Patent Abstract:
An arrayed waveguide grating may include a plurality of waveguides, each associated with a different channel in a wavelength division multiplexed system. Each incoming signal channel in a node of a wavelength division multiplexed network may be of an arbitrary wavelength and is provided to one of the wavelength converters attached on the arrayed waveguide grating. Each converter also receives one of blank light channels of different wavelengths on a grid. The converters convert each of the incoming wavelength signals to one of the distinct new wavelength signals on the grid of wavelengths, and these new wavelength signals are multiplexed into a fiber.

Full Description:
BACKGROUND 
   This invention relates generally to optical networks and, particularly, to wavelength division multiplexed networks. 
   In wavelength division multiplexed (WDM) optical networks, several signals are transmitted at different wavelengths over a single fiber. In a variety of circumstances, various wavelengths may be added to an existing network along the way or removed from the network along the way. As a result, conflicts may arise where several channels of the same wavelength are delivered to the same network node and must be sent along the same fiber. 
   To overcome these conflicts, one signal of the duplicate wavelengths needs to be converted to a different wavelength. Existing wavelength converters operate as standalone devices, converting a signal from one incoming channel to a signal of a different wavelength in one outgoing channel. An intricate management of the network is needed to multiplex the signals and to avoid channel conflicts in subsequent nodes. 
   Thus, there is a need for better ways to handle the issue of wavelength conflicts in optical networks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic depiction of one embodiment of the present invention; and 
       FIG. 2  is a cross-sectional depiction of a wavelength converter which is part of the embodiment shown in  FIG. 1  in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   An arrayed waveguide grating (AWG)  10 , sometimes also called a waveguide grating router (WGR) or a phasar, may be formed as an integrated optical circuit. The AWG  10  may include a plurality of input waveguides  18  that leads to a star coupler  12   a , an array of waveguides  14  between the star coupler  12   a  and the star coupler  12   b , and an output waveguide  19  coupled to the coupler  12   b . The length of each arrayed waveguide  14  in the array of waveguides  14  may be distinguished from its adjacent waveguide by a length difference (ΔL). 
   A channel of certain wavelength enters the AWG in one of the input waveguides  18 . The input coupler  12   a  splits the light in the channel among the arrayed waveguides  14 . Each portion of the input light traveling through an arrayed waveguide  14  includes any wavelength that has entered the AWG  10  in any of the input channels  18 . Each wavelength then acquires an individual phase shift. In addition, each wavelength for each channel receives phase shifts in the input and output star couplers  12 . Therefore, each portion of light of a given wavelength requires different phase shifts, and all these portions interfere at the output coupler  12   b . That leads to the property of an AWG that the light channel focuses on one of the output waveguides  19  depending on the position of an input waveguide  18  and the wavelength of the channel. In order to multiplex the light channels from input waveguides  18   a , . . .  18   d  into the same output waveguide  19 , these channels must be set on a wavelength grid λ 1 , . . . λ N . This grid is usually such that the frequency difference between adjacent channels λ n  and λ n+1  is constant. 
   In an arbitrary situation in a WDM network, the input channels do not satisfy this condition. Some of the input channels occupy the same wavelength. Besides this situation changes dynamically as traffic patterns in the network changes. 
   In the embodiment shown in  FIG. 1 , each of a plurality of lasers  32 , such as a continuous wave laser, generates one of N signals that are placed into the input waveguide  18 . Each laser  32  generates a constant intensity light of a single wavelength from λ 1  through λ N . Each channel includes a wavelength converter  20 . Thus, the laser  32   a , at wavelength λ 1 , generates a light signal that enters a wavelength converter  20   a . The resulting output signal is passed to the coupler  12   a.    
   Each converter  20  converts the input light signal, that comes in at some wavelength from a fiber  30   a , to a different wavelength. In accordance with one embodiment of the present invention, a regular grid of wavelengths with regular spacing there between is defined by the array of lasers  32 . The incoming wavelengths on the incoming channels indicated by the fibers  30  are then converted to the appropriate grid of wavelengths. In particular, the signal that comes in on each input fiber  30  is modulated so as to carry the same information, but using a light signal having a different wavelength. 
   Again, referring to the example shown in  FIG. 1 , a laser  32   a  produces light of a wavelength λ 1 . The input signal from another optical component comes in over the fiber  30   a  at a wavelength λ 3 . The output signal from the converter  20   a  carries the information that came in on the fiber  30   a , but provides it using the wavelength λ 1  supplied by the laser  32   a . The wavelength λ 1  is then provided to an optical fiber coupled to the output waveguide  19   a.    
   The same operation occurs in each of the other channels. Thus, for example, if the input signal has a potential wavelength conflict, for example, because the input signal on the fibers  30   a  and  30   d  are at the same wavelength (λ 3 ), the resulting converted signals all have different wavelengths. For example, the wavelength that came in on the fiber  30   a  is converted to the wavelength λ 1  and the signal that came in on the fiber  30   d  is converted to a wavelength λ N . 
   As a result, a regular grid of distinct wavelength channels is generated for all the incoming signals, regardless of their original wavelength. The resulting output signal coming out of the output waveguide  19  has the regular grid of distinct wavelengths preordained by the array of lasers  32 . Outgoing wavelength channels are then directed into a single fiber connected to the AWG  10 . This avoids the possibility of wavelength conflict. 
   In accordance with one embodiment of the present invention, the converters  20  may each receive a blank optical channel from a different laser  32  in the plane of the integrated circuit forming the AWG  10 . The incoming signal from the fiber  30  may be brought vertically into the converter  20 . 
   In one embodiment, the conversion may occur in a group III-V semiconductor material wavelength converter  20 , as shown in FIG.  2 . The converter  20  comprises a PIN detector diode  34  on top of a PIN diode modulator  36 . The PIN diode modulator  36  includes a p-type region  42 , an intrinsic region  40 , and an n-type region  38 . The AWG  10  may include an upper silica layer  46  over a substrate  42  that may be silicon in one embodiment. In the upper layer  46 , the germanium doped buried-channel silica waveguide  44  are formed. The wavelength converter  20  sits in a trench  48  formed in the substrate  42 . The layer of silica waveguide  44  is aligned to the modulator  38 . 
   In one embodiment, the input signal from a fiber  30  is absorbed in the PIN detector  20 , thereby creating free carriers and changing the voltage on the modulator  36 . The blank light is then modulated due to a change in absorption caused by the voltage change. 
   In another embodiment, the wavelength conversion may occur due to cross-gain modulation between the two laser beams. 
   The modulated blank signal is coupled to a silica output waveguide  18  which then passes on to a star coupler  12   a  as shown in FIG.  1 . The waveguides  18  are actually formed in the silica layer  46  (and are positioned in the page in  FIG. 2 ) behind the p-type region  42  and intrinsic region  40  of the PIN diode modulator  36 . 
   In accordance with some embodiments of the present invention, the AWG  10  may be defined by lithographic methods and fabricated of a III-V semiconductor material in a single process to include the waveguides  14 , the couplers  12 , and the converters  20 . As a result, the cost of optical components may be reduced because the cost of optical components is largely driven by fiber interfacing and aligning of the devices and the cost of testing them. Combining multiple devices into a single integrated circuit may significantly decrease cost in some embodiments. Also, the integrated approach may decrease losses of optical power in the network since most of optical losses occur in the interfaces between fibers and integrated circuits. 
   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.

Technology Classification (CPC): 6