Abstract:
Apparatus for producing an interference pattern from an input optical beam, including a first optical element for separating the input optical beam into a plurality of divergent optical sub-beams and a second optical element including a first surface and a second surface. The first surface is optically coupled to the first optical element to receive at least two of the plurality of sub-beams. In addition, the second optical element is capable of redirecting via total internal reflection at least one of the sub-beams received at the first surface such that at least two sub-beams emerge from the second surface along respective paths intersecting one another outside the second optical element at a distance from the second surface. Advantageously, the use of total internal reflection in the second optical element for sub-beam redirection removes any requirement for a mirror to perform this function, results in only minimal power losses as the sub-beams reflect internally and the location at which they intersect is easily adjustable.

Description:
This application is a Continuation of U.S. application Ser. No. 10/296,079, now U.S. Pat. No. 6,990,272 having a filing/371 (c) date of Jan. 16, 2003, which is the National Stage of PCT/CA02/01184, filed Jul. 26, 2002 and which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention relates to the generation of optical interference patterns, which can be of particular use in producing changes in the index of refraction of a glass medium such as the core of an optical fiber. 
   BACKGROUND OF THE INVENTION 
   Certain types of glass have optical properties that can be altered when they are exposed to radiation. In particular, some index of refraction variations can be permanently inscribed in these types of glass with ultraviolet radiation. In Bragg grating writing by flood exposure, an interference pattern is permanently written in the form of index of refraction variations in an optical fiber. Typically, a laser beam in the ultraviolet (UV) region of the optical spectrum is split into two sub-beams. The two sub-beams are then recombined to produce an interference pattern which is shone on the core of the optical fiber for a period of time. After the laser beam is turned off, the index of refraction variations stay inscribed in the optical fiber. 
   PCT Application 00/02068 filed on Jun. 30, 1999 by Bhatia et al. describes an apparatus to write Bragg gratings in an optical fiber. The apparatus includes a laser, which produces a laser beam. The laser beam is split into two sub-beams by a beam splitter. Then, the two sub-beams are each reflected by a plurality of mirrors to make them converge at a certain location in space. The two converging beams interfere and therefore produce an interference pattern at the certain location. The optical fiber is positioned at the certain location to write the grating. 
   Apparatuses such as the one described in the above-referenced PCT application present many disadvantages. First, the mirrors have to be precisely aligned to produce the desired interference pattern. Also, the whole apparatus has to be very rigid and isolated from external vibrations to keep the interference pattern at a precise location in space. If the interference pattern is displaced during the writing process, the grating will be veiled and may eventually be useless. In addition, the surface of the mirrors has to be kept clean in order to bring as much energy as possible to the certain location where the interference pattern is produced. 
   Two properties that are often required of Bragg gratings are apodization and balance. Apodization relates to having an interference pattern including a plurality of bright fringes and a plurality of dark fringes wherein the bright fringes are not uniformly bright across the whole interference pattern. Therefore, if an apodized interference pattern with fringes having a low intensity close to the extremities of the grating is used to produce the Bragg grating, the index of refraction differences will also be apodized, which is desirable in some Bragg gratings used as optical filters. Balance relates to having indices of refraction in the grating which vary alternatively above and below an average value. In the apparatus described above, only variations in indices of refraction in one direction are possible as dark fringes in the interference pattern produce no variation in the index of refraction inside the optical fiber and bright fringes all produce variations in the index of refraction inside the optical fiber having a same sign. Once again, it is often desirable to have variations above and below an average value when the gratings are used as optical filters. 
   To produce a balanced grating, two exposures are required in the apparatus described above. In a first exposure, the beams, eventually apodized, are used to create the variations in index of refraction as described above. In the second exposure, the sub-beams are slightly offset to provide a uniform increase in index of refraction along the grating. However, this two-step process is time consuming as two exposures have to be made. In addition, the apodization is performed through collimators and spatial filters which need to be precisely aligned with the rest of the apparatus. 
   In view of the above, there is a need in the industry to provide new apparatuses and method for writing features in or on a photosensitive medium. 
   More particularly, the invention relates to the use of an interference pattern between two coherent light beams to induce changes in the index of refraction of the medium wherein the two light beams are produced by splitting a first light beam and propagated in a prism through total internal reflection. 
   SUMMARY OF THE INVENTION 
   According to a first broad aspect, the present invention provides an apparatus for producing an interference pattern from an input optical beam. The apparatus includes a first optical element for separating the input optical beam into a plurality of divergent optical sub-beams and a second optical element including a first surface and a second surface. The first surface is optically coupled to the first optical element to receive at least two of the plurality of sub-beams. In addition, the second optical element is capable of redirecting via total internal reflection at least one of the sub-beams received at the first surface such that at least two sub-beams emerge from the second surface along respective paths intersecting one another outside the second optical element at a distance from the second surface. 
   Advantageously, the use of total internal reflection in the second optical element for sub-beam redirection removes any requirement for a mirror to perform this function. This improves not only physical robustness but also sensitivity to dust and grease. Moreover, there are only minimal power losses as the sub-beams reflect internally and the location at which they intersect is easily adjustable. 
   In addition, the present invention has the capability to pass zeroth and first order diffraction sub-beams, which allows a balanced grating to be produced in a single exposure. 
   According to a second broad aspect, the present invention provides a method of writing an interference pattern on a photosensitive medium with a laser beam. The method includes receiving at least two of the sub-beams at an optical element; redirecting at least one of the received sub-beams via total internal reflection such that at least two sub-beams emerge from the optical element along respective paths that intersect in a region of intersection; and placing the photosensitive medium at least partly in the region of intersection. 
   These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  shows an optical apparatus for producing an interference pattern on a photosensitive substrate in accordance with an embodiment of the present invention; 
       FIG. 2  shows a region of space wherein the interference pattern is produced by the optical apparatus shown in  FIG. 1 ; and 
       FIG. 3  shows a valiant of the optical apparatus shown on  FIG. 1  including a curve surface. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an optical apparatus  100  for producing an interference pattern on a photosensitive medium in the form of a core  105  of an optical fiber  110 . While the optical apparatus  100  is described herein in the context of Bragg grating writing in an optical fiber  110 , the reader skilled in the art will readily appreciate that the apparatus could be used in other contexts without departing from the spirit of the invention. In specific examples of implementation, the apparatus could be used to produce an interference pattern illuminating other photosensitive media, including discrete optical fibers, optical fibers mounted in a module and integrated optics components. 
   The optical apparatus  100  includes a laser  120 , a diffractive element  130  and a transmissive block  140  as described in further detail herein below. The laser  120  produces a coherent beam of light  125 . In the case of Bragg grating writing in an optical fiber, the laser  120  may produce light at a wavelength between 193 nm and 300 nm and is either pulsed or continuous. In an even more specific case, the laser  120  produces light at a wavelength between 193 nm and 260 nm. However, it will be understood that a laser  120  producing a beam  125  having a wavelength outside of the mentioned interval can be used in the apparatus  100 . 
   The beam  125  may be shaped and collimated by a lens assembly  127 . The shaped and collimated beam is then defected by a mirror  129 , that can be used to optimally align the beam  125  with the core  105 . The mirror  129  may be movable in order to permit precise alignment to be controlled by a user or a feedback control circuit. The reader skilled in the art will recognize that while the laser  120 , the lens assembly  127  and the mirror  129  are preferably used in the optical apparatus  100 , other sources of coherent light could be used without departing from the spirit of the invention, with or without the lens assembly  127 . Also, the apparatus of the present invention may be used in cases where it is desirable to produce an interference pattern from non-coherent light. 
   The beam  125  arrives at the diffractive element  130 , which produces a diffraction pattern including a plurality of sub-beam  135   k , k=0, ±1, ±2, . . . . Each pair of sub-beams  135   ±k  corresponds to a diffraction order k of a diffraction pattern produced by the diffractive element  130 . The sub-beams  135   k  diverge from each other, each at a respective divergence angle measured with respect to the sub-beam  135   0 . In a specific example of implementation, the diffractive element  130  can be an apodized holographic phase mask producing sub-beams  135   −1  and  135   1  diverging from the sub-beam  135   0  at an angle between 7° and 23°. The exact number of sub-beams  135   k  produced and the value of their respective divergence angle depend on the specific diffractive element used and on the wavelength of the beam  125  produced by the laser  120 . 
   In a variant, the diffractive element  130  can be replaced by a beam splitter in the optical apparatus  100 . However, a typical beam splitter does not produce an order 0 sub-beam  135   0 . As it will be detailed below, the order 0 sub-beam  135   0  is preferably present to produce a balanced grating in a single exposure. 
   The transmissive block  140  is composed of a material having an index of refraction higher than its surroundings and which is transparent or nearly transparent at the wavelength of the beam  125  produced by the laser  120 . In a very specific example of implementation, the transmissive block  140  can be made of quartz. In a specific example of implementation, shown on  FIG. 1 , the transmissive block  140  is a cubic prism having homogenous optical properties and including two planar lateral faces  141  and  142 , one planar front face  143  and one planar back force  144 . In a very specific example of implementation, suitable dimensions of the cubic prism may be 3 cm×3 cm×15 cm located approximately 2 cm from the diffractive element  130  and approximately 2 cm from the core  105 . However, the reader skilled in the art will appreciate that these dimensions can vary considerably, depending on the interference pattern to be produced. 
   In an embodiment of the present invention, the transmissive block  140  is adapted to propagate only the zeroth and first orders of diffraction produced by the diffractive element  130 , namely, the sub-beams  135   −1 ,  135   0  and  135   1 . Sub-beams corresponding to other orders of diffraction can be avoided by suitably dimensioning and positioning the transmissive block  140  so that it is clear of the path taken by the sub-beams corresponding to these other orders of diffraction. In other embodiments of the invention, undesired orders of diffraction are filtered by the transmission block  140 . 
   In one embodiment of the present invention, the two first order sub-beams  135   ±1  are reflected inside the transmissive block  140  through total internal reflection and subsequently converge on the core  105  to produce an interference pattern. In other embodiments of the present invention, one of the first order sub-beams may pass straight through the transmissive block  140 , while the other of the first order sub-beams may be totally internally reflected and redirected towards the sub-beam that was not totally internally reflected. Intersection of at least two sub-beams exiting a back surface of the transmissive block  140  occurs outside the transmissive block  140  at a distance away from its back surface. 
   In this specific example of implementation, the sub-beams  135   −1 ,  135   0  and  135   1  enter the transmissive block  140  through the front face  143 . Since the sub-beams  135   −1  and  135   1  are not perpendicular to the front face  143 , they will be refracted when entering the transmissive block  140 , in opposition to the sub-beam  135   0  which enters the transmissive block  140  perpendicularly to the front face  143  and is therefore not refracted. 
   Inside the transmissive block  140 , the sub-beam  135   0  is propagated in a straight line to the back face  144 . However, the dimensions of the transmissive block  140  are such that the two sub-beams  135   −1  and  135   1  arrive to the lateral faces  141  and  142  before arriving to the back face  144 . Since the index of refraction inside the transmissive block  140  is larger than the index of refraction outside the transmissive block  140 , the two sub-beams  135   −1  and  135   1  are reflected through total internal reflection at the lateral surfaces  141  and  142  of the transmissive block  140 . Also, the transmissive block  140  has dimensions such that the two sub-beams  135   −1  and  135   1  will arrive to the back face  144  before intersecting. 
   When exiting the transmissive block  140  through the back face  144 , the two sub-beams  135   −1  and  135   1  are refracted and converge at a certain location in space. Since the two sub-beams  135   −1  and  135   1  have been reflected inside a single rigid piece of material, there are only minimal losses in a power carried by the two sub-beams  135   −1  and  135   1  and the location at which they intersect is easily adjustable. 
   Meanwhile, the zeroth order sub-beam  135   0  emerges from the transmissive block  140  without having been deflected and the zeroth order sub-beam may be focused by a focusing lens  150 . The relative position of the focusing lens  150  with respect to the optical fiber  110  determines an intensity of the zeroth order beam  135   0  illuminating the optical fiber  110 , which allows to write a balanced grating on the optical fiber in a single exposure. The reader skilled in the art will readily appreciate that the focussing lens  150  alleviates the need for a specialized diffractive element that is capable of producing balanced order 1 and 0 sub-beams  135   −1 ,  135   0  and  135   1 . 
   The reader skilled in the art will readily appreciate that many shapes of the transmissive block  140  can be designed so as to select only orders 0 and 1 of diffraction and make two sub-beams of first order converge at the certain location in space through total internal reflection. In addition, transmissive blocks selecting other orders of diffraction can be used in the optical apparatus  100  without departing from the spirit of the invention. 
   In a variant, the front face  143  of the transmissive block  140  is partially coated with an opaque layer to block the sub-beam  135   0 . This may be desirable in processes wherein the sub-beam  135   0  is not required. 
   It will be appreciated that since the transmissive block  140  is a self-contained unit for redirecting the sub-beams  135   k , it can be readily exchanged with another transmissive block with only minimal realignment requirements, which affords flexibility in the use of the apparatus  100 . 
   It will also be appreciated that the distance between the focusing lens  150  and the core  105  determines the intensity of the sub-beam  135   0  at the location of the core  105 . Alternatively, the focusing lens  150  could be interchanged with another lens having a different focal distance to vary intensity of the sub-beam  135   0  at the location of the core  105 . The reader skilled in the art will appreciate that the exact value of the distance between the focusing lens  150  and the core  105  and the exact value of the focal distance of the focusing lens  150  required to produce a balanced Bragg grating depend on many characteristics of the apparatus  100 . Accordingly, it is preferable to adjust these parameters for each particular grating written, either through theoretical calculations or through measurements of intensity using an optical power meter. Such methods for adjusting these parameters are well known in the art and will not be discussed in further detail. 
   It will further be appreciated that in those instances when the sub-beam  135   0  is undesired, the sub-beam  135   0  can be blocked by replacing the focussing lens  150  by a piece of all opaque material. 
   It will also be appreciated that the distance between the transmissive block  140  and the core  105  regulates a length of grating written in the optical fiber  110 . As shown on  FIG. 2 , the interference pattern produced by the two sub-beams  135   −1  and  135   1  is present in a diamond-shaped region of space  210  in which the two sub-beams  135   −1  and  135   1  intersect. Depending on the exact position of the core  105  in the diamond-shaped region of space  210 , the length of a portion of the core  105  exposed to the interference pattern will vary, which will therefore change the length of the grating produced. 
   In a further variant, shown on  FIG. 3 , the transmissive block  140  includes a curved surface  197 A instead of the front face  143  shown in  FIG. 1 , which was planar. Backward or forward shifting the curved surface  197 A can be used to change the angle at which the sub-beams  135   −1  and  135   1  enter the transmissive block  140 , which changes the angle at which the sub-beams  135   −1  and  135   1  leave the transmissive block  140 , which changes the period of the Bragg grating produced at the location of intersection of the sub-beams  135   −1  and  135   1 . 
   Specifically, changing the distance between the curved surface  197  and the diffractive element  130  (i.e., moving from A to B in  FIG. 3 ) produces a variation in the location along the surface  197  at which the divergent sub-beams  135   −1  and  135   1  enter the transmissive block  140 . Due to the surface  197  not being planar, the angle of incidence with which the sub-beams  135   −1  and  135   1  arrive at the curved surface  197  varies with the distance between the curved surface  197  and the diffractive element  130 . This variation in the angle of incidence at which the sub-beams  135   −1  and  135   1  enter the transmissive block  140  produces a variation in the angle at which the two sub-beams  135   −1  and  135   1  are propagated in the transmissive block  140  further to being refracted at the surface  197 . 
   This may lead to an intersection occurring at a different region in space for case A than case B. However, the distance between the back face  144  and the region of intersection can be controlled by appropriately selecting the distance between the diffractive element  130  and the curved surface  197 . Specifically, by appropriately shifting the position of the diffractive element, the sub-beams  135   −1  and  135   1  exiting the transmissive block  140  can be made to intersect at the same region in space in both A and B. In this way, it is possible to change the period of the Bragg grating by merely changing the transmissive block  140  without having to change any other component in the apparatus  100 . 
   It should be appreciated that in some embodiments, the transmissive block  140  may be composed of a basic block in the shape of a prism, to which it is possible to optically couple any of a set of curved attachment blocks, each having a curved face and a particular length. The curved face may have the same curvature profile or it may be different for different attachment blocks of different lengths. Also, it is within the scope of the present invention to provide attachment blocks of roughly the same length, with different curvature profiles in order to achieve different angles of intersection and hence different Bragg periodicity. Those skilled in the art will be capable of determining what shift, if any, is required in the position of the diffraction element  130  in order to maintain the distance between the transmissive block  140  and the region of intersection of the first order sub-beams. 
   Those skilled in the art will appreciate that the apparatus  100  would work in sensibly the same way if the inside walls of the transmissive block  140  are not parallel or if the curved surface  197  is located on the side of the transmissive block  140  through which the light exits the transmissive block  140 . 
   While specific embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims.