Note: Descriptions are shown in the official language in which they were submitted.
CA 02357428 2001-09-18
CANADA
PATENT APPLICATION
PIASETZKI & NENNIGER
File No.: AN1002
Title: METHOD AND APPARATUS FOR COMPENSATING FOR
POLARIZATION MODE DISPERSION IN OPTICAL DEVICES
Inventor:
Jingyun Zhang
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IN1~ENTIONTITLE: Method and Apparatus for Compensating for Polarization Mode
Dispersion in Optical Devices.
1. Field of the Invention
This invention relates generally the filed of optical signal processing and
more
particularly to optical signal processing in which optical signals undergo
polarization mode
dispersion or PMD during signal processing and manipulation. Most particularly
this
invention relates to methods and apparatuses to compensate for such PMD.
2. Description of Prior Art
PMD is a well-known phenomenon that occurs in optical signals of the type
used,
for example, in telecommunications. Many signal-propagating materials exhibit
a property
of birefringence, which means that the refractive index of the material
differs for optical
signals or light of different polarization. Because of the birefringence, the
two polarization
IS components TE and TM of the incoming light suffer dispersion, since they
propagate
through the signal-propagating materials at different speeds, resulting in
their separation in
time as they exit the material. In the earlier period of optical fiber
telecommunication, the
data rate of the optical signals, was not very high ( typically a few hundred
Mbit/sec). At
low data rates, any birefringent impact caused by such materials used in
optical devices
2o and networks, for example, is limited. Further, thin components such as
beam displacers,
beam splitter/combiners and waveplates define a short signal path. In such
cases the
amount of PMD arising is small enough that it is not a problem in further
signal
processing.
More recently however other uses have been developed for such signal-
propagating
25 birefringent materials which involve longer signal paths. Also, in high-
speed digital
optical telecommunications networks the temporal separation between two
successive
impulses (bits) of a signal can be of the order of a few tens of psec. For
example, an optical
signal of 12.5 Gbit/sec data rate has a temporal separation of 80 psec., while
for a 40
Gbit/sec data rate the temporal separation will be 25 psec. In such new
materials and at
3o such speeds PMD can be a limiting design factor. To control the quality of
data
transmission, it is desired that the degradation of temporal separation
between two
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successive impulses should not be larger than 10% for the receiver to detect
the incoming
signal with acceptable bit error rate (BER).
In practice implementing an optical network system with devices and components
made from materials having birefringence will always introduce polarization
mode
dispersion (PMD) into the signal. More specifically such PMD is a phase
displacement of
the two polarization components of the same bit. 'This situation is worse when
a number of
such devices or components are arranged in cascade, which causes a
superimposition
between successive bits, resulting in a higher BER. PMD is calculated by the
following
equation:
PMD= ~x(nt-no)
where L is the path length of birefringence crystal, C is the speed of light,
ne and no are the
refractive indices of crystal for the extra-ordinary beam and ordinary beam,
respectively.
If such PMD limitations can be overcome, the data quality of optical signals
can be
improved significantly. Thus, there have been a number of attempts in the past
to try to
overcome unacceptable PMD arising in optical devices and Garners. Some of the
prior art
attempts to deal with the problems associated with PMD include the teachings
of the
following references:
"Polarization mode dispersion compensation via an automatic tracking of a
principal state of polarization", Cao; Xiang-Dong, US patent No. 6,130,766,
issued on
January 7, 1999. Compensation of PMD is through an active control process
implemented
by polarization beam sputter and polarization controller. This is costly,
complex and
difficult to reliably implement.
"Method and apparatus for providing high-order polarization mode dispersion
compensation using temporal imaging", Cao; Xiang-Dong, US patent No.
6,104,515,
issued on February 1, 1999. Compensation of PMD is through an active control
process
implemented by phase modulator. This suffers from the same limitations as the
previous
one.
"Method and apparatus for automatic compensation of first-order polarization
mode dispersion (PMD)", Fishman; Daniel A., US patent No. 5,930,414, issued on
September 16, 1997. Compensation of PMD is achieved using a birefringent
compensator,
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in which the compensator automatically and adaptively generates a level of
differential
time delay that substantially equals the differential time delay that the
optical signal
experiences, but of different sign, and, therefore, essentially cancels out
the undesired
delay. It is very difficult and expensive to develop the automatic, adaptive
compensation
taught in this reference.
"Acousto-optical waveguide device with compensation of polarization mode
dispersion", Morasca; Salvatore, US patent No. 5,850,492, issued on December
15, 1998.
An acousto-optical waveguide device with compensation of polarization mode
dispersion
(PMD) comprises at least one first and one second optical path in an optical
wave-guide
to and at least one compensation optical path connected to the first and
second optical path.
The compensation optical path has a prefixed PMD such that the passage time of
a first
polarization component of an optical signal in the first optical path and in
the
compensation optical path is substantially equal to the passage time of a
second
polarization component in the second optical path and in the compensation
optical path.
This teaches setting up a parallel but separate compensation path which is
awkward and
expensive and introduces additional alignment issues.
"Fiber-optic transmission polarization-dependent distortion compensation",
Haas;
Zygmunt. US patent No. 5,311,346, issued on May 10, 1994. The polarization-
dependent
distortion of an optical signal transmitted through an optical fiber is
reduced by aligning
2o the polarization of the optical signal to minimize the received signal
distortion. A
polarization controller (a device which can change the polarization of light
in an optical
fiber) may be located at either the input or output end of a long haul optical
fiber system
and is used to align the polarization of the signal to minimize the received
signal distortion.
Automatic operation of the polarization controller can be obtained by using a
steepest-
descent method based on a distortion measure of the received signal for the
optical signal
transmitted through the optical fiber to generate control signals which are
used to control
the polarization controller. While such alignment may work in some
circumstances, such
as long haul, it is not really an appropriate solution for short hops such as
within a metro
area network.
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Most of the foregoing attempts involve complex adaptive systems, which are
expensive, difficult to control, and unwieldy. What is needed is a simple yet
effective
solution to overcome PMD induced limitations, which solution is also reliable
and
inexpensive.
3. Summary of the Invention
According to the present invention compensation of Polarization Mode
Dispersion
(PMD) can be achieved in an optical device, such as, a Polarization
Independent (PI) ultra-
fast optical switch. Such an optical switch may be, for example, comprised of
a
to birefringent electro-optic crystal, such as LiNb03. When a block of a
different crystal of an
opposite birefringence with proper length, or a block of the same type of
crystal with equal
length but finished at orthogonal cut, is incorporated into the signal path
between the input
and output of the device, the PMD introduced by the switching crystal will be
reduced
significantly. An object of the present invention therefore is to compensate
for the PMD
introduced in the switching crystal by substantially reducing the PMD in a
compensating
crystal. A fiarther object is to achieve this PMD compensation for at least a
specified
wavelength as well as over a range of wavelengths such as the C-band
telecommunications
spectrum.
Thus, an aspect of the present invention is to provide techniques and methods
for
2o compensating for such PMD occurring, for example, in such optical switches
made from
electro-optic crystal of strong birefi-ingence, like LiNbO3. The usage of
electro-optic
crystal makes it possible to build electro optic devices (EOD) such as ultra-
fast switches in
bulk optic form. A typical crystal in a working EOD is usually z-cut and has a
three
dimensional structure, which includes dimensions which are in x and y
directions. An
external electric field can be applied for example in a z direction for
controlling the
variation of refractive indices through electro-optic co-efficiencies. Optical
signal
propagating inside EOD through a certain length in x-y plane will suffer no
PMD when the
wave has only either TE or TM component, but will suffer PMD when the wave has
both
TE and TM components.
3o Therefore according to a general aspect of the invention, there is provided
an optic
device for manipulating an optical signal, said optic device comprising:
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A signal beam path passing through at least one birefringent material;
said signal beam path having at least a first signal path portion
having a first birefringence; and
at least a second signal path portion having a second birefi~ingence,
s wherein said second birefringence is substantially equal to and opposite to
said first birefi-ingence;
whereby polarization mode desperation occurring in said signal as said signal
passes through said first path portion is substantially reversed as said
signal passes through
said second path portion.
to Optical fiber carriers typically have indeterminate signal path lengths,
since the signal
may be routed in any one of a number of ways. Thus, PMD will vary greatly from
one
data stream to the next. For polarization independent (PI) wave guide devices
which have
a complicated structure any PMD introduced is generally unpredictable. In
contrast, any
PMD introduced by the EOD in a bulk optical switch can be determined from the
EOD
is design and thus can be compensated for. The present invention comprehends a
number of
specific approaches, all comprehended by the general aspect of the invention
noted above,
by which such PMD compensation can be achieved, including:
1. In a first more specific aspect of the present invention, when the sign and
magnitude of PMD introduced by the EOD is known, a PMD compensator made
2o from another type of crystal can be designed and used, as long as:
- 'The compensating crystal has the opposite sign of that of EOD crystal
- The compensating crystal has the same cut of that of EOD crystal
- The length L ' of the compensating crystal (with ne' and no ~ is related to
that
L of EOD crystal (with ne and no) by: L x (ne - no ) + L'x(ne - no ) = 0
2s - The installation of the said compensator in the optical path will cancel
out
the PMD
2. In a second aspect of the present invention when the sign and magnitude of
PMD
introduced by the EOD is known, a compensator made from the same type of
crystal can be designed and used as long as:
30 - The compensating crystal has an orthogonal cut of that of EOD crystal
- The length of the compensating crystal is the same as that of EOD crystal
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- The installation of the said compensator in the optical path will cancel out
the PMD
3. In a third aspect of the present invention when the sign and magnitude of
PMD
introduced by the EOD is known, its compensation can be achieved by use of a
compensator and a'/~ ~. wave plate:
- The compensating crystal of the same material as the EOD has the same
cut of that of EOD crystal
- The length of the compensating crystal is the same as that of EOD crystal
- A'h ~, wave plate is placed behind the EOD
to - The installation of the said compensator after'/Z ~, wave plate in the
optical
path will cancel out the PMD
4. In a forth aspect of the present invention when the sign and magnitude of
PMD
introduced by the EOD is known, its compensation can be achieved by EOD itself
with the use of a '/ ~. wave plate with mirror coating:
- The % ~. wave plate is placed behind the EOD
- Optical wave propagates through EOD first, then the % ~, wave plate, and is
reflected back to the % ~, wave plate and the EOD
- The PMD suffered by the wave on its first half way into EOD is cancelled
on its second half way back through EOD
2o With respect to the foregoing aspects of the invention, the sign and
magnitude of
the PMD do not need to be calculated or even measured in some cases. In most,
knowing
the length of the signal path will be enough, and in the fourth aspect the
length of the
signal path does not even need to be known. It will be appreciated by those
skilled in the
art that an optical device which is polarization independent (PI) is desirable
and one which
is both PI and PMD free is even more desirable.
4. Brief Description of the Drawings)
Reference will now be made, by way of example only, to various figures, which
illustrate preferred embodiments of the present invention, and in which:
3o Figure 1 is schematic of a switch having a YV04 crystal block PMD
compensator;
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Figure 2 is a schematic of a switch having a LiNb03 crystal block as PMD
compensator
Figure 3 is a schematic of a switch having a PMD compensated via %2 ~, wave
plate
Figure 4 is a schematic of a switch having a. PMD compensated via '/ ~. wave
plate
with reflection
Figure 5 is a schematic of a switch having PMD compensated by crystal of PMD
opposite to that of EOD;
Figure 6 is a schematic of PMD compensation employing a %Z ~, plate at the
optimum wavelength; and
to Figure 7 is a schematic of PMD compensation employing a 'h ~, wave plate at
a
wavelength slightly different from the optimum wavelength.
5. Description of the Preferred Embodiment
Output collimator 2
is Figure 1:1 x 2 switch with YV04 block as PMD compensator
Figure 1 is a schematic drawing of 1 x 2 switch composed of LiNb03 EOD and PMD
compensator, which is a piece of YV04 crystal. The white and gray areas of
LiNb43 have
opposite domain. When there is no external switching electric field, optical
signal exits
from output collimator 1; when there is an external switching electric field,
the domain
20 inversion interface functions as a mirror through the Total Internal
Reflection (TIR) and
optical signal exits from output collimator 2. The PMD compensation principle
is same as
Approach 1 described above.
It will be understood that a PMD compensator according to this aspect of the
present invention provides a first signal path portion through the LiNb03 and
a second
25 signal path portion through the YV04 crystal. For both switch
configurations, namely
when the signal path is from input collimator 1 to output collimator 1 or from
input
input commator
Output collimator 1
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collimator 1 to output collimator 2 the first signal path length does not
change nor does the
second signal path length. Thus, if PMD is compensated for in one switch
orientation it
will also be compensated for in the other. In this way no adjustment to PMD
compensation is required if the switch changes from one connection
configuration to the
other. Thus, the present invention provides a simple and effective way to
compensate for
PMD that would otherwise arise in such a switch. Further such PMD compensation
remains effective throughout all switch modes or settings. It should be
pointed out that the
compensating crystal can be made from other types of materials, of high
birefringence like
rutile (same cut) and calcite (orthogonal cut).
to
T.iNhOz_ z-cut Output collimator 2
Figure 2: 1 x 2 switch with LiNb03 block as PMD compensator
Figure 2 is similar in configuration to Figure 1, except that both the switch
element and the
PMD compensator are made from the same crystal namely, LiNb03 . In this aspect
of the
present invention the 1 x 2 switch is composed of LiNb03 EOD and PMD
compensator,
which is a piece of LiNb03 crystal of the same length, but with orthogonal cut
as compared
to the EOD. The PMD compensation principle is same as second specific aspect
of the
invention described above. Thus, since the first and second signal path
lengths are the
same, and since they are through the same crystal material, and since they are
different
2o only in the cut of one being orthogonal to the other, any PMD introduced by
the first signal
path portion will be removed by the second signal path portion. Again it will
be noted that
this result applies no matter what the switch connection configuration is and
is constant
regardless of switching operations occurring in the EOD. For both the
embodiments
shown in Figures 1 and 2, the compensating crystal can be cut into any shape,
such as the
shape of a prism, to direct the optical path through a more compact
configuration, as long
as the total length of optical path within the shaped crystal remains the
desired length for
PMD compensation.
~..r... ....______~_.._
LiNb03, x or y-cut Ou~ut collimator 1
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'h wave plate
Figure 3: 4 x 4 switch with PMD compensated via'/ ~, wave plate
Figure 3 is a schematic drawing of 4 x 4 switch composed of two pieces of
LiNb03 EODs
with same cut and '/2 ~, wave plate. The 2°d piece EOD functions as PMD
compensator as
well. The PMD compensation principle is same as Approach 3 described above.
There are
five regions inside LiNb03 where gray and white areas have double TIR
structures,
allowing basic 2 x 2 switching.
It will be appreciated by those skilled in the art that the foregoing
embodiment
1o relies on the'/z ~, wave plate, to rotate the two orthogonal polarization
components TE and
TM by 90° around the axis of beam propagation or the beam path. Such a
wave plate
functions precisely at a single wavelength of its design (or central
wavelength). Thus, at
the design wavelength, where the majority of the signal intensity is located,
a$er
compensation, the signal is substantially free of PMD (see Fig. 6). At
wavelengths slightly
greater or slightly less other than the design wavelength the TE and TM
components are
rotated by angles slightly larger or slightly smaller than 90°,
resulting PMD free signal
after compensation, but with some residual noise (see Fig. 7). Such noise is
carried
forward and displaced by PMD, but usually has very low intensity. In general,
the
acceptable level of bandwidth noise is application dependent. For fiber
devices and
components working over a spectral window of less than 30nm, such residual
noise is
negligible.
Figure 4 is a schematic drawing of 2 x 2 switch composed of one piece of
LiNb03 EODs
and % ~, wave plate. The PMD compensation principle is same as Approach 4
described
LiNb03 of same length and same cut
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above. Gray and white areas inside LiNb03 have double TIR structures, allowing
basic 2 x
'/4 D wave plate
/1
2 switching.
LiNb03, z-cut Mirror coating
Figure 4: 2 x 2 switch with PMD compensated via'/ ~. wave plate
and double pass through reflection
s
One example of a lireferred embodiment will be to construct a 2 x 2 switch,
which
is PMD free using this invention, as shown in Figure 5. Light is launched into
the switch
via Port 1 and Port 2. The double TIR structure built up by the external
switching electric
field will switch the outputs of Port 3 and Port4 from each other.
1o As a light wave propagates through the switching EOD, it will be degraded
by
PMD since LiNb03 is a negative crystal of strong birefringence. Inside LiNbOj
the extra-
ordinary beam propagates faster than the ordinary beam and PMD is negative.
For a length
of 40 mm, the PMD will be around -11 psec. at wavelength of 1310 nm.
When the same light wave propagates through the YV04 compensator, its PMD
1s will be cancelled, this is because YV04 is a positive crystal of even
stronger birefringence,
and inside YV04 the extra-ordinary beam propagates slower than the ordinary
beam and
PMD is positive. For a length of 15 mm, its PMD will be around +11 psec. at
wavelength
1310 nm.
The use of crystal with opposite birefringence also brings the lateral walk-
off
2o between TE and TM components to zero.
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Port 1 Port 4
Figure 5: 2 x 2 switch with PMD compensated by crystal of PMD opposite to that
of
EOD
This invention describes techniques for compensating PMD occurring in electro-
optic crystal switches. The usage of electro-optic crystal makes it possible
to build ultra-
fast switches, which are desirable essential devices in optical fiber
telecommunication
systems with high & optimum data handling capacity, but must be PMD free.
'This
invention provides a solution for achieving that goal.
It will be understood by those skilled in the art that while reference is made
to a
1o first signal path portion and a second signal path portion, the order of
what happens is not
that relevant. The present invention comprehends that the signal switching
could happen
in either the first signal path portion or the second signal path portion. The
switch might
thus start with a PMD compensating crystal, and then finish with switch
element in reverse
to that shown in the drawings without departing from the broad spirit of the
invention.
15 It will be further understood by those skilled in the art that while the
examples of
EOD used were forms of switches, the present invention can also be used on all
kinds of
crystal devices which might otherwise induce PMD into a signal to the
detriment of the
BER. Thus, the present invention comprehends all forms of crystal devices that
as a result
of birefringence introduce PMD into a signal. Switches are merely one form of
crystal
2o devices that can benefit from such PMD compensation.
It will be further understood that although reference has been made to a few
specific materials having birefringence (LiNb03, YVOa) and many other
materials
exhibiting birefringence are also comprehended by the present invention.
Essentially as
long as the crystal device introduces PMD because of the signal propagating
materials used
25 to form the crystal devices the present invention will have application.
While the foregoing description has been provided in relation to specific
examples
of preferred embodiments of the invention it will be understood that various
alterations and
Port 2 LiNb03 ~Oa Port 3
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modifications can be made without departing from the broad scope of the
invention as
defined by the appended claims. Certain of these variations have been
discussed above
and others will be apparent to those skilled in the art.
