We review the recent developments in the area of optical fiber grating sensors, including quasi-distributed strain sensing using Bragg gratings, systems based on chirped gratings, intragrating sensing concepts, long period-based grating sensors, fiber grating laser-based systems, and interferometric sensor systems based on grating reflectors.

Fiber grating sensors

Recent research on fibre optic long-period gratings (LPGs) is reviewed with emphasis placed upon the characteristics of LPGs that make them attractive for applications in sensing strain, temperature, bend radius and external index of refraction. The prospect of the development of multi-parameter sensors, capable of simultaneously monitoring a number of these measurands will be discussed.

https://iopscience.iop.org/article/10.1088/0957-0233/14/5/201/pdf

Optical fibre long-period grating sensors: characteristics and application

An optical attenuator for attenuating a collimated beam of light propagating along an optical path while preserving the composition of polarization of the collimated beam of light is disclosed. The optical attenuator comprises a beam attenuator for attenuating a portion of the collimated beam of light when a portion of the beam attenuator is disposed within the optical path. The beam attenuator has a cross section along a plane perpendicular to the direction of propagation of the collimated beam of light of the portion of the attenuator in the shape of a wedge. The attenuation is varied using a controller for moving the beam attenuator in order to vary a size of the portion of the wedge within the optical path.

Adjustable optical attenuator

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically. Bragg gratings formed in optical fibers in multimode propagation show multiple reflection peaks or multiple transmission dips in the reflection or transmission spectra, respectively. For standard graded-index multimode fiber, the number of reflection peaks of a Bragg grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 /spl mu/m is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze the phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 /spl mu/m in three-mode propagation shows three reflection peaks in the reflection spectrum. The temperature dependence of each reflection peak is similar to that of a conventional Bragg grating in single-mode fiber. Polarization dependence measured on a Bragg grating in multimode graded-index fiber is negligible. An advantage of Bragg gratings in multimode fiber (MMF) and the applications are discussed.

Bragg gratings in multimode and few-mode optical fibers

This chapter deals with the properties of highly non-linear fibers. It reviews the techniques used to measure the non-linear parameter ( γ ). The four kinds of fibers that are developed to enhance the non-linear effects are also reviewed. The three major non-linear effects occurring inside the optical fibers—the self-phase modulation (SPM), the cross-phase modulation (XPM), and the four-wave mixing (FWM), are governed by a single non-linear parameter γ. This parameter is fixed for each glass material. The chapter elaborates that the use of the non-silica glasses provides an alternative approach to designing non-linear fibers. The SPM technique uses the broadening of the pulse spectrum. It measures the maximum value of the non-linear phase shift. In place of deducing the SPM-induced phase shift from spectral broadening, a related technique deduces it from the spectral changes occurring when light from two lasers operating at slightly different wavelengths is transmitted through the fiber. Measurements in such an approach can be performed with CW lasers. The chapter summarizes that the combination of unusual dispersive properties and a high value of γ leads to a variety of novel non-linear effects.

Highly Nonlinear Fibers

Long-period fiber gratings are promising components as gain equalizers for erbium-doped fiber amplifiers, band rejection filters and sensors. It has been reported that these devices have a high temperature sensitivity, typically 0.05 nm//spl deg/C at room temperature. Judkins et al., showed a method to eliminate the temperature sensitivity by optimizing the refractive-index profile of the host fiber. In this paper, we show a novel method to compensate the temperature sensitivity using a boron codoped germanosilicate core fiber as a host fiber.

A novel temperature-insensitive long-period fiber grating using a boron-codoped-germanosilicate-core fiber

The present invention relates to an optical fiber filter provided with a portion which selectively reflects or transmits light of a specific wavelength, this portion comprised of a multimode optical fiber in which the refractive index of the core changes periodically along the longitudinal direction thereof. Furthermore, this multimode optical fiber has an input end and an output end, with at least the input end of the multimode optical fiber being connected to a single mode optical fiber. As a result of forming an optical fiber filter in this way, it is possible to realize an optical fiber filter that exhibits no transmission loss at wavelengths other than the center wavelength.

optical fiber filter

An optical waveguide grating with radiative mode-coupling properties, with exceptional stability and reliability as an optical component, wherein the central wavelength of the rejection band has a low temperature dependence, due to the use of silica glass doped with germanium and boron for the core. The rejection bandwidth can be narrowed without increasing the grating length by forming the radiative mode-coupled optical waveguide grating in an optical waveguide wherein the mean relative refractive index difference of the core is greater than that of optical communication waveguides. The rejection can be increased by reducing the occurrence of cases wherein propagation modes of the core coupled to cladding modes once again couple to the core in palnar optical waveguide gratings formed by making periodic changes in the waveguide structure along the direction of propagation of light in an optical waveguide having a cladding with a lower refractive index than the core surrounding the core, by making the thickness of the cladding at least thirteen times the thickness of the core.

Optical waveguide grating and production method therefor

A printed wiring board is provided with a base material, which has at least one wiring and is composed of an adhesive insulating base material and a conductive layer formed on one plane of the insulating base material; a penetrating electrode which is connected to the conductive layer, penetrates the insulating base material and is composed of a conductive paste; and an IC chip having a rewiring section. An IC chip is embedded in an interlayer adhesive material of the base material having the wiring, by connecting the rewiring section with the penetrating electrode. A supporting substrate is arranged on a plane opposite to the rewiring section of the IC chip through the adhesive layer, and the rewiring section and the base material having the wiring constitute a rewiring layer. Therefore, the multilayer printed wiring board having fine components mounted thereon is provided by simple process without increasing the cost and deteriorating the yield.

Printed wiring board and method for manufacturing printed wiring board

The nonlinear refractive index coefficient n2 dependence on the GeO2 concentration in optical fibers is quantitatively shown for SiO2 core fibers and GeO2-SiO2 core/SiO2 clad fibers with Δ = 0.35–2.2% using an XPM measurement method where the phase change in the fiber of a probe signal caused by a pump signal is found from the delay self-heterodyne spectrum. The GeO2 concentration distribution which determines the n2 distribution in the fiber is not uniform, and the measured n2 is the average of the product of the local n2 and the fourth power of the electric field in the fiber. In this work, this relationship of the measured n2, the cross sectional distribution of n2, and the electric field distribution in the fiber is used to define an effective GeO2 concentration, ηeff. This effective GeO2 concentration level is used to evaluate n2. The measured n2 increases proportionally to ηeff by the relationship n2[x 10–20 m2/W] = 0.0552°eff[mol%] + 2.44. Furthermore, the measured n2 in the fiber agrees well with the n2 of bulk glass obtained a semiempirically for the nonlinear refractive index coefficient derived from linear refractive index data.

Satoshi Okude

Papers

A novel temperature-insensitive long-period fiber grating using a boron-codoped-germanosilicate-core fiber

optical fiber filter

Optical waveguide grating and production method therefor

Printed wiring board and method for manufacturing printed wiring board

Geo2 concentration dependence of nonlinear refractive index coefficients of silica‐based optical fibers