We propose and experimentally demonstrate a technique using a composite-double-probe-pulse (CDPP) to eliminate the effect of polarization fading for phase-sensitive optical time-domain reflectometry (Φ-OTDR) based on ultra-weak FBG (UWFBG) array. The CDPP is composed of two optical pulses whose spatial interval is equal to twice the spatial interval of adjacent UWFBGs in the UWFBG array. One optical pulse is a long optical pulse, and the other optical pulse is composed of two continuous short optical pulses, whose polarization states are orthogonal to each other. The width of the short pulse is equal to half of the width of the normal pulse and their frequencies are different from the long pulse. By using such a method to perform the sensing for the UWFBG array, distributed quantitative measurement can be realized with only direct detection scheme and the influence of polarization fading in the demodulation of signal is thoroughly eliminated.
We developed a novel lab-on-a-chip device with the capability of rapidly antibody determination that use nano-beads as the solid carriers. The device combines a plasmon-assisted optical conveyor belt in the main microfluidic channel, which is made of gold nano-ellipses perpendicular to each other. In the presence of an external uniform electric field, the hot spots in the belt function as optical tweezers can trap and transport properly sized nano-beads with target antibody combined along a fix direction through rotating the polarization. Several branch channels intersecting the main microfluidic channel at right angles are used to transport smaller antigen modified nano-beads, which can be labeled with fluorescent dyes. When arriving at the crossings, the smaller nano-beads would be trapped by hotspots on the surface of two-dimensional ellipses arrays around the conveyor belt and can’t be transported between two ellipses due to their smaller size. So, the antibody modified nano-beads would be transported along the optical conveyor belt and encounter the trapped antigen modified ones in the ellipses arrays successively. Only those ones with specific antigen combined that stick to the antibody to be measured can be dragged by bigger nano-beads and transport with it. In light of that, we can determinate the antibody by identifying the fluorescence-labelled nano-beads at the exit of the main channel. With the capacity for parallel detection, our design offers an attractive scheme for rapid, high throughput determination of antibody in microfluidic channels, which are also ease to operate.
An improved data processing and analysis method is proposed to realize simultaneous monitor of multiple vibrations using polarization optical time domain reflectometry system. In our method, a differential trace of the frequency component along the fiber is got by doing subtraction of the distance traces with different number of vibrations at a certain frequency, and the vibrations vibrating at different time can be located by analyzing the response power variation of the differential trace. For multiple vibration points vibrating with the same frequency and at the same time, multiple vibration events can also be distinguished by extracting the modulated frequency component, and the frequency component is obtained at the starting or ceasing state of the vibrations because the initial phases of vibration sources are unsteady and different. With our method, a POTDR sensing system which can simultaneously monitor multiple vibration points over 3km with 10m spatial resolution is demonstrated.
A distributed vibration sensing technique using double-optical-pulse based on phase-sensitive optical time-domain reflectometry (ϕ-OTDR) and an ultraweak fiber Bragg grating (UWFBG) array is proposed for the first time. The single-mode sensing fiber is integrated with the UWFBG array that has uniform spatial interval and ultraweak reflectivity. The relatively high reflectivity of the UWFBG, compared with the Rayleigh scattering, gains a high signal-to-noise ratio for the signal, which can make the system achieve the maximum detectable frequency limited by the round-trip time of the probe pulse in fiber. A corresponding experimental ϕ-OTDR system with a 4.5 km sensing fiber integrated with the UWFBG array was setup for the evaluation of the system performance. Distributed vibration sensing is successfully realized with spatial resolution of 50 m. The sensing range of the vibration frequency can cover from 3 Hz to 9 kHz.
A polarization optical time-domain reflectometer (POTDR) can distributedly measure the vibration of fiber by detecting the vibration induced polarization variation only with a polarization analyzer. It has great potential in the monitoring of the border intrusion, structural healthy, anti-stealing of pipeline and so on, because of its simple configuration, fast response speed and distributed measuring ability. However, it is difficult to distinguish two vibrations with the same frequency for POTDR because the signal induced by the first vibration would bury the other vibration induced signal. This paper proposes a simple method to resolve this problem in POTDR by analyzing the phase of the vibration induced signal. The effectiveness of this method in distinguishing two vibrations with the same frequency for POTDR is proved by simulation.
We demonstrate a novel fiber-optic hydrophone that use a fiber Bragg grating (FBG) as
a sensing element. The operation principle is based on the modulation of birefringence of the FBG
by high-frequency ultrasound. By measuring the amplitude of the first Stokes parameters of the
transmitted light in FBG using a in line polarimeter, the amplitude and frequency of acoustic
pressure can be determined. The FBG hydrophone has a linear response to acoustic pressure, and
the sensitivity and dynamic range of the sensor are studied.
A distributed fiber sensor which can simultaneously measure the strain and vibration of fiber is proposed. Only the Brillouin scattering is used for the measurement with a heterodyne detection scheme. In the Brillouin scattering, the Brillouin frequency shift is utilized for the strain measurement and the state of polarization is utilized for the vibration measurements. Experimental results proved a distributed fiber strain and vibration sensor which can achieve 11.5 km sensing distance, 10 m spatial resolution, 26 Hz frequency measurement range, 0.8 Hz frequency resolution and 0.1 MHz uncertainty of Brillouin frequency measurement.
The technology of Polarization Optical Time Domain Reflectometer (POTDR) can be used to obtain the external
events' information by measuring the change of state of polarization (SOP) of the Rayleigh backscattering in fiber. When
the fiber is disturbed at two different positions simultaneously, we analyze the frequency spectrums of the change of
Rayleigh backscattering light which are obtained by POTDR theoretically for ideal fiber, and by numerical simulation
for single mode fibers. We find that the frequency spectrums between the first and second events contain the first
vibration's frequency and its frequency multiplication. The frequency components of the spectrums after the second
event are the linear combination of the first and the second events' frequencies. So we can obtain the location and the
frequency information of the two events by analyzing the frequency spectrums. In addition, the frequency distribution in
the frequency spectrums from different positions are different because of the different initial SOPs at different positions.
So all the actual frequency information can not be obtained from only one frequency spectrum. We add up the frequency
spectrums from the positions within a beat length to obtain the perturbation's frequency and the method can reduce the
misdiagnosis rate because the sum of the frequency spectrums contains all the initial SOP within a beat length.
Using a short optical pulse with finite extinction ration (ER) in Brillouin optical time domain analysis (BOTDA)
system can achieve high spatial resolution and frequency resolution at the same time. However, the fluctuation
after the pulse generated by the pulse generator can result in a same fluctuation to the optical pulse modulated by
electro-optic modulator (EOM). Theoretical and experimental results demonstrate that this fluctuation can result
in a reverse peak which has a 60MHz dip in the detected Brillouin spectrum. This makes it impossible to
perform the sensing successfully when the Brillouin frequency shift (BFS) induced by the strain or temperature
is within the dip. By adjust the DC bias of the EOM and the power of the electrical pulse, the influence of the
fluctuation is eliminated successfully. A 15MHz BFS induced by strain on 20cm section over a 50m fiber is
successfully detected with a frequency error of 0.8MHz.
Photoelastic effect which is the result of the external stress on the fiber causes inductive birefringence. In general, the photoleastic effect induces the differential group delay (DGD) between two orthogonal states of polarization (PSPs). Then the polarization mode dispersion (PMD) can be shown by DGD. In this paper, a digital emluator is present to validate a new method which uses PMD to pinpoint stress-location of optical-fiber cable .The major point about this new location method is how to pick up signal from a mass of random polarization states. Based on a waveplate model and Monte Carlo analysis, a more physical digital emulator is shown to find the possibility of the stress-location in optical fiber.