A handheld mid-infrared methane sensor using a dual-step differential method for additive/multiplicative noise suppression

A miniature mid-infrared (mid-IR) methane (CH₄) sensor system was developed by employing a wide-band wire-source and a semi-ellipsoid multi-pass gas cell. A dual-step differential method instead of the traditional one-step differential method was adopted by this sensor to tune measuring range/zero point and to suppress the additive/multiplicative noise. This method included a first subtraction operation between the two output signals (including a detection signal and a reference signal) from the dual-channel detector and a second subtraction operation on the amplitudes of the first-subtraction signal and the reference signal, followed by a ratio operation between the amplitude of the second-subtraction signal and the reference signal. Detailed experiments were performed to assess the performance of the sensor system. The detection range is 0–50 k ppm, and as the concentration gets larger than 12 k ppm, the relative detection error falls into the range of −3% to +3%. The Allan deviation is about 4.65 ppm with an averaging time of 1 s, and such value can be further improved to 0.45 ppm with an averaging time of 124 s. Due to the cost-effective incandescence wire-source, the small-size ellipsoid multi-pass gas cell and the miniature structure of the sensor, the developed standalone device shows potential applications of CH₄ detection under coal-mine environment.     

Chromatic aberration free liquid crystal adaptive optics for flood illuminated retinal camera

A high-resolution, flood-illumination retinal camera using liquid crystal (LC) adaptive optics (AO) is presented. The retinal camera uses light at 780 nm for ocular aberration measurement while light at 655 nm and 593 nm for retinal imaging. In order to avoid chromatic aberrations due to wavelength dependence of LC, we adopt an open-loop technique, in which dynamic correction of aberrations is applied only to the imaging light. A compensation pattern projected on the LC wavefront corrector is adjusted to provide phase wrapping of 2 π for illumination light. We confirmed feasibility of this technique by performing in vivo retinal imaging experiments. Photoreceptors were clearly revealed at both imaging light at 655 nm and 593 nm. Feasibility of the technique was also supported by comparison of the retinal images taken by the present open-loop technique with those taken by the conventional closed-loop one and by analysis of the spatial distribution of the photoreceptors.     

Experimental implementation of an all-optical OFDM system based on time lens

A real time all-optical orthogonal frequency division multiplexing (OOFDM) system based on two time lenses is experimentally demonstrated. At the transmitter, groups of initial optical pulses are transformed into OOFDM symbols by a continuous inverse Fourier transform (IFT) device, which consists of the first a quadratic phase modulator and two high dispersive elements. After transmission, a continuous Fourier transform (FT) device, consisting of another quadratic phase modulator and two high dispersive elements, rebuilds the initial optical pulses at the receiver. The electrical sinusoidal waveform employed in the FT module at the receiver is obtained directly from the transmitter with certain phase shift. In the system, the guard interval is necessary for each OOFDM symbol to ensure the initial pulses are operated FT/IFT under a parabolic waveform driving and to have more Fourier transformation time window. Experiment results show that 8 × 2.5 Gb/s signals successfully transmit through 200-km-G.655 fibers without any dispersion compensation, with the BER being 10^? 12.     

A low communication and large time step explicit finite-volume solver for non-hydrostatic atmospheric dynamics

An explicit finite-volume solver is proposed for numerical simulation of non-hydrostatic atmospheric dynamics with promise for efficiency on massively parallel machines via low communication needs and large time steps. Solving the governing equations with a single stage lowers communication, and using the method of characteristics to follow information as it propagates enables large time steps. Using a non-oscillatory interpolant, the method is stable without post-hoc filtering. Characteristic variables (built from interface flux vectors) are integrated upstream from interfaces along their trajectories to compute time-averaged fluxes over a time step. Thus we call this method a Flux-Based Characteristic Semi-Lagrangian (FBCSL) method. Multidimensionality is achieved via a second-order accurate Strang operator splitting. Spatial accuracy is achieved via the third- to fifth-order accurate Weighted Essentially Non-Oscillatory (WENO) interpolant.We implement the theory to form a 2-D non-hydrostatic compressible (Euler system) atmospheric model in which standard test cases confirm accuracy and stability. We maintain stability with time steps larger than CFL = 1 (CFL number determined by the acoustic wave speed, not advection) but note that accuracy degrades unacceptably for most cases with CFL > 2. For the smoothest test case, we ran out to CFL = 7 to investigate the error associated with simulation at large CFL number time steps. Analysis suggests improvement of trajectory computations will improve error for large CFL numbers.     

10.6 μm Infrared light photoinduced insulator-to-metal transition in vanadium dioxide

Vanadium dioxide has excellent phase transition characteristic. Before or after phase transition, its optical, electrical, magnetic characteristic hangs hugely. It has a wide application prospect in many areas. Now, the light which can make vanadium dioxide come to pass photoinduced phase transition range from soft X-ray to medium infrared light (6.9 μm, 180 meV). However, whether 10.6 μm (117 meV) long wave infrared light can make vanadium dioxide generate photoinduced phase transition has been not studied. In this paper, we researched the response characteristic of vanadium dioxide excited by 10.6 μm infrared light. We prepared the vanadium dioxide and test the changes of vanadium dioxide thin film’s transmittance to 632.8 nm infrared light when the thin film is irradiate by CO₂ laser. We also test the resistivity of vanadium dioxide. Excluding the effect of thermal induced phase transition, we find that the transmittance of vanadium dioxide thin film to 632.8 nm light and resistivity both changes when irradiating by 10.6 μm laser. This indicates that 10.6 μm infrared light can make the vanadium dioxide come to pass photoinduced phase transition. The finding makes vanadium has a potential application in recording the long-wave infrared hologram and making infrared detector with high resolution.     

Potential-induced structure transitions in self-assembled monolayers: II. Propanethiol on Au(1 0 0)

The structure of propanethiol self-assembled monolayers (SAMs) on Au(1 0 0) in 0.1 M H₂SO₄ has been investigated as a function of electrode potential by in situ scanning tunnelling microscopy (STM). These studies reveal a potential-induced, reversible transition between an in essence quadratic and a distorted hexagonal structure. We suggest that a thiolate-driven surface reconstruction, similar to Au(1 0 0)-(hex), is responsible for the hexagonal SAM structure, whereas the quadratic one refers to the unreconstructed substrate.     

A study on wavelength dependence and dynamic range of the quadratic response of commercial grade light emitting diodes

Experimental results of a study on the wavelength dependence and the dynamic range of the quadratic response of commercial grade light emitting diodes (LEDs) are reported over a broad spectral range of 680 nm to 1080 nm using ~ 100 fs duration laser pulses from cw mode locked laser oscillator. A large dynamic range of the quadratic response has been demonstrated in a reverse biased LED. The observed dynamic range compares well with that obtained using a biased photomultiplier tube with large internal gain.     

Adaptive phase plates for optical encoding systems invariant to second-order aberrations

Certain optical aberrations of imaging systems yield low resolution images. Wavefront coding has proven to minimise this problem by means of hybrid optical-digital imaging systems. The optical part usually involves a phase plate described in terms of cubic polynomials whose shape is a linear combination of (x³ + y³) and (x²y + xy²). Optimization is achieved by seeking the most appropriate linear combination with respect to the optical system's constraints. Here, we propose the shape of two pairs of phase plates such that by means of relative rotations they allow for variation of the linear combination of the cubic terms. This will enable adaptive optimization of the cubic phase to the optical system's constraints when these vary in time. Results will be illustrated with numerical simulations.     

Statistical modeling of laser welding of DP/TRIP steel sheets

In this research work, a statistical analysis of the CO₂ laser beam welding of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets was done using response surface methodology. The analysis considered the effect of laser power (2–2.2 kW), welding speed (40–50 mm/s) and focus position (?1 to 0 mm) on the heat input, the weld bead geometry, uniaxial tensile strength, formability limited dome height and welding operation cost. The experimental design was based on Box–Behnken design using linear and quadratic polynomial equations for predicting the mathematical models. The results indicate that the proposed models predict the responses adequately within the limits of welding parameters being used and the welding speed is the most significant parameter during the welding process.     

Application of thermal wave imaging and phase shifting method for defect detection in Stainless steel

This paper presents an experimental arrangement for detection of artificial subsurface defects in a stainless steel sample by means of thermal wave imaging with lock-in thermography and consequently, the impact of excitation frequency on defect detectability. The experimental analysis was performed at several excitation frequencies to observe the sample beginning from 0.18 Hz all the way down to 0.01 Hz. The phase contrast between the defective and sound regions illustrates the qualitative and quantitative investigation of defects. The two, three, four and five-step phase shifting methods are investigated to obtain the information on defects. A contrast to noise ratio analysis was applied to each phase shifting method allowing the choice of the most appropriate one. Phase contrast with four-step phase shifting at an optimum frequency of 0.01 Hz provides excellent results. The inquiry with the effect of defect size and depth on phase contrast shows that phase contrast decreases with increase in defect depth and increases with the increase in defect size.     

A study on electromagnetic wave absorber for the collision-avoidance radar

In this paper, we designed and fabricated the EM wave absorber for eliminating false image and system-to-system interference in a collision-avoidance radar using Permalloy and CPE, the mixing ratio of which was Permalloy:CPE = 70:30 wt%. The EM wave absorption ability was simulated according to different thicknesses of the EM wave absorber using measured permittivity and permeability, and then the EM wave absorber was fabricated based on the simulated design. The simulated results agree well with the measured ones. As a result, we suggest Permalloy for use as a new EM wave absorber in W-band, and the fabricated EM wave absorber with the thickness of 1.4 mm has absorption ability more than 20 dB in frequency range of 76–77 GHz.     

Lens Sag Measurement of Microlens Array Using Optical Interferometric Microscope

This study attempted to develop a detection system for lens sag of the microlens array in real time using an optical automatic inspection framework to link with the computer through a camera. An image processing technique was applied to detect the spherical microlens array, and then, the results were compared.The system light source used laser light and applied CCD to collocate with the microscope array to form an automatic optical detection system for an optical interferometric microscope. It applied the principle of the Fizeau interferometer, illuminated the surface of microlens array, and formed the phase difference required by the interference of two lights through the laser light reflected by the reference plane and the surface of the microlens array, thus, forming an interference fringe.When the sag of the microlens was much longer than the wave length of the detection light source, the fringe would be densely distributed, thus, only a few central fringes were clear in the microscopic image. An image processing method was used to search the center of the interference fringe and a creative algorithm was utilized to obtain the lens sag of the microlens. As proved by the experiment, lens sag of 4 microlens arrays were detected in real time, with a minimum detection error of 0.08 μm, and a maximum detection error of 4 μm (error value 1 ~ 9%), according to different sample processes. This system featured a simple structure and is applicable to non-contact detection and detection of different-sized microlens arrays.     

Simultaneous demodulation and dispersion compensation of WDM DPSK channels using optical ring resonator

We have introduced and comprehensively analyzed a novel scheme of simultaneous demodulation and dispersion compensation of wavelength division multiplexed (WDM) non-return-to zero (NRZ) differential phase shift keying (DPSK) optical link using an optical ring resonator (ORR) based filter. Using extensive numerical simulation we have demonstrated the transmission of 10.7 Gb/s WDM DPSK channels having 50 GHz and 100 GHz spacing over 400 km of unrepeatered reach at 20 dB optical-signal-to-noise-ratio (OSNR) to achieve a bit error rate (BER) of 10^? 3.     

Mechanistic insight into sonochemical biodiesel synthesis using heterogeneous base catalyst

The beneficial effect of ultrasound on transesterification reaction is well known. Heterogeneous (or solid) catalysts for biodiesel synthesis have merit that they do not contaminate the byproduct of glycerol. In this paper, we have attempted to identify the mechanistic features of ultrasound–enhanced biodiesel synthesis with the base–catalyst of CaO. A statistical design of experiments (Box–Behnken) was used to identify the influence of temperature, alcohol to oil molar ratio and catalyst loading on transesterification yield. The optimum values of these parameters for the highest yield were identified through Response Surface Method (with a quadratic model) and ANOVA. These values are: temperature = 62 °C, molar ratio = 10:1 and catalyst loading = 6 wt.%. The activation energy was determined as 82.3 kJ/mol, which is higher than that for homogeneous catalyzed system (for both acidic and basic catalyst). The experimental results have been analyzed vis–à–vis simulations of cavitation bubble dynamics. Due to 3–phase heterogeneity of the system, the yield was dominated by intrinsic kinetics, and the optimum temperature for the highest yield was close to boiling point of methanol. At this temperature, the influence of cavitation bubbles (in terms of both sonochemical and sonophysical effect) is negligible, and ultrasonic micro–streaming provided necessary convection in the system. The influence of all parameters on the reaction system was found to be strongly inter–dependent.     

An optical OFDM multiplexer and 16 × 10 Gb/s OOFDM system using serrodyne optical frequency translation based on LiNbO3 phase modulator

This paper proposes an all-optical orthogonal frequency division multiplexing (OFDM) multiplexer based on sawtooth wave driving LiNbO₃ phase modulators by using serrodyne optical frequency translation. This multiplexer has high integration ability. The designing concept and implementation method have been discussed. A 16 × 10 Gb/s optical OFDM system is designed based on this multiplexer. Transmission characteristics, including tolerances of polarization mode dispersion (PMD) and nonlinear impairments have been studied numerically. Simulations show that the PMD tolerance is about 42 ps. The spectral efficiency reaches 1 bit/s/Hz with binary modulation format.     

Design of a super wide-band EM wave absorber for a general purpose anechoic chamber

In order to construct an anechoic chamber satisfying international standards for EMI testing, it has been recognized that the absorption characteristics of the EM wave absorber must be higher than 20 dB over the frequency band from 30 MHz to 18 GHz. In this paper, an EM wave absorber with super wide-band frequency characteristics is proposed and designed in order to satisfy the above requirements by using the EMCM and FDTD. As a result, the proposed absorber has absorption characteristics higher than 20 dB over the frequency band from 30 MHz to more than 20 GHz.     

Performance evaluation of 64 × 10 Gbps and 96 × 10 Gbps DWDM system with hybrid optical amplifier for different modulation formats

In this paper, we investigated the performance of multi terabits DWDM system consisting of hybrid optical amplifier RAMAN-EDFA for different data format such as non-return to zero (NRZ), return to zero (RZ) and differential phase shift keying (DPSK). We find that in 64 × 10 and 96 × 10 Gbps, RZ is more adversely affected by nonlinearities, where as NRZ and DPSK is more affected by dispersion. We further show that RZ provide good quality factor (13.88 dB and 15.93 dB for 64 and 96 channels), less eye closure (2.609 dB and 3.191 dB for 64 and 96 channels) and acceptable bit error rate (3.89 × 10⁸ and 1.24 × 10⁹ for 64 and 96 channels) at the respective distance as compare to other existing modulation format. We further investigated the maximum single span distance covered by using existing data formats.     

Can two chaotic systems give rise to order?

The recently discovered Parrondo's paradox claims that two losing games can result, under random or periodic alternation of their dynamics, in a winning game: “losing + losing = winning”. In this paper we follow Parrondo's philosophy of combining different dynamics and we apply it to the case of one-dimensional quadratic maps. We prove that the periodic mixing of two chaotic dynamics originates an ordered dynamics in certain cases. This provides an explicit example (theoretically and numerically tested) of a different Parrondian paradoxical phenomenon: “chaos + chaos = order”.     

Adsorption of chlorine on Ru(0001)—A combined density functional theory and quantitative low energy electron diffraction study

Chlorine adsorption on Ru(0001) surface has been studied by a combined density functional theory (DFT) and quantitative low energy electron diffraction (LEED) approach. The (√3 × √3)R30°-Cl phase with Θ_Cl = 1/3 ML and chlorine sitting in fcc sites has been identified by DFT calculations as the most stable chlorine adsorbate structure on Ru(0001) with an adsorption energy of ? 220 kJ/mol. The atomic geometry of (√3 × √3)R30°-Cl was determined by quantitative LEED. The achieved agreement between experimental and simulated LEED data is quantified by a Pendry factor of r_P = 0.19 for a fcc adsorption site with a Cl-Ru bond length of 2.52 Å. At chlorine coverages beyond 1/3 ML LEED reveals diffuse diffraction rings, indicating a continuous compression of the hexagonal Cl overlayer with a preferred average Cl–Cl distance of 4.7 Å in the (√3 × √3)R30°-Cl, Θ_Cl = 1/3 ML phase towards 3.9 Å at saturation coverage of 0.48 ML.     

A phase stable short pulse generator using a DPMZM and phase modulators for application in 160 GBaud DQPSK systems

A method of 40 GHz phase stable short pulses generation is experimentally demonstrated. It is based on a dual parallel Mach–Zehnder modulator (DPMZM) driven by only one electrical sinusoidal clock and two cascaded phase modulators. The generated pulses are characterized with full-width-at-half-maximum pulse width of 1.9 ps, extinction ratio of 27 dB, timing jitter of 36 fs and signal to noise ratio over 30 dB. The high quality and phase stability of the pulses are further experimentally verified in a 4 × 40 GBaud differential quadrature phase shift keying (DQPSK) optical-time-division-multiplexing (OTDM) system.