Sensors based on the localized surface plasmon resonance (LSPR) effect are known as sensitive methods for refractive
index measurements or for detection of specific binding reactions in biosensing due to a variation in layer thickness.
Fiber based arrangements are able to perform such measurements with very small analyte volumina. A generalization of
this concept for a multiplexed measurement of different biomolecules in a single fiber could be an attractive approach.
We discuss the possibility of selectively sensitizing the inner surfaces of a suspended core fiber with two different
metallic nanoparticle types in monolayers for such multiplexed measurements.
Optical resonance coupling between a side-polished fiber and thin film waveguides has been investigated in the presence
of biochemical adsorbates. The shift of the resonance wavelengths was found to be highly sensitive to the capture of
target DNA recognition elements with Au nanoparticle markers, allowing for a sensitivity limit of 10 particles on the
side-polished fiber core area (2000 μm²) during on-line measurements using a polychromator spectrometer.
Microstructured optical fibers (MOFs) represent a promising platform technology for new biosensing devices. Using
MOFs with adapted cavity diameters of about 20 to 30 μm, they can be used to carry the biofluids of analytical interest.
Such cavities with their walls coated by transducer material form in combination with adequate microfluidic chips a
platform for fully integrated next generation plasmonic devices. This paper describes the use of a dynamic chemical
nanoparticle layer deposition (NLD) technique to demonstrate the wet chemical deposition of gold and silver
nanoparticles (NP) within MOFs with longitudinal, homogenously-distributed particle densities. The plasmonic
structures were realized on the internal capillary walls of a three-hole suspended core fiber. Electron micrographs, taken
of the inside of the fiber holes, confirm the even distribution of the NP. With the proposed procedure fiber lengths of
several meters can be coated and afterwards cut up into small pieces of desired lengths. Accordingly, this procedure is
highly productive and makes the resulting MOF-based sensors potentially cost efficient. In proof-of-principle
experiments with liquids of different refractive indices, the dependence of the localized surface plasmon resonance
(LSPR) on the surroundings was confirmed. Comparing Raman spectra of NP coated and uncoated MOFs, each filled
with crystal violet, a significant signal enhancement demonstrates the usability of such functionalized MOFs for surfaceenhanced
Raman spectroscopy (SERS) experiments.
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.
Advantages of optical fiber Bragg grating sensors at low temperatures, are electrical isolation, low electro-magnetic interference,
low thermal conductivity to a large number of multiplexed sensors. They show negligible thermo-optic and magnetooptic
effects in cryogenic environment. These properties make them attractive for temperature surveillance and structural
health monitoring of cryogenic systems, or for the testing of material properties and system components at low temperatures.
Microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next generation
plasmonic devices. This paper details the use of a dynamic chemical deposition technique to demonstrate the wet
chemical deposition of gold and silver nanoparticles (NP) within MOFs with longitudinal, homogenously-distributed
particle densities. The plasmonic structures were realized on the internal capillary walls of a three-hole suspended core
fiber. The population density of the NP on the surface, which directly influences the usable / necessary sensor length, can
be tailored via the controlled pre-treatment of the fiber. With the proposed procedure we can coat several meters of fiber
and, afterwards, cut the fiber into the desired lengths. Accordingly, this procedure is highly productive and makes the
resulting MOF-based sensors potentially very cheap. Electron microscope micrographs, taken of the inside of the fiber
holes, confirm the even distribution of the NP. A transversal through-light setup was used for the non-destructive layer
characterization. In proof-of-principle experiments with liquids of different refractive indices, the LSPR dependence on
the surroundings was confirmed and compared with Mie-theory based calculations.
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.
Fiber Bragg grating sensors in side-polished optical fiber are sensitive to an external analyte by evanescent
field interaction. Deposition of sensor-specific transducer layers can advance such fiber Bragg grating refractometer
to the optochemical monitoring of specific substances: absorbed gases, vapors, and adsorbed biomolecules
as well. Refractive index range of highest sensitivity can be adjusted to the analyte medium of interest
by proper construction of the multilayer waveguide. A specific application example aims at hydrogen
detection using a palladium thin film transducer.
A fiber optic Bragg grating sensor system has been installed in the blades of a wind turbine and was successfully
tested for several years. We report the requirements, system design and construction parameters of a
sensor system for continuous on-line monitoring of bending loads of the rotor blades, and provide characteristic
examples of monitoring results.
We present a compact and fast wavelength monitor capable of resolving pm wavelength changes. A photosensor array or
position detector element is coated with a linear variable filter, which converts the wavelength information of the
incident light into a spatial intensity distribution on the detector. Differential read-out of two adjacent elements of the
photosensor array or the position detector is used to determine the centroid of this distribution. A wavelength change of
the incident light is detected as a shift of the centroid of the distribution. The performance of this wavelength detector
was tested with a wavelength tunable light source. We have demonstrated that our device is capable of detecting
wavelength changes as small as ~0.1 pm. The wavelength monitor can be used as read-out unit for any optical sensor
that produces a wavelength shift in response to a stimulus. In particular, changes in the reflection properties of one and
two-dimensional photonic crystals can been detected. The performance of this interrogation method has been tested for
the case of temperature and strain sensors based on Fiber Bragg Gratings (FBG).
Hollow glass micro-spheres, first used to make fibre optic sensors for high hydrostatic pressure, have been interrogated
using a high-precision CCD spectrometer, to give far better precision than earlier. It is found that these simple, low-cost
micro-sensors have excellent sensitivity to both static and dynamic pressure, and have the advantage of being hermetically
sealed. Many other application areas are foreseen for these low-cost sensors.
In order to identify defects of the electrical infrastructure during train operation, a fiber Bragg grating based sensor
system performs measurements of the distribution of short time force changes in vertical and horizontal (driving)
direction between current collector and overhead contact line. The actual model calculations and the practical design of
a 2-dimensionally arranged strain sensor network have been especially enhanced to the calculation of impact directions.
The well-known advantages of fiber-optic sensors - embedding capability in the composite carbon/aluminum collector
strip, multiplexing of distributed sensor networks, electrical isolation - are of particular importance for detection and
characterization of fast impacts immediately at the position of incidence. Tests under everyday operating conditions
with trains on high-speed tracks as well as under high load in mountain regions proved the application of this sensing
technology. Problems and solutions for the sensor network embedment, the fast Bragg sensor interrogation algorithms,
and actual lab test results with their application-orientated analysis will be presented.
Fibre side-polishing technology allows for refractive index measurements using optical fibre Bragg grating sensors. Evanescent field interaction between the guided fibre light mode and the analyte near to the core of the side-polished optical fibre yields a Bragg wavelength shift in dependence on the refractive index of the analyte. This refractometric sensor technique has been extended to opto-chemical measurements with specific sensitivity using transducer layers with selective absorption properties deposited on the side-polished fibre core region. Experimental results are presented for the case of a hydrogen gas sensor (palladium transducer) and a pH sensor (polyaniline transducer).
A fibre Bragg grating sensor system has been installed in a horizontal axis wind turbine and was successfully tested for already more than one year. We report the requirements, system design and realisation of the sensor system for continuous on-line load monitoring of the rotor blades, and provide examples of strain measurement results.
A sensor network consisting of unified fiber grating based temperature and strain sensor pads has been developed for integration in carbon/aluminum composite current collector strips, and has been tested in electrical trains on commercial railways. The fiber optic sensor network measures value and position of both con-tact forces and impacts under real-time conditions, immediately at the high voltage location of the interface between overhead contact line and current collector.
In a German slab track system (Feste Fahrbahn FF, system Boegl) for speeds up to 300 km/h and more different fiber optic sensors have been embedded in several levels and locations of the track system. The track system consists of prestressed precast panels of steel fiber concrete which are supported by a cat-in-situ concrete or asphalt base course. The sensors are to measure the bond behavior or the stress transfer in the track system. For that, tiny fiber-optic sensors - fiber Fabry-Perot and Bragg grating sensors - have been embedded very near to the interface of the layers. Measurements were taken on a full scale test sample (slab track panel of 6.45 m length) as well as on a real high speed track. The paper describes the measurement task and discusses aspects with regard to sensor design and prefabrication of the sensor frames as well as the embedding procedure into the concrete track. Results from static and dynamic full scale tests carried out in the testing laboratory of BAM and from measurements on a track are given.
Evanescent field interaction in side-polished fibre Bragg gratings (FBG) provides a sensor for measuring refractive indices of liquid analytes and thin films with spectral read-out. The specific advantages of FBG sensors - networking capability, independence of variable signal transmission losses - give the potential to a higher diversity and accuracy of opto-chemical measurements than the measurement of transmission intensities through evanescent field sensitive fibre structures.
In a fiber-optic multi-point sensor network basing on optical fiber Bragg gratings, simultaneous measurements of refractive indices and of temperature are performed. The Bragg wavelength of a side-polished fiber grating is determined by the refractive index of the surrounding fluid, and is measured using a high-resolving low-cost compact spectrometer. The influence of temperature is separated considering the temperature-induced Bragg wavelength shift in an adjacent non-polished part of the fiber grating. The sensor network can be applied for on-line process control in chemical, biochemical and petrol technologies, and for environmental and geotechnical monitoring, especially in flammable or corrosive surroundings, and in electromagnetic fields. A buffer layer between fiber core and analyte allows to adjust the sensor characteristic to appear with high sensitivity in the refractive index range of the interesting fluids.
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