GRATING WAVEGUIDE CONFIGURATION FOR PHASARS

Abstract: A grating is provided having a generally s-shaped waveguide configuration. The grating includes a plurality of waveguides arranged adjacent to one another to form an array. The array (32) includes a first portion (34) bending in a counter-clockwise direction (36), a second portion (38) bending in a clockwise direction (40), and a third portion (38') extending in a straight direction. The arc length of each waveguide along the counter-clockwise cause some waveguides to be longer than others. This results in a clear separation of the individual waveguides. Further, the overall dimension of the grating is smaller than conventional devices which leads to reduced cross-talk.

Description: GRATING WAVEGUIDE CONFIGURATION FOR PHASARS Background Of The Invention 1. Technical Field The present invention generally relates to wavelength division multiplexing systems and, more particularly, to an improved grating configuration for a phasar. 2. Technical Background The simultaneous transmission of several channels along a single path is known as wavelength division multiplexing. This technology is important in fiber based systems due to its impact on system configuration, performance and cost. One major advantage of this technology is its ability to increase system capacity by increasing the number of channels that can be carried per fiber. With the ability to increase capacity, existing systems can be upgraded rather than replaced. In wavelength division multiplexing systems, each channel operates at a different wavelength. The individual wavelengths are combined into a single path by a multiplexer and are separated from one another by a demultiplexer. Multiplexers and demultiplexers normally take the form of a dispersive element such as a diffraction grating, prism, or hologram. When using a grating, demultiplexing is accomplished by transmitting the multiplexed signal through the grating which separates the individual wavelengths from one another and diffracts each in a slightly different direction. Multiplexing is accomplished by utilizing the same device in reverse. As wavelength division multiplexing technology has evolved, the need for more complex photonic components such as gain-flattening filters, variable attenuators, and add-drop multiplexers has increased. An efficient way to package such components is to use a planar photonic device. Planar photonic devices are chosen for their ability to implement many optical functions on a common wafer. In addition, active devices can be added to the wafer in order to create hybrid packages delivering many of the functions (e.g., switching, attenuating, monitoring, multiplexing and demultiplexing, etc.) needed in optical networks in very compact packages. When combining many different elements on the same wafer, noise or crosstalk generated by each element sometimes interferes with the other elements. This greatly decreases the performance of the overall system. Cross-talk is an especially critical consideration in phasars for dense wavelength division multiplexing applications where more and more signals with different wavelengths travel through the network. Each wavelength must be correctly routed to the correct detector or fiber, and must not interfere with the other wavelengths. Other requirements for phasars have also increased dramatically in the last decade. In addition to having less cross-talk, phasars must have better insertion losses, work with a greater number of channels, and have denser spectral spacing. These specifications have resulted in the development of larger and larger devices. Since cross-talk is dependent on the length traveled in phasars, such larger devices are more sensitive to cross-talk. In the case of refractive index, core layer, or etching non- uniformity in the wafer, the accumulated phase error along each grating waveguide increases dramatically with length. Conventional gratings for phasars include a plurality of adjacently arranged waveguides forming an array. The array of waveguides includes a first portion extending in a straight direction, a second portion bending in a clockwise direction, and a third portion extending in a straight direction. The second, clockwise bend portion has a fixed or variable radius of curvature bringing the direction of the array to the horizontal. This array is then repeated in a mirror image for the opposite side of the phasar. Although such conventional grating configurations have achieved great success, there is room for improvement in the art. For example, the first straight portion of the grating forces wavelengths to travel a longer distance than is necessary thereby increasing the potential for cross-talk interference. As such, it would be desirable to provide a compact grating configuration for reducing cross-talk interference in wavelength division multiplexing systems by decreasing the distances that wavelengths must travel. Summary Of The Invention The above and other objects are provided by a grating having a generally s- shaped waveguide configuration. The grating includes a plurality of waveguides arranged adjacent to one another to form an array. The array includes a first portion bending in a counter-clockwise direction, a second portion bending in a clockwise direction, and a third portion extending in a straight direction. The bends have either a fixed or a variable radius of curvature. The arc length of each waveguide along the counter-clockwise bend portion varies from one waveguide to the next. The arc length of the first counter-clockwise bend is nearly zero. The different arc lengths along the counter-clockwise bend cause some waveguides to be longer than others. This results in a clear separation of the individual waveguides. Further, the overall dimension of the grating is smaller than conventional devices which leads to reduced cross-talk. Brief Description Of The Drawings In order to appreciate the manner in which the advantages and objects of the invention are obtained, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings only depict preferred embodiments of the present invention and are not therefore to be considered limiting in scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: FIG.1 is a schematic illustration of a phasar for a wavelength division multiplexing system including a grating in accordance with the present invention; FIG. 2 is a more detailed view of the input waveguide and input slab of FIG. 1 ; FIG. 3 is a more detailed view of the input slab of FIG. 2; FIG. 4 is a more detailed view of the grating of FIG. 1 ; FIG. 5 is a more detailed view of the output slab and output waveguide array of FIG. 1 ; FIG. 6A is schematic illustration of a preferred embodiment grating for the phasar of FIG. 1 ; FIG. 6B is another schematic illustration of a preferred embodiment grating for the phasar of FIG. 1 ; FIG. 7 is a schematic illustration of the phasar of FIG. 1 having dimensional reference symbols inserted therein; and FIG. 8 is a schematic illustration of the input slab and a single waveguide of the phasar of FIG. 1 with dimensional reference symbols inserted therein. Detailed Description Of The Preferred Embodiments The present invention is directed towards a waveguide multiplexing system including a grating having an s-shaped waveguide configuration. The grating includes a plurality of waveguides arranged adjacent to one another to form an array. Each half of the array includes a first portion forming a counter-clockwise bend. The arc length of each waveguide along the first portion varies from the first waveguide in the array to the last. This grating configuration improves upon known gratings by reducing the total distance that all wavelengths must travel. This reduces the potential for cross-talk and increases efficiency. Turning now to the drawing figures, FIG. 1 illustrates a phasar 10 for use in a wavelength division multiplexing system. Although phasar 10 is illustrated, it is to be understood that this environment is merely exemplary of the type of system in which the grating of the present invention could be employed. The phasar 10 is preferably formed in a single planar wafer of glass and includes an input waveguide 12 coupled to an input slab 14 so as to optically communicate therewith. The input slab 14 is coupled to a grating 16 which is coupled to an output slab 18. The grating 16 enables optical communication between the input slab 14 and output slab 18. The output slab 18 is coupled to an output waveguide array 20 so as to optically communicate therewith. Preferably, the input waveguide 12, input slab 14, grating 16, output slab 18 and output waveguide array 20 are formed by reactive ion etching through a photolithography process. Referring now to FIG. 2, a more detailed view of the input waveguide 12 and input slab 14 of FIG. 1 is illustrated. The input waveguide 12 delivers a multiplexed signal 22 to the input slab 14. The trapezoidal input slab 14 diffracts the multiplex signal 22 along its length such that the signal's E-field changes from a narrow profile 24a to a broad profile 24b. As illustrated in FIG. 3, after diffracting, the broad profile E-field 24b splits into individual narrow profile E-fϊelds or modes wavelengths 26a-g which enter the grating 16. A plurality of funnels 28 on the input slab 14 help to split the broad profile E-field 24b into the separate waveguides 30a-g in the grating 16 with minimal energy loss. Turning now to FIG. 4, a more detailed view of the grating of FIG. 1 is illustrated. After exiting the input slab 14, the modes 26a-d travel along the waveguides 30a-d to the output slab 18. The length of each waveguide 30a-d is selected such that the modes 26a-d arrive at the output slab 18 in phase for a specific wavelength. As described in greater detail below, a constant length increment is provided between consecutive waveguides 30a-d. Although not illustrated, a plurality of funnels interposed between the grating 16 and output slab 18 help direct the individual modes 26a-d therein. As shown in FIG. 5, the trapezoidal output slab 18 diffracts the signal 22' such that the modes 26a-d combine into a broad profile E-field 24b' which changes to the narrow profile E-field 24a'. The signal 22' is then delivered to the output waveguide array 20. Referring now to FIG. 6A, a more detailed view of the first half of the grating 16 is illustrated. One skilled in the art will appreciate that the other half of the grating 16 has been omitted for simplicity of illustration. As shown in FIG. 6B, the second half of the grating 16 is the mirror opposite of the half illustrated in FIG. 6A. The purpose of the grating 16 is to direct the diffracted signal from the input slab 14 through different paths which vary in length thereby dividing the diffracted signal into a plurality of modes with a desired phase pattern. The grating 16 includes a plurality of waveguides 30a-n (wherein n = the number of waveguides) formed adjacent one another so as to define an array 32. The array 32 includes a first arcuate portion 34 in the form of a counter-clockwise bend 36. The first arcuate portion 34 transitions into a second arcuate portion 38 in the form of a clockwise bend 40. As such, the second arcuate portion 38 curves opposite the first arcuate portion 34. The second arcuate portion 38 brings the array 32 to the horizontal. The second arcuate portion 38 transitions into a portion 42 in the form of a straight 44 which leads to the symmetry plane 46. As illustrated in FIG. 6B, the grating 16 continues beyond the symmetry plane 46 with a second straight portion 42', a third arcuate portion 38' in the form of a clockwise bend, and a fourth arcuate portion 34' in the form of a counter-clockwise bend which is coupled to the output slab 18. The arc length of each individual waveguide 30a-n along the first arcuate portion 34 (and fourth arcuate portion 34')increases a constant amount from a minimum at the first waveguide 30a to a maximum at the last waveguide 30n. Preferably, the arc length of the first waveguide 30a along the first arcuate portion 34 is nearly zero. By incrementally increasing the arc length of consecutive waveguides 30a-n, the length of each waveguide 30a-n increases from a minimum at the first waveguide 30a to a maximum at the last waveguide 30n. This provides clear separation of individual modes propagating through the grating 16. Further, due to the s-shape of the grating 16 provided by the first arcuate portion 34, the overall dimension of the grating 16 is smaller than conventional designs. For example, in a 16 channel phasar at 100 GHz, a conventional grating configuration (i.e., straight/clockwise bend/straight) has a first grating arm length of 14,670 mm whereas the present grating configuration (i.e., counter-clockwise bend/clockwise bend/straight) has a first grating arm length of 13,772 mm. In a 40 channel phasar at 50 GHz, a conventional grating configuration has a first grating arm length of 13,645 mm whereas the present grating configuration has a first grating arm length of 12,729 mm. This compact configuration reduces crosstalk and increases efficiency. For instance, in the 16 channel comparison described above, the level of cross-talk was 8dB better with the present grating configuration. The efficiency of the grating 16 can be more completely understood from the following model. When light propagates through both symmetrical portions of the grating 16, its phase shift from the input slab 14 to the output slab 18 is equal to: Phase[k]= ff[k]( + kdl) ( 1 ) wherein: k is the waveguide 30 for which the phase shift is being sought; Phasefk] is the phase shift from the input slab 14 to the output slab 18 along waveguide 30k; neff[k] is equal to the mean effective index in the waveguide 30k; and (Lo + kdl) is equal to the length of the waveguide 30k. Theoretically, neff[k] is uniform. In that case: neff[k]=neff 0 (2) Phase [k]=neffo( + kdl) (3) The phase difference between two waveguides 30 is constant and equal to Phase[k]-Phase[k-l]=ntϊϊodl (4) With process variations, the effective index varies from one waveguide 30 to another: neff[k]=neffo + dneff[k] (5) The phase difference between two adjacent waveguides 30 is no longer constant. The phase errors are amplified for long Lo, and generate cross-talk in the spectral transmission of the phasar 10. The phase difference between two waveguides 30 and the phase increment variance becomes: Phase[k]-Phase[k-l]=neff 0 d\ + (L 0 + ω/jdneff[k]-( +(k-l)dl)dneff[k-l] (6) <Phase Increment>=(Lo + n/2dl)<dneff> (7) where n is the number of waveguides 30. As the phase errors are proportional to the length of the grating, the grating 16 is designed to reduce Lo as much as possible. Referring now to FIGS. 7 and 8, an optimum s-bend grating configuration will be described. In FIG. 7, F is the length of the input slab 14, L is the input origin of the input slab 14, Xd the distance between L and the symmetry plane 46, R the bend radius, and θo the tilt angle of the input slab 14. The symmetrical part is repeated on the right side. In FIG. 8, Lt is the length of the funnels 28, a k the arc length of the counter-clockwise bend portion 36, b k the arc length of the clockwise bend portion 40, I k the length of the straight portion 44, and p the pitch between two waveguides 30. To optimize the s-Bend configuration, a coordinate system is defined starting from I 0 . The angle between two grating waveguides 30 is fixed: (5t? = 2arctan(— ) 2/ The orientation of the waveguide 30k is equal to: θ k = θ ϋ + (k -~)δθ where n is the number of waveguides 30. For a fixed slab orientation θo and a position of the symmetry plane Xd, the parameters a k , b and I k are calculated by solving the following set of three equations: straight portion 38 perpendicular to symmetry plane 46; b k = θ k + a k all waveguides 30 finishing on the symmetry plane 46; and Xd = (F + lt) cos(^ ) + R(s (θ k + a k ) - sin(0 λ ) + sin(b k )) + l k path difference between two waveguides 30 equal to dl. l k + R(a k + b k ) - /,_, - R(a k _, + b k _ t ) = — For a fixed slab orientation, the plane position Xd is optimized so that the length of the shortest straight portion 38 in the grating 16 is equal to zero (0). The position of the symmetry plane 46 is just offset of all the path lengths. Finally, the slab orientation θ 0 is chosen to minimize Lo. L 0 is equal to: L 0 - l 0 + Rb 0 (note - ao is equal to 0 for the first waveguide 30a). According to the foregoing calculations, the grating 16 starts on the circular interface portion of the input slab 14, ends on the circular interface portion of the output slab 18, has a constant length increment between waveguides 30, and is as small as possible. The gradually increasing counter-clockwise bend is advantageous over the initial straight section of conventional "straight/clockwise bend/straight" configuration. While both configurations lead to a constant increment of travel for the different modes in the grating, the s-shaped configuration partially eliminates the distance that must be traveled by all of the modes. This total decrease in the distance that modes must travel in the grating is beneficial in that cross-talk and other interference significantly decreases as devices and waveguides become shorter. Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.

Claims: What is claimed is: 1. A phasar comprising: an input waveguide; an input slab coupled to said input waveguide; a grating coupled to said input slab; an output slab coupled to said grating; and an output waveguide array coupled to said output slab, wherein said grating includes two symmetrical halves, each of said halves having a generally s-shaped configuration 2. The phasar of claim 1 wherein said grating further comprises: a first arcuate portion coupled to said input slab. 3. The phasar of claim 2 wherein said grating further comprises: a second arcuate portion extending from said first arcuate portion, said second arcuate portion curving opposite said first arcuate portion. 4. The phasar of claim 3 wherein said gating further comprises: a first straight portion extending from said second arcuate portion. 5. The phasar of claim 4 wherein said gating further comprises a second straight portion extending from said first straight portion. 6. The phasar of claim 5 wherein said grating further comprises: a third arcuate portion extending from said second straight portion. 7. The phasar of claim 6 wherein said grating further comprises: a fourth arcuate portion extending from said third arcuate portion and coupled to said output slab, said fourth arcuate portion curving opposite said third arcuate portion. 8. The phasar of claim 6 wherein said third arcuate portion is a mirror opposite of said second arcuate portion. 9. The phasar of claim 7 wherein said fourth arcuate portion is a mirror opposite of said first arcuate portion. 10. A grating comprising: a plurality of waveguides arranged adjacent to one another so as to form an array, said array having a first s-shaped configuration wherein said plurality of waveguides have incrementally increasing lengths from a minimum at one edge of said array to a maximum at an opposite edge of said array. 1 1. The grating of claim 10 wherein said first s-shaped configuration further comprises: a first arcuate portion, a second arcuate portion extending from said first arcuate portion, and a straight portion extending from said second arcuate portion, said second arcuate portion curving opposite said first arcuate portion. 12. The grating of claim 11 wherein said first arcuate portion bends in a counterclockwise direction. 13. The grating of claim 11 wherein said second arcuate portion bends in a clockwise direction. 14. The grating of claim 10 wherein said array has a second s-shaped configuration extending opposite said first s-shaped configuration. 15. The grating of claim 14 wherein said second s-shaped configuration is a mirror opposite of said first s-shaped configuration. 16. The grating of claim 14 wherein said second s-shaped configuration further comprises: a first arcuate portion, a second arcuate portion extending from said first arcuate portion, and a straight portion extending from said second arcuate portion, said second arcuate portion curving opposite said first arcuate portion. 17. The grating of claim 16 wherein said first arcuate portion bends in a counterclockwise direction. 18. The grating of claim 16 wherein said second arcuate portion bends in a clockwise direction. 19. A compact grating comprising: a plurality of waveguides arranged adjacent to one another to form an array, said array including, a first counterclockwise bend portion wherein said plurality of waveguides have incrementally increasing arc lengths, a first clockwise bend portion extending from said first counterclockwise bend portion, and a first straight portion extending from said first clockwise portion. 20. The compact grating of claim 19 wherein the array further comprises: a second straight portion extending from said first straight portion; a second clockwise portion extending from said second straight portion; and a second counter-clockwise portion extending from said second clockwise portion wherein said plurality of waveguides have incrementally increasing arc lengths.

Inventors: FONDEUR BARTHELEMY, FONDEUR, BARTHELEMY
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2000
Main classification: G02B6
IPC codes: G02B6