High sensitivity axial strain and temperature sensor based on dual-frequency optoelectronic oscillator using PMFBG Fabry-Perot filter.

A dual-frequency optoelectronic oscillator (OEO) incorporating a polarization-maintaining fiber Bragg grating (PMFBG) Fabry-Perot filter for high-sensitivity and high-speed axial strain and temperature sensing is proposed and experimentally demonstrated. In the OEO loop, two oscillation frequencies are determined by a PMFBG Fabry-Perot filter with two ultra-narrow notches and two laser sources which operate as a dual-passband microwave photonic filter. The fiber birefringence affected by axial strain is far less than the temperature. Through monitoring the variations of two oscillating frequencies and beat frequency, the simultaneous measurement for the axial strain and temperature is realized. The sensitivities of the proposed OEO sensor for axial strain and temperature are experimentally measured to be as high as 100.6 or 100.5 MHz/µÎµ and -41 MHz/°C, respectively.

Fiber ring laser based on MMF-PMFBG-MMF filter for three parameters sensing.

A dual-wavelength fiber ring laser based on multimode fiber-polarization maintaining fiber Bragg grating-multimode fiber (MMF-PMFBG-MMF) filter for simultaneously axial strain, temperature and refractive index (RI) sensing is proposed and experimentally demonstrated. In the ring laser, stable dual-wavelength lasing is determined by the MMF-PMFBG-MMF filter with two polarization states. The fiber birefringence affected by axial strain is far less than the effect of the temperature. Through monitoring the variations of each wavelength shift and output power, the simultaneous measurement for the axial strain, temperature and RI is realized. In our experiment, the proposed fiber laser sensor exhibits an axial strain sensitivity of 1.16 × 10-3nm/µÎµ and an RI sensitivity of 81.2dB/RIU. Meanwhile, the temperature sensitivities of two wavelengths are experimentally measured to be 9.74 × 10-3nm/°C and 9.2 × 10-3nm /°C, respectively.

Stable single-polarization single-longitudinal-mode linear cavity erbium-doped fiber laser based on structured chirped fiber Bragg grating.

A novel linear cavity erbium-doped fiber (EDF) laser based on a structured chirped fiber Bragg grating (CFBG) filter is proposed for stable single-polarization (SP) single-longitudinal-mode (SLM) operation. For the first time, to the best of our knowledge, a structured CFBG filter with an ultranarrow transmission band which is generated by tapering directly on CFBG is used to select the laser longitudinal mode. The SLM operation is obtained by using the structured CFBG together with an unpumped EDF acting as a saturable absorber. The fluctuations of the laser peak power and center wavelength are less than 0.07 dB and 1 pm in 1 h, respectively. The stable SP operation is achieved by using the inline broadband polarizer. The measured 20 dB laser linewidth is about 27.7 kHz, which indicates the laser linewidth is approximately 1.39 kHz FWHM.

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.

Fiber Bragg grating sensor interrogator based on 2D imaging system.

We propose and demonstrate a fiber Bragg grating sensor interrogator based on a 2D imaging system using a virtually imaged phased array (VIPA) and infrared camera. There are no moving parts, and the interrogator has good stability and reliability. The absolute wavelength accuracy of the interrogator is better than that of traditional diffraction grating and a 1D-imaging-system-based method, thanks to the VIPA's high spectral resolution and large angular dispersion. The wavelength resolution of the interrogator is about 7 pm, and the spectral range of this interrogator is more than 30 nm. The optimum setup for the best wavelength resolution is analyzed.

DFB laser based on single mode large effective area heavy concentration EDF.

A π phase shifted distributed feedback (DFB) laser based on single mode large effective area heavy concentration erbium-doped fiber (EDF) has been demonstrated. The homemade EDF was fabricated by the modified chemical-vapor deposition (MCVD) technique, and the 13cm long π phase shifted fiber grating was written in the intracore of the EDF. The erbium-doped concentration is 4.19 × 10(25) ions/m(3), the mode field diameter of the fiber is 12.2801 um at 1550 nm, the absorption coefficients of the fiber are 34.534 dB/m at 980 nm and 84.253 dB/m at 1530 nm. The threshold of the DFB laser is 66 mW, and the measured maximum output power is 43.5 mW at 450 mW pump power that corresponding to the slope efficiency of 11.5%. The signal-to-noise ratio (SNR) of the operating laser at 200 mW input power is 55 dB, and the DFB laser has a Lorentz linewidth of 9.8 kHz at the same input pump power.

Single-polarization, switchable dual-wavelength erbium-doped fiber laser with two polarization-maintaining fiber Bragg gratings.

An improved erbium-doped fiber laser configuration for achieving single-polarization, switchable dual-wavelength of orthogonal polarizations oscillations at room temperature is proposed. For the first time, two fiber Bragg gratings (FBGs) directly written in a polarization-maintaining (PM) and photosensitive erbium-doped fiber (PMPEDF) as the wavelength-selective component are used in a linear laser cavity. Due to the polarization hole burning (PHB) enhanced by the polarization-maintaining FBG (PMFBG), the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller (PC). Each lasing line shows a single polarization with a polarization extinction ratio of >25 dB under different pump levels. The optical signal-to-noise ratio (OSNR) is greater than 50 dB. The amplitude variation with 16 times scans in nearly one and half an hour is less than 0.5 dB at both operating wavelength.

Analysis of radiation-mode coupling in reflective and transmissive tilted fiber Bragg gratings.

Radiation-mode coupling is stronger and more efficient in tilted fiber Bragg gratings than in other fiber gratings; it can be used to good advantage in such fields as optical communication and optical sensors. A simplified coupled-mode theory (CMT) approach to the analysis of radiation-mode coupling is proposed for what we believe to be the first time, whose validity and accuracy is demonstrated by comparing its simulation results with that of the complete CMT equations and the volume current method (VCM). With the simplified CMT approach, a theoretical spectral analysis of coupling from core mode to radiation modes in both reflective and transmissive tilted fiber Bragg gratings is presented. The influence of tilt angle on the transmission spectrum characteristics is comprehensively investigated. The different dependences between s-polarized and p-polarized radiation-mode coupling on grating tilt angle are discussed, and an analysis is performed on how to obtain a high-performance in-fiber polarizer that involves a compromise between polarization extinction and insertion loss.

Proposal and analysis of two-cavity Fabry-Perot structures based on fiber Bragg gratings.

An all-fiber two-cavity Fabry-Perot (FP) configuration based on fiber Bragg gratings (FBGs) is proposed. The characteristics of transmission spectra for the two-cavity FP structure are theoretically analyzed and comprehensively modeled. The explicit expression of the transmission coefficient for the structures is derived. The general conditions for the lengths of two cavities and reflectivities of FBGs are presented to produce the single resonant transmission peak at the central wavelength in the FBG stop band. Based on the theoretical analysis, the transmission spectra of symmetric and asymmetric two-cavity FP structures are simulated, and the simulation results are discussed and explained qualitatively. The design guidelines of the device, including the choices of cavity lengths, grating lengths, and index modulation depths, are concluded. The results show that when the increasing of the cavity length of a single-cavity FP structure results in multiple resonant peaks in the stop band, the two-cavity FP structures of the same length can inhibit the secondary resonant peaks and keep the main peak without degrading the performance through appropriately designing the cavity lengths and FBGs. Finally, the fabrication error tolerances of the structures, including inaccurate cavity lengths and reflectivities of FBGs, are calculated and discussed.