A torsion sensor based on the near-helical (NH) long period fiber grating (LPFG) is fabricated by using a high frequency pulsed CO₂ laser. Each groove of the NH-LPFG is spirally written in the four sides of a single-mode fiber. The NH-LPFG has a helical periodic vertical index modulation. This is different from the screw-type index modulation of the common helical LPFGs (H-LPFGs) fabricated in a twisted fiber. The torsion and temperature characteristics of the NH-LPFG are experimentally investigated. The temperature sensitivity is about 0.0668 nm/°C. The torsion sensitivity is 0.103 nm/(rad/m) and independent of the polarization state of incident light.

We propose a passive compensation fiber-optic radio frequency (RF) transfer scheme with a nonsynchronized RF stable source during a round-trip time, which can avoid high-precision phase-locking and efficiently suppress the effect of backscattering only using two wavelengths at the same time. A stable frequency signal is directly reproduced by frequency mixing at the remote site. The proposed scheme is validated by the experiment over a 40 km single mode fiber spool using nonsynchronized common commercial RF sources. The influence of the stability of nonsynchronized RF sources on the frequency transfer is investigated over different length fiber links.

We demonstrate a fiber refractive index (RI) sensor based on an excessively tilted fiber grating (ExTFG) immobilized by large-size plasmonic gold nanoshells (GNSs). The GNSs are covalently linked on ExTFG surface. Experimental results demonstrate that both the intensity of the transverse magnetic (TM) and transverse electric (TE) modes of ExTFG are significantly modulated by the localized surface plasmon resonance (LSPR) of GNSs due to the wide-range absorption band. The wavelength RI sensitivities of the TM and TE modes in the low RI range of 1.333–1.379 are improved by ～25% and ～14% after GNSs immobilization, respectively, and the intensity RI sensitivities are ～599%/RIU and ～486%/RIU, respectively.

A few-mode erbium-doped fiber (FM-EDF) is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm, respectively. The refractive index difference is 0.98%, numerical aperture (NA) is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating (LPFG) is fabricated, which can convert LP₀₁ mode to LP₁₁ mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum (OAM) is also generated by combining the LPFG and polarization controller (PC). Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP₁₁ mode and first-order OAM beams, is built up. Its on-off gain of the LP₁₁ mode beam is 37.2 dB at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP₁₁ mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme, which can be used in the optical communication fields.

Stokes vectors direct detection (SV-DD) is an effective solution for short-reach optical communications. In this Letter, we investigate two-dimensional-modulation direct-detection systems based on three Stokes vector receivers (SVRs). The influences of three key factors including the states-of-polarization (SOP), the splitting ratio of the coupler, and the excess loss (EL) are studied in detail. It is shown that the splitting ratio for achieving optimum performance will be changed with SOP and EL conditions. Among these SVRs, the 3×3 coupler-based receiver with its optimal splitting ratio shows the best bit error rate performance and stability against the change of SOP.

In liquid crystal spatial light modulator (SLM)-based holographic projection, the image is usually displayed at a distant projection screen through free space diffraction from a computer-generated hologram (CGH). Therefore, it allows for removing of the projection lens for the sake of system simplification and being aberration free, known as the “lensless holographic projection”. However, the maximum size of the optical projected image is limited by the diffraction angle of the SLM. In this Letter, we present a method for the implementation of image magnification in a lensless holographic projection system by using convergent spherical wave illumination to the SLM. The complete complex amplitude of the image wavefront is reconstructed in a lensless optical filtering system from a phase-only CGH that is encoded by the off-axis double-phase method. The dimensions of the magnified image can break the limitation by the maximum diffraction angle of the SLM at a given projection distance. Optical experiment results with successful image magnification in the lensless holographic projection system are presented.

In this Letter, a test method based on oblique incidence is practically implemented in the interferometric measurement process. Three sets of wavefront data are achieved through cavity interference measurement with a Fizeau interferometer and one oblique incidence measurement. An iterative algorithm is applied to retrieve the absolute surface shape of the test flat. By adding two sets of measurements, the absolute surface error of the interferometer’s reference flat can be obtained. The new method can not only calibrate the reference flat error of interferometer, but also provide the absolute measurement method for high precision optical components applied in high power laser systems.

This study describes a novel fringe-shaping technique developed to alleviate the fringe truncation problem engendered by the acquired saturated and/or weak fringe images from high-/low-reflectance surfaces of three-dimensional (3D) objects in phase-shifting profilometry. The particle swarm optimization algorithm is employed to perform the recovery of the truncated fringes with optimal fitting for compensation after single-trial acquisition. The results show that the proposed method improves phase recovery accuracy to accomplish 3D surface reconstruction with only one set of phase-shifting fringes under different truncation sceneries.

Optical delay lines (ODLs) are one of the key enabling components in photonic integrated circuits and systems. They are widely used in time-division multiplexing, optical signal synchronization and buffering, microwave signal processing, beam forming and steering, etc. The development of integrated photonics pushes forward the miniaturization of ODLs, offering improved performances in terms of stability, tuning speed, and power consumption. The integrated ODLs can be implemented using various structures, such as single or coupled resonators, gratings, photonic crystals, multi-path switchable structures, and recirculating loop structures. The delay tuning in ODLs is enabled by either changing the group refractive index of the waveguide or changing the length of the optical path. This paper reviews the recent development of integrated ODLs with a focus on their abundant applications and flexible implementations. The challenges and potentials of each type of ODLs are pointed out.

We demonstrate a high-efficiency and high-power quasi-three-level laser based on a trapezoidal composite slab architecture with a 270 μm-thick Yb-doping surface. The design of a surface-doped slab architecture, temperature effects, laser oscillator model, and laser oscillator experiments with a surface-doped slab as a laser host medium have been presented. By theoretical calculation, the temperature rise in the surface-doped slab is only one seventh of that in the bulk-doped slab at the same maximum pump power of 30 kW. Finally, in the laser oscillator experiments, an output energy of 21.6 J is obtained when the pump energy is 48 J with a repetition rate of 5 Hz and a pulse width of 1 ms. The optical-optical efficiency is 45%.

In order to improve the morphology of microchannels fabricated by femtosecond laser ablation, the thermal process was introduced into the post-treatment processing. It was found that the thermal process cannot only decrease the roughness but also the width and depth of the microchannel. The change rates of width, depth, and roughness of the microchannel increase with processing temperature. When we prolong the time of constant temperature, the change rate of the width decreases at the beginning, and then it tends to be stable. However, the change rates of depth and roughness increase, and then they tend to be stable. In this Letter, we discuss the reasons of the above phenomena.

In order to fabricate a large-aperture continuous phase plate (CPP) using atmospheric pressure plasma processing (APPP) with high efficiency and precision, the position dwell mode and velocity mode were proposed and the iterative calculation method was developed for the non-constant removal rate. Two 320 mm × 320 mm × 2 mm CPPs were fabricated with two processing modes. The experiment results show that the velocity mode is capable of significantly reducing the processing time and shape error. The total processing time is decreased from 13.2 h to 9.3 h, and the surface shape error is decreased from 0.158λ to 0.119λ (λ = 632.8 nm) (root mean square).

In this Letter, we have demonstrated significant electric field induced (EFI) optical rectification (OR) effects existing in the surface layers of germanium (Ge) and measured the distributions of EFI OR signals along the normal directions of surface layers of Ge samples. Based on the experimental results, the ratios of the two effective second-order susceptibility components χzzz(2eff)/χzxx(2eff) for Ge(001), Ge(110), and Ge(111) surface layers can be estimated to be about 0.92, 0.91, and 1.07, respectively. The results indicate that the EFI OR can be used for analyzing the properties on surface layers of Ge, which has potential applications in Ge photonics and optoelectronics.

We present a specific-window method to subtract the interference of water vapor on terahertz frequency-domain spectroscopy (THz-FDS) at ambient temperature and pressure. A continuous-wave spectrometer based on photomixing was utilized to obtain THz-FDS of methanol vapor in the range of 50–1200 GHz. The distinctly spaced absorption features in the neighborhood of atmospheric windows of transparency were selected to perform linear fitting versus the calculated absorption cross section and obtain the concentration of methanol. Furthermore, the gradually decreased methanol vapor was quantified to demonstrate the reliability of the method.

In this Letter, we study the molecular alignment and orientation driven by two elliptically polarized laser pulses. It is shown that the field-free molecular alignment can be achieved in a three-dimensional (3D) case, while the field-free molecular orientation is only along the x and y directions, and that the field-free alignment and orientation along different axes are related to the populations of the rotational states. It is demonstrated that changing the elliptic parameter is efficient for controlling both in-pulse and post-pulse molecular alignment and orientation. The delay time also has an influence on the field-free molecular alignment and orientation.

The temporal profiles of high-power short-pulse lasers reflected from self-induced plasma mirrors (PMs) were measured with high temporal resolution in the sub-picosecond window. The leading front shape of the laser pulse is found to depend sensitively on the laser fluence on the PM surface. Spectral modulation plays a key role in pulse profile shaping. Our findings will extend our knowledge on properly using PMs.