Optical waveguides represent the key element of integrated planar photonic circuitry having revolutionized many fields of photonics ranging from telecommunications, medicine, environmental science and light generation. However, the use of solid cores imposes limitations on applications that demand controlling strong light-matter interaction within low permittivity media such as gases or liquids, which has triggered substantial interest towards the development of hollow core waveguides. Here, we introduce the concept of the on-chip hollow core light cage that consists of free standing arrays of cylindrical dielectric strands surrounding a central hollow core implemented by 3D nanoprinting. The cage operates by the anti-resonant guidance effect and exhibits extraordinary properties such as (1) diffraction-less propagation in “quasi-air” over more than a centimetre distance within the ultraviolet, visible and near-infrared spectral domains, (2) unique side-wise direct access to the hollow core via open spaces between the strands speeding up gas diffusion times by at least a factor of 10.000, and (3) an extraordinary high fraction of modal fields in the hollow section (> 99.9%). With these properties, the light cage can overcome the limitations of current planar hollow core waveguide technology, allowing unprecedented future on-chip applications within quantum technology, ultrafast spectroscopy, bioanalytics, acousto-optics, optofluidics and nonlinear optics.
Microstructured optical fibers offer different possibilities for infiltration with unconventional fiber materials. By this way
the propagation properties of the guided light can be modified in a very flexible way and new functionality in sensing or
modulation can be introduced in optical fiber structures.
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.
The usability, advantages and limitations of suspended core fibres and hollow core band gap fibres for gas sensing in the
NIR will be discussed and demonstrated. Suspended core fibres of various geometries and hollow core photonic band
gap fibres with different transmission properties have been investigated with respect to their relative sensitivity and their
usable spectral bandwidth, using combustion gases as test substances. It has been found that, despite of the more than an
order lower sensitivity of suspended core fibers, both kinds of fibre may found use in different practical gas sensing
applications.
We filled a refractive index matching liquid into the air holes of a Ge-doped solid-core microstructured optical fiber
(MOF) with a fiber Bragg grating (FBG) to investigate its switching functions. A type of thermo-optic in-fiber switch
based on the tunable bandgap effect was demonstrated in the fluid-filled FBG at the Bragg wavelength of 830nm, and its
extinction ratio depends strongly on the reflectivity of the FBG. Another type of optical switch with an extinction ratio of
30 dB was developed in the fluid-filled MOF at a long wavelength of 1200 or 1400nm, attributing to the absorption of
the filled liquid. Such two types of switches can turn on/off the light transmission via a small temperature adjustment of
±5 or ±10ºC, respectively, and will find useful applications in all-fiber optical communication systems.
Liquid core wave guides (LCW) are particularly suited as flow-through cells for highly sensitive spectral analysis of
small liquid sample volumes as the light propagation is constrained to the liquid core. We report on LCW formed by
inside coating of glass capillaries with Teflon(R) AF 1600 (n ≈ 1.31). The wave guiding losses are between 0.2 and 0.3
dB/cm. On the base of fiber-coupled LCW flow-through cells both, a spectrophotometer and a fluorescence detector
have been developed and tested for in-situ water analysis, especially for nitrate and phosphate detection in sea water.
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.
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.
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.
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