Sub-ppm Methane Detection with Mid-Infrared Slot Waveguides.

Hybrid integration of photonic chips with electronic and micromechanical circuits is projected to bring about miniature, but still highly accurate and reliable, laser spectroscopic sensors for both climate research and industrial applications. However, the sensitivity of chip-scale devices has been limited by immature and lossy photonic waveguides, weak light-analyte interaction, and etalon effects from chip facets and defects. Addressing these challenges, we present a nanophotonic waveguide for methane detection at 3270.4 nm delivering a limit of detection of 0.3 ppm, over 2 orders of magnitude lower than the state-of-the-art of on-chip spectroscopy. We achieved this result with a Si slot waveguide designed to maximize the light-analyte interaction, while special double-tip fork couplers at waveguide facets suppress spurious etalon fringes. We also study and discuss the effect of adsorbed humidity on the performance of mid-infrared waveguides around 3 µm, which has been repeatedly overlooked in previous reports.

Performance improvement in a supercontinuum fiber-coupled system for near infrared absorption spectroscopy.

Accurate, in-field-compatible, sensing based on near infrared spectroscopy (NIRS) requires development of instrumentation with low noise and long-term stability. Here, we present a fully fiber-optic spectroscopy setup using a supercontinuum source in the long-pulse regime (2 ns) and a balanced detector scheme to demonstrate high-accuracy NIRS-based sensing. The noise sources of the system are studied theoretically and experimentally. The relative intensity noise was reduced from typical values up to 6% to less than 0.1% by deploying a balanced detector and averaging. At well-balanced wavelengths, the system without transmission cells achieved a signal to noise ratio (SNR) above 70 dB, approaching the shot noise limit. With transmission cells and long-term measurements, the overall SNR was 55 dB. Glucose in physiological concentrations was measured as a model system, yielding a root mean square error of 4.8 mM, approaching the needed accuracy for physiological glucose monitoring.

Multiplexed Mach-Zehnder interferometer assisted ring resonator sensor.

A Mach-Zehnder interferometer assisted ring resonator configuration (MARC)-based multiplexed photonic sensor with a large measurement range is experimentally demonstrated. The presented MARC sensor consists of a balanced Mach-Zehnder interferometer and a ring resonator acting as a sensing component. It produces transmission responses with unique spectral signatures, which depend on the physical angular separation between the through port and the drop port waveguides. These unique spectral signatures enhance the effective free spectral range of ring resonators. Hence MARC sensors with a large measurement range are realized. We experimentally demonstrated that the measurement range from the MARC with 135° angular separation is 8x larger than a standard ring resonator. Moreover, by utilizing the MARC, we distinguished the responses from two and three ring resonators multiplexed together. These results verify proof-of-principle for the MARC-based sensors. This inexpensive compact multipurpose device holds promise for numerous applications.

Micro-Lensed Negative-Curvature Fibre Probe for Raman Spectroscopy.

We developed a novel miniature micro-lensed fibre probe for Raman spectroscopy. The fibre probe consists of a single negative-curvature fibre (NCF) and a spliced, cleaved, micro-lensed fibre cap. Using a single NCF, we minimized the Raman background generated from the silica and maintained the diameter of the probe at less than 0.5 mm. In addition, the cap provided fibre closure by blocking the sample from entering the hollow parts of the fibre, enabling the use of the probe in in vivo applications. Moreover, the micro-lensed cap offered an improved collection efficiency (1.5-times increase) compared to a cleaved end-cap. The sensing capabilities of the micro-lensed probe were demonstrated by measuring different concentrations of glucose in aqueous solutions.

Spectral shaping of ring resonator transmission response.

We present a Mach-Zehnder interferometer assisted ring resonator configuration (MARC) to realize resonator transmission spectra with unique spectral signatures and significantly large effective free spectral ranges. Transmission spectra with unique spectral signatures are generated by changing the angular separation between the through port and the drop port waveguides of the ring resonator (RR). These spectral signatures are comprised of several distinct resonance lineshapes including Lorentzian, inverse Lorentzian and asymmetric Fano-like shapes. One of the spectral signatures generated from the MARC device is utilized for the temperature sensing measurement to demonstrate a MARC-based sensor with high Q-factor and wide measurement range.

Investigation of the effect of clinically relevant interferents on glucose monitoring using near-infrared spectroscopy.

Near infrared spectroscopy (NIR) is a promising technique for continuous blood glucose monitoring for diabetic patients. Four interferents, at physiological concentrations, were introduced to study how the glucose predictions varied with a standard multivariate calibration model. Lactate and ethanol were found to interfere strongly with the glucose predictions unless they were included in the calibration models. Lactate was mistaken for glucose and gave erroneously high glucose predictions, with a dose response of 0.46 mM/mM. The presence of ethanol resulted in too low glucose predictions, with a dose response of -0.43 mM/mM. Acetaminophen, a known interferent in the glucose monitoring devices used for diabetes management today, was not found to be an interferent in NIR spectroscopy, nor was caffeine. Thus, interferents that may appear in high concentrations, such as ethanol and lactate, must be included in the calibration or model building of future NIR-based glucose measurement devices for diabetes monitoring.

Beyond the 2D limit: étendue-squeezing line-focus solar concentrators.

Line-focus solar concentrators are commonly designed by extruding a two-dimensional concentrator in the third dimension. For concentration in air, these concentrators are, by the nature of their design, limited by the two-dimensional solar concentration limit of 212×. This limit is orders of magnitude lower than the 45000× concentration limit for three-dimensional solar concentrators. Through the use of étendue squeezing, we conceptually show that it is possible to design line-focus solar concentrators beyond this 2D limit. This allows a concentrator to benefit from a line focus suitable for heat extraction through a tubular receiver, while reaching concentration ratios and acceptance angles previously unseen for line-focus concentrators. We show two design examples, achieving simulated 75× concentration and 218× concentration ratios, with a ±1∘ acceptance angle. For comparison, the 2D concentration limit is 57× at this acceptance angle. Étendue-squeezing line-focus solar concentrators, combined with recent developments in tracking integration, may enable the development of a new class of concentrated solar power systems.

Infrared measurements of glucose in peritoneal fluid with a tuneable quantum cascade laser.

Fast and accurate continuous glucose monitoring is needed in future systems for control of blood glucose levels in type 1 diabetes patients. Direct spectroscopic measurement of glucose in the peritoneal cavity is an attractive alternative to conventional electrochemical sensors placed subcutaneously. We demonstrate the feasibility of fast glucose measurements in peritoneal fluid using a fibre-coupled tuneable mid-infrared quantum cascade laser. Mid-infrared spectra (1200-925 cm-1) of peritoneal fluid samples from pigs with physiological glucose levels (32-426 mg/dL, or 1.8-23.7 mmol/L) were acquired with a tuneable quantum cascade laser employing both transmission and attenuated total reflection (ATR) spectroscopy. Using partial least-squares regression, glucose concentrations were predicted with mean absolute percentage errors (MAPEs) of 8.7% and 12.2% in the transmission and ATR configurations, respectively. These results show that highly accurate concentration predictions are possible with mid-infrared spectroscopy of peritoneal fluid, and represent a first step towards a miniaturised optical sensor for intraperitoneal continuous glucose monitoring.

Waveguide asymmetric long-period grating couplers as refractive index sensors.

A highly sensitive integrated photonic transducer is designed by utilizing asymmetric long-period gratings on a silicon waveguide. These gratings are formed by periodic perturbation of the waveguide width, leading to coupling between the fundamental mode and the 1st order asymmetric leaky mode. The coupled modes are studied via finite-element and finite-difference time-domain methods. Only a single fabrication step is required to realize this novel design. The device is utilized as a refractive index sensor in liquid, yielding a sensitivity of 5078 nm/RIU. The design is a unique combination of being highly sensitive, easily fabricated and highly compact.

High-performance stationary solar tracking through multi-objective optimization of beam-steering lens arrays.

Beam-steering lens arrays enable solar tracking using millimeter-scale relative translation between a set of lens arrays. This may represent a promising alternative to the mechanical bulk of conventional solar trackers, but until now a thorough exploration of possible configurations has not been carried out. We present an approach for designing beam-steering lens arrays based on multi-objective optimization, quantifying the trade-off between beam divergence and optical efficiency. Using this approach, we screen and optimize a large number of beam-steering lens array configurations, and identify new and promising configurations. We present a design capable of redirecting sunlight into a <2° divergence half-angle, with 73.4% average yearly efficiency, as well as a simplified design achieving 75.4% efficiency with a <3.5° divergence half-angle. These designs indicate the potential of beam-steering lens arrays for enabling low-cost solar tracking for stationary solar concentrators.

Impact of Silanization Parameters and Antibody Immobilization Strategy on Binding Capacity of Photonic Ring Resonators.

Ring resonator-based biosensors have found widespread application as the transducing principle in "lab-on-a-chip" platforms due to their sensitivity, small size and support for multiplexed sensing. Their sensitivity is, however, not inherently selective towards biomarkers, and surface functionalization of the sensors is key in transforming the sensitivity to be specific for a particular biomarker. There is currently no consensus on process parameters for optimized functionalization of these sensors. Moreover, the procedures are typically optimized on flat silicon oxide substrates as test systems prior to applying the procedure to the actual sensor. Here we present what is, to our knowledge, the first comparison of optimization of silanization on flat silicon oxide substrates to results of protein capture on sensors where all parameters of two conjugation protocols are tested on both platforms. The conjugation protocols differed in the chosen silanization solvents and protein immobilization strategy. The data show that selection of acetic acid as the solvent in the silanization step generally yields a higher protein binding capacity for C-reactive protein (CRP) onto anti-CRP functionalized ring resonator sensors than using ethanol as the solvent. Furthermore, using the BS3 linker resulted in more consistent protein binding capacity across the silanization parameters tested. Overall, the data indicate that selection of parameters in the silanization and immobilization protocols harbor potential for improved biosensor binding capacity and should therefore be included as an essential part of the biosensor development process.

Surface-Enhanced Absorption Spectroscopy for Optical Fiber Sensing.

Visible and near-infrared spectroscopy are widely used for sensing applications but suffer from poor signal-to-noise ratios for the detection of compounds with low concentrations. Enhancement by surface plasmon resonance is a popular technique that can be utilized to increase the signal of absorption spectroscopy due to the increased near-field created close to the plasmons. Despite interest in surface-enhanced infrared absorption spectroscopy (SEIRAS), the method is usually applied in lab setups rather than real-life sensing situations. This study aimed to achieve enhanced absorption from plasmons on a fiber-optic probe and thus move closer to applications of SEIRAS. A tapered coreless fiber coated with a 100 nm Au film supported signal enhancement at visible wavelengths. An increase in absorption was shown for two dyes spanning concentrations from 5 × 10-8 mol/L to 8 × 10-4 mol/L: Rhodamine 6G and Crystal Violet. In the presence of the Au film, the absorbance signal was 2-3 times higher than from an identically tapered uncoated fiber. The results confirm that the concept of SEIRAS can be implemented on an optical fiber probe, enabling enhanced signal detection in remote sensing applications.

Infrared Spectroscopy with a Fiber-Coupled Quantum Cascade Laser for Attenuated Total Reflection Measurements Towards Biomedical Applications.

The development of rapid and accurate biomedical laser spectroscopy systems in the mid-infrared has been enabled by the commercial availability of external-cavity quantum cascade lasers (EC-QCLs). EC-QCLs are a preferable alternative to benchtop instruments such as Fourier transform infrared spectrometers for sensor development as they are small and have high spectral power density. They also allow for the investigation of multiple analytes due to their broad tuneability and through the use of multivariate analysis. This article presents an in vitro investigation with two fiber-coupled measurement setups based on attenuated total reflection spectroscopy and direct transmission spectroscopy for sensing. A pulsed EC-QCL (1200-900 cm - 1 ) was used for measurements of glucose and albumin in aqueous solutions, with lactate and urea as interferents. This analyte composition was chosen as an example of a complex aqueous solution with relevance for biomedical sensors. Glucose concentrations were determined in both setup types with root-mean-square error of cross-validation (RMSECV) of less than 20 mg/dL using partial least-squares (PLS) regression. These results demonstrate accurate analyte measurements, and are promising for further development of fiber-coupled, miniaturised in vivo sensors based on mid-infrared spectroscopy.

Glucose sensing by absorption spectroscopy using lensed optical fibers.

The bulkiness of common transmission spectroscopy probes prevents applicability at remote locations such as within the body. We present the fabrication and characterization of lensed fibers for transmission spectroscopy in the near-infrared. Eigenmode simulations and measurements of the coupling efficiency are presented and applied to design the setup corresponding to the sample absorption. Sensing capabilities are demonstrated on aqueous glucose samples ranged 80 to 500 mM, obtaining a mean absolute percentage error of calibration of 4.3%. With increased flexibility, transmission spectroscopic sensors at remote locations may be achievable, for example, applied to in vivo continuous glucose monitoring.

Micro-lensed optical fibers for a surface-enhanced Raman scattering sensing probe.

We present the fabrication and characterization of a novel sensing configuration based on surface-enhanced Raman scattering (SERS) and two micro-lensed optical fibers. The first micro-lensed fiber is used to excite surface plasmon resonance in a gold film deposited over a mono-layer of nano-sphere surface (AuFON), and the second lensed fiber is used to collect the SERS signal. The sensing capabilities of the fabricated device are demonstrated by measuring different concentrations of Rhodamine 6G in a water solution.

Heterodyne interferometer for absolute amplitude vibration measurements with femtometer sensitivity.

A heterodyne interferometer for highly sensitive vibration measurements in the range 100 kHz - 1.3GHz is presented. The interferometer measures absolute amplitude and phase. The signal processing of the setup is analyzed and described in detail to optimize noise suppression. A noise floor of 7.1 fm/Hz(1/2) at 21 MHz was achieved experimentally where the bandwidth is the inverse of all time needed for filter settling and signal sampling. To demonstrate the interferometer, measurements up to 220 MHz were performed on arrays of capacitive micromachined ultrasonic transducers (CMUTs). The measurements provided detailed information e.g. about the frequency response, vibration patterns and array uniformity. Such measurements are highly valuable in the design process of ultrasonic transducers.

Speckle contrast of the sum of N partially correlated speckle patterns.

In this paper a general method is presented for calculating the theoretical speckle contrast of a sum of correlated speckle patterns, motivated by the need to suppress the presence of speckle in laser projection displays. The method is applied to a specific example, where correlated speckle patterns are created by sequentially passing light through partially overlapping areas on a diffuser, before being projected onto a screen. This design makes it possible to find a simple expression for the correlation between speckle patterns. When the set of correlations involves symmetry, it is shown that the expression for the speckle contrast becomes simpler. The difference in performance between discretely and continuously varying speckle patterns is also investigated. In an example with speckle reduction by a rotating sinusoidal grating, it is found that continuous variation gives a speckle contrast that is 0.61 times the contrast obtained by discretely summing the maximum number of independent patterns.

Hyperspectral imaging of atherosclerotic plaques in vitro.

Vulnerable plaques constitute a risk for serious heart problems, and are difficult to identify using existing methods. Hyperspectral imaging combines spectral- and spatial information, providing new possibilities for precise optical characterization of atherosclerotic lesions. Hyperspectral data were collected from excised aorta samples (n = 11) using both white-light and ultraviolet illumination. Single lesions (n = 42) were chosen for further investigation, and classified according to histological findings. The corresponding hyperspectral images were characterized using statistical image analysis tools (minimum noise fraction, K-means clustering, principal component analysis) and evaluation of reflectance/fluorescence spectra. Image analysis combined with histology revealed the complexity and heterogeneity of aortic plaques. Plaque features such as lipids and calcifications could be identified from the hyperspectral images. Most of the advanced lesions had a central region surrounded by an outer rim or shoulder-region of the plaque, which is considered a weak spot in vulnerable lesions. These features could be identified in both the white-light and fluorescence data. Hyperspectral imaging was shown to be a promising tool for detection and characterization of advanced atherosclerotic plaques in vitro. Hyperspectral imaging provides more diagnostic information about the heterogeneity of the lesions than conventional single point spectroscopic measurements.

CMUT characterization by interferometric and electric measurements.

Capacitive micromachined ultrasonic transducers (CMUTs) with 5.7 mum radius, realized by wafer bonding, have been characterized by both optical and electrical measurements. These measurements are performed by our heterodyne interferometer and a network analyzer, respectively. The results from this article will be used to improve the fabrication of next-generation CMUTs. We have investigated the spread in resonance frequency of different CMUT membranes along the array. Q-factors have been obtained using both measurement methods, and the results from the 2 methods have been numerically compared. The relation between applied dc voltage and resonance frequency has been studied. Temperature measurements show that increasing temperature leads to a small decrease in the resonance frequency of the CMUTs; however, the decrease is small enough to ensure stable operation with small variations in room temperature. The heterodyne interferometer is used to inspect the vibration pattern of the CMUTs' higher harmonic modes. These modes are located at approximately 60 MHz in air. To the authors' knowledge, vibration modes at frequencies >40 MHz have not been previously studied.

Wide frequency range measurements of absolute phase and amplitude of vibrations in micro- and nanostructures by optical interferometry.

A heterodyne interferometer has been built in order to characterize vibrations on micro- and nanostructures. The interferometer offers the possibility of both absolute phase and high resolution absolute amplitude vibrational measurements. By using two acousto-optic modulators (AOMs) in one of the interferometer arms and varying the frequency inputs of both, the setup is designed to measure vibrations in the entire frequency range 0 - 1.2GHz. The system is here demonstrated on Capacitor Micromachined Ultrasonic Transducers (CMUTs) and a PZT transducer to show measurements from 5kHz up to 35MHz. We have measured absolute amplitudes with picometer resolution.