Thermal regeneration of fiber Bragg gratings in photosensitive fibers

The Impact of 1030 nm fs-Pulsed Laser on Enhanced Rayleigh Scattering in Optical Fibers

This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping.

Thermal tuning of a fiber-integrated Fabry-Pérot cavity

Here, we present the thermal tuning capability of an alignment-free, fiber-integrated Fabry-Pérot cavity. The two mirrors are made of fiber Bragg gratings that can be individually temperature stabilized and tuned. We show the temperature tuning of the resonance wavelength of the cavity without any degradation of the finesse and the tuning of the individual stop bands of the fiber Bragg gratings. This not only permits for the cavity's finesse to be optimized post-fabrication but also makes this cavity applicable as a narrowband filter with a FWHM spectral width of 0.07 ± 0.02 pm and a suppression of more than -15 dB that can be wavelength tuned. Further, in the field of quantum optics, where strong light-matter interactions are desirable, quantum emitters can be coupled to such a cavity and the cavity effect can be reversibly omitted and re-established. This is particularly useful when working with solid-state quantum emitters where such a reference measurement is often not possible once an emitter has been permanently deposited inside a cavity.

Nanofiber-based high-Q microresonator for cryogenic applications

We demonstrate a cryo-compatible, fully fiber-integrated, alignment-free optical microresonator. The compatibility with low temperatures expands its possible applications to the wide field of solid-state quantum optics, where a cryogenic environment is often a requirement. At a temperature of 4.6 K we obtain a quality factor of (9.9 ± 0.7) × 106. In conjunction with the small mode volume provided by the nanofiber, this cavity can be either used in the coherent dynamics or the fast cavity regime, where it can provide a Purcell factor of up to 15. Our resonator is therefore suitable for significantly enhancing the coupling between light and a large variety of different quantum emitters and due to its proven performance over a wide temperature range, also lends itself for the implementation of quantum hybrid systems.

Fiber Bragg grating regeneration at 450°C for improved high temperature sensing

Type-I fiber Bragg gratings photo-inscribed in hydrogen-loaded B/Ge co-doped silica single-mode optical fibers have been regenerated efficiently at 450°C, which is the lowest temperature reported so far. The mechanical strength of the annealed fiber is preserved while ensuring temperature sensing of the regenerated gratings up to 900°C. Unlike low temperature cycles (≤600°C), an annealing process at higher temperatures revealed faster regeneration for strong gratings. Changes in grating strength were also measured before the regeneration cycle. These behaviors suggest the contribution of different mechanisms to the regeneration process with different relative dynamics.

Fiber Bragg grating characterization using factorial design

The design of the experiment is a scientific approach that provides the maximum amount of information with the minimum number of experiments. It is applicable in scientific and industrial research. We represent a three-variable two-level factorial design to assess fiber Bragg grating properties under simultaneous temperature, humidity, and strain stimuli. Three uniform gratings were inscribed in a single-mode standard optical fiber using the interferometric technique. Two gratings were recoated, one with acrylate and another with a layer of polyimide, while the third grating remained bare. With only eight measuring points, the sensitivities of temperature, strain, and humidity were computed. Moreover, with this technique, the cross-sensitivities between temperature and strain, temperature and humidity, humidity and strain, and between all three factors could also be quantified. We have proven that the results of this design are comparable with those of the classic method. For all gratings, the temperature and strain sensitivities were obtained in the order of 10 pm/°C and 1.1 pm/με, respectively. The humidity sensitivity of the polyimide recoated grating was estimated to be in the order of 4.47 pm/%RH at room temperature.

Effect of two annealing processes on the thermal regeneration of fiber Bragg gratings in hydrogenated standard optical fibers

In this work, we demonstrate the thermal regeneration of fiber Bragg gratings written in the hydrogenated standard communication optical fibers by two annealing processes. The first annealing process is done at an intermediate temperature (500°C, 700°C, and 900°C) for a specific period of time before cooling down to room temperature. The second annealing is at 1000°C in which the thermal regeneration is attained. The experimental results show that the regenerated gratings that are preannealed at 700°C have charted a reflectivity larger than 65%. They have higher thermal stability compared to that of the standard annealing process. Meanwhile the difference in temperature sensitivity is very small. The temperature sensitivities of regenerated gratings, which have undergone only two annealing processes, are 16.1 pm/°C and 15.8 pm/°C, respectively.

Flexible femtosecond inscription of fiber Bragg gratings by an optimized deformable mirror

The period of fiber Bragg gratings is adapted by shaping the wavefronts of ultrashort laser pulses applied in a phase mask inscription technique. A specially designed deformable mirror, based on a dielectric substrate to withstand high peak powers, is utilized to deform the wavefront. A shift of about 11 nm is demonstrated for a Bragg wavelength around 1550 nm.

Observations from direct UV-written, non-hydrogen-loaded, thermally regenerated Bragg gratings in double-clad photosensitive fiber

In this Letter, experimental evidence is provided for an enhanced thermal sensitivity for a double thermal regeneration feature in fiber Bragg gratings fabricated by direct ultraviolet (UV) writing. Here 47 gratings of varying fluence and wavelength were written along a double-clad, germanium-doped core fiber. Subsequently thermal processing without hydrogen loading the fiber was performed and thermal treatment was carried out in a pure oxygen environment. Thermal sensitivity for the double regeneration increased from 13.6±0.3  pm/°C to 21.3±0.2  pm/°C. Furthermore, one of the highest nominal fluence gratings, #45, exhibited a regeneration factor of 1.73.

An Improved Metal-Packaged Strain Sensor Based on A Regenerated Fiber Bragg Grating in Hydrogen-Loaded Boron-Germanium Co-Doped Photosensitive Fiber for High-Temperature Applications

Local strain measurements are considered as an effective method for structural health monitoring of high-temperature components, which require accurate, reliable and durable sensors. To develop strain sensors that can be used in higher temperature environments, an improved metal-packaged strain sensor based on a regenerated fiber Bragg grating (RFBG) fabricated in hydrogen (H₂)-loaded boron–germanium (B–Ge) co-doped photosensitive fiber is developed using the process of combining magnetron sputtering and electroplating, addressing the limitation of mechanical strength degradation of silica optical fibers after annealing at a high temperature for regeneration. The regeneration characteristics of the RFBGs and the strain characteristics of the sensor are evaluated. Numerical simulation of the sensor is conducted using a three-dimensional finite element model. Anomalous decay behavior of two regeneration regimes is observed for the FBGs written in H₂-loaded B–Ge co-doped fiber. The strain sensor exhibits good linearity, stability and repeatability when exposed to constant high temperatures of up to 540 °C. A satisfactory agreement is obtained between the experimental and numerical results in strain sensitivity. The results demonstrate that the improved metal-packaged strain sensors based on RFBGs in H₂-loaded B–Ge co-doped fiber provide great potential for high-temperature applications by addressing the issues of mechanical integrity and packaging.

Fiber Bragg Grating Sensors for the Oil Industry

With the oil and gas industry growing rapidly, increasing the yield and profit require advances in technology for cost-effective production in key areas of reservoir exploration and in oil-well production-management. In this paper we review our group’s research into fiber Bragg gratings (FBGs) and their applications in the oil industry, especially in the well-logging field. FBG sensors used for seismic exploration in the oil and gas industry need to be capable of measuring multiple physical parameters such as temperature, pressure, and acoustic waves in a hostile environment. This application requires that the FBG sensors display high sensitivity over the broad vibration frequency range of 5 Hz to 2.5 kHz, which contains the important geological information. We report the incorporation of mechanical transducers in the FBG sensors to enable enhance the sensors’ amplitude and frequency response. Whenever the FBG sensors are working within a well, they must withstand high temperatures and high pressures, up to 175 °C and 40 Mpa or more. We use femtosecond laser side-illumination to ensure that the FBGs themselves have the high temperature resistance up to 1100 °C. Using FBG sensors combined with suitable metal transducers, we have experimentally realized high- temperature and pressure measurements up to 400 °C and 100 Mpa. We introduce a novel technology of ultrasonic imaging of seismic physical models using FBG sensors, which is superior to conventional seismic exploration methods. Compared with piezoelectric transducers, FBG ultrasonic sensors demonstrate superior sensitivity, more compact structure, improved spatial resolution, high stability and immunity to electromagnetic interference (EMI). In the last section, we present a case study of a well-logging field to demonstrate the utility of FBG sensors in the oil and gas industry.

Low loss Type II regenerative Bragg gratings made with ultrafast radiation

A novel type of fiber Bragg grating is produced by annealing a type I-like grating that is written with multiple infrared femtosecond laser pulses through a phase mask under conditions that are typically used to fabricate thermally stable type II gratings. This new grating is created through a process similar to a regenerative one and displays low loss and high resilience in a 1000 °C ambient environment. Such gratings are ideally suited for quasi-distributed sensing at high temperatures.

Influence of pre-annealing on the thermal regeneration of fiber Bragg gratings in standard optical fibers

A detailed study of the dynamics during thermal regeneration of fiber Bragg gratings, written in hydrogen-loaded standard single-mode fibers using a ns pulsed 213 nm UV laser, is reported. Isothermal pre-annealing performed in the range 85 °C to 1100 °C, with subsequent grating regeneration at 1100 °C, resulted in a maximum refractive index modulation, Δn(m) ~1.4⋅10(-4), for gratings pre-annealed near 900 °C while a minimum value of Δn(m) ~2⋅10(-5) was achieved irrespective of pre-annealing temperature. This optimum denote an inflection point between opposing thermally triggered processes, which we ascribe to the reaction-diffusion mechanism of molecular water and hydroxyl species in silica. The results shed new light on the mechanisms underlying thermal grating regeneration in optical fibers.

High temperature sensing with fiber Bragg gratings in sapphire-derived all-glass optical fibers

A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.

Fast thermal regeneration of fiber Bragg gratings

In this Letter we report a fast thermal regeneration of Type I fiber Bragg gratings inscribed with a UV laser in up to four different optical fibers: hydrogenated standard fiber, hydrogenated highly Ge-doped fiber, hydrogenated photosensitive fiber, and nonhydrogenated fiber. The thermal treatment consists in directly introducing the optical fiber into a preheated oven. The preheat temperature depends on the type of fiber used and is high enough to erase the grating and regenerate it afterward. The best results are obtained with hydrogenated photosensitive fiber and highly Ge-doped fiber, whereas no satisfactory results were obtained with hydrogenated standard fiber and nonhydrogenated photosensitive fiber. A regenerated grating with only 1.6 dB of loss was obtained in 10 min, reducing the time needed by a factor of 5.7. By adjusting the temperature of the oven, regenerated gratings of 13.7 dB of loss in 31 s and 5.8 dB of loss in 3 min were obtained. The factors of improvement in time are 110.3 and 19, respectively.

Spectroscopy, manipulation and trapping of neutral atoms, molecules, and other particles using optical nanofibers: a review

The use of tapered optical fibers, i.e., optical nanofibers, for spectroscopy and the detection of small numbers of particles, such as neutral atoms or molecules, has been gaining interest in recent years. In this review, we briefly introduce the optical nanofiber, its fabrication, and optical mode propagation within. We discuss recent progress on the integration of optical nanofibers into laser-cooled atom and vapor systems, paying particular attention to spectroscopy, cold atom cloud characterization, and optical trapping schemes. Next, a natural extension of this work to molecules is introduced. Finally, we consider several alternatives to optical nanofibers that display some advantages for specific applications.

Inscription of first-order sapphire Bragg gratings using 400 nm femtosecond laser radiation

The paper describes the implementation of fiber Bragg gratings inscribed by femtosecond laser pulses with a wavelength of 400 nm. The use of a Talbot interferometer for the inscription process makes multiplexing practicable. We demonstrate the functionality of a three-grating multiplexing sensor and the temperature stability up to 1200 °C for a single first-order Bragg grating.

Nanofiber Fabry-Perot microresonator for nonlinear optics and cavity quantum electrodynamics

We experimentally realize a Fabry-Perot-type optical microresonator near the cesium D2 line wavelength based on a tapered optical fiber, equipped with two fiber Bragg gratings that enclose a subwavelength diameter waist. Owing to the very low taper losses, the finesse of the resonator reaches F=86 while the on-resonance transmission is T=11%. The characteristics of our resonator fulfill the requirements of nonlinear optics and cavity quantum electrodynamics in the strong coupling regime. These characteristics, combined with the demonstrated ease of use and advantageous mode geometry, open a realm of applications.

Production of optical notch filters with fine parameter control using regenerated fiber Bragg gratings

A new method to create a controlled notch filter for attenuation of signals based on regenerated fiber Bragg gratings is experimentally demonstrated. A fine adjustment of the notch depth is achieved by controlling the time of regeneration phenomenon in strongly saturated Bragg gratings written in standard ITU G.652 single-mode fiber. This method can used to produce tailored notch filters for several photonic applications, such as optical RF filtering, subcarrier processing or radio over fiber systems.

Post-hydrogen-loaded draw tower fiber Bragg gratings and their thermal regeneration

The idea of Bragg gratings generated during the drawing process of a fiber dates back almost 20 years. The technical improvement of the draw tower grating (DTG) process today results in highly reliable and cost-effective Bragg gratings for versatile application in the optical fiber sensor market. Because of the single-pulse exposure of the fiber, the gratings behave typically like type I gratings with respect to their temperature stability. This means that such gratings only work up to temperatures of about 300 °C. To increase temperature stability, we combined DTG arrays with hydrogen postloading and a thermal regeneration process that enables their use in high-temperature environments. The regenerated draw tower gratings are demonstrated to be suitable for temperatures of more than 800 °C.

A study of regenerated gratings produced in germanosilicate fibers by high temperature annealing

In light of recent proposals linking structural change and stresses within regenerated gratings, the details of regeneration of a seed Type-I Bragg grating written in H2 loaded germanosilicate fiber annealed at high temperatures (~900°C) are systematically explored. In particular, the influence of the strength of the grating, the effect of GeO2 doping concentration and the annealing conditions on regeneration are studied. We show that the role of dopants such as Ge and F contribute nothing to the regeneration, consistent with previous results. Rather, they may potentially be detrimental. Strongest regenerated gratings with R ~35% from a 5mm seed grating could be obtained in fibres with the lowest GeO2 concentrations such as standard telecommunications-compatible grade fibre.

Arrays of regenerated fiber bragg gratings in non-hydrogen-loaded photosensitive fibers for high-temperature sensor networks

We report about the possibility of using regenerated fiber Bragg gratings generated in photosensitive fibers without applying hydrogen loading for high temperature sensor networks. We use a thermally induced regenerative process which leads to a secondary increase in grating reflectivity. This refractive index modification has shown to become more stable after the regeneration up to temperatures of 600 °C. With the use of an interferometric writing technique, it is possible also to generate arrays of regenerated fiber Bragg gratings for sensor networks.