Abstract:
An integrated optical signal wavelength demultiplexing device, which may simultaneously demultiplex and detect an optical signal, is discussed. The integrated device features a waveguide structure to carry an optical signal, a photodetector in close proximity to the waveguide structure, and a wavelength limiting grating structure integrated with the photodetector and coupling the waveguide structure to the photodetector. The grating structure is fabricated within the photodetector and is used to transmit only a selected wavelength onto the photodetector.

Description:
RELATED APPLICATION  
       [0001]     This application is a continuation of U.S. Application filed on Jun. 30, 2006, attorney docket no. 3230.1008-001, entitled “Integrated Thin Film MSM Photodetector/Grating for WDM”, inventor Zhaoran Huang, which claims the benefit of U.S. Provisional Application No. 60/696,478, filed on Jul. 1, 2005. The entire teachings of the above application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     In optical communication systems, wavelength division multiplexing (WDM) is commonly used to transport information. Optical WDM is a technology where multiple sources of information are combined, resulting in a multi-channel signal, on a single optical fiber by using different wavelengths of laser light to carry the different signals. A demultiplexer is typically used at a receiver to separate the signal into its respective wavelengths. WDM systems allow for an expansion of the capacity of a network without increasing the amount of optical fiber utilized.  
         [0003]     An optical communication system  100  is shown as  FIG. 1 . Various chips  1 - 4  are interconnected with the use of optical waveguides. The chips  0 - 4  all comprise photo-detection devices as well as wire bonding pads. Multi-channel signals are transmitted to the optical communication system via optical inputs  1  and  2 . Optical waveguide splitters  101  are then used to branch off or distribute the optical signals to the various chips  1 - 4 . Once a multi-channel signal reaches a photo-detection device  103 , the photo-detection device  103  demultiplexes the multi-channel signal and detects the selected wavelength. The wire bonding pads  105  may be used to supply electronic power to the various chips  1 - 4 .  
         [0004]     A typical optical waveguide  200 , as shown in  FIG. 2 , comprises three regions, an upper cladding  204 , a core  205 , and an under cladding  206 . The main purpose of the optical waveguide is to guide light waves through the use of total internal reflection. In order for total internal reflection to occur, the core of the waveguide must have a higher refractive index than the upper and under claddings and the angle of incidence of the light beam must be at an angle less than a critical angle. Thus, the optical signal may travel through the core of the waveguide while reflecting from the top and bottom surfaces of the core. As is shown in  FIG. 2 , a multi-channel signal comprising various wavelengths,  201  and  203 , may be transmitted through the core  205  of the waveguide by total internal reflection.  
         [0005]     As was previously mentioned, in order to detect a signal of a particular wavelength from a multi-channel signal, the multi-channel signal may be demultiplexed. One method of demultiplexing involves the use of a diffraction grating. A diffraction grating  300 , shown in  FIG. 3 , consists of multiple gratings or peaks  302 . When a polychromatic light source impinges on a diffraction grating, each wavelength is diffracted at a different angle and therefore to a different point in space. As an incoming multi-channel signal  301  approaches the grating  300 , the signal is reflected and separated into its respective wavelengths  303  and  305 .  
         [0006]     Upon demultiplexing, photo-detection may be performed. Many photo-detection devices may be used in the detection of the optical signal; as examples a p-i-n, avalanche or metal-semiconductor-metal (MSM) photodetector may be employed. An MSM photodetector is shown in  FIGS. 4A and 4B . The MSM photodetector  400  comprises a metal contact superimposed on various semiconductor layers. The MSM photodetector semiconductor layers  408  comprise a cap layer  404 , an absorbing layer  402 , a buffer layer  405 , and a thinned substrate layer  401 . The MSM photodetector  400  also features MSM electrodes  406  which are interdigitated Schottky metal contacts on top of the MSM cap layer  404 . Once the active or absorbing layer  402  is illuminated by an optical signal or light  407 , electron-hole pairs or carriers  403  are generated within the layer. The carriers are transported to the contact pads  407  which are supplied with a voltage. The MSM photodetector detects photons by collecting electric signals generated by photo-excited electrons and holes in the semiconductor  408  that drift to respective interdigitated fingers under the electrical field applied between the interdigitated fingers  406 .  
         [0007]     Detailed examples of a photo-detection device, as shown in  FIG. 1 , are displayed in  FIGS. 5A and 5B . The photo-detection device of  FIG. 5A  comprises an optical polymer waveguide  501  superimposed over an MSM photodetector  400 . As was discussed above, the MSM photodetector  400  comprises a thinned substrate  401 , a cap layer  404 , an absorbing layer  402 , a buffer layer  405 , and electrodes  406 . The polymeric waveguide  501  comprises an upper cladding  502 , a core  504 , and an under cladding  505 . The upper cladding  502  further comprises a diffraction grating  503 . As was discussed above in relation to  FIG. 3 , the diffraction grating is used to demultiplex the multi-channel signal traveling in the core of the optical polymer waveguide. The grating is designed such that only light of one particular wavelength (in the example provided by  FIG. 5A , λ 1 ) will be selected at angle such that the signal of that wavelength will be reflected into the MSM photodetector  400 .  
         [0008]     The photo-detection device of  FIG. 5B  works in a manner similar to that of the device shown in  FIG. 5A . The photo-detection device of  FIG. 5B  comprises a polymeric waveguide  506  and an MSM photo-detector  400 , similar to the MSM photodetector of  FIG. 5A . The polymeric waveguide comprises an upper cladding  507 , a core  508 , and an under cladding  509 . The most striking difference between the photo-detection devices featured in  FIGS. 5A and 5B  is that the diffraction grating featured in waveguide  506  is fabricated within the core  508  of the waveguide instead of the upper cladding  507 . Both the gratings in  FIGS. 5A and 5B , may be used to reflect light of a selected waveguide into the MSM photodetector  400 .  
       SUMMARY OF THE INVENTION  
       [0009]     An integrated optical signal wavelength demultiplexing device and method is discussed. The device comprises a waveguide structure to carry an optical signal, a photodetector in close proximity to the waveguide structure, and a wavelength limiting grating structure integrated with the photodetector and coupling the waveguide structure to the photodetector. The photodetector may be in the form of a metal-semiconductor-metal (MSM) photodetector, the MSM photodetector may further comprise a cap layer, an absorbing layer, a buffer layer and a substrate, wherein all these layers may be formed in semiconductor material with a grating structure formed in a side of the MSM photodetector opposite of the electrodes. The MSM photodetector may also be backside illuminated.  
         [0010]     The waveguide structure may comprise a top cladding, an optical signal carrier core, and an under-cladding layer. The waveguide structure may also be formed in a material comprising a lower index of refraction than the photodetector and the grating structure, for example polymer. The grating structure may be filled with material of the waveguide. The optical signal may be evanescently coupled from the waveguide to the photodetector. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.  
         [0012]      FIG. 1  is a schematic of an optical communication system;  
         [0013]      FIG. 2  is a diagram for a multi-channel signal traveling through an optical waveguide;  
         [0014]      FIG. 3  is a schematic of an illustrated example of the use of a diffraction grating;  
         [0015]      FIGS. 4A and 4B  are a top and cross-sectional view, respectively, of an MSM photodetector;  
         [0016]      FIGS. 5A and 5B  are schematics of photo-detection devices according to the prior art.  
         [0017]      FIGS. 6A and 6B  are cross section views of a photo-detection device according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     A description of preferred embodiments of the invention follows.  
         [0019]     Many problems may arise when fabricating a diffraction grating from a polymer substance such as the polymeric waveguides of  FIGS. 5A and 5B . In particular, long term reliability is often an issue for polymer based optical devices. Since the molecular arrangement of a polymeric material is not very solid, over time the grating may deform.  
         [0020]     A high-speed demultiplexing and detection embodiment of the present invention is shown in  FIG. 6A .  FIG. 6A  displays an MSM photodetector  400  mounted on a substrate  602 . The MSM photodetector  400  comprises a cap layer  404 , an absorbing layer  402 , a buffer layer  405 , and electrodes  406 . It should be appreciated that the MSM photodetector  400  may include a thinned substrate layer above the buffer layer  405 . The MSM photodetector  400  is arranged in a manner to allow for backside illumination. In backside illumination, an optical signal is directed to the absorbing layer  402  through the buffer layer  405 , or alternatively the thinned substrate layer  401 ; while in frontside illumination, the signal is directed to the absorbing layer  402  through the electrodes  406  and cap layer  404 .  
         [0021]     When employing frontside illumination, a problem of finger shadowing commonly occurs. Since the electrode fingers  406  are not transparent, during frontside illumination a portion of the optical signal may be reflected off of the fingers  406 , thus causing loss which is also known as finger shadowing. Backside illumination prevents finger shadowing and thereby also reduces the amount of loss suffered by the incoming optical signal. The buffer layer  405  does not absorb the incoming signal but instead reduces defect density between the substrate and the absorbing layer, thus reducing leakage current and increasing response speed.  
         [0022]     The buffer layer  405 , shown in  FIGS. 6A and 6B , comprises an MSM grating  603 . Unlike the diffraction gratings shown in  FIGS. 5A and 5B , the MSM grating  603  is fabricated completely out of a semiconductor material (i.e. the MSM buffer layer  405 ). Thus, the issue of long term reliability is no longer a problem as the molecular arrangement of a semiconductor material is much more solid and durable than that of a polymer. Furthermore, as seen in  FIG. 6B , the optical polymer waveguide  605  is superimposed on the buffer layer  405  of the MSM photodetector  400 . Thus, the MSM grating  603  is further protected by being embedded in the material of the waveguide  605 . It should be appreciated that a semiconductor waveguide may be used. In that case, the grating gaps may be filled with waveguide semiconductor or left open.  
         [0023]     The MSM grating  603  shown in  FIGS. 6A and 6B  does not function in the same manner as the diffraction gratings shown in  FIGS. 3, 5A  and  5 B. As explained above, diffraction gratings reflect and separate a multi-channel signal into various waveguide components at different angles. An MSM grating instead transmits a single wavelength in to the absorption layer  402  of the MSM photodetector  400 . Therefore, as the multi-channel signal travels through the optical waveguide  605 , the selected wavelength is evanescently coupled into the active region  402  of the MSM photodetector  400 . Wavelength selection may be determined by the spacing of the grating peaks  601 .  
         [0024]     The photo-detection device of  FIGS. 6A and 6B  also allows for simultaneous demultiplexing and detection of an optical signal. Thus, with use of the device featured in  FIGS. 6A and 6B , it is possible to increase the speed of optical communication networks by combining the steps of demultiplexing and detection into a single step. The fabrication of the MSM grating  603  in the semiconductor material may also be easily obtained compared to the fabrication of the diffraction grating in the polymer waveguide.  
         [0025]     Furthermore, the semiconductor material generally has a refractive index above n=3, for example, n=3.2 for Si, n=3.5 for InP, and n=3.65 for InGaAs; whereas, the refractive index of polymeric material is in the range of n=1.4˜1.8. Therefore, a high contrast of refractive index is obtained at the interface of the grating structure with the polymeric waveguide. The high index difference at the grating structure  603  is desirable for easily creating a long period grating pitch, or ensuring that the grating period is larger than the wavelength of the selected wavelength. A high extraction ratio, or the amount of the optical signal which may be coupled, may also be obtained with a high contrast of refractive index.  
         [0026]     Although the gratings have been shown in the buffer layer  405  of the MSM detector  400 , it should be appreciated that other alterations may be possible. For example; the grating structure  603  may be fabricated in the thinned substrate  401 , the grating structure may be fabricated within the thinned substrate  401  and the buffer layer  405 , the thinned substrate layer and the buffer layer may be removed from the MSM photodetector with the grating structure fabricated directly in the absorbing layer  402 , or the grating structure may be fabricated in the buffer and absorbing layers, with the thinned substrate layer being removed.  
         [0027]     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. For example it should be appreciated that other means of photo-detection may be employed, for example pin or avalanche photodetectors.