The spectral, interferometric and polarization properties of mode coupling fiber gratings under various external perturbations are described. The grating is formed by photosensitivity using modal interference in the two-mode fiber. Theoretical and experimental results show the possibilities of using the fiber grating for optical switches, tunable filters, interferometric sensors and polarizers.
In-line fiber optic gratings formed by photosensitivity are finding wide application areas. With the development of highly efficient gratings and side writing technology, the permanently photo-induced grating devices are expected to replace some of the conventional fiber-optic components for telecommunications and fiber sensors. Compared to the reflective gratings written in single mode fibers, the mode coupling gratings in birefringent fibers or few-mode fibers are inherently transmitting devices that can be useful for many applications.
The refractive index change with the longitudinal period of the modal beatlength ($L_B$), breaking the symmetry of the cross-sectional index profile of the core, introduces coupling between the symmetric ($LP_{01}$) and antisymmetric ($LP}^{even}_{11$) modes. The periodic index change was accomplished by launching the writing optical beam with equal power for the two modes in the same polarization direction, producing a periodically varying modal interference pattern with the period of $L_B$ in the fiber. With high enough writing beam power, the interference pattern induces a permanent refractive index change in the core by photosensitivity. Once written, the grating remains permanently and can couple small optical signals between the two modes at similar wavelengths as the writing beam.
The fiber used in this work was an elliptical core (4.1μm × 2.2μm) germanium doped two-mode fiber that supports $LP_{01}$ and $LP^{even}_{11}$ modes with stable intensity lobe orientation. The cut off wavelength for the $LP^{even}_{11}$ mode was 633 nm.
The maximum coupling occurs when $L_B$ of probe beam matches with peirod of grating (phase matching) When a fiber grating is elongated, the changes of grating period($\Lambda$) and beatlength $L_B$ are different, and that also occurs for the grating under temperature change. This difference can be advantageously utilized to control the amount of light coupling. The experimental results indicate that the two mode fiber grating can be used for a digital optical switch operated with strain or temperature. The nonlinear behavior of the fiber grating by strong optical pulse was also investigated theoretically. Nonlinear optical effect at grating by high intensity pump beam can also induce phase mismatch between the L$_B$ of probe beam and the period of the grating. This effect can be utilized to controll the amount of light coupling at high speed. The simulation result showed a possibility of all optical switch.
The wavelength and the polarization properties of the grating in response to strain were investigated for its use as wavelength and polarization filters. For greater tuning range, the grating was formed while the two mode fiber was under strain that made it possible to tune the grating period both in positive and negative directions. The resolution of the grating was about 5 nm. For two orthogonal polarization states, the fiber grating described above required different amount of strain for maximum coupling. When used as a polarization filter this device demonstrated 11 dB of extinction ratio.
An in-line modal interferometer was also demonstrated. The interferometer configuration can be used as an interferometric sensor for various physical parameter.
