Fiber-optic Interferometric Sensor Research Articles

High sensitivity Mach-Zehnder interferometric fiber-optic humidity sensor based on multimode interference enhancement

Variations in humidity can significantly disrupt the proper functioning of precision instruments and meters across various industrial, medical, and scientific applications, thereby necessitating real-time humidity monitoring. Here, a high sensitivity fiber-optic relative humidity (RH) sensor based on Mach-Zender interferometer (MZI) is proposed and demonstrated. The humidity sensor consists of single-mode fiber (SMF), peanut structure (PS), and multi-mode fiber (MMF). A multi-mode interference (MMI) enhanced fiber-optic MZI was constructed using a SMF-PS-SMF-MMF-SMF-PS-SMF cascade structure to detect small changes in the refractive index (RI) and expansion force of gelatin film caused by humidity variation. PS, as a beam splitter and combiner of MZI, provides the possibility for simple, economical, and efficient manufacturing of the fiber-based part of humidity sensors. The addition of MMF will excite higher-order core and cladding modes, laying the foundation for achieving high-sensitivity fiber-optic humidity sensors. The fiber-optic RH sensor prototype demonstrate a high sensitivity of about 0.633 nm/% RH in the RH range of 50 % RH to 80 % RH, and a low temperature cross-sensitivity of 0.165 % RH/°C. This sensor has the advantages of safe and easy manufacturing process, low manufacturing cost, and high sensitivity, providing continuous high accuracy humidity measurement results for humidity sensing in different fields.

Fiber-optic Michelson interferometric trace dimethyl methyl phosphate sensor based on MnO2/ZnO composites film

A dimethyl-methyl phosphonate (DMMP) sensor based on MnO2/ZnO composite film integrated fiber-optic Michelson interference structure is proposed. The sensing structure is formed by a thick taper between a single-mode fiber (SMF) and a four-core fiber (FCF), and then the no-core fiber (NCF) is spliced at the other end of the FCF. To enhance reflection, the silver film was deposited on the end of the NCF, and the fiber optic Michelson interference structure is formed. MnO2/ZnO composite sensing film was deposited on the FCF surface, and the structure, morphology, and properties of the sensing material were analyzed by x-ray diffraction, Scanning electron microscopy, Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, etc. The sensitivity and the response time of the sensor are 0.3478 dB/ppm and 180 s, respectively. The sensor has good selectivity and stability, and it has a good application prospect in trace DMMP detection with high sensitivity.

Intermodal Fiber Interferometer with Spectral Interrogation and Fourier Analysis of Output Signals for Sensor Application

Interferometric fiber-optic sensors provide very high measurement accuracy and come with many other benefits. As such, the study of signal processing techniques for fiber-optic interferometers in order to extract information about external perturbation is an important area of research. In this work, the method of Fourier analysis was applied to extract information from the output signals of an intermodal fiber interferometer with spectral interrogation. It is shown that the external perturbation can be measured by obtaining the phase spectrum of the spectral transfer function of an intermodal fiber interferometer and determining the phase difference of a certain pair of mode groups. A mathematical model of this approach was developed, taking into account the parameters of the laser and the optical fiber, the number of excited mode groups, and the parameters of external perturbation. The theoretically considered method of Fourier analysis was experimentally verified, and it was proved to provide a linear response to external perturbation in a wide dynamic range.

Temperature-Decoupled Single-Crystal MgO Fiber-Optic Fabry-Perot Vibration Sensor Based on MEMS Technology for Harsh Environments.

A high-temperature-resistance single-crystal magnesium oxide (MgO) extrinsic Fabry-Perot (FP) interferometer (EFPI) fiber-optic vibration sensor is proposed and experimentally demonstrated at 1000 °C. Due to the excellent thermal properties (melting point > 2800 °C) and optical properties (transmittance ≥ 90%), MgO is chosen as the ideal material to be placed in the high-temperature testing area. The combination of wet chemical etching and direct bonding is used to construct an all-MgO sensor head, which is favorable to reduce the temperature gradient inside the sensor structure and avoid sensor failure. A temperature decoupling method is proposed to eliminate the cross-sensitivity between temperature and vibration, improving the accuracy of vibration detection. The experimental results show that the sensor is stable at 20-1000 °C and 2-20 g, with a sensitivity of 0.0073 rad (20 °C). The maximum nonlinearity error of the vibration sensor measurement after temperature decoupling is 1.17%. The sensor with a high temperature resistance and outstanding dynamic performance has the potential for applications in testing aero-engines and gas turbine engines.

All-sapphire fiber-optic sensor for the simultaneous measurement of ultra-high temperature and high pressure

An all-sapphire fiber-optic extrinsic Fabry-Perot interferometric (EFPI) sensor for the simultaneous measurement of ultra-high temperature and high pressure is proposed and experimentally demonstrated. The sensor is fabricated based on all-sapphire, including a sapphire fiber, a sapphire capillary and a sapphire wafer. A femtosecond (fs) laser is employed to drill a through hole at the side wall of the sapphire capillary to allow gas flow. The sapphire fiber is inserted from one side of the sapphire capillary. The sapphire wafer is fixed at the other side of the sapphire capillary. The first Fabry-Perot (FP) cavity, composed of the end face of the sapphire fiber and the front surface of the sapphire wafer, is used for measuring pressure, while the second FP cavity, composed of the two surfaces of the sapphire wafer, is used for measuring temperature. Experimental results show that the sensor can simultaneously measure ultra-high temperature and gas pressure within the temperature range of 20 - 1400 °C and the pressure range of 0 - 5 MPa. The temperature sensitivity is 0.0033 µm/°C, and the pressure sensitivity decreases as the temperature increases, reaching 1.8016 µm/MPa and 0.3253 µm/MPa at temperatures of 20 °C and 1400 °C, respectively.

Four-wavelength laser interferometry for the demodulation of dual-cavity fiber-optic extrinsic Fabry-Perot interferometric sensors

A four-wavelength passive demodulation algorithm is proposed and experimentally demonstrated for the interrogation of the one cavity in a dual-cavity extrinsic Fabry-Perot interferometric (EFPI) sensor. The lengths of two cavities are adjusted to generate four quadrature signals for each individual cavity. Both simulation and experimental results are presented to validate the performance of this technique. The experimental results demonstrate that dynamic signals at frequencies of 100 Hz, 200 Hz, and 300 Hz with varying amplitude are successfully extracted from a dual-cavity EFPI sensor with initial lengths of 93.4803 µm and 94.0091 µm. The technique shows the potential application to measure dynamic signals in dual-cavity fiber-optic EFPI sensors.

Nonlinear error elimination using the fusion of PGC-DCM, geometric fitting, and Atan algorithms.

A phase generated carrier (PGC) demodulation scheme is always accompanied by nonlinear errors. We propose a fusion of PGC differential and cross multiplying (PGC-DCM), geometric fitting, and arctangent (Atan) algorithms for fiber optic interferometric sensors to eliminate nonlinear errors. The output amplitude of the PGC-DCM algorithm is used to judge whether the Lissajous figure of quadrature signals is larger than 1/2 ellipse arc. When the Lissajous figure exceeds 1/2 ellipse arc, the contaminated quadrature signals are corrected by the ellipse correction parameters calculated from the geometric fitting; otherwise, the previous fitting parameters are employed for correction. Geometric fitting is realized by minimizing the Sampson error, and its failure problem under small signals is solved by using the temporary stability of fitting results. Finally, desired signals are extracted from the corrected quadrature signals by the Atan algorithm. Experimental results show that the fusion combines the merits of the three algorithms and expands the application of the geometric fitting in PGC demodulation schemes.

Adaptive Filtering Noise Suppression Technique for EFA-Based Interferometric Fiber Optic Sensors

Adaptive Filtering Noise Suppression Technique for EFA-Based Interferometric Fiber Optic Sensors

Hybrid Integrated Optical Transceiver for Interferometric Fiber-Optic Sensors

Hybrid Integrated Optical Transceiver for Interferometric Fiber-Optic Sensors

Investigation of CO2 measurement of ZIF-8-based F–P interferometric optical fiber sensor

In this paper, a set of ZIF-8 (Zeolitic Imidazolate Framework-8)-based Fabry-Perot (F–P) interferometric optical fiber sensor was designed to measure the carbon dioxide gas with different concentrations. The optical path difference of our sensing element was mainly caused by the F–P cavity filled with ZIF-8 instead of gas cavity. We analyzed the operating principle and verified the feasibility of the sensor. We gave the exact relation between the concentration of carbon dioxide and each parameter of the F–P cavity. The experimental results show that the spectrum will be red-shifted with the increase of the adsorption of carbon dioxide by ZIF-8 at a certain concentration of carbon dioxide. In addition, after applying Fourier transform to the spectrum, the frequency spectrum displayed two main peaks which changed to the lager value with the increase of concentration of carbon dioxide. The sensor has linear wavelength and phase sensitivities with the concentration of carbon dioxide under 5 × 104 ppm. Besides, the response time becomes shorter as the concentration of carbon dioxide decreases. To sum up, we presented a fiber optic gas sensor that is both simple to manufacture and has high selectivity, making it an effective method for measuring carbon dioxide.

Ultrahigh-sensitivity optical fiber sensor based on the virtual harmonic vernier effect

This paper proposes a fiber-optic sensor based on the virtual harmonic Vernier effect (VHVE). The VHVE is obtained by processing the spectrum of the traditional Vernier effect (TVE). We demonstrate that the periodic outer envelope and the high-frequency fringes in the TVE can be used as the spectra of the virtual interferometer and sensing interferometer of the VHVE, respectively. Thus, the VHVE-based spectrum will be viewed as a “superposition” of the spectra of the virtual interferometer and sensing interferometer. The optical path length of the virtual interferometer is multiple times different from that of the sensing interferometer, they meet the excitation condition of the HVE, and they can use the same spectral demodulation method. Thus, this is called the “VHVE.” A novel parallel-structured dual Fabry–Perot interferometer fiber-optic sensor is employed to detect gas refractive index. The spectral demodulation methods of the VHVE and TVE are used to track the same set of spectral data, and the obtained sensitivities are 1,158,000 nm/RIU and 39,000 nm/RIU, respectively. Experimental results show that through the VHVE, the sensitivity of the sensor is increased to 106 orders of magnitude. Compared with the HVE, the VHVE can easily increase the harmonic order. In this experiment, the harmonic order reaches 28. Meanwhile, based on the proposed VHVE principle, any sensor based on the TVE can be converted into a VHVE-based sensor to significantly enhance the sensitivity without changing the existing sensor structure. This new sensing mechanism has significant potential in improving sensor sensitivity.

Three-dimensional sound source localization system based on fiber optic sensor array with an adaptive algorithm

Sound Source Localization (SSL) based on microphone arrays nowadays is widely used in the fields of machinery rattle localization, weapon navigation, and robot hearing. However, there is also a need for SSL in some special environments, such as long-distance gas pipelines where there is a risk of leakage. In these environments, arrays built with ordinary microphones can be a safety hazard, which can be compensated by the External Fabry-Perot interferometer (EFPI) fiber-optic sensor's advantages of being uncharged, inexpensive, and highly sensitive. To expand the application areas of microphone array-based SSL techniques, this paper using four EFPI fiber-optic sensors to constructs a 1 m × 1 m × 1 m sensor array, which is used for real-time localization of sound sources based on three-wavelength adaptive intensity demodulation methods and Time Difference Of Arrival (TDOA) localization techniques. An improved method is proposed for the three-wavelength adaptive intensity demodulation method to avoid the occurrence of signal flips. Finally, formulas are provided for this sensor array to quickly calculate the sound source position based on the arrival delay. Several experiments show that the system can achieve sound source localization over a wide range with an average error of less than 3 cm.

Low-Coherence Homodyne Interferometer for Sub-Megahertz Fiber Optic Sensor Readout.

This study proposes a method for interferometric fiber optic sensor readouts. The method utilizes the advantages of the active homodyne demodulation technique and low-coherence interferometry. The usage of the tandem low-coherence interferometer enables modulating the reference interferometer without any changes to the sensor. This achieves high sensitivity, high stability, and a wide frequency band. A sensitivity of up to 0.1 nm (RMS) in the frequency range of 5 kHz is demonstrated by detecting acoustic signals with a fiber Michelson interferometer as a sensor.

Improved PGC demodulation algorithm for fiber optic interferometric sensors.

An improved phase generated carrier arctangent demodulation algorithm based on harmonic mixing and phase quadrature technology (PGC-Arctan-HP) is proposed in this paper, which can eliminate the effects of modulation depth shift, carrier phase delay, and light intensity disturbance (LID) on the demodulation results. The simulation results are consistent with theoretical analysis, and indicate that the PGC-Arctan-HP algorithm can achieve optimal demodulation compared with other demodulation algorithms. The results of interferometric experiments show that the demodulated waveforms of the improved algorithm are relatively stable with an amplitude error of 0.0294%. The total-harmonic-distortion (THD) and the signal-to-noise-and-distortion (SINAD) can reach -60.0286 dB and 59.5388 dB.

Ameliorated 3 × 3 coupler-based demodulation algorithm using iteratively reweighted ellipse specific fitting.

Passive demodulation scheme using 3 × 3 coupler has been widely used in phase-sensitive optical time-domain reflectometry (φ-OTDR), interrogation of fiber Bragg gratings or fiber optic interferometric sensors, and sensor multiplexing. However, the asymmetry of the 3 × 3 coupler in real applications affects the demodulation performance seriously. We proposed an ameliorated 3 × 3 coupler-based demodulation algorithm using iteratively reweighted ellipse specific fitting (IRESF) to overcome the drawback. IRESF combines iterative reweight technology with ellipse specific fitting, which decreases the weights of high noise points and always outputs ellipse solutions. Any two output signals from the 3 × 3 coupler-based interferometer are fitted by the IRESF and then corrected as a pair of quadrature signals. The stability of the fitting parameters is utilized to resolve the failures of IRESF under small signals. A real-time 1/4 ellipse arc judging module is designed, if the Lissajous figure is larger than 1/4 ellipse arc, IRESF is executed to offer ellipse correction parameters. Otherwise, the fixed parameters preset in the algorithm are used. The fixed parameters are mean values of the fitting parameters of IRESF under a large stimulus. The desired phase signal is finally extracted from the corrected quadrature signals. Experimental results show that the ameliorated algorithm does not require strict symmetry of the 3 × 3 coupler and can work under small signals. The noise floor of the proposed algorithm is -112 dB re rad/√Hz and the demodulated amplitude is 23.15 dB (14.37 rad) at 1 kHz when THD is 0.0488%. Moreover, the response linearity is as high as 99.999%. Compared to the algorithm using direct least squares, the proposed demodulation algorithm is more robust and precise, which has broad application prospects.

Features of Application of Adaptive Interferometric Fiber Optic Sensors of Acoustic Emission to Monitor the Condition of Polymer Composite Materials

Features of Application of Adaptive Interferometric Fiber Optic Sensors of Acoustic Emission to Monitor the Condition of Polymer Composite Materials

Large Dynamic Range Interrogation Technique for Fiber-Optic Interferometric Sensor Based on AWG and Deep Learning Algorithm

Large Dynamic Range Interrogation Technique for Fiber-Optic Interferometric Sensor Based on AWG and Deep Learning Algorithm

Mach-Zehnder interferometer fiber optic sensor coated with UiO-66 metal organic framework for creatinine detection

An in-line Mach-Zehnder fiber optic sensor based is proposed and fabricated to measure creatinine (CR) concentration in the physiological range for clinical trials. The shift in valleys wavelength is monitored to assess the sensor response to the variation of CR concentration. Single-mode fiber as the sensing part is coated with UiO-66 metal–organic framework (MOF) adsorbent by dip coating to enhance the performance of the sensor. For further improvement, the length of the SMF and the coating time of the UiO-66 are optimized. The sensitivity and limit of detection (LOD) of the optimized bare sensor are 1530 pm/(mg/dL) and 0.4 mg/dL, respectively. The sensor with optimized UiO-66 coating thickness has a sensitivity of 2400 pm/(mg/dL) and an LOD of 0.2 mg/dL indicating the high performance. The sensors offer fast response, high sensitivity, and proper linear response at low concentrations of CR which reveal their potential in bio and chemical applications.

Ameliorted algorithm for PGC to eliminate the influence of carrier phase delay with FFT

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 1θ0, 2θ0, 3θ0, 4θ0 from the first to fourth harmonic signals of the interference signal, then introduce 1θ0, 2θ0, 3θ0, 4θ0 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.

Temperature Demodulation for an Interferometric Fiber-Optic Sensor Based on Artificial Bee Colony–Long Short-Term Memory

Demodulation methods play a critical role in achieving high-performance interferometric fiber-optic temperature sensors. However, the conventional passive 3 × 3 coupler demodulation method overlooks certain issues, such as the non-1:1:1 splitting ratio of the coupler, resulting in a non-ideal phase difference in the three output interference signals. These problems significantly impact the measurement results of interferometric temperature sensors. In this paper, we propose a novel arc-tangent method based on a 3 × 3 coupler and a demodulation algorithm combining long short-term memory (LSTM) with an artificial bee colony (ABC). The arc-tangent method is employed to enhance the input phase signal of the ABC-LSTM network model and establish a nonlinear mapping between the phase signal and temperature, effectively preventing the influence of the spectral ratio and phase difference of the 3 × 3 coupler on temperature demodulation. The proposed ABC-LSTM method achieves high-resolution measurements with an interval of 0.10 °C, and the absolute error is below 0.0040 °C within the temperature range of 25.00–25.50 °C. To demonstrate the stability and adaptability of the proposed method under long-term constant temperature conditions, we conducted measurements for approximately three hours in a controlled temperature environment set at 25.00 °C. Experimental results indicate that the maximum error of LSTM-ABC method remains around 0.0040 °C, outperforming the conventional algorithm (0.0095 °C). Furthermore, when comparing the average error values of the conventional passive 3 × 3 coupler method (0.0023 °C), LSTM model (0.0019 °C), and ABC-LSTM model (0.0014 °C), it is evident that the demodulation results of the ABC-LSTM method exhibit the highest level of stability. Therefore, the ABC-LSTM method enhances the accuracy and reliability of interferometric fiber-optic temperature-sensing systems.