Photoinduced refractive-index changes in two-mode, elliptical-core fibers: sensing applications

All-fiber fused directional coupler for highly efficient spatial mode conversion

We model and demonstrate a simple mode selective all-fiber coupler capable of exciting specific higher order modes in two- and few-mode fibres with high efficiency and purity. The coupler is based on inter-modally phase-matching the propagation constants in each arm of the asymmetric fused coupler, formed by dissimilar fibres. At a specific coupler diameter, the launched fundamental LP(01) mode is coupled into the higher order mode (LP(11), LP(21), LP(02)) in the other arm, over a broadband wave-length range around 1550 nm. Unlike other techniques, the demonstrated coupler is composed of a multimode fiber that is weakly fused with a phase matched conventional single mode telecom fiber (SMF-28). The beating between the supermodes at the coupler waist produces a periodic power transfer between the two arms, and therefore, by monitoring the beating while tapering, it is possible to obtain optimum selection for the desired mode. High coupling efficiencies in excess of 90% for all the higher order modes were recorded over 100 nm spectral range, while insertion losses remain as low as 0.5 dB. Coupling efficiency can be further enhanced by performing slow tapering at high temperature, in order to precisely control the coupler cross-section geometry.

Intermodal beat length measurement with Fabry-Perot optical fiber cavities

We present a new technique for measuring the intermodal beat length of a two-mode optical fiber. We formed a Fabry-Perot fiber cavity by depositing reflective mirrors on the fiber tips. As the fiber is stretched, two series of resonance peaks are observed. One series is due to the resonance of LP(01 q) modes and the other is due to the LP(11 q) modes. From the separation of resonance peaks as a function of cavity length, we deduce the intermodal beat length of the fiber. The measurement principle and the experimental confirmation are discussed.

Attometer resolution of wavelength shifts by use of fiber modal interferometers

A method for on-line, real-time detection of wavelength drifts in laser diodes is proposed. This technique uses the wavelength dependence of the differential propagation constant between the LP(01)/LP(even)(11) or LP(01)/LP(02) modes in two-mode fibers to measure wavelength changes that manifest themselves as intensity modulation in the output far field. Theoretical calculations show that wavelength drifts of the order of 10(-18) m can be resolved by this technique.

Spatially weighted, grating-based, two-mode, elliptical-core optical fiber vibration sensors

Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers affect the beat length and the sensor sensitivity. Chirped gratings are written by attaching such fibers to cantilever beams positioned in a strained state. We show that fibers with in-line chirped gratings, with the chirp being shaped in the form of a vibration-mode shape, can be used as spatially weighted fiber sensors for vibration analysis. We demonstrate enhanced detection of the first and second modes of vibration of a cantilever beam using this process; vibration mode suppression of the order of 10 dB is obtained.