Abstract:
A method and apparatus for reducing power saturation in a photodetector is provided. The photodetector includes a plurality of parallel absorption channels that receive and split incident light into plural segments. The parallel absorption channels operate as multi mode interference couplers.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to devices and methods used in fiber optics networks and more particularly, to semiconductor photodetectors.  
           [0003]    2. Background  
           [0004]    Conventional waveguide type, photodetectors (hereinafter referred as “photodetector” or “photodetectors”) are used extensively in fiber optics networks. FIG. 1A shows a top level block diagram of a typical fiber optics network  100 , which includes a transmitter  100 A that receives an electrical input (not shown) and converts it to an optical output  100 B using a laser diode (not shown). Optical signal  100 B is transmitted via fiber (not shown) and is received by optical amplifier  100 C. Optical amplifier  100 C amplifies optical signal  100 B and the amplified signal  100 D is transmitted to photodetector  100 F, via filter  100 E.  
           [0005]    Conventional photodetectors utilize a waveguide for guiding incident light to an absorption layer located between p and n-type semiconductor layers. FIGS. 1B and 1C, described below, show a cross-sectional and perspective view, respectively, of a typical waveguide photodetector.  
           [0006]    Turning in detail to FIG. 1B, a laminated structure is sequentially formed by a n-type cladding layer  104 , an absorption layer  103 , a p-type cladding layer  102  and an ohmic contact layer  101 , on a semiconductor substrate  105 . Electrodes (not shown) are mounted on ohmic contact layer  101  and on the back surface of layer  105 . If a reverse voltage is applied between layer  102  and layer  104 , incident light (not shown) guided to absorption layer  103  is converted into a photoelectric signal because electric field is maintained within a depletion layer created within absorption layer  103 . Excited carriers within the depletion layer are detected as photoelectric current.  
           [0007]    Turning in detail to FIG. 1C, is a perspective view of a conventional photodetector  106  with a cut-out cross-sectional view showing absorption layer  103  between layers  102  and  104 . In photodetector  106 , the total optical power generated by absorbed incident light is exponentially dependent upon the distance that incident light has to travel in absorption layer  103 . Typically, most of the incident light (not shown, perpendicular to the paper surface of FIG. 1C) is absorbed in the front area  103 A of absorption layer  103 . High concentration of absorbed photons result in high density of generated current carriers, resulting in reduced efficiency and power saturation of photodetector  106 .  
           [0008]    One common solution to the foregoing problem is to reduce the confinement factor for the waveguide design, by reducing absorption layer  103 &#39;s thickness (“T”, as shown in FIG. 1C) with respect to the overall waveguide thickness (“T 1 ”, as shown in FIG. 1C), and hence reducing the effective absorption coefficient. However, to offset the reduction in thickness, the length l (FIG. 1C) of the photodetector must be increased to absorb the same amount of incident light, which will result in higher capacitance due to increase in the waveguide sectional area, which ultimately reduces the overall photodetector efficiency. Furthermore, in a longer photodetector the velocity mismatch between optical and electric waves will produce noise in the detected optical signal.  
           [0009]    Therefore, there is a need to reduce power saturation in a photodetector without increasing the overall length of the photodetector.  
         SUMMARY OF THE INVENTION  
         [0010]    There is provided in accordance with one aspect of the present invention a method and apparatus to reduce power saturation in a photodetector without increasing the photodetector length. The present invention provides a photodetector with plural parallel absorption channels (N) that split incident light received from optical fiber into N segments. Because the absorption channels are parallel to each other, the overall length of the photodetector is not increased to absorb more incident light.  
           [0011]    In accordance with another aspect of the present invention, there is provided a method and apparatus wherein the photodetector efficiency is improved without increasing channel length or capacitance. Furthermore, since absorption channels are connected in parallel, the overall series resistance is reduced by a factor of N (number of plural absorption channels).  
           [0012]    This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1A, as described above, is a block diagram of a conventional fiber optics network.  
         [0014]    [0014]FIG. 1B, as described above, is a cross-sectional view of a conventional photodetector.  
         [0015]    [0015]FIG. 1C, as described above, is a perspective view of a conventional photodetector.  
         [0016]    [0016]FIG. 2 illustrates a top view of a photodetector with parallel absorption channels, according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 3 illustrates a process flow diagram for a photodetector using parallel absorption channels, according to an embodiment of the present invention. 
     
    
       [0018]    Features appearing in multiple figures with the same reference numeral are the same unless otherwise indicated.  
       DETAILED DESCRIPTION  
       [0019]    In one aspect of the present invention, plural parallel absorption channels are provided such that incident light that enters the optical path of a photodetector is absorbed by those plural parallel absorption channels. Because plural parallel absorption channels are used, the overall length of the photodetector is not increased which does not increase the overall capacitance of the photodetector.  
         [0020]    Turning in detail to FIG. 2, is waveguide  200  of a photodetector (not shown) with incident light  201  entering optical path  202 . Incident light  201  is absorbed by N parallel absorption channels  203  of a multi mode interference coupler  203 A that utilize properties of multi mode interference couplers (“MMI”) to split incident light  201  into N segments, and thereafter absorb incident light  201 . Since incident light  201  is split into N segments its power density is reduced by a factor of N, which reduces power saturation of the photodetector. Power density is defined as optical power, P, within the waveguide cross-section, divided by the waveguide cross-sectional area.  
         [0021]    In another aspect of the present invention, the length of the plural absorption channels of waveguide  200  is chosen such that the junction capacitance of waveguide  200  and  106  [FIG. 1B] is substantially similar. The length l of waveguide  106  is given by: 2(Γ 0 α)- 1  where Γ 0 α is the effective absorption coefficient of the waveguide channel and Γ 0  is the confinement factor of the waveguide. To maintain the junction capacitance for waveguide  200 , substantially similar to that of the single channel waveguide  106  with length l, the length L  204  for N parallel absorption channels  203  is given by:  
         
       L=l/N  
     
         [0022]    The foregoing relationship maintains the same capacitance as that of a series channel absorber shown in FIG. 1B, with length l and absorbs more incident light without increasing the overall channel length.  
         [0023]    In yet another aspect of the invention, referring to FIG. 3, a process is provided such that incident light that enters the optical path leading to a photodetector waveguide is absorbed by plural parallel absorption channels. Because plural parallel absorption channels are used, the overall capacitance of the photodetector is not increased, while the plural parallel absorption channels compared to photodetectors with a single absorption channel absorb more light.  
         [0024]    The process flow diagram of FIG. 3 comprises of: directing incident light to N absorption channels; splitting the incident light into N segments, wherein the light is split by plural parallel absorption channels operating as MMI couplers; and absorbing the split incident light.  
         [0025]    Turning in detail to FIG. 3, in Step S 301 , incident light is directed to N parallel absorption channels  203  [FIG. 2]. Incident light  201  enters optical path  202 .  
         [0026]    In Step S 302 , incident light  201  is split into plural segments. N absorption channels  203  operate as MMI couplers, as described above, and split incident light  201  into N segments.  
         [0027]    In step S 303 , incident light that is split into N segments is absorbed by N absorption channels  203 .  
         [0028]    In yet another aspect of the present invention the photodetector efficiency is improved without increasing channel length or increasing capacitance.  
         [0029]    In another aspect of the present invention, the overall series resistance is reduced by a factor of N since absorption channels are all connected in parallel,  
         [0030]    While the present invention is described above with respect to what is currently consider its preferred embodiments, it is to be understood that the invention is not limited to that described above. To the contrary, the invention is intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims.