MXPA97002200A - Method to form a grid in an opt wave guide - Google Patents

Abstract

A method and apparatus for inscribing a Bragg grid in a photosensitive optical waveguide is described. The novel method consists of an interferometer that has precisely adjustable light gathering and direction components. A phase mask illuminated with ultraviolet light is used as the novel source of the interferometer. The adjustability of the interferometer allows the recording of Bragg gratings for filtering wavelengths on the Bragg scale for filtering wavelengths on the scale of 1275 nm to 1575 nm while adjusting the appropriate elements of the interferometer for only +/- 0.75ocada u

Description

METHOD STOP FORMING A GRID IN AN OPTICAL WAVE GUIDE BACKGROUND OF THE INVENTION The invention is directed to a method for forming a Bragg reflecting grid in an optical waveguide filament. Very particularly the method includes the use of a tunable phase mask to make the grid. The sensitivity of optical waveguide filaments to light of a certain wavelength and intensity since the late 1970s has been known. It was found that the loss characteristic and the refractive index of a filament could be permanently changed. of waveguide by exposing the waveguide to the light of given wavelength and intensity. A publication describing the effect and how it can be used is "Light-sensitive optical fibers and planar waveguides" Kashyap et al., BR Techno., 1, Vol. 11, No. 2, April 1993. The publication deals with the elaboration of light-induced reflecting grids, page 150, section 2.1, and indicates that the amount of refractive index change as the wavelength of light is reduced from 600 nm to 240 nm, where the photosensitivity of the guide of waves seems to reach its maximum. The first made grids that used photosensitivity, used light beams coherent with appropriate in the filament. The resulting interference pattern produced a periodic variation in the refractive index of the waveguide that behaved like a Bragg reflecting grid. A physical explanation of the photosensitive effect is given in the Kashyap document on page 151, section 3. Such a Bragg grid is useful as a wavelength filter. A Bragg grid in a waveguide filament will selectively filter the propagated light having wavelength which is twice the period of the grid. Since systems employing waveguide filaments can use multiplexing with wavelength division, a method and apparatus for forming Bragg filters, for a wavelength scale, is very desirable. In addition, such an apparatus and method must be efficient, economical, precise and simple to guarantee the speed, repeatability and accuracy of the method. Such a method lends itself to a production environment in which it is possible to produce, in volume, narrow band filters, effective at pre-selected wavelengths for use in sophisticated systems such as those using wavelength communication signals with wavelength division. . There are, of course, a lot of ways in which the filtering capacity of a reflecting grid can be used to produce information. The strain gauge, discussed in U.S. Patent 4,725,110, Glenn et al., Is an example. The method for forming a grid within a waveguide filament described in U.S. Patent 4,725,110, Glenn et al., Is illustrated in Figure 1 of that document, Here a Bragg grid is formed by irradiating the filament. of waveguide from the side in comparison with the method in which the light propagates through the waveguide, cited above. In addition, the formation of the grid is done using a terferometer. A disadvantage of this method for forming a grid is that, if grids of different period are required, the positions of the interferon source and the optical elements must be adjusted. In addition, the source of the terferometer described in the patent are two beams emerging from a beam splitter. The efficiency, luminous control and flexibility of a beam divider source are limited compared to a phase mask source. A publication, "Deep-UV spatial-frequency do? Bling by combining multilayer irrors with diffraction grating", Hawrylu et al., 3. Vac. Sci., B, Vol 1, No. 43, Oct-Dec 1983, shown, in Figure 3, page 1202, is a basic disposition of? Nterferometer having a grid as the input light source to the terferometer. The method and apparatus for forming grids of this publication does not address the problem of forming grids of different period in a waveguide filament. In the North American Patent 5, 327, 515, Anderson and others, a light source is directed onto a phase mask by means of a lens. Different kinds of diffraction emerge from the phase mask and are directed over a waveguide filament by means of a second lens, as shown in Figure 2 of that patent. The different diffraction classes interfere to produce a fringe model on the waveguide. The grid period of the fae mask is an integer multiple of the period of the fringe model that illuminates the waveguide. The integer refers to the highest of the two kinds of interference diffraction model. The sharpness of the grid produced by the method of? 515, Anderson and others, is limited by the quality of the lens or the lenses used to focus the fringe pattern on the optical waveguide. The lenses absorb light, thus placing more stringent requirements on the source. In addition, the possible number of grid periods is limited by the fact that the luminous intensity decreases rapidly with the increasing class of diffraction. Anderson et al. '515 and US Pat. No. 5,367,588, Hill et al., Both make reference to the use of phase masks incorporating at least two sections which have different periods from one another thus allowing at least two to form gratings of different period in a waveguide filament that uses a single phase mask. The inherent difficulties that one encounters using a phase mask grid that has at least two periods are: the phase mask grid is complex and therefore difficult to manufacture with accuracy and precision; and, the spatial separation of the gratings formed in the waveguide is limited by the spatial dimensions of the phase mask. There is therefore a need for a method and apparatus for forming gratings in the filament in a waveguide which: uses a single phase phase mask to form gratings in a waveguide filament, in the which grids thus formed can have a scale of periods; - form waveguide grids that have interfacial areas with clearly defined refractive index; Y, - uses a means to adjust the grid period which is simple in design so that the accuracy and precision in the manufacture of the grid is easily realizable.

DEFINITIONS A grid for optical waveguide is a periodic variation in the refractive index in the waveguide on the longitudinal axis of the waveguide. - Photosensitivity is an interaction between certain selected glass compositions and wavelengths of light in which the incident light changes the refractive index or the loss characteristics of the radiated glass i. - Lateral inscription is a technique for forming a grid in an optical waveguide filament in which the light is caused to form an interference pattern next to the waveguide filament and on a portion of the longitudinal axis of? N waveguide filament. The periodic light intensity model, produced by luminous interference, will induce a periodic change in the refractive index over a portion of the longitudinal axis of the waveguide filament. - A phase mask is essentially a diffraction grating for transmission. More particularly, in the context of this document, a phase mask is a material, transparent to a selected scale of wavelengths, transformed into a plate having a pair of flat, parallel and opposite sides, in which one of these sides has inscribed in them a set of separated lines periodically. The depth of the inscribed lines can be chosen to provide a phase variation that is an integer multiple of t, thereby suppressing the zero-order diffraction pattern. Alternatively, the zero-order diffraction pattern can be eliminated by means of a beam obstacle located between the phase mask and the waveguide filament. The angular extent of the divergent light rays emerging from the phase mask at any point on the linear grid can be characterized by a 2p angle.

BRIEF DESCRIPTION OF THE INVENTION The novel method and apparatus of this application satisfy the need for a method to form an exact and precise grid in an optical waveguide filament, wherein the method is simple and the apparatus is simple and durable, making them so Both practical for s? Use in a manufacturing environment. The invention of this application provides a method and apparatus for using a high power ultraviolet light source of relatively low coherence length, i.e., approximately 300 μm, together with a phase mask and an adjustable interferometer for laterally inscribing grid in an optical waveguide filament. Laser beams for inscription of coherence length as low as 10 μm are effective to form gratings. However, sources that have longer coherence lengths provide more flexibility in terms of the grid length that can be registered and the contrast of the grid. Coherence lengths of 10 meters are contemplated, as is possible using a laser beam of argon ion of duplicate frequency. The adjustability of the interferometer makes it possible to adjust the periodicity of the interference fringes formed on an interference plane. The longitudinal axis of a waveguide filament is located in the interference plane and oriented so that the interference fringes, formed in the interference plane, intersect the longitudinal axis of the filament of the waveguide. The photosenebility of an optical waveguide glass effectively copies or inscribes the pattern of fringes on the longitudinal axis of the waveguide. That is, the incident light on the waveguide increases the refractive index over the area of a constructive interference fringe, while the refractive index remains unchanged over the area of a destructive interference fringe. Therefore a first aspect of the invention is a method for laterally inscribing a Bragg grating in an optical waveguide. A phase mask is placed to receive light from a light source and diffract the light to an interferometer. The elements of the interferometer gather the light rays to the phase mask and direct them in convergence, that is, they overlap on a plane called the interference plane. It is therefore caused that the light rays coming out of the phase mask, which has an optical path difference less than or equal to the coherence length of the source, produce interference fringes on the interference plane. Since the gathering and direction elements of the interferometer are adjustable, the interference pattern is adjustable. In particular, the period of the fringe model is adjustable. In this way Bragg gratings, which filter a wavelength or a narrow band of selected wavelengths of a wavelength scale, can be inscribed in a waveguide filament. A high intensity monochromatic light source, in the scale of the ultraviolet wavelengths, ie, in the scale from about 100 to 400 nm, is particularly suitable for laterally inscribing a grid in an optical waveguide filament based on silica. However, the light wave lengths of 600 nm are effective for inscribing a grid in a waveguide filament. The coherence length of a light source of approximately 100 μrn produces a grid in the waveguide of sufficient length to provide high efficiency filtration, greater than 99%. However, a coherence length of 10 μm is sufficient to produce useful grids with waveguide filaments. An excited dimer laser beam is an excellent source of high intensity ultraviolet light, and, typically, has a coherence length of approximately 300 μm. An excited dimer laser beam can have a coherence length on a scale of about 10 μm to about 2.5 cm. An excited dimer laser beam which operates at various wavelengths including, 193 nm, 248 nm, 308 nm and 351 nm can be constructed. The meeting and direction elements of the interferometer are selected from a group of elements that include lenses, prisms, mirrors and combinations thereof. Mirrors are preferred because of s? power efficiency. In addition, the separation of fringes can be easily adjusted in the case in which the interferometer includes a pair of opposite j? Stablee mirrors. The amount of adjustment can be described by the angle between the plane of the mirror and the plane of the phase mask, angle 24 of FIG. 2. For a characteristic ultraviolet light source, which has the required coherence length, one adjustment of the included angle of approximately +/- Q.75 ° C, of each of two mirrors, in relation to an initial angle of 90 °, to produce gratings with waveguide filaments that are effective filters on the scale of wavelengths from about 1275 nm to 1575 nm, which scale includes two important wave scales of telecommunication signals near 1300 nm and 1550 nm, respectively. You can register this family of filters using a single mask. It will be understood that the invention can be used to inscribe grids or different wavelength scales by selecting a phase mask having an appropriate period. For example, the invention can inscribe gratings on the wavelength scale from 500 nm to 1275 nm, thereby covering an important part of the wavelength scale of the waveguide filament communications. The period of the inscribed grid changes from approximately 435 nm to 540 nm on the wavelength scale from 1275 nm to 1575 nm. One mode of the phase mask is a flattened object made of a material that is transparent to light on the scale of ultraviolet wavelengths. A series of slots uniformly spaced on the flattened surfaces of the object is formed to provide the structure of a grid. To eliminate the zero-order diffraction pattern, a beam obstacle can be used. As an alternative to the beam obstacle, you can choose the depth of the grooves, multiplied by the refractive index of the substrate, which is an integer multiple of * to eliminate the diffraction pattern of zero order, as is done in the conventional technique of the phase mask in the which mask is essentially in contact with the photosensitive surface. A second aspect of the invention is an apparatus for laterally inscribing a Bragg grating on an optical waveguide filament, in which the apparatus is adjustable to produce effective gratings on a wavelength scale, i.e. have a preselected period. The device consists of a light source that illuminates the phase mask. The light comes out of the phase mask co or a series of divergent light rays. The optical means to gather and direct light are placed in the trajectories of the rays to cause the rays to overlap at points in space that define a plane called the plane of interference.

The light rays that transpose that have differences in the length of s? optical path less than or equal to the coherence length will interfere, producing a pattern of stripes on the interference plane. The apparatus includes a device for maintaining the longitudinal axis of the waveguide filament in the interference plane. The zero order of the diffraction can be eliminated by any of the methods indicated above. In a preferred embodiment of this aspect, the optical means for gathering and directing the light emerging from the phase mask are a pair of mirrors arranged symmetrically around the phase mask. The mirrors are mounted with pivots so that the angle between the plane of the mirror and the plane of the phase mask can be changed, thereby changing the spacing between the fringes of the fringe pattern. To produce effective gratings on a scale of wavelengths of several hundred nanometers, each mirror must have a minimum setting of approximately +/- 0.75 ° around an initial included angle that was chosen to be nominally 90 °. A preferred source is an excited dimer laser beam operating on the ultraviolet wavelength scale from about 100 nm to 400 nm. Using an ultraviolet excited laser beam source, a symmetrical change in the respective mirror angles of about 0.75 ° produces a change in the strip spacing on the scale from about 435 nm to 540 nm. These and other aspects of the invention will be discussed with the help of the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The figure shows the basic elements of the new lateral inscription device. Fig. Ib shows a waveguide filament in the interference plane with interference strips on sunsets. Figure 2 illustrates the adjustability of the mirrors. Figure 3 is an illustration of a grid formed in a phase mask.

DETAILED DESCRIPTION OF THE INVENTION The novel method combines a simple thermometer with a phase mask to inscribe laterally, in an optical waveguide filament, a Bragg grid that has a preselected period. The period of the grid is adjusted by pivoting the elements of the interferometer. It will be understood that the method can be used to laterally inscribe gratings in light-sensitive objects of glass other than the filaments of a waveguide. Some examples of such objects are planar waveguides or planar structures such as dividers. The method is sufficiently robust for use in a manufacturing environment, because the elements of the interferometer are fixedly fixed except for the pivoted elements. Means are also required to place the waveguide in the interference plane. Such means, including vacuum nozzles mounted on a precision transducer carrier, are known in the art of waveguide filaments. The intimate relation between the method and the apparatus arises from the requirement of sharpness of the structure of the grid, that is to say, the sharpness or the contrast between the bright stripes and lae obecurae, and from the requirement that a scale of periods of the grid using a very stable apparatus, that is to say, an apparatus that is placed in a considerably fixed manner. The interference phenomenon provides sharpness between the illuminated and unlighted portions of the waveguide. The fixed position of the light source and the axes of the elements of meeting and direction of the light of the interferornetro guarantee the flexibility of the apparatus, in terms of the period of the grid, as well as the stability and the reliability. The basic elements of the apparatus are shown in figure la. The light source 2 directs a light beam onto the phase mask. The elements to collect and direct the light from the phase mask are shown, as mirror planes 10. It is understood, however, that these elements can be other simple or composite optical devices for gathering and directing light, such as prisms, lenses, non-flat mirrors or combinations of these. The mirror element is preferred because it provides simplicity and reliability of interferometer design. The elements 10 direct the multiple rays 8 to overlap on the plane of interference 12, where the rays interfere to produce a pattern of stripes. The length 26 shows the linear dimension of the striped pattern. The angle 6 is half the angle of the beam that comes out of the fae mask. The optional beam obstacle 11 is also shown. The device is displayed in its initial position. That is, it is shown that the angle 14, between the flat mirror 10 and the phase mask 4, ee of 90 °. The magnitude of the length 26, the length of the fringe model, is of interest when determining the filtering capacity of the photoinduced grid in a waveguide by the fringe model. If the length 26 is too short, the filtration may be insufficient to satisfy a particular requirement of the system. However, it is noted that filtration efficiency greater than 99% with grid lengths of a few millimeters, length precisely within the apparatus capacity described herein, is possible. In Figure Ib, a waveguide filament 18 with its longitudinal axis is shown in the interference plane 12. The stripes 22 are incident on the waveguide, thereby producing, through the photosensitivity effect, periodic changes in the waveguide. the refractive index on the longitudinal axis of the waveguide. The locations in the waveguide that receive the bright stripes are increased in their refractive index while those locations that do not actually receive light remain unchanged. Therefore, the interference fringes effectively inscribe a Bragg reflecting grid in the waveguide filament. The novel apparatus, as illustrated in Figure 2, shows the pivotal movement feature of the light gathering and directional elements of the interferometer. For the case shown, each plane mirror pivots about an axis fixed to a plane mirror edge and extends perpendicular to the plane of the drawing. The pivot angle of rotation is marked 24. As the angle of rotation in pivot 24 increases, the length of the pattern of stripes 28 in the interference plane decreases. The length of the fringe pattern and therefore the length of the photoinduced grid in the waveguide is more than efficient for the purposes of making the Bragg filters described herein.

EXAMPLE BRAGG GRIDS COVERING THE TRANSPARENCY SCALE OF WAVELENGTH LENGTHS FROM 1275 NM TO 1575NM The excited dimer laser, which has a coherence length of about 300 μm and a wavelength at the center of 248 nm, is used together with the apparatus of Figure 2 to laterally inscribe a grid in the central part of the Waveguide. The grid separation of the phase mask and the parameters of the interferometer are chosen, fixing the angle 24 in figure 2, equal to zero, to produce a pattern of fringes in the interference plane of the period 1425 nm / 2 (1.46 ) = 488 nm, where 1.46 is the refractive index of the silica. This configuration of the apparatus allows a suitable Bragg grid to filter 1425 nm light to be inscribed on the central part of the waveguide. The spatial period of the grid inscribed on the waveguide is essentially the same as the period of the stripe model of the near field of the phase mask. By changing the angle 24 of both mirrors symmetrically by 0.75 °, a suitable grid can be inscribed to filter 1575 rm on the central part of the waveguide. By changing the angle 24 of both mirrors by -0.75 °, a suitable grid can be inscribed to filter 1275 nm. For the present apparatus, a positive angular change is made by pivoting the angle of the right side clockwise and the mirror of the left side counterclockwise about its respective axis. Therefore, the apparatus of the example is capable of describing the Bragg gratings to filter selected wavelengths somewhere in the wavelength range from 1275 nm to 1575 nm making very small changes in the angle 24. This slippage is advantageous for at least two reasons. First, it is generally true that better mechanical accuracy and precision can be achieved when the distance of travel of a component, which can be moved by a very precise and repeatable amount, is relatively small. Second, the pattern length of stripes in the interference plane is still very large at the maximum required angle 24. A Bragg grating made at a larger adjustment of angle 24, that is, a grid suitable for filtering 1535 nm, may be the length in the waveguide of more than 2 cm for the configuration of the apparatus of this example. The grid length can be calculated from the general equation, 2d = 21cos (p - 2t) / tan (2t), where 2d is the length 28 of the fringe model in Figure 2, p is half the angle 6 in FIG. 2, t is the angle 24 in FIG. 2, and 1 is the coherence length of the source, q? E is approximately 300 μm for the laser source of excited dimers, used in the example. A characteristic phase mask is illustrated as 36 in Figure 3. The width of the slot 32 is equal to the width of the plateau 30 between the slots. The depth of slot 34, multiplied by the refractive index of the substrate, can be made equal to a multiple integer to go to provide destructive interference of the distraction model of zero order. Other embodiments of the invention will be apparent to those skilled in the art. For example, the pivot axis of the mirror should not necessarily be at the edge of the mirror. Also, adjustable thermometer elements that are not mirrors can be used. Although particular embodiments of the invention have been disclosed and described herein, the invention is nevertheless limited only by the following claims.

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for laterally inscribing a grid in a photosensitive optical waveguide filament consisting of the steps: a) placing a phase mask, having first and second sides which are flat and parallel to each other, to receive, on the first side, light from a light source, light retracting through said phase mask and leaving the second side as a disposition of diverging light rays, placed the second place of said phase mask as the light source for a interferometer, in which the interferometer causes the light rays to overlap and form a pattern of interference fringes on an interference plane, the interferometer having adjustable optical elements to vary the spacing between the fringes in the interference plane; b) adjusting the optical elements to provide a preselected strip separation, wherein said optical elements include means for eliminating the zero order of diffraction; and c) placing the longitudinal axis of the filament of an optical waveguide in the interference plane in such a way that the pattern of the interference fringes is incident on the waveguide filament on the longitudinal axis.

2. The method of claim 1, further characterized in that the light of the light source has a coherence length q? E is at least 10 μm and / or because the wavelength of the light of the light source is on the scale of approximately 100 nm to 600 n, or because the coherence length is on the scale of 10 μm to 10 meters, or because the coherence length of the light source is approximately 100 μm and the scale of the length of wave of the light source is from about 100 nm to 400 n.

3. The method of claim 1, further characterized in that said adjustable optical elements are two planes and the angle between each plane mirror and the phase mask is adjustable, or because the angle included is initially 90 ° and because it can adjust at least +/- 0.75 °, or because the separation between the corresponding points on the adjacent fringes from about 465 nm to about 540 nm can be adjusted.

4. The method of claim 1, further characterized in that said means of eliminating the zero-order diffraction pattern ee a phase mask including the substrate and separate slots periodically formed therein, each slot having a depth of which each depth, multiplied by the refractive index of the substrate material, is chosen because it is an integer multiple of ir.

5. The method of claim 1, further characterized in that said means for eliminating the diffraction pattern of zero order is a beam obstacle.

6. An apparatus for laterally inscribing a grid of preselected period in a light-sensitive optical waveguide filament consisting of: a light source; a phase mask positioned to receive light from said light source and transmit the light as a l-ray array? diverging inos; optical means to eliminate the zero-order deflection model; optical means to intercept and redirect the rays lurninosoe di ergentee so that they overlap in a series of points in the space that gave rise to an interference plane, causing the overlap of light rays that forms a pattern of interference fringe on the plane of interference, in which optical means are adjustable so that the separation between the interference fringes is adjustable; means for permanently placing the longitudinal axis of an optical waveguide filament in the interference plane so that the interference pattern is incident on the waveguide filament about the longitudinal axis.

7. The apparatus of claim 6, further characterized in that said optical means for intercepting and redirecting the divergent light rays are a pair of flat mirrors positioned symmetrically with respect to said phase mask and forming an angle between the coated mirrors. planes and said phase mask which is adjustable around a central value of 90 °, in which optionally the amount of adjustment of the angle comprised ee of at least +/- 0.75 °.

8. - The apparatus of claim 7, further characterized in that the spacing between the stripes ranges from about 435 n to 540 nm as each angle included is symmetrically varied between 0.75 ° or 0.75 °.

9. The apparatus of claim 6, further characterized in that the light source has a wavelength in the range of 100 nm to 600 nm.

10. The apparatus of claim 9, characterized in that the light source is an excited dimer laser beam, optionally said excited dimer laser beam has a coherence length in the scale of about 100 μm to 2.5 cm, or having a wavelength on the scale of ap approximately 100 nm to 400 nm and the coherence length of approximately 100 μrn.