Wavelength Width Research Articles

Highly coherent supercontinuum generation in CS2-infiltrated single-core optical fiber

As is generally known, the broadband near-infrared supercontinuum (SC) is significant for applications such as pump-probe spectroscopy, nonlinear spectroscopy, frequency combs and nonlinear optical parametric amplification. We present the first detailed demonstration of approximate near-infrared, octave-spanning SC generation in an all-normal dispersion CS2 single-hole liquid core optical fiber (LCOF). By solving the generalized nonlinear Schrödinger equation, the SC can be calculated at different parameters. The influence of important parameters—including but not limited to the fiber core diameter, pump wavelength, pump power and pulse width—on the supercontinuum generation (SCG) of a CS2-infiltrated LCOF can be analyzed for the optimization of near-infrared SCG. The results show that at low pump power (such as P0 = 4 kW), the SC generated by using two pump wavelengths (1.55 μm and 1.95 μm) has spectral flatness, good coherence and is octave-spanning. However, the bandwidth is severely affected by the large pulse duration T0, and the coherence is seriously influenced by the pulse duration T0 > 1200 fs at the pump wavelength of 1.55 μm. The effect of pulse duration on the first-order coherence is greater at short pump wavelengths (1.55 μm) than long wavelengths (1.95 μm). Under high power conditions (such as P0 = 20 kW), the coherence of the SC generated remains good at the two-pump wavelength. This study explores an optimized scheme for obtaining a broadband SC with high coherence and fine spectral flatness suitable for specific applications.

Spectral variations of Lyman $\alpha$ emission within strongly lensed sources observed with MUSE

ABSTRACT We present an analysis of ${\rm H\,\rm{\small {I}}}$ Lyman $\alpha$ emission in deep VLT/MUSE observations of two highly magnified and extended galaxies at $z=3.5$ and 4.03, including a newly discovered, almost complete Einstein ring. While these Lyman $\alpha$ haloes are intrinsically similar to the ones typically seen in other MUSE deep fields, the benefits of gravitational lensing allow us to construct exceptionally detailed maps of Lyman $\alpha$ line properties at sub-kpc scales. By combining all multiple images, we are able to observe complex structures in the Lyman $\alpha$ emission and uncover small ($\sim120$ km s−1 in Lyman $\alpha$ peak shift), but significant at $ \gt $4 $\sigma$, systematic variations in the shape of the Lyman $\alpha$ line profile within each halo. Indeed, we observe a global trend for the line peak shift to become redder at large radii, together with a strong correlation between the peak wavelength and line width. This systematic intrahalo variation is markedly similar to the object-to-object variations obtained from the integrated properties of recent large samples. Regions of high surface brightness correspond to relatively small line shifts, which could indicate that Lyman $\alpha$ emission escapes preferentially from regions where the line profile has been less severely affected by scattering of Lyman $\alpha$ photons.

Lamellar structure/processing relationships and compressive properties of porous Ti6Al4V alloys fabricated by freeze casting

Lamellar pores have superior biocompatibility due to their similarity to the lamellar structure of natural bones. In the present work, porous Ti6Al4V alloys with lamellar pores were successfully fabricated by directionally freeze casting. The lamellar structure/processing relationships were systematically studied through analyzing the interaction between ice front and alloy powders. The structural feature of translamella bridges is observed in the lamellar structure. The volume shrinkage of porous Ti6Al4V alloys is in the range of 44-60%. This is much higher compared with that of the porous ceramics. The solid content in the slurry exerts a strong influence on the porosity, while the freezing ice front velocity affects the structural wavelength and pore width. With the increase in ice front velocity, the structural wavelength decreases by an exponential function. The lamella formation mechanism and porosity gradient along the freezing direction were discussed. Young's modulus and yield stress of porous Ti6Al4V alloys fall in the range of 2-12 GPa and 40-300 MPa, respectively. The dominant compressive deformation mode is lamella buckling and splitting. The fabricated porous Ti6Al4V alloys possess higher relative yield stress.

An energy window study of light transmission-disorder relationship in 1D photonic structures

While the light transmission of photonic crystals is characterized by the photonic band gap, the one of disordered photonic structures is typified by a multiplicity of transmission depths. The total transmission over a range of wavelengths is related to the width of such range, but also to the type of disorder. Less homogeneous disordered structures transmit more light than the ordered counterpart regardless of the wavelengths range width. More homogeneous disordered structures transmit more light than the ordered counterpart only above a certain value of the width. We studied this behaviour with a statistical analysis over 5000 permutations of structures for six wavelength widths and for two different homogeneity degrees (Shannon-Wiener index).

Experimental studies of 1.5 – 1.6 μm high-power asymmetric-waveguide multimode lasers

The current – voltage, light – current, and spectral characteristics of high-power multimode semiconductor lasers emitting at wavelengths of 1.5 – 1.6 μm are studied experimentally. It is shown that the cw output power of a C-mount laser with a cavity length of 2.6 mm and a mesa-stripe contact width of 100 μm exceeds 4 W at a pump power of 15 A. The output power in a pulsed regime amounts to 20 W at a pulse duration of 100 ns, a pulse repetition rate of 5 kHz, and a pump current no higher than 80 A. The dependences of the laser wavelength and spectral width on the pump current are presented. The current – voltage and light – current characteristics, as well as the characteristic temperatures of the lasers, are calculated.

Statistical luminescence method for determining the region of origin of emeralds

Large samples of emeralds from eight deposits in Brazil, China, Zambia, Russia, Afghanistan, Colombia, and Tanzania were studied using pulsed cathodoluminescence to develop a nondestructive method for determining their region of origin. The average wavelength, variance, and inverse effective width of the luminescence band in the 660–870 nm wavelength range of these minerals followed normal distributions with intensities characteristic of each of the regions of origin. Based on this result, a statistical luminescence method for determining the region of origin of the minerals was proposed and tested. This method uniquely determined the region of origin for more than 80% of the emeralds.

Fe:ZnSe laser pumped by a 2.93- Cr, Er:YAG laser**Project supported by the National Natural Science Foundation of China (Grant No. 61405047).

We demonstrated an Fe:ZnSe laser pumped by a 2.93- Cr, Er:YAG laser at liquid nitrogen and room temperature in single-shot free-running operation for the first time. The xenon flash lamp pumped Cr, Er:YAG laser had a maximum single pulse energy of 1.414 J, and the threshold and slope efficiency were 141.70 J and 0.70% which were respectively reduced by 29.3% and increased by 52.2% compared with the Er:YAG laser. At liquid nitrogen temperature of 77 K, the maximum single pulse energy of the Fe:ZnSe laser was 197.6 mJ, corresponding to a slope efficiency of 13.4%. The central wavelength and full width at half maximum (FWHM) linewidth were 4037.4 nm and 122.0 nm, respectively. At room temperature, the laser generated a maximum single pulse energy of 3.5 mJ at the central wavelength of 4509.6 nm with an FWHM linewidth of 171.5 nm.

Wavelength, repetition rate, and pulse width tunable high average power narrow linewidth 1.5 μm fiber amplifier

Abstract Here, a wavelength, repetition rate, and pulse width tunable high average power narrow linewidth 1.5 μm all-fiber amplifier has been achieved by directly modulating a continuous-wave single-frequency diode laser using an acousto-optic modulator. The laser wavelength can be precisely tuned from 1530 nm to 1570 nm. The repetition rate and pulse width are also conveniently tunable. The maximum output average power is more than 2.5 W and the laser linewidth is at the range of 150–500 MHz. It has wide applications in tunable mid-infrared hollow-core fiber gas lasers, mid-infrared laser generation by nonlinear frequency conversion, fiber radar, coherent combination, and so on.

Impacts of spectral variations in multiwavelength excitation method on measurements of fluorescently whitened papers

AbstractThe multiwavelength excitation (MWE) method for measuring the colorimetric properties of fluorescent whitening agent (FWA)‐treated specimens illuminated by a standard daylight illuminant computationally approximates not the spectral power distribution (SPD) of the illuminant but the luminescent SPD excited thereby by a weighted sum of the luminescent SPDs excited by a few different narrow‐band illuminations. The weights are optimized for the actual SPDs of those illuminations and the bispectral luminescent radiance factors of typical FWA‐treated paper specimens. Since the latter is invariant among instruments once provided as the common numerical data, the variations of the narrow‐band SPDs give major impacts to the reproducibility of this method. The weights optimized for the varied SPDs, however, mitigate the impacts. For investigating how they impact, one basic illumination system and its 16 simple variation systems were built virtually. The basic system consists of three narrow‐band LEDs and one blue‐excited white LED, whereas the individual simple variation system has either the peak wavelength or spectral width of one of the four LEDs (including the blue LED in the white LED) varied. With those systems, seven FWA‐treated papers with the known bispectral radiance factors were measured computationally by simulating the procedure of the MWE method. The differences in the colorimetric values measured with the simple variation systems from those with the basic system are far below the just noticeable difference, which indicates that the MWE method can be a practical solution for better reproducibility in measuring FWA‐treated papers.

All-optical switching based on soliton self-trapping in dual-core high-contrast optical fibre

A systematic numerical study of ultrafast nonlinear directional coupler performance based on soliton self-trapping in a novel type of dual-core optical fibre is presented. The considered highly nonlinear fibre structure is composed of a real, intentionally developed soft glass-pair with high refractive index contrast at the level of 0.4 in the near infrared. Nonlinear propagation of picojoule level femtosecond pulses was studied numerically with the aim to identify the best switching performance in input parameter space of 1400–1800 nm in terms of excitation wavelengths, and of 75–150 fs in terms of pulse width, respectively. For every combination of excitation wavelength and pulse width, the switching energies together with the optimal fibre length were determined and their relation to the input and switching parameters is discussed. The highest switching contrast of 46 dB in the time window of the ultrashort soliton was predicted at combination of 1500 nm excitation wavelength and 75 fs pulse width considering 43 mm fibre length. These results represent significant improvement both from point of view of switching contrast and switching energies, which are only at level of 20 pJ, in comparison to the previously published case of air-glass dual-core photonic crystal fibre. Moreover, the simpler fibre design without cladding microstructure together with the all-solid approach holds promise of improved dual-core symmetry and therefore offers high probability of the successful realization of a low power, compact and simple switching device.

Accurate manipulation of optogenetic proteins with wavelength tunable femtosecond laser system

Photoactivated proteins controlled by optogenetic tools have broad application prospects in cell biology, neuroscience, and brain science. However, due to the narrow excitation wavelength width and the inflexibility of spatiotemporal operations, conventional sources such as visible light severely limit the further application of optogenetics. In this work, a femtosecond laser-operated system based on the optogenetic application was designed to address these limitations. The interaction between the photoreceptor and its partner protein can be triggered by a wavelength-tunable femtosecond laser. The results indicated that this process can be used to accurately manipulate optogenetic proteins in cells, which met spectral flexibility (700–1040 nm) and operational flexibility in time and space (a single cell to multiple cells). To demonstrate the practical applications of this process, the apoptotic signaling pathway of cancer cells was taken as an example. We believe that this wavelength-tunable femtosecond laser system will promote the development of optogenetics, making optics and even physics more powerful tools in biology.

Parallel-Integrated Fiber Bragg Gratings Inscribed by Femtosecond Laser Point-by-Point Technology

Conventional grating-inscription technology usually inscribes only one fiber Bragg grating (FBG) in the core of an optical fiber. A novel femtosecond laser point-by-point technology was demonstrated to parallel-inscribe multiple FBGs, i.e., so-called parallel-integrated FBGs (PI-FBGs), with either different or the same reflection wavelengths in the core of a standard single-mode fiber. Single- or multiwavelength PI-FBGs exhibited one or multiple peaks in the reflection spectrum, respectively. The length of PI-FBGs can be shortened to hundreds of micrometers to increase the spatial resolution of the FBG-based sensors. Our ultrashort PI-FBGs with a length of 500 μ m could be used to realize ultrahigh temperature sensing with a high spatial resolution of less than 1 mm, a linear sensitivity of 15.0 pm/°C, and a large measurement range of up to 1100 °C, which overcomes the shortcomings of conventional FBG-based sensors with a low spatial resolution of more than 1 cm and a small measurement range of up to 300 °C. Moreover, the grating parameters, such as reflectivity, reflection wavelength, polarization-dependent loss, and full width at half maximum, can be improved to achieve desired PI-FBGs by changing the spatial distribution and grating number of the PI-FBGs. The PI-FBGs are insensitive to bending, which reduces bend-induced measurement errors. Therefore, our PI-FBGs will find potential applications in the fields of optical fiber sensors, communications, and lasers.

Synchronous Luminescence Spectroscopy as a Tool in the Discrimination and Characterization of Oral Cancer Tissue.

High incidence of oral cancer is primarily due to ongoing tobacco epidemic. In this work, synchronous luminescence spectroscopy (SLS) has been used to characterize and discriminate oral cancer tissue. Spectral deconvolution method is employed to compute the fluorescence intensity, peak wavelength, and full width half maxima for different endogenous fluorophores. The fluorescence measurements were made on 21 normal and 88 oral squamous cell carcinoma biopsy tissues. Besides, variations in relative concentration of collagen, NADH, and FAD, peak shifts and broadening of peaks are observed for tryptophan, NADH, and FAD, in oral cancer tissues indicating both biochemical and micro environmental changes at cellular level. Linear discriminant analysis showed that oral cancer tissue is discriminated with a sensitivity and specificity of 100% and 95.2% respectively.

Silicon nitride waveguide as a power delivery component for on-chip dielectric laser accelerators.

We study the weakly guided silicon nitride waveguide as an on-chip power delivery solution for dielectric laser accelerators (DLAs). We focus on the two main limiting factors on the waveguide network for DLAs: the optical damage and nonlinear characteristics. The typical delivered fluence at the onset of optical damage is measured to be ∼0.19 J/cm2 at a 2μm central wavelength and 250fs pulse width. This damage fluence is lower than that of the bulk Si3N4 (∼0.65 J/cm2), but higher than that of bulk silicon (∼0.17 J/cm2). We also report the nonlinearity-induced spectrum and phase variance of the output pulse at this pulse duration. We find that a total waveguide length within 3mm is sufficient to avoid significant self-phase modulation effects when operating slightly below the damage threshold. We also estimate that one SiNx waveguide can power 70μm silicon dual pillar DLAs from a single side, based on the results from the recent free-space DLA experiment.

Free-Space to Fiber Coupling of Electromagnetic Gaussian Schell-Model Beams in Turbulent Marine Atmospheric Channel

We present a method to analyze the fiber coupling efficiency η of electromagnetic Gaussian Schell-model (EGSM) beams propagating through marine atmospheric turbulence. The expression for η is derived from the relation of field distributions between EGSM beam and fiber mode. The influence factors of η are discussed, including the properties of EGSM beam, focusing lens, and turbulence. We find that η declines as the turbulence enhances. A large-aperture lens and an EGSM beam with high coherence, long wavelength, and optimum beam width are able to achieve a higher η in turbulent links. Beam polarization also takes effect on η but the effect is complex. Our investigation has applications in the fiber coupling of vectorial beams in communication systems.

Near-infrared laser irradiation of a multilayer agar-gel tissue phantom to induce thermal effect of traditional moxibustion

Traditional moxibustion therapy can stimulate heat and blood-vessel expansion and advance blood circulation. In the present study, a novel noncontact-type thermal therapeutic system was developed using a near-infrared laser diode. The device allows direct interaction of infrared laser light with the skin, thereby facilitating a controlled temperature distribution on the skin and the deep tissues below the skin. While using a tissue-mimicking phantom as a substitute for real skin, the most important optical and thermal parameters are the absorption/attenuation coefficient, thermal conductivity, and specific heat. We found that these parameters can be manipulated by varying the agar-gel concentration. Hence, a multilayer tissue-mimicking phantom was fabricated using different agar-gel concentrations. Thermal imaging and thermocouples were used to measure the temperature distribution inside the phantom during laser irradiation. The temperature increased with the increase in the agar-gel concentration and reached a maximum value under the tissue phantom surface. To induce a similar thermal effect of moxibustion therapy, controlled laser-irradiation parameters such as output power, wavelength and pulse width were obtained from further analysis of the temperature distribution. From the known optothermal properties of the patient’s skin, the temperature distribution inside the tissue was manipulated by optimizing the laser parameters. This study can contribute to patient-specific thermal therapy in clinics.

Broadband zero backward scattering by all-dielectric core-shell nanoparticles.

Efficiently controlling the direction of optical radiation at nanoscale dimensions is essential for various nanophotonics applications. All-dielectric nanoparticles can be used to engineer the direction of scattered light via overlapping of electric and magnetic resonance modes. Herein, we propose all-dielectric core-shell SiO2-Ge-SiO2 nanoparticles that can simultaneously achieve broadband zero backward scattering and enhanced forward scattering. Introducing higher-order electric and magnetic resonance modes satisfies the generalized first Kerker condition for breaking through the dipole approximation. Zero backward scattering occurs near the electric and magnetic resonant regions, this directional scattering is therefore efficient. Adjusting the nanoparticles' geometric parameters can shift the spectral position of the broadband zero backward scattering to the visible and near-infrared regions. The wavelength width of the zero backward scattering could be enlarged as high as 142 and 63 nm in the visible and near-infrared region. Due to these unique optical features the proposed core-shell nanoparticles are promising candidates for the design of high-performance nanoantennas, low-loss metamaterials, and photovoltaic devices.

Electropulsing-Induced Microstructural Changes and Their Effects on Electrical Conductivity of Thin Films of an Al-doped ZnO

Electropulsing-induced phase decompositions and microstructural changes in AZO-5 thin films were studied by X-ray diffraction, scanning electron microscopy, atomic force microscopy, Hall effect measurement and photoluminescence (PL) measurement techniques. It was found that the electropulsing induced phase decomposition started with spinodal decomposition, which was accompanied by discontinuous precipitation in the AZO-5 thin films. Both circular phase decompositions and the crystal orientation occurred. Inappropriate electropulsing might damage zones, which resulted in tremendous decrease in electrical conductivity. Circular changes in both the peak position and the width of the PL wavelength were observed in the EPT AZO-5 thin films. Formation of zones favored reducing the roughness of the thin film.

Temperature and excitation wavelength-dependent photoluminescence of CH3NH3PbBr3 crystal.

We have investigated temperature and excitation wavelength-dependent luminescent properties of CH3NH3PbBr3 (MAPbBr3) perovskite crystal using steady-state and time-resolved photoluminescence (PL) spectroscopy. Optical spectroscopic results indicate that the PL intensity, peak wavelength, and full width at half maximum are jumped due to the phase transition from orthorhombic to tetragonal and cubic. As temperature increases, the peaks of above-bandgap and intrinsic PL have blueshift and redshift, respectively. The above-bandgap PL emerges under one-photon laser excitation and vanishes under the excitation of near-infrared femtosecond laser due to reabsorption. The initial redshift of PL peak and lifetime change at various wavelengths reveals the existence of a trap-assisted excitonic state. It is found that upconversion PL in all phases is composed of a photoinduced thermal-assisted and an intrinsic excitonic state, which produces the observation of biexponential dynamics. Our results can facilitate the understanding of the thermal effect of exciton recombination in hybrid perovskite crystal.

Peculiarities of the coherence time of a spectral supercontinuum generated in microstructured fibers with two zeros in the group-velocity dispersion

We used a numerical simulation to calculate the coherence time of the spectral supercontinuum generated in a microstructured fiber in which the group velocity dispersion as a function of wavelength and pump pulse width has two zeros. We determined the contribution to the total coherence time from each of the separate substructures in the resulting spectral supercontinuum for various models of the initial pulse parameters. We show that, near the zeros in the group-velocity dispersion, the spectral supercontinuum coherence time is observed to be a rapidly increasing function of the central wavelength of the initial pulse. This is because the primary contribution to the coherence time near the zeros in the group-velocity dispersion in such cases comes from regular structures formed during generation of the supercontinuum. At intermediate wavelengths between the zeros in the group dispersion, the contribution to the interference signal from the regular structures shifts from one substructure to another and depends on the central wavelength of the emission.