GB2190264A

GB2190264A - Calibrating a reflectometer; characterizing an optical fibre device

Info

Publication number: GB2190264A
Application number: GB08709898A
Authority: GB
Inventors: Marc Foucault; Jean-Luc Kemystetter
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-04-28
Filing date: 1987-04-27
Publication date: 1987-11-11
Anticipated expiration: 2007-04-27
Also published as: FR2597986B1; FR2597986A1; GB2190264B; GB8709898D0

Abstract

A coupler 2 has two outputs connected to optical fibres f1 and f2 of different lengths. The backscatter curve (Fig. 10) produced by reflectometer 4 (connected to the input of the coupler 2) includes a step characteristic of the coupler 2, the amplitude of which represents a standard attenuation. The attenuation of an optical element 28 connected to the longer fibre is determined by comparison of its corresponding step in the backscatter trace with that corresponding to the coupler. Alternative arrangements calibrate the reflectometer (Fig. 1, 2) or the coupler (Fig. 3, 4) in a similar way. <IMAGE>

Description

SPECIFICATION Optical coupler device for calibrating or standardizing a reflectometer system for echometry and processes for characterizing a coupler and for measuring attenuations using this device The present invention relates to an optical coupler device, usable for calibrating or standardizing an optical reflectometer, as well to a system for optical echometery, a process for characterizing an optical coupler and a process for measuring optical attenuations, which utilize this device. It is used, in particular, in the field of the transmission of information by optical fibres and, in particular, in the field of the installation or the maintenance of video communication networks.

The growth of the transmissions of information in optical form, by means of optical fibre links, has entailed the development of various measurement techniques for determining the characteristics of these links before use.

One of the fundamental characteristics of the passive elements which are included in these links is the optical attenuation which they cause. It is therefore important to measure, in particular, the optical attenuation of each one of the various passive elements of an optical link in order to clarify that the attenuations due to these elements do not exceed specified limits, with a view to obtaining a transmission of good quality.

Among the known techniques for determining optical attenuations, measurements by backscatter which are carried out by means of optical reflectometers have become established both in metrology laboratories and at locations where optical links are found, either for the installation of these links, or for maintenance thereof.

Among the features of the performance of an optical reflectometer, the most significant is the accuracy of the measurements of optical attenuation which they permit to be carried out.

In fact, it is important to measure, with accuracy, the attenuations due to the various passive elements (optical fibres, connectors, splices . .) of an optical link.

Before carrying out measurements of attenuation with a reflectometer, it is therefore necessary to verify that this reflectometer exhibits correct indications and, if this is not the case, to calibrate it, that is to say to modify the setting thereof in order that it should provide such indications.

It may likewise be necessary to standardize a reflectometer, that is to say to ensure that it gives the results which are required from it as a function of optical attenuations the value of which is known.

Now, the techniques which are in current use for checking the reflectometers rely upon optical attenuators, the insertion losses of which are at least equal to 3 dB.

Accordingly, these known techniques do not permit verification of the indications given by a reflectrometer with a view to using the latter for the purpose of checking optical links, certain elements of which are to exhibit attenuations below a few tenths of a decibel.

This is, in particular, the case with regard to video communication networks, the splices of which are to have attenuations below 0.3 dB.

The object of the present invention is to remedy the above drawbacks by proposing a device permitting, in particular, calibration or standardization of an optical reflectometer, with regard to the optical attenuation measurements which the latter is intended to carry out, with a view to being able to measure, by means of this reflectometer, optical attenuations not exceeding a few tenths of a decibel (and indeed, likewise, attenuations of larger values).

Expressed in precise terms, the subject of the present invention is, first of all, an optical device characterized in that it comprises: at least one optical coupler which is formed from optical fibres and which comprises an input and at least two outputs, and at least two optical fibres, and in that one end of each optical fibre which is included in the device is connected to at least one output of the coupler, and in that the lengths of the optical fibres which are included in the device are not all identical and are selected in such a manner as to be able to distinguish, on a backscatter curve which an optical reflectometer is capable of providing when it is connected to the input of the coupler and which includes steps, each step characteristic of the coupler, and to measure the amplitude of this step, this amplitude being moreover calculable and thus constituting a standard of attenuation for the reflectometer.

By comparing the calculated value of the standard attenuation with the value measured by the reflectometer, it is possible to see whether this reflectometer provides correct indications and consequently to regulate it if this is not the case.

The device forming the subject of the invention likewise permits regulation of the reflectometer in order that it should give an attenuation value equal to the value of the standard when this reflectometer is connected to the device.

The present invention permits the checking of a reflectometer with a view to measuring, by means of the latter, optical attenuations not exceeding a few tenths of the decibel if only the coupler of the device forming the subject of the invention is appropriately selected.

The couplers which are commercially available permit the obtaining of a wide range of attenuations but, should the need arise, it is possible to produce (in a known manner, for example by draw welding of optical fibres) particular couplers corresponding to the attenuations of particular values.

The device forming the subject of the invention is a simple, low-cost device which has a small space requirement and which is passive, which imparts to it a high degree of stability and which is usable both in a metrology laboratory and on a site of installation of optical links.

The fibres from which the coupler is made and the fibres which are included in the device may be of the same type, that is to say they may have the same coefficient of reflection and the same coefficient of attenuation per unit length, which, as will be seen subsequently, permits simplification of the formulae giving the standard attenuations, or may be of different types.

The device forming the subject of the invention may comprise another optical fibre, one end of which is connected to the input of the coupler, the reflectometer then being connected to the other end of this fibre in order to carry out the measurement.

The fibres which are included in the device may have different lengths in pairs, or certain ones of the fibres may have the same length.

In the case where, for example, two fibres have the same length, there is, corresponding to them, a single step in the backscatter curve, whereas if the two fibres did not have the same length, this curve would include a supplementary step, and the amplitude of the sole step obtained is the sum of the amplitudes of the distinct steps which the back-scatter curve would include if the two fibres did not have the same length. The utilization of optical fibres, certain ones of which have the same length, may thus permit an increase in the range of values of standard attenuations which may be required.

In this connection, it is even possible to make a device, according to the invention, comprising optical fibres at least one of which has an end which is connected to two outputs of the coupler or more.

The selection of the optical fibres which are included in the device depends, in effect, upon the separating power of the reflectometer: given that the length of the interval separating two adjacent peaks of a backscatter curve is substantially equal to the difference between two neighbouring lengths of the increasing sequence which is formed by the lengths of the optical fibres which are included in the device, it is desirable that the length of the shortest optical fibre which is included in the device and each difference between two neighbouring lengths of the said sequence should be at least equal to approximately twice the value of the separating power of the reflectometer. The length of the shortest optical fibre and each difference may, for example, be equal to approximately three times the value of this separating power.

Preferabiy, in order to achieve good results with the device forming the subject of the invention, the type of the optical fibres which are included in the device and the type of the optical fibres from which the coupler is made are identical to the type of emission of the reflectometer.

This means that the optical fibres which are included in the device and the optical fibres from which the coupler is made are preferably monomode (or multimode respectively), that is to say intended for the measurement of installations including fibres which are monomode (or muitimode respectively).

According to a particular embodiment of the device forming the subject of the invention, this device comprises a single optical coupler, the number of outputs of which is equal to the number of optical fibres which are included in the device, each one of these fibres being connected to a specified output.

According to another particular embodiment, the device comprises n optical couplers, n being an integer at least equal to two, the inputs of n- 1 of the couplers are respectively connected to n- 1 outputs of couplers selected from among the n couplers, and the optical fibres which are included in the device are connected to the remaining outputs.

In this way, it is possible to obtain the standard attenuations which are desired, by means of an appropriate combination of couplers.

According to another particular embodiment, the outputs of at least one coupler are respectively provided with variable optical attenuators.

This permits the obtaining of standard attenuations, the values of which may be very large or, on the other hand, very small.

According to another particular embodiment, the input of at least one coupler is provided with a variable optical attenuator.

Such a device permits knowledge of the variation of the accuracy of a reflectometer as a function of the level of the backscatter signal at a point on the backscatter curve, this being a point at which there is a step, of which it is desired to determine the amplitude.

The subject of the present invention is likewise a system for optical echometry, which system is characterized in that it comprises an optical reflectometer and a device for calibrating or for standardizing this reflectometer, with regard to the optical attenuation measurements which the latter is intended to carry out, in that the device is in conformity with the device which likewise forms the subject of the invention, and in that the reflectometer is intended to be connected to the input of a coupler which is included in the latter device.

The present invention further concerns a process for characterizing an optical coupler which is made from optical fibres and which comprises an input and n outputs, n being an integer at least equal to two, characterized in that it comprises the following successive steps: connection of the n outputs respectively to n optical fibres of differing lengths in twos, establishment of a link between the input of the coupler and a standardized optical reflectometer, formation, by means of this reflectometer, of a backscatter curve relative to the coupler, the respective lengths of the fibres being selected in such a manner that this curve includes n- 1 steps which are characteristic of the coupler and a step corresponding td the insertion loss of the coupler, and that the steps have measurable amplitudes, determination of the respective amplitudes of the steps, and determination, from these amplitudes, of the luminous powers which are recoverable at the n outputs of the coupler when a luminous flux of given power is injected at the input of this coupler.

Finally, the present invention likewise concerns a process for measuring the optical attenuation due to an optical element, characterized in that it comprises the following successive steps: an installation is constructed, comprising an optical reflectometer and a device comprising: an optical coupler which is made from optical fibres and which comprises an input and two outputs, and two optical fibres respectively connected by one end to the outputs of the coupler, one of the two fibres being longer than the other and the lengths of these fibres being selected in such a manner as to obtain, when the reflectometer is connected to the input of the coupler, a backscatter curve including a step which represents a reference optical attenuation, the value of this attenuation being calculable, the element is connected to the other end of the longest fibre and the reflectometer to the input of the coupler, which permits the obtaining of a backscatter curve including two steps corresponding respectively to the reference attenuation and to the attenuation due to the optical element, and the optical attenuation due to this element is measured by comprising this attenuation with the reference attenuation.

The present invention will be better understood on reading the description which follows, of exemplary embodiments which are given on a purely indicative and in no sense limitative basis, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a particular embodiment of the device forming the subject of the invention, Figure 2 represents a backscatter curve obtained with the device represented in Fig. 1, Figures 3 and 4 illustrate a method of standardization of an optical coupler forming part of a device according to the invention, with a view to knowing the standard attenuations thereof, Figure 5 illustrates a method of characterizing an optical coupler, utilizing a device according to the present invention, Figures 6 to 8 are schematic views of other particular embodiments of the device forming the subject of the invention, Figure 9 schematically illustrates an installation permitting the measurement of the attenuation of an optical element, and Figure 10 is a backscatter curve relative to this installation.

Fig. 1 shows schematically a particular embodiment of the device forming the subject of the invention. The device represented in Fig. 1 comprises a coupler 2 made from optical fibres and comprising an input branch B0 and n output branches B,, B2 ..., B,, this coupler being intended to perform the function of a differentiator.

The device likewise includes n optical fibres F1, F2, ..., Fn of respective lengths 1" 12 ..., In, these lengths forming an increasing sequene.

The fibres Fj, j varying between 1 and n, are respectively connected, by one end, to the output branches B, by means of optical connectors, of splices or of welds.

The device represented in Fig. 1 likewise includes, although this is not necessary in the present invention, an optical fibre F0 of length 1o, one end of which is connected, for example by a weld, to the input B0 and the other end of which is intended to be connected to an optical reflectometer 4 which it is desired to check with the device of the invention, with a view to seeing whether the attenuation indications given by the reflectometer are correct.

The fibres F,, F1, F2 ..., Fn as well as the fibres from which the coupler 2 is made, are monomode (or multimode respectively) when the reflectometer 4 is monomode (or multimode respectively).

Furthermore, the lengths of the fibres F1, F2, ., Fn are selected in order that the length 1" the shortest length of the series 1" í2, .., I,, as well as each different li,,--li, i varying between 1 and n-- 1, are greater than approximately three times the separating power Ps of the reflecto meter to be checked. The lengths 1" 12, 13, s n In may thus be respectively selected to be equal to 3Ps, 6Ps, 9Ps, ..., (3n)Ps.

The present invention is based on the creation of optical attenuations in a fictitious manner, by utilizing the actual principle of the measurements of reflectometry.

Accordingly, the reflectometer 4 is caused to operate in such a manner as to inject a light pulse into the fibre Fo, this being a pulse which then propagates in the device according to the invention, each point of the device supplying a backscattered signal. The reflectometer detects the sum of the backscattered signals originating from these points.

The reflectometer is capable of providing a backscatter curve of the type of that which is shown in Fig. 2, this curve representing the variations of the relative level NR (in decibels) of the intensity of the signal detected by the reflectometer, as a function of the distance D (expressed, for example, in metres) to this reflectometer.

In order to simplify the situation, the peaks due to the reflections of the light at the free end of the fibres F1, F2 ., F0 have not been shown.

The backscatter curve further includes a step of amplitude A0 which is due to the insertion loss of the differentiator 2, as well an n- 1 steps of respective amplitudes A1, A2 . . . , An~l, which are respectively situated at distances 10+11, 10+12 ..., lo+ln~, and the amplitudes of which constitute standard attenuations.

It should be noted that the step of amplitude A0 would be observed even if the fibre F0 were absent, the reflectometer then being directly connected to the branch B0.

The value of each of the amplitudes A0, A1, A2 ..., An~1 will now be determined.

There is designated by P0 the power of the light pulse of the input B0 by P" j varying between 1 and n, the power of the pulse at the output B, and by P, j, i varying between 1 and n-l, the power of the same pulse, in the fibre F, and at the distance Ii.

Furthermore, the ratio P,/PO is noted as xj.

The amplitude A0 is given by the following formula: A0 = 10 log (SO/PO) in this formula log represents a decimal logarithm and So is such that: So=P1+P2+ .. + Pn The following is accordingly obtained: Ao = 10 log (x1+x2+ . + Xn) (1) The following may likewise be written: A1=(1/2) > c10 log (S1l/S2i (la) in this formula the coefficient 1/2 corresponding to the fact that the attenuation detected between two points by the reflectometer corresponds to an outward and return traverse of the light, and the following being applicable: S1i = P1'l+Pl'l+2+ . . +Pl'n S2i=Pl'l+Pl'l+1+ ... +Pl'n Assuming that the fibres are of the same type, the following is obtained: A,=(1/2)x10 log (T1/T2) in this formula the following being applicable: T1,=P, 1+P,2+ .. +Pn T2=P,+P,1+ . - +Pn it being possible for this formula to be written as: A,=5 log ((x,e1+x,+2+ . . +xn)/(x,+x,+1+ It will be noted that Ao is zero in the case of a perfect differentiator, for which the following is applicable: P0--P1+P2+ +Pn that is to say: x,+x2+ ... + Xn=1.

In the case of a differentiator referred to as "uniform" or "balanced", for which the dispersion of the attenuations between channels is zero, the numbers x 1 . . . , xn are equal and formula (2) becomes: A,=5 log ((n-i)/(n-i+1)) (3) Figs. 3 and 4 show stages of a method permiting the standardization of the device shown in Fig. 1, that is to say the determination of the coefficients x, of this device, j varying between 1 and n.

According to this method, first of all a luminous flux of power P0 is injected into the optical fibre F0 by means of an appropriate light source 6 comprising, for example, a laser diode, and there are successively measured, by means of the appropriate photodetector 8, the luminous powers P0j, j varying between 1 and n, respectively emerging from the fibres F1, F2 ..., F0 (Fig.

3).

There is then subsequently injected, by means of the source 6, the luminous flux of power P0 into the free ends of the fibres F1, F2 ..., Fn, while measuring on each occasion, by means of the photo detector 8, the luminous power Pjo emerging from the free end of the fibre Fo, (Fig.

4).

It is then possible to calculate the following quantities yj, j varying between 1 and n: y1=(1/2)x(10Iog(p01/0)+ lOlog(pjO/pO)) (4) while bearing in mind that the attenuation relative to the branch of rank j of the coupler is the average of the attenuations relative to this branch, for the "concentrator" and "differentiator" senses of the coupler.

It is then possible to calculate the coefficients xj by designating by a the attenuation per unit length of the fibres F0 and Fj which are assumed to be of the same type, j varying between 1 and n, by the following formula: 10 log xj=yi-a(lj+lo) (5) Knowing the values x;, j varying between 1 and n, it is then possible to calculate the values Ao and A, i varying between 1 and n-1, by using the formulae (1) and (2).

When F1, F2 ..., F0 are not of the same type, A0, A" ..., An~1 are still calculable. It would then be necessary to take account of the respective attenuations per unit length of the fibres in the formulae (lea) and (5).

Fig. 5 shows schematically an installation permitting the characterization of an optical coupler 10 comprising an input branch b0 and n output branches b,, b2 ..., b0. The input branch b0 is connected, for example by means of an optical connecting block Co, to one end of an optical fibre f0, the other end of which is connected is to a standardized optical reflectometer 12.

Each branch bj, j varying between 1 and n, is connected, for example by means of an optical connecting block c, to one end of an optical fibre fj, the fibres f1, f2, . , f0 being of the same type.

When the coupler is made from optical fibres which are monomode (or multimode respectively), a reflectometer which is monomode (or multimode respectively) and optical fibres fO, f1, f2 ..., f0 which are monomode (or multimode respectively) are selected.

Moreover, the lengths of the fibres f1, f2 , f0 are selected such that they form an increasing sequence, such that the length of the shortest fibre as well as each difference between two adjacent lengths of the sequence are at least equal to approximately three times the separating power of the reflectometer.

The characterization of the coupler 10 consists in the determination of the coefficients x1, x2, ..., xn of the latter.

To this end, the reflectometer 12 is employed to measure the amplitudes Ao, A, A2 ..., An 1 of the steps which are present on the backscatter curve corresponding to the coupler 10 equipped with the optical fibres fj, j varying between 1 and n, when this coupler is connected to the reflectometer 12.

By utilizing the formulae (1) and (2), and by inserting log aO=Ao/10 log (1+t,)=A,/5 for i varying between 1 and n-i, it is possible to write the following: x1+x2+ . . + xn=ao (6) x,=tl (xlE 1+Xl+2+ . . . +xn) (7) The equations (7) permit the obtaining of the coefficients x1, x2 ..., xn~1 as a function of xn, and the equation (6) then permits calculation of xn.

Knowing xn, it is possible to calculate subsequently xn-1, xn-2, ..., x, by using equations (7).

Measurements have been made with multimode reflectometers of respective wavelengths 850 nanometres and 900 nanometres and with devices in accordance with the invention and each comprising a coupler with two output branches B1 and B2 and one input branch B0, as well as a fibre F0 having a length of 100 metres, a fibre F1 having a length of 760 metres and a fibre F2 which is longer, by approximately 100 metres, than the fibre F1, the reflectometers each having a separating power of the order of 30 metres.

For such a balanced coupler, the coefficient A, has a value of approximately - 1.5 dB, and a correctly standardized reflectometer actually gives this value of - 1.5 dB.

With a device having a coupler which is not balanced, with two output branches, which is equipped with a fibre of length 1 of the order of 760 metres and with another fibre of length 12 of the order of 860 metres, the reflectometer is employed to measure of an amplitude A1 of the order of -0.95 dB. By reducing the length 12 by approximately 200 metres, the amplitude obtained is then -2.18 dB, these values being compatible with formula (3) given above and permitting the calculation of the coefficients x, and x2 relative to this coupler by also measuring the amplitude A,.

Measurements were also made with a device comprising a differentiator with one input and 8 outputs, to which 8 optical fibres were respectively welded, the lengths 1" i2, i3, i4, i5, 16, 17, and 18 of which have values of approximately 100m, 200m, 300m, 400m, 500m, 600m, 700m and 1 200m respectively.

This device was subsequently characterized with two multimode reflectometers of respective wavelengths 850 nanometers and 900 nanometres and having a resolution of 0.1 dB.

Table 1 given at the end of the present description permits a comparison of the theoretical values of the attenuations A1, A2 ..., A7 corresponding to the differentiator assumed to be balanced, these values being calculated using formula (3), with the values measured respectively at 850 nanometres and 900 nanometres, each value measured being the difference between two measurements made to the left and to the right of the corresponding step respectively.

The results obtained show a good degree of agreement between the theoretical values and the measured values. The differences which are found are due to the fsct that the differentiator employed is not perfect and that there is a non-zero dispersion of the attenuations between channels. This dispersion can be aggravated if the attenuations due to the welds between the fibres and the coupler are not homogeneous.

The standardization described with reference to Figs. 3 and 4 permits the elimination of this dispersion.

The results obtained also show that the values of the amplitudes of the attenuations are independent of the wavelength. The values measured at 850 nanometres and at 900 nanometres are, in fact, substantially identical. The only differences which are found originate from the resolution of the equipment used (of the order of 0.1 dB).

It would likewise be possible to use monomode reflectometers and other wavelengths, for example 1300 nm or 1500 nm.

In order to calibrate or to standardize a reflectometer intended for checking installations devised for transmitting a luminous flux of given wavelength, it is, however, preferable to employ a device according to the invention, the fibres of which (those of the coupler and those which are connected to the outputs of this coupler ) are able to transmit a luminous flux of wavelength at least equal to this given wavelength.

Fig. 6 shows schematically a device according to the present invention, comprising a first optical coupler 14 at an input which can be connected to an initiating fibre f10 and, for example, four outputs, one of which is connected to the input of a second optical coupler 16 comprising, for example, two outputs. The latter are connected respectively to optical fibres f11 and f12, while the three remaining outputs of the coupler 14 are connected to optical fibres f13, f14 and fis.

The fibres f11 to f15 and the fibres from which the couplers are made are of the same type (monomode or multimode) as the reflectometers intended to be checked with the device.

Moreover, the lengths 111 to 115 of the fibres f11 to f15 form an increasing sequence such that the length 111 and the difference between two neighbouring lengths of the sequence are greater than approximately three times the separating power of the reflectometers. The amplitudes of the standard attenuations corresponding to the couplers 14 and 16 can be determined in the manner indicated herein above.

The devices of the type of that which is represented in Fig. 6 permit-by appropriately selecting the number of couplers, the number of outputs of the latter and the connections between couplers-the obtaining of standard attenuations of predetermined values which may be, for example, of the order of 0.1 dB in absolute value, while the devices of the type of that which is represented in Fig. 1 are made from commercial couplers and do not always permit the obtaining of such low values.

Fig. 7 shows schematically another device according to the present invention, comprising one optical coupler 18 with one input and, for example, two outputs. An initiating fibre f0 may be connected to the input of the coupler, while the outputs of the latter are respectively connected to an optical fibre F1 by means of a first variable optical attenuator 20, and to an optical fibre F2 by means of a second variable optical attenuator 22.

The optical fibres F,, F1, F2 as well as those from which the coupler 18 is made are selected in such a manner as to have the same type (monomode or multimode) as the reflectometers with which the device of Fig. 7 is intended to be used.

Furthermore, the fibres F, and F2 have lengths which are selected in such a manner that that of the fibre F, and the difference between the lengths of the fibres F, and F2 are at least equal to approximately three times the separating power of these reflectometers.

The devices of the type of that which is represented in Fig. 7 permit the obtaining of attenuations which are variable between very low values (close to O dB) and very high values (substantially infinite).

Such devices permit the evaluation of the dynamics of measurement of a reflectometer, that is to say the maximum attenuation which is measurable with the latter.

Fig. 8 shows schematically another device according to the present invention. This device comprises an optical coupler 24 comprising an input and, for example, five outputs. A variable optical attenuator 26 is connected to the said input. It is likewise possible to provide an initiating optical fibre F0 such that the latter is connected to the input by means of the variable attenuator 26. The device further comprises optical fibres F1, F2 ..., F5 which are connected, by one end, respectively to the outputs of this device. The condition relative to the type of the optical fibres of the device is identical to that given hereinabove for the device of Figs. 6 and 7.

Moreover, the lengths 1, tio 15 of the fibres F1 to F5 form an increasing sequence and are selected in such a manner that the length 1, and each difference between two neighbouring lengths of the sequence are greater than approximately three times the separating power of the reflectometers intended to be used with the device represented in Fig. 8.

Such a device permits knowledge of the variation of the accuracy of a reflectometer as a function of the level of the signal at a point of a backscatter curve, this point corresponding to a step.

In accordance with the present invention, it is likewise possible to construct devices resulting from a combination of the devices of the type of those which are represented in Figs. 6, 7 and 8.

Fig. 9 shows schematically an installation permitting the determination of the variations of the attenuation of a (passive) optical element as a function of the time.

This installation comprises a device 30 of the type of that which has been described with reference to Fig. 1, the number n being equal to 2. The coupler 2 of the device is, for example, assumed to be balanced, and the fibre F0 is connected on the one hand to the reflectometer 4 and on the other hand to the input of the coupler.

The element 28 to be tested, a connector for example, is connected on the one hand to the free end of the longest fibre F2, while it is connected on the other hand, to an end of a fibre 32, the other end of which is free or equipped with an optical reflecting means; in this case, this permits knowledge of the attenuation of the element 28 in the two senses.

The environment of the element 28 can be monitored: for example, the element 28 can be enclosed in an oven 34 in which the temperature is maintained constant.

The backscatter curve obtained by virtue of the reflectometer (Fig. 10) comprises two steps, one of which, which has an amplitude A1, corresponds to the device 30 and serves as reference value, and the other of which, which has an amplitude AE, corresponds to the attenuation of the element 28.

It is thus possible to observe the development of A8 in the course of time by comparing this value A8 with the value A1.

The device is selected in such a manner that the reference value A1 is of the order of magnitude of the attenuation or of the variation of the attenuation or of variation of the attenuation of the element under investigation.

The benefit of the installation represented in Fig. 9 resides, in particular, in the fact that the measurements of the attenuations respectively due to the device 30 and to the element 28 to be tested are in all cases made with the same level of the signal.

It is possible to contemplate the integration of a device according to the invention in a reflectometer, for example in a protective cap of the latter. Such a system permits the calibration of the reflectometer with respect to the two axes (attenuation and distance). The amplitudes of the steps relative to the device and the lengths of the fibres of this integrated device being known and stable, the parameters of the corrections which may possibly be caused to be effected may be input by means of the keyboard or of potentiometers of the reflectometer, depending upon the type of reflectometer employed, with a view to obtaining correct indications on the part of the latter.

TABLE I

Theoretical values Values measured Values measured at 850 nm at 900 nm A1 5 log (7/8) = -0.29 dB -0.2 dB -0.1 dB A2 5 log (6/7) = -0.33 dB -0.2 dB -0.2 dB A3 5 Log (5/6) = -0.40 dB -0.3 c -0.3 de A4 5 log (4/5) = -0.48 dB -0.5 dB -0.4 dB A5 5 log (3/4) = -0.62 dB -0.6 dB -0.6 d8 A6 5 log (2/3) = -0.88 dB -0.9 dB -0.9 dB A7 5 Log (1/2) = -1.50 dB -1.5 dB -1.6 dB

Claims

1. Optical device characterized in that it comprises: at least one optical coupler (2; 14, 16; 18; 24) which is formed from optical fibres and which comprises an input and at least two outputs, and at least two optical fibres (F" ..., Fn; fl1, , f15), in that one end of each optical fibre which is included in the device is connected to at least one output of the coupler, and in that the lengths of the optical fibres which are included in the device are not all identical and are selected in such a manner as to be able to distinguish, on a backscatter curve which an optical reflectometer (4) is capable of providing when it is connected to the input of the coupler and which includes steps, each step which is characteristic of the coupler, and to measure the amplitude of this step, this amplitude being moreover calculable and thus constituting a standard of attenuation for the reflectometer.

2. Device according to Claim 1, characterized in that the length of the shortest optical fibre (F,) which is included in the device and each difference between two neighbouring lengths of the increasing sequence which is formed by the lengths of the optical fibres (F" ..., Fn) which are included in the device are at least equal to approximately twice the value of the separating power of the reflectometer (4)

3. Device according to either one of Claims 1 and 2, characterized in that the type of the optical fibres (F1, .., Fn) which are included in the device and the type of the optical fibres from which the coupler (2) is constructed are identical to the type of emission of the reflectometer (4).

4. Device according to any one of Claims 1 to 3, characterized in that it comprises a single optical coupler (2), the number of outputs of which is equal to the number of optical fibres (F1, ., fun) which are included in the device, each of one of these fibres being connected to a specified output.

5. Device according to any one of Claims 1 to 3, characterized in that it comprises n such optical couplers (14, 16), n being an integer at least equal to two, in that the inputs of n-i of the couplers (16) are respectively connected to n-i outputs of couplers (14) selected from among the n couplers, and in that the optical fibres (f1, ., f,5) which are included in the device are connected to the remaining outputs.

6. Device according to any one of Claims 1 to 5, characterized in that the outputs of at least one coupler (18) are respectively provided with variable optical attenuators (20, 22).

7. Device according to any one of Claims 1 to 6, characterized in that the input of at least one coupler (24) is provided with a variable optical attenuator (26).

8. System for optical echometry, characterized in that it comprises an optical reflectometer (4) and a device (2, F" . . ., Fn) for calibrating or for standardizing this reflectometer, with regard to the optical attenuation measurements which the latter is intended to carry out, in that the device is in conformity with the device according to any one of Claims 1 to 7, and in that the reflectometer is intended to be connected to the input of a coupler (2) which is included in the latter device.

9. Process for characterizing an optical coupler (10) which is constructed from optical fibres and which comprises an input and n outputs, n being an integer at least equal to two, characterized in that it comprises the following successive steps: connection of the n outputs respectively to n optical fibres (1, -, f5) of differing lengths in twos, establishment of a link between the input of the coupler and a standardized optical reflectometer (12), formation, by means of this reflectometer, of a backscatter curve relative to the coupler, the respective lengths of the fibres being selected in such a manner that this curve includes n- 1 steps which are characteristic of the coupler and one step corresponding to the insertion loss of the coupler, and that the steps have measurable amplitudes, determination of the respective amplitudes of the steps, and determination from these amplitudes, of the luminous powers which are recoverable at the n outputs of the coupler when a luminous flux of given power is injected at the input of this coupler.

Process for measuring the optical attenuation due to an optical element (28), characterized in that it comprises the following successive steps: an installation is constructed, comprising an optical reflectometer (4) and a device comprising: an optical coupler (2) which is constructed from optical fibres and which comprises an input and two outputs and two optical fibres (F1, F2) which are respectively connected by one end to the outputs of the coupler, one of the two fibres being longer than the other and the lengths of these fibres being selected in such a manner as to obtain, when the reflectometer is connected to the input of the coupler, a backscatter curve including a step which represents a reference optical attenuation, the value of this attenuation being calculable, the element (28) is connected to the other end of the longest fibre and the reflectometer to the input of the coupler, which permits the obtaining of a backscatter curve including two steps corresponding to the reference attenuation and to the attenuation due to the optical element respectively, and the optical attenuation due to this element is measured by comparing this attenuation with the reference attenuation.

11. Optical coupler device substantially as hereinbefore described with reference to any of Figs. 1, 6-9 of the accompanying drawings.

12. A process for measuring the optical attenuation due to an optical element, the process being substantially as hereinbefore described with reference to the accompanying drawings.