GB2042165A

GB2042165A - Improvements in or relating to methods of and circuit arrangements for measuring the attenuation of optical fibers

Info

Publication number: GB2042165A
Application number: GB7941340A
Authority: GB
Original assignee: Felten and Guilleaume Carlswerk AG
Current assignee: Felten and Guilleaume Carlswerk AG
Priority date: 1979-02-08
Filing date: 1979-11-30
Publication date: 1980-09-17
Also published as: IT1126360B; FR2448715A1; IT7927456A0; NL8000323A; DE2904703A1; FR2448715B1; DE2904703C2

Abstract

In order to measure the attenuation of an optical fibre 3, a light pulse from a source 2 is coupled into the fiber 3 and a proportion of the light scattered back by dispersion is coupled out of the fibre 3 by a beam splitter 4 to a detector 5. The detector 5 supplies via an amplifier 6 two gate circuits 7, 9 whose outputs are connected via integrators 12 to a logarithmic divider 13. A control unit 1, 8, 10, 11 triggers the light source 2 to supply the light pulse and, after a first predetermined delay t1, opens the first gate circuit 7 for a predetermined period. After a second predetermined delay t2 from the opening of the first gate circuit 7, the control unit 1, 8, 10, 11 opens the second gate circuit for another predetermined period. The output of the logarithmic divider 13 is supplied via an amplifier 14 to a display 15, which can display directly the attenuation of the fibre 3 per unit length. <IMAGE>

Description

SPECIFICATION Improvements in or relating to methods of and circuit arrangements for measuring the attenuation of optical fibres The present invention relates to methods of and circuit arrangements for measuring the attenuation of optical fibres. In such a method, a light pulse is coupled into an optical fibre and the proportion of light which is scattered back by dispersion is coupled out of the fibre and recorded by an optical detector and the electrical signal formed by the detector is supplied via at least one gate circuit to an integrating circuit and the gate circuit is driven so that after specific delay time, proceeding from the light pulse, said gate circuit is opened for a short time to key out the light energy which is scattered back from the place on the optical fibre defined by the delay time and the pulse signal transit time on the optical fibre.

In known back scatter measuring procedures, which permit non-destructive measurement of attenuation distribution as well as determination of the fibre length, the location of fractures and the investigation of the fibre length, the location of fractures and the investigation of fibre connections, use is made of the fact that the change per unit length of the light power which is scattered back to the beginning of the fibre by microstructures, discontinuities and impurities, can be regarded as a measure of the local attenuation of the fibre. The initially described procedure has already been proposed. In this procedure, slow continuous increase of the delay time makes it possible to scan the fibre over its entire length.The method value is then obtained in the form of a curve which represents the energy carried at each place of the fibre and the attenuation can be obtained from the slope of said curve. Integration of the signals which leave the gate circuit is necessary in order to distinguish the signal from the noise. The advantages of this method are that precise measurement is made possible without destruction of the measured fibre and that all measurements can be performed from one side of the fibre. Furthermore it is advantageous that imperfect places within the fibre can also be recognized. A disadvantage of this method is more particularly a relatively long measuring time which is required since it is necessary to traverse over the entire fibre length by means of the gate.Moreover, a substantial amount of apparatus is required, for example a recorder or the like and this in turn calls for a likewise relatively complicated evaluation procedure.

According to one aspect of the invention, there is provided a method of measuring the attenuation of an optical fibre, in which a light pulse is coupled into the optical fibre and the proportion of light which is scattered back by dispersion is coupled out of the fibre and recorded by an optical detector, the electrical signal formed by the detector being supplied via a first gate circuit to an integrating circuit and the first gate circuit being driven so that after a predetermined delay time from the light pulse the first gate circuit is opened for a pedetermined time to pass a signal corresponding to the light energy which is scattered back from the place on the optical fibre define by the delay time and the light pulse transit time on the optical fibre, there being provided in parallel with the first gate circuit a second gate circuit which opens after a further predetermined delay, corresponding to a predetermined fibre length, with respect to the first gate circuit, the output signals of the first and second gate circuits being supplied to a logarithmic divider whose output is subsequently displayed.

It is possible to provide a method in which the measuring time can be shortened and the amount of apparatus can be reduced.

A preferred method results in a short measuring time because complete scanning of the entire travel I- ing length is not necessary and instead the plain integration time is regarded as the measuring time.

Attenuation of the fibre over a specific fibre length may therefore be directly displayed since the logarithmic divider can display a value in dB. Afibre length of, for example, 100 m or the like can be used as a unit. The delay time of the first gate defines the beginning of the measured distance and the delay time of the second gate defines the measured length of the fibre. As already mentioned, this length can be standardized, i.e. it is possible to measure the attenuation of the fibre per 10 m, per 100 m, per 1 km or the like. It is not necessary to provide additional apparatus and simple evaluation is permitted.

The delay which is effected between the two gate circuits can preferably be variably set. Matching to different lengths of optical fibres is therefore possible. The delay can also be entered into a scaler device so that a display, referred to, for example, 1 km, is possible in every case provided the scale means are calibrated to 1 km.

Preferably, both gates, with a fixed time delay between them, can be displaced along the measured fibre by changing the delay time of he gate circuits with respect to the trigger pulse for the light source.

The change of attentuation over the entire length of the optical fibre can thus be directly determined by this method.

According to another aspect of the invention, there is provided a circuit arrangement for performing the method according to the invention, comprising a light source, a beam splitter, a detector, an amplifier, first and second gate circuits for signal processing whose outputs are connected to respect integrators and whose inputs are connected together to the amplifier, a logarithmic divider whose inputs are connected to the integrators and whose output is connected to a display unit, and a control unit for the light source and the first and second gate circuits.

The invention will be further described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a circuit for performing a method constituting a preferred embodiment of the invention; and Figure 2 is a graph of attentuation on an optical fibre with the gate times plotted and with the associated time axis for delay times of the two gates with corresponding pulses.

Figure 1 is a block circuit diagram of a circuit or device for performing a method constituting a preferred embodiment of the invention. A laser diode 2, triggered by a pulse generator 1, generates a light pulse of short duration and high intensity. It is convenient to use a Ga-As (gallium arsenide) laser diode to this end. The light pulse leaving the laser diode is coupled into an optical fibre 3 and the light scattered back thereby is supplied via a beam splitter 4to a photodiode 5. The electric signal generated thereby is amplified by an amplifier 6. A glass fibre branch of the kind described in the German Patent Application P 2738 050.3 is used as beam splitter.

The output signal of the amplifier 6, which should have a wide band width with low internal noise, is supplied to a first gate circuit 7. The said gate circuit 7 is driven and triggered by a delay element 8.

Furthermore, the amplifier- signal 6 is supplied to a second gate circuit 9, triggered via a delay element 10 which is connected in series with the delay element 8. A respective pulse generator 11 is also connected between each of the two delay elements 8 and 10 and the associated gate circuit 7, 9 to ensure a rapid closing of the gate circuits 7, 9. The outputs of the gate circuits 7, 9 are connected to respective integrators 12 whose outputs are connected in common to a logarithmic divider 13. An amplifier 14 is connected to the logarithmic divider and the amplifier output is connected to display means 15.

The display means 15 can be calibrated, for example, in dB km (decibels per kilometre).

Figure 2 shows the graph of a light pulse on an optical fibre with plots of the opening times associated with the two gate circuits 7, 9. The times or delay times of the two gate circuits are plotted on a parallel time axis in relation to the signal which triggers the light pulse. As disclosed by this graph, a pulse signal 16, applied by the pulse generator 1 to the laser diode 2, triggers a light pulse 17 the amplitude of which diminishes over the length of the optical fibre. A trigger pulse 18 which briefly opens the gate 7 is generated after a delay time t1, calculated from the time of the trigger pulse, and the pulse value of the light pulse on the optical fibre is therefore measured after the delay time t1 which corresponds to a specific transit time of the pulse and therefore corresponds to a specific fibre length.

After a delay time t2, calculated from the trigger time of the gate 7, the gate 9 is driven by a trigger pulse 19 and is briefly opened so that the light pulse value at that time is measured. The pulse difference between the opening times of the two gates is then measured by means of the logarithmic divider and attentuation of the fibre over the optical fibre portion between the two trigger times of the two gates is thus measures.

It is possible to vary the distance between the two trigger times and therefore the measured fibre length by varying the delay time t2. It is also possible to scan the entire optical fibre by shifting both gates along the said optical fibre with a fixed distance between the two gates.

Claims

1. A method of measuring the attentuation of an optical fibre, in which a light pulse is coupled into the optical fibre and the proportion of light which is scattered back by dispersion is coupled out of the fibre and recorded by an optical detector, the electrical signal formed by the detector being supplied via a first gate circuit to an integrating circuit and the first gate circuit being driven so that after a predetermined delay time from the light pulse the first gate circuit is opened for a predetermined time to pass a signal corresponding to the light energy which is scattered back from the place on the optical fibre defined by the delay time and the light pulse transit time on the optical fibre, there being provided in parallel with the first gate circuit a second gate circuit which opens after a further predetermined delay, corresponding to a predetermined fibre length, with respect to the first gate circuit, the output signals of the first and second gate circuits being supplied to a logarithmic divider whose output is subsequently displayed.

2. A method as claimed in claim 1, in which the further predetermined delay time between opening of the first and second gate circuits is adjustable.

3. A method as claimed in claim 1 or 2, in which by changing the delay times of the first and second gate circuits with respect to the light pulse, the gating performed by the first and second gate circuits is displaced along the optical fibre with a fixed delay between the respective gating.

4. A circuit arrangement for performing the method as claimed in claim 1, comprising a light source, a beam splitter, a detector, an amplifier, first and second gate circuits for signal processing whose outputs are connected to respective integrators and whose inputs are connected together to the amplifier, a logarithmic divider whose inputs are connected to the integrators and whose output is connected to a display unit, and a control unit for the light source and the first and second gate circuits.

5. A circuit arrangement as claimed in claim 4, in which the control unit comprises a pulse generator for generating trigger pulses for the light source and connected via a first delay element to the first gate circuit and via a second delay element connected in series with the first delay element to the second gate circuit.

6. A circuit arrangement as claimed in claim 5, in which further pulse generators are disposed between the gate circuits and the associated delay elements.

7. A method of measuring the attentuation of an optical fibre, substantially as hereinbefore described with reference to the accompanying drawings.

8. A circuit arrangement for measuring the attenuation of an optical fibre, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.