A novel demodulation method for interferometric fiber sensor is proposed in this paper. The physical parameters to be measured by the sensor is obtained by calculating the phase variation of the interference components. The phase variation is computed with the assist of the fast Fourier analysis. For fiber interferometers, most of the energy is contained in the few spatial frequencies corresponding to the components that produce the interference. Therefore, the information of the interference fringe can be presented by the Fourier results at those intrinsic frequencies. Based on this assumption, we proposed a novel method to interrogate the fiber interferometer by calculating the Fourier phase at the spatial frequency. Theoretical derivation proves that the Fourier phase variation is equal to the phase change of the interferometer. Simulation results demonstrate the ability of noise resistance of the proposed method since the information of all wavelength sampling points are adopted for the demodulation process. A Sagnac interferometer based on a section of polarization-maintaining photonic crystal fiber is utilized to verify the feasibility of the phase demodulation technique by lateral pressure sensing. Experimental results of -0.069rad/kPa is acquired.
A novel special fiber fabrication method based on a common single mode fiber (SMF) for dual-parameters measurement has been proposed and experimentally demonstrated. The fabrication setup is based on a three dimensional electric displacement platform which can realize the function of twisting and tapering at the same time. The proposed novel structure simultaneously undergoes the aforementioned two processes. Then a twisted-tapering fiber structure is formed. There are two dominant resonant wavelengths in the spectrum. Thus, simultaneous measurement for strain and temperature can be achieved. The following result shows that the strain measurement can be achieved by intensity demodulation, with the sensitivity of -0.01565 dB/με and 0.00705 dB/με corresponding to the dip1 and dip2, respectively. Therefore, the total sensitivity of the strain is 0.0227 dB/με. Moreover, the cross impacts of the wavelength shift are - 0.772 pm/με and 0.895 pm/με. Similarly, the wavelength demodulation is selected to temperature measurement. The temperature sensitivity of 50.53pm/°C and 45.12pm/°C are obtained. The cross sensitivity of the intensity variation are 0.04058dB/°C and 0.02031 dB/°C. As a result, the dual-parameters can be described to a cross matrix of the sensitivity value. The proposed sensor has a great potential for engineering applications due to its compact structure, simple manufacture and low cost.
A highly sensitive twist sensor without temperature cross sensitivity based on tapered single mode-thin core-single mode fiber offset structure is proposed and experimentally demonstrated. The two parameters mentioned above can be measured simultaneously without cross sensitivity. The twist sensitivity of 0.12dB/° is achieved by tracking power variation of the resonant wavelength, and the wavelength shift of the spectrum is ±0.01nm. The temperature sensitivity of 0.12nm/°C can be achieved by wavelength demodulation, and the power fluctuation of the spectrum is ±0.015dB. Therefore, the twist and temperature can be detected by the power and wavelength demodulation method, respectively.
In this article, we propose a fiber displacement sensor based on a few mode fiber loop sandwiched between two single mode fibers (SMF). The proposed sensor is flexible due to the tunable resolution and dynamic range. The FMF is coiled to a fiber loop by making a knot. The in-line MZI sensing structure is fixed on a two dimensional (2D) translation stages. By moving one stage while another stage is fixed, the displacement is applied on the sensing structure. The resolution of the translation stage is 10μm. The few mode fiber loop acts as the transducer for the displacement sensing. The displacement will change the radius of the few mode fiber loop, which leads to a wavelength shift of the interference pattern. When the fiber loop has different initial radius, the same displacement will cause a different curvature variation. So the sensitivity of the wavelength shift to the displacement is dependent on the initial radius. A smaller initial radius of the loop will lead to a larger sensitivity, higher resolution but smaller dynamic range, so it is proper for micro displacement sensing. On the contrary is the lager initial radius that is proper for sensing in a large dynamic range. By simply adjusting the initial radius of the transducer loop, different sensitivity and resolution can be reached. Experimental results show the sensitivities of 0.267nm/mm, 0.384nm/mm, 0.749nm/mm and 1.06nm/mm for initial loop radius of 1.9cm, 1.5cm, 1cm and 0.75cm, respectively.
KEYWORDS: Sensors, Demodulation, Acoustics, Signal processing, Fabry–Perot interferometry, Tunable filters, Optical filters, Temperature metrology, Head, Aluminum, Positron emission tomography, Signal to noise ratio
A phase demodulation method for short-cavity extrinsic Fabry-Perot interferometer (EFPI) based on two orthogonal wavelengths via a tunable optical filter is proposed in this paper. A broadband light is launched into the EFPI sensor and two monochromatic beams with 3dB bandwidth of 0.2nm are selected out from the reflected light of the EFPI sensor. A phase bias is induced between the two interferential signals due to the wavelength difference of the two beams. The wavelength difference will have an affect on the sensitivity of demodulated signal, which has been theoretically and experimentally demonstrated. The maximum sensitivity can be obtained when the phase bias is 0.5π corresponding to the wavelength difference of 1/4 FSR of the EFPI spectrum. The acoustic wave induced phase variation can be interrogated through an optimized differential cross multiplication (DCM) method. A normalization process is induced into the traditional DCM method to eliminate the influence of ambient temperature and pressure fluctuation induced spectrum shift on output signal. This means that, once the wavelength difference is fixed, the wavelength variation of each individual beam will have little influence on the amplitude of demodulated signal. The EFPI sensing head is formed by a 3μm-thick aluminum diaphragm, which has a SNR of more than 53dB. Through the proposed demodulation scheme, a large dynamic range and good linearity is acquired and Q-point drift problem of traditional EFPI sensor can be solved. The demodulation scheme can be applied to other kinds of short-cavity EFPI based acoustic sensors.
Due to the overwhelming advantages compared with traditional electronicsensors, fiber-optic acoustic sensors have arisen enormous interest in multiple disciplines. In this paper we present the recent research achievements of our group on fiber-optic acoustic sensors. The main point of our research is high sensitivity interferometric acoustic sensors, including Michelson, Sagnac, and Fabry-Pérot interferometers. In addition, some advanced technologies have been proposed for acoustic or acoustic pressure sensing such as single-mode/multimode fiber coupler, dual FBGs and multi-longitudinal mode fiber laser based acoustic sensors. Moreover, our attention we have also been paid on signal demodulation schemes. The intensity-based quadrature point (Q-point) demodulation, two-wavelength quadrature demodulation and symmetric 3×3 coupler methodare discussed and compared in this paper.
KEYWORDS: Fiber Bragg gratings, Sensors, Acoustics, Signal detection, Fiber optics sensors, Signal to noise ratio, Aluminum, Temperature metrology, Titanium, Head
We propose and fabricate a new type fiber acoustic sensor based on dual fiber Bragg gratings (FBGs) configuration. The acoustic sensor head is constructed by putting the sensing cells enclosed in an aluminum cylinder space built by two Cband FBGs and a titanium diaphragm of 50 um thickness. One end of each FBG is longitudinally adhered to the diaphragm by UV glue. Both of the two FBGs are employed for reflecting light. The dual FBGs play roles not only as signal transmission system but also as sensing component, and they demodulate each other’s optical signal mutually during the measurement. Both of the two FBGs are pre-strained and the output optical power experiences fluctuation in a linear relationship along with a variation of axial strain and surrounding acoustic interference. So a precise approach to measure the frequency and sound pressure of the acoustic disturbance is achieved. Experiments are performed and results show that a relatively flat frequency response in a range from 200 Hz to 1 kHz with the average signal-to-noise ratio (SNR) above 21 dB is obtained. The maximum sound pressure sensitivity of 11.35mV/Pa is achieved with the Rsquared value of 0.99131 when the sound pressure in the range of 87.7-106.6dB. It has potential applications in low frequency signal detection. Owing to its direct self-demodulation method, the sensing system reveals the advantages of easy to demodulate, good temperature stability and measurement reliability. Besides, performance of the proposed sensor could be improved by optimizing the parameters of the sensor, especially the diaphragm.
A novel curvature sensor comprize a section of multi-mode fiber and a up-taper is proposed and demonstrated experimentally. The whole fabrication process is quite simple and the sensor head is cost effective. Measurement results show that it has a maximum curvature sensitivity of -61.877nm/m-1 at 1.1718m-1 and -9.2115nm/m-1 from 1.1718m-1 to 1.6583m-1. Temperature sensitivity of 89.01 pm/°C within the range of 20~80 °C has also been achieved, which implies the possibility for measurement of temperature. High sensitivity and low-cost make it a preferable candidate for curvature sensing in practical applications.
In this paper, we propose two optical fiber sensing applications based on the pulse selection technique, in which an acousto-optic modulator (AOM) is employed to generate optical pulses and to choose the pulses traveling back to it. Firstly, we propose an interferometer configuration with optical path difference (OPD) amplified, which offers an alternative way to increase the interrogation resolution and develops some potential sensing applications which require high sensitivity. By adjusting the repetition interval, we can select the pulses which travel along the corresponding arms of the interferometer for any given trips. Secondly, we designed a Fiber Bragg Grating (FBG) sensing system with intelligent sensor management, which offers two work modes and improves the availability of information.
Two kinds of thulium-doped fiber ring lasers based on a spatial mode beating filter and comb filtering effect are presented and experimentally demonstrated, which all show multiwavelength laser spectrum around 2 μm. In the implementation of the first type of experiment configuration by the use of a piece of multimode fiber (MMF) as a spatial mode beating filter, dual-,triple-, and quadruple-wavelengths appeared whose extinction noise ratio is 25 dB by adjusting the angle of polarization controller. Different wavelength spaces are obtained by inserting different lengths of MMF. The second type is achieved by inserting a Sagnac loop mirror, which was constructed by a 3-dB coupler and a piece of polarization maintaining fiber. Seven stable wavelengths with channel spacing of 0.65 nm and an extinction ratio of 35 dB was achieved. These systems are simple and easy to construct, which can be useful for 2 μm wavelength-division-multiplexed applications.
A novel arrayed-waveguide grating (AWG) based on unbent waveguides is proposed. Two graded-index planar waveguides (GISLAB) are used as input and output planar waveguides, respectively, and the arrayed waveguides are replaced by unbent waveguides, which are photosensitive waveguides exposed to UV light twice. Next, the 2-D semivectorial beam propagation method is adopted to simulate the light transmitting process in the device, and the simulation shows that this device and the design are feasible.
A novel arrayed-waveguides grating (AWG) used for demultiplexer is proposed. Two graded index planar waveguides (GRIPW) were used for input and output planar waveguide respectively, and the arrayed waveguides were replaced by unbent waveguides that were directly fabricated through UV Written. Theoretical analysis and numerical simulation showed that this device and the design were feasible. The design of the device was also optimized.
To realize practical wavelength division multiplexing (WDM) system, a high-performance AWG with an all-optical polarization state controller (AOPSC) is introduced. This device is polarization-independence based on asymmetric planar waveguide in the silicon-oxynitride material system with a high-refractive-index difference n =0.7 % and large a/b (here, a is the width and b is the thickness of the core layer waveguide). Only TE0 mode can exist in the device, the TM0 mode is cutoff. Finally, an eight-channel AWG using 100GHz channel spacing (the center wavelength is 1552.52nm) is designed. In order to get more realistic results, the BPM method was used to simulate the light propagation in the AWG. The simulation was calculated for both TE and TM. Using BPM method, and we found only TE0 can propagate through the device. No polarization dependence was measured in the wavelength response.
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