A METHOD FOR DETECTING FAULTS OR BREAKS IN OPTICAL FIBER LINKS

Abstract: In an optical fiber transmission, a laser diode (1), acting as transmittor in one of the stations of the system, is biased slightly below threshold value, just after the sending of a light pulse in an optical fiber (2). The pulse, which is backscattered by a fault or break in the optical fiber section, is re-injected in the laser diode (1), which amplifies this pulse, after being biased below the threshold, up to about 20dB and is then detected by a detecting means (3), such a monitoring photodiode, positioned in the front of the rear facet of the laser. The signal detected by said detecting means (3) is sent through a line (4) to a control means, such an oscilloscope or other processing unity, provided with a grafic recorder, by means of which the position and the nature of the fault or break of the optical fiber can be detected.

Description: - I - A METHOD FOR DETECTING FAULTS OR BREAKS IN OPTICAL FIBER LINKS The present invention relates to a method and apparatus for detecting the faults in optical fiber links. The fault location in an optical fiber link is commonly achieved by employing the OTDR Technique (Optical Time Domain Reflectometer) , which requires the direct observation of the light pulse backscattered by the break. However , this method which can be easily used in a Laboratory and which is extremely sensitive , implies the modification of the transmission apparatus , consisting in a separation of the input light beam from the backscattered one. More in particular , between the transmitter or laser and the optical fiber must be interposed a kind of directional coupler consisting of a set of lenses and of a ray divider or semire fleeting mirror by means of which the backscattered signal is applied to a photodiode, which is connected , for instance , to a processing unit, connected in turn to a graphic recorder. The measurement of the time which elapses between the detection by the photodiode , of the signal reflected from the input surface of the optical fiber and that which has been reflected by the break of said fiber, when the refractive index is known, enables to localize the break point. As has already mentioned this method has the inconvenience of requiring a substantial modification of the whole transmission system. Even if that can be easily carried out in some repeater stations, that becomes absolutely impossible in other stations , i.e. in those stations which are placed in zones difficult to be approached , as, for instance , in the case of submarine cables . In order to overcome the aforementioned obstacle according to the present invention a method for detecting the breaks in an optical fiber link is provided , according to which a light pulse sent in an optical fiber by a laser diode, when backscattered by a break or fault in a link section, is amplified by the laser diode which is biased, lightly below the threshold value, immediatly after the light pulse emission thereof, and which will be detected by a detecting means, as, for instance , a monitoring photodiode, mounted in the front of the rear facet of the laser, the signal emitted from said photodiode being then sent to a control means, such as an oscilloscope, so as to allow to localize the break point in said fiber link section, on the base of the delay of the backscattered pulse with regard to the emitted pulse, when the speed of the light pulse in the optical fiber is known. This method presents very important advantages . For carr ing out the line control, only the various devices usually employed for such a purpos&s are necessary , which are employed in a trasmission system using optical fibers for accomplishing transmission function and without varying their different disposition , since the same transmitter can be easily changed over by the use of only electronic means, so as to let be able to act as a line cotrol means. It is not necessary to intervene for modifying the coupling between the transmitter and the optical fiber link, by employing semire fleeting mirrors or directional couplings in order to allow to insert means fσr detecting the break, since a monitoring photodiode is commonly already provided near the rear facet of the laser diode in order to control the operation thereof , as transmitter. According to another aspect of this invention, relating the localization of the break, the use of a monitoring photodiode could be also avoided, since it is s-ufficient to verify the variation of the voltage in the laser diode, due to the backscattered -pulse created by the break or faults and re-inj ected toward the laser diode. The characteristics of the present invention will be better understood from the following description of an embodiment of the invention and from an experimental test, reference being made to the accompanying drawings , in which: Figure 1 is a block diagram of a part of an apparatus embody ing the method of the present invention for an optical trasmission of a light beam; Figure 2 is a diagrammatic view of an experimental embodiment disposition for carrying out the method of the present invention; Figure 3 shows a diagram of the pulse coming from the laser output, in particular exciting conditions, i.e. the light pulse emitted by the laser, and the backscattered pulse which is amplified by the same laser; Figures 4a and 4b_ show the diagram of the shapes of the pulses amplified by a reference system similar to that of Figure 3, and in which the curves of Figure 4a and of Figure 4b_ are vertically shifted one below the other for an easier inspection; Figure 5 shows diagrammatically the diagram of the wavelength shift as a function of the time t during the emission of the light pulse. Referring to the Figure 1, 1 indicates a semiconductor laser diode, and 2 indicates an optical fiber link, 3 is a monitoring photodiode and 4 is a tramsission line of the detected pulse. Figure 1 represents diagrammatically an intermediate station, fed through the line la and lb for the driving and the bias of the laser 1, similar to any of other repeater stations of a system of optical fiber transmission . In order to control the integrity of the optical fiber along the line section concerning the shown transmitter , the laser diode 1 must biased slightly below the threshold value just after the emission of a short light pulse. The light pulse partially reflected when a break takes place or on account of any other defect present in a certain point of the fiber section , will be re-injected in the emission source, i.e. in the laser diode 1, where said light pulse is amplified , owing to the amplification capacity of the laser, when biased below the threshold value. Such an aplified signal can be then detected by the photodiode 3 and sent through the line 4 to a control means, which, owing to the typical characteristics of the signal of the intermediate station concerned , and to the information given by said signal, the break position can be localized. The localisation of the optical fiber break could be also carried out, without the use of the photodiode 3, when a control instrument is inserted capable of measuring the current of the laser diode 1, while the effect of the presence of the backscattered pulse can bβ verified and applied to the laser diode, which has been biased slightly below the threshold value. This method has been proved by the use of the experimental apparatus shown in Figure 2. In this latter the reference numbers , used in Figure 1, designate the corresponding elements . Furthermore 5a and 5b_ are lenses for the collimation and focusing of the light beam; 6 is a light beam divider , 7 is a neutral density filter, 8 is a coupling filter, 9 is a mirror, 10 is an APD (avalanche photodiode) and 11 is an oscilloscope. The exciting source 1 is a gain guided semiconductor laser diode of the LCW 10 type of the ITT, working at a wavelength of 850 nm and having a threshold current of 61.8 raA. From the laser diode 1, before its bias below threshold , a light signal is sent to the monomode optical fiber 2 having a lenght of 400 m and possessing a cut off wavelength of 750 nm. A prism 8, in contact through an index-matching liquid, with the input face of the optical fiber 2, is used, in order to avoid coupling back in the laser the Fresnel reflections . A break or fault of the optical fiber 2 is simulated by its end face opposite to the input face, where is mounted the mirror 9, which has the task to enhance the reflected signal intensit ; the set of the neutral density (ND) filters 7 has the purpose of attenuating the signal. The beam divider too is a semiref lending mirror. Owing to such a disposition , the signal, reflected by the mirror 9 passes again through the other ones, and in particular the beam divider 6 from which it is partially deviated , and is re-injected in the laser diode 1, biased below threshold. Since the employed laser 1 has not a direct access to the rear facet, the amplified signal is received, through the semireflective mirror 6, in the avalanche photodiode 10, followed by the oscilloscope 11. It was possible to study in what a manner the amplification of the backscattered signal depends on the intensity of this latter, when the laser diode is biased slightly below the threshold at various levels of current and which is driven by pulses of rectangular shape, and of variable amplitude and width. Figure 3 shows the diagram of a typical laser output exhibiting, together with the exciting pulse, the presence of a lower pulse due to the amplified backscattered radiation, and spaced apart from the main pulse of a distance of 4, 3 JUs. The reference system is that of an oscilloscope, in which the axis of the abscissae is the time axis in nsec /div , while the ordinate axis relates to the voltage detected on the photodiode . The operative conditions are the following ones: Bias current intensity = 61.6 mA, Peak current intensity = 2.25 mA, Horizontal scale = 500 nsec /div. The zero is 3 divisions above the top of the picture. Figures 4a and 4b show how the shape of the reflected pulse depends on the value of the bias current and on the driving pulse amplitude . R ferring to Figure 4a the ope ative conditions of the various curves, which are vertically shifted for an easier inspection , are respectively the following ones, starting from above: Bias current intensity = 61.5 mA; 61.0 mA; 60.5 mA; 60.0 mA; 59.5 mA and 59.0 mA. Peak current intensity = 7mA for all the curves. Horizontal scale = 100 nsec /div . The variation in the shape of the amplified backscattered pulses, as a function of the driving condition , can be understood considering the wavelength selectively of a Fabry Perot amplifier (see: J.C. Simon "Light Amplifiers in Optical Communication System" ) and the frequency chirping of the transmitted pulses (see: D.D. Cook and F.R. Nash -"Gain-induced Guiding and Astigmatic Output Beam of GaAs lasers "-Appl .Phys . 1975, 46 pages 1660- 1672) . More in particular, the amplification of the re-injected optical signal has a maximum, when its wavelength equals the resonance wavelength of the laser cavity in its actual bias condition and decreases as the wavelength moves away from this condition (as stated by J.C. Simon and by A. Yariv- "Quantum Electronics" 2.nd Edition -John Wiley and Sons, New York, 1975). Figure 4b_ shows, that, when, on the contrary , the bias is fixed and the pulse amplitude is increased toward larger and larger values, although the initial wavelength shift remains constant, the corresponding increase-rate of the temperature causes again an effect similar to the previous one, as can be well seen in said Figure 4b. The operating conditions, concerning Figure 4b_, starting from above, are the following ones: Bias current intensity = 61.5 mA for all the curves. Peak current intensity = 7.0 mA; 4.45 mA; 3.15 mA; 2.25 mA and 1.8 mA. Horizontal scale = 100 nsec /div . In Figure 5 on the ordinate axis are indicated the wavelengths Λ and on the ascissa axis are indicated the times . /{o is the emission wavelength in the bias- conditions; (when the bias current intensity increases, λo is shifted downwards and vice versa. Δλ is the initial wavelength the total wavelength shifts for two different amplitudes of the pulse; is the pulse duration and t' and t" are the instants at which the wavelenght , during the pulse emission , concides with λo for the two reported cases. In this Figure 5 is shown, as the wavelength of the transmitted pulse changes during the application of the electric pulse, due to two counter facing phenomena: at first the initial growth of the carrier density causes a lowering of the emission wavelength with respect to the bias conditions (see the already cited report of Cook and Nash); then the temperature increase , which follows and which is proportional to the pulse amplitude, causes a progressive increasing of the emission wavelength. This mechanism implies that the maximum amplification condition has to be fulfilled only at a well defined instant , i.e. at the time t' or t" in the one or the other of the represented cases ( Λ of the r flected pulse = Λo of the resonance cavity ) . The above argument allows to understand the different behaviours of the curves, shown in Figures 3a, 3b. In fact, by varying the bias current starting from the lower values towards the threshold value, the difference in the carrier density (i.e. the wavelength shift), just before and after the application of the driving pulse, tends to be lower and lower. On the other hand, since the temperature swing is almost insensitive to the b_ias conditions , the instant , at which the wavelength of the laser in the bias state, and that of the backscattered pulse concide- (points t' t" in Figure 5), as a result of the bias current increasing, it will shift towards the beginning of the re-injected pulse. This behaviour is experimentall , shown in Figure 4a. On the contrar , when the bias is fixed and the pulse amplitude is increased toward larger and larger values as above already mentioned, although the initial wavelength remains constant, corresponding increase-rate of the temperature causes again tin effect similar to the previous one, as has shown in Figure 4b. In order to estimate the method sensitivity , it is to be noted, r ference being made to the Figure 3, that the power re-injected into the laser 1 has been measured to be about 5xl0~ times the power emitted by the laser 1; from that, considering a coupling efficiency of 40% between the transmitter and the fiber 2, and a reflection coefficient of 0.04 from the supposed break along the fiber length, one can estimate an allowable loss of 5.5 dB (single way) of the fiber 2, to get a result equal to that shown in Figure 3. Using the above value for the ratio between the power, re-injected into the laser 1, and that emitted, one can also evaluate the gain of the laser, used in the conditions to which the Figure 3 concerns . From the inspection of the curve in Figure 3, it turns to be of about 21 dB (said value being in good agreement with the results reported by J.C. Simon, as has been already before mentioned) . As a conclusion , the experiment shows the feasibility of using the same laser as transmitter, to perform amplification of the backscattered pulse due to a fiber fault or break. The amplified pulse can be easily detected by the monitoring photodiode usually present in the optical transmission system. This gives two distint advantages : the use of lossy semire fleeting mirrors or directional couplers between the laser and the fiber is avoided , and the same trasmitter device can be easily switched only by electronic means, to perform a line monitoring function .

Claims: CLAIMS 1. A method for detecting faults or breaks in an optical fiber link of a transmission system, comprising a plurality of repeating stations , each including , as a transmitter, a semiconductor laser diode, connected to an optical fiber (2), and provided , in correspondance of its rear facet a detecting means, such as a monitoring photodiode (3), characterized by the fact that the laser photodiode (1) is associated with biasing means, which is capable of creating a bias slightly below the threshold value, just after the emission of a light pulse, so that the light pulse emitted in the optical fi'ber (2), when backscattered by a fiber break or fault is re-injected into the laser diode (1) , which is now biased below the threshold so as to act as an amplifier, capable of amplifying the backscattered pulse, detected by a monitoring means, such as a monitoring photodiode (3). 2. A method according to claim 1, wherein through a telecontrol system of the plurality of the transmitters , an exciting control is sent to the transmitter concerning the optical fiber link section to be controlled (2), for producing a ligth pulse and for changing the bias condition thereof , while subsequently in the central control station the signal is detected by a monitoring photodiode capable of localizing the break point on the base of the time delay of the amplified backscattered signal with respect to the time of the emitted pulse, knowing the light speed in the optical fiber link (2) . 3. A method for detecting the faults or breaks in an optical fiber link of a transmission system according to claim 1, comprising a plurality of repeater stations inserted in the link, each including, as transmitter source, a semiconductor laser diode (1) connected to an optical fiber (2), characterized by the fact that the fault or break localisation is obtained by the analysis of t-he variation of the exciting voltage or current of the laser diode (1) caused by a backscattered pulse, re- injected in the same laser.

Inventors: DAINO BENEDETTO, SPANO PAOLO, DAINO, BENEDETTO, SPANO, PAOLO
Applicants: BORDONI UGO FONDAZIONE, FONDAZIONE UGO BORDONI
Application year: 1985
Main classification: G01M11
IPC codes: G01M11