Formation of a refractive index grating

Abstract

An apodised refractive index grating is recorded in a photosensitive optical fiber by forming first and second component interference patterns with different pitches, that are recorded in the grating such as to result in apodisation. The component patterns are spatially in phase in a central region and move progressively out of phase towards the ends of the patterns. The patterns may be recorded sequentially or concurrently. The fiber may be stretched once or cylically.

1 . A method of recording an apodised refractive index grating in a photosensitive optical medium with a pattern of optical radiation, that comprises producing a plurality of spatially periodic component optical patterns for recording a sequence of elements that form the grating, with a relative spatial phase which varies along the sequence in such a manner is to result in apodisation of the grating recorded in the optical medium.  
     
     
         2 . A method according to    claim 1    wherein the effective refractive index (n eff ) of the optical medium is substantially constant along the recorded grating.  
     
     
         3 . A method according to    claim 1    or    2    wherein said relative phase progressively changes in directions away from an intermediate region of the component patterns towards the ends thereof.  
     
     
         4 . A method according to    claim 3    wherein the component patterns have zero relative spatial phase in the intermediate region.  
     
     
         5 . A method according to    claim 4    wherein the component patterns have a relative phase of ±π/2 in respect to the spatial periodicity of the patterns, at opposite ends thereof.  
     
     
         6 . A method according to    claim 3   ,    4   , or  5 , wherein the intermediate region is disposed centrally of the component patterns.  
     
     
         7 . A method according to any preceding claim wherein the component patterns are formed sequentially.  
     
     
         8 . A method according to any preceding claim wherein the component patterns are optical interference patterns.  
     
     
         9 . A method according to    claim 8    including causing beams of optical radiation to interfere to produce a first of the component interference patterns, and thereafter introducing a phase shift across the width of at least one of said beams, so as to form a second of the component interference patterns.  
     
     
         10 . A method according to    claim 8    including introducing a wedge of optically transparent material into one of said beams so as to produce said phase shift for said second component interference pattern.  
     
     
         11 . A method according to    claims 1    to    10    including forming an interference pattern, arranging the optical medium in a first disposition relative to said interference pattern so as to provide the first component interference pattern to be recorded in the medium, and thereafter arranging the medium in a second disposition relative to the interference pattern so as to provide the a second component interference pattern to be recorded in the medium.  
     
     
         12 . A method according to    claim 11    including rotating the optical medium relative to the interference pattern between the first and second dispositions.  
     
     
         13 . A method according to    claim 11    including altering the length of the optical medium in order to achieve said first and second dispositions.  
     
     
         14 . A method according to    claim 13    including stretching the medium.  
     
     
         15 . A method according to    claim 13    or    14    including stretching the optical medium cyclically.  
     
     
         16 . A method according to    claim 13   ,    14    or  15  including stretching the medium from one end only of the recorded pattern.  
     
     
         17 . A method according to    claim 13    including compressing the medium.  
     
     
         18 . A method according to  13 ,  14 ,  15  or  16  including repeatedly recording the pattern along the medium.  
     
     
         19 . A method according to any one of    claim 13    to  13  wherein the pattern is provided from a phase mask.  
     
     
         20 . A method according to    claim 19    including altering the length of the phase mask so as to produce the component patterns.  
     
     
         21 . A method according to    claim 19    wherein the optical medium is stretched symmetrically about the centre of the resultant grating.  
     
     
         22 . A method according to    claim 19    wherein a beam of said radiation is scanned along the phase-mask and the optical medium is cyclically stretched such that the condition:  
       
         
           
             
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       is met, where ƒ is the frequency of stretching, υ is the scanning speed of the beam and ω is the diameter of the beam spot.  
     
     
         23 . A method according to any one of    claims 1    to    6    wherein the component patterns are formed concurrently.  
     
     
         24 . A method according to    claim 23    wherein beams of optical radiation are caused to interfere to produce said component patterns, said beams having a predetermined spectral content whereby light at different wavelengths interferes to produce said patterns.  
     
     
         25 . A method according to any one of    claims 1    to    6    wherein a phase mask that includes first and second mask patterns is placed adjacent to the waveguide and light is directed through the phase mask so as to produce the component patterns to be recorded in the optical medium in the waveguide.  
     
     
         26 . A method according to    claim 25    including selectively illuminating the mask patterns in the phase mask sequentially.  
     
     
         27 . A method according to any preceding claim including recording the refractive index grating in an optical waveguide that is photosensitive to the optical radiation.  
     
     
         28 . A method according to    claim 27    wherein the waveguide comprises an optical fibre.  
     
     
         29 . A waveguide including an apodised refractive index grating formed by a method according to any preceding claim.  
     
     
         30 . An optical chirp filter comprising a grating which is a composite of a first component grating and a second component grating, the phase difference between the first and second component gratings increasing from the centre of the said grating to produce apodisation thereof.  
     
     
         31 . An optical wavelength division multiplexed communications system comprising a transmitting station and a receiving station coupled by an optical waveguide, the receiving station including a plurality of chirp filters according to    claim 27   , each filter being arranged to compensate for dispersion of an optical signal in a different WDM channel received from the waveguide.