APPARATUS AND METHOD FOR THE REDUCTION OF POLARIZATION SENSITIVITY IN DIFFRACTION GRATINGS USED IN FIBER OPTIC COMMUNICATIONS DEVICES

Abstract: A diffraction grating (10') for use in multiplexing and demultiplexing optical signals in optical communication systems having reduced polarization sensitivity has a plurality of reflective step surfaces (16') separated by a plurality of riser surfaces (18') defining grooves of the grating. The step surfaces (16') have a reflective coating (22') and the riser surfaces (18') do not have a conductive coating. A method of making a reflective diffraction grating includes forming a plurality of grooves in a substrate, the grooves having a reflective surface for reflecting an incident beam and non-reflective surface and providing a reflective coating on the reflective surface and not on the non-reflective surface.

Description: APPARATUS AND METHOD FOR THE REDUCTION OF POLARIZATION SENSITIVITY IN DIFFRACTION GRATINGS USED IN FIBER OPTIC COMMUNICATIONS DEVICES TECHNICAL FIELD The present invention is directed toward optical communications, and more particularly toward reduction of polarization sensitivity in optical multiplexers/demultiplexers using bulk diffraction gratings. BACKGROUND ART At the inception of fiber optic communications, typically a fiber was used to carry a single channel of data at a single wavelength. Dense wavelength division multiplexing (DWDM) enables multiple channels at distinct wavelengths within a given wavelength band to be sent over a single mode fiber, thus greatly expanding the volume of data that can be transmitted per optical fiber. The wavelength of each channel is selected so that the channels do not interfere with each other and the transmission losses to the fiber are minimized. Typical DWDM allows up to 40 channels to be simultaneously transmitted by a fiber. DWDM requires two conceptually symmetric devices: a multiplexer and a demultiplexer. A multiplexer takes multiple beams or channels of light, each at a discrete wavelength and from a discrete source and combines the channels into a single multichannel or polychromatic beam. The input typically is a linear array of waveguides such as a linear array of optical fibers, a linear array of laser diodes or some other optical source. The output is typically a single waveguide such as an optical fiber. A demultiplexer spacially separates a polychromatic beam into separate channels according to wavelength. Input is typically a single input fiber and the output is typically a linear array of waveguides such as optical fibers or a linear array of photodetectors. In order to meet the requirements of DWDM, multiplexers and demultiplexers require certain inherent features. First, dispersive devices must be able to provide for a high angular dispersion of closely spaced channels so that individual channels from a multi-channel or multiplexed beam can be separated sufficiently over relatively short distances to couple with a linear array of single channel fibers. Multiplexers and demultiplexers are preferably reversible so that a single device can perform both multiplexing and demultiplexing functions (hereinafter, a "(demultiplexer"). Furthermore, the (de)multiplexer must be able to accommodate channels over a free spectral range commensurate with fiber optic communications bandwidth. Moreover, the devices must provide high resolution to minimize cross talk and must further be highly efficient to minimize signal loss. The ideal device would also be small, durable, inexpensive, and scalable. Diffraction grating based multiplexers and demultiplexers have significant advantages over other technologies for dense wavelength division multiplexing applications because of their relatively low cost, high yield, low insertion loss and cross talk, uniformity of loss as well as their ability to multiplex a large number of channels concurrently. Representative diffraction grating based (de)multiplexer configurations are disclosed on applicant's commonly assigned co-pending U.S. Patent Application Serial No. 09/628,774, filed July 29, 2000, entitled "Echelle Grating Dense Wavelength Division Multiplexer/Demultiplexer", the contents of which are incorporated herein in their entirety. However, diffraction gratings have an intrinsic polarization sensitivity that can limit their usefulness in (de)multiplexing applications. That is, an optical signal propagating through an optical fiber has an indeterminate polarization state, requiring that the (de)multiplexer be substantially polarization insensitive so as to minimize polarization dependent losses, a measure of diffraction efficiency that is dependent on the polarization state of the optical signal. There are numerous methods and apparatus for reducing the polarization sensitivity of diffraction grating fiber optic (de)multiplexers. Chowdhury, U.S. Patent Nos.5,966,483 and 6,097,863 (collectively "Chowdhury"), the disclosure of which is incorporated in its entirety by reference, describes a diffraction grating with reduced polarization sensitivity. Chowdhury teaches that polarization sensitivity can be minimized by orienting the reflective faces of a diffraction grating at a blaze angle "θ b " for retro-reflecting normal incident light of a wavelength "λ t> " that is different from a median wavelength "λ 0 " of a transmission bandwidth "Δλ". The blaze angle θ b is chosen to reduce the difference between first and second diffraction efficiencies of a wavelength λ within the transmission bandwidth Δλ. This solution for minimizing differences in diffraction efficiency can be of limited utility because it places limitations on election of blaze angles and blaze wavelengths that can inhibit the overriding goal of providing a diffraction grating for a (de)multiplexer accommodating a large number of closely spaced channels with high resolution, minimal cross talk and little signal loss. Chowdhury further teaches that diffraction grating polarization sensitivity can be reduced by providing concave and convex corners between adjacent reflective steps and risers of a diffraction grating. More particularly, Chowdhury teaches that polarization sensitivity can be reduced by varying the radius of concave corners between adjacent steps and risers. While this proposal has the advantage of not placing an unwarranted restraint on selection of a blaze wavelength and blaze angle for a grating, accurately controlling the concave and convex radii on a nanometer scale could be both difficult and expensive. It can also limit the absolute efficiency of the grating. Chowdhury also teaches that maximizing the pitch (or groove spacing) can help to minimize polarization sensitivity. However, as with Chowdhury's proposal of manipulating blaze angle and blaze wavelength to minimize polarization sensitivity, this proposal puts constraints on grating pitch that can degrade other important objectives of the diffraction grating, such as achieving suitable channel separation for DWDM signals. McMahon, U.S. Patent No. 4,736,360, teaches that polarization sensitivity in a bulk optic grating can be minimized by assuring that the width of the reflective surface is sufficiently large as compared to the operating wavelength of the grating. This is effectively similar to maximizing pitch as taught by Chowdhury. While this solution may have limited application, it also places what can be an unnecessary restraint on grating design choices and thus may limit the ability of the grating to perform its wavelength division (de)multiplexing function for signals having a close channel spacing. He, U.S. Patent No. 5,937,113, teaches yet another way to minimize polarization dependent losses for an optical waveguide diffraction grating. He teaches a diffraction grating device having an output region with a plurality of predetermined light receiving locations. A first slab waveguide region has a first birefringence, the first slab guide region being optically coupled with input and output regions of the device. A second slab waveguide region adjacent to the first slab waveguide region has a predetermined shape and predetermined dimensions providing a second different birefringence than the first slab waveguide region to provide polarization compensation for the device. This solution requires providing first and second slab waveguides and thus is not readily applicable to bulk optic devices. In any event, providing at least two slab waveguides increases product complexity and cost. Another known method for reducing polarization sensitivity is providing a polarization separator followed by a half wave plate on one of the separated beams between a collimating optic and a grating. The polarization separator splits an incident beam into first and second beams of light, with each beam being linearly polarized along different orthogonal directions. The half wave plate located on one of the beams results in both beams having the same orthogonal polarization. While this method has the advantage of not placing limitations on the design of the diffraction grating so as to limit its utility for performing DWDM, both the polarization beam splitter and the half wave plate tend to degrade the overall efficiency of the (de)multiplexer and add to part count and device complexity. The use of a polarization beam splitter for minimizing polarization sensitivity is taught in Nicia, U.S. Patent No. 4,741,588; Martin, U.S. Patent No. 6,084,695; Doerr, U.S. Patent No. 5,809,184; and Boord, WO 99/41858. The present invention is intended for overcoming one or more of the problems discussed above. SUMMARY OF THE INVENTION A first aspect of the present invention is a diffraction grating for multiplexing and demultiplexing optical signals in an optical communication system. The diffraction grating has a plurality of grooves formed in a substrate, with each groove having a groove surface including a reflective step surface. The reflective step surfaces have a reflective coating and the remainder of the groove surfaces do not have a reflective coating. The groove surfaces may further include a transverse riser between reflective step surfaces of adjacent grooves. The reflective coating may be an electrically conductive metallic coating, preferably consisting of gold. Alternatively, the coating may be a dielectric, such as a multi-layer dielectric coating. Another aspect of the present invention is a method of making a reflective diffraction grating for diffracting optical signals in an optical communications system. The method includes forming a plurality of parallel grooves in a substrate, the parallel grooves comprising steps and transverse risers. A reflective coating is provided on the steps and not on the risers. The reflective coating may be applied to the grooves, excluding the risers, by ion beam sputtering or the reflective coating may be applied to both the steps and the risers and then etched from the risers. The reflective coating may be an electrically conductive metallic coating, preferably gold. Alternatively, the coating may be a dielectric, such as a multi-layer dielectric coating. The apparatus and method for reducing the polarization sensitivity of diffraction gratings in accordance with the present invention allows the blaze angle and groove spacing of the grating to be chosen to optimize such things as angular dispersion, overall efficiency and resolution for dense channel spacing (0.4 nm or less) over a relatively wide bandwidth. Polarization insensitivity is then provided by providing a reflective conductive coating on the reflective steps of the diffraction grating and not on the risers. Alternatively, a dielectric coating may be applied to both the steps and the risers or to only the steps. The present invention allows for the reduction of polarization sensitivity without introducing additional components or complexity into a multiplexer/demultiplexer utilizing a diffraction grating and without materially limiting grating design choices. The modifications to the diffraction grating necessary to practice the present invention are both minor and inexpensive, having essentially no effect on the cost or complexity of the grating itself. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic profile of the groove pattern of a prior art diffraction grating; Fig. 2 is a graphical representation of the field strength of s polarized and p polarized light refracted from the reflective surfaces of the grating steps of Fig. 1 as a function of distance from the riser; Fig. 3 is a schematic profile of the groove pattern of the diffraction grating of Fig. 1 with only the steps having a conductive reflective coating in accordance with the present invention; Fig. 4 is a schematic profile of the groove pattern of an exemplary diffraction grating with a reflective coating on the entire surface of the grating; Fig. 5 is a schematic profile of the groove pattern of the diffraction grating of Fig. 4 without a reflective coating on the risers in accordance with the present invention; Fig. 6 is a graph of diffraction efficiency varying as a function of wavelength within a select bandwidth for the TM and TE components of the refracted optical signal of the grating of Fig. 4; and Fig. 7 is similar to the graph of Fig. 6 showing the effect of not having a reflective coating on the risers of the grating of Fig. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 is the schematic profile of the groove pattern of a prior art diffraction grating 10. Grating 10 consists of a substrate 12 having a plurality of grooves 14 formed therein. The grooves are defined by adjacent transverse, planar steps 16 and risers 18. The planar steps 16 are a reflective surface in that they reflect an incident light beam. The planar risers 18 are a non-reflective surface in that they do not reflect an incident light beam with the grating configured to receive an incident beam as illustrated by the sine wave 24 of Fig. 1. Adjacent steps and risers have an apex 20 directed away from the substrate with a select angle (α) between the adjacent steps and risers 16, 18. As illustrated in Fig. 1, both the steps and the risers are coated with a conductive reflective coating, typically a metallic reflective coating 22 such as gold. A sine wave 24 is drawn in Fig. 1 to represent the oscillating electric field of (s) polarized light in the "TM" (transverse magnetic) direction of polarization. The incident light is reflected off the reflective surface or planar step 16. This electric field oscillates in a plane perpendicular to the grating grooves and perpendicular to the plane of riser 18. The orthogonal oscillating reflective field of (p) polarized light or the "TE" (transverse electric) direction of polarization oscillates parallel to the grating grooves. It is known that the diffraction efficiency for the TE and TM components of the diffracted light are different in diffraction gratings. While there is no intention to limit the scope of this disclosure or the appended claims by this statement of theory, one at least partial explanation for the different diffraction efficiency of the orthogonal components of polarized light is that the conductive coating on the riser surface interferes with the electric field. This phenomenon is illustrated graphically in Fig. 2. Fig. 2 is a representation of field strength on the Y axis versus distance from the riser on the X axis. The field strength E s at the riser is zero. In contrast, the electric field vector of the (p) polarized light, which is parallel to the grating rulings and perpendicular to the plane of the (s) polarized light, does not sample the region near the grating riser. Thus, there are minimal boundary conditions imposed by the grating riser and at the riser E p ≠ 0. It is the change in boundary conditions that is believed to be one factor increasing the polarization dependence of the grating. Fig. 3 illustrates a modification to the grating 10 of Fig. 1 for minimizing polarization dependent loss of the grating. The grating 10' has a reflective coating 22' on the reflective steps 16' only, and no reflective coating on the risers 18', which form a non-reflective surface that does not reflect an incident beam. This eliminates the boundary condition imposed by the conductive coating on the grating riser as illustrated in Fig. 2, and for that reason is believed to decrease polarization dependent loss. An alternative to eliminating the boundary condition imposed by the riser (or non-reflective surface) by not applying a conductive coating to the riser is to use a multi-layer dielectric coating in place of the conductive reflective coating 22 in the prior art embodiment depicted of Fig. 1. The multi-layer dielectric coating could be made of any one of a number of highly reflective multi-layer dielectric coatings known in the art, including layered titanium dioxide (TiO 2 ) and silicon dioxide (SiO 2 ); layered tantalum pentoxide (Ta 3 Os) and silicon dioxide (SiO 2 ); and layered halfhium dioxide (HfO 2 ) and silicon dioxide (SiO 2 ). The multi-layer dielectric coatings have the advantage of being more highly reflective than simple metal coatings, possibly exceeding 99.9% efficiency. Because these dielectric coatings are necessarily nonconductive, they ensure that the TE and TM components of the electric field sample similar boundary conditions. Thus, application of the dielectric coating offers the potential of higher efficiency and decreased polarization dependent loss. The dielectric coating may also be applied only to the reflective surface as is the case with the conductive reflective coating 22' depicted in Fig. 3. Similarly, the dielectric coating may be used in any other embodiment employing a reflective conducting coating such as those shown in Figs. 4 and 5 or holographic gratings (which are not illustrated). The gratings disclosed herein may be formed from one of several known methods. For example, it may be formed from an epoxy layer deposited on a glass substrate into which a master die defining the grooves is pressed. The grooves may also be precision machined directly into a glass or silicon substrate by an interferametrically controlled ruling engine. A further option is the use of photolithographic techniques described in McMahon, U.S. Patent No. 4,736,360, the contents of which are hereby expressly incorporated by reference in its entirety. Coating of only the steps (i.e., the reflective surface) can be accomplished using coating techniques that employ a highly directional beam of reflective coating material (e.g. , ion beam sputtering) or by coating the entire grating surface with the reflective coating using known techniques and ion etching the coating from the risers (or non- reflective surface). The reflective coating may be any suitably reflective material, and is typically a metallic conductive reflective coating such as gold, although as discussed above dielectric coatings may be preferred. While not intending to be limiting on the scope of the disclosure, the following example illustrates that providing the reflective coating on the reflective steps of a grating and not on the riser can be effective in reducing polarization dependent loss. Fig. 4 is a schematic profile of groove pattern of a diffraction grating 28. The grating 28 consists of a plurality of grooves 30 formed in a substrate 32. Each groove is defined by a transverse step 34 and riser 36 that are joined in the groove trough by a flat 38. In this example, the entire surface of the grooves, including the reflective step 34, the flat 38 and the riser 36 are covered by a conductive reflective coating 40 of gold. The groove density is 171.4 grooves per millimeter , the blaze angle, θ b , is 31°, the groove depth is about 2500.0 nm, the flat 38 is about 713.0 nm long, and the apex angle, α a , is 80°. Fig. 5 is a profile of the groove pattern of a diffraction grating 28' which is identical to the diffraction grating 28 in all respects except the surface of the risers 36' does not have the conductive reflective coating 40'. Thus, the reflective step and the flat 38' are the only parts of the grating surface that are coated. Fig. 6 is a graph of efficiency versus wavelength for the TE and TM components of a refracted optical signal as a function of wavelength. The efficiency is the ratio of the energy content of the diffracted light of the respective TE and TM component and the energy content of the light incident on the grating. For the C band of wavelengths which is currently used for optical communication (λ = 1528 - 1565 nm) the polarization dependent loss, measured as the difference in efficiency between the TE and TM components, varies between about 10 - 16 percent. Fig. 7 illustrates the polarization dependent loss for the grating 28' illustrated in Fig. 5 where only the reflective steps 34' are coated. Here the polarization dependent loss is sharply reduced, varying between about 2.5 - 5 percent. Not providing a reflective coating on the riser of a grating in accordance with the present invention decreases polarization dependent loss inherent in bulk diffraction gratings. It does not require alteration of the grating profile which could detrimentally effect the ability of the grating to provide necessary channel separation, resolution and efficiency. Use of a dielectric reflective coating can yield similar advantages. Furthermore, these advantages can be provided to known grating profiles including but not limited to the embodiments illustrated herein and holographic gratings, with minimal effort and expense.

Claims: CLAIMS What is claimed is: 1. A diffraction grating for use in multiplexing and demultiplexing optical signals in optical communications systems, the diffraction grating comprising a plurality of grooves formed in a substrate with each groove comprising a reflective surface for reflecting an incident light beam and a non-reflective surface which does not reflect the incident beam, the non-reflective surface being non-conductive. 2. The diffraction grating of claim 1 wherein the reflective surface and the non-reflective surface have a reflective dielectric coating. 3. The diffraction grating of claim 1 wherein the reflective surface and the non-reflective surface have a multi-layer dielectric coating. 4. The diffraction grating of claim 1 wherein the reflective surface has a reflective coating and the non-reflective surface does not have reflective coating. 5. The diffraction grating of claim 4 wherein the reflective coating is electrically conductive. 6. The diffraction grating of claim 4 wherein the reflective coating is metallic. 7. The diffraction grating of claim 4 wherein the reflective coating comprises gold. 8. The diffraction grating of claim 4 wherein the reflective coating is non- conductive. 9. The diffraction grating of claim 4 wherein the reflective coating is a dielectric. 10. The diffraction grating of claim 4 wherein the reflective coating is a multi-layer dielectric. 11. The diffraction grating of claim 4 wherein the reflective coating is a multi-layer dielectric selected from the group comprising the following combinations: layered titanium dioxide and silicon dioxide, layered tantalum pentoxide and silicon dioxide or layered halfnium dioxide and silicon dioxide. 12. The diffraction grating in claim 1 wherein each groove comprises a reflective step surface and a non-reflective riser surface transverse the reflective step surface, the non-reflective riser surface lying between the reflective step surfaces of adj acent grooves . 13. A method of making a reflective diffraction grating for diffracting optical signals in an optical communications system, the method comprising: a) forming a plurality of parallel grooves in a substrate, the parallel grooves comprising steps and risers; and b) providing a reflective coating on the steps and not on the risers. 14. The method of claim 13 wherein step b) is performed by ion beam sputtering. 15. The method of claim 13 wherein step b) is performed by first coating the steps and risers with a reflective coating and then etching the coating from the risers. 16. The method of claim 13 wherein the reflective coating comprises an electrically conductive reflective coating. 17. The method of claim 13 wherein the reflective coating is a dielectric. 18. A diffraction grating for use in multiplexing and demultiplexing optical signals in optical communications systems, the diffraction grating comprising a plurality of grooves formed in a substrate with each groove having a groove surface including a reflective step surface, the reflective step surfaces having a reflective coating and the remainder of the groove surfaces not having a reflective coating. 19. The diffraction grating of claim 18 wherein each groove surface further comprises a transverse riser between reflective step surfaces of adjacent grooves. 20. The diffraction grating of claim 18 wherein the reflective coating comprises an electrically conductive coating. 21. The diffraction grating of claim 18 wherein the reflective coating is metallic. 22. The diffraction grating of claim 18 wherein the reflective coating comprises gold. 23. The diffraction grating of claim 18 wherein the reflective coating is non- conductive. 24. The diffraction grating of claim 18 wherein the reflective coating is a dielectric. 25. The diffraction grating of claim 18 wherein the reflective coating is a multi-layer dielectric. 26. The diffraction grating of claim 18 wherein the reflective coating is multi-layer dielectric selected from the group comprising the following combinations: layered titanium dioxide and silicon dioxide, layered tantalum pentoxide and silicon dioxide or layered halfhium dioxide and silicon dioxide. 27. A diffraction grating for use in multiplexing and demultiplexing optical signals in an optical communication system, the diffraction grating comprising a plurality of parallel grooves formed in a substrate, each groove having a planar reflective step surface and a riser transverse the reflective step surface, the riser lying between reflective step surfaces of adjacent grooves, the reflective step surfaces having a reflective coating and the risers not having a reflective coating. 28. The diffraction grating of claim 27 wherein the reflective coating is metallic. 29. The diffraction grating of claim 27 wherein the reflective coating is a dielectric. 30. A method of making a reflective diffraction grating for diffracting optical signals in an optical communications system, the method comprising: a) forming in a substrate a plurality of grooves having a reflective surface for reflecting an incident light beam and a non-reflective surface which does not reflect the incident beam; and b) applying a non-conductive reflective coating to the reflective surface. 31. The method of claim 30 wherein step b) further comprises not applying a non-conductive reflective coating to the non-reflective surface. 32. The method of claim 30 wherein in step b) the non-conductive reflective coating comprises a dielectric. 33. The method of claim 30 wherein in step b) the non-conductive reflective coating comprises a multi-layer dielectric. 34. The method of claim 30 further comprising: c) applying a non-conductive reflective coating to the non-reflective surface.

Inventors: SAPPEY ANDREW D, SAPPEY, ANDREW, D.
Applicants: SAPPEY ANDREW D, ZOLO TECHNOLOGIES INC, SAPPEY, ANDREW, D., ZOLO TECHNOLOGIES, INC.
Application year: 2001
Main classification: G02B27
IPC codes: G02B5