Abstract:
A tunable filter may be utilized to successively tune to different wavelengths. As each wavelength of the wavelength division multiplexed signal is extracted, it may be successively power monitored. Thus, power monitoring may done without requiring separate power monitors for each channel. This results in considerable advantages in some embodiments, including reduced size, reduced complexities in fabrication, and reduced yield issues in some embodiments.

Description:
BACKGROUND 
     This invention relates generally to power monitors for optical circuits. 
     In wavelength division multiplexing applications, a number of channels, each of a different wavelength, may be multiplexed over a single optical path, such as a waveguide or fiber. Channel power monitoring becomes increasingly important with more channels because it is necessary to ensure that each channel has sufficient power. 
     Power monitoring may be done through a free space optical approach where a bulk reflection grating is used to disperse light of different wavelengths for different channels. Alternatively, a planar lightwave circuit approach may be used where each channel is monitored by one tap plus one power monitor after the multiplexer. Although both approaches work well in terms of optical functionality, the former suffers from a large form factor and less integrateability with planar optical devices. The latter poses substantial process and yield challenges since a large number of monitors may be needed for high channel counts, especially in dense wavelength division multiplexing. 
     Thus, there is a need for better ways to monitor power in an optical wavelength division multiplexed network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic depiction of one embodiment of the present invention; 
         FIG. 2  is a top plan view of the filter shown in  FIG. 1  in accordance with one embodiment of the present invention; and 
         FIG. 3  is an enlarged cross-sectional view taken generally along the line  3 - 3  in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a tunable power monitor circuit  10  may include a channel waveguide  12 , a tap coupler  14 , a tunable filter  18 , and a single power monitor  20  in one embodiment. In one embodiment, a small proportion of the light on the waveguide  12  is tapped out through the tap coupler  14 . For example, in one embodiment, five percent of the overall power may be tapped for power monitoring purposes. 
     The tapped light, which includes all the wavelengths that are multiplexed on the waveguide  12 , is sent to a tunable filter  18 . The tunable filter  18  selects one channel of one wavelength at a time and couples the selected channel to a power monitor  20  where the channel power is measured. 
     In this way, the monitor circuit  10  behaves like a fixed wavelength single channel power monitor. However, because the monitor circuit  10  is tunable, the need for a monitor  20  for each channel may be avoided in some embodiments. This makes the monitor  10  smaller, and more integrateable in some applications. 
     The tunable filter  18  may be implemented in one of a variety of ways, including in the form of a grating coupler, as shown in  FIG. 2 . By burying the grating coupler in a polymer well  28 , and using local heaters  24   a  and  24   b  to thermally change the polymer&#39;s refractive index, the grating coupler can be made tunable through thermo-optical principles. The change of the polymer&#39;s refractive index changes the wavelength of the grating coupler. As a result, different channels are coupled to the power monitor  20  in  FIG. 1  for a power measurement. By appropriate design, a wide range of channels can be sequentially scanned over time to the power monitor  20 . For example, one channel at a time may be sequentially scanned to the power monitor  20 . 
     Channels may be selected by varying the heat applied to the filter  18  to select a particular channel. The bond pads  26  may be coupled to variable power supplies  34  ( FIG. 1 ) to vary the resistive heating of each heater  24 . This enables selection of a desired channel by the thermo-optical effect on refractive index of the well  28 . 
     The filter  18  may be made of two asymmetric single mode waveguide cores  16  or a single twin mode waveguide core, as two examples. In either example, the cores  16  may be made of germanium and silicon dioxide. 
     Referring to  FIG. 3 , the gap G between the cores  16   a  and  16   b  may be zero in a twin mode embodiment. A tilted reflection grating can be inscribed in either the core  16  or the gap G region for a twin core design or in the waveguide core for a twin mode design. In both cases, the two cores  16  of the coupler may be asymmetric. Local heaters  24   a  and  24   b  of low power consumption may be introduced for changing the temperature of the polymer well  28  and, therefore, its refractive index. The heaters  24  may be coupled through bond pads  26  and metallization  34  to one of the power supplies  34  of  FIG. 1 . 
     The power monitor  20  may be made through trenching and flip-chip bonding processes in one embodiment. For example, an angular trench may be made which reflects light from a waveguide upwardly to an overlying photodetector flip-chip bonded to the trenched substrate. The reflective trench may be formed by etching at an angle and covering the angled, etched surface with a reflective material. 
     In accordance with other embodiments of the present invention, the tunable filter  18  may be a Mach-Zehnder interferometer-based coupler, a phase-shifted long period grating coupler, or a grating assisted ring-like coupler, to mention a few examples. A thermo-optic tuning mechanism may, for example, be used with a local heating scheme and each application may be similar to that described above. 
     As a result, a large number of monitors are not needed, reducing the process and yield challenges in some embodiments. This may reduce the size of the power monitor and make it more amenable to planar optical devices. Thus, in some embodiments, the filter  18  may be formed on a planar light circuit having a cladding  30  and a semiconductor substrate  32 . In one embodiment the cladding  30  may be silica and the substrate  32  may be silicon. 
     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.