Method and apparatus for writing grating structures using controlled phase delay between beams

Abstract

A method of writing a grating structure with at least one of predetermined amplitude, period and phase properties in a photosensitive waveguide, the method comprising providing at least two light beams which overlap in an overlap region to form an interference pattern; moving the photosensitive waveguide through the overlap region; and modulating the phase of at least one of the light beams relative to the phase of the other light beams using a non-mechanical beam modulator so that the interference pattern appears to move through the overlap region, the apparent movement being variably controlled in response to the movement of the photosensitive waveguide such that a grating structure is written with the at least one of predetermined amplitude, period and phase properties. The apparent movement of the interference pattern may be variably controlled to match the movement of the waveguide, or to be deliberately detuned. The grating structure may be chirped or apodized.

1 . A method of writing a grating structure with at least one of predetermined amplitude, period and phase properties in a photosensitive waveguide, the method comprising: 
 providing at least two light beams which overlap in an overlap region to form an interference pattern;    moving the photosensitive waveguide through the overlap region; and    modulating the phase of at least one of the light beams relative to the phase of the other light beams using a non-mechanical beam modulator so that the interference pattern appears to move through the overlap region, the apparent movement being variably controlled in response to the movement of the photosensitive waveguide such that a grating structure is written with the at least one of predetermined amplitude, period and phase properties.    
   
   
       2 . A method as claimed in  claim 1 , further comprising sensing the movement of the photosensitive waveguide relative to the apparent movement of the interference pattern and variably controlling the apparent movement of the interference pattern in response to the sensed movement of the photosensitive waveguide.  
   
   
       3 . A method as claimed in  claim 1 , in which the apparent movement of the interference pattern is variably controlled to be substantially the same as the movement of the photosensitive waveguide over at least a portion of the grating structure.  
   
   
       4 . A method as claimed in  claim 1 , in which the apparent movement of the interference pattern is variably controlled to be different to the movement of the photosensitive waveguide over at least a portion of the grating structure.  
   
   
       5 . A method as claimed in  claim 1 , in which the apparent movement of the interference pattern is variably controlled to be different to the movement of the photosensitive waveguide over at least a portion of the grating structure and to be substantially the same as the movement of the photosensitive waveguide over at least another portion of the grating structure.  
   
   
       6 . A method as claimed in  claim 2 , in which the sensing and the variably controlling comprises a feedback loop for improving noise properties of the grating structure.  
   
   
       7 . A method as claimed in  claim 1 , in which the at least two light beams are provided by splitting a single coherent light beam and directing the at least two light beams to overlap in the overlap region.  
   
   
       8 . A method as claimed in  claim 7 , in which the modulation of the phase of the at least one of the light beams is performed before the splitting of the single coherent light beam.  
   
   
       9 . A method as claimed in  claim 7 , in which the modulation of the phase of the at least one of the light beams is performed after the splitting of the single coherent light beam.  
   
   
       10 . A method as claimed in  claim 7 , in which the modulation of the phase of the at least one of the light beams is performed during the splitting of the single coherent light beam.  
   
   
       11 . A method as claimed in  claim 1 , in which the non-mechanical beam modulator comprises at least one of a group selected from: 
 an electro-optic phase modulator;    a magneto-optic phase modulator;    a frequency shifter;    an acousto-optic frequency shifter;    a controllable optical retarder; and    an optical delay line.    
   
   
       12 . A method as claimed in  claim 1 , in which: 
 two of the light beams have substantially orthogonal polarization states;    the non-mechanical beam modulator modulates the phase of at least one of the polarization states relative to the other polarization states; and    the method further comprises aligning the polarization states of the light beams after the modulation to form the interference pattern.    
   
   
       13 . A method as claimed in  claim 1 , in which the grating structure is written during a single continuous writing process.  
   
   
       14 . A method as claimed in  claim 1 , in which at least a portion of the grating structure includes at least one of an apodized structure and a chirped structure.  
   
   
       15 . An apparatus for variably controlling the apparent movement of an interference pattern with reference to a moving photosensitive waveguide to write a grating structure in the photosensitive waveguide, the apparatus comprising: 
 at least one light beam source for providing at least two light beams;    a beam director for directing at least one of the light beams so that the light beams overlap in an overlap region to form the interference pattern;    at least one non-mechanical beam modulator for modulating the phase of at least one of the light beams relative to the phase of the other light    beams so that the interference pattern appears to move through the overlap region; and    a beam modulator controller for controlling the modulation of the beam modulator so that the apparent movement of the interference pattern is variably controlled in response to the movement of the photosensitive waveguide.    
   
   
       16 . An apparatus as claimed in  claim 15 , the apparatus further comprising a waveguide mover for moving the photosensitive waveguide through the overlap region.  
   
   
       17 . An apparatus as claimed in  claim 15 , the apparatus further comprising a position sensor for sensing the position of the interference pattern relative to the photosensitive waveguide.  
   
   
       18 . An apparatus as claimed in  claim 15 , in which the beam modulator controller is configurable to variably control the apparent movement of the interference pattern to be substantially the same as the movement of the photosensitive waveguide over at least a portion of the grating structure.  
   
   
       19 . An apparatus as claimed in  claim 15 , in which the beam modulator controller is configurable to variably control the apparent movement of the interference pattern to be different to the movement of the photosensitive waveguide over at least a portion of the grating structure.  
   
   
       20 . An apparatus as claimed in  claim 17 , in which the beam modulator controller includes a feedback arrangement for improving the noise properties of the grating structure by controlling the modulation of the at least one non-mechanical beam modulator in response to the movement of the photosensitive waveguide, the feedback arrangement being in communication with the position sensor.  
   
   
       21 . An apparatus as claimed in  claim 15 , in which the at least one beam source includes a beam splitter for splitting a single coherent light beam to form the at least two light beams.  
   
   
       22 . An apparatus as claimed in  claim 17 , in which the at least one beam splitter and the at least one non-mechanical beam modulator are arranged so that, in use, the modulation of the phase of the at least one of the light beams is performed after the splitting of the single coherent light beam.  
   
   
       23 . An apparatus as claimed in  claim 17 , in which the at least one beam splitter and the at least one non-mechanical beam modulator are arranged so that, in use, the modulation of the phase of the at least one of the light beams is performed before the splitting of the single coherent light beam.  
   
   
       24 . An apparatus as claimed in  claim 17 , in which the at least one beam splitter and the at least one non-mechanical beam modulator are arranged so that, in use, the modulation of the phase of the at least one of the light beams is performed during the splitting of the single coherent light beam.  
   
   
       25 . An apparatus as claimed in  claim 15 , in which: 
 the at least one beam source includes a polariser for polarising a single coherent light beam into at least two polarisation states and a polarization beam splitter for splitting the polarized single coherent light beam into the at least two light beams;    the at least one non-mechanical beam modulator is arranged to modulate the phase of at least one of the at least two polarised states relative to the other polarisation states; and    the apparatus further comprises a polarisation manipulation element for aligning the polarisation states of the at least two light beams after the modulation.    
   
   
       26 . An apparatus as claimed in  claim 15 , in which the non-mechanical beam modulator comprises at least one of: 
 an electro-optic phase modulator;    a magneto-optic phase modulator;    a frequency shifter;    an acousto-optic frequency shifter;    a controllable optical retarder; and    an optical delay line.