We propose an approach for tuning the three-dimensional polarization of a focusing subwavelength spot by a high numerical aperture objective. The incident beams are composed of a radially polarized beam, an azimuthally polarized beam, and a linearly polarized beam with three different weighting factors, respectively. A specially designed adjustable amplitude angular selector is also inserted at the back aperture of the objective for tuning the polarization azimuthally. It is shown that any desired overall polarization orientation can be obtained. We calculated the overall polarization orientation in the focal volume. It is found that the polar angle of the overall polarization orientation can be arbitrarily tuned by the combination of a radially polarized beam and a linearly polarized beam with different weighting factors, and the azimuthal angle can be tuned by rotating the orientation of the linearly polarized beam azimuthally.

In this Letter, a 16 channel 200 GHz wavelength tunable arrayed waveguide grating (AWG) is designed and fabricated based on the silicon on insulator platform. Considering that the performance of the AWG, such as central wavelength and crosstalk, is sensitive to the dimension variation of waveguides, the error analysis of the AWG with width fluctuations is worked out using the transfer function method. A heater is designed to realize the wavelength tunability of the AWG based on the thermo-optic effect of silicon. The measured results show that the insertion loss of the AWG is about 6 dB, and the crosstalk is 7.5 dB. The wavelength tunability of 1.1 nm is achieved at 276 mW power consumption, and more wavelength shifts will gain at larger power consumption.

We experimentally obtain cylindrical vector beams (CVBs) in a passively mode-locked fiber laser based on nonlinear polarization rotation. A mode-selective coupler composed of both a single-mode fiber (SMF) and a two-mode fiber (TMF) is incorporated into the cavity to act as a mode converter from LP01 mode to LP11 mode with broad spectral bandwidth. CVBs in different mode-locked states including single-pulse, multi-pulse, and bound pulse are obtained, for the first time to our best knowledge. The ultrafast CVBs with different operation states have potential applications in many fields such as laser beam machining, nanoparticle manipulation, and so on.

Induced loss at 633 nm is tested in Yb3+/Al3+ co-doped silica fiber by a core pumped with a 974 nm laser and probed with a 633 nm laser. The fiber is prepared by the modified chemical vapor deposition method combined with solution doping. Different power scales of pump light and probe light are used in the tests. It is found that there is a dynamic equilibrium between photobleaching induced by 633 nm probe light and photodarkening (PD) induced by 974 nm pump light. For the first time to our knowledge, the effect of 633 nm probe laser power on an induced loss test of Yb3+/Al3+ co-doped silica fiber is studied quantitatively. It suggests that as long as the 633 nm probe light power is less than 0.2 mW, the induced loss is mainly contributed by the PD effect of pumping light, and the deviation of induced loss is less than 5%.

A polarization-maintained coupled optoelectronic oscillator (COEO) with its performance significantly improved by a short-length unpumped erbium-doped fiber (EDF) is reported and experimentally investigated. A 10 GHz optical pulse train with a supermode suppression ratio of 61.8 dB and a 10 GHz radio frequency signal with a sidemode suppression ratio of 94 dB and a phase noise of 121.9 dBc/Hz at 10 kHz offset are simultaneously generated. Thanks to saturable absorption of the 1 m unpumped EDF, which introduces relatively large cavity loss to the undesired modes and noise, the supermode suppression ratio and the phase noise are improved by 9.4 and 7.9 dB, respectively.

In order to make the fiber-based frequency synchronization system suitable for the use of large-scale scientific and engineering projects in which the ambient temperature of the fiber links change dramatically, we design a non-harmonic frequency dissemination system immune to strong temperature fluctuation. After the lab tests, in which the ambient temperature of the fiber fluctuates 40°C/day and 20°C/h, respectively, the relative frequency stabilities of this system reaches 4.0 × 10 14/s and 3.0 × 10 16/104 s. It is demonstrated that the proposed non-harmonic scheme shows a strong robustness to complicated working environment with strong temperature fluctuation.

We propose a cavity length demodulation method that combines virtual reference interferometry (VRI) and minimum mean square error (MMSE) algorithm for fiber-optic Fabry–Perot (F-P) sensors. In contrast to the conventional demodulating method that uses fast Fourier transform (FFT) for cavity length estimation, our method employs the VRI technique to obtain a raw cavity length, which is further refined by the MMSE algorithm. As an experimental demonstration, a fiber-optic F-P sensor based on a sapphire wafer is fabricated for temperature sensing. The VRI-MMSE method is employed to interrogate cavity lengths of the sensor under different temperatures ranging from 28°C to 1000°C. It eliminates the “mode jumping” problem in the FFT-MMSE method and obtains a precision of 4.8 nm, corresponding to a temperature resolution of 2.0°C over a range of 1000°C. The experimental results reveal that the proposed method provides a promising, high precision alternative for demodulating fiber-optic F-P sensors.

In order to improve the precision of the laser–radio-frequency (RF) synchronization system from sub-picosecond to femtosecond (fs), a synchronization system between a picosecond laser and a 1.3 GHz RF generator has been developed based on a fiber-loop optical-microwave phase detector (FLOM-PD). Synchronization with fs-level (3.8 fs) rms jitter, integrated from 10 Hz to 1 MHz, is achieved for the first time, to the best of our knowledge, in this kind of configuration. This system will be used for the superconducting RF accelerator at Peking University.

Polarization fluctuation induced noise and backscattering-induced noise are the dominant noises in resonant fiber optic gyroscopes. This Letter proposes a new method to suppress the carrier and backscattering induced noise by the sideband locking technique. Besides choosing an optimized modulation depth and different clockwise and counterclockwise modulation frequencies, the sideband is locked to the cavity resonance. With the proper modulation frequency, the carrier frequency component locates at a position far away from the resonant frequency, and then it is suppressed by the cavity itself, which can be taken as a bandpass filter. The amplitude of the carrier frequency can be suppressed by 20–25 dB additionally by the cavity and the total intensity suppression ratio can reach 115.74 dB. The backscattering induced noise can be eliminated for the adoption of different frequencies. The method can realize a stable and high suppression ratio without high requirements for parameter accuracy or device performance.

A stable, passively Q-switched thulium fluoride fiber laser (TFFL) using a multi-walled carbon nanotube (MWCNT)-based saturable absorber (SA) for operation in the S-band region is proposed and demonstrated. The proposed TFFL has a central lasing wavelength of 1486.4 nm and an input power range of 87.1–126.6 mW. The output pulses have a repetition rate and pulse width range of 30.1–40.0 kHz and 9.0–3.2 μs, respectively, with a maximum pulse energy of 28.9 nJ. This is the first time, to the author’s knowledge, of the successful demonstration of a passively Q-switched S-band TFFL using an MWCNT-based SA.

In this Letter, the effects of material/structure parameters of photonic crystal (PhC) parallel waveguides on the coupling length are investigated. The results show that, increasing the effective relative permittivity of the PhC leads to a downward shift of the photonic bandgap and a variation of the coupling length. A compact PhC 1.31/1.55 μm wavelength division multiplexer (WDM)/demultiplexer with simple structure is proposed, where the output power ratios are more than 24 dB. This WDM can multiplex/demultiplex other light waves efficiently.

We evaluate the effects of the holes geometry drilled by a femtosecond laser on a stainless alloy with various defocused irradiation time, which ranges from 0 min to 1 h. The laser ablation efficiency is increased by a factor of 3 when the irradiation time is elevated from 0 to 30 min. Also, the morphology of the hole is observed by a scanning electron microscope, where the result indicates that the defocused irradiation time has a significant influence on the morphology changes. The reason for such changes is discussed based on the pretreatment effect and the confined plasma plume. As an application example, the microchannel is fabricated by a femtosecond laser combined with the defocused irradiation to demonstrate the advantage of the proposed method in fabricating functional structures.

In this Letter, the loss and gain characteristics of an unconventional InxGa1 xAs/GaAs asymmetrical step well structure consisting of variable indium contents of InxGa1 xAs materials are measured and analyzed for the first time, to the best of our knowledge. This special well structure is formed based on the indium-rich effect from the material growth process. The loss and gain are obtained by optical pumping and photoluminescence (PL) spectrum measurement at dual facets of an edge-emitting device. Unlike conventional quasi-rectangle wells, the asymmetrical step well may lead to a hybrid strain configuration containing both compressive and tensile strains and, thus, special loss and gain characteristics. The results will be very helpful in the development of multiple wavelength InGaAs-based semiconductor lasers.

A solid-state green-light-emitting upconversion coherent random laser was realized by pumping macroporous erbium-doped lithium niobate with a 980 nm laser. The lasing threshold was determined to be about 40 kW/cm2. Above the threshold, the emission intensity increased sharply with the increasing pump intensity. Moreover, a narrow multi-peaks structure was observed in the green-light-emission band, and the positions of lasing lines were various at different angles. The results were the direct evidences of coherent random lasing emission from macroporous erbium-doped lithium niobate. These phenomena were attributed to the coexistence of upconversion emission and a multiple scattering feedback mechanism.

The stress damage process of a single crystal silicon wafer under millisecond laser irradiation is studied by experiments and numerical simulations. The formation process of low-quality surface is monitored in real-time. Stress damage can be observed both in laser-on and -off periods. Plastic deformation is responsible for the first stress damage in the laser-on period. The second stress damage in the laser-off period is a combination of plastic deformation and fracture, where the fundamental cause lies in the residual molten silicon in the ablation hole.

We report experimental realization of Raman spectra enhancement of copper phthalocyanine, using an on-chip metallic planar waveguide of the sub-millimeter scale. The oscillating ultrahigh order modes excited by the direct coupling method yield high optical intensity at resonance, which is different from the conventional strategy to create localized “hot spots.” The observed excitation efficiency of the Raman signal is significantly enhanced, owing to the high Q factor of the resonant cavity. Furthermore, effective modulation of the Raman intensity is available by adjusting the polymethyl methacrylate (PMMA) thickness in the guiding layer, i.e., by tuning the light–matter interaction length. A large modulation depth is verified through the fact that 10 times variation in the enhancement factor is observed in the experiment as the PMMA thickness varies from 7 to 23 μm.

For better night-vision applications using the low-light-level visible and infrared imaging, a fusion framework for night-vision context enhancement (FNCE) method is proposed. An adaptive brightness stretching method is first proposed for enhancing the visible image. Then, a hybrid multi-scale decomposition with edge-preserving filtering is proposed to decompose the source images. Finally, the fused result is obtained via a combination of the decomposed images in three different rules. Experimental results demonstrate that the FNCE method has better performance on the details (edges), the contrast, the sharpness, and the human visual perception. Therefore, better results for the night-vision context enhancement can be achieved.

An optomechanical cavity embedded with a V-type three-level atom is exploited to control single-photon transport in a one-dimensional waveguide. The effects of the atom–cavity detuning, the optomechanical effect, the coupling strengths between the cavity and the atom or the waveguide, and the atomic dissipation on the single-photon transport properties are analyzed systematically. Interestingly, the single-photon transmission spectra show multiple double electromagnetically induced transparency. Moreover, the double electromagnetically induced transparency can be switched to a single one by tuning the atom–cavity detuning.

Using a measurement system based on fluorescence induced by variable pulse light, photosynthesis parameters of chlorella pyrenoidosa are obtained, employing single-turnover and multiple-turnover protocols under dark-adapted and light-adapted conditions. Under the light-adapted condition, σPSII′ is larger, and Fv′/Fm(ST)′ and Fv′/Fm(MT)′ are smaller than those of the dark-adapted condition, but the corresponding parameters possess good linear correlations. Fm(MT), Fm(MT)′, Fv/Fm(MT), and Fv′/Fm(MT)′, which are measured using the multiple-turnover protocol, are larger than those of the single-turnover protocol. The linear correlation coefficient between Fm(ST) and <inline-for

The femtosecond laser pulses reflected from the self-induced plasma mirror (PM) surface are characterized. More than two orders of magnitude improvement on intensity contrast both in nanosecond and picosecond temporal scales are measured. The far-field distribution, i.e., focusability, is measured to degrade in comparison with that without using a PM. Experiments on proton accelerations are performed to test the effect of the balance between degraded focusability and increased reflectivity. Our results show that PM is an effective and robust device to improve laser contrast for applications.