Optical fiber monitoring by detection of polarization variations

The invention concerns apparatus for monitoring optical fibers by detecting variations in the polarization of light being transmitted through the fibers. In one embodiment the optical fibre carries non-coherent light and the incoming light is split by a passive splitter (2) connected to a polarization beam splitter (5). Variations in the output of this polarization beam splitter are detected, amplified and filtered so that they can be compared with a preset reference voltage. A counter (12) is used to detect variations. Another embodiment relates to the detection of polarization variations in coherent systems.

These are becoming increasingly important in the field of data 
transmission. In optical fiber data transmission systems physical movement 
of the fibers can cause the transmission characteristics of the fibers to 
vary and it is becoming increasingly important to be capable of monitoring 
the outputs of optical fibers in such a manner as to be able to detect 
when the fibers have undergone unwanted perturbations. It is already known 
that movements of fibers carrying optical signals causes variation in the 
polarization of the light carrying the signals. The present invention 
proposes using this to monitor the fibers. 
Accordingly the invention comprises means for detecting variations in the 
polarization of light being transmitted through an optical fiber being 
monitored. 
SUMMARY OF THE INVENTION 
In order that the present invention may be more readily understood various 
embodiments thereof will now be described by way of example and with 
reference to the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing the embodiments it will be appreciated that data 
transmission down an optical fiber can be carried out either with 
non-coherent light or with coherent light the polarization of which is 
controlled. 
FIG. 1 shows a polarization fluctuation detector for monitoring the optical 
fibers of a non-coherent system. In FIG. 1 an incoming signal is shown at 
1 and this signal is sampled by a 10:1 passive splitter 2. Such a splitter 
introduces less than 1 dB loss into the signal path. The main signal path 
continues at 3 to a main data receiver which is conventional and which is 
not shown in the drawing. The low ratio arm 4 of the splitter 2 is taken 
to a polarization beam splitter 5. As the polarisation of the signal light 
varies the power exiting the polarization beam splitter 5 from either arm 
will vary from 0 to the maximum beam splitter 5 power. Only one arm 6 of 
the polarization beam splitter 5 has to be monitored. The exiting light 
from this arm 6 is detected with a photodiode 7 and a transimpedance 
pre-amplifier 8 is used to convert the photo current into voltage 
fluctuations. This pre-amplifier 8 can have a much lower bandwidth than 
the main signal amplifier because only polarization fluctuation 
frequencies below 1 kHz are of interest. A high pass filter 9 removes 
frequencies below 10 Hz. This removes all polarization fluctuations caused 
by natural changes in the fiber characteristic which occur at frequencies 
below 1 Hz. The filtered signal is amplified in a gain stage indicated at 
10 and the amplified signal compared with a pre-set reference voltage by a 
comparator 11. Under normal circumstances the output of comparator 11 will 
be constant. When the optical fibre being monitored is disturbed the 
comparator output will rapidly switch as the disturbances cause 
polarization fluctuation in the light signal. The changes of state of the 
comparator 11 are counted in a counter 12 and a timer 13 is triggered by 
the first output change of the comparator. After a pre-determined delay 
the timer 13 causes the conductor to be read. If the count of the counter 
12 exceeds a preset value than an alarm 14 is triggered. The purpose of 
the counter and its associated timer together with filter 9 is to ensure 
that no alarm is triggered unless the fiber polarization changes at above 
10 Hz for the time set by the timer. This is intended to eliminate false 
alarms. 
Referring now to FIG. 2 of the accompanying drawings this shows a system 
for monitoring polarization fluctations in optical fiber of a coherent 
optical transmission system. In such a system polarization fluctuations in 
the data signal have to be prevented to stop the fluctuations causing data 
errors. In FIG. 2 the incoming coherent signal is shown at 20 and this 
signal is split into two orthogonal states of polarization by appropriate 
polarization beam splitters generally indicated at 21. The output of a 
local oscillator 22 is also split by polarization beam splitters 23 into 
two orthogonal states. The outputs from the beam splitters 21 and 23 are 
mixed to provide two signals at 24 and 25 which fall upon two receivers 26 
and 27, the receiver 26 being for the horizontally polarized signals and 
the receiver 27 for the vertically polarized signals. The outputs of the 
receivers 26 and 27 are combined in a maximum ratio combiner 28 the output 
29 of which is the data output. The system just described is intended to 
compensate for fluctuations in polarization so that the data can be 
extracted correctly. It will be appreciated that as the polarization 
fluctuates the signals received by the receivers 26 and 27 will vary 
inversely with respect to each other. Accordingly in the present 
embodiment the outputs of the receivers 26 and 27 are monitored at 28 
which detects large changes in the state of polarization of a frequency 
greater than a fixed value, which value may for example be 10 Hz. 
Referring now to FIG. 3 of the drawings this also relates to a coherent 
light system but in this embodiment the incoming signal 30 is left 
unaffected. In the FIG. 3 embodiment a local oscillator 31 and a 
polarization control device 32 is used to change the state of polarization 
of the local oscillator 31 to that of the signal 30. The polarization 
control device 32 can take many forms such as fiber squeezers, fiber 
stretchers, liquid crystal cells or Lithium Niobate elements. In any of 
these forms the control device 32 consists of several elements and each 
element has a control voltage applied with a small dither signal. A 
slightly different dither frequency is applied to each element. The 
application of the dither frequencies is controlled by a micro-processor 
controller 33 which calculates how to adjust each element in the 
polarization control device 32 to match the local oscillators polarization 
state to that of the incoming signal 30. In order to do this the output of 
the polarization control device 32 and the incoming signal 30 are combined 
in an optical fiber coupler 34 and taken to a balanced receiver 35 the 
output of which is amplified at 36 to provide the data output 37 with the 
amplifier 36 also being connected to a dither frequency detection circuit 
38. In this arrangement the microprocessor controller 33 continually 
monitors the state of polarization of the incoming signal. Accordingly the 
microprocessor controller 33 is also connected to an alarm circuit 39 and 
is programmed to raise an alarm when the input signal varies in a 
predetermined manner. The set of predetermined conditions to be met before 
an alarm is raised can include the rate of change of polarization, the 
magnitude of change and the duration of change. 
Both of the techniques used in the embodiments of FIGS. 2 and 3 have the 
advantage that the signals to be used for the alarm are already present 
and thus implementation of the alarm is relatively simple.