Method of and device for measuring the attenuation of optical fibers by means of the backscatter method

The invention relates to a method for the time-dependent, automatic measurement of the backscattered portions of light pulses coupled into an optical fiber under test, said portions being received by an optical detector and converted into electrical measurement signals, followed by amplification by a gain factor adapted to the measurement range, application to an electronic evaluation circuit and storage in a memory wherefrom they can be fetched for display on a display device. In the course of an automatic measurement operation at least two time-adjacent time domains (.DELTA.t1, .DELTA.t2) of measurement signals (6) are measured with different gain factors in order to be stored in memories (S1, S2) which are associated with the respective gain factors and which can be independently addressed.

BACKGROUND OF THE INVENTION 
The invention relates to a method for the time-dependent, automatic 
measurement of the backscattered portions of light pulses coupled into an 
optical fibre under test, said portions being received by an optical 
detector and converted into electrical measurement signals, followed by 
amplification by a gain factor adapted to the measurement range, 
application to an electronic evaluation circuit and storage in a memory 
wherefrom they can be fetched for display on a display device. 
A method of this kind is known from IEEE Journal of Quantum Electronics, 
Vol. QE-17, Pages 1264 to 1269. Therein, selected time domains of the 
optical backscattered signals are examined using an adapted gain factor, 
so that either the available dynamic range of an amplifier can be fully 
utilized or, in the case of a given dynamic range, a longer measurement 
length of an optical fibre can be examined. However, the known method is 
only capable of covering a part of the length of an optical fibre in the 
case of an automatic measurement. The dynamic range expansion will not be 
obtained when the entire length of a long optical fibre is to be measured 
in a single operation and the variation of the attenuation values as a 
function of location along the entire length of the optical fibre is to be 
subsequently represented as a single, continuous curve. 
SUMMARY OF THE INVENTION 
It is an object of the invention to modify the described method so that 
uniform measurement of the attenuation values of a long optical fibre and 
a continuous display of the measured backscatter curve can be realized 
with great accuracy, without imposing excessively high requirements as 
regards the dynamic range of the electrical components required. 
This object is achieved in that in the course of an automatic measurement 
operation at least two time-adjacent time domains of measurement signals 
are measured with different gain factors in order to be stored in memories 
which are associated with the respective gain factors and which can be 
independently addressed. 
In the present context, time domains are to be understood to mean periods 
of time during which signals scattered back from more or less remote 
length portions of the optical fibre are received with different delays. 
Thus, these time domains correspond to assignable local portions of an 
optical fibre to be tested. 
In accordance with the invention, two or more preferably digital additional 
memory devices and also additional amplifiers or one switchable amplifier 
are required. However, because use can be made of amplifiers and 
analog-to-digital converters having a substantially smaller dynamic range, 
the overall expenditure will be less. 
On the other hand, it will be apparent that, when an as large as possible 
dynamic range is maintained, the range of the measurement method can be 
substantially increased in accordance with the invention. 
The independently addressable memories can be combined so as to form a 
single unit. It is essential that data groups assigned to different time 
domains and stored with a different scale can be independently addressed. 
The different data groups can be electronically displayed on a display 
device, for example an x-y-plotter or video monitor, in the form of a 
uniform, continuous curve in a manner known to those skilled in the art, 
so that the operator cannot distinguish the display of measurement data 
from that obtained by means of known methods. 
A preferred version of the method in accordance with the invention is 
characterized in that, when use is made of a single amplifier whose gain 
factor can be switched over, the various time domains are measured with 
backscatter signals which originate from different light pulses. 
The time-controlled automatic switching over of the gain factor of an 
amplifier requires a stabilization period during which the amplifier can 
reach the new stable state and during which, of course, no measurement 
data can be derived. However, in order to ensure that neighbouring time 
domains can still be covered without gaps, the associated stored data are 
determined on the basis of backscatter signals originating from different 
light pulses. 
Backscatter signals of later time domains, scattered by remote length 
portions of the optical fibre to be tested, have a comparatively low level 
and are processed with a correspondingly higher gain factor. In order to 
prevent high signal levels from prematurely reaching the amplifier, thus 
overloading the amplifier, in a preferred version in accordance with the 
invention at least the measurement signals which exceed the permissible 
level are blocked by means of an optical switch during the measurements 
utilizing the higher gain factor. The amplifier then operates with optimum 
accuracy in the relevant time domain. 
For the measurement of backscatter curves, a measurement operation consists 
of a plurality of individual measurements wherefrom mean values are 
electronically determined. Accordingly, substantial measurement periods 
occur. Especially short measurement periods are possible in that an 
amplifier having a different gain factor is associated with each memory, 
said amplifiers simultaneously receiving the measurement signals in 
parallel. This version of the method in accordance with the invention 
necessitates an own amplifier for each time domain, but the measurement 
data of each time domain can then be measured during a common sequence of 
backscatter signals. In comparison with the previously described solution, 
the measurement time will then be a factor 1/n smaller in the case of a 
number of n time domains to be covered. 
Notably in the case of the latter solution it is advantageous when the 
amplifier having the higher gain factor is preceded by a level limiter in 
order to prevent overloading of notably the amplifiers associtated with 
the later time domains. 
In a preferred version of the method in accordance with the invention, the 
time domains of the stored data overlap in time. Consequently, on the one 
hand it is ensured that measurement data are obtained over the entire 
length of an optical fibre to be examined without gaps. On the other hand, 
this also offers the attractive possibility of determining, from at least 
one pair of values of the data stored in the memories, measured for the 
same backscatter points within the overlapping region, an evaluation 
constant by means of an arithmetic circuit in order to convert the data 
stored in the second memory, after which they can be fetched with the same 
scale as the data stored in the first memory. 
Because of unavoidable tolerances and because of fluctuating environment 
conditions, there is a risk that the attenuation values stored for one and 
the same backscatter location in memories associated with different time 
domains do not correspond exactly, so that unstable locations may occur on 
a display device at the boundary of two domains, said instabilities unduly 
indicating a fault in the optical fibre. As a result of the automatic 
arithmetical adaptation of the backscatter curves in the overlapping 
region, there will always be obtained a smooth curve as well as values 
which have absolutely exactly the same scale in the neighbouring domains. 
When direct measurement values are stored, the evaluation constant is 
preferably chosen as a mean value of the quotient of the pairs of values 
averaged over a large number of pairs of values in the ovelapping region. 
However, when the measurement signals are stored in logarithmic form, the 
evaluation constant is preferably formed by a difference between the 
measurement signals in logarithmic form whereby the values of a next time 
domain must be reduced, for example in the case of representation in the 
form of a curve. 
A measuring device for carrying out the method in accordance with the 
invention is characterized in that it comprises at least two mutually 
independent digital memory elements which are connected to outputs of an 
electronic evaluation circuit whose input is connected to an optical 
detector, the device comprising an arithmetic circuit whose input is 
connected to the memories and whose output is connected to a display 
device. These are the components which are specific of the invention and 
which must be used to supplement known devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The measuring device shown in FIG. 1 serves to measure the attenuation 
behaviour of a test length of an optical fibre 1 which is connected to the 
measuring device via a coupling 2. 
A series of light pulses 3 is coupled into the optical fibre, via the beam 
splitter 5, by the laser L which is controlled by the pulse generator 4. 
During a period of time which corresponds to the length of the optical 
fibre 1, backscatter signals 6 from each light pulse 3 are deflected to an 
avalanche photodiode APD by the beam splitter 5, via the switch AOD which 
is constructed as an acousto-optical deflector. The electrical output 
signal thereof, constituting the measurement signal 7, is amplified and 
processed in a customary manner in an electronic evaluation circuit W; it 
is notably split into a plurality of digital measurement data by analog to 
digital converters, said data being associated with different backscatter 
locations and being stored in the memory S1 or S2. The data can be fetched 
therefrom via the arithmetic circuit C for display on a display device D, 
notably in the form of a curve. 
The memory S1 stores the less amplified data of those measurement signals 
which arise by backscattering from the starting zone of the optical fibre 
1. The memory S2 stores the more amplified measurement signals which 
appear from the end zone of the optical fibre after a longer delay. The 
timer circuit T, is provided for the time-wise coordination of the 
electronic operations and for the correct timing of the actuation of the 
optical switch AOD. 
The analog values s of the digital data stored in the memory S1 
(backscatter curve R1) and in the memory S2 (backscatter curve R2) are 
shown in FIG. 2 as a function of the backscatter time t following a 
transmission pulse. 
the backscatter curve R1 is based on a lower gain factor. The associated 
digital data were measured with a constantly open (not acoustically 
excited) optical switch AOD and stored in the memory S1 during the first 
time domain .DELTA.t1. During the next pulse 3 of the laser L, the gain 
factor was increased. During the reflection time of the light reflected 
from the starting zone of the optical fibre 1 the AOD was acoustically 
excited so that its output beam no longer reached the optical detector 
APD. Thus, the amplifier could not be overloaded. Measurement and storage 
of the digital data in the time domain .DELTA.t2 in the memory S2 took 
place only after the instant t2. 
The arithmetic circuit forms the respective quotient of the values of both 
backscatter energies for a plurality of instants in the overlapping 
regiona .DELTA.tc of the backscatter curves R1 and R2. The mean value 
serves as an evaluation constant whereby, for example, the data of the 
memory S2 which are applied to the display device D are multiplied. The 
display device then displays the backscatter curve R2 as a smooth 
continuation R2 of the backscatter curve R1.