1. Field of the Invention
The present invention relates in general to fiber-optic devices, and more particularly to a fiber-optic device prepared using a mode coupling fiber grating and being suitable for using as a wavelength filter, a polarizer, an optical switch, a wavelength division multiplexer, a strain sensor, a temperature sensor and etc.
2. Description of the Prior Art
It is well known to those skilled in the art that varieties of materials are used to increase the refractive index of an optical fiber core in preparation of an optical fiber. One of them, such as germanium doped in core, is exposed to an intensive light, its refractive index is changed permanently and, in this regard, such a material is named as a photo-sensitive material. Also, an optical fiber core doped with such a photosensitive material changes its refractive index when it is exposed to such an intensive light. And also, the change of the refractive index of the fiber core is permanent, so that the fiber core remains its changed refractive index even when the optical fiber does not receive the intensive light any longer. Here, when the optical fiber is an optical fiber through which at least two modes propagate, it is possible to induce a change of the refractive index of the fiber core due to an interference of and a propagation constant difference between, and a polarization of the modes, thus to provide an optical fiber in which the refractive index of the core is periodically and spatially changed. Such an optical fiber in which the refractive index of the fiber core is periodically and spatially changed is called a fiber grating which is used in preparation of varieties of optical fiber devices.
A schematic illustration of a mode coupling fiber grating will be given with reference to FIG. 1 showing a mode beat of and a fiber grating preparation theory of a conventional two-mode optical fiber.
When at least two modes propagate through an optical fiber, these modes propagate with different propagation constants, respectively. Especially in the elliptical-core two-mode optical fiber, two modes propagate stably through the optical fiber with different propagation constants. The LP.sub.01 mode of the two mode optical fiber shows an electric field distribution of the incident light which is symmetric radially while the electric field distribution of the LP.sub.11 mode is antisymmetric in the long-axis direction. During the propagation of the two modes through the two-mode optical fiber, these modes cause a beating in the optical fiber due to a relative phase difference at a point between them and the beat length L.sub.B is changed in accordance with a wavelength and a polarization of an incident light. At this time, the electric field distribution of the lights propagated through the optical fiber causes a constructive interference at one side of the optical fiber core as well as a destructive interference at the other side of the optical fiber core in accordance with the electric field distribution, thus causing intensive light parts to alternately appear at an upper section and a lower section of the optical fiber core at regular intervals equal to the beat lengths L.sub.B. Here, when the incident light has a sufficient intensity, the refractive index of the optical fiber core is permanently changed by the photo-sensitive material doped in the optical fiber core as described above, thus to periodically induce asymmetric refractive index distributions .DELTA.n along the optical fiber.
When the fiber grating, of which the refractive index is permanently changed, is exposed to a weak light, the two modes are coupled due to the periodic distribution of permanent refractive index change of the optical fiber, thereby to result in a mode coupling, i.e., a power exchange of the modes. Let this be represented as following equation (1). EQU A.sub.(Z) =B.times.K.sup.2 /{K.sub.2 +(.theta./2).sup.2 }sin.sup.2 {K.sup.2 +(.theta./2).sup.2 Z} (1)
wherein A is the coupling amount, B is the quantity of the incident light, K is a coupling coefficient, .theta. is the amount of phase mismatch i.e., a phase difference between the fiber grating period and the beat of the incident light, and Z is the length of the fiber grating.
As will be noted to those skilled in the art, the coupling efficiency of the fiber grating depends upon the phase mismatch .theta.. In other words, the phase mismatch, .theta.=0, results in a complete mode coupling while the phase mismatch, .theta.=0, results in an incomplete mode coupling.
For example, when a strain is applied to the fiber grating, the phase mismatch .theta. will be represented by the following equation (2). EQU .theta.=2.pi.[1/.lambda..sub.0 {1-(.delta.l/l.sub.0)}-1/LB.sub.0 {1-.alpha.(.delta.l/l.sub.0)}] (2)
wherein .delta.l is an elongated length of the fiber grating and .lambda..sub.0 is a grating period.
From the above equation (2), it is noted that the grating period .lambda.o is changed as much as .delta..lambda./.lambda..sub.0 =.delta.l/l.sub.0 by the strain, and the beat length L.sub.B of the modes of the light propagating through the optical fiber is changed by .delta.L.sub.B /L.sub.B0 =.alpha.(.delta.l/l).
As a result, the phase mismatch .theta. depends upon the change of the fiber grating period .lambda.o as well as the change of the mode beat length L.sub.B of the incident light. Here, it should be noted that the change of the fiber grating period .lambda.o is different from the change of the mode beat length L.sub.B of the incident light, i.e., .alpha. is smaller than 1. Owing to such a difference between the change of the fiber grating period and the change of the mode beat length of the incident light, it is possible to control the coupling amount of the fiber grating by adjusting the phase mismatch .theta..
In addition, the fiber grating is determined in its characteristics, such as the grating period, in accordance with a wavelength of and a polarization of a writing beam serving to fabricate the fiber grating, and the phase mismatch .theta. is different in accordance with a wavelength of and a polarization of a probe beam passing through the fiber grating, thus to change the coupling amount of the fiber grating.