Optical wavelength router based on polarization interferometer

A method and apparatus for optical wavelength routing separates even and odd optical channels from an input WDM signal. The input beam is first converted to at least one pair of orthogonally-polarized beams. A split-mirror resonator has a front mirror with two regions having different reflectivities, and a reflective back mirror spaced a predetermined distance behind the front mirror. Each of the orthogonally-polarized beams is incident on a corresponding region of the front mirror of the split-mirror resonator. A portion of each beam is reflected by the front mirror, while the remainder of each beam enters the resonator cavity where it is reflected by the back mirror back through the front mirror. The group delay of each reflected beam is strongly dependent on wavelength. The two reflected beams from the resonator are combined and interfere in a birefringent element (e.g., a beam displacer or waveplates) to produce a beam having mixed polarization as a function of wavelength. The polarized components of this beam are separated by a polarization-dependent routing element (e.g., a polarized beamsplitter) to produce two output beams containing complementary subsets of the input optical spectrum (e.g., even optical channels are routed to output port A and odd optical channels are routed to output port B).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to the field of optical
 communications systems. More specifically, the present invention discloses
 an optical wavelength router for wavelength division multiplex (WDM)
 optical communications.
 2. Statement of the Problem
 Wavelength division multiplexing is a commonly used technique that allows
 the transport of multiple optical signals, each at a slightly different
 wavelength, on an optical fiber. The ability to carry multiple signals on
 a single fiber allows that fiber to carry a tremendous amount of traffic,
 including data, voice, and even digital video signals. As an example, the
 use of wavelength division multiplexing permits a long distance telephone
 company to carry thousands or even millions of phone conversations on one
 fiber. By using wavelength division multiplexing, it is possible to
 effectively use the fiber at multiple wavelengths, as opposed to the
 costly process of installing additional fibers.
 In wavelength division multiplexing techniques, multiple wavelengths can be
 carried within a specified bandwidth. It is advantageous to carry as many
 wavelengths as possible in that bandwidth. International
 Telecommunications Union (ITU) Draft Recommendation G.mcs, incorporated
 herein by reference, proposes a frequency grid which specifies various
 channel spacings including 100 GHz and 200 GHz. It would be advantageous
 to obtain 50 GHz spacing. Separating and combining wavelengths with these
 close spacings requires optical components which have high peak
 transmission at the specified wavelengths and which can provide good
 isolation between separated wavelengths.
 One technique which has been developed to accomplish the demultiplexing of
 closely spaced wavelengths is to cascade a series of wavelength division
 demultiplexing devices, each device having different wavelength separating
 characteristics. A typical application involves cascading an
 interferometric device such as an arrayed waveguide device having a narrow
 spacing of transmission peaks (e.g., 50 GHz) with a second interferometric
 device which has a coarser spacing and correspondingly broader
 transmission peaks (e.g., 100 GHz spacing). The cascade of devices
 provides the separation of wavelengths by subdividing the wavelengths once
 in the first device, typically into a set of odd and even channels, and
 then separating wavelengths in the subsets in following devices in the
 cascade.
 Arrayed waveguide, fused biconical taper, fiber Bragg grating, diffraction
 grating, and other interferometric wavelength demultiplexing devices can
 be constructed to have the appropriate characteristics for the first or
 second stage devices in the cascade. However, traditional interferometric
 devices have the characteristic that as the spacing of the channels is
 decreased, the transmission peaks become narrower, and are less flat over
 the wavelength region in the immediate vicinity of each peak than a device
 with wider channel spacings. As a result, when using a traditional device
 in the first stage of a cascade, the transmission peaks may not have a
 high degree of flatness, and any drift or offset of a wavelength from its
 specified value may result in significant attenuation of that wavelength.
 In addition, the isolation between wavelengths is frequently unsuitable
 with conventional interferometric devices and can result in unacceptable
 cross-talk between channels.
 With increasing numbers of wavelengths and the close wavelength spacing
 which is utilized in dense wavelength division multiplexing systems,
 attenuation and cross-talk must be closely controlled to meet the system
 requirements and maintain reliable operations. As an example, 40 or 80
 wavelengths can be generated using controllable wavelength lasers, with
 transmission signals modulated onto each laser. It is desirable to be able
 to demultiplex these channels. Although the lasers can be controlled and
 the wavelengths stabilized to prevent one channel from drifting into
 another, there is always some wavelength drift which will occur.
 For the foregoing reasons, there is a need for a wavelength division
 demultiplexing device which tolerates wavelength drift, maintains a high
 degree of isolation between channels, and is able to separate large
 numbers of wavelengths.
 3. Prior Art
 FIG. 1 illustrates a prior art interferometer that shares some of the basic
 principles employed in the present invention. An input laser beam is split
 into two beams by a beamsplitter 10. One beam propagates toward a mirror
 14 and is reflected back by this mirror. The other beam propagates toward
 a resonator 12 and is also reflected back. The resonator 12 is a
 Fabry-Perot cavity with a partially-reflective front mirror and a
 totally-reflective back mirror. The resonator 12 reflects substantially
 all of the incident optical power back regardless of wavelength, but the
 group delay of the reflected light is strongly dependent on wavelength.
 The two reflected beams from the mirror 14 and from the resonator 12
 interfere at the beamsplitter 10 and the resulting output is split into
 two beams, one at output A, and the other in a different direction at
 output B. The two output beams contain complementary subsets of the input
 optical spectrum, as shown for example in FIG. 2. Such a wavelength router
 concept has been proposed by B. B. Dingle and M. Izutsu, "Multifunction
 Optical Filter With A Michelson-Gires-Tournois Interferometer For
 Wavelength-Division-Multiplexed Network System Applications," Optics
 Letters, vol. 23, p.1099 (1998) and the references therein.
 The two output ports A and B divide the spectral space evenly with
 alternating optical channels being directed to each output port (i.e.,
 optical channels 1, 3, 5, 7, etc. are directed to output port A, while
 channels 2, 4, 6, etc. are directed to output port B). This function has
 sometimes been called an optical interleaver.
 4. Solution to the Problem
 The present invention address the problems associated with the prior art
 using a polarization-based interferometer to implement an optical
 interleaver capable of separating closely spaced optical channels with
 minimal cross-talk.
 SUMMARY OF THE INVENTION
 This invention provides a method and apparatus for optical wavelength
 routing in which an input beam is converted to at least one pair of
 orthogonally-polarized beams. A split-mirror resonator has a front mirror
 with two regions having different reflectivities, and a reflective back
 mirror spaced a predetermined distance behind the front mirror. Each of
 the orthogonally-polarized beams is incident on a corresponding region of
 the front mirror of the resonator. A portion of each beam is reflected by
 the front mirror, while the remainder of each beam enters the resonator
 cavity where it is reflected by the back mirror back through the front
 mirror. The group delay of each reflected beam is strongly dependent on
 wavelength. The two reflected beams from the resonator are combined and
 interfere in a birefringent element (e.g., a beam displacer or waveplates)
 to produce a beam having mixed polarization as a function of wavelength.
 The polarized components of this beam are separated by a
 polarization-dependent routing element (e.g., a polarized beamsplitter) to
 produce two output beams containing complementary subsets of the input
 optical spectrum (e.g., even optical channels are routed to output port A
 and odd optical channels are routed to output port B).
 These and other advantages, features, and objects of the present invention
 will be more readily understood in view of the following detailed
 description and the drawings.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 3 is a diagram showing a one possible implementation of the present
 optical wavelength router based on a polarization interferometer. A
 collimated beam from an optical fiber propagates along the Z axis and is
 incident into the first beam displacer 31. For example, a birefringent
 element consisting of a material such as calcite, rutile, lithium niobate,
 YVO.sub.4 -based crystals, and the like could be used as the beam
 displacers in the present invention. The first beam displacer 31 splits
 the input beam into two beams having orthogonal polarizations (e.g., along
 the X and Y directions, respectively). A half-wave plate (.lambda./2) 32
 rotates the polarization of one of these beams by 90 degrees, so that both
 beams have the same polarization. For example, both beams exiting the
 half-wave plate 32 in FIG. 3 are polarized along the Y axis.
 Both beams then pass through a polarized beamsplitter (PBS) 33 without
 significant attenuation. A second beam displacer 34 splits the Y-polarized
 beam pair into two pairs of beams that are orthogonally polarized in the
 XY plane. One pair of these beams is polarized at 45 degrees relative to
 the X axis, while the other pair is polarized at 135 degrees relative to
 the X axis. The two pairs of beams are incident onto and reflected by a
 split-mirror resonator (SMR) 35.
 FIGS. 4(a) and 4(b) show the structure of the split mirror resonator 35 in
 FIG. 3. The resonator 35 is formed by a front mirror 41 and a back mirror
 43 separated a predetermined distance by a center spacer 42. The front
 mirror 41 is a split mirror in which part of the surface is coated with a
 high-reflectivity coating and part of the surface is only partially
 reflective (e.g., 18% reflectivity). The degree of reflectivity of both
 regions is a matter of design. For example, the high-reflectivity region
 can be 100% reflective, or only partially reflective so long as it is more
 reflective than the other region of the front mirror 41. For example, this
 can be accomplished by applying a split coating to the front mirror 41.
 The second mirror 43 has a high reflectivity.
 Returning to FIG. 3, the second beam displacer 34 produces two pairs of
 orthogonally-polarized beams. The first beam pair strikes the
 highly-reflective region of the front mirror 41 and is largely reflected
 back along the Z axis to the second beam displacer 34 without propagating
 through the resonator 35. In contrast, the second beam pair strikes the
 partially-reflective region of the front mirror 41 and is partially
 transmitted through the front mirror 41 into the resonator cavity between
 the front and back mirrors 41 and 43. A portion of the second beam pair is
 also reflected back along the Z axis to the second beam displacer 34
 without propagating through the resonator 35. The transmitted portions of
 the first and second beam pairs are reflected by the back mirror 43
 through the front mirror 41 of the resonator 35 toward the second beam
 displacer 34. The split-mirror resonator 35 reflects substantially all of
 the incident optical power back regardless of wavelength, but the group
 delay of the reflected beams is strongly dependent on wavelength.
 Thus, both pairs of reflected beams from the split mirror resonator 35
 back-propagate along the negative Z axis (moving toward the left in FIG.
 3) and are recombined into one pair of beams by the second beam displacer
 34. Due to the birefringence of the second beam displacer 34, a difference
 in the optical path lengths between the two beam pairs is generated. As a
 result, the polarization state of the back-propagating beam pair exiting
 the second beam displacer 34 is a function of optical wavelength. In other
 words, this back-propagating beam pair has mixed polarization as a
 function of the optical wavelengths carried by the beams.
 The back-propagating beam pair enters the polarized beamsplitter 33. The
 components of the beam pair that are polarized along the Y axis are
 transmitted through the polarized beamsplitter 33 toward the first beam
 displacer 31, while those components that are polarized along the X axis
 are reflected by the polarized beamsplitter 33 toward a third beam
 displacer 37, as illustrated in FIG. 3. It should be expressly understood
 that other types of polarization-dependent routing elements could be
 employed to separate the components of the back-propagating beam pair. For
 example, an angled beamsplitter, beam displacer, or other birefringent
 element could substituted for this purpose.
 One of the beams in the transmitted beam pair passes through the half-wave
 plate 32 which rotates its polarization by 90 degrees, so that the
 transmitted beams have orthogonal polarizations. These beams are then
 recombined by the first beam displacer 31 into a single beam at output
 port A. Similarly, one of the beams in the reflected beam pair passes
 through a half-wave plate 36 which rotates its polarization by 90 degrees,
 so that the reflected beams become orthogonally polarized. These beams are
 recombined by the third beam displacer 37 into a single beam at output
 port B.
 Thus, this device functions as an optical interleaver. The outputs beams at
 output ports A and B contain two complementary subsets of the input
 optical spectrum, similar to those shown in FIG. 2, with alternating
 optical channels in the input spectrum being routed to each output port.
 If desired, this device can be extended in a cascade architecture with
 multiple stages of optical interleavers to progressively separate
 individual channels or groups of channels.
 The embodiment of the split-mirror resonator shown in FIGS. 4(a) and 4(b)
 has advantages in certain applications. This embodiment can decrease the
 device size. More importantly, it allows the two beam pairs to share a
 common path, thereby minimizing the effects of vibration, air turbulence,
 and temperature change.
 Ring-Shaped Resonator Structures. Alternatively, the split-mirror resonator
 can be implemented as a ring structure with more than two mirrors. For
 example, FIG. 5(a) shows a resonator with three mirrors 51, 52, and 53.
 Here, the first mirror 51 is a split mirror, similar to the example shown
 in FIG. 4 (b). The other mirrors 52 and 53 are coated with a high
 reflectance coating. FIG. 5(b) extends this concept to a resonator with
 four mirrors 51-54 in a ring structure.
 FIG. 6 shows an alternative embodiment of an optical wavelength router
 using the ring resonator structure from FIG. 5(a). The input optical
 signal passes through a polarizer 61 that converts the random polarization
 of the input beam to a known linear polarization. For example, the
 polarizer 61 can be implemented as a birefringent element 31 and half-wave
 plate 32 as shown in FIG. 3 that converts the input beam into a pair of
 beams having the same polarization. Alternatively a simple polarization
 filter can be employed to produce a single polarized beam as shown in FIG.
 6.
 The polarized beam is then separated into two orthogonally-polarized beams
 by a first beam displacer 62. As before, one of these beams strikes the
 highly reflective region of the first mirror 51 and is reflected to the
 second beam displacer 63. The other beam passes through the partially
 reflective region of the first mirror 51 and is reflected in turn by the
 second and third mirrors 52 and 53 before being reflected back through the
 first mirror 51 toward the second beam displacer 63. The beams exiting the
 ring resonator 51-53 are combined by the second beam displacer 63. Here,
 again, the difference in the optical path lengths between the beams due to
 the birefringence of the first beam displacer 62 and the second beam
 displacer 63 produces interference between the beams and results in an
 output beam having a polarization state that is a function of optical
 wavelength. A polarized beamsplitter 64 (or other polarization-dependent
 routing element) separates the polarized components of the output beam
 from the second beam displacer 63 to output ports A and B, respectively,
 to produce two complementary subsets of the input optical spectrum,
 similar to those shown in FIG. 2.
 Wavelength Router Using Waveplates and a Zero-Order Beam Displacer. FIG. 7
 shows another alternative embodiment of the present optical wavelength
 router. In this device, one or more waveplates 71 are used to generate
 birefringence and thereby produce a predetermined difference in the
 optical path lengths between different optical polarizations. The
 waveplates 71 are oriented such that the optical axis for each one is at
 45 degrees relative to the polarizing axis of the beamsplitter 33.
 However, the waveplates 71 do not disturb the net beam propagation
 direction. The waveplates 71 can be one piece of birefringent material
 oriented at 45 degrees, or a plurality of birefringent elements that are
 all oriented at 45 degrees.
 The first beam displacer 31 splits the input beam into two
 orthogonally-polarized beams. A half-wave plate 32 rotates the
 polarization of one of these beams by 90 degrees, so that both beams have
 the same polarization. Both beams then pass through a polarized
 beamsplitter 33 without significant attenuation. The waveplates 71 cause a
 50/50 split of the incident optical power of both beams into two
 orthogonal polarizations as a result of the 45 degree orientation of the
 waveplates' axis. After the waveplates 71, a second beam displacer 72
 spatially separates the two orthogonal polarizations in the beam pair to
 create two pairs of beams as illustrated in FIG. 7.
 A split-mirror resonator 35, as describe above and shown in FIGS. 4(a) and
 4(b), reflects both beams pairs beams back along the negative Z axis so
 that they are recombined into one pair of beams by the second beam
 displacer 72. Due to the birefringence of the waveplates 71, a difference
 in the optical path lengths between the orthogonally polarized beams is
 generated. As a result, the polarization state of the back-propagating
 beam pair exiting the waveplate 71 is a function of optical wavelength.
 The back-propagating beam pair enters the polarized beamsplitter 33 (or
 other polarization-dependent routing element). The components of the beam
 pair that are polarized along the Y axis are transmitted through the
 polarized beamsplitter 33 toward the first beam displacer 31, while those
 components that are polarized along the X axis are reflected by the
 polarized beamsplitter 33 toward a third beam displacer 37. One of the
 beams in the transmitted beam pair passes through a half-wave plate 32
 that rotates its polarization by 90 degrees, so that the transmitted beams
 have orthogonal polarizations. These beams are then recombined by the
 first beam displacer 31 into a single beam at output port A. Similarly,
 one of the beams in the reflected beam pair passes through a half-wave
 plate 36 that rotates its polarization by 90 degrees, so that the
 reflected beams become orthogonally polarized. These beams are recombined
 by the third beam displacer 37 into a single beam at output port B.
 The second beam displacer 72 in FIG. 7 is preferably constructed as shown
 in greater detail in FIG. 8. Two beam displacers 81 and 82, made of
 similar materials and having similar thicknesses, are aligned so that
 their optical axes are 90 degrees relative to one another as shown in FIG.
 8. The two beam displacers 81, 82 are then bonded together to form one
 piece. When an optical beam passes through this assembly, the two input
 polarizations are spatially separated, but there is no net difference in
 the optical path lengths through the beam displacers 81 and 82 between the
 two polarizations. In other words, FIG. 8 demonstrates a "pure" beam
 displacer (i.e., a zero-order beam displacer), in which the orthogonal
 input polarizations are spatially separated but at most only a negligible
 amount of birefringence is added to the beams.
 A zero-order beam displacer can also be implemented as depicted in FIG. 9.
 Here, a zero-order half-wave plate 92 is placed between beam displacers 91
 and 93. The two displacers 91, 93 can be identical pieces but have their
 respective optical axes rotated 90 degrees from one another as shown in
 FIG. 9. FIG. 10 shows another arrangement to construct a zero-order
 displacer 72 with two identical pieces 101, 102 of conventional displacer
 using a different crystal orientation.
 The embodiment illustrated in FIG. 7 is of practical importance because of
 the reduced difficulty of optical alignment. In general, either a
 waveplate or beam displacer can be used to generate birefringence in an
 optical beam. However, the birefringence of a conventional beam displacer
 is very sensitive to its orientation. To achieve a given amount of path
 delay between two polarizations, the position of a conventional displacer
 must be controlled to within very tight tolerances, making it difficult to
 initially align and to maintain proper alignment over a range of operating
 conditions, including temperature changes and mechanical vibration.
 In contrast to a beam displacer, the amount of birefringence from a
 waveplate is much less sensitive to its orientation. There are two reasons
 for this difference in sensitivity. In a conventional beam displacer as
 used in FIG. 3, the optical beam usually propagates at about 45 degrees
 from the optical axis of the crystal. In this configuration, the index of
 refraction of the extraordinary beam is very sensitive to the exact angle
 between propagation direction and the optical axis. In a waveplate, the
 optical beam propagates at 90 degrees from the optical axis. In this
 configuration, the index of refraction of the extraordinary beam is
 relatively insensitive to the angle between the propagation direction and
 the optical axis. The second reason is that in a beam displacer, the
 ordinary and extraordinary rays exit the crystal with a spatial
 separation. When the crystal is tilted, the physical distance between the
 ordinary ray and the extraordinary ray travel become different. In
 contrast, the physical distances that the ordinary ray and extraordinary
 ray travel in a waveplates remain almost unchanged.
 These two effects combine to make the embodiment of the present invention
 shown in FIG. 3 much more sensitive to the perturbations to the position
 of the beam displacer 34. In contrast, the implementation shown in FIG. 7
 using waveplates 71 as the interferometer is very robust.
 In addition to the advantages associated with waveplates 71, the zero-order
 displacer 72 introduces at most a negligible amount of birefringence and
 is therefore easy to initially align and to maintain alignment. In the
 device shown in FIG. 7, the waveplates 71 can be easily tuned to achieve a
 desire amount of birefringence and optical path length difference. Such a
 design makes it possible to produce a compact, reliable, and low-cost
 wavelength router for WDM communications. The zero-order displacer can
 further be used to implement a beam displacer with at most negligible
 inherent differential group delay (DGD). Such zero-DGD displacers also
 have zero polarization mode dispersion (PMD) and is a very important
 feature for a polarization-based wavelength router.
 The above disclosure sets forth a number of embodiments of the present
 invention. Other arrangements or embodiments, not precisely set forth,
 could be practiced under the teachings of the present invention and as set
 forth in the following claims.