Air pollution by particles of aerodynamic diameter less than 2.5 μm (PM2.5) is currently an important environmental issue. As standard methods for monitoring and characterization are time-consuming and expensive, there is need for a simple, effective and inexpensive device for real-time PM2.5 monitoring. Although optical methods based on the scattering of laser beams exist, they do not provide independent measurement of particle size and refractive index. In systems based on detection in an evanescent optical field, particles tend to adhere to the surface, strongly limiting the device lifetime. Here we report a new approach to airborne particle characterization that makes use of hollow-core photonic crystal fibre (HC-PCF). Employing optical gradient and radiation forces to trap and propel airborne particles along the hollow core, it provides in situ particle counting, sizing and refractive index measurement in real-time with effectively unlimited device lifetime. We show that the transmission drop and time-of-flight can be unambiguously mapped to particle size and refractive index.
The properties of noise induced by stimulated Brillouin scattering (SBS) in long-range interferometers and their influences on the positioning accuracy of dual Mach-Zehnder interferometric (DMZI) vibration sensing systems are studied. The SBS noise is found to be white and incoherent between the two arms of the interferometer in a 1-MHz bandwidth range. Experiments on 25-km long fibers show that the root mean square error (RMSE) of the positioning accuracy is consistent with the additive noise model for the time delay estimation theory. A low-pass filter can be properly designed to suppress the SBS noise and further achieve a maximum RMSE reduction of 6.7 dB.
We report long-range optical binding of multiple polystyrene nanoparticles (100-600 nm in diameter) at fixed interparticle distances that match multiples of the half-beat-lengths between the lowest order modes of a hollow-core photonic crystal fiber. Analysis suggests that each nanoparticle converts the incoming optical mode into a superposition of co-propagating modes, within the beat pattern of which further particles can become trapped. Strikingly, the entire particle arrangement can be moved over a distance of several cm, without changing the inter-particle spacing, by altering the ratio of backward-to-forward optical power. Potential applications are in multi-dimensional nanoparticle-based quantum optomechanical systems.
We report light-driven instability and optomechanical self-oscillation of a fused silica “nanospike” at low gas pressures.
The nanospike (tip diameter 400 nm), fabricated by thermally tapering and HF-etching a single mode fiber (SMF), was
set pointing at the endface of a hollow-core photonic crystal fiber (HC-PCF) into the field created by the fundamental
optical mode emerging from the HC-PCF. At low pressures, the nanospike became unstable and began to self-oscillate
for optical powers above a certain threshold, acting like a phonon laser or "phaser". Because the nanospike is robustly
connected to the base, direct measurement of the temporal dynamics of the instability is possible. The experiment sheds
light on why particles escape from optical traps at low pressures.
Fiber inhomogeneity induced random birefringence in single mode fiber (SMF) can lead to the polarization fading effect in distributed fiber vibration sensor using Mach-Zehnder interferometry. By considering the contributions of both visibility and phase noises in interference output signal, the influence of polarization fading effect on the position accuracy of distributed vibration sensor using cross-correlation algorithm is theoretically derived. A new term of equivalent signal to noise ratio (SNRe) is defined in the model. Based on the theory, a novel method for decreasing the positioning error of low-frequency vibration signal is given and verified by field test results.
This paper research an ultra long distance distributed fiber-optic intrusion detects system. Laser source is amplified and
modulated before input to the dual Mach-Zehnder interferometers. This ensures the system can detect longer distance
with less loss and nonlinearity than continuous laser input. The cross correlation algorithm is considered to locate the
vibration event after the polarization controlled of the optical source, and the locate precision is also discussed. Finally,
an experiment result is given as the sensing fiber length is 112km with the location precision about 160m.
A novel all-metal fiber-optic accelerometer with Michelson interferometric configuration is proposed, using mass blocks
and flexible reed structure. The sensing arm is configured to directly transform the applied acceleration to fiber
deformation in axial direction, which increases the acceleration sensitivity of the sensor. The axis of the reference arm is
set orthogonal to that of the sensing arm in order to make it insensitive to applied acceleration, and thus decrease the
phase noise of the sensor. Using the flexible reed of 1mm thick and the mass block of 208g, an acceleration sensitivity of
556.7rad/g(54.9dB) is achieved by this structure. In the case of 10-4 rad/rt Hz(100Hz)noise floor, the minimum detectable
acceleration is 200ng/rt Hz.
The phase generated carrier (PGC) arctangent algorithm needs to be compensated when the carrier modulation depth C
deviates from 2.63 rad. Parameters measurement error can induce an incomplete compensation in the arctangent
algorithm. The demodulation distortion due to parameters estimation error is analyzed quantitatively in this paper. The
relative amplitude error (RAE) and harmonic suppression ratio (HSR) are considered, which indicate a linear distortion
and a nonlinear one separately. Theoretical analysis shows that the fluctuation range of RAE has a linear relation with an
absolute value of δC which is the estimation error of the modulation depth C. The fluctuation range of RAE also has a
square relation with δφ1, δφ2 which are the estimation error of the phase delay Δφ1Δφ2 of fundamental and second
harmonic carriers respectively. Similarly, minimum HSR decreases by 6dB along with a doubled absolute value of δC
and by 12dB with that of δφ1. Experimental results are presented to corroborate the theory.
A novel noise suppression method is proposed for the pressure surveillance of heavy oil thermal recovery well using
white light extrinsic Fabry-Perot interferometric (EFPI) fiber-optic sensor. The moving grey model GM(1, 1) algorithm
is applied to suppress the non-stationary noise and disturbance in downhole environment. Both theoretical analysis and
field test results show that there exists a threshold effect for the moving window length selection, the optimized value of
window length occurs at the threshold point which best removes the high frequency disturbance as well as retaining the
fast change of actual pressure signal. Field test results show that the proposed method can increase the signal to noise
ratio (SNR) of pressure profile with 10.4dB, and then can effectively suppress the large deviation in the forecasting result
of daily oil production by combining temperature information.
For a fiber with complex index profile, its density fluctuations change with position and it also introduces
birefringence, hence the Brillouin frequency changes with position due to variation of modal index, and sound
velocity has a range instead of being a constant. As a result, the single mode fibers support multiple Brillouin
resonances varying in position, even with polarization scramblers (PS) of the pump and probe waves. For a Brillouin
optical time domain analysis (BOTDA), at a specific location, because of the spatial resolution, the measured
Brillouin frequency still gives a range, although PS can help to reduce this fluctuation, as the spatial resolution is
much smaller than the beat length of single mode fiber (SMF). The measured Brillouin frequency variations in one
position and its location dependence reflect the non-uniformity of the optical fiber, rather than the systematic error
of the sensor detection system. When a fiber supports elliptical birefringence, then four Brillouin resonances can be
found for an acoustically uniform fiber based on theoretical calculation of their eigenmodes. The beat of different
Brillouin peak frequencies and their magnitudes change with temperature and strain, which can be used to measure
temperature and strain simultaneously in LEAF fiber without the need of the sweeping Brillouin spectrum.
A novel method for distributed fiber beat length, birefringence and differential group delay (DGD) measurement based
on Brillouin optical time domain analysis (BOTDA) technique is proposed. The difference of local maximum and
minimum Brillouin gain is determined by scanning input state of polarization (SOP) of pulsed probe wave to recover
local beat length. The average beat length, birefringence and DGD measured on 100m SMF28 at 1550nm wavelength
with 1m spatial resolution is 11.8m, 1.4×10-7 and 0.21ps respectively.
KEYWORDS: Interferometry, Sensors, Signal detection, Linear filtering, Fiber optics sensors, Baryon acoustic oscillations, Current controlled current source, Environmental sensing, Interference (communication), Signal to noise ratio
Dual Mach-Zehnder interferometric vibration sensor is an appropriate solution for submarine cable security application. While in this application the detected vibration signal is always narrow-bandwidth and short-duration subject to environmental constraints, which makes correlation based vibration positioning algorithm a poor robustness to noise. A preprocessing method focusing on expanding signal 3dB bandwidth before correlation is proposed in purpose of reducing ultimate positioning mean square error. A high pass filter is imposed to enhance the weight of high frequency components by attenuating low frequency main lobe. Field test results indicate a significant positioning error reduction when using this novel method as long as the cutoff frequency of high pass filter is selected in a valid region where positive effect of bandwidth extension is larger than effect of SNR reduction, and MSE reduction value in valid region agrees well with theoretical prediction.
KEYWORDS: Signal to noise ratio, Interferometry, Bandpass filters, Electronic filtering, Signal detection, Optical filters, Fiber optics sensors, Signal processing, Linear filtering, Polarization
Dual Mach-Zehnder interferometric fiber sensor is one of the appropriate solutions to the application of submarine cable
security. While the seabed environment makes the vibration output a narrow bandwidth lowpass interference signal and
these property makes the correlation based event positioning algorithm hard to design. A preprocessing method intends
to increase signal power spectral range before correlation is proposed. A bandpass filter is imposed to enhance the effect
of high frequency components, despite the filtered signal is different from the original one. A post-integration SNR
threshold is calculated to determine the proper center frequency and bandwidth of the filter. Experiment results show a
good accordance with theoretical prediction.
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