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

Transition from purely elastic to viscoelastic behavior of silica optical fibers at high temperatures characterized using regenerated Bragg gratings

In this study, the response of regenerated fiber Bragg gratings (RFGBs) to axial forces was investigated in a temperature range from room temperature to 900 °C. For the first time, the transition from pure elastic to viscoelastic behavior around 700 °C of a standard SMF28 optical fiber was measured with an inscribed RFBG. An elastic model with linear temperature dependencies of Young's modulus and Poisson's ratio was established, and showed good agreement with the measurements up to temperatures of ∼500 °C. In the temperature range up to 900 °C, the RFBG response could be well described with a simple, single-material approach and a Burgers model that consists of a Kelvin and a Maxwell part. Based on the elastic parameter of the Maxwell part, the temperature-dependent force sensitivity of the RFBG was determined, and it showed a linear decrease in the range from room temperature to ∼500 °C, constant values in the range between ∼500 °C and ∼600 °C, and a strong increase at higher temperatures. While fulfilling the condition to operate in the elastic domain of the silica fiber, the investigations demonstrate that RFBGs can be used as force sensors up to temperatures of ∼600 °C - the range in which temperature-dependent force sensitivities have to be considered. The temperature-dependent parameters of the effective single-material model (elastic and viscoelastic part) are essential to describe the effective mechanical behavior of the optical fiber at high temperatures.

Fiber-Optic Multipoint Sensor System with Low Drift for the Long-Term Monitoring of High-Temperature Distributions in Chemical Reactors

A low-drift fiber-optic sensor system, consisting of 24 regenerated fiber Bragg gratings (RFBG), equally distributed over a length of 2.3 m, is presented here. The sensor system can monitor spatially extended temperature profiles with a time resolution of 1 Hz at temperatures of up to 500 °C. The system is intended to be used in chemical reactors for both the control of the production ramp-up, where a fast time response is needed, as well as for production surveillance, where low sensor drifts over several years are required. The fiber-optic sensor system was installed in a pilot test reactor and was exposed to a constant temperature profile, with temperatures in the range of 150–500 °C for more than two years. During this period, the temperature profile was measured every three to five months and the fiber-optic temperature data were compared with data from a three-point thermocouple array and a calibrated single-point thermocouple. A very good agreement between all temperature measurements was found. The drift rates of the 24 RFBG sensor elements were determined by comparing the Bragg wavelengths at a precisely defined reference temperature near room temperature before and after the two-year deployment. They were found to be in the range of 0.0 K/a to 2.3 K/a, with an average value of 1.0 K/a. These low drift rates were achieved by a dedicated temperature treatment of the RFBGs during fabrication. Here, the demonstrated robustness, accuracy, and low drift characteristics show the potential of fiber-optic sensors for future industrial applications.

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.

High temperature resistant ultra-short DBR Yb-doped fiber laser

We present a distributed Bragg reflector (DBR) Yb-doped fiber laser based on a pair of type IA fiber Bragg gratings (FBGs). The high temperature resistant gratings are fabricated in high absorption hydrogen loaded Yb-doped silica fiber by use of a 244 nm argon laser and phase mask method. The DBR laser, with only 10 mm cavity length, exhibits high signal-noise ratio (SNR) of over 50 dB and can survive at 450°C in a long term with stable output power and central wavelength. Besides, the laser has a relatively low temperature sensitivity of 8.9 pm/°C, which contributes to cross-sensitization of stress and temperature.

Regenerated grating produced in a multimaterial glass-based photosensitive fiber with an ultrahigh thermal regeneration ratio

This work demonstrates thermal regeneration of gratings inscribed in a new type of multi-material glass-based photosensitive fiber. And isothermal annealing procedure has been carried out on a type-I seed grating (SG) imprinted in erbium-doped zirconia-yttria-alumina-germanium (Er-ZYAG) silica glass-based fiber, which is initiated from room temperature of 25°C up to 900°C. The findings show that the created regenerated grating (RG) has an ultrahigh thermal regeneration ratio with a value of 0.72.

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.

A IR-Femtosecond Laser Hybrid Sensor to Measure the Thermal Expansion and Thermo-Optical Coefficient of Silica-Based FBG at High Temperatures

In this paper, a hybrid sensor was fabricated using a IR-femtosecond laser to measure the thermal expansion and thermo-optical coefficient of silica-based fiber Bragg gratings (FBGs). The hybrid sensor was composed of an inline fiber Fabry-Perot interferometer (FFPI) cavity and a type-II FBG. Experiment results showed that the type-II FBG had three high reflectivity resonances in the wavelength ranging from 1100 to 1600 nm, showing the peaks in 1.1, 1.3 and 1.5 μm, respectively. The thermal expansion and thermo-optical coefficient (1.3 μm, 1.5 μm) of silica-based FBG, under temperatures ranging from 30 to 1100 °C, had been simultaneously calculated by measuring the wavelength of the type-II FBG and FFPI cavity length.

Potential to Detect Hydrogen Concentration Gradients with Palladium Infused Mesoporous-Titania on D-Shaped Optical Fiber

A distributed sensing capable high temperature D-shaped optical fiber modified with a palladium nanoparticle sensitized mesoporous (∼5 nm) TiO₂ film, is demonstrated. The refractive index of the TiO₂ film was reduced using block copolymer templating in order to realize a mesoporous matrix, accommodating integration with optical fiber. The constructed sensor was analyzed by performing direct transmission loss measurements, and by analyzing the behavior of an integrated fiber Bragg grating. The inscribed grating should reveal whether the refractive index of the composite film experiences changes upon exposure to hydrogen. In addition, with frequency domain reflectometry the distributed sensing potential of the developed sensor for hydrogen concentrations of up to 10% is examined. The results show the possibility of detecting chemical gradients with sub-cm resolution at temperatures greater than 500 °C.

Development of regenerated fiber Bragg grating sensors with long-term stability

The effect of annealing cycle on regeneration efficiency was investigated through isothermal treatments between 700 and 1000°C. We determined an inverse relationship between the recovery rate of the peak reflectivity and temperature. A regeneration efficiency of 85.2% and long-term stability at 1000°C for 500 hours were achieved via a slow regeneration process. Thermal sensors developed by isothermal regeneration were determined to be reliable up to 1000°C (±2 °C). Experimental findings suggest the involvement of both diffusion related phenomena and stress variation through densification of the fiber core in type-I FBG during the thermal regeneration process.

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.

A thermally annealed Mach-Zehnder interferometer for high temperature measurement

An in-fiber Mach-Zehnder interferometer (MZI) for high temperature measurement is proposed and experimentally demonstrated. The device is constructed of a piece of thin-core fiber (TCF) sandwiched between two short sections of multimode fiber (MMF), i.e., a MMF-TCF-MMF structure. A well-defined interference spectrum is obtained owing to the core-mismatch, and the interference dips are sensitive to the ambient temperature. The experimental results show that the proposed interferometer is capable of high temperature measurement up to 875 °C with a sensitivity of 92 pm/°C over repeated measurements. The explored wavelength drop point may limit the measurement range, which can be improved by repeated thermal annealing.

Fiber-optic flow sensors for high-temperature environment operation up to 800°C

This Letter presents an all-optical high-temperature flow sensor based on hot-wire anemometry. High-attenuation fibers (HAFs) were used as the heating elements. High-temperature-stable regenerated fiber Bragg gratings were inscribed in HAFs and in standard telecom fibers as temperature sensors. Using in-fiber light as both the heating power source and the interrogation light source, regenerative fiber Bragg grating sensors were used to gauge the heat transfer from an optically powered heating element induced by the gas flow. Reliable gas flow measurements were demonstrated between 0.066  m/s and 0.66  m/s from the room temperature to 800°C. This Letter presents a compact, low-cost, and multiflexible approach to measure gas flow for high-temperature harsh environments.

Engineering metal oxide nanostructures for the fiber optic sensor platform

This paper presents an effective integration scheme of nanostructured SnO2 with the fiber optic platform for chemical sensing applications based on evanescent optical interactions. By using a triblock copolymer as a structure directing agent as the means of nano-structuring, the refractive index of SnO2 is reduced from >2.0 to 1.46, in accordance with effective medium theory for optimal on-fiber integration. High-temperature stable fiber Bragg gratings inscribed in D-shaped fibers were used to perform real-time characterization of optical absorption and refractive index modulation of metal oxides in response to NH3 from the room temperature to 500 °C. Measurement results reveals that the redox reaction of the nanostructured metal oxides exposed to a reactive gas NH3 induces much stronger changes in optical absorption as opposed to changes in the refractive index. Results presented in this paper provide important guidance for fiber optic chemical sensing designs based on metal oxide nanomaterials.

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.

Regenerated distributed Bragg reflector fiber lasers for high-temperature operation

This Letter presents distributed Bragg reflector (DBR) fiber lasers for high-temperature operation at 750°C. Thermally regenerated fiber gratings were used as the feedback elements to construct an erbium-doped DBR fiber laser. The output power of the fiber laser can reach 1 mW at all operating temperatures. The output power fluctuation tested at 750°C was 1.06% over a period of 7 hours. The thermal regeneration grating fabrication process opens new possibilities to design and to implement fiber laser sensors for extreme environments.

Regeneration of fiber Bragg gratings under strain

The effect of strain on both the index modulation, Δn(mod), and average index, Δn, during grating regeneration within two types of fibers is studied. Significant tunability of the Bragg wavelength (λ(B)>48 nm) is observed during postannealing at or above the strain temperature of the glass. The main reason for the grating wavelength shift during annealing with load is the elongation of the fiber. As well, the observed Moiré interference cycling through regeneration indicates the presence of two gratings.

Bulk regeneration of optical fiber Bragg gratings

The reliability and reproducibility of regenerated gratings for mass production is assessed through simultaneous bulk regeneration of 10 gratings. The gratings are characterized and variations are compared after each stage of fabrication, including seed (room-temperature UV fabrication), regeneration (annealing at 850°C), and postannealing (annealing at 1100°C). In terms of Bragg wavelength (λ(B)), the seed grating variation lies within Δλ(B)=0.16 nm, the regenerated grating within Δλ(B)=0.41 nm, and the postannealed grating within Δλ(B)=1.42 nm. All the results are within reasonable error, indicating that mass production is feasible. The observable spread in parameters from seed to regenerated grating is clearly systematic. The postannealed spread arises from the small tension on the fiber during postannealing and can be explained by the softening of the glass when the strain temperature of silica is reached.

Regenerated gratings in air-hole microstructured fibers for high-temperature pressure sensing

We present thermally regenerated fiber Bragg gratings in air-hole microstructured fibers for high-temperature, hydrostatic pressure measurements. High-temperature stable gratings were regenerated during an 800 °C annealing process from hydrogen-loaded Type I seed gratings. The wavelength shifts and separation of grating peaks were studied as functions of external hydrostatic pressure from 15 to 2400 psi, and temperature from 24 °C to 800 °C. This Letter demonstrates a multiplexible pressure and temperature sensor technology for high-temperature environments using a single optical fiber feedthrough.