Abstract:
Optical gratings having a range of possible Bragg wavelengths can be produced using a single phase mask by exposing the mask to a non-collimated spatially filtered beam of light. A spatial filter removes high spatial frequency components from the beam, and a focusing system directs the filtered beam to a phase mask. A rate at which the beam is focused and a spacing between the phase mask and a photo-sensitive waveguide are varied to produce gratings in the waveguide having a range of possible periods.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application, Ser. No. 06/106,512, filed Oct. 30, 1998 entitled WAVELENGTH TUNING PHOTO-INDUCED GRATINGS, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to optical gratings manufactured in photosensitive media of optical waveguides by patterned exposures to actinic radiation. The patterns which include alternating bands of light are generally produced by interference fringes and can have periods of less than one micron. 
     BACKGROUND 
     Interference patterns for making optical waveguide gratings, particularly fiber Bragg gratings, can be produced using an interferometer or a phase mask. Interferometers divide a coherent beam of light into two separate beams that are angularly recombined at the waveguides for producing a desired interference pattern. Phase masks, which are themselves diffraction gratings, divide a similarly coherent beam into different diffraction orders that are recombined at the waveguides for producing a similar interference pattern. 
     For purposes of manufacturing, phase masks are often preferred because interferometers can be less stable and difficult to use in production environments. Phase masks are more stable but have less flexibility for adjusting the period of their resulting interference patterns. The interference period between two collimated beams is a function of the wavelength of the interfering beams and the angle at which the beams are combined. Any change in the wavelength of a beam divided by a phase mask also changes the diffraction angles through which the divided beams are recombined, so a different phase mask is often needed for each desired interference pattern. 
     A few techniques have been developed to adjust the effect of photoinduced waveguide patterns produced by phase masks. Bragg wavelengths (center wavelengths) of the resulting grating responses are a function of both the grating period and the average refractive index of the waveguides. Small adjustments to the Bragg wavelength have been made by pre-straining waveguides (i.e., optical fibers) and by illuminating phase masks with converging or diverging beams. 
     The former technique is described in a paper entitled “Tuning Bragg Wavelength by Writing Gratings on Prestrained Fibers” by Quin Zhang et al., published in Photonics Technology Letters, Vol., 6, No. 7, July 1994. A photosensitive fiber is exposed to an interference pattern produced by a phase mask while under strain. When the strain is relieved, the Bragg wavelength is down-shifted with respect to a similarly exposed unstrained fiber. Only a limited amount of strain can be tolerated by fibers and other waveguides, so the amount of adjustment by this technique is limited. 
     A paper entitled “Magnification of Mask Fabricated Fibre Bragg Gratings” by J. D. Prohaska et al., published in Electronics Letters, Vol. 29, No. 18, Sep. 2, 1993, proposes to illuminate phase masks with converging or diverging beams to adjust the magnification of interference patterns incident to photosensitive fibers. The power of a converging or diverging lens, the distance between the lens and the phase mask, and the distance between the phase mask and fiber can be changed to adjust the magnification of the interference pattern within the Fresnel near field of the light passing through the phase mask. However, only small changes in periodicity are practical because interference patterns produced at a distance from the phase mask are limited by spatial coherence of the illuminating beam. 
     U.S. Pat. No. 5,327,515 to Anderson et al. mounts a lens between a phase mask and a photosensitive fiber to project an image of a interference pattern formed at the phase mask onto the fiber. The lens projection system can be arranged to provide magnification or reduction of the interference pattern projected onto the fiber. However, like the known interferometer arrangements, issues of stability and alignment render this technique less practical in a production environment. 
     SUMMARY OF INVENTION 
     Our invention provides more flexibility in the manufacture of optical waveguide gratings with phase masks. Adjustments can be made in grating period and grating length, as well as apodization profiles. Small changes can be made to the gratings for purposes of tuning to compensate for other design variations, or large changes can be made to manufacture gratings with different specifications. One grating can be written over another with the same phase mask, which is particularly useful for making compound sensors. Grating chirp can also be controlled to support more complex spectral responses. 
     Our preferred embodiment includes a spatial filter that removes spatially incoherent light from a beam of light (i.e., actinic radiation). A phase mask divides the filtered beam of light into two interfering beams that form an interference pattern with an average fringe period along a waveguide. A focusing system directs the beam of light approaching the spatial filter as a converging beam and further directs the filtered beam of light as a noncollimated beam impinging upon the phase mask. A waveguide support positions the waveguide at a distance from the phase mask to adjust the average fringe period of the interference pattern formed along the waveguide. 
     The spatial filter filters incoherent light from at least a first of two orthogonal directions transverse to an axis of beam propagation. This is the same direction in which the waveguide is oriented. The focusing system converges the beam of light in the first orthogonal direction through a first focal line located at the spatial filter. Approaching the waveguide, the focusing system diverges or converges the filtered beam in the first orthogonal direction. By locating a diverging or converging element between the spatial filter and the phase mask, the effective center of curvature of the beam impinging on the phase mask can be varied. A diverging beam impinging on the phase mask in the first orthogonal direction increases the average fringe period formed along the waveguide, and a converging beam impinging on the phase mask in the same first orthogonal direction decreases the average fringe period. To minimize optical components requiring alignment, the diverging beam can have a center of curvature on the first focal line, which is located at the spatial filter. 
     The focusing system preferably provides separate control over beam shape in the two orthogonal directions. In the first direction, which is filtered to enhance spatial coherence, the beam is shaped to influence the magnitude of a change in the fringe period associated with a given spacing between the phase mask and waveguide. In the second direction, a separate focusing optic can be used to concentrate light energy on the waveguide. For example, the filtering system can be arranged to diverge the filtered beam along the first orthogonal direction approaching the phase mask for increasing the average fringe period and to converge the same beam along the second orthogonal direction approaching the phase mask for concentrating more light energy on the waveguide. 
     The spatial filter permits the phase mask and the waveguide to be separated through larger distances while still producing an interference pattern with good fringe contrast. A similar fringe period can be obtained at more than one distance between the phase mask and the waveguide by adjusting the rate of convergence or divergence of the filtered beam. Other variables affected by the distance separating the phase mask and the waveguide include the length of overlap between interfering beams emerging from the phase mask and the intensity profile of the recombined beams. The length of overlap controls the length of grating written into the waveguide. The intensity profile affects apodization issues for obtaining the desired spectral response. 
     The waveguide can also be tilted in an axial plane that includes the first orthogonal direction for producing a linear chirp in the grating, evident as a grating period that varies from one end of the grating to the other. Shorter focal lengths of the diverging or converging beam upon the phase mask (e.g., a shorter distance between the first focal line at the spatial filter and the phase mask) can produce a quadratic chirp in the grating, evident as a grating period that varies as a function of distance from the center of the grating. Such quadratic chirp can also influence apodization issues. 
    
    
     DRAWINGS 
     FIG. 1 is a diagram of apparatus for photo-inducing a grating in a waveguide using a phase mask. 
     FIG. 2 is a diagram of the waveguide inclined with respect to the phase mask for producing a grating with a linearly chirped period. 
     FIG. 3 is a diagram of an interferometer for photo-inducing a grating in a waveguide. 
     FIG. 4 is a diagram of an amplitude mask arranged together with a waveguide for photo-inducing a long period grating in the waveguide. 
    
    
     DETAILED DESCRIPTION 
     The arrangement  10  shown in FIG. 1 can be used for forming an optical grating in a waveguide  12 , which is shown as an optical fiber, but can also be configured in other forms including planar or channel waveguides. The waveguide  12  has an exposed portion  14  that includes a photosensitive core surrounded by a cladding. An exemplary photosensitive core is made from a combination of silica and germanium, while the cladding can be composed of silica alone. Hydrogen loading can be used to enhance photosensitivity. 
     A laser source  16 , such as an excimer laser operating in a wavelength range between 150 and 250 nm, produces a beam  18  of actinic, temporally coherent radiation. Other lasers and other wavelengths can also be used in combination with material sensitive to the alternative wavelengths and power regimes. Pulsed or continuous wave radiation can be used. 
     Following a turning mirror  20 , a first aperture stop  22  strips extraneous portions from the beam  18  in advance of a first focusing lens  24  (e.g., a cylindrical lens) that converges the beam through a first line focus  26 . Referenced with respect to an orthogonal coordinate system in which a coordinate “Z” extends in the direction of beam propagation and coordinates “X” and “Y” extend in a plane transverse to the direction of beam propagation, the convergence of the beam  18  through the line focus  26  takes place in the “X” coordinate direction. The line focus  26  extends in the “Y” coordinate direction, which is normal to the drawing plane of FIG.  1 . 
     A spatial filter  30  in the vicinity of the line focus  26  diverts high spatial frequency components of the beam  18  for enhancing the beam&#39;s spatial coherency. Details of our preferred spatial filter  30  are disclosed in U.S. Provisional Application No. 60/047,859 entitled “Spatial Filter for High Power Laser Beam”, which is hereby incorporated by reference. Leaving the spatial filter  30 , the beam  18  has a sinc 2  intensity profile, which is stripped of side lobes by a second aperture stop  34 . A second focusing lens  36  (e.g., another cylindrical lens) converges the beam  18  in the “Y” coordinate direction through a second line focus  38  approximately coincident with an axis  39  of the waveguide  10 . Both the line focus  38  and the axis  39  of the waveguide  10  extend in the “X” orthogonal direction. 
     In contrast to the focused convergence in the “Y” coordinate direction, the beam  18  continues to diverge in the “X” coordinate direction through the focusing lens  36  until the beam  18  strikes a phase mask  40  at angles of incidence that vary as a function of distance from a center of the beam  18 . A total divergence of the beam  18  from the first focal line  26  to the phase mask  40  takes place through a distance “z”. An adjustable mount  42  for the phase mask  40  translates in the “Z” coordinate direction to adjust the distance “z”. The phase mask  40 , which is itself a diffraction grating, diffracts the beam  18  into two interfering beams  44  and  46  through two different preferably opposite sign diffraction orders (e.g., +1 and −1 orders). Other combinations of orders can also be used including a combination of zero and first orders, but the two first orders are preferred. 
     Instead of positioning the phase mask  40  directly against the exposed portion  12  of the waveguide  10  in accordance with usual practices, the phase mask  40  is spaced apart from the exposed portion  12  through a distance “d” along the “Z” coordinate axis. The waveguide  10  is supported on an adjustable mount  48  that is translatable in the “Z” coordinate direction for varying the distance “d” between the phase mask  40  and the waveguide  10  independently of the distance “z” between the line focus  26  and the phase mask  40 . Given the enhanced spatial coherence of the beam  18 , distances “d” of one centimeter or more are possible, which provide significant additional control over pitch spacing of the resulting fringe pattern on the waveguide  10  as well as the corresponding Bragg wavelength of the resulting optical grating  50  formed in the exposed portion  12  of the waveguide  10 . Also, positioning the waveguide out of contact with the phase mask avoids unnecessary interactions that can affect quality and consistency of results. 
     A change in the Bragg wavelength “Δλ B ” as a function of the variables “z” and “b” is given by the following equation: 
     
       
         Δλ B =( n   eff λ write /sin θ){[1+( d/z ) 2 +2( d/z )cos θ] ½ −1} 
       
     
     where “λ write ” is the wavelength of the illuminating beam  18 , “n eff ” is the effective average index of the core of the waveguide  10 , and “θ” is the half angle between the opposite sign orders of the interfering beams  44  and  46 . 
     The above equation can be approximated for many practical purposes where “z” is much larger than “d” as follows: 
      Δλ B   ≡n   eff λ write ( d/z )cot θ 
     From the equation just above, it is apparent that decreasing “z” or increasing “d” increases the change in the Bragg wavelength of the grating. The distance “z”, which is related to the divergence rate of the beam  18  in the “X” coordinate direction, controls the rate at which changing the distance “d” affects the amount of overlap between the interfering beams  44  and  46  as well as the combined intensity profile of the interfering beams  44  and  46 . The amount of overlap controls the length of grating that is written into the waveguide  10 . Increasing spacing between peak intensity profiles of the interfering beams  44  and  46  can be used to reduce intensity variations within the range of overlap to provide better control over the resulting spectral response of the optical grating  50 . A further explanation of the apodization effects of spacing the waveguide  10  at a distance “d” from the phase mask  40  is provided in U.S. Provisional Application No. 60/091,547 filed on Jul. 1, 1998 with the title of “Apodization of Optical Filters Formed in Photosensitive Media”, and this application is hereby incorporated by reference. 
     For example, changes to the Bragg wavelength of the optical grating  50  can be made in combination with control over the length of grating  50  and the combined intensity profile of the interfering beams  44  and  46  by exploiting various combinations of the distances “z” and “d”. Shorter distances “z” can also be used to provide the grating  50  with a quadratic chirp, namely, a progressive variation of the grating pitch as a function of distance from the center of the grating. 
     Tilting the waveguide  10  in a plane that includes the “X” and “Z” coordinate directions as shown in FIG. 2 can be used to provide the grating  50  with a progressive linear chirp in accordance with the following equation: 
     
       
         chirp=δ(Δλ B )/ L =( n   eff λ write   /z )cot θ sin α 
       
     
     where “δ(Δλ B )” is the amount of change in the center wavelength along a length “L” of the grating  50  and “α” is the angle through which the waveguide  10  is tilted with respect to the phase mask  40 . Chirp otherwise introduced by a chirped phase mask can be further increased, decreased, or even removed by similarly tilting the waveguide  10  in the “X-Z” plane. The waveguide  10  can also be rotated in the “X-Y” plane about the “Z” axis to write blazed gratings. 
     Control over convergence in the “Y” coordinate direction can also be used to regulate the intensity profile along the inclined axis  39  of the waveguide  10 . Regardless of inclination, the intensity profile along the waveguide axis  39  can also be further controlled by an amplitude mask, which is preferably located between the spatial filter  30  and the phase mask  40 . More than one spatial filter  30  can be arranged in series to further enhance the spatial coherence of the beam  18 . Adding another spatial filter in series with the spatial filter  30  converts the sinc 2  intensity profile from the first spatial filter to a sinc 4  intensity profile from the second spatial filter. 
     One example that demonstrates some of the possibilities of this invention has the following values: 
     z=3.0 meters 
     d=1.0 centimeters 
     λ write =248 nanometers 
     n eff =1.45 
     θ=13.4 degrees 
     The resulting change in the Bragg wavelength “Δλ B ” equals 5.05 nanometers. Small changes in the Bragg wavelength can be used for such purposes as tuning to compensate for manufacturing variations or anticipated environmental effects. Large changes can be used to manufacture different gratings in the same or different waveguides using the same phase mask. For example, multiple gratings can be overwritten on the same waveguide with different Bragg wavelengths to support a plurality of functions such as compound optical sensors. A series of different gratings can also be produced along the waveguide to provide such functions as compound filtering or demultiplexing. 
     Although the invention is primarily applicable to the use of phase masks for producing optical gratings in waveguides, some overlapping benefits can be obtained when using interferometers for producing similar gratings. FIG. 3 depicts such an interferometer  60 . A similar light source  62  is used along with a similar combination of focusing optics  64  and  66 , an aperture stop  68 , and a spatial filter  72  to fashion a spatially coherent beam  74  with a desired shape and intensity profile. 
     In contrast to the preceding embodiment, however, a beamsplitter block  76  divides the beam  74  into two separate beams  78  and  80  that are angularly oriented through angle “β” by combinations of respective turning mirrors  82 ,  84  and  86 ,  88 ,  90 . Each beam  78  and  80  is finally shaped by a focusing optic  92  and  94  prior to impinging on an exposed portion  96  of a waveguide  100 . 
     Similar to the preceding embodiment, the beams are shaped differently in a plane (i.e., the drawing plane of FIG. 3) within which the exposed portion  96  of the waveguide  100  is aligned with respect to a normal plane transverse to the waveguide  100 . Within the plane of the waveguide  100 , the beams  78  and  80  progressively converge upon the waveguide  100 . (The same beams  78  and  80  could also be arranged to diverge upon the waveguide  100  similar to the preceding embodiment.) In the transverse plane, which includes a focal line  102  at the spatial filter  72 , the beams  78  and  80  also preferably converge to concentrate more light energy on the waveguide  100 . 
     The beams  78  and  80  have central axes  104  and  106  that cross in a position offset from the waveguide  100 . A distance from a focus (e.g.,  102 ) of the beams  78  and  80  in the waveguide plane to a crossing point  108  of the beam axes  104  and  106  is comparable to the distance “z” in the preceding embodiment. A distance from the crossing point  108  to the waveguide  100  is comparable to the distance “d” of the preceding embodiment. The distance “d” in this embodiment, however, can have a negative or positive value depending whether the waveguide  100  is located before or after the crossing point  108 . The angle “β” corresponds to twice the angle “θ” of the preceding embodiment. 
     The distances “z” and “d” can be adjusted to further regulate the average period of the resulting grating  110  formed in the waveguide  100  as well as issues of chirp and apodization. A waveguide mount  112  adjusts the distance “d” and can also be used to tilt the waveguide  100  to vary grating pitch in a more linear fashion. Since grating period can be adjusted by varying the angle “β”, the distances “z” and “d” are expected to be more useful contributing to issues of chirp or apodization. 
     Our invention is also applicable to the manufacture of long period gratings. A setup similar to FIG. 1 can be used. However, the phase mask  40  is replaced by a rectangular amplitude mask  120 . An additional focusing optic  122  is located between the spatial filter  40  and the amplitude mask to change the rate of divergence at which the beam  18  approaches the amplitude mask  120 . Such focusing optics  122  can be used in any of the embodiments to move the effective center of curvature of the impinging beam  18  and to correspondingly shorten or lengthen the distance “z”. 
     In contrast to the earlier embodiments that rely on the mechanism of interference to produce a pattern of light bands along waveguides  10  and  100 , a single beam  124  emerges from the amplitude mask  120  containing a varying intensity pattern that produces the desired bands of light on an exposed portion  126  of a waveguide  128 . Both a length “L” and a pitch spacing of a resulting long period grating  130  are controlled by the rate of divergence through the amplitude mask  120  (measured by the distance “z A ” from an effective center of curvature  136 ) and a distance “d A ” between the amplitude mask  120  and the exposed portion  126  of the waveguide  128 . 
     A mask mount  132  is translatable in the “Z” coordinate direction to vary the distances “Z A ” and “d A ”, and a waveguide mount  134  is translatable in the same direction to vary the distance “d A ” independently of the distance “z A ”. The waveguide mount is also tiltable about the “Y” or “Z” coordinate directions to further influence the chirp or blaze of the long period grating  130 . 
     Other applications, uses, and arrangements of the invention will be apparent to those of skill in the art. The gratings produced according to our invention are particularly useful for communications systems but also include uses as sensors, dispersion compensators, and laser pump stabilizers.