We have fabricated long-period fiber gratings by use of a novel technique using focused irradiation of infrared femtosecond laser pulses. We investigate the thermal stability of the fabricated fiber gratings. The values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500°C are the same as initial values before heat treatment. The fiber gratings that were fabricated by this technique have a high resistance to thermal decay. We propose that this technique mill be useful for fabrication of fiber gratings with a superior aging characteristic.

Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses

Although mainstream grating writing, more often than not using single photon excitation of germanosilicate based defects with CW 244 nm light, remains the key technology for complex devices it is now being complemented by a whole host of processes which can enhance and tailor the properties of both conventional and not-so-conventional fibre Bragg gratings. Further, processes for writing of gratings in non-germanosilicate fibres have also continued to develop and include multi-photon excitation directly into the band edge of the glass. It is now possible to custom tailor a gratings property based on the application and the nature of production as well as custom tailor the grating writing process to suit the type of fibre and application. Examples and suggestions where these can benefit sensors and lasers are outlined.

Fibre gratings and devices for sensors and lasers

A fiber-optic temperature sensor based on the interference of selective higher-order modes in circular optical fibers is described. The authors demonstrate that by coupling the LP01 mode in a standard single-mode fiber to the LP0m modes in a multimode fiber, and utilizing the interference of the higher-order modes, a fiber-optic temperature sensor which has an extremely simple structure and is suitable for high-temperature measurements can be constructed. The sensing principle, temperature measurement experiments, and results are presented.

/pdf/fiber-optic-temperature-sensor-based-on-interference-of-bykkyirfe2.pdf

Fiber-optic temperature sensor based on interference of selective higher-order modes

Fiber Bragg grating (FBG) temperature sensor and sensor arrays were applied widespread particularly in harsh environments. Although FBGs are often referring to permanent refractive index modulation in the fiber core, exposure to high-temperature environments usually results in the bleach of the refractive index modulation. The maximum temperature reported for the conventional FBG temperature sensor is around 600 degC due to its weak bonds of germanium and oxygen. In this paper, we report design and development of a novel high-temperature resistance FBG temperature sensor, based on the hydrogen-loaded germanium-doped FBG. The refractive index modulation in the FBG is induced by the molecular water. The results of our experiments have shown that the stability of the device is substantially increased at high temperature range. Due to the high bonds energy of hydroxyl and the low diffusivity of the molecular water, the thermal testing results of this temperature sensor show the thermal stability of hydrogen-loaded FBG can be increased by using annealing treatment; moreover, the highest erasing temperature for the device could reach to 1100 degC or more. The reflectivity of this new FBG depends on the concentration of Si-OH and indirectly related to the reflectivity of hydrogen-loaded FBG. Furthermore, the experimental results have provided a better understanding of the formation of the hydrogen-loaded FBGs and the chemical transfers at elevated temperatures in the fiber core

High-Temperature Resistance Fiber Bragg Grating Temperature Sensor Fabrication

In this topical review, the recent progress on radiation-hardened fiber-based technologies is detailed, focusing on examples for space applications. In the first part of the review, we introduce the operational principles of the various fiber-based technologies considered for use in radiation environments: passive optical fibers for data links, diagnostics, active optical fibers for amplifiers and laser sources as well as the different classes of point and distributed fiber sensors: gyroscopes, Bragg gratings, Rayleigh, Raman or Brillouin-based distributed sensors. Second, we describe the state of the art regarding our knowledge of radiation effects on the performance of these devices, from the microscopic effects observed in the amorphous silica glass used to design fiber cores and cladding, to the macroscopic response of fiber-based devices and systems. Third, we present the recent advances regarding the hardening (improvement of the radiation tolerance) of these technologies acting on the material, device or system levels. From the review, the potential of fiber-based technologies for operation in radiation environments is demonstrated and the future challenges to be overcome in the coming years are presented.

https://iopscience.iop.org/article/10.1088/2040-8986/aad271/pdf

Recent advances in radiation-hardened fiber-based technologies for space applications

Fiber Bragg gratings (FBGs) have been written in silica fiber codoped with boron and germanium, using a continuous-wave (CW) 244-nm laser. The thermal decay of the gratings written with and without hydrogen loading is examined over a period in excess of 5000 h. It is shown that the thermal decay of the nonhydrogen loaded gratings is well explained by the power law model proposed by Erdogan. Gratings written in hydrogen loaded fiber, however, do not obey the power law model. A new model is presented based on a log time representation which can be used to predict the decay characteristics of gratings written in hydrogen loaded fiber.

Thermal decay of fiber Bragg gratings written in boron and germanium codoped silica fiber

A novel electron-beam exposure strategy which has been used in the fabrication of very high quality chirped fiber gratings for use as dispersion compensators is described. A key stage of the processing is the patterning and relief etching of a quartz phase mask, which by ultraviolet exposure, is used sequentially to form long lengths of continuously chirped grating on optical fiber. The use of electron lithography has been restricted by the occurrence of stitching errors which produce prohibitively large phase errors. These tend to be even more pronounced in generating chirped gratings. For practical operation errors of only a few nanometers are probably required. The measured performance of chirped fiber gratings is presented to illustrate the degree of improvement we have achieved by use of two techniques used in the electron-beam exposure of the phase mask, i.e., compensation for residual field distortion, and the delocalization of stitch error. Long chirped gratings produced in this way on a mature e-beam system at Nortel Networks have demonstrated enhanced performance, while similar devices produced by Leica Microsystems on a state-of-the-art machine provided a significantly closer approximation to a perfectly chirped grating.

High perfection chirped grating phase masks by electron-beam definition

In current high bit rate optical links (10 Gb/s), one of the major limitations of system performance and reach is chromatic dispersion. An essential feature of multi-span systems is the dispersion compensating module, of which dispersion compensating fibre (DCF) is one option and dispersion compensating gratings (DCG’s) another.

Fabrication of extremely long fibre gratings by phase matched concatenation of multiple short sections

The thermal decay of fibre Bragg gratings is examined over a period in excess of 5000 hours. A new model is presented to predict the decay characteristics of gratings written in hydrogen loaded fibre.

Thermal decay of fibre Bragg gratings

Closer channel spacings in WDM systems demand a filter technology which can provide the required bandpass and isolation. We describe a high performance 4-channel device based on fused fibre Mach-Zehnders and UV written gratings.

http://ieeexplore.ieee.org/iel4/5806/15506/00715637.pdf

D. Goodchild

Papers

Thermal decay of fiber Bragg gratings written in boron and germanium codoped silica fiber

High perfection chirped grating phase masks by electron-beam definition

Fabrication of extremely long fibre gratings by phase matched concatenation of multiple short sections

Thermal decay of fibre Bragg gratings

A low loss 4-channel wavelength demultiplexer based on fibre Bragg gratings