Abstract:
The present invention relates to an array waveguide grating and a method of manufacture thereof, in which a substantially smooth wavelength-loss characteristics is obtained in a pass band, rather than increasing the loss. In an array waveguide grating ( 10 ), wavelength multiplex division signal light is demultiplexed by a channel waveguide array ( 15 ), reflected by a Fabry-Perot resonator array ( 16 ), and passed through the channel waveguide array ( 15 ) again, so that a substantially smooth wavelength-loss characteristic is obtained in the pass band, rather than increasing the loss.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an array waveguide grating and a manufacturing method of the same.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 6 of the accompanying drawings is a plan view showing an example of conventional array waveguide gratings.  
           [0005]    The array waveguide grating  1  shown in this figure is includes a substrate  2 , an input channel waveguide  3  formed on top of the substrate  2  for accepting wavelength-multiplexed signal light at one end thereof (in the figure, the left end), an input-side fan-shaped slab waveguide  4  connected to the input channel waveguide  3  at one end thereof, an array of a plurality of channel waveguides  5 , one end of which array is connected to the other end (in the figure, the right end) of the input-side fan-shaped slab waveguide  4  such that lengths of the channel waveguides  5  become successively longer by a desired variation, an output-side fan-shaped slab waveguide  6 , one end of which is connected to the other ends of the array channel waveguides  5 , and a plurality of output channel waveguides  7 , each of which has one end connected to the other end of the output side fan-shaped slab waveguide  6  for outputting signal lights which have been split.  
           [0006]    Signal light of various wavelengths (λ1, λ2, . . ., λn) introduced to the input channel waveguide  3  is spread out in the input-side slab waveguide  4  by diffraction because it is not trapped in the transverse direction (surface direction of the substrate  2 ) in the input-side fan-shaped slab waveguide  4 , and is propagated along the array of channel waveguides  5 . In this array  5 , the channel waveguides extend different lengths (successively elongated by a desired length) so that equiphase planes of the signal light are inclined in accordance with the wavelengths of the signal light. Thus, the points of convergence of the signal light shift at the connecting face between the output-side fan-shaped slab waveguide  6  and the output channel waveguides  7 .  
           [0007]    Therefore, the signal light of different wavelengths is incident on the output channel waveguides  7  respectively, and the signal light of different wavelengths (λ1, λ2, . . ., λn) is outputted from the output ends of the output channel waveguides  7  respectively.  
           [0008]    [0008]FIGS. 7A and 7B are diagrams showing the relationship between converging beam fields of the output-side fan-shaped slab waveguide and fields of the output channel waveguides of the array waveguide grating shown in FIG. 6. FIGS. 8A and 8B are diagrams of wavelength-loss characteristics of the array waveguide grating  1  shown in FIG. 6, in which the horizontal axis indicates wavelength and the vertical axis indicates loss.  
           [0009]    The loss of the array waveguide grating  1  is proportional to the overlap integral (hatched portion) of the electric field distribution φf of the converging beam and the electric field distribution φo of the output waveguides as shown in FIGS. 7A and 7B. Because the electric field of the output channel waveguide  7  has a single-peaked distribution, the loss is lowest at the wavelength that matches the peak of the converging beam φf, making for a peaky wavelength-loss characteristic (FIG. 8A).  
           [0010]    Accordingly, a waveguide structure that uses Y-branching or a parabolic shape is employed such that the electric field distribution of the input channel waveguide  3  (or the output channel waveguide  7 ) has a dual-peaked shape as shown in FIG. 7B. In this case, even if the points of convergence of the beams shift to a certain extent, the value of the overlap integral hardly changes, making for a flat wavelength-loss characteristic (FIG. 8B).  
           [0011]    [0011]FIG. 9 is an enlarged view of part of the wavelength-loss characteristic of the array waveguide grating shown in FIG. 6. The horizontal axis indicates the wavelength, and the vertical axis indicates the loss.  
           [0012]    When the electric field distribution of the output waveguide (or the input waveguide) has a dual-peaked shape, it is difficult to achieve total flatness in the wavelength-loss characteristics distribution. Further, 2 to 3 dB of additional loss is generated to realize the flatness. When application in a wavelength-multiplexed optical telecommunications system is considered, this loss increase is a problem and loss improvement must be made. When a yet flatter wavelength-loss characteristics is sought in this structure, another problem arises; a ripple appears in the transmission region as shown in FIG. 9, greatly affecting transmission characteristics.  
           [0013]    Accordingly, an object of the present invention is to provide an array waveguide grating, which solves the above-mentioned problems, and which has approximately flat wavelength-loss characteristics in the transmission region without causing an increase in loss, and a manufacturing method of the array waveguide grating.  
         SUMMARY OF THE INVENTION  
         [0014]    An array waveguide grating of the present invention comprises a substrate, an input channel waveguide formed on top of the substrate for receiving wavelength-multiplexed signal light at one end thereof; a plurality of output channel waveguides disposed parallel to the input channel waveguide for outputting split output signal light from respective one ends thereof; a fan-shaped slab waveguide connected to the other end of the input channel waveguide at one end thereof and to the other ends of the output channel waveguides at the same one end thereof, an array of a plurality of channel waveguides connected to the other end of the fan-shaped slab waveguide at one end thereof such that lengths of the channel waveguides are successively elongated by a desired variation ΔL, and an array of Fabry-Perot resonators connected to the other end of the array of channel waveguides at one end thereof such that lengths of the resonators are successively elongated by twice the channel waveguide length variation ΔL.  
           [0015]    The signal light obtained by splitting the wavelength-multiplexed signal light in the array of channel waveguides is reflected inside the Fabry-Perot resonators, and this split signal light is caused to pass through the array of channel waveguides once again. Therefore, the array waveguide grating provided by the invention has approximately flat wavelength-loss characteristics in the transmission region without increasing loss.  
           [0016]    Preferably a reflectance of the Fabry-Perot resonator array at its face connecting with the channel waveguide array is approximately 11%, and a reflectance at its face opposite the connecting face is approximately 100% (full reflection).  
           [0017]    The face of the Fabry-Perot resonator array in contact with the channel waveguide array may be coated with a metallic film having a reflectance of approximately 11%, and the opposite face may be coated with either a metallic film having a reflectance of approximately 100%, or a multilayer dielectric film of TiO 2  and SiO 2  having a reflectance of approximately 100%.  
           [0018]    The Fabry-Perot resonator array may be constituted from an array of semicircular concentric channel waveguides, respective one ends of which are connected to the other ends of the array channel waveguides such that lengths of the semicircular concentric channel waveguides become successively longer by twice the waveguide length variation ΔL, a reflective film formed between the channel waveguide array and semicircular channel waveguide array and having a reflectance of approximately 11%, and another reflective film formed at the other end of the semicircular channel waveguide array and having a reflectance of approximately 100%.  
           [0019]    Alternatively the Fabry-Perot resonator array may be constituted from a second array of channel waveguides, respective one ends of which are connected to the other ends of the first array of channel waveguides such that lengths of the second channel waveguides are successively elongated by twice the first waveguide length variation ΔL, a first groove transversely crossing the first and second channel waveguide arrays at the connecting portion of these arrays, a reflective film fitted in the first groove and having a reflectance of approximately 11%, a second groove transversely extending across the second channel waveguide array midway, and another reflective film fitted in the second groove and having a reflectance of approximately 100%.  
           [0020]    The reflective film of the array waveguide grating may be formed by vapor deposition of a metallic film of silicon, gold or similar, on top of a thin film of polyimide.  
           [0021]    In the array waveguide grating, preferably the polyimide reflective film, on which the metallic film has been deposited by vapor deposition, is affixed using optical resin.  
           [0022]    In the array waveguide grating, preferably the channel waveguides are formed either on top of a quartz substrate or a silicon substrate, and constituted by a material having quartz glass as a principal constituent.  
           [0023]    Preferably the channel waveguides are primarily made from InP.  
           [0024]    A method of manufacturing an array waveguide grating according to the present invention includes the steps of forming, in parallel on top of a substrate, an input channel waveguide adapted to receive wavelength-multiplexed signal light at one end thereof, and a plurality of output channel waveguides adapted to output split output signal light from one end thereof forming a fan-shaped slab waveguide on the same substrate such that one end thereof is connected to the other end of the input channel waveguide and the other ends of the output channel waveguides; forming at the other end of the fan-shaped slab waveguide on the same substrate an array of a plurality of channel waveguides, which becomes successively longer by a desired waveguide length variation ΔL; forming, on top of a second substrate, an array of Fabry-Perot resonators which become successively longer by twice the waveguide length variation ΔL; and connecting the first substrate to the second substrate such that one end of the Fabry-Perot resonator array connects to the other end of the channel waveguide array.  
           [0025]    In the manufacturing method for the array waveguide grating according to the present invention, the Fabry-Perot resonator array may be constituted from an array of semicircular concentric channel waveguides formed on top of the second substrate such that lengths of the semicircular concentric channel waveguides become successively longer by twice the waveguide length variation ΔL, a reflective film formed at one end of the array of semicircular concentric channel waveguides and having a reflectance of approximately 11%, and another reflective film formed at the other end of the array of semicircular concentric channel waveguides and having a reflectance of approximately 100%, and the first substrate may be connected to the second substrate such that the reflective film having a reflectance of approximately 11% is inserted between the other end of the array of channel waveguides and the one end of the array of the semicircular concentric channel waveguides.  
           [0026]    Alternatively, in the manufacturing method for the array waveguide grating according to the present invention, the Fabry-Perot resonator array may be constituted from a second array of channel waveguides which are formed on the same substrate such that lengths of these channel waveguides are successively elongated by twice the waveguide length variation ΔL and such that respective one ends thereof are connected to the other ends of the first array of channel waveguides, a first groove intersecting with both the first and second arrays of channel waveguides at a connecting portion of the first and second arrays of channel waveguides, a reflective film having a reflectance of approximately 11% and fitted in the first groove, a second groove intersecting with the second array of channel waveguides midway, and another reflective film having a reflectance of approximately 100% and fitted in the second groove.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1A is a plan view showing an embodiment of an array waveguide grating of the present invention;  
         [0028]    [0028]FIG. 1B illustrates a cross-sectional view taken along the line A-A in FIG. 1A;  
         [0029]    [0029]FIG. 1C also illustrates a cross-sectional view taken along the line A-A but depicts another embodiment of an array waveguide grating as shown in FIG. 1A;  
         [0030]    [0030]FIG. 2 is a diagram showing phase changes relative to wavelengths of light propagating each channel of the array waveguide grating shown in FIG. 1.  
         [0031]    [0031]FIG. 3 is a diagram showing the wavelength-loss characteristics of the array waveguide grating shown in FIG. 1;  
         [0032]    [0032]FIG. 4 is a plan view showing another embodiment of an array waveguide grating according to the present invention;  
         [0033]    [0033]FIG. 5A is a plan view showing still another embodiment of an array waveguide grating according to the present invention;  
         [0034]    [0034]FIG. 5B is a structural diagram of a reflective film inserted in a first groove shown in FIG. 5A;  
         [0035]    [0035]FIG. 5C illustrates a structure of a reflective film inserted in a second groove shown in FIG. 5A;  
         [0036]    [0036]FIG. 6 is a plan view showing an example of conventional array waveguide gratings;  
         [0037]    [0037]FIG. 7A is a diagram showing relationship between a converging beam field in an output-side fan-shaped slab waveguide and an output channel waveguide field having a single-peaked field distribution, in the array waveguide grating shown in FIG. 6;  
         [0038]    [0038]FIG. 7B is a diagram showing relationship between the converging beam field in the output-side fan-shaped slab waveguide and the output channel waveguide field having a dual-peaked field distribution, in the array waveguide grating shown in FIG. 6;  
         [0039]    [0039]FIG. 8A illustrates a diagram of wavelength-loss characteristics of an array waveguide grating, in which are formed output channel waveguides having the single-peaked field distribution shown in FIG. 7A;  
         [0040]    [0040]FIG. 8B illustrates a diagram of wavelength-loss characteristics of an array waveguide grating, in which are formed output channel waveguides having the dual-peaked field distribution shown in FIG. 7B; and  
         [0041]    [0041]FIG. 9 illustrates an enlarged view of part of a wavelength-loss characteristics of the array waveguide grating shown in FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    Embodiments of the present invention will be described in reference to the drawings.  
         [0043]    An array waveguide grating  10  shown in FIG. 1A includes a substrate  11 , an input channel waveguide  12  formed on top of the substrate  11  for receiving wavelength-multiplexed signal light at one end thereof, a plurality of output channel waveguides  13  disposed in parallel to the input channel waveguide  12  for outputting split output signal light from respective one ends thereof, a fan-shaped slab waveguide  14  connected to the other end of the input channel waveguide  12  at one end thereof and to the other ends of the output channel waveguides  13  at the same end thereof, an array of a plurality of channel waveguides  15  connected to the other end of the fan-shaped slab waveguide  14  at one end thereof such that lengths of the channel waveguides  15  are successively elongated by a desired variation ΔL, and an array of Fabry-Perot resonators  16  connected to the other end of the array of channel waveguides  15  at one end thereof such that lengths of the resonators  16  are successively elongated by twice the channel waveguide length variation ΔL.  
         [0044]    In this array waveguide grating  10 , because most of the light from the array of channel waveguides  15  is reflected by the latter stage (right side of the figure) Fabry-Perot resonator array  16  and returns, the split signal light is outputted from the output channel waveguides  13 , which are positioned on the same side as the input channel waveguide  12 . The Fabry-Perot resonator array  16  has a second array of channel waveguides  17 , a reflective film (made of silicon)  18   a  having a reflectance of approximately 11%, and another reflective film (made of TiO 2 /SiO 2 )  18   b  having a reflectance of approximately 100%. The right end face of the channel waveguide array  15  and the left end face of the Fabry-Perot resonator array  16  are connected to each other by an optical adhesive  19 .  
         [0045]    The lengths of the channel waveguides in the array  17  of the Fabry-Perot resonator array  16  become successively longer by ΔP, as is the case with the array  15 . The relationship between the waveguide length variation ΔL and the waveguide length variation ΔP is expressed in Equation (1):  
         Δ P= 2 ×ΔL    (1)  
         [0046]    Here, ΔP is the waveguide length variation in the second channel waveguide array  17 , and ΔL is the waveguide length variation in the first channel waveguide array  15 .  
         [0047]    Furthermore, both the arrays of channel waveguides  15  and  17  have a structure as shown in FIG. 1A wherein cores  20  are formed on top of a quartz substrate  11   g , and these cores  20  are covered by a cladding  21 , or a structure as shown in FIG. 1B wherein a buffer layer  22  is formed on top of a silicon substrate  11   s , cores  20  are formed on top of the buffer layer  22 , and these cores  20  are covered by a cladding  21 . The buffer layer  22  is provided for suppressing the leakage of light to the silicon substrate  11   s  from the cores  20 .  
         [0048]    The operating principle of the array waveguide grating shown in FIG. 1A will be described below.  
         [0049]    Signal light of various wavelengths (λ1, λ2, . . ., λn) introduced to the input channel waveguide  12  is spread out by diffraction because it is not trapped in the transverse direction in the fan-shaped slab waveguide  14 , and the signal light is propagated in the array of channel waveguides  15 . The signal light emitted from the array of channel waveguides  15  is partially reflected by the reflective film  18  formed on the Fabry-Perot resonator array  16 , and the remaining signal light is propagated along the array of channel channel waveguides  17  and reflected approximately 100% by the reflective film  18 . The reflected signal light travels back and forth in the Fabry-Perot resonator array  16  numerous times and ultimately returns to the fan-shaped slab waveguide  14 . “S” indicates the equiphase plane of the signal light inside the array of channel waveguides  15 , which has been reflected and returned from the Fabry-Perot resonator array  16 . Because the signal light progresses inside the fan-shaped slab waveguide  14  in a perpendicular direction to this equiphase plane S, it converges at the one end of the fan-shaped slab waveguide  14  and enters respective ones of the output channel waveguides  13  before it is output.  
         [0050]    It is understood from FIG. 2 that the stepwise phase change occurs relative to the wavelength change. This signifies that the gradient of the equiphase plane changes stepwise relative to the wavelength. That is, the point of convergence of the light stays within a certain range of wavelengths, but immediately moves when the wavelength is deviated from that range.  
         [0051]    It is understood from FIG. 3 that compared to a conventional array waveguide grating, loss is not sacrificed, and a rectangular wavelength-loss characteristic without ripples is achieved in the transmission region.  
         [0052]    Next, another embodiment of the array waveguide grating according to the present invention will be described.  
         [0053]    The difference between this embodiment and the embodiment shown in FIG. 1 lies in that an array of channel waveguides  17   a  in a Fabry-Perot resonator array  16   a  is formed in a concentric semicircular shape.  
         [0054]    Specifically, the Fabry-Perot resonator array  16   a  is constituted from the semicircular array of concentric channel waveguides  17   a , respective ends of which are connected to the other ends of the array channel waveguides  15  such that the waveguides  17   a  become successively longer by a length of twice the waveguide length variation ΔL, a reflective film  18   a  having a reflectance of approximately 11% and formed between the first array of channel waveguides  15  and the second array of channel waveguides  17 , and another reflective film  18   b  having a reflectance of approximately 100% and formed at the other end of the array of channel waveguides  17   a.    
         [0055]    By employing such a Fabry-Perot resonator array  16   a , a compact-structure array waveguide grating  10   a  is achieved.  
         [0056]    Next, still another embodiment of an array waveguide grating according to the present invention will be described.  
         [0057]    The difference between this embodiment and the embodiment shown in FIG. 1 is that a Fabry-Perot resonator array  16   b  is integrated with an array waveguide grating  10   b.    
         [0058]    That is, this array waveguide grating  10   b  includes a substrate  11   a , an input channel waveguide  12  formed on top of the substrate  11   a  for accepting wavelength-multiplexed signal light at one end thereof, an output channel waveguide  13  disposed in parallel to the input channel waveguide  12  for outputting split output signal light from one end thereof, a fan-shaped slab waveguide  14  connected to the other end of the input channel waveguide  12  at one end thereof, and to the other end of the output channel waveguide  13  at the same end thereof, an array of a plurality of channel waveguides  15 , one end of which is connected to the other end of the fan-shaped slab waveguide  14  such that lengths of the channel waveguides  15  are successively elongated by a desired variation ΔL, another array of channel waveguides  17 , one end of which is connected to the other end of the first channel waveguide array  15  such that lengths of the second channel waveguides  17  are successively elongated by twice the channel waveguide length variation ΔL, a first groove  23  transversely extending across the channel waveguide arrays  15  and  17  at the connecting portion of these arrays, a reflective film  18   a  fitted in the first groove  23  with a reflectance of approximately 11%, a second groove  24  extending transversely across the second channel waveguide array  17  midway, and a reflective film  18   b  inserted in the second groove  24  with a reflectance of approximately 100%.  
         [0059]    The reflective film  18   a  includes a polyimide film  25  of a thickness of about 10 μm, and an Si film  26  of a submicron thickness formed on top of the polyimide film  25 . The reflective film  18   b  is constituted from the polyimide film  25  of a thickness of about 10 μm, and a multi-layered film  27  of TiO 2 /SiO 2  formed on top of the polyimide film  25 .  
         [0060]    The same advantages as with the array waveguide grating  10  shown in FIG. 1 are also achieved with the array waveguide grating  10   b . Further, since the Fabry-Perot resonator array  16   b  is integrally formed in the array waveguide grating  10   b , optical axis alignment and connection between the first array of channel waveguides  15  and the second array of channel waveguides  17  are not required, enabling the number of production processes to be reduced.  
         [0061]    This application claims priority of Japanese Patent Application No. 11-350257 filed Dec. 9, 1999, the entire disclosure of which is incorporated herein by reference.  
       INDUSTRIAL APPLICABILITY  
       [0062]    The present invention is applicable to an array waveguide grating and a manufacturing method therefor.