We propose and demonstrate a novel scheme of semi-open-loop polarization control (SOL-PC), which controls the state of polarization (SOP) with high accuracy and uniform high speed. For any desired SOP, we first adjust the initial SOP using open-loop control (OLC) based on the matrix model of a three-unit piezoelectric polarization controller, and quickly move it close to the objective one. Then closed-loop control (CLC) is performed to reduce the error and reach precisely the desired SOP. The response time is three orders faster than that of the present closed-loop polarization control, while the average deviation is on par with it. Finally, the SOL-PC system is successfully applied to realize the suppression of the polarization mode dispersion (PMD) effect and reduce the first-order PMD to near zero. Due to its perfect performance, the SOL-PC energizes the present polarization control to pursue an ideal product that can meet the future requirements in ultrafast optical transmission and quantum communication.

We propose here a novel method for position fixing in the micron scale by combining the convolutional neural network (CNN) architecture and speckle patterns generated in a multimode fiber. By varying the splice offset between a single mode fiber and a multimode fiber, speckles with different patterns can be generated at the output of the multimode fiber. The CNN is utilized to learn these specklegrams and then predict the offset coordinate. Simulation results show that predicted positions with the precision of 2 μm account for 98.55%. This work provides a potential high-precision two-dimensional positioning method.

In order to achieve the accurate measurement of displacement, this Letter presents a self-mixing interference displacement measurement method suitable for the speckle effect. Because of the speckle effect, the amplitude of the self-mixing interference signal fluctuates greatly, which will affect the measurement accuracy of displacement. The ensemble empirical mode decomposition is used to process the interference signal, which can filter out high-frequency noise and low-frequency noise at the same time. The envelope of the self-mixing interference signal is extracted by Hilbert transform, and it is used to realize the normalization of the signal. Through a series of signal processing, the influence of speckle can be effectively reduced, and the self-mixing interference signal can be transformed into standard form. The displacement can be reconstructed by fringe counting and the interpolation method. The experimental results show that the method is successfully applied to the displacement measurement in the presence of speckle, which verifies the effectiveness and feasibility of the method.

Photonic waveguide arrays provide a simple and versatile platform for simulating conventional topological systems. Here, we investigate a novel one-dimensional (1D) topological band structure, a dimer chain, consisting of silicon waveguides with alternating self-coupling and inter-coupling. Coupled mode theory is used to study topological features of such a model. It is found that topological invariants of our proposed model are described by the global Berry phase instead of the Berry phase of the upper or lower energy band, which is commonly used in the 1D topological models such as the Su–Schrieffer–Heeger model. Next, we design an array configuration composed of two dimer patterns with different global Berry phases to realize the topologically protected waveguiding. The topologically protected propagation feature is simulated based on the finite-difference time-domain method and then observed in the experiment. Our results provide an in-depth understanding of the dynamics of the topological defect state in a 1D silicon waveguide array, and may provide different routes for on-chip lightwave shaping and routing.

We demonstrate absorption spectroscopy of water vapor for the determination of gas temperature. An adaptive dual-comb detection system is utilized to obtain precise spectroscopic data in a broadband range from 7143 to 7240 cm^?1 with a spectral resolution of 0.049 cm^?1. The measured spectra are in accordance with the simulated results from the HITRAN (high-resolution transmission molecular absorption) database. Several measurements are investigated in the temperature range of 500–1000 K, revealing relative deviations of less than 5% compared to the thermocouple. This broadband and accurate adaptive dual-comb spectral detection method could be a powerful tool for non-invasive combustion diagnosis.

In this work, SF₆ as a Raman-active medium is investigated to generate a multispectral Raman laser by the combination of cascade stimulated Raman scattering (SRS) and four wave mixing. The Raman frequency comb from the 10th-order anti-Stokes to the 9th-order Stokes was generated, and its spectral range covered 377–846 nm. The photon conversion efficiency of 16.4% for the first Stokes was achieved, and the Raman gain coefficient at 1.5 MPa of SF₆ under the 532 nm pump laser was calculated to be 0.83 cm/GW by the SRS threshold comparison with H₂. Using helium as the carrier gas, the thermal effect of the SF₆ Raman laser was improved dramatically under a repetition rate of 10 Hz.

The unamplified spontaneous emission (SE) is one of the important physical processes of the light–matter interaction in a diode laser in terms of Einstein’s theory. The recent research on a kind of new indium-rich cluster (IRC) laser structure did not reveal SE characteristics of the IRC structure, as its unusual quantum confined structure made it difficult to acquire correctly the SE spectra through theoretical simulation or previous experimental techniques. Thus, in this Letter, we firstly established a convenient and effective experimental approach to acquire SE spectra of the IRC structure by the measurement of amplified SEs from dual facets of a single edge-emitting chip with little sample processing. With the proposed method, the special SE spectra due to the IRC effect were observed. Then, the SE formation mechanism and characteristics in the IRC structure were analyzed by comparing the experimental data with theoretical SE spectra using a standard InGaAs/GaAs quantum well with similar material composition. This research provides a useful tool to investigate the SE characteristics of any non-standard diode laser structure and is very meaningful to develop a new type of IRC lasers.

The ultimate capacity of a cladding-pumped 10/130 Tm:fiber is experimentally investigated with a 793 nm laser diode bidirectionally pumped amplifier. The laser system works stably at the output powers of 52 W, 65 W, and 87 W. Eventually, the damage of the amplifier occurs when the output power reaches about 103.5 W with a total incident pump power of 176.8 W. Considering the incident seed power of 12.3 W, the amplifier conversion efficiency is estimated to be about 51.6% before it is damaged. With valuable exploration, we achieve the first air-cooling 60 W Tm:fiber laser at 1945.845 nm with a spectral linewidth of 0.4 nm. The laser power stability reaches 1.24% during a continuous test time of >65 h. The beam quality is measured as Mx2=1.16 and My2=1.14.

Laser-induced discharge plasmas (LDPs) have the potential to be inspection and metrology sources in extreme ultraviolet (EUV) lithography. An LDP EUV source was developed to avoid tin electrode erosion in which a tin pool was used as a cathode. A CO₂ pulse laser was focused on the liquid tin target surface, and then a breakdown occurred in a very short time. The voltage-current characteristics of the discharge oscillated, lasting for several microseconds, and an RLC fitting model was used to obtain the inductance and resistance. An intensified charge-coupled device (ICCD) camera was used to investigate the dynamics of LDP, which can explain the formation of a discharge channel. The EUV spectra of laser-induced liquid tin discharge plasma were detected by a grazing incident ultraviolet spectrometer, compared with a laser-produced tin droplet plasma EUV spectrum. To explain the EUV spectrum difference of laser-induced liquid tin discharge plasma and laser-produced tin droplet plasma, the collision radiation (CR) model combined with COWAN code was used to fit the experimental EUV spectrum, which can estimate the electron temperature and density of the plasma.

To introduce ordered nano-structures inside a transparent amorphous matrix with superior optical and mechanical properties bears scientific and technological importance, yet limited success has been achieved. Here, via simple melting-quenching and subsequent thermal activation, we report the successful preparation of transparent nano-structured glass-ceramics embedded in Sr₂LuF₇ nano-crystals (～26 nm), as evidenced by X-ray diffraction, transmission electron microscopy (TEM), and high resolution TEM. The successful incorporation of dopants into formed Sr₂LuF₇ nano-crystals with low phonon energy results in highly tunable blue–green photoemission, which depends on excitation wavelength, dopant type, and temperature. We found that Eu³⁺ and Eu²⁺ ions co-exist in this hybrid optical material, accompanied by the broadband blue emission of Eu²⁺ and sharp red emissions of Eu³⁺. A series of optical characterizations are summoned, including emission/excitation spectrum and decay curve measurement, to reveal the reduction mechanism of Eu³⁺ to Eu²⁺. Furthermore, near green–white photoemission is achieved via the enrichment of Tb³⁺/Eu³⁺ into crystallized Sr₂LuF₇ nano-crystals. The temperature-dependent visible photoemission reveals thermal activation energy increases with the precipitation of Sr₂LuF₇ nano-crystals in a glass matrix, suggesting better thermal stability of glass-ceramics than precursor glasses. These results could not only deepen the understanding of glass-ceramics but also indicate the promising potential of Eu³⁺/Tb³⁺-ions-doped Sr₂LuF₇ glass-ceramics for UV pumped white light emitting diodes (WLEDs) with good thermal stability.

In this Letter, the surface-enhanced Raman scattering (SERS) signal of early breast cancer (BRC) patient serum is obtained by a composite silver nanoparticles (Ag NPs) PSi Bragg reflector SERS substrate. Based on these advantages, the serum SERS signals of 30 normal people and 30 early BRC patients were detected by this substrate. After a baseline correction of the experimental data, principal component analysis and linear discriminant analysis were used to complete the data processing. The results showed that the diagnostic accuracy, specificity, and sensitivity of the composite Ag NPs PSi Bragg reflector SERS substrate were 95%, 96.7%, and 93.3%, respectively. The results of this exploratory study prove that the detection of early BRC serum based on a composite Ag NPs PSi Bragg reflector SERS substrate is with a stable strong SERS signal, and an unmarked and noninvasive BRC diagnosis technology. In the future, this technology can serve as a noninvasive clinical tool to detect cancer diseases and have a considerable impact on clinical medical detection.

A high repetition rate, picosecond terahertz (THz) parametric amplifier with a LiNbO₃ (LN) crystal has been demonstrated in this work. At a 10 kHz repetition rate, a peak power of 200 W and an average power of 12 μW have been obtained over a wide range of around 2 THz; at a 100 kHz repetition rate, a maximum peak power of 18 W and an average power of 10.8 μW have been obtained. The parametric gain of the LN crystal was also investigated, and a modified Schwarz–Maier model was introduced to interpret the experimental results.

A grating-coupled surface plasmon resonance sensor based on bilayer aluminum nanowire arrays is fabricated by laser interference lithography. The device presents impressive reflective sharp peaks by lateral surface plasmon resonances even for aluminum thicknesses of merely several nanometers. Distinct reflective peaks and dramatic color shifts under different analytes are observed within a wide range of incident angle, metal thickness, and refractive index. The sensitivity of 307 nm per refractive index unit is experimentally obtained. The reflective-peak-type surface plasmon resonance sensors are suitable for practical applications because of easy fabrication, low cost, wide range, and high signal visibility.

A polarization-insensitive plasmonic absorber is designed consisting of Au fishnet structures on a TiO₂ spacer/Ag mirror. The fishnet structures excite localized surface plasmon and generate hot electrons from the absorbed photons, while the TiO₂ layer induces Fabry–Perot resonance, and the Ag mirror acts as a back reflector. Through optimizing the TiO₂ layer thickness, numerical simulation shows that 97% of the incident light is absorbed in the Au layer. The maximum responsivity and external quantum efficiency of the device can approach 5 mA/W and ～1%, respectively, at the wavelength of 700 nm.

A switchable microwave photonic filter (MPF) using a phase modulator (PM) and a silicon-on-insulator micro-ring resonator (MRR) is proposed and demonstrated. By adjusting the polarization controller between the PM and the MRR, the filtering function of the MPF can be switched between a band-stop filter and a band-pass filter. In a proof-of-concept experiment, an MPF with a rejection ratio of 30 dB (or 15 dB) for the band-stop (or band-pass) response and a frequency tuning range from 9.6 to 20.5 GHz is achieved.

In this Letter, we propose a broadband near-infrared (NIR) absorber based on the phase transition material VO₂. By designing different arrangements of the VO₂ square lattice at high and low temperatures on fused silica substrates, the absorption rate reaches more than 90% in the entire 1.4–2.4 μm range. Using a finite-difference time-domain simulation method and thermal field analysis, the results prove that the absorber is polarization-independent and has wide-angle absorption for incident angles of 0°–70°. The proposed absorber has a smoother absorption curve and is superior in performance, and it has many application prospects in remote sensing geology.

Quantum walks, a counterpart of classical random walks, have many applications due to their neoteric features. Since they were first proposed, quantum walks have been explored in many fields theoretically and have also been demonstrated experimentally in various physical systems. In this paper, we review the experimental realizations of discrete-time quantum walks in photonic systems with different physical structures, such as bulk optics and time-multiplexed framework. Then, some typical applications using quantum walks are introduced. Finally, the advantages and disadvantages of these physical systems are discussed.