Photonic variable delay devices based on optical birefringence

Abstract

Optical variable delay devices for providing variable true time delay to multiple optical beams simultaneously. A ladder-structured variable delay device comprises multiple basic building blocks stacked on top of each other resembling a ladder. Each basic building block has two polarization beamsplitters and a polarization rotator array arranged to form a trihedron; Controlling an array element of the polarization rotator array causes a beam passing through the array element either going up to a basic building block above it or reflect back towards a block below it. The beams going higher on the “ladder” experience longer optical path delay. An index-switched optical variable delay device comprises of many birefringent crystal segments connected with one another, with a polarization rotator array sandwiched between any two adjacent crystal segments. An array element in the polarization rotator array controls the polarization state of a beam passing through the element, causing the beam experience different refractive indices or path delays in the following crystal segment. By independently control each element in each polarization rotator array, variable optical path delays of each beam can be achieved. Finally, an index-switched variable delay device and a ladder-structured variable device are cascaded to form a new device which combines the advantages of the two individual devices. This programmable optic device has the properties of high packing density, low loss, easy fabrication, and virtually infinite bandwidth. The device is inherently two dimensional and has a packing density exceeding 25 lines/cm 2 . The delay resolution of the device is on the order of a femtosecond (one micron in space) and the total delay exceeds 10 nanosecond. In addition, the delay is reversible so that the same delay device can be used for both antenna transmitting and receiving.

1. An index-switched optical variable delay device for varying a path length of an optical beam, comprising:
 A slab of birefringent crystal having a first birefringent axis and a second birefringent axis, a first array of corner reflectors being placed at a first side of said slab, a second array of corner reflectors being placed at an opposite side of said slab, and a multiple of polarization rotators each independently operable to rotate a first polarization state of an input light beam to a second polarization state which is substantially orthogonal to the first polarization state when being activated and leave the first polarization state unaffected when being de-activated; said two corner reflector arrays being such arranged that the optical beam entering from the first side of the slab towards the opposite side is reflected back by a first corner reflector on the opposite side towards a second corner reflector at the first side; the beam being reflected again by the second corner reflector towards a third corner reflector on the opposite side, and continuing being reflected back and forth across the slab by successive corner reflectors until exiting; the polarization rotators each being placed between the slab and a selected corner reflector.    
     
     
       2. The index-switched optical variable delay device of  claim 1  further comprising multiple lenses each having a focal length being placed at a selected position to allow said optical beam to pass through each lens; a distance between two successive lenses being substantially twice said focal length. 
     
     
       3. The index-switched optical variable delay device of  claim 1  wherein more than one such device being stacked together to form a multiple channel delay device. 
     
     
       4. The index-switched optical variable delay device of  claim 3  wherein a pair of electrodes being placed across each slab so that an electric field can be applied to the slab. 
     
     
       5. A method of changing an optical path length of an optical beam comprising the steps of:
 placing in a path of said optical beam a first polarization rotator operable to rotate a first polarization state of said optical beam to a second polarization state which is substantially orthogonal to the first polarization state when being activated and leave the first polarization state unaffected when being de-activated,    after said first polarization rotator, placing a first birefringent crystal segment having a first birefringent axis and a second birefringent axis;    making said optical beam propagate in said first birefringent crystal segment substantially perpendicularly to said first and second birefringent axis,    de-activating said polarization rotator so that said optical beam experiences a first refractive index,    activating said polarization rotator to rotate said first polarization state to said second polarization state to make said optical beam experience a second refractive index,    connecting multiple birefringent crystal segments with one another, with a polarization rotator sandwiched between any two adjacent crystal segments;    activating and de-activating each polarization rotator independently to make said optical beam experience different refractive indices in each birefringent crystal segment, thereby varying the path length of said optical beam passing through all said birefringent crystal segments.    
     
     
       6. The method of  claim 5  wherein a polarization rotator array having multiple polarization rotators is placed in front of each said birefringent crystal segment to accept multiple optical beams. 
     
     
       7. An optical delay device to vary a path length of an optical beam, said delay device comprising:
 a first polarization rotator configured to rotate the optical beam to a first polarization state when active and a second polarization state when inactive; and    a first birefringent crystal segment having a first end coupled with the first polarization rotator, said first birefringent crystal segment including a first birefringent axis substantially aligned with the first polarization state and a second birefringent axis substantially aligned with the second polarization state.    
     
     
       8. The optical delay device of  claim 7  further comprising a second polarization rotator coupled with a second end of said first birefringent crystal segment. 
     
     
       9. The optical delay device of  claim 8  further comprising a second birefringent crystal segment having an input end coupled to the second polarization rotator, a first birefringent axis of the second birefringent crystal segment substantially aligned with the first polarization state and a second birefringent axis of the second birefringent crystal segment substantially aligned with the second polarization state. 
     
     
       10. The optical delay device of  claim 9  further comprising a plurality of polarization rotators and a plurality of birefringent crystal segments coupled with one another in an alternating order. 
     
     
       11. The optical delay device of  claim 10  wherein the optical path difference between said first polarization state and said second polarization state in different birefringent crystal segments differs by a factor of two. 
     
     
       12. The optical delay device of  claim 10  wherein different birefringent crystal segments having different birefringences. 
     
     
       13. The optical delay device of  claim 9  further comprising a first polarization beamsplitter coupled with the first birefringent crystal segment;
 a first external polarization rotator placed between the first polarization beamsplitter and the first birefringent crystal segment;    a second polarization beamsplitter coupled with an output end of the second birefringent crystal segment; and    a second external polarization rotator placed between the second polarization beamsplitter and the second birefringent crystal.    
     
     
       14. The optical delay device of  claim 7  wherein said first polarization rotator is fabricated with a material selected from the group consisting of liquid crystals, birefringent crystals, magneto-optic materials, and electro-optic crystals. 
     
     
       15. A multi-channel optical device to independently control path lengths for a plurality of optical beams, the multi-channel optical device comprising:
 a first polarization rotator array having at least two polarization rotation elements, each polarization rotation element configured to rotate a corresponding optical beam in the plurality of optical beams to a first polarization state when active and to rotate the corresponding optical beam to a second polarization state when inactive, and    a first birefringent crystal segment having a first end coupled with the first polarization rotator array, said first birefringent crystal segment including a first birefringent axis substantially aligned with the first polarization state and a second birefringent axis substantially aligned with the second polarization state.    
     
     
       16. The multi-channel optical delay device of  claim 15  further comprising a second polarization rotator array coupled with a second end of said first birefringent crystal segment. 
     
     
       17. The multi-channel optical delay device of  claim 16  further comprising a second birefringent crystal segment having an input end coupled to the second polarization rotator array, said second birefringent crystal segment including a first birefringent axis of the second birefringent crystal segment substantially aligned with the first polarization state and a second birefringent axis of the second birefringent crystal segment substantially aligned with the second polarization state. 
     
     
       18. The multi-channel optical device of  claim 17  further comprising a plurality of polarization rotator arrays and a plurality of birefringent crystal segments coupled with one another in an alternating order. 
     
     
       19. The multi-channel optical device of  claim 18  wherein an optical path difference between said first polarization state and said second polarization state in different birefringent crystal segments increases by a factor of two. 
     
     
       20. The multi-channel optical device of  claim 18  wherein different birefringent crystal segments having different birefringences. 
     
     
       21. The multi-channel optical device of  claim 17  further comprising:
 a photodetector array having multiple photodetectors each operable to convert an optical signal to an electrical signal;    an electrical signal combiner having multiple input ports and operable to combine the electrical signals from the multiple photodetectors;    said photodetector array being coupled to an output end of the second birefringent crystal segment with each photodetector receiving an optical signal from each channel;    said electrical signal combiner with each input port being coupled to a corresponding photodetector on the photodetector array.    
     
     
       22. The multi-channel optical delay device of  claim 17  further comprising:
 a first polarization beamsplitter coupled with an input end of the optical delay device;    a first external polarization rotator array placed between the first polarization beamsplitter and the input end of the optical delay device;    a second polarization beamsplitter coupled with an output end of the second birefringent crystal segment; and    a second external polarization rotator array placed between the second polarization beamsplitter and the second birefringent crystal segment.    
     
     
       23. The multi-channel optical device of  claim 15  further comprising:
 A pair of electrodes being placed across said first birefringent crystal segment for applying a voltage in a predetermined direction.    
     
     
       24. The multi-channel optical device of  claim 15  wherein an output end of the device being connected to an input end of a different multi-channel variable delay device, with each channel of the device aligned with each channel of the different device to form a cascaded multi-channel variable delay device. 
     
     
       25. A method of changing an optical path length of an optical beam comprising the steps of:
 receiving the optical beam in a first polarization rotator;    adjusting the first polarization rotator to polarize the optical beam to a first desired polarization, and transmitting the output of said first polarization rotator through a first segment of birefringent crystal having a first birefringent axis and a second birefringent axis;    inputting the optical beam output by said first segment of birefringent crystal into a second polarization rotator;    adjusting the second polarization rotator to polarize the optical beam to a second desired polarization, and transmitting the output of said second polarization rotator through a second segment of birefringent crystal having a first birefringent axis and a second birefringent axis.    
     
     
       26. The method of  claim 25  wherein said first and second polarization rotators are selected from the group consisting of liquid crystal polarization rotator, half-wave plate polarization rotator, magneto-optic polarization rotator, and electro-optic crystal based polarization rotator. 
     
     
       27. The method of  claim 25  further comprising the steps of:
 determining the desired optical path length;    applying a first activation signal to set the first polarization rotator and a second activation signal to set the second polarization rotator; said first and second activation signals determined by desired optical path lengths calculated in the determining step.    
     
     
       28. The method of  claim 25  wherein one of the first desired polarization and the second desired polarization is aligned with one of the first and the second birefringent axes. 
     
     
       29. The method of  claim 25  wherein each of the first polarization rotator and the second polarization rotator has at least two independent polarization rotating elements to accept at least two optical beams. 
     
     
       30. An optical delay device for varying a path length of an optical beam comprising:
 a birefringent crystal including a first birefringent axis and a second birefringent axis;    a first corner reflector coupled to a first side of the birefringent crystal at a first position;    a polarization rotator coupled to the birefringent crystal and positioned to receive the optical beam from the first corner reflector, the polarization rotator configured to switch the polarization of the optical beam between a first polarization state and a second polarization state;    a second corner reflector positioned to reflect the optical beam output by the polarization rotator back into said birefringent crystal.    
     
     
       31. A variable optical delay device, comprising a plurality of variable optical delay units cascaded to form an optical path through which an optical beam is directed, each variable optical delay unit producing a variable optical delay and comprising:
   a polarization rotator operable to control a polarization of received light in response to a unit control signal;        a birefringent segment formed of a birefringent material and located in said optical path to receive output light from said polarization rotator and to transmit received light along said optical path; and        a unit control element, coupled to said polarization rotator to supply said unit control signal, to control light received by said birefringent segment in a first polarization state to cause a first optical delay in light output by said birefringent segment and in a second polarization state to cause a second, different optical delay in light output by said birefringent segment.     
     
     
       32. The device as in  claim 31 , wherein said birefringent material includes a birefringent crystal. 
     
     
       33. The device as in  claim 31 , wherein said birefringent material includes a PM fiber. 
     
     
       34. The device as in  claim 31 , wherein different birefringent segments in different variable optical delay units have different lengths along said optical path. 
     
     
       35. The device as in  claim 34 , wherein two adjacent different birefringent segments differ in length by a constant factor. 
     
     
       36. The device as in  claim 35 , wherein said constant factor is  2 . 
     
     
       37. The device as in  claim 34 , wherein lengths of said different birefringent segments increase successively along said optical path from a first variable optical delay unit that receives an optical beam to a last variable optical delay unit that outputs said optical beam. 
     
     
       38. The device as in  claim 34 , wherein said variable optical delay units includes a first variable optical delay unit whose birefringent segment is formed of a first birefringent material and a second variable optical delay unit whose birefringent segment is formed of a second birefringent material that has birefringence different from said first birefringent material. 
     
     
       39. The device as in  claim 34 , wherein at least one birefringent segment in one variable optical delay unit is formed of a birefringent material that is responsive to an index- control signal to change a refractive index and thus an associated optical delay therein to fine tune a total variable optical delay.   
     
     
       40. The device as in  claim 31 , wherein said variable optical delay units includes a first variable optical delay unit whose birefringent segment is formed of a first birefringent material and a second variable optical delay unit whose birefringent segment is formed of a second birefringent material that has birefringence different from said first birefringent material. 
     
     
       41. The device as in  claim 31 , wherein at least one birefringent segment in one variable optical delay unit is formed of a birefringent material that is responsive to an index- control signal to change a refractive index and thus an associated optical delay therein to fine tune a total variable optical delay.   
     
     
       42. The device as in  claim 41 , wherein said birefringent material exhibits an electro- optic effect and said index - control signal is an electric field, and wherein said one variable optical delay unit further includes a pair of electrodes coupled to said birefringent segment to supply said electric field.   
     
     
       43. The device as in  claim 41 , wherein different birefringent segments in different variable optical delay units have different lengths along said optical path. 
     
     
       44. The device as in  claim 41 , wherein birefringent segments in at least two variable optical delay units are formed of different birefringent materials. 
     
     
       45. The device as in  claim 31 , further comprising a ladder- structured optical module optically coupled in said optical path to receive an output beam from said plurality of variable optical delay units, wherein said ladder - structured optical module includes:      a plurality of ladder units stacked over one another to form a first optical path along which said output beam is received and a second optical path along which said output beam is exported to produce an additional variable optical delay; and        a common corner reflector coupled to said plurality of ladder units to reflect transmitted light from said first optical path to said second optical path,      
       
         wherein each ladder unit comprises:  
         
           a first polarization beamsplitter located in said first optical path to transmit light in a transmitting polarization and to reflect light in a reflecting polarization orthogonal to said transmitting polarization;  
         
           a second polarization beamsplitter located in said second optical path and coupled to said first polarization beamsplitter to receive light reflected from said first polarization beamsplitter and to direct received light to said second optical path, said first and said second polarization beamsplitters forming a polarization - sensitive corner reflector which transmits light of said transmitting polarization along said first optical path towards said common corner reflector and directs light of said reflecting polarization along said second optical path away from said common corner reflector;    
           a first polarization rotator in said first optical path and adjacent to said first polarization beamsplitter to control a polarization of light entering said first polarization beam splitter to vary an optical delay of said light when exiting said ladder - structured optical module;    
         
           a second polarization rotator in said second optical path and adjacent to said second polarization beamsplitter to control a polarization of light exiting said second polarization beam splitter in a manner identical to said first polarization rotator; and  
         
         
           a control unit coupled to control said first and said second polarization rotators. 
         
       
     
     
       46. The device as in  claim 45 , wherein at least a portion of successive ladder units are spaced from one another by different distances. 
     
     
       47. The device as in  claim 46 , wherein a distance between two adjacent ladder units in said portion increase successively by a factor of  2 . 
     
     
       48. The device as in  claim 31 , wherein said polarization rotator in each variable optical delay unit is selected from a group consisting of a liquid crystal polarization rotator, a half- wave plate polarization rotator, a magneto - optic polarization rotator, and an electro - optic polarization rotator.   
     
     
       49. A variable optical delay device, comprising a plurality of variable optical delay units arranged relative to one another to form an optical path through which an optical beam is directed, each variable optical delay unit comprising:
   a polarization rotator operable to control a polarization of received light in response to a unit control signal;        a PM fiber segment located in said optical path to receive output light from said polarization rotator and to transmit received light along said optical path; and        a unit control element, coupled to said polarization rotator to supply said unit control signal, to control light received by said PM fiber segment in a first polarization state to cause a first optical delay in light output by said PM fiber segment and in a second polarization state to cause a second, different optical delay in light output by said PM fiber segment.     
     
     
       50. The device as in  claim 49 , wherein each PM fiber segment in one variable optical delay unit has a length along said optical path different from lengths of other PM fiber segments in other variable optical delay units. 
     
     
       51. The device as in  claim 50 , wherein said polarization rotator in each variable optical delay unit is selected from a group consisting of a liquid crystal polarization rotator, a half- wave plate polarization rotator, a magneto - optic polarization rotator, and an electro - optic polarization rotator.   
     
     
       52. The device as in  claim 50 , wherein lengths of PM fiber segments along said optical path of two adjacent variable optical delay units are different by a constant factor of  2 . 
     
     
       53. The device as in  claim 50 , wherein at least two PM fiber segments in two different variable optical delay units exhibit different amounts of birefringence. 
     
     
       54. The device as in  claim 50 , wherein at least one PM fiber segment in one variable optical delay unit is configured to change a refractive index in response to a control signal, and wherein said one variable optical delay unit further includes an index control element coupled to supply said control signal to vary said refractive index and thus adjust an optical delay in light output by said one variable optical delay unit in addition to a control of said optical delay by said polarization rotator. 
     
     
       55. The device as in  claim 54 , wherein said one PM fiber segment exhibits an electro- optic effect and said index - control signal is an electric field, and wherein said one variable optical delay unit further includes a pair of electrodes.   
     
     
       56. A method for producing a variable optical delay in an optical beam, comprising:
   causing the optical beam to transmit through a plurality of birefringent segments along an optical path;        causing polarization states of the optical beam upon respective entries of the plurality of birefringent segments to be controlled at a first set of polarization states, respectively, to produce a first optical delay in the optical beam upon exiting the plurality of birefringent segments; and        causing a polarization state of the optical beam upon entry of at least one of the plurality of birefringent segments to be changed to produce a second, different optical delay in the optical beam upon exiting the plurality of birefringent segments.     
     
     
       57. The method as in  claim 56 , further comprising causing the plurality of birefringent segments to have different lengths along the optical path. 
     
     
       58. The method as in  claim 57 , wherein lengths of two adjacent birefringent segments are different by a factor of  2 . 
     
     
       59. The method as in  claim 56 , further comprising causing at least two of the plurality of birefringent segments to be formed of different birefringent materials with different amounts of birefringence. 
     
     
       60. The method as in  claim 56 , further comprising:
   causing at least one birefringent segment to include a birefringent material that changes a refractive index in response to an index control signal; and        causing the index control signal to be applied to the birefringent material to modify the second optical delay.     
     
     
       61. The method as in  claim 60 , further comprising causing each of the birefringent segments to include a PM fiber segment. 
     
     
       62. The method as in  claim 56 , further comprising causing the birefringent segments to include PM fiber segments. 
     
     
       63. The method as in  claim 62 , further comprising causing two different PM fiber segments to have different amounts of birefringence. 
     
     
       64. A device having a variable optical delay mechanism, comprising:
   a plurality of variable optical delay units cascaded to form a plurality of parallel optical paths, each variable optical delay unit comprising  (   1   )  a polarization rotator array of a plurality of polarization rotators respectively located in said parallel optical paths, and  (   2   )  a birefringent segment formed of a birefringent material and located in said parallel optical paths to receive and transmit output light from said polarization rotator array, wherein each polarization rotator is operable to control a polarization of received light in a first polarization state to cause a first optical delay in light output by said birefringent segment and in a second polarization state to cause a second, different optical delay in light output by said birefringent segment; and        a detector array of a plurality of optical detectors respectively located in said parallel optical paths to receive output beams output from said plurality of variable optical delay units to produce a plurality of detector signals corresponding to said output beams of said parallel optical paths.     
     
     
       65. The device as in  claim 64 , wherein different birefringent segments have different lengths along said parallel optical paths. 
     
     
       66. The device as in  claim 65 , wherein lengths of two adjacent birefringent segments are different by a factor of  2 . 
     
     
       67. The device as in  claim 64 , wherein at least two of said plurality of birefringent segments are formed of different birefringent materials with different amounts of birefringence. 
     
     
       68. The device as in  claim 64 , further comprising a laser array of lasers responding to a plurality of electrical signals to produce a plurality of laser beams respectively directed into said parallel optical paths, wherein said detector signals respectively represent said electrical signals with different delays optically produced by said plurality of variable optical delay units. 
     
     
       69. The device as in  claim 64 , further comprising:
   a laser driven by an electrical signal to produce a laser beam that carries information in said electrical signal; and        a lens located between said laser and said plurality of variable optical delay units to expand said laser beam and to direct said expanded laser beam to cover said parallel optical paths formed by said plurality of variable optical delay units, wherein different parts of said expanded laser beam undergo different optical delays through said plurality of variable optical delay units and said detector signals are replica of said electrical signal with different delays.     
     
     
       70. The device as in  claim 69 , further comprising a grid amplifier that amplifies said detector signals. 
     
     
       71. The device as in  claim 69 , further comprising an electrical signal combiner coupled to said detector array to combine said detector signals to produce a single detector output that represents a filtered result of said electrical signal. 
     
     
       72. A method, comprising:
   causing generation of an optical beam to carry information of an input electrical signal;        causing the optical beam to be expanded to allow for different parts of the optical beam to transmit through different optical paths that go through a plurality of birefringent segments;        causing polarization states of the different parts of the expanded optical beam upon entry of the plurality of birefringent segments to be controlled to produce different optical delays on the different parts of the expanded optical beam upon exiting the plurality of birefringent segments; and        causing different parts of the expanded optical beam to be converted into a plurality of electrical output signals.     
     
     
       73. The method as in  claim 72 , further comprising causing the electrical output signals to be combined into a single electrical output signal that represents a filtered result of the input electrical signal. 
     
     
       74. The method as in  claim 72 , further comprising causing the plurality of birefringent segments to have different lengths along the parallel optical paths. 
     
     
       75. The method as in  claim 72 , wherein lengths of two adjacent birefringent segments are different by a factor of  2 . 
     
     
       76. The method as in  claim 72 , further comprising causing at least two of the plurality of birefringent segments to be formed of different birefringent materials with different amounts of birefringence. 
     
     
       77. A method, comprising:
   causing generation of a plurality of optical beams to carry information of a plurality of input electrical signals;        causing the optical beams to transmit through different optical paths that go through a plurality of birefringent segments;        causing polarization states of the optical beams upon entry of the plurality of birefringent segments to be controlled to produce different optical delays on the different optical beams upon exiting the plurality of birefringent segments; and        subsequently causing different optical beams to be converted into a plurality of electrical output signals that represent the input electrical signals with different delays.     
     
     
       78. The method as in  claim 77 , further comprising causing the plurality of birefringent segments to have different lengths along the parallel optical paths. 
     
     
       79. The method as in  claim 78 , wherein lengths of two adjacent birefringent segments are different by a factor of  2 . 
     
     
       80. The method as in  claim 77 , further comprising causing at least two of the plurality of birefringent segments to be formed of different birefringent materials with different amounts of birefringence.