Apparatus and methods for depolarizing polarized light in optical fibers and in free space

Abstract

Techniques and devices for depolaring polarized light by using one or more birefringent optical elements in the optical path of light to scramble the state of polarization of light. Examples of devices for coupling with a fiber or a fiber device, and their applications in various fiber environments are also described.

What is claimed is:  
     
         1 ) A system to depolarize a beam of polarized light comprising: 
 a first birefringent crystal wedge to receive the beam of polarized light at an input first surface, the first birefringent crystal wedge to output spatially depolarized light from an output surface.    
     
     
         2 ) The system of  claim 1  wherein the output surface of the first birefringent crystal wedge includes a plurality of small steps, each step including a subsurface parallel to the input surface.  
     
     
         3 ) The system of  claim 1  wherein the input surface of the first birefringent wedge includes a plurality of small steps, each step including a subsurface parallel to the output surface.  
     
     
         4 ) The system of  claim 2  wherein a height of each step is less than 0.1 mm.  
     
     
         5 ) The system of  claim 1  wherein an optic axis of the first birefringent crystal wedge is oriented perpendicular to the propagation direction of the beam of polarized light.  
     
     
         6 ) The system of  claim 1  further comprising: 
 A second birefringent crystal wedge positioned adjacent to the first birefringent crystal wedge, the second birefringent crystal wedge to receive the output of the first birefringent crystal wedge and to further randomize the polarization of light output by the first birefringent crystal wedge.  
 
     
     
         7 ) The system of  claim 6  wherein the output surface of the first birefringent crystal wedge and an input surface of the second birefringent crystal wedge each include a plurality of steps, each step including a subsurface parallel to the input surface of the first birefringent crystal wedge.  
     
     
         8 ) A system to depolarize a beam of polarized light comprising: 
 an array of birefringent crystal chips arranged to from a plane, each crystal chips having an optic axis oriented such that a substantial component of the optic axis is oriented perpendicular to the propagation direction of the beam of light being depolarized, each chip dimensioned to be substantially smaller than a cross section of the beam of polarized light.    
     
     
         9 ) The system of  claim 8  wherein each birefringent crystal chip is a quarter wave plate.  
     
     
         10 ) The system of  claim 8  wherein the birefringent crystal chips are mounted on a plate of optically transparent material.  
     
     
         11 ) The system of  claim 8  wherein the birefringent crystal chips are made of mica.  
     
     
         12 ) The system of  claim 8  wherein the birefringent crystal chips are formed from cross sections of at least one rod of birefringent material.  
     
     
         13 ) The system of  claim 8  wherein the space between birefringent crystal chips is filled with an optically opaque material.  
     
     
         14 ) The system of  claim 8  wherein the birefringent crystal chips are square in shape, thereby minimizing spacing between adjacent birefringent crystal chips.  
     
     
         15 ) A system to depolarize light from a fiber comprising; 
 a collimating lens to collimate light from a first fiber,    a depolarizing element to spatially depolarize the collimated light output by the collimating lens; and    a focusing lens to focus the depolarized light output by the depolarizing element into a second fiber segment.    
     
     
         16 ) The system of  claim 15  wherein the depolarizing element is a wedge of birefringent crystal.  
     
     
         17 ) The system of  claim 16  wherein a first surface of the wedge of birefringent crystal includes a plurality of small steps, each small step including a subsurface parallel with a second surface of the wedge of birefringent crystal.  
     
     
         18 ) The system of  claim 17  wherein the depolarizing element further includes a second wedge of birefringent crystal.  
     
     
         19 ) The system of  claim 15  wherein the depolarizing element includes an array of birefringent chips positioned such that the collimated light passes through a significant number of the birefringent crystal chips.  
     
     
         20 ) The system of  claim 17  wherein an optical axis of the wedge of birefringent crystal is perpendicularly oriented to a direction of propagation of the collimated light.  
     
     
         21 ) the system of  claim 16  further comprising a polarizer to polarize the collimated light to a predetermined orientation prior to input into the depolarizing element.  
     
     
         22 ) The system of  claim 21  wherein the polarizer is oriented at an approximately 45° angle to an optical axis of the wedge of birefringent crystal.  
     
     
         23 ) A method of depolarizing light comprising: 
 collimating light output from an optical fiber segment into a polarized beam of light;    depolarizing the beam of light by directing the beam of light through a birefringent crystal to generate a depolarized beam of light; and    focusing the depolarized beam of light into a second optical fiber segment.    
     
     
         24 ) The method of  claim 23  wherein the birefringent crystal includes a plurality of birefringent crystal chips.  
     
     
         25 ) A method of forming a depolarizer comprising: 
 forming a plurality of rods of birefringent material;    bundling together the plurality of rods;    and cutting a section of the bundled rods to form a plane of birefringent chips.    
     
     
         26 ) The method of  claim 25  wherein the birefringent chips have a product of thickness and birefringnece approximately equal to one quarter of a wavelength of light.  
     
     
         27 ) The method of  claim 25  wherein a cross section of at least one rod of the plurality of rods of birefringent material is circular.  
     
     
         28 ) The method of  claim 25  wherein a cross section of at least one rod of the plurality of rods of birefringent material is square.  
     
     
         29 ) An optical fiber communication system comprising: 
 a light source;    a depolarizer to depolarize an output of the light source and output depolarized light;    a modulator to modulate the depolarized light; and a receiver to receive an output of the modulator.    
     
     
         30 ) The optical fiber communication system of  claim 29  wherein the depolarizer further comprises a wedge of birefringent crystal.  
     
     
         31 ) The optical fiber communication system of  claim 29  wherein the depolarizer further comprises a plurality of birefringent chips.  
     
     
         32 ) A communication system comprising: 
 a plurality of depolarizers, each depolarizer to receive an output from a corresponding light source;    a wavelength division multiplexer to combine the depolarized light outputs of the depolarizers;    an electro-optic modulator to modulate the multiplexed signal.    
     
     
         33 ) The system of  claim 32  further comprising a wavelength division demultplexer to demultiplex an output of the electro-optic modulator.  
     
     
         34 ) The optical fiber communication system of  claim 32  wherein the depolarizer further comprises a wedge of birefringent crystal.  
     
     
         35 ) The optical fiber communication system of  claim 32  wherein the depolarizer includes a plurality of birefringent chips.  
     
     
         36 ) A coherent heterodyne communication system comprising: 
 a modulator to modulate a laser light signal; and    a depolarizer to depolarize light output by the modulator.    
     
     
         37 ) The coherent heterodyne communication system of  claim 36  further comprising a coupler to combine an output of the depolarizer with a local oscillator light from a local laser.  
     
     
         38 ) The coherent heterodyne communication system of  claim 37  further comprising a polarizer to polarize light from the depolarizer.  
     
     
         39 ) The coherent heterodyne communication system of  claim 37  further comprising a receiver to receive the output of the coupler.  
     
     
         40 ) The optical fiber communication system of  claim 36  wherein the depolarizer further comprises a wedge of birefringent crystal.  
     
     
         41 ) The optical fiber communication system of  claim 36  wherein the depolarizer includes a plurality of birefringent chips.