Variable optical fiber Bragg filter arrangement

Abstract: A variable light filtering arrangement (10) includes at least one optical fiber section (11) including a waveguiding core (13), and at least one permanent Bragg grating region (12) in the optical fiber section. The grating region includes a plurality of grating elements constituted by periodic refractive index variations of a predetermined initial periodicity and cumulatively redirecting, of the light launched into the core for guided propagation therein, that having an axial wavelength within a narrow band around a central wavelength that is determined by the periodicity and refractive index variations of the grating elements. At least one of the periodicity and refractive index variations of the grating region is controlledly modified in such a manner as to temporarily change the central wavelength within a predetermined wavelength range.

Description: Technical Field The present invention relates to optical fiber embedded Bragg gratings in general, and more particularly to arrangements for controlledly modifying the effect of such gratings on light propagating through the optical fiber core. Background Art There are already known various constructions of optical filters, among them so-called Bragg filters. On type of a Bragg filter, which has been developed for sensing stress and/or temperature changes in structures, is incorporated or embedded in the core of an optical fiber by a method disclosed, for instance, in the commonly owned US-A-4,807,950. As discussed in this patent, permanent periodic gratings of this kind can be provided or impressed in the core of an optical fiber by exposing the core through the cladding to the interference pattern of two coherent ultraviolet light beams that are directed against the optical fiber symmetrically to a plane normal to the fiber axis. This results in a situation where the material of the fiber core has permanent periodic variations in its refractive index impressed therein by the action of the interfering ultraviolet light beams thereon, with the individual grating elements (i.e. the periodically repetitive regions of the core exhibiting the same refractive index behavior) being oriented normal to the fiber axis so as to constitute the Bragg grating. The embedded Bragg grating of this kind reflects, of the light launched into the fiber core for guided propagation therein in a propagation direction, only that having a wavelength within a very narrow range around a central wavelength that is dependent on the grating element periodicity, back along the fiber axis opposite to the original propagation direction while being substantially transparent to light at wavelengths outside the aforementioned narrow band so that it does not adversely affect the further propagation of such other light. In effect, this type of grating creates a narrow notch in the transmission spectrum and, by the same token, a similarly narrow peak in the reflection spectrum. In the applications for which this kind of Bragg filter has been developed, any stresses or temperature changes encountered at the location of the structure in which the Bragg filter is embedded affect the grating and change its periodicity, or the index of refraction of the material of the core, or both, so that the position of the central wavelength in the spectrum is shifted, thus providing an indication of the stresses or temperature changes existing or taking place in the structure being monitored at the location of the grating. From Optical Fiber Sensors Topical Meeting, 1988, Technical Digest Series, New Orleans, Louisiana, 27-29 January 1988, volume 2, part 1, Optical Society of America, (Washington, D.C., U.S.) G. Meltz et al.: "In-fiber Bragg-Grating sensors", pages 163-166, it has become known to use permanent in-fiber Bragg gratings as wavelength selective temperature or strain sensors as a Bragg grating acts as a very narrowband reflection filter the line center of which shifts with temperature or applied strain. There is no disclosure as to how the temperature or strain to be sensed can be applied to the Bragg grating. As advantageous as these Bragg gratings are in the above-mentioned applications, there are other areas which would benefit from the use of such gratings and especially from the wavelength selectivity or narrow band response thereof, but for which these gratings are not suited in the form proposed in the above patent. Such other areas include, for instance, applications in which it is required to tune the Bragg grating stopband to a desired frequency (by correspondingly shifting the central wavelength), or maintain the grating response at the desired frequency, or to scan a frequency range, such as in the optical fiber communication field, in the optical laser frequency control field, and the like. From US-A-3,916,182 and from EP-A-0 221 560 it has become known to use temporary Bragg gratings as filters the filter characteristics of which can be tuned or modulated by varying the grating pattern temporarily produced in a fiber material which is adapted to from a grating patter only as long as it is exposed ro radiation causing altering the index of refraction of the fiber material. Tuning or modulating the filter characteristics is done by varying the condition of exposing the fiber material to the grating producing radiation. Temporary gratings impose narrow restrictions as to suitable fiber materials and require a relatively complex and costly exposing apparatus for each grating filter. Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art. More particularly, it is an object of the present invention to provide a Bragg filter arrangement which does not possess the disadvantages of the known arrangements of this kind. Still another object of the present invention is so to develop the filter arrangement of the type here under consideration as to be able to control the optical wavelength or wavelength band to which the Bragg filter is responsive without restrictions and expenditure associated to temporary gratings. It is yet another object of the present invention to devise an arrangement of the above type which is capable of operating at relatively high speeds in response to applied external signals. A concomitant object of the present invention is to design the arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation. Disclosure of the Invention In keeping with these objects and others which will become apparent hereafter, the present invention provides a variable light filtering arrangement as defined in claim 1. Embodiments of the invention are defined in the dependent claims. In a variable light filtering arrangement according to the present invention, at least one of the periodicity and refractive index variations of a permanent grating region is controlledly modified as to temporarily change the central wavelength within a predetermined wavelength range. Brief Description of the Drawing The present invention will be described in more detail below with reference to the accompanying drawing in which: Fig. 1 is a perspective view of an optical fibers section incorporating a Bragg grating in its core and provided with a wavelength modifying device including an external layer responsive to acoustic field changes on the fiber section; and Fig. 2 is a side elevational view of an arrangement including the grating of Figure 1 but using an acoustic wave applicator as the modifying device. Best Mode for Carrying Out the Invention Referring now to the drawing in detail, it will be appreciated that the same reference numerals, possibly supplemented with reference characters where appropriate, have been used throughout to identify corresponding parts. Turning first to Figure 1 as a basis for the following general explanation, it may be seen that the reference numeral 10 has been used therein to identify a variable optical filtering arrangement of the present invention in its entirety. The arrangement 10 includes an optical fiber section 11 that has a Bragg (or periodic) grating 12 embedded therein. The grating 12 may be, and advantageously is, of the type, and made in accordance with the method, disclosed in US-A-4,807,950 the disclosure of which is incorporated herein by reference to the extent necessary to provide additional explanation of the type of the Bragg grating 12 that is particularly suited for use in the variable optical or light filtering arrangement 10. As more fully explained in the above patent, gratings 12 of this type act as wavelength selective filters. An embedded grating 12 of specified periodicity will partially or wholly reflect or block in transmission a specific wavelength of an optical signal carried by the fiber section 11. In general, it is proposed in accordance with the present invention to change or shift the reflected or transmission blocked wavelength, by altering the effective spacing of the grating elements or the index of refraction through externally applied forces or actions on the fiber section 12, by an amount that is related to the intensity of the external force or action. As a consequence, the external force or action can be used to tune or adjust the wavelength of the Bragg grating filter 12. By increasing the intensity of the external action, the wavelength of the filter 12 can be scanned. Furthermore, by applying a periodic action, the wavelength of the filter 12 can be modulated. External actions that can change the effective spacing or resonances of the Bragg filter 12 are mechanical strain by acoustic and ultrasonic interactions. The fiber section 11 can also be coated with various materials that can enhance the interaction between the applied action or force and the Bragg filter 12, and thus affect the wavelength tuning, scanning, or modulation. Moreover, by applying a traveling wave action to the fiber section 11 containing the filter 12, a frequency shift can be applied to the optical signal carried in the fiber section 11. An approach to mechanically affecting the fiber section 11 for wavelength tuning of the grating 12 is to use special coatings on the fiber section 11 over the region of the Bragg filter 12, as shown in Figure 1 of the drawing. The coating 15d is sensitive to modulating acoustic fields. In the case that the electric fields coating 15d is sensitive to modulation of the Bragg filter 12 could be effective to frequencies of a few hundred MHz utilizing the standing acoustic waves generated in the fiber by the action of the electric field on the coating 15d. Figure 2 illustrates how a traveling wave modulation of the optical signal carried by the fiber 16 can be achieved by passing acoustic flexure waves along the fiber section 11 containing the Bragg filter 12. Such acoustic flexure waves result from the application to the fiber section 11 of acoustic waves generated by a piezoelectric driver 15e driven by an electric source 17 and concentrated by an acoustic horn 18 onto the fiber section 11 which in this instance is mounted between two vibration dampers 19 and 20. This acoustic action shifts the frequency of the optical signal by generating side bands. Figure 1 illustrates the frequency shifter. The thus generated flexure wave travels along the fiber section 11 in the direction of the optical signal and through the Bragg filter 12. Modulation of the filter 12 by the flexure wave also modulates the optical signal. For a flexure wave traveling with the optical signal, the frequency of the optical signal is incremented by the frequency of the flexure wave. Another feature for wavelength tuning would be to apply a spatially varying or gradient force or action along the fiber section 11 at the embedded grating 12. This action would create a variable grating spacing or "chirped" grating 12. The variable spacing would broaden the bandwidth of the Bragg reflection. Thus, the central wavelength of the Bragg reflection can be adjusted, as discussed previously, with a constant action along the fiber section 11 and/or the bandwidth of the Bragg reflection can be varied by using a varying action or component along the fiber section 11. It is further contemplated in accordance with the present invention to use at least some of the above-discussed approaches in conjunction with gratings of the type redirecting light a given axial wavelength between a first path extending longitudinally of the fiber core and a second path extending through the fiber cladding to the exterior of the fiber. In this case, the perioditicy and/or refractive index changes caused by the application of longitudinal stresses to the grating in accordance with the present invention commensurately change the central wavelength of the light that is redirected by the grating from the first into the second path, or vice versa. It is to be understood that all references made herein to a filter arrangement containing a Bragg filter, refer not only to a single embedded grating filter 12, but also, by extension, to a more complex multiple grating filter. While the present invention has been illustrated and described as embodied in particular constructions of optical filtering arrangements, it will be appreciated that the present invention is not limited to these particular examples; rather, the scope of protection of the present invention is to be determined solely from the attached claims.

Claims: A variable light filtering arrangement comprising: a) at least one optical fiber section (11) including a waveguiding core (13); b) at least one permanent Bragg grating region (12) in said optical fiber section (11) constituted by periodic refractive index variations of a predetermined initial periodicity and adapted for cumulatively redirecting of the light launched into said core (13) for guided propagation therein, that light having an axial wavelength within a narrow band around a central wavelength that is determined by the periodicity and refractive index variations of said grating region (12); and c) wavelength tuning, scanning or modulating means (including 15d;15e) for controlledly modifying at least one of the periodicity and refractive index of said grating region (12); d) said modifying means (including 15d;15e) including means for mechanically affecting the optical fiber section (11) with acoustic fields, with attendant changes in said central wavelength. The variable light filtering arrangement as defined in claim 1, wherein said optical fiber section (11) includes a cladding surrounding (14) said core (13) and being at least substantially coextensive with said grating region (12); and wherein the modifying means (15d) includes a layer of material surrounding said cladding (14) and changing its dimensions in response to changes in acoustic field applied thereto, and means for varying the acoustic field applied to the layer. The variable light filtering arrangement as defined in claim 1, wherein the modifying means (15e) includes means for passing acoustic flexure waves along the optical fiber section (11) with attendant shifting of the frequency of the central wavelength. The variable light filtering arrangement as defined in claim 3, wherein the means for passing acoustic flexure waves includes a piezoelectric driver (15e) driven by an electric source (17). The variable light filtering arrangement as defined in claim 4, wherein the means for passing acoustic flexure waves includes an acoustic horn (18) concentrating the acoustic waves generated by the piezoelectric driver onto the optical fiber section (11). The variable light filtering arrangement as defined in claim 5, wherein the acoustic horn (18) is mounted between two vibration dampers (19, 20).

Inventors: GLENN WILLIAM H, LEONBERGER FREDERICK J, MELTZ GERALD, MOREY WILLIAM W, GLENN, WILLIAM H., LEONBERGER, FREDERICK J., MELTZ, GERALD, MOREY, WILLIAM W.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1990
Main classification: G02F1
IPC codes: G02B6, G02F1