Measured Wavelength Shifts Research Articles

The Temperature Dependence of Hot Prograde Flows in Solar Active Regions

Using simultaneous observations from the EUV Variability Experiment (EVE) and imaging from the Atmospheric Imaging Assembly (AIA), we characterise the temperature dependence of apparent hot flows in solar active regions. The EVE instrument performs Sun-as-a-star spectroscopy and is composed of two spectrographs: MEGS-A and MEGS-B (Multiple EUV Grating Spectrograph-A, -B). It is known that EVE can measure wavelength shifts and thus can observe relative Doppler velocities in solar atmospheric plasmas over an extended temperature range. However, MEGS-A is affected by a known astigmatism effect (Chamberlin: Solar Phys.291, 1665, 2016); inhomogeneities in EUV brightness on the solar surface result in purely instrumental wavelength errors. We validate our methods by independently quantifying this effect and comparing to Chamberlin’s results, and we explore the wavelength dependence as an extension of his formula as derived for He ii 304 Å. MEGS-B is unaffected by this instrumental effect in any case, and this has allowed us to find evidence of hot prograde flows in active regions. Using our image-based models for the astigmatism and flows, we independently confirm our original MEGS-B result. We now extend our knowledge of the temperature dependence of these flows via the additional Fe emission lines available in MEGS-A. We find a monotonic increase of apparent flow speed with temperature up through lines of Fe xvi, nominally formed at about 6.4 MK.

Simultaneous Measurement of Torsion and Temperature Based on π-Phase-Shifted Long-Period Fiber Grating Inscribed on Double-Clad Fiber.

In this work, we experimentally demonstrated an optical fiber sensor capable of performing simultaneous measurement of torsion and temperature using a π-phase-shifted long-period fiber grating (LPFG) inscribed on double-clad fiber (DCF), referred to as a PS-DC-LPFG. The fabricated PSDC- LPFG showed split attenuation bands near its resonance wavelength, and the two dips in these bands were selected as sensor indicators, denoted as Dips A and B, for the simultaneous measurement of torsion and temperature. The torsion and temperature responses of the two indicators were investigated in a twist angle range from -360° to 360° and a temperature range from 30 to 120 °C, respectively. When the twist angle increased from 0° to 360° (clockwise) at room temperature, both Dips A and B showed redshifts. On the contrary, when the twist angle decreased from 0° to -360° (counterclockwise), the two dips showed blueshifts. In terms of temperature responses, both dips showed redshifts with increasing ambient temperature while the sensor head (i.e., the PS-DC-LPFG) remained straight without any applied torsion. Owing to their linear and independent responses to torsion and temperature, the changes in torsion and temperature applied to the PSDC- LPFG could be simultaneously estimated from the measured wavelength shifts and calculated sensitivities of the two indicator dips.

Bend-Insensitive Discrimination of Strain and Temperature Based on Fiber Transmission Grating Inscribed on High Birefringence Photonic Crystal Fiber.

In this paper, we propose a bend-insensitive optical fiber sensor capable of separately measuring strain and temperature by incorporating a fiber transmission grating (FTG) inscribed on high birefringence photonic crystal fiber (HBPCF) with a CO2 laser. The FTG was fabricated by exposing unjacketed HBPCF to CO2 laser pulses using the line-by-line technique. The FTG inscribed on HBPCF, referred to as the HBPC-FTG, has two resonance dips with different wavelengths depending on input polarization. These two resonance dips were utilized as sensor indicator dips denoted by a shorter wavelength dip (SD) and a longer wavelength dip (LD). The strain and temperature responses of the SD and LD were investigated in a strain range of 0 to 3105 μ and a temperature range of 30 to 85 °C, respectively. The measured strain sensitivities of the SD and LD at room temperature (25 °C) were approximately -0.46 and -0.58 pm/μ, respectively. Similarly, the measured temperature sensitivities of the SD and LD without applied strain (0 μ) were ˜5.99 and ˜9.89 pm/°C, respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the HBPC-FTG can be simultaneously estimated from the measured wavelength shifts of the two indicator dips (i.e., SD and LD) using their predetermined strain and temperature sensitivities. Moreover, bend-induced spectral variations of the SD and LD were also examined in a curvature range of 0-4.705 m-1, and it was observed that both dips showed little wavelength shift due to applied bending. Thus, it is concluded from the experimental results that the fabricated HBPC-FTG can be employed as a cost-effective sensor head for bend-insensitive discrimination of strain and temperature.

Strain-Insensitive Simultaneous Measurement of Bending and Temperature Based on Cascaded Long-Period Fiber Gratings Inscribed on Double-Clad Fiber

We demonstrated an optical fiber sensor capable of performing strain-insensitive simultaneous measurement of bending and temperature using cascaded long-period fiber gratings (LPFG’s) inscribed on double-clad fiber with a CO2 laser at $10.6~\mu \text{m}$ . In the interference fringe spectrum of the fabricated cascaded LPFG’s, we selected three transmission dips as sensor indicators for the simultaneous measurement of bending and temperature. The measured bending sensitivities of the three indicator dips were −427.63, −911.29, and 233.33 pm/ $\text{m}^{-1}$ , and their measured temperature sensitivities were 60.49, 56.44, and 74.61 pm/°C. Owing to their linear and uncorrelated responses to bending and temperature, we could simultaneously estimate applied bending and temperature with two of the three indicators by converting the measured wavelength shifts of the two selected dips with their pre-determined bending and temperature sensitivities. In particular, our sensor showed a very low strain sensitivity less than 0.05 pm/ $\mu \varepsilon $ , 25 times less than that of a conventional LPFG, which can significantly reduce strain-induced measurement uncertainty. With two optimal indicator dips chosen considering the measured perturbation responses of the three indicators, we carried out the simultaneous measurements of bending and temperature for arbitrary sixteen points of applied bending and temperature. The standard deviations of the measured bending and temperature values were $\sim 0.059\,\,\text{m}^{-1}$ and ~0.85 °C, respectively.

Wavelength tuning in the purple wavelengths using strain-controlled AlxGa1–xN/GaN disk-in-wire structures

AlxGa1–xN/GaN disk-in-wire polar nanostructures were fabricated, and their optical properties were studied. Wavelength tuning was observed by locally controlling the strain in each nanopillar via its diameter. The measured wavelength shift was in an excellent agreement with a one-dimensional strain relaxation model considering only the elastic and piezoelectric properties of the material. The inhomogeneous broadening decreases and internal quantum efficiency increases with a decreasing nanopillar diameter. The potential extension of strain-induced wavelength tuning across ultraviolet through near infrared was also discussed.

Simultaneous measurement of strain and temperature based on a long-period fiber grating written on high birefringence photonic crystal fiber concatenated with a Faraday rotator mirror

Here we report simultaneous measurement of strain and temperature, carried out by incorporating a long-period fiber grating (LPFG) written on high birefringence photonic crystal fiber (HBPCF), referred to as an HBPC-LPFG, and a Faraday rotator mirror (FRM) concatenated in series with the LPFG. The HBPC-LPFG used as a sensor head was fabricated by irradiating CO2 laser pulses to unjacketed HBPCF with line-by-line technique. The HBPC-LPFG connected in series with the FRM exhibits a polarization-independent wavelength-dependent loss spectrum in its reflection spectrum, which has two resonance dips (RDs) with different cladding-mode orders, designated as RDs 1 and 2. As the two RDs in the fabricated HBPC-LPFG ended with the FRM have dissimilar cladding-mode orders, their strain sensitivities and their temperature sensitivities are different from each other. For the two RDs with resonance wavelengths of λ1 = ∼1470.5 nm and λ2 = ∼1495.1 nm, strain and temperature responses were investigated in an applied strain range from 0 to 1790 μe (step: 179 μe) and an ambient temperature range from 30°C to 95°C (step: 5°C), respectively. The strain sensitivities of the RDs 1 and 2 were measured to be approximately −0.77 and −1.07 pm / μe at room temperature, and the temperature sensitivities of the RDs 1 and 2 were measured as ∼10.44 and ∼8.56 pm / ° C without applied strain, respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the HBPC-LPFG can be simultaneously estimated from the measured wavelength shifts of the RDs 1 and 2 using their predetermined strain and temperature sensitivities.

Simultaneous Measurement of Strain and Temperature Using Long-Period Fiber Grating Written on Polarization-Maintaining Photonic Crystal Fiber.

Here we propose a novel optical fiber sensor capable of simultaneous measurement of strain and temperature by utilizing a long-period fiber grating (LPFG) inscribed on polarization-maintaining photonic crystal fiber (PMPCF) as a sensor head. The sensor head was fabricated by irradiating CO₂ laser pulses to one side of PMPCF with line-by-line technique. The LPFG written on PMPCF (referred to as the PMPC-LPFG) exhibits two different wavelength-dependent loss bands, obtained at two orthogonal input polarization states. For two resonance wavelengths of these two wavelength-dependent loss bands, designated as Dips A and B, strain and temperature responses were investigated in a strain range of 0 to 2058 μɛ with a step of 98 μɛ and a temperature range of 30 to 85 °C with a step of 5 °C. Strain sensitivities of Dips A and B were measured and found to be approximately -0.82 and -1.43 pm/μɛ, respectively, at room temperature (25 °C). Similarly, temperature sensitivities of Dips A and B were measured and found to be ~7.89 and ~4.76 pm/°C without applied strain (0 μɛ), respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the PMPC-LPFG can be simultaneously estimated from the measured wavelength shifts of the two resonance dips (Dips A and B) using their premeasured strain and temperature sensitivities. The experimental results prove that the PMPC-LPFG can be used as a sensor head for simultaneous measurement of strain and temperature.

Fiber Bragg Grating High Impact Force Sensors With Adjustable Sensitivity and Dynamic Range

This paper presents the development of a fiber Bragg grating sensor system for loads applied perpendicularly to the fiber axis. The sensor design allows for adjustable sensitivity and high impact resistance. Two designs were tested, each containing a fiber Bragg grating embedded in a polymer material and encased in aluminum. Testing with a rate controlled load press was used to show the dependence of the response on loading conditions with loads reaching kN levels. Projectile drop tests further showed the response of both types of sensors with forces applied over millisecond durations. The magnitudes of the measured wavelength shift loading and unloading rates ranged from 0.203 to 0.507 nm/ms. Static structural simulations were completed and are in agreement with the experimental results. Loading rates of 1.667 mm/s with contact areas ranging from 38.5 to 113 mm2 resulted in respective sensitivities ranging from 0.154 to 0.064 nm/kN. Simulations and experiments further demonstrated the ability to increase the sensitivity up to 99% through modifications of lid thickness, cavity diameter, and loading area.

Wavelength Shift Measurement Method Without Use of Spectrometer: The New Way for Environment Photonic Sensors

The photonic sensors have shown very effectively for measuring the toxic contents in the liquid and air environments. In principle, the photonic sensors based on measurement of wavelength shift between reference condition and testing environments that we need use the spectrometer with high cost. In this paper, we present new configurations of photonic devices for measuring wavelength shift without use of spectrometer, which has a large potential for application in sensing technique with low cost. There are two configurations of photonic sensors are presented: i) first of them is based on fiber Bragg grating (FBG) combined with DFB laser diode with controlling wavelength emission by laser temperature and ii) second one is used the fiber ring laser from Erbium-doped fiber and two FBG operated as reference and sensing probe. The etched-fiber Bragg grating (e-FBG) as sensing probe is suitable for bio- and/ or chemical sensors. A novel photonic sensor can increase sensitivity and measuring accuracy of device by the narrow line-width of reflection spectra from laser and the sensor can determine a refractive index variation of 2x10-4, which is similarly for high resolution spectrometer. The experimental results show that this sensing method could determine different mixing ratios of organic solvents in liquid environment with good repeatability, high accuracy and rapid response.Keywords:

Real-time compensation of errors in refractive index shift measurements of microring sensors using thermo-optic coefficients.

We report a method for compensation of errors caused by temperature fluctuations in refractive index measurements using Silicon photonic microring sensors. The method involves determination of resonance wavelength shifts caused by thermal fluctuations using real-time measurement of on-chip temperature variations and thermo-optic coefficient (TOC) of analyte liquids. Resistive metal lines patterned around Silicon microrings are used to track temperature variations and TOC of analyte is calculated by measuring wavelength shifts caused by controlled increments in device temperature. The TOC of de-ionized water is determined to be -1.12 × 10-4/°C, with an accuracy of ±8.26 × 10-6/°C. In our system, chip-surface temperature variations were measured with an instrument limited precision of 0.004 °C yielding a factor of 16 enhancement in tracking accuracy compared to conventional, bottom-of-chip temperature measurement. We show that refractive index detection limit of the microring sensor is also improved by the same factor.

Novel method of dual fiber Bragg gratings integrated in fiber ring laser for biochemical sensors

Optical sensors have been shown to be very effective for measuring the toxic content in liquid and air environments. Optical sensors, which operate based on the wavelength shift of the optical signals, require an expensive spectrometer. In this paper, we propose a new configuration of the optical sensor device for measuring wavelength shift without using a spectrometer. This configuration has a large potential for application in biochemical sensing techniques, and comes with a low cost. This configuration uses dual fiber Bragg gratings (FBGs) integrated in a fiber ring laser structure of erbium-doped fiber, in which one FBG is used as a reference to sweep over the applicable spectrum of the etched-Bragg grating. The etched-FBG as a sensing probe is suitable for bio- and/or chemical sensors. The sensitivity and accuracy of the sensor system can be improved by the narrow linewidth of emission spectra from the laser, the best limit of detection of this sensor is 1.5 × 10−4 RIU (RIU: refractive index unit), as achieved by the optical sensor using a high resolution spectrometer. This sensor system has been experimentally investigated to detect different types of organic compounds, gasoline, mixing ratios of organic solvents in gasoline, and nitrate concentration in water samples. The experimental results show that this sensing method could determine different mixing ratios of organic solvents with good repeatability, high accuracy, and rapid response: e.g. for ethanol and/or methanol in gasoline RON 92 (RON: research octane number) of 0%–14% v/v, and nitrate in water samples at a low concentration range of 0–50 ppm. These results suggest that the proposed configuration can construct low-cost and accurate biochemical sensors.

Insensitivity to Humidity in Fabry–Perot Sensor With Multilayer Graphene Diaphragm

We developed a novel open-cavity Fabry-Perot (F-P) interferometer with multilayer graphene diaphragm suspended on the endface of a ferrule. The insensitivity to relative-humidity (RH) ranging from ~20%RH to 70%RH was investigated in terms of the wavelength shift and power change in interference spectrum, along with the balanced air pressure on both sides of the graphene membrane. The measured wavelength shift of 0.02 nm/%RH and maximum power change of 0.02 dB/%RH demonstrated the humidity-insensitive response of at least one order lower than those for previously reported F-P humidity sensors using porous hydrophilic films. Furthermore, the fitted coupled interaction between dominated temperature and minor humidity yielded a good fitting R-square of about 99.5%. These measurements could shed light on the environmental adaptability of F-P sensors using graphene or other 2-D nanomaterials.

Long-period fiber grating: a specific design for biosensing applications.

In this paper, a detailed investigation on the modeling of long-period fiber grating (LPFG) sensors is discussed with the aim of providing a more realistic solution for their use in biosensing. Add-layer sensitivity, i.e., sensitivity of the sensor to an additional layer adhered onto the fiber surface, is quantified and a clear and complete analysis about the influence of the average thickness of the deposited biological sensing layers, as well as the change in refractive index of these layers, on the resonant wavelength of the cladding modes of an LPFG is provided. Add-layer sensitivity of LPFG sensors close to mode transition (MT) and also at turn-around point (TAP) are taken into account. Adsorbed layer thicknesses, as estimated from measured wavelength shifts of the LPFG, are found to have a good match with the values obtained through other measurement techniques.

Optical sensing with Anderson-localised light

We show that fabrication imperfections in silicon nitride photonic crystal waveguides can be used as a resource to efficiently confine light in the Anderson-localised regime and add functionalities to photonic devices. Our results prove that disorder-induced localisation of light can be utilised to realise an alternative class of high-quality optical sensors operating at room temperature. We measure wavelength shifts of optical resonances as large as 15.2 nm, more than 100 times the spectral linewidth of 0.15 nm, for a refractive index change of about 0.38. By studying the temperature dependence of the optical properties of the system, we report wavelength shifts of up to about 2 nm and increases of more than a factor 2 in the quality factor of the cavity resonances, when going from room to cryogenic temperatures. Such a device can allow simultaneous sensing of both local contaminants and temperature variations, monitored by tens of optical resonances spontaneously appearing along a single photonic crystal waveguide. Our findings demonstrate the potential of Anderson-localised light in photonic crystals for scalable and efficient optical sensors operating in the visible and near-infrared range of wavelengths.

Measuring Solar Doppler Velocities in the He ii 30.38 nm Emission Using the EUV Variability Experiment (EVE)

The EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory has provided unprecedented measurements of the solar EUV irradiance at high temporal cadence with good spectral resolution and range since May 2010. The main purpose of EVE was to connect the Sun to the Earth by providing measurements of the EUV irradiance as a driver for space weather and Living With a Star studies, but after launch the instrument has demonstrated the significance of its measurements in contributing to studies looking at the sources of solar variability for pure solar physics purposes. This paper expands upon previous findings that EVE can in fact measure wavelength shifts during solar eruptive events and therefore provide Doppler velocities for plasma at all temperatures throughout the solar atmosphere from the chromosphere to hot flaring temperatures. This process is not straightforward as EVE was not designed or optimized for these types of measurements. In this paper we describe the many detailed instrumental characterizations needed to eliminate the optical effects in order to provide an absolute baseline for the Doppler shift studies. An example is given of a solar eruption on 7 September 2011 (SOL2011-09-07), associated with an X1.2 flare, where EVE Doppler analysis shows plasma ejected from the Sun in the He ii 30.38 nm emission at a velocity of almost \(120~\mbox{km}\,\mbox{s}^{-1}\) along the line-of-sight.

Measurement of small wavelength shifts based on total internal reflection heterodyne interferometry

This Letter presents a method of an optical sensor for measuring wavelength shifts. The system consists of a diffraction grating and a total internal reflection heterodyne interferometer. As a heterodyne light beam strikes a grating, the first-order diffraction beam is generated. The light penetrates into a total internal reflection prism at an angle larger than the critical angle. A wavelength variation will affect the diffractive angle of the first-order beam, thus inducing a phase difference variation of the light beam emerging from the total internal reflections inside the trapezoid prism. Both the experimental and theoretical results reveal that, for the first-order diffractive beam, the sensitivity and resolution levels are superior to 5°/nm and 0.006 nm, respectively, in the range of wavelength from 632 to 634 nm, and are superior to 3.1°/nm and 0.0095 nm in the range from 632 to 637 nm. For the theoretical simulation of the fourth-order diffractive beam, they are superior to 6.4 deg/nm and 0.0047 nm in the range from 632 to 637 nm.

A Compact Sagnac Loop Based on a Microfiber Coupler for Twist Sensing

We present a compact highly sensitive microfiber coupler-based twist sensor formed by fusing and tapering two optical fibers and then connecting two of the pigtails to form a Sagnac loop. The device is miniature, low cost, and easily fabricated. The sensor has high twist sensitivities of $\sim 0.9$ nm/° when measuring wavelength shift and $\sim 0.16$ dB/° when measuring transmission power, both of which confirm that the sensor has good potential for application in the structural health monitoring and other areas.

Nanoimprinted distributed feedback lasers comprising TiO2 thin films: Design guidelines for high performance sensing

AbstractDesign guidelines for optimizing the sensing performance of nanoimprinted second order distributed feedback dye lasers are presented. The guidelines are verified by experiments and simulations. The lasers, fabricated by UV‐nanoimprint lithography into Pyrromethene doped Ormocomp thin films on glass, have their sensor sensitivity enhanced by a factor of up to five via the evaporation of a titanium dioxide (TiO2) waveguiding layer. The influence of the TiO2 layer thickness on the device sensitivity is analyzed with a simple model that accurately predicts experimentally measured wavelength shifts induced by varied superstrate refractive indices. The superstrate refractive index is additionally shown to determine which of the possible waveguiding modes dominates for lasing, indicating a method to flexibly select the polarization of the laser. The detection limit of the sensor system is further discussed, finding an optimum at 7.5· 10−6 RIU. Wavelength changes caused by dye bleaching must be taken into account for long‐term measurements.

Curvature sensor based on side-leakage photonic crystal fiber with high sensitivity and broad linear measurement range

Based on Sagnac interferometer by incorporating a segment of side-leakage photonic crystal fiber (SLPCF), a curvature sensor with high sensitivity and broad linear measurement range was proposed and demonstrated experimentally. Experimental results show that high sensitivity to curvature can be achieved by measuring group modal birefringence and wavelength shift of transmission dip when the linear side-leakage defect is in the same direction as the bending, but the linear measuring range is not large enough to cover the small curvature. When the linear side-leakage defect is in the vertical direction to the bending, the sensitivity can be as high as 10.798 nm·m-1 and the linear measurement range as wide as 0–5.03 m-1 by measuring wavelength shift of transmission. Combined with the introduction of the measurement matrix, the measurement of temperature and curvature can be realized simultaneously, which would offer a way to eliminate the perturbation of the environment temperature to the measurement values of the curvature. Under the same conditions, though the sensitivity obtained by measuring group modal birefringence is low, it can be regarded as a curvature sensor which is insensitive to temperature.

Slope Stability Analysis Based on Measured Strains along Soil Nails Using FBG Sensing Technology

In the past few decades, slope stability analysis using numerical methods is becoming a hot issue, but it is based on extremely ideal assumptions. Soil nailing technique, as one of the most cost-effective reinforcing methods, has already been widely used for reinforcing slopes. In this study, to evaluate the safety factor of a slope, the strains on soil nails were measured by fiber Bragg grating (FBG) sensor. Strains along soil nails in the same cross section of a slope can be computed using the measured wavelength shifts of FBG sensors. In order to evaluate the stability of a slope, an optimal model was proposed to search the potential slip surfaces based on measured strain values. Maximum sum of strains on soil nails at different elevations of the same cross section was taken as the objective. Positions of soil nails, circular slip surface, and boundary conditions of the soil nails were summarized and taken as constraints. Finally, safety factors can be computed using the searched slip surface regarding the axial stress of soil nails. This method combines the limit equilibrium methods with measured axial strains on site which can reflect the actual condition of field slopes.