In-fiber Mach-Zehnder Interferometer Research Articles

Thermally stable vector torsion sensing based on cascaded long-period fiber gratings in two-mode fiber

We demonstrated a torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of two identical cascaded long-period fiber gratings (CLPFGs) inscribed in two-mode fiber (TMF) by high-frequency CO2 laser. The transmission and mode coupling performances, as well as the spectral properties have been proved theoretically and experimentally. The proposed TMF-CLPFGs could not only be used as an efficient multi-wavelength LP01-LP11 mode converter, but also achieve the vector torsion measurements. As the twist induced, the proposed TMF-CLPFGs present vector torsion ability through the intensity of interference dips varying. The torsion sensitivity of TMF-CLPFGs with spacing length 1 cm can reach −1.07 dB/(rad/m) in counterclockwise direction and 0.9 dB/(rad/m) in clockwise direction. Moreover, the temperature sensitivity of TMF-CLPFGs was measured to be 0.055 dB/°C, which could be negligible to torsion measurement. Therefore, the proposed TMF-CLPFGs could be potential in few-mode fiber-based communication systems and sensing fields.

High sensitivity and low temperature cross-talk magnetic field sensor with ferrofluid-filled in-fiber Mach-Zehnder interferometer

A ferrofluid-filled in-fiber Mach-Zehnder interferometer (MZI) is presented and validated for magnetic field sensing. It is constructed by embedding a short capillary tube filled with ferrofluid which is sandwiched between two SMFs through a short MMF at its both ends, and the device is experimentally proved having extreme high refractive index (RI) sensitivity. When the sensor is placed in an external magnetic field, the RI of the ferrofluid changes, causing a shift in the transmission spectrum of the interferometer. Experimental results show that the proposed sensor exhibits a high sensitivity of 8.9657 nm/mT and 4.2642 nm/mT for longitudinal and vertical magnetic field within the range of 0–1.5 mT and 0.8–2.2 mT, respectively. Furthermore, the sensor exhibits excellent low temperature cross-talk in longitudinal and transverse magnetic fields as low as 0.014 mT/°C and 0.030 mT/°C, respectively. The sensor offers advantages such as stable operation, compact size, and robust mechanical structure.

Magnetic field sensor utilizing U-Shaped Cavity based on in-fiber Mach–Zehnder interferometer

Magnetic field sensor utilizing U-Shaped Cavity based on in-fiber Mach–Zehnder interferometer

In-fiber Mach-Zehnder interferometer for wearable respiration measurement

An in-fiber Mach-Zehnder interferometer based on single-mode fiberup-tapermultimode fiberup-tapersingle-mode fiber structure for respiration measurement is presented. The sensor probe is imbedded in a textile belt and fixed on the individual’s abdomen as a wearable solution. The system is used to detect the respiration rate of six volunteers, the respiration under different states is experimentally investigated. Attribute to the robust structural design and signal demodulation technique, the interference of temperature and body movement is reduced and the measurement accuracy and stability is distinctly improved. This simple, high accuracy, cost-effective wearable respiration sensor has great potential in real-time respiration monitoring.

Switchable multi-wavelength fiber lasers based on asymmetric biconical fiber tapers

In this paper, switchable multi-wavelength erbium-doped fiber lasers (MW-EDFLs) are proposed and experimentally demonstrated. An in-fiber Mach–Zehnder interferometer (IFMZI) composed of asymmetric biconical fiber taper (ABFT) pairs is integrated into the all-fiber ring cavity for multi-wavelength filtering. This laser can stably emit single-, dual-, or triple-wavelength outputs by fine-tuning the polarization controller. A high optical signal to noise ratio of 58.5 dB is obtained for the output spectra. The maximum wavelength shift is 0.08 nm and the power fluctuation in all laser emissions is recorded at 1.64 dB.

Mode transition phenomena into an in-fiber Mach-Zehnder interferometer

This work reports on a novel in-fiber Mach-Zehnder interferometer (MZI) device which is able to exploit the mode transition phenomena to tailor the sensitivity to external refractive index (RI). The proposed MZI structure is extremely simple: it consists of a small section of single mode double cladding fiber (DCF) in between two sections of standard single mode fiber using the core-offset splicing approach. The DCF presents a W-shaped RI profile with the outer cladding having a greater RI than the inner cladding. Therefore, the mode transition of cladding modes from outer to inner cladding of the DCF can be induced by tailoring the cladding thickness, in turn enhancing the sensitivity to surrounding refractive index. Specifically, wet chemical etching employing hydrofluoric acid is used to reduce the outer cladding diameter. Here, we report a comprehensive study of the phenomena while fiber diameter is decreased by 30 µm. Moreover, a specific study case with DCF diameter of 112 µm demonstrates a sensitivity gain of 2.5 over unetched fiber in the refractive index range of 1.33–1.39. Temperature and mechanical stress cross sensitivities are also evaluated. The suggested sensing device's advantages of considerable compactness, low manufacturing cost, simple fabrication technique, and higher sensitivity will be useful for future applications in the bio-chemical sensing and others.

In-fiber Mach-Zehnder interferometer as a promising tool for optical nose and odor prediction during the fermentation process.

In this paper, we present an in-fiber Mach-Zehnder interferometer (MZI) applied to coffee bean fermentation monitoring. Two MZIs, based on a combination of a fiber taper cascaded by a micro-tapered long-period fiber grating, were installed in a fermentation barrel to monitor the liquids and gases released during the fermentation process. During this process, a variety of odors arise due to the yeast activity and their classification is important to decide when to stop the fermentation process. In this work, we show that the in-fiber MZIs are good candidates for optical noses in this scenario.

High-performance vector torsion sensor based on high polarization-dependent in-fiber Mach-Zehnder interferometer.

We propose a high-performance vector torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of a straight waveguide inscribed in the core-cladding boundary of the SMF by a femtosecond laser in only one step. The length of the in-fiber MZI is 5 mm, and the whole fabrication time does not exceed 1 min. The asymmetric structure makes the device have high polarization dependence, and the transmission spectrum shows a strong polarization-dependent dip. Since the polarization state of the input light entering the in-fiber MZI varies with the twist of the fiber, torsion sensing can be achieved by monitoring the polarization-dependent dip. Torsion can be demodulated by both the wavelength and intensity of the dip, and vector torsion sensing can be achieved by setting the appropriate polarization state of the incident light. The torsion sensitivity based on intensity modulation can reach 5763.96 dB/(rad/mm). The response of dip intensity to strain and temperature is weak. Furthermore, the in-fiber MZI retains the fiber coating, so it maintains the robustness of the complete fiber structure.

In-fiber Mach–Zehnder interferometer based on polarization-maintaining fiber for displacement and temperature sensing

A displacement sensor based on polarization-maintaining fiber has been proposed and proved in experiment. The polarization-maintaining fiber (PMF) is sandwiched with two graded-index multimode fibers (GI MMF), which form the Mach–Zehnder interferometer (MZI) sensor. Graded-index multimode fiber serve as an optical coupler for modes conversion. The results show that with the increase of displacement, the spectrum moves to the long wavelength direction, but when temperature increases, the spectrum has a red shift, which means that the displacement and temperature can be measured separately according to the wavelength drift direction. The sensor consists of 4 mm GI MMF and 14 mm PMF, which can exhibit the displacement sensitivity of –9.275 pm/μm in the range of 0–600 μm. In addition, temperature will also affect the sensitivity of displacement measurement, so the sensitivity of the sensor to temperature is also measured. The results show that the selected monitoring dip provides a better temperature sensitivity of 33.605 pm/℃ in the range of 35–75℃. The sensor is easy to fabricate and does not has any functional coating, which make it become a good candidate in the industrial field.

Seven-core fiber based in-fiber Mach–Zehnder interferometer for temperature-immune curvature sensing

An in-fiber Mach–Zehnder interferometer (MZI) based on a brief coreless fiber (CLF) and a section of seven-core fiber (SCF) sandwiched in single mode fiber (SMF) is proposed and demonstrated. A supermode interference is created in the SCF that forms interference spectrum to perceive external curvature changes. A linear enhanced sensitivity of -26.5517 dB/m−1 for temperature-immune curvature measurement within the curvature range of 0.527–1.395 m−1 is experimentally achieved. Moreover, the strain response of the MZI is experimentally investigated. This MZI exhibits accurate and high sensitivity for curvature detection by economic intensity detection, which indicates its potential applications in the engineering field.

Strain and Temperature Discrimination Based on a Mach-Zehnder Interferometer With Cascaded Single Mode Fibers

An in-fiber Mach-Zehnder interferometer is proposed for the discrimination of strain and temperature. The sensor is based on two cascaded standard single mode fibers using three peanut tapers fabricated by simple splicing. The cascaded structure excites more frequency components, which induce four sets of interference dips in the transmission spectrum. One set of the spectrum dips have different sensitivities to temperature and strain from those of the other three. The sensor can discriminate strain and temperature by monitoring the wavelength shifts of two spectrum dips. Repeated experiments are taken both for strain and temperature increasing and decreasing scenarios. Experimental results show that Dip 1 has an average strain sensitivity of −0.911 pm/µε and an average temperature sensitivity of 49.98 pm/°C. The strain sensitivity for Dip 2 is negligible and its average temperature sensitivity is 60.52 pm/°C The strain and temperature resolutions are ±3.82 µε and ±0.33 °C.

In-fiber Mach-Zehnder interferometer based on hollow optic fiber for metal ion detection.

A new, to the best of our knowledge, in-fiber Mach-Zehnder interferometric sensor is proposed and experimentally demonstrated for detecting Cu2+ ions in an aqueous environment. The sensor is fabricated simply and cost-effectively by arc-fusing a short section of hollow optical fiber between two standard single-mode fibers and is functionalized by depositing chitosan and poly(acrylic acid) bilayers using electrostatic self-assembly. The proposed sensor shows a linear response with sensitivity of 42 nm/mM in the Cu2+ ion concentration from 0 to 40 µM. Moreover, the fiber sensor exhibits good reusability and repeatability and is a promising candidate for contamination detection in drinking water and industrial waste water.

Curvature sensor based on femtosecond laser-inscribed straight waveguide in FMF

We propose an ingenious method for higher-order mode excitation in few-mode fiber (FMF), in which a femtosecond laser is utilized to inscribe a straight waveguide in FMF. Based on this method, a new-type in-fiber Mach-Zehnder interferometer (MZI) for bending sensing is proposed. The in-fiber MZI is achieved through femtosecond laser directly writing a straight waveguide in a very short section of FMF between two standard single mode fibers. The MZI is caused by the interference between the fundamental mode and the higher-order mode excited in the modified FMF. When the device is bent in specific four orthogonal orientations, the direction of its spectrum shift is different. The device has a large curvature measurement range of 0–10 m−1. The curvature sensitivity can reach 2 nm/m−1 and the temperature sensitivity is as low as 2.34 pm/°C. Moreover, the device has the superiorities of miniature size, simple manufacturing, high reproducibility, low insertion loss and high spectral contrast.

Optical curvature sensor with high resolution based on in-line fiber Mach-Zehnder interferometer and microwave photonic filter.

Curvature measurement plays an important role in structural health monitoring, robot-pose measuring, etc. High-resolution curvature measurement is highly demanded. In this paper, an optical curvature sensor with high resolution based on in-fiber Mach-Zehnder interferometer (MZI) and microwave photonic filter (MPF) is proposed and experimentally demonstrated. The in-fiber MZI is constructed with a ring-core fiber (RCF) which is fusion spliced between two coreless fibers (CLFs). The structure of CLF-RCF-CLF is then sandwiched between two segments of single-mode fiber (SMF), making the whole interferometer structure of SMF-CLF-RCF-CLF-SMF. The operating principle is that different curvatures will cause the variations of the interference spectrum of MZI due to elastic-optic effect, and then the variations are converted into the frequency-shift of the MPF. The factors affecting the visibility of the interference spectrum are researched. A preliminary exploration of the multiplexing demodulation for the in-fiber-MZIs is also investigated and discussed, which is for the first time to the best of our knowledge, holding great potential to pave the way for constructing the sensing network composed of interferometric sensors. The curvature measurement sensitivity is -147.634 MHz/m-1, and the resolution is 6.774 × 10-6 m-1 which is the highest value up to now.

Highly Modulated In-Fiber Mach-Zehnder Interferometer Based on an Ultracompact Leaky-Guided Liquid Core.

We proposed a novel sensor based on an ultracompact leaky-guided liquid core fiber Mach–Zehnder interferometer (LLCFMZI) for high modulation of an interference spectrum. The sensor structure is based on a micro-sized hollow-core fiber (HCF) splicing a tilt end face single-mode fiber (SMF) to create a miniature oblique gap for the effective access of different liquids. The liquid core with a relatively lower refractive index (RI) than the cladding can achieve a leaky-mode optical waveguide (LMOW) mechanism, and its volume is only approximately 7.85 pL. In addition, the utilized micro-length HCF can reduce the energy loss of core in the LMOW to obtain an acceptable extinction ratio (>30 dB) with high temperature (T) sensitivity in the interference spectra. Experimental results show that the interference spectra can be highly modulated within the wide measurement range of 1250–1650 nm with a steadily linear response for thermal effect. The measured temperature sensitivities (T-sensitivities) of various liquids of DI water, ethanol, and Cargille-liquid (nD = 1.305) are 0.8869, 4.4754, and 4.8229 nm/°C, and the corresponding measured thermal optics coefficient (TOC) are −4.16 × 10−5, −2.11 × 10−4, and −3.6 × 10−4 °C−1, respectively. Measurement results demonstrate that the used liquids with a higher TOC can obtain better T-sensitivity modulation. The highest experimental sensitivity of the liquid-core filled with Cargille-liquid (nD = 1.40) is up to +13.87 nm/°C with a corresponding TOC of −4.07 × 10−4 °C−1. Furthermore, the experimental and theoretical values are in good agreement according to FSR the measuring scheme that investigates the effectiveness of the proposed LLCFMZI.

Compact Fiber Curvature and Temperature Sensor Inscribed by Femtosecond Laser Through the Coating

An integrated structure in the original single mode fiber (SMF) which can measure curvature and temperature simultaneously, is proposed and demonstrated. The key components of the device include an in-fiber Mach–Zehnder interferometer (MZI) and a fiber Bragg grating (FBG), which are integrally fabricated in a coating-unstripped SMF by femtosecond laser direct writing. The curvature sensing of the device is based on the wavelength shift of the MZI interference fringe dips. It is found that the bending response is closely related to the bending orientation. The measured curvature sensitivities are 2.324 and 2.491 nm/m−1 for the 0° and 180° orientation in the range of 0–10 m−1, respectively. Under different bending states, the temperature response of the FBG resonant wavelength is almost the same and the mean temperature sensitivity is 9.72 pm/°. The advantages of compact, simple fabrication, low insertion loss, no assembly, wide operating range of curvature measurement and real-time temperature sensing make our device very attractive in practical engineering monitoring and somatosensory posture recognition.

Ammonia Gas Sensor Based on Graphene Oxide-Coated Mach-Zehnder Interferometer with Hybrid Fiber Structure.

A graphene oxide-coated in-fiber Mach-Zehnder interferometer (MZI) formed with a multimode fiber-thin core fiber-multimode fiber (MMF-TCF-MMF) is proposed and experimentally demonstrated for ammonia gas (NH3) sensing. The MZI structure is composed of two segments of MMF of length 2 mm, with a flame-tapered TCF between them as the sensing arm. The MMFs act as mode couplers to split and recombine light owing to the core diameter mismatch with the other fibers. A tapered TCF is formed by the flame melting taper method, resulting in evanescent wave leakage. A layer of graphene oxide (GO) is applied to the tapered region of the TCF to achieve gas adsorption. The sensor operates on the principle of changing the effective refractive index of the cladding mode of a fiber through changing the conductivity of the GO coating by adsorbed NH3 molecules, which gives rise to a phase shift and shows as the resonant dip shifts in the transmission spectrum. So the concentration of the ammonia gas can be obtained by measuring the dip shift. A wavelength-shift sensitivity of 4.97 pm/ppm with a linear fit coefficient of 98.9% is achieved for ammonia gas concentrations in the range of 0 to 151 ppm. In addition, we performed a repetitive dynamic response test on the sensor by charging/releasing NH3 at concentration of 200 ppm and a relative humidity test in a relative humidity range of 35% to 70%, which demonstrates the reusability and stability of the sensor.

Study of asymmetric biconical fiber tapers for in-fiber Mach-Zehnder interferometers and applications in single-frequency fiber lasers.

In this paper, asymmetric biconical fiber tapers (ABFTs) for in-fiber Mach-Zehnder interferometers (IFMZIs) are proposed and analyzed to enhance the interference effect. The proposed ABFT-IFMZIs are fabricated, tested, and demonstrated in applications of single-frequency (SF) emissions when incorporated into an all-fiber laser cavity as the frequency selecting component. In comparison with the traditional IFMZIs composed of all symmetric biconical fiber tapers (SBFTs), higher average transmittance and fringe contrast have been demonstrated with the ABFT-based IFMZIs. When applied to the SF fiber laser emission, lower pump threshold and higher slope efficiency have also been confirmed with the ABFT-IFMZI device. The theoretical and experimental results have indicated that the interference effect of IFMZIs can indeed be improved by the designated asymmetry of conical taper angles in ABFTs that can offer an extra flexibility in fiber taper design, fabrication, and applications.

Highly sensitive hydrogen sensor based on an in-fiber Mach-Zehnder interferometer with polymer infiltration and Pt-loaded WO3 coating

A highly sensitive fiberized hydrogen sensor based upon Mach-Zehnder interference (MZI) is experimentally demonstrated. The hydrogen sensor consists of an MZI realized by creating an air cavity inside the core of a half-pitch graded-index fiber (GIF) by use of femtosecond laser micromachining. Thermosensitive polymer was filled into the air cavity and cured by UV illumination. Subsequently, the external surface of the polymer-filled MZI was coated with Pt-loaded tungsten trioxide (WO3). The exothermic reaction occurs as Pt-loaded WO3 contacts the target of the sensing, i.e. hydrogen in the atmosphere, which leads to a significant local temperature rise on the external surface of the coated MZI sensor. The sensor exhibits a maximum sensitivity up to -1948.68 nm/% (vol %), when the hydrogen concentration increases from 0% to 0.8% at room temperature. Moreover, the sensor exhibits a rapid rising response time (hydrogen concentration increasing) of ∼38 s and falling response time (hydrogen concentration decreasing) of ∼15 s, respectively. Thanks to its small size, strong robustness, high accuracy and repeatability, the proposed in-fiber MZI hydrogen sensor will be a promising tool for hydrogen leakage tracing in many areas, such as safety production and hydrogen medical treatment.

High-Performance Bending Sensor Based on Femtosecond Laser-Inscribed in-Fiber Mach–Zehnder Interferometer

A novel bending sensor based on an in-fiber Mach–Zehnder interferometer (MZI) has been fabricated by femtosecond laser. The MZI is formed by an off-axial straight waveguide inscribed in a no-core fiber spliced between two standard single mode fibers. The in-fiber MZI is formed due to the interference of a fundamental mode and a higher-order mode which excited in the no-core fiber. Analysis results of the structure agree well with the measured transmission spectra. Experimental results show that the device has low temperature crosstalk and high curvature sensitivity of 14.91 nm/m−1 and 12.86 nm/m−1 for the 0° and 180° orientations in the range of 0–3 m−1, respectively. We can distinguish the bending orientation of the device by the shift direction of the spectrum. The advantages of high sensitivity, low temperature crosstalk, compact structure, and easy fabrication make the device attractive for actual engineering bend sensing.