Partial optical circulators

A partial optical coupler of the present invention comprises first, second and third walk-off devices having a non-reciprocal polarization interchanger unit positioned between the first and second and the second and third walk-off devices. The first and third walk-off devices are matched in terms of the amount of walk-off and the direction of walk-off. A further embodiment includes a reflector which allows the light beam to pass through each walk-off device and each non-reciprocal polarization interchanger unit twice.

FIELD OF THE INVENTION 
This invention relates generally to the field of optical devices and more 
particularly to optical devices, such as circulators, isolators and 
switches, that are used in optical communications systems. 
BACKGROUND OF THE INVENTION 
Optical communications systems, as well as other optical technologies, 
require apparatus that manipulates optical signals that are in the form of 
light rays. One of the best known apparatus for such manipulation is the 
silica based optical fiber which is widely used for both short and long 
distance optical transmission. Additionally, other well known apparatus 
include couplers and multiplexers that have been developed to couple one 
or more optical signals into one or more optical fibers or waveguides. 
Some applications for transmission of optical signals also desirably use 
non-reciprocal apparatus or devices, where the propagation characteristics 
of light within the apparatus depends upon the direction of light 
propagation within the apparatus. 
One type of such non-reciprocal apparatus, commonly referred to as an 
optical isolator, permits light to pass through the apparatus in one 
direction but not in the reverse direction. Another type of such 
non-reciprocal apparatus is termed an optical circulator. An optical 
circulator has 3 or more ports which permit light to pass from a first to 
a second port, for example, but not from the second port to the first 
port. Instead light entering the second port passes to a third port. If 
there is a fourth port, then light entering the third port exits at this 
fourth port. Generalizing, port n .fwdarw. port (n+1) if ports n and (n+1) 
exist. A partial circulator is a circulator as described above but where 
port n.sub.max does not lead to port 1. A complete or full circulator 
couples light entering port n.sub.max to port 1. An isolator may be 
thought of as a two port partial circulator. However, the term isolator is 
the preferred term. 
Many types of optical devices, including circulators and isolators, have 
been developed for commercial applications. For many applications, such a 
device should be polarization independent to the external world; that is, 
device operation should not depend upon the polarization of the incoming 
light. For example, Fujii in Journal of Lightwave Technology, Vol. 10, pp. 
1226-1229, September 1992, describes a polarization independent apparatus 
that as stated can be used as an optical circulator. Although stated to be 
useful over a wide range of wavelengths, preferred operation of the 
apparatus depends upon precise orientation of the components with respect 
to each other. Another optical circulator is described in U.S. Pat. No. 
5,204,771 issued to Koga on Apr. 20, 1993. The essence of the Koga 
circulator appears to be the use of a birefringent plate followed by 
non-reciprocal optical rotators. The birefringent plate splits the 
incoming beam into two parallel beams, and the optical rotators change the 
polarization of each of the two beams by 45 degrees so that there are two 
parallel beams with orthogonal polarization. 
Consideration of the devices described in the previous paragraph reveals 
aspects that make their use disadvantageous in some situations. As 
mentioned, the Fujii circulator depends upon precise relative orientation 
of the components. This orientation may be difficult to achieve in 
practice and still more difficult to maintain for extended periods of 
time. The Koga device is extremely complicated. The complexity arises not 
only because there are many components, where the individual components 
are made up of multiple connected plates, but also because the large 
number of components necessarily has a large number of surfaces. Any one 
of these surfaces can produce undesired reflections. The devices must be 
designed to either eliminate the reflections or to compensate for them. 
Additionally, the optical rotators must be precisely aligned so that one 
beam passes through the top half of the rotator and the other beam passes 
through the bottom half of the rotator. The small beam sizes and small 
separation of the beams will likely make this difficult. Accordingly, 
there is a need for an optical circulator device which is simpler to 
manufacture and maintain than those devices found in the prior art. 
SUMMARY OF THE INVENTION 
According to one advantageous embodiment of the invention an optical 
apparatus of the present invention comprises first, second and third 
walk-off devices having a polarization interchanger unit positioned 
between the first and second walk-off devices and the second and third 
walk-off devices. The first and third walk-off devices are matched in 
terms of the amount of walk-off and the direction of walk-off. A further 
embodiment includes a reflector which allows the light beam to pass 
through each walk-off device and each polarization interchanger unit twice 
.

DETAILED DESCRIPTION 
Referring to FIG. 1, there is shown an exemplary embodiment of a partial 
optical circulator 100 in accordance with the present invention. As shown, 
partial optical circulator 100 is comprised of a first walk-off device 
110, a second walk-off device 130 and a third walk-off device 150. A first 
non-reciprocal polarization interchanger unit 120 is disposed between 
first walk-off device 110 and second walk-off device 130 and a second 
non-reciprocal polarization interchanger unit 140 is disposed between 
second walk-off device 130 and third walk-off device 150. In general, 
walk-off devices can either separate an incoming unpolarized light beam 
into parallel outgoing beams of orthogonal polarization or combine 
incoming parallel beams of orthogonal polarization into an unpolarized 
light beam. For example, walk-off devices split an incoming light beam 
into "ordinary" and "extraordinary" beams which are designated as O and E 
polarized beams, respectively. Walk-off devices may be fabricated from 
birefringent materials such as calcite or rutile and also by use of 
polarization separation coatings. As explained below, the term 
non-reciprocal polarization interchanger, in general, is intended to cover 
a wide variety of devices which transform polarization from one state to 
another for beam propagation in one direction and not transform in the 
same manner for beam propagation in the reverse direction. For example, a 
non-reciprocal polarization interchanger can be an odd-multiple half wave 
plate, a half wave plate, combinations of Faraday rotators (each of which 
rotate the plane of polarization.+-.45.degree.) or any other 90.degree. 
rotators. The materials and methods for making the walk-off device and the 
non-reciprocal polarization interchange units are well known and can be 
readily fabricated by those skilled in the art. 
Although partial optical circulator 100 is shown in FIG. 1 as having ports 
1, 2, 3 . . . 10, it would be understood that the number of ports is 
arbitrary and can be either an even or odd number of ports. For example, 
the number of ports could be three. In general, an unpolarized light beam 
entering at port 1 will emerge at port 2 as an unpolarized beam. An 
unpolarized beam entering at port 2 will emerge as an unpolarized beam at 
port 3, etc. In other words, light entering ports 1-9 will emerge at ports 
2-10. Note that there is no facility in this arrangement for light 
entering at some port and emerging at port 1 or for entering at port 10 
and emerging at some port. This is indicated in FIG. 1 by unused path 160 
and lost path 170, respectively. Thus partial optical circulator 100 
differs from a full circulator in that the circular path is broken. As 
would be understood, a N port partial circulator can be converted into a 
full N-2 port circulator by connecting port N to port 1 using, for 
example, mirrors, lenses and/or fiber. The full N-2 port circulator would 
then utilize ports 2, 3, . . . (N-1) for optical communications purposes. 
As is evident from FIG. 1, horizontally directed, unpolarized light beams 
will enter and leave ports 1, 2, . . . N. However, the unpolarized light 
beam undergoes separation and then combination prior to emerging at the 
destination port. The above preferably should be accomplished with minimum 
polarization mode dispersion ("PMD"). Minimizing PMD requires that the two 
alternate polarization related optical paths are equalized in their 
optical delay. The most straight forward means to achieve this is to use 
symmetry. A method that does not use symmetry requires that the F and B 
sections, comprising the polarization interchanger units as described 
below, have unequal delays to compensate for unequal paths produced by the 
choice of and orientation of the first and third walk-off devices 110 and 
150, respectively. However, this method is more complex and is not 
generally preferred if symmetry can be used. 
In order to ensure that PMD is minimized between the input beam and output 
beam, first walk-off device 110 and third walk-off device 150 must be 
matched in the magnitude of the walk-off the beam will experience and in 
the walk-off sense of direction, where the latter term refers to a binary 
sense of direction. That is, whether the walk-off devices have, for 
example, a top/down, or right/left walk-off sense of direction. In 
contrast, a walk-off direction refers to having the same magnitude 
relative to a line. For example, if a light beam enters port 1 and leaves 
at port 2, minimal PMD will result if the paths followed by the now split 
light beam forms a parallelogram 180 and that units 120 and 140 form 
equivalent delays for equivalent regions (B, F). That is, if the amount of 
walk-off or the magnitude of walk-off in first walk-off device 120 and 
third walk-off device 150 is equal, then divergent path 181 and convergent 
path 182 will also be equal and the resulting shape, together with 
parallel paths 183,184, will form parallelogram 180. If first walk-off 
device 110 and third walk-off device 150 are not matched, then a 
trapezoidal path shape results, which may correspond to greater PMD. Note 
that having equal walk-off magnitudes does not mean that the dimensions of 
first walk-off device 110 and third walk-off device 150 have to be equal. 
The dimensions of each walk-off device could be different, depending upon 
the material used for fabricating the specific walk-off device. As would 
be understood, the fabrication materials used, as well as the direction of 
their optic axis orientation, each have different walk-off 
characteristics. As long as the magnitude of the walk-off, which is a 
combination of the material, its orientation and the dimensions of the 
material, is equivalent in terms of walking apart and walking toward each 
other then proper recombination of both polarization paths is achieved. 
Summarizing, there are two issues that must be addressed. First, the two 
polarization paths must be recombined such that good coupling is achieved 
for both polarizations simultaneously. If not, there is a potential for 
recombining mismatch. Assuming that the first issue is addressed, then, 
secondly, the two optical path lengths preferably should be matched or 
made equal, i.e., PMD =0, since the PMD is proportional to the difference 
in the optical path lengths. Note that in FIG. 1, the plane of the page 
contains the walk-off directions of the first walk-off device 110, the 
second walk-off device 130 and the third walk-off device 150. 
First non-reciprocal polarization interchanger unit 120 and second 
non-reciprocal polarization interchanger unit 140 are, for example, 
combinations of Faraday rotators, which rotate the plane of polarization 
.+-.45.degree., and reciprocal elements, such as suitably oriented half 
wave plates or optically active materials, e.g., a crystalline quartz 
along its optic axis. These combinations are labeled in FIGS. 1 and 2 as 
"F" and "B", which describe two types of behavior, as explained further 
below. A polarized light beam, with either E or O polarization, emerging 
from walk-off device 110 and traveling left to right through an F 
combination will not undergo any polarization change due to the F 
combination prior to entering second walk-off device 130. However, due to 
the non-reciprocal nature of Faraday rotators, an E or O polarized light 
beam traveling right to left through the F combination will undergo a 
polarization change. That is, an E or O polarized light beam will become 
an O or E polarized light beam, respectively. In a similar but opposite 
manner, the B combination will effect the light beam's polarization when 
traveling in a left to right direction, but will not effect the 
polarization when traveling from a right to left direction. 
Referring now to FIG. 2, a partial optical circulator 200 is shown in 
accordance with the present invention. In a similar manner to partial 
optical circulator 100, partial optical circulator 200 comprises a first 
walk-off device 210, a second walk-off device 230 and a third walk-off 
device 250. As above, a first non-reciprocal polarization interchanger 
unit 220 and a second non-reciprocal polarization interchanger unit 240 
are disposed between first and second walk-off devices 210 and 230 and 
second and third walk-off devices 230 and 250, respectively. In this 
configuration, the positioning of the F and B combinations in first and 
second non-reciprocal polarization interchanger units 220 and 240 are 
reversed as shown in FIG. 2. This change reflects the fact that the 
optical axis of second walk-off device 230 has been inverted with respect 
to second walk-off device 130 of FIG. 1. In other words, the embodiment of 
FIG. 1 illustrates the case where the E polarized light beam travels in 
the same walk-off sense of direction in each walk-off device and FIG. 2 
illustrates the case where the E polarized beam travels in the other 
walk-off sense of direction in the second walk-off device. 
Although the following discussion is with respect to FIG. 1, it would be 
understood that the discussion is equally applicable to the embodiment 
shown in FIG. 2. Referring now to FIG. 1, a light beam entering at port 1 
of first walk-off device 110 separates into two polarization components, E 
and O polarization, which are walked far enough apart so that the two 
resulting paths can be treated differently. The E polarized light beam 
travels through the B combination and by the assumptions made above, gets 
converted to the O polarization. As a result, both beams are incident upon 
second walk-off device 130 as O polarized light beams and travel 
horizontally through second walk-off device 130 in parallel paths. As the 
O polarized light beams exit second walk-off device 130, the originally O 
polarized light beam gets converted by an B combination to E polarization 
prior to entering third walk-off device 150 and the previously converted O 
polarized light beam remains the same as it passes through an F 
combination. Third walk-off device 150 walks the two light beams with 
opposite polarization towards each other so that they converge to a common 
position and emerge at port 2 as an unpolarized light beam. 
As shown below, an unpolarized light beam entering at port 2 does not 
emerge at port 1, but does emerge as an unpolarized light beam at port 3. 
Note that a structure with this functionality (ignoring port 3) is 
generally referred to as a two-stage isolator(s). The isolation 
finctionality results because a light beam entering at port 2 does not 
retrace a path to port 1 due to the non-reciprocal nature of 
non-reciprocal polarization interchanger unit 140 (and also unit 120). As 
the two beams pass through non-reciprocal polarization interchanger unit 
140, the O polarized light beam is converted to an E polarized light beam 
by an F combination and the E polarized light beam remains the same as it 
passes through an B combination. As a result, both beams are incident upon 
second walk-off region 130 as E polarized light beams. The two light beams 
will travel parallel paths through second walk-off device 130 which walk 
away from port 1 and walk toward port 3. It is "two-staged" in that light 
entering at port 2 should be converted solely to E polarization by 
polarization interchanger unit 140 and hence lead to isolation by the 
walk-off direction in walk-off device 130. Trace amounts of O 
polarization, due to imperfect polarization in interchange unit 140, will 
travel toward port 1 through walk-off device 130. However, polarization 
interchanger unit 120 converts the current polarization so that the light 
leads away from port 1 when it is traversing walk-off device 110, thus 
resulting in a second stage of isolation. Upon leaving second walk-off 
device 130, the originally E polarized light beam passes through an F 
combination and changes to O polarization and the converted E polarized 
light beam remains the same as it passes through an B combination. The 
oppositely polarized light beams are converged to a point by first 
walk-off device 110 and emerge at port 3. As it would readily apparent, 
this process can continue from port 3 to port 4 in a manner analogous to 
the ray tracing for port 1 to port 2. Similarly light beams going from 
port 4 to port 5 would be analogous to the port 2 to port 3 path. This 
process continues until the highest numbered port is reached. 
Referring now to FIG. 3, a reflexive partial optical circulator 300 is 
shown in accordance with the present invention. This embodiment is 
implemented by folding the embodiments illustrated in FIG. 1 and FIG. 2 or 
by substituting other walk-off devices. As shown in FIG. 3, folding is 
accomplished by, for example, subdividing second walk-of device 130 into 
two equal pieces 330,331 and using, for example, a porro prism 380 to pass 
the beams through first and third walk-off devices 310,350 and through 
first and second non-reciprocal polarization interchanger unit 320,340 
twice. As it would be understood, a mirror arrangement or other similar 
device could be used to pass the light beams through each of the devices 
twice as required. In this embodiment, although the length of the required 
material is halved, the width of the remaining material is doubled since 
all of the ports are on the same side. 
In the actual implementation of the device of FIG. 3, the walk-off angles 
of the birefringent materials are much smaller than as illustrated, so 
that there would be less vertical separation of the ports and/or greater 
thickness for the birefringent plates. Reflexive partial optical coupler 
300 operates as before, where the solid lines are used to trace a light 
path from port 1 to port 2 and the dotted lines are used to trace a path 
from port 2 to port 3. For sake of clarity, the dotted lines are displaced 
in FIG. 3 where they would overlap solid line paths traveling in the 
opposite direction. Although a three port reflexive partial optical 
circulator is shown in FIG. 3, it would be understood that additional 
ports can be added if space permits. As it would be understood, porro 
prism 380 and walk-off device 330,331 could be integrated to form a single 
unit. In a further embodiment, the ports could interdigitate, but with an 
increased risk in cross coupling. 
Referring to FIG. 4, a fiber optic-dual parallel beam converter 400 is 
shown in accordance with the present invention. This optical device 
permits conversion between fiber optic input/output lines and parallel 
traveling beams with parallel linear polarization. Although not explicitly 
detailed in FIG. 4, the conversion from collimated beams to fiber optics 
can be accomplished using a lensing system, such as a GRIN, or a spherical 
or aspheric lens. A disadvantage of converter 400 is the overall length of 
the optical device, which includes the ends of fiber optic pigtails 410, 
collimating optics 420, full length of walk-off section 430, 
non-reciprocal polarization interchanger unit 440 and a remaining portion 
of the optical system 450. Remaining portion of optical system 450, for 
example, could have the arrangement as shown in FIG. 3, it could have a 
reflexive configuration where porro prism 380 and walk-off device 330,331 
are an integrated unit, or remaining portion of optical system 450 could 
have an arrangement analogous to that shown in FIG. 7, which, for 
instance, has a horizontal discontinuity between plates 710 and 720. 
Referring now to FIGS. 5(a), 5(b), 6(a) and 6(b), by using a reflexive 
configuration for walk-off section 430, a reduction is realized in the 
overall size of the optical device. This length reduction has the 
advantage of decreasing chances of mis-alignment due to warpage of the 
package, where warpage tends to increase with increasing package length. A 
further advantage is that the dimensions of the birefringent material are 
reduced and therefore there is also a corresponding reduction in the 
volume of the optical system package. 
Referring to FIGS. 5(a) and 5(b), a top and side view is shown of reflexive 
fiber optic-dual parallel beam converter 500. Converter 500 comprises a 
reflexive walk-off device 530, which is coupled to a collimating device 
520 on the top and a non-reciprocal polarization interchanger unit 540 on 
the bottom. Collimating device 520 is further coupled to a fiber optic 
pigtails 510 and non-reciprocal polarization interchanger unit 540 is 
further coupled to a remaining portion 550 of the optics system. Note that 
reflexive walk-off device 530 could also have an arrangement analogous to 
that shown in FIG. 7. Although the following is functionally described 
with respect to FIG. 6(a), it would be understood that the explanation is 
equally applicable to FIG. 6(b). Referring now to FIG. 6(a), where 
identical parts are identically numbered, a light beam enters reflexive 
walk-off device 530, where it is initially separated into E and O 
polarized beams. The beams emerge and enter non-reciprocal polarization 
interchanger unit 540, where the beams pass through either an F or B 
combination and either undergo or do not undergo polarization conversion. 
The beams then travel to the rest of the optics system. 
Actual implementation of the reflexive converter embodiments may prove 
easier by using birefringent materials having a maximum thickness of, for 
example, 3 millimeters. If the reflexive walk-off device needs to be 
thicker, it is possible to stack a multiple of the maximum thickness 
sheets together to obtain the required thickness. A disadvantage of this 
arrangement is that although the plates may have comparable thickness, the 
optical axes of the plates are orientated differently, one has to be 
careful to control the orientation of each plate individually. A further 
embodiment is shown in FIG. 7, where a portion of an optical circulator 
700 consists of two plates 710 and 720, which together could, for example, 
represent the elements denoted as 330 (331) and 380 in FIG. 3. 
Numerous modifications and alternative embodiments of the invention will be 
apparent to those skilled in the art in view of the foregoing description. 
Accordingly, this description is to be construed as illustrative only and 
is for the purpose of teaching those skilled in the art the best mode of 
carrying out the invention. Details of the structure may be varied 
substantially without departing from the spirit of the invention and the 
exclusive use of all modifications which come within the scope of the 
appended claim is reserved.