Tunable periodic filter

Abstract

A polarization-interferometry based tunable periodic filter includes polarization defining components such as polarizing beam splitters or polarizing beam displacers located on the input and output sides of a phase retarder such as a birefringent crystal. A polarization independent input consisting of multiple optical channels having a periodic frequency spacing is converted to a branched output of optical channels in which each branch has a periodic frequency spacing that is different from that of the input, and which are interleaved with each other. The output period is tunable by adjusting the phase delay of orthogonal polarization components. A contrast ratio of ≧20 dB can be realized. The device allows the mux/demux of up to 200 WDM channels with a 50 GHz frequency spacing. Applications of the device include a band splitter, a wavelength selective cross-connect, and a wavelength monitor.

We claim:  
     
         1 . A tunable optical channel routing device, comprising: 
 an input for a plurality of optical channels having a frequency period;    first means for separating a polarization state of the input optical channels into orthogonal (s and p) polarization states;    means for temporally retarding one of the orthogonal polarization states with respect to the other orthogonal polarization state for producing an ordinary beam and an extraordinary beam for each of the polarization states;    second means for separating the polarization states of an output from the retarding means into orthogonal (s and p) polarization states; and    an output for the plurality of optical channels from the second means wherein each channel of a first group of output channels has a center frequency and the first group of output channels has a frequency period that is different from the frequency period of the input channels, and wherein each channel of a second group of output channels has a center frequency and the second group of output channels has a frequency period different from the frequency period of the input channels and is interleaved with the first group;    further wherein the device has a contrast ratio ≧20 dB over a spectral band from about 1520 nm to 1570 nm.    
     
     
         2 . The device of  claim 1 , wherein the first and second means for separating the polarization states each comprises at least one of a polarizing beam splitter and a polarizing beam displacer and further wherein the means for temporally retarding one of the orthogonal polarizations with respect to the other orthogonal polarization comprises at least one of a birefringent crystal and a birefringent optical fiber.  
     
     
         3 . The device of  claim 2 , further comprising means for fine-tuning the center frequency of the output channels.  
     
     
         4 . The device of  claim 2 , further comprising means for changing the length of the birefringent fiber for tuning the frequency period of the output channels.  
     
     
         5 . The device of  claim 1 , further comprising an optical path length compensator positioned in a propagation path of at least one of the first group of output channels and the second group of output channels.  
     
     
         6 . The device of  claim 5 , wherein the optical path length compensator is an achromatic half-wave plate.  
     
     
         7 . The device of  claim 5 , wherein the optical path length compensator is a transparent material having an index of refraction and a physical thickness sufficient to adjust an optical path length difference between the first group of output channels and the second group of output channels.  
     
     
         8 . The device of  claim 1 , wherein the input comprises multiple groups of input channels.  
     
     
         9 . The device of  claim 1 , wherein the first and second groups of output channels each have a frequency spacing ≦200 GHz.  
     
     
         10 . The device of  claim 1 , wherein the first and second groups of output channels each have a frequency spacing ≦100 GHz.  
     
     
         11 . The device of  claim 1 , wherein the first and second groups of output channels each have a frequency spacing ≦50 GHz.  
     
     
         12 . An optical channel routing device, comprising a concatenated plurality of devices according to  claim 8 , wherein at least one of a set of inputs and outputs of one device is connected to a respective one of a set of outputs and inputs of another device.  
     
     
         13 . The device of  claim 2 , comprising two birefringent crystals such that each one of the orthogonal polarization components passes through a respective birefringent crystal.  
     
     
         14 . The device of  claim 2 , wherein the birefringent crystal is one of YVO 4 , calcite and rutile.  
     
     
         15 . The device of  claim 1 , wherein the frequency period of the output channels is substantially constant over the wavelength range from about 1520 nm to 1570 nm.  
     
     
         16 . The device of  claim 2 , wherein the means for retarding one of the orthogonal polarizations with respect to the other orthogonal polarization comprises a polarization maintaining fiber pigtailed to an end of the birefringent fiber.  
     
     
         17 . A tunable optical channel routing device, comprising: 
 an input for a plurality of optical channels having a frequency period;    a first polarization beamsplitter in an optical path of the input channels for separating and transmitting an output comprising one of an orthogonal (s or p) polarization state of the input and for reflecting the other orthogonal (p or s) polarization state of the input;    means for steering the reflected polarization state of the input without changing the polarization state;    a birefringent crystal in the first polarization beamsplitter output optical paths having a thickness L and a c-axis oriented at 45° with respect to the s and p polarization states such that the crystal propagates an output comprising an ordinary beam and an extraordinary beam;    means for steering an output from the birefringent crystal without changing the polarization state;    a second polarization beamsplitter in the optical path of the output from the birefringent crystal for separating and transmitting a first group of output channels comprising a portion of the orthogonal (s and p) polarization states of the output and for reflecting a second group of output channels comprising another portion of the orthogonal (p and s) polarization states of the output;    wherein each channel of a first group of output channels has a center frequency and the first group of output channels has a frequency period that is different from the frequency period of the input channels, and wherein each channel of a second group of output channels has a center frequency and the second group of output channels has a frequency period different from the frequency period of the input channels and is interleaved with the first group;    further wherein the device has a contrast ratio ≧20 dB over a spectral band from about 1520 nm to 1570 nm.    
     
     
         18 . The device of  claim 17  wherein the crystal is rotatable about the c-axis for fine tuning the center frequency of the output channels.  
     
     
         19 . The device of  claim 17  further comprising a phase compensator located in an optical path after the first polarization beam splitter for fine tuning the center frequency of the output channels.  
     
     
         20 . The device of  claim 19  wherein the phase compensator is a liquid crystal.  
     
     
         21 . The device of  claim 17  wherein said device is a microoptic assembly.  
     
     
         22 . A tunable optical channel routing device, comprising: 
 an input for a plurality of optical channels having a frequency period;    a first polarization beam displacer having a thickness, d, in an optical path of the input channels for separating and transmitting an output comprising orthogonal (s and p) polarization states of the input;    a birefringent crystal in the first polarization beam displacer output optical path having a thickness L and a c-axis oriented at 45° with respect to the s and p polarization states such that the crystal propagates an output comprising at least one of an ordinary beam and an extraordinary beam;    a second polarization beam displacer having a thickness, d, in the crystal output optical path for separating and transmitting an output comprising orthogonal (s and p) polarization states of the second polarization beam displacer output, one of which is a second group of output channels;    an optical path length compensator located in an optical path of one of the s and p polarization states output from the second polarization beam displacer;    a third polarization beam displacer having a thickness, 2d, in the second polarization beam displacer output optical path of one of the s and p polarizations for combining and transmitting a first group of output channels,    wherein each channel of a first group of output channels has a center frequency and the first group of output channels has a frequency period that is different from the frequency period of the input channels, and wherein each channel of a second group of output channels has a center frequency and the second group of output channels has a frequency period different from the frequency period of the input channels and is interleaved with the first group;    further wherein the device has a contrast ratio ≧20 dB over a spectral band from about 1520 nm to 1570 nm.    
     
     
         23 . The device of  claim 22  wherein the optical path length compensator is an achromatic half-wave plate.  
     
     
         24 . The device of  claim 17  wherein the means for steering the reflected polarization states are right angle prisms.  
     
     
         25 . A tunable optical channel routing device, comprising: 
 an input for a plurality of optical channels having a frequency period;    a first polarization beamsplitter in an optical path of the input channels for separating and transmitting an output comprising one of an orthogonal (s or p) polarization state of the input and for reflecting the other orthogonal (p or s) polarization state of the input;    a first birefringent fiber coupled to the first polarization beamsplitter output for propagating a portion of the orthogonal (s and p) polarizations such that the fiber propagates an output comprising an ordinary beam and an extraordinary beam;    a second birefringent fiber coupled to the first polarization beamsplitter output for propagating another portion of the orthogonal (s and p) polarizations such that the fiber propagates an output comprising an ordinary beam and an extraordinary beam;    a second polarization beamsplitter coupled to both the first birefringent fiber output and the second birefringent fiber output for separating and transmitting a first group of output channels comprising orthogonal (s and p) polarization states of the output and for reflecting a second group of output channels comprising orthogonal (s and p) polarization states of the output;    wherein each channel of a first group of output channels has a center frequency and the first group of output channels has a frequency period that is different from the frequency period of the input channels, and wherein each channel of a second group of output channels has a center frequency and the second group of output channels has a frequency period different from the frequency period of the input channels and is interleaved with the first group;    further wherein the device has a contrast ratio ≧20 dB over a spectral band from about 1520 nm to 1570 nm.    
     
     
         26 . The device of  claim 25  wherein the first and second birefringent fibers are coupled to the first and second polarization beamsplitters by polarization maintaining fiber pigtails.  
     
     
         27 . The device of  claim 26  wherein the first and second birefringent fibers have polarization axes and the polarization maintaining fiber pigtails have polarization axes wherein the birefringent fiber axes are oriented at 45 degrees to the polarization maintaining fiber axes.  
     
     
         28 . The device of  claim 25  further comprising means for stretching at least one of the birefringent fibers for fine-tuning the center frequency of the output channels.