SYSTEM, METHOD AND DEVICE FOR MONITORING A FIBER OPTIC CABLE

Abstract: The invention relates to a system for monitoring a fiber optic cable (2) connected between optical signal transmitting (TX) and receiving (RX) equipment, and comprising first (3) and second (4) electro-optical devices for measuring optical power being both connected to a central control unit (14) over a bus (13) and permanently connected to the beginning and the end, respectively, of the fiber optic cable (2), at said transmitting (TX) and receiving (RX) equipment. The invention also relates to a compact electro-optical device (3) for measuring the operating optical power (P) of a fiber optic cable (2), having an optic input (5) and optic output (7), as well as an electric input (9) and electric output (11) and being of the type which comprises an optical power measuring circuit (20) having an input (19) connected to the optic input (5) and an output (21) connected to the electric output (11). The measuring device (3) further comprises an optical splitter (15) connected between the optic input (5) and the optic output (7). The optical splitter (15) has a secondary output (16) connected, through an electro-optical detector (17), to the optical power measuring circuit (20) and separates, on the secondary output (16), a fraction (PTX) of the operating optical power (P) according to a predetermined splitting ratio (RR). The fraction (PTX) of the operating optical power is processed by the optical power measuring circuit (20) and presented at the electric output (21) of the measuring circuit.

Description: "System, method and device for monitoring a fiber optic cable" DESCRIPTION Field of the Invention The present invention relates to a system for monitoring a fiber optic cable. Specifically, the invention concerns a system for monitoring a fiber optic cable connected between a transmitting equipment and a receiving equipment of optical signals, which system is connected to a central control unit through a bus and comprises compact electro¬ optical devices for measuring optical power which are permanently connected to the opposite ends of the fiber optic cable. The invention further relates to a method of monitoring a fiber optics link which includes the above monitoring system. The invention further relates to a compact electro-optical device for measuring optical power which is adapted to be permanently connected into a network of fiber optic cables, which devices have an optic input and optic output as well as an electric input and electric output, and are of a type which comprises an optical power measuring circuit having an input connected to the optic input and an output to the electric output, the electric output providing the value of the operating optical power being flowed through the fiber optic cable. Background Art As is well known, fiber optic cables are widely used in telecommunications networks. As a matter of fact, information exchanged on such optical carriers can improve the overall quality of telecommunications systems and their operational speed. In the specific field of this invention, a need exists for a quick troubleshooting procedure. Of special interest is an ability to prevent the occurrence of failures by spotting variations in the attenuation through each fiber optic cable, regarded as a sure indication of a deteriorating cable link. For the purpose, arrangements for monitoring fiber optic cable systems have been available which can provide useful information to the evaluation of the cables characteristics. A first prior art solution provides a measurement of the attenuation through a fiber optic cable "out-of-service" , that is: a cable which has been disconnected from the equipment to which it is normally connected. This prior solution consists of using a measuring system which simulates a transmitter/receiver system and inputs a test signal to the fiber optic cable. The cable performance can be checked upon the measures of the trasmitted and received signals. However, this first solution does not allow the performance of fiber optic cables "in-service", that is during their normal operation, to be checked. In addition, the monitoring of each cable and the network as a whole is time consuming. A second prior art solution provides instead a measurement of the attenuation through a fiber optic cable using a reflectometer or OTDR (Optical Time Domain Reflectometer) . The reflectometer is connected to a first end of a fiber optic cable and caused to send an optical test signal therethrough. From this first end, the strength and time of arrival of the optical wave reflected back through the cable are measured; from these values, one can work back to the attenuation value of the fiber optic cable and/or the location of a possible fault. Measurements with an OTDR test set are easily made on "out- of-service" cables, and are feasible on cables in operation. In the latter case, the optical test signal is added to the operating optical signal by means of a suitable optical coupler. It is on this account that the wavelength of the optical test signal and that of the operating optical signal must lie far apart, so as to minimize interference on said signal. Although achieving its objective, not even this second prior solution is entirely devoid of drawbacks, a major one being the bulk and high cost of an OTDR test set. Due to these limitations, one OTDR test set is usually shared by a plurality of cables which, therefore, can only be monitored on a cyclic basis. This involves, however, the use of a large number of optical switches. In addition, the presence of two optical signals with discrete wavelengths may cause noise to appear at the receiving end which cannot be completely suppressed neither using an optical filter. The technical problem underlying this invention is to provide an electro-optical measuring device, and relating system for the continual monitoring of a fiber optic cable, which have such constructional and functional features as to overcome the drawbacks with which prior art monitoring systems are still beset. „„„„ 97/10 - 4 - Summary of the Invention The solvent idea on which this invention is predicated is that of providing for continual measurement over time of the attenuation of a fiber optic, using compact electro- optical measuring devices which are permanently installed at the ends of the fiber optic. Based on the above solvent idea, the technical problem is solved by a system for monitoring a fiber optic cable connected between a transmitting equipment and a receiving equipment, as defined in Claim 1. The technical problem is also solved by a method of monitoring a fiber optic cable, wherein the attenuation through the fiber is found by measuring predetermined fractions of the optical power propagated through the fiber, at the beginning and the end of the fiber, as defined in Claim 4. The problem is further solved by the use of electro-optical devices for measuring optical power, which are provided in a highly compact form and tap off predetermined fractions of the operating optical signal at the beginning and the end of a fiber under test to calculate the average value of the power propagated through the fiber, as defined in Claim 15. The features and advantages of the monitoring system and the electro-optical measuring devices of this invention will be apparent from the following detailed description of embodiments thereof, given by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a diagramatic view of a monitoring system of a bidirectional connection of fiber optic cables according to the invention; Figure 2 is a diagramatic view showing schematically the monitoring system of Figure 1; Figure 3 shows diagramatically an electro-optical device for measuring optical power according to the invention, in a highly compact embodiment thereof; and Figure 4 shows diagramatically a modified embodiment of an electro-optical device for measuring optical power according to the invention. D tailed Description Referring to the drawing figures, generally shown at 1 is a system for monitoring a bidirectional connection of fiber optic cables 2 laid between a transmitting equipment TX and a receiving equipment RX. In particular, the fiber optic cables 2 are connected to an optic input 80 of an optic/electric converter O/E included in the receiving equipment RX and an optic output 50 of a laser transmitter LD included in the transmitting equipment TX. The most straightforward monitoring method of checking attenuation through a fiber optic cable 2 is to calculate the ratio (or the difference if the values are expressed in dBm) between a value of the transmitted operating optical power PT and a corresponding value of the received operating optical power PR. Advantageously, in the nethod according to the invention, this ratio between optical powers is provided on fractions, PT and PRX, of the overall optical power flowing through _,, O 97/10651 - o - the fiber optic cable 2, to which the fractions stand in a predetermined fixed splitting ratio, so that the value of the attenuation through the fiber optic cable can be correctly found. To that aim, the monitoring system 1 comprises first 3 and second 4 electro-optical devices for measuring optical power which are respectively connected to the beginning and the end of the fiber optic cable 2, at the transmitting equipment TX and the receiving equipment RX. The electro-optical devices 3 and 4 for measuring optical power have at least two optic inputs 5, 6 and at least two optic outputs 7, 8, as well as respective electric inputs 9, 10 and electric outputs 11, 12. Specifically, the electro-optical measuring device 3 has its optic input 5 connected to the output 50 of the transmitting equipment TX and its optic output 7 to the fiber optic cable 2. In a similar way, the electro-optical measuring device 4 has its optic input 6 connected to the fiber optic cable 2 and its optic output 8 connected to the input 80 of the receiving equipment RX. As shown in Figures 1 and 2, the monitoring system 1 is connected, through a bus 13, to a central control unit 14 which is external, or remotely located, from the monitoring system 1 and in which the attenuation through the fiber optic cable 2 is evaluated. Shown in Figure 3 is a preferred embodiment of an electro¬ optical device for measuring optical power according to this invention, e.g. the device 3. The electro-optical device 3 comprises an optical splitter 15 which is inserted between the optic input 5 and the optic output 7 and has a secondary output 16 connected to an electro-optical detector 17, in turn connected to an input 19 of an optical power measuring circuit 20. In particular, the optical splitter 15 taps, according to a predetermined splitting ratio RR, a fraction P X of the operating optical power P off the output 50 of the transmitting equipment TX. In a preferred embodiment, the optical splitter 15 is effective to split the optical power present on the optic input 5 between the optic output 7 and the secondary output 16, at a splitting ratio RR of 90:10. Advantageously, the electro-optical device 3 uses an optical splitter 15 having a low power additional insertion loss, i.e. of less than 0.25 dB. Advantageously, according to the invention, by the use of a high-sensitivity type of electro-optical detector 17 and a high-impedance pre-amplifier 18 the electro-optical device 3 is able to detect optical powers having a level lower than -50 dB. Thus, the measuring circuit 20 will supply, on an output 21, the value of the optical power fraction PTX present on the input 19. The output 21 is connected to an analog-to- digital converter 22 which converts the value of the optical power fraction P X into a binary code DPTX and transmits it, by means of a transmit protocol processor circuit 23, to the electric output 11 of the electro¬ optical measuring device 3, and thence to the central control unit 14. The transmit protocol processor circuit 23 is connected, in a one way manner, to a receipt protocol processor circuit 24 which is in its turn connected to the electric input 9 of the electro-optical device 3 and to the central control unit 14. The transmit protocol processor circuit 23 transmits, to the central control unit 14, the binary value of the outgoing optical power fraction measurement DPTX along with an indentifying binary code COD3 associated with the electro-optical measuring device 3 and recognizable by the central control unit 14. In a similar way, the receipt protocol processor circuit 24 can recognize the identifying binary code COD3 associated therewith and being transmitted from the central control unit 14. Advantageously, according to the invention, the electro¬ optical measuring device 3 further includes a remote- powering circuit 25 which uses a DC value Ic, transmitted over the bus 13, to generate a voltage Vs effective to power the electro-optical measuring device 3. Thus, additional devices can advantageously be connected to the bus 13 as adapted to the same function or to measure different magnitudes of the optical power, all are remotely powered and controlled through the bus 13 itself. The remote-powering circuit 25 is connected to the electric input 9 and the electric output 11 through a first resistor Rl and second uncoupling resistor R2, respectively. The uncoupling resistors, Rl and R2, separate all the useful electric signals, such as COD3 or DPTX, present on the bus 13 from the DC remote-powering current Ic. The operation of the monitoring system 1 according to the invention will now be described. The electro-optical measuring devices 3 and 4 are interrogated by the central control unit 14 using a polling or interrupt type of procedure. In either case, the Q O 97/10651 - 9 - electro-optical measuring devices 3 and 4 are input an identifying code COD 3 , COD4 and will output a binary value DPT / DPRX corresponding to the measured optical power fractions PTX PRX from the point where the electro- optical measuring device 3, 4 is connected, and the identifying code COD3 or COD4 of the answering measuring device. Particularly in the preferred embodiment of the invention, the exchange of information with the central control unit 14 takes place using a protocol of the serial type HDCL (High level Data Link Control) , as is appropriate for a configuration of the monitoring system wherein the electro¬ optical measuring devices 3, 4 are connected to each other and to the central control unit 14 through a single bus 13. Based upon the values of the received fractions of the optical power, DPTX and DPRX, the central control unit 14 can calculate the attenuation through the fiber optic cable 2 and, accordingly, check it for the performances or the possible faults. In actual practice, telecommunications networks often use bidirectional links, in the form of a pair of fiber optic cables 2, 2' connected to corresponding pairs of receiving RX, RX' and transmitting TX, TX' equipment, as shown in Figure 1. The central control unit 14 would receive, in this case, pairs of binary values of incoming DPRX, DP'RX and outgoing DPTX/ DP'TX optical power fractions, and evaluate the overall attenuations through the bidirectional links 2, 2' for both directions. The monitoring system of this invention is also applicable to fiber optics transmissions with connections in a point- to-point unidirectional configuration or a point-to- multipoint undirectional configuration. .„„, O 97/10651 - 10 - Of special importance is the application of ■ the monitoring system 1 to optical distribution networks with a tree type of architecture. The highly complicated construction of the optical links in such networks requires, in fact, that any deteriorations of which the variations of the attenuation in the flowing optical power through the connection is the most significant indication, be timely located therein. It would also be possible to have the electro-optical measuring devices of this invention connected to the outputs of the optical amplifiers as well, used in the point-to-multipoint distribution networks, so as to improve the reliability of the whole system. Figure 4 illustrates a modified embodiment of an electro¬ optical measuring device 3" according to the invention which can be used at the output of transmitting equipment TX in telecommunications networks equipped with automatic optical protection for personell safety. In such systems, in fact, the operating optical source, which supplies the operating optical power P and is connected within the transmitting equipment TX (and has been omitted from Figure 4 because conventional) , is turned off automatically on the occurrence of an interruption in the fiber due to breakage or disconnection. In a condition of no operating optical power P from the transmitting equipment TX it is impossible to tell failures due to broken fiber optic cables (which would have caused the optical source of the transmitting equipment TX to go off) from failures which have occurred in the transmitting equipment TX itself. To overcome this limitation, the electro-optical measuring device 3" further includes an optical coupler 26 connected across the optical splitter 15 and the optic output 7. The optical coupler 26 is used to add, to the operating optical signal with power P being propagated through the fiber optic cable 2, an auxiliary optical signal with power PA whose level lies below the danger threshold for any human operators of the system. The optical coupler 26 is connected to an auxiliary optical source 27, such as a low-power optical transmitter or LED, or a low-power laser, in turn connected to the output 21 of the measuring circuit 20 through a mean power detector 28. The operation will now be discussed of this modified embodiment of the electro-optical measuring device 3" according to the invention. Any time that there is no operating optical power P, the mean power detector 28 would turn on the auxiliary optical source 27 to supply an auxiliary power PA, at a much lower level than the operating power, to the link formed by the fiber optic cable 2. In this way, it becomes possible to discriminate among the following operational conditions: • Normal operation -- The received optical power fraction PRX has a value close to that of a fraction FP of the operating power P obtained in conformity with the splitting ratio RR introduced by the optical splitter 15. • Cable failure -- No received optical power fraction PRX- • Transmitter failure -- The received optical power fraction PRX has a reduced value determined by the auxiliary optical power PA. The electro-optical measuring device 3" of this invention is, therefore, capable of evaluating the attenuation of a fiber optic cable, and telling malfunctions occurred in the transmitting equipment from those due to a broken fiber optic. Advantageously, the electro-optical measuring devices 3, 3" of this invention can be implemented in a highly compact form on a single miniaturized module or single integrated circuit, thereby facilitating their installation within telecommunications networks and making for reduced cost and bulk. The electro-optical measuring devices 3 and 3" can be implemented with hybrid technology, using optical component and semiconductor circuit chips integrated on an alumina or silicon substrate. In particular, the optical splitter 15 and optical coupler 26 can be implemented with "fused fiber" technology, to afford very low insertion loss attenuations approximating theoretical design values. Where a silicon substrate is used, the optical splitter 15 and optical coupler 26 may be implemented using "optical wave guide" techniques wherein the light guides are formed on the same substrate. To summarize, the electro-optical devices for optical power measuring and the system for monitoring fiber optic cables provided by the invention allow the attenuation through, and hence the possible deterioration of, links formed by fiber optic cables to be checked while in normal operation, without interrupting or in any way disturbing an operating signal being transmitted. Furthermore, the electro-optical devices for optical power measuring, useful with telecommunications networks which incorporate automatic optical protection, according to this invention, can tell malfunctions due to a broken fiber optic cable from those due to a failure in the transmitting equipment.

Claims: CLAIMS 1. A system for monitoring a fiber optic cable (2) connected between an optical signal transmitting (TX) and receiving (RX) equipment, characterized in that it comprises first (3) and second (4) electro-optical devices for measuring optical power being both connected to a central control unit (14) over a bus (13) and permanently connected to the beginning and the end, respectively, of the fiber optic cable (2) in proximity of said transmitting (TX) and receiving (RX) equipment. 2. A monitoring system according to Claim 1, characterized in that it has respective optic inputs (5,6) connected to an output (50) of said transmitting equipment (TX) and to said fiber optic cable (2) , respective optic outputs (7,8) connected to said fiber optic cable (2) and to an input (80) of said receiving equipment (RX) , and respective electric inputs (9,10) and electric outputs (11,12) connected to said bus (13) . 3. A monitoring system according to Claim 2, characterized in that additional measuring devices are connected to said bus (13), and that said coupling resistors (R1,R2) separate each useful signal present on the bus (13) from a supply current (Ic) able to remotely power said additional measuring devices. 4. A method of monitoring a fiber optics link (2) established between optical signal transmitting (TX) and receiving (RX) equipment, being of the type which provides a measurement of the operating optical power (P) attenuation through the fiber optics link (2) , characterized in that the power attenuation is measured as the ratio between optical power fractions (PTX PRX) piched up on the transmit and receive ends according to a predetermined fixed splitting ratio (RR) to evaluate the performances of the fiber optics cable (2) . 5. A monitoring method according to Claim 4 and implemented by a monitoring system (1) comprised of first (3) and second (4) electro-optical measuring devices permanently connected to the beginning and the end of the fiber optics link (2) , at optical signal transmitting (TX) and receiving (RX) equipment, and connected to a central control unit over a bus (13), characterized in that, - the central control unit (14) interrogates the electro-optical measuring devices (3,4) to supply, to an electric input (9,10), a binary code (COD 3 ,COD 4 ) identifying the measuring device; the electro-optical measuring devices (3,4) supply, to an electric output (11,12) connected to the central control unit (14), first and second binary codes (DPTX / DPRX) respectively corresponding to the first and the second optical power fraction ( TX PRX) as measured at the point where the electro-optical measuring device (3,4) is connected, and the identification code (COD 3 ,COD4) of the answering device; and the central control unit (14) evaluates the attenuation of the fiber optics link (2) as the ratio between the transmit optical power fraction (PTX) measured by the first electro-optical measuring device (3) and the receive optical power fraction (PRX) measured by the second electro-optical measuring device (4) . 6. A monitoring method according to Claim 5, characterized in that the exchange of information with the central control unit (14) takes place through a transmit protocol processor circuit (23) and a receive protocol processor (24) , both processors using a protocol of the HDLC type . 7. A monitoring method according to Claim 5, characterized in that the central control unit (14) interrogates the electro-optical measuring devices (3,4) by a polling or an interrupt procedure. 8. A monitoring method according to Claim 4 and adapted to be applied even when an automatic protection for the optical safety in a network of fiber optics links is operated, characterized in that, with no operating optical power (P) on the fiber optics link (2) , an auxiliary power (PA) of lower level than the operating power (P) is input to the link (2) to discriminate among the following three operational conditions: normal operation -- the receive optical power fraction (PRX) has a value close to that of a fraction (FP) of the operating power (P) obtained in conformity with the splitting ratio RR; fiber optics link failure (2) -- no receive optical power fraction (PRX) ; and - transmitting equipment (TX) failure -- the receive optical power fraction (PRX) has a reduced value determined by the auxiliary optical power (PA) . 9. A monitoring method according to Claim 8 and implemented by a monitoring system (1) comprised of first (3) and second (4) electro-optical measuring devices permanently connected to the beginning and the end of the fiber optics link (2) , at optical signal transmitting (TX) and receiving (RX) equipment, and connected to a central control unit over a bus (13) , characterized in that said first electro-optical measuring device (3) comprises a mean power detector (28) connected to an auxiliary optical „„„„.„ - 17 - PCT/IT95/00150 O 97/10651 source (27) adequate to supply said auxiliary optical power (PA) . 10. A monitoring method according to Claim 9, characterized in that said mean power detector (28) turns on said auxiliary optical source (27) whenever no operating optical power (P) is present on the fiber optics link (2) . 11. A monitoring method according to Claim 4, characterized in that said fiber optics link (2) comprises a fiber optic cable. 12. A monitoring method according to Claim 4, characterized in that said fiber optics link (2) is of the bidirectional type and comprises a pair of fiber optic cables (2,2') connected between corresponding pairs of optical signal transmitting (TX,TX') and receiving (RX,RX') equipment. 13. A monitoring method according to Claim 12, characterized in that said bidirectional link has a unidirectional point-to-point configuration. 14. A monitoring method according to Claim 12, characterized in that said bidirectional fiber optic connection has a unidirectional point-to-multipoint configuration. 15. A compact electro-optical device (3) for measuring the operating optical power (P) of a fiber optic cable (2) , having an optic input (5) and optic output (7) as well as an electric input (9) and electric output (11) and being of the type which comprises an optical power measuring circuit (20) having an input (19) connected to said optic input (5) and an output (21) connected to said electric output (11) , characterized in that it comprises an optical splitter (15) connected between said optic input (5) and said optic output (7) and having a secondary output (16) connected, through an electro-optical detector (17) , to said optical power measuring circuit (20) , said optical splitter (15) being operative to split on said secondary output (16) a fraction (PTX) of the operating optical power (P) according to a predetermined splitting ratio (RR) , and said fraction (PTX) of the operating optical power being processed by the optical power measuring circuit (20) and presented at the electric output (21) of said circuit. 16. A device according to Claim 15, characterized in that the output (21) of said optical power measuring circuit (20) is connected to an analog-to-digital converter (22) operative to convert the value of the optical power fraction (PTX) to a binary code (DPTX) which is transmitted to the electric output (11) along with a further binary code (COD 3 ) identifying the measuring device (3). 17. A device according to Claim 16, characterized in that said binary codes (DPTX) are transmitted through a transmit protocol processor circuit (23) connected to a receive protocol processor (24) connected, in turn, to said electric input (9) for receiving the identification binary code (COD 3 ) . 18. A device according to Claim 15, characterized in that said optical splitter (15) is connected to said optical power measuring circuit (20) through a pre-amplifier (18) . 19. A device according to Claim 15, characterized in that it comprises a remote-powering circuit (25) operative to generate an adequate voltage (Vs) to power the electro¬ optical measuring device (3) and being connected to the electric input (9) and the electric output (11) through first (Rl) and second (R2) blocking resistors, respectively. 20. A device according to Claim 15, characterized in that said optical splitter (15) performs a division of the optical power present on the optic input (5) to the optic output (7) and the secondary output (16) in a splitting ratio (RR) of 90:10. 21. A device according to Claim 18, characterized in that said electro-optical detector (17) is a high-sensitivity type and said pre-amplifier (18) is a high-impedance type. 22. A device according to Claim 15, characterized in that said optical splitter (15) has an addittional insertion loss lower than 0.25 dB. 23. A device according to Claim 19 and of a type which remains active even after an automatic optical safety for a network of fiber optic cables (2) has been operated, characterized in that it comprises an optical coupler (26) connected between the optical splitter (15) and the optic output (7) and connected to said optical power measuring circuit (20) , said optical coupler (26) adding an auxiliary optical power signal (PA) , whose level lies below a danger threshold, to the operating power signal (P) being propagated through the fiber optic cable (2) . 24. A device according to Claim 23, characterized in that said optical coupler (26) is connected to an auxiliary optical source (27) in turn connected to the output (21) of the measuring circuit (20) through a mean power detector (28) . 25. A device according to Claim 24, characterized in that said auxiliary optical source (27) is a low-power optical transmitter LED or low-power laser.

Inventors: LEALI SERGIO, LEALI, SERGIO
Applicants: LEALI SERGIO, PLLB ELETTRONICA S P A, LEALI, SERGIO, PLLB ELETTRONICA S.P.A.
Application year: 1995
Main classification: H04B10
IPC codes: G01M11, H04B10