The phase-sensitive time-domain reflectometer (φ-OTDR) has been popularly used for events detection over a long period of time. In this study, the events classification methods based on convolutional neural networks (CNNs) with different features, i.e., the temporal-spatial features and time-frequency features, are compared and analyzed comprehensively in φ-OTDR. The developed CNNs aim at distinguishing three typical events: wind blowing, knocking, and background noise. The classification accuracy based on temporal-spatial images is higher than that based on time-frequency images (99.49% versus 98.23%). The work here sets a meaningful reference for feature extraction and application in the pattern recognition of φ-OTDR.

We experimentally built a W-band photonics-aided millimeter-wave radio-over-fiber transmission system and demonstrated the delivery of up to 8192-ary quadrature amplitude modulation (QAM) signal. Discrete multitone signals are converted into 1-bit data streams through delta-sigma modulation and then modulated onto a 76.2 GHz carrier. An envelope detector is used at the receiver side for direct detection. The results prove that our proposed system can support 2048QAM and 8192QAM transmission while meeting the hard decision forward error correction threshold of 3.8×10-3 and the soft decision forward error correction threshold of 4.2×10-2, respectively. We believe this cost-effective scheme is a promising candidate for future high-order QAM millimeter-wave downlink transmission.

Usually, a multilens optical system is composed of multiple undetectable sublenses. Wavefront of a multilens optical system cannot be measured when classical transmitted phase measuring deflectometry (PMD) is used. In this study, a wavefront measuring method for an optical system with multiple optics is presented based on PMD. A paraxial plane is used to represent the test multilens optical system. We introduce the calibration strategy and mathematical deduction of gradient equations. Systematic errors are suppressed with an N-rotation test. Simulations have been performed to demonstrate our method. The results showing the use of our method in multilens optical systems, such as the collimator and single-lens reflex camera lenses show that the measurement accuracy is comparable with those of interferometric tests.

Laser ranging with frequency comb intermode beats (IMBs) has been suffering from random phase drifts (RPDs) for two decades. In this study, we reveal the influence of signal transmission path on the RPDs and propose a real-time suppression method using two IMBs of similar frequencies from different combs. As the two IMBs obtain similar RPDs during their transmission through same signal paths, the RPD of the original probing signal IMB is suppressed by deducting the RPD of the newly added local IMB in real time. In our experiments, a real-time suppression of RPDs is achieved using IMBs of 1001 and 1000 MHz. For the sampling time of 100 s, the effect of 19-fold suppression has been achieved. The proposed method provides a new solution for the long-standing phase drift problem in laser ranging with comb IMBs.

Fabrication of high-quality optics puts a strong demand on high-throughput detection of macroscopic bulk defects in optical components. A dark-field line confocal imaging method is proposed with two distinct advantages: (i) a point-to-line confocal scheme formed by a columnar elliptical mirror and an optical fiber bundle breaks through the constraint on light collection angle and field of view in the traditional line confocal microscopy using an objective, allowing for an extended confocal line field of more than 100 mm while maintaining a light collection angle of 27°; (ii) the bulk defects are independently illuminated as a function of time to eliminate the cross talk in the direction of the confocal slit, thus preserving point confocality and showing the optical section thicknesses to be 162 µm in the axial direction, and 19 and 22 µm in the orthogonal transverse directions. The experimental results verify that the method has a minimum detectable bulk defect of less than 5 µm and an imaging efficiency of 400mm2/s. The method shows great potential in high-throughput and high-sensitivity bulk defects detection.

The self-mixing interferometry (SMI) technique is an emerging sensing technology in microscale particle classification. However, due to the nature of the SMI effect raised by a microscattering particle, the signal analysis suffers from many problems compared with a macro target, such as lower signal-to-noise ratio (SNR), short transit time, and time-varying modulation strength. Therefore, the particle sizing measurement resolution is much lower than the one in typical displacement measurements. To solve these problems, in this paper, first, a theoretical model of the phase variation of a single-particle SMI signal burst is demonstrated in detail. The relationship between the phase variation and the particle size is investigated, which predicts that phase observation could be another alternative for particle detection. Second, combined with continuous wavelet transform and Hilbert transform, a novel phase-unwrapping algorithm is proposed. This algorithm can implement not only efficient individual burst extraction from the noisy raw signal, but also precise phase calculation for particle sizing. The measurement shows good accuracy over a range from 100 nm to 6 μm with our algorithm, proving that our algorithm enables a simple and reliable quantitative particle characteristics retrieval and analysis methodology for microscale particle detection in biomedical or laser manufacturing fields.

We propose a high-sensitivity bidirectional torsion sensor using a helical seven-core fiber taper embedded in multimode fiber (MHSTM). Sensors with different taper waists and helical pitches are fabricated, and their transmission spectra are obtained and analyzed. The waist and length of the sandwiched seven-core fiber are finally determined to be 68 µm and 3 mm, respectively. The experimental results show that the clockwise and counterclockwise torsion sensitivities of the proposed sensor are 2.253 nm/(rad/m) and -1.123 nm/(rad/m), respectively. When tapered waist diameter reduces to 48 µm, a superior torsion sensitivity of 5.391 nm/(rad/m) in the range of 0–4.24 nm/(rad/m) is obtained, which is 46 times as large as the traditional helical seven-core fiber structure. In addition, the MHSTM structure is also relatively stable to temperature variations.

Wavelength-tunable dissipative solitons and amplifier similaritons have been obtained by inserting all-fiber Mach–Zehnder interferometer (MZI) filters with different free spectral ranges (FSRs) in a Yb-doped mode-locked fiber laser. The MZI filter is fabricated by splicing one segment of seven-core fiber (SCF) between two segments of single-mode fibers. The bandwidth of the filter depends on the FSR of the modulated interference curve and consequently depends on the tapered fiber diameter. Inserting MZI filters with bandwidths in a fiber laser and applying a tensile strain on the tapered SCF, both wavelength-tunable dissipative solitons and amplifier similaritons have been obtained.

In this Letter, we report on widely tunable pulse generation from a red-diode-clad-pumped mid-infrared (mid-IR) Er3+/Dy3+ codoped ZrF4 fiber laser, for the first time, to the best of our knowledge. Using a Fe2+:ZnSe crystal, continuously tunable Q-switched pulses across the range of 3.06–3.62 µm have been attained, which not only represents the widest range (in wavelength domain) from a pulsed rare-earth-doped fiber laser at any wavelength, but also almost entirely covers the strong absorption band of C-H bonds in the mid-IR, providing a potential way for gas detection and polymer processing. In addition, the commercial InAs quantum-well-based saturable absorbers (SAs) have been employed instead, and the obtained longest Q-switching wavelength of 3.39 µm is slightly shorter than 3.444 µm determined by its nominal direct bandgap of 0.36 eV.

The development of laser systems leads to an increasing threat to photoelectric imaging sensors. A cubic phase plate wavefront coding imaging system is proposed to reduce the risk of damage owing to intense laser radiation. Based on the wavefront coding imaging model, the diffracted spot profile and the light intensity distribution on the observation plane are simulated. An experimental device is set up to measure the laser-induced damage thresholds and investigate the morphology of laser-induced damage patterns of the conventional and the wavefront encoding imaging system. Simulations and experimental results manifest the superior laser suppression performance of the proposed method, which can help diminish the undesirable effects of laser irradiation on an imaging sensor.

We report continuous-wave deep red lasers at 696.6 and 698.6 nm in a Pr3+:YLF crystal pumped by an InGaN laser diode. A Lyot filter was inserted into the cavity as a birefringent filter to select wavelength; the lasers at 696.6 and 698.6 nm were obtained with a maximum output power of 1.36 and 3.11 W, separately. To the best of our knowledge, the output powers of these two lasers are the highest to date, and this is the first scaling of the output power of the Pr3+:YLF laser to the watt level at around 696 nm. In addition, the corresponding theoretical analysis and simulation were carried out to explain the experimental phenomena.

We demonstrated an electrically pumped InP-based microcavity laser operating in continuous-wave mode. The active region is designed with antimony surfactants to enhance the gain at 2 μm, and a selective electrical isolation scheme is used to secure continuous-wave operation for the microcavity laser at room temperature. The lasers were fabricated as a notched elliptical resonator, resulting in a highly unidirectional far-field profile with an in-plane beam divergence of less than 2°. Single-mode emission was obtained over the entire dynamic range, and the laser frequencies were tuned linearly with the pumping current. Overall, these directional lasers pave the way for portable and highly integrated on-chip sensing applications.

We first study the effect of cavity modes propagating in the lateral dimension on high-power semiconductor lasers with a large stripe width. A sidewall microstructure was fabricated to prevent optical feedback of lateral resonant modes. Theoretically, we demonstrate the existence of lateral resonant modes in the Fabry–Perot cavity with a large stripe width. Experimentally, we design the corresponding devices and compare them with conventional broad-area diode lasers. About a 15% reduction in threshold current and a 27% increase in maximum electro-optical conversion efficiency are achieved. The amplified spontaneous emission spectrum is narrowed, which proves that lateral microstructures suppress optical feedback of lateral resonant modes. Under a large continuous-wave operation, the maximum output power of laser device is 43.03 W, about 1 W higher than that of the standard broad-area laser at 48 A.

Perovskite-structured barium strontium titanate (Ba_1-xSr_xTiO₃, x = 0.1–0.9) films have been epitaxially fabricated by using a pulsed-laser deposition technique. The third-order nonlinear optical properties were studied through a z-scan method, allowing the resolution of the nonlinear refractive and absorptive contributions to the responses. Although all the samples show almost the same value of nonlinear absorption coefficient, the extracted nonlinear refractive index of the sample of x = 0.3 is apparently larger than that of other samples. Dependency of the nonlinear optical properties on the Ba/Sr ratio is discussed in terms of the crystal phase transformation and metal-oxygen bond length of the selected materials.

A kind of optical data storage medium based on electron-trapping materials, Y3Al5O12:Ce3+ fluorescent ceramic, was developed by vacuum sintering technology. The medium shows sufficiently deep traps (1.67 and 0.77 eV). The properties of trap levels were researched by thermoluminescence curves, and the optical storage mechanism based on Ce3+ ion doping was proposed. More importantly, the data can be written-in by 254 nm UV light, and readout by heating (300°C). This work expands the application fields of fluorescent ceramics, and it is expected to promote the development of electron-trapping materials.

Controlling architecture of hierarchical microstructures in liquid crystals (LCs) plays a crucial role in the development of novel soft-matter-based devices. Chiral LC fingerprints are considered as a prospective candidate for various applications; however, the efficient and real-time command of fingerprint landscapes still needs to be improved. Here, we achieve elaborate rotational fingerprint superstructures via dual photopatterning semifree chiral LC films, which combine the photoalignment technique and a dynamic light patterning process. An intriguing spatial-temporal rotational behavior is presented during the patterning of chiral superstructures. This work opens new avenues for the applications of chiral LCs in soft actuators, sensing, and micromanufacturing.

ZnGeP₂ (ZGP) crystals have attracted tremendous attention for their applications as frequency conversion devices. Nevertheless, the existence of native point defects, including at the surface and in the bulk, lowers their laser-induced damage threshold by increasing their absorption and forming starting points of the damage, limiting their applications. Here, native point defects in a ZGP crystal are fully studied by the combination of high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and optical measurements. The atomic structures of the native point defects of the Zn vacancy, P vacancy, and Ge-Zn antisite were directly obtained through an HAADF-STEM, and proved by photoluminescence (PL) spectra at 77 K. The carrier dynamics of these defects are further studied by ultrafast pump-probe spectroscopy, and the decay lifetimes of 180.49, 346.73, and 322.82 ps are attributed to the donor V_p⁺ → valence band maximum (VBM) recombination, donor Ge_Zn⁺ → VBM recombination, and donor–acceptor pair recombination of V_p⁺ → V_Zn^-, respectively, which further confirms the assignment of the electron transitions. The diagrams for the energy bands and excited electron dynamics are established based on these ultrahigh spatial and temporal results. Our work is helpful for understanding the interaction mechanism between a ZGP crystal and ultrafast laser, doing good to the ZGP crystal growth and device fabrication.

Sb-doped β-Ga2O3 crystals were grown using the optical floating zone (OFZ) method. X-ray diffraction data and X-ray rocking curves were obtained, and the results revealed that the Sb-doped single crystals were of high quality. Raman spectra revealed that Sb substituted Ga mainly in the octahedral lattice. The carrier concentration of the Sb-doped single crystals increased from 9.55×1016 to 8.10×1018cm-3, the electronic mobility depicted a decreasing trend from 153.1 to 108.7cm2·V-1·s-1, and the electrical resistivity varied from 0.603 to 0.017 Ω·cm with the increasing Sb doping concentration. The un-doped and Sb-doped β-Ga2O3 crystals exhibited good light transmittance in the visible region; however, the evident decrease in the infrared region was caused by increase in the carrier concentration. The Sb-doped β-Ga2O3 single crystals had high transmittance in the UV region as well, and the cutoff edge appeared at 258 nm.

Relativistic electrons moving over a periodic metal grating can lead to an intriguing emission of light, known as Smith–Purcell radiation (SPR), the precursor of the free-electron laser. The speed of light plays a critical role in the far-field emission spectrum. Inspired by this photonic SPR, here we experimentally demonstrate a photoacoustic phased array using laser-induced shock waves. We observe acoustic radiation spectrum in the far field, perfectly predicted by a universal theory for the SPR. This scheme provides a tool to control the acoustic radiation in the near field, paving the way toward coherent acoustic wave generation and microstructure metrology.

Discriminating two spatially separated sources is one of the most fundamental problems in imaging. Recent research based on quantum parameter estimation theory shows that the resolution limit of two incoherent point sources given by Rayleigh can be broken. However, in realistic optical systems, there often exists coherence in the imaging light field, and there have been efforts to analyze the optical resolution in the presence of partial coherence. Nevertheless, how the degree of coherence between two point sources affects the resolution has not been fully understood. Here, we analyze the quantum-limited resolution of two partially coherent point sources by explicitly relating the state after evolution through the optical systems to the coherence of the sources. In particular, we consider the situation in which coherence varies with the separation. We propose a feasible experiment scheme to realize the nearly optimal measurement, which adaptively chooses the binary spatial-mode demultiplexing measurement and direct imaging. Our results will have wide applications in imaging involving coherence of light.

Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols. The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching. Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam. The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW). Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source. This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.

Remarkable progress has been made in satellite-based quantum key distribution (QKD), which can effectively provide QKD service even at the intercontinental scale and construct an ultralong-distance global quantum network. But there are still some places where terrestrial fiber and ground stations cannot be constructed, like harsh mountainous areas and air space above the sea. So the airborne platform is expected to replace the ground station and provide flexible and relay links for the large-scale integrated communication network. However, the photon transmission rate would be randomly reduced, owing to the randomly distributed boundary layer that surrounds the surface of the aircraft when the flight speed is larger than 0.3 Ma. Previous research of airborne QKD with boundary layer effects is mainly under the air-to-ground scenario in which the aircraft is a transmitter, while the satellite-to-aircraft scenario is rarely reported. In this article, we propose a performance evaluation scheme of satellite-to-aircraft QKD with boundary layer effects in which the aircraft is the receiver. With common experimental settings, the boundary layer would introduce a ∼31dB loss to the transmitted photons, decrease ∼47% of the quantum communication time, and decrease ∼51% of the secure key rate, which shows that the aero-optical effects caused by the boundary layer cannot be ignored. Our study can be performed in future airborne quantum communication designs.

This paper investigates the combination of laser-induced breakdown spectroscopy (LIBS) and deep convolutional neural networks (CNNs) to classify copper concentrate samples using pretrained CNN models through transfer learning. Four pretrained CNN models were compared. The LIBS profiles were augmented into 2D matrices. Three transfer learning methods were tried. All the models got a high classification accuracy of >92%, with the highest at 96.2% for VGG16. These results suggested that the knowledge learned from machine vision by the CNN models can accelerate the training process and reduce the risk of overfitting. The results showed that deep CNN and transfer learning have great potential for the classification of copper concentrates by portable LIBS.

We demonstrated a scheme to differentiate the high-harmonic generation (HHG) originating from the surface states and bulk states of the topological insulator Bi2Se3. By adopting two-color mid-infrared laser fields on Bi2Se3, we found that the nonlinear response sensitively depends on the relative phase of the driving fields. The even harmonics arise from the surface states with a clear signature, whose modulation period equals the cycle of the second-harmonic generation (SHG) field. We reveal that the weak SHG perturbs the nontrivial dipole phase of the electron-hole pair in surface states, and thus leads to the modulation of HHG. It provides a means to manipulate the ultrafast dynamics in surface states through adopting a weak perturbing laser field.

The plasma mirror system was installed on the 1 PW laser beamline of Shanghai Superintense Ultrafast Laser Facility (SULF) for enhancing the temporal contrast of the laser pulse. About 2 orders of magnitude improvement on pulse contrast was measured on picosecond and nanosecond time scales. The experiments show that high-contrast laser pulses can significantly improve the cutoff energy and quantity of proton beams. Then different target distributions are assumed in particles in cell simulations, which can qualitatively assume the expansion of nanometer-scale foil. The high-contrast laser enables the SULF-1PW beamline to generally be of benefit for many potential applications.