Abstract:
A planar light wave circuit may include a directional coupler with two waveguides come close to one another in a so-called gap region. Polarization dependent coupling may be reduced by forming trenches on either side of the gap region to reduce birefringence.

Description:
BACKGROUND  
         [0001]    This invention relates generally to planar light circuits for optical systems.  
           [0002]    Optical systems may communicate using light signals. In some cases, light signals of different wavelengths are multiplexed together in what is known as a wavelength division multiplexing system. A planar light wave circuit is an optical device formed using conventional semiconductor fabrication techniques.  
           [0003]    A coupler is an optical device that enables light from different sources to be combined. A directional coupler is a coupler that only allows light to pass in one direction. Directional couplers may be used, for example, when optical power is to be split into two routes in a predetermined ratio, when combining optical power from two different channels into a single channel, and for tapping power for power monitoring purposes, as well as routing signals into different channels.  
           [0004]    An evanescent coupler is a type of directional coupler with relatively low insertion loss using silica on silicon technology. Unfortunately, the coupling coefficient of evanescent couplers is sensitive to polarization of the incoming light wave. As a result, the output power in either of the output ports of the coupler varies, generating different ratios of power at different output ports as the incoming polarization changes.  
           [0005]    Thus, there is a need for a way to reduce the dependence of couplers on polarization of the incident light. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a top plan view of one embodiment of the present invention; and  
         [0007]    [0007]FIG. 2 is an enlarged cross-sectional view taken generally along the line  2 - 2  in FIG. 1 in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0008]    Referring to FIG. 1, a planar light circuit (PLC)  10  may be formed using semiconductor integrated circuit fabrication techniques. A 2×2 coupler  12  is shown for illustration purposes only.  
         [0009]    The circuit  10  includes a first waveguide  14  and a second waveguide  16 . The waveguides  14 ,  16  may be hour glass shaped in one embodiment. The waveguides  14  and  16  begin in a region A on an input side and end in the region A on the output side. In the regions A, the waveguides  14  and  16  are widely separated from one another. The waveguides  14 ,  16  taper towards one another in the region B on the input side and taper away from one another in the region B on the output side. In the region C, the waveguides  14  and  16  are relatively close to one another.  
         [0010]    A significant source of polarization dependent coupling in a coupler  12  is due to a mismatch of stress induced birefringence between the cores of the waveguides  14  and  16  and the region between them. With silica on silicon couplers, the thermal coefficient of expansion of the substrate is large compared to the overlying areas of doped and undoped silica. The mismatch in thermal coefficient of expansion gives rise to highly compressive stresses in the upper layers along the plane of the coupler. Such mismatches of thermal coefficients of expansion in multilayer systems result in polarization dependent coupling.  
         [0011]    The stress in the uppermost layer in the vertical direction is comparatively less, as the uppermost layer can expand or contract more freely in the vertical direction than in the lateral direction because of the lateral boundary. This arrangement is true to a lesser degree with respect to the core and the lower cladding. This mismatch in stress between the lateral and the vertical directions generates what is known as a birefringence or change of refractive index along X and Y directions.  
         [0012]    In evanescent couplers, the problem is more complicated as the two coupled waveguides  14 ,  16  have to be very close together in the region C in FIG. 1. The high thermal coefficient of expansion in the upper cladding material cannot reduce the stress in the lower cladding material, which may be thermal oxide. The highly stressed lower cladding generates a highly stressed region between two comparatively low stress core waveguides. When the two core waveguides are placed very close to one another in the region C, the stress on the cores in the gap region C may be six to seven times larger than in the rest of the core in one embodiment. The effective refractive index contrast between the core and the gap region becomes different in the X and Y directions, giving rise to a large polarization dependent coupling.  
         [0013]    The gap stress may be reduced by introducing trenches  18   a  and  18   b  on the two outer sides of the waveguides  14  and  16 . The stress in the cores and the gap region can be relaxed in the lateral direction due to the presence of the side trenches. This helps reduce the birefringence in the gap region, and increase the birefringence in the core slightly and thereby reduce the difference in stress levels between the cores and the gap region.  
         [0014]    By means of illustration only and without limitation, in a coupler with a 5 micron×6 micron core, trenches  18  having a width of 15 microns, a depth of 30 microns, and a distance from the core of 15 microns, may significantly reduce birefringence in the gap region.  
         [0015]    In some embodiments it may also be desirable to boron dope the core material to further reduce birefringence.  
         [0016]    Referring to FIG. 2, a semiconductor substrate  12  may be covered by a lower cladding  22 . An upper cladding  20  may be positioned over the cores  14  and  16 . The trenches  18   a  and  18   b  may penetrate through the upper and lower cladding  20  and  22  in one embodiment.  
         [0017]    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.