Abstract:
A flat-top arrayed waveguide grating with wideband transmission spectrum may be produced by integrating a series of directional couplers to the output slab waveguide coupler of a dual channel-spacing arrayed waveguide grating having Gaussian spectral profile. The primary channel spacing of the Gaussian arrayed waveguide grating determines the spectral width of the resultant wideband device, whereas the secondary channel spacing determines the wavelength separation between the adjacent output channels. In such a structure, a wideband or flat transmission spectral profile may be achieved without excessive losses.

Description:
BACKGROUND  
       [0001]     This invention relates generally to optical filters that may be useful for multiplexing and demultiplexing optical signals in wavelength division multiplexed communication networks.  
         [0002]     In wavelength division multiplexed optical signals, a plurality of different optical signals, each having a different wavelength, may be multiplexed over the same optical link. At intended destinations, one or more of the wavelength signals may be separated using a demultiplexing technique.  
         [0003]     An arrayed waveguide grating, also called a phased arrayed waveguide or phaser, works like a diffraction grating. It may be fabricated as a planar structure including input and output waveguides, input and output slab waveguides, and arrayed waveguides. The length of any arrayed waveguide may differ from adjacent waveguides by a constant ΔL.  
         [0004]     The input slab waveguide splits the wavelength channels among the arrayed waveguides. Each portion of the input light traveling through the arrayed waveguide includes all of the wavelengths that have entered the grating. Each wavelength in turn is individually phase shifted. As a result of that phase shift and phase shifts at the input/output slab waveguides, every portion of light at a given wavelength acquires different phase shifts. These portions may interfere at the output slab waveguide, producing a set of maximum light intensities. The direction of each maximum intensity depends on its wavelengths. Thus, each wavelength is directed to an individual output waveguide.  
         [0005]     Commercially available arrayed waveguide gratings have Gaussian transmission spectral transfer functions that are easy to manufacture. However, high speed applications usually involve flat or wideband profiles. Currently, such flat spectral shapes may be achieved by introducing a horn taper of various profiles, such as parabolic, exponential, sinc, Y-splitter, and the like, at the free propagation region of the arrayed waveguide grating. However, this approach leads undesirably to higher losses than conventional arrayed waveguide gratings and poses manufacturing challenges since horn tapers are very sensitive to fabrication tolerances.  
         [0006]     Thus, there is a need for low loss, wideband or flat top arrayed waveguide gratings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is top plan view of one embodiment of the present invention;  
         [0008]      FIG. 2  is an enlarged plan view of a portion of the embodiment shown in  FIG. 1 ;  
         [0009]      FIG. 3  is a calculated plot of transmission versus wavelength for the input to the coupler  22  of  FIG. 1  in one embodiment;  
         [0010]      FIG. 4  is a calculated plot of transmission versus wavelength for the output from the coupler  22  shown in  FIG. 1  in accordance with one embodiment of the present invention; and  
         [0011]      FIG. 5  is a top plan view of a portion of an alternate embodiment in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]     Referring to  FIG. 1 , a planar lightwave circuit  10  may include an arrayed waveguide grating. An input waveguide  12  is coupled to an input slab waveguide  14   a . The output waveguides  24  are coupled to an output slab waveguide  14   b  through a directional coupler  22 . A slab waveguide, also called a free propagation region, confines light in one dimension, usually the vertical dimension, and does not significantly confine the light in another dimension, typically the horizontal dimension, such as the plane of the circuit  10 .  
         [0013]     The directional coupler  22  is coupled to the output slab waveguide  14   b  through the waveguides  18  and  20 . In one embodiment, the directional coupler  22  may be approximately a 3-dB coupler. The waveguide array  16 , connecting the slab waveguides  14   a  and  14   b , may include a plurality of waveguides. The difference in length of the successive waveguides in the array is ΔL.  
         [0014]     The arrayed waveguide grating may be a dual channel spacing device with Gaussian transmission spectral profile. Referring to  FIG. 2 , the dual channel spacing device may consist of a primary channel spacing  26  (for example that between the waveguides  18   a  and  20   a ) and a secondary channel spacing  28  (that between the waveguides  18   a  and  18   b ). An array of directional couplers  22  are integrated with the arrayed waveguide grating to achieve the desired transmission spectral profile. The waveguide separation between the adjacent channel pairs coupled to the same couplers  22 , i.e., the spacing between the waveguides  18   a  and  20   a ,  18   b  and  20   b , on the output slab waveguide  14   b , determines the overall spectral width of the transmission profile of the arrayed waveguide grating. This separation is chosen to be an appropriate fraction of the secondary channel spacing to achieve the desired balance between bandwidth, cross-talk, and insertion loss.  
         [0015]     The phase difference between the optical beams entering the directional couplers  22  is controlled by choosing appropriate path length difference between the corresponding output waveguides  18  and  20  of the arrayed waveguide grating. As a result, light exits from the intended output waveguide  24  of the directional coupler  22 .  
         [0016]     For example, in order to get a flat spectral shape, two successive output waveguides  18  and  20  of the arrayed waveguide grating that are input to the couplers  22 , have a length difference equal to approximately (2 m+1)λ c /4n eff , where m is an integer, λ c  is the average center wavelength, and n eff  is the effective refractive index of the two respective waveguides.  
         [0017]      FIG. 3  shows a calculated plot of transmission versus wavelength. The signals A 1  and A 2 , centered on approximately 1554 nanometers, in this case, correspond to the signals from the waveguides  18   a  and  20   a . Similarly, the spectra B 1  and B 2  are the signals from the waveguides  18   b  and  20   b  and, likewise, the signals C 1  and C 2  are the signals from the next pair of output waveguides.  
         [0018]     The primary channel spacing  26  between the signals A 1  and A 2  determines the width of the resulting signal, shown in  FIG. 4 , from the couplers  22 . That is, the signal A is the resultant of the signals A 1  and A 2 . Thus, the spacing between the signals A 1  and A 2  determines the width of the signal A. Similarly, the signal B is the resultant of the signals B 1  and B 2  by the coupler  22   b.    
         [0019]     The secondary channel spacing  28 , between the signal A and the signal B, is determined by the spacing between the waveguide  18   a  and the waveguide  18   b . This secondary channel spacing determines the wavelength separation between the adjacent output channels of the overall integrated multiplexer/demultiplexer device.  
         [0020]     Thus, in one embodiment, the first and second waveguides  18 ,  20  have a primary channel spacing  26  that is about one-quarter the secondary channel spacing between the first and third waveguides  18 . In other embodiments, different primary and secondary channel spacings may be desirable, but in a variety of cases, it may be desirable to space the individual waveguides  18 ,  20  of a pair by a smaller separation than successive waveguides  18  are spaced from one another.  
         [0021]     Referring to  FIG. 4 , calculated results from a representative device of the type shown in  FIG. 1  are presented for the coupler  22  output. The secondary channel spacing  28  of the Gaussian arrayed waveguide grating is 100 GHz in this example, whereas the primary spacing between the waveguides  18  and  20 , coupled to the coupler  22 , is four times smaller, i.e., 25 GHz, in this example. As shown in  FIG. 2 , the ability to obtain a flat-top spectral shape of the arrayed waveguide grating by the technique described above is demonstrated. The simulated results indicate only about a 1-dB excess loss compared to the Gaussian arrayed waveguide grating and approximately 40-dB cross-talk.  
         [0022]     The engineering of the spectral shape of an arrayed waveguide grating can be implemented by monolithic or hybrid integration approaches. The arrayed waveguide grating and directional coupler structures may be fabricated on the same chip in the monolithic approach, while they are fabricated separately and later bonded together in the hybrid approach.  
         [0023]     Referring to  FIG. 5 , in accordance with another embodiment of the present invention, the slab waveguides  15   b  may be coupled to a multi-mode interference (MMI) coupler  34  in another embodiment of the present invention. In this embodiment, the MMI coupler  34  replaces the directional coupler  22 . In this embodiment, the output waveguides  30  and  32  need not be of different lengths. However, the primary and secondary channel spacings may be as described previously in accordance with the embodiment shown in  FIG. 2 . As before, the coupler  34  may be coupled to an output waveguide  36 .  
         [0024]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.