In-fibre Bragg grating (FBG) sensors are one of the most exciting developments in the field of optical fibre sensors in recent years. Compared with conventional fibre-optic sensors, FBG sensors have a number of distinguishing advantages. Significant progress has been made in applications to strain and temperature measurements. FBG sensors prove to be one of the most promising candidates for fibre-optic smart structures. This article presents a comprehensive and systematic overview of FBG sensor technology regarding many aspects including sensing principles, properties, fabrication, interrogation and multiplexing of FBG sensors. It is anticipated that FBG sensor systems will be commercialized and widely applied in practice in the near future due to the maturity of economical production of FBGs and the availability of cost effective interrogation and multiplexing techniques.

https://iopscience.iop.org/article/10.1088/0957-0233/8/4/002/pdf

In-fibre Bragg grating sensors

Fiber optic sensor technology has experienced tremendous growth since its early beginnings in the 1970's with early laboratory demonstrations of fiber optic gyros and acoustic sensors and the introduction of the first commercial intensity and spectrally based sensors. These early efforts were followed by a tremendous growth of interest in the 1980's when the number of workers in the field increased from perhaps a few hundred to thousands. The result was the introduction in the 1990's of the first mass produced fiber optic sensors that are being used to support navigation and medical applications. The number of fiber optic sensor products can be expected to grow tremendously in the years to come as rapid progress continues to be made in the related optoelectronic and communication fields. This paper provides an overview of some of the technologies being used to support fiber optic sensor development and how they are being applied.

Fiber optic smart structures

Aspects of the subject disclosure may include, for example, receiving, by a feed point of a dielectric antenna, electromagnetic waves from a dielectric core coupled to the feed point without an electrical return path, where at least a portion of the dielectric antenna comprises a conductive surface, directing, by the feed point, the electromagnetic waves to a proximal portion of the dielectric antenna, and radiating, via an aperture of the dielectric antenna, a wireless signal responsive to the electromagnetic waves being received at the aperture. Other embodiments are disclosed.

Method and apparatus for coupling an antenna to a device

An all-fiber-optic frequency shifter is demonstrated that uses mode coupling between the LP(01) and LP(11) modes by a traveling acoustic flexural wave guided along the optical fiber. The input and output leads of this frequency shifter are single-mode fibers. Unity mode-conversion efficiency for cw operation is achieved at 8-MHz frequency shift with about 0.25 W of electrical input power. Carrier and image sideband suppression of 15 and 35 dB, respectively, are demonstrated.

All-fiber acousto-optic frequency shifter

A distributed antenna and backhaul system provide network connectivity for a small cell deployment. Rather than building new structures, and installing additional fiber and cable, embodiments described herein disclose using high-bandwidth, millimeter-wave communications and existing power line infrastructure. Above ground backhaul connections via power lines and line-of-sight millimeter-wave band signals as well as underground backhaul connections via buried electrical conduits can provide connectivity to the distributed base stations. An overhead millimeter-wave system can also be used to provide backhaul connectivity. Modules can be placed onto existing infrastructure, such as streetlights and utility poles, and the modules can contain base stations and antennas to transmit the millimeter-waves to and from other modules.

Backhaul link for distributed antenna system

The four complex, almost degenerate, second-order eigenmodes of a two-mode fiber having a circular core are reduced to two nondegenerate, linearly polarized second-order eigenmodes with stable-intensity lobe positions in a highly elliptical core fiber. Existing two-mode-fiber devices can be improved by this stabilization of the second-order modes. Practical sensors employing the two spatial modes as the two arms of an interferometer are described. The two arms of an interferometer of this type can have the same group delays, while the difference in phase delays is large.

Use of highly elliptical core fibers for two-mode fiber devices.

An apparatus utilizes a two-mode optical waveguide with a non-circular core to provide stable spatial intensity patterns in both propagation modes for light propagating therein. The light has a wavelength, and the non-circular core has cross-sectional dimensions selected such that (1) the waveguide propagates light at that wavelength in a fundamental mode and a higher order mode, and (2) substantially all of the light in the higher order mode propagates in only a single, stable intensity pattern. Embodiments of the invention include, for example, modal couplers, frequency shifters, mode selectors and interferometers. One of the interferometer embodiments may be used as a strain gauge.

Apparatus using two-mode optical waveguide with non-circular core

Acoustic modes propagating in cylindrical solid rods are considered. A review of the properties of several mode groups is given, and computed results are shown for a wide range of material properties. The lower mode of each group is relevant for an all-fiber-optic frequency shifter recently demonstrated. For these modes, dispersion relations, mode patterns, power relations, and acoustooptic coupling strength are calculated. Some experimental results are compared with the calculations, and alterations to improve the frequency shifter are proposed. >

Propagation and optical interaction of guided acoustic waves in two-mode optical fibers

The propagation of both fundamental and second-order modes in highly elliptical core two-mode fibers under external perturbations is investigated. Theoretical and experimental results are presented for the differential phase shift between two spatial modes in each eigenpolarization and the polarization behavior of each spatial mode. The perturbations include axial strain, radial strain, hydrostatic strain, temperature change, twisting, squeezing, and bending. An application of the modal behavior of these fibers to simultaneous measurement of strain and temperature is described. >

James N. Blake

Papers

All-fiber acousto-optic frequency shifter

Use of highly elliptical core fibers for two-mode fiber devices.

Apparatus using two-mode optical waveguide with non-circular core

Propagation and optical interaction of guided acoustic waves in two-mode optical fibers

Perturbation effects on mode propagation in highly elliptical core two-mode fibers