Wenxing Jin

Tunable orbital angular momentum generation in optical fibers

We present a method in this Letter to generate optical vortices with tunable orbital angular momentum (OAM) in optical fibers. The tunable OAM optical vortex is produced by combining different vector modes HE2,meven (HE2,modd) and TE0,m (TM0,m) when l=1 or combining HEl+1,meven (HEl+1,modd) and EHl-1,modd (EHl-1,meven) when l>1 with a π/2 phase shift. The vortex can be regarded as a result of overlapping two orthogonal optical vortex beams of equal helicity but opposite chirality with a π/2 phase shift. We have experimentally demonstrated the smooth variation of OAM from l=-1 to l=+1 by adjusting a polarizer at the output end of the fiber.

Switchable Optoelectronic Oscillator Using an FM-PS-FBG for Strain and Temperature Sensing

By incorporating a phase-shifted fiber Bragg grating (PS-FBG) inscribed in few-mode fiber (FMF) named FM-PS-FBG for the first time, a novel frequency-switchable optoelectronic oscillator (OEO) to perform simultaneous strain and temperature measurement is proposed and experimentally demonstrated. In the OEO loop, two switchable oscillation frequencies are determined by the reflected bands of FM-PS-FBG based on different mode couplings and corresponding notches. Due to the characteristics of modes, two OEO-generated microwave signals exhibit different strain and temperature sensitivities. Therefore, through monitoring the frequency shifts of microwave signals, the simultaneous strain and temperature sensing can be realized by the proposed OEO sensor with high-speed and high-resolution interrogation.

Tunable Orbital Angular Momentum Generation Based on Two Orthogonal LP Modes in Optical Fibers

We present a method to generate tunable orbital angular momentum (OAM) based on linearly polarized (LP) modes in optical fibers. The tunable OAM is produced by combining two even (odd) LP modes with orthogonal polarization directions. The tunability of OAM is realized by controlling the power proportion of the two LP modes through a polarizer. From another perspective, the generated mixed vortex can also be regarded as a result of overlapping two orthogonal optical vortices of equal helicity but opposite chirality, then the tunable OAM can be achieved through filtering the mixed mode using a polarizer.

Generation of the Tunable Second-Order Optical Vortex Beams in Narrow Linewidth Fiber Laser

We experimentally demonstrate a narrow linewidth single-longitudinal-mode erbium-doped fiber ring laser with an output of tunable second-order optical vortex beams (OVB). The laser is based on a mode selective all-fiber fused coupler, which is composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF–FMF coupler inserted in the cavity of the laser acts as a mode converter, which converts LP01 mode to LP21 mode group. The tunable OVB is produced by combing two even LP21 modes with orthogonal polarization directions. The tunability of second order OVB in the fiber laser is experimentally realized by adjusting a polarizer at the output end of the fiber. The 20 dB linewidth of the fiber laser is approximate 7.2 kHz.

Two-dimensional tunable orbital angular momentum generation using a vortex fiber

We demonstrate the two-dimensional tunable orbital angular momentum (OAM) generation in a ring-core (vortex) fiber. The LP11 mode generated by an all fiber fused coupler is coupled into a vortex fiber. Because the vector modes of the LP11 mode group in the vortex fiber are no longer degenerate, the mode status will change between linearly polarized modes (LPMs) and complex OAM modes periodically during propagation. The generated OAM can be tuned smoothly by filtering the mixed mode with different polarization directions or changing the wavelength at a certain polarization directions. The two-dimensional tuning of OAM from l=-1 to l=+1 is experimentally demonstrated in an all fiber OAM generator.

Compact Mach–Zehnder interferometer-based no-core fiber hollow-core fiber no-core fiber structure

We propose and experimentally demonstrate a compact Mach–Zehnder interferometer (MZI)-based no-core fiber hollow-core fiber no-core fiber (NHN) structure. Two segments of the no-core fiber embedded on two ends of the hollow-core fiber are employed as beam splitter and combiner, which realize an MZI to measure temperature and strain. With the variation of the temperature and strain, the measurement is achieved by monitoring the transmission spectrum shift of the MZI. The results show that the proposed sensor has a temperature sensitivity of 30.92  pm/°C in the range from 30°C to 90°C, and strain sensitivity of 0.652  pm/μϵ in the range from 0 to 700  μϵ, respectively. The NHN structure exhibits the advantages of compactness, easy fabrication, and low cost, which shows the potential sensing applications of the proposed fiber structure.

Measuring Vector Modal Content of Vortex Fibers

We present a method based on spatial and spectral (S2) imaging technique to measure the vector modal content of vortex fibers. Different vector modes can be told apart and their modal contents can be determined by implementing the S2 imaging process four times with four different polarizer directions. Moreover, the orbital angular momentum modes (OAMs) can be observed directly through the spiral phase patterns without interference process, and the OAM mode purity can be obtained from the intensity and phase patterns.

Low-threshold dual-wavelength erbium-doped fiber laser based on ring core fiber with core-offset structure

A low-threshold dual-wavelength erbium-doped fiber laser is proposed and tested in an experiment. The filter is based on a ring-core fiber with a core-offset structure. The threshold of the laser is as low as 16 mW. The operating wavelengths are 1560.02 and 1564.75 nm, respectively. Meanwhile, both of the side-mode suppression ratios of the operation wavelengths are higher than 50 dB when the pump power is 100 mW. In addition, the strain sensitivities are 0.7574 and 0.6676 pm μe −1, respectively.