Abstract:
Waveguide photodetector apparatus and methods employing an optical waveguide having a tapered section, which may be horizontally tapered, vertically tapered, or both. The apparatus also includes a photodetector with an intrinsic region, which in one embodiment may be tapered in a manner corresponding to a horizontal taper of the tapered section. The photodetector is arranged adjacent the tapered section such that the intrinsic region is coupled to the optical waveguide via an evanescent wave of a guided lightwave. The tapered section serves to force energy carried in the guided lightwave from the optical waveguide into the intrinsic region of the photodetector via the evanescent wave, thereby shortening the photodetector length.

Description:
TECHNICAL FIELD  
         [0001]    The technical field of the invention pertains to photodetectors and, in particular, to waveguide-based high-speed photodetectors.  
         BACKGROUND INFORMATION  
         [0002]    There are many lightwave applications, such as optical telecommunications and chip interconnects, that involve transmitting optical signals and converting them to electrical signals at high data rates. Systems for performing such transmission and conversion usually require a photodetector compatible with the speed and bandwidth of the optical signal. Preferred photodetectors are typically PIN (p-type/intrinsic/n-type) semiconductor (e.g., Si or Ge) detectors, as such detectors can have a fast (i.e., GHz) frequency response.  
           [0003]    Certain high-speed photodetectors utilize optical waveguides as a conduit for providing light to the intrinsic region of a PIN photodetector. An optical waveguide is a planar, rectangular or cylindrical structure having a high-index core surrounded by a low-index cladding. Light is trapped in the waveguide mostly within the high-index core, with a small portion of the light propagating in the cladding as an evanescent wave. When the intrinsic region of a PIN photodetector is located sufficiently close to the optical waveguide, light can be coupled to the intrinsic region via the evanescent wave. This phenomenon is referred to as “evanescent coupling.” 
           [0004]    To form a high-speed waveguide-based photodetector, the light traveling in the optical waveguide must be efficiently coupled to the intrinsic region of the photodetector. This light is then converted to photon-generated carriers, which then diffuse out to the electrodes (i.e., the p+ and n+ regions of the PIN detector). The result is an electrical signal (e.g., a photocurrent) that corresponds to the detected light.  
           [0005]    The speed of the detector is related to the time it takes for the photon-generated carriers to reach the electrodes. This time is referred to as the “transit time.” The narrower the intrinsic region, the shorter the transit time and the faster the detector. A fast photodetector allows for the detection and processing of high-speed optical signals.  
           [0006]    Often, the width and length of the intrinsic region of a photodetector is dictated by the width and length of the waveguide. However, the waveguide is typically designed for optimally transmitting a particular wavelength of light rather than for optimizing the detector speed. For low-index waveguides, the intrinsic region width can be quite wide (e.g., greater than 1 micron) and also quite long (e.g., greater than 50 microns).  
           [0007]    The intrinsic region of a PIN detector is typically silicon (Si) or germanium (Ge), both of which have a high refractive index (e.g., about 3.5) as compared to the typical optical waveguide index (e.g., about 1.5 for SiO x N y ). This results in a mismatch between the optical waveguide and the PIN detector with respect to the propagation constant and the waveguide mode of the guided lightwave. This mismatch leads to inefficient optical coupling. In some cases, a relatively lengthy waveguide-detector interface may be used to make up for the coupling inefficiency and to ensure that sufficient light is coupled to the detector. However, a lengthy interface is undesirable because it results in a large detector. Further, in many cases, the interface length needed to make up for the coupling inefficiency is too long for practical purposes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a plan view of an embodiment of the photodetector apparatus of the present invention, wherein the optical waveguide has a tapered section with a horizontal taper;  
         [0009]    [0009]FIG. 2 is a cross-sectional view of the photodetector apparatus of FIG. 1;  
         [0010]    [0010]FIG. 3 is a cross-sectional view of a photodetector apparatus similar to that of FIG. 1, except that the optical waveguide tapered section has a vertical taper;  
         [0011]    [0011]FIG. 4 is a perspective end view of a photodetector similar to that of the embodiment shown in FIG. 1, except that the optical waveguide tapered section has a vertical and horizontal (i.e., “double”) taper;  
         [0012]    [0012]FIG. 5 is an end-view of the photodetector apparatus of FIG. 1;  
         [0013]    [0013]FIG. 6 is a close-up plan view of an example embodiment of a PIN photodetector having a tapered intrinsic region;  
         [0014]    [0014]FIG. 7A is a plot based on a simulation of the time-averaged intensity I (arbitrary units) of the light coupled into the intrinsic region of a PIN detector versus the distance D (microns) along the intrinsic region for a photodetector apparatus similar to that of FIG. 1, but without a tapered waveguide section;  
         [0015]    [0015]FIG. 7B is a plot similar to FIG. 7A, except that the photodetector apparatus includes a horizontal tapered section; and  
         [0016]    [0016]FIG. 8 is a schematic diagram of an embodiment of an electrical-optical system that employs any one of the embodiments of the photodetector apparatus of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]    In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice them, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from their scope. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
         [0018]    [0018]FIG. 1 is a plan view of an embodiment of a photodetector apparatus  10  of the present invention, wherein the optical waveguide  14  has a tapered section  70  with a horizontal taper.  
         [0019]    [0019]FIG. 2 is a cross-sectional view of the photodetector apparatus of FIG. 1. In FIG. 2, the cross-sectional view is taken in the Y-Z plane of FIG. 1 along axis Al.  
         [0020]    Referring now to both FIGS. 1 and 2, apparatus  10  includes optical waveguide  14  having an input end  16 , an upper surface  18 , and a lower surface  20 . In an example embodiment, optical waveguide is a rectangular waveguide, as shown. Optical waveguide  14  also includes a core  22  surrounded by a cladding  24 . Core  22  has a core width in the X-direction of W X  (FIG. 1) and a core width in the Y-direction of W Y  (FIG. 2). The index of refraction of core  22  is greater than that of cladding  24 .  
         [0021]    In example embodiments, core  22  is made of Si 3 N 4  for transmission of light having a wavelength of 850 nm, or it is made of intrinsic silicon for wavelengths of 1 micron or greater. Further in the example embodiments, cladding  24  is made of SiO 2 , which has a relatively low refractive index (about 1.5) as compared to that of Si 3 N 4  (about 3.5) at near-infrared and infrared wavelengths. By using materials that provide a large index contrast between the core and the cladding (e.g., Si 3 N 4  and SiO 2 , respectively), the core dimensions W X  and W Y  can be made small, e.g., W X , W Y &lt;1 micron.  
         [0022]    The high-index-core/low-index-cladding geometry is necessary for optical waveguide  14  to guide a lightwave  50 . Lightwave  50  includes a central portion  56  that propagates in core  22  and an evanescent wave  58  that propagates in cladding  24 . In an example embodiment, lightwave  50  represents or carries an optical signal.  
         [0023]    Optical waveguide  14  further includes a tapered section  70 . Tapered section  70  has a length L1 as measured along the Z-direction starting at a point  76  along the waveguide and terminating at a narrow end  82 . In the example embodiment of apparatus  10  illustrated in FIG. 1, tapered section  70  is tapered in the horizontal (X-Z) plane and is thus referred to herein as a “horizontal taper.” 
         [0024]    [0024]FIG. 3 is a cross-sectional view of a photodetector apparatus  10  similar to that of FIG. 1, except that the optical waveguide tapered section  70  has a vertical taper. This embodiment is referred to herein as a “vertical taper.” 
         [0025]    [0025]FIG. 4 is a perspective end view of a photodetector  110  similar to that of the embodiment shown in FIG. 1, except that the optical waveguide tapered section has a vertical and horizontal (i.e., “double”) taper. This embodiment is referred to herein as a “double taper.” 
         [0026]    The role of tapered section  70  in its various forms is discussed in greater detail below. For the sake of illustration, the discussion below continues with the horizontal taper example embodiment of apparatus  10  as shown in FIG. 1  
         [0027]    [0027]FIG. 5 is an end-view of the photodetector apparatus  10  of FIG. 1. Apparatus  10  further includes a PIN photodetector  110  having opposing p-type and n-type electrodes  116  and  120  separated by an intrinsic region  126 . Intrinsic region  126  is made from a semiconductor material, and in example embodiments it comprises either silicon or germanium. Intrinsic region  126  has a width W1 as well as a length L2 as measured between a leading end  134  and a terminating end  140  (FIG. 2). In an example embodiment, width W1 is variable.  
         [0028]    PIN photodetector  110  is arranged adjacent core  22  so that waveguide  14  and intrinsic region  126  optically communicate via evanescent wave  58 . In other words, the waveguide and intrinsic region of the PIN detector are evanescently coupled. In an example embodiment, width W1 of intrinsic region  126  corresponds to core width W X  of waveguide  110 , which can be constant (e.g., for a vertical taper) or variable (e.g, for a horizontal taper). Further, in an example embodiment, instrinsic region width W1 is equal to or substantially equal to core width W X . In yet another example embodiment, intrinsic region length L2 is equal to or substantially equal to the tapered section length L 1 .  
         [0029]    In an example embodiment, intrinsic region  126  is formed beneath waveguide  14  in combination with the self-aligned formation of p-type and n-type electrodes  116  and  120 , formed with respect to waveguide core  22 .  
         [0030]    [0030]FIG. 6 is a close-up plan view of an example embodiment of a PIN photodetector having a tapered intrinsic region  126 . A tapered intrinsic region is most appropriate when tapered section  70  (FIGS.  1 - 4 ) includes a horizontal taper. In an example embodiment, tapered intrinsic region  126  matches the taper of tapered section  70  of waveguide  14  (FIGS.  1 - 4 ). In an example embodiment, the p-type and n-type electrodes  116  and  120  are shaped to accommodate a tapered intrinsic region  126 .  
         [0031]    In operation, referring to FIGS.  1 - 3 , lightwave  50  is inputted into input end  16  of optical waveguide  14  and propagates down the waveguide. Eventually, lightwave  50  reaches starting point  76  of tapered section  70 , which in an example embodiment is also the location of leading end  134  of photodetector  110 . At this point, evanescent wave  58  of lightwave  50  evanescently couples to intrinsic region  126  (FIGS.  2 - 6 ), and light (energy) is transferred to the intrinsic region  126 . To facilitate the coupling, in an example embodiment intrinsic region  126  is intimately contacted directly to core  22  in tapered section  70  (FIGS.  2 - 3 ).  
         [0032]    Still referring to FIGS.  1 - 3 , lightwave  50  continues propagating towards narrow end  82  of tapered section  70  as power is coupled to the intrinsic region. For a purely horizontal taper (e.g. FIGS.  1 - 2 ), the core diminishes in size from point  76  to narrow end  82  only in the X-direction (i.e., core width W X  varies in the Z-direction, while core width W Y  remains constant). For a purely vertical taper (e.g. FIG. 3), the core diminishes in size only in the Y-direction while the core width W X  remains constant.  
         [0033]    Referring to FIG. 4, for a combination vertical and horizontal (i.e., a double) taper, the core diminishes in size in both the horizontal and vertical directions, so that core widths W X  and W Y  (not shown in FIG. 4) both vary in the Z-direction.  
         [0034]    Referring once again to FIGS.  1 - 3 , the reduction in the size of core  22  in tapered section  70  causes the energy carried in lightwave  50  to spread out from central portion  56  into evanescent wave  58 . The increase in energy in the evanescent wave leads to more and more energy being coupled into intrinsic region  126  from lightwave  50 . Thus, as lightwave  50  continues propagating through tapered section  70  toward narrow end  82 , more and more energy from the lightwave is evanescently coupled into intrinsic region  126 . The coupled light energy creates photon-generated carriers in intrinsic region  126 , which diffuse to the electrodes  116  and  120  (FIGS. 4 and 5), creating an electrical signal  168 , such as a photocurrent.  
         [0035]    Still referring to FIGS.  1 - 3 , tapered section  70  is designed such that when lightwave  50  reaches narrow end  82 , the amount of energy left in the lightwave is negligible. This is to prevent energy from being reflected backwards and traveling back up optical waveguide  14 . Also, in an example embodiment, the degree of taper of tapered section  70  is selected such that the transfer of energy from waveguide  14  to intrinsic region  126  is adiabatic, i.e., occurs with minimal reflection or loss of energy other than the evanescent coupling to the intrinsic region. The optimal design of tapered section  70  for a given set of parameters (e.g., wavelength of light, desired length of intrinsic region, relative indices of refraction of the core, cladding and intrinsic region, etc.) can be accomplished by computer modeling using commercially available simulation software. An example of such simulation software is the rSoft BPM simulator, currently available from rsoft, Inc., at www-rsoft-com (to avoid inadvertent hyperlinks the periods in the preceding URL have been replaced by dashes).  
         [0036]    Thus, tapered section  70  makes for efficient optical coupling by forcing the energy in lightwave  50  into intrinsic region  126 . This allows length L2 (FIGS.  2 - 3 ) of intrinsic region  126  to be shorter than if tapered section  70  were not present. This in turn makes for a more compact and efficient photodetector apparatus.  
         [0037]    [0037]FIG. 7A is a plot based on a simulation of the time-averaged intensity I (arbitrary units) of the light coupled into the intrinsic region of a PIN detector versus the distance D (microns) along the intrinsic region for a photodetector apparatus similar to that of FIG. 1, but without a tapered waveguide section.  
         [0038]    [0038]FIG. 7B is a plot similar to FIG. 7A, except that the photodetector apparatus includes a horizontal tapered section.  
         [0039]    In FIGS. 7A and 7B, the distance required to transfer substantially all of the energy from guided lightwave  50  (e.g. in FIG. 2) to intrinsic region  126  (e.g. in FIG. 2) is referred to herein as the “energy transfer distance” and is denoted D T . The rsoft BPM simulator referred to above was used to perform the simulations to obtain the data for the plots.  
         [0040]    From FIG. 7A, it is seen that the energy transfer distance D T  for the “no taper” case is about  25  microns. On the other hand, from FIG. 7B, it is seen that the energy transfer distance D T  for the horizontal taper case is about 2.5 microns. Thus, use of a horizontal tapered section  70  in apparatus  10  can result in a very large (i.e., about an order of magnitude) reduction in the energy transfer distance D T . Similar results can be obtained for the vertical and double taper embodiments. As a consequence, the PIN detector section of apparatus  10  can be made almost two orders of magnitude smaller than prior art apparatus.  
         [0041]    Besides increasing the coupling efficiency and reducing the energy transfer distance D T , an increase in detection speed can be realized with embodiments of apparatus  10 , particularly those embodiments having a horizontal taper component in tapered section  70 . As discussed above in connection with FIG. 6, a horizontal component to tapered section  70  allows for intrinsic region  126  to be correspondingly tapered so that the intrinsic region is, on the average, narrower then a conventional intrinsic region. This results in a shorter distance between the p-type and n-type electrodes  116  and  120  as compared to a conventional PIN. This, in turn, translates into a shorter transit time for photon-generated carriers and thus a faster detector speed.  
         [0042]    Electrical-Optical System  
         [0043]    [0043]FIG. 8 is a schematic diagram of an embodiment of an electrical-optical system  200  that employs any one of the embodiments of photodetector apparatus  10  of the present invention. System  200  includes an optical or opto-electronic device  210  optically coupled to optical waveguide  14  at input end  16 . Device  210  is capable of generating an optical signal carried by or otherwise represented by lightwave  50 . In an example embodiment, device  210  includes a microprocessor (not shown) and a light-emitting device (not shown) such as a diode laser or a light-emitting diode.  
         [0044]    System  200  further includes an electronic or optoelectronic device  230  electrically coupled to photodetector  110  via a wire  236 . Device  230  is any device capable of receiving and processing electrical signal  168 , such as but not limited to, for example, a microprocessor, a filter, an amplifier, or any combination thereof. Device  230  could include any other type of signal-processing element or circuit.  
         [0045]    In operation, device  210  emits an optical signal represented by or carried by lightwave  50 , which is coupled into optical waveguide  14 . Lightwave  50  propagates in waveguide  14  to tapered section  70 . In tapered section  70 , the energy in lightwave  50  is forced by the taper into intrinsic region  126  (refer to FIGS. 2, 3,  5 , and  6 ) of photodetector  110  as the lightwave continues propagating toward narrow end  82 . The light in intrinsic region  126  is converted to photon-generated carriers, which diffuse to electrodes  116  and  120  (refer to FIGS.  4 - 6 ), resulting in electrical signal  168 . Electrical signal  168  is then carried by wire  236  to device  230 , which then processes the electrical signal.  
         [0046]    The various elements depicted in the drawings are merely representational and are not drawn to scale. Certain proportions thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various implementations of the invention, which can be understood and appropriately carried out by those of ordinary skill in the art.  
         [0047]    While certain elements have been described herein relative to “upper” and “lower”, and “horizontal” and “vertical”, it will be understood that these descriptors are relative, and that they could be reversed if the elements were inverted, rotated, or mirrored. Therefore, these terms are not intended to be limiting.  
         [0048]    It is emphasized that the Abstract is provided to comply with 37 C.F.R. §1.72(b) requiring an Abstract that will allow the reader to quickly ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.  
         [0049]    In the foregoing Detailed Description, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate preferred embodiment.  
         [0050]    While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.