Zhibo Liu

Refractive index sensing characterization of a singlemode-claddingless-singlemode fiber structure based fiber ring cavity laser.

This paper firstly demonstrated the refractive index (RI) characteristics of a singlemode-claddingless-singlemode fiber structure filter based fiber ring cavity laser sensing system. The experiment shows that the lasing wavelength shifts to red side with the ambient RI increase. Linear and parabolic fitting are both done to the measurements. The linear fitting result shows a good linearity for applications in some areas with the determination coefficient of 0.993. And a sensitivity of ~131.64nm/RIU is experimentally achieved with the aqueous solution RI ranging from 1.333 to 1.3707, which is competitively compared to other existing fiber-optic sensors. While the 2 order polynomial fitting function, which determination relationship is higher than 0.999, can be used to some more rigorous monitoring. The proposed fiber laser has a SNR of ~50dB, and 3dB bandwidth ~0.03nm.

Tunable and switchable dual-wavelength single polarization narrow linewidth SLM erbium-doped fiber laser based on a PM-CMFBG filter

A tunable and switchable dual-wavelength single polarization narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser based on polarization-maintaining chirped moiré fiber Bragg grating (PM-CMFBG) filter is proposed and demonstrated. For the first time as we know, the CMFBG inscribed on the PM fiber is applied for the wavelength-tunable and-switchable dual-wavelength laser. The PM-CMFBG filter with ultra-narrow transmission band (0.1 pm) and a uniform polarization-maintaining fiber Bragg grating (PM-FBG) are used to select the laser longitudinal mode. The stable single polarization SLM operation is guaranteed by the PM-CMFBG filter and polarization controller. A tuning range of about 0.25 nm with about 0.075 nm step is achieved by stretching the uniform PM-FBG. Meanwhile, the linewidth of the fiber laser for each wavelength is approximate 6.5 and 7.1 kHz with a 20 dB linewidth, which indicates the laser linewidth is approximate 325 Hz and 355 Hz FWHM.

A Static Axial Strain Fiber Ring Cavity Laser Sensor Based on Multi-Modal Interference

A novel and simple static strain fiber ring cavity laser sensor based on all-fiber single-mode-multimode-single-mode (SMS) structure filter is proposed and experimentally demonstrated. When an axial strain is applied on the SMS filter, the pass-band varies, resulting in shifts of the lasing central wavelength. Thus, the value of axial strain imposed on the structure can be calculated by the monitored central wavelength of the system via the function relationship between these two parameters. Compared with the conventional axial strain sensing system based on an SMS fiber structure, the proposal exhibits additional advantages in terms of high optical signal to noise ratio (~ 50 dB) and narrow 3 dB bandwidth apart from simple fabrication and low cost. A good linearity is obtained between the axial strain applied on the SMS filter and central wavelength of the proposed laser sensing system with R2 of 0.99954, and high sensitivity of 2.3 pm/με in the range 0-2333 με is successfully achieved.

Temperature-Independent and Strain-Independent Twist Sensor Based on Structured PM-CFBG

In this letter, we present a structured polarization-maintaining chirped fiber Bragg grating (PM-CFBG) for temperature-independent and strain-independent twist measurement. The structured PM-CFBG, which has two transmission peaks, is created by tapering directly on the PM-CFBG. The grating transmissivity of structured PM-CFBG changes linearly with twist while it is insensitive to strain and temperature. The wavelength interval changes proportionally to the temperature, but remains the same as the strain increases. This novel sensor can be used to measure temperature, strain, and twist simultaneously.

Interferometric vibration sensor using phase-generated carrier method

An interferometric fiber-optic vibration sensing system using the phase-generated carrier (PGC) method is proposed and experimentally demonstrated. The sensing section consists of a Sagnac interferometer combined with a Mach-Zehnder interferometer, a length of sensing fiber is shared between the two interferometers. The PGC demodulation scheme is used to demodulate the time-varying phase shifts induced by vibrations. Spatial information can be extracted from the demodulated results. A prototype sensing system with a 628 m long sensing fiber has been tested and a spatial resolution better than 12 m is successfully achieved.

Detection of Liquid Level With an MI-Based Fiber Laser Sensor Using Few-Mode EMCF

This letter firstly and experimentally demonstrated a fiber laser sensing system using Michelson interferometer (MI) based on few-mode elliptical multilayer-core fibers (EMCFs) for liquid level and refractive index (RI) measurement. The fiber sensing system has high optical signal-to-noise ratio (~45 dB) and narrow 3-dB bandwidth (<;0.1 nm). Four water-glycerin solutions of different RI are used for this experiment, and the higher sensitivity is acquired for the higher RI liquid. The sensitivities for the liquids with indices of 1.333, 1.353, 1.373, and 1.393 are 30.5, 35.6, 41.1, and 50.6 pm/mm within a depth range of 0-45 mm, respectively. Moreover, the RI of liquid can also be measured, and a sensitivity of 329.222 pm/mm/RIU is experimentally achieved.

Single-Frequency and Single-Polarization DFB Fiber Laser Based on Tapered FBG and Self-Injection Locking

A stable single-polarization narrow linewidth single-frequency distributed feedback (DFB) fiber laser is proposed and demonstrated. For the first time, to our knowledge, the tapered FBG written in the photosensitive large-mode-area (LMA) high-concentration erbium-doped fiber (EDF) is applied in the DFB fiber laser. Self-injection locking is used to make the proposed DFB fiber laser operate in the stable single-polarization state and reduce the linewidth of the DFB fiber laser. The LMA high-concentration EDF is fabricated by the modified chemical vapor deposition technique. The tapered FBG can be seen as the equivalent phase shift FBG, and the equivalent π phase shift is produced by tapering the uniform FBG directly. The threshold of the DFB fiber laser is about 145 mW, and the maximum output power is 43.55 mW at 450-mW pump power. The measured and modified slope efficiencies of the DFB fiber laser are approximately 14.27% and 15.82%, and the optical signal-to-noise ratio (OSNR) of the laser is over 55 dB. The stable single-polarization and single-frequency operation of the fiber laser is realized, and the measured 20-dB linewidth of DFB fiber laser with self-injection locking and without self-injection locking are approximately 10.3 and 52.3 kHz, respectively.

In-fiber liquid-level probe based on Michelson interferometer via dual-mode elliptical multilayer-core fiber

Abstract An in-fiber liquid-level probe fabricated from homemade dual-mode elliptical multilayer-core fiber (EMCF) was proposed and experimentally demonstrated. The proposed sensor simply consists of a segment of the EMCF with one end coated with silver film, and a Michelson interferometer is roughly established when light from single-mode fiber incident from the other end. The detected interference patterns, rather clean due to the few-mode property, shift as the liquid level due to strong interaction between high-order modes and measurands through evanescent waves. Both the propagation characteristics and operation principle of such a sensor were demonstrated in detail, and sensitivities of 33.48, 43.35, and 48.93 pm/mm corresponding to liquid indices of 1.333, 1.353, and 1.373 were successfully achieved with a 50-mm EMCF probe, respectively. Moreover, the proposed sensor had the potential to discriminate measurand index after proper calibration.

Switchable Dual-Wavelength SLM Fiber Laser Using Asymmetric PMFBG Fabry–Perot Cavities

A ring cavity erbium-doped fiber laser based on asymmetric structure filter with two polarization-maintaining fiber Bragg grating (PMFBG) Fabry-Perot cavities is proposed and demonstrated for switchable dual-wavelength single-polarization single-longitudinal-mode (SLM) operation. The asymmetric structure filter consists of three FBGs written in a photosensitive PM fiber to form asymmetric two Fabry-Perot cavities with different cavity lengths, which can effectively decrease the transmission passband bandwidth of the asymmetric structure filter and restrain the secondary resonance peak. The fluctuations of each operating wavelength and output power of the proposed fiber laser in single-wavelength operation are less than 1 pm and 0.1 dB. The stable single polarization SLM operation is guaranteed by the asymmetric structure filter with two PMFBG Fabry-Perot cavities and the uniform PMFBG. Meanwhile, the Lorentz fitting with a 3-dB linewidth of the fiber laser for each wavelength is <;5 kHz.

Characteristics of HE11-HE21 mode elliptical multilayer-core fiber with low nonlinearity

Abstract. An elliptical multilayer-core fiber (EMCF) with low nonlinearity for wide-band dual-mode operation was proposed. The EMCF, in which only the HE11 and HE21 modes could be propagated, supported a distinct dual-mode operation. The characteristics of the EMCF, including modal properties, the operating wavelength, the transmission loss, the effective area, and the bending loss, were systematically investigated. For EMCFs, a large modal diameter (33.48  μm) was achieved to reduce the fiber nonlinear penalty; meanwhile, the bending loss could be kept in a lower level. In addition, the single-polarization property in EMCFs could be realized by combining stress applying elements.