Abstract:
According to this disclosure, embodiments of the present invention include photonic integrated circuits having active and passive geometric regions geometrically arranged to provide for more compact integrated photonic integrated circuits which, in turn, leads to higher chip yields and lower fabrication costs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/110,605 filed Nov. 2, 2008 which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to photonic integrated circuits and, more particularly, to photonic integrated circuits which have compact architectures. 
         [0004]    2. Description of the Related Art 
         [0005]    Monolithic photonic integrated circuits (PICs), also sometimes referred to as planar lightwave circuits (PLCs), are increasingly deployed in modern optical telecommunication systems. These devices provide the integration of both active and passive optical components on a single substrate and are integrated with other optical components to form a multi-functional optical device for use in such systems. The gravitation to PICs is strong because it leads to the utility of providing an entire system function, let alone a component function, in a single chip in a single package. Compared to the deployment of discrete optical components, such monolithic PIC chips can significantly reduce the size of optical components necessary in the optical system, as part of a transmitter photonic integrated circuit (TxPIC) or a receiver photonic integrated circuit (RxPIC) for example, as well as significantly reduce the overall costs in a system. Examples of such TxPICs and RxPICs are disclosed in U.S. Pat. Nos. 7,283,694; 7,116,851; 7,079,715; and 7,058,246, all of which are incorporated herein in their entirety by reference. 
         [0006]    What is needed is a photonic integrated circuit having on-chip active and passive elements located within corresponding active and passive geometric regions arranged to provide for more compact photonic integrated circuits, ultimately leading to higher chip yields and lower fabrication costs. 
       SUMMARY OF THE INVENTION 
       [0007]    According to this disclosure, embodiments of the present invention include photonic integrated circuits having an active geometric region geometrically arranged with respect to other passive elements as to provide for more compact integrated photonic integrated circuits which, in turn, leads to higher chip yields and lower fabrication costs. 
         [0008]    In the various embodiments of the present invention, a photonic integrated circuit includes a substrate having a first area and an active geometric region having a second area, where the ratio of the second area to the first area is greater than, or equal to, 0.5. In certain embodiments of the present invention the active geometric region has a first boundary defined by a first one of a plurality of signal channels and a second boundary defined by a second one of a plurality of signal channels, and remaining ones of the plurality of signal channels are positioned between the first and second ones of the plurality of signal channels. In still other embodiments of the present invention, the photonic integrated circuit includes an optical combiner, the plurality of signal channels and the optical combiner arranged such that a line extending through a first of a plurality of optical elements in each of the plurality of signal channels substantially normal to a longitudinal axis of each of the corresponding plurality of signal channels, also extends through a portion of the optical combiner. In certain embodiments, the first of the plurality of optical elements is a laser source. In certain other embodiments the output of the photonic integrated circuit is substantially normal to a longitudinal axis of a signal channel, as part of the active geometric region. In still other certain embodiments disclosed herein, radio-frequency (RF) bond pads of certain active elements as part of the active geometric region are positioned closer to associated electronic circuitry to allow for single component-level circuits and shorter transmission lines between such bond pads and associated electronic circuitry. Other various embodiments of the present invention apply both to TxPICs as well as RxPICs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    In the drawings wherein like reference symbols refer to like parts: 
           [0010]      FIG. 1  is a general circuit diagram depicting a first arrangement of integrated components. 
           [0011]      FIG. 2  is a circuit diagram depicting an arrangement of integrated components in accordance with various embodiments of the present invention. 
           [0012]      FIG. 3  is a circuit diagram depicting another arrangement of integrated components in accordance with various embodiments of the present invention. 
           [0013]      FIG. 4  is a circuit diagram depicting still another arrangement of integrated components in accordance with various embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Reference is now made to  FIG. 1 , a general circuit diagram depicting a first arrangement of integrated components or elements will be discussed in greater detail.  FIG. 1  depicts a photonic integrated circuit  100  generally comprising a substrate  101 , an active region  110 , depicted in dashed-line, and additional passive elements including an optical combiner  120  and the various waveguide structures utilized to guide optical energy along the photonic integrated circuit  100 , as discussed in greater detail below. For purposes of the discussion herein, the active geometric region indicates that portion of the photonic integrated circuit devoted to the placement of active components. The active geometric region  110  includes a number of optical signal channels  112 . The active geometric region  110  has a first boundary  110 ( 1 ) and a second boundary  110 ( 2 ) along a first dimension, e.g. a width, of the active geometric region  110 , and a third boundary  110 ( 3 ) and a fourth boundary  110 ( 4 ) along a second dimension, e.g. a length, of the active geometric region  110 . Each of the optical signal channels  112  comprise a plurality of optical elements  114  for creation and proper propagation of optical signals throughout the photonic integrated circuit  100 . 
         [0015]    The output of each optical signal channel  112  of active geometric region  110  is provided on one of a plurality of inputs  122  of the combiner  120  via a corresponding optical waveguide  116 , such as waveguide  116 ( 1 ) associated with optical signal path of channel  1  for example. The optical signal propagating along waveguide  116 ( 1 ) passes through an input free space region  128 ( 1 ), grating arms  130 , and provided at an output  124  of the combiner via an output free space region  128 ( 2 ). The optical combiner  120  combines each of the optical signals received, e.g. all the optical signals associated with optical signal channels  1  through  10 , into a wave division multiplexed (WDM) signal which is then output on a plurality of output waveguides  126 . In turn, optical waveguides  126  each provide the WDM signal to a corresponding one of a plurality of outputs  127  along one of a plurality of facets  102  bordering the photonic integrated circuit, a facet  102 ( 1 ) of the photonic integrated circuit  100  for example. 
         [0016]    In the photonic integrated circuit  100  of  FIG. 1 , the active geometric region  110 , generally outlined in dashed line, occupies less than half of the total area of the photonic integrated circuit  100  when the optical elements  114  of the signal channels  112  and the optical combiner  120  are arranged as depicted in  FIG. 1 . 
         [0017]    With respect to  FIG. 2 , a circuit diagram depicting an arrangement of integrated components in accordance with various embodiments of the present invention is depicted. More specifically,  FIG. 2  depicts a photonic integrated circuit  200  including similar components as circuit  100 , as well as an active geometric region  210  generally outlined in dashed-line. Photonic integrated circuit  200  comprises a plurality of optical signal channels  212  provided on a substrate  201 , each including a plurality of optical elements  214 . The outputs of each of the optical signal channels  212  are directed to an optical combiner  220 , provided on the substrate  201 , via a plurality of optical waveguides  216 . While depicted as a arrayed waveguide grating (AWG), the combiner  220  can be any suitable combiner, such as an Echelle grating for example. The optical combiner  220  combines each of the optical signals received, e.g. all the optical signals associated with optical signal channels  1  through  10 , into an WDM signal which is then output on a plurality of output waveguides  226 . In turn, optical waveguides  226  each provide the WDM signal to a corresponding one of a plurality of outputs  227  along one of a plurality of facets  202  bordering the photonic integrated circuit, output waveguides  226 ( 1 ) providing the WDM signal to an output  227 ( 1 ) along a facet  202 ( 1 ) of the photonic integrated circuit  200  for example. Second and third exemplary outputs are also depicted. More specifically, output waveguides  226 ( 2 ) can provide a plurality of outputs  227 ( 2 ) on a second facet  202 ( 2 ) or a plurality of output waveguides  226 ( 3 ) can provide a plurality of outputs  227 ( 3 ) on a third facet  202 ( 3 ). As with other embodiments discussed herein, in accordance with the present invention, a plurality of output waveguides  226  are provided. Only one of such outputs obtained from one of the plurality of output waveguides  226  may be utilized, for example the one most suitable for transmission over a network infrastructure. The remaining ones of the plurality of output waveguides  226  may be utilized for other purposes such as signal monitoring. 
         [0018]    While ten such optical signal channels  212  are depicted, for example optical channels  1  through  10  identified as  212 ( n ), it should be apparent that more or less of such optical channels could be present. Such optical elements  214  within each signal channel  212  include, but are not limited to, laser sources, modulated laser sources, modulators such as electro-absorption modulators or Mach-Zehnder modulators, semiconductor optical amplifiers (SOA), variable optical attenuators, photodetectors or photodiodes, or any other optical element which can be used for signal conditioning or monitoring of the optical signals for transport through the photonic integrated circuit. For illustration purposes, each optical signal channel  212  may include a laser source  214 ( 1 ), an SOA  214 ( 2 ), and a photodetector  214 ( 5 ), among other elements. It is important to note that every optical signal channel  212  need not comprise the same optical elements, nor the same optical elements in the same order as described with respect to signal channel  212 ( 1 ). 
         [0019]    Active geometric region  210  has a first boundary  210 ( 1 ) and a second boundary  210 ( 2 ) generally defined by a first signal channel  212 ( 1 ) and a second signal channel  212 ( 10 ), and a third boundary  210 ( 3 ) and a fourth boundary  210 ( 4 ) extending from the first boundary to the second boundary, as depicted. Each signal channel  212  has a geometric longitudinal axis, for example a line L 1  of a plurality of lines L n  extends through the longitudinal axis along signal channel  212 ( 1 ). In similar fashion, lines L 7  and L 9 , extend through the longitudinal axis along signal channels  212 ( 7 ) and  212 ( 9 ), respectively, are depicted as well. Each of the lines L n , where n is the number of optical signal channels present, extending through the longitudinal axis of each of the signal channels  212  are substantially parallel to each other, and thus substantially parallel to the first and second boundaries  210 ( 1 ),  210 ( 2 ) of the active geometric region  210 , and substantially perpendicular to the third and fourth boundaries  210 ( 3 ),  210 ( 4 ) of the active geometric region  210 . As depicted, the first and second boundaries  210 ( 1 ),  210 ( 2 ) have a length which is shorter than the third and fourth boundaries  210 ( 3 ),  210 ( 4 ). The optical combiner  220  is positioned along a shorter one of the boundaries, that is, along the second boundary  210 ( 2 ) in the exemplary configuration of  FIG. 2  which provides for a more efficient design. For illustration purposes, the optical combiner  220  may be positioned with respect to the active geometric region  210  such that a line L D1  which extends through each of the laser sources  214 ( 1 ) substantially normal to each of the lines L n  extending through the corresponding longitudinal axis of each signal channel  212 , also extends through a portion of the combiner  220 . The active geometric region  210  of the photonic integrated circuit  200 , while the same general size as the active geometric region  110  of circuit  100  since both comprise similar optical elements  114 ,  214  in each corresponding signal channel  112 ,  212 , assumes a larger portion of the total circuit  200  area when compared to the active geometric region  110 . That is, the ratio of the active geometric region  210  area to the total area of the substrate  201 , e.g. the photonic integrated chip area, is larger than the ratio of the active geometric region  110  to the total area of the substrate  101 . Preferably, the ratio of the active geometric region  210  area to the total area of the substrate  201  is greater than or equal to about 0.5. This increase in the ratio of active geometric region area to total substrate area is achieved, as discussed above, by positioning the combiner  220  adjacent the shortest of the boundaries  210 ( 1 )- 210 ( 4 ) of the active geometric region  210 . 
         [0020]    The optical combiner  220  comprises a first free space region  228 ( 1 ) and a second free space region  228 ( 2 ) and a plurality of waveguides  230  coupling the first free space region  228 ( 1 ) to the second free space region  228 ( 2 ). Optical waveguides  216  have a first end coupled to a corresponding one of the optical signal channels  212  and a second end coupled to the first free space region  228 ( 1 ). The first end of each of the output waveguides  216  has a longitudinal axis equivalent to the longitudinal axis of the corresponding signal channel. For example, the line L 1  extending through the longitudinal axis of the signal channel  212 ( 1 ) also extends through the longitudinal axis of the first end of output waveguide  226 ( 1 ). With the optical combiner  220  oriented as shown in  FIG. 2 , the second end of each of the output waveguides  116  couples to the first free space region  228 ( 1 ) having a longitudinal axis generally parallel to a line L FS1 . As such, the longitudinal axis of each of the first ends of each of the optical waveguides  216  is substantially parallel to the longitudinal axes of each of the second ends of the optical waveguides  216 . Each of the output waveguides  226  have a first end coupled to the second free space region  228 ( 2 ) of the optical combiner  220 . Each of the first ends of the output waveguides  226  couples to the second free space region  228 ( 2 ) having a longitudinal axis generally parallel to a line L FS2 . A line L O1  extends parallel to the longitudinal axis of a second end of each of the output waveguides  226 ( 1 ), a line L O2  extends parallel to the longitudinal axis of a second end of each of the alternative output waveguides  226 ( 2 ), and a line L O3  extends parallel to the longitudinal axis of a third end of each of the alternative output waveguides  226 ( 3 ). The line L FS2  being substantially parallel to the line L O1 , and substantially perpendicular to line L O1  and L O2 . When used herein as a comparison of longitudinal axes corresponding to first and second ends of optical waveguides  216 , first and second ends of output waveguides  226 , as well as first and second ends of like elements in the embodiments of  FIGS. 3 and 4 , the term “substantially parallel” means more parallel than perpendicular and the term “substantially perpendicular” means more perpendicular than parallel. 
         [0021]    As should be readily understood, the geometric arrangement of photonic integrated circuit  200  of  FIG. 2 , as well as the exemplary embodiments of  FIGS. 3 and 4  discussed below, results in a greater area ratio of the active geometric region  210  with respect to the total area of circuit  200 . This, in turn, leads to more dense wafers and higher chip yields, as well as a corresponding reduction in associated fabrication costs. The geometric arrangement depicted in  FIG. 2  has additional advantages. First, a reduction in transmission line effects as between electrical circuitry external to circuit  200  and one or more optical elements  214  is reduced, electrically connected through the use of bond pads for example. One or more optical elements  214  may require an electrical interface to provide an electrical connection to electrical circuitry located proximate to the photonic integrated circuit  200 . The geometric structure, or overall shape, of the photonic integrated circuit  200  reduces the distance between each of the elements  214  and the perimeter of the circuit  200  as defined by the various output facets  202 , such as facet  202 ( 4 ). 
         [0022]    Second, a reduction in scattered light can also be achieved through selection of the appropriate output signals. Utilizing the outputs  127 ( 2 ) at the output facet  202 ( 2 ) or the outputs  127 ( 3 ) at the output facet  202 ( 3 ) will reduce the scattered light which may impact the output signals, as well as provide an overall shorter output path leading to facet  202 ( 2 ) or facet  202 ( 3 ), respectively, which also results in power savings. 
         [0023]    As should be apparent to one of ordinary skill in the art, while the active geometric region  210  comprising the optical signal channels  212  represents a majority of active elements on the photonic integrated circuit  200 , the active geometric region  210  may not comprise each and every active device as part of photonic integrated circuit  200 . 
         [0024]    With reference to  FIG. 3 , a circuit diagram depicting another arrangement of integrated components in accordance with various embodiments of the present invention is shown.  FIG. 3  depicts a photonic integrated circuit  300  similar to the circuit  200  of  FIG. 2 , however includes a combiner  320  which is rotated 90° counter-clockwise with respect to the orientation of combiner  220  of circuit  200  of  FIG. 2 . Photonic integrated circuit  300  comprises a plurality of optical signal channels  312  provided on a substrate  301 , each including a plurality of optical elements  314 . The outputs of each of the optical signal channels  312  are directed to an optical combiner  320 , provided on the substrate  301 , via a plurality of optical waveguides  316 . The optical combiner  320  comprises a first free space region  328 ( 1 ) and a second free space region  328 ( 2 ) and a plurality of waveguides  330  coupling the first free space region  328 ( 1 ) to the second free space region  328 ( 2 ). The optical combiner  320  combines each of the optical signals received, e.g. all the optical signals associated with optical signal channels  1  through  10 , into an WDM signal which is then output on a plurality of output waveguides  326 . In turn, optical waveguides  326  each provide the WDM signal to a corresponding one of a plurality of outputs  327  along one of a plurality of facets  302  bordering the photonic integrated circuit  300 , output waveguides  326 ( 1 ) providing the WDM signal to an output  327 ( 1 ) along a facet  302 ( 1 ) of the photonic integrated circuit  300  for example. A second exemplary output is also depicted. More specifically, output waveguides  326 ( 2 ) can provide a plurality of outputs  327 ( 2 ) on a second facet  302 ( 2 ). 
         [0025]    Active geometric region  310 , in dashed-line, has a first boundary  310 ( 1 ) and a second boundary  310 ( 2 ) generally defined by a first signal channel  312 ( 1 ) and a second signal channel  312 ( 10 ), and a third boundary  310 ( 3 ) and a fourth boundary  310 ( 4 ) extending from the first boundary to the second boundary, as depicted. Similar to the photonic circuit  200  of  FIG. 2 , each signal channel  312  has a geometric longitudinal axis, for example a line L 1  of a plurality of lines L n  extends through the longitudinal axis along signal channel  312 ( 1 ). In similar fashion, lines L 7  and L 9 , extend through the longitudinal axis along signal channels  312 ( 7 ) and  312 ( 9 ), respectively, are depicted as well. Each of the lines L n , where n is the number of optical signal channels present, extending through the longitudinal axis of each of the signal channels  312  are substantially parallel to each other, and thus substantially parallel to the first and second boundaries  310 ( 1 ),  310 ( 2 ) of the active geometric region  310 , and substantially perpendicular to the third and fourth boundaries  310 ( 3 ),  310 ( 4 ) of the active geometric region  310 . As depicted, the first and second boundaries  310 ( 1 ),  310 ( 2 ) have a length which is shorter than the third and fourth boundaries  310 ( 3 ),  310 ( 4 ). The optical combiner  320  is positioned along a shorter one of the boundaries, that is, along the second boundary  310 ( 2 ) in the exemplary configuration of  FIG. 3  which provides for a more efficient design. For illustration purposes, the optical combiner  320  may be positioned with respect to the active geometric region  310  such that a line L D2  which extends through each of the laser sources  314 ( 1 ) substantially normal to each of the lines L n  extending through the corresponding longitudinal axis of each signal channel  312 , also extends through a portion of the combiner  320 . More specifically, for example the line L D2 , as shown, may extend between the first free space region  328 ( 1 ) and the second free space region  328 ( 2 ) of the combiner  320 , and further extend through the plurality of waveguides  330 . With the active geometric region  310  area generally equal to the active geometric region  210  area, and the total area of the substrate  301  generally equal to the total area of the substrate  201 , a ratio of the active geometric region  310  area to the total area of the substrate  301  of photonic integrated circuit  300  is greater than or equal to about 0.5. 
         [0026]    Optical waveguides  316  have a first end coupled to a corresponding one of the optical signal channels  312  and a second end coupled to the first free space region  328 ( 1 ). The first end of each of the optical waveguides  316  has a longitudinal axis equivalent to the longitudinal axis of the corresponding signal channel. For example, the line L 1  extending through the longitudinal axis of the signal channel  312 ( 1 ) also extends through the longitudinal axis of the first end of optical waveguide  316 ( 1 ). With the optical combiner  320  oriented as shown in  FIG. 3 , the second end of each of the optical waveguides  316  couples to the first free space region  228 ( 1 ) having a longitudinal axis generally parallel to a line L FS3 . As such, the longitudinal axis of each of the first ends of each of the optical waveguides  316  is substantially perpendicular to the longitudinal axes of each of the corresponding second ends of the optical waveguides  316 . Each of the output waveguides  326  have a first end coupled to the second free space region  328 ( 2 ) of the optical combiner  320 . Each of the first ends of the output waveguides  326  couples to the second free space region  328 ( 2 ) having a longitudinal axis generally parallel to a line L FS4 . A line L O4  extends parallel to the longitudinal axis of a second end of each of the output waveguides  326 ( 1 ) and a line L O5  extends parallel to the longitudinal axis of a second end of each of the alternative output waveguides  326 ( 2 ). The line L FS4  being substantially parallel to the line L O4  and line L O5 . 
         [0027]    With this arrangement, while providing for a more compact photonic integrated circuit, the signal channel waveguides  316  are distanced from output waveguides  326  further reducing optical noise as between the optical signals propagating therethrough.  FIG. 3  further depicts outputs  327 ( 1 ) provided on a first facet  302 ( 1 ) as well as exemplary outputs  327 ( 4 ) provided on a second chip facet  302 ( 4 ). 
         [0028]    With respect to  FIG. 4 , a circuit diagram depicting another arrangement of integrated components in accordance with various embodiments of the present invention is depicted. More specifically,  FIG. 4  depicts a photonic integrated circuit  400  comprising similar components as circuit  200 , an active geometric region  410  identified in dashed-line. As with circuit  200 , circuit  400  depicts ten signal channels, e.g. signal channels  1  through  10 , of a plurality of associated optical signal channels  412 . 
         [0029]    Photonic integrated circuit  400  comprises a plurality of optical signal channels  412  provided on a substrate  401 , each including a plurality of optical elements  414 . The outputs of each of the optical signal channels  412  are directed to an optical combiner  420 , provided on the substrate  401 , via a plurality of optical waveguides  416 . The optical combiner  420  includes a free space region  428  having a first end  432 ( 1 ) which includes an input portion optically coupled to the plurality of waveguides  416 , and an output portion optically coupled to a plurality of output waveguides  426 . The free space region  420  also includes a second end  432 ( 2 ) which is optically coupled to a plurality of waveguides  430 . As depicted, the distal ends of waveguides  430  interface with the facet  402 ( 3 ). The surface of facet  402 ( 3 ) is configured to reflect the optical signal passing through each of the corresponding waveguides  430  such that the optical signals are redirected back into the free space region  428 . The reflected optical signals are combined into a WDM signal which is then provided to the first plurality of outputs  427 . More information regarding the reflective combiner  440  can be found in U.S. Pat. No. 7,444,048, which is incorporated herein in its entirety by reference. The output waveguides  426  each provide the WDM signal to a corresponding one of a plurality of outputs  427  along one of a plurality of facets  402  bordering the photonic integrated circuit  400 , output waveguides  426 ( 1 ) providing the WDM signal to an output  427 ( 1 ) along a facet  402 ( 1 ) of the photonic integrated circuit  400  for example. Second and third exemplary outputs are also depicted. More specifically, alternative output waveguides  426 ( 2 ) can provide a plurality of outputs  427 ( 2 ) on a second facet  402 ( 2 ), and alternative output waveguides  426 ( 3 ) can provide a plurality of outputs  427 ( 3 ) on a third facet  402 ( 3 ). 
         [0030]    Active geometric region  410 , in dashed-line, has a first boundary  410 ( 1 ) and a second boundary  410 ( 2 ) generally defined by a first signal channel  412 ( 1 ) and a second signal channel  412 ( 10 ), and a third boundary  410 ( 3 ) and a fourth boundary  410 ( 4 ) extending from the first boundary to the second boundary, as depicted. Similar to the photonic circuit  200  of  FIG. 2 , each signal channel  412  has a geometric longitudinal axis, for example a line L 1  of a plurality of lines L n  extends through the longitudinal axis along signal channel  412 ( 1 ). In similar fashion, lines L 7  and L 9 , extend through the longitudinal axis along signal channels  412 ( 7 ) and  412 ( 9 ), respectively, are depicted as well. Each of the lines L n , where n is the number of optical signal channels present, extending through the longitudinal axis of each of the signal channels  412  are substantially parallel to each other, and thus substantially parallel to the first and second boundaries  410 ( 1 ),  410 ( 2 ) of the active geometric region  410 , and substantially perpendicular to the third and fourth boundaries  410 ( 3 ),  410 ( 4 ) of the active geometric region  410 . As depicted, the first and second boundaries  410 ( 1 ),  410 ( 2 ) have a length which is shorter than the third and fourth boundaries  410 ( 3 ),  410 ( 4 ). The optical combiner  420  is positioned along a shorter one of the boundaries, that is, along the second boundary  410 ( 2 ) in the exemplary configuration of  FIG. 4  which provides for a more efficient design. For illustration purposes, the optical combiner  420  may be positioned with respect to the active geometric region  410  such that a line L D3  which extends through each of the laser sources  414 ( 1 ) substantially normal to each of the lines L n  extending through the corresponding longitudinal axis of each signal channel  412 , also extends through a portion of the combiner  420 . More specifically, the line L D3 , as shown, extends through the plurality of waveguides  430 . With the active geometric region  410  area generally equal to the active geometric region  210  area, and the total area of the substrate  401  generally equal to the total area of the substrate  201 , or perhaps somewhat smaller since the reflective combiner  420  takes up less area than the combiner  220 , a ratio of the active geometric region  410  area to the total area of the substrate  401  of photonic integrated circuit  400  is greater than or equal to about 0.5. 
         [0031]    Optical waveguides  416  have a first end coupled to a corresponding one of the optical signal channels  412  and a second end coupled to the free space region  428 . The first end of each of the optical waveguides  416  has a longitudinal axis equivalent to the longitudinal axis of the corresponding signal channel. For example, the line L 1  extending through the longitudinal axis of the signal channel  412 ( 1 ) also extends through the longitudinal axis of the first end of optical waveguide  416 ( 1 ). With the optical combiner  420  oriented as shown in  FIG. 4 , the second end of each of the optical waveguides  416  couples to the free space region  428  having a longitudinal axis generally parallel to a line L FS5 . As such, the longitudinal axis of each of the first ends of each of the optical waveguides  416  is substantially perpendicular to the longitudinal axes of each of the corresponding second ends of the optical waveguides  416 . Each of the output waveguides  426  have a first end coupled to the free space region  428  of the optical combiner  420 . Each of the first ends of the output waveguides  426  couples to the free space region  428  having a longitudinal axis generally parallel to a line L FS6 . A line L O6  extends parallel to the longitudinal axis of a second end of each of the output waveguides  426 ( 1 ), a line L O7  extends parallel to the longitudinal axis of a second end of each of the alternative output waveguides  426 ( 2 ), and a line LO 8  extends parallel to the longitudinal axis of a second end of each of the alternative output waveguides  426 ( 3 ). The line L FS6  being substantially perpendicular to the line L O6  and L O8 , and substantially parallel to line L O7 . 
         [0032]    While the invention has been described in conjunction with several specific embodiments, it is evident to those skilled in the art that many further alternatives, modifications and variations will be apparent in light of the foregoing description. Thus, the invention described herein is intended to embrace all such alternatives, modifications, applications and variations as may fall within the spirit and scope of the appended claims.