Note: Descriptions are shown in the official language in which they were submitted.
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This is a divisional of Canadian Patent Application
No. 511,830, which was filed on June 18, 1986.
The present invention relates to the transmission
of digital information in which electro-magnetic radiation is
modulated in accordance with the information.
Various detection methods have been proposed in the
past for demodulating the transmitted radiation including
coherent detection and direct detection. Although coherent
detection has major advantages over direct detection, it has
the drawback of polarization sensitivity. This problem could,
in principle, be eliminated if a transmission medium could be
developed which was polarization holding.
In the case of optical radiation, special optical
fibres have been developed which are substantially
polarization holding but these have complex structures and
much higher losses (and are more expensive) when compared to
standard circular symmetric monomode fibres. Furthermore,
since large quantities of standard optical fibre have already
been installed, are being installed, and are planned for the
telecommunications network, which initially use polarization
insensitive to direct detection, it is desirable to devise a
transmission method and system which is compatible with these
fibre coherent networks.
So far two schemes have been proposed that will
enable coherent detection to be used with standard fibre;
these are active polarization control and polarization
diversity. The former is capable of eliminating all
polarization penalties. However, an extra opto-mechanical or
electro-optic device is required in either the local
oscillator or signal path at the receiver. This complicates
the receiver and could also result in an insertion loss
penalty. Polarization diversity reception eliminates the need
for extra optical control devices in the receiver but requires
the addition of a polarizing beam splitter and a second
photodiode, amplifier chain and intermediate frequency (I.F.)
demodulator. With polarization diversity reception there can
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be up to 3dB receiver sensitivity penalty for certain
combinations of input polarization and local oscillator
polarization states when the outputs of the two I.F.
demodulators are simply combined (although it may be possible
to reduce this penalty to about ldB with more complex post
demodulator processing).
These two methods of overcoming polarization
problems both result in a more complex receiver which although
possibly acceptable in a long distance high capacity point-to-
point transmission link could introduce a significant costpenalty in a local wideband distribution scheme or LAN/MAN
type application.
The invention, instead of making use of the absolute
polarization of radiation, detects changes in polarization
state between adjacent bits at the receiver. The absolute
polarization does not matter.
We have realized that although the output state of
polarization of a long transmission path using standard
optical fibre fluctuates, it does so only slowly. For cable
buried under the ground, where the temperature remains fairly
stable, significant fluctuations may not occur over several
hours.
According to one aspect of the present invention
there is provided a method of transmitting digital information
comprising: frequency modulating a polarized electromagnetic
signal in response to said information to produce a frequency
modulated electromagnetic signal, causing said frequency
modulated electromagnetic signal to be incident on a
birefringent medium so as to generate a polarization modulated
signal having a polarization which corresponds to the
frequency of said frequency modulated electromagnetic signal,
and transmitting said polarization modulated signal.
According to a further aspect of the present
invention, there is provided a digital information
transmission system comprising: means for modulating a
polarized electromagnetic signal in response to said
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information to produce a frequency modulated electromagnetic
signal at an output thereof, means for transmitting a
polarization modulated electromagnetic signal including a
birefringent medium which is coupled to said output of said
means for modulating, and means for causing said frequency
modulated electromagnetic signal to be incident on said
birefringent medium so as to generate a polarization modulated
signal having a polarization which is dependent on the
frequency of said frequency modulated electromagnetic signal.
The polarization modulation could be based on any
of the following known effects: Kerr, Faraday rotation or TE
to TM conversion.
The detection of the transmitted wave may be
achieved in any conventional manner. Preferably, however, the
detecting step comprises combining the transmitted modulated
wave with a detection signal of fixed polarization to generate
a wave with an intermediate frequency, and detecting changes
in phase and/or amplitude of the intermediate frequency wave
to regenerate the information.
In some examples, the modulation step may comprise
switching the polarization of the wave between two values at
for example 90. Alternatively, a change in polarization
within a clock period may be achieved by ramping the
polarization between two polarizations (i.e., by more
gradually changing polarization within a clock period, such
as by linearly changing polarization with respect to time over
a substantial portion of a clock period thus defining a "ramp"
if polarization is plotted versus time).
Although the invention is applicable to electro-
magnetic radiation with a variety of wavelengths, it isparticularly applicable to wavelengths in the optical domain.
In this specification, the term optical is intended to refer
to that part of the electro-magnetic spectrum which is
generally known as the visible region together with those
parts of the infra-red and ultra-violet regions at each end
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of the visible region which are capable of being transmitted
by dielectric optical waveguides such as optical fibres.
It is particularly advantageous if the method
further comprises additionally modulating one or more of the
phase, amplitude, and frequency of the polarized electro-
magnetic wave in accordance with the digital information.
This facility can be used in two ways. Firstly,
double the amount of information can be sent on the
transmitted wave thus doubling the transmission rate, or
alternatively the digital information used to modulate the one
or more of the phase, amplitude and frequency of the polarized
electro-magnetic wave may be the same information which is
used to modulate the polarization of the wave. The latter
possibility provides a way of reducing the chances of error
in detecting the transmitted information.
In accordance with a particular aspect of the
present invention, a digital information transmission system
comprises a source of polarized electro-magnetic radiation;
modulating means responsive to the information for causing in
successive clock periods changes in polarization of the
radiation in accordance with the information; transmitting
means for transmitting the modulated radiation; and detecting
means for receiving the transmitted modulated radiation and
for detecting changes in polarization of the radiation to
regenerate the information.
The source of electro-magnetic radiation may
comprise for example a laser.
In one particularly convenient arrangement in which
the radiation has a wavelength in the optical domain, the
apparatus further comprises a birefringent medium; and means
for modulating the frequency of the polarized electro-magnetic
radiation in accordance with the information, the frequency
modulated radiation being incident on the birefringent medium
whereby corresponding changes in polarization of the radiation
are caused.
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This provides a particularly elegant arrangement
which would eliminate the need for an external modulator and
thus avoid the losses associated with such a modulator. The
output polarization from the birefringent medium, such as a
short length of optical fibre, is dependent on the optical
frequency of the source and can therefore be modulated as the
source frequency is modulated.
We believe that the invention is particularly
applicable to overcoming the problem of long term polarization
stability in coherent transmission systems using monomode
fibre. The invention enables considerable simplification of
the detecting means such a heterodyne receiver. This will be
of benefit in future wideband distribution schemes. There may
also be some scope for using the invention in optical networks
that have all optical sources centrally located. In this case
it may be possible to provide polarization modulators at
remote terminals fed by continuous wave light from a central
laser bank.
Some examples of methods and systems in accordance
with the invention will now be described with reference to the
accompanying drawings, in which:-
Figure 1 illustrates schematically one example ofa transmission system;
Figure 2 illustrates the receiver of Figure 1 in
more detail;
Figure 3 illustrates a second example of a
transmission system;
Figure 4 illustrates graphically a detection method;
and,
Figures 5 and 6 illustrate the waveforms of input
signals, local oscillator signals, and IF signals in two
different examples.
The system shown in Figure 1 comprises a
semiconductor laser 1 which generates a linearly polarized
beam of optical radiation. The beam is fed to a polarization
modulator 2 of conventional form which is controlled via a
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data input 3. At successive clock periods, data is applied
to the modulator 2 which causes either a change or no change
in the polarization of the incoming beam. For example, a
binary digit "1" may cause a 90 switch in polarization
whereas a binary digit "0" will cause no change. The
modulated radiation is then fed into a conventional monomode
optical fibre 4 defining a transmission path.
At a receiving station, the optical fibre 4 is
connected to an optical coupler 5 having a second input
connected to a local oscillator 6 constituted by a
semiconductor laser which generates circularly polarized
optical radiation. The optical coupler 5 combines the
incoming modulated optical signal with the local oscillator
signal and the resultant IF signal is fed to a detector 7.
Information is contained in both the differential
phase, i.e. the change in phase between clock periods, and the
differential amplitude of the IF signal, that is the change
in amplitude between clock periods.
The relative magnitude of the demodulated phase
signal to the demodulated amplitude signal will depend on the
relationship of the received state of polarization to that of
the local oscillator polarization. For certain combinations
of input signal to local oscillator polarization there will
be no useful amplitude information. Take, for instance, the
case when the input polarization is switching between two
linear orthogonal states (Figure 5A) and the local oscillator
is circular (Figure 5B). With this combination although the
IF amplitude will remain constant and the IF phase will switch
in sympathy with the input signal's polarization (Figure 5C).
In contrast, consider the case, again with a circular local
oscillator (Figure 6B), where the input signal is switching
between right circular and left circular (Figure 6A). This
time the IF envelope switches completely (Figure 6C).
Therefore to determine that a polarization change has taken
place it is necessary to process both the demodulated
differential phase and envelope signals together. To give
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optimum performance in some cases it may be better not to
represent symbols by step changes in polarization states but
by some other function; for example a polarization ramp (e.g.,
a more gradual polarization change with respect to time via
a conventional ramp circuit 3a).
The detector 7 which includes a filter generates an
output signal which is fed in parallel to a differential phase
demodulator 8 and a differential amplitude demodulator 9. The
output signals from these demodulators 8, 9 are fed to a
micro-processor 10 which provides an output signal
representing the original data. The micro-processor 10 could
select between the signals from the phase demodulator 8 and
amplitude demodulator 9 the signal with the largest magnitude
or it could add the two signals to produce a resultant signal.
Figure 2 illustrates one way in which the
demodulators 8, 9 could be implemented. The detector 7
includes a sensor 11 such as a photodiode whose output is fed
to an amplifier 12 and then to a filter 13 and a further
amplifier 14. The amplitude demodulator 9 is constituted by
a conventional envelope detector 15 whose output is split and
fed in parallel to the inverting and non-inverting inputs of
a differential amplifier 16. The path length to the non-
inverting input is longer than that to the inverting input so
that a single clock period (or bit period) delay is applied
to that bit enabling comparison of signal levels between
adjacent bits to take place. In a similar manner the phase
change between adjacent clock periods or bits is determined
by splitting the path from the amplifier 14 into two 17, 18,
delaying one path 18 by a single bit period, and multiplying
the two signals in a double balanced modulator 19.
The micro-processor 10 determines what weighting
should be given to each of the two demodulated signals. In
the simplest case it may be possible to take the signal which
has the largest peak-to-peak level.
A second example is illustrated in Figure 3. In
this example the frequency of the optical radiation generated
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by the semiconductor laser 1 is modulated directly by the
digital data. This frequency modulated radiation, of fixed
polarization, is fed to a short length of high birefringence
fibre 20. Preferably, the frequency modulated beam is
launched at ~/4 to the birefringent axis of the fibre 20. The
output polarization from this short length of fibre 20 will
be dependent on the optical frequency of the source and can
therefore be modulated as the laser frequency is modulated.
The beam output from the fibre 20 is then coupled to the main
optical fibre 4.
Frequency modulation of a semiconductor laser can
be achieved directly by control of injection current or by
acoustic wave interaction. In the simple case of a laser
directly frequency shift keyed between fl and f2 (where the
difference between these optical frequencies is much greater
then the data rate) it is only necessary to demodulate just
one of the two frequencies to determine the symbol
transmitted; this single filter detection of FSK gives the
same performance as ASK. If it is now arranged that the
frequency shift is sufficient for the two signals to have
orthogonal polarizations we now have a choice of two signals
that could be detected at the distant receiver and either
signal containing the transmitted information. The local
oscillator frequency at the receiver could be tuned to
whichever signal presented the best polarization match.
Moreover, by careful selection of the IF frequency
with respect to fl - f2 it may be possible to site the signal
associated with the orthogonal polarization state near the
image band of the detected signal. Therefore under this
condition where the local oscillator frequency is positioned
just off centre of fl ~ f2 either signal would automatically
appear in the receiver IF bandwidth, individually or together
depending on the received polarization state. In this case
polarization diversity may be possible without returning the
frequency of the receiver local oscillator laser.
