Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
  • G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
  • G01M11/31—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter and a light receiver being disposed at the same side of a fibre or waveguide end-face, e.g. reflectometers
  • G01M11/3109—Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR
  • G01M11/3145—Details of the optoelectronics or data analysis

Definitions

• 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.
• OTDR Technique Optical Time Domain Reflectometer
• 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.
• 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.
• a method for detecting the breaks in an optical fiber link 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.
• a control means such as an oscilloscope
• 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.
• 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.
• FIG. 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 .
• the laser diode 1 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.
• 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 b ⁇ 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.
• the reference numbers used in Figure 1, designate the corresponding elements .
• 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 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.
• 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:
• 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) .
• 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.
• Horizontal scale 100 nsec /div .
• / ⁇ o is the emission wavelength in the bias- conditions; (when the bias current intensity increases, ⁇ o is shifted downwards and vice versa.
• ⁇ 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 ⁇ o for the two reported cases.
• 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 .

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Optics & Photonics (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• General Physics & Mathematics (AREA)
• Optical Communication System (AREA)

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• 1984
  • 1984-06-13 IT IT48394/84A patent/IT1179209B/it active
• 1985
  • 1985-06-11 WO PCT/IT1985/000007 patent/WO1986000134A1/en unknown
  • 1985-06-11 EP EP85903032A patent/EP0216773A1/de not_active Withdrawn
  • 1985-06-11 AU AU44319/85A patent/AU4431985A/en not_active Abandoned

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