Orbital angular momentum (OAM) holds significant potential for achieving extremely high communication capacity, attributed to its orthogonality and infinite modes. Employing convolutional neural networks (CNN) for OAM mode recognition is an effective strategy to mitigate the effects of turbulence. However, recognition accuracy can be compromised when the training dataset is limited. To address this, we leveraged a conditional generative adversarial network (cGAN) for data augmentation (DA). The well-trained cGAN generated abundant augmented data with mode information, thereby enhancing the performance of the CNN. Experimental results clearly demonstrate that cGAN-based DA is an effective method for boosting recognition accuracy, resulting in a significant increase in recognition accuracy, rising from 24% to more than 99%. In addition, analyzing the relationship between the degree of DA and accuracy was instrumental in finding a balance between generation time cost and accuracy improvement. In addition, the application of cGAN-based DA to decomposed OAMs from the vortex array further validates its applicability in enhancing recognition performance.
In recent years, rhenium disulfide (ReS2) has attracted much attention in polarization dependent photonic applications due to its anisotropic optoelectronic response. To reveal the anisotropic response mechanisms, single crystal layered ReS2 flake with lateral size of about 10 μm is obtained using mechanical exfoliation method, and the ultrafast carrier dynamics of an individual flake is studied using transient absorption microscopy. The excited carrier concentration of layered ReS2 flake exhibits obvious polarization dependence of the pump light, which is caused by the anisotropic linear absorption and electron mobility rates in different orientations. These results could provide more references for the application of ReS2 in polarization sensitive detectors and optoelectronic devices.
A split-step birefringence simulation method is proposed to investigate the gating efficiency and intensity distribution of the Kerr signal field considering the evolution of the switch beam and probe beam in their path. Using this simulation method, we investigated the switch-beam power-dependent gating efficiency and conducted an experiment to prove its reliability. Furthermore, we analyzed the optical intensity distribution of the Kerr signal exiting the Kerr medium under different switch-beam powers. This study provides an effective theoretical tool for the design and optimization of optical Kerr gates.
We study the optical limiting (OL) property of a newly emerged MXene material, niobium carbide (Nb2CTx) nanosheets, using nanosecond laser Z-scan measurement. The nanosheets exhibited excellent OL property, and the OL thresholds are estimated to be about 0.35 and 0.44 J / cm2 for 532 and 1064 nm, respectively. Compared with the traditional OL material, reduced graphene oxide, Nb2CTx nanosheets show a better OL property. The nonlinear scattering (NLS) measurements indicate that the excellent OL behavior of the nanosheets originates mainly from the strong NLS effect due to the good photothermal property of the material.
We studied the structural and elemental evolutions during the femtosecond laser percussion drilling of high-aspect-ratio diamond microholes. Microholes 225-μm-deep having an aspect ratio of 15 were drilled with an exposure time of 100 s and a laser power of 60 mW. It is found that when the specimen was machined by femtosecond laser at low power and short exposure time, the laser-affected zone (LAZ) may be still solid in the interior rather being void, even it reached the bottom of the diamond. A clear crack appeared between the solid portion and pristine diamond. Elemental analysis revealed that oxygen was incorporated into the solid portion of the LAZs, and its atomic percentage reached 6.5% for a laser power of 10 mW at initial position and decreased as the depth increased in the solid portion. The wall of the void contained nearly no oxygen. Furthermore, nanoripples were observed on the sidewall surface of the hole.
Laser ablation of zircon can be used to analyze its composition for geological history. However, the effect of laser properties on nanoparticle size has not been studied extensively. The effect of laser fluence and pulse duration on the diameter of zircon nanoparticles was analyzed using field-emission scanning electron microscopy and energy dispersive spectroscopy. The results showed that the diameters of the zircon nanoparticles induced by a femtosecond laser increased with increasing laser fluence, and that these particles were smaller than those induced by a nanosecond laser with the same laser fluence. Furthermore, the mechanism of zircon nanoparticle formation induced by laser ablation has been discussed. The explosion mechanism is the primary mechanism of nanoparticle generation. In particular, the zircon nanoparticles induced by the femtosecond laser were the result of Coulomb explosion, while phase explosion contributed to the zircon nanoparticles induced by the nanosecond laser. Therefore, the nature of zircon nanoparticles induced by laser ablation is mainly determined by the pulse duration.
The Ti-6Al-4V polished by femtosecond laser processing is first investigated. The surface nanoparticles of Ti-6Al-4V induced by femtosecond laser have been characterized by scanning electron microscopy and energy dispersive spectrometry, and the influence of laser fluence and the overlapping rate of laser beam on surface roughness have also been analyzed. Moreover, the relationship between the laser parameters and the surface roughness of Ti-6Al-4V has been revealed, and the fine surface roughness of Ti-6Al-4V is obtained based on the optimized femtosecond laser processing parameters.
Stray light is the main noise source for planar imaging measurement technique, which can affect the accuracy of results directly. A method of Structured Laser Illumination Planar Imaging (SLIPI) was used to solve this issue. The key of SLIPI is periodic modulation of laser spatial intensity and implementation of post filtering algorithm. In this paper, cylindrical micro lens array was used to modulate the spatial intensity of laser periodically, which was compared with Ronchi ruling. The post filtering algorithm adopts phase-locked detection method. The signal results can be separated from the noisy image using only one measurement image by this method. SLIPI method has been used in Temperature Sensitive Paints measurement experiments. A diagnostic optical path combining cylindrical micro lens array and cylindrical mirror was designed for the need of surface light source irradiation. The results show that the method of SLIPI can be applied to most planar imaging measurement techniques, and the accuracy of two-dimensional parameter measurement can be further improved.
A theoretical model based on rate equations for actively Q-switched Er3+-doped ZBLAN fiber laser is built. The operation behaviors and output characteristics of the actively Q-switched fiber laser at 2.8 μm are analyzed. Effects of some important laser parameters, such as pump power levels, reflectivity of the laser output coupler, fiber lengths, Er concentrations, etc., on laser output were investigated. The model and simulating results are useful for design and optimization of actively Q-switched fiber laser at 2.8-μm region.
We employ a supercontinuum (SC) illumination to image the high-pressure fuel sprays in the near-nozzle region. The effect of speckles in the images is significantly mitigated using the SC illumination to improve the identifiability of the microstructures in the spray. The microstructures in the near-nozzle region, i.e., lobes, holes, ligaments, and bridges, are clearly imaged for different fuel pressures and nozzle orifice diameters. The shadowgraphs captured in the experiments also show the spray cone angle of spray is strongly dependent on the injection pressures and nozzle orifice diameters.
We report on the recent progress on high power pulsed 2.8 μm Er3+-doped ZBLAN fiber laser through techniques of passively and actively Q-switching in our research group. In passively Q-switched operation, a diode-cladding-pumped mid-infrared passively Q-switched Er3+-doped ZBLAN fiber laser with an average output power of watt-level based on a semiconductor saturable absorber mirror (SESAM) was demonstrated. Stable pulse train was produced at a slope efficient of 17.8% with respect to launched pump power. The maximum average power of 1.01 W at a repetition rate of 146.3 kHz was achieved with a corresponding pulse energy of 6.9 μJ. The maximum peak power was calculated to be 21.9 W. In actively Q-switched operation, a diode-pumped actively Q-switched Er3+-doped ZBLAN fiber laser at 2.8 μm with an optical chopper was reported. The maximum laser pulse energy of up to 130 μJ and a pulse width of 127.3 ns at a repetition rate of 10 kHz with an operating wavelength of 2.78 μm was obtained, yielding the maximum peak power of exceeding 1.1 kW.
In order to achieve the two-dimensional (2-D) velocity measurement of a flow field at extreme condition, a 2-D interferometric Rayleigh scattering (IRS) velocimetry using a multibeam probe laser was developed. The method using a multibeam probe laser can record the reference interference signal and the flow interference signal simultaneously. What is more, this method can solve the problem of signal overlap using the laser sheet detection method. The 2-D IRS measurement system was set up with a multibeam probe laser, aspherical lens collection optics, and a solid Fabry–Perot etalon. A multibeam probe laser with 0.5-mm intervals was formed by collimating a laser sheet passing through a cylindrical microlens arrays. The aspherical lens was used to enhance the intensity of the Rayleigh scattering signal. The 2-D velocity field results of a Mach 1.5 air flow were obtained. The velocity in the flow center is about 450 m/s. The reconstructed results fit well with the characteristic of flow, which indicate the validity of this technique.
In order to obtain the velocity with high dependability at extreme combustion condition, a 2-D interferometric Rayleigh scattering (IRS) velocimetry based on detecting the Doppler frequency shift of molecular scattering with Fabry–Perot etalon was developed. The 2-D IRS measurement system was set up with a multi-beam probe laser, aspherical lens collection optics, and a solid Fabry-Perot etalon. A multi-beam probe laser with 0.5mm intervals was formed by collimating a laser sheet passing through a cylindrical microlens arrays. The aspherical lens was used to enhance the intensity of Rayleigh scattering signal. The 2-D velocity results of a Mach 1.5 air flow were obtained. The velocity in the flow center is about 450 m/s. The reconstructed results are in accordance with the characteristic of flow, which indicates the validity of this technique.
Fabrication of grating structures on surfaces of wide-bandgap semiconductors, namely silicon carbide (SiC) and gallium nitride (GaN), was achieved using a femtosecond laser and a phase mask. The phase mask was used to produce stable interference patterns from the focused femtosecond laser to form the grating structures on the bulk materials. The effects of the irradiation power and time on the Bragg grating morphology that was formed on the SiC surface were studied. By optimizing the fabrication parameters, we successfully produced grating structures with uniform periods of 1.07 μm on SiC and GaN. The threshold powers necessary for grating structure formation on wide-bandgap semiconductors were investigated. It was found that the threshold powers for SiC and GaN were much smaller than those for silica glass. The reason for this difference is that the absorption of the incident laser light in SiC and GaN is a lower-order nonlinear absorption process compared to that in silica glass.
The report proposed a saturable absorber based on a D-shaped fiber embedded in a single-walled carbon nanotube solution. Such a saturable absorber solution method with a D-shaped fiber has the virtues of good antioxidant capacity, excellent scattering resistance, high heat dissipation, and high damage threshold. The nonsaturable loss of this kind of saturable absorber was evaluated to be 3%. To the best of our knowledge, this is the lowest value compared with other carbon nanotube saturable absorbers. By incorporating the saturable absorber into a Yb-doped fiber laser cavity, a mode-locked fiber laser was achieved with a central wavelength of 1054.16 nm. The repetition rate was 23 MHz with a signal-to-noise ratio of 60 dB, and the pulse duration was measured to be 194 ps. The long-term working stability of working is also good. The results indicated that the solution method with a D-shaped fiber possesses a potential for fiber laser stability applications.
An all-normal dispersion passively mode-locked Yb-doped fiber laser with a Bi2Te3 absorber is presented. The modulation depth of this kind of saturable absorber was measured to be 8.4%. By incorporating a Bi2Te3/PVA film into a Yb-doped fiber laser oscillator, a mode-locked fiber laser oscillator was achieved. The repetition rate and the central wavelength are 25.6 MHz and 1052.7 nm, respectively. The 3-dB spectral width is 0.45 nm and the pulse duration is 417 ps. The results indicate that topological insulator Bi2Te3 possesses the potential for ultrafast fiber laser application.
We demonstrated two ballistic imaging for an object hidden behind turbid media using the optical Kerr gate (OKG) and spatial filtering (SF), respectively. The influence of the scattering parameters of the turbid media on the image contrast was investigated. The experimental results showed that the image contrast of the SF imaging decreased significantly with increasing optical density and scattering particle size of the turbid media. Compared to the SF imaging, the OKG imaging showed a higher and more stable image contrast as scattering photons in the optical gated imaging case were more effectively eliminated.
Using a femtosecond time-resolved optical polarigraphy (FTOP) imaging technique, we measured the ultrafast propagation dynamics of femtosecond laser pulses in transparent materials, CS2 and fused silica, respectively. The FTOP images showed different profiles in these two media due to their different nonlinear response time. Based on the FTOP technique, a femtosecond time-resolved single-shot optical Kerr effect measurement was demonstrated, which can be accomplished using a single-laser shot and has a time resolution of about 100 fs. The polarization dependence of the image intensity indicated that the FTOP images were mainly induced by the transient birefringence effect induced by the pump pulse.
An instantaneous three-dimensional imaging technique using a chirped supercontinuum and an ultrafast optical Kerr gate, in which a sapphire plate and a TeO2-ZnO-Na2O oxide glass were used to generate the chirped supercontinuum and the ultrafast optical Kerr gate, respectively, is demonstrated. This technique is applicable to ultrafast shape measurement, such as shape imaging of moving objects, or imaging of laser-induced refractive index changes in transparent media.
We demonstrate a facile and flexible method to fabricate close-packed microlens arrays (MLAs). Glass molding
templates with concave structures are produced by a femtosecond (fs)-laser point-by-point exposures followed by a
chemical treatment, and convex MLAs are subsequently replicated on Poly(methyl methacrylate) [PMMA] using a hot
embossing system. As an example, a microlens array (MLA) with 60-μm rectangular-shaped spherical microlenses is
fabricated. Optical performances of the MLAs, such as focusing and imaging properties are tested, and the results
demonstrate the uniformity and smooth surfaces of the MLA. We also demonstrated that the shape and alignment of the
arrays could be controlled by different parameters.
Femtosecond laser interference is a promising tool for micro-fabrication and micromachining of periodical structures on
the surface of samples or inside transparent materials, but femtosecond laser pulses are very hard to interfere due to their
spectrum widths may reach to several tens of nanometers, and their spectrum widths will be stretched by shorting the
duration according to the Fourier transform. We realized two 25 fs pulses interference and encoded micro-gratings on
Au-Cr thin films using this interference pattern. The interference patterns of two laser pulses with different pulse
durations in sub-hundred femtosecond time domain were calculated to explore the influence of pulse durations on
processing qualities of encoded micro-gratings. The results show that, the shorter pulses are preferable to fabricate
micro-gratings with fine resolution on intractable materials, and longer pulses are helpful to improve encoding efficiency
and contrast ratio of bright & dark interfered fringes. The differences between encoded micro-gratings on Au-Cr thin film
using these interference patterns validated our analysis, which are hardly observed when pulse duration is longer than
100 fs mainly because the size of interfered area is larger than the focal spots. Moreover, the distance between two focal
spots also has been chosen to identify our calculations, and the experimental results are agreement with the calculations.
To develop high nonlinear optical fibers for all-optical switching applications, 7.5 wt% AgNO3 was incorporated into
tellurite glasses with composition of
75TeO2-20ZnO-5Na2CO3 (TZN75) under precisely-controlled experimental
conditions to form 7.5Ag-TZN75 glass. Surface Plasmon resonance absorption peak of Ag nanocrystals embedded in
7.5Ag-TZN75 glass was found to center at 552 nm. By degenerated
four-wave mixing method, the non-resonant
nonlinear refractive index, n2, of 7.5Ag-TZN75 glass was measured to be 7.54×10-19 m2•W-1 at 1500 nm, about 3 times of
the reference TZN75 glass without any dopant and 27 times of the silicate glasses and fibers, and the response time is
about 1 picosecond.
The ultrafast third-order nonlinear optical properties of C60+ vinyltriethoxysilane H2C=CHSi(OC2H5)3 (C60+VTES) sol-gel were investigated by the femtosecond optical Kerr gate (OKG) technique at 800 nm. The third-order optical susceptibility was measured to be 5.42 × 10−14 esu for C60+VTES at a weight concentration of 0.06 wt.%. Using C60+VTES as the OKG material, we acquired a series of narrow bandwidth and symmetric gated spectra continuously from the chirped white light continuum generated in water with femtosecond laser pulses. The gated spectra obtained using the C60+VTES OKG have distinct superiorities compared with CS2.
An ultrafast optical Kerr gate (OKG) in femtosecond time scale was used to determine the scattering coefficients of intralipids, in which the BI2O3-B2O3-SiO2 oxide glass was employed as the Kerr medium. Because of the joint action of the time gate and a transient spatial gate that was induced in the Kerr materials by the gating beam, more precise scattering coefficients could be obtained. Our experimental results show that, for low turbid media, the scattering coefficients measured using the OKG method are similar to those measured using the collimated transmittance (CT) approach, while for highly turbid media, the results obtained using the OKG method are bigger than those using the CT approach.
Fabrication of microstructures embedded in silica glasses using a femtosecond (fs)-laser-assisted chemical etching
technique is systematically studied in this work. By scanning the laser pulses inside samples followed by the treatment of
5%-diluted hydrofluoric (HF) acid, groups of straight channels are fabricated and the relationship between the etching
rate and processing parameters, including laser power, scanning speed, scanning time and laser polarization, is
demonstrated. Based on the optimization of these parameters, complicated microstructures such as channels, cavities and
their combinations are manufactured. The work has great potential applications in microelectromechanical systems,
biomedical detection and chemical analysis.
Theory about the Thermal Infrared Imaging Fourier Transform Spectrometer has been discussed,
and then we found the Interference efficiency is an important factor related to SNR of Thermal
Infrared Imaging Fourier Transform. The Interference efficiency involved in transverse shear
splitting. After study of this kind of beam splitting, some formulas about Thermal Infrared
Imaging Fourier Transform Spectrometer has been found, then the simulation modes were done.
At the end, Interference efficiency of Imaging Fourier Transform Spectrometer was calculated.
The relationship between interference efficiency and SNR was simply given.
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