Huifeng Wei

Ambient refractive index-independent bending vector sensor based on seven-core photonic crystal fiber using lateral offset splicing

A novel, simple, and compact optical fiber directional bending vector sensor based on Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The device consists of a piece of seven-core photonic crystal fiber (PCF) sandwiched between two single mode fibers (SMFs) with a lateral offset splicing joint that covering two cores of PCF. Bending sensitivity of the seven-core PCF based MZI is changed by an axial rotation angle, which shows its capacity for recognizing positive and negative directions. Within a curvature range of -7.05 m-1 to 7.05 m-1, the calculated bending sensitivities of two resonant central wavelengths with opposite fiber orientations are 1.232 nm/m-1 and 1.174 nm/m-1, respectively. This novel MZI is formed by invoking interference between the LP01-like supermode and other higher order supermodes in the core, which leads to insensitive to ambient refractive index (ARI). We have also investigated the transmission characteristics of the sensor with the temperature change.

Multimodal coherent anti-Stokes Raman spectroscopic imaging with a fiber optical parametric oscillator

We report on multimodal coherent anti-Stokes Raman scattering (CARS) imaging with a source composed of a femtosecond fiber laser and a photonic crystal fiber (PCF)-based optical parametric oscillator (FOPO). By switching between two PCFs with different zero dispersion wavelengths, a tunable signal beam from the FOPO covering the range from 840 to 930 nm was produced. By combining the femtosecond fiber laser and the FOPO output, simultaneous CARS imaging of a myelin sheath and two-photon excitation fluorescence imaging of a labeled axons in rat spinal cord have been demonstrated at the speed of 20 μs per pixel.

Flat supercontinuum generation in cascaded fibers pumped by a continuous wave laser

We realize the flattening and extending of a CW-pumped supercontinuum with a high spectral intensity peak at the pump region. It is achieved by cascading a long zero-dispersion wavelength high-nonlinearity fiber with the output photonic crystal fiber, in order to improve the conversion efficiency of residual pump energy to long-wavelength continuum based on the effect of cascaded stimulated Raman scattering. Compared with the non-flattened continuum of 10.3 W with 3 dB bandwidth of 62 nm and 10 dB bandwidth of 360 nm, a flat continuum of 8 W with 3 dB spectral range of 340 nm and 10 dB spectral range of 420 nm is obtained. The spectral peak at the pump region decreases more than 5 dB, below the level of long-wavelength spectral intensity. Also, the long-wavelength edge has been extended by 60 nm.

Polarization dependent visible supercontinuum generation in the nanoweb fiber

We have fabricated a novel nanoweb fiber with web-like bundle of the fused-silica membranes with different thickness in its cross section. We pumped the 0.55-μm-thick membrane with 200-fs laser pulse at 800-nm just adjacent to its second-zero-dispersion wavelength, and demonstrated the polarization dependent visible supercontinuum (SC). The mode patterns were recorded in detail and analyzed at different polarization angles of incident pulse. The broadband spectrum range from ~350 nm to 950 nm is achieved for TM mode excitation. The tunable visible SC in the nanoweb fiber may be used in the substrate integrated waveguide for sensing.

Fiber optical parametric oscillator for sub-50 fs pulse generation: optimization of fiber length

We demonstrate generation of 48fs pulses with linear chirp using a short (27mm) fiber optical parametric oscillator (FOPO), which is synchronously pumped by a mode-locked ytterbium-doped fiber laser. We also study the pulse quality for both the short- and long-wavelength operation where the fiber length inside of the oscillator varies from 17 to 61mm. The optimal pulse duration is observed only in the short-wavelength operation. Furthermore, we model the FOPO system as a single-pass parametric amplifier including dispersive pulse broadening and walk-off between the pump and output. The optimal condition arises from the minimization of the walk-off and dispersion. When walk-off is large, the parametric amplification process is most efficient over some reduced effective fiber length, leading to an upper limit in the amount of the observed pulse broadening.

Theoretical and experimental analysis of splicing between the photonic crystal fiber and the conventional fiber using grin fibers

Photonic crystal fibers (PCFs) are widely used in all-fiber, high-power lasers and supercontinuum sources. However, the splice loss between PCFs and conventional fibers limits its development. Grin fibers and coreless fibers were used as a fiber lens to achieve low-loss, high-strength splicing between PCFs and single-mode fibers (SMFs). The beam propagation method was used to optimize the lengths of grin fibers and coreless fibers for a minimum splice loss. The splice loss changing with the lengths of grin fiber, coreless fiber, and the air-hole collapsed region was systematically studied theoretically and experimentally. Ultimately, a minimum splice loss of 0.26 dB at 1064 nm was realized between a high-nonlinear PCF and a conventional SMF with this method.

Polarization-maintaining photonic crystal fiber based quarter waveplate for temperature stability improvement of fiber optic current sensor

Fiber optic quarter waveplate (QWP) is widely used in all kinds of optical fiber systems such as fiber sensing systems. Specifically, in fiber optic current sensor (FOCS) system, which is now applied in power grids to measure current and monitor operation, fiber optic QWP is a key device and has serious impact on temperature stability and accuracy of the system. We fabricated a QWP using polarization-maintaining photonic crystal fiber (PM PCF), and studied its impact on the temperature characteristic of a home-made FOCS system. We also made a QWP with conventional polarization-maintaining fiber (PMF) for comparison purposes. The measured system error of the FOCS prototype using a QWP made of PM PCF between −40°C and 70°C was 8.45‰, which is five times smaller than with a QWP made of conventional PMF.

A broadband ultra-low loss all-solid photonic bandgap fiber

We describe the fabrication and characteristic of an all-solid photonic bandgap fiber with loss as low as 0.41dB/km at 1550nm within the first-order bandgap. The control of bandgaps, dispersion and bend characteristic has been investigated.

Improved large-mode-area Bragg fiber

A novel large-mode-area Bragg fiber (BF) is proposed for selectively suppressing the amplified spontaneous emission (ASE) of Yb. Confinement loss can be effectively lowered by adding a layer of F-doped glass near the core of this fiber. The BF can achieve effective suppression of ASE of Yb when the bend radius is 0.15 m at wavelength lower than 1.13 \mu m in theory, and eliminate LP11 mode in mode competition in wavelength range of 1.15-1.2 \mu m.

Long-Period Fiber Gratings Fabricated in All-Solid Photonic Bandgap Fibers by Femtosecond Laser

LPFGs in all-solid photonic bandgap fiber are inscribed by femtosecond laser pulses. The resonance peaks appear in the forbidden band of AS-PBGF, which LP01-like supermode can be guided. Mode coupling between the guided LP01-like supermodes can be obtained.