Fiber-optic Mach-Zehnder Interferometer Research Articles

GO/MoS2@PVA composite membrane-based optical fiber Mach-Zehnder interferometer for humidity sensing and its non-contact sensing application.

A humidity sensor based on an optical fiber Mach-Zehnder interferometer (MZI) coated with a GO/MoS2@PVA composite membrane was investigated for non-contact sensing. MoS2 was used as a nanospacer to enhance the humidity-sensitive properties of GO, and the adhesion and stability of the composite membrane on the fiber surface could be increased by PVA. The proposed sensor shows a maximum sensitivity of 0.26 dB/%RH with average response and recovery times of 1.62 and 1.11 s, respectively. In non-contact sensing applications, the sensor can effectively recognize a maximum distance of 10 mm for the proximity of a human finger with a distance variation interval of 3 mm. The proposed sensor is expected to be applied in non-contact distance detection and localization or as a non-contact human-computer interaction panel.

Highly Sensitive Optical Fiber MZI Sensor for Specific Detection of Trace Pb2+ Ion Concentration

A novel chitosan (CS) functionalized optical fiber sensor with a bullet-shaped hollow cavity was proposed in this work for the trace concentration of Pb2+ ion detection in the water environment. The sensor is an optical fiber Mach–Zehnder interferometer (MZI), which consists of a sequentially spliced bullet-shaped hollow-core fiber (HCF), thin-core fiber, and another piece of spliced bullet-shaped HCF. The hollow-core fiber is caused to collapse by adjusting the amount of discharge to form a tapered hollow cavity with asymmetric end faces. The bullet-like hollow cavities act as beam expanders and couplers for optical fiber sensors, which were symmetrically spliced at both ends of a section of thin core fiber. The simulation and experiments show that the bullet-like hollow-core tapered cavity excites more cladding modes and is more sensitive to variation in the external environment than the planar and spherical cavities. The ion-imprinted chitosan (IIP-CS) film was fabricated with Pb2+ ion as a template and uniformly coated on the surface for specific recognition of Pb2+. Experimental verification confirms that the developed sensor can achieve high-sensitivity Pb2+ ion detection, with a sensitivity of up to −12.68 pm/ppm and a minimum Pb2+ ion detection concentration of 5.44 ppb Meanwhile, the sensor shows excellent selectivity, repeatability, and stability in the ion detection process, which has huge potential in the direction of heavy metal ion detection in the future.

Frequency multiplexed dual-pulse fiber-optic Φ-OTDR for a fading-free distributed fiber sound sensor

Distributed fiber-optic sensor utilizing optical interference signal detection of the Rayleigh backscattering (RBS) light along the fiber under test (FUT) has greatly progressed over the past 20 years. It can now realize real-time sound signal recovery along the FUT with a good fidelity and high sensitivity. However, the performance of the practical distributed fiber-optic sound sensor is still limited by the fading problem, as invalid spatial channels at unknown positions arise due to the low signal intensity at these positions and the resulting near-zero signal-to-noise ratio (SNR). In this paper, we propose a novel, to the best of our knowledge, fading-free fiber sensor structure by incorporating an acousto-optic frequency shift (AOFS) loop into the light source in a dual-pulse phase optical time domain reflectometry (Φ-OTDR). As a result of stable frequency shifting and feedback within the loop, the loop source can output a high-repetition-rate narrow pulse train consisting of multiple separated frequencies. This pulse train is further shaped into two pulse bursts by an acousto-optic modulator (AOM) and an in-balanced fiber-optic Mach–Zehnder interferometer (MZI). The pulse bursts are then sent into the sensing fiber, while the RBS light is detected using an Avalanche photodiode after an erbium-doped fiber amplifier. The experimental results show that the proposed structure is capable of vibration signal recovery with a good SNR all along the FUT. The fading-free performance is evaluated under different AOFS loop gains, showing significant potential in practical fiber sensor network applications.

Hilbert transform for modulation of interference fringes of liquid substances

The demodulation of interferometric signals presents a challenge in the study of object, especially when working in environments with disturbances, instability or noise. This document presents a procedure to demodulate a pair of highly noisy interferograms obtained with a fiber optic Mach Zehnder interferometer, the procedure is based on using the reconfiguration of the interference fringes as a result of Hilbert Transform. The method shown is fast, it does not present susceptibilities at high frequencies, it avoids demodulating interference patterns of closed fringes and with little carrier signal, in addition, it allows to verify the mathematical equivalence of the spectral analysis using Fourier Transform. As an application model, samples of liquid substances were used, where it was possible to know changes between substances according to their density

Optimization of nanoparticles for application in optical sensors

This study explored the influence of crystallite size on a nanoparticle-coated fiber optic Mach-Zehnder Interferometer (MZI). The primary aim was to enhance the magnetic properties of the nanoparticles for application in fiber optic sensors designed for magnetic field sensitivity. The motion of the nanoparticles within the magnetic field induced alterations in the sensor's transmission, creating an imbalance in the optical signals between the interferometer arms. Nickel ferrites (NiFe2O4) with average crystallite sizes of 3.3 nm, 51.9 nm, and 74.3 nm were used. The nanoparticles were identified by X-ray diffraction, VSM magnetic measurements, and Mössbauer spectroscopy. Structural parameters obtained from X-ray diffraction underwent refinement using the Rietveld method, and the Scherrer equation was applied to determine the average crystallite size. Magnetic sensor performance was assessed for sensitivity, precision, and accuracy for different nanoparticle sizes. The relationship between output power and applied magnetic field exhibited linearity between 87.6% and 99.2%. Sensitivity varied from 1.65 dB/Oe to 3.17 dB/Oe for different particle sizes, indicating an increase in sensitivity with nanoparticle magnetization. This study establishes a correlation between crystallite size and nanoparticle magnetization, enhancing sensor responsiveness. This optimization facilitates the development of tailored electric current and magnetic field sensors, considering nanoparticle size and their distinctive magnetic properties. Although the primary focus was on the optical sensor, a comprehensive nanoparticle characterization was crucial as it directly influenced sensor performance.

Fiber-optic Mach–Zehnder interferometer based on α-Fe2O3@MoS2 functionalization for Cd2+ trace detection

Fiber-optic Mach–Zehnder interferometer based on α-Fe2O3@MoS2 functionalization for Cd2+ trace detection

SnO2/Graphene incorporated optical fiber Mach-Zehnder interferometer for methane gas detection

Detecting methane gas in the environment quickly and accurately is critical to preventing poisoning and leaks. This study describes a methane (CH4) gas optical fiber sensor that is coated with SnO2/graphene. The SnO2/graphene nanocomposites were prepared by a solution-based method. An microfiber bi-taper structure is fabricated by the flame melting biconical method. And the previously manufactured SnO2/graphene methane sensitive material was then coated in the microfiber interferometer (MFI) area using the drop coating method. Methane molecules can be absorbed by the SnO2/graphene material on the fiber’s surface. And it can provide the conditions for charge transfer, which leads to the change of transmission spectrum intensity. The sensing response of the sensor for methane concentration is investigated in the range of volume fraction 0–35%. The SnO2/graphene sensitive material coated fiber sensor with a low-cost and simple fabrication process provide prospects in applications widely.

Trace cadmium ion detection using optical fiber Mach–Zehnder interferometer coated with PVA/TEOS/APTES

Abstract A cadmium ion (Cd2+) sensor based on polyvinyl alcohol (PVA)/tetraethyl orthosilicate (TEOS)/3-aminopropyl triethoxysilane (APTES) film integrated Mach–Zehnder interferometer was fabricated and evaluated. The three core fiber (TCF) is spliced with two single mode fibers (SMFs), one end of TCF and SMF are spliced with enlarged taper, and the other end of TCF is directly spliced with another SMF. PVA/TEOS/APTES composite film is coated on the surface of TCF. When the Cd2+ is adsorbed by the sensing film, the relative refractive index of TCF cladding changes, the output interference trough wavelength shifts. By monitoring the wavelength shift information of the interference trough, the concentration of Cd2+ will be obtained. In the concentration range of 0–0.4 μM, with the increase of Cd2+ concentration, the interference trough shows a red shift. The sensitivity and the detection limit of the sensor are 7 nm/μM and 0.0143 μM (1.6 ppb), respectively. The sensor has excellent selectivity and good pH stability. It has the advantages of easy preparation, simple structure, and has potential application for the detection of trace Cd2+.

Group delay manipulation in a passive microwave-photonic fiber optic Mach-Zehnder interferometer.

Conventional slow and fast light (SFL) systems are based on active or nonlinear media or photonic band-gap systems. Radio-frequency (RF) interferometric systems were recently explored for sustaining fast and slow light, owing to their simple structures and passive operation. This Letter introduces a simple, effective, and efficient SFL system based on a linear and passive microwave-photonic (MWP) fiber optic Mach-Zehnder interferometer (MZI). By optically tuning the light transmission ratio of the two fiber beams of the MZI (i.e., the optical carrier), we demonstrate that both delay and advancement of a RF signal (i.e., the envelope signal) can be realized. The magnitude of the signal group delay can also be adjusted using the optical tuning method. Therefore, high-degree manipulation of signal group delay can be achieved using the proposed MWP fiber optic MZI system.

Experimental analysis for refractive index sensing by using a compact, simple and robust Mach-Zehnder interferometer based on an air gap inside of a fiber

• Fabrication of optical fiber devices with air gap inside optical fiber with a simple procedure. • High sensitivity values for refractive index sensing. • Compact (a few μ m ) and simple optical fiber device for refractive index sensing. Compact and straightforward construction of optical fiber Mach-Zehnder interferometer and its application as a refractive index sensor is presented. The interferometer is composed of an air gap inside an ultra-high Numerical Aperture (NA) fiber. As the light propagates through the air gap, the fundamental mode travels in the air gap. Simultaneously, high-order modes are excited in the section constructed by the air gap and the outside air-cladding interface. The propagating high-order modes are sensitive to the surrounding media because their intensity distribution can extend to the surrounding media. Cargille calibrated oils of different refractive indexes are used to test the refractive index change response of various interferometers with several air gap lengths over refractive index changes in the surrounding media. A maximum sensitivity response of 226.8 nm/RIU within the range from 1.43 to 1.45 for a sample device with 12 min of etching time and inconsistent behavior of the intensity (dBm/RIU) for the different devices are reported.

Optical fiber Mach-Zehnder interferometer based on a pair of microholes

An optical fiber in-line Mach-Zehnder interferometer based on a pair of inner microholes in the fiber core is proposed and demonstrated for refractive index and temperature sensing. The microhole is fabricated by firstly inscribing a short line structure by femtosecond, and then discharging the short line area by using a fusion splicer. The incident light beam on the first microhole is divided into two parts, one remains traveling in the fiber core and the other is directed into the cladding and then collected by the second microhole after multiple reflections at the air/cladding interface. The two parts recombines in the fiber core, thus forming a Mach-Zehnder interferometer. By controlling the separation of the two microholes, transmission spectrum with different free spectral range can be obtained. The sensitivities achieved for refractive index and temperature are ∼ 68.2 nm/RIU and 57.8 pm/°C, respectively. Such a device is robust in structure, easy in fabrication, and convenient in operation.

High sensitivity strain sensor based on a novel offset-core single micro-tapered fiber-optic Mach-Zehnder interferometer

A novel offset-core single micro-tapered fiber-optic Mach-Zehnder interferometer (MZI) with high strain sensitivity is proposed and experimentally demonstrated. This novel structure is fabricated only by using ordinary single mode fiber (SMF) and a fiber fusion splicer, of which the offset-core distance is about 12 μm and micro-tapered fiber length is about 490 μm. A high strain sensitivity of −34.61 pm/με is obtained, which is 10 times higher than that (3 pm/με) of common inline optic-fiber MZI and 6 times higher than that (6 pm/με) of common inline optic-fiber Fabry-Perot interferometer (FPI). In addition, the length (∼490 μm) of this novel MZI is much smaller than that of an ordinary inline optic-fiber MZI (∼2 cm) with advantages of high sensitivity, low-cost, and easy fabrication. The novel optic-fiber MZI proposed in this paper enriches the manufacturing methods for fiber-optic MZIs, which has a good application prospect for strain sensing.

Direction-Discriminating torsion sensor based on optical fiber Mach-Zehnder interferometer

Optical fiber torsion sensors play a pivotal role in artificial intelligence, construction engineering, structural health monitoring and other fields. Research on low cost, simple structures and measurement methods has become a priority for researchers. Here, we propose an intensity-modulated and direction-recognizing torsion sensor based on two mode fiber with a sandwich structure, that serves as a mode interferometer formed by fundamental modes LP01 and second order core modes LP11 excited by means of offset fusion splicing. The capacity of intensity modulation and torsional direction recognition comes from the antisymmetric phase of fiber mode, which makes the sensor work like Marius’ law. The antisymmetric phase of the LP11 mode leads to an asymmetric transmission spectrum, whose direction enables the sensor to distinguish the torsion direction. Peaks and troughs in the measured spectrum interchange when the torsion direction is reversed. The initial angle between two offset directions enables control of the zero-crossing point within in the twisting range. The torsion sensitivity reaches 364%/(rad/cm) with a measurement accuracy of ± 0.22 rad/m.

Real‐Time Monitoring of Hydrogen Production by Water Electrolysis Using a Tapered Polarization‐Maintaining Optical Fiber Mach–Zehnder Interferometer

Gas microbubbles easily cover surface active sites of the solid electrodes in most gas evolution reactions, hindering the transfer of mass and energy and declining the reaction efficiency. However, online monitoring of gas bubbles’ behaviors on the surface of solid electrodes accurately in real time remains challenging. Herein, a tapered polarization‐maintaining fiber Mach–Zehnder interferometer for in situ monitoring of H2 bubbles detachment in water electrolysis is reported. In the experiment, the H2 bubble released from the platinum microelectrode can be detected by recording the variation of transmitted optical power in the optical fiber. By comparing it with the electrochemical measurement and the charge‐coupled device imaging, the measurement accuracy can be regarded as near 100%, and the time resolution is determined to be ≈ms. It is found that the size of the detached bubbles on the microelectrode surface is positively correlated with the electrolysis voltage. The increasing electrolysis time causes a long detachment time. Three different charged surfactants are all proved to accelerate the H2 detachment. This method is adapted to study gas bubble behaviors in various complex gas‐involving reaction systems, even for the invisible gas bubbles in a dark environment.

Broadband Fiber-Optic Acoustic Sensors

A broadband fiber-optic acoustic sensing system is proposed and demonstrated, which is configured by a fiber Bragg grating (FBG) and an erbium-doped fiber (EDF) ring laser. As the acoustic waves can shift the FBG reflective spectrum, the wavelength of the laser output shifts accordingly and this shift is subsequently converted into a corresponding phase shift and demodulated by an unbalanced fiber-optic Mach-Zehnder interferometer (MZI). The sensitivity of the presented sensing system is also investigated, which is a function of the optical path difference of the MZI and the linewidth of the FBG. Due to the adaptability and high sensitivity of the system, it can detect signals with frequencies from tens of hertz to hundreds of kilohertz, which shows the potential for acoustic emission detection in real applications.

High sensitivity and fast response optical fiber nucleic acid sensor

A label-free optical fiber Mach-Zehnder interferometer (MZI) based biosensor was proposed for the specificity detection of single-stranded nucleic acid (NA). The MZI was made of a fiber sandwich structure by a core-offset fusion splicing means. The piranha solution, APTES (3-Aminopropyltriethoxylsilan) and GA (Glutaraldehyde) were used for the chemical modification of the optical fiber surface, and then the single-stranded probe NA with the known sequence was fixed on the optical fiber surface by covalent fixation method. The specificity of complementary target NA could be detected through the changing of the spectra of the MZI. Thanks to the high sensitivity of the MZI and the high fixation efficiency of the covalent fixation method, the optical fiber MZI based NA sensor showed a fast response of 30 s and a high sensitivity of 0.175 dB/nM with a minimum detection concentration of 1 nM.

High Fidelity MZI-BCG Sensor With Homodyne Demodulation for Unobtrusive HR and BP Monitoring

An all-optical active homodyne detection aided fiber-optic Mach-Zehnder interferometer (MZI) vital signs monitoring system is proposed and investigated. The active homodyne detection is mainly realized by employing a wavelength-tunable vertical-cavity surface-emitting laser (VCSEL) and automatic closed-loop control system (CLCS). It can effectively maintain the MZI sensor to operate around the quadrature point and successfully eliminate the fading effect of output signal induced by low frequency drift. Therefore, high fidelity and high stableness of the ballistocardiograph (BCG) signal is guaranteed. The results show that according to the Bland-Altman and the confidence ellipse analysis, the heart rate (HR) monitoring performance reveals high accuracy and good consistence. What’ more, a multiple linear regression-based blood pressure (BP) estimation model is presented, which shows that the BP is highly correlated with I-J-K complex oriented features, including IJ interval, JK interval and JK amplitude. It indeed offers a potential approach to achieve cuff-less BP estimation merely based on the high-fidelity BCG signal. In conclusion, the proposed MZI-BCG monitoring system is simple to use, and it shows great potential in future unobtrusive cardiac monitoring both in home and hospital.

Optical Fiber Interferometer Based on Inner Air-Cavity With an Open Channel for Gas Pressure Sensing

An optical fiber Mach-Zehnder interferometer based on an inner air-cavity with an open micro-channel structure is presented. The device is fabricated by using femtosecond laser to inscribe a cavity structure in the optical fiber core, followed by discharging the cavity area with a fusion splicer to form an inner air-cavity in the fiber core, and finally create an open micro-channel with the help of hydrofluoric acid corrosion. The fiber in-line interferometric device is miniature, robust, and stable in operation and exhibits a high pressure sensitivity of ~3351 pm/MPa, with an extremely low temperature cross-sensitivity of 2.58 kPa/°C.

High-sensitivity optical fiber hydrogen sensor based on the metal organic frameworks of UiO-66-NH2.

A hydrogen sensor with high sensitivity was demonstrated by coating the metal organic frameworks of ${\rm{UiO}}\! -\! {\rm{66 \!-\! N}}{{\rm{H}}_2}$ on an optical fiber Mach-Zehnder interferometer (MZI). The MZI was made of a fiber mismatch structure by using a core-offset fusion splicing method. The effective refractive index of the ${\rm{UiO}}\! -\! {\rm{66\! -\! N}}{{\rm{H}}_2}$ film varied with the absorption and release of hydrogen, and the interference resonant dip wavelength and the intensity of the MZI changed with the variations of the concentration of hydrogen. The experimental results showed that the proposed sensor had a high hydrogen sensitivity of 8.78 dB/% in the range from 0% to 0.8%, which is almost seven times higher than the existing similar hydrogen sensor.

Magneto-refractive properties and measurement of an erbium-doped fiber.

The magneto-refractive properties of an erbium-doped fiber (EDF) are investigated by theoretically analyzing the change in mode characteristics with a magnetic field and experimentally measuring it based on a fiber-optic Mach-Zehnder interferometer (MZI). The numerical results indicate that the mode effective refractive index (RI) increases as the magnetic field increases, and the mode field intensity distribution tends to be more concentrated in the core region with an increasing magnetic field. The variation in the mode effective RI of the fundamental mode with the magnetic field is greater than that of the higher-order modes. A magneto-refractive measurement system based on a fiber-optic MZI is set up to analyze the magneto-refractive effect of the EDF. The changes in the mode effective RI measured with a direct-current (DC) magnetic field and with a 100 Hz alternating-current (AC) magnetic field are 4.838×10-6 and 4.245×10-6 RIU/mT, respectively. The experimental results are in reasonable agreement with the theoretical analysis. Furthermore, the error between the experimental and numerical results is discussed. The magneto-refractive properties of the EDF exhibit potential in all-fiber magnetic field or current sensing area.