Optical time-domain reflectometry apparatus

Abstract: An optical time domain reflectometry (OTDR) apparatus uses Raman scattering light as a light source. A light pulse having a high intensity generated from laser diode pumped solid state laser (11) is launched into optical fiber (12) from its one end to generate induced Raman scattering. The induced Raman scattering light emitted from the other end of optical fiber (12) has a wide spectrum of wavelengths with strong intensity. Thus obtained induced Raman scattering light is introduced into optical fiber under test (9) from its one end. Subsequently, the backward scattering light returning to the aforementioned one end is taken out by means of optical coupler or optical switch (3) to brought into optical detectors (5), which, in turn, detects the light so as to be measured on the intensity of the particular wave components by every period of time by means of digital averaging circuit (7) and computer (8).

Description: Optical Time-Domain Reflectometry Apparatus The present invention relates to an optical time-domain reflectometry apparatus capable of measuring characteristics such as optical losses at any point along the longitudinal direction of an optical fiber. In the following a brief description is given about previous optical time-domain reflectrometry apparatus, as is described in "A Review of Optical Reflectrometry Data" in "Anritsu technical", No. 54, pp 117-123, September 1987. In an optical time-domain reflectometry (which will be abbreviated as OTDR hereinafter) apparatus, the loss distribution along the longitudinal direction of an optical fiber can be acquired by the following procedures. A light pulse generated from a light source is launched into an optical fiber under test from one end thereof by means of an optical coupler or optical switch. The light scattered inside the optical fiber under test, returning backward and outgoing from the aforementioned one end (known as backward scattering light), is split from the incident light by means of the optical coupler or optical switch. The backward scattering light, thereafter, is led to an optical receiver to be transformed into electric signals, and thus the intensity of the backward scattering light is obtained in correspondence with the passage of time. The variations in the intensity of the backward scattering light intensity are analyzed with respect to the passage of time from the time at which the light pulse is generated, in order to obtain the loss distribution along the longitudinal direction of the optical fiber. Since the intensity of the Rayleigh scattering light detected as the backward scattering light is weaker than one-ten thousandth that of the incident light, use of a laser diode (will be abbreviated as LD hereinafter) which is compact and has a high intensity of light has been made. For this light source, the LD devices respectively having wavelengths of 0.85 µm, 1.3 µm, and 1.55 µm are usually employed since these have been brought into commercial use for communication equipment. The conventional OTDR apparatus, however, has a problem that it is impossible to measure the loss with respect to arbitrary wavelengths in the wide wavelength range, except at 0.85 µm, 1.3 µm, and 1.55 µm. In other words, an LD with a wavelength other than the aforementioned wavelengths is hard to use practically since it is not available due to the lack of production, or due to extremely high cost. For these reasons, it is impossible to measure the loss increase at a wavelength of 1.38 µm which may arise when the optical fiber includes impurities such as water or the like. Accordingly, it is impossible to determine whether or not an optical fiber has any locally increased loss by being mixed with impurities in the course of manufacturing, and whether or not an optical fiber has any locally increased loss by water breaking thereinto when the optical fiber installed is submerged. Consequently the conventional apparatuses have difficulty evaluating important factors of optical fibers. On the other hand, there has been provided another conventional method which makes it possible to measure the loss characteristic of an optical fiber in a wavelength range other than 0.85 µm, 1.3 µm, and 1.55 µm, in such a manner that by leading light having a certain wavelength to one end of the optical fiber with the help of a white light source and spectroscope, the output light from the other end is detected to measure the total loss along the longitudinal direction of the long optical fiber. However with this method, the measurement cannot be made on the loss at each point along the longitudinal direction of the optical fiber, so that it is impossible to measure the loss caused by impurities existing locally along the longitudinal direction. From Frequenz, 35 243 et seq. (1981), H.F. Schlaak et al.: Optical Fiber Length Measurement by Pulsereflectometry, an OTDR apparatus is known comprising a light source for generating a light pulse. An introducing and extracting means for introducing the light pulse into an optical fiber under test and extracting the backward scattered light emitted from the optical fiber under test is provided. Optical detectors for transforming the backward scattering light into electrical signals is provided. A processing means analyzes the detected signal from the optical detectors with the passage of time to determine the length of the optical fiber under test. From US-A-4 063 106, it is known to use the stimulated Raman-scattering process in an optical fiber to generate several orders of Stokes radiation in an optical oscillator cavity. A plurality of wavelength peaks can be generated. From EP-A-0 405 553, an OTDR apparatus is known comprising a light source for generating a light pulse for an optical fiber to be tested. Three operation modes are given, light oscillation, light amplification, and light attenuation. A laser is used as the light source, which can be replaced by a fiber Raman amplifier. The wavelength of the laser is determined by a rare-earth element doped optical fiber. From IEEE, Journal of Quantum Electronics, QE-18, No. 10, October 1982, 1509 et seq., B. Costa et al.: Phase Shift Technique for the Measurement of Chromatic Dispersion in Optical Fibers Using LED's, it is known to use a single-mode fiber, though such a single-mode fiber is described as suffering from jitter and optical triggering instabilities. Therefore, it is suggested to get rid of any wavelength-dependent phase shift other than that due to the fiber to have a monochromator. From Journal of Lightwave Technology, Vol. LT-4, No. 7, 884-888, July 1986 a OTDR apparatus is known which forms the generic portion of claim 1. In the known OTDR apparatus a Nd:Yag laser is used, the pulses thereof being coupled with a simple lens into a single-mode fiber, 400 meter long, to generate stimulated Raman scattering. At the output of this fiber one obtains a typical multiply stimulated Raman spectrum, with four recognizable Stokes lines. It is therefore an object of the present invention to provide an OTDR apparatus at low cost which is readily capable of conducting measurement of an optical fiber on the loss characteristic at any wavelength within a wide range of wavelength. This object is solved by an OTDR apparatus with the features of claim 1. Preferred embodiments of the OTDR apparatus are the subject of the subclaims. In the OTDR apparatus of the present invention, a light pulse having a high intensity is projected into the optical fiber from one end thereof for generating induced Raman scattering wavelength-shifted light to be emitted from the other end thereof. The induced Raman scattering light is used as a light source to be incident to the optical fiber under test from one end thereof. In the case where a dispersion-shift fiber is employed as the aforementioned optical fiber, the induced Raman scattering light is to comprise Stokes light waves of first order, second, third, ... and higher orders. On the other hand, in a case where a lengthy single mode fiber is used as the optical fiber, the light is to have a continuous spectrum of wavelengths without any peak. In either case, the scattering light is to arise across the wide range of wavelengths longer than that of the incident light. The induced Raman scattering light thus generated has a considerable intensity and a high enough intensity required for the light source of the OTDR apparatus as some tens hundreds mili-watts. Accordingly, use of the induced Raman scattering light as incident light to the optical fiber under test from one end thereof, makes it possible to measure the loss characteristic and the like at arbitrary wavelength within a wide range of wavelengths. In addition, measurements on the wide range of wavelengths can be made by this apparatus alone. Moreover the application of the induced Raman scattering light to this apparatus decreases the cost of the apparatus. Fig.1 is a block diagram showing an embodiment of an OTDR apparatus according to the present invention; Fig.2 is a graph plotting spectral characteristics; Fig.3 is a schematic diagram showing a construction example of a spectroscopic device; Fig.4 is a block diagram showing another embodiment of an OTDR apparatus according to the present invention; Fig.5 is a graph showing several kinds of light-emission spectra under the influence of the non-linear optical effect; and Fig.6 is a graph showing characteristics of transmission loss caused by an optical fiber under test. As shown in Fig.1, a fiber Raman laser apparatus 1 includes a laser diode (LD) pumped solid state laser device 11, and an optical fiber 12 connected thereto. A light pulse generated by fiber Raman laser apparatus 1 is incident to the optical fiber under test 9 through an optical attenuator 2 and optical coupler or optical switch 3. As optical switch 3, for example, a sound effect optical switch and the like can be used. The Rayleigh backward scattering light generated inside this optical fiber under test 9 is split by the optical coupler or optical switch 3 to be introduced into a spectroscopic device 4. In this embodiment, two kinds of light waves having different wavelengths are taken out from the spectroscopic device 4, and respectively transformed into electric signals by an optical detector 5. The transformed electric signals are amplified by an amplifier 6 to be inputted to a digital averaging circuit 7, which is connected to, and interactively communicates with, a computer 8. The digital averaging circuit 7 transmits a trigger signal for generating a light pulse for the LD pumped solid state laser device 11. Synchronizing with a trigger signal, the digital averaging circuit 7 starts sampling of the input signals and conducting an A/D conversion to collect data as to the intensity of the backward scattering by every unit time from the time at which the light pulse entered the optical fiber under test 9. In addition, the digital averaging circuit 7 is adapted to take an arithmetical average of the data obtained by repetition of the light-pulse incidences, to thereby improve the S/N ratio. The data obtained are transmitted to the computer 8 to be subjected to operation processes. Subsequently, the computer 8 displays these data on a display device (not shown in the figure), plotting, for example, data magnitudes in the y-direction against data sampling times in the x-direction. Such a graph is used to illustrate the loss characteristic along the longitudinal direction of the measured optical fiber 9. When the noise level is low, the treatment of taking an arithmetical average of data becomes unnecessary, and consequently the electric signals obtained by the optical detectors 5 can be also observed directly through an oscilloscope. Next will be an explanation of the operation of the OTDR apparatus configurated as described above. The optical fiber 12 of the fiber Raman laser apparatus 1 is adapted to be provided with a pulse light having a higher output than a threshold above which induced Raman scattering occurs. To achieve this, the output of the light pulse should be necessarily more than some watts, but it is difficult for the usual LD to emit light with such a large output power. It is true that a solid state laser source as well as a gas laser source may emit a high output light pulse, but such apparatuses are bulky and need accessory systems such as a cooling system and the like, thus resulting in impracticality. To overcome this difficulty, the solid state laser device 11 using LD excitation which has recently been brought into commercial use is employed as the light source of this embodiment. This device is one in which a flesh lamp as a light source for exciting the conventional solid state laser is replaced with an LD, which has a good efficiency and can be miniaturized. As commercially available products of this device, there are known two kinds;one of which has for laser oscillation a solid crystal consisting of Nd-doped YAG (yttrium aluminum garnet; Y₃Al₅O₁₂) having an oscillating wave length of 1.06 um; the other of which has a solid crystal consisting of Nd-doped YLF (yttrium lithium tetra-fluoride; YLiF₄) having an oscillating wave length of 1.32 µm. In this embodiment the latter one is used for the LD pumped solid state laser device 11. When a high output light pulse having a light intensity equal to a threshold or more is incident to the optical fiber 12, the induced Raman scattering occurs. Generally, when light is introduced into an optical fiber, there occurs a Rayleigh scattering light wave having the same wavelength as that of the incident light and the Raman scattering light waves having shifted wavelengths. The intensity of these scattering light waves increases in proportion to that of the incident light (linear effect). However, when the light intensity of the incident light exceeds a certain threshold, there appears non-linear effect, that is, a sharp increase in the light intensity of the Raman scattering light. Subsequently, in the non-linear effect region, the higher order Stokes light waves, which are too faint to be observed in the linear region, become extremely large. Also, in the non-linear effect region, the components of the Rayleigh light and anti-Stokes light decrease largely, losing their energies in Stokes Light. Hence, the distribution of the induced Raman scattering light exhibits an aspect as shown by the dashed curve in Fig.2, having first order, second, third and higher order Stokes light waves generated on the side of longer wavelength than that of the incident light as shown by the solid curve. The incident light has a wavelength of 1.32 µm as stated above, and the wavelengths of the Stokes light waves of the first order, second order, third order and the fourth order, are respectively 1.403 µm, 1.495 µm, 1.600 µm and 1.721 µm. As is understood from Fig.2, this induced Raman scattering light has and therefore can provide in the wide range of wavelengths an intensity required for the OTDR apparatus greater than some hundreds mili-watts. Also as is apparent from the dashed curve in Fig.2, the intensity of the light is considerably high in wavelengths around the troughs and especially in the wavelengths around the peaks. With regard to an optical fiber, since the incident light is confined within the narrow space inside the core of the optical fiber, the threshold at which the induced Raman scattering becomes dominant is rather low, specifically some tens of watts for multi-mode optical fibers and some watts for single-mode optical fibers. The shifts for Stokes light waves of respective orders are determined depending on constituent molecules of the optical fiber such as Si-O and the like. The light having a wide range of wavelengths generated in the fiber Raman laser apparatus 1 is provided into the optical fiber 9 under test via the attenuator 2 and the optical coupler or optical switch 3. Here, the attenuator 2 is used for regulating the intensity of the incident light lower than the threshold in order to prevent the induced Raman scattering from arising inside the optical fiber under test 9. The spectroscopic device 4 is composed of, for example a diffraction grating, an optical filter film having dielectric multi-layers or the like. In the case of a diffraction grating being used, advantages as follows can be obtained: (a) a high insertion loss, (b) capability of narrowing the range of wavelengths of transmitted light, (c) freely setting up the range of wavelengths of the transmitted light by adjusting positions of the slit and the optical fiber on the light receiving side, and (d) feasibility of setting up a plurality of wavelengths for transmitted light waves at the same time by arranging a plurality of slits and optical fibers on the light receiving side. On the other hand, when an optical filter film is employed, the insertion loss is low, so that it is impossible to make markedly narrow the range of wavelengths of the transmitting light, but the structure is simple so that the device can be miniaturized. In this embodiment, the spectroscopic device 4 is structured with a diffraction grating 41 as shown in Fig.3. This diffraction grating 41 receives light having passed through an optical fiber 43 and a lens 44, and diffracts light to the receiving side. On the receiving side, two slits 42 are disposed to extract light waves from the light coming from the diffraction grating 41. The extracted light waves are introduced using respective optical fibers 43 via respective lenses 44. This arrangement makes it possible to pick up and transmit two kinds of light waves having different wavelengths. By setting up, for example, at 1.38 µm one wavelength of the transmitted light, whereas the other at any other wavelength, it is possible to detect the presence of an impurity of a water component, and at the same time, to measure the loss characteristic, with respect to any point along the longitudinal direction of the optical fiber under test 9. In the embodiment described above, the induced Raman scattering light having a wide range of wavelengths emitted from the optical fiber 12 of the fiber Raman laser apparatus 1 is incident to the optical fiber 9 under test, which in turn radiates Rayleigh scattering light having a wide range of wavelengths. Thus obtained backward scattering light is split by the optical coupler or optical switch 3 to be introduced to the spectroscopic device 4. Then the spectroscopic device 4 takes out particular narrowed ranges of wavelengths of the backward scattering light desired to be measured. However, it is possible to construct an apparatus in which the light emitted from the optical fiber 12 of the fiber Raman laser apparatus 1 is attenuated by the optical attenuator 2 to be directly subjected to a spectroscopic device 14. This spectroscopic device 14 extracts particular narrowed ranges of wavelengths desired to be measured from the induced Raman scattering light emitted from the optical fiber 12 having a wide range of wavelengths. The extracted light is introduced to optical fiber under test 9, which, in turn, radiates Rayleigh scattering light having the particular narrowed ranges of wavelengths. The obtained backward scattering light is split by the optical coupler or optical switch 3 to be introduced to the optical detector 5. As discussed heretofore, the OTDR apparatus of the present embodiment makes it possible to measure the loss of an optical fiber in a range of wavelengths in which the measurement used to be difficult, with a simple method at low cost. Moreover, it is possible for this apparatus alone to perform the measurement in a wide range of wavelengths. Next will be described another embodiment of the present invention. In the embodiment described above, a dispersion-shift fiber is employed as optical fiber 12, which produces under the influence of the non-linear optical effect the induced Raman scattering light having peaks of the first order Stokes light wave, the second order Stokes light wave, etc. (refer to Fig.2). The generated induced Raman scattering light is taken out and introduced into the optical fiber under test 9. However, when a single mode fiber is employed as the optical fiber 12, scattering light having a continuous spectrum of wavelength can be obtained due to its non-linear optical effect. Fig.5 shows several kinds of light-emission spectra under the influence of the non-linear optical effect, taking the wavelength of light emitted from optical fiber 12 as abscissa and the light intensity (with arbitrary unit) as the ordinate. In the figure, plot L1 shows the spectrum of the light-emission when a dispersion shift fiber (of 50 m length) is used, plot L2 shows the spectrum when a single mode fiber (of 50 m length) is used, and plot L3 shows the spectrum when a single mode fiber (of 5 m length) is used. Here, the incident light is a light pulse having a wavelength of 1.32 µm and a peak output power of about 100 watts with 30 ns in its pulse width. In the case of the dispersion shift fiber as shown by plot L1, a peak of the first order Stokes light due to the induced Raman scattering is observed. Even though a single mode fiber is used, in the case where the fiber is not longer than 10 m, there exists, as shown by plot L3, a peak to be considered as an induced Raman scattering. On the contrary, in the case where a single mode fiber having a length of 50 m is employed as the case of plot L2, with respect to the emitted light, no peaks are observed, but an even or flat light-emission spectrum is observed ranging from 1.34 to 1.7 µm in wavelength. Here, the emitted light has an intensity of several tens watts or more. Thus, a lengthy single mode fiber produces light having a continuous spectrum of wavelengths ranging from 1.34 to 1.7 µm, so that the use of an optical fiber of this kind makes it possible to construct an OTDR apparatus in which wavelength is variable. For example, light emitted from this non-linear optical fiber 12 and having a continuous spectrum of wavelengths ranging from 1.34 to 1.7 µm, was made to pass through a diffraction grating to obtain light waves each having a wavelength width of 5 nm. The obtained light was introduced into the optical fiber under test 9, and the transmission loss was calculated from the backward scattering light. Fig.6 shows a characteristic of transmission loss caused by an optical fiber under test, taking the wavelength of the incident light into the optical fiber under test 9 as abscissa and the transmission loss as ordinate. As apparent from Fig.6, there is an absorption at the wave length of 1.38 µm, and the loss is minimized in the wavelength band of 1.55 µm, whereas the loss increases with the wavelengths of 1.65 µm or more.

Claims: An OTDR apparatus characterized by comprising: a light source (11) for generating a light pulse; an optical fiber (12) having a first end and a second end for receiving said light pulse at the first end and emitting from the other end thereof wavelength-shifted light having wavelengths shifted from said received light pulse under the influence of the non-linear optical effect; said optical fiber (12) being made of a lengthy single mode optical fiber such that said wavelength-shifted light has a continuous spectrum of wavelengths without substantive peaks; an introducing and extracting means (3) for introducing said wavelength-shifted light into an optical fiber under test (9) and extracting the backward scattering light emitted from said optical fiber under test (9); optical detectors (5) for transforming said backward scattering light into electric signals; and a processing means (6,7,8) for analyzing detected signal from said optical detectors with the passage of time, to obtain a loss characteristic of said optical fiber under test. An OTDR apparatus according to claim 1, characterized in that said light source is a solid state laser device (11) which excites a laser beam by a laser diode. An OTDR apparatus according to claim 1 or 2, characterized in that said introducing and extracting means includes an optical coupler or optical switch (3) which introduces said wavelength-shifted light into said optical fiber under test (9), and splits said backward scattering light emitted from said optical fiber under test (9) to be introduced to said optical detectors (5). An OTDR apparatus according to claim 3, characterized in that said introducing and extracting means includes a first spectroscopic device (4) which separates said backward scattering light into its spectral components and extracts light waves having predetermined wavelengths to be introduced to said optical detectors (5). An OTDR apparatus according to claim 3, characterized in that said introducing and extracting means includes a second spectroscopic device (14) which separates said wavelength-shifted light into its spectral components and extracts light waves having predetermined wavelengths to be introduced to said optical coupler or optical switch (3). An OTDR apparatus according to claim 1, characterized in that said processing means determines the loss characteristic of said optical fiber under test (9) by sampling said detected signal from said optical detectors (5) by every unit period of time, and analyzing the obtained data with respect to the passage of time. An OTDR apparatus according to claim 6, characterized in that said processing means (7) transmits to said light source (11) a trigger signal for generating said light pulse, and starts processing of said backward scattering light with the passage of time by adopting the moment when said trigger signal is generated as an original time point. An OTDR apparatus according to claim 7, characterized in that said processing means (7) process data by taking an arithmetical average of measurements on backward scattering light corresponding to a plurality of light pulses. An OTDR apparatus according to any of claims 1 to 8, comprising an attenuator (2) for regulating the intensity of said wavelength- shifted light to be incident to said optical fiber under test (9).

Inventors: WADA FUMIO, WADA, FUMIO
Applicants: FUJIKURA LTD, FUJIKURA LTD.
Application year: 1992
Main classification: G01M11
IPC codes: G01M11