Xing Zhong

In-Line Mach–Zehnder Interferometric Sensor Based on a Linear Five-Core Fiber

A linear five-core fiber was sandwiched in between two single-mode fibers to construct an all-fiber Mach-Zehnder interferometer (MZI). The linear five-core fiber can support multiple supermodes due to a small distance between two adjacent cores. The interferences between the fundamental supermodes, between the fundamental supermodes and the high-order core modes, and between the fundamental supermodes and the low-order cladding modes are investigated. The proposed MZI can realize strain, temperature, and curvature fiber sensors. The experimental results show both the interference between the core modes and the interference between the core modes and the cladding modes have approximately equal temperature sensitivity. The interference between the core modes is insensitive to the axial strain and the bending. In contrast, the interference between the core and the cladding modes is highly sensitive to the strain and the bending perpendicular to the arranged plane of the cores, the corresponding strain and bending sensitivities are 0.8 pm/με and 10.37 nm/m-1.

Long period fiber grating in two-core hollow eccentric fiber.

Long period fiber gratings (LPGs) in a two-core hollow eccentric fiber (TCHF) have been demonstrated experimentally. Two LPGs have been fabricated into the respective core of the TCHF by a high frequency CO2 laser. The coupling characteristics in the TCHF-LPG have been studied using the coupling mode theory (CMT). The resonant peak is mainly caused by the coupling between the core mode LP01 and cladding mode LP81. The experimental results agree well with the simulation results. Furthermore, the sensing properties of the TCHF-LPG have been investigated with respect to bending, temperature and axial strain. Compared with the LPG in the single mode fiber (SMF), the experimental results indicate that the sensitivity of the TCHF-LPGs to bending curvature is low and even very small at some bending directions. In addition, TCHF-LPGs are insensitive to the axial strain while sensitive to the temperature. Therefore, the proposed TCHF-LPGs can efficiently sense the changing temperature that is independent of the strain. Moreover, the TCHF-LPGs can also be applied to two-channel filters without signal crosstalk between two cores.

Bending characteristics of a long-period fiber grating in a hollow eccentric optical fiber.

In this paper, the bending characteristics of a long-period fiber grating (LPFG) in a hollow eccentric optical fiber (HEOF) have been investigated. Two samples fabricated under different laser exposure directions have been studied experimentally. The results show that the HEOF-LPFG can distinguish specific bending directions. The resonant peak shifts linearly with increasing curvature, but its sensitivity is lower than that of the LPFGs in single-mode fibers. In addition, the exposure direction shows a great influence on the bending characteristics due to the asymmetry of the fiber core. Furthermore, the responses of the HEOF-LPFG to temperature, axial strain, and external refractive index have been measured.

Mach-Zehnder interferometric based on a 5-core fiber

A linear 5-core fiber was sandwiched in between two single mode fibers (SMF) to construct an all fiber Mach-Zehnder interferometer (MZI). The interferences between the fundamental supermodes, between the fundamental supermodes and the high order core modes, and between the fundamental supermodes and the low order cladding mode are investigated. The experimental results show both the interference between the core modes and the interference between the core modes and the cladding modes have approximately equal temperature sensitivity. The interference between the core modes is insensitive to the axial strain.

Fabrication of fiber Bragg gratings in embedded-core hollow optical fiber

A novel Bragg fiber grating (FBG) in an embedded-core hollow optical fiber (ECHOF) has been proposed and experimentally demonstrated. The high-quality FBG fabricated with phase-mask technique by using 248 nm ultraviolet laser, has a resonant wavelength of ~943.1 nm and a dip of ~24.2 dB. Subsequently, the dependences of the resonant peak on the temperature and the axial strain were studied. Experimental results show that the temperature and axial stain sensitivity are 6.5 pm/°С and 1.1 pm/με, respectively. In addition, a 0.03 nm shift of the transmission dip can be obtained when the polarization state changes from X polarization to Y polarization.

Fabrication of long period fiber gratings based on embedded-core hollow optical fiber

We propose and experimentally demonstrate a novel long period fiber grating (LPFG) in an embedded-core hollow optical fiber (ECHOF). Without the structural deformation of the air hole and the fiber core, the high-quality LPFG can be fabricated within a few scanning cycles by a high-frequency CO2 laser with a low energy density of 0.896J/mm2. The ECHOF LPFG reveals a high temperature sensitivity of 50.2 pm/°C and a low strain sensitivity of 0.4 pm/με. Due to the good performance and easy fabrication, the ECHOF LPFG will be important to develop novel in-fiber devices.

Polarization characteristics of graphene-coated surface core fiber

A technique to enhance the response of Brillouin distributed sensors is proposed and experimentally validated. The method consists in creating a multi-frequency pump pulse interacting with a multi-frequency continuous-wave probe. The power of each pulse at a distinct frequency is lower than the threshold for nonlinear effects, while the sensor response remains given by the total power of all pulses. Distinct frequency pulses are delayed to avoid temporal overlapping and cross-interaction; this requires to smartly reconstruct the traces before photo-detection. The method is validated in a 50 km-long sensor using 3 frequencies, demonstrating a signal-to-noise ratio enhancement of 4.8 dB.

Long period fiber grating sensor in hollow eccentric optical fiber

The bending characteristics of the long period fiber grating (LPFG) in hollow eccentric optical fiber (HEOF) were investigated experimentally. The results indicate that the HEOF-LPFG is insensitive to the bending and the biggest sensitivity only is 1.3nm/m-1 in the range of 0~5m-1. More than that, the dependences of the resonant peak on the temperature and the axial strain were also studied, obtaining the response of 56.7 pm/°С and 0.3 pm/με, respectively. Obviously, the HEOF-LPFG is more sensitive to the temperature and immune to the curvature and the strain. The HEOF-LPFG can be employed to measure the single parameter and simplify the measurement equipment in the practical application.