Waveguide grating router having a predetermined composite amplitude spectrum

<u>Field of the Invention</u>

The invention relates generally to optical devices and, more particularly, to waveguide grating routers.

<u>Background of the Invention</u>

Waveguide grating routers in conventional lightwave systems are used as optical switches, multiplexers, demultiplexers, detectors, add/drop filters, one by N (1 X N) and N by one (N X 1) splitters and N by N (N X N) arrays. Typically, such waveguide grating routers include an interconnection apparatus having a plurality of closely spaced input waveguides communicating with an input of a star coupler. An output of the star coupler communicates with an optical grating comprising a series of optical waveguides in which each of the grating waveguides differ in length with respect to its nearest neighbor by a predetermined fixed amount. The optical grating is further connected to an input of a second star coupler, the outputs of which form outputs of the switching, multiplexing, and demultiplexing apparatus. Waveguide grating routers are also frequently referred to as "frequency routing devices" and are further described in <patcit id="pcit0001" dnum="US5002350A"><text>U. S. Patent No. 5,002,350, issued March 26, 1991 to C. Dragone</text></patcit>, entitled "Optical Multiplexer/Demultiplexer" (hereinafter "Dragone 1") and <patcit id="pcit0002" dnum="US5136671A"><text>U. S. Patent No. 5, 136,671, issued August 4, 1992 to C. Dragone</text></patcit>, entitled "Optical Switch, Multiplexer, and Demultiplexer" (hereinafter "Dragone 2"), both of which are hereby incorporated by reference.

A known characteristic of these waveguide grating routers is that they do not efficiently provide a predetermined composite amplitude spectrum for each of the output signals that may comprise many wavelengths or frequencies. Rather, these routers tend to attenuate signals in outer waveguides more than signals in the inner waveguides. Thus, the amplitude response of the router across the spectrum of wavelengths in the waveguides is nonuniform.

Attempts have been made to design a waveguide grating router with a uniform composite amplitude spectrum, yet prior designs have proven to be inefficient in deriving a predetermined composite amplitude spectrum, such as a uniform output spectrum, or such designs suffer severe loss. For example, one method uses a "loop-back" optical path with the waveguide grating router. "Loop-back" optical path connections are connections from the output signal fed back into a predetermined input port as described in an article entitled "<nplcit id="ncit0001" npl-type="s"><text>Loss Imbalance Equalization of Arrayed Waveguide Grating Add-Drop Multiplexer," written by Osamu Ishida, et. al., that appeared in Electronics Letters, Vol. 30, No. 14, July 7, 1994</text></nplcit>. However, this method has not been effective to equalize loss in waveguide routers.

Another method used for achieving a uniform output spectrum averages the waveguide grating router loss over the cascaded routers by shifting the port connections between adjacent routers, as described in an article entitled "<nplcit id="ncit0002" npl-type="s"><text>Loss Imbalance Equalization in Arrayed Waveguide Grating (AWG) Multiplexer Cascades" written by Osamu Ishida, et al., that appeared in Journal of Lightwave Technology, Vol. 13, No. 6, June 1995</text></nplcit>. However, such a system suffers severe loss due to the cascading of routers.

Still another method to achieve greater channel uniformity was described in an article entitled "<nplcit id="ncit0003" npl-type="s"><text>Waveguide Grating Router with Greater Channel Uniformity" written by J.C. Chen and C. Dragone, that appeared in Electronics Letters, Vol. 33, No. 23, pp. 1951-2, November 6, 1997</text></nplcit>. The method disclosed therein, which is incorporated by reference herein, uses auxiliary waveguides between adjacent grating arms in a waveguide grating router. Although such a system does not add loss, it also does not provide a predetermined composite amplitude spectrum, such as a uniform output spectrum. Specifically, the auxiliary waveguides simply help to shape the radiation pattern of the star coupler within the waveguide grating router to reduce channel non-uniformity. Thus, there is a need for a more effective means of providing a waveguide grating router having a predetermined composite amplitude spectrum thereby reducing the complexity of lightwave systems.

<u>Summary of the Invention</u>

A waveguide grating router having a predetermined composite amplitude spectrum is achieved in accordance with the principles of the present invention by introducing loss into the waveguide grating router's waveguides. When the desired predetermined composite amplitude spectrum is a uniform amplitude spectrum, this same loss can produce loss equalization by the complete elimination of gain nonuniformity in all the output signals. The invention however is not limited to the production of a flat amplitude spectrum. For example, the predetermined composite amplitude spectrum may be sinusoidal, triangular, stepwise discrete, sloped, logarithmic, etc.

According to one aspect of the invention, a waveguide grating router is provided that includes at least one input waveguide and a plurality of output waveguides. A variable loss element is introduced to a predetermined group of output waveguides to produce a predetermined composite amplitude spectrum of the output signals.

According to another aspect of the invention, a waveguide grating router is provided that includes a plurality of input waveguide and a plurality of output waveguides. The variable loss element is introduced loss to a predetermined group of both input and output waveguides to yield output signals that exhibit the predetermined composite amplitude spectrum. In both types of waveguide grating routers, loss may be introduced in a variety of ways, including varying the cross-section of an input waveguide with respect to that of an output waveguide.

<u>Brief Description of the Drawing</u>

The invention will be more readily understood after reading the following detailed description taken in conjunction with the accompanying drawing, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> depicts a block diagram of a prior art waveguide grating router;</li><li><figref idref="f0002">FIG. 2</figref> illustrates a graph of the composite amplitude spectrum of the transmitted power of the signal's output from the various waveguides of the waveguide grating router illustrated in <figref idref="f0001">FIG. 1</figref> at specified wavelengths (prior art);</li><li><figref idref="f0003">FIG. 3</figref> is a block diagram of an embodiment of an N X N waveguide grating router with loss equalization;</li><li><figref idref="f0002">FIG. 4</figref> depicts a graph of the transmitted power of the signal's output from the various waveguides of the waveguide grating router illustrated in <figref idref="f0003">FIG. 3</figref> at specified wavelengths;</li><li><figref idref="f0004">FIG. 5</figref> shows an illustrative embodiment of an N X N router with loss equalization; and</li><li><figref idref="f0005">FIG. 6</figref> shows another illustrative embodiment of an N X N router with loss equalization.</li></ul>

<u>Detailed Description</u>

A brief description of the prior art waveguide grating router is provided in order to develop a better understanding of the present invention and its departure from the prior art. As noted earlier, a waveguide grating router is also known as a frequency routing device. The latter term describes the action of light at different frequencies taking different paths through the router. In the description which follows, the terms "frequency" and "wavelength" may be used interchangeably when referring to the operation of the router.

<figref idref="f0001">FIG. 1</figref> shows a prior art waveguide grating router 100 that includes a plurality of input waveguides 102_i, i=1,2, ... N, a first and second free space region 104 and 108 respectively, an optical grating 106, and a plurality of output waveguides 110_k, k=1,2, ... N. As illustrated, the plurality of input waveguides 102_i, i= 1, 2, ... N, is connected to free space region 104. The plurality of output waveguides 110_k, k=1,2, ... N, extends from free space region 108 which is coupled to optical grating 106. Optical grating 106 includes a plurality of unequal length waveguides that provide a predetermined amount of path length difference to the corresponding plurality of input waveguides 102_i. In operation, if a signal of amplitude A is applied to one of the input waveguides 102_i, for example, input waveguide 102₁, then signals of amplitudes AT₁₁, AT₁₂, ...AT_1N are produced at output waveguides 110_k where T_ik is the value of the transmission coefficient for input waveguide 102₁ and the plurality of output waveguides 110_k.

As illustrated in <figref idref="f0002">FIG. 2</figref>, output signals from the plurality of output waveguides 110_k of the waveguide grating router form a Gaussian amplitude spectrum. With continued reference to <figref idref="f0002">FIG. 2</figref>, the composite amplitude spectrum of the output signals as a function of wavelength is shown illustrating the corresponding transmission coefficients T_ik for the routing device. The details of the routing devices' constriction and operation are more fully described in Dragone 1, Dragone 2, and <patcit id="pcit0003" dnum="US5243672A"><text>U. S. Patent No. 5,243,672, issued September 7, 1993 to C. Dragone</text></patcit>, entitled "Planar Waveguide Having Optimized Bend", (hereinafter "Dragone 3"), which is hereby incorporated by reference.

<figref idref="f0003">FIG. 3</figref> illustrates an embodiment of a waveguide grating router 300 in accordance with the present invention. Waveguide grating router 300 includes all of the elements of <figref idref="f0001">FIG. 1</figref> including a plurality of input waveguides 302, a first free space region 304, an optical grating 306, a second free space region 308, and a plurality of output waveguides 310. However, unlike the prior art, waveguide grating router 300 also includes a plurality of variable loss elements 320. The plurality of variable loss elements 320 are optically coupled to a predetermined set of the output waveguides 310 to produce a predetermined composite amplitude spectrum in the output signals. Alternatively (not shown), variable loss elements 320 may be optically coupled to a predetermined set of input waveguides 302, instead of output waveguides 310, to produce a predetermined composite amplitude spectrum in the output signals. Variable loss permits shaping of the output signals from each waveguide independent of each other.

Functionally, the waveguide grating router of <figref idref="f0003">FIG. 3</figref> directs a multiwavelength-input signal through a predetermined one of the input waveguides 302, where the input signal is a multiplexed signal with wavelengths, λ₁, λ₂, ... λ_n. The input signal is demultiplexed into component wavelengths with each of the component wavelengths directed to a predetermined one of the output waveguides 302 of the router. Specifically, the input signal is transmitted through the first free space region 304 to the optical grating 306. Due to differing lengths and curvatures of each waveguide that are separated by a fixed amount within the optical grating 306, the optical grating 306 phase-shifts each signal transmitted through each waveguide. Ultimately, the multiwavelength signal is directed through the router and separates into a number of output signals each exhibiting a wavelength component of the original input signal. Variable loss elements 320 apply loss to each output signal in a predetermined amount to achieve the desired predetermined composite amplitude spectrum. Optionally, as illustrated in <figref idref="f0003">FIG 3</figref>, an amplifier 322 may be optically coupled to the plurality of output waveguides 310 to amplify the output signals after the loss is applied. The amplifier 322 will receive and amplify these output signals. Amplification is desirable in cases where a specific power level is required.

With simultaneous reference to <figref idref="f0003">FIG 3</figref> and <figref idref="f0002">FIG 4</figref>, where a uniform amplitude spectrum is shown, loss elements 320 of <figref idref="f0003">FIG 3</figref> can be configured to alter the output signals, producing such a uniform composite amplitude spectrum. For example, if a 1 X N waveguide grating router exhibiting a Gaussian amplitude spectrum, as shown in <figref idref="f0002">FIG. 2</figref>, is desired, the greatest loss should be introduced at the output waveguides closest to the center and less loss introduced at each adjacent port waveguide progressing toward the outermost waveguides.

Loss may be introduced in a variety of ways. One way is to vary the cross-sectional area of the output waveguides to the cross-sectional area of the input waveguide. This may be implemented, for example, by varying the waveguide widths, heights, cross-sectional shapes, position, and angular tilt. In addition, offsetting each connection of the output waveguides, for example, offset centers or intentional misalignment of the optical fiber which links the routing device to the rest of the multiwavelength lightwave system creates the same effect. Similarly, loss can be introduced by an output waveguide having different cross-sectional areas at different sections of the same output waveguide. For example, in <figref idref="f0004">Fig. 5</figref>, output waveguide 510 can have a larger cross-sectional area near the free space region 508 and a smaller cross-sectional area at the opposite end.

Other methods of introducing loss are also available. For example, loss can be introduced by "siphoning light" into a "dummy" waveguide. Such a dummy waveguide can be placed near the output waveguides so that a portion of the multiwavelength signal couples to the dummy waveguide. This is accomplished with directional coupler or a Y-branch splitter/tap. Still another way to introduce loss is through wavelength selective filters, such as asymmetrical Mach Zehnder interferometers or gratings, such as long period, Bragg and/or chirped.

Further, integrated loss may be made variable by placing an electrode over each output waveguide. Then, by electrically addressing each electrode with an appropriate signal, the loss, via refractive index changes, could be variably tuned. Optionally, the incorporation of a dopant similar to rare earth dopants for optical fibers, such as erbium, may be useful for tuning the loss where the loss elements are semiconductor devices.

One aspect of the invention is shown by the embodiment of the waveguide grating router in <figref idref="f0004">FIG. 5</figref>, which generates a uniform amplitude spectrum. The waveguide grating router 500 shown therein includes a plurality of input waveguides 502, a first free space region 504, an optical grating 506, a second free space region 508, and a plurality of output waveguides 510 - 518. The output waveguides 510 - 518 are designed in such a way as to differ in cross-sectional width from that of the input waveguides 502.

Loss is generated due to the different cross-sectional widths of the output waveguide. To produce output signals that exhibit a uniform amplitude spectrum, the greatest loss is introduced at the center waveguide and correspondingly less loss introduced at each adjacent waveguide thereafter. For example, in <figref idref="f0004">FIG 5</figref>, waveguide grating router 500 includes a plurality of output waveguides 510 - 518 having a center waveguide 514 and adjacent waveguides 512 and 516, as illustrated. The center waveguide 514 has a width of α, the adjacent waveguides 512 and 516 have a width of δ, and waveguides 510 and 518 have a width of η, where α&gt;δ&gt;η and each coordinate is adjusted in such a way that the router's output signals exhibit a uniform amplitude spectrum. Accordingly, the center waveguide 514 has the greatest cross-sectional width. Next, waveguides 512 and 516 adjacent to the center waveguides are smaller in cross-section than waveguide 514 and the waveguides 510 and 518 are smaller in cross-section than waveguides 512 and 516. These output signals incur loss proportional to the cross-sectional width of each output waveguide, thus, a uniform amplitude spectrum is achieved.

As discussed above, loss may be introduced by varying the cross-sectional area of an input waveguide with respect to that of an output waveguide. Variation in cross-sectional width of an input waveguide and an output waveguide with reference to one another generates a difference in field and, thus, proportional radiation losses. Mathematically, the loss is depends on an overlap integral between the fields at the input waveguides, for example, 702_i, i=1, 2, 3, ... N, in <figref idref="f0005">FIG 6</figref>, and the output waveguides, for example, 730_k, k=1, 2, 3, ...N, in <figref idref="f0005">FIG 6</figref>. The overlap integral, also known as the transmission coefficient between waveguides, for example i and k, is governed by: <maths id="math0001" num=""><math display="block"><msub><mi mathvariant="normal">T</mi><mrow><mi>i</mi><mo>,</mo><mi>k</mi></mrow></msub><mo>=</mo><mo>∬</mo><msub><mi>f</mi><mi>i</mi></msub><mfenced separators=""><mi>x</mi><mo>⁢</mo><mi>y</mi></mfenced><mo>⁢</mo><msub><mi>f</mi><mi>k</mi></msub><mfenced separators=""><mi>x</mi><mo>⁢</mo><mi>y</mi></mfenced><mo>⁢</mo><mi mathvariant="italic">dxdy</mi></math><img file="EP2273293A2_D0001.tif" /></maths> where f_i and f_k are the radiation fields at the cross-section of the input waveguide i and output waveguide k and x and y are the spatial coordinates that span the cross-section. Accordingly, the better the match or overlap the lower the loss therein. Normally, the fields between two waveguides are a combination of radiation fields and waveguide modes, whose number, extent, shape, character, etc., depend on the size, shape, and index of the waveguide. This overlap integral can also describe relative differences in angular tilt and spatial position. Thus, in accordance with the present invention, loss elements can be introduced to the input waveguides with respect to the loss elements introduced to the output waveguides to produce the desired predetermined composite amplitude spectrum.

Since the transmission coefficients T_ik of a N X N waveguide grating router form a N X N matrix, there are N² independent values that must be accounted for to introduce the desired losses to produce the desired predetermined composite amplitude spectrum. However, since the transmission T_ik coefficients values are interdependent, they can be modified (or equalized to achieve a uniform composite amplitude spectrum) by using only 2N adjustments to the waveguides. By choosing 2N radiation fields (or 2N cross-sectional widths in the waveguides) the N² transmission coefficients can be satisfied because not all of the radiation fields are unique and not all of the transmission coefficients are unique. Advantageously, complete equalization of a N X N waveguide grating router with N² possible transmission T_ik coefficients can be accomplished with 2(N-2) adjustments to the waveguides, as shown in <figref idref="f0005">FIG. 6</figref>.

As illustrated in <figref idref="f0005">FIG. 6</figref>, an embodiment of a N X N waveguide grating router 700 having N input waveguides and N output waveguides is shown. The waveguide grating router includes a plurality of input waveguides 702_i, a first free space region 704, an optical grating 706, a second free space region 708, and a plurality of output waveguides 730_k. A plurality of multiwavelength signals is received by the plurality of input waveguides 702. The signals are coupled at the first free space region 704. The coupled signal is launched into the optical grating 706. Due to differing lengths and curvature of each waveguide within the optical grating 706, where each waveguide is separated by a fixed amount, the optical grating 706 phase-shifts each signal transmitted through each output waveguide. Ultimately, the multiwavelength signals received by the plurality of input waveguides are transmitted through the router and separated into output signals, each having a specific wavelength component of the signals received. Since the cross-sectional widths of the plurality of input waveguides 702_i differ from the cross-sectional widths of the plurality of output waveguides 730_k, the signals suffer loss such that a uniform amplitude spectrum is achieved. The widths of the input waveguides 702_i are set in such a fashion as to allow the center input waveguide to experience the greatest loss and, correspondingly, each adjacent waveguide thereafter experiences less loss. The widths of the plurality of output waveguides 730_k are set such that the center output waveguide experience the least loss and, correspondingly, each adjacent waveguide thereafter experiences greater loss.

The exemplary embodiment described above is but one of a number of alternative embodiments of the invention that 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. Various other alternatives can be devised by one skilled in the art without departing from the teachings of this invention.