EP0477189A1 - Device and process for investigating the damping cycle of a light-waveguide - Google Patents
Device and process for investigating the damping cycle of a light-waveguideInfo
- Publication number
- EP0477189A1 EP0477189A1 EP19900906903 EP90906903A EP0477189A1 EP 0477189 A1 EP0477189 A1 EP 0477189A1 EP 19900906903 EP19900906903 EP 19900906903 EP 90906903 A EP90906903 A EP 90906903A EP 0477189 A1 EP0477189 A1 EP 0477189A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- signal
- optical
- optical waveguide
- examined
- output signal
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
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
Definitions
- the invention relates to a device for examining the attenuation curve of an optical waveguide with an optical transmitter with a signal input, to which test pulses from a pulse generator can be applied, with an optical branching device via which one end of the optical waveguide to be examined is connected to the optical transmitter , with an optical receiver, which is connected via the optical splitter to one end of the optical waveguide, and with a filter unit downstream of the optical receiver, at the output of which an analog output signal can be tapped.
- an optical transmitter is excited by test pulses from a pulse generator to emit light pulses.
- the light pulses are coupled into an optical waveguide to be examined via an optical splitter. Because of the attenuation (Rayleigh backscattering) in the optical waveguide and at light fiber interference points (Fresnel reflections), reflected light signals pass through the optical splitter to an optical receiver formed by a photodiode.
- a signal processing branch downstream of the optical receiver contains, in addition to amplifier elements, an integrating element which acts as a filter unit designed as a low-pass filter.
- an analog output signal can be tapped, which depicts the attenuation curve of the optical waveguide to be examined.
- this output signal can be described by a summation of time-limited e-functions as a result of the Rayleigh backscattering, at whose joints Dirac-I pulses may be due to Fresnel reflections.
- the mapping of the attenuation curve can be signal-theoretically taking into account the spectral noise power density of the optical receiver as the convolution product of the impulse response of a low-pass filter. filters with a sum formed from the spectral noise power density of the optical receiver and the convolution product of the test pulse with the attenuation profile of the optical waveguide to be examined (cf. Eq. 1).
- n R ⁇ (t) spectral noise power density of the optical receiver h.
- p - r (t) impulse response of the low-pass filter
- imaging errors occur, the duration of which is determined by the duration of the convolution product of the test pulse with the impulse response of the low-pass filter.
- the impulse response of a system is understood to mean the system reaction (ie the output signal) of a system when a Dirac impulse is applied to the input (see, for example, Otto Mildenberger "Fundamentals of System Theory for Telecommunications Engineers", Hänser Verlag, 1981, p. 48-50).
- the following contradictory requirements are therefore placed on the low-pass filter: on the one hand, the low-pass filter should have a low cut-off frequency in order to achieve the highest possible signal-to-noise ratio.
- a high cut-off frequency of the low-pass filter is desired.
- the measurement error resulting from the low-pass filtering is particularly large where the backscatter signal - at jump points at which the attenuation curve of the optical waveguide to be examined changes suddenly, or at impurities causing Fresnel reflections - experiences large changes in value. This can be done in the familiar direction used low-pass filter represent only a compromise of the above requirements.
- the invention has for its object to provide a device for examining the attenuation curve of optical fibers of the type mentioned, whose output signal has the best possible signal-to-noise ratio with a small measurement error and with which statements about the attenuation curve also in the area of large changes in value of the backscatter signal of the optical fiber to be examined can be made.
- the filter unit is a signal-matched filter.
- a signal-adapted filter is to be understood as a filter whose impulse response is a time-inverse image of the signal to be filtered.
- An advantage of the device according to the Invention is that the imaging error determined by the signal-matched filter is exactly limited to a certain period of time. Since the duration of the imaging error is determined by the duration of the convolution product of the test pulse and the impulse response of the signal-adapted filter, the image is also due to the time limitation of the impulse response of a signal-matched filter to twice the pulse duration of the test pulse The duration of the application error is limited to twice the pulse duration.
- the temporal extent of the imaging error of the device according to the invention therefore does not depend on the course of the signal as a result of a fault or jump point of the optical fiber to be examined.
- the device according to the invention is also advantageously characterized by an extraordinarily favorable signal-to-noise ratio of the output signal. This leads to a considerable reduction in the required measuring times.
- the signal-adapted filter is a convolver which has a first input connected to the optical receiver and a further input which can be acted upon by the time-inverse test pulses of the pulse generator.
- a so-called surface wave convolver can be used, for example, which has two signal inputs, each of which is formed by an input of an interdigital transducer.
- Each interdigital transducer converts the input signals into surface waves which move along a track defined by an integration electrode.
- An output signal which can be tapped at the integration electrode corresponds to the convolution product of the input signals and is therefore a measure of the correlation of the input signals.
- Such a convolver is characterized by a simple and robust construction.
- the impulse response of the convolver used as a matched filter is determined by the input signal present at its further signal input.
- the convolver thus lends the device according to the invention to
- a device for examining optical waveguides is known (F. Sischka, Electronics 3 / 05.02.88, pages 76 ff), in which a correlator is used to evaluate the signals scattered back from the optical waveguide to be examined. is applied;
- pulse trains formed from complementary codes are coupled into the optical waveguide to be examined and the signals scattered back from the optical waveguide to be examined are amplified and fed to an A / D converter. Only after this A / D conversion is there a correlation with the original codes.
- the correlation technique used in the known device cannot be used in the area of interference points causing Fresnel reflections, because the relatively high backscatter signal that occurs in this case leads to an overflow of the A / D converter.
- the imaging error in the device according to the invention is limited to twice the pulse duration of the test pulse.
- the optical receiver of the device according to the invention is only operated in a narrow area of its characteristic curve, no optical receiver with a highly linear characteristic curve is required in a wide area;
- an inexpensive and powerful AP diode which has no highly linear characteristic curve, can advantageously be used as the optical receiver.
- the device according to the invention also has the advantage in terms of circuit complexity that it allows the examination of the entire optical waveguide to be examined in a single operating mode and thus with a single circuit arrangement. Since the impulse response of the signal-matched filter is determined by the signal present at the further input of the convolver, temperature fluctuations of the signal-matched filter have no influence on its impulse response and the output signal of the device according to the invention.
- a method for examining an optical waveguide with the device provides that an auxiliary variable is formed by logarithmization and time derivation of the output signal of the signal-matched filter, that it is checked whether a time interval exists in which the auxiliary variable is not constant and that when such a time interval corresponding to twice the pulse duration of the test pulses is found, based on knowledge of the attenuation curve of the optical waveguide to be examined, following the time interval, the attenuation curve in the section of the light waveguide to be examined corresponding to the time interval by extra polation is determined.
- the imaging error of the device according to the invention which occurs as a result of a fault or jump point in the optical waveguide to be examined is limited exactly to twice the pulse duration of the test pulses, the presence of a jump point or a Fresnel can be derived from the progression of the logarithmic output signal over time -Reflection causing reflections are closed if this course is not constant in a time interval corresponding to twice the pulse duration of the test pulse. Since the determined attenuation curve of the optical waveguide no longer has any imaging errors in the signal-adapted filter after the time span, extrapolation can be used to draw a largely error-free conclusion on the course within the section corresponding to the time interval.
- An advantageous further development of the method according to the invention consists in that when a time interval exceeding twice the pulse duration of the test pulses is found, the pulse duration of the test pulses is reduced. If the time interval exceeds twice the pulse duration of the test pulses, then this is a reliable indication of the presence of at least one further defect, the at least two defects not being resolvable with the local resolution predetermined by the pulse duration of the test pulses.
- This finding can be used in an advantageous manner to reduce the pulse duration of the test pulses to such an extent that a spatial resolution is achieved with which the defects can be distinguished.
- the method according to the invention can be automated in a simple manner in such a way that starting from a predetermined one
- the pulse duration of the test pulses is initially reduced until all the time intervals during which the auxiliary variable is not constant correspond in their interval duration to the double pulse duration of the test pulses that is then set, and then the extrapolation to the jump and / or Impurities occur.
- FIG. 2 shows a schematic sequence of the method according to the invention and FIG. 3 shows the output signal and the time derivative of the logarithmic output signal of the device according to the invention, with a given attenuation profile of an optical waveguide to be examined.
- 1 contains a device 1 for examining the attenuation profile of an optical waveguide 10 with a
- Damping profile d e ⁇ (t) and a disturbance point 12 causing Fresnel reflections an optical transmitter 15, the input 16 of which can be acted upon by rectangular test pulses p (t) with a pulse duration T of a pulse generator 17.
- Light pulses emitted by the optical transmitter 15, which correspond in time to the test pulses p (t), are coupled into an end 20 of the optical waveguide 10 to be examined via an optical splitter 18.
- the attenuation profile -. (T) Rayleigh backscattering
- due to the impurity 12 Fresnel reflection
- the output signal of the optical receiver 22 is sent to an input 25 a surface wave convolver 26 out.
- Another input 27 of the surface wave convolver 26 is connected to another output 28 of the pulse generator 17, at which time-inverse test pulses p (-t).
- An output signal g Mp (t) can be tapped at the surface wave convolver 26 on the output side.
- the surface wave convolver 26 represents a signal-adapted filter, the impulse response of which is determined by the input signal applied to its input 27, which is formed by the test pulses p (t) of the pulse generator 17, with which the trigger input 16 of the optical transmitter 15 is applied. If the test pulses used are symmetrical I pulse shapes, the time-inverse test pulses p (-t) correspond to the test pulses p (t). When a test pulse p (t) occurs, the optical transmitter 15 emits a corresponding light pulse, which is coupled into the optical waveguide 10 to be examined via the optical splitter 18.
- the imaging error that occurs in signals as a result of jumps or interferences is twice the length of the Pulse duration T of the test pulses p (t) exactly limited. It can be shown that the imaging error of the attenuation profile of the optical waveguide to be examined after a period of twice the pulse duration T of a test pulse p (t) from the time of occurrence of the Fresnel reflection is only a constant amplification error which is the square of the Pulse duration T is approximately proportional.
- a so-called timing condition When using the surface wave convolver 26 as a signal-adapted filter, a so-called timing condition must be taken into account. sight.
- the physical condition of the convolution is reflected in the surface wave convolver 26 in the timing condition; the electrical signals present at its inputs 25 and 27 are converted into surface waves by interdigital transducers which move towards one another on a track predetermined by an integration electrode.
- the physical length of the integration electrode determines the integration time T. of the folding process.
- the input signals to be processed In order to obtain an output signal g MF (t) proportional to the convolution product of the input signals, the input signals to be processed must be completely converted as surface waves under the integration electrode. This results in a time limitation of the duration T BE of the output signal of the optical receiver 22 to be processed, which is present at the signal input 25 of the surface wave convolver, according to the formula:
- T R F Duration of the backscatter signal
- T. duration of integration of the surface wave convolver 26
- T pulse duration of the test pulse p (t).
- FIG. 2 shows a flowchart of a method for examining the attenuation curve of an optical waveguide 10 with a device 1 as described in FIG. 1.
- a light pulse is injected into the optical waveguide 10 to be examined and the output signal g MF (t) corresponding to its attenuation profile d j (t) is tapped at the output of the device 1.
- an auxiliary variable D MF (t) is then formed - if necessary after averaging over several measurements - by logarithmizing the amount of the output signal g- v) p (t) and then deriving it over time.
- the auxiliary variable D MF (t) is then examined in a discrimination stage 33 to determine whether it is not constant during a (or more) time interval T F that is greater than or equal to twice the pulse duration T of the test pulse p (t).
- auxiliary variable D MF (t) is constant, this is indicated by a display device 40. This is indicated in FIG. 2 by an examination result N of the discrimination level 33, which is fed to the display device 40 according to arrow 35. If the time interval T is exactly twice the pulse duration T, the damping curve up to the jump point or the fault point causing the Fresnel reflection is determined by extrapolation in an extrapolation device 38 and the result is fed to the display device 40.
- FIG. 3 an (ideal) course of the attenuation profile d -. (T) of the optical waveguide to be examined is shown in the upper diagram in FIG. 3 (cf. FIG. 1).
- a pronounced peak can be seen at a time t as a result of a Fresnel reflection at an impurity. Since the transit times of the backscattered signals and the regions of the optical waveguide to be examined which cause their backscattering are directly related, in connection with the explanation of FIG. 3 details of specific locations of the optical waveguide to be examined are given by specifying the corresponding times of origin of the backscattered Signals.
- the temporal course of the damping profile d, (t) represents areas between a point in time and a point
- Time t "e represents an e-function.
- the lower diagram in FIG. 3 shows the real output signal g MF (t) of the device 1 according to FIG. 1.
- the amount of the auxiliary variable D MF (t) formed by the time derivative of the logarithm 1 of the amount of the output signal g MF (t) ) dash-dotted in its qualitative course.
- a jump in the output signal g MF (t) can be seen in the range between the time t and a time t 1, which is caused by the coupling of the light pulse into the optical waveguide to be examined.
- the output signal g MF (t) drops continuously from a maximum value g 3 .M, r r m m a x at the time t 1 to the time t.
- the auxiliary variable D fMF (t) is constant in the interval t- to t.
- time t eg at an impurity 12 (see FIG. 1)
- a Fresnel reflection occurs, which leads to a discontinuity in the output signal g MF (t).
- Tp 2T
- the output signal g MF (t) continues its course at a time t 2 .
- the auxiliary quantity D Mp (t) is not constant.
- the time interval T F is limited exactly to twice the pulse duration T. It can thus be seen that in the area of the instant t the influence of only one fault point (causing Fresnel reflections) comes into play.
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- Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- General Physics & Mathematics (AREA)
- Testing Of Optical Devices Or Fibers (AREA)
Abstract
En vue de l'analyse d'un guide d'ondes lumineuses (10), celui-ci est soumis à une impulsion de photon; des signaux rediffusés par suite de son profil d'atténuation (réflexion de Rayleigh) et d'irrigularités (12) (réflexion de Fresnel) sont amenés, par l'intermédiaire d'un dérivateur optique (18), à un récepteur optique (22). Le signal de sortie de ce dernier est amplifié et amené, par l'intermédiaire d'un filtre passe-bas, au traitement de signal ultérieur. La grande gamme dynamique des signaux rediffusés et le fait que par suite de la réflexion de Rayleigh le signal rediffusé est inférieur au bruit propre du récepteur optique (22) entraînent des exigences contradictoires pour la fréquence limite du filtre passe-bas, de sorte que ce dernier ne représente qu'un compromis insatisfaisant. Le dispositif selon l'invention présente un filtre (26) adapté au signal qui est constitué de préférence d'un convolutionneur (26) à ondes de surface; elle présente ainsi un rapport signal/bruit considérablement amélioré et, en présence de réflexions de Fresnel, un défaut d'image limité précisément dans le temps. Application: analyse de guides d'ondes lumineuses.For the analysis of a light waveguide (10), the latter is subjected to a photon pulse; signals replayed as a result of its attenuation profile (Rayleigh reflection) and irregularities (12) (Fresnel reflection) are brought, via an optical derivator (18), to an optical receiver (22 ). The output signal of the latter is amplified and brought, via a low-pass filter, to further signal processing. The large dynamic range of the re-emitted signals and the fact that as a result of Rayleigh reflection the re-emitted signal is less than the inherent noise of the optical receiver (22) places contradictory demands on the limit frequency of the low-pass filter, so that this the latter represents only an unsatisfactory compromise. The device according to the invention has a filter (26) adapted to the signal which preferably consists of a surface wave convolutionalor (26); it thus presents a considerably improved signal / noise ratio and, in the presence of Fresnel reflections, an image defect limited precisely in time. Application: analysis of light waveguides.
Description
Claims
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
DE3920169 | 1989-06-16 | ||
DE19893920169 DE3920169A1 (en) | 1989-06-16 | 1989-06-16 | DEVICE AND METHOD FOR EXAMINING THE DAMPING PROCESS OF A LIGHT WAVE GUIDE |
Publications (1)
Publication Number | Publication Date |
---|---|
EP0477189A1 true EP0477189A1 (en) | 1992-04-01 |
Family
ID=6383155
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP19900906903 Withdrawn EP0477189A1 (en) | 1989-06-16 | 1990-05-11 | Device and process for investigating the damping cycle of a light-waveguide |
Country Status (4)
Country | Link |
---|---|
EP (1) | EP0477189A1 (en) |
AU (1) | AU5648090A (en) |
DE (1) | DE3920169A1 (en) |
WO (1) | WO1990015978A1 (en) |
Families Citing this family (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CA2075961C (en) * | 1990-02-15 | 1999-01-05 | Peter John Keeble | Optical test apparatus |
US5066118A (en) * | 1990-04-12 | 1991-11-19 | Minnesota Mining And Manufacturing Company | Optical fault finder using matched and clipping filters |
US5069544A (en) * | 1990-04-12 | 1991-12-03 | Minnesota Mining And Manufacturing Company | Adaptive pulse width optical fault finder |
GB2292495B (en) * | 1994-08-17 | 1998-03-25 | Northern Telecom Ltd | Fault location in optical communication systems |
JP2914225B2 (en) * | 1995-06-30 | 1999-06-28 | 安藤電気株式会社 | Optical fiber test method |
Family Cites Families (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2092743A (en) * | 1981-02-10 | 1982-08-18 | Ceat Cavi Spa | Apparatus for Measuring Attenuation in an Optical Fibre |
-
1989
- 1989-06-16 DE DE19893920169 patent/DE3920169A1/en not_active Withdrawn
-
1990
- 1990-05-11 WO PCT/DE1990/000356 patent/WO1990015978A1/en not_active Application Discontinuation
- 1990-05-11 AU AU56480/90A patent/AU5648090A/en not_active Abandoned
- 1990-05-11 EP EP19900906903 patent/EP0477189A1/en not_active Withdrawn
Non-Patent Citations (1)
Title |
---|
See references of WO9015978A1 * |
Also Published As
Publication number | Publication date |
---|---|
AU5648090A (en) | 1991-01-08 |
WO1990015978A1 (en) | 1990-12-27 |
DE3920169A1 (en) | 1990-12-20 |
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