Note: Descriptions are shown in the official language in which they were submitted.
12634~5
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This invention relates to optical fibre communication
systems.
In such communication systems polarised amplitude or
phase (or frequency) modulated coherent light signals may be
S transmitted over optical fibres terminating at an optical
receiver including signal detecting/demodulating means.
In order to cope with the noise content of incoming
signals and thermal noise of the receiver the signal-to-
noise ratio of the receiver may be improved by arranging
that optical output signal from a relatively high power
local oscillator at the receiver is mixed with the modulated
incoming light signals, the frequency of the local
oscillator signal being the same as, or different from the
carrier of the incoming light signals according to whether
homodyning or heterodyning of the signal is required.
However, the basic problem which arises in such
communication systems is that since the optical fibre
communication path may extend over large distances (e.g.
hundreds of kilometres) it is not possible in a simple
homodyne receiver system to maintain phase tracking between
the carrier signal launched into the optical fibre at the
transmitter end of the communication system and the local
oscillator signal in order to correct for fluctuations in
the communication path length due to environmental changes.
~263445
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Accordingly, noise results in a homodyne system, whereas in
the case of a heterodyne receiver system the intermediate or
difference frequency which is generated by the mixing of the
local oscillator signal and the incoming signal wi]l contain
phase noise.
The present invention seeks to at least alleviate this
problem by providing an optical fibre communication system
in which modulator means is provided at the transmitter end
of the system for modulating the relative phase between
orthogonally polarised signals derived from a coherent light
source in accordance with data or information to be
transmitted over an optical fibre communication path to the
receiver end of the system where the signals will be
demodulated. The communication path preferably includes
sections of polarisation maintaining optical fibre which are
angularly rotated through 90 or thereabouts relatively to
one another so that the overall delay for the respective
polarised signals over the entire length of the
communications path is substantially the same.
In this way the delay differential between the
polarised signals in one section of polarisation-maintaining
optical fibre is compensated for by an opposite or reverse
delay differential in another section. The phase
differential between the polarised signals at the receiver
end corresponds to phase modulations embodying the
tran~mitted data or information.
1263~4S
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In one method of carrying out the present invention the
optical fibre communication path may simply comprise two
contiguous optical fibre sections of equal length which are
rotated through 90 relatively to one another about their
common axis. In this way although the orthogonally
polarised light components in the first section of optical
fibre may be subjected to different delays due, for
instance, to imperfect spatial coherence of the light
generated by the light source, the differential delay
between the polarised light components in the second optical
fibre section will be effectively reversed due to the 90
angular displacement of this second section relative to the
first optical fibre section (i.e. the orthogonal axes of
polarisation in the second section are aligned in parallel
with respect to the opposite axes in the first section).
The compensating effect produced by the second optical fibre
section affords equalisation of the delays experienced by
the polarised light signals over the entire length of the
optical fibre communication path.
For the purpose of modulating the relative optical
phase of the orthogonal polarised signals in order to encode
the data or information to be transmitted over the
communication path as differential phase information a
plane-polarised laser light source may be arranged to launch
light through an electro-optic modulator crystal into the
optical fibre communication path so that equal intensities
lZ6;~445
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of light are launched into each of the two polarisation
modes. By applying to the modulator crystal a voltage which
varies in accordance with the information to be transmitted
over the communication path a corresponding relative phase
modulation is produced between the two polarised signals.
Alternatively, the transmitter end of the system may
comprise two separate plane-polarised laser light sources
which are arranged to launch light through beam splitter
means into the optical fibre communication path in the
respective polarisation modes. The average frequencies of
the light sources may be locked together by the use of
frequency lock-loop techniques and the data or information
to be transmitted may be generated as relative phase
perturbations or transient frequency perturbations of the
two light sources.
The receiver end of the system may comprise a local
oscillator which produces an output signal which is mixed by
mixing means with the received signal after polarisation of
the former at 45 to the two orthogonal polarisation axes of
the communication path optical fibre. In the case of a
heterodyne receiver the frequency of the local oscillator
will be different from that of the transmitted light signal
and consequently the output from the mixing means (e.g.
photo-diode) will contain the frequency difference between
the transmitted and local oscillator signals but it will
also contain phase and amplitude modulation dependent on the
12634~;
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relative phase modulation between the polarised signals
applied to the communication path at the transmitter end of
the system and representing the data or information
transmitted. In the case of a homodyne receiver the local
oscillator will, by means of a conventional feedback loop,
be frequency locked to the average frequency of the received
signal and again the amplitude of the mixing means may be
detected for deriving the transmitted data or information.
As an alternative to the receiver arrangement just
described the two polarisation mode signals arriving at the
receiver may, by the use of a Wollaston Prism for example,
be directed on to separate mixer/detectors where they are
mixed with a relatively high local oscillator signal of
different average frequency to the received signals. The
respective outputs from the mixer/detectors may then be
compared in phase by a phase comparator in order to obtain
the data or information encoded into the polarised inputs at
the transmitter end of the system. By way of example the
present invention will now be described with reference to
the accompanying drawings in which;
Figure 1 shows a simple diagram of a known optical
fibre communication system;
Figure 2 shows a communication system according to the
present invention;
Figure 3 is a diagram depicting the delay differential
which may be produced between polarised components in the
system of Figure 2 and means for compensating for such delay
differential; and,
1263445
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Figure 4 shows two alternative arrangements of optical
fibre sections to provide full compensation over the entire
communication path for differences in velocity of the
polarised light components.
Referring to Figure 1 the optical fibre communication
system illustrated comprises a coherent light source (e.g.
semiconductor laser) 1 the output from which is suitably
amplitude or frequency (or phase) modulated by optical
modulating means 2 for the encodement into the light carrier
signal of data or information to be transmitted over an
optical fibre 3 (e.g. polarisation-maintaining or single
polarisation monomode fibre or stress-free conventional
monomode fibre) to an optical receiver 4 including a local
oscillator 8.
The output from the modulating means 2 would usually
be polarised along one of the orthogonal axes of
polarisation of the optical fibre 3.
The modulated signal transmitted down the optical fibre
3 will experience propagation delay. Therefore, due to the
imperfect spatial coherence of both the light source 1 and
oscillator light source, and any propagation delay changes
due to acoustic or seismic stressing of the fibre or fibre
thermal fluctuations the received signal and the local
oscillator may be unintentionally phase displaced relative
to one another at the receiver end of the system giving rise
to noise.
~263~S
In order to improve the signal/noise ratio the receiver
~ the received signal is mixed by optical mixing means 7
with the optical output signal of the local oscillator 8.
The frequency of the oscillator signal may either be the
same as, or different from, the frequency of the light
source output signal according to whether the receiver 4 is
a homodyne or heterodyne receiver.
The mixed signal then passes to a detector 5 of the
receiver for demodulation of the encoded signal.
This system suffers from the introduction of phase
noise in both the case of the heterodyne and homodyne
receivers.
Referring now to Figure 2 of the drawing this shows an
optical fibre communication system according to the
invention. The system comprises a plane-polarised laser
coherent light source 9 which launches light into an
electro-optic crystal 10 at a suitable angle to permit
launching of orthogonally polarised signals of equal
intensity into an optical fibre communication path 11. A
variable voltage V is applied to the modulator crystal 10
and this voltage which corresponds to the data or
information to be transmitted over the fibre path 11
produces modulation of the relative phase between the
outputs from the crystal 10. This data is accordingly
transmitted as "differential phase" information.
~263~
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In order to ensure that the relative phase between the
polarised signals is preserved on exit from the entire fibre
section without the intrusion of phase noise (which may
arise with imperfectly coherent light sources due to any
differential delays produced between orthogonal polarised
signals transmitted over the optical fibres), the optical
fibre communication path comprises in the present example
two sections 12 and 13 of equal length. As can be seen from
the enlarged fibre diagram the two sections are rotated
through 90 about their axes relative to one another so that
the opposite polarisation axes of the fibre sections are
aligned. Such an arrangement serves to equalise the delays
suffered by the respective orthogonally polarised signals so
that the signals arrive in a similar relative phase
condition at the receiver end of the optical fibre
communication path to that initially launched. This
equalisation of the delays is depicted in Figure,~ where it
can be seen that the polarised components P3 and P4 meet at
point M (i.e. no phase differential in the absence of
encoded data) after travelling distance L from the
transmitter. Without this means of compensation, there
would be a propagation time difference X between the two
polarisation components Pl and p2 which gives rise to phase
noise with incoherent sources.
The output from the optical fibre 11 is then mixed by a
photodetector (diode) 15 with the output of a local
126344~
g
oscillator 16 polarised at 45 to the two polarisation axes
of the fibre. The electrical output from the photodetector
15 will contain amplitude and phase modulation dependent
upon the phase modulation injected by the electro-optic
crystal 10 at the transmitter end of the system. This
modulation is detected by a demodulator 17 which produces an
output corresponding to the data or information transmitted
over the optical fibre communication path 11.
As will readily be appreciated, although the optical
fibre 11 is shown as consisting of two fibre sections many
different combinations of fibre sections are possible to
achieve equalisation of the delays suffered by orthogonal
polarised signals over the full length of the communications
path. Two such combinations are shown in Figure 4 of the
lS drawings.
It will be apparent from the foregoing that by the use
of differential phase modulation techniques, and by
providing delay equalisation over the communication path,
the aforesaid problems experienced with known systems due
inter alia to imperfect spatial coherence of the light
source are overcome.
By the additional use of heterodyne techniques in the
receiver, the requirement for accurate phase tracking of the
local oscillator is overcome.
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