Reinhardt Willsch

Fibre optic sensor network for spacecraft health monitoring

Close meshed instrumentation or sensor networks applying conventional sensors for temperature and strain monitoring may result in excessive penalties in terms of weight constraints, reliability and sensitivity to environmental conditions, and complex interfaces. The fibre-optic sensor network described in this paper is a multiplexed system of fibre Bragg grating (FBG) strain and temperature sensors and was designed and developed for a demanding space environment, but it can also be emphasized as a promising sensor technology with high potential for non-space applications. The FBG sensor network measures both strain and temperature at the measuring conditions of the structural core of the X-38 spacecraft, by means of wavelength shifts due to tensile stress on a Bragg grating. Dependent on the fixation of the fibre, either isolated from or mechanically coupled to the structure, local thermal or mechanical loads can be determined in the temperature range from -40 to +190°C, and in the strain range from -0.1 % to +0.3%. Short-term resolution and repeatability of the strain measurement amount to 5 μe and 25 μe, respectively. The FBG sensor network is very suitable for structural health monitoring of large structures, i.e. to determine thermal and mechanical load profiles during operation, to assess residual strength of structural elements or to detect irregular working conditions. In comparison to conventional sensors like thermocouples and strain gauges, an FBG sensor network significantly reduces the amount of required front end electronics (FEE) and harness.

Splicing Ge-doped photonic crystal fibers using commercial fusion splicer with default discharge parameters.

A novel technique for splicing a small core Ge-doped photonic crystal fiber (PCF) was demonstrated using a commercial fusion splicer with default discharge parameters for the splicing of two standard single mode fibers (SMFs). Additional discharge parameter adjustments are not required to splice the PCF to several different SMFs. A low splice loss of 1.0 approximately 1.4 dB is achieved. Low or no light reflection is expected at the splice joint due to the complete fusion of the two fiber ends. The splice joint has a high bending strength and does not break when the bending radius is decreased to 4 mm.

Inscription and characterization of Bragg gratings in single-crystal sapphire optical fibres for high-temperature sensor applications

We report the fabrication and high-temperature characteristics of Bragg gratings in single-crystal sapphire fibres inscribed by infrared femtosecond laser pulses. For stable read-out of the multimode Bragg reflection signal, the sapphire fibre was illuminated by broadband light from a superluminescent diode applying a long section of a step-index multimode silica fibre. This evenly distributed excitation of many guided modes in the sapphire fibre resulted in the polychromator measurement of stable reflection peak shapes and allowed for multiplexed temperature sensing up to 1745 °C at remote read-out with repeatability better than 1 °C. During the temperature tests an annealing process of the sapphire grating was observed at temperatures above 1400 °C. Possibilities for further improvement of sensors performance are discussed with a view to avoiding instabilities from the lack of a fibre cladding that shields the sapphire waveguide from the environment.

Optical switch based on a fluid-filled photonic crystal fiber Bragg grating

We report the implementation of an in-fiber optical switch by means of filling a fluid into the air holes of a photonic crystal fiber with a fiber Bragg grating. Such a switch can turn on/off light transmission with an extinction ratio of up to 33 dB within a narrow wavelength range (Bragg wavelength) via a small temperature adjustment of +/-5 degrees C. The switching function is based on the temperature-dependent coupling between the fundamental core mode and the rod modes in the fluid-filled holes resulting from the thermo-optic effect of the filled fluid.

Toward photonic crystal fiber based distributed chemosensors

Different structures of photonic crystal fibers (PCF) have been investigated for application as intrinsic optical gas sensors. The fiber is used both as waveguiding structure and as sample containment. Advantages and drawbacks of solid core and hollow core PCF structures will be discussed. Theoretical assessment for the sensitivity of the investigated fiber types will be given. The calculated sensitivity of the solid core PCF will be reviewed using the fiber as methane sensor. A laser micro-drilling technology is used to perform first samples for quasi-distributed PCF chemical sensing.

Optical fiber grating sensor networks and their application in electric power facilities, aerospace and geotechnical engineering

Five German companies in collaboration with IPHT developed potential low-cost strain, vibration and temperature sensor systems based on fiber Bragg grating arrays. Performance has been demonstrated in field tests for generators, aircraft, and rock-bolts.

Combined time and wavelength multiplexing technique of optical fiber grating sensor arrays using commercial OTDR equipment

The use of a commercial optical time domain reflectometry device for simultaneous wavelength and time domain multiplexing of fiber-optic Bragg grating sensors is proposed and demonstrated. The realization of such systems provides an inexpensive way of interrogating several hundreds of sensors in a single fiber. Strain resolution of 30 /spl mu/m/m over a strain measuring range of 0.5% was obtained.

Thermo-Optic Switching Effect Based on Fluid-Filled Photonic Crystal Fiber

We report a thermo-optic switching effect with a high extinction ratio of 30 dB by means of filling a fluid into air holes of a solid-core photonic crystal fiber (PCF). Such an effect can perform a turn on-off operation of the transmitted light via a small temperature adjustment of ±10°C. The switching function attributes to the absorption of the filled fluid in combination with the interaction between the core mode and the excited ¿fluid rod¿ modes, resulting from the thermo-optic effect of the filled fluid.

Optical Fiber Micro-Taper with Circular Symmetric Gold Coating for Sensor Applications Based on Surface Plasmon Resonance

We describe a novel, fully symmetrical deposition method, based on a sputtering technique and use of a gold ring target for deposition of a gold layer uniform around the taper waist. With such a circular symmetric coating, the plasmon resonance effect in the fiber taper becomes completely independent of the polarization of the illuminating light. We have measured the complex refractive index of such sputtered gold layers. On this basis, model calculations have been performed to describe the plasmon resonance as a function of taper waist diameter, gold layer thickness, analyte interaction length, and analyte refractive index. Optimized parameters especially for measurement in aqueous solutions are derived from these theoretical calculations. Experimental results are shown to be in good agreement with the theoretical analysis.

ORMOCER Coated Fiber-Optic Bragg Grating Sensors at Cryogenic Temperatures

ORMOCER coated Fiber-Bragg-Gratings (FBGs) were investigated at cryogenic temperatures. Below the Bragg wavelength of uncoated FBG is nearly independent on temperature. ORMOCER coated FBG are temperature dependent over the whole temperature range investigated from 10 to 300 K. For 50-300 K, the ORMOCER coating contributes to an additional linear temperature shift of the Bragg wavelength of 2.4 pm/K. Below 40 K the temperature dependence decreases to 1.0 pm/K. ORMOCER coated FBGs can be used as sensor at cryogenic temperatures.