In-line Mach-Zehnder Interferometer Research Articles

A temperature-insensitive refractive index sensor based on in-line Mach–Zehnder interferometer with micro spindle structures and photonic crystal fibers

A high-sensitivity refractive index (RI) sensor is proposed and designed, which is an in-line Mach–Zehnder interferometer (IMZI) with a photonic crystal fiber (PCF) and multilevel micro spindle structures fabricated by the arc-discharging and tapering method. To improve the sensitivity, a tapered multimode fiber (MMF) is inserted into the PCF. The performance of RI and temperature detection has been analyzed and discussed. The maximum RI sensitivity of −2958.875 nm/RIU is achieved by detecting the NaCl solution. In addition, the maximum temperature sensitivity is 6.4 pm/oC in the range of 25 oC–65 oC, and the maximum measurement error changes caused by temperature is −0.0170‰/oC. It shows that the sensor is very sensitive to RI yet insensitive to temperature.

Tapered fiber optic sensor for arterial pulse wave monitoring

Arterial pulse wave monitoring has been recognized as an effective medical diagnostic tool for various cardiovascular diseases. To overcome the barriers in traditional, complex, and expensive detection configurations, new electronic and optical sensors have been developed with their unique advantages. Particularly, various configurations of fiber-optic sensors have been proposed. This paper demonstrates the use of a tapered optical fiber structure to create a sensitive sensor that can detect carotid arterial pulse waves on the skin surface. The demonstrated fiber sensor probe utilizes an in-line Mach-Zehnder interferometer configuration and liquid-gold film coating on the sensor tip to enhance its sensitivity and demonstrate its use by encapsulating the sensor with a flexible elastomer material for detecting arterial pulse waves with a simple testing configuration. Considering the simple fabrication and high sensitivity without a complex demodulation scheme, the proposed sensor has broad implementation in biomedical health monitoring systems.

Experimental Study of White Light Interferometry in Mach-Zehnder Interferometers Based on Standard Single Mode Fiber.

In this work, we experimentally analyzed and demonstrated the performance of an in-line Mach-Zehnder interferometer in the visible region, with an LED light source. The different waist diameter taper and asymmetric core-offset interferometers proposed used a single-mode fiber (SMF). The visibility achieved was V = 0.14 with an FSR of 23 nm for the taper MZI structure and visibilities of V = 0.3, V = 0.27, and V = 0.34 with FSRs of 23 nm, 17 nm, and 8 nm and separation lengths L of 2.5 cm, 4.0 cm, and 5.0 cm between the core-offset structure, respectively. The experimental investigation of the response to the temperature sensor yielded values from 50 °C to 300 °C; the sensitivity obtained was 3.53 a.u./°C, with R2 of 0.99769 and 1% every 1 °C in the transmission. For a range of 50 °C to 150 °C, 20.3 pm/°C with a R2 of 0.96604 was obtained.

A simultaneous audio and temperature sensing system based on in-line Mach–Zehnder interferometer

A system integrating audio and temperature sensing capabilities has been developed for simultaneous detection. Within this system, an in-line Mach–Zehnder interferometer (IMZI) sensor has been explored. Manufactured using the electro-discharge method, the sensor comprises a structure connecting single mode fiber (SMF) to polarization-maintaining fiber (PMF) and then back to SMF, with distinctive peanut-shaped junctions. The sensor’s sensitivity to temperature and curvature has been assessed to determine its potential for simultaneous audio and temperature detection. Within the temperature range of 15 °C–48 °C and a curvature range of 0 m−1–0.06654 m−1, the peak sensitivities observed were -0.1072 nm/°C and 63.5569 nm/m−1, respectively. Experimental verification confirmed the system’s audio performance using the proposed IMZI sensor. The system operates over a broad frequency range from 20 Hz to 20,000 Hz. The maximum frequency response and minimum detectable pressures (MDP) are 1804.92 mV/Pa and 11.74 μPa/Hz1/2 at 800 Hz, respectively, indicating notable acoustic sensitivity. Based on both theoretical analyses and empirical methodologies, the system successfully achieves simultaneous audio and temperature detections. This is accomplished by processing the detected data of curvature and temperature variations via high-speed sampling and dual-wavelength demodulation. Notably, this approach addresses and overcomes the sensitivity reduction experienced by acoustic sensors based on interferometers due to temperature-induced Q point drifts.

Highly sensitive vibration frequency sensing utilising serial in-line Mach-Zehnder interferometer

A serial in-line Mach-Zehnder interferometer (ILMZI) based on a hybrid of micro-bottle structures and fibre taper structures is demonstrated for vibration frequency sensing applications. The proposed structure shows an improved vibration sensing sensitivity and accurate vibration frequency measurement. The sensing performance was demonstrated using the proposed structure mounted on a cantilever beam. The vibration frequency interrogation was based on Fast Fourier Transform (FFT) spectral peak detection. Compared to the conventional single ILMZI structure, the proposed device offers relatively much wider detection range of up to 224 Hz based on the specific experiment setup. This structure allows superior sensing performance for ILMZI, thus making it more practical to be accepted in many industrial sensing applications.

Ultra-low bending and temperature crosstalk directional torsion sensor by few-mode fiber based Mach-Zehnder interferometer

In this paper, a few-mode-fiber based in-line Mach-Zehnder interferometer is proposed and completed for directional torsion sensing by core-offset splicing technique. The optimal offset and fiber length for fabrication are gained and the comprehensive tests are conducted in terms of torsion, bending and temperature. The experimental results show that our sensor has obvious intensity variation with the applied shear strain. The torsion sensitivity reaches ∼1.1 dB/(rad/m) in the range from −13.08 rad/m to 13.08 rad/m, but with the wavelength shift of 0.102 nm/(rad/m). In addition, thanks for the tiny bending and temperature responses of −0.055 dB/m−1 and 0.014 dB/℃, the ultra-low cross-sensitivities of 4.7 × 10−2 (rad/m)/m−1 and 1.2 × 10−2 (rad/m)/℃ are gained simultaneously, which dramatically improves the practicality of fiber-optic torsion sensor.

Optical fiber in-line Mach-Zehnder interferometer based on down-up tapers for refractive index measurement

An optical fiber in-line Mach-Zehnder interferometer based on down-up tapers is proposed and demonstrated for refractive index measurement. The device is fabricated by splicing two segments of multimode fibers in a heating and pushing process to form an up taper and followed by heating and pulling of the fiber adjacent to the up taper area to form a down taper. The down-up fiber taper structure constitutes a Mach-Zehnder interferometer and the whole device length is only 3 mm. The experimental results show that the refractive index sensitivity is ∼109.71 nm/RIU (refractive index unit) in the refractive index range of 1.340 – 1.370. Such a device is compact in size, robust in structure, good in sensitivity, convenient in operation and low in cost, and has high potential for chemical and biomedical sensing applications.

Fiber laser with dual-channel and a selectable comb filter based on Vernier effect of parallel-connected two Mach–Zehnder interferometers

We propose and demonstrate the implementation and application of a new configuration of an all-fiber comb filter based on the Vernier effect produced by parallel-connected two in-line Mach–Zehnder interferometers (MZIs). Each in-line MZI was fabricated by fusion splicing a section of panda-type polarization maintaining fiber (PMF) with peanut-shaped tapers between two single-mode fibers. These two in-line MZIs, respectively, form reference and sensing interferometers, which are parallel-connected by two 3-dB optical fiber couplers to realize the Vernier effect. By incorporating the proposed comb-filter into the erbium-doped fiber laser cavity, two output channels at 1533.2 and 1558.2 nm have been achieved. Further, the combination of the parallel-connected two in-line MZIs and polarization controllers (PC1 and PC2) promotes the lasing in a switchable and selective way. To the best of the authors’ knowledge, this is the first demonstration of a comb filter that employs paralleled two peanut-shaped MZIs in PMF. The experimental results indicate that the proposed filter has the potential to be used in communication systems.

Fiber laser with intracavity parallel Mach-Zehnder interferometers towards Vernier refractive index sensing.

In this paper, dual-wavelength laser emission of an erbium-doped fiber laser (EDFL) with a tunable distinct wavelength selection of the simultaneously produced laser lines was achieved by applying a parallel comb filter configuration based on the optical Vernier effect. The intracavity inserted proposed comb filter consists of two parallel branches to generate the Vernier effect. Each branch is an in-line Mach-Zehnder interferometer (MZI) filter, which is composed of a polarization-maintaining fiber fusion spliced between single-mode fibers with sphere shapes at both ends. The tunability of the selected laser wavelength was realized by submerging the proposed filter in different sodium chloride/water mixtures. The proposed comb filter-based Vernier effect was used to independently achieve the selection of the dual-wavelength EDFL lines and for refractive index (RI) sensing applications. The in-line M Z I 1 and M Z I 2 structures show a wavelength shift sensitivity to RI variations of -88 and 79nm/RIU, respectively. Our proposed MZI structure presents a reliable, straightforward, and low-cost spectral comb filter for separate tunable dual-wavelength laser generation in the c-band region. Furthermore, the proposed filter structure-based Vernier effect presents a new perspective and method in the RI sensing application.

Low-volume label-free SARS-CoV-2 detection with the microcavity-based optical fiber sensor

Accurate and fast detection of viruses is crucial for controlling outbreaks of many diseases; therefore, to date, numerous sensing systems for their detection have been studied. On top of the performance of these sensing systems, the availability of biorecognition elements specific to especially the new etiological agents is an additional fundamental challenge. Therefore, besides high sensitivity and selectivity, such advantages as the size of the sensor and possibly low volume of analyzed samples are also important, especially at the stage of evaluating the receptor-target interactions in the case of new etiological agents when typically, only tiny amounts of the receptor are available for testing. This work introduces a real-time, highly miniaturized sensing solution based on microcavity in-line Mach–Zehnder interferometer (μIMZI) induced in optical fiber for SARS-CoV-2 virus-like particles detection. The assay is designed to detect conserved regions of the SARS-CoV-2 viral particles in a sample with a volume as small as hundreds of picoliters, reaching the detection limit at the single ng per mL level.

A Reliable Anti-Interference Temperature Sensor Based on a Four-Hole Microstructure Optical Fiber Mach–Zehnder Interferometer

In this paper, an in-line Mach-Zehnder interferometer (ILMZI)-based temperature sensor of a sandwich-like composite structure was proposed and experimentally demonstrated. Using simple cleaving and splicing operations, ILMZI was formed by compactly sandwiching a section of four-hole microstructure optical fiber (FHMF) of ultra-short length between two segments of multi-mode fibers and single-mode fibers in turn. Thanks to the large thermo-optic coefficient difference between the silica and air modes in FHMF, the sensor achieves a comparable sensitivity of 67.29 pm/°C over a wide dynamic range of 20.8-100.4 °C. Because the sensing area length is only a few hundred microns, the long-term stability and anti-interference of the sensor is greatly enhanced. In addition, the large airhole design confines the propagation mode in silica medium by total internal reflection, reducing the interaction between the liquid sample and the sensing element, making the sensor inert to the ambient refractive index. The maximum response deviation of the sensor under micro-vibration disturbances is 3.51×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> nm/°C, and the maximum cross-sensitivity under strain disturbances is only 0.0018 °C/με. The proposed sensor is excellent in performance, easy in fabrication, simple in structure and low in cost, which can be expected to be used in practical temperature sensing applications without any additional packaging.

High-Sensitivity and High-Resolution RI Sensor With Ultrawide Measurement Range Based on NCF With Large Offset Splicing and MPF Interrogation

A high-sensitivity and high-resolution refractive index (RI) sensor with ultrawide measurement range based on no core fiber (NCF) with large offset splicing and microwave photonic filter (MPF) interrogation is demonstrated theoretically and experimentally for the first time. In the sensor, an inline Mach–Zehnder interferometer (MZI) based on NCF with large offset splicing is used as the sensing head, and a single-passband MPF is designed to interrogate the sensing signal. Compared with the traditional interrogation in optical domain, the MPF interrogation converts the periodic interference pattern in optical spectrum into the nonperiodic bandpass frequency response in radio frequency (RF) domain, so the measurement range is unlimited in principle. Owing to the large offset splicing in the inline MZI structure, the sensitivity of the sensor as high as −1.1054 GHz/RIU is obtained in the experiment. More importantly, MPF interrogation improves the resolution by an order of magnitude to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9\times10$ </tex-math></inline-formula> −7 RIU. Meanwhile, the sensitivity and the resolution of the sensor are easily adjusted by changing the length of dispersive media. The high sensitivity and high resolution of the sensor have potential applications in ultralow concentration biochemical detection field.

Refractive Index Measurement Based on Thin-Core Fiber In-Line Mach-Zehnder Interferometer and Differential Intensity Demodulation

A compact and reliable thin-core fiber (TCF) based in-line Mach-Zehnder interferometric (MZI) sensor for refractive index (RI) measurement is proposed and experimentally demonstrated. A section of TCF is sandwiched between a multi-mode fiber (MMF) and a single-mode fiber (SMF) to form the sensing head. In addition, the use of the differential intensity demodulation method not only greatly reduces the system cost, but also solves the problem of power fluctuation in the RI interrogation system. Three samples with different sensing lengths have been made, and the maximum sensitivity of 59.9 dB/RIU has been obtained in the range of 1.332–1.411 RIU, which means the measurement resolution is about 1.669 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> RIU. Besides, the proposed MZI-based RI sensor is less sensitive to temperature, thus the cross-sensitivity effect can be greatly weakened.

Tunable and switchable erbium-doped fiber laser based on the parallel filter of the FBGs embedded in Sagnac ring and the in-line MZI

A tunable and switchable erbium-doped fiber laser (EDFL) based on the parallel filter is proposed and experimentally verified. The parallel filter is composed of two branches. One branch is double fiber Bragg gratings (FBGs) embedded in Sagnac ring filter, which can realize the selection of two wavelengths; the other branch is an in-line Mach Zehnder interferometer (MZI) filter, which is composed of a 10 cm polarization-maintaining fiber (PMF) fused in the middle of single mode fibers (SMFs) with spheres at both ends, which can realize the selection of five wavelengths. When the parallel filter is inserted into the fiber laser cavity and the polarization controllers (PCs) are adjusted, the laser excitation from single-wavelength to quadruple-wavelength can be realized, including 7 different single-wavelength laser states, 16 different dual-wavelength laser states, 11 different triple-wavelength laser states and 2 different quadruple-wavelength laser states. In addition, the intervals of the dual-wavelength laser states are relatively regular, and the dual-wavelength laser intervals are obtained every ∼ 2 nm from 7 nm to 18 nm. And the optical signal-to-noise ratios (OSNRs) are more than 43 dB, and the highest OSNR is 60 dB. Finally, by changing the temperature applied to the FBG, tunability of dual-wavelength intervals can be achieved with a range of 1.1 nm. Due to the variety of lasing laser states of the fiber laser, easy switching, and regular intervals between the dual-wavelength laser states, the fiber laser proposed can be better applied to wavelength division multiplexing (WDM)system.

In-line Mach-Zehnder interferometer and Bragg grating integrated by femtosecond laser for discrimination of temperature and directional torsion.

Femtosecond laser micromachining has been considered as a powerful tool for fabricating versatile fiber devices and received increasing attention in recent years. Here, we report on a compact sensor by integrating a bridge-like waveguide inside a single-mode fiber to construct an in-line Mach-Zehnder interferometer and then inscribing a second-order Bragg grating in the core of the same fiber. The interference dip shows good performance in torsion sensing - the maximum torsion sensitivity of 1.5573 nm/(rad/m), the ability to identify the torsion direction, and low perturbation of axial strain. In order to compensate the cross impact of temperature, the fiber Bragg grating dip is employed as the second indicator and combined with the interference dip for discriminating temperature and directional torsion simultaneously. The proposed device also has the merits such as compact size, high thermal stability, and so on.

Fiber In-Line Mach-Zehnder Interferometer Based on a Pair of Optical Fiber Diffraction Gratings

Optical fiber in-line Mach-Zehnder interferometer for strain and refractive index sensing is proposed based on a pair of multi-column diffraction gratings. In the device operation, part of the light traveling in the fiber core diffracted to the cladding region through the first set of multi-column diffraction gratings, and directed back into the fiber core by the second set of multi-column diffraction gratings after being reflected at the cladding-air interface. The rest of the light remains traveling along the core, and the two parts of light recombine and produce interference. Thus, the multi-column diffraction gratings pair essentially forms an optical fiber in-line Mach-Zehnder interferometer. The transmission spectra of different free spectral ranges can be obtained by controlling the separation of two set of multi-column diffraction gratings. The sensitivities achieved for strain and refractive index are ~−1.43 pm / <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> and 363.41 nm/RIU (refractive index unit), respectively. As the device has a low temperature sensitivity 8.03 pm/°C, the temperature cross sensitivity can be greatly reduced. Such a device is small in size, robust in structure, and convenient in operation, and has potential in photonic applications.

Fiber-Optic Axial-Strain Sensor with Sensitivity Enhancement and Temperature Compensation

In this paper, we report a tapered thin-core fiber based in-line Mach-Zehnder interferometer to improve the response of axial-strain. With the varied diameters of taper waist, the light field distributions are studied by beam propagation method, and the structures are fabricated by arc-discharged lateral offset splicing and tapering techniques. The comprehensive tests are then conducted and compared in terms of axial-strain and temperature. The experimental results show that, by reducing the diameter of taper waist, more than 400% enhancement of wavelength sensitivity can be gained, and the maximum reaches 4.07 pm/µε with the measured error of 3.6%. Moreover, owing to high consistency of temperature response, the near-zero crosstalk is presented by differential compensation method. Furthermore, owing to the merit of high repeatability and stability, our sensor is very practical and promising in the high-precision measurement and engineering monitoring.

(INVITED)Investigation of liquids with microcavity in-line Mach-Zehnder interferometers – impact of the microcavity shape on the sensing performance

In this work, we review microcavity in-line Mach-Zehnder Interferometers (µIMZI) obtained in optical fibers using femtosecond (fs) laser micromachining. These structures can be considered as a great solution satisfying the requirements mentioned above for small-volume RI sensing applicable in label-free biosensing. Furthermore, application of the femtosecond laser facilitates tailoring of the microcavity's shape with high degree of flexibility. Over the years, various µIMZI have been reported, where RI sensing has been mainly analyzed but no impact of the microcavity shape has been shown up to date. Thus, on top of the review on µIMZIs, in this work, we discuss the impact of the shape of the on the sensing performance of the device. We use two representative examples of microcavity shapes, i.e., U-shape and V-trench, made in a standard single-mode fiber. Despite different shapes, both structures offer similar and high RI sensitivity (exceeding 13,000 nm/RIU in the RI range 1.333–1.340 RIU). However, the performance of the structures in microfluidic systems is different. Based on the experimental results and numerical simulations, the advantages and disadvantages of different shapes are discussed for their application in investigations of liquids and biosensing.

Life in an optical fiber: Monitoring of cell cultures with microcavity in-line Mach-Zehnder interferometer

Monitoring cell adhesion and growth are crucial for various applications involving drug screening, cytotoxicity, and cytocompatibility studies. However, acquiring accurate information about the growing state and responsiveness to a treatment of a cell system in a real-time and label-free manner is still a challenge. This work presents the first research on direct, real-time, and label-free adherent cell culture monitoring using a microcavity in-line Mach–Zehnder interferometer (μIMZI) fabricated in an optical fiber. The sensing solution based on μIMZI offers a great advantage over many other monitoring concepts tracking the changes taking place on the microcavity's bottom surface and within its volume, thus offering a greater penetration depth. In this study, we verified performance of the approach using a non-cancer bone marrow stromal cell line HS-5. The results demonstrate that the changes of the acquired signal are closely related to the different states of cells' adhesion, proliferation, morphology, and variation of mass. Thus, this label-free, real-time μIMZI-based monitoring technique gives a great promise to the analysis or monitoring of relevant new treatments in future scientific, as well as clinical applications.

Research on in-line Mach–Zehnder interferometer concentration sensor based on no-core fiber with lateral offset structure

An in-line Mach–Zehnder interferometer (MZI) based on no-core fiber (NCF) with lateral offset for concentration sensing is presented and demonstrated. The in-line MZI is formed by splicing a section of NCF between two single-mode fibers (SMFs) with a lateral offset. The theoretical and simulation analysis results indicate that the interference dip originates from the mode coupling between the high-order mode LP01 and the high-order mode LP26. In the concentration ranges of 0% to 37%, 37% to 66%, and 66% to 82%, the glycerol concentration sensitivities are 0.166, 0.378, and 1.44 nm/%, respectively. The temperature sensitivity is 0.081 nm / ° C from 30°C to 80°C. Our sensor has the advantages such as ease of fabrication, high glycerol concentration sensitivity, and great stability, providing a practical and competitive solution for chemical sensing and food processing.