In this paper, an all-fiber dual-parameter sensor based on reflective Lyot filter (RLF) and fiber Bragg gating (FBG) structure is proposed and demonstrated for simultaneous measurement of temperature and strain. The RLF consists of an in-line polarizer and a polarization maintaining (PM) reflector, and then an FBG is fused in front of the RLF to compose the reflective sensing head. Due to the different wavelength responses of RLF and FBG to temperature and strain variations, simultaneous measurement of temperature and strain can be achieved using a dual-parameter matrix. Experimental results show that in the temperature range of 30-80 °C, the temperature sensitivities of the RLF and FBG are −1.19 nm/°C and 9.22 pm/°C, respectively. In addition, for a strain adjustment of 0-1600 με, the strain sensitivity of the RLF is 6.13 pm/με, while that of the FBG is 0.359 pm/με. Meanwhile, the regression coefficients of linear fitting are both large than 0.992, which indicated that the sensor possesses an excellent linearity. Moreover, the sensor we proposed has a large temperature and strain detection range and is simple, easy to fabricate, which demonstrating potential applications in dual-parameter detection.
In this paper, alcohol solution is added into the center hole of PCF-SPR sensor to realize the dual parameter sensor of temperature and refractive index (RI) simultaneously. Firstly, the external temperature is fixed at 20 °C, and the RI of the alcohol solution is changed. The resonance wavelength sensitivity is -6015.4 nm/RIU and the linearity is 0.99985 when the RI of alcohol solution is from 1.34 to 1.3654 with a resonance-loss sensitivity of -2640 dB/RIU and a linearity of 0.99475, respectively. When the concentration of alcohol solution is fixed at 70.4% and the external temperature of PCF is changed, the resonance loss sensitivity is 1.0375 dB/°C and the linearity is 0.976 when the external temperature is changed between 20 °C and 80 °C, its resonance loss sensitivity is 2.4219 nm/°C and the linearity is 0.9997. Finally, the sensor matrix of temperature and refractive index with respect to the shift of resonant wavelength and the change of resonant loss is obtained.
In this paper, we propose an optical fiber pH sensor with homemade poly (acrylic acid-co-acrylamide) hydrogel coating based on Mach-Zehnder interferometer (MZI), which is designed via concatenating no-core fiber, few-mode fiber and no-core fiber in sequence. The fiber surface of the sensing region is silanizated to enhance the adhesion of the hydrogel onto the fiber. When the pH value of the solution varied, the amount of free carboxylic acid ions in the hydrogel changes. This change causes the hydrogel to expand or contract, resulting in a change in refractive index. Such a refractive-index change of the hydrogel alters the effective refractive index of the cladding layer of the NFN MZI, leading to a spectral shift of the MZI. The sensor can be employed to measure a wide pH range of 2 to 12. According to the relationship between the pH values and spectral dips, the corresponding sensitivities are -0.346 nm/pH and -64 pm/pH at the pH ranges of 2-5 and 5-12, respectively. Meanwhile, the coefficients of determination of the fitting lines are both large than 0.973, which indicated that the sensor possesses an excellent linearity. Moreover, the response time of is 21.78 s. The results suggest that the proposed sensor exhibits a good repeatability and stability and holds considerable potential in fields such as environmental monitoring and chemical analysis.
An all-fiber sensor for heart rate monitoring is proposed and demonstrated based on the disturbance of the evanescent field in the no-core fiber (NCF). The sensing structure is realized through splicing a piece of single mode fiber (SMF) at the ends of the NCF, respectively. When a broad-band light is injected into the structure, the vibration of the pulse signal applied to the sensing structure will lead to the disturbance of the evanescent field in the NCF and modulate the intensity of the optical output power. Therefore, when the sensing structure is placed at the wrist of a human, it can be used to monitor the heart rate. It is demonstrated that a standard electrocardiogram (ECG) signal can be obtained when 30-mm long NCF is used in monitoring the heart rate. According to the measured ECG signals, the proposed sensor can have a response to the heart pulse at different rates ranging from 60 beats per minute (bpm) to 120 bpm.
An all-fiber optic high-sensitivity displacement sensor based on 45°-spliced PM Lyot filter is proposed and its sensing performance is investigated experimentally. According to the relationships between the dips and the displacements, the sensor has a good linearity in passive mode, whose R square is larger than 0.998, and the highest sensitivity of 132.55 pm/μm is obtained in the range of 200 μm displacement variation. Moreover, it can be compatible to an intracavity displacement sensing system, achieving narrow linewidth, high signal-to-noise ratio (SNR), and high resolution. It can be found that the sensitivity of the intracavity displacement sensor can be 60 pm/μm when the PMF fiber length is about 20 cm with a linewidth narrower than 0.05 nm and a SNR higher than 55 dB.
In this paper, we propose and demonstrate a high-integration intracavity displacement-sensor through inserting U-shape single-mode fiber interferometer (U-SMFI) into an Er-doped fiber ring laser. Considering that the U-SMFI can be realized only through bending a SMF, its characteristic of easy fabrication can reduce the cost of manufacturing process in contrast with those wavelength-modulated displacement sensors based on fiber Bragg grating, long-period grating and surface plasmon resonance. When the U-SMFI is inserted into the laser cavity, the variation of the bending radii can modulate the cavity loss so as to have an effect on the spectrum of the output laser. The proposed sensor has a higher signal to noise ratio and a narrower full width at half maximum. Through measuring the change of the spectrum, a high-resolution displacement sensor can be realized. The experimental results indicate that the sensitivities are 39 pm/μm when the bending radius is 6.5 mm.
An optical fiber sensor for simultaneous measurement of refractive index (RI) and temperature is achieved by using an optical fiber Mach–Zehnder interferometer (MZI) with two core-offset trapezoid cone structures, in which part of the core mode light will be coupled into cladding modes when passing through the first trapezoid cone and the two parts of light will combine in the second one and interfere with each other over there. The transmission spectrum would vary with the RI and the temperature. The experimental data show that the RI sensitivities of the two interference valleys are −35.93 and −51.14 nm / RIU, respectively. The temperature sensitivities of them are 0.05 and 0.04 nm / ° C, respectively. Therefore, it can be realized to gauge the RI and the temperature simultaneously. The proposed sensor can also be applied to chemistry and biology which has broad application prospects.
We obtain a switchable and tunable dual-wavelength single-frequency Er-doped ring fiber laser. In order to realize single-longitudinal output, two saturable-absorber-based tracking narrow-band filters are formed in 3- meter-long unpumped Er-doped fiber to narrow the linewidth via using the PM-FBG as a reflection filter. The maximum output power is 2.11 mW centered at 1550.16 nm and 1550.54 nm when the fiber laser operates in dual-wavelength mode. The corresponding linewidths of those two wavelengths are measured to be 769 Hz and 673 Hz, respectively. When the temperature around the PM-FBG is changed from 15 °C to 55 °C, the dual-wavelength single-frequency fiber laser can be tuned from 1550.12 nm to 1550.52 nm and from 1550.49 nm to 1550.82 nm, respectively.
We propose a sensitivity-enhanced intracavity-absorption gas sensor based on the phenomenon of mode competition in the dual-wavelength ring fiber laser. The laser configuration possesses the sensing and reference wavelengths as 1530.372 nm and 1532.168 nm, respectively. When the hollow-core photonic crystal fiber (HC-PCF) is filled with 1000-ppmv acetylene, a sudden change on absorption intensity of more than 30 dB can be achieved by adjusting the optical loss in the laser cavities, resulting from the mode competition between the sensing and reference wavelengths. The minimum detectable acetylene concentration (MDAC) of 29.53 ppmv is obtained in experiment, one order of magnitude higher than former works.
A dual-parameter optical fiber sensor, which is fabricated by sandwiching a segment of few-mode fiber (FMF) with two down-tapers between two segments of standard single-mode fibers (SMFs), is investigated theoretically and experimentally. The two down-tapers on the FMF can enhance the evanescent field, making the sensor more sensitive to changes in the external environment. The refractive index (RI) and temperature are measured simultaneously using the different sensitivities of the two dips in this experimental interference spectrum. The measured temperature sensitivities are 0.097 and 0.114 nm/°C, and the RI sensitivities are −97.43 and −108.07 nm/RIU, respectively. Meanwhile, the simple SMF-FMF-SMF structure is also measured. By comparing the experimental results of the two structures, the sensitivities of the proposed structure based on the dual-taper FMF are significantly improved. In addition, the sensor is easy to fabricate and cost effective.
Temperature-insensitive pressure measurement using a single fiber Bragg grating (FBG) based on reflection spectrum
bandwidth modulation and optical power detection is proposed. A specially designed double-hole cantilever beam
(DHCB) is used to provide pressure induced axial strain gradient along the sensing FBG and further modulate the
reflection bandwidth of the grating. The bandwidth modulation immunes to spatially uniform temperature effect and the
pressure can be unambiguous determined by reflected optical power measurement, avoiding the complex wavelength
interrogation system. The thermal fluctuate of the system is less than 1.2 % F.S. for temperature range from -10°C to 80
°C without any temperature compensation.
A technique for temperature-insensitive displacement measurement using a single fiber Bragg grating (FBG) based on
strain-gradient-induced reflection spectrum bandwidth modulation and optical power detection is demonstrated. Linear
displacement measurement up to 20mm with displacement resolution of 0.054 mm and thermal stability of < 0.8 % F.S.
for temperature range from 20 °C to 80 °C are respectively achieved without any temperature compensation.i
The design and development of a digital liquid level sensor system based on parallel fiber sensor heads are presented. A
novel parallel fiber sensor heads based on side-polished common single-mode optical fiber is designed and applied.
When the sensor head contacts with the liquid, we observe a sharp decrease in the output intensity. The digital liquid
level sensor system based on 16 sensor points is operated by using an optical source with limited power. The
measurement resolution and range of the liquid level sensor can be selected by adjusting the distance between the sensor
heads. The advantages of this sensor system include low cost, high sensitivity, simple structure, excellent real-time
character, high stability, and temperature-insensitiveness in normal temperature and pressure.
Time-division-multiplexing(TDM) demodulation technique of Fiber Bragg Grating(FBG) sensor arrays based on a
tunable pulsed laser is studied. A tunable fiber laser based on a matched FBG is applied, and the wavelengths of the
sensing FBGs are obtained by determining the maximum voltages by using a photodiode. The advantages of this scheme
include simple structure, high signal-to-noise ratio, and the sensing signals are obtained by determining the maximum
voltages.
Temperature-independent micro-displacement measurement using a single fiber Bragg grating based on broadened reflection spectrum is proposed and experimentally demonstrated. The structure of specially designed bending cantilever beam (BCB) is proposed. The BCB induces axial strain gradient along the sensing FBG, resulting in a Bragg bandwidth modulation. The broadening of FBG spectrum bandwidth and the reflected optical power correspond to micro-displacement changes, insensitive to spatially uniform temperature variations. For a displacement variation of 20mm and a temperature change from 20°C to 100°C, the micro-displacement measurement deviation error is ±0.12mm without any temperature compensation. Through optical power detecting by a pin photodiode (PD), the micro-displacement sensor avoids complex demodulation process and potentially costs little.
An innovative algorithm based on the evolutionary programming (EP) method is developed for recovering distributions of axial strain along a fiber grating from its reflection spectral response. The proposed method exhibits a number of attractive features that prove to be effective for solving the inverse problems. The basics of EP are reviewed and the detailed programming procedures of the proposed algorithm are presented. By combining EP and the matrix method for calculating the reflection spectrum of a FBG, we obtain a new method for the distributed sensing. The numerical simulations show good agreements between the original and the reconstructed strain profiles.
Simultaneous measurement of temperature and force using a single fiber Bragg grating based on broadened reflection spectrum is proposed and demonstrated. The wavelength peak shift and the bandwidth broadening with the change of temperature and force allow discrimination between the temperature and force effects. Standard deviation errors of 1.8°C and 0.16N have been obtained with temperature and force ranges of up to 20~100°C and 0~8N, respectively.
Design and construction of temperature-insensitive fiber Bragg grating (FBG) liquid level sensor based on bending cantilever beam (BCB) is proposed and demonstrated. The BCB induces spatially gradient strain on the unique sensing FBG, resulting in a Bragg bandwidth modulation. The broadening of FBG spectrum bandwidth and the reflected optical power are corresponded to liquid level changes, insensitive to spatially uniform temperature variations. In the liquid-level range of 500 mm and temperature change from 20oC to 80oC, the liquid level measurement fluctuates less than 2% without any temperature compensation. By a pin-photodiode (PD) optical power detecting, the liquid-level sensor avoids expensive and complex demodulation techniques and potentially costs low.
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