Zhifang Wu

Unique characteristics of a selective-filling photonic crystal fiber Sagnac interferometer and its application as high sensitivity sensor.

We demonstrate a Sagnac interferometer (SI) based on a selective-filling photonic crystal fiber (SF-PCF), which is achieved by infiltrating a liquid with higher refractive index than background silica into two adjacent air holes of the innermost layer. The SF-PCF guides light by both index-guiding and bandgap-guiding. The modal birefringence of the SF-PCF is decidedly dependent on wavelength, and the modal group birefringence has zero value at a certain wavelength. We also theoretically and experimentally investigate in detail the transmission and temperature characteristics of the SI. Results reveal that the temperature sensitivity of the interference spectrum is also acutely dependent on wavelength and temperature, and an ultrahigh even theoretically infinite sensitivity can be achieved at a certain temperature by choosing proper fiber length. An ultrahigh sensitivity with -26.0 nm/°C (63,882 nm/RIU) at 50.0 °C is experimentally achieved.

All-fiber multiwavelength thulium-doped laser assisted by four-wave mixing in highly germania-doped fiber

An all-fiber multiwavelength Tm-doped laser assisted by four-wave mixing (FWM) in highly Germania-doped highly nonlinear fiber (HG-HNLF) has been experimentally demonstrated. Benefiting from the high nonlinearity of the HG-HNLF, intensity-dependent gain caused by FWM is introduced into the laser cavity to mitigate the gain competition in Tm-doped fiber. Thanks to a 50-m HG-HNLF, 9, 22, and 36 lasing lines with considering 10-dB, 20-dB, and 30-dB bandwidth, respectively is obtained at room temperature with wavelength spacing of 0.86 nm. More than 30-nm broad-band lasing can be obtained. The stability of the proposed fiber laser has also been studied. Repeat measurements show the power fluctuations and wavelength drifts of the lasing lines are less than 1.6 dB and 0.05 nm, respectively. The laser performances without the assistance of HG-HNLF have fewer center wavelengths lasing, which indicates that FWM in HG-HNLF plays an important role for the multiwavelength laser operation.

In-line Mach-Zehnder interferometer composed of microtaper and long-period grating in all-solid photonic bandgap fiber

We report a compact in-line Mach-Zehnder interferometer combining a microtaper with a long-period grating (LPG) in a section of all-solid photonic bandgap fiber. Theoretical and experimental investigations reveal that the interferometer works from the interference between the fundamental core mode and the LP01 cladding supermodes. The mechanism underlying the mode coupling caused by the microtaper can be attributed to a bandgap-shifting as the fiber diameter is abruptly scaled down. In addition, the interferometer designed to strengthen the coupling ratio of the long-period grating has a promising practical application in the simultaneous measurement of curvature and temperature.

Simultaneous temperature and force measurement using Fabry-Perot interferometer and bandgap effect of a fluid-filled photonic crystal fiber.

A novel fiber sensor capable of simultaneously measuring force and temperature is proposed and investigated. A section of high-index-fluid-filled photonic bandgap fiber (HIFF-PBGF) is inserted in a fiber loop to act as the sensing head. Photonic bandgap effect of the HIFF-PBGF as well as Fabry-Perot interferometer (FPI) introduced by controlling the splicing between the HIFF-PBGF and single mode fiber is used for achieving force and temperature discrimination. Taking advantage of the bandgap being high sensitivity to the temperature, a high temperature sensitivity of more than -1.94 dB/°C is achieved, which is the highest based on the intensity measurement, to our best knowledge. Meanwhile, a force sensitivity of 3.25 nm/N (~3.9 pm/με) is obtained, which could be enhanced by controlling the FPI shape. The device also has the strong points of easy fabrication, compact structure and high interference fringe contrast.

Efficient one-third harmonic generation in highly Germania-doped fibers enhanced by pump attenuation

We provide a comprehensive study on one-third harmonic generation (OTHG) in highly Germania-doped fiber (HGDF) by analyzing the phase matching conditions for the step index-profile and optimizing the design parameters. For stimulated OTHG in HGDF, the process can be enhanced by fiber attenuation at the pump wavelength which dynamically compensates the accumulated phase-mismatch along the fiber. With 500 W pump and 35 W seed power, simulation results show that a 31% conversion efficiency, which is 4 times higher than the lossless OTHG process, can be achieved in 34 m of HGDF with 90 mol. % GeO2 doping in the core.

Fiber Bragg gratings in heterogeneous multicore fiber for directional bending sensing

We present the fabrication of fiber Bragg gratings (FBGs) in a trench-assisted heterogeneous multicore fiber (MCF). Two obviously different Bragg reflection peaks are obtained due to the slight difference of refractive indices between the center core and the outer cores. To investigate the reflections of the two FBGs simultaneously, only a segment of multimode fiber is inserted between the lead-in single mode fiber and the MCF. The experimental results confirm that the curvature sensitivity of the FBG in the outer core is a sinusoidal function of the bending orientation angle. The maximum linear curvature sensitivity is about 0.128 nm/m?1. The cross sensitivity to temperature or externally applied axial strain can be eliminated by discriminating the different responses of FBGs inscribed in outer cores and the center core. Thus this MCF with FBGs can be utilized as a directional bending sensor. Moreover, the proposed sensor offers several advantages, such as low cost and flexibility in fabrication.

Temperature- and strain-insensitive curvature sensor based on ring-core modes in dual-concentric-core fiber

We report on a high-performance curvature sensor based on a long-period grating (LPG) in a dual-concentric-core fiber (DCCF). The LPG is inscribed to couple light from the fundamental mode of the central core to the ring-core modes, resulting in the generation of a series of resonant dips. Two adjacent dips shift toward each other when the LPG is bent. By monitoring the variation of the wavelength interval between these two dips, this LPG can be applied in curvature measurement with a sensitivity as high as -9.046  nm/m(-1). More importantly, such a wavelength interval is almost immune to the cross impacts of temperature and axial strain, since the sensitivities to temperature and axial strain are only 2.6 pm/°C and 0.083 pm/με, respectively.

Design and analysis of surface plasmon resonance sensor based on high-birefringent microstructured optical fiber

Optical fiber based surface plasmon resonance (SPR) sensors are favored by their high sensitivity, compactness, remote and in situ sensing capabilities. Microstructured optical fibers (MOFs) possess microfluidic channels extended along the entire length right next to the fiber core, thereby enabling the infiltrated biochemical analyte to access the evanescent field of guided light. Since SPR can only be excited by the polarization vertical to metal surface, external perturbation could induce the polarization crosstalk in fiber core, thus leading to the instability of sensor output. Therefore for the first time we analyze how the large birefringence suppresses the impact of polarization crosstalk. We propose a high-birefringent MOF based SPR sensor with birefringence larger than 4 × 10−4 as well as easy infiltration of microfluidic analyte, while maintaining sensitivity as high as 3100 nm/RIU.

In-line optofluidic refractive index sensing in a side-channel photonic crystal fiber

An in-line optofluidic refractive index (RI) sensing platform is constructed by splicing a side-channel photonic crystal fiber (SC-PCF) with side-polished single mode fibers. A long-period grating (LPG) combined with an intermodal interference between LP01 and LP11 core modes is used for sensing the RI of the liquid in the side channel. The resonant dip shows a nonlinear wavelength shift with increasing RI over the measured range from 1.3330 to 1.3961. The RI response of this sensing platform for a low RI range of 1.3330-1.3780 is approximately linear, and exhibits a sensitivity of 1145 nm/RIU. Besides, the detection limit of our sensing scheme is improved by around one order of magnitude by introducing the intermodal interference.

Coupling-length phase matching for efficient third-harmonic generation based on parallel-coupled waveguides.

We study third-harmonic generation (THG) in parallel-coupled waveguides where the spatial modulation of the mode intensity provides quasi-phase matching, called coupling-length phase matching (CLPM), for efficient nonlinear frequency conversion. Different types of CLPM are investigated for THG, and it is found that two sets of CLPM conditions can be practically implemented with traditional waveguides. These two CLPM conditions are further investigated by considering nonlinear phase modulations, which can degrade the CLPM-based THG conversion. However, up to 45% efficiency is still possible in this scheme. The greatest significance of this approach is that the requirement of perfect phase matching in a single waveguide is no longer necessary, leading to an alternative waveguide design for THG.