Steffen Lochmann

Feedback Stabilized Interrogation Technique for EFPI/FBG Hybrid Fiber-Optic Pressure and Temperature Sensors

This paper discusses a Single Wavelength Interrogation (SWI) technique used to measure fast changing pressure related signals and over a large operational temperature range. The novel technique is based on a fiber-optic pressure and temperature hybrid sensor, and a feedback stabilization technique for a tunable laser source. The fiber-optic hybrid sensor consists of a miniature diaphragm based all-silica Extrinsic Fabry-Perot Interferometric (EFPI) Fiber-Optic Pressure Sensor (FOPS) which additionally incorporates a Fiber Bragg Grating (FBG) temperature sensor. The FBG temperature sensor is used as a feedback element to stabilize the output wavelength of the tunable laser source to operate always in the linear range of the EFPI FOPS.

Optical fibre cavity for ring-down experiments with low coupling losses

A fibre-cavity design based on highly reflective gold coatings, vapour deposited to the two end faces of a large core multimode waveguide, is presented. In contrast to common fibre-cavity approaches, the laser pulses are not coupled through the reflective coatings into the cavity but through a micro hole in one of the fibre end faces, which reduces the commonly very high coupling loss to less than 1%. Accordingly, lower demands on the source power and the sensitivity of the receiver circuit result in lower costs of ring-down sensor approaches. The scope of the paper is on the proof of principle and the characterization of the novel fibre-cavity design. Nevertheless, potential applications are briefly addressed at the end of the paper.

Fibre optic pressure and temperature sensor for geothermal wells

In this paper a fibre optic sensor is developed and tested to measure pressure and temperature under simulated wellbore conditions. The sensor consists of a miniature all-silica fibre optic Extrinsic Fabry-Perot Interferometer (EFPI) sensor which has a novel embedded Fibre Bragg Grating (FBG) reference sensor element to determine both pressure and temperature at the point of measurement. The sensor head is completely fabricated from fused-silica components, i.e. completely made of glass, utilizing a 200µm outer diameter silica glass fibre, a silica glass capillary and a Single-Mode FBG. All silica-glass components were spliced together using a conventional fusion splicer to obtain a robust sensor structure.

A neural network based approach for determination of optical scattering and absorption coefficients of biological tissue

The propagation of light in biological tissue depends on the absorption and reduced scattering coefficient. The aim of this project is the determination of these two optical properties using spatially resolved reflectance measurements. The sensor system consists of five laser sources at different wavelengths, an optical fibre probe and five photodiodes. For these kinds of measurements it has been shown that an often used solution of the diffusion equation can not be applied. Therefore a neural network is being developed to extract the needed optical properties out of the reflectance data. Data sets for the training, validation and testing process are provided by Monte Carlo Simulations.

A neural networks based approach for determining fouling of multi-point optical fibre sensors in water systems

An optical fibre sensor system for monitoring contamination in water supplies is presented. The sensor comprises a number of individual sensor elements on a single fibre loop. It is addressed using optical time domain reflectometry so that the required spatial resolution of 1 metre is achieved. Analysis of the signals at the receiving end is performed using artificial neural networks coupled with pattern recognition techniques, thus allowing external influences such as the degree of sensor fouling to be detected. In this investigation limescale build-up in hard water is investigated as the interfering parameter.

Fabrication of a high temperature-resistance optical fibre micro pressure sensor

A micro diaphragm-based Extrinsic Fabry-Perot Interferometric (EFPI) pressure sensor with high temperature immunity is described. The EFPI pressure sensor consists of a single mode silica glass optical fiber, a glass capillary and a glass diaphragm, which are fused together using a conventional fusion splicer. This eliminates the need for the use of other joining materials, e.g. epoxy, allows any thermal mismatch between different materials to be almost completely eliminated and hence the sensor is capable of high temperature operation. In addition, the EFPI pressure sensor is cheap and relatively easy to produce as only splicing, polishing and wet etching are necessary. Moreover, due to its small size, a frequency response of several MHz can be achieved

Fibre-optic evanescent-wave field fluid concentration sensor

A fibre-optic fluid concentration sensor based on refractive index measurements is reported. Its working principle is validated for some simple aqueous primary alcohol mixtures, such as Acetone, Ethanol, Methanol, 1-Propanol, and 2-Pro-panol, covering a concentration range from 0%vol to 10%vol. With a simple and low cost LED based measurement setup a resolution of about 0.00005 refractive index units was achie-ved, which approximately corresponds to 0.2% methanol, 0.75% ethanol and acetone, and 0.05% 1-propanol and 2-propanol. Moreover, described sensing method is applicable for all fluids that show different refractive indices at different concentrations, which by far is the case for almost all fluids, since the refractive index of a mixture is related to its density.

Measuring of exhaust gas emissions using absorption spectroscopy

This paper describes an optical fibre sensor for the detection of NOx (NO2 and NO) and CO2 in the exhaust system of a road vehicle. The measurement is based on a free path interaction zone which is interrogated using UV and NIR light guiding optical fibres and collimated lenses. Results are presented in the absorption spectra of the gases in the UV region for the NOx gases and NIR region for CO2. These demonstrate that using this method it is feasible to identify the individual CO2, NO and NO2 species as well as other gases in the exhaust system. Measurement of concentrations to the level of tens parts per million (ppm) have been demonstrated for the NOxx gases.

A fibre optic sensor for the in situ determination of rock physical properties

To understand the behaviour of rocks under changing load or temperature conditions, the determination of physical parameters like pore pressure or temperature within the pore space is essential. Within this study, the implementation of a novel fibre optic point sensor for pressure and temperature determination into a high pressure / high temperature triaxial cell is presented. For the first time, pressure was measured directly within the pore space of a Flechtinger sandstone specimen during a hydrostatic compression test at up to 70 MPa. The sensor used within this study consists of a miniature all-silica fibre optic Extrinsic Fabry-Perot Interferometer (EFPI) sensor which has an embedded Fibre Bragg Grating (FBG) reference sensor element to determine temperature and pressure directly at the point of measurement.

Temperature compensated miniature all-glass fibre optic pressure sensor

In this paper the low pressure response and resolution of a temperature compensated miniature all-glass Fibre Optic Pressure Sensor (FOPS) is evaluated. The active temperature compensation of a diaphragm based all-silica Extrinsic Fabry Perot Interferometric (EFPI) FOPS is established by encapsulating a Fibre Bragg Grating (FBG) within the EFPI cavity. The evaluation is performed by using an extended White Light Interrogation (WLI) system. Due to its several advantages the reported FOPS has already been successfully applied in measuring pressure in an exhaust system of a vehicle as well as in a simulated geothermal well.