Hartmut Lehmann

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.

Experimental results on an optical pH measurement system for bioreactors

Abstract Properties of a pH-sensitive membrane have been tested with respect to its possible application in bioreactors. The influences of ionic strength, buffer type, buffer capacity, color and turbidity of the solution on the response of the membrane are studied. The developed membrane shows high reproducibility and long-term stability. The pH-sensing membrane can be sterilized chemically and by using heat. With the present opto-electronic set-up, an accuracy of ±0.1 pH between pH 6 and 10 can be achieved.

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.

Capillary waveguide optrodes: an approach to optical sensing in medical diagnostics

Glass capillaries with a chemically sensitive coating on the inner surface are used as optical sensors for medical diagnostics. A capillary simultaneously serves as a sample compartment, a sensor element, and an inhomogeneous optical waveguide. Various detection schemes based on absorption, fluorescence intensity, or fluorescence lifetime are described. In absorption-based capillary waveguide optrodes the absorption in the sensor layer is analyte dependent; hence light transmission along the inhomogeneous waveguiding structure formed by the capillary wall and the sensing layer is a function of the analyte concentration. Similarly, in fluorescence-based capillary optrodes the fluorescence intensity or the fluorescence lifetime of an indicator dye fixed in the sensing layer is analyte dependent; thus the specific property of fluorescent light excited in the sensing layer and thereafter guided along the inhomogeneous waveguiding structure is a function of the analyte concentration. Both schemes are experimentally demonstrated, one with carbon dioxide as the analyte and the other one with oxygen. The device combines optical sensors with the standard glass capillaries usually applied to gather blood drops from fingertips, to yield a versatile diagnostic instrument, integrating the sample compartment, the optical sensor, and the light-collecting optics into a single piece. This ensures enhanced sensor performance as well as improved handling compared with other sensors.

In-Line Gas Sensor Based on a Photonic Bandgap Fiber With Laser-Drilled Lateral Microchannels

The fabrication, characterization, and gas sensor application of a hollow core photonic bandgap fiber (HC-PBGF) with laser drilled lateral microchannels will be described. The HC-PBGF was tailor-made for gas sensing applications in the near infrared region from 1.5 to 1.7 μm wavelengths, covering the first harmonic absorptions of quite a number of natural gas components. Laser-drilled, lateral microchannels makes access to the light guiding core of the HC-PBGF and offers the ability to realize fast-responding, distributed gas sensor cells with large optical path lengths. The application of those cells in a distributed sensor arrangement using white light spectroscopy combined with chemometrical methods as interrogation method will be demonstrated.

Fabry–Pérot Cavity Based on Hollow-Core Ring Photonic Crystal Fiber for Pressure Sensing

An interferometric Fabry-Pérot cavity based on hollow-core ring photonic crystal fiber (HCR-PCF) for pressure sensing is proposed. The sensing head is formed by splicing a small section of HCR-PCF to standard single mode fiber. The spectral response depends on the cavity length due to the geometry of the HCR-PCF. The sensing head is subjected to methane pressure variations, where it exhibits a sensitivity of 0.82 nm/MPa. Its response to nitrogen pressure variation is also studied. The sensing heads intrinsic sensitivity to the nitrogen refractive index variations inside the hollow-core is also estimated. Finally, temperature measurement is performed and a sensitivity of 3.77 pm/°C is obtained for temperatures below 200°C.

Optical sensors based on inhomogeneous waveguiding in the walls of capillaries (‘capillary waveguide optrodes’)

Glass capillaries with a chemically sensitive coating on the inner surface are used for optical chemical sensing in gaseous and liquid samples. The light from an LED or a laser is coupled into and out of the capillary under a well-defined angle. The coated glass capillary acts as an inhomogeneous waveguide, in which light is guided in both the glass and the coating. The portion of the light that is absorbed in the chemically sensitive coating is a function of the concentration of a chemical analyte. Typical relative light-intensity signal changes with this type of optical interrogation are as high as 98%. Such capillary waveguide optrodes possess several attractive features: (a) they can serve as sample cavities or flow cells for gases and liquids; (b) they are suitable for direct sampling; (c) their relative signal change can be optimized by adjusting the optical pathlength; (d) the optical pathlength is much longer compared to other optrode designs based on absorption or reflection, therefore the coatings can be made thinner, and less loaded with chemically active components; (e) the sensors are hardly cross-sensitive to color and turbidity of the sample; (f) cheap light sources and detectors can be used in connection with these novel capillary waveguide optrodes.

Phase-sensitive polarimetric sensing in the evanescent field of single-mode fibres

A new approach for using optical fibres as sensing elements by exploiting the evanescent field of the guided wave is proposed. The optical principle is based on the differential phase modulation between the two orthogonal polarization modes of a single-mode fibre via evanescent field interaction with sensitive thin films deposited on the side-polished fibre. A new theoretical method to calculate the matching of an anisotropic fibre-optic waveguide to a planar layer system is shown. As a practical example, a sensitive polarimetric acetone sensor is presented.

Fiber optic sensor for adsorption studies using surface plasmon resonance

An optical fiber sensor for absorption studies based on surface plasmon resonance (SPR) interrogation in the wavelength domain is proposed. The SPR wavelength is measured with high resolution which makes it possible to detect very small changes in SPR owing to the absorption of molecules onto the sensing surface. As a model case, the absorption of water molecules on silicon dioxide was studied experimentally.

Phase-sensitive fibre-optic monoptodes for chemical sensing

Abstract A highly sensitive measurement method and miniaturized all-fibre sensing device for the detection of small changes in refractive index caused by chemical or biochemical reactions are proposed. The optical principle is based on the differential phase modulation between the two orthogonal polarization modes of a single-mode fibre via evanescentfield interaction with sensitive thin films deposited on the side-polished fibre, a so-called ‘monoptode’. Using the matrix method for an approximated planar five-layer waveguide, the influence of the main parameters of the system, such as film thickness, refractive indices and wavelength of operation, on the polarimetric interferometer efficiency has been theoretically studied. As an example, the method is applied to optimize theoretically the waveguide parameters of a humidity-sensing optode using porous TiO2-SiO2 thin film.