Yuanhong Yang

An embedded pressure sensor based on polarization maintaining photonic crystal fiber

An embedded optical fiber pressure sensor was made by sandwiching a piece of polarization maintaining photonic crystal fiber (PM-PCF) between two polymer foils. The excess birefringence induced by the transverse pressure applied in different directions with respect to the birefringent axes of the PM-PCF was numerically calculated and the slow axis was identified to be the most sensitive direction. An experimental test was carried out by incorporating the embedded PM-PCF sensor in a Sagnac interferometer; the output interference fringes were found to shift linearly with the applied transverse pressure with a peak wavelength to pressure sensitivity of 1.764 nm MPa–1, which corresponds to a birefringence to pressure sensitivity of 2.476 × 10−6 MPa−1.

Dynamic and static strain gauge using superimposed fiber Bragg gratings

This paper demonstrates a simple and fast interrogation method for the dynamic and/or static strain gauge using a reflection spectrum from two superimposed fiber Bragg gratings (FBGs). The superimposed FBGs are designed to decrease nonequidistant space of generated a sensing pulse train in a time domain during dynamic strain gauge. By combining centroid finding with smooth filtering methods, both the interrogation speed and accuracy are improved. A four times increase in the interrogation speed of dynamic strain, by generating a 2 kHz optical sensing pulse train from a 500 Hz scanning frequency, is demonstrated experimentally. The interrogation uncertainty and total harmonic distortion characterization of superimposed FBGs are tested and less than 4 pm standard deviation is obtained.

Quasi-distributed acoustic sensing based on identical low-reflective fiber Bragg gratings

A quasi-distributed acoustic sensing (QDAS) scheme based on identical low-reflective fiber Bragg grating is proposed and analyzed theoretically and experimentally. We realize the acoustic demodulation of different location and different frequency simultaneously by using imbalanced Michelson interferometer of -OTDR and Phase Generated Carrier technology with 600 identical low-reflective fiber Bragg gratings(FBGs) written on-line during drawing of the ordinary signal mode fibers in an equal separation of 2 m. We further obtain the 1.4 dB of frequency response flatness at the range of 200 Hz–1500 Hz and proportional character of demodulated intensity of acoustic sources with different drive voltage of underwater speaker in the experiment.

Fast digital envelope detector based on generalized harmonic wavelet transform for BOTDR performance improvement

We propose a fast digital envelope detector (DED) based on the generalized harmonic wavelet transform to improve the performance of coherent heterodyne Brillouin optical time domain reflectometry. The proposed DED can obtain undistorted envelopes due to the zero phase-shift ideal bandpass filter (BPF) characteristics of the generalized harmonic wavelet (GHW). Its envelope average ability benefits from the passband designing flexibility of the GHW, and its demodulation speed can be accelerated by using a fast algorithm that only analyses signals of interest within the passband of the GHW with reduced computational complexity. The feasibility and advantage of the proposed DED are verified by simulations and experiments. With an optimized bandwidth, Brillouin frequency shift accuracy improvements of 19.4% and 11.14%, as well as envelope demodulation speed increases of 39.1% and 24.9%, are experimentally attained by the proposed DED over Hilbert transform (HT) and Morlet wavelet transform (MWT) based DEDs, respectively. Spatial resolution by the proposed DED is undegraded, which is identical to the undegraded value by HT-DED with an allpass filter characteristic and better than the degraded value by MWT-DED with a Gaussian BPF characteristic.

Polarization maintaining photonic crystal fiber IFOG

Polarization maintaining (PM) photonic crystal fiber (PCF) and Er-doped PCF technologies are investigated for interferometer fiber optic gyroscope (IFOG) applications. A PM-PCF optimized to achieve low-loss and high polarization extinction splice-joints with conventional PM fiber pigtails of a multifunction integrated optical circuit (MIOC) was drawn and tested under different temperature conditions. The loss and polarization crosstalk properties of a fiber coil made from the PM PCF were measured and compared with conventional PM fiber. An erbium-doped PCF was designed and fabricated, with which a superfluorescent fiber source (SFS) was made and tested. The results show that an IFOG with the PM-PCF coil and the Er-doped PCF source could achieve better temperature performance. An experimental PCF-based prototype IFOG was built and tested , and preliminary results was obtained.

Experimental investigation of birefringence of solid core polarization maintaining photonic crystal fibers

We characterized the temperature sensitivity of birefringence of typical solid core polarization maintaining photonic crystal fiber (PM-PCF). Mainly the temperature dependence of birefringence is determined by two factors which are thermo-optic effects and thermal expansion. Numerical simulation with ANSYS and RSOFT show that the birefringence temperature susceptibility can be described as a polynomial function of Λ (distance between small holes) and zero temperature coefficient may be obtained at optimum Λ. A high precision birefringence measure setup was built and the birefringence of five PM-PCFs samples with different Λ were measured under different temperature. The experimental result agreed well with the simulation.

Fourier transform-based absolute phase interrogation algorithm for Sagnac interferometer-based PMF sensors

A high resolution and absolute phase interrogation algorithm for Sagnac interferometer based polarization maintaining fiber (PMF) sensor is proposed. Taking wavenumber instead of wavelength as the function variable, the output spectrum will be a standard cosine spectrum function. After Fourier transform, the frequency and relative phase of this spectrum function can be obtained and the precise absolute phase due to optical path difference between the two polarization modes in sensing PMF can be determined. A typical Sagnac interferometer based PMF temperature sensor was built and the absolute phase was determined by use of the proposed algorithm. Experiments show that 0.1°C temperature resolution and up to 900°C temperature can be measured with good linearity.

Additive non-uniform random sampling in superimposed fiber Bragg grating strain gauge

This paper demonstrates an additive non-uniform random sampling and interrogation method for dynamic and/or static strain gauge using a reflection spectrum from two superimposed fiber Bragg gratings (FBGs). The superimposed FBGs are designed to generate non-equidistant space of a sensing pulse train in the time domain during dynamic strain gauge. By combining centroid finding with smooth filtering methods, both the interrogation speed and accuracy are improved. A 1.9 kHz dynamic strain is measured by generating an additive non-uniform randomly distributed 2 kHz optical sensing pulse train from a mean 500 Hz triangular periodically changing scanning frequency.

Phase shifted FBG fabricated with arc discharge erasing technique

An Arc Discharge Erasing (ADE) technique was proposed to erase the refractive index modulation at selective area in Fiber Bragg Gratings (FBGs) and was taken to fabricate Phase Shift FBGs (PSFBGs). The ADE effect and the PSFBGs fabrication technology was demonstrated experimentally first. The capability and fabrication technique was investigated and the full control of the transmission spectral characteristics of PSFBGs, including the peak wavelength position, intensity and bandwidth, was demonstrated. To get optimum technique parameters, the experimental studies were done under different discharge electric current, time and position, and the effect of annealing processing were investigated experimentally too. With this technique, an in-line distributed feedback fiber laser with 1.16 kHz line-width was made.

Measure and redress of mode field diameter of polarization maintaining photonic crystal fibers

The measurement technology of mode field diameter (MFD) of polarization maintaining photonic crystal fiber (PM PCF) was demonstrated and experimentally investigated based on near field spot image. The PM PCFs MFD evolution under arc discharge was investigated and it was found that the MFD can be redressed effectively with this method. Experimental studies proved that the low loss splicing can be achieved after redressing the MFD of PM PCFs appropriately by controlling the arc discharge current.