KR19990071841A - Ring Interferometer Structure for Diffraction Grating Formation - Google Patents

Abstract

A method and apparatus for forming a structure such as a diffraction grating in the photosensitive material 40 is disclosed. The method and apparatus form at least two coherent light beams 33, 38 and surround a series of reflective elements 35, 37 along a substantially equivalent path such that the coherent beams interfere in the structural staging area 45. The beams are reflected by the back propagation method. The structural staging area can then be used to form diffractive structures and the like with improved quality.

Description

Ring Interferometer Structure for Diffraction Grating Formation

FIG. 1 shows a first known installation 1 for forming a diffraction grating on an optical waveguide 3 which may comprise optical fibers. The plant 1 has an ultraviolet laser 4 for generating a UV beam 5, which is split into two coherent beams 7, 8 by a beam splitter 6. . Beam splitter 6 may comprise a translucent mirror or a phase mask. The UV beams 7, 8 are reflected by mirrors 10, 11 reflecting the coherent beams 12, 13. The two beams 12, 13 interfere with each other to form an interference fringe projected onto the photosensitive optical fiber 3 and also form a diffraction grating 2.

Since the photosensitive optical fiber 3 is only affected by the bright fringes of the interference fringes, the fringes are converted into a permanent refractive index diffraction grating. The fiber 3 is photosensitive in that the physical properties of the fiber are permanently changed by the action of UV light. In the case of an optical fiber it is the refractive index that is changed but the invention is not limited thereto but extends to other properties such as absorbance, density, chemical composition or volume.

In the arrangement of FIG. 1, it is also known to add one more mirror to one of the paths of the beams 7, 8 to invert the profile of one of the beams. When only two fold mirrors 10, 11 are used, one beam is not inverted relative to the other, and the two beam profiles 12, 13 will be identical at the intersection 2. It is also known to produce an expanded diffraction grating using three mirror arrangements for scanning narrow beams along the entire opening defined by the beam splitter 6 and the mirrors 10, 11.

2 shows a second known apparatus 20 for forming a diffraction grating on an optical fiber such as (21). In this device, the optical fiber 21 is located immediately after the phase or amplitude mask 22. The UV beam 23 is diffracted by the phase mask 22, producing a diffraction order such as (24, 25). The diffraction orders 24 and 25 interfere with each other to produce an interference fringe 26 directly behind the phase mask 22 where the photosensitive optical fiber 21 is located. In this structure 20, the diffraction grating period will be determined by the period of the phase mask 22. The Bragg wavelength of the diffraction grating in the optical fiber 21 can only be changed by, for example, stretching the diffraction grating while the diffraction grating is formed.

As described above, it is possible to use the phase mask 22 as a beam splitter that provides multiple diffraction orders including orders 24 and 25. Therefore, the phase mask 22 can also be used as the beam splitter 6 by changing the orientation of the beam splitter 6 and the UV laser light source 4 even in the apparatus of FIG. 1.

In another known device, Kashyap replaces the two mirrors 10, 11 of FIG. 1 with a solid silica block so that the entire UV beam is reflected internally at the side of the block. When using this structure, all diffraction coming other than the two to be used for the interference fringe can be blocked. This structure is also more mechanically stable, but the angle between the interfering beams cannot be easily adjusted.

In all of the above interference structures, whether beam splitters or phase masks are used to split the beams, the resulting two beams 7, 8 intersect other physical elements before they cross to form the diffraction grating 2. Follow the path. Therefore, a general problem with the apparatus of FIG. 1 is that a slight mechanical shake in the mirrors 10, 11 reverses the phase of the interference fringe during exposure which causes the diffraction grating 2, resulting in loss of contrast and / or interference fringe. It causes the overall "washing out" of. It has thus been found that great care is required to stabilize the device 1 from any vibrations, such as from air flow or movement, and such stabilization requirements severely limit the use of the device of FIG.

The present invention relates to the formation of a pattern on such materials by placing a photosensitive optical material, such as a waveguide, such as an optical fiber, in an interference pattern resulting from the intersection of at least two light beams, preferably ultraviolet light beams. The period or interference fringe of the diffraction grating is determined by the angle of incidence and the nature of the intersecting beam.

While there may be any other forms within the scope of the invention, the preferred form of the invention will be described with reference to the accompanying drawings by way of example only.

1 shows the first method for forming a diffraction grating according to the prior art;

2 shows a second method for forming a diffraction grating according to the prior art;

3 is a schematic diagram of a device of a preferred embodiment of the present invention;

4 is a perspective view of the apparatus of FIG. 3;

5 shows a schematic diagram of an alternative embodiment using three mirrors.

It is therefore an object of the present invention to provide an improved method and apparatus for forming a diffraction grating-like structure on a photosensitive optical waveguide.

In a preferred aspect of the present invention, a method of forming a structure such as a diffraction grating in a photosensitive optical material such as an optical fiber is provided. The method includes the following steps: (1) making the first two coherent light beams, preferably made by a method of passing one coherent beam through a diffraction grating, (2) two beams Back propagating two beams around an optical circuit comprising a plurality of reflective elements so as to interfere at this predetermined position; (3) placing the photosensitive optical waveguide at a predetermined position to create a desired structure in the optical waveguide.

Although the present invention is useful for forming many different types of structures, it is most useful for making diffraction grating structures.

Coherent beams can be made using a phase mask, a diffraction grating or a partially transparent mirror. Optical materials may include optical waveguides or optical fibers. The coherent beam can be swept along the phase mask to form an extended structure in the photosensitive optical material. Additionally, the refractive element can be moved or rotated to produce different structures, such as chirped diffraction gratings or apodized diffraction gratings. The optical material may be positioned at various angles to produce a blazed diffraction grating or the like.

In another alternative embodiment, a method of making an extended diffraction grating structure, a chirped diffraction grating or an apodized diffraction grating, or the like will be described. The optical material may be positioned at various angles to produce other effects such as an apodized diffraction grating.

In a second aspect of the invention, there is provided an apparatus for making a structure on a photosensitive material, the apparatus comprising: (1) coherent light generating means for generating at least two coherent light beams; (2) a plurality of reflective elements for reflecting the coherent beam along substantially the same path propagating backwards such that the coherent beam interferes in the structure staging area; And (3) structural staging means for installing the photosensitive material in the structural staging area.

Preferably, the coherent light generating means further comprises coherent light input means for inputting coherent light into the coherent light generating means, in which case the coherent light input means is scanned along the coherent light generating means so that the photosensitive It is possible to create corresponding structures that extend to the material.

3 shows a device 30 of a preferred embodiment. In this arrangement, the ultraviolet laser beam 31 impinges on the phase or amplitude mask 32, which acts as a diffractive element and has a symmetrical direction about an axis defined by the beam 31, for example (33, Produces many diffraction orders such as 38). In Fig. 3 only the first order diffraction orders (+1, -1) are shown. Any further diffraction coming that may be generated may be blocked and do not form any further part of the installation 30. The first diffraction order beam 33 is first reflected 34 by the mirror 35 and then reflected 36 by the mirror 37. The reflected beam 36 then impinges on the photosensitive optical waveguide 40. As shown better in FIG. 4 and described below, the optical waveguide 40 is in the horizontal plane of the line (33, 34, 36) from the point 41 where the UV beam 31 strikes the phase mask 32. Displaced in a direction perpendicular to it.

The second primary diffraction beam 38 passes through the opposite path in that it is first reflected 42 by the mirror 37 and then by the mirror 43. The two mirrors 35, 37 can be adjusted such that the two beams 36, 43 strike on the same point of the optical fiber waveguide 40, thereby creating a diffraction grating 45 with an interference effect. Alternatively, the optical fiber waveguide can be moved to the overlapping position of the interfering beams. The correct placement of the fiber 40 and the angle of the mirrors 35, 37 are determined first by using a visible light source and by aligning the instruments, but it should be taken into account that the visible light source will be diffracted at a different angle than the UV light source. do.

Since the two traveling beams pass through substantially the same optical path but move in the opposite direction, any movement, vibration or shaking of one of the mirrors 35 and 37 will affect both beams in substantially the same shape, As a result, the relative phase of the two interfering beams is maintained. Thus, the interference fringe remains substantially stable, and the effects of "washing" of the diffraction grating that otherwise occur do not occur.

4 shows a perspective view of the device of FIG. 3. The UV beam 31 strikes the phase mask 32 to produce diffracted beams 33 and 38. Only the continuous path of the diffracted beam 33 is shown. This beam is reflected by the mirror 35 and mirror 37 to form a beam 36 which strikes on the fiber 40 at a point vertically above the path of the UV beam 31. The path of the beam 38 is similar in symmetry.

Various modifications are possible in addition to the preferred device 30. For example, a reflective diffraction grating can be used in place of the phase mask 32. Other beam orders can be used for reflection. It is possible to change the small inclination in the vertical direction. Preferably, the optical fiber 40 is located in a vertical position as close as possible to the vertical position of the UV beam 31.

The waveguide 40 may also be placed at a constant distance in front of or behind the diffractive element 32, with the angles of the reflecting mirrors 35, 37 adjusted accordingly to overlap the beams intersecting on the waveguide 40. You can do it at maximum. This has the effect of providing a tunable grating period, which will vary with the angle of the intersecting beams 36,43.

Furthermore, in an alternative arrangement, more mirrors than two mirrors 35, 37 may be used if the two beams travel along substantially the same path in the opposite direction. Preferably, the number of mirrors is even if no diffraction grating is used because in this case the spatial correlation of the two beam profiles will be allowed.

It is clear that the device of the preferred embodiment has a number of significant advantages. This includes the following:

(1) Since any movement in the mirrors 35 and 37 will affect both beams, the device 30 will ensure inherent stability, and consequently the movement of the stripes of the interference fringe will be minimized. Therefore, no special care is necessary to ensure the mechanical stability of the device 30.

(2) If any undesired diffraction beams can be blocked, and if the diffraction beams used have equal intensity, the interfering beams on the optical waveguide 40 will also be reflected by the same mirror to have equal intensity. Thus, the diffraction grating will have high contrast when the waveguide is located within the maximum overlap region of the intersecting beams 36, 43.

(3) The grating period can be adjusted by adjusting the angles of the reflecting mirrors 35, 37 and thereby adjusting the intersection of the beams 36, 43. When varying the angles of the mirrors 35, 37, the maximum overlapping area of the intersecting beams 36, 43 may move towards or away from the phase mask 32. Therefore, the position of the waveguide 40 must be adjusted accordingly in order to obtain the maximum stripe contrast. For each minor change, it was found that the inherent insensitivity of the interference fringe was maintained. Mathematical analysis shows that the sensitivity to the movement of one mirror appears only to introduce a second error correction period.

(4) One advantage of the preferred embodiment is that the two intersecting beams are not spatially inverted with respect to each other. Thus, the high spatial coherence of the beam is not required because the same parts of the beam interfere with the parts themselves and do not interfere with other parts of the beam that are symmetrically opposite as in the case of the prior art method. to be. As a result, in a detailed preferred embodiment, it can be used to form a long diffraction grating by scanning a narrow beam along the phase mask 32. The degree of transient coherence is however required for longer diffraction gratings, where this is the path difference between the beams.

(5) In an alternative embodiment, the diffraction grating period can be changed dynamically by varying the angles of the mirrors 35 and 37 while scanning the narrow beam. This can be used to give the diffraction grating a “chirp”, ie a change in period along its length. For small chirps, the effect on the striped contrast is minimal. Stripe contrast, on the other hand, depends on the overlap of the beams 36,43 on the optical fiber 40, and this overlap is varied by adjusting the angles of the mirrors 35, 37, which is why the apothesis is controlled by simultaneously adjusting both the period and the stripe contrast. It can be advantageously used to obtain a diced diffraction grating reflection spectrum.

(6) By positioning the optical waveguide at an angle to the interference fringe, the resulting diffraction grating can be " blazed " in that the diffraction grating stripes can be made with an angle to the plane transverse to the waveguide. .

FIG. 5 shows an alternative device 50 similar to the device of FIG. 3 but using a “three mirror system”. Mirrors 51, 52 and 53 are placed to form a Sagnac Loop. The coherent UV beam 31 is transmitted through the phase mask 32 to form the first and second coherent beams 55, 56. Beam 56 is reflected to mirror 51 to produce beam 57, which in turn is reflected to mirror 52 to produce beam 58, which in turn is reflected by mirror 53 to beam 59. ) In FIG. 5, the two beams 59, 55 are shown overlapping, thus showing a technically more accurate depiction of the actual system than in FIG. 3. The beam 55 undergoes a corresponding series of reflections in the mirror 53, mirror 52 and mirror 51 before reaching the interference region 60 where the optical fiber 40 is located in accordance with the techniques described above.

The device 50 is complicated by adding more mirrors, but when the UV beam 31 is displaced from the center of symmetry, the two beams, for example (55, 56), travel along the exact same path, thereby allowing a high degree of It has the advantage of having uniformity and vibration insensitivity. This should be distinguished from the device 30 of FIG. 3. When moving the UV beam 31 of FIG. 3 to the left, the interference region 45 of FIG. 3 moves in the opposite direction, ie to the right. This is the result of the two beams 33 and 38 of FIG. 3 traveling along different paths. This should be distinguished from the apparatus of FIG. 5 in which the interference zone 60 moves in the same direction as the UV beam 31 moves to the left 62. The device 50 therefore has a detrimental effect of the actions that cause the two backpropagating beams to travel along almost exactly the same path as the input beam 31 is swept along the phase mask 32 and thereby cause vibrations, airflow and other shakes. It has many advantages in that it enables self-clearing. Moreover, the path lengths of the two beams 55 and 56 will be substantially the same so that the coherence of the interference fringes will be maintained. This allows the generation of a substantially longer diffraction grating compared to the apparatus of FIG. 3.

Furthermore, apparently any one or more of the mirrors 51-53 can be rotated to provide a change in wavelength of conventional chirping and diffraction gratings formed. Thus, by using an odd number of mirrors, the side displacement of the input UV beam 31 does not cause the movement of the interference region in the opposite direction, which would result in undesirable separation of the backpropagation beam path. In contrast, the device 50 causes the interference region 61 to move in line with the UV beam 31. The use of the phase mask 32 also advantageously results in symmetry of the interfering beams superimposed on top of each other in the correct orientation, so that the phase mask 32 is preferred over the beam splitter.

It will be appreciated by those skilled in the art that numerous changes and / or improvements can be made in the present invention as shown in a particular embodiment without departing from the broader spirit and scope of the invention. Embodiments of the invention are therefore to be considered in all respects only as illustrative and not restrictive.

Claims (28)

CLAIMS 1. A method of forming a structure in a photosensitive material, comprising: (a) forming two coherent light beams; (b) backpropagating the two beams around the plurality of reflective elements such that the two beams interfere at a predetermined location; (c) positioning the photosensitive material at the predetermined position such that the structure is made at the predetermined position of the photosensitive material. The method of claim 1, wherein the structure is a diffraction grating structure. 3. The method of claim 1 or 2, wherein forming the two coherent light beams comprises passing one coherent light beam through a phase mask to produce two diffracted coherent light beams. Characterized in that. 3. A method according to claim 1 or 2, wherein the step of forming two coherent light beams comprises the reflection of one coherent beam in a diffraction grating. The method according to any one of claims 1 to 4, wherein the number of reflective elements is a multiple of two. The method of claim 1 wherein the photosensitive material comprises a photosensitive optical waveguide. 7. The method of claim 6, wherein the optical waveguide is an optical fiber. 4. The method of claim 3, wherein the one coherent light beam is scanned along the phase mask to produce an extended structure in the photosensitive material. 9. A method according to claim 8, wherein said interference at a predetermined position is moved to maintain a predetermined spatial relationship with said one coherent beam. 10. The method of claim 1 or 9, wherein the number of reflective elements is odd. The method of claim 10 wherein the number of reflective elements is three. 5. The method of claim 4, wherein the one coherent light beam is scanned along the diffraction grating to produce an extended structure in the photosensitive material. 13. A method according to any one of the preceding claims, wherein at least one of the reflective elements undergoes rotation or spatial movement in the creation of the structure. The method of claim 13, wherein the structure is a chirped diffraction grating. 15. The method of claim 14, wherein the structure is located in the plane of the interference beams substantially perpendicular to the bisector of the interference beam. 15. The method of claim 14, wherein the structure is located in the plane of the interference beam at an angle not perpendicular to the bisector of the interference beam. 15. The method of claim 1 or 14, wherein the photosensitive material is substantially in line with the plane of the two interfering beams and is not perpendicular to the bisector of the beams. 18. The method of claim 17, wherein the diffraction grating is a blazed diffraction grating. 15. The method of claim 1 or 14, wherein the photosensitive material is rotated about the plane of the two interference beams. 20. The method of claim 1 or 19, wherein the structure is an apodized diffraction grating. An apparatus for forming a structure in a photosensitive material, comprising: (a) coherent light generating means for generating at least two coherent light beams; (b) a plurality of reflective elements for reflecting the coherent light along a substantially equal path propagating back to cause interference of the coherent light in a structure staging area; and (c) structural staging means for installing said photosensitive material in said structural staging area. 22. The apparatus of claim 21 wherein the structure is a diffraction grating structure. 22. The apparatus of claim 21, wherein the number of reflective elements is even. 22. The apparatus of claim 21 wherein the structure is a photosensitive waveguide. 22. The method of claim 21, wherein the coherent light generating means further comprises coherent light input means for inputting coherent light into the coherent light generating means, and generating an extended structure corresponding to the photosensitive material. Device for scanning said coherent light input means along said coherent light generating means. 26. The apparatus of claim 25, wherein the interference in the structural staging area is translated with the scanning of the coherent light input means in order to maintain a spatial relationship between the coherent light input means and the structural staging area. Device. 27. An apparatus according to claim 21 or 26, wherein the number of reflective elements is odd. 28. An apparatus according to claim 27, wherein the number of reflective elements is three.