Abstract:
A photodetector for power monitoring purposes may be positioned directly on a planar light circuit. The photodetector may be protected by hermetically sealing a localized region over the planar light circuit corresponding to the position of the photodetector. The remainder of the planar light circuit may remain unsealed.

Description:
BACKGROUND  
         [0001]    This invention relates generally to planar light circuits that transmit signals for optical communication systems.  
           [0002]    Optical communication systems may convey a plurality of channels multiplexed as different wavelengths over a communication path. A plurality of signals multiplexed as different wavelengths may be conveyed to an intended destination. At the intended destination, the signals may be demultiplexed and/or split to form a plurality of output signals that may be transmitted to subscribers or other end users.  
           [0003]    Thus, it may be important to know whether each channel has sufficient power. To this end, the demultiplexed signals may be conveyed through planar light circuits. Planar light circuits are integrated circuits with waveguides formed using semiconductor processing techniques. At periodic intervals, a trench may be formed into the planar light circuit so that light traveling in a core within that circuit is reflected upwardly. The upwardly reflected light may then be detected by an onboard photodetector.  
           [0004]    Conventionally, the planar light circuit and the photodetector or power monitor are separate devices coupled by a fiber optic cable. By placing the photodetector directly on the planar light circuit, considerable efficiencies can be achieved.  
           [0005]    However, power monitoring devices, such as photodiodes, exposed on top of planar light circuits may suffer from moisture exposure. Such exposure may cause increased dark current.  
           [0006]    To this end, the entire planar light circuit and photodiode may be encapsulated within a container. But this is particularly expensive and makes connections to components on the planar light circuit more difficult.  
           [0007]    Thus, there is a need for better ways to protect power monitors on planar light circuits. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS FIG.  1  is a schematic, partial, enlarged cross-sectional view of one embodiment of the present invention;  
       [0008]    [0008]FIG. 2 is an enlarged, partial, cross-sectional view of still another embodiment of the present invention;  
         [0009]    [0009]FIG. 3 is an enlarged, cross-sectional view of one embodiment of the present invention;  
         [0010]    [0010]FIG. 4 is a cross-sectional view taken generally along the line  4 - 4  in FIG. 3, in accordance with one embodiment of the present invention;  
         [0011]    [0011]FIG. 5 is a cross-sectional view taken generally along the line  5 - 5  in FIG. 3, in accordance with one embodiment of the present invention;  
         [0012]    [0012]FIG. 6 is a cross-sectional view corresponding to FIG. 5 of still another embodiment of the present invention; and  
         [0013]    [0013]FIG. 7 is a cross-sectional view corresponding to FIG. 4 in accordance with still another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    Referring to FIG. 1, in accordance with one embodiment of the present invention, instead of enclosing the entire planar light circuit  12 , a localized region  36  may be hermetically sealed by a cover  34 . Thus, only a portion of the exposed upper surface of the planar light circuit  12  is hermetically sealed, while the remainder remains unsealed and accessible for connections as necessary.  
         [0015]    More particularly, a core  20  may convey a light signal A which may correspond to one channel of a particular wavelength of a previously multiplexed wavelength division multiplexed signal. This signal A, traveling through the core  20 , may be subjected to power monitoring to determine whether that particular channel has the desired power characteristics.  
         [0016]    The core  20  may be defined within an upper cladding  16  and a lower cladding  18  over the planar light circuit substrate  12 . An interface or trench  14  may be defined through the cladding  16  and the cladding  18  in alignment with an end of the core  20 . When the light signal A passes from the core  20  into the trench  14 , it is reflected by a reflective surface  22 , which may be angled with respect to the direction of incident light. As a result, the reflected light may be deflected upwardly to a photodetector  24 .  
         [0017]    The photodetector  24  may be mounted directly on the upper surface of the planar light circuit  12 , for example, by an adhesive connection  32 . The photodiode  24  may have an active area  26  that detects the incident light. In the case shown in FIG. 1, a bottom illumination system  10  is illustrated where the light passes through the photodiode  24  to the active area  26  on a side of the photodiode  24  opposite to the side adjacent the planar light circuit  12 . Electrical contacts  28  may be coupled by wire bondings  30  to appropriate anode/cathode connections.  
         [0018]    The region  36  may be encapsulated by a cap or lid  34 , which in one embodiment may include a cylindrical wall  37  closed by a top  35 . The cylindrical wall  37  may be secured to the planar light circuit  12  and, particularly, to the cladding  16 , by a sealant  38 .  
         [0019]    The sealant  38  is effective to maintain a hermetically sealed region  36  within the cover  34 . Generally, the sealant  38  is a preform or paste, which then may be melted when the entire assembly is put together, either in a furnace or using laser illumination. In some embodiments, the laser activated sealant may be more effective because there may be less stress applied through localized heating. In general, however, the sealant  38  is heated to seal the cover  34  to the planar light circuit  12 .  
         [0020]    In one embodiment, the sealant  38  is simply a vitreous glass layer. The vitreous glass layer directly bonds the cap or lid  34  to the planar light circuit  12 . In one embodiment the cap  34  may be formed of aluminum nitride ceramic.  
         [0021]    Alternatively, a soft solder or lead based solder may be used as the sealant  38 . As still another alternative, hard solder, which is gold based, may be used as the sealant  38 .  
         [0022]    The top  35  may be joined to the wall  37  using a gold-tin preform in one embodiment when the top  35  and wall  37  are ceramic. As another alternative, a Kovar ring may be used to enable laser metal-to-metal welding between the top  35  which may be metal such as Kovar and the wall  37  which may be a ceramic such as aluminum nitride, in one embodiment. The Kovar ring may be brazed to the wall  37  that may be made of a non-metal such as a ceramic material. Then the top  35  is laser welded to wall  37  via the Kovar ring. Kovar is an alloy of nickel, cobalt, and iron.  
         [0023]    Referring to FIG. 2, a top illumination system  10   a  is illustrated. In this case, the photodiode  26  is exposed for direct illumination by the light A. A housing  44  may support an anode  50  and cathode  54  over the photodiode  26 . External electrical contacts may be made to the anode and cathode  50  and  54 .  
         [0024]    A sealant  46  may be utilized between the wall  44  and the planar light circuit  12 . The chamber  48  is again hermetically sealed. The photodiode  26  may be die attached by the adhesive  42 . The adhesive  42  may be a silver filled glass, epoxy, soft solder, or hard solder, in some embodiments of the present invention for securing the die to the housing  44 .  
         [0025]    Referring to FIG. 3, in accordance with another embodiment of the present invention, the planar light circuit  72  may be sealed to the photodetector  70 . In this example, there is no need for an encompassing cover  34  because the electrical connection between the planar light circuit  72  and the photodiode  70  also creates a localized, hermetically sealed chamber  75  at the single die level. In this case, the trench  82  receives the light signal A within the planar light circuit  72  and reflects it up to the photodetector  70 .  
         [0026]    Referring to FIG. 4, the planar light circuit  72  includes a ring pad  76  that includes an extension  78  for external electrical connections. The ring pad  76  acts as an anode. An additional pad  74  acts as a cathode coupled by a connector  80  to the exterior. The ring  76  may circle the trench  82 , as shown in FIGS. 6 and 5. The pads  76  may be in any closed geometric shape having a central opening including circles, squares, rectangles, and ovals, as a few examples.  
         [0027]    As shown in FIG. 5, the photodetector  70  includes a corresponding ring  76  that matches the ring  76  on the planar light circuit  72 . It may also include a contact  74  that matches the contact  74  on the planar light circuit  72 .  
         [0028]    Thus, the gold wire bonding pads on the photodiode  26 , for example, may be changed to a ring pad  76  for the anode and an additional pad  74  in the corner for the cathode in one embodiment. The pads  74  and  76  may be gold pads deposited on the planar light circuit  72  and photodetector  70  in one embodiment. The two pieces ( 70 ,  72 ) are then bonded metallurgically at the interface of the ring  76  and contact  74  using flip chip or other surface mount techniques along with thermal compression.  
         [0029]    As a result, not only may the wire bonding process from the photodetector to the planar light circuit be eliminated in some cases, but localized hermetic sealing may also be achieved. In some embodiments, lower costs may be achieved by eliminating the need for a hermetic package at the component level and also eliminating splicing and fusing between the planar light circuit and the photodetectors. In addition, in some cases, the gold wire bonding process for bonding the photodetector to the planar light circuit pads may be eliminated. The light traveling distance may be shortened by placing the photodetector directly on top of the planar light circuit while protecting the photodetector from exposure to extremes of humidity.  
         [0030]    Referring to FIGS. 6 and 7, another arrangement for the anodes and cathodes is illustrated. In this case, the cathode  90  may be an outer open ring  90  and the anode  92  may be an inner, closed, concentric ring on the photodiode  70 . Meanwhile, the planar light circuit  72  may include a cathode ring  90 , a anode ring  92 , and contact extensions  94  and  96  out to the edge for appropriate electrical connections. Each of the rings  90 ,  92  may be gold rings in one embodiment of the present invention.  
         [0031]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.