Center Wavelength Shift Research Articles

Real-time collision dynamics of vector solitons in a fiber laser

Particle-like structures of solitons, as a result of the balance between dispersion and nonlinearity, enable remarkable elastic and inelastic soliton collisions in many fields. Despite the experimental observation of temporal vector-soliton collisions in birefringent fibers, collision dynamics of vector solitons in fiber lasers have not been revealed before, to the best of our knowledge. Here, the real-time spectral evolutions of vector solitons during collisions in a dual-comb fiber laser, which generates vector solitons with slightly different repetition rates, are captured by a time-stretch dispersive Fourier transform technique. We record the whole process of vector-soliton collisions, including the formation of weak pulses induced by cross-polarization coupling, opposite central wavelength shifts of both vector solitons, distinct intensity redistribution and dissipative energy, and gradual recovery to initial states. Furthermore, extreme collisions with strong four-wave mixing sidebands are observed by virtue of coherent coupling between the orthogonal polarization components of vector solitons. Numerical simulations match well with the experimental observations. The experimental and numerical evidences of vector-soliton collision dynamics could give insight into the understanding of nonlinear dynamics in fiber lasers and other physical systems, as well as the improvement of laser performance for application in dual-comb spectroscopy.

An Optical Chiral Sensor Based on Weak Measurement for the Real-Time Monitoring of Sucrose Hydrolysis.

A chiral sensor with optical rotation detection based on weak measurement for the kinetic study of sucrose hydrolysis is presented. Based on the polarization modulation to the pre-selection state, the optical rotation of chiral sample was accurately determined through the central wavelength shift of the output spectrum. With this approach, the concentration response curves of sucrose and its hydrolysis products, i.e., fructose and glucose, were experimentally obtained for the hydrolysis analysis. By collecting the output spectrum with a frequency of 100 Hz and fitting the central wavelength shift synchronously during the measurement, the sucrose hydrolysis process was monitored in real time. Different hydrolysis conditions with varied concentration of invertase enzyme and citrate were implemented in this work. As a consequence, the real-time hydrolysis curves of the hydrolysis process with distinct velocities was achieved and analyzed. Such a kinetic monitoring about sucrose hydrolysis with optical rotation detection technology played a critical role in the researches involving sucrose, and also revealed the great potential of weak measurement in intersections, such as food safety inspection and chemical analysis.

Optical Filter Bank Modeling and Design for Multi-Color Visible Light Communications

Multi-color visible light communication (MC-VLC) can achieve a multi-fold rate by taking advantage of multiple color channels. The optical filters for separating different colors are often designed for the normal incidence case only. However, the center wavelength (CWL) of the optical filter shifts as the angle of incidence (AOI) varies, leading to performance degradation. To address the issue caused by the CWL shift, we propose a novel concept of the optical filter bank that uses a group of narrow-band optical filters and divides the whole visible light spectrum into many narrow bands of wavelengths. The optical filter bank can flexibly combine the useful signal of each monochromatic color and effectively avoid the undesired interference, even if the CWL shifts. Based on the optical filter bank, we establish the channel model and signal model of the MC-VLC system, and then jointly design the linear precoding and equalization for MC-VLC. As illustrated by simulations, the proposed optical filter bank transceiver can achieve a high rate and stable performance at varying AOIs.

Evaluation of a Novel Spectral Pair of a Chirp FBG Embedded in a Cantilever Beam for Simultaneous Temperature and Transverse Forces Measurement

An evaluation of a novel spectrum features combining the distant longer and shorter wavelengths of a single chirped fiber Bragg grating (CFBG) for temperature and transverse forces discrimination is experimentally demonstrated. The shift of the two distant wavelengths’ pair is compared with the conventional pair combining the bandwidth modulation and center wavelength shift. The CFBG sensor is simply bonded to a cantilever beam and subjected to transverse loading (four times) and a heating-cooling cycle. The transverse forces calibration results show a repeatability of 3.9 pm and 1.7 pm for the bandwidth’s and center wavelength’s responses, respectively, while the distant wavelengths’ show a repeatability of 2.37 pm and 3.01 pm, respectively. The cantilever CFBG sensor exhibits high correlation coefficients of 0.9 between the two heating and cooling data sets, except for the bandwidth, which only had a lower coefficient of 0.75. The linear model of both pairs for calculating temperature and transverse forces can provide an accurate estimate, with the longer-shorter wavelengths’ pair having an advantage over the pair of the bandwidth-centre-wavelength. The study has demonstrated the feasibility of the method proposed by our group in a previous work, by which three physical quantities can be measured with a single custom FBG.

Integrated Ultrahigh-Sensitivity Temperature Sensor Based on Asymmetric Mach–Zehnder Interferometer and Stress Deformation of Aluminum-SiO2

This paper proposes an integrated high-sensitivity temperature sensor based on SiO2-asymmetric Mach-Zehnder interferometer (AMZI) and thermal expansion induced stress of aliminium-SiO2. Mechanical analysis of this structure is carried out. The result shows that the SiO2 chip is compressed, and the waveguide length is decreased, leading to the central wavelength shift of the sensor. The sensitivity of the sensor can be customized by different arm lengths of the AMZI. The chip is fabricated using standard planar lightwave circuit, and the aluminium alloy plate is glued to the chip using α-cyanaloc acrylic resin adhesive. The sensitivities of two sensors with different arm lengths are tested. The test results show the sensitivity reaches 6.86 nm∙K-1 and 54.2 nm∙K-1, respectively, which coincides with the theoretical analysis. With low cost and customizable sensitivity, this new device is applicable in many fields.

Research on end force measurement method of soft robot based on fiber Bragg grating

The measurement of the terminal operating force of the surgical robot is the key to achieve the precise control of the robot, which is crucial to ensure the safety of the surgical operation. In this paper, a method of measuring the three-dimensional force of soft robot terminal end based on fiber Bragg grating was proposed to meet the requirement of minimally invasive surgery. Based on the principle of fiber grating sensing, the sensing characteristics of fiber sensor embedded in soft robot were analyzed and a soft robot terminal force decoupling model based on linear calibration with least square method and nonlinear compensation with Bernstein polynomial was established. Then the relationship between the central wavelength shift of the fiber Bragg grating and the three-dimensional force at the end of the soft robot was studied. The results show that the average repeatability of FBG sensor is 1.5 pm. The measurement accuracy error of the end force in the three directions of XYZ is lower than 5% of the full range, and the residual distribution is mostly concentrated in the reliable range, with good repeatability. The proposed FBG decoupling algorithm provides an effective method for the precise measurement of the terminal force of soft robot in minimally invasive surgery, and has a promising application prospect in the measurement of the terminal force of soft robot in biomedicine.

An optical fiber sensor based on polyimide coated fiber Bragg grating for measurement of relative humidity

Humidity is an important parameter in many industry fields and accurate humidity measurement is of great significance to improve production efficiency and ensure product quality. This paper presents a sensing model and scheme for utilizing thermoplastic polyimide (PI) coated fiber Bragg grating (FBG). The humidity sensitive PI layer was firstly prepared and coated on the FBG. The humidity calibration experiment was designed to verify the sensor performance based the saturated salt solution method. The experimental results show this FBG humidity sensor can work steady in the relative humidity (RH) condition ranging from 11%RH to 83% RH, with the sensitivity of the sensor less than 2 pm/% RH and measurement uncertainty of ±1% RH in the temperature range of 30 °C to 70 °C. In order to solve the problem that the PI-coated FBG is both sensitive to the humidity and temperature (cross sensitivity problem), a novel method of temperature compensation has been proposed. The method uses the character that the coefficients of expansion of the PI-coated layer for humidity and temperature are linearly superimposed and obtains the temperature compensation coefficient by linear interpolation of the function relationship between the measured temperature and the FBG central wavelength shift. The analysis of experimental data shows that within the temperature variation range of 20 °C, this temperature compensation method can effectively guarantee the average wavelength errors within 0.03 nm without reference grating. In this way, it can extend the FBG humidity measurement under environmental temperature up to 90 °C making it possible for application in the measurement the humidity in the high temperature environment.

Selective fiber Bragg grating inscription in four-core fiber for two-dimension vector bending sensing.

The paper presents selective fiber Bragg grating (FBG) inscription in four-core fiber based on a phase mask scanning method. The inscription factors are systematically investigated, which involves fiber core position and focused laser beam size in fiber, etc. Several specific inscriptions (including individual, dual and all inscriptions) are demonstrated. Two orthogonally positioned cores are selectively inscribed and applied to two-dimension vector bending measurement. The measured bending sensitivities of two FBGs range from -54.3 pm/m-1 to 52.2 pm/m-1 and -53.7 pm/m-1 to 52.8 pm/m-1, respectively. More importantly, it has been revealed that their sensitivities versus bending direction follow regular cosinoidal and sinusoidal distribution. The direction and amplitude of the vector bending can be recovered using measured central wavelength shifts of those two FBGs.

Buildup dynamics of asynchronous vector solitons in a polarization-multiplexed dual-comb fiber laser.

Polarization-multiplexed dual-comb fiber lasers enable significant applications in dual-comb spectroscopy and optical sensing. However, the complexity of the underlying formation dynamics of dual-comb solitons has not been unveiled so far. Here, we capture the real-time spectral evolutions of both vector solitons from the initial fluctuations, with the help of the time-stretch dispersive Fourier transform technique. Both vector solitons experience the relaxation oscillation, quasi-mode-locking, beating dynamics, and mode locking, accompanying central wavelength shifts in opposite directions, which might be induced by the gain saturation during their buildup processes. Moreover, polarization-dependent gain in the gain fiber leads to the different buildup time of both vector solitons. Our findings open new perspectives for dual-comb buildup dynamics and might impact laser design for applications.

Determination of Stark Shifts and Widths Using Time Resolved Laser-Induced Breakdown Spectroscopy (LIBS) Measurements.

Stark broadening parameters have been estimated for resonant lines of Al(I) using time resolved measurements. The relation between the various emission line characteristics at different phases of opacity have been utilized to obtain the value of plasma temperature and Stark width parameters from the experimental data. The observed value of the center line intensity and Lorentzian component of the line width are compared against a simulated value of these parameters for optically thin case. The plasma temperature and Stark broadening parameters are obtained for the best fit condition by matching the experimentally observed and the simulated values of intensity and line widths. The time resolved measurements result in much better estimates for Stark parameters by allowing multiple points for fitting keeping the number of variables limited. The Stark shift parameters are also obtained from the slope of the plot of observed central wavelength shifts versus observed electron number density which is measured as a function of time. Hence, a method utilizing multiple-time observation data to obtain the Stark broadening parameters for lines showing self-absorption has been demonstrated.

Fabrication of Multiple Superimposed Fiber Bragg Gratings for Multiple Parameter Sensing

Multiple superimposed fiber Bragg gratings (FBGs) are fabricated based on a non-uniform single-core As2Se3-Poly methyl methacrylate (PMMA) tapered fiber, by which superimposition of up to nine chirped FBGs is achieved. Multiple superimposed uniform FBGs are first inscribed on the nonuniform waveguide and multiple superimposed chirped FBGs are obtained by stretching the non-uniform waveguide. Simultaneous measurement of temperature and strain is realized based on two superimposed chirped FBGs at wavelengths of 1540 nm (FBG1) and 1550 nm (FBG2), respectively. The central wavelength shifts and bandwidth changes of the two superimposed chirped FBGs, when temperature and strain change, are measured, respectively. Four matrices are defined to predict the temperature and strain change, and the one defined by measuring the bandwidth change of FBG1 and the central wavelength shift of FBG2 gives the most accurate measurement with the sensitivities of 29.1 pm/°C for temperature measurement and 0.373 pm/μe for strain measurement by measuring the bandwidth change of FBG1, and 75.1pm/°C for temperature measurement and 0.245 pm/μe for strain measurement by measuring the central wavelength shift of FBG2, respectively. The resolutions for temperature and strain measurement are enhanced by more than a factor of 10, respectively, compared with those using the matrix defined by measuring the wavelength shifts of FBG1 and FBG2. A large strain change from 0 to 22000 μe is achieved due to the low stiffness of As2Se3-PMMA taper, which is far beyond the operating range of silica materials.

Bandwidth modulation and centre wavelength shift of a single FBG for simultaneous water level and temperature sensing

Abstract A diaphragm-based hydrostatic level transducer with the use of a single Fibre Bragg grating (FBG) is presented for simultaneous measurement of water level and temperature. In order to distinguish the mismatch between water level and temperature sensitivity, a bandwidth modulation and centre wavelength shift measurement technique was introduced. The change in bandwidth is sensitive towards water level variations with a sensitivity of 0.0015 nm/cm. Meanwhile, the change of centre wavelength is sensitive towards water level and temperature variations, with sensitivities of 0.0008 nm/cm and 0.0365 nm/°C, respectively. By expressing the obtained sensitivities in a matrix equation approach, simultaneous water level and temperature measurement can be predicted accurately.

Analysis of the smoothing performance of a rapid polarization rotation smoothing scheme.

In order to improve the irradiation uniformity on the target plane and reduce the parametric backscattering in indirect-drive inertial confinement fusion facilities, a rapid polarization rotation smoothing scheme is proposed. In this scheme, there is a central wavelength shift between adjacent beams in a laser quad. Two of these beams are transformed into left-rotating circularly polarized beams by using polarization control plates, and the other two are transformed into right-rotating circularly polarized beams. Moreover, conjugate spiral phase plates are adopted to transform the beams into conjugate Laguerre-Gaussian beams. As a result of the frequency beat among the beams with counter-rotating polarization and conjugate spiral phases, both the intensity distribution and polarization of the focal spot would rotate rapidly on the picosecond timescale. The physical model of the rapid polarization rotation smoothing scheme is built up, and the smoothing performance affected by the parameters of spiral phase plates, polarization control errors, wavelength shift, and wavefront distortions is analyzed in detail. The results indicate that the rapid polarization rotation smoothing scheme has a relatively wide tolerance to the polarization errors and the spiral phase plates, and can improve the smoothing performance effectively with the combination of the smoothing by spectral dispersion scheme.

Wide-Range Fiber Bragg Grating Tilt Sensor Based on a Cam Structure

This work presents a fiber Bragg grating (FBG) wide-range tilt sensor based on a cam structure. Two FBGs are fixed on a specially-designed sensor shell. When one FBG is stretched by the rotating cam driven by tilt, the other FBG moves along a standard semicircle with a constant radius, and compensates for temperature variations of the stretched FBG. By detecting the difference of the central wavelength shifts of two FBGs, variations of the tilt angle can be measured. The detailed mechanical structure design and measurement theory of the sensor is presented, and the sensor prototype is discussed. Testing results of comprehensive performance show that there is a good linear relationship between the tilt angle and the central wavelength of the FBG over three intervals from -90° to 90°. The repeatability and creep resistance of the FBG tilt sensor are good, the measuring accuracy is 1.516°, and the measuring resolution is 0.003°. The measurement range is much larger than comparable existing sensors, and it avoids cross-sensitivity between temperature and strain for FBG. The sensitivity of the sensor can be easily adjusted by changing the outline of the cam, or the length of the optical fiber between two fixed points.

Experimental Investigation and Error Analysis of High Precision FBG Displacement Sensor for Structural Health Monitoring

High precision structural displacement monitoring is challenging, but an effective method for structural health monitoring and particularly damage evaluation. In this paper, a high precision fiber Bragg grating (FBG) displacement sensor with embedded spring is developed to monitor structural displacement variation even at very small ranges. The principle of such monitoring is based on the central wavelength shifts in accordance with the displacement between measuring points. Calibration experiments are conducted to examine the performance of the innovative displacement sensor. The result indicates that the sensor has excellent linearity and repeatability, with the sensitivity coefficient being 23.96[Formula: see text]pm/mm, and the static relative error is 4.94% after three loading and unloading cycles. The displacement sensor therefore shows an excellent sensitivity and high precision for application. Moreover, it has been verified that this sensor is suitable and applicable for displacement monitoring in quasi-static experiment of structures.

Simultaneous measurement of temperature and strain using double-cladding fiber based hybrid Bragg grating

A novel hybrid fiber Bragg grating (FBG) for simultaneous measurement temperature and strain is proposed. This sensor is fabricated by inscribing fusion splice joint between two types of fibers using excimer laser and phase mask. Due to the different characteristics of the two fibers, the two resonance peaks have different responses to temperature and strain, and both physical parameters can be demodulated by analyzing the central wavelength shifts. Practical measuring results show that the hybrid FBG proposed in this paper can precisely achieve simultaneous measurement of temperature and strain. This work provides a new scheme for multi-parameter simultaneous sensing.

Temperature and Emissivity Inversion Accuracy of Spectral Parameter Changes and Noise of Hyperspectral Thermal Infrared Imaging Spectrometers

The emergence of hyperspectral thermal infrared imaging spectrometers makes it possible to retrieve both the land surface temperature (LST) and the land surface emissivity (LSE) simultaneously. However, few articles focus on the problem of how the instrument’s spectral parameters and instrument noise level affect the LST and LSE inversion errors. In terms of instrument development, this article simulated three groups of hyperspectral thermal infrared data with three common spectral parameters and each group of data includes tens of millions of simulated radiances of 1525 emissivity curves with 17 center wavelength shift ratios, 6 full width at half maximum (FWHM) change ratios and 6 noise equivalent differential temperatures (NEDTs) under 15 atmospheric conditions with 6 object temperatures, inverted them by two temperature and emissivity separation methods (ISSTES and ARTEMISS), and analyzed quantitatively the effects of the spectral parameters change and noise of an instrument on the LST and LSE inversion errors. The results show that: (1) center wavelength shifts and noise affect the inversion errors strongly, while FWHM changes affect them weakly; (2) the LST and LSE inversion errors increase with the center wavelength shift ratio in a quadratic function and increase with FWHM change ratio slowly and linearly for both the inversion methods, however they increase with NEDT in an S-curve for ISSTES while they increase with NEDT slightly and linearly for ARTEMISS. During the design and development of a hyperspectral thermal infrared instrument, it is highly recommended to keep the potential center wavelength shift within 1 band and keep NEDT within 0.1K (corresponding LST error < 1K and LSE error < 0.015) for normal applications and within 0.03K (corresponding LST error < 0.5K and LSE error < 0.01) for better application effect and level.

Temperature induced modulation of near-infrared photoluminescence in BaTiO3:Er

Due to the shielding effect of the outermost electronic layer 5s and 5p of rare earth, the near-infrared photoluminescence (PL) of Er3+ can not be easily affected by the external environment. However, we have regulated the near-infrared PL of Er3+ by changing temperature, which is ascribed to the phase transition of BaTiO3 from tetragonal to cubic phase and the presence of impurity. It also found that the redshift of wavelength in samples containing tetragonal phase BaTiO3 can reach 13 nm, which is never observed in other materials. The center wavelength shifts from 1530 nm to 1543 nm as the temperature rises from 20 to 200 °C. Moreover, the full width at half maximum (FWHM) of samples containing cubic phase has a growth of 6 nm (up to 85 nm) with increasing temperature. It indicated that temperature-induced phase transition can be an important method to modulate the PL of Er3+ in BaTiO3 with impurity.

Resonant Wavelength Thermal Stability of Fiber Bragg Gratings Produced by Femtosecond Laser

This study is focused on the fiber Bragg gratings central resonant wavelength thermal stability, fabricated by the point-by-point inscription method, using an infrared femtosecond laser. A new methodology is proposed to analyze the refractive index decay using the central wavelength shift of fiber Bragg gratings, executing stepped and continuous annealing regimes up-to a maximum temperature of 800 °C. The obtained thermal refractive index decay follows a typical power-law function, and from the theoretical model fit to the experimental data was possible to simulate and predict the refractive index decay over a large time period, up-to 1000 years. With the proposed approach it is possible to adjust the fiber Bragg gratings pre-treatment conditions to ensure effective operation during a certain time period, at a specific working temperature, without any degradation. An experimental method to mitigate this decay is proposed, and the results demonstrate the high potential of using infrared femtosecond inscribed fiber Bragg gratings for high temperature monitoring.

Real-time in situ monitoring of internal stress of the electroplating processes using FBG sensors

To study the internal stress during the electroplating processes, the Fiber Bragg Grating (FBG) sensors were electroplated with Ni, Zn, Cu films, respectively. During the electroplating, the current density was set to be 24 mA/cm2. The changes of internal stress of the metal films can be calculated according to the shift of central wavelength of the FBG sensors, blue shift showing a compressive stress and red shift showing a tensile stress. Results show: (i) the central wavelength of the FBG sensor blue shifted 2.26 nm after electroplating Ni for 420 min, the associated compressive stress in FBG sensor was calculated to be 551 MPa; (ii) the central wavelength of the FBG sensor red shifted 0.29 nm after electroplating Zn for 420 min, the associated tensile stress in FBG sensor was calculated to be 69 MPa; (iii) the central wavelength of the FBG sensor red shifted 0.04 nm after electroplating Cu for 420 min, the associated tensile stress in FBG sensor was calculated to be 10 MPa; (iv) a model characterizing the relationship between the internal stress and the central wavelength shift of the FBG was proposed, the feasibility and associated conclusions are verified by the nanoindentation tests.