BRIEF DESCRIPTION OF THE DRAWINGS
The present invention shall become better understood by reference to the following detailed description and considered in connection with the accompanying drawings wherein:
FIG. 1 is a schematic illustration of a setup used for writing FBG having modulated refractive index using an amplitude mask according to the method and system of the prior art.
FIG. 2 is a schematic illustration of a setup used for writing FBG having modulated refractive index using an amplitude mask according to the method and system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 2, the setup 200 used in the method and system of the present invention is shown. As can be noted, FIG. 2 comprises two amplitude masks. A first “real” amplitude mask 270, is positioned between the cylindrical lens 220 and the optical fiber 240 as per the invention. The second amplitude mask 210, shown in phantom lines, is the equivalent, at least in ray optic, of the first amplitude mask 270 should the amplitude mask 270 be placed before the cylindrical lens 220 (as in the prior system of FIG. 1).
As stated above, in ray optic, both amplitude masks 210 and 270 are equivalent. However, in wave optic, both amplitude masks 210 and 270 are not equivalent. In other words, the apodization profile defined by the amplitude mask 210 would be different from the apodization profile defined by the amplitude mask 270. This difference is one of the bases of the present invention.
As explained above, contrary to conventional FBG apodization method setup (see FIG. 1), in this invention, the amplitude mask 270 is placed along the focused laser beam propagation path 260, between the cylindrical lens 220 and the focus point where the optical fiber 240 is mounted with means known in the art. According to the laws of ray optic, the amplitude mask 270 has a substantially equivalent amplitude mask 210 in the laser beam propagation path 250 before the cylindrical lens 220. The converse is also true. Therefore, amplitude mask 210 positioned before the cylindrical lens 220 and along the laser beam propagating path 250 has a substantially equivalent amplitude mask 270 along the focused beam propagating path 260 and located between the cylindrical lens 220 and the optical fiber 240. According to the laws of ray optic, both masks 210 and 270 are mutual images of each other. Therefore, still according to ray optic, these equivalent amplitude masks should provide the same apodization profile on the optical fiber 240. From the optical fiber 240 position point of view and according to wave optic, the apodization profiles provided by both masks 210 and 270 are different and even irreversible.
Referring now to FIG. 1, we can see a prior art setup 100 for apodization method and system. In this setup, the amplitude mask 110 is placed before the cylindrical lens 120 and along the laser beam propagation path 150. In setup 100, the amplitude mask 110 can provide only one power modulation profile on the optical fiber 140 even though it has many substantially equivalent masks along the focus beam propagation path 160.
In optimized focus condition, the total laser power that reaches the optical fiber 140 is constant. In the case of an amplitude mask 110 having a small aperture, since most of the laser power is blocked out by the amplitude mask 110, this limits the strength of the grating writing when apodization is used. Understandably, the laser power blocked by the amplitude mask 110 cannot be used to write the grating.
Moreover, after having passed through the small aperture of the amplitude mask 110, the laser still has a relatively long distance to travel before reaching the optical fiber 140. Laser beam which travels over long distance after going through a small aperture induces distortion in the apodization profile because of Fraunhofer diffraction. Similarly, strong apodization with narrow-high amplitude mask induces distortion in the apodization profile because of Fresnel diffraction which are caused when the laser beam travels a long distance after going through a slit-like aperture.
However, as shown in FIG. 2 and as explained above, if an amplitude mask 270 is placed along the focus laser beam propagation path 260 between the cylindrical lens 220 and the optical fiber 240, the amplitude mask 270 has an equivalent mask 210 before the cylindrical lens 220. Moreover, by displacing the amplitude mask 270 along the focus laser beam propagating path 260 (see arrow 290), the amplitude mask 270 can define a series of different equivalent amplitude masks 210 depending the mask 270 position. These equivalent masks will have the same width but different height and therefore, different apodization profile. Thus, only by varying the position of a single amplitude mask 270, it is possible to create a plurality of apodization profile.
Also, by placing the amplitude mask 270 along the focused laser beam propagation path 260, the laser power inherently blocked by the mask 270 is significantly lower than if the mask 270 was equivalently placed before the cylindrical lens 220. By blocking less laser power, the amplitude mask 270 allows for the writing of stronger FBG with the apodization method and system.
Furthermore, by placing the amplitude mask 270 nearer the optical fiber 240, the distance traveled by the laser beam after going through a small aperture or a narrow slit-like opening is substantially reduced. This results is a tremendous reduction of the distortions in the apodization profile caused by Fraunhofer diffraction and/or Fresnel diffraction.
The skilled addressee will understood that the setup 200 of the present invention can obviously further comprises a phase mask 230, known in the art, placed substantially near the optical fiber 240 as in the prior art setup 100 (see FIG. 1). Phase mask 230 is used for creating an interference pattern on the grating writing area of the optical fiber 240.
Although preferred embodiments of the invention have been described in detail herein and illustrated in the accompanying figures, it is to be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention.
Patent Application
20070280596
