Hartmut Bartelt

Optical sapphire fiber Bragg gratings as high temperature sensors

Fiber Bragg gratings (FBG) were inscribed in single crystalline sapphire fibers by fs-laser irradiation. Due to the used multi-mode air clad fiber a sapphire-FBG spectra showa a wide asymmetric peak with a half width of 7 nm. Different mathematical peak functions were tested to determine a fiber Bragg wavelength. It was shown that the shift of the calculated Bragg wavelengths in dependence on temperature is identical for the different peak functions. The determination of the fiber Bragg wavelength shift with a resolution of 10pm allows temperature measurements within an accuracy of ±1°C in the temperature range up to 1500°C. Sapphire FBG were used to measure the temperature distribution and thermal fluctuations within an inductive heated furnace in the range from 100°C to 1500°C.

Draw tower fiber Bragg gratings and their use in sensing technology

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.

Microstructured index-guiding fibers with large cladding holes for evanescent field chemical sensing

Design, fabrication and application of small solid-core microstructured optical fibers with large cladding holes for evanescent field chemical sensing of gases and liquids will be presented. Such steering-wheel fiber structures give a high mode-field overlap in the holey region, they show low losses over a broad spectral range and they are easier to fabricate than hollow-core bandgap-guiding photonic crystal fibers.

Optical fiber sensor research and industry in Germany: review and outlook

Since more than 30 years, the increased research, technology development and commercialization of optical fiber sensors combined with their continuously growing technical applications have become a story of success worldwide and in Germany as well. German fiber sensor research and industry achieved remarkable milestones in the 1980ies and 1990ies, such as first field tests of magneto-optic current sensors in power facilities or of micro-bending fiber strain sensors in a highway bridge. Recent progress and the state of the art of optical fiber sensing in Germany are demonstrated by examples of advanced fiber Bragg grating and distributed sensor system applications, fiber gyroscopes and other interferometric sensors, chemical and bio-medical sensors, and sensors based on polymer fibers as well. In context with the growing international cooperation, the potential of German research and industry will be discussed in terms of novel fiber-optic sensor system concepts, of increasing maturity and reliability of this exciting sensor technology and of new applications and markets.

Distributed gas sensor based on a photonic bandgap fiber cell with laser-drilled, lateral microchannels

The fabrication, characterization, and use of a laser-drilled hollow core photonic band gap fiber (HC-PBGF) as a gas sensor in the near infrared region, from 1.5 μm to 1.7 μm wavelengths, are discussed. HC- PBGFs with laser-drilled, lateral micro channels have the ability to realize fast-responding, distributed gas sensor cells with large optical path lengths. By using white light spectroscopy as a sensor interrogation method, together with chemometric methods, not only the detection of individual gases but also the quantification of composed gas mixtures is possible.

Preparation and characterization of microstructured silica holey fibers filled with high-index glasses

Silica based microstructured holey fibers offer the possibility for filling with unconventional fiber materials. Of special interest are chalcogenide glasses due to their high refractive index and their nonlinear optical properties. We demonstrate two types of fibers: an index guiding fiber type with high-index glass core and silica cladding and a fiber with silica core surrounded by a periodic, hexagonal high-index glass structure giving antiresonant guiding properties. We prepared such fibers filled with arsenic sulphide glass and arsenic selenide glass by a pressurized infiltration technique. The manufacturing process is modelled on the basis of viscous glass flow parameters and is compared with experimental results obtained from the filled fibers. The propagation and spectral transmission properties of such fibers are measured and discussed.

PCF interferometer based temperature sensor with high sensitivity

In this work, the use of a photonic crystal fiber (PCF) with a highly Germanium (Ge) doped core is exploited as temperature sensor for the first time (to our knowledge). The PCF has an outer diameter of 125 μm and consists of a microstructured cladding with an average pitch and hole diameter of Λ=4.6 μm and d=1.0 μm, respectively. A short PCF stub (~2.0 mm) is used for the preparation of an interferometer. The PCF is spliced between single mode fibers (SMF), meaning that the PCF holes are fully collapsed in the splicing region while the Ge-doped core is still present. The splice parameters were changed to make a short collapse region of (200±30) μm. The first splice is used to excite the fundamental core mode and multiple higher order cladding modes by applying a core-to-core offset. The second splice acts as spatial filter to detect only the light which is guided in and near the core. The interferometer is heated up to 500°C and the total wavelength shift with the temperature variation found to be 74 pm/°C which is more than 5 times higher than a fiber Bragg grating at 1550 nm (13 pm/°C). The PCF interferometer preparation requires only a few steps, cleaving and splicing the fibers. The short length, the high thermal sensitivity and stability of the structure make the device attractive for many sensing applications including high temperature ranges.

Regenerated single pulse fiber Bragg gratings for high temperature sensing

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.

Interferometric inscription of volume Bragg gratings in a commercial high-refractive index glass (S-TIH53) by 400 nm femtosecond (fs) laser pulses

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.

Plasmonic nanoparticles for optical biosensing

Metal nanoparticles exhibit a large potential for the development of innovative and cost-effective sensing devices. They fulfill key requirements for biosensors such as the potential for miniaturization as well as for high parallelization, and they are compatible with the molecular world for the required biofunctionalization approaches. Their optical properties based on the localized surface plasmon resonance (LSPR) are well adjustable from the UV- to the infrared spectral range using chemical synthesis. Due to the strong influence of the surrounding dielectrics on the resonant properties these particles offer a high potential for sensing of minimal changes in the surrounding media. Additionally, plasmon nanoparticles can induce a local field-enhancement and so a signal amplification such as for fluorescence or Raman-spectroscopy. In general, plasmon nanoparticles are well suited as label or as transducer for different optical detection techniques. We will give an overview about recent developments in this field, and will present different sensing strategies at single particle or ensemble level and based on planar or fiber-based systems aiming for ultrasensitive point-of care applications in bioanalytics.