Polarization-maintaining fibers and their applications are reviewed. The classification of high-birefringent fibers and low-birefringent fibers and their fabrication methods and characteristics are discussed in Section II. Analytical methods and numerical methods for fiber design on the birefringence are presented in Section III. Degradation factors of polarization maintenance expressed as crosstalk or mode-coupling parameters caused by internal origins such as structural imperfections, wavelength, and nonlinear effects, and by external origins such as temperature fluctuations, mechanical perturbations, and electromagnetic effects, are discussed in Section IV. Characterization methods on beat length, mode coupling, stress distribution, and mechanical strength are presented in Section V. Applications to the fiber devices and nonlinear effects, and splicing methods for the polarization-maintaining fibers are described in Sections VI and VII.

Polarization-maintaining fibers and their applications

New experimental studies on the effects of high hydrostatic pressure on highly birefringent optical fibers are presented. Using Rayleigh scattering, the effect of beat length modification in highly birefringent singlemode bow tie fibers under high pressure up to 100 MPa was investigated. The results indicate that the beat length decreases with pressure, with the mean coefficient of 1/L(B0)|dL(B)/dp| = 0.15%/MPa. A similar decrease of beat length was observed when uniaxial longitudinal stress was applied to the bow tie fiber. A more developed semiphenomenological interpretation of these experimental results is also presented in the paper.

Influence of high hydrostatic pressure on beat length in highly birefringent single-mode bow tie fibers.

Optical-frequency-domain reflectometry is used to measure the group-index difference and the refractive-index difference (i.e., beat length) between the fast and slow modes in polarization-maintaining optical fiber. The Rayleigh scatter normally present in the fiber is measured in reflection. This measurement, in turn, enables a distributed measurement of the fiber's birefringence that is rapid and completely nondestructive

Characterization of Polarization-Maintaining Fiber Using High-Sensitivity Optical-Frequency-Domain Reflectometry

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

A three-section fiber model has been used to derive the formula for measuring the birefringence of single-mode optical fibers. This analysis relieves the problems associated with measurements of the birefringence of optical fibers with very short beat length. The distortion of the measured curves for fibers with nonuniform birefringence is discussed. A newly developed nulling scheme makes the measurement accurate and stable.

Measuring the birefringence of single-mode fibers with short beat length or nonuniformity: a new method.

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.

Zong-Qi Lin

Papers

Measuring the birefringence of single-mode fibers with short beat length or nonuniformity: a new method.

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