are presented in Byte 124. When the μOTDR is 
               
               
                   
                   
                   
                   
                 Enabled (bit 6) the every new measurement will overwrite the values with 
               
               
                   
                   
                   
                   
                 the actual number of reflection detected. 
               
             
          
           
               
                 124 
                 1 
                 OTDR_Status 
                 This Byte is user Read 
               
             
          
           
               
                   
                   
                   
                 Bit 
                 Description 
               
               
                   
                   
                   
                 7 
                 1 data was not read; 0 data was read. The Bit is set any time fresh data 
               
               
                   
                   
                   
                   
                 is introduced to 125-127, after μOTDR test and after a command per bit 7 
               
               
                   
                   
                   
                   
                 Byte 123. The bit is reset to 0 every time Byte 126 is read. 
               
               
                   
                   
                   
                 6 
                 OTDR status (1 enabled or 0 disabled) 
               
               
                   
                   
                   
                 5 
                 1 indicates this unit has μOTDR software 
               
               
                   
                   
                   
                 4 
                   
               
               
                   
                   
                   
                 0- 
                 Numeric integer (4 bits representing an integer 0 to 15, bit 0 is LSB). The 
               
               
                   
                   
                   
                 3 
                 INDEX of the fault depicted in Bytes 125-127. Default is the farthest 
               
               
                   
                   
                   
                   
                 location - the INDEX is the reflection count. 0 means NO Reflections 
               
             
          
           
               
                 125-  
                 2 
                 Dist_Fault 
                 Distance to fault - value of 0 to 65536 meters - 2 bytes Byte 125 is MSB (This 
               
               
                 126 
                   
                   
                 Byte is user Read). The scaling to distance from the timing data generated above 
               
               
                   
                   
                   
                 is done by the MICROCONTROLLER 102 prior to storage in the memory 115. 
               
               
                 127 
                 1 
                 ORL_Fault 
                 Reflection Amplitude - positive number for the ORL that represents 0.25 dB/count 
               
               
                   
                   
                   
                 (This Byte is user Read) the value generated by the peak detector 230 and stored 
               
               
                   
                   
                   
                 in the memory 115 by the MICROCONTROLLER 102. 
               
               
                   
               
             
          
         
       
     
     The data in Bytes 124-126 updates after each measurement, following the record in Memory  115  after proceeding through the No branch  448   b  of decision point  448 . While the unit is NOT LINKED reading multiple reflections results via control of Byte  123  necessitates disabling the μOTDR (Bit  6  Byte  123  should be 0) throughout the process of reading multiple reflections, otherwise the new fresh data of μOTDR measurement will overwrite previous results while the user is in the process of reading the data. 
     Referring now to  FIG. 5 , there is shown a  STATE MACHINE  flow diagram for Reflection Immune Operation of a single fiber single wavelength optical transceiver. 
     The diagram represents the functional flow of an additional implementation that can be added to μOTDR operation. This additional innovation relates exclusively to single fiber single wavelength transceivers. The additional independent innovation as described herein applies to any single fiber single wavelength transceiver (with or without the μOTDR) and enables Reflection Immune Operation. The functionality of a regular  TRANSCEIVER  is not described here, except when monitored as part of the logic operation. It is important to note that in this example, the validity of incoming optical signal into the “standard” elements of the  TRANSCEIVER  is designated by lack of LOS  120  where there is a presence of reflection during μOTDR operation is designated here as Signal Detect (SD)  220  and the output function delivered to the  HOST E QUIPMENT  by the  TRANSCEIVER  (i) via a designated  OUTPUT PIN    215  and (ii) a readout field via the IIC, status kept in  MEMORY    115 . This advanced implementation enables detection of input power change, and initiates the μOTDR measurement based on detection of such change. The change can be if the  FIBER  is bent abruptly hence creating a reflection, if someone is trying to couple power out of the fiber link for monitoring or when the  FIBER  is opened and the  CONNECTORS  are not angle polished so the reflection form an open fiber connector (−14 dB is the reflected power intensity for non-angled termination) is still within the sensitivity range of the  RECEIVER . The detection of power change also solves one of the main deficiencies of single wavelength operation—data in the reflected signal can be processed by the  TRANSCEIVER  as a valid dataflow from the other side of the link (while in reality it is the data that the same  TRANSCEIVER  is sending and is reflected when a non-angled connector is open and there is no link, or any other reflection source including a non-powered transceiver at the other side of the link). 
     The situations when reflection can affect the link and/or confuse the operator are: i)  TRANSCEIVER  is plugged into an equipment but the  FIBER LINK  is not completed, ii) a link is completed but the other side is not powered—the reflection can come back from the far non-powered  TRANSCEIVER , and iii) the link is open, has a bad splice, a dirty connector, a broken adaptor, or broken fiber and the point of disconnect has significant optical reflection. When an active fiber link, (dataflow goes both directions) is disconnected by opening of any non-angled connector the power reflected from the connector is changing—even very slightly when the steady state reflected power from a connector (at the receiver) is the same as the power that was received from the other side while the link was active. The reason for the minute change is the instantaneous interference that can actually spike the power when two flat optical terminations are separated by a very small distance. Since the mechanical action of opening a connector is measured in mSec time frame, the optical power undergoes variations between two equal steady state conditions. This variation can be detected as described herein thus enabling comprehensive Reflection Immune Operation. The detection of an “event” (small power change) can be used to trigger a μOTDR measurement (sense priority), or to check if the link is still valid (link priority). In link priority, the possibility to generate a link with the other side is checked first such that the link will be recovered as soon as possible. In sense priority, a single μOTDR measurement will take place before trying to establish a link, hence keeping a record of the disturbance if detected. The implementation is depicted such that it can be implemented without μOTDR operation, just improving standard single fiber single wavelength transceiver operation. With this  STATE MACHINE    214  operation in conjunction with the μOTDR measurement, the  TRANSCEIVER  can distinguish between the reflected signal and the incoming signal following a break. This is another means by which the present invention distinguishes between the optical power of a transmitter and an incoming optical data stream from a remote transceiver. 
     The Reflection Immune Operation (RIO) depicted in this patent application is applicable to any standard single fiber single wavelength transceiver. The functionality depicted above, that of detecting change in the incoming optical power, is called Delta Power Detector (DPSO) and it can be enabled or disabled via user control in bit  5  byte  123  (see Table 1). 
     RIO Operation 
     The reflection immune operation (RIO) as described below, is comprised of two elements: i) detection upon LOS High if the local transmitter power is reflected—(in the absence of incoming power)—but still allowing for normal link to be recognized, and ii) delta power measurements during normal linked status. Each of the features can work independently but combined they cover all the operational conditions of single fiber single wavelength  TRANSCEIVER.    
     The sequence of operation is depicted in the STATE MACHINE diagram illustrated in  FIG. 5 . When the  TRANSCEIVER  is powered the  INITIAL STATE  ( 539 ) is Tx-Disable (the local transmitter is shut down-no output power) and LOSout  120 —a binary state that reports to the  HOST EQUIPMENT , e.g. S WITCH ) is high indicating that the received signal is missing or inappropriate. The unit at decision point  540  is looking for detecting a valid input power indicated by LOS Low for at least 10 mSec, LOS (Loss of Signal is an internal variable that reports the received signal state at the Quantizer/PostAmp  110 —low mean good signal for data communication)—if LOS is low for at least 10 mSec (indicating external steady source of light) the unit goes to check whether there is an external control (e.g. from the switch) that Disables the Transmitter at  STATE    542 . If not, then at  STATE    542   b —the Transmitter is enabled. The LOS state is now becoming normal operation LOSout=LOS the output LOS is directly indicating the status of the Quantizer/PostAmp  110  and the unit returns to  STATE    540 . When LOS is low the Quantizer/PA  110  that generates it and employs squelch operation release the data stream to the output of the transceiver. Without the Quantizer squelch the data at the output is noisy and unpredictable when the signal is at very low levels, usually below the level sufficient to trigger SD. At this condition the  STATE MACHINE    214  is checking through the loop described above and data is presented at the output of the  TRANSCEIVER.    
     If at  STATE    540  the LOS is High for more than 100 mSec the unit branch through the No  540   b    BRANCH  and enables the Tx —data (or idle signal) from the switch is transmitted to the fiber when Tx Enable is High and the initial LOS state is High—that was the condition leading into this branch. The Tx Enable can be conditional on The Soft Tx Disable or the Tx Disable pin that are defined in the MSA for SFP  TRANSCEIVERS.    
     The unit will transmit data for a random time of 0.22-0.66 sec. Afterwards the Tx is Disabled and the LOS is queried at STATE  541 . If it is LOW that means that a remote source is generating the power (meaning that there is an external source of optical energy) and the unit follow to STATE  542  into normal operation as depicted above. If the LOS is High the  TRANSCEIVER  can move into the actual μOTDR operation not depicted in  FIG. 5  or back to  540  to continue to monitor the incoming power—waiting for the other side. This NO branch  540   b  is enabling transmission to the other side to generate a link even if there are some reflections. The other side, if equipped with the same features, will detect the incoming light and will go through similar steps to  542  and  540  for normal operation. If the other side is a regular transceiver it transmits all the time and it will be detected at  STATE    540  and the link will be established. The random time ensures that both sides, equipped with this feature, will not synchronize. Such operation is useful for units without μOTDR to avoid locking on its own reflected signal. This is the first element of reflection immune operation. 
     In the following, the delta power detector is described. The flow diagram in  FIG. 5  depicts simple independent loop starting at  559  that resets the value of the LOS&lt;Rx&gt; counter in step  558  to 48 as an example. This initial value, that can be designed to be much smaller (e.g. 8) or bigger (e.g. 128), represents the number of sampling events that must pass before another monitoring function starts. LOS&lt;Rx&gt; is a counter that counts down every time a new input power is measured. Rx Power is the input power measurement value and it is typically implemented in fiber optic transceivers and is not explained here. 
     In order to measure changes in the incoming optical power, which may have originated due to fiber handling, disconnect, etc., an algorithm depicted below is used to eliminate measurement noise. Other digital filtration and/or smoothing can be used and what is depicted here is for example only. Every cycle of the internal micro-controller  102  the input optical power is measured (designated measure Rx Power) and stored, in flow box  550 . Individual measurements are noise susceptible so a smoothing filter is used. In the example. there is used a smooth/8 filter that generates an internal variable rxPwr that represents a smoothed measurement value:
 
 rx Pwr= rx Pwr+(measured Rx  Power− rx Pwr)/8  (Equation 1)
 
     After each measurement cycle all the previous values are retained internally in sequence. Up to n values of smoothed input power are kept in memory  102 . N can be 48 for example. The latest value of the smoothed power rxPwr is rxPwr(0) and the “oldest” in our example is in a generalized form rxPwr(n−1). The LOS&lt;Rx&gt;counter is checked to see if the minimum number of measurements (48 in our example) was completed at decision box  551 . If not, through the No branch  551   b  the LOS&lt;Rx&gt;counter is decremented and the loop returns to  550 . After the minimum measurements were accumulated the LOS&lt;Rx&gt;counter is set to 0, and it will remain 0 until LOS High will be detected in  559 . Once the amount of measurements are sufficient the rxPwr(0)—latest smoothed value is compared with rxPwr(n) to check if the difference is bigger than a set threshold ( 552 ). 
     The formula depicted is:
 
ABS( rx Pwr[ n]−rx Pwr[0])&gt;MIN( rx Pwr[ n],rx Pwr[0])/ Q   (Equation 2)
 
     The formula depicted ( 552  in  FIG. 5 ) uses Q variable to scale in reference to the smallest power of the two, rxPwr(0) and rxPwr(n). 
     If the difference is bigger than the set threshold—the decision point  552 , through the Yes branch  552   a , will return the transceiver to flow-box  539 —monitoring the input power and causing the restart of the μOTDR operation, if implemented. The variable Q effectively sets the relative power change. Here are some values for different value of Q: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Q 
                 dB change 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 3.01 
               
               
                   
                 2 
                 1.76 
               
               
                   
                 4 
                 0.97 
               
               
                   
                 8 
                 0.51 
               
               
                   
                 16  
                 0.26 
               
               
                   
                   
               
             
          
         
       
     
     If the change of power was caused by opening the optical fiber at a point with high reflection (open UPC connector for example) the detection at  540  will indicate no incoming power and the loop will proceed in the same way as in the case when the transceiver was powered with no incoming light. Without this algorithm the reflected power will be detected by the Quantizer/PA  110  LOS  120  will stay low and the output signal will be exactly the input signal—which is a case we want to avoid. 
     Referring now to  FIG. 6 , there is shown an alternate embodiment of the state machine flow of the present invention. 
     The sequence of operation is depicted in the state machine diagram.  FIGS. 6   a ,  6   b  and  6   c  is based on the prior depiction in  FIGS. 4   a  and  4   b  combined with the features of  FIG. 5  with additional refinements. All flow diagrams are self explanatory and anyone familiar with the art of controlling optical transceiver via software routines can understand and implement those. For further clarity, there is provided a detailed explanation throughout the application. 
     In the implementation described here includes the following capabilities: 
     Measuring multiple reflections (counter j) 
     Repeating each measurement a few times to increase validity (counter n) 
     Recovering from a bad measurement and retrying a few times (counter k) 
     Recognizing self reflection when there is no input power from the other side 
     Enabling the DPS to initiate a μOTDR measurement 
     Enabling external Tx Disable to initiate one μOTDR measurement 
     In  FIG. 6  the STATE numbers are depicted in a hexagonal border. Flow boxes important for the explanation below, as in previous figures, are also numbered. Connections from page to page are indicated in a number within an ellipse. To provide further clarification there is provided the following state table: 
     
       
         
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 STATE 
                 DESCRIPTION 
               
               
                   
               
             
             
               
                 601 
                 Power ON 
               
               
                 602 
                 Start LOSCHECK 
               
               
                 603 
                 CHECK LOS 
               
               
                 604 
                 WAIT RANDOM DELAY 
               
               
                 605 
                 WAIT QUITE (ensure all reflected power from afar is gone) 
               
               
                 606 
                 SETUP 41 
               
               
                 607 
                 CHECK LOS (low LOS w/o Tx) 
               
               
                 608 
                 Tx ENABLED 
               
               
                 609 
                 START LOSCHECK 1 ( = START LOSCHECK) 
               
               
                 610 
                 START Tx DISABLE 
               
               
                 611 
                 START DPS 
               
               
                 612 
                 START μOTDR Measurement 
               
               
                 613 
                 RUN μOTDR 
               
               
                   
               
             
          
         
       
     
     The operation starts in  FIG. 6   a . When the transceiver is powered TsC clock (a clock that measure the time since the unit was powered or since the latest μOTDR measurement whichever is the most recent) is initiated and the flow moves to the STATE  601  and immediately to I NITIALIZATION    639  the initial conditioning of the μOTDR flow, that is Tx-Disable (the local transmitter is shut down—no output power) and LOSout (Los of Signal Output—a binary state that reports to the host equipment, e.g. Switch) is high indicating that the received signal is missing or inappropriate. STATE  602  (Start LOS Check) is at the entrance to  DECISION POINT    640 . At  DECISION POINT    640  the state machine is looking for detecting a valid input power indicated by LOS Low for at least 10 mSec. LOS  120  (Loss of Signal  120  is an internal variable that reports the received signal state at the Q UANTIZER /P OSTAMP    110 —low mean good signal for data communication)—if LOS  120  is low for at least 10 mSec (indicating external steady source of light), STATE  603 , Y ES BRANCH    640   a , the unit goes to check whether there is an external control (e.g. from the switch) that Disables the Transmitter at  STATE    642  and the T RANSMITTER  is conditionally enabled (if Tx Enable is allowed) and the flag Flg1Tdone to 0. The Flg1Tdone indicates if at least one μOTDR measurement was successfully conducted. The LOS state is now becoming normal, LOSout=LOS, the output LOS is directly indicating the status of the Q UANTIZER /P OST A MP    110  and the unit returns to STATE  602  and  DECISION POINT    640 . When LOS is low the Q UANTIZER /PA  110  that generates it and employs squelch operation release the data stream to the output of the transceiver. Without the Q UANTIZER  squelch the data at the output is noisy and unpredictable when the signal is at very low levels (below the level sufficient to trigger SD). At this condition the state machine is checking through the loop described above and data is presented at the output of the transceiver. 
     The external input into the transceiver that controls the transmitter—called Tx Enable (external) is provided by an input hardware line and/or via IIC  119  as a soft Tx Disable—is constantly monitored in  660 . As long as external override is not detected the local loop keeps checking at  660 . If external control is detected the unit checks at  661  if more than prescribed time (e.g. 2 sec) AND the Run μOTDR_once control is set (see bit  4  byte  123  in Table 1) the TsC clock will be reset and the flow will go to State  612  (Start μOTDR measurement). If the TsC clock time is less than the prescribed time, OR the Run OTDR_once is not enabled, the flow will return to  STATE    602  and to  DECISION POINT    640 . This sequence enables forcing of μOTDR measurement with external control of Tx Disable. 
     If at  DECISION POINT    640  the LOS is High for more than 100 mSec the unit branch through No leg  640   b , checks again if external Tx Disable was not introduced, and enable the Tx—thus data (or idle) from the host equipment is transmitted to the fiber when Tx Enable is High—and the initial LOSout is High—that was the condition leading into  DECISION POINT    640 . The Tx Enable is conditional on The Soft Tx Disable or the Tx Disable pin that are defined in the MSA for SFP transceivers. 
     The No  640   b  leg out of  DECISION POINT    640  is enabling transmission (if there is no outside Tx Disable) to the other side to generate a link even if there are some reflections. The other side will detect the incoming light and will go through similar steps to  642  and  640  for normal operation. The random time ensures that both sides will not synchronize. Such operation is useful for units without μOTDR to avoid locking on its own reflected signal. 
     The unit will transmit data for a random time of 0.22-0.66 sec, STATE  604 . Afterwards the Tx is Disabled and the units wait for an additional 1 msec to ensure that any reflections from the fiber from remote location will already be gone, and then the LOS is queried ( 641 ). If it is LOW that means that a remote source is generating the power (meaning that there is an external source of optical energy) and the unit through the Y ES BRANCH    641   a  follows to box  642 , STATE  608 , into normal operation as depicted above. If the LOS is High the transceiver can move via the No branch  641   b  into the actual μOTDR operation, starting at STATE  612 . 
     The μOTDR operation starts with initialization: the TFR (internal variable that represents the round trip time of last reflection) is set to 0, the repeat measurement if failed k is set to 0, the flag FlgOTDRsuccess is set to 0, and the reflection counter j is also set at 0. Next step, the state machine checks if the μOTDR is disabled (can be disabled via bit  6  byte  123  in Table 1 for example). If it is disabled the unit returns to, STATE  602 , if not it enters, STATE  609  checking for LOS  120 . To enable measurement it should be High—no incoming power—because incoming power will interfere with operation and indicate an operational transceiver at the other side of the link. The LOS  120  check continuously for, as an example, 0.78 sec (longer then the random wait states Tx enable). If LOS  120  remains low for longer time the unit returns to, STATE  602 . Once LOS  120  is high the unit enters, STATE  613  RUN μOTDR, the k counter is incremented and TFR is set to the last recorded reflection time ( 0  if it is the first entry to STATE  613 ). Connector  3  on  FIG. 6   a  continues to  3  on  FIG. 6   b . Further delay is introduced to ensure that optical line does not have reflections from a previous pulse (in the case we got to this stage after a failed measurement pulses travelling in the fiber must be allowed to purge)—for example 800 mSec. Next the HPP (High Power Pulse) counter (n) is set to 0 since no pulse was transmitted yet for this measurement cycle. High Power Pulse (for example 489 nSec wide with peak power of 15 dBm) is sent via the driver circuitry ( 215   a  and  215   b ). An internal timer starts at the end of the HPP after waiting for TFR, such waiting ignores the reflections from closer faults already accounted for. 
     If no echo is detected, the output of the comparator  219 , SD  220  will not turn high and Tn will be set to 0. 
     If SD is Detected the timer records the time from end of HPP to the time a signal is received as Tn. 
     After the Tn is recorded the STATEMACHINE verifies if n (3 in the example) HPP were sent in the current test. If not—n is incremented and another HPP is transmitted. After transmitting the pulse n (e.g. 3) times the three records of the time to reflection (Tn) are compared (same time to reflection within a tolerance of 450 nSec—less than 50 in variation)—if they are the same—hence valid measurement—the average Tn from the n measurements (n is 3 in this example) is recorded as TRj—time to jth reflection and the FlgOTDRsuccess is set to High and the TRj time is converted to distance. It is possible that the n measurements will all be 0 no reflection detected after the previous one, actually EVERY measurement cycle is expected to end with n consecutive measurements of 0 no reflections after the once already recorded (or none at all). If the TRj  0  it means that the measurement did not detect any reflection. If TRj is valid (&gt;0) internal parameter TFR is set to TRj+1 uSec (ready to detect reflection 100 m farther away), k is set to 1 and j is incremented, and the measurement loop repeats by continuing from 44 and repeating the cycle. By adding the 1 uSec to the TFR we ensure that the next measurement will start sufficiently after the last measured reflection and thus be a valid new measurement. 
     The k counter is the counter of failed measurements. After each failed measurement the cycle re-starts (connection  4  between  FIG. 6   b  and  FIG. 6   a ) through, STATE  613  where k is incremented. If the three Tn measured were different (the test T 1 =T 2 = . . . =Tn had failed) the k counter is checked. If it is smaller than a preset number (e.g. 4) another try, through the loop to connector  4 , to the μOTDR Disable cheek and eventually to, STATE  613 , will commence. If it reaches the set number it means that there is a problem (for example a regular—non μOTDR—transceiver that transmits intermittently from the other side) and no further measurement can take place. 
     The j counter is the counter of valid reflections. The present invention can get to the check of j=0? If TRj=0 OR if k exceeds the pre-set value. 
     If j is 0—no reflection. OR measurement cannot be completed, RESET current reflection data to 0 reflections and the FlgOTDRsuccess is checked. If FlgOTDRsuccess is one it means that at least one successful measurement took place in a prior cycle—and since this measurement had failed something has changed. The FlgOTDRsuccess=1 it means that all n times were 0 and the branch will reset all current data (since we got here when j was 0). If we do not have an FlgOTDRsuccess we set j to  15  and none of the existing data will be re-written—we could not conclude this measurement cycle. 
     If j is &gt;0 (j=0 test results in No)—at least one reflection measurement cycle was concluded successfully so only data for j+1 and higher will be reset. Next all data is saved in memory  115 , and the farthest reflection will be stored in our example at a location (byte  124 ) page A2. Through connector  5  the state machine will return to, STATE  602  and enter  640 . 
       FIG. 6   c  depicts the DPS (Delta Power Sensor) and its integration with the μOTDR operation. Simple independent loop starting at 659 that resets the value of the LOS&lt;Rx&gt; counter in step  658  to S times 6 (S*6) is our example. We will use S as a scaling factor for various measurements in the following. This initial value represents the number of sampling events that must pass before another monitoring function starts. LOS&lt;Rx&gt; is a counter that counts down every time a new input power measurement. Rx Power is the input power measurement and it is typically implemented in fiber optic transceivers and is not explained here. 
     Delta Power Measurement (DPS) description: In order to measure changes in the incoming optical power, which may have originated due to fiber handling, disconnect, etc., an algorithm depicted below is used to eliminate measurement noise. STATE  611  is the start of DPS measurement. Other digital filtration and/or smoothing can be used and what is depicted here is for example only. Every cycle of the internal micro-controller  102  the input optical power is measured (designated measure Rx Power) and is present in memory  115 . Individual measurements are noise susceptible so a smoothing filter is used. In the example we use smooth parameter S to designate a generic family of smoothing filters. rxPwr that represents a smoothed measurement value, for example:
 
 rx Pwr= rx Pwr+(measured Rx  Power− rx Pwr)/ S   (Equation 3)
 
     Other smoothing filters are well known to a person knowledgeable in the art of digital signal processing. After each measurement cycle all the previous values are retained internally in sequence. Up to 3*S values of smoothed input power are kept in memory  102 . The latest value of the smoothed power rxPwr is rxPwr(0) and the “oldest” in our example is in a generalized form rxPwr(3*S−1). The LOS&lt;Rx&gt;counter is checked to see if the minimum number of measurements (6*S) in our example was completed. If not the LOS&lt;Rx&gt;counter is decremented and loop returns to  650 . After the minimum measurements were accumulated the LOS&lt;Rx&gt;counter is set to 0, and it will remain 0 until LOS or LOSout High will be detected in  659 . Once the amount of measurements are sufficient the rxPwr(0)—latest smoothed value is compared with rxPwr(m), m is the oldest measurement (3*S−1 measurements ago) to check if the difference is bigger that a set threshold at decision point  652 . 
     The formula depicted is:
 
ABS( rx Pwr[ m]−rx Pwr[0])&gt;MIN( rx Pwr[ m],rx Pwr[0])/ Q  
 
and
 
ABS( rx Pwr[ m]−rx Pwr[0])&gt; Q +2  (Equation 4)
 
     Equation 4 depicted above ( 52  in  FIG. 6   b ) uses Q variable to scale in reference to the smallest power of the two, rxPwr(0) and rxPwr(m). The condition of the absolute value of the change must be bigger than Q by 2 in this example to avoid noisy measurements for very low power values. The variable Q effectively sets the relative power change. Table 2 can be used to decide how much delta one wants to detect. A typical operating example can be S=8 and Q=16. 
     If the difference is bigger than the set threshold—the decision point  652  will result in Yes and further test—is μOTDR measurement after DPS is enabled (for example by bit  5  byte  123 ) if it is not enabled OR the TsC timer is smaller then set time between the last measurement (2 sec in this example) the  TRANSCEIVER  to,  STATE    602  box  639  (through connector  1  between  FIG. 6   c  and  FIG. 6   a )—monitoring the input power and causing the restart of the input signal evaluation. If the μOTDR measurement after DPS is enabled AND TsC is bigger than the set time, the TsC clock is reset and the  STATE MACHINE  proceeds to,  STATE    612  (through  CONNECTOR    2  between  FIG. 6   c  and  FIG. 6   a ) and the actual μOTDR measurement cycle will commence. Enabling μOTDR measurement after DPS ensure that at least an attempt to measure reflection will be conducted. When the 2  TRANSCEIVERS  at the two ends of the  LINK  are configured like that each can go through the same process. If only one detects the DPS and is attempting μOTDR measurement the Tx disable at  STATE    612  will cause the other end to have LOS high which restart that transceiver at  STATE    602   BOX    639  for its μOTDR operation to commence.