Tunable add/drop optical filter

Abstract

In all-optical networks, optical switching and routing become the most important issues for interconnecting the transport network layers. This invention describes a novel tunable optical add/drop filter for the all-optical wavelength-division-multiplexing (WDM) network applications. This filter can add or drop part of the high transmission capacity signals of a WDM link. It can be used to decentralized access point in the access network or as small core network node to realizing branching points in the network topology. It works in both wavelength and space domains. It has the advantages of: 1) High throughput and low voltage operation; 2) Wide tuning range and therefore, high channel capacity; 3) High isolation and high directivity between input and output ports; 4) Compact device packaging is possible as compares to the conventional grating and mechanical switching type of add/drop filter; 5) Multiple ports add/drop tunable filters can be realized with this invention to interconnect multiple WDM networks. This novel add/drop filter can be used in various WDM topologies. It enhances the performance of the conventional tunable filter by re-routing the rejected wavelengths back to network, which not only save the precious optical energy, but also cut down the return loss of the device.

I claim:  
     
       1. A tunable add/drop filter which comprises  comprising: 
       a nonreciprocal setup for optical channels return to WDM network, a tunable filter for predetermined optical channel selection, and a reciprocal optical add/drop setup for add/drop operations, where, the non-reciprocal optical setup is a combination of an optical birefringent element which has a thickness of d, a Faraday rotator that rotates eigen polarization of input light beam by 45°, a polarization beam combiner and a right angle prism placed at an entrance side of the birefringent element to recombine returned optical signals; the tunable filter for selectively dropping or adding the optical channels; and the reciprocal optical add/drop setup comprises two optical birefringent elements with thickness of (1/2)d, and with their polarization eigen planes oriented at 45° and −45° relatively to a polarization eigen plane of the birefringent element in the non-reciprocal setup, respectively.   
       wherein said non-reciprocal optical setup comprises a combination of an optical birefringent element, which has a thickness of d, and a Faraday rotator that rotates eigen polarization of input light beams by  45 °, and  
       
         wherein said non-reciprocal optical setup further comprises a polarization beam combiner and a right angle prism placed at an entrance side of said birefringent element to recombine returned optical signals. 
       
     
     
       2. The tunable add/drop filter of claim  1 , wherein said tunable filter is  comprises a liquid crystal based, birefringent filter, a pair of halfwave plates are added to the front and back facets of the filter; said two halfwave plates are placed to intersect two orthogonally polarized incident light paths, respectively; a first one of the halfwave plates is placed on a first light path that rotates an ordinary light wave into extraordinary light wave that aligns a polarization of the light wave to an optical axis of the liquid crystal film; a second one of the halfwave plates is placed on a second light path that rotates the extra-ordinary light wave into ordinary light wave. 
     
     
       3. The tunable add/drop filter of claim  2 , wherein said tunable birefringent filter is a patterned liquid crystal based filter with a plurality of sections; each filter section can selectively filter through optical channel and said tunable add/drop filter forms a tunable multiple-port add/drop filter. 
     
     
       4. The tunable add/drop filter of claim  1 , wherein said reciprocal optical add/drop setup comprises two optical birefringent elements. 
     
     
       5. The tunable add/drop filter of claim  1 , wherein said tunable filter comprises a filter selected from the group consisting of: piezo- tuned, Fabry - Perot tunable filters; liquid crystal based Fabry - Perot tunable filters; tunable polarization interference filters; and acoustooptic tunable filters.   
     
     
       6. The tunable add/drop filter of claim  1 , wherein said tunable filter further comprises a pair of halfwave plates, each of said pair located on opposite sides of said filter, respectively. 
     
     
       7. A tunable add/drop filter comprising: 
       
         a tunable filter which is selectively adjustable to pass only desired frequencies of light waves;  
       
       
         means for splitting an optical input polarization into two eigen orthogonal polarizations;  
       
       
         means for rotating said two orthogonal eigen polarizations by  45 ° and passing the rotated orthogonal eigen polarizations to a first side of said tunable filter;  
       
       
         means for spatially combining an optical output from a second side of said tunable filter;  
       
       
         means for spatially combining undesired frequencies of said two orthogonal eigen polarizations which are rejected by said tunable filter; and  
       
       
         means for outputting a combination of said undesired frequencies. 
       
     
     
       8. The tunable add/drop filter of claim  7 , further comprising means for inputting an optical signal to said second side of said tunable filter; and 
       
         means for splitting said optical signal into two eigen orthogonal polarizations and passing the split optical signal to said second side of said tunable filter; and  
       
       
         means for spatially combining desired frequencies of said split optical signal which are outputted from said first side of said tunable filter with said undesired of frequencies of said two orthogonal eigen polarizations which are rejected by said tunable filter and outputting a combined signal through said means for outputting. 
       
     
     
       9. A tunable add/drop filter comprising:  
       selectively adjustable means for passing only desired frequencies of light waves;  
       
         means for splitting an optical input signal into two eigen orthogonal polarizations;  
       
       
         means for rotating said two orthogonal eigen polarizations by  45 ° and passing the rotated orthogonal eigen polarizations to a first side of said selectively adjustable means;  
       
       
         means for spatially combining an optical output from a second side of said selectively adjustable means;  
       
       
         means for redirecting frequencies of said two orthogonal polarizations, which are rejected by said selectively adjustable means, through said means for splitting such that said rejected frequencies follow different paths than the split optical input signal; and  
       
       
         means for spatially combining said rejected frequencies of said two orthogonal eigen polarizations. 
       
     
     
       10. The tunable add/drop filter of claim  9 , further comprising means for outputting the combination of said rejected frequencies. 
     
     
       11. The tunable add/drop filter of claim  9 , further comprising: 
       
         means for splitting a second optical input signal into two eigen orthogonal polarizations and passing the split second optical signal to said second side of said selectively adjustable means;  
       
       
         means for directing frequencies of said split optical signal, which pass through said selectively adjustable means, along said different paths and with the same polarizations as said rejected frequencies of said first optical signal; and  
       
       
         means for spatially combining said rejected frequencies of said first optical signal and the passed frequencies of said second optical signal. 
       
     
     
       12. The tunable add/drop filter of claim  11 , further comprising means for outputting the combination of said rejected frequencies of said first optical signal and the passed frequencies of said second optical signal. 
     
     
       13. A tunable add/drop filter comprising: 
       
         at least two input ports for inputting optical signals;  
       
       
         selectively adjustable means for passing only desired frequencies of light waves;  
       
       
         means for splitting an optical input signal from each said input port into two eigen orthogonal polarizations;  
       
       
         means for rotating each pair of said two orthogonal eigen polarizations by  45 ° and passing the rotated orthogonal eigen polarizations to a first side of said selectively adjustable means;  
       
       
         means for spatially combining outputs received from a second side of said selectively adjustable means to form optical outputs, wherein each optical output corresponds to a respective pair of said two orthogonal eigen polarization inputted to said first side of said selectively adjustable means;  
       
       
         means for redirecting frequencies of each of said pairs of said two orthogonal polarizations, which are rejected by said selectively adjustable means, through said means for splitting such that said rejected frequencies follow different paths than the split optical input signals; and  
       
       
         means for spatially combining said rejected frequencies, wherein each spatial combination of rejected frequencies corresponds to a respective pair of said two orthogonal eigen polarizations rejected from said first side of said selectively adjustable means. 
       
     
     
       14. The tunable add/drop filter of claim  13 , further comprising at least two output ports for outputting said spatial combinations of rejected frequencies. 
     
     
       15. The tunable add/drop filter of claim  14 , wherein a number of said input ports equals a number of said output ports. 
     
     
       16. The tunable add/drop filter of claim  13 , further comprising: 
       
         at least two input ports for inputting optical add signals;  
       
       
         means for splitting said optical add input signal into two eigen orthogonal polarizations each, and passing the split optical add signals to said second side of said selectively adjustable means;  
       
       
         means for directing frequencies of said split optical add signals, which pass through said selectively adjustable means, along said different paths and with the same polarizations as said rejected frequencies of said optical input signals; and  
       
       
         means for spatially combining said rejected frequencies of said optical input signal and the passed frequencies of said optical add signals, wherein each spatial combination of rejected frequencies and passed frequencies corresponds to a respective pair of said input ports for inputting optical signals and said input ports for inputting optical add signals. 
       
     
     
       17. The tunable add/drop filter of claim  16 , further comprising means for outputting said spatial combinations of said rejected frequencies and said passed frequencies. 
     
     
       18. The tunable add/drop filter of claim  17 , wherein said means for outputting comprises at least two output ports. 
     
     
       19. The tunable add/drop filter of claim  18 , wherein a number of said input ports for inputting optical signals, said input ports for inputting optical add signals, and said output ports are all the same. 
     
     
       20. A tunable add/drop filter comprising 
       
         a nonreciprocal setup for optical channels return to WDM network, a tunable filter for predetermined optical channel selection, and a reciprocal optical add/drop setup for add/drop operations,  
       
         wherein said reciprocal optical add/drop setup comprises two optical birefringent elements, and wherein said two optical birefringent elements of said reciprocal optical add/drop setup have polarization eigen planes oriented at  45 ° and − 45 ° relative to a polarization eigen plane of said birefringent element in said non - reciprocal setup, respectively.   
     
     
       21. The tunable add/drop filter of claim  20 , wherein said non- reciprocal optical setup comprises a combination of an optical birefringent element, which has a thickness of d, and a Faraday rotator that rotates eigen polarization of input light beams by  45 °.   
     
     
       22. The tunable add/drop filter of claim  21 , wherein said non- reciprocal optical setup further comprises a polarization beam combiner and a right angle prism placed at an entrance side of said birefringent element to recombine returned optical signals.   
     
     
       23. The tunable add/drop filter of claim  21 , wherein said reciprocal optical add/drop setup comprises two optical birefringent elements, each having a thickness of (   1 / 2   ) d, and with their polarization eigen planes oriented at  45 ° and − 45 ° relative to a polarization eigen plane of said birefringent element in said non - reciprocal setup, respectively.   
     
     
       24. The tunable add/drop filter of claim  23 , wherein said tunable filter further comprises a pair of halfwave plates, each of said pair located on opposite sides of said liquid crystal based, birefringent filter, respectively. 
     
     
       25. The tunable add/drop filter of claim  20 , wherein said tunable filter comprises a liquid crystal based, birefringent filter. 
     
     
       26. The tunable add/drop filter of claim  25 , wherein said tunable filter comprises a patterned liquid crystal based, birefringent filter with a plurality of sections. 
     
     
       27. The tunable add/drop filter of claim  20 , wherein said tunable filter comprises a filter selected from the group consisting of: piezo- tuned, Fabry - Perot tunable filters; liquid crystal based Fabry - Perot tunable filters; tunable polarization interference filters; and acoustooptic tunable filters.   
     
     
       28. A tunable add/drop filter comprising 
       
         a nonreciprocal setup for optical channels return to WDM network,  
       
       
         a tunable filter for predetermined optical channel selection, and a reciprocal optical add/drop setup for add/drop operations,  
       
         wherein said non - reciprocal optical setup comprises a combination of an optical birefringent element, which has a thickness of d, and a Faraday rotator that rotates eigen polarization of input light beams by  45 °, and    
         wherein said reciprocal optical add/drop setup comprises two optical birefringent elements, each having a thickness of  (   1 / 2   ) d, and with their polarization eigen planes oriented at  45 ° and − 45 ° relative to a polarization eigen plane of said birefringent element in said non - reciprocal setup, respectively.   
     
     
       29. The tunable add/drop filter of claim  28 , wherein said tunable filter further comprises a pair of halfwave plates, each of said pair located on opposite sides of said liquid crystal based, birefringent filter, respectively. 
     
     
       30. The tunable add/drop filter of claim  28 , wherein said tunable filter comprises a filter selected from the group consisting of: piezo- tuned, Fabry - Perot tunable filters; liquid crystal based Fabry - Perot tunable filters; tunable polarization interference filters; and acoustooptic tunable filters.