Youchao Jiang

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.

Linearly polarized orbital angular momentum mode purity measurement in optical fibers

We presented a simple method for measuring the mode purity of linearly polarized orbital angular momentum (OAM) modes in optical fibers. The method is based on the analysis of OAM beam projections filtered by a polarizer. The amplitude spectrum and phase spectrum of a data ring derived from the beam pattern are obtained by Fourier transform. Then the coefficients of the mixed electric field expression can be determined and the mode purity can be obtained. The proposed method is validated and it is experimentally demonstrated in a two-mode fiber.

Simultaneous measurement of pressure and temperature based on processed capillary tube and fiber Bragg grating

A hybrid optical fiber structure sensor for simultaneous measurement of temperature and low pressure based on Fabry–Perot (FP) interference and fiber Bragg grating (FBG) is proposed. The FP cavity is fabricated with a capillary pure silica tube, whose one end is surface processed to result in an open FP cavity. The measurement of refractive index change of different pressures has been achieved by monitoring the wavelength shift of the interference pattern in the reflection spectrum. The FP cavity is integrated with the FBG to obtain temperature measurement simultaneously.

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.

Multilayer-core fiber with a large mode area and a low bending loss

We present a single-mode multilayer-core fiber with a large mode area (LMA) and a low bending loss in this Letter. A low equivalent core-cladding refractive index difference is achieved by exploiting the multilayer structure. The multilayer structure has a better bending performance than a traditional step-index core and this structure also contributes to realizing different curved refractive index profiles that have a better bending performance. An index trench is also introduced to dramatically reduce the bending loss. The experimental results show that, at a wavelength of 1550 nm, the mode area of the fabricated fiber is about 215.5 μm2 and the bending loss is 0.58 dB/turn at a 10 mm bending radius. The LMA and excellent bending performance can be obtained simultaneously with the proposed fiber.

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.

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.

Ultralow Bending-Loss Trench-Assisted Single-Mode Optical Fibers

An ultralow bending-loss trench-assisted single-mode optical fiber is designed and fabricated in this letter. The experimental results show that the fiber can support an ultralow bending loss of 0.05 dB/turn at a sharp bend radius of 5 mm at 1550 nm. This letter is considered to be of significance for the design and fabrication of bend insensitive fibers (BIFs), specifically for verifying the value of trench-assisted BIF designs. Furthermore, the fast growth of fiber-to-the-home applications will benefit from this letter.

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.