Machine learning can effectively accelerate the runtime of a computer-generated hologram. However, the angular spectrum method and single fast Fresnel transform-based machine learning acceleration algorithms are still limited in the field-of-view angle of projection. In this paper, we propose an efficient method for the fast generation of large field-of-view holograms combining stochastic gradient descent (SGD), neural networks, and double-sampling Fresnel diffraction (DSFD). Compared with the traditional Gerchberg–Saxton (GS) algorithm, the DSFD-SGD algorithm has better reconstruction quality. Our neural network can be automatically trained in an unsupervised manner with a training set of target images without labels, and its combination with the DSFD can improve the optimization speed significantly. The proposed DSFD-Net method can generate 2000-resolution holograms in 0.05 s. The feasibility of the proposed method is demonstrated with simulations and experiments.

In this Letter, we have experimentally verified a low-complexity subcarrier pairwise-averaging (SPA)-enhanced channel estimation (CE) method for small-size fast Fourier transform (FFT) non-Hermitian symmetric orthogonal frequency-division multiplexing (NHS-OFDM) transceivers. Compared with intra-symbol frequency averaging (ISFA), more than 20% look-up tables and 10% logic power consumption can be saved. The least-square (LS), ISFA, and SPA CE methods are compared by offline and real-time digital signal processing approaches. The results show that the receiver sensitivity of the SPA NHS-OFDM transmission system with 64/128-point FFT can be improved by more than 1 dB at the bit error rate of 3.8 × 10^-3 compared to the LS.

The spectral linewidth of a transversely excited pulsed CO2 laser is broadened at high working pressures. This phenomenon causes a decrease in the upper-level lifetime such that the pulse width is significantly compressed. Although the tail part of CO2 laser pulses owns a non-negligible proportion of total energy, it has minor effects during the interaction process between photons and materials due to its low amplitude. Thus, it is of great significance to yield the tail part and generate a narrow pulse in most applications. In this study, a continuously tunable pulsed CO2 laser with a low nitrogen proportion in the mixture is developed to generate tail-free short pulses; a minimum pulse width of 30.60 ns with a maximum pulse energy of 481 mJ is synchronously achieved at a pressure of 7 atm, and the estimated peak power is above 15 MW. A numerical simulation is also conducted for comparison with the experimental results. The contribution of the spectral gain toward the compression of the pulse width is discussed in the last section of this paper.

We fabricate a high-performance Bi/Er/La co-doped silica fiber with a fluorescence intensity of -33.8dBm and a gain coefficient of 1.9 dB/m. With the utilization of the fiber as a gain medium, a linear-cavity fiber laser has been constructed, which exhibits a signal-to-noise ratio of 74.9 dB at 1596 nm. It has been demonstrated that the fiber laser has a maximum output power of 107.4 mW, a slope efficiency of up to 17.0%, and a linewidth of less than 0.02 nm. Moreover, an all-fiber single-stage optical amplifier is built up for laser amplification, by which the amplified laser power is up to 410.0 mW with pump efficiency of 33.8%. The results indicate that the laser is capable of high signal-to-noise ratio and narrow linewidth, with potential applications for optical fiber sensing, biomedicine, precision measurement, and the pump source of the mid-infrared fiber lasers.

We report VO_x/NaVO3 nanocomposite as a novel saturable absorber for the first time, to the best of our knowledge. The efficient nonlinear absorption coefficient and the modulation depth are determined by the Z-scan technology. As a saturable absorber, a passively Q-switched Nd-doped bulk laser at 1.34 µm is demonstrated, producing the shortest pulse duration of 129 ns at a repetition rate of 274 kHz. In the passively Q-switched Tm:YLF laser with the prepared saturable absorber, the shortest pulse duration was 292 ns with a repetition rate of 155 kHz. Our work confirmed the saturable absorption in VO_x/NaVO3 for possible optical modulation in the near-infrared region.

In this study, an excellent polarization optical crystal LiNa5Mo9O30 with wide transmission range and high laser damage threshold was researched in detail. The laser damage threshold of the LiNa5Mo9O30 crystal was measured to be 2.64GW/cm2, which was the highest among polarized optical crystals. The birefringence in the range of 0.435–5 μm was larger than 0.14, while the wedge angle between 31.94° and 32.12° would satisfy the application in this waveband. The extinction ratio of the fabricated prism with the wedge angel of 31.09° was larger than 15,000:1. The results show that the LiNa5Mo9O30 prism is an excellent polarization device, especially in the mid-infrared range and high-power applications.

We report an experimental generation of few-cycle pulses at 53 MHz repetition rate. Femtosecond pulses with pulse duration of 181 fs are firstly generated from an optical parametric oscillator (OPO). Then, the pulses are compressed to sub-three-cycle with a hybrid compressor composed of a commercial single-mode fiber and a pair of prisms, taking advantage of the tunability of the OPO and the numerical simulating of the nonlinear compression system. Our compressed optical pulses possess an ultrabroadband spectrum covering over 470 nm bandwidth (at -10dB), and the output intensity fluctuation of our system is less than 0.8%. These results show that our system can effectively generate few-cycle pulses at a repetition rate of tens of megahertz with excellent long-term stability, which could benefit future possible applications.

This study proposes a method based on material dispersion models to computationally simulate terahertz (THz) time-domain spectroscopy signals. The proposed method can accurately extract the refractive indices and extinction coefficients of optically thin samples and high-absorption materials in the THz band. This method was successfully used to extract the optical constants of a 470-μm-thick monocrystalline silicon sample and eliminate all errors associated with the Fabry–Pérot oscillation. When used to extract the optical constants of a 16.29-mm-thick polycarbonate sample, our method succeeded in minimizing errors caused by the low signal-to-noise ratio in the extracted optical constants.

Palladium-based hydrogen sensors have been typically studied due to the dielectric function that changes with the hydrogen concentration. However, the development of a reliable, integral, and widely applicable hydrogen sensor requires a simple readout mechanism and an optimization of the fast detection of hydrogen. In this work, optical fiber hydrogen sensing platforms are developed using an optimized metasurface, which consists of a layer of palladium nanoantennas array suspended above a gold mirror layer. Since the optical properties of these palladium nanoantennas differ from the traditional palladium films, a high reflectance difference can be achieved when the sensor based on the metasurface is exposed to the hydrogen atmosphere. Finally, the optimized reflectance difference ΔR of ∼0.28 can be obtained when the sensor is exposed in the presence of hydrogen. It is demonstrated that this integrated system architecture with an optimized palladium-based metasurface and a simple optical fiber readout system provides a compact and light platform for hydrogen detection in various working environments.

Photon nanosieves, as amplitude-type metasurfaces, have been demonstrated usually in a transmission mode for optical super-focusing, display, and holography, but the sieves with subwavelength size constrain optical transmission, thus leading to low efficiency. Here, we report reflective photon nanosieves that consist of metallic meta-mirrors sitting on a transparent quartz substrate. Upon illumination, these meta-mirrors offer the reflectance of ∼50%, which is higher than the transmission of visible light through diameter-identical nanoholes. Benefiting from this configuration, a meta-mirror-based reflective hologram has been demonstrated with good consistence between theoretical and experimental results over the broadband spectrum from 500 nm to 650 nm, meanwhile exhibiting total efficiency of ∼7%. Additionally, if an additional high-reflectance layer is employed below these meta-mirrors, the efficiency can be enhanced further for optical anti-counterfeiting.

Optical line tweezers have been an efficient tool for the manipulation of large micron particles. In this paper, we propose to create line traps with transformable configurations by using the transverse electromagnetic mode-like laser source. We designed an optical path to simulate the generation of the astigmatic beams and line traps with a series of lenses to realize the rotational transformation with respect to the rotation angle of cylindrical lenses. It is shown that the spherical particles with diameters ranging from 5 μm to 20 μm could be trapped, aligned, and revolved in experiment. The periodical trapping forces generated by transformable line traps might open an alternative way to investigate the mechanical properties of soft particles and biological cells.

We present a discovery of an unusual unidirectionally rotating windmill scattering of electromagnetic waves by a magnetized gyromagnetic cylinder via an analytical theory for rigorous solution to fields and charges and an understanding of the underlying mathematical and physical mechanisms. Mathematically, the generation of nonzero off-diagonal components can break the symmetry of forward and backward scattering coefficients, producing unidirectional windmill scattering. Physically, this windmill scattering originates from the nonreciprocal unidirectional rotation of polarized magnetic charges on the surface of a magnetized gyromagnetic cylinder, which drives the scattering field to radiate outward in the radial direction and unidirectionally emit in the tangential direction. Interestingly, the unidirectional electromagnetic windmill scattering is insensitive to the excitation direction. Moreover, we also discuss the size dependence of unidirectional windmill scattering by calculating the scattering spectra of the gyromagnetic cylinder. These results are helpful for exploring and understanding novel interactions between electromagnetic waves and gyromagnetic materials or structures and offer deep insights for comprehending topological photonic states in gyromagnetic systems from the aspect of fundamental classical electrodynamics and electromagnetics.