Note: Descriptions are shown in the official language in which they were submitted.
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POLARIZATION DEPENDENT SIGNAL MIXING DEVICES
The present invention relates to optical devices, and in particular to optical
devices that use the state of polarization (SOP) of the sub-beams of a
composite beam of
light to determine the path taken by the light through a set of birefringent
crystals.
Background of the Invention
Optical circulators and switches are used to control optical signals on
to communication fibers. Multiple signals are placed on a single fiber in
order to increase
the amount of data that can be sent down such fibers. Polarization dependent
optical
circulators and switches have been taught in U.S. Patents Nos. 5,923,472
issued to
Bergmann on July 13, 1999; 5,930,039 issued to Li et al on July 27, 1999; and
5,204,771
issued to Koga on April 20, 1993 (previously published in The Journal of
Lightwave
Technology, Vol. 10, pp. 1210-1216, 1992). Referring to Figure 1, all such
systems are
based on the ability of a birefringent crystal (calcite, rutile, etc.) to
cause a spatial
displacement, with respect to the optical axis of the crystal, of the a
(extraordinary)
polarized sub-beam of light as it travels through the crystal. The o
(ordinary) polarized
sub-beam of light travels through the crystal with no spatial displacement.
The spatial
2o displacement (walk-off) that an a sub-beam is subjected to depends upon the
composition
of the birefringent crystal, the crystallographic orientation of the
birefringent crystal, and
the size of the crystal available to the a sub-beam, i.e. the length. Further
these systems
are based on properties that are specific to the composition of the
birefringent material
used to make the crystal and the limitations the composition imposes, as well
as the
chosen optical axes direction of walk-off for the a sub-beam. However, the
devices
disclosed in Li et al and Bergmann are relatively complicated and limited to 3-
port
circulators. While the Koga reference discloses a 4-port circulator, this
device requires
relatively large birefringent crystals, which greatly increase the cost of the
device. None
of the prior art references discloses overlapping the paths of sub-beams of
input
3o composite beams, enabling the sub-beams of one composite beam mix with the
sub-
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beams of another composite beam. This particular arrangement enables much
smaller
devices to be constructed, which has many advantages, not the least of which
is cost.
Summary of the Invention
An object of the present invention is to overcome the shortcomings of the
prior art
by providing a compact optical device for mixing the polarized sub-beams of
input
composite beams, which can also be used in a variety of other optical devices.
Accordingly the present invention relates to a method of mixing two beams of
to light comprising the steps of:
a) receiving a first composite beam, having a first polarization component and
an
orthogonal second polarization component, at a first location;
b) separating the first and second polarization components of the first beam;
c) rotating the first polarization or the orthogonal second polarization
component
of the composite first beam to provide two first sub-beams having a same
polarization;
d) receiving a second composite beam, having a first polarization component
and
an orthogonal second polarization component, at a second location;
e) separating the first and second polarization components of the second
composite beam;
f) rotating the first or the second polarization component of the second
composite
beam to provide two second sub-beams having a same polarization direction
oriented orthogonally to the two first sub-beams;
g) mixing one of the two first sub-beams with one of the two second sub-beams
to
provide a first mixed sub-beam at a third location; and
h) mixing the other of the two first sub-beams with the other of the two
second
sub-beams to provide a second mixed sub-beam at a fourth location.
A further aspect of this invention is to a polarization dependent optical
signal
mixing device comprising:
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first input means for receiving a first composite beam having a first
polarization
component and an orthogonal second polarization component;
first splitting means for separating the first and second polarization
components of
the first composite beam;
first polarization rotating means for rotating the first or the second
polarization
component of the first composite beam to provide two first sub-beams having a
same polarization;
second input means for receiving a second composite beam having a first
polarization component and an orthogonal second polarization component;
1o second splitting means for separating the first and second polarization
components of the second composite beam;
second polarization rotating means for rotating the first or the second
polarization
component of the second composite beam to provide two second sub-beams
having a same polarization, which is orthogonal to the polarization of the
first
sub-beams;
first mixing means for mixing one of the first sub-beams with one of the
second
sub-beams to provide a first mixed beam at a third location; and
second mixing means for mixing the other of the first sub-beams with the other
of
the second sub-beams to provide a second mixed beam at a fourth location.
Another embodiment of this invention relates to an optical circulator device
comprising:
a first port for inputting a first composite beam having a first polarization
component and an orthogonal second polarization component, and for outputting
a fourth
composite beam having a first polarization component and an orthogonal second
polarization component, input from a fourth port;
a third port for inputting a third composite beam having a first polarization
component and an orthogonal second polarization component, and for outputting
a
second composite beam having a first polarization component and an orthogonal
second
3o polarization component, input from a second port;
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first mixing means for mixing one of the components of the first composite
beam
with one of the components of the third composite beam forming a first mixed
beam, for
mixing the other of the components of the first composite beam with the other
of the
components of the third composite beam forming a third mixed beam, and for
recombining the components of the second and fourth composite beams from
second and
fourth mixed beams for output at the third and first ports, respectively;
non-reciprocal rotating means for making the components of the second and
fourth mixed beams entering therein orthogonal to those passing therethrough,
while
unaffecting the polarity of the components of the first and third mixed beams;
and
second mixing means for mixing one of the components of the second composite
beam with one of the components of the fourth composite beam forming the
second
mixed beam, for mixing the other of the components of the second composite
beam with
the other of the components of the fourth composite beam forming the fourth
mixed
beam, and for recombining the components of the first and third composite
beams from
the first and third mixed beams for output at the second and fourth ports,
respectively.
Another aspect of the present invention relates to an optical switch
comprising:
a first input port for receiving a first composite beam having a first
polarization
component and an orthogonal second polarization component;
second input port for receiving a second composite beam having a first
polarization
2o component and an orthogonal second polarization component;
mixing means for mixing one of the components of the first composite beam with
one of
the components of the second composite beam forming a first mixed beam, and
for
mixing the other of the components of the first composite beam with the other
of the
components of the second composite beam forming a second mixed beam;
adjustable rotating means either for rotating all of the components by a first
angle or for
rotating all of the components by a second angle, which is 90° from
said first angle; and
recombining means for recombining the first and second components of the first
composite beam at a first output port and the first and second components of
the second
composite beam at a second output port when the adjustable rotating means
rotates the
3o components by the first angle, or for recombining the first and second
components of the
first composite beam at the second port and the first and second components of
the
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second composite beam at the first port when the adjustable rotating means
rotates the
components by the second angle.
Brief Description of the Drawings
The invention will be described in greater detail with reference to the
accompanying drawings, which illustrate a preferred embodiment of the
invention,
wherein:
Fig. 1 is a schematic representation of the paths traveled by o and a
polarized sub-beams
in a typical polarization beam sputter;
Fig. 2 is a schematic representation of the device of the present invention
that uses the
State of Polarization (SOP) of the sub-beams of two input optical signals to
separate and
mix the sub-beams to form two new output signals;
Fig. 3 a schematic representation of a second embodiment of the device of Fig.
2;
Fig. 4 is a schematic representation of a third embodiment of the device of
Fig. 2;
Fig. 5 is an isometric view of a fourth embodiment of the device of Fig. 2;
Fig. 6 is schematic representation of a four port optical circulator according
to the present
invention using the device of Fig. 2;
Fig. 7 is a schematic representation of a second embodiment of the four port
optical
circulator according to the present invention using the device of Fig. 3;
Fig. 8 is a schematic representation showing the optical paths of the o and a
sub-beams
3o through the optical components presented in Figure 7 for input at ports 1
and 3;
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Fig. 9 is a schematic representation showing the optical paths of the o and a
sub-beams
through the optical components presented in Figure 7 for input at ports 2 and
4;
Fig. 10 is a schematic representation of a three port optical circulator
according to the
present invention using the device of Fig. 3;
Fig. 11 is a schematic representation of a switch according to the present
invention; and
Fig. 12 is a schematic representation of an add/drop switch according to the
present
l0 invention.
Detailed Description of the Drawings
Figure 1 is a schematic diagram of a birefringent optical device (e.g. a
rutile or a
calcite crystal) that is used to split an input beam of light into two
orthogonally polarized
sub-beams, labeled o for ordinary and a for extraordinary. In a birefringent
material, with
a slow and a fast axis, the ordinary polarized beam will travel straight
through while the
extraordinary sub-beam will be displaced spatially (walked-off) depending upon
the
crystallographic axis orientation.
2o
With reference to Figure 2, an optical signal S 1 is launched into a
birefringent
component 1 through a top port 2 and is divided into 0 3 and a 4 sub-beams.
The sub-
beams 3 and 4 are spatially displaced from each other by an amount and a
direction
dependent upon the composition and orientation of the birefringent component
1. The
two sub-beams 3 and 4 exit the birefringent component 1 and sub-beam 3 passes
through
polarization rotator 5, typically a'/2 waveplate, which changes the
polarization of the sub-
beam 3 so that sub-beam 3 and 4 have the same polarization. Since sub-beam 4
has been
displaced, it does not pass through the polarization rotator 5 and therefore
maintains its
original polarization. The two sub-beams 3 and 4 (now both a sub-beams) then
pass
3o through birefringent component 6, which displaces both, directing them to
output ports 7
and 8, respectively.
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Similarly, optical signal S2 is launched into birefringent component 1 through
a
port 9 and is split into o sub-beam 11 and a sub-beam 12. Note that the input
ports 2 and
9 for the optical signals S1 and S2 are selected such that the optical paths
do not overlap
in the space between optical components 5 and 13. Sub-beam 11 passes through
birefringent component 1, but does not pass through polarization rotator 13,
which has
the exact optical characteristics of polarization rotator 5. Sub-beam 12 is
displaced from
the original path and passes through polarization rotator 13. Now both of the
sub-beams
11 and 12 are identically polarized. Accordingly, due to the crystallographic
orientation
to of the birefringent component 6, sub-beams 11 and 12 (now both o sub-beams)
pass
straight through birefringent component 6 and exit through output ports 7 and
8,
respectively. In this arrangement sub-beams 3 and 11 are combined at output
port 7, and
sub-beams 4 and 12 are combined at output port 8. In this arrangement,
assuming that
both of the birefringent components 1 and 6 have the same characteristics,
birefringent
component 6 must be longer than birefringent component 1 to ensure that the
walk off of
sub-beams 3 and 4 is sufficient to meet sub-beams 11 and 12.
Figure 3 illustrates a further embodiment of the present invention, in which
two
2o birefringent components 1 and 6 have the same length. Accordingly, the
paths for sub-
beams 4 and 11 become overlapped. Otherwise the optical functions are the same
as in
Figure 2 except for a glass support piece 16 which is included to support the
polarization
rotators 5 and 13.
Figure 4 illustrates another embodiment of the present invention in which the
optical path of sub-beam 11 does not overlap the optical path of sub-beam 4.
The
difference in approaches lies in the fact that the optical axis of the second
birefringent
component 6 is inverted relative to the optical axis of the first birefringent
component,
and accordingly only one waveplate is necessary. Thus the walk-off direction
(up) of the
3o second birefringent component is opposite to the walk-off direction (down)
of the first
birefringent component.
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Referring to Figure 4, the optical signal S 1 has optical sub-beams 3 and 4.
Sub-
beam 3 passes straight through optical component 1, bypassing polarization
rotator 17,
and passes straight through birefringent component 6 to output 18. Sub-beam 4
is
displaced by birefringent component 1 and passes through birefringent
component 17
thereby changing its polarization enabling it to pass straight through
birefringent
component 6 to output port 19. The optical signal S2 has optical sub-beams 11
and 12.
Sub-beam 11 passes straight through birefringent component 1 and into
polarization
rotator 17, which rotates the polarization of sub-beam 11 by 90°.
Accordingly the sub-
1o beam 11 is displaced in birefringent component 6 and is directed to output
18. Since the
birefringent component 6 has been inverted relative to the birefringent
component 1 e-
polarized sub-beams are displaced upwardly instead of downwardly. Sub-beam 12
is
displaced in birefringent component 1, its optical path bypasses polarization
rotator 17.
Accordingly sub-beam 12 is again displaced by birefringent component 6,
whereby it
exits birefringent component 6 at output port 19. Therefore the output signals
are O1 =
sub-beam 3 + sub-beam 11 and 02 = sub-beam 4 + sub-beam 12.
In another embodiment illustrated in Figure 5, the two birefringent components
have optical axes that are perpendicular to each other, i.e. the walk-off
direction (down)
of the first birefringent component 1 is perpendicular to the walk-off
direction (right side)
of the second birefringent component 6. As a result, the position of the
polarization
rotators 5 and 13 must be adjusted, whereby they are vertically and laterally
offset, and
the position of the output ports are adjusted, whereby they are superposed
instead of
adj acent.
Referring to Figure 5, S 1 has two sub-beams 3 and 4. Sub-beam 3 (o sub-beam)
travels straight through birefringent component 1, and through polarization
rotator 5,
which change the polarization thereof. Since the polarization of sub-beam 3
has changed
(e sub-beam), it gets displaced by birefringent component 6 due to the
crystallographic
orientation thereof. However, since the optical axis of birefringent component
6 has been
rotated by 90°, sub-beam 3 is displaced to the right, not vertically,
and exits output port
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21. Sub-beam 4 is vertically displaced downwardly by birefringent component 1,
thereby
bypassing polarization rotators 5 and 13. Accordingly, sub-beam 4 is laterally
displaced
by birefringent component 6 and exits output port 22.
Signal S2 has two sub-beams 11 and 12. Sub-beam 11 travels straight through
birefringent component 1, missing optical component 13, and straight through
birefringent component 6 exiting output port 21. Sub-beam 12 is vertically
displaced by
birefringent component 1 and passes through polarization rotator 13, which
results in a
polarization change. Sub-beam 12 then travels straight through birefringent
component 6
1o to output port 22. Accordingly output signals O1 = sub-beam 3 + sub-beam 11
while 02
= sub-beam 4 + sub-beam 12.
Figure 6 illustrates an optical circulator utilizing two of the previously
described
devices aligned in series with a non-reciprocal 90° rotator, typically
a Faraday non-
reciprocal 45° rotator and a half waveplate, therebetween. Each
provides a 45° shift so
that when sub-beams are travelling in the forward direction (from left to
right) this
combination results in a cumulative change in the SOP. However, when the sub-
beams
are traveling in the reverse direction (from right to left) the result is no
change in the SOP
of the sub-beams, since the non-reciprocal Faraday rotator has a -45°
shift and the
2o waveplate a 45° shift.
Accordingly, when signals are launched into ports P1 and P3 sub-beams 33 and
34 get mixed with sub-beams 41 and 42 forming a first mixed beam, containing
sub-
beams 33 and 41, at output port 37, and a third mixed beam, containing sub-
beams 34
and 42, at output port 38. The cumulative effect of passing through the
waveplate and the
Faraday non-reciprocal rotator, in the forward direction, is that the
components of the
mixed beams are rotated by 90°. Thereafter sub-beams 33 and 34 end up
recombining at
port P2, and sub-beams 41 and 42 end up recombining at port P4.
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When optical signals are fed into ports P2 and P4, of Figure 6, we can use the
same reference numerals 33, 34, 41 and 42 to identify the sub-beams travelling
through
birefringent components 51 and 56.
However, since there is zero cumulative effect on the sub-beams after passing
through the Faraday non-reciprocal rotator 25 and'h waveplate 26 in the
reverse
direction, we must renumber the sub-beams when travelling through birefringent
components 31 and 36. Therefore sub-beam reference numerals for the reverse
direction
are shown in brackets.
to
Due to the zero cumulative effect of elements 25 and 26, sub-beams (33), (34),
(41) and (42) travel different paths in the reverse direction than they did in
the forward
direction. As a result sub-beam (33) and (34) recombine at port P3 and sub-
beams (41)
and (42) recombine at port P1.
To summarize the embodiment presented in Figure 6, an optical signal placed on
the input of port 1 passes through the optical system to port 2. An optical
signal that is
placed on port 2 as input passes through the optical system to port 3. An
optical signal
that is placed on port 3 as input passes through the optical system to port 4.
An optical
2o signal that is placed on port 4 as input passes through the optical system
to port 1.
Therefore, this system is a closed 4 port optical circulator.
Figures 7 to 9 b) are schematic representations of an in-line 4 port optical
circulator using two of the basic structures defined in Fig. 3, positioned
back-to-back
with two'/a pitch lenses 87 and 88, a Faraday non-reciprocal rotator 89 and
a'/z
waveplate 90, positioned therebetween. This particular lens arrangement
results in a'/2
pitch lens system that enables the paths of the sub-beams to cross. The
lenses, combined
with the inverting of the optical elements on the right side of the optical
system with
respect to the elements on the left side of the optical system, results in a
compact, closed,
3o in-line, four port optical circulator. The optical component layout on the
right side has
the same optical effect as going from right to left in the optical component
layout on the
to
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left side. Thus an a sub-beam is displaced down the page by optical components
71 and
76 and up the page by optical components 91 and 96. In other words, the walk-
off
direction of components 91 and 96 is opposite to the walk-off direction of
components 71
and 76.
The specific paths taken by each sub-beam will be discussed in greater detail
with
reference to Fig 8a, 8b, 9a, and 9b.
Figures 8 and 9 are a more detailed schematic of the sub-beam paths for the
l0 optical system presented in Figure 7. Figures 8 a) and 8 b) show the sub-
beam paths for
input signals at ports 1 and 3, respectively while Figures 9 a) and 9 b) show
the sub-beam
paths for inputs at ports 2 and 4, respectively. Figures 8 a) and 8 b)
illustrate the sub-
beams travelling left to right, in the optical system, and Figures 9 a) and 9
b) illustrate the
sub-beams travelling right to left, in the optical system. Faraday non-
reciprocal rotator
89 and'/z waveplate 90 cause no change in SOP for sub-beams travelling from
left to
right. However, there is a change in the SOP for sub-beams travelling from
right to left
in the optical system.
With reference to Figure 8 a), a signal S1 is input to port Pl and splits into
sub-
2o beams 73 and 74. Sub-beam 73 travels straight through birefringent
component 71 and
impinges on polarization rotator 75, which changes the polarization thereof.
Subsequently sub-beam 73 passes straight through the glass support element 86
but is
displaced by birefringent component 76. The sub-beam 73 is inverted by the
lenses 87
and 88, however the optical elements 89 and 90 cause no SOP change therein.
Sub-beam
73 is displaced by birefringent element 91, passes through glass element 106,
bypasses
polarization rotators 95 and 103, is displaced by birefringent component 96
and exits the
system at output port P2.
Sub-beam 74 is displaced by birefringent element 71, bypasses polarization
3o rotators 75 and 83, passes through glass element 86, and is displaced by
birefringent
element 76. Sub-beam 74 is inverted by optical lens 87 and 88 but optical
element 89
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and 90 cause no SOP change therein. Accordingly, sub-beam 74 is displaced by
birefringent element 91, passes through glass element 106, and passes through
polarization rotator 95, which causes a SOP change therein. Accordingly, sub-
beam 74
passes straight through birefringent element 96 and exits output port P2 along
with sub-
beam 73.
In Figure 8 b), a signal S3 is launched through port P3 and divided into sub-
beams
81 and 82. Sub-beam 81 travels straight through birefringent component 71 and
straight
1o through glass element 86, bypassing polarization rotators 75 and 83. Sub-
beam 81 then
travels straight through birefringent component 76 and is inverted by optical
lens 87 and
88. As before, optical elements 89 and 90 cause no change in SOP in the
forward
direction. Sub-beam 81 travels straight through birefringent component 91 and
glass
element 106, impinging upon polarization rotator 103, which causes a SOP
therein.
Accordingly, sub-beam 81 is displaced by birefringent element 96 and exits
output port
P4.
Sub-beam 82 is displaced by birefringent element 71 and impinges on
polarization rotator 83, which causes a SOP change therein. Accordingly, sub-
beam 82
2o passes through glass element 86 and straight through birefringent component
76.
Subsequently sub-beam 82 is inverted by optical lens 87 and 88 with no change
in SOP
by optical elements 89 and 90. Therefore, sub-beam 82 then travels straight
through
birefringent component 91 and glass element 106, bypassing polarization
rotators 95 and
103. Sub-beam 82 then travels straight through birefringent component 96 and
exits
output port P4, along with sub-beam 81.
With reference to Figure 9 a), a signal S2 is launched through port P2 and is
divided into sub-beams 113 and 114. The sub beam 113 travels straight through
the
birefringent component 96 and impinges on the polarization rotator 95, causing
a SOP
3o change therein. Accordingly, sub-beam 113 passes through glass element 106
and then is
displaced by birefringent component 91. Subsequently, sub-beam 113 is inverted
by
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optical lenses 88 and 87 and undergoes a change in SOP via optical elements 90
and 89.
Therefore, sub-beam 113 travels straight through birefringent component 76,
glass
element 86, and polarization rotator 83, which causes a SOP change therein.
Accordingly, sub-beam 113 is displaced by birefringent component 71 and exits
output
port P3.
The sub-beam 114 is displaced by the birefringent component 96, bypasses the
polarization rotators 95 and 103, and travels through glass element 106. Sub-
beam 114 is
then displaced by birefringent element 91, inverted by optical lens 87 and 88,
and
to undergoes a change in SOP due to optical elements 89 and 90. Therefore, sub-
beam 114
travels straight through birefringent component 76 and glass element 86,
bypassing
polarization rotators 75 and 83, and then travels straight through
birefringent component
71, exiting at output port P3, along with sub-beam 113.
With reference to Figure 9 b), a signal S4 is launched on port P4 and is
divided
into sub-beams 121 and 122. The sub-beam 121 travels straight through the
birefringent
component 96 and the glass element 106, bypassing the polarization rotators 95
and 103.
Then sub-beam 121 travels straight through birefringent component 91, and is
inverted
by optical lens 87 and 88. A change in the SOP occurs due to optical
components 89 and
2o 90. Accordingly, sub-beam 121 is displaced by birefringent component 76 and
travels
straight through glass element 86, bypassing polarization rotators 83 and 75.
Subsequently, sub-beam 121 is displaced by birefringent component 71, and
exits at
output port P1.
The sub-beam 122 is displaced by the birefringent component 96 and impinges on
the polarization rotator 103, which changes the SOP thereof. Accordingly, sub-
beam 122
travels through glass component 106 and straight through birefringent
component 91.
Sub-beam 122 is inverted by optical lens 87 and 88 and undergoes a change in
SOP due
to optical components 89 and 90. Therefore, sub-beam 122 is displaced by
birefringent
3o component 76, travels through glass component 86, and impinges on
polarization rotator
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75, which changes the SOP thereof. As a result, sub-beam 122 travels straight
through
birefringent component 71 and exits output port P1, along with sub-beam 121.
To summarize the embodiment presented in Figures 7 to 9 b), an optical signal
placed on the input of port 1 passes through the optical system to port 2. An
optical signal
that is placed on port 2 as input passes through the optical system to port 3.
An optical
signal that is placed on port 3 as input passes through the optical system to
port 4. An
optical signal that is placed on port 4 as input passes through the optical
system to port 1.
1o Figure 10 is a schematic representation of a 3 port circulator using the
devices of
Fig. 3 in series with a single birefringent component 131 having the two
lenses 87 and 88,
the Faraday non-reciprocal rotator, and the'/z waveplate 90, thereinbetween.
The optical
axis of the birefringent component 131 is inverted relative to the optical
axis of
birefringent components 71 and 76, i.e. the walk-off direction of the
birefringent
component 131 is the opposite to the walk-off direction of birefringent
components 71
and 76. Therefore, an a sub-beam that travels down the page in birefringent
components
71 and 76 travels up the page in birefringent component 131.
With reference to Figure 10 a), an optical signal S1 is again launched on port
P1
2o and is divided into sub-beams 73 and 74. Sub-beam 73 travels straight
through
birefringent component 71 and impinges on polarization rotator 75, which
changes the
SOP thereof. Accordingly, the sub-beam 73 travels through glass element 86 and
gets
displaced by birefringent component 76. Subsequently sub-beam 73 is inverted
by
optical lens 87 and 88. The SOP of the sub-beam 73 is unchanged by optical
elements 89
and 90. Sub-beam 71 bypasses polarization rotator 135, is then displaced by
birefringent
component 131, and exits at output port P2.
Sub-beam 74 is displaced by birefringent component 71 and travels through
glass
element 86, bypassing polarization rotators 75 and 83. Sub-beam 74 is further
displaced
by birefringent component 76, and inverted by optical lens 87 and 88. The SOP
is
unchanged by optical elements 89 and 90. Sub-beam 74 impinges on polarization
rotator
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135, which changes the SOP thereof, and then travels straight through
birefringent
component 131, exiting at output port P2, along with sub-beam 73.
When an optical signal S2 is input at port P2 sub-beams 141 and 142 are
created
in birefringent component 131. Sub-beam 141 travels straight through
birefringent
component 131 and impinges on polarization rotator 135, which changes the SOP
thereof. Sub-beam 141 is then inverted by optical lens 87 and 88 and undergoes
another
SOP change in elements 89 and 90. Accordingly, sub-beam 141 travels straight
through
birefringent component 76 and through glass element 86, impinging on
polarization
to rotator 83, which causes a change in the SOP thereof. Therefore, sub-beam
141 is
displaced by birefringent component 71 and exits output port P3.
The sub-beam 142 is originally displaced by birefringent component 131, which
enables it to bypass polarization rotator 135. Then the sub-beam 142 is
inverted by
optical lens 87 and 88 and undergoes a change in the SOP due to optical
elements 89 and
90. Accordingly, sub-beam 142 travels straight through birefringent component
76 and
glass element 86, bypassing polarization rotators 75 and 83. Therefore, sub-
beam 142
travels straight through birefringent component 71 and exits output port P3
with sub-
beam 141.
The optical elements 89 and 90 can be replaced by any component that performs
the same function such as a liquid crystal rotator.
With reference to Figure 11, the polarization dependent mixing devices of the
previous embodiments are used in a switching device, which includes
birefringent
elements 201, 202 and 203, polarization rotators (half waveplates) 204 and
205, a non-
reciprocal rotator 206 defined by a Faraday rotator with a lh waveplate, and
an adjustable
rotator 207. The adjustable rotator 207 preferably comprises a liquid crystal
cell with a
reflective surface for reflecting incident sub-beams. The liquid crystal cell,
which is
3o sensitive to different input voltages, variably adjusts the polarization of
the sub-beams.
In a first position the sub-beams pass through the liquid crystal cell
resulting in a
CA 02326421 2000-11-22
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cumulative change in the SOP of 90°. Alternatively, the liquid crystal
cell can be
arranged whereby no cumulative change in the polarization of the sub-beams
results. Of
course, it is also possible to have other arrangements including one, which
rotates both
sub-beams by different amounts.
When a signal S11, is launched into an input port P11 incident sub-beams 208
and
209 are created by the birefringent element 201. Sub-beam 208 travels straight
through
birefringent component 201, bypassing polarization rotators 204 and 205,
through
birefringent element 202, and non-reciprocal rotator 206. Sub-beam 208 is
unaffected by
1o non-reciprocal rotator 206, and travels straight through birefringent
element 203. If the
adjustable rotator is set to provide no change in polarization, the incident
sub-beam 208 is
reflected with no change in the SOP. Output sub-beam 218 then travels back
through
birefringent element 203 and, when it passes through non-reciprocal rotator
206,
undergoes a change in SOP. Accordingly, output sub-beam 218 is displaced by
birefringent element 202 and impinges polarization rotator 204, which causes a
change in
the SOP. Accordingly, output sub-beams 218 travels straight through the
birefringent
element 201 and exits on port P0.
Incident sub-beam 209 is displaced by birefringent element 201 and impinges on
polarization rotator 205, which causes a change in the SOP. Accordingly, sub-
beam 209
travels straight through birefringent elements 202 and 203 and non-reciprocal
rotator 206.
Since we already assumed that the adjustable rotator was set to have no effect
on the
incident sub-beams, sub-beam 209 is reflected by optical element 207 with no
change in
the SOP. Output sub-beam 219 travels straight back through birefringent
element 203,
whereby the SOP is changed by the non-reciprocal rotator 206. Accordingly,
output sub
beam 219 is displaced by birefringent element 202 and bypasses polarization
rotators 205
and 204. Output sub-beam 219 is then further displaced by birefringent element
201 and
exits port P0, along with sub-beam 218. A first output path is defined by sub-
beams that
travel straight through the birefringent element 203 and get displaced by the
birefringent
3o element 202.
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Assuming now that the adjustable rotator is set to rotate the incident sub-
beams
by 90°, the SOP of both incident sub-beams 208 and 209 will be changed.
This change in
the SOP causes output sub-beams 228 and 229 to be displaced in birefringent
element
203. Non-reciprocal rotator 206 causes another change in the SOP to occur
which enables
output sub-beams 228 and 229 to travel straight through birefringent element
202. The
path traveled by sub-beams 228 and 229 is defined as the second output path.
Output sub-
beam 228 bypasses the polarization rotators 204 and 205, and travels straight
through
birefringent element 201, while output sub-beam 229 travels through
polarization rotator
204 which changes the polarization and causes sub-beam 229 to be displaced in
1o birefringent element 201. As a result both output sub-beams 228 and 229
exit output port
P90.
With reference to Figures 6 and 7, if the non-reciprocating rotators, defined
by
Faraday rotators 25 and 89 with ih waveplates 26 and 90, are replaced by
adjustable
switches, i.e. liquid crystal cells, 2 x 2 switches can be constructed. In
this case, when
the adjustable rotator is set to maintain the polarization of the various sub-
beams
constant, a composite beam entering P1 will exit P2 and vice versa. Similarly,
a
composite beam entering P3 will exit P4 and vice versa. However, if the
adjustable
rotator is set to rotate all of the sub-beams by 90°, a composite beam
launched into P1
will exit P4 and vice versa. Moreover a composite beam launched into P3 will
exit P2
and vice versa.
Figure 12 illustrates a preferred embodiment of this type of Add/Drop switch
in
the "folded" arranged, which includes a mirror to reflect the mixed beams
after passing
through the adjustable rotator . The Add/Drop switch includes birefringent
elements 301,
302 and 303, polarization rotators (waveplates) 304, 305 and 306, non-
reciprocal rotator
(Faraday and'/a waveplate) 307, and adjustable rotator 308. First we will
assume that the
adjustable rotator 308 is set to provide no change in the SOP of the sub-
beams.
Light enters the In port, divides into incident sub-beams 311 and 312, which
pass
3o through birefringent element 301. Subsequently, sub-beam 312 travels
through
polarization rotator 306, which changes the SOP thereof to be parallel to sub-
beam 311.
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Accordingly both sub-beams 311 and 312 travel straight through birefringent
elements
302 and 303, and non-reciprocal rotator 307 unaffected. Since the adjustable
rotator 308
is set at zero, the incident sub-beams are reflected without a change in the
SOP.
Therefore, output sub-beams 321 and 322 pass back through birefringent element
303
along the same path as incident sub-beams 311 and 312. When the sub-beams pass
through non-reciprocal rotator 307, their SOP changes and accordingly when
they pass
through birefringent element 302 they get displaced. This output path is
defined as the
first output path. Subsequently, output sub-beam 321 passes through
polarization rotator
305, which changes the SOP, so that sub-beams 321 and 322 have orthogonal
1o polarizations. Accordingly as output sub-beams 321 and 322 pass through
birefringent
element 301 they recombine at the Out port.
Light enters the ADD port and divides into incident sub-beams 331 and 332 as
it
passes through birefringent element 301. The SOP of sub-beam 331 changes as it
passes
through polarization rotator 304 which enables both sub-beams 331 and 332 to
be
displaced similarly as they traverse birefringent elements 302, 303 and non-
reciprocal
rotator 307. Since the adjustable rotator is set at zero, the output sub-beams
341 and 342
are reflected back, retracing the paths of incident sub-beams 331 and 332
until impinging
non-reciprocal rotator 307. In this direction the non-reciprocal rotator 307
provides a
2o change in the SOP which enables the output sub-beams 341 and 342 to travel
straight
through birefringent element 302. This path is referred to as the second
output path.
Subsequently, sub-beam 342 traverses polarization rotator 305, which changes
the
polarization thereof and enables sub-beams 341 and 342 to converge at the Drop
port
after passing through birefringent element 301.
If the adjustable rotator 308 is set to rotate the incident sub-beams by
90°, incident
sub-beams 311 and 312 will follow the second output paths previously defined
by output
sub-beams 341 and 342, respectively. Similarly incident sub-beams 331 and 332
will
follow the first output paths of output sub-beams 321 and 322, respectively.
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Accordingly, when the adjustable rotator 308 is set at 0° a signal
entering the Add
port exits the Drop port and the signal entering the In port exits at the Out
port.
Moreover, when the adjustable rotator 308 is set at 90° a signal
entering the Add port
exits the Out port, and a signal entering the In port exits the Drop port.
to
20
30
19
