Patent Application
20130272656
The tunable FBG filters are the key components in many applications including optical communications as the tunable DWDM add-drop multiplexers, channel monitoring, tunable fiber lasers, optical fiber sensors, and so on.
The central wavelength of a fiber Bragg grating can be tuned by modifying the fiber refraction index or by changing the grating period. These variations can be induced thermally [1] or by mechanical stresses [2]. Due to the good silica behavior under stress, the mechanical compression or strain is normally preferred over thermal tuning. In particular, compression could provide more tuning span compared to the stretching which is limited by the tensile strength of the fiber.
Various approaches are proposed to compress the fiber [2-5]. For instance, a flexible beam is used in [3] to make the curvature in the fiber which could be controlled manually or by a motorized actuator. Axial compression of the fiber Bragg grating as described in U.S. Pat. No. 5,469,520 [2] and analyzed in [4-5] is another technique that received much attention. In this technique, the FBG is confined in a ferrule to prevent from bending during the compression. The fiber is fixed on one end to the stage while the other end is fixed on a movable stage which controls manually or by a motorized actuator. While the concept of the axial compression is the same, different approaches are focused on the choose of system parameters like grating and ferrule sizes, the gap between ferrules, maximum strain, etc to make a uniform strain over the FBG length and to prevent the fiber from local or global buckling. A fiber buckling creates non uniformities in the fiber strain which causes a non uniform variation in the grating period. As a result, the shape of grating filter is no longer the same; the higher the strain, the wider the bandwidth.
In all aforementioned approaches, the FBG is fixed from one end on a fixed stage and from other end on a movable stage. The unguided fiber length is at least equal to the maximum axial displacement. On the other hand, the maximal allowable length without bending of an unguided fiber is limited by a critical length based on the buckling theory of columns [4]. The critical length is a function of the stressed length and the axial displacement and in some cases, especially for long gratings limits the maximum shift in the central wavelength.
In this invention, we present a novel technique to compress or strain the FBG from both sides in order to get the more uniform force distribution along the grating and also the smaller unguided fiber length. As the axial displacement can be assumed to be on the both sides of the grating, the unguided fiber length is divided by a factor of two, which could double the maximum possible shift in the Bragg central wavelength. The first above-mentioned factor results a wider shift without changing the spectral shape of the filter compared to the conventional techniques.
An object of the present invention is to provide a novel technique to compress or strain the FBG from both sides.
Another object of the present invention is to compress the long gratings uniformly over the wide spectrum.
Another object of the present invention is to increase the shift in the FBG central wavelength without changing the spectral shape of FBG.
Still another object of the present invention is to more uniformly distribute the strain along the grating.
Still another object of the present invention is to change the grating period uniformly.
Still another object of the present invention is to reduce the unguided fiber length.
Still another object of the present invention is to prevent the fiber bending in the unguided fiber section.
Still another object of the present invention is to increase the maximum supportable strain by the FBG in the unguided section before breakage.
According to the present invention, a fiber grating is compressed from the both sides simultaneously or individually. The present innovation represents a significant improvement over the prior wavelength tunable fiber grating techniques by dividing the axial displacement length in two sections at both ends of the fiber grating. In some applications, the tunable fiber grating filters with a narrow bandwidth (less than 0.1 nm) and high extinction ratio (>30 dB) is needed. To meet these requirements, the grating length could be up to 20 mm or even longer. The compression of such a long grating over a wide spectrum needs a large axial displacement which could be larger than the maximal allowable length without bending of an unguided fiber. According to the present innovation, the long fiber gratings can be compressed uniformly and at least the same central wavelength shift as for the short gratings can be obtained without remarkable changes in the bandwidth or the spectral filter shape.
For a better understanding of the present invention, reference is made to the following detailed description and the attached figures, where:
With reference to the annexed drawings the preferred embodiment of the present invention will be herein described for indicative purpose and by no means as of limitation.
Referring to
The gap 106 between the fixed ferrule 105 and moving ferrules 103 contains the unguided fiber and permits to compress the grating when the ferrule 103 moves toward ferrule 105. The gap 107 between the fixed ferrule 105 and moving ferrules 104 contains the unguided fiber and permits to compress the grating when the ferrule 104 moves toward ferrule 105. The ceramic tubes 108 and 109 with a length of 10 mm are used to cover and protect the unguided fibers and also to align the ferrules during the displacement. When ferrule 104 moves toward ferrule 105, the ceramic tube 109 keeps them well aligned all the time.
The gaps 106 and 107 could be the same or different. If the ferrules 103 and 104 move simultaneously as it will be explained later in the present innovation, the gaps sizes should be the same. The total gap length of 106 and 107 determines the maximum axial displacement in the fiber, ΔL. It is related to the total shift in the central wavelength, Δλ by: Δλ/λ=α ΔL/L, where λ is the FBG center wavelength and L is the fiber length before strain. α is a parameter related to photoelastic coefficient of the fiber and is about 0.79 [4]. By having the fiber length, center wavelength and the shift in the center wavelength, the fiber axial displacement length can be calculated from the above equation. The gap length 106 and 107 are simply half of the fiber displacement length.
For the long gratings, the fiber axial displacement length could be in the order or longer than the maximal allowable length without bending of an unguided fiber and the compress technique as described in U.S. Pat. No. 5,469,520 is not able to provide a big shift in the center wavelength without changing the spectral shape due to fiber buckling and the fiber breakage.
Referring now to
By rotating the shaft 16 clockwise, the support 13 moves to the left and at the same time, the support 14 moves to the right compressing the FBG from both sides. The shaft 16 could be rotated manually or by using a motorized actuator. By rotating the shaft 16 counter-clockwise, the support 13 moves to the right and at the same time, the support 14 moves to the left relaxing the grating from both sides.
Referring now to
The manual rotation of shafts is shown in
[1] L. Eldada, et al., “Thermo-optic Planar Polymer Bragg Grating OADM's with Broad Tuning Range,” Photonics Tech. Let., vol. 11, no. 4, April 1999.
[2] Morey et al., “Compression-Tuned Fiber Grating,” U.S. Pat. No.: 5,469,520, Nov. 21, 1995.
[3] M. R. Mokhtar, et al., “Fiber Bragg grating compression-tuned over 110 nm,” Electron. Lett. Vol.39, 509, 2003.
[4] A. locco, et al., “Bragg Grating Fast Tunable Filter for Wavelength Division Multiplexing,” J. Lightwave Technol., vol. 17, no. 7, pp. 1217-1221, July 1999.
[5] N. Mohammad, et al., “Analysis and Development of a Tunable Fiber Bragg Grating Filter based on Axial Tension/Compression,” J. Lightwave Technol., vol. 22, no. 8, pp. 2001-2013, Aug. 2004.
