Femtosecond-Laser-Assisted Fabrication of Radiation-Resistant Fiber Bragg Grating Sensors

This paper demonstrates the fabrication of radiation-resistant fiber Bragg grating (FBG) sensors using infrared femtosecond laser irradiation. FBG sensors were written inside acrylate-coated fluorine-doped single-mode specialty optical fibers. We detected the Bragg resonance at 1542 nm. By controlling the irradiation conditions, we improved the signal strength coming out from the FBG sensors. A significant reduction in the Bragg wavelength shift was detected in the fabricated FBG sensors for a radiation dose up to 105 gray, indicating excellent radiation resistance capabilities. We also characterized the temperature sensitivity of the radiation-resistant FBG sensors and detected outstanding performance.

Hyperspectral Satellite Remote Sensing of Aerosol Parameters: Sensitivity Analysis and Application to TROPOMI/S5P

Precise knowledge about aerosols in the lower atmosphere (optical properties and vertical distribution) is particularly important for studying the Earth’s climatic and weather conditions. Measurements from satellite sensors in sun-synchronous and geostationary orbits can be used to map distributions of aerosol parameters in global or regional scales. The new-generation sensor Tropospheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor (S5P) measures a wide variety of atmospheric trace gases and aerosols that are associated with climate change and air quality using a number of spectral bands between the ultraviolet and the shortwave infrared. In this study, we perform a sensitivity analysis of the forward model parameters and instrument information that are associated with the retrieval accuracy of aerosol layer height (ALH) and optical depth (AOD) using the oxygen (O2) A-band. Retrieval of aerosol parameters from hyperspectral satellite measurements requires accurate surface representation and parameterization of aerosol microphysical properties and precise radiative transfer calculations. Most potential error sources arising from satellite retrievals of aerosol parameters, including uncertainties in aerosol models, surface properties, solar/satellite viewing geometry, and wavelength shift, are analyzed. The impact of surface albedo accuracy on retrieval results can be dramatic when surface albedo values are close to the critical surface albedo. An application to the real measurements of two scenes indicates that the retrieval works reasonably in terms of retrieved quantities and fit residuals.

Correction of Temperature Variation with Independent Water Samples to Predict Soluble Solids Content of Kiwifruit Juice Using NIR Spectroscopy

Using the framework of aquaphotomics, we have sought to understand the changes within the water structure of kiwifruit juice occurring with changes in temperature. The study focuses on the first (1300–1600 nm) and second (870–1100 nm) overtone regions of the OH stretch of water and examines temperature differences between 20, 25, and 30 °C. Spectral data were collected using a Fourier transform–near-infrared spectrometer with 1 mm and 10 mm transmission cells for measurements in the first and second overtone region, respectively. Water wavelengths affected by temperature variation were identified. Aquagrams (water spectral patterns) highlight slightly different responses in the first and second overtone regions. The influence of increasing temperature on the peak absorbance of the juice was largely a lateral wavelength shift in the first overtone region and a vertical amplitude shift in the second overtone region of water. With the same data set, we investigated the use of external parameter orthogonalisation (EPO) and extended multiple scatter correction (EMSC) pre-processing to assist in building temperature-independent partial least square regression models for predicting soluble solids concentration (SSC) of kiwifruit juice. The interference component selected for correction was the first principal component loading measured using pure water samples taken at the same three temperatures (20, 25, and 30 °C). The results show that the EMSC method reduced SSC prediction bias from 0.77 to 0.1 °Brix in the first overtone region of water. Using the EPO method significantly reduced the prediction bias from 0.51 to 0.04 °Brix, when applying a model made at one temperature (30 °C) to measurements made at another temperature (20 °C) in the second overtone region of water.

Особенности одночастотной генерации в квантово-каскадных лазерах спектрального диапазона 7.5-8.0 μm с малой длиной резонатора

The possibility of realizing single-mode emission in quantum-cascade lasers due to modulation of output optical losses in a Fabry-Perot cavity is demonstrated. For the active region of the 7.5–8.0 μm spectral range, two-phonon resonance design we used thus the 50 stages and waveguide layers based on indium phosphide made it possible to realize single-mode lasing at 7.765 μm and at temperature of 292 K. Side-mode suppression ratio was about 24 dB and remained the same with an increase in the current pumping up to 1.2 of the threshold current values. The coefficient of wavelength shift with temperature (temperature tuning) in the single-mode lasing regime was 0.56 nm / K.

Opto-Mechatronics System for Train-Track Micro Deformation Sensing

In this paper, we proposed and experimentally demonstrated an opto-mechatronics system to detect the micro-deformation of tracks caused by running trains. The fiber Bragg grating (FBG) array acting as sensing elements has a low peak reflectivity of around −40 dB. The center wavelengths were designed to alternate between 1551 nm and 1553 nm at 25 °C. Based on dual-wavelength, wavelength-division multiplexing (WDM)/time-division multiplexing (TDM) hybrid networking, we adopted optical time-domain reflectometry (OTDR) technology and a wavelength-scanning interrogation method to achieve FBG array signal demodulation. The field experimental results showed that the average wavelength shift of the FBG array caused by the passage of the lightest rail vehicle was −225 pm. Characteristics of the train-track system, such as track occupancy, train length, number of wheels, train speed, direction, and loading can be accurately obtained in real time. This opto-mechatronics system can meet the requirements of 600 mm spatial resolution, long distance, and large capacity for monitoring the train-track system. This method exhibits great potential for applications in large-scale train-track monitoring, which is meaningful for the safe operation of rail transport.

Monolithic, Optically Coupled, Multi-Section Mid-IR Quantum Cascade Lasers

Mid-infrared (mid-IR λ ≈ 3–12 μm), single-mode-emission Quantum Cascade Lasers (QCLs) are of significant interest for a wide range of applications, especially as the laser sources are chosen for laser absorption spectroscopy. In this work, we present the design, fabrication and characterization of multi-section, coupled-cavity, mid-IR quantum cascade lasers. The purpose of this work is to propose a design modification for a coupled-cavity device, yielding a single-mode emission with a longer range of continuous tuning during the pulse, in contrast to a 2-section device. This effect was obtained and demonstrated in the work. The proposed design of a 3-section coupled-cavity QCL allows for a single-mode emission with 35 dB side-mode suppression ratio. Additionally, the time-resolved spectra of the wavelength shift during pulse operation, show a continuous tuning of ~3 cm−1 during the 2 μs pulse. The devices were fabricated in a slightly modified, standard laser process using dry etching.

Simultaneous Detection of Relative Humidity and Temperature Based on Silicon On-Chip Cascaded Photonic Crystal Nanobeam Cavities

In this paper, we propose and numerically demonstrate a novel cascaded silicon-on-insulator (SOI) photonic crystal nanobeam cavity (PCNC) dual-parameter sensor for the simultaneous detection of relative humidity (RH) and temperature. The structure consists of two independent PCNCs supporting two different resonant modes: a dielectric-mode and an air-mode, respectively. The dielectric-mode nanobeam cavities (cav1) are covered with SU-8 cladding to increase the sensitivity ratio contrast between RH sensing and temperature sensing. The air-mode nanobeam cavities (cav2) are coated with a water-absorbing polyvinyl-alcohol (PVA) layer that converts the change in RH into a change in refractive index (RI) under different ambient RH levels, thereby inducing a wavelength shift. Due to the positive thermo-optic (TO) coefficient of silicon and the negative TO coefficient of SU-8 cladding, the wavelength responses take the form of a red shift for cav2 and a blue shift for cav1 as the ambient temperature increases. By using 3D finite-difference time-domain (3D-FDTD) simulations, we prove the feasibility of simultaneous sensing by monitoring a single output transmission spectrum and applying the sensor matrix. For cav1, the RH and temperature sensitivities are 0 pm/%RH and −37.9 pm/K, while those of cav2 are −389.2 pm/%RH and 58.6 pm/K. The sensitivity ratios of temperature and RH are −1.5 and 0, respectively, which is the reason for designing two different resonant modes and also implies great potential for realizing dual-parameter sensing detection. In particular, it is also noteworthy that we demonstrate the ability of the dual-parameter sensor to resist external interference by using the dual wavelength matrix method. The maximum RH and temperature detection errors caused by the deviation of resonance wavelength 1 pm are only 0.006% RH and 0.026 K, which indicates that it achieves an excellent anti-interference ability. Furthermore, the structure is very compact, occupying only 32 μm × 4 μm (length × width). Hence, the proposed sensor shows promising prospects for compact lab-on-chip integrated sensor arrays and sensing with multiple parameters.

Study of a MOEMS XOR gate based on optical ring resonator

Based on ring resonator with Microelectromechanical systems, optical XOR logic gate is proposed in this paper to realize the optical logic gate application. The proposed gate is basically structured on an optical ring resonator with 7µm radius and resonance wavelength of 1.55µm which is placed on the edge of a thin SiC circular diaphragm. In order to apply input voltages to electrodes, two very thin circular gold layers with 50nm air gap spacing are deposited under the diaphragm. Input voltages are considered as logic inputs and resonance wavelength shift as logic output. When an input DC voltage is applied across the diaphragm, an attractive electrostatic force is created between two electrodes. As a result, the diaphragm is deformed and an internal stress is created. This in turn changes the resonator refractive index due to the photoelastic effect and thus shifts its resonance wavelength about 35nm. COMSOL Multiphysics and MATLAB are carried out to verify FEA and numerical analysis of the designed structure, respectively. A good agreement between the simulations and analytical results is obtained. Enhancement of the wavelength shift and FSR are resulted. The proposed structure is used as an optical XOR gate for the first time.

Hollow Core Bragg Fiber-Based Sensor for Simultaneous Measurement of Curvature and Temperature

In this paper, the hollow core Bragg fiber (HCBF)-based sensor based on anti-resonant reflecting optical waveguide (ARROW) model is proposed and experimentally demonstrated for simultaneous measurement of curvature and temperature by simply sandwiching a segment of HCBF within two single-mode fibers (SMFs). The special construction of a four-bilayer Bragg structure provides a well-defined periodic interference envelope in the transmission spectrum for sensing external perturbations. Owing to different sensitivities of interference dips, the proposed HCBF-based sensor is capable of dual-parameter detection by monitoring the wavelength shift. The highest curvature sensitivity of the proposed sensor is measured to be 74.4 pm/m−1 in the range of 1.1859–2.9047 m−1 with the adjusted R square value of 0.9804. In the meanwhile, the best sensitivity of temperature sensing was detected to be 16.8 pm/°C with the linearity of 0.997 with temperature range varying from 25 to 55 °C. Furthermore, with the aid of the 2 × 2 matrix, the dual demodulation of curvature and temperature can be carried out to realize the simultaneous measurement of these two parameters. Besides dual-parameter sensing based on wavelength shift, the proposed sensor can also measure temperature-insensitive curvature by demodulating the intensity of resonant dips.