A complete analysis of symmetric 2 × 2 fused-fiber couplers in terms of supermode beating is made. The tranverse and longitudinal geometry of the couplers has been carefully modeled to take into account the fabrication parameters. The first two scalar supermodes and their polarization corrections have then been calculated for each degree of fusion and taper ratio, resulting in powerful tools for the design of particular couplers. The good agreement between the observed and the predicted responses for slightly and strongly fused couplers confirms the validity of each step of this analysis.

Modeling of symmetric 2 × 2 fused-fiber couplers

This paper describes the design and development of a novel fibre optic sensor to detect ultrasound. It is based on a fused-tapered optical fibre coupler, incorporating mechanical strain amplification in the sensor design. Strain amplification allows the sensor to be used for detecting low acoustic energy, such as acoustic emission emanating from composite materials and structures. This fibre optic ultrasonic sensor has a frequency response range from several tens of kHz to several hundred kHz. The sensitivity of the sensor system was found to be 0.56 V mbar−1 with an acoustic noise floor of 18 µbar at 155 kHz when evaluated in a water bath. The bandwidth of the electrical circuit was in the range of 10 kHz up to 1 MHz.

https://iopscience.iop.org/article/10.1088/0957-0233/15/8/010/pdf

A novel ultrasound fibre optic sensor based on a fused-tapered optical fibre coupler

We propose and study an optical microfiber coupler (OMC) sensor working near the turning point of effective group index difference between the even supermode and odd supermode to achieve high refractive index (RI) sensitivity. Theoretical calculations reveal that infinite sensitivity can be obtained when the measured RI is close to the turning point value. This diameter-dependent turning point corresponds to the condition that the effective group index difference equals zero. To validate our proposed sensing mechanism, we experimentally demonstrate an ultrahigh sensitivity of 39541.7 nm/RIU at a low ambient RI of 1.3334 based on an OMC with the diameter of 1.4 μm. An even higher sensitivity can be achieved by carrying out the measurements at RI closer to the turning point. The resulting ultrasensitive RI sensing platform offers a substantial impact on a variety of applications from high performance trace analyte detection to small molecule sensing.

/pdf/ultrasensitive-optical-microfiber-coupler-based-sensors-5cgos8nvr7.pdf

Ultrasensitive optical microfiber coupler based sensors operating near the turning point of effective group index difference

This article describes a model and the process technology of realizing fused fiber coupler-based branching components through the use of an experimental setup of a PC-controlled fabrication rig. To simulate the fabrication process and to interpret the experimental characteristics of such devices, a simple but realistic model based on well known super-mode-beating analysis is presented. The key results of the analysis embedded into the control software of the fabrication rig has ensured achieving repeatability in the targeted specifications of these components. Typical results of the modeling and a comparison with experimental results are also presented. The model should be very useful in defining the design parameters for realizing these fused coupler-based branching components.

Fabrication and Modeling of Fused Biconical Tapered Fiber Couplers

Fiber optic acoustic emission sensor and its applications in the structural health monitoring of CFRP materials

Internal fields in fused couplers have been investigated for the first time by measuring the near field and far field patterns at different cross sections of the couplers. Detailed information concerning the internal fields in the couplers provides experimental evidence for the validity of the hybrid mode theory of fused couplers.

Jian-Luo Zhang

Papers

Measurements and analyses of fields in fused tapered single-mode fiber couplers