Optical refractive index of air: dependence on pressure, temperature and composition

Evaluation of atmospheric coherent length of free-space optical links by using phase fluctuation

Atmospheric coherence length is one of the most crucial parameters for free-space optical (FSO) links, which can reflect the level of phase and amplitude fluctuations caused by the atmospheric turbulence. In this paper, we study the evaluation of the atmospheric coherence length of the FSO links. The analytical expression of atmospheric coherence length is rendered based on the phase fluctuation resulting from atmospheric turbulence by using the most realistic Bump model. The proposed method is validated theoretically with the Monte Carlo phase screen. Also, the experimental setup with respect to FSO links is established with the spatial light modulator to validate the method experimentally, wherein the fluctuated phase is collected by Shack-Hartmann sensor. The results show that the evaluation of atmospheric coherence length by the analytical expression is consistent with the theoretical prediction as well as the experimental measurement. Thus, the proposed method enables the accurate evaluation of atmospheric coherence length under various turbulence conditions, which can assist the performance analysis as well as design of free-space optical communication systems.

Synthetic dispersion interferometry for relative atmospheric pressure sensing

We present a modified version of the two-arm, two-color, single second harmonic generation heterodyne dispersion interferometer, as introduced by Irby et. al. [Rev. Sci. Instrum.70, 699 (1999) 10.1063/1.1149489]. The amount of optical elements is reduced and digital in-phase and quadrature demodulation is used to retrieve the phase shift from a single photodetector signal. The intrinsic system noise and drift for this device are analyzed by measuring the Allan deviation. We investigate the use of this device for relative atmospheric pressure measurement. Relative pressure measurements are performed in a pressure chamber and referenced against a piezoresistive pressure transceiver to demonstrate the concept. It was found that the deviation was less than 150 Pa and an error estimation has been derived.

Terahertz Nondestructive Testing with Ultra-Wideband FMCW Radar

This paper presents the development, performance, integration, and implementation of a 150 GHz FMCW radar based on a homodyne harmonic mixing scheme for noncontact, nondestructive testing. This system offers high-dynamic-range measurement capabilities up to 100 dB and measurement rates up to 7.62 kHz. Such interesting characteristics make this system attractive for imaging applications or contactless sensing. Numerous samples of different materials and geometries were imaged by taking advantage of the radar's performance. By taking into account the nonionizing capability of the system, new applicative fields such as food industry and pharmaceutical packaging were explored.

Filamentation in low pressure conditions

Filamentation is favorable for many long-range outdoor laser applications, some of which require propagation to or at high altitudes. Understanding how the filamentation process and filament properties are impacted by the low pressure conditions present at high altitudes is essential in designing effective applications. The scaling of filament preconditions with pressure is considered. An increase in critical power and decrease in transition numerical aperture (NA) is predicted to occur with a drop in pressure, indicating that nonlinear pulse propagation and filamentation at high altitudes requires higher energy and a longer assisted focal length than sea level filamentation. A summary of pressure-scaled filament properties is also presented. New simulations demonstrate filamentation at pressures as low as 0.0035 atm (38.5 km altitude) is possible.

Flow-Enhanced Photothermal Spectroscopy

Photothermal spectroscopy (PTS) is a promising sensing technique for the measurement of gases and aerosols. PTS systems using a Fabry-Pérot interferometer (FPI) are considered particularly promising owing to their robustness and potential for miniaturization. However, limited information is available on viable procedures for signal improvement through parameter tuning. In our work, we use an FPI-based PTS configuration, in which the excitation laser irradiates the target collinearly to the flowing gas. We demonstrate that the generated thermal wave, and thus the signal intensity, is significantly affected by the ratio between excitation modulation frequency and gas flow velocity towards another. We provide an analytical model that predicts the signal intensity with particular considerations of these two parameter settings and validate the findings experimentally. The results reveal the existence of an optimal working regime, depending on the modulation frequency and flow velocity.

Real-Time Monitoring of Gas-Phase and Dissolved CO2 Using a Mixed-Matrix Composite Integrated Fiber Optic Sensor for Carbon Storage Application

Novel chemical sensors that improve detection and quantification of CO₂ are critical to ensuring safe and cost-effective monitoring of carbon storage sites. Fiber optic (FO)-based chemical sensor systems are promising field-deployable systems for real-time monitoring of CO₂ in geological formations for long-range distributed sensing. In this work, a mixed-matrix composite integrated FO sensor system was developed with a purely optical readout that reliably operates as a detector for gas-phase and dissolved CO₂. A mixed-matrix composite sensor coating consisting of plasmonic nanocrystals and hydrophobic zeolite embedded in a polymer matrix was integrated on the FO sensor. The mixed-matrix composite FO sensor showed excellent reversibility/stability in a high humidity environment and sensitivity to gas-phase CO₂ over a large concentration range. This remarkable sensing performance was enabled by using plasmonic nanocrystals to significantly enhance the sensitivity and a hydrophobic zeolite to effectively mitigate interference from water vapor. The sensor exhibited the ability to sense CO₂ in the presence of other geologically relevant gases, which is of importance for applications in geological formations. A prototype FO sensor configuration, which possesses a robust sensing capability for monitoring dissolved CO₂ in natural water, was demonstrated. Reproducibility was confirmed over many cycles, both in a laboratory setting and in the field. More importantly, we demonstrated on-line monitoring capabilities with a wireless telemetry system, which transferred the data from the field to a website. The combination of outstanding CO₂ sensing properties and facile coating processability makes this mixed-matrix composite FO sensor a good candidate for practical carbon storage applications.

A Review of Computer Vision-Based Structural Deformation Monitoring in Field Environments

Computer vision-based structural deformation monitoring techniques were studied in a large number of applications in the field of structural health monitoring (SHM). Numerous laboratory tests and short-term field applications contributed to the formation of the basic framework of computer vision deformation monitoring systems towards developing long-term stable monitoring in field environments. The major contribution of this paper was to analyze the influence mechanism of the measuring accuracy of computer vision deformation monitoring systems from two perspectives, the physical impact, and target tracking algorithm impact, and provide the existing solutions. Physical impact included the hardware impact and the environmental impact, while the target tracking algorithm impact included image preprocessing, measurement efficiency and accuracy. The applicability and limitations of computer vision monitoring algorithms were summarized.

Integration of a Metal-Organic Framework Film with a Tubular Whispering-Gallery-Mode Microcavity for Effective CO2 Sensing

Carbon dioxide (CO₂) sensing using an optical technique is of great importance in the environment and industrial emission monitoring. However, limited by the poor specific adsorption of gas molecules as well as insufficient coupling efficiency, there is still a long way to go toward realizing a highly sensitive optical CO₂ gas sensor. Herein, by combining the advantages of a whispering-gallery-mode microcavity and a metal-organic framework (MOF) film, a porous functional microcavity (PF-MC) was fabricated with the assistance of the atomic layer deposition technique and was applied to CO₂ sensing. In this functional composite, the rolled-up microcavity provides the ability to tune the propagation of light waves and the electromagnetic coupling with the surroundings via an evanescent field, while the nanoporous MOF film contributes to the specific adsorption of CO₂. The composite demonstrates a high sensitivity of 188 nm RIU^-1 (7.4 pm/% with respect to the CO₂ concentration) and a low detection limit of ∼5.85 × 10^-5 RIU. Furthermore, the PF-MC exhibits great selectivity to CO₂ and outstanding reproducibility, which is promising for the next-generation optical gas sensing devices.

Hysteresis and temperature drift compensation for FBG demodulation by utilizing adaptive weight least square support vector regression

Hysteresis and temperature drift deteriorate the demodulation performance of tunable Fabry-Perot (F-P) filters. This study addresses a novel adaptive weight least square support vector regression (AWLSSVR) to compensate for the hysteresis and temperature drift of F-P filters. The temperature drift of a referent fiber Bragg grating(FBG) is used to estimate the temperature drifts of other three sensing FBGs, and a novel adaptive weighting strategy with an asymmetric noise interval is proposed, to eliminate the effects of noise in the training dataset. The experimental results show that when the temperature-changing modes of the training and testing datasets were close to each other, the error of the proposed method is reduced to 8.7 pm, while the errors of the other three conventional methods based on LSSVR are more than 10.8 pm. Further, when the temperature-changing modes of the training and testing datasets were partly different, the error of the proposed method was reduced to 5.4 pm, while the errors of other methods were more than 11.9 pm. It was verified experimentally that the proposed AWLSSVR method is more accurate and robust than other versions of WLSSVR for training samples with noise, requires no additional hardware, and covers the entire C band.

Millisecond-long suppression of spectroscopic optical signals using laser filamentation

Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.

Dual trace inter-pulse interferometer for measurement of phase stability of ultra short laser pulse train

A dual trace intra-pulse and inter-pulse spatio-spectral interferometer has been set up to study the temporal stability of a ∼200 fs duration laser pulse train from a cw mode-locked laser oscillator. Simultaneous recording of twin interferograms helps identify the phase error in inter-pulse interferograms due to the diagnostic setup kept in a standard laboratory environment. Applicability of inter-pulse tilted pulse-front interferograms has been demonstrated to constitute an alternative inexpensive method for visual detection and estimation of phase slippage and pulse repetition frequency of an ultra short pulse train. The effect of pump beam intensity on the repetition rate of pulses due to accumulated intra-cavity non-linear phase shifts is also presented.

Mid-IR refractive index sensor for detecting proteins employing an external cavity quantum cascade laser-based Mach-Zehnder interferometer

Novel laser light sources in the mid-infrared region enable new spectroscopy schemes beyond classical absorption spectroscopy. Herein, we introduce a refractive index sensor based on a Mach-Zehnder interferometer and an external-cavity quantum cascade laser that allows rapid acquisition of high-resolution spectra of liquid-phase samples, sensitive to relative refractive index changes down to 10^-7. Dispersion spectra of three model proteins in deuterated solution were recorded at concentrations as low as 0.25 mg mL^-1. Comparison with Kramers-Kronig-transformed Fourier transform infrared absorbance spectra revealed high conformance, and obtained figures of merit compare well with conventional high-end FTIR spectroscopy. Finally, we performed partial least squares-based multivariate analysis of a complex ternary protein mixture to showcase the potential of dispersion spectroscopy utilizing the developed sensor to tackle complex analytical problems. The results indicate that laser-based dispersion sensing can be successfully used for qualitative and quantitative analysis of proteins.

Reconfigurable polymer-templated liquid crystal holographic gratings via visible-light recording

Polymer-templated nematic liquid crystal (LC) holographic gratings via visible-light recording are presented in the presence of reactive mesogens (RMs) and rose bengal (RB)/N-phenylglycine (NPG) photoinitiation systems. By optimizing the concentration of RMs in the polymer-templated LC gratings, the template after being washed out can be refilled with suitable fluidic components. And the dependence of the first-order diffraction efficiency (DE) on the concentration of RB and NPG molecules was discussed in detail. The polarization-dependency of diffraction properties was also investigated. It is revealed that the diffractive behaviors of polymer-templated LC gratings can be dynamically reconfigured by varying temperature or refilling organic solutions with different refractive index (RI). Furthermore, the potential for recording holograms using green light is explored. We expect that the reconfigurable polymer-templated LC gratings fabricated via visible-light interference would provide a facile approach to regulate the diffraction properties of holographic gratings apart from electric field, thus paving a way towards a class of novel anti-counterfeiting devices.

Ultrasensitive Gas Refractometer Using Capillary-Based Mach-Zehnder Interferometer

In this paper, we report a capillary-based Mach–Zehnder (M–Z) interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The sensing mechanism is quite straightforward. Cladding and core modes of a capillary are simultaneously excited by coupling coherent laser beams to the capillary cladding and core, respectively. An interferogram would be generated as the light transmitted from the core interferes with the light transmitted from the cladding. Variations in the refractive index of the air filling the core lead to variations in the phase difference between the core and cladding modes, thus shifting the interference fringes. Using a photodiode together with a narrow slit, we could interrogate the fringe shifts. The resolution of the sensor was found to be ~5.7 × 10⁻⁸ RIU (refractive index unit), which is comparable to the highest resolution obtained by other interferometric sensors reported in previous studies. Finally, we also analyze the temperature cross sensitivity of the sensor. The main goal of this paper is to demonstrate that the ultra-sensitive sensing of gas refractive index could be realized by simply using a single capillary fiber rather than some complex fiber-optic devices such as photonic crystal fibers or other fiber-optic devices fabricated via tricky fiber processing techniques. This capillary sensor, while featuring an ultrahigh resolution, has many other advantages such as simple structure, ease of fabrication, straightforward sensing principle, and low cost.

Membrane-free acoustic sensing based on an optical fiber Mach-Zehnder interferometer

Traditional optical fiber acoustic sensors are mostly based on mechanical diaphragms and use indirect coupling between the acoustic and optical signals. The detectable frequency range and sound pressure range of such a sensor have limitations because they are influenced by the membrane or a mechanically deformable material. In this paper, a Mach-Zehnder interferometer-based membrane-free acoustic sensing method is developed. The sensing principle relies on direct detection of sound-pressure-induced changes of the refractive index in the open cavity. This enables an inherently flat frequency response over a broad bandwidth. Simulation and experiment were carried out to verify and demonstrate the idea. The results show that the membrane-free acoustic sensor has a flat frequency response from 500 Hz to 20 kHz.

The Performance of Different Mapping Functions and Gradient Models in the Determination of Slant Tropospheric Delay

Global navigation satellite systems (GNSSs) have become an important tool for remotely sensing water vapor in the atmosphere. In GNSS data processing, mapping functions and gradient models are needed to map the zenith tropospheric delay (ZTD) to the slant total tropospheric delay (STD) along a signal path. Therefore, it is essential to investigate the spatial–temporal performance of various mapping functions and gradient models in the determination of STD. In this study, the STDs at nine elevations were first calculated by applying the ray-tracing method to the atmospheric European Reanalysis-Interim (ERA—Interim) dataset. These STDs were then used as the reference to study the accuracy of the STDs that determined the ZTD together with mapping functions and gradient models. The performance of three mapping functions (i.e., Niell mapping function (NMF), global mapping function (GMF), and Vienna mapping function (VMF1)) and three gradient models (i.e., Chen, MacMillan, and Meindl) in six regions (the temperate zone, Qinghai–Tibet Plateau, Equator, Sahara Desert, Amazon Rainforest, and North Pole) in determining slant tropospheric delay was investigated in this study. The results indicate that the three mapping functions have relatively similar performance above a 15° elevation, but below a 15° elevation, VMF1 clearly performed better than the GMF and NMF. The results also show that, if no gradient model is included, the root-mean-square (RMS) of the STD is smaller than 2 mm above the 30° elevation and smaller than 9 mm above the 15° elevation but shows a significant increase below the 15° elevation. For example, in the temperate zone, the RMS increases from approximately 35 mm at the 10° elevation to approximately 160 mm at the 3° elevation. The inclusion of gradient models can significantly improve the accuracy of STDs by 50%. All three gradient models performed similarly at all elevations and in all regions. The bending effect was also investigated, and the results indicate that the tropospheric delay caused by the bending effect is normally below 13 mm above a 15° elevation, but this delay increases dramatically from approximately 40 mm at a 10° elevation to approximately 200 mm at a 5° elevation, and even reaches 500–700 mm at a 3° elevation in most studied regions.

Research on the Control Point Extraction Method of Chinese Gaofen 7 Satellite Using Altimeter and Footprint Camera

Satellite laser altimetry is one of the most advanced information acquisition technologies in Earth observation system. It can provide high-accuracy elevation information, however, due to the lack of detail intensity information, its planimetric accuracy is usually worse than the elevation accuracy. Gaofen 7 (GF-7) satellite scheduled for launch in 2019 will be equipped with laser altimeter, footprint camera, stereo mapping camera, etc. The laser altimeter together with the footprint camera was designed to provide high accuracy ground control point of satellite mapping. The laser altimeter can provide the high-accuracy elevation information and the joint processing of footprint camera and stereo mapping camera can provide high-accuracy planimetric information. Therefore, this paper mainly studies the technology of extracting high-accuracy control points based on GF-7 satellite’s altimeter, footprint camera and stereo mapping camera using a simulated dataset extracted from Quickbird image and ICESat altimetric data.

Effects of self-phase modulation (SPM) on femtosecond coherent anti-Stokes Raman scattering spectroscopy

The effects of self-phase modulation (SPM) on the power spectra of femtosecond (fs) pulses and the consequent impact on N₂ chirped-probe-pulse (CPP) fs coherent anti-Stokes Raman scattering (CARS) spectra are discussed in this paper. We investigated the pressure dependence of CPP fs CARS for N₂ in a room-temperature gas cell at pressures ranging from 1 to 10 bar, and in our initial experiments the CPP fs CARS spectrum changed drastically as the pressure increased. We found that the spectra of the near-Fourier-transform-limited, 60-fs pump and Stokes pulses at the exit of the gas cell changed drastically as the pressure increased due to self-phase-modulation (SPM). This effect was examined in detail in further experiments where the pulse energies of the pump and Stokes pulses were controlled using a combination of a half-wave plate and a linear polarizer. Along with the generated CARS spectrum, the spectra of pump and Stokes pulses were measured at the entrance and exit of the gas cell. The extent of SPM effects for a particular spectrum was characterized by the least squares difference between that spectrum and a spectrum recorded at low enough pressure and laser intensities that SPM was negligible. SPM effects were investigated for N₂, O₂, CO₂, and CH₄, for pressures ranging from 1 to 10 bar, and for pump and Stokes pulse energies ranging from 10 to 60 µJ. We found that SPM effects in N₂ were much weaker than for O₂, CO₂ and CH₄.

On-chip simultaneous sensing of humidity and temperature with a dual-polarization silicon microring resonator

It is still challenging to realize an on-chip optical sensor that can detect humidity and temperature at the same time. In this paper, we demonstrate a silicon-based dual-polarization micro-ring resonator (MRR) with a polyvinyl-alcohol (PVA) upper-cladding, which enables the simultaneous measurement of humidity and temperature. Due to the significant polarization-dependence of the silicon-on-insulator (SOI) nanophotonic waveguide, the transverse electric (TE) and transverse magnetic (TM) polarization modes have quite different sensitivities towards the changes of ambient relative humidity (RH) and temperature. Sensitivity, resolution, stability and cross-sensitivity are analyzed for the present dual-parameter sensor. The RH and temperature response sensitivity are measured to be 97.9 pm/%RH, 325.1 pm/%RH, 69.0 pm/°C and 30.6 pm/°C for TE and TM polarizations, respectively. To the best of our knowledge, it is the first on-chip optical sensor enabling the simultaneous measurement of RH and temperature.

Demonstration of opposing thermal sensitivities in hollow-core fibers with open and sealed ends

The output phase and propagation time of an optical signal propagating through a hollow-core optical fiber (HCF) drift with changes in environmental temperature significantly less than in conventional optical fibers. In all earlier experimental studies, however, the simplifying assumption was made that the thermo-optic effect of air was negligible. In this Letter, we present, to the best of our knowledge, the first experimental demonstration that the air inside a HCF core can make an appreciable contribution to the fiber's thermal sensitivity with the performance depending on whether the fiber is open to the atmosphere or sealed at both ends (e.g., spliced to solid fiber pigtails). We measure both the sensitivity of the accumulated phase as well as the signal propagation time for both open and sealed HCF and show that these are opposite in sign. Most importantly, we show that the thermal sensitivity contribution from the air inside an open HCF has the sign opposite to the effect of fiber elongation (which is otherwise the dominant effect responsible for the overall thermal sensitivity of HCF). We then go on to show that these two effects can be used to balance each other out in order to achieve zero thermal sensitivity for both accumulated phase and propagation time. We demonstrate this property experimentally over a large spectral range.

Malaria-Infected Red Blood Cell Analysis through Optical and Biochemical Parameters Using the Transport of Intensity Equation and the Microscope's Optical Properties

The accuracy, reliability, speed and cost of the methods used for malaria diagnosis are key to the diseases' treatment and eventual eradication. However, improvement in any one of these requirements can lead to deterioration of the rest due to their interdependence. We propose an optical method that provides fast detection of malaria-infected red blood cells (RBCs) at a lower cost. The method is based on the combination of deconvolution, topography and three-dimensional (3D) refractive index reconstruction of the malaria-infected RBCs by use of the transport of intensity equation. Using our method, healthy RBCs were identified by their biconcave shape, quasi-uniform spatial distribution of their refractive indices and quasi-uniform concentration of hemoglobin. The values of these optical and biochemical parameters were found to be in agreement with the values reported in the literature. Results for the malaria-infected RBCs were significantly different from those of the healthy RBCs. The topography of the cells and their optical and biochemical parameters enabled identification of their stages of infection. This work introduces a significant method of analyzing malaria-infected RBCs at a lower cost and without the use of fluorescent labels for the parasites.

Scalar potential reconstruction method of axisymmetric 3D refractive index fields with background-oriented schlieren

Deflection angles of light rays passing through a refractive index field can be measured by the background-oriented schlieren (BOS) technique. Assuming that the deflection angle is sufficiently small and the paraxial approximation can apply to the light rays, a vector consisting of deflection angles in two orthogonal directions is shown to be derived from a gradient of a scalar potential. The scalar potential can be written as an integration of the refractive index field over the light ray path. Thus, a method to reconstruct an axisymmetric 3D refractive index field with the scalar potential is proposed here. An arbitrary measured deflection angle vector, however, is generally written not only with a scalar potential but with a vector potential. Thus, the Poisson's equation is derived to extract a scalar potential from a measured deflection angle vector. The axisymmetric 3D refractive index field is able to be reconstructed using the Abel transformation [1] of the scalar potential derived by applying the 2D Fourier transformation to the Poisson's equation. The scalar potential reconstruction method is validated by reconstructing a spherically symmetric refractive index field where a deflection angle vector field is able to be calculated accurately.

Ray tracing calculations in simulated propellant flames with detailed chemistry

Optical measurements in propellant flames are necessary to understand the combustion physics, yet these conditions provide challenges in probing the flame and may introduce uncertainty into the measurement. This work reports the use of simulations of an ammonium perchlorate propellant flame with finite rate chemistry to understand the role of ammonium perchlorate particle size and pressure on the uncertainty of imaging-based measurements on propellant flames. A two-dimensional ray tracing code was developed to incorporate the effects of the species concentration and temperature gradients on ray refraction within propellant flames. It was determined that the effects of the flame structure based upon pressure and oxidizer particle size increases the amount of ray deflection particularly at high pressures explaining a cause for challenges of aluminum agglomerate measurements at elevated pressure. This framework shows promise for understanding limitations and uncertainties of optical measurements for reacting and turbulent flows.

Reconstruction method of axisymmetric refractive index fields with background-oriented schlieren

A refractive index field can be quantitatively visualized by using a background-oriented schlieren (BOS) technique to measure the displacements of background dot patterns of a 2D image captured by a camera. In BOS, the displacement is caused by the deflection of the light ray passing through the refractive index field. However, obtaining depth information of refractive index fields is often difficult because dot-pattern displacement results from deflection of light rays integrated over their path. A method of reconstructing depth information of the refractive index field using an inverse matrix method with the BOS technique is proposed here under the assumption of an axisymmetric refractive index field. A field, generated by laser irradiation inside a glass, is reconstructed by the proposed method and is also theoretically calculated using the heat diffusion equation and thermal stress theory. The reconstructed field agrees well with the theoretically calculated one, which validates the proposed method. The proposed method is a promising candidate for measuring axisymmetric refractive index fields of various media.

A pressure-tuned Fabry Pérot interferometer for laser frequency stabilization and tuning

We present a simple, inexpensive pressure-tuned Fabry Pérot interferometer which can be used to frequency stabilize and tune a laser when no suitable atomic reference is available. Our design, made largely from off-the-shelf parts, yields a tuning range of about 4 GHz and offers an absolute tuning accuracy of better than 1 MHz. The interferometer, which uses air as its working medium, is characterized by a low thermal drift rate of order 1 MHz per hour.

Metal-Organic Framework Thin Film Coated Optical Fiber Sensors: A Novel Waveguide-Based Chemical Sensing Platform

Integration of optical fiber with sensitive thin films offers great potential for the realization of novel chemical sensing platforms. In this study, we present a simple design strategy and high performance of nanoporous metal-organic framework (MOF) based optical gas sensors, which enables detection of a wide range of concentrations of small molecules based upon extremely small differences in refractive indices as a function of analyte adsorption within the MOF framework. Thin and compact MOF films can be uniformly formed and tightly bound on the surface of etched optical fiber through a simple solution method which is critical for manufacturability of MOF-based sensor devices. The resulting sensors show high sensitivity/selectivity to CO₂ gas relative to other small gases (H₂, N₂, O₂, and CO) with rapid (<tens of seconds) response time and excellent reversibility, which can be well correlated to the physisorption of gases into a nanoporous MOF. We propose a refractive index based sensing mechanism for the MOF-integrated optical fiber platform which results in an amplification of inherent optical absorption present within the MOF-based sensing layer with increasing values of effective refractive index associated with adsorption of gases.

Air wedge with variable refractive index for precise laser beam steering in a small range

This paper provides the results of work on a new method for metrological laser beam steering for the purpose of angular stabilization. Frequency stabilized metrological lasers are used for interference measurements of length and angle. Angular deviation of the beam may cause cosine errors when measuring displacements. This paper presents a method for metrological laser beam deflection with an air wedge of variable refractive index. The change in the index is achieved by changing the air pressure in the wedge volume. The implemented system produced a range of angular displacements of the 0.1 mrad (10^-4  rad; 100 μrad) beam with a resolution of 10^-7  rad.

Temperature dependence of Brewster's angle

In this work, a dielectric at a finite temperature is modeled as an ensemble of identical atoms moving randomly around where they are trapped. Light reflection from the dielectric is then discussed in terms of atomic radiation. Specific calculation demonstrates that because of the atoms' thermal motion, Brewster's angle is, in principle, temperature-dependent, and the dependence is weak in the low-temperature limit. What is also found is that the Brewster's angle is nothing but a result of destructive superposition of electromagnetic radiation from the atoms.

Optical thermal sensor based on cholesteric film refilled with mixture of toluene and ethanol

We demonstrate an optical thermal sensor based on cholesteric film refilled with mixture of toluene and ethanol. The thermal response mechanism is mainly based on the thermal expansion effect induce by toluene, where the ethanol is used for refractive index adjustment to determine the initial refection band position of cholesteric film. The ethanol-toluene mixture was used to adjust the color tunability with the temperature in relation with the habits of people (blue as cold, green as safe and red as hot). A broad temperature range of 86 °C and highly sensitivity of 1.79 nm/ °C are achieved in proposed thermal sensor, where the reflective color red-shifts from blue to red when environmental temperature increases from -6 °C to 80 °C. This battery-free thermal sensor possesses features including simple fabrication, low-cost, and broad temperature sensing range, showing potential application in scientific research and industry.

Thin SnOx films for surface plasmon resonance enhanced ellipsometric gas sensing (SPREE)

Background: Gas sensors are very important in several fields like gas monitoring, safety and environmental applications. In this approach, a new gas sensing concept is investigated which combines the powerful adsorption probability of metal oxide conductive sensors (MOS) with an optical ellipsometric readout. This concept shows promising results to solve the problems of cross sensitivity of the MOS concept. Results: Undoped tin oxide (SnOx) and iron doped tin oxide (Fe:SnOx) thin add-on films were prepared by magnetron sputtering on the top of the actual surface plasmon resonance (SPR) sensing gold layer. The films were tested for their sensitivity to several gas species in the surface plasmon resonance enhanced (SPREE) gas measurement. It was found that the undoped tin oxide (SnOx) shows higher sensitivities to propane (C3H8) then to carbon monoxide (CO). By using Fe:SnOx, this relation is inverted. This behavior was explained by a change of the amount of binding sites for CO in the layer due to this iron doping. For hydrogen (H2) no such relation was found but the sensing ability was identical for both layer materials. This observation was related to a different sensing mechanism for H2 which is driven by the diffusion into the layer instead of adsorption on the surface. Conclusion: The gas sensing selectivity can be enhanced by tuning the properties of the thin film overcoating. A relation of the binding sites in the doped and undoped SnOx films and the gas sensing abilities for CO and C3H8 was found. This could open the path for optimized gas sensing devices with different coated SPREE sensors.

Hybrid Photonic Cavity with Metal-Organic Framework Coatings for the Ultra-Sensitive Detection of Volatile Organic Compounds with High Immunity to Humidity

Detection of volatile organic compounds (VOCs) at parts-per-billion (ppb) level is one of the most challenging tasks for miniature gas sensors because of the high requirement on sensitivity and the possible interference from moisture. Herein, for the first time, we present a novel platform based on a hybrid photonic cavity with metal-organic framework (MOF) coatings for VOCs detection. We have fabricated a compact gas sensor with detection limitation ranging from 29 to 99 ppb for various VOCs including styrene, toluene, benzene, propylene and methanol. Compared to the photonic cavity without coating, the MOF-coated solution exhibits a sensitivity enhancement factor up to 1000. The present results have demonstrated great potential of MOF-coated photonic resonators in miniaturized gas sensing applications.

Hierarchical photonic structured stimuli-responsive materials as high-performance colorimetric sensors

Hierarchical photonic structures in nature are of special interest because they can be used as templates for fabrication of stimuli-responsive photonic crystals (PCs) with unique structures beyond man-made synthesis. The current stimuli-responsive PCs templated directly from natural PCs showed a very weak external stimuli response and poor durability due to the limitations of natural templates. Herein, we tackle this problem by chemically coating functional polymers, polyacrylamide, on butterfly wing scales which have hierarchical photonic structures. As a result of the combination of the strong water absorption properties of the polyacrylamide and the PC structures of the butterfly wing scales, the designed materials demonstrated excellent humidity responsive properties and a tremendous colour change. The colour change is induced by the refractive index change which is in turn due to the swollen nature of the polymer when the relative humidity changes. The butterfly wing scales also showed an excellent durability which is due to the chemical bonds formed between the polymer and wing scales. The synthesis strategy provides an avenue for the promising applications of stimuli-responsive PCs with hierarchical structures.

Higher order correlation beams in atmosphere under strong turbulence conditions

Higher order correlation beams, that is, two-photon beams obtained from the process of spontaneous parametric down-conversion pumped by Hermite-Gauss or Laguerre-Gauss beams of any order, can be used to encode information in many modes, opening the possibility of quantum communication with large alphabets. In this paper we calculate, analytically, the fourth-order correlation function for the Hermite-Gauss and Laguerre-Gauss coherent and partially coherent correlation beams propagating through a strong turbulent medium. We show that fourth-order correlation functions for correlation beams have, under certain conditions, expressions similar to those of intensities of classical beams and are degraded by turbulence in a similar way as the classical beams. Our results can be useful in establishing limits for the use of two-photon beams in quantum communications with larger alphabets under atmospheric turbulence.

Performance analysis of the retractable dome for the Chinese Large Telescope

In order to quantitatively assess the influence of the retractable dome on the observational performance of the 4-m Chinese Large Telescope (CLT), an integrated analysis method based on computational fluid dynamics (CFD) and sub-harmonic phase screen is proposed in this paper. The pressure, the temperature, and the speed of air surrounding the retractable dome are attained by CFD simulations, and then the fluctuation of refractive index of air is calculated. Based on sub-harmonic phase screen algorithm, three kinds of performance evaluation parameters are presented: irradiance, phase of the target, and Full Width Half Maximum (FWHM). The wind tunnel tests (WT) with a 1:120 scaled model of the retractable dome for the CLT are conducted to verify the calculated precision of the CFD. The results show that the fluctuation of air refractive index surrounding the CLT is mainly caused by the inhomogeneous distribution of temperature and speed, and with the help of pier's height the impact of inhomogeneous air temperature from the ground layer on the fluctuation of air refractive index can be effectively decreased. Furthermore, the lower of the air speed is, the better performance of the retractable dome will be, and when the speed of air is less than 5m/s, the dome seeing induced by the retractable dome on the observational wave front is less than 0.13 arcsec.

Fiber-tip gas pressure sensor based on dual capillaries

A micro-cavity fiber Fabry-Perot interferometer based on dual capillaries is proposed and demonstrated for gas pressure measurement. Such a device is fabricated by fusion splicing of a tiny segment of a main-capillary with a feeding-capillary on one end, and a single mode fiber on the other, to allow gas enters the main-capillary via the feeding-capillary. The reflection spectrum of the interferometer device shifts with the variation of gas pressure due to the dependence of gas refractive index on the pressure applied. During the device fabrication process, a core-offset fusion splicing method is adopted, which turns out to be highly effective for reducing the detection limit of the sensor. The experimental results obtained show that the proposed device exhibits a high gas pressure sensitivity of 4147 pm/MPa, a low temperature cross-sensitivity of less than 0.3 KPa/°C at atmospheric pressure, and an excellently low detection limit down to ~4.81 KPa. The robust tip structure, ultra-compact device size and ease of fabrication make the device an attractive candidate for reliable and highly sensitive gas pressure measurement in a precise location.

Air refractive index measurement using low-coherence interferometry

This paper presents a theoretical analysis and an experimental verification of a direct method for a refractive index of air measurement combining low-coherence interferometry and laser interferometry. The method is based on monitoring optical path changes in a measuring arm of the Michelson interferometer caused by the different optical environment in a double-spaced glass cell. This article presents a set of experimental results in comparison with the results obtained by a couple of reference techniques and proves the ability of the designed method to measure the refractive index of air with accuracy in the order of 10^-8.

Investigation of a macrobending micro-plastic optical fiber for refractive index sensing

This paper investigates a refractive index (RI) sensor based on a macrobending micro-plastic optical fiber (m-POF), which is used as a sensitive probe. The m-POFs are fabricated from commercial POFs using a heating and pulling method. The m-POFs have diameters of approximately 20-40 μm and act as multimode fibers at visible wavelengths. The macrobending structure of the m-POFs is simulated and optimized using the ray tracing method. The RI sensitivity and the range of RI measurements are affected by the structure parameter R/ρ, which is the ratio of the radius of curvature of the macrobending fiber to the radius of the fiber itself. A linear RI sensing response is obtained when R/ρ=20 and the sensitivity reaches 500%/RIU. The experimental results coincide well with the simulation results.

Hybrid optical fiber Fabry-Perot interferometer for simultaneous measurement of gas refractive index and temperature

We present a hybrid miniature optical fiber Fabry-Perot interferometer for simultaneous measurement of gas refractive index and temperature. The interferometer is fabricated by cascading two short sections of capillary tubes with different inner diameters. One extrinsic interferometer is based on the air gap cavity formed by the capillary tube with large diameter. Another section of capillary tube with small inner diameter performs as an intrinsic interferometer and also provides a channel enabling gas to enter and leave the extrinsic cavity freely. The experiment shows that the different dips or peaks in fringe exhibit different responses to the changes in gas refractive index and temperature. Owing to this feature, simultaneous measurement of the gas refractive index and temperature can be realized.

High pressure and temperature optical flow cell for near-infra-red spectroscopic analysis of gas mixtures

A new optical flow cell with a new optical arrangement adapted for high pressures and temperatures using glass fibres to connect light source, cell, and spectrometer has been developed, as part of a larger project comprising new methods for in situ analysis of bio and hydrogen gas mixtures in high pressure and temperature applications. The analysis is based on measurements of optical, thermo-physical, and electromagnetic properties in gas mixtures with newly developed high pressure property sensors, which are mounted in a new apparatus which can generate gas mixtures with up to six components with an uncertainty of composition of as little as 0.1 mol. %. Measurements of several pure components of natural gases and biogases to a pressure of 20 MPa were performed on two isotherms, and with binary mixtures of the same pure gases at pressures to 17.5 MPa. Thereby a new method of analyzing the obtained spectra based on the partial density of methane was investigated.

Validation of refractive index structure parameter estimation for certain infrared bands

Variation of the atmospheric refraction index due to turbulent fluctuations is one of the key factors that affect the performance of electro-optical and infrared systems and sensors. Therefore, any prior knowledge about the degree of variation in the refractive index is critical in the success of field studies such as search and rescue missions, military applications, and remote sensing studies where these systems are used frequently. There are many studies in the literature in which the optical turbulence effects are modeled by estimation of the refractive index structure parameter, C(n)(2), from meteorological data for all levels of the atmosphere. This paper presents a modified approach for bulk-method-based C(n)(2) estimation. According to this approach, conventional wind speed, humidity, and temperature values above the surface by at least two levels are used as input data for Monin-Obukhov similarity theory in the estimation of similarity scaling constants with a finite difference approximation and a bulk-method-based C(n)(2) estimation. Compared with the bulk method, this approach provides the potential for using more than two levels of standard meteorological data, application of the scintillation effects of estimated C(n)(2) on the images, and a much simpler solution than traditional ones due to elimination of the roughness parameters, which are difficult to obtain and which increase the complexity, the execution time, and the number of additional input parameters of the algorithm. As a result of these studies, Atmospheric Turbulence Model Software is developed and the results are validated in comparison to the C(n)(2) model presented by Tunick.

Fabrication, characterization, and simulation of a cantilever-based airflow sensor integrated with optical fiber

In this paper, we present the fabrication and packaging of a cantilever-based airflow sensor integrated with optical fiber. The sensor consists of a micro Fabry-Perot (FP) cavity including a fiber and a micro cantilever that is fabricated using the photolithography method. Airflow causes a small deflection of the micro cantilever and changes the cavity length of the FP, which makes the fringe shift. The pressure distribution and velocity streamlines across the cantilever resulted from the airflow in the channel have been simulated by the finite element method. The experimental results demonstrate that the sensor has a linear sensitivity of 190 [fringe shift (pm)] per (l/min) and a minimum detectable airflow change of 0.05 (l/min).

Performance and characterization of the prototype nm-scale spatial resolution scanning multilayer Laue lenses microscope

Synchrotron based x-ray microscopy established itself as a prominent tool for noninvasive investigations in many areas of science and technology. Many facilities around the world routinely achieve sub-micrometer resolution with a few instruments capable of imaging with the spatial resolution better than 100 nm. With an ongoing effort to push the 2D/3D resolution down to 10 nm in the hard x-ray regime both fabrication of the nano-focusing optics and stability of a microscope become extremely challenging. In this work we present our approach to overcome technical challenges on the path towards high spatial resolution hard x-ray microscopy and demonstrate the performance of a scanning fluorescence microscope equipped with the multilayer Laue lenses focusing optics.

Ultrahigh-sensitivity temperature fiber sensor based on multimode interference

The proposed sensing device relies on the self-imaging effect that occurs in a pure silica multimode fiber (coreless MMF) section of a single-mode-multimode-single-mode (SMS)-based fiber structure. The influence of the coreless-MMF diameter on the external refractive index (RI) variation permitted the sensing head with the lowest MMF diameter (i.e., 55 μm) to exhibit the maximum sensitivity (2800  nm/RIU). This approach also implied an ultrahigh sensitivity of this fiber device to temperature variations in the liquid RI of 1.43: a maximum sensitivity of -1880  pm/°C was indeed attained. Therefore, the results produced were over 100-fold those of the typical value of approximately 13  pm/°C achieved in air using a similar device. Numerical analysis of an evanescent wave absorption sensor was performed, in order to extend the range of liquids with a detectable RI to above 1.43. The suggested model is an SMS fiber device where a polymer coating, with an RI as low as 1.3, is deposited over the coreless MMF; numerical results are presented pertaining to several polymer thicknesses in terms of external RI variation.