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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.

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.

The idea of fabricating fiber Bragg gratings during the drawing process of an optical fiber dates back almost 20 years. The application of a transverse holographic writing method on a fiber draw tower offers a promising solution for a highly effective Bragg grating production. Because of the high technology requirements it took more than 10 years to develop the method into a reliable process. The improvements in the technical development during the last five years enable today a cost efficient industrial production of draw tower grating (DTG®) arrays. In this paper we report about new possibilities of the improved process with respect to the grating type (type I gratings, type II gratings), the coating type (2ORMOCER®, metals) and the fiber diameter (125μm, 80μm and below). Furthermore, we present an example for the application of draw tower fiber Bragg gratings in sensing technologies for medical applications.

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.

The idea of fabricating fiber Bragg gratings already during the drawing of a fiber dates back almost 20 years. The application of a transverse holographic writing method on a draw tower offers a promising solution for a highly effective Bragg grating production. Because of the high technology requirements it took more than 10 years to develop the method into a reliable process. During the last five years the improvements in the technical development enables cost effective industrial production of draw tower gratings (1DTG®). In this paper we report about new possibilities of the improved process with respect to the grating type (type I gratings, type II gratings), the coating type (2ORMOCER®, metals) and the fiber type and diameter (125μm, 80μm and below). Furthermore, we present examples for the application of draw tower fiber Bragg gratings in sensing technologies for medical applications.

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.