Center Wavelength Shift Research Articles

33.2: A Tunable Filter with Single Layer Twist Structure Liquid Crystals

A bandwidth tunable filter and a multi‐pitch filter based on twist structure liquid crystals (LCs) were proposed. The LC filters can be fabricated by multiple templating and refilling process, which shows large bandwidth and good extensibility. The bandwidth of the twist structure LC filter can be tuned from 111 nm to 244 nm with central wavelength shift of 16 nm. Furthermore, the bandwidth can be broadened to a great extent theoretically by the simple fabrication process. Three‐pitch twist structure LC filter was obtained, and the pitch number can be increased by utilizing multiple templating technique. The fabricated tunable twist structure LC filters show a potential application in optical communication device and spectral systems.

Estimation of the nonlinear dependence of the indications of a fiber Bragg grating on temperature and strain from experimental data

The readings of the Bragg grating are determined based on the optical radiation reflected from it. A quantitative characteristic of this radiation is the wavelength at which the maximum power of the optical signal is achieved. This characteristic is called the central wavelength of the grating. The central wavelength shift depends on temperature and strain. As a rule, a linear approximation of this dependence is used. However, from the available literature it is known that, the grating wavelength shift demonstrates a strong nonlinear dependence on temperature at 5<T<200K and a weak quadratic dependence close to room temperature. Thus far, the authors have not found studies that consider all terms in the quadratic expansion of the central wavelength of the Bragg grating as a function of temperature and strain at near-room temperatures. Our work is intended to fill this gap. The article describes an experiment in which an optical fiber with Bragg grating was subjected to loading using three different weights. A step-wise temperature change from 5 to 100 0С was realized for each weight. Based on these data, all terms of the quadratic expansion of the desired function are determined. The contribution of each term is estimated.

Surface exciton polariton resonances (SEPR)–based sensors

A new type of resonance in the development of sensors using long-range surface exciton polariton (LRSEP) phenomena has been coined: surface exciton polariton resonance (SEPR). The resonance was obtained in the reflected spectrum of a Kretschmann-Raether setup with a two-coupled-interface structure composed of 412 nm magnesium fluoride and 50 nm chromium thin films. The roles of different parameters such as thicknesses of the films and the incidence angles have been simulated. Some preliminary experimental results show a promising performance with a shift of the resonance central wavelength with changes in the incidence angle of -136.52 nm/° and a sensitivity of 23,221 nm/refractive index unit.

Theoretical analysis of beam quality degradation induced by laser spectral characteristics in spectral beam combining system

We establish a model based on the dispersion of the diffractive grating to investigate the influence of the spectral envelope and wavelength stability on the combined beam quality M 2 of the spectral beam combining (SBC) system. We obtain the analytic expressions of combined beam quality corresponding to the second-moment-spectral width of the beamlets, which can be used to forecast the combined beam quality for any integrable spectral envelope shape of beamlets in an SBC system. Additionally, we find that the center wavelength shift of beamlets has a significant impact on the combined beam quality. The second-moment-based width of the center wavelength shift distribution among all beamlets should be less than 0.5 times the spectral width. Here, the spectral width is the weighted root mean square of all beamlets’ spectral width, where the weight is the power of each single channel.

Thermo-optic tunable optical filters with GHz-bandwidth and flat-top passband on thin film lithium niobate platform.

Lithium niobate on insulator (LNOI) is a new photonic integrated platform that provides high optical confinement and retains the inherent excellent properties of lithium niobate (LN). Tunable filters are one of the indispensable devices for integrated optics. Here we design and fabricate a thermo-optic (TO) tunable optical filter using two cascaded racetrack microring resonators (MRRs) based on LNOI. The filter shows a narrow and flat top passband with intra band ripple less than 0.3 dB, 3 dB bandwidth of 4.8 GHz and out-of-band rejection of about 35 dB. The insertion loss of the filter is about -14 dB, including grating coupling loss about -6.5 dB and on-chip loss less than -1 dB. The heating power for center wavelength shift of the filter is about 89.4 mW per free spectral range (FSR). Relevant applications of such filters include optical information processing and microwave photonics.

Real-time monitoring the hydrolysis of chiral molecules by optical weak measurement

As one of important optical properties of chiral molecules, the real-time monitoring of the optical rotation is of great significance to study the reaction characteristics of chiral molecules. In this study, we have built an optical frequency domain weak measurement system with linear common-path method for chiral sensing, in which the response between optical rotation and central wavelength shift can reach more than 1200 nm/rad. The high-precision measurement of the optical rotation of chiral molecules and the high-sensitivity real-time monitoring of the decomposition rate in the acidic hydrolysis of sucrose under different concentrations of catalysts are realized using this system. The experimental results are basically consistent with the theoretical prediction. This study shows that optical weak measurement chiral sensing can monitor the hydrolysis degree of chiral molecules in real time without separating and labeling molecules. It has great application potential in the production and research of chiral compounds, especially chiral medicine.

Pulsating dynamics in a pure-quartic soliton fiber laser.

We numerically investigate the pulsating dynamics of pure-quartic solitons (PQSs) in a passively mode-locked fiber laser. The bifurcation diagrams show that the PQS can alternate between the stable single soliton and pulsating regimes multiple times before transiting into the chaotic state. This multi-alternation behavior can be attributed to energy redistribution across the central part and the oscillating tails of the PQS, which is caused by an imperfect counterbalance between self-phase modulation (SPM)-induced and fourth-order dispersion (FOD)-induced phase shifts. Soliton creeping behavior can be observed during the pulsating process, accompanied by periodic asymmetric temporal profiles and central wavelength shifts of the PQS. These findings give new insights into the dynamics of PQSs in fiber lasers.

Size effects of AlGaInP red vertical micro-LEDs on silicon substrate

To study the size effect of AlGaInP red micro-LEDs on the silicon substrate, we fabricated five AlGaInP red micro-LEDs with different pixel sizes (160 × 160, 80 × 80, 40 × 40, 20 × 20, and 10 × 10 µm2) and studied their electrical and optical properties. Smaller micro-LEDs have smaller leakage current and larger series resistance and can withstand higher current density without the current crowding effect. Due to the larger perimeter-to-area ratio of small-sized micro-LEDs, non-radiative recombination increases, which leads to a lower EQE. But smaller micro-LEDs can alleviate the problem of the high-current efficiency droop. In addition, because of a better heat dissipation under a high injection current, smaller micro-LEDs (>80 µm) have a smaller center wavelength shift. These experimental results provide important data for designing and fabricating red micro-LEDs with different pixel sizes for diverse future applications.

Profile Morphology of Fiber Bragg Gratings During Initial Cracking: A Case Study on Proof Load Testing of an Arch Spandrel Segment

The occurrence of “initial cracking” in bridges during proof load testing (PLT) is a relatively brief procedure, taking only a few hours. Its long-term effects may play a role in future understanding of the future behavior of the bridge. In this paper, we document a brief period of PLT via interrogation an FBG sensor (central wavelength of 1554.2nm) in an arch spandrel bridge which was constructed before opening for public use. We logged data before, during and after the load testing session using a broadband light source and a tunable laser. Based on the data, we discuss the wavelength shift profiles, and its morphology during the period. The main findings of this study reveals that the initial cracking procedure exhibits unique behavior indicated by the sudden changes of the FBG spectra, though exhibiting some latency. Prior to the PLT session (acclimatizing window), there were already perturbations to structure causing up to 0.2nm shift in the FBG central wavelength, while during the PLT session, the central wavelength snapped between ~1554.1nm to ~1554.3nm. The fluctuations stopped after the PLT, confirmed with tunable lasers’ profile as well. These findings demonstrates that not only initial crackings most likely can be detected via PLT, but it may also occur before the PLT exercise itself due to its own intrinsic load.

Ultra-Short Pulse Generation From Optical Parametric Oscillators With a Cavity-Length Detuning

We report that the pulse compression mechanism in cavity-length-detuned optical parametric oscillators (OPOs) could be used to generate high-quality, soliton-like optical pulses with sub-100 fs duration. By numerical simulations, we illustrated the pulse and spectral evolution in an OPO with respect to cavity-length detuning, including central wavelength shift, spectral broadening and pulse compression. Experimentally, from an OPO pumped by 180 fs pulses, we obtained 42 fs signal pulses with a time-bandwidth product of 0.32. At elevated pump powers, the formation of multiple pulses was observed. Nonetheless, we demonstrated that the high-quality optical pulses could be obtained by simply slightly increasing the cavity-length detuning.

A Low-Frequency Magnetic Field Sensor Based on Fiber Bragg Gratings

A new type of low-frequency magnetic-field sensor based on fiber Bragg gratings (FBGs) was experimentally demonstrated for measuring the DC to low-frequency magnetic field. The operating mechanism of this AC magnetic sensor is based on the strain exerted by a loaded magnet on the sensing structure, which causes center-wavelength shifts of FBG. The achieved sensitivity was 8.16 pm/G with a resolution of 3 Gauss at ambient conditions. The proposed FBG magnetic sensor is easy to use, compact, and suitable for DC to low-frequency magnetic sensing applications.

The Influence of Temperature and Photobleaching on Irradiated Sodium Chloride at Europa-like Conditions

Abstract Europa’s leading-hemisphere chaos regions have a spectral feature at 450 nm that has been attributed to absorption by crystal defects in irradiated sodium chloride, known as F-centers. Some discrepancies exist between the laboratory data of irradiated sodium chloride and the observations, including a ∼10 nm shift in central wavelength of the F-center band and the lack of the prominent 720 nm absorption on Europa from M-centers, which result from the coalescence of pairs of F-centers. Here, we perform irradiation experiments on sodium chloride in an attempt to understand these discrepancies. We show that careful control of the temperature of the sample at 120 K yields F-centers with an absorption wavelength comparable to that of Europa. In addition, we measure the effect of photobleaching—the destruction of F-centers by photons—and show that at the energetic particle and photon flux on Europa, an equilibrium will be reached where only a modest F-center absorption develops. The density of F-centers never reaches high enough values for the creation of secondary M-centers. Our experiments predict that F-centers grow during the night on Europa in the absence of photobleaching and then partially decay during the daytime. We show observations from the Hubble Space Telescope consistent with this prediction. All observations of the 450 nm F-center on Europa are now consistent with laboratory measurements of sodium chloride, confirming the presence of this salt on Europa.

Highly sensitive nonlinear temperature sensor based on soliton self-frequency shift technique in a microstructured optical fiber

A novel fiber-optic soliton self-frequency shift (SSFS) temperature sensor fabricated using an in-house-made microstructured optical fiber was proposed. Based on this sensor, SSFS-based sensing was systematically investigated with the variation of average pump power and pump wavelength. By detecting the central wavelength shift of the 3-dB bandwidth of the soliton with the change in temperature, the sensing performance of the proposed sensor was evaluated experimentally and theoretically, subject to the average pump power and pump wavelength. At the generation of the fundamental soliton, when the pump wavelength was fixed, the higher the average pump power, the higher the temperature sensitivity. When the average pump power was fixed, the longer the pump wavelength, the higher the temperature sensitivity. The maximum temperature sensitivity of the proposed sensor was 1.759 nm/℃ at an average pump power of 300 mW and pump wavelength of 1600 nm. This temperature sensor exhibited excellent properties, such as high sensitivity, a simple structure, an easy fabrication process, good mechanical strength, and low cost, rendering it highly applicable in fields such as food quality control, environmental monitoring, and biomedical testing.

Microstructured optical fiber filled with glycerin for temperature measurement based on dispersive wave and soliton

A nonlinear temperature sensor was proposed drawing on dispersive wave (DW) and soliton in an in-house made silica microstructured optical fiber (MOF). Glycerin with high thermo-optical coefficient was filled into the MOF air holes as the thermal medium. Using a 1050 nm femtosecond laser as the pump source, DW and soliton were experimentally and theoretically explored for temperature sensing by detecting their central wavelength shift of 3-dB bandwidth. The results showed that the higher the average pump power, the higher the temperature sensitivity of DW and soliton. When the DW blue-shifted relative to the soliton, the temperature sensitivity of DW was higher than that of soliton, and the experimental maximum values were 0.928 nm/℃ (DW) and -0.923 nm/℃ (soliton). Our work explored the nonlinear phenomenon for optical fiber-based sensing technology and verified that both DW and soliton can be used to realize temperature sensors of high sensitivity and good mechanical strength. The sensing characteristic difference of DW and soliton revealed in this work might provide a new way for solving the cross-sensitivity of dual-parameter sensing in biological engineering, disease detection and environmental monitoring without introducing extra structural complexity.

Simultaneous AC and DC measurement based on an FBG-magnetostrictive fiber sensor.

Simultaneous alternating current (AC) and direct current (DC) measurement is demonstrated with a new fiber sensor that combines fiber Bragg gratings (FBGs) with giant magnetostrictive materials (GMMs). Deformation of GMMs, proportional to the central wavelength shift of FBGs, is different when applying DC and AC to a solenoid where the whole sensing structure is located. DC introduces a constant deformation while AC causes a periodic variation. Therefore, the central wavelength of FBG has a periodic harmonic variation, which makes it possible to demodulate the values of DC and AC at the same time. In addition, the proposed method has potential significance for reducing temperature cross talk caused by the thermal sensitivity of FBGs. The experimental results show that the sensitivity of the AC current can reach up to 0.090576 nm/A in the range of 0 to 1 A, and in different AC environments, the sensitivity of the DC current can reach up to 0.2378 nm/A.

Tunable erbium-doped-fiber laser based on the Double-Superimposed Fiber Bragg Gratings

Multi-wavelength D-SIFBG has the advantages of small size, wavelength interval and flexible reflectivity, so it is suitable for tunable EDFL. Therefore, a single wavelength tunable EDFL based on multi-wavelength D-SIFBG is proposed. The pump source of the laser is a 980 nm semiconductor laser, and the resonator is composed of SIFBG1 with a reflectivity higher than 91% and SIFBG2 with a reflectivity higher than 87%. The gain medium is erbium-doped fiber. Based on gain equalization technology, stable single-wavelength tunable laser output is obtained at room temperature. The test results show that the 3 dB line width is less than 0.020 nm, the optical signal-to-noise ratio is more than 60.4 dB, the central wavelength shift and maximum output power fluctuation are less than 0.004 nm and 1.25 dBm respectively, and the maximum tunable range is 5.796 nm. The single-wavelength laser output from this multi-wavelength laser has good stability.

The 3.3 μm Infrared Emission Feature: Observational and Laboratory Constraints on Its Carrier

Abstract We examine the self-consistency of laboratory and observational data for potential carriers of the 3.3 μm infrared emission feature (IEF), a member of the ubiquitous family of strong interstellar IEFs at 3.3, 3.4, 6.2, 7.7, 8.6, 11.2, and 12.7 μm. Previous studies have shown that most Galactic sources (reflection nebulae, H ii regions, and planetary nebulae) show 3.3 μm IEFs displaying similar central wavelengths, FWHM, and profiles. Our study is focused on the band profile designated as Class A, the most prevalent of four classes of observed band profiles. In contrast to the observations, laboratory spectra for gas phase polycyclic aromatic hydrocarbons (PAHs), the widely assumed carriers of the IEFs, display central wavelength shifts, widths, and profiles that vary with temperature and PAH size. We present an extrapolation of the laboratory band shifts and widths for smaller PAHs (≤32 carbon atoms) to the larger PAHs (>50 carbon atoms) that are thought to be the IEF carriers. The extrapolation leads to tight constraints on the sizes of the putative PAH carriers. Reconciling the observations with the implications of the laboratory spectra pose a significant challenge to the PAH and other IEF carrier hypotheses.

Optical schemes of super-angular AOTF-based imagers and system response analysis

We propose a super-angular aperture scheme instead of the circular aperture scheme for AOTF spectral imager to increase radiation flux of acousto-optic tunable filter (AOTF). At the same time, we quantitatively calculate the radiation flux variation caused by the super angular aperture scheme and the AOTF element response at oblique incidence. We analyze the system response of the two optical schemes and verify the simulation results through the system frequency response to compare the two AOTF optical schemes, where one is more suitable for super aperture angle imaging. We believe that the collimated optical path is more suitable for the super-angular aperture scheme because its spectral bandwidth is better than that of confocal optical scheme, and the center wavelength shift can be corrected by model or calibration.

A Fiber Bragg Grating acceleration sensor with temperature compensation

A Fiber Bragg Grating(FBG) accelerometer based on a composite beam is proposed. Owing to the opposite movement of the two FBGs, the influence of temperature can be reduced and the sensitivity can be improved by the difference operation of FBGs’ center wavelength shift. Through theoretical analysis and simulation of the proposed sensor, the sensor structure parameters are determined. The theory of the proposed sensor is verified by experiments, and the experimental results show that the temperature impact on the sensor structure is reduced to 0.47 pm/℃ in the range of 25–75 ℃. The accelerometer has a good response flat region at 40–140 Hz and the sensitivity is 234.5 pm/g. The orthogonal crosstalk of the accelerometer is −21.01 dB. The proposed FBG acceleration sensor has a good application prospect in low-frequency oil and gas seismic exploration.

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.