Multi-port optical circulator utilizing imaging lens and correction optical element

A nonreciprocal optical device, preferably an optical circulator, and a method of transferring optical signals utilize a compensating lens coupled optically to a focusing lens. The compensating lens operates to correct misalignments caused by the focusing lens. The focusing lens and the compensating lens provide efficient coupling of optical fibers. Preferably, the compensating lens has a forward face with a number of flat surfaces that can refract light in a desired manner. In a first embodiment, two compensating lenses in optical series between two focusing lenses are utilized. In a second embodiment, only one compensating lens is utilized, but a mirror assembly is introduced so that polarization components of a light beam propagate through the compensating lens twice. In the first embodiment, the circulatory functions are accomplished by two optical assemblies and a shift plate. On the other hand, the circulatory functions in the second embodiment are performed by only one optical assembly and a shift plate. The single optical assembly is utilized twice in single transmission of a light signal, providing a compact optical device.

TECHNICAL FIELD 
The invention relates generally to nonreciprocal optical devices and more 
particularly to an optical circulator that can accommodate transmissions 
among multiple optical fibers. 
DESCRIPTION OF THE RELATED ART 
Continuing innovations in the field of fiber optic technology have 
contributed to the increasing number of applications of fiber optics in 
different technologies. The flexibility and reliability of communication 
networks based upon transmission of light signals via optical fibers are 
greatly increased by the availability of assemblies such as optical 
circulators and isolators. Optical circulators enable a bidirectional 
fiber to be coupled to both an input optical fiber and an output optical 
fiber. Optical isolators provide forward propagation of light signals from 
an input optical fiber to an output optical fiber, while inhibiting light 
from propagating in the rearward direction from the output optical fiber 
to the input optical fiber. 
U.S. Pat. No. 5,204,771 to Koga describes an optical circulator having 
three birefringent crystals and two nonreciprocal rotator assemblies. Each 
of the two nonreciprocal rotator assemblies is positioned between two 
adjacent birefringent crystals. The nonreciprocal rotator assemblies are 
comprised of a Faraday rotator, a left half-wave plate, and a right 
half-wave plate. The left half-wave plate provides a rotation of 
polarization components of light beams in a direction that is opposite to 
the direction of rotation caused by the right half-wave plate. The optical 
circulator of Koga includes an input/output port on one face of the 
optical circulator assembly. On the opposite face, an output port and an 
input port are positioned such that the input port is located above the 
output port. The optical circulator of Koga operates to transmit light 
signals received by the input port to the input/output port. However, 
light signals that are received by the input/output port are directed to 
the output port, instead of being directed back to the input port. 
Another optical circulator of interest is described in U.S. Pat. No. 
5,471,340 to Cheng et al. In an attempt to reduce the number of required 
components for achieving optical isolation or circulation, the optical 
circulator of Cheng et al. utilizes a mirror at one end of an optical 
assembly. Adjacent input/output ports are situated at the end of the 
assembly opposite to the mirror. The optical assembly includes a first 
birefringent crystal, upper and lower half-wave plates, a first Faraday 
rotator, a second birefringent crystal, and a second Faraday rotator. In 
operation, a light beam enters the optical assembly from one of the 
input/output ports. The first birefringent crystal divides the light beam 
into two polarization components. The adjacent nonreciprocal Faraday 
rotator and the upper and lower half-wave plates properly align the 
polarization components for lateral displacement (walk-off) of one or both 
of the polarization components by the center birefringent crystal. The 
polarization components are rotated by the second Faraday rotator twice as 
the light beams are reflected by the mirror. The first birefringent 
crystal plate then recombines the two polarization components for output 
to a different input/output port. 
There are a number of factors that must be considered in the design of 
optical circulators and isolators. U.S. Pat. No. 5,319,483 to Krasinski et 
al. identifies insertion loss and crosstalk as two performance-related 
design considerations. Insertion loss is the difference in power between 
input light and the light that exits the optical assembly. The primary 
causes of insertion loss are identified as absorption of light and 
imperfections of polarization separation and recombination. Crosstalk in 
an optical circulator is the transmission of light from an input fiber to 
a fiber which is not the intended output fiber. Krasinski et al. assert 
that the primary cause of crosstalk in optical circulators is 
back-reflection from the various optical elements in the assembly. The 
system described in the patent utilizes birefringent crystals instead of 
polarization splitting cubes in an attempt to provide more complete 
polarization separation, thereby reducing insertion loss and crosstalk. 
Moreover, the system is one in which the optical elements of the assembly 
are in optical contact with each other, thereby reducing back-reflections. 
Another cause for insertion loss is dispersion of light beams when 
propagating through optical elements, which hinders efficient 
fiber-to-fiber coupling. One means to alleviate this concern is to use 
lenses to focus the light beams. Patent Cooperation Treaty application No. 
PCT/AU96/00800by Frisken, published on Jun. 19, 1997, International 
Publication No. WO 97/22034, describes an optical circulator having a pair 
of lenses between two optical assemblies. The pair of lenses operates to 
focus light beams propagating through the optical circulator. Each of the 
two optical assemblies includes a Faraday rotator and a half-wave plate 
positioned between two birefringent crystals. The two optical assemblies 
perform separation, rotation, and recombination operations on polarization 
components of propagating light beams to facilitate circulation of light 
beams to and from optical fibers. 
In addition to the above-identified performance-related concerns, there are 
manufacturing-related concerns. Preferably, the assembly is physically 
small, providing advantages with respect to the cost of materials and the 
ability to house a number of such assemblies. If there is an array of 
input/output ports at one side of an optical circulator or isolator, the 
core-to-core spacing between the ports (i.e., "pitch") may determine the 
width and the length of the assembly of optical elements. Conventionally, 
there is a pitch of at least 2 mm in order to accommodate the use of 
collimators. The minimum width of the assembly is the product of the pitch 
and the number of ports in the array. Rutile is a common material for 
forming the birefringent crystals that provide the desired walk-off 
displacements within the assembly. For each 1 mm of walk-off, the rutile 
crystal must have a thickness of approximately 10 mm. The thicknesses of 
the other optical elements in the assembly, e.g. the Faraday rotator, add 
to the total thickness dimension of the assembly. 
While known optical circulators operate well for their intended purposes, 
improvements in performance and reduction in fabrication cost are desired 
in a design of optical circulators. What is needed is an optical 
circulator for coupling multiple optical fibers for transmitting signals, 
such as light signals within a communication network, with a high density 
of input/output ports and with a significant cost efficiency in the 
fabrication process. 
SUMMARY OF THE INVENTION 
A nonreciprocal optical device, preferably an optical circulator, and a 
method of transferring optical signals utilize a compensating lens coupled 
optically to a focusing lens. The compensating lens operates to correct 
misalignment caused by the focusing lens. Preferably, the compensating 
lens has a forward face with number of flat surfaces that can refract 
diverging light received from the rearward face of the compensating lens 
into parallel light. In addition, the compensating lens is able to receive 
parallel light on the forward face and refract the parallel light such 
that the light is emitted from the rearward face in a converging manner. 
In a first embodiment of the invention, the nonreciprocal optical device 
includes two compensating lenses in optical series between two focusing 
lenses. The two focusing lenses are preferably identically shaped 
converging lenses. Although not critical to the invention, the focusing 
lenses may be configured to provide 1:4 imaging. Similar to the focusing 
lenses, the two compensating lenses are physically identical. The two 
compensating lenses are positioned such that the forward faces of the 
compensating lenses are face-to-face. Preferably, the four lenses are 
positioned such that the distance between the first focusing lens and the 
first compensating lens is equal to the distance between the second 
compensating lens and the second focusing lens. The focusing lenses and 
the compensating lenses provide efficient coupling of optical fibers. The 
circulatory functions are accomplished by two optical assemblies and a 
shift plate. 
The first optical assembly includes a walk-off crystal, upper and lower 
half-wave plates, and a Faraday rotator. The first optical assembly 
operates to separate a light beam into two orthogonal polarization 
components and rotates one of the two polarization components such that 
both polarization components are aligned to be shifted laterally, i.e., 
positive or negative x-direction, by the shift plate. The second optical 
assembly operates to recombine the two polarization components for output 
via a predetermined optical fiber. Preferably, the first and second 
optical assemblies are structurally identical. The only difference between 
the two optical assemblies is the orientation of the second optical 
assembly with respect to the first optical assembly. The second optical 
assembly is rotated 180.degree., such that forward and rearward faces of 
the second optical assembly are switched with respect to the faces of the 
first optical assembly. 
The optical elements in the first optical assembly, and consequently the 
second optical assembly, can be configured in a number of alternative 
arrangements without affecting the operations of the first optical 
assembly and the second assembly. First, the walk-off crystal may have a 
walk-off direction in either vertical direction, i.e., positive or 
negative y-direction, to separate the polarization components of a light 
beam. Second, the upper and lower half-wave plates can be positioned 
either in front of the Faraday rotator or rearwardly of the Faraday 
rotator. The only criterion is that the half-wave plates are arranged such 
that one polarization component of a light beam travels through the upper 
half-wave plate, while the other polarization component travels through 
the lower wave plate. Preferably, each of the Faraday rotator and the 
half-wave plates provides a 45.degree. rotation of polarization 
components. If the walk-off crystal has a walk-off direction in the 
positive y-direction, the Faraday rotator and the upper half-wave plate 
can provide clockwise rotations of a forward propagating polarization 
component, while the lower half-wave plate provides a counter-clockwise 
rotation. Alternatively, the Faraday rotator and the upper half-wave plate 
can provide counter-clockwise rotations, while the lower half-wave plate 
provides a clockwise rotation. If the walk-off crystal has a walk-off 
direction in the negative y-direction, the rotations of the upper and 
lower half-wave plates are reversed. 
In the preferred embodiment, the shift plate is positioned between the two 
compensating lenses. The shift plate only displaces forward propagating 
polarization components in a lateral direction. This is due to the 
orientation of the polarization components caused by the first optical 
assembly. In the forward direction, all of the polarization components are 
aligned horizontally when propagating through the shift plate. However, in 
the rearward direction, the polarization components are aligned 
vertically. 
The nonreciprocal optical device of the first embodiment operates to couple 
first and second optical fibers to a single third optical fiber. For 
example, the first and second optical fibers can be positioned adjacent to 
the second optical assembly, while the third optical assembly is 
positioned adjacent to the first optical assembly. A light beam from the 
third optical fiber will be transmitted to the second optical fiber, while 
a light beam from the first optical fiber will be transmitted to the third 
optical fiber. However, a light beam from the second optical fiber will 
not be transmitted to the third optical fiber. 
In operation, a forward propagating light beam enters the first optical 
assembly from a first optical fiber. The light beam is separated into 
first and second polarization components by the walk-off crystal in the 
first optical assembly. The first polarization component has a vertical 
polarization state, while the second polarization component has a 
horizontal polarization state. The first polarization component is then 
rotated 90.degree. by the first optical assembly. The first and second 
polarization components then exit the first optical assembly and encounter 
the first focusing lens. The focusing lens initially converges the 
polarization components to a focal point of the first focusing lens. 
However, after the focal point, the polarization components begin to 
diverge. In addition, the relative positions of the polarization 
components are inverted by the converging and diverging process. Next, the 
polarization components impinge upon the first compensating lens. Each 
polarization component is refracted at a flat surface of the compensating 
lens, such that the polarization components cease to diverge and propagate 
in parallel with respect to the other polarization component. 
Next, the polarization components propagate through the shift plate, which 
displaces the polarization components in either the positive or the 
negative x-direction. The polarization components are then caused to 
converge by the second compensating lens. Again, each polarization 
component is refracted at a flat surface of the second compensating lens. 
The polarization components converge to a focal point of the compensating 
lens and begin to diverge. The polarization components then travel through 
the second focusing lens, which alters the propagation direction of the 
polarization component such that the polarization components are again 
propagating in parallel. The second optical assembly then rotates the 
first polarization again by 90.degree. and recombines the first and second 
polarization components. The combined polarization components of the light 
beam are transmitted to a second optical fiber. 
A rearward propagating light beam from the second optical fiber is not 
transmitted to the first optical fiber. In the rearward direction, the 
rotation caused by the second assembly is such that the rearward 
propagating polarization components of the light beam are not displaced by 
the shift plate. Therefore, the polarization components follow a different 
propagation path. The two compensating lenses and the two focusing lenses 
operate on rearward propagating polarization components in the exact 
opposite manner as on forward propagating polarization component. The 
second focusing lens now converges the polarization components, while the 
first focusing lens alters the polarization components to propagate in 
parallel. In addition, the converging function is performed by the first 
compensating lens, instead of the second compensating lens. 
In a second embodiment of the invention, the second optical assembly, the 
second focusing lens, and the second compensating lens are removed. 
Instead, a mirror assembly is placed rearwardly of the shift plate. In 
this embodiment, a forward propagating light beam emitted from a first 
optical fiber in an array of fibers is transmitted to an adjacent second 
optical fiber in the array of fibers in a rearward direction. A light beam 
emitted from the second optical fiber is transmitted to a third optical 
fiber, etc. 
The mirror assembly includes a mirror and a Faraday rotator. The Faraday 
rotator operates to change the polarization states of polarization 
components, so that reflected polarization components are not shifted a 
second time by the shift plate. The Faraday rotator rotates polarization 
components by 45.degree. before reflection and again after reflection. The 
overall effect of the Faraday rotator is a 90.degree. rotation of the 
polarization components. 
In operation, the first optical assembly separates polarization components 
of a light beam from a first optical fiber and rotates one of the 
polarization components by 90.degree.. The polarization components then 
travel through the first focusing lens, which causes the polarization 
components to diverge. The compensating lens then adjusts the propagating 
directions of the polarization components, such that they are propagating 
in parallel. Next, the polarization components are displaced by the shift 
plate and travel through the mirror assembly. The Faraday rotator in the 
mirror assembly rotates the polarization components by 45.degree.. The 
polarization components are then reflected by the mirror. The polarization 
components again travel through the Faraday rotator, which rotates the 
polarization components by another 45.degree.. 
Next, the polarization components propagate through the shift plate. 
However, because the polarization components have been rotated 
perpendicularly, the polarization components are not displaced by the 
shift plate. The polarization components then travel through the first 
compensating lens and first focusing lens. Operating in an opposite 
manner, the compensating lens converges the polarization components, and 
the focusing lens then adjust the polarization components such that they 
are propagating in parallel. In the rearward direction, the first optical 
assembly rotates one of the polarization components and recombines the 
polarization components. The recombined polarization components are then 
transmitted to an adjacent optical fiber in the array of fibers with 
respect to the first optical fiber. 
A method of transferring circulating optical signals from multiple optical 
fibers utilizes the nonreciprocal optical device in accordance with the 
invention. First, a light beam is received by an optical assembly of the 
multi-port optical circulator from a first optical fiber in an array of 
fibers. The light beam is then separated into polarization components by 
the optical assembly. Next, one of the polarization components is rotated, 
such that both polarization components have a common polarization state. 
The polarization components are then diverged, such that the polarization 
components are moving away relative to each other. In the process of 
diverging the polarization components, the polarization components are 
also inversely projected. 
Next, the polarization components are redirected, such that the 
polarization components are propagating in a parallel manner. After being 
redirected, the polarization components are laterally displaced. In one 
embodiment, the polarization components are reflected toward the array of 
fibers. In addition, the polarization components are rotated 
perpendicularly. The displaced polarization components are then converged, 
such that the separation distance of the polarization components is 
decreasing. Next, the converging polarization components are again 
redirected to propagate in a parallel manner. Propagating in the parallel 
manner, one of the polarization components is rotated perpendicularly. 
Lastly, the polarization components are recombined and transmitted to a 
second optical fiber. 
An advantage of the invention is that fiber-to-fiber coupling efficiency is 
improved with the use of compensating lens(es). As a result, more fibers 
may be coupled using the invention. In addition, the optical assemblies 
utilized in both embodiments of the invention are compact. Still another 
advantage of the invention is that in the first embodiment of the 
invention the first and second optical assemblies are physically 
identical, lowering the cost of fabrication. Finally, the use of the 
mirror assembly in the second embodiment of the invention greatly reduces 
the overall size of the device, while maintaining the improved 
fiber-to-fiber coupling.

DETAILED DESCRIPTION 
In FIG. 1, a multi-port optical circulator 10 in accordance with a first 
embodiment of the invention is shown. The multi-port optical circulator 10 
includes a first optical assembly 12, a second optical assembly 14, a pair 
of focusing lenses 16 and 18, a pair of compensating lenses 20 and 22, and 
a shift plate 24. The multi-port optical circulator 10 is positioned 
between two arrays of optical fibers 26 and 28. The array of fibers 26 
includes optical fibers 30, 32, 34 and 36. The optical fibers 30-36 are 
positioned in place by a fiber holder 38. Similarly, the array of fibers 
28 includes optical fibers 40, 42, 44 and 46 in a fiber holder 48. 
Preferably, the optical fibers 30-36 and 40-46 are thermally expanded core 
(TEC) fibers having mode field diameters (MFDs) of approximately 20 .mu.m. 
Although the fiber arrays 26 and 28 are shown as having four optical 
fibers, the fiber arrays 26 and 28 may include more or fewer optical 
fibers. While not critical to the invention, the TEC fiber arrays 26 and 
28 have a pitch of 250 .mu.m. 
The optical assembly 12 includes a walk-off crystal 50, an upper half-wave 
plate 52, a lower half-wave plate 54, and a Faraday rotator 56. The 
walk-off crystal 50 provides a displacement in the positive y-direction of 
vertical polarization components of light beams that are propagating in 
the forward direction, i.e., positive z-direction. The walk-off crystal 50 
may be made of rutile (titanium dioxide-TiO.sub.2) or yttrium vanadate 
(YVO.sub.4). In addition, inexpensive Lithium Niobate (LiNbO.sub.4) may be 
used to form the walk-off crystal 50. The thickness of the walk-off 
crystal 50 depends on the type of the optical fibers 30-36 and 40-46, due 
to the difference in MFDs. Wider MFDs require greater spatial displacement 
by the walk-off crystal 50. 
The upper half-wave plate 52 and the lower half-wave 54 are positioned such 
that the displaced polarization components propagate through the upper 
half-wave plate 52, while the horizontal polarization components propagate 
through the lower-half wave plate 54. The upper half-wave plate 52 and the 
Faraday rotator 56 operate to provide 90.degree. rotation of polarization 
components propagating in the forward direction. However, due to the 
nonreciprocal nature of the Faraday rotator 56, the upper half-wave plate 
52 and the Faraday rotator 56 provide 0.degree. rotation for polarization 
components propagating in a rearward direction, i.e., negative 
z-direction. Conversely, the lower half-wave plate 54 and the Faraday 
rotator 56 operate to provide 0.degree. rotation for polarization 
components of forwardly propagating light beams and 90.degree. rotation 
for rearwardly propagating light beams. 
The focusing lenses 16 and 18 are configured to focus polarization 
components traveling in either the forward or rearward direction. 
Preferably, the two focusing lenses 16 and 18 are physically identical 
converging lenses. While not critical to the invention, the focusing 
lenses 16 and 18 may provide 1:4 imaging. Preferably, the four lenses 16, 
18, 20 and 22 are positioned such that the distance between the focusing 
lens 16 and the compensating lens 20 is equal to the distance between the 
compensating lens 22 and the focusing lens 18. In operation, the focusing 
lens 16 inversely projects polarization components propagating in the 
forward direction onto the compensating lens 20. The polarization 
components are projected at angles that depend upon the impinging position 
of a polarization component on the focusing lens 16. Similarly, the 
focusing lens 18 inversely projects polarization components propagating in 
the rearward direction onto the compensating lens 22. The compensating 
lenses 20 and 22 receive polarization components from either the focusing 
lens 16 or focusing lens 18 and refract the polarization components such 
that the angles caused by the focusing lenses 16 and 18 are countered. The 
result is that polarization components are propagating in a parallel 
manner between the compensating lenses 20 and 22, regardless of the 
propagating direction. Preferably, the compensating lens 20 is positioned 
at a distance from the focusing lens 16 that is greater than the focal 
length of focusing lens 16. In addition, the compensating lens 22 is 
positioned at a distance from the focusing lens 18 that is greater than 
the focal length of focusing lens 18. 
The shift plate 24 is positioned between the compensating lenses 20 and 22. 
Preferably, the shift plate 24 is a walk-off crystal having a walk-off 
direction parallel to the x-axis. By properly orientating polarization 
components of light beams in both the forward and rearward directions, the 
shift plate 24 provides displacement of polarization components 
propagating in only one direction. 
The second optical assembly 14 also includes a walk-off crystal 58, an 
upper half-wave plate 60, a lower half-wave plate 62, and a Faraday 
rotator 64. Preferably, the second optical assembly 14 is structurally 
identical to the first optical assembly 12, except for the orientation of 
the second optical assembly 14 with respect to the first optical assembly 
12. The second optical assembly 14 is a mirror image of the first optical 
assembly 12. In other words, the second optical assembly 14 is the first 
optical assembly 12 that has been rotated 180.degree. about the y-axis. 
For forward propagating light beams, the Faraday rotator 64 and the upper 
half-wave plate 60 rotate polarization components by 90.degree., while the 
Faraday rotator 64 and the lower half-wave plate 62 provide 0.degree. 
rotation. Conversely, the Faraday rotator 64 and the upper half-wave plate 
60 provide 0.degree. rotation, while the Faraday rotator 64 and the lower 
half-wave crystal 62 provide 90.degree. rotation for rearward propagating 
light beams. The walk-off crystal 58 provides displacement in the negative 
y-direction for vertical polarization components propagating in the 
forward direction. 
Turning to FIGS. 2, 3 and 4, a compensating lens 66 having eight surfaces 
68, 70, 72, 74, 76, 78, 80 and 82 on a forward face is shown from various 
points of view. The upper surfaces 68-74 operate on displaced polarization 
components of light beams caused by either walk-off crystal 50 or 58. The 
lower surfaces 76-82 operate on the other non-displaced polarization 
components. An upper surface and a lower surface form a pair of surfaces 
that operates on polarization components of a single light beam. For 
example, if one polarization component of a light beam travels through the 
upper surface 70, the other polarization component will travel through the 
lower surface 78. Similarly, surfaces 68 and 76, 72 and 80, and 74 and 82 
form the remaining pairs. The surfaces 68-82 are symmetrical about the 
horizontal centerline 84 and the vertical centerline 86. 
The compensating lens 66 can be positioned such that the eight surfaces 
68-82 are facing either the positive or negative z-direction. The 
compensating lens 20 of FIG. 1 is a compensating lens that is facing the 
positive z-direction. The compensating lens 22 of FIG. 1 is a compensating 
lens that is facing the negative z-direction. In operation, the 
compensating lens 66 can refract diverging polarization components of 
light beams to propagate in a parallel manner. Conversely, the 
compensating lens 66 can receive polarization components that are 
propagating in parallel and redirect the polarization components to 
propagate in a converging manner. 
The fiber holders 38 and 48 of FIG. 1 can be composed of semiconductor 
substrates. Preferably, the fiber holders 38 and 48 are etched to form 
V-shaped grooves to properly position the input and output optical fibers 
30-36 and 40-46. FIG. 5 illustrates optical fibers 88 positioned on 
V-shaped grooves 90 that are etched on a substrate, such as a silicon 
wafer 92. Conventional integrated circuit fabrication techniques may be 
utilized to form the grooves 90. For example, the grooves may be formed 
photolithographically, using a mask to define the grooves and using 
chemical etchant. While not critical, the angle of one of the V-shaped 
grooves 90 relative to the other wall is preferably 70.5.degree.. The 
fiber holders 38 and 48 may also include another etched silicon wafer 94 
that is affixed to the lower silicon wafer 92 by a layer of adhesive 96, 
as shown in FIG. 6. The use of an adhesive layer is not critical to the 
invention. Alternatively, wafer bonding may be used to attach the two 
silicon wafers 92 and 94. 
In FIG. 7, a top view of the multi-port optical circulator 10 is 
illustrated. Also shown in FIG. 7 are four propagation paths 98,100,102 
and 104. The four propagation paths 98-104 may represent paths taken by 
the displaced polarization components caused by either of the walk-off 
crystals 50 or 58. However, the four propagation paths 98-104 may also 
represent paths taken by the other non-displaced polarization components. 
The reason for the dual representation is that on an x-z plane the two 
polarization components follow the same path. The only difference between 
the paths taken by the two polarization components of a light beam is in 
the y-direction. The difference in paths with respect to the y-axis is 
illustrated below with reference to FIG. 8. 
In a rearward direction, a light beam from any one of the optical fibers 
40-46 follows the same referenced propagation paths 98-104. For example, a 
light beam from the optical fiber 40 would propagate through the optical 
assembly 14 in a negative z-direction following the propagation path 98. 
The propagation path 98 is initially located above the other propagation 
paths 100-104. The focusing lens 18 refracts the light beam, such that the 
propagation path 98 is now below the other propagation paths. The 
compensating lens 22 redirects the light beam, such that once again the 
propagation path 98 is in the negative z-direction. 
Propagating through the shift plate 24, the light beam is not affected by 
the walk-off properties of the shift plate 24. This is due to rotation of 
the rearward propagating light beam by the optical assembly 14, such that 
the polarization components are aligned orthogonally to the walk-off 
direction of the shift plate 24. Therefore, displacing paths 106, 108,110 
and 112, illustrated within the shift plate 24, are not applicable for 
rearward propagating light beams. The light beam is then refracted in a 
converging manner by the compensating lens 20 and redirected by the 
focusing lens 16. Following the compensating lens 20 and focusing lens 16, 
the propagation path 98 of the light beam is back to a location above the 
other propagation paths 100-104. The propagation path 98 leads to the 
optical fiber 30. Therefore, light beams from the optical fiber 40 are 
coupled to the optical fiber 30. Similarly, light beams from optical 
fibers 42, 44 and 46 are coupled to optical fibers 32, 34 and 36, 
respectively. 
In a forward direction, polarization components of a light beam are 
affected by the walk-off properties of the shift plate 24. Therefore, the 
propagation path of the light beam is shifted by the shift plate 24. For 
example, a light beam from the optical fiber 36 follows the propagation 
path 104. However, the light beam is displaced by the shift plate 24, 
because the operation of the optical assembly 12 aligns the polarization 
components of the light beam with the walk-off direction of the shift 
plate 24. The light beam travels through the displacing path 106, and then 
follows the propagation path 102. Thus, the light beam from the optical 
fiber 36 is coupled to the optical fiber 44. Similarly, light beams from 
the optical fibers 34 and 32 are coupled to the optical fibers 42 and 40, 
respectively. However, a light beam from the optical fiber 30 is not 
transmitted to any of the optical fibers 40-46. The light beam from the 
optical fiber 30 is displaced by the shift plate 24 to follow the 
displacing path 112 that is not aligned with any of the optical fibers 
40-46. 
FIG. 8 illustrates a side view of the multi-port optical circulator 10. 
Also shown in FIG. 8 are two propagation paths 114 and 116 with respect to 
the y-axis. The two propagation paths 114 and 116 represent paths taken by 
polarization components of any light beam from one of the optical fibers 
30-36 and 40-46 through the multi-port optical circulator 10. 
In the forward direction, a light beam enters the optical assembly 12 from 
an optical fiber of the fiber array 26. The vertical polarization 
component of the light beam is displaced in the positive y-direction by 
the walk-off crystal 50. Therefore, the vertical polarization component 
will follow the propagation path 114, while the horizontal polarization 
component will follow the propagation path 116. The two polarization 
components are recombined by the walk-off crystal 58 and transmitted to an 
optical fiber of the array of optical fibers 28. However, as stated above, 
a light beam from the optical fiber 30 will not be transmitted to any 
optical fiber of the array of optical fibers 28. Rearward propagating 
polarization components of a light beam will also follow the same paths 
114 and 116 in the same manner. 
FIGS. 9-18 illustrate the operation of the multi-port optical circulator 10 
on polarization components of light beams from the optical fibers 30-36. 
For simplicity, only two light beams from the optical fibers 32 and 34 
that are propagating in the forward direction, i.e., positive z-direction, 
are illustrated. Each of the ten figures is an illustration of the 
relative positions of the polarization components of the two light beams 
before and after traveling through one of the optical elements in the 
multi-port optical circulator 10, as viewed from the position of the fiber 
array 26. 
In FIG. 9, a first pair of orthogonal polarization components 118 and 120 
and a second pair of orthogonal polarization components 122 and 124 that 
are about to enter input ports, i.e., windows, at the forward face of the 
optical assembly 12, are shown. The polarization components 118 and 120 
represent a light beam from the optical fiber 32 that is about to enter 
the input port positioned at location 126. The polarization components 122 
and 124 represent a light beam from the optical fiber 34 that is about to 
enter the other input port positioned at location 128. The other two 
locations 130 and 132 are positions of ports on the rearward face of the 
optical assembly 12. Preferably, the locations 130 and 132 are also 
positions of ports on the forward face of the optical assembly 14. In 
addition, the locations 126 and 128 preferably represent positions of 
ports on the rearward face of the optical assembly 14 that are aligned 
with the optical fibers 42 and 44, respectively. As will be described in 
detail below, the first light beam from the optical fiber 32 will be 
transmitted to the optical fiber 40, while the second light beam from the 
optical fiber 34 will be transmitted to the optical fiber 42. 
The light beams enter the first optical assembly 12, encountering the 
walk-off crystal 50. As the light beams travel through the walk-off 
crystal 50, the aligned polarization components 118 and 122 are displaced 
in the positive y-direction, as indicated by the arrow in the lower left 
corner of FIG. 9. As shown in FIG. 10, the polarization components 118 and 
122 have been displaced to locations 130 and 132, respectively. Next, the 
polarization components 118 and 122 travel through the upper half-wave 
plate 52, which rotates the polarization components 118 and 122 in the 
clockwise direction by 45.degree., as shown in FIG. 11. The other 
polarization components 120 and 124 travel through the lower half-wave 
plate 54, which rotates the polarization components 120 and 124 in the 
counter-clockwise direction, also shown in FIG. 11. The polarization 
components 118,120,122 and 124 then travel through the Faraday rotator 56, 
which rotates all the polarization components 118-124 in the clockwise 
direction by 45.degree., as shown in FIG. 12. The overall effect of the 
upper half-wave plate 52 in conjunction with the Faraday rotator 56 is a 
90.degree. rotation of the polarization components 118 and 122 in the 
clockwise direction. On the other hand, the overall effect of the lower 
half-wave plate 54 and the Faraday rotator 56 is a 0.degree. rotation o 
components 120 and 124. 
In FIG. 12, the polarization components 118-124 are shown that are about to 
enter the focusing lens 16. The focusing lens 16 initially refracts the 
propagating paths of the polarization components 118-124, such that the 
polarization components 118-124 are propagating in a converging manner. 
However, the polarization components 118-124 begin to diverge after 
reaching a focal point of focusing lens 16. When the polarization 
components 118-124 reach the compensating lens 20, the polarization 
components 118-124 have been inversely projected onto the compensating 
lens 20. The compensating lens 20 does not affect the relative positions 
of the polarization components 118-124. However, the compensating lens 20 
does stop the divergence of the polarization components 118-124. 
The effect of the focusing lens 16 is shown in FIG. 13, which illustrates 
the polarization components 118-124 prior to entering the shift plate 24. 
Four new locations 134, 136, 138 and 140 are shown in FIG. 13. The 
polarization component 118, which was positioned in the upper left section 
at location 130 in FIG. 12, is now positioned in the lower right section 
at location 136. Similarly, the relative positions of the polarization 
components 120,122 and 124 have been changed from locations 126,128 and 
132 to locations 140, 134 and 138, respectively. 
From the locations shown in FIG. 13, the polarization components 118-124 
travel through the shift plate 24. The polarization components 122 and 124 
are displaced to locations 136 and 140, respectively. The polarization 
components 118 and 120 are displaced to two new locations 142 and 144, 
respectively. The polarization components 118-124 next encounter the 
compensating lens 22 and the focusing lens 18. The compensating lens 22 
and the focusing lens 18 operate to reverse the effects of the focusing 
lens 16 and the compensating lens 20. The compensating lens 22 inversely 
projects the polarization components 118-124 onto the focusing lens 18. 
The focusing lens 18 refracts the polarization components 118-124 from the 
compensating lens 22 to a direction parallel to the z-axis. The overall 
effect of the compensating lens 22 and the focusing lens 18 is to 
reposition the polarization components 118-124 back to relative positions 
prior to entering the focusing lens 16. 
The polarization components 122 and 124 are now in locations 130 and 126, 
respectively, as shown in FIG. 15. In addition, the polarization 
components 118 and 120 are positioned at two new locations 146 and 148, 
respectively. The polarization components 118-124 then travel through the 
Faraday rotator 64. In FIG. 16, the polarization components 118-124 have 
been rotated by 45.degree. in the counter-clockwise direction by the 
Faraday rotator 64. Next, the polarization components 118 and 122 are 
rotated by the upper half-wave plate 60 in the counter-clockwise direction 
by 45.degree., while the other polarization components 120 and 124 are 
rotated by the lower half-wave plate 62 in the clockwise direction by 
45.degree., as shown in FIG. 17. The overall effect of the Faraday rotator 
64 and the upper half-wave plate 60 is a 90.degree. rotation of the 
polarization components 118 and 122 in the counter-clockwise direction. On 
the other hand, the overall effect of the Faraday rotator 64 and the lower 
half-wave plate 62 is a 0.degree. rotation of the polarization components 
120 and 124. 
In FIG. 18, the polarization components 118-124 are recombined by the 
walk-off crystal 58 in front of the fiber holder 48. The polarization 
component 118 is displaced to location 148 to recombine with the 
polarization component 120. In addition, the polarization component 122 is 
displaced to location 126 to recombine with the polarization component 
124. As stated above, the optical fiber 42 is aligned with the location 
126. Thus, the polarization components 118 and 120 will be transmitted to 
the optical fiber 42. Furthermore, the location 148 is aligned with the 
optical fiber 40. Therefore, the polarization components 122 and 124 will 
be transmitted to the optical fiber 40. In a similar manner, a light beam 
from the optical fiber 36 will be transmitted to the optical fiber 44. 
The rearward propagation of light beams from the optical fibers 40 and 42 
to the optical fibers 30 and 32, respectively, is illustrated in FIGS. 
19-28. When applicable, the same reference numerals will be used for 
illustrating the various locations of the light beams along the multi-port 
optical circulator 10 in the rearward direction, i.e., negative 
z-direction. Turning to FIG. 19, a rearward propagating light beam having 
polarization components 150 and 152 exits from the optical fiber 40 and is 
about to enter the second optical assembly 14 at location 148. In 
addition, a second rearward propagating light beam having polarization 
components 154 and 156 exits from the optical fiber 42 and is about to 
enter the optical assembly 14 at location 126. Shown in FIG. 20, the 
polarization components 150-156 have traveled through the walk-off crystal 
58. Walk-off crystal 58 has displaced the polarization components 150 and 
154 in the positive y-direction to locations 146 and 130, respectively, as 
shown in FIG. 20. 
After the walk-off crystal 58, the polarization components 150 and 154 
travel through the upper half-wave plate 60, while the polarization 
components 152 and 156 travel through the lower half-wave plate 62. The 
polarization components 150 and 154 are rotated by 45.degree. in the 
clockwise direction by the upper half-wave plate 60, as shown in FIG. 21. 
However, the polarization components are rotated by 45.degree. in the 
counter-clockwise direction by the lower half-wave plate 62, as shown in 
FIG. 21. Next, the polarization components 150-156 are all rotated by 
45.degree. in the counter-clockwise direction by the Faraday rotator 64. 
As shown in FIG. 22, the polarization components 150-156 are now in a 
vertical position. The polarization components 150-156 then travel through 
the focusing lens 18. 
Identical to the effect of the focusing lens 16 and the compensating lens 
20 on forward propagating polarization components, the focusing lens 18 
has inversely projected the polarization components 150-156 onto the 
compensating lens 22, as shown in FIG. 23. In front of the compensating 
lens 22, the polarization components 150, 152, 154 and 156 are now at 
locations 142, 144, 136 and 140, respectively. In FIG. 24, the 
polarization components 150-156 have traveled through the shift plate 24. 
The shift plate 24 does not affect any of the polarization components 
150-156, because the polarization states of the polarization components 
150-156 are orthogonal to the walk-off direction of the shift plate 24. 
Next, the polarization components 150-156 travel through the compensating 
lens 20 and the focusing lens 16. The effect of polarization components 
traveling through the compensating lens 20 and the focusing lens 16 in the 
rearward direction is identical to the effect of the compensating lens 22 
and the focusing lens 18 on polarization components propagating in the 
forward direction. The compensating lens 22 inversely projects the 
polarization components 150-156 onto the focusing lens 16. The 
polarization components 150-156 then travel through the focusing lens 16. 
In FIG. 25, the polarization components 150,152,154 and 156 are shown at 
locations 146, 148, 130 and 126, respectively, in front of the focusing 
lens 16. The focusing lens 16 has redirected the polarization components 
150-156, such that the propagation paths of the polarization components 
150-156 are parallel to the z-axis. The polarization components 150-156 
then enter the optical assembly 12, encountering the Faraday rotator 56. 
In FIG. 26, the polarization components 150-156 have been rotated by 
45.degree. in the clockwise direction by the Faraday rotator 56. Next, the 
polarization components 150 and 154 are rotated by the upper half-wave 
plate 52 in the counter-clockwise direction by 45.degree., while the other 
polarization components 152 and 156 are rotated by the lower half-wave 
plate 54 in the clockwise direction by 45.degree., as shown in FIG. 27. 
The overall effect of the Faraday rotator 56 and the upper half-wave plate 
52 is a 0.degree. rotation of the polarization components 150 and 154. On 
the other hand, the overall effect of the Faraday rotator 56 and the lower 
half-wave plate 54 is a 90.degree. rotation of the polarization components 
152 and 156. 
The polarization components 150 and 154 are then displaced in the negative 
y-direction by the walk-off crystal 50 to locations 148 and 126, 
respectively, as shown in FIG. 28. As stated above, the optical fiber 30 
is aligned with location 148. In addition, the optical fiber 32 is aligned 
with location 126. Thus, the polarization components 150 and 152 will be 
transmitted to the optical fiber 30, coupling the optical fiber 40 to the 
optical fiber 30 in the rearward direction. Similarly, the polarization 
components 154 and 156 will be transmitted to the optical fiber 32, 
coupling the optical fiber 42 to the optical fiber 32 in the rearward 
direction. In a similar manner, the optical fiber 44 is coupled to the 
optical fiber 34 and the optical fiber 46 is coupled to the optical fiber 
36 for rearward transmission of light beams. 
The optical elements in the optical assembly 12 can be configured in a 
number of alternative arrangements without affecting the operation of the 
first optical assembly. Again, the second optical assembly 14 is 
structurally identical to the first optical assembly 12. Therefore, the 
arrangement of the optical assembly 12 will affect the arrangement of the 
optical assembly 14. First, the walk-off crystal 50 may have a walk-off 
direction in either vertical direction, i.e., the positive or negative 
y-direction, to separate the polarization components of a light beam. 
Second, the upper and lower half-wave plates 52 and 54 can be positioned 
in front of the Faraday rotator 56 or rearwardly of the Faraday rotator 
56. The only concern is that one polarization component of a light beam 
travels through the upper half-wave plate 52, while the other polarization 
component travels through the lower wave plate 54. Preferably, each of the 
Faraday rotator 56 and the half-wave plates 52 and 54 provides a 
45.degree. rotation of polarization components. If the walk-off crystal 50 
has a walk-off direction in the positive y-direction, the Faraday rotator 
56 and the upper half-wave plate 52 can provide clockwise rotations of a 
forward propagating polarization component, while the lower half-wave 
plate 54 provides a counter-clockwise rotation. Alternatively, the Faraday 
rotator 56 and the upper half-wave plate 52 can provide counter-clockwise 
rotations, while the lower half-wave plate 54 provides a clockwise 
rotation. If the walk-off crystal 50 has a walk-off direction in the 
negative y-direction, the rotations of the upper and lower half-wave 
plates 52 and 54 are reversed. 
Although the multi-port optical circulator 10 is shown coupling eight 
optical fibers, the multi-port optical circulator 10 can be slightly 
modified to couple additional optical fibers. The only substantive 
modification needed to accommodate additional optical fibers is the 
surface configuration of the compensating lenses 20 and 22. Each two 
additional optical fibers would require a pair of new surfaces on the 
compensating lenses 20 and 22. 
Turning to FIG. 29, a perspective view of a multi-port optical circulator 
160 in accordance with the second embodiment of the invention is shown. 
The multi-port optical circulator 160 includes an optical assembly 162, a 
focusing lens 164, a compensating lens 166, and a shift plate 168. The 
optical assembly 162 is identical to the optical assembly 12 of the 
multi-port optical circulator 10. The optical assembly 162 includes the 
walk-off crystal 50, the upper and lower half-wave plates 52 and 54, and a 
Faraday rotator 56. Also shown in FIG. 29 is an array of optical fibers 
170. The array 170 contains four optical fibers 172, 174,176 and 178 in a 
fiber holder 180. Similar to the multi-port optical circulator 10, the 
multi-port optical circulator 160 can be modified to accommodate more or 
fewer optical fibers. The multi-port optical circulator 160 further 
includes a mirror assembly 182. The mirror assembly 182 is comprised of a 
Faraday rotator 184 and a mirror 186. 
The multi-port optical circulator 160 operates to transmit light beams 
emitted from one of the optical fibers 172-176 to an adjacent optical 
fiber. For example, a light beam from the optical fiber 172 will propagate 
through the multi-port optical circulator 160 and be transmitted to the 
optical fiber 172. In this configuration, the optical fiber 172 is a 
unidirectional input fiber and the optical fiber 178 is a unidirectional 
output fiber. However, the optical fibers 174 and 176 are bidirectional 
input/output fibers. 
In operation, a light beam enters the optical assembly 162 from one of the 
optical fibers 172-176. The light beam is initially separated into 
polarization components within the optical assembly 162. The polarization 
components then travel through the rest of optical assembly 162, the 
focusing lens 164, the compensating lens 166, and the shift plate 168 in 
the same manner as polarization components of a light beam traveling 
through the optical assembly 12, the focusing lens 16, the compensating 
lens 20, and the shift plate 24 of the multi-port optical circular 10. 
However, unlike the multiport optical circulator 10, the polarization 
components are reflected back by the mirror 186 of the multi-port optical 
circulator 160. Therefore, the polarization components will propagate 
through the shift plate 168, the compensating lens 166, the focusing lens 
164, and the optical assembly 162 a second time. 
The functions of the Faraday rotator 184 of the mirror assembly 182 is to 
change the polarization states of the polarization components so that the 
reflected light is not shifted a second time by the shift plate 168. This 
is achieved by rotating polarization components of a light beam twice by 
45.degree. in either the clockwise or the counter-clockwise direction. 
Because of the nonreciprocal nature of a Faraday rotator, the polarization 
components are first rotated by 45.degree. when propagating through the 
Faraday rotator 184 in the forward direction, and further rotated by 
45.degree. in the same direction when propagating through the Faraday 
rotator 184 in the rearward direction. 
In FIG. 30, a top view of the multi-port optical circulator 160 is 
illustrated. Also shown in FIG. 30 are four propagation paths 188,190,192 
and 194. Each of the propagation paths 180-194 represents a potential path 
taken by both polarization components of a single light beam from one of 
the optical fibers 172-176. Similar to FIG. 7, four displacing paths 
196,198, 200 and 202 are shown within the shift plate 168. These paths are 
taken by only forward propagating polarization components. In a rearward 
direction, the polarization components are not affected by the shift plate 
168, because they have been rotated perpendicularly by the Faraday rotator 
184. In this manner, a light beam from one of the optical fibers 172-176 
is transmitted to an adjacent optical fiber. 
FIG. 31 illustrates a side view of the multi-port optical circulator 160. 
Also shown in FIG. 31 are two propagation paths 204 and 206 with respect 
to the y-axis. The two propagation paths 204 and 206 represent paths taken 
by polarization components of any light beam from one of the optical 
fibers 172-178 of the array of fibers 170 through the multi-port optical 
circulator 160. Polarization components of a light beam will follow the 
paths 204 and 206 in the forward direction as well as in the rearward 
direction, after being reflected by the mirror 186 of the mirror assembly 
182. 
FIGS. 32-38 illustrate the operations performed upon polarization 
components of forward propagating light beams through the multi-port 
optical circulator 160, before being reflected back by the mirror 186. 
Again for simplicity, only two light beams from the optical fibers 174 and 
176 are illustrated. Each of the seven figures is an illustration of the 
relative positions of the polarization components of the two light beams 
before and after traveling through one of the optical elements in the 
multi-port optical circulator 160, as viewed from the position of the 
array of fibers 170. 
In FIG. 32, a first pair of orthogonal polarization components 208 and 210 
and a second pair of orthogonal polarization components 212 and 214 that 
are about to enter input ports, i.e., windows, at the forward face of the 
optical assembly 162 are shown. The polarization components 208 and 210 
represent a light beam from the optical fiber 174 that is about to enter 
the input port positioned at location 216. The polarization components 212 
and 214 represent a light beam from the optical fiber 176 that is about to 
enter the input port positioned at location 218. The optical fibers 174 
and 176 are aligned with locations 216 and 218, respectively. Location 220 
is aligned with the optical fiber 178. The other locations 222, 224 and 
226 are positions of ports on the rearward face of the optical assembly 
162. 
The light beams enter the first optical assembly 162, encountering the 
walk-off crystal 50. The polarization components 210 and 214 are displaced 
to locations 222 and 224 by the walk-off crystal 50, as shown in FIG. 33. 
The polarization components 210 and 214 are then rotated 90.degree. by the 
upper half-wave plate 52 and the Faraday rotator 56, as shown in FIGS. 34 
and 35. However, the polarization components 208 and 212 are rotated 
0.degree. by the lower half-wave plate 54 and the Faraday rotator 56. 
Next, the polarization components 208-214 propagate through the focusing 
lens 164 and the compensating lens 166 in the same manner as the 
polarization components 118-124 through the focusing lens 16 and the 
compensating lens 20 of the multi-port optical circulator 10. The effects 
of the focusing lens 164 and the compensating lens 166 on polarization 
components 208-214 are illustrated in FIG. 36. The polarization components 
208, 210, 212 and 214 are positioned at new locations 234, 230, 232, and 
228, respectively. The polarization components 208-214 then travel through 
the shift plate 168, which laterally displaces the polarization components 
208-214 in the positive x-direction, as shown in FIG. 37. The polarization 
components 212 and 214 are displaced to locations 234 and 230, 
respectively. The other two polarization components 208 and 210 are 
displaced to two new locations 236 and 238, respectively. In FIG. 38, the 
polarization components 208-214 have been rotated by 45.degree. in a 
clockwise direction by the Faraday rotator 184. The polarization 
components 208-214 are then reflected by the mirror 186. 
FIGS. 39-45 illustrate the operations performed upon polarization 
components of rearward propagating light beams through the multi-port 
optical circulator 160 of FIG. 29, after being reflected by the mirror 186 
at the rearward face of the multi-port optical circulator 160. Again, each 
of the seven figures is an illustration of the relative positions of the 
polarization components of the two light beams before and after traveling 
through one of the optical elements in the multi-port optical circulator 
160, as viewed from the position of the fiber array 170. 
In FIG. 39, the polarization components 208, 210, 212 and 214 have been 
reflected by the mirror 186 and are propagating in the rearward direction 
toward the array of fibers 170. The polarization components 208-214 then 
travel through the Faraday rotator 184, which further rotates the 
polarization components by 45.degree. in the clockwise direction, as shown 
in FIG. 40. Next, the polarization components 208-214 travel through the 
shift plate 168. However, in the rearward direction, the polarization 
components 208-214 are not displaced by the shift plate 168, because the 
polarization states of the polarization components 208-214 are orthogonal 
to the walk-off direction of the shift plate 168. 
Next, the polarization components 208-214 propagate through the 
compensating lens 166 and the focusing lens 164. The compensating lens 166 
and the focusing lens 164 operate on the polarization components 208-214 
in the identical manner as the focusing lens 18 and the compensating lens 
22 of the multi-port optical circulator 10 on the polarization components 
118-124, as shown in FIGS. 14 and 15. After passing through the focusing 
lens 164, the polarization components 208, 210, 212 and 214 are positioned 
at locations 218, 224, 220 and 226, respectively, as shown in FIG. 42. The 
polarization components 208-214 then propagate through the Faraday rotator 
56. In FIG. 43, the polarization components 208-214 have been rotated by 
45.degree. in the clockwise direction by the Faraday rotator 56. The 
polarization components 208 and 212 are further rotated by 45.degree. in 
the clockwise direction by the lower half-wave plate 54, as shown in FIG. 
44. However, the polarization components 210 and 214 are re-rotated by 
45.degree. in the counter-clockwise direction by the upper half-wave plate 
52. 
Lastly, the polarization components 210 and 214 are displaced to locations 
218 and 220, respectively, by the walk-off crystal 50. Thus, the 
polarization components 208 and 210 have been recombined by the walk-off 
crystal 50 and are transmitted to the optical fiber 176, which is aligned 
with location 218. Similarly, the polarization components 212 and 214 are 
recombined and transmitted to the optical fiber 178, which is aligned with 
location 220. In a similar manner, a light beam from the optical fiber 172 
is transmitted to the optical fiber 174. 
A method of transferring circulating optical signals from multiple optical 
fibers utilizing a multi-port optical circulator in accordance with the 
invention is illustrated as a flow diagram in FIG. 46. First, a light beam 
is received at step 300 by an optical assembly of the multi-port optical 
circulator from a first optical fiber in an array of fibers. The light 
beam is then separated in step 310 into polarization component by the 
optical assembly. Next, one of the polarization components is rotated, 
such that both polarization components have a common polarization state. 
The polarization components are then diverged in step 320 such that the 
polarization components are moving away relative to each other. In the 
process of diverging the polarization components, the polarization 
components are also inversely projected. 
Next, the polarization components are redirected in step 330 such that the 
polarization components are propagating in a parallel manner. After being 
redirected, the polarization components are laterally displaced, as shown 
at step 340. In one embodiment, the polarization components are reflected 
toward the array of fibers as indicated at step 350. In addition, the 
polarization components are rotated perpendicularly in step 360. The 
displaced polarization components are then converged in step 370 such that 
the separation of the polarization components are decreasing. Next, the 
converging polarization components are again redirected to propagate in a 
parallel manner. Propagating in the parallel manner, one of the 
polarization components is rotated perpendicularly. Lastly, the 
polarization components are recombined in step 380 and transmitted at step 
390 to a second optical fiber.