Polymer Waveguide Sensor Based on Evanescent Bragg Grating for Lab-on-a-Chip Applications.

In this work, an evanescent Bragg grating sensor inscribed in a few-mode planar polymer waveguide was integrated into microchannel structures and characterized by various chemical applications. The planar waveguide and the microchannels consisted of epoxide-based polymers. The Bragg grating structure was postprocessed by using point-by-point direct inscription technology. By monitoring the central wavelength shift of the reflected Bragg signal, the sensor showed a temperature sensitivity of -47.75 pm/K. Moreover, the functionality of the evanescent field-based measurements is demonstrated with two application examples: the refractive index sensing of different aqueous solutions and gas-phase hydrogen concentration detection. For the latter application, the sensor was additionally coated with a functional layer based on palladium nanoparticles. During the refractive index sensing measurement, the sensor achieved a sensitivity of 6.5 nm/RIU from air to 99.9% pure isopropyl alcohol. For the gas-phase hydrogen detection, the coated sensor achieved a reproducible concentration detection up to 4 vol% hydrogen. According to the reported experimental results, the integrated Bragg-grating-based waveguide sensor demonstrates high potential for applications based on the lab-on-a-chip concept.

QEPAS sensor in a butterfly package and its application.

Molecular gases are highly relevant in healthcare, production control, safety, and environmental monitoring. They often appear in small concentrations. The measurement of trace gases has increasingly become a key technique in those domains. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a suitable method that can provide the required low detection limits in such applications at comparatively low cost and small size. For mobile implementation, the size of an entire sensor unit matters. In this paper, we present a QEPAS sensor that fits into a standard butterfly package, its characterization, and its application on CH4 and CO2.

Artificial neural network for the reduction of birefringence-induced errors in fiber shape sensors based on cladding waveguides gratings.

Cladding waveguide fiber Bragg gratings (FBGs) provide a compact and simple solution for fiber shape sensing. The shape sensing accuracy is limited by birefringence, which is induced by bending and the non-isotropic FBG structure (written by femtosecond laser point-by-point technique). An algorithm based on an artificial neural network for fiber shape sensing is demonstrated, which enables increased accuracy, better robustness, and less time latency. This algorithm shows great potential in the application of high-accuracy real-time fiber shape measurements.

From Femtosecond to Nanosecond Laser Microstructuring of Conical Aluminum Surfaces by Reactive Gas Assisted Laser Ablation.

A conical microstructure is one of the most versatile surface textures obtained by ultrashort laser micromachining. Besides an increased surface area, unique surface properties such as superhydrophilicity, increased absorptivity; and thermal emissivity can be tailored. On metals, usually ultrashort laser pulses in the femtosecond to low picosecond range are used to obtain these surface structures, whereas nanosecond laser pulses favor melting processes. Herein, we report on an investigation of reactive gas atmospheres such as oxygen, steam, and halogens during laser micromachining of aluminum with 6 ns laser pulses. At a reduced pressure of 20 hPa (air) with additional iodine vapor as reactive species, we found a perfectly microconical structured surface to be formed with nanosecond laser pulses. The resulting surface structures were proven to be free of residual halogens. The application of nanosecond instead of femtosecond laser pulses for the surface structuring process allows to apply significantly less complex laser sources.

Manufacturing and Characterization of Femtosecond Laser-Inscribed Bragg Grating in Polymer Waveguide Operation in an IR-A Wavelength Range.

Optical sensors, such as fiber Bragg gratings, offer advantages compared to other sensors in many technological fields due to their outstanding characteristics. This sensor technology is currently transferred to polymer waveguides that provide the potential for cost-effective, easy, and flexible manufacturing of planar structures. While sensor production itself, in the majority of cases, is performed by means of phase mask technique, which is limited in terms of its degrees of freedom, other inscription techniques enable the manufacture of more adaptable sensor elements for a wider range of applications. In this article, we demonstrate the point-by-point femtosecond laser direct inscription method for the processing of polymer Bragg gratings into waveguides of the epoxy-based negative photoresist material EpoCore for a wavelength range around 850 nm. By characterizing the obtained grating back-reflection of the produced sensing element, we determined the sensitivity for the state variables temperature, humidity, and strain to be 45 pm/K, 19 pm/%, and 0.26 pm/µÎµ, respectively. Individual and more complex grating structures can be developed from this information, thus opening new fields of utilization.

Comparison of the spectral excitation behavior of methane according to InP, GaSb, IC, and QC lasers as excitation source by sensor applications.

The MIR wavelength regime promises better gas detection possibilities than the NIR or the visible region because of the higher absorbencies simulated by HITRAN. In the MIR region are many important absorption lines of significant gases, which are relevant in healthcare, production supervision, and safety and environmental monitoring. One of those gases is methane. CH4 shows significant variations in absorbance with a maximum at 3.3 µm, which results in low detection limits in the range of low ppm. Interband-cascade- and quantum-cascade-based lasers emit at higher wavelengths, where the absorbencies of methane are higher. The comparison is done by analyzing the performance of two spectroscopy applications: tunable diode laser absorption spectroscopy and quartz-enhanced photoacoustic spectroscopy.

Molecularly Imprinted Sol-Gel for TNT Detection with Optical Micro-Ring Resonator Sensor Chips.

A sensor for trinitrotoluene (TNT) detection was developed by using a combination of optical micro-ring technology and a receptor coating based on molecularly imprinted sol-gel layers. Two techniques for deposition of receptor layers were compared: Airbrush technology and electrospray ionization. A concentration of less than 5 ppb for TNT in the gas-phase, using electrospray deposition of the receptor layer, was detected. The cross-sensitivities to organic substances and further nitro-based explosives were compared. As a result, the sensitivity to TNT is about one order of magnitude higher in comparison to the explosives 2,4-dinitrotoluene (DNT) or 1,3-dinitrobenzene (DNB) and about four orders of magnitude higher than the organic substances phenol, ethanol, and acetone. The signal response of the sensor is fast, and the compact sensor design enables the deposition of different receptor layers on multiple optical micro-rings on one chip, which allows a more precise analysis and reduction of side effects and false alarms.

Rapid prototyping of all-polymer AWGs for FBG readout using direct laser lithography.

This Letter describes the design, production, and characterization of a 1×2 arrayed waveguide grating (AWG) for fiber Bragg grating (FBG) readout, centered at 850 nm, with a channel spacing of 1 nm. The employed manufacturing process is laser direct lithography, a low-cost, rapid-prototyping capable, maskless method that allows for short iteration cycles and simple migration of a successful design to mask-based high throughput methods. We also consider the achievable AWG performance if used for strain or temperature measurements with FBG sensors.

Direct inscription and evaluation of fiber Bragg gratings in carbon-coated optical sensor glass fibers for harsh environment oil and gas applications.

In this research work, we show the successful inscription of fiber Bragg gratings into carbon-coated pure silica as well as germanium-doped glass fibers by applying the pulsed laser point-by-point manufacturing technique. First, the parameters used for the Ti:sapphire femtosecond laser process are demonstrated. Without removing the polymeric carbon coating, destruction-free formation of highly reflective Bragg gratings is performed with selected types of hermetically enclosed fibers. We demonstrate the advantage of the carbon coating by long-term exposure to a pure hydrogen atmosphere at an elevated temperature. Such harsh conditions exist in the oil and gas industry, which means there is high application potential for technologically advanced optical sensors. Compared to the also examined standard glass fibers with a distinct signal attenuation, carbon-coated fibers show no significant degradation. Finally, we analyze the mechanical stability of the processed fibers via standardized tensile tests. No substantial decrease in strength occurs among the sensor-integrated samples.

QEPAS sensor for breath analysis: a behavior of pressure.

The measurement of trace gases has increasingly become a key technique in healthcare and other medical applications. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a suitable method that can provide the required characteristics in such applications for a comparatively low cost and small size. The quantitative detection and a low detection limit are also required by applications. In this paper, we present new results on sensing biomedically relevant gases using the on-beam QEPAS technique with some newly developed tunable high-power single-mode laser diodes based on GaSb material. The data processing and detection limit determination are done by a field programmable gate array device, as well as an automatic measurement of the resonance frequency.

Multiple off-axis fiber Bragg gratings for 3D shape sensing.

Point-by-point femtosecond laser processed fiber Bragg gratings are arranged around the edge of a standard single-mode optical fiber core. The relative amplitudes of at least three such fiber Bragg gratings are utilized to detect the central position of the mode field within the fiber core and calculate the local curvature of the fiber. An analytical approximation is given, and an experimental validation is performed.

Active Molybdenum-Based Anode for Dehydrogenative Coupling Reactions.

A new and powerful active anode system that can be operated in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) has been discovered. In HFIP the molybdenum anode forms a compact, conductive, and electroactive layer of higher-valent molybdenum species. This system can replace powerful but stoichiometrically required MoV reagents for the dehydrogenative coupling of aryls. This electrolytic reaction is more sustainable and allows the conversion of a broad scope of activated arenes.

Single mode GaSb diode lasers for sensor applications in a long wavelength regime.

The wavelength, λ, range of 1.8 µm≤λ≤3.5 µm contains strong spectral absorption lines of many gases used in health, industry, safety, and medicine and whose sensitive and quantitative detection is desirable. However, the performance of InP diode lasers markedly deteriorates beyond λ∼2 µm. In this paper we present new results on developing tunable high power single mode laser diodes based on the GaSb material system with emission in the wavelength range of 1.8 µm≤λ≤2.2 µm.

All-polymer arrayed waveguide grating at 850 nm: design, fabrication, and characterization.

In this Letter, a novel all-polymer arrayed waveguide grating (AWG) device with an operating wavelength around 850 nm is reported. The all-polymer AWG consists of polymer ridge waveguides fabricated on a thin poly(methyl methacrylate) foil via microscope projection photolithography. The developed device is suitable to be integrated into optical circuits, e.g., a planar polymer foil and, along with other optical integrated devices, to be used for different sensing applications. The functionality of the device is demonstrated by using a fiber Bragg grating sensor and performing strain measurements.

Femtosecond laser writing of Bragg grating waveguide bundles in bulk glass.

Waveguide bundles in bulk glass materials, consisting of several parallel scans of refractive index modifications, have been generated with a low-repetition femtosecond laser. Additionally, Bragg grating (BG) structures for 840 and 1550 nm have been introduced by segmentation of the central scan. A spectral loss in the transmission signal of >36 dB was achieved at 1550 nm with a second-order Bragg grating waveguide (BGW) in fused silica, which corresponds to an intrinsic grating efficiency of >16 dB/cm. This is to our knowledge the strongest BG structure realized in glass with a femtosecond laser. The BGW were proven to be stable up to a temperature of 250°C in fused silica. The diameter of the waveguide bundles can be adapted very easily for a broad range of wavelengths and have been demonstrated for diameters between 1 and 50 µm. The transmission properties of the waveguide bundles are affected minorly by the insertion of BG structures, which opens the ability for adjusting the BGW for a broad range of wavelength in single-mode or multimode optical circuits. BGW have been realized successfully in fused silica, borosilicate glass (BOROFLOAT 33), and AF 32 eco Thin Glass from Schott.

Plasmonic nanoshell functionalized etched fiber Bragg gratings for highly sensitive refractive index measurements.

A novel fiber optical refractive index sensor based on gold nanoshells immobilized on the surface of an etched single-mode fiber including a Bragg grating is demonstrated. The nanoparticle coating induces refractive index dependent waveguide losses, because of the variation of the evanescently guided part of the light. Hence the amplitude of the Bragg reflection is highly sensitive to refractive index changes of the surrounding medium. The nanoshell functionalized fiber optical refractive index sensor works in reflectance mode, is suitable for chemical and biochemical sensing, and shows an intensity dependency of 4400% per refractive index unit in the refractive index range between 1.333 and 1.346. Furthermore, the physical length of the sensor is smaller than 3 mm with a diameter of 6 µm, and therefore offers the possibility of a localized refractive index measurement.

Cladding waveguide gratings in standard single-mode fiber for 3D shape sensing.

Femtosecond laser pulses were used for the direct point-by-point inscription of waveguides into the cladding of standard single-mode fibers. Homogeneous S-shaped waveguides have been processed as a bundle of overlapping lines without damaging the surrounding material. Within these structures, FBGs have been successfully inscribed and characterized. A sensor device to measure the bending direction of a fiber was created by two perpendicular inscribed cladding waveguides with FBG. Finally, a complete 3D shape sensor consisting of several bending sensor planes, capable of detecting bending radii even below 2.5 cm is demonstrated.

LED-Absorption-QEPAS Sensor for Biogas Plants.

A new sensor for methane and carbon dioxide concentration measurements in biogas plants is presented. LEDs in the mid infrared spectral region are implemented as low cost light source. The combination of quartz-enhanced photoacoustic spectroscopy with an absorption path leads to a sensor setup suitable for the harsh application environment. The sensor system contains an electronics unit and the two gas sensors; it was designed to work as standalone device and was tested in a biogas plant for several weeks. Gas concentration dependent measurements show a precision better than 1% in a range between 40% and 60% target gas concentration for both sensors. Concentration dependent measurements with different background gases show a considerable decrease in cross sensitivity against the major components of biogas in direct comparison to common absorption based sensors.

Point-by-point inscription of phase-shifted fiber Bragg gratings with electro-optic amplitude modulated femtosecond laser pulses.

Femtosecond laser pulses were used for the direct point-by-point inscription of phase-shifted fiber Bragg gratings (FBGs) in a single fabrication step without postprocessing. An electro-optic amplitude modulator is used in the setup to generate a defined delay between two identical laser pulse trains for the grating inscription. The grating structure with a central phase shift is formed by focusing the modulated laser pulses into the core of a fiber, while the fiber is translated with a constant velocity. The induced phase shift leads to a narrow transmission band with a bandwidth considerably below 10 pm within the stop band of the FBG. The inscribed FBGs show a birefringence of 3.9×10(-5) whereas their temperature and strain sensitivities are 10.4 pm/K and 1.4 pm/µstrain, respectively. The fabrication process is fast and offers a high grade of flexibility for the control of all grating parameters.

Arrayed waveguide grating interrogator for fiber Bragg grating sensors: measurement and simulation.

A fiber Bragg grating (FBG) interrogation system based on an intensity demodulation and demultiplexing of an arrayed waveguide grating (AWG) module is examined in detail. The influence of the spectral line shape of the FBG on the signal obtained from the AWG device is discussed by accomplishing the measurement and simulation of the system. The simulation of the system helps to create quickly and precisely calibration functions for nonsymmetric, tilted, or nonapodized FBGs. Experiments show that even small sidebands of nonapodized FBGs have strong influences on the signal resulted by an AWG device with a Gaussian profile.