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This article presents a new interrogation technique for serial fiber optical sensor networks. A hybrid code-division multiplexing (CDM)-wavelength-division multiplexing (WDM) scheme allows interrogating 2000 serial fiber-Bragg grating (FBG) sensors in a network with a length of 50 m. Furthermore, an interrogation of 477 sensors over a length of 150 m is shown. The work focuses on influences given by the above-mentioned sensor networks, such as reflectance, multiple reflections, and overlapping FBG spectra, as well as on specifications of used equipment. Since CDM relies on orthogonal codes, an autocorrelation function of a code is presented that suits for the interrogation of the previous mentioned sensor networks. A simulation of the interrogation scheme allowed for the study of the overall sensor network performance and the impact of its different components. The promising results were reproduced in a testbed. Furthermore, strain calibration measurements showed a high level of accuracy. The proposed hybrid CDM-WDM interrogation scheme can deal with sensor network lengths from a few meters to the range of a few kilometers with massive numbers of freely distributable sensors in the fiber. It is a promising approach in the field of FBG based optical sensing.

The influence of hydrogen gas on Fiber Bragg Grating (FBG)-based optical fiber sensors has been validated experimentally. More in particular, the focus was on FBGs written in the so-called Butterfly Micro Structured Fiber that targets simultaneous pressure and temperature monitoring with a minimum in cross-sensitivity to be used in, for example, downhole applications for the oil and gas market. The hydrogen-induced pressure and temperature errors from this type of sensor have been quantified as a function of the partial hydrogen pressure. The induced errors can be related to the diffusion of the hydrogen into the microstructure and to refractive index changes due to the presence of the hydrogen in the micro holes and penetration of it into the fiberglass. Furthermore, we have also shown that the hydrogen-induced errors scale with the partial hydrogen pressure. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

In this paper, we evaluate different thermal treatments in order to stabilize fiber Bragg gratings written by a femtosecond pulsed laser in specialty highly birefringent micro-structured optical fiber, targeting pressure monitoring at high pressure and high temperature environments. We have obtained a pressure sensitivity of 3.30 pm/bar up to 1400 bar and 290 degrees C. An effective thermal treatment has been experimentally implemented, yielding a nearly unchanged reflectivity at high temperature in combination with stable temperature and pressure readings: a standard deviation of 0.42 bar in the pressure reading was observed over 7 days at 280 degrees C. (C) 2017 Optical Society of America

In this paper, we demonstrate that femtosecond laser pulse written fiber Bragg gratings (FBGs) fabricated in specialty highly birefringent micro-structured optical fiber (MSF) can be used for high pressure and high temperature monitoring in downhole applications. The design of the micro-structure allows encoding the pressure information into the spectral separation between the two Bragg peaks reflected by the obtained MS-FBG. We obtained a differential pressure sensitivity of 3.30 pm/bar over a pressure range from atmospheric up to 1400 bar and at temperatures between 40 °C and 290 °C. Owing to the negligible differential pressure-temperature cross-sensitivity of 6.06E-3 bar/°C, the proposed MS-FBG sensor is an ideal candidate for pressure monitoring in the presence of high temperature transients.

Fiber Bragg grating inscription with a deep ultraviolet (DUV) femtosecond laser and two-beam interferometry allows grating inscription in germanium-free fibers without classical UV photosensitivity. Therefore such a writing technology is a promising solution for the fabrication of fiber Bragg gratings in the active fiber material to realize a fully integrated all-fiber solution. In this paper we report on the use of the DUV femtosecond laser fiber Bragg grating technology to build a grating-stabilized fiber laser with high spectral purity. Output powers up to 158 W at 1083 nm and a slope efficiency of 76% have been demonstrated.

Using a combination of phase mask and interferometric inscription technique we have generated 1st order fiber Bragg gratings (FBGs) with resonance wavelengths in a wide range of the visible spectrum (VIS). We use a modified Talbot interferometer which offers great flexibility in Bragg wavelength, spatial grating length and reflectivity. Gratings with shortest grating lengths of less than 1mm and with reflectivities of more than 70% have been produced. Peak reflectivities of up to 99% have been achieved in the visible spectrum. FBGs in the visible spectral range require structure sizes in the sub 100 nm scale. Potential applications are in the field of laser and sensor technology.

The ability to decouple strain from temperature is being explored using Bragg gratings written in highly birefringent fiber in combination with a high accuracy interrogator. Both the birefringent preform as well as the interrogator have been optimized in order to reach maximum measurement accuracy. The results from calibration measurements will be presented together with the estimated stability.

Thermally regenerated fiber Bragg gratings written in photosensitive fibers with nanosecond laser pulses without hydrogen loading have been shown to become temperature stable up to 600 °C. In this paper, we give an experimental analysis of the refractive index development for fabricating the gratings and for the thermal regeneration process. Furthermore, we identify a thermal induced type IIa (type In) behavior of the refractive index to be responsible for the regeneration. The variation of refractive index modulation and the average index in the grating region is discussed. With additional annealing it is possible to improve the temperature stability beyond 1000 °C. © 2010 Elsevier B.V. All rights reserved.

Today fiber Bragg gratings are commonly used in sensing technology as well as in telecommunications. Numerous requirements must be satisfied for their application as a sensor such as the number of sensors per system, the measurement resolution and repeatability, the sensor reusability as well as the sensor costs. In addition current challenges need to be met in the near future for sensing fibers to keep and extend their marketability such as the suitability for sterilization, hydrogen darkening or the separation of strain and temperature (or pressure and temperature). In this contribution we will give an outlook about trends and future of the fiber Bragg gratings in sensing technologies. Specifically, we will discuss how the use of draw tower grating technology enables the production of tailored Bragg grating sensing fibers, and we will present a method of separating strain and temperature by the use of a single Bragg grating only, avoiding the need for additional sensors to realize the commonly applied temperature compensation.

We demonstrate volume Bragg gratings inscribed in S-TIH53 glass. S-TIH53 is in the proper meaning not photosensitive; therefore we used a fs-laser system for the inscription process. The grating structure was formed in a Talbot interferometer and was investigated with help of the external Bragg reflection method. With this method we could measure the reflectivity profile and thereto the size of the grating. To ensure that the generated gratings are no surface or absorption gratings the probes were investigated by a microscope and absorption measurements and heating experiments were done.

Regeneration of fiber Bragg gratings has been shown to be an effective method for improving the temperature stability well beyond the limit of conventional gratings. Strong gratings, which require a high number of laser pulses, have been used mostly in the past for the additional regeneration process. Specific production methods such as draw tower inscription allow only single laser pulse illumination. Such a process can provide, however, versatile and cost effective Bragg grating arrays for sensor applications. Therefore, a combination of single pulse gratings and a regeneration process is of great practical interest. We have demonstrated that an increase of the temperature stability up to 800°C for arrays of single pulse gratings is possible. Furthermore we observe a stronger regeneration for 800 nm wavelength gratings with considerably higher reflectivity after the thermal process compared to gratings for the 1550nm wavelength range.

This Letter presents simulation and experimental results that explore bending insensitivity of fiber Bragg gratings in suspended-core optical fibers. The implementation of thin silica bridge in the fibers enhances index contrast of the fiber core and reduces bending-induced strain transfer to the fiber core. This fiber design lead to a reduction of over 7 times in strain-induced fiber Bragg grating resonant peak shifts in the suspended-core fiber compared with that in standard telecommunication fiber, and an 0.14 dB bending loss at a bending radius of 6.35 mm.

Fiber Bragg gratings (FBGs) are attractive as reflector elements in fully integrated all-fiber laser systems. Furthermore, FBGs made with femtosecond laser technology allow to reduce splice connections in the fiber, since no special photosensitive fibers are required. Fiber Bragg grating inscription with deep ultraviolet femtosecond laser (267 nm) and two beam interferometry allows to target germanium-free and non-photosensitive fibers while maintaining versatility in the choice of the output wavelength of the fiber laser. This concept offers the potential of gratings with high spatial resolution, great flexibility and good homogeneity and complements the methods of point-by-point inscription at 800 nm or of phase-mask inscription with 400 nm femtosecond laser exposure. We report on the application of the interferometric fiber Bragg grating inscription technology to build a grating-stabilized fiber laser with high beam purity. Output powers up 160 W have been achieved.

The idea of Bragg gratings, generated during the drawing process of a fiber dated back almost 20 years ago. The technical improvement of the DTG process results today in high reliable and cost effective Bragg gratings for versatile application in the optical fiber sensor market. Due to 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 lower than 300 °C. To increase temperature stability we combined DTG arrays with hydrogen loading and a thermal regeneration process which enables their use in high temperature environment. The regenerated draw tower gratings are suitable for temperatures up to 900°C.

This paper presents simulation and experimental results of orientation-dependent transverse load fiber sensors using fiber Bragg gratings written in four-hole suspended core fibers. Resonant peak shifts and splitting of fiber Bragg gratings were studied as functions of the applied transverse load and its orientation. Both simulation and experimental results revealed that response of fiber Bragg grating in suspended core fibers are sensitive to both orientation and magnitude of transverse strains.

Various spectral-encoded fiber optic sensor concepts and advanced system solutions for application in energy facility monitoring have been investigated. The technological maturity, high performance and reliability of multiplexed fiber Bragg grating (FBG) sensor arrays and networks for the measurement of temperature, dynamic strain, air flow, and magnetic field distributions in electric power generators increasing their efficiency will be demonstrated by selected examples of field testing under harsh environmental conditions. For high-temperature combustion monitoring in gas turbines, beside silica FBGs with enhanced temperature stability also sapphire FBGs and Fabry-Perot sensors have been tested and evaluated as well as fiber-based black-body thermal radiation sensors. Finally, the potential of FBG sensors for application in cryo-energetic facilities such as super-conductive high-power motors and experimental nuclear fusion reactors will be discussed.

Fiber Bragg grating based fiber lasers are promising for stable all fiber laser solutions. Standard methods for fiber Bragg gratings in fiber lasers apply germanium doped passive fibers which are connected to the amplifier section of the fiber laser with a splice. The connection is usually recoated using a low-index polymer coating to maintain guidance properties for the pump light. At high pump powers the spliced connections are affected by absorbed pump light and are prone to thermal degradation. Fiber Bragg gratings made with femtosecond laser exposure allow the direct inscription of resonator mirrors for fiber lasers into the amplifying section of the fiber laser. Such a technology has a number of advantages. The number of splices in the laser cavity is reduced. Fiber Bragg grating inscription does not relay on hydrogenation to increase the photosensitivity of the fiber. This is of special interest since hydrogen loading in large mode area fibers is a time consuming procedure due to the diffusion time of hydrogen in silica glass. Finally, one gets direct access to fiber Bragg gratings in air-clad fibers. In this paper we use a two beam interferometric inscription setup in combination with an frequency tripled femtosecond laser for grating inscription. It allows to write fiber Bragg gratings in rare earth doped fibers with a reflection wavelength span that covers the Ytterbium amplification band. Reflections with values higher than 90% have been realized.

We report about a thermal regeneration of fiber Bragg gratings written in photosensitive fibers without hydrogen loading and with the use of UV nanosecond laser pulses. We observe a complex regenerative process which indicates a secondary grating growth in an optical fiber by thermal activation. This process leads to an increased temperature stability of the gratings up to 600 °C which differs from the commonly known Type I gratings. With the use of an interferometric writing technique it is possible to generate arrays of regenerated fiber Bragg gratings (RFBGs) for sensor networks. The writing conditions of such new type of gratings are investigated and the temperature behavior of these RFBGs is analyzed. This type of gratings is suitable for high temperature sensor networks by combining the attributes of good spectral shape and high reflectivity with high temperature stability showing no drift or hysteresis.

Numbers of >1000 fiber Bragg gratings have been realized in sensor fibers for monitoring applications by on-line inscription during the fiber drawing process. The method is useful for realization of strain sensor chains.

We report about a thermal regeneration of fiber Bragg gratings in photosensitive fibers without hydrogen loading. We observe a complex regenerative process which indicates a secondary grating growth in an optical fiber by thermal activation.

We report about a thermal regeneration of fiber Bragg gratings written in photosensitive fibers with nanosecond laser pulses. We observe a complex regenerative process in a photosensitive fiber without hydrogen loading which indicates a secondary grating growth in an optical fiber by thermal activation. This process is more temperature stable than the commonly known color center modifications. The writing conditions of such new grating types are investigated and the temperature behavior of these regenerated fiber Bragg gratings (RFBG) is analyzed. The main application options are foreseen in the field of high temperature sensor systems by combining the good spectral shape of a Type I grating with the high temperature stability of a Type II grating.

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.

We report about a thermal regeneration of fiber Bragg gratings written in photosensitive fibers with nanosecond laser pulses. We observe a regenerative process in a highly photosensitive fiber without hydrogen loading which indicates a secondary grating growth in an optical fiber by thermal activation. This process is more temperature stable than the commonly known gratings produced by color center modifications. The writing conditions of such new type of gratings are investigated and the temperature behavior of these regenerated fiber Bragg gratings is analyzed. The application possibilities are in the field of high temperature sensor systems by making use of the combination of good spectral shape of a Type I grating with a Type II like temperature stability.

The combination of fiber Bragg grating inscription with femtosecond laser sources and the usage of the Talbot interferometer setup not only gives access to the fabrication of Bragg gratings in new types of materials but also allows, at the same time, to keep the high flexibility of an interferometric setup in choosing the Bragg grating wavelength. Since the spatial and temporal coherence properties of the femtosecond laser source differ strongly from those of conventional laser sources, specific limits and tolerances in the interferometric setup have to be considered. Such limits are investigated on the basis of an analytical ray tracing model. The results are applied to tolerance measurements of fiber Bragg grating reflections recorded with a DUV sub-picosecond laser source at 262 nm. Additionally we demonstrate the wavelength versatility of the two-beam interferometer setup for femtosecond inscription over a 40 nm wavelength band. Inscription experiments in Al/Yb doped silica glasses are demonstrated as a prove for the access to non-photosensitive fibers.

Fiber Bragg gratings (FBGs) are today fundamental components in fiber optics. They can be used as sensors, in signal processing, e.g. telecom applications, as wavelength stabilizers in fiber lasers or in dispersion compensators. However, there are applications where the demand for fiber Bragg gratings is not compatible with standard photosensitivity techniques like germanium doping or hydrogen loading. Examples are their use as laser-mirrors in spliceless all fiber fiber-laser solutions or the fiber Bragg grating inscription in suspended core all silica fibers for evanescent field sensing. Fiber Bragg grating inscription with femtosecond-laser exposure is a challenging new method to realize grating structures for waveguides made of materials which do not provide UV-photosensitivity. Especially fs-IR-inscription has been demonstrated for Bragg grating inscription in a variety of material systems such as boron-silica glass, sapphire and pure silica glass. The feasibility of the phase mask FBG inscription technique with DUV femtosecond lasers was also shown, which allows grating inscription even in pure silica microstructured fibers. The phase mask inscription method requires that the fiber will be placed directly behind the phase mask. While the laser beam should be focused onto the fiber to support nonlinear material interaction, this inscription method also leads to phase mask degradations, presumably due to non-bridging oxygen holes (NBOH). Our solution to avoid the mask degradation is to increase the space between fiber and phase mask by using a Talbot-interferometer. Another advantage is the wavelength versatility of this inscription setup. Due to the short temporal coherence length of the femtosecond pulses, the angular alignment variability of the interferometer mirrors is limited and restrictions concerning the wavelength versatility of the interferometer arise. Grating arrays in pure silica suspended core fibers are demonstrated as an example for the versatility of the inscription arrangement.

The inscription of fiber Bragg gratings with femtosecond laser radiation gives access to a wide range of new materials beyond conventional UV-photosensitive and special hydrogen loaded materials. Typically, infrared wavelengths have been used until now for femtosecond Bragg grating inscription. The use of UV femtosecond laser radiation gives more direct access to two-photon absorption in materials with high band-gaps and could achieve gratings with high spatial resolution. The refractive index modulation process for UV femtosecond pulses is then obtained as a combination of classical color center absorption and high intensity structural modification. Besides the choice of the inscription laser wavelength, also the inscription method is of great practical importance for the grating properties. The most commonly used phase mask technique suffers from mask degradation effects in case of high intensity laser pulses and gives only limited flexibility in variation of the Bragg reflection wavelengths. We have therefore investigated the use of an interferometric technique in combination with UV femtosecond laser pulses. The relevant parameters of spatial and temporal coherence have been considered and the wavelength tuning and reflection properties have been tested for several different types of fibers.

We have generated fiber Bragg gratings with Bragg wavelengths in VIS from 700 nm down to 480 nm. Peak reflectivities of over 99 % were achieved. Potential applications are in the field of sensor technology.

This Letter presents simulation and experimental results of orientation-dependent transverse stress fiber sensors using fiber Bragg gratings (FBGs) inscribed in four-hole suspended-core fibers. Resonant peak shifts and splitting of FBGs were studied as functions of the applied transverse load and fiber orientation. Both simulation and experimental results revealed that the response of FBGs in suspended-core fibers is sensitive to both the orientation and magnitude of an applied transverse stress.

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. © 2011 Optical Society of America.

Fiber Bragg grating inscription with deep ultraviolet femtosecond exposure and two beam interference is a promising technology to achieve first order fiber Bragg gratings in materials which do not show photosensitivity at conventional FBG inscription technology. It allows bypass the hydrogen loading process, which facilitates the FBG inscription process in microstructured fibers and fibers with large outer diameters. Fiber Bragg gratings in suspended core fibers and rare earth doped fibers are demonstrated.