Zu-g Guan

Coherence Multiplexing of Distributed Sensors Based on Pairs of Fiber Bragg Gratings of Low Reflectivity

A coherence multiplexing scheme for distributed sensors based on fiber Bragg grating (FBG) pairs is introduced. Each pair of identical FBGs forms a Fabry-Perot (FP) interferometer (FPI) and induces an additional optical path difference (OPD) that is proportional to the center-to-center interval between the two FBGs (one for sensing and the other for reference). The interference intensity reaches its maximum when the FPI-induced OPD is compensated by scanning one arm of a Michelson interferometer to a certain position. The variation of the measurand induces a mismatch between the central reflection wavelengths of two FBGs and consequently reduces interference intensity. To separate the interferometric signals for demultiplexing, the intervals between the two FBGs are preset to different values for different sensors. In order to improve the multiplexing ability of the system and reduce the crosstalk among the sensors, we use gratings of low reflectivity. Temperature sensing is demonstrated to show the high sensitivity (-1.92%/degC) and low crosstalk of our distributed sensing system.

Multiplexing Scheme for Self-Interfering Long-Period Fiber Gratings Using a Low-Coherence Reflectometry

A novel multiplexing scheme based on a low-coherence reflectometry (LCR) is proposed for a sensing array (in parallel) of self-interfering long-period fiber gratings (SI-LPGs). Each SI-LPG sensor consists of an LPG and a section of fiber with a highly reflective end (mirror). The spectral information of each LPG is sensitive to some parameters of the surrounding measurand and can be reconstructed from the corresponding subreflectograms (obtained by the LCR) through a fast Fourier transformation. The sensing signals of multiple SI-LPG sensors can be multiplexed if the length of the fiber section in each sensor is set to a different value. Experiments of measuring the surrounding temperature at different sensors are demonstrated to show the good performance of our multiplexing system.

Optimization and fabrication of stitched long-period gratings for gain flattening of ultrawide-band EDFAs

We present the optimization and fabrication of the stitched long-period fiber gratings (SLPGs) for gain flattening of an ultrawide-band erbium-doped fiber amplifier (EDFA). With an improved genetic algorithm, the gain spectrum of a practical EDFA with bandwidth of 80 nm is flattened within /spl plusmn/0.69 dB by an optimized SLPG. Furthermore, the SLPG is designed specially for a point-by-point grating writing technique. A superior control of the fabrication procedure of the designed SLPG has been demonstrated in the experiment.

Interrogation technique for a fiber Bragg grating sensing array based on a Sagnac interferometer and an acousto-optic modulator

We propose and experimentally demonstrate a novel real-time interrogation technique for a fiber Bragg grating (FBG) sensing system that is based on a frequency-shifted asymmetric Sagnac interferometer. FBG sensors are connected to the Sagnac loop by an optical coupler, and an acousto-optic modulator (AOM) is asymmetrically placed in the Sagnac loop. By linearly sweeping the driving frequency of the AOM, the environmental variation around each FBG sensor can be determined by measuring the spectrum of the interference signals of the two counterpropagating light beams reflected by the corresponding FBG. The system has the advantages of low cost and real-time sensing.

Optical low-coherence reflectometry based on long-period grating Mach-Zehnder interferometers

The optical low-coherent interferometric technology for long-period grating (LPG) Mach-Zehnder interferometers is described. By including the coupling and recoupling behaviors of a LPG pair, a numerical model is developed to analyze the output reflectogram of the system. The effects of the grating interval, grating length, grating strength, and light source on the output reflectogram have been comprehensively discussed, which reveals that the low-coherence reflectometry offers the capability of interrogating the multiplexed sensors based on LPG pairs. A comparison of the calculated and experimental results is presented, and an excellent agreement between the simulation and the measurement is shown.

Wavelength Detection of Coherence-Multiplexed Fiber-Optic Sensors Based on Long-Period Grating Pairs

Multiplexed fiber-optic sensors based on long-period grating pairs (LPGPs) are presented. With a low-coherence reflectometry, the spectral responses of the specially configured LPGP sensors are precisely quantified from the corresponding sub-reflectograms through a fast Fourier transformation (FFT). By monitoring the wavelength shift of the LPGP sensors, temperature is measured experimentally, and good performance is obtained

Optimization of step-changed long-period gratings for gain-flattening of EDFAs

We report the use of the genetic algorithm to optimize the parameters of our previously proposed step-changed long-period gratings for gain-flattening of erbium-doped fiber amplifiers (EDFAs). The gain spectrum of an EDFA is flattened within /spl plusmn/0.35 dB over 30 nm through a suitable selection of the cladding mode and the optimization of the grating parameters. An amplitude-mask structure is proposed to simplify the fabrication process of the optimal grating structure.

Coherence Multiplexing System Based on Asymmetric Mach–Zehnder Interferometers for Faraday Sensors

An optical coherence multiplexing system for Faraday sensors is proposed. With an asymmetric Mach-Zehnder (M-Z) interferometer, the linearly polarized light from a Faraday sensor is decomposed into two orthogonal modes, which are interrogated by a path-scanning Michelson interferometer at different path positions. The variations in the intensities of these two modes are utilized to deduce the rotation of the polarization plane, which is related to the measurand tested by the Faraday sensor. By employing multiple asymmetric M-Z interferometers with arm-lengths carefully controlled, the sensing signals of the multiplexed Faraday sensors are obtained and well separated in one scan of the Michelson interferometer.

Low Coherent Optical Frequency Domain Reflectometry Interrogates Fiber Bragg Grating Sensors

A novel fiber optical reflectometry is proposed for long distance measurements and sensing applications. The reflectrometric information is included in the phase difference of two interfering light beams reflected at the same scattering point on the fiber under test (FUT). The reflection position and local information can be determined by modulating the frequency difference and analyzing the reflecting signal in frequency domain. Theoretical analysis and experimental demonstration are presented. Two adjacent reflection points 50 km away from the reflectometry with a gap of 11 meters between each other is distinguished. It is estimated that the operating range could be over 500 km theoretically by employing a light source with 0.3 nm bandwidth. This novel reflectometry is applied as a fiber Bragg grating interrogation solution as an example.

Distributed Sensing System of Optical Low-Coherence Reflectometry Using an Array of Identical Fiber Bragg Gratings

The optical low-coherent interferometric technology for Fiber Bragg grating (FBG) sensing array is described. The sensing array is formed by a series of identical FBGs with low reflectivity (5.25%), where the first FBG acts as a reference (shielded from the measurand), and the other FBGs work as sensors. Each sensing-FBG and the reference-FBG can form a Fabry-Perot (F-P) interferometer with a specific cavity length. By scanning a home-made optical low-coherence reflectometry (OLCR), the signals corresponding to different sensing-FBGs (F-P interferometers) can be obtained and well distinguished. A measurement of temperature is demonstrated and it shows good performance of the scheme.