This paper provides a thorough review of state-of-the-art current sensing techniques. It catalogues the current sensors according to the underlying physical principle in order to point out their strengths and weaknesses.

Current Sensing Techniques: A Review

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

Origins and control of polarization effects in single-mode fibers (A)

Recent research on fibers with very small or very large birefringence for polarization-dependent applications is reviewed. The nature of random coupling between normal modes of polarization is analyzed and discussed in connection with various applications.

Polarization in optical fibers

The polarization state of light in single-mode fibers is very sensitive to any perturbation which is not symmetric about the fiber axis. While this is a source of noise, drift, or signal fading in some applications, it can also be exploited in novel guided-wave devices. The basic birefringences that couple the two modes and change the polarization state along the fiber are reviewed. The three cases of uniform, phase-matched, and random coupling are considered. Polarization preservation in both low- and high-birefringence fibers is achieved by reducing this coupling. In addition to polarization-state changes, bireftingent fibers can quickly reduce the polarization degree of nonmonochromatic light if both modes are excited, a characteristic that greatly simplifies evaluation of the degree of polarization preservation in these fibers. Current evaluations of the birefringence and the polarization-holding ability of state-of-the-art fibers are discussed, and it is concluded that fibers with good polarization-holding properties are becoming available.

Origins and control of polarization effects in single-mode fibers

The principles of measuring currents in high-voltage lines by magnetooptical means are described. Particular attention is payed to using an optical fiber both as a transmission line and a measuring sensor. The influence of birefringence on the measuring signal is discussed.

Magnetooptical current transformer. 1: Principles

A number of applications require single-mode optical fi­ bers in which the state of polarization of the guided light is sustained. The solid-core optical fibers described in the literature generally exhibit strong birefringence and also usually have a depolarizing effect. In this Letter we describe experiments on single-mode fi­ bers that were prepared under carefully controlled conditions and studied for their polarization behavior. The most fa­ vorable results were noted for fibers prepared by chemical vapor deposition (CVD). First, a Si02-B2O3 glass was de­ posited from the gas phase as cladding on the inside wall of a fused-silica tube, followed by pure quartz glass as the core material. The tube was then collapsed into a rod by raising the temperature and thereafter drawn into a fiber. Directly after leaving the furnace, the fiber was coated with silicon resin. Through precise control of the various step of the process, especially during the collapsing of the tube, a circular cross section in preform and fiber were realized. Figure 1 shows a transmission micrograph of a cross section of the preform. A comparison of round and oval fibers made of glass of the same composition showed a circularly symmetrical cross

Low-birefringence single-mode optical fibers: preparation and polarization characteristics.

The present invention is directed to an improvement in a magneto-optical device having a light waveguide for conducting light characterized by the light waveguide being a cladded optical fiber having a core of quartz glass and a cladding of synthetic material. This improvement provides an improved method of using the magneto-optical device by suppressing distortions of polarized light by reducing birefringence in the waveguide by using a waveguide comprising an optical fiber having a quartz glass core and a cladding of synthetic material.

Waveguides with a low birefringence

In this second part, the most important components of a magnetooptical current transformer are described and their suitability is discussed Today He–Ne gas lasers and laser diodes seem to meet best the requirements of the light source Single-mode optical fibers are capable of transmitting polarized light without depolarization A fiber coil wound on a special coil former is used as a measuring sensor Two P-I-N photodiodes, together with an analog electronic circuit, transform the optical into an electrical measuring signal

Magnetooptical current transformer. 2: Components.

A laboratory setup of a magnetooptical current transformer was established. A single-mode fiber acts as both a measuring sensor and a transmission line. The overall measurement accuracy at room temperature within a primary current range 50–1200 A is better than 0.24%, the SNR at 1000 A is 85 dB. Current measurements of high accuracy were also performed in the −20–+45°C temperature range.

Alfred Papp

Papers

Magnetooptical current transformer. 1: Principles

Low-birefringence single-mode optical fibers: preparation and polarization characteristics.

Waveguides with a low birefringence

Magnetooptical current transformer. 2: Components.

Magnetooptical current transformer. 3: Measurements