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In the real environment, the carrier phase delay (θ) is difficult to accurately maintain at nπ (n is an integer), which results in the demodulation error of the phase generation carrier (PGC) demodulation algorithm. In this paper, the fast Fourier transform (FFT) technique is used to extract 1 θ 0 , 2 θ 0 , 3 θ 0 , 4 θ 0 from the first to fourth harmonic signals of the interference signal, then introduce 1 θ 0 , 2 θ 0 , 3 θ 0 , 4 θ 0 into the mixed signal as the initial phase, and finally perform a mixing operation to real time eliminate the carrier phase delay. The experiments show that when the carrier delay is in the range of 0 to π, the SINAD of the ameliorated algorithm reaches 43.66 dB, which is 19.31 dB higher than the PGC-Arctan algorithm on average. The THD of the ameliorated algorithm can be stabilized at 0.183 %, which always maintains the best level of the PGC-Arctan algorithm. The ameliorated algorithm eliminates the carrier phase delay and does not have the phase singularity, thus expanding the application range of the fiber optic interferometer sensor. [ABSTRACT FROM AUTHOR]

A sensitivity-enhanced temperature and strain sensor based on a fiber Sagnac interferometer (FSI) cascaded with a suspended-core microstructured optical fiber (SCMOF) Fabry-Perot probe is proposed and experimentally demonstrated. The FSI consisted of a certain length of high birefringence ultra-fine polarization maintaining fiber (UFPMF) as the sensing interferometer. The cascaded Fabry-Perot interferometer (FPI) is fabricated by fusion splicing a SCMOF tip with a single mode fiber (SMF) as the reference interferometer. By adjusting the length of the UFPMF in the FSI, the free spectral ranges (FSRs) of the FSI and FPI can be easily made similar, thus creating a Vernier effect and achieving enhanced sensitivity. The experimental results show that under the influence of the Vernier effect, the temperature sensitivity and strain sensitivity of the cascaded sensors are 30.909 nm/℃ and −52.57 pm/με, respectively. With the advantages of simple production, easy fabrication, and high sensitivity, this sensor has a wide application prospect in the fields of aerospace and precision measurement. [Display omitted] • A sensitivity-enhanced temperature and strain sensor based on vernier effect is proposed. • The FPI is an ideal reference interferometer that is insensitive to temperature and strain compared to the FSI. • The length of the sensing fiber for FSI sensing is significantly reduced due to the high birefringence coefficient of UFPMF. • The production of both interferometers requires only simple fusion splicing and cleaving. • The temperature sensitivity and strain sensitivity of the cascaded sensors are 30.909 nm/℃ and −52.57 pm/με, respectively. [ABSTRACT FROM AUTHOR]

For clinical applications, a sensor of cancer cells should exhibit high sensitivity, have a small and robust sensing structure, and exhibit good anti-interference. This paper presents a novel biosensor for the recognition and concentration detection of biotin receptor (BR)-overexpressed cancer cells. This biosensor uses a hydrogel composed of biotin-quantum dots (BQDs) grafted onto oxidized sodium alginate with carboxymethyl chitosan (BQDs-g-OSA/CMCS) and a double optical fiber ball (DOFB) sensing structure. The developed BQDs-g-QSA/CMCS hydrogel exhibited specific recognition capabilities for cancer cells and an excellent fluorescence stability. Meanwhile, the proposed DOFB structure was small and presented a strong robustness and integrated fluorescence excitation and reception, which are favorable for in vivo sensing. In addition, in-depth study of the sensing performance of the sensor was conducted by utilizing streptavidin (SV) protein and BR-overexpressed cancer cells (leukemia cells, K562). The experimental results revealed a linear relationship between the photoluminescence intensity of the sensor and concentrations of SV and K562, with minimum detection concentrations of 1.62 nM and 1.0 × 103 cells/mL, respectively. In addition, the specificity of the sensor was confirmed through investigation of the its response to another BR-overexpressed cancer cell type (lung cancer cells, A549) and normal cells (human gastric mucosal epithelial, GES-1). The sensor exhibited similar responses to A549 as to K562 cells but not to GES-1 cells, which indicates its specificity toward cancer cells with BR overexpression. In summary, the proposed sensor offers important potential for applications in cancer diagnosis and treatment. • A QD/ hydrogel coated double fiber optic ball (DOFB) cancer cell sensor with high sensitivity, strong specificity and good stability was developed. • The proposed DOFB biosensor is small in size, strong in robustness, integrated fluorescence excitation and reception, which are particularly suited for in vivo sensing. • The prepared QDs-doped hydrogel not only can specifically recognize cancer cells, but also has good fluorescence stability. • The DOFB biosensor can effectively and specifically detect the biotin receptor protein and the biotin receptor-overexpressed cancer cells A549, the minimum detection concentrations are 1.62 nM and 1.0×103/mL, respectively. [ABSTRACT FROM AUTHOR]

• The geometric dimension of the package shell of the sensing unit are studied by theory and experiment, and improper design of the package housing may degrade the frequency response characteristics of the sensing unit. • The influence of prestress on the frequency response characteristic of sensor is studied. • The geometric dimension of the package shell and the prestress of the sensing unit are reasonably optimized to make the working frequency of the sensor avoid the resonant frequency of the shell. • The paper provides a theoretical basis for the design and manufacture of the sensors with wide frequency response and high sensitivity in different engineering applications, such as high frequency small amplitude and low frequency large amplitude. In coal mine, military, oil well monitoring and other fields, the traditional electrical accelerometers based on piezoelectric and capacitive can not meet the needs of high-performance applications. The optical fiber acceleration sensor with high precision, large dynamic, wide frequency response, anti-electromagnetic interference and other unique advantages, can meet a variety of applications. The complete optical fiber acceleration sensor consists of a package housing and an internal sensing unit. Some factors, including the vibration and deformation of the housing itself, and the prestress of the sensing unit will directly affect the frequency response characteristics of the sensor. The experimental results show that the resonant frequency of the package housing is outside the operating frequency band of the sensor through reasonable design of shell parameters; in a certain torque range (0.2 N . m ∼ 0.8 N . m), the resonant frequency increases with the increase of the torque. The research provides a theoretical basis for the design and manufacture of the sensors with wide frequency response and high sensitivity in different engineering applications, such as high frequency small amplitude and low frequency large amplitude. [ABSTRACT FROM AUTHOR]

• Fourier characteristics of wavelength-scanned optical spectrum of fiber optical Fabry-Perot acceleration is investigated theoretically. • The interference spectrum can be regarded as carrier signal in the wavelength domain. • The dynamic spectrum demodulation scheme can pick up and recover low-frequency vibration signals (6 Hz) experimentally. • The low cost and simple operation make the demodulating system has a potential application in the detection of low-frequency vibration signal in the fields of building healthy monitoring, earthquake prediction, etc. A dynamic spectral demodulation scheme for the low-frequency vibration signal detection based on extrinsic Fabry-Perot vibration sensor is proposed and demonstrated. The scanned optical spectrum of the vibration sensor shows a ripple waveform in response to the vibration signals and can be regarded as a carrier signal in the wavelength domain, which will introduce sideband components in the Fourier domain. The spatial frequency spectrum and sideband components contain the vibration signal to be measured. The experimental results show that the left and right sideband frequencies are symmetrical with respect to the signal frequency, which is completely consistent with the theoretical analysis; the peak powers of the left and right sidebands are linearly increasing with the amplitude of vibration signal by nearly the same sensitivities of 17.0 mW/g and 16.8 mW/g with very high linearity of 0.99845 and 0.99899 respectively; the system detection frequency is as low as 6 Hz. This proposed scheme shows that low cost and simple operation make the demodulating system has a potential application in the detection of low-frequency vibration signal in the fields of building healthy monitoring, earthquake prediction, geological exploration, oil well survey, etc. [ABSTRACT FROM AUTHOR]

• A highly sensitivity temperature sensor based on Lyot filter with Vernier effect has been demonstrated. • Because the interference spectra of Fabry-Perot interferometer and the Lyot filter are both relatively uniform, it is easy to achieve the Vernier effect. • By adjusting the length of the Ultra-fine polarization maintaining fiber (UFPMF) in the Lyot filter, it is easy to achieve higher magnification based on Vernier effect. • The reference interferometer used is insensitive to temperature. • Ultra-fine polarization maintaining fiber (UFPMF) in sensing interferometers features a high birefringence (B), which effectively reduces the size of the sensor. A highly sensitive fiber-optic temperature sensor based on a Lyot filter cascaded with a Fabry-Perot interferometer (FPI) is proposed and demonstrated in this study. The FPI consists of a segment of hollow-core photonic crystal fiber (HCPCF) spliced between two single-mode fibers (SMFs), which is insensitive to temperature and strain and is an ideal reference interferometer. The Lyot filter is composed of two polarizers with an uiltra-fine polarization maintaining fiber (UFPMF) of a certain length for temperature sensing. By adjusting the length of the UFPMF, the free spectral range (FSR) of the Lyot filter is close to FPI, resulting in an optical Vernier effect and improved sensitivity. Experimental results show that with the Vernier effect, the temperature sensitivity is enhanced from −1.145 nm/℃ to −59.31 nm/℃, and the sensitivity amplification factor is about 51.8. The proposed sensor with the Vernier effect has good performance in temperature sensing and is suitable for applications fields where high sensitivity temperature measurement is required. [ABSTRACT FROM AUTHOR]

A new cascaded optical fiber temperature and strain sensor based on polarization mode interferometer (PMI) and Fabry-Perot interferometer (FPI) is proposed and demonstrated experimentally. The PMI made by fusion splicing a segment of polarization maintaining fiber (PMF) after a polarizer is used as a sensing interferometer. Integrating a FPI as a reference interferometer cascaded after the PMI to generate the Vernier effect and improve sensitivity. The FPI is made from a small segment of hollow core photonic crystal fiber (HCPCF), which is fusion spliced to PMF and multi-mode fiber (MMF) at each end to form two reflective surfaces of FPI. Based on numerical simulations, a suitable free spectral range (FSR) of the sensor is produced. Experimental results show that the temperature sensitivity can be improved from −1.095 nm/℃ to −22.753 nm/℃ and the strain sensitivity can be improved from 2.17 pm/με to 38.89 pm/με by using the Vernier effect. The measured amplification factors are about 21 and 18,respectively. The high sensitivity of PMI and FPI cascaded sensors with the Vernier effect offers a new option for simultaneous measurements in the field of temperature and strain. [ABSTRACT FROM AUTHOR]

• We proposed and experimentally demonstrated a novel sensitivity-enhanced temperature sensor utilizing core offset and hollow core Bragg fiber. • The introduction of core offset provides a new method for improving the temperature sensitivity of traditional air-core Bragg fiber temperature sensors. • Based on improving the temperature sensitivity, the extremely low strain sensitivity is unchanged. • The temperature sensitivity increases from 10.27 pm/°C to 30.03 pm/°C in the temperature range of 30–50 °C. • The proposed optical fiber temperature sensor does not have the problem of cross-sensitivity of temperature and strain. A novel sensitivity-enhanced optical fiber temperature sensor (OFTS) utilizing core offset and hollow core Bragg fiber (HCBF) is proposed and experimentally investigated. The OFTS was fabricated by sandwiching the HCBF between two single-mode fibers (SMFs). The operation principle of temperature sensitivity enhancement is to excite inter-modal interference by lateral offset splicing fibers. The four-cladding structure unique to HCBF induces periodic interference envelopes in the transmission spectrum, which can be used to monitor changes in the external environment. The experimental results show that the temperature and strain sensitivities of OFTS is 10.27 pm/°C and −4.03 × 10−4 nm/με, respectively. After introducing different core offsets, the temperature sensitivities are improved to 16.29 pm/°C, 18.29 pm/°C, and 30.03 pm/°C, respectively. The strain sensitivities are −1.98 × 10−4 nm/με, −4.33 × 10−4 nm/με, and −4.96 × 10−4 nm/με, respectively. Moreover, due to the high-temperature sensitivity, strain insensitivity, and compact structure, the proposed sensor has broad application prospects in complex environments such as aerospace. [ABSTRACT FROM AUTHOR]

• We proposed and experimentally demonstrated a novel high-sensitivity optical fiber temperature sensor which is composed of the Lyot filter cascading a polarization mode interferometer. • A simple method for adjusting the free spectral range based on polarization mode interferometer is proposed. • The Lyot filter is used as a reference interferometer, which provides a new strategy and method for the development of Vernier effect-based sensors. • A high sensitivity of 25.562 nm/°C for temperature is achieved in the range of 31–35 °C. • The proposed sensor has no special manufacturing process, only requiring the cleaving and fusion splicing methods. We proposed and experimentally demonstrated a novel high-sensitivity optical fiber temperature sensor (OFTS) which is composed of the Lyot filter cascading a polarization mode interferometer (PMI). The Lyot filter is fabricated by inserting a polarization-maintaining fiber (PMF) between two linear fiber polarizers at two 45-degree angles. By simply cleaving the end fiber of the PMI, the free spectral range (FSR) can be maximized to approach the FSR of the Lyot filter, which introduces the Vernier effect. Experimental results show that the cascaded sensor can provide high-temperature sensitivity of 10.666 nm/°C and 26.562 nm/°C, which are 6.71 and 12.168 times that of single PMI, respectively. The method is demonstrated to achieve a stable and controlled Vernier effect obtaining ultra-high temperature sensitivity, which agrees well with the theoretical simulation results. The proposed sensor has the advantages of high sensitivity, good stability, and easy fabrication. [ABSTRACT FROM AUTHOR]

Abstract: We have demonstrated experimentally a long period fiber grating (LPFG) written in a hollow eccentric optical fiber (HEOF). A high-quality LPFG with no structural deformation was fabricated using a high-frequency CO2 laser with a low energy density of 0.896J/mm2. The LPFG writing parameters showed only a weak dependence on orientation of the fiber core to the writing beam. The liquid-filled HEOF LPFG has a high refractive index sensitivity of 1005nm/RIU over a refractive index range of 1.422–1.441. The good performance and easy fabrication of the HEOF LPFG indicate that it is a promising platform for novel in-fiber microfluidic devices. [Copyright &y& Elsevier]

• An intrinsically secure fiber optic MS monitoring system for rockburst is developed. • Performance of the optical system is superior to commercialized electrical system. • A large number of effective MS signals have been obtained in field tests. By capturing the micro-crack signals of rock mass structure, the microseismic (MS) monitoring technology is a good candidate for the forecasting and early warning of dynamic disaster. In this paper, MS sensors based on fiber optic interferometric scheme are studied and fabricated, and authors develop an 8-channel highly sensitive fiber optic MS system based on space division multiplexing technology to monitor the whole process of rock mass from micro-fracture to macro-damage. Field test of the system is carried out in Hanjiang River to Weihe River Water Diversion Project. The results show that it can realize continuously monitored with high accuracy and reliability in the tunnel and obtain the characteristic of temporal-spatio-strong of MS events, which are superior to the commercialized electrical monitoring equipment. This work shows that using fiber optic interference technology to develop accelerometer for MS monitoring system is both advantageous and feasible with the character of intrinsically safe. [ABSTRACT FROM AUTHOR]

• A novel high spatial resolution quantum dots (QDs) optical fiber micro-probe temperature sensor is developed. • The sensor can high-efficiently excite and collect fluorescence. • The sensor production only requires 400 pL of QDs ink, which greatly saves expensive QDs materials and improves the spatial measurement accuracy. • The sensor based on selective etching and fiber end-face inkjet printing has a simple structure and good consistency, which has great application potential. A novel quantum dots (QDs) optical fiber micro-probe temperature sensor fabricated using selective etching and fiber end-face inkjet printing is proposed. The high-efficiency fluorescence excitation and collection of QDs micro-probe (QMP) are enabled using ingenious combination of controllable selective etching and precise inkjet printing. The production of the micro-probe only requires 400 pL of QDs ink. The small sensing point significantly saves expensive QDs materials and improves spatial measurement accuracy. The fabricated QMP sensors exhibit outstanding consistency and fluorescence stability. High sensitivity of 113 pm/°C and good repeatability are obtained. The performance of the QMP sensor is demonstrated by comparing it with that of an infrared thermometer. The proposed sensor provides a new fluorescence sensing strategy that allows the design of high-spatial-resolution sensors with stable and reliable performance and offers a great potential for medical diagnosis, environmental measurement, and industrial production. [ABSTRACT FROM AUTHOR]

• The TCF can better excited cladding modes. Due to the small cladding diameter of UTF, a high sensitivity liquid level sensor is realized. • Simultaneous measurement of liquid level and temperature can be obtained through the demodulation matrix. • A high sensitivity of −231.67 pm/mm for liquid level is achieved in the range of 0–8 mm. And the temperature sensitivity is 77.86 pm/°C. • The proposed sensor has no special manufacturing process, only requiring the cleaving and fusion splicing methods. A compact optical fiber sensor (OFS) based on an inline Mach-Zehnder interferometer for simultaneous measurement of liquid level and the temperature is proposed and demonstrated experimentally. The proposed sensor is formed by fusion splicing a thin core fiber (TCF) with an ultra-thin fiber (UTF) with core offset, then TCF and UTF are sandwiched between two standards of single mode fibers (SMFs). The liquid level or temperature change will lead to the dips shifts in the interference spectrum, and the liquid level and temperature sensitivities can be measured independently by monitoring the resonance dips shifts. Then the simultaneous measurement of liquid level and temperature can be obtained through the demodulation matrix. Four sensors with different UTF length are fabricated based on numerical simulation. The experimental results show that the proposed sensor with a UTF length of 8 mm has the optimal liquid level and temperature sensitivities of −231.67 pm/mm and 77.86 pm/°C, respectively. The measuring liquid level and temperature ranges are 0–8 mm and 30–60 °C. The proposed sensing configuration has practical application potential in various industries due to the advantages of high sensitivity, simple fabrication and low cost. [ABSTRACT FROM AUTHOR]

• The in-fiber directional coupler-based relative humidity sensor is proposed and demonstrated for the first time. • The maximum and average RH sensitivities of the sensor reach −18.94 nm/%RH near the 90%RH and −7.005 nm/%RH in the RH range of 70 %–90 %, which is the highest as far as we know. • The proposed sensor has a low temperature crosstalk and a good stability. We demonstrate the first experimental realization of in-fiber directional coupler-based relative humidity (RH) sensor. A segment of single eccentric hole-assistant dual-core fiber (SEHADCF) is directly spliced between two single mode fibers (SMFs) to realize an in-fiber directional coupler (IFDC), and the suspended core of the SEHADCF is exposed to the external environment by using the side-polishing process. Gelatin is covered on the exposed suspended core to form the cladding of the suspended core and acts as a humidity sensitive material. Compared to the previous literatures, we report the highest sensitivity of the RH sensor to our best knowledge. In the RH range of 70 %–90 %, the average and maximum RH sensitivities are −7.005 nm/%RH and −18.94 nm/%RH, respectively. In addition, the temperature response and stability of the IFDC is also experimentally measured. Due to the super-high sensitivity and low temperature crosstalk, the proposed sensor will be expected to be a promising candidate for highly sensitive RH sensing applications in high humidity environment. [ABSTRACT FROM AUTHOR]