This invention relates to a wavelength selective beam splitter/combiner and, more particularly, to a method of and apparatus for implementing a polarized wavelength selective beam splitter/combiner. 
One technique for increasing the capacity of a wavelength division multiplexed (WDM) transmission system is to increase the number of optical wavelengths that can be processed by the system.  As the number of wavelengths increases, the wavelength separation between adjacent wavelength channels decreases resulting in an increase in the four wave mixing that occurs between adjacent wavelength channels.  Moreover, because four wave mixing increases as a function of power level and distance these additional conditions must be considered in any WDM system design. 
It is known that an optical signal outputted from an optical fiber has both a transverse electric, TE, and a transverse magnetic TM, modes.  The TE and TM components could also be modulated and used as separate signal channels.  Typically, however, optical systems are designed to modulate and process the merged TE and TM components as one signal rather than to separately modulate and process the TE and TM components of the signal.  With reference to FIG.  1, prior systems did not separately process the TE and TM components because a separate polarizing beam splitter (PBS) 105 would be needed for each wavelength being demultiplexed by the system. 
Because of the increasing demand for data transmission capacity, there is a continuing need to further increase the capacity of optical transmission systems. 
In accordance with the present invention, we implement a wavelength selective polarization beam splitter/combiner using waveguide grating routers (WGRs) having birefringent grating waveguides. 
More particularly, in accordance with our invention, a birefringent wavelength grating router (WGR) comprises (1) a first star coupler having an input and a first plurality of outputs, the input for receiving an N channel wavelength division multiplexed, WDM, signal; (2) a second star coupler having a first plurality of inputs and N pairs of outputs, each output pair associated with different orthogonal polarization states of one wavelength of the WDM signal and each output pair having a first predetermined spatial wavelength separation therebetween and a second predetermined spatial wavelength separation from an adjacent output pair; and (3) an optical grating including a first plurality of unequal length birefringent grating waveguides connecting the first plurality of first star coupler outputs and the first plurality of second star coupler inputs, the path length difference of adjacent grating waveguides determining the first and second spatial wavelength separations. 
In other embodiments, the spatial separation between the output ports of the second star coupler can be made the same or unequal.  In another embodiment, each wavelength uses only one polarization and adjacent wavelengths are orthogonally polarized to reduce four wave mixing of the wavelengths.  In a further embodiment, the WGR uses birefringent grating waveguides having a uniform cross section core while in another embodiment different parts of the waveguides have different cross sections.