GB2115253A - Transmission properties of light guides; fault location - Google Patents

Abstract

The system comprises a laser (10) launching short adjustable light pulses into one end of the light guide (11); a light deflector (12) for picking up the backscattered light from the guide and feeding it to a detector (13), a control unit including a timing unit (15) to control the laser and the detector; and a multichannel analyzer connected to the detector and the control and timing unit. The analyzer includes a plurality of storage cells or channels - each representing a time interval, the length of which is determined by the required resolution - for integrating in said plurality of parallel channels the digitized signals received from the detector and in which said plurality of parallel channels represent a time window. The system also comprises means (14) for adjusting the output power of the laser so that the received backscatter or reflected light in an actual time window exhibits an optimum intensity as defined in this context; means (14) for making corrections for the implied digital overflow introduced by the optimizing process; and means (14) for linking together data sets from measurements in mutually overlapping time windows to achieve, by a linear regression operation, a correct transmission of measurement data from one time window to a following one and to make corrections for background level and for different output powers of the laser. <IMAGE>

Description

SPECIFICATION A system for measuring transmission properties in light guides This invention relates to optical fibres or light guides and more particularly to a system for measuring important opitical transmission properties of fibre optic light guides using a single apparatus.

An apparatus of the abovementioned kind is often in the literature named an Optical Time Domain Reflectometer or simply an OTDR. The ODTR-technique or "backscatter"-technique which may be regarded as a sort of optical "radar"-technique, is a convenient nondestructive technique for rapidly locating breaks and other defects in an optical fibre or light guide, the last-mentioned term being preferred in the following. This technique is also employed for evaluating the transmission properties of a light guide, such as its attenuation, as well as measuring its length. Typically, in these prior art systems or reflectometers, lasers are used to intermittently launch intensive light pulses into one end of the light guide to be tested. Subsequently, a photodetector normally an avalanche photodiode, detects the reflected light.The amplified signal is then passed through a filter and displayed on the CRT of an oscilloscope, where it appears as an exponentially decreasing function of time. If the tested light guide is faultless then the received backscattered signal will substantially appear as a continuous curve without sudden changes in amplitude. That which is shown on the oscilloscope screen is the so-called Rayleigh backscattered light which is due to density variations of the light guide material or its composition. This is by low loss guides the major cause of transmission loss. Additionally, reflections may occur from boundaries, such as fibre ends or fracture surfaces, bad splices or cracks in the light guide material. A perfect fracture can thus reflect 4% of the incident energy.The numerical examples mentioned in the following refer to a specific case in which a wavelength of 850 nm is employed in a standard, multimode fibre or light guide.

For comparison it can be stated that a five meter long piece of light guide reflects due to Rayleigh backscattering only 10-5 times the incident energy or expressed in another way; the backscatter level lies 36 dB below the level of a 4% reflection, typically.

The distance x to the point of reflection in the light guide can be determined from the elapsed time t between the launching of the light pulse and the return of the reflection by the relationship x = 0,5 v.t., where v is the velocity of light. The velocity of light in a light guide is approximately 200 minus. That means that a timelag of 1 us corresponds to a guide length of 100 m. Accordingly, the relationship between the spatial resolution Ax and the pulse width T of the transmitted light pulse is given by Ax = 0,5 v.T, which means that a pulse duration or pulse width of 10 ns corresponds to a spatial resolution of 1 m.In order not to reduce this resolution during detection the bandwidth ofthe detector and the oscilloscope should equal the reciprocal of the pulse width. At this value the mostfavourablesignal-to-noise ratio is obtained.

A system of the type sketched above is capable of detecting Rayleigh scattered light from an optical depth which corresponds to a one-way attenuation of 12-15 dB. A higher sensitivity can be provided by utilizing a more sophisticated signal processing such as a sampling and integration of the reflected signal prior to its display. A system of this kind which is capable of detecting backscattered light from an optical depth of 20 dB, is disclosed in British patent application No. 79 21610. The principle here is to divide the guide length into a plurality of short sections-so-called range cells-the lengths of which are determined by the required spatial resolution.The measurement process is carried out for one range cell at a time and the integration time is determined by the required signal-to-noise ratio, or in short the SNR. The measurement for one range cell is concluded before the measurement in the following range cell is started. Hence, the sensitivity has been increased at the expense of consumption of time of the measuring process. Further, in order two keep the SNR low for each sample it is absolutely necessary to use in a system like this highly stable and very linear integrators. Additionally, the laser pulses should, owing to the extended measuring periods, be kept constant and the detector side must not drift.

A still further sophisticated system is disclosed in Electronics Letters, Vol. 17, No.20,1. October1981, pp 751-752, in which use is made of a multichannel, photon-counting technique to observe Rayleigh backscattered signals in very long lengths of monomode fibre. Briefly, the signal processing part of the system comprises a digital correlatorfollowed by a multichannel linear digital integrator. Multichannel averaging makes best use of the available information content of the signal returning between successive operations of the laser and is therefore very time efficient. Muitichannel averaging is also simple and inexpensive to implement with photon-counting because the signal already is in digital form and only requires one bit per location to identify its value.

This system, however, is also subjected to some iimitations regarding its dynamic range and measuring speed. The intensity of light is kept at low levels in order to prevent overflow which results in a limited dynamic range for a single measurement which means that the lengths of the scanned blocks become limited. Further, the lengths of the scanned blocks are also determined by the memory capacity and the required spatial resolution. The outcome from each cycle of operation may be displayed on a chart recording paper, but if the response from the entire length of light guide has to be shown, then it is necessary to manually adjoin all the charts from the individual fibre blocks. Certainiy, this procedure further extends the time spent to get an overview of the entire length of light guide.

Accordingly, it is an object of the invention to provide a system for measuring optical transmission characteristics of optical fibres or light guides which overcome difficulties of previously used systems. It is a further object of the invention to provide a system for observing Rayleigh backscattered signals in very long lengths of multimode optical fibres or light guides based on a multichannel, photon-counting technique, in which the light guide is scanned in blocks or sections of a certain length and in which the measured data from each scanning are linked together to obtain a response curve covering the entire length of light guide.It is still further object of the invention to provide a system for measuring optical transmission characteristics of optical fibres or light guides based on a multichannel, photon-counting technique, combined with a scanning of the light guide in blocks corresponding to time windows, in which each cycle of operation is opened by optimizing the output power of the light source in order to minimize the measuring time.

Briefly summarized, the above and additional objects are accomplished by an adequate signal processing in the control unit taking into account that sets of measuring data belonging to mutually overlapping scanning blocks or time windows exhibit in the overlapping region a linear relationship and by permitting a slight digital overflow at an initial stage of a cycle of measurements, which overflow is corrected at the stages of data processing in the control unit. The two parameters defining the linear relationship are evaluated by means of a linear regression process in an arithmetic unit comprised in the control unit. This technique also permits an optimization of the light source output power for each cycle of measurement in a time window, since differences in signal levels of the reflected signals are corrected by utilizing the abovementioned two parameters.

Accordingly, the system in accordance with the invention provides for an automatic adjoinment of measurement data from different times windows to secure a correct transition from one time window to the following causing a reduced measurement time.

It further provides for an optimization facility for the light source output power which further reduces the measurement time and greatly increases the dynamic range of the system.

The abovementioned and additional objects, adventages and applications of the invention and a further understanding of the invention will be apparent after consideration of the following detailed description in conjunction with the accompanying drawing.

FIG. lisa schematic representation of a system in accordance with the invention, FIG. 2 shows a number of different waveforms occuring at various locations in the system, FIG. 3 is a schematic representation of a multichannel analyzer and parts of a control unit, and FIG. 4 is a representative of a chart recording showing adjoined traces from various time windows thus forming the entire trace for a length of light guide between two repeater stations.

The system schematically shown in FIG.1 compris es a laser 10 which at a fixed repetition rate can launch an intensive light pulse into a optical fibre or light guide 11 shown schemalicaliy in the drawing.

The launched as well as the reflected light is transmitted through a so-called beam splitter 12 operating as a directional coupler so that only backscattered light from the light guide 11 is directed towards a photon detector 13. The laser 10 and the photon detector 13 are controlled by a control unit together with a timing unit 15. The control of the units making up the system can best be illustrated by means of a timing scheme for the signals occurring at various locations within the system.

This is illustrated in FIG. 2 in which the different waveforms are identified by the letters a through e, which letters again are found in FIG. 1 at the locations where they occur. The photon detector is preferably a photomulipliertube. A certain fraction of the photons (the so-called quantum efficiency) incident on the photocathode will cause a voltage pulse to occur at the anode of the photomultiplier tube. The signal is then transmitted from the detector 13 to a discriminating means 16 which emits a standard pulse when it senses a pulse the amplitude of which lies between predetermined levels. The elimination of almost all thermal noise is ensured by this measure. The signal processing after the discriminating means is purely digital.

As mentioned above the photon detector is preferably a photo-multiplier tube. However, an analogue light detector followed by an analogue-to-digital converter may as well by employed instead of the photon counter. This is suggested in FIG. 1 by means of dash lines. The photon detector 13 and the discriminator means 16 can be replaced by an analogue light detector 13' followed by an analogue-to-digital converter 16'. The signal processing after these means is in principle the same, the only practical difference being that the resolution in the analogue case is of the magnitude of more than one bit, e.g. 8 bits, unlike the digital case, where the resolution at the front end of the apparatus is 1 bit (photonlno photon). The signal is then conducted from the discriminator means 16 to a multichannel analyzer 17.Also the analyzer is controlled by the timing unit 15. The control unit 14 and the multichannel analyzer 17 are further interconnected through a data-bus 18, which is to be described further in connection with FIG. 3. Finally, a graphic display unit 19 is connected to the control unit 14 together with a keyboard 20 which is used by the operator to set up the measurement in a manner well known to people with ordinary skill in the art.

A blanking signal represented by waveform a in FIG. 2 is applied the photon detector 13 by the timing unit 15 to ensure that it is shut-off when the laser 10 is fired by a trigger signal from the timing unit 15, cf.

waveform bin FIG. 2. By this measure the detector is prevented from an overload due to reflections from the front end of the light guide, and an extended dynamic range is achieved additionally. The detector13 does not generate pulses before the blanking signal ceases, cf. waveform cin FIG. 2, and the discriminator 16 generates simultaneously a standard pulse, cf. waveform e in FIG. 2. The multichannel analyzer 17 contains a plurality of numbered storage cells, also called channels. Upon receiving a gating signal from the timing unit 15, cf.

wavefonn din FIG. 2, a clock incorporated in a clock'syncronizing unit 21 shown in FIG. 3 is started.

The time of arrival tfor each standard pulse received after the start is registered and the content of channel No. t/l is increased by one unit of counting (T represents the laser pulse duration). Hence, each channel corresponds to a range cell in the light guide.

From the example illustrated in the drawing, cf.

waveform e in FIG. 2, it appears that the content of channel No. 2, 7 and 12 is increased by one unit of counting. Thus, the countings of photons are distributed with respect to time. After the elapse of some integration time, during which the laser 10 is triggered at a highest possible repetition rate, the content or count in the various channels represents the amount of light reflected from the corresponding range cell. The moment at which the gating signal d has to be applied to the multi-channel analyzer 17 is determined by the timing unit 15 which gives the possibility of choosing the position in the light guide from where the measuring process has to start.

The total attenuation between repeater stations will in a buried cable of light guides typically range from 35 to 45 dB. The system according to the invention as well as all other prior art systems do not have a dynamic range which makes it possible to scan an entire cable length in one single measurement. The cable or fibre length is therefore scanned in blocks or sections, the lengths of which are determined by the actual attenuation of the fibre section is question which attenuation at the same time corresponds to a manageable dynamic range of say 5 dB. In other words, the cable or fibre length is divided into a number of time windows.This involves two processes, viz. an optimization of the power of light from the light source or laser according to the actual measuring depth, and a linking of the time windows including the correction for differences in transmitted light power and for background level of light.

A case is described in the example above in which each of the detected photon pulses is allocated a channel of its own. By higher intensities of light, however, there is an increasing probability of more than one photon falling in the same channel during a single laser shot (launch of a laser pulse). It will then be necessary to make corrections for the counts registered, since the count in the multichannel analyzer is increased by one unit only per channel per shot even if more than one photon are received or detected. Hence, the total of measured counts No in a channel will be less than the real number N of detected photons.

A theoretical analysis employing calculus of probability proves, provided the number of pulses received by a channel per laser shot are Poisson-distributed, that N=-Kln(1- NK where K is the number of shots.

Evidently, the signal-to-noise ratio decreases to wards zero as the light intensity approaches zero. On the other hand, it is not worth while letting the intensity increase unlimitedly. The fraction No/K goes against the value 1 (the probability of receiving one or more photons per channel per shot goes against the value 1) as the intensity of light in- creases, which results in a great uncertainty in determining the value N from the above equation. In this context the optimum intensity of light is defined as the intensity by which a predetermined or required signal-to-noise ratio can be obtained with as few shots as possible. By computing the variance of N it can be shown that K for a required signal-to-noise ratio is given by K = const.

where m is the mean number of photons per shot and where the required SNR is contained in the constant.

This function of m has a flat minimum form = 1,6 (approximately) which means that the optimum intensity of light corresponds to the reception of approx. 1,6 photons (average figure) per channel per shot; however, at most 1 photon is counted. Similar conditions apply to the situation in which an analogue photodetector follower byan analogue-to-digital converter are employed. The conversion then has a limited bit-resolution.

Regarding the linking of time windows mentioned above this is based on the fact that in a region where two time windows overlap there is a linear relationship between the two sets of data. Thus, the total attenuation in the light guide can be provided by linking together measurements from a sequence of mutually overlapping time windows.

Before measuring in a time window the intensity of the light received is optimized by adjusting the output power of the transmitter or laser. The two parameters defining the above mentioned linear relationship are determined by linear regression and are used to correct for different transmitter output powers and for background level. Thereby a correct transition between data sets from two overlapping time windows is achieved. The linking process is carried out in that part of the system which is illustrated by way of a block diagram in Fig. 3. The signal efrom the discriminator 16 is applied to the multichannel analyzer 17, where it is received by a receiving unit 22, integrated in an integrating unit 23 and finally stored in a memory 24 especially adapted for this purpose.During the measurements in a time window a concurrent transfer of partial results to a memory 25 associated with a central processing unit 26 occurs. The two lastmentioned units are incorporated in the control unit 14. The CPU 26 communicates with an arithmetic unit 27 in which various arithmetical operations are carried out, including the abovementioned linear regression process, and the corrections for digital overload.Simultaneously, a new sequence of measurements in a contiguous or overlapping time window can be initiated, as the memory 24 of the multichannel analyzer includes two buffer stores 24' and 24" each being capable of storing data for an entire time window. Having concluded two sequences of measurement for two mutually overlapping time windows after which the data sets are transferred through the databus 18 and stored in the CPU-memory 25, they are linked together in the arithmetic unit 27 in accordance with the algorithm put into the CPU-memory 25. Subse 'quently, the adjusted data sets are returned to the CPU-memory 25 which is capable of storing data sets from all actual time windows.

The measurement data stored in the CPU-memory 25 may concurrently be transferred to be displayed on the graphic display unit 19 associated with the control unit 14. A plot of the final outcome may look as illustrated in Fig. 4. It shows the result of a real measurement made on a piece of light guide 5,8 km in length. The curve represents the measurements carried out for 8 time windows. The accumulated attenuation amount to 42 dB.

The duration of the measurement process is reduced considerably by sampling the signal in parallel channels and the problems of drift in the power of light is also avoided, but this is further supported by the present invention according to which some digital overflow is permitted at an initial stage and corrected at a final stage. In addition to further shorten the duration of measurement the linking process extends substantially the dynamic range of the system and simultaneously makes it possible to correct for background noise. The digital signal processing ensures that an upper limit of the integration time doses not exist.

While the fundamental novel features of the invention have been shown and described and pointed out as applied to particular embodiments by way of example, it will be appreciated by those skilled in the art that various substitutions and changes may be made within the scope of the invention as expressed in the appended claims.

Claims (15)

1. A system for measuring the transmission properties of a light guide or optical fibre and locating breaks, discontinuities or other defects within the light guide in which the measurements are based on Rayleigh backscattered signals, the system comprising means for launching short adjustable light pulses into one end of the light guide; light deflecting means for picking up the ibackscattered light from the guide and conducting it to a detecting means; a control means including a timing unit means to control the light pulses launching means and said detecting means; a multichannel analyzer means connected to said detecting means and said control and timing unit means and including a plurality of storage cells or channels each representing a time interval, the length of which is determined by the required resolution-for integrating in said plurality of parallel channels the digitized signals received from the detecting means and in which said plurality of parallel channels represent a time window; the system further comprising means for adjusting the output power of the light pulses launching means so that the received backscattered or reflected light in an actual time window exhibits an optimum intensity as defined in this context; and means for making corrections for the implied digital overflow introduced by the opti mizing process.

2. A system of claim 1 wherein. said correcting means comprises a microprocessor means with associated memory means and arithmetic means for performing said corrections.

3. A system of claim 1 wherein the multichannel analyzer means comprises a receiving unit means for receiving standard pulses from said detecting means; an integrating unit means for integrating the signals received by the receiving unit means; a memory means for storing the integrated pulses; and a clocktsyncronization unit means for controlling the operations of said receiving, integrat.ng and storing means so that a received pulse is allocated an appropriate storage ce!l or channel.

4. A system for measuring the transmission properties of a light guide or optical fibre and locating breaks, discontinuities or other defects within the light guide in which the measurements are based on Rayleigh backscattered signals, the system comprising means for launching short adjustable light pulses into one end of the light guide; light deflecting means for picking up the backscattered light from the guide and conducting it to a detecting means; a control means including a timing unit means to control the light pulses launching means and said detecting means; a multichannel analyzer means connected to said detecting means and said control and timing unit means and including a plurality of storage cells or channels each representing a time interval, the length of which is determined by the required resolution, - for integrating in said plurality of parallel channels the digitized signals received from the detecting means and in which said plurality of parallel channels represent a time window; the system further comprising means for linking together data sets from measurements in mutually overlapping time windows to achieve, by a linear regression operation, a correct transition of measurement data from one time window to a following one and to make corrections for background level and for different output powers of said light pulses launching means.

5. A system of claim 4 wherein said linking means comprises a microprocessor means with associated memory means and arithmetic unit means for performing said linear regression operation and said corrections.

6. A system of claim 4 wherein the multichannel analyzer means comprises a receiving unit means for receiving standard pulses from said detecting means; an integrating unit means for integrating the signals received by the receiving unit means; a memory means for storing the integrated pulses; and a clockisyncronization unit means for controlling of the operations of said receiving, integrating and storing means so that a received pulse is allocated an appropriate storage cell or channel.

7. Asystem of claim 4 wherein said memory means contains buffer stores having a capacity of temporarily storing measurements data or data sets from at least tow overlapping time windows.

8. A system for measuring the transmission properties of a light guide or optical fibre and locating breaks, discontinuities or other defects within the light guide in which the measurements are based on Rayleigh backscattered signals, the system comprising means for launching short adjustable light pulses into one end of the light guide; light deflecting means for picking up the backscattered light from the guide and conducting it to a detecting means; a control means including a timing unit means to control the light pulses launching means and said detecting means; a multichannel analyzer means connected to said detecting means and said control and timing unit means and including a plurality of storage cells or channelseach representing a time interval, the length of which is determined by the required resolution,-for integrating in said plurality of parallel channels the digitized signals received from the detecting means and in which said plurality of parallel channels represent a time window; the system further comprises means for adjusting the output power of the light pulses launching means so that the received backscattered or reflected light in an actual time window exhibits an optimum intensity as defined in this context; means for making corrections for the implied digital overflow introduced by the optimizing process; and means for linking together data sets from measurements in mutually overlapping time windows to achieve, by a linear regression operation, a correct transition of measurement data from one time window to a following one and to make corrections for background level and for different output powers of said light pulses launching means.

9. A system of claim 8 wherein said correcting means comprises a microprocessor means with associ- ated memory means and arithmetic means for performing said corrections.

10. A system of claim 8 wherein said linking means comprises a microprocessor means with associated memory means and arithmetic unit means for performing said linear regression operation and said corrections.

11. A system of claim 8 wherein the multichannel analyzer means comprises a receiving unit means for receiving standard pulses from said detecting means; an integrating unit means for integrating the signals received by the receiving unit means; a memory means for storing the integrated pulses; and a clock/syncronization unit means for controlling of the operations of said receiving, integrating and storing means so that a received pulse is allocated an appropriate storage cell or channel.

12. A system of claim 8 wherein said memory means contains buffer stores having a capacity of temporarily storing measurement data or data sets from at least two overlapping time windows.

13. A method of measuring the transmission properties of a light guide or an optical fibre and locating breaks, discontinuities or other defects within the light guide and in which the measurements are based on Rayleigh backscattered signals, said method comprising: A. Launching a plurality of short light pulses into one end of said fibre, B. Detecting reflected or backscattered photons incident to said launching step, C. Selecteing a time window by deferring said detection step a predetermined period of time from said moment of launching, D. Measuring the elapsed time between the launching of a pulse and the return of the reflected signal, and E.Allocating the detected photon an appropriate storage position or channel number of a memory in accordance with said time measurement and thereby increasing the content of said storage position by one unit of counting.

14. The method of claim 13 and including the additional step of adjusting the output power of the light pulses launching means so that- in the selected time window- approximately 1,6 photons are received per channel per laser shot so that an optimum intensity of light is provided and subsequently correcting for the digital overflow introduced by this optimizing process.

15. The method of claim 13 and including the additional step of storing the measurement data or data sets from two overlapping time windows in a buffer store and performing a linear regression operation on these data sets to achieve the correct transition of measurement data from one time window to a following one and a correction for background level and for different output powers of the light pulses launching means.