Abstract:
A set of three gratings may be operated in a vernier loop fashion to select a particular wavelength from a wavelength division multiplexed system. As a result, an optical add/drop multiplexer may be provided that can be tuned to select a desired wavelength. In one embodiment, the tuning may be done thermo-optically.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 11/151,000, filed on Jun. 13, 2005, now U.S. Pat. No. 7,110,622 which is a divisional of U.S. patent application Ser. No. 10/609,837, filed on Jun. 30, 2003, now U.S. Pat. No. 6,928,208. 

   BACKGROUND 
   This invention relates generally to add/drop multiplexers useful in optical communication networks. 
   Many optical communication networks operate as wavelength division multiplexed networks wavelength channels. In such networks, a number of information containing light signals (channels), of different wavelengths, may be multiplexed over a single optical communication path. At desired points, signals of a given wavelength may be added or removed from the optical path. An add/drop multiplexer is utilized to add and/or remove a particular wavelength channel from the optical path. 
   Tunable add/drop networks would offer a number of operating advantages to the service provider. Existing add/drop multiplexer technologies may be combined in ways that accomplish tunability. However, such a functionality on its own may be of little use in a network because of the impact on existing data streams while the components are being tuned. What is needed is a “hitless” tunable add/drop with the ability to tune without affecting or “hitting” the existing data flows. Hitless tunability may be achieved using paired analog switches to reroute the data stream through a bypass optical circuit, thereby protecting the data flow during the switching time of the add/drop multiplexer, or by using a full crossbar switch between pairs of wavelength demultiplexers. 
   These existing approaches may have cost, structural, and control complexities due to the use of paired analog switches or crossbar switches. In addition, they may have cost, environmental sensitivity, and reliability issues related to the use of mechanical structures. 
   Thus, there is a need for a better way to provide a tunable add/drop multiplexer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic depiction of one embodiment of the present invention; and 
       FIG. 2  is a schematic depiction of another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In some embodiments, the present invention provides an optical add/drop multiplexer using a combination of at least three sampled gratings, thermally tuned over a narrow frequency range and operated in a vernier loop configuration to select one or no wavelength channels to add or drop across a wide frequency range. 
   In a waveguide optical integrated circuit, three non-resonant parallel couplers, each containing optical sampled gratings, may be disposed as three grating assisted couplers in a vernier loop configuration to add and drop a single wavelength channel. A grating assisted coupler is a coupler with asymmetric and non-resonant waveguides that may be of different propagation constants. Resonance may be achieved using a grating written into the coupler in a region where the light in both waveguides overlaps. The grating may be sampled in one embodiment. A sampled grating may have a multi-peak optical spectrum with a given spectral peak separation and may be thermally tunable over a range at least as large as the peak separation. Two grating designs may be used with slightly different spectral peak separations so that over a relatively wide frequency range, only one pair of peaks may be made to coincide. The width of the individual grating peaks is selected to be narrow compared to the spectral peak separations so that some tuning states allow no peak overlap within the desired operating range in some embodiments of the present invention. 
   Referring to  FIG. 1 , in one embodiment, a principal waveguide  24  may include an input path  12  and an output path  14 . The waveguide  24  may be part of a planar lightwave circuit or may be an optical fiber, as two examples. A pair of identical grating assisted couplers  16  and  18  may be provided on the principal waveguide  24  in one embodiment. A third grating assisted coupler  20 , with slightly different peak separation, may be located on the bypass waveguide  26  that forms a loop with the principal waveguide  24 , intersecting the principal waveguide  24 . 
   The couplers  16 ,  18 , and  20  may have a thermal actuator  32  for tuning the gratings. The two actuators  32  on the principal waveguide  24  may be actuated in the same way, for example, by a controlled power supply  34   b . The thermal actuator  32  on the bypass waveguide  26  may be controlled independently by a controlled power supply  34   a . The waveguide circuit  10  may have an optical input port  12 , an output port  14 , an add port  30 , and a drop port  28 . 
   In one embodiment, the gratings in  16 ,  18 , and  20  may be entirely reflective and the optical paths defined by the waveguides  24  and  26  may be equal. The couplers  16  and  18  may be aligned to couple a multiplicity of wavelength channels across the secondary waveguide  26 . Isolation may be achieved in some embodiments because any leaking light along the principal waveguide  24  is filtered out twice. Light in the secondary waveguide  26  is then totally coupled back into the output port  14  by the coupler  18 . 
   If the two path lengths (coupled and non-coupled channels) are the same, as illustrated, the configuration may be hitless because the data packets go either direction without experiencing any relative time delay. A phase control  22  may be provided in some embodiments to reduce destructive interference upon recombination. In one embodiment, the path length around the bend is exactly equal on the waveguides  24  and  26 , so a single phase setting may be adequate for all channels. 
   When the couplers  16  and  18  are tuned, the effective reflection depth into the grating changes, dynamically unbalancing the equal (at full reflection) path lengths. This effect can be compensated dynamically by changing the phase control  22  during tuning. 
   The coupler  20  accomplishes the dropping or adding of a single wavelength channel, leaving the other channels unaffected. In operation, the majority of channels pass straight through the device  10  and through the phase control  22 . A first set of wavelength channels is selected by the coupler  16  to pass through the secondary waveguide  26 . These channels are reinserted into the output port  14  by the coupler  18 , with the exception of the unique channel that is dropped or added at the coupler  20 . 
   The gratings may be designed to operate in a vernier fashion so that either a single wavelength channel (or no channels) are selected out for drop or add. The spectra of the (identical) gratings in the couplers  16  and  18  have a first peak spacing whereas the spectrum of the grating in the coupler  20  has a different peak spacing. Channels that lie within the spectrum given by the product of the two grating spectra are dropped (and/or added). Because of the different peak spacing, the product spectrum typically has only a single peak. The adjacent peaks do not overlap. 
   The single peak may be tuned to drop and/or add an individual wavelength channel. By tuning the gratings together at the same rate, the product spectrum tunes continuously from one channel to the next. By tuning the grating spectrum of coupler  20  relative to the spectrum of couplers  16  and  18 , the product spectrum tunes in jumps between adjacent peaks of the sampled grating spectra, skipping over many channels at a time. Thus, by appropriate tuning of the two spectra (together and/or relative), any individual channel may be selected over a broad tuning range. 
   By making the width of the individual grating spectra sufficiently narrow, tuning states are enabled in which no wavelength channel is selected. Further, tuning states may be chosen in which the product grating spectrum peak lies between channels so that, again, no channel is selected. Hitless operation may be achieved by selecting only no-channel states between channel-tuned states. 
   Referring to  FIG. 2 , multiple independent multiplexers  10  may be provided in any convenient geometry, provided that crossings are sufficiently isolated and thermal effects are well controlled. In the embodiment shown in  FIG. 2 , the output of a first multiplexer  10   a  is provided as the input of a second multiplexer  10   b  and the output of the second multiplexer  10   b  is provided as the input of a third multiplexer  10   c.    
   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.