Fiber In-line Mach-Zehnder Interferometer Research Articles

Investigation of an inline fiber Mach-Zehnder interferometer based on hybrid joints of core-offset and peanut-shape for refractive index sensing

An inline fiber Mach-Zehnder Interferometer (MZI) that consisted of a standard single-mode fiber sandwiched between a core-offset and a peanut-shape joint is fabricated for the purpose of refractive index (RI) sensing. After one of the core-offset joints is replaced by a peanut-shape joint in an inline fiber MZI with the core-offset pair, the influence of the interference performance caused by the relative offset direction of the two core-offsets is avoided. The RI sensing characteristics under different core-offset displacement and interference arm length are studied. After bending the inline fiber MZI, the interference spectra become more stable, and the RI sensing performance is markedly improved. The RI sensitivity of −167.27 nm/RIU with a resolution of 2.39 × 10-4 RIU is achieved within the measured RI range of 1.333–1.373.

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.

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.

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.

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.

Femtosecond laser processing for a high sensitivity fiber MZI microcavity.

An ultra-compact fiber inline Mach-Zehnder interferometer sensor based on femtosecond laser micromachining technology is demonstrated. It is found that the microstructure has an ultra-high refractive index sensitivity of 16660 nm/RIU when a femtosecond pulsed laser is used to remove the upper cladding and part of the core of a standard single-mode fiber. However, its temperature sensitivity is not much different from that of most pure quartz fibers and can be as high as 7.934 nm/°C when the microcavity is coated with a low-refractive-index ultraviolet adhesive, which was originally used for bonding glass. With this coating, however, it demonstrates excellent robustness.

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.

Optical Fiber Diffraction Gratings

Optical fiber diffraction gratings with periodic structure across the fiber section and fabricated by femtosecond laser are proposed and demonstrated. The diffraction patterns can be clearly observed and greatly enhanced by adopting multiple layer gratings. An optical fiber in-line Mach-Zehnder interferometer can be constructed by use of multiple layer diffraction grating pairs. The fiber diffraction gratings represent a new type of optical fiber device featured with high robustness, good operation stability and great potential for many photonics applications.

Temperature dependence of a refractive index sensor based on a bent core-offset in-line fiber Mach-Zehnder interferometer

A bent core-offset in-line fiber Mach-Zehnder Interferometer (MZI) as a refractive index (RI) sensor was fabricated by using a commercial fusion splicer. The temperature dependence on the so-obtained sensor was investigated. It showed that not only the RI sensitivity, but also the temperature sensitivity could be enhanced by reducing the bending radius. The variation of RI and the temperature hardly affected the sensitivity of each other within the margin of error. The wavelength-related measurement was used to realize the simultaneous measurement of RI and temperature. The highest temperature sensitivity of 0.11745 nm/°C and the highest RI sensitivity of −115.5646 nm/RIU was obtained by the sensor with a length of 30 mm, a core-offset of 6 µm, and a bending radius of 25.42 mm.

Optical Fiber In-Line Mach-Zehnder Interferometer Based On an Inner Air-Cavity With Long Cavity Length

An optical fiber in-line Mach-Zehnder interferometer based on an inner air-cavity with long cavity length is demonstrated. The device is fabricated by using femtosecond laser to inscribe a waveguide structure in the optical fiber core, followed by discharging the waveguide area with a fusion splicer. The inner air-cavity structure is highly robust, and the relatively long cavity length corresponds to a small free spectral range, which makes it possible to implement accurate measurement. Such an inner air-cavity device fabricated in single mode fiber has good high temperature sustainability. The device is also “open” to the external environment when being fabricated in multimode fiber, thus supporting refractive index measurement. The proposed device is attractive in many optical fiber sensing applications.

Ultrasensitive Refractive Index Sensor Based on Mach–Zehnder Interferometer and a 40μm Fiber

An ultra-high sensitivity refractive index (RI) sensor, based on an in-line fiber Mach-Zehnder interferometer (MZI) and utilizing a special 40 μm fiber which has been specifically designed for the purpose, supporting only the LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes propagating in the fiber, has been proposed and numerically demonstrated in this work. The all-fiber MZI, based on LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode interference, was fabricated by utilizing a special design with two gradually tapered joints at both sides of this multi-clad thin diameter fiber (TDF). When the TDF diameters varied from 40 μm to 5 μm, a sudden change in the direction of the wavelength shift in the transmission spectrum of the TDF-MZI devices was observed both in liquid RI and air relative humidity (RH) monitoring experiments, as the surrounding RI or RH was seen to increase monotonically. Ultra-high sensitivity values of 5942.8nm/RIU (1.335-1.342) and 21292.2nm/RIU (1.4000-1.4025) were obtained in the experiments carried out, through the use of a tapered TDF-MZI device with TDF diameters of ~5 μm and ~9 μm, respectively. The maximum RH sensitivity of 1.084nm/%RH was obtained by the use of the TDF-MZI device with a TDF diameter of ~12 μm. A compact all-fiber TDF-MZI sensor was thus created with an overall sensing length of <; 4mm, showing the advantages of high sensitivity, low loss, and flexibility in the tunable monitoring direction of the wavelength shift. This design is well suited to various applications, where the high sensitivity RI and RH measurements are required at precise locations.

Fiber in-line Mach-Zehnder interferometer for simultaneous measurement of transverse loading and temperature based on multi-core fiber

A fiber in-line Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated for simultaneously measuring transverse loading and temperature. The MZI is fabricated by simply splicing a segment multicore fiber (MCF) with two short sections of multimode fibers (MMFs). The sensing principle is theoretically analyzed and the transverse loading and temperature characteristics of the sensor are investigated in experiment. The results show that the multimode interference among the center core mode (CCM), the cladding modes (CM) and outer core mode (OCM) of the MCF enables the sensor to realize the measurements of transverse loading and temperature. The transmission spectra will shift when the external transverse loading and temperature variation, and different spectral responses of the resonant dips are observed, which indicates that the sensor can implement the simultaneous transverse loading and temperature measurement by monitoring the wavelength changes of two resonant dips. The obtained sensitivities of transverse pressure and temperature can reach up to −165 pm/N and 45 pm/°C, respectively. The proposed sensor has the potential application in the fields where both temperature and transverse loading measurements are required.

A CONCATENATED UP AND DOWN TAPERED FIBER FOR SIMULTANEOUS MEASUREMENT OF STRAIN AND TEMPERATURE

A novel fiber optical sensor based on in-line fiber Mach-Zehnder interferometer for simultaneous measurement of strain and temperature is proposed and demonstrated experimentally. The interferometer is simple, extremely robust and highly sensitive and consists of two concatenated parts; one is a down-tapered fiber (DTF) and the other is an up-tapered fiber (UTF). UTF and DTF sections of the sensor are fabricated by using a commercial fiber splicer and a non-commercial setup based on heating and stretching a portion of a standard single-mode fiber, respectively. While UTF section behaves as a beam splitter to decompose the injected light into core and cladding modes, DTF section provides evanescent field to access the surrounding environment. Experimental results indicate that the resolutions of 0.83 °C and 45.80 micro-epsilon were achieved in temperature and strain, respectively, for simultaneous measurement with a 10 pm of wavelength resolution.

Fiber in-line interferometer based on air-cavity with suspended fiber-core for sensing

We demonstrate a new type of fiber in-line Mach–Zehnder interferometer (MZI). The device is fabricated by firstly inscribing a ring structure surrounding the fiber core at the ends of two single-mode fibers using a femtosecond laser micromachining technique and then fusion-splicing the two fibers with end structures. Due to the high temperature produced in the fusion splicing process, the ring structure expands rapidly and results in an inner air-cavity with a suspended fiber-core. The incident light traveling in the fiber core is split into two parts at the edge of the inner air-cavity, one propagating through the air cavity while the other remains traveling along the suspended fiber core; both are collected by the fiber core at the air-cavity end, thus forming an in-line MZI. The device has good high-temperature sustainability up to 900 °C. Moreover, by drilling a hole on top of the air-cavity, an open micro-channel can be created for air pressure measurement with a pressure sensitivity of ~−8214.68 pm MPa−1.

Influence of temperature on the refractive index sensor based on a core-offset in-line fiber Mach-Zehnder interferometer

Abstract The influence of temperature on a refractive index (RI) sensor based on an in-line fiber Mach-Zehnder interferometer (MZI) was investigated in both simulation and experiment. The RI sensing performance of the sensor was investigated at different temperature (25–60 °C) over an RI range of 1.33–1.37, and the RI sensitivity of the sensor was around −20.00 nm/RIU at different temperature. The results showed that the temperature variation led to a RI deviation in the RI measurement, but hardly affect the RI sensitivity. To eliminate the RI deviation in the measurement, the RI correction was given.

High Temperature (Up to 950 °C) Sensor Based on Micro Taper In-Line Fiber Mach–Zehnder Interferometer

A high temperature (up to 950 °C) sensor was proposed and demonstrated based on a micro taper in-line fiber Mach–Zehnder interferometer (MZI) structure. The fiber MZI structure comprises a single mode fiber (SMF) with two micro tapers along its longitudinal direction. An annealing at 1000 °C was applied to the fiber sensor to stabilize the temperature measurement. The experimental results showed that the sensitivity was 0.114 nm/°C and 0.116 nm/°C for the heating and cooling cycles, respectively, and, after two days, the sensor still had a sensitivity of 0.11 nm/°C, showing a good stability of the sensor. A probe-type fiber MZI was designed by cutting the sandwiched SMF, which has good linear temperature responses of 0.113 nm/°C over a large temperature range from 89 to 950 °C. The probe-type fiber MZI temperature sensor was independent to the surrounding refractive index (RI) and immune to strain. The developed sensor has a wide application prospect in the fields of high temperature hot gas flow, as well as oil and gas field development.

Simultaneous Curvature and Temperature Sensing Based on a Novel Mach-Zehnder Interferometer

A novel fiber inline Mach-Zehnder interferometer (MZI) is proposed for simultaneous measurement of curvature and temperature. The sensor composes of single mode-multimode-dispersion compensation-multimode-single mode fiber (MMF-DCF-MMF) structure, using the direct fusion technology. The experimental results show curvature sensitivities of −12.82 nm/m−1 and −14.42 nm/m−1 in the range of 0–0.65 m−1 for two resonant dips, as well as temperature sensitivities of 57.6 pm/°C and 74.3 pm/°C within the range of 20 °C–150 °C. In addition, the sensor has unique advantages of easy fabrication, low cost, high fringe visibility of 24dB, and high sensitivity, which shows a good application prospect in dual-parameters of sensing of curvature and temperature.

Enhancement of Refractive Index Sensitivity by Bending a Core-Offset In-Line Fiber Mach–Zehnder Interferometer

We proposed a bent core-offset in-line fiber Mach–Zehnder interferometer for refractive index (RI) sensing. It was fabricated by the splicing method and consists of a standard single-mode fiber sandwiched between two core-offset joints, with a core-offset distance of about 6 $\mu \text{m}$ . The RI sensing performance of this so-obtained interferometer was evaluated and compared with that of straight ones. Straight interferometers exhibited a relatively low sensitivity (15.326 nm/RIU), inadequate for RI measurements. However, this sensitivity was greatly improved by simply bending them into U-shaped probes. The RI sensitivity was enhanced as the curvature radius was reduced; the highest sensitivity (358.039 nm/RIU) was achieved with an interferometer having a length of 30 mm and a bend radius of 16.42 mm. Hence, the experimentally confirmed high sensitivity of our proposed sensor, combined with its simple and compact structure, easy fabrication, and low cost, makes it a promising candidate for RI sensing applications.

Ultrahigh sensitivity with different taper geometries of thin-core fiber modal interferometer for refractive index sensing

A highly sensitive refractive index sensor based on a fiber in-line Mach–Zehnder interferometer sensor based on single-mode–tapered thin-core–single-mode fiber is proposed. We developed a theoretical model using wide-angle beam propagation method in cylindrical coordinates to simulate light propagation performance of such fiber devices. Tapered structures using different taper geometries (linear, exponential, and parabolic taper) with waist diameters as low as 30 μm are presented. The proposed sensor has an ultrahigh sensitivity of 996.4 nm/refractive index unit in glycerin solution with different concentrations in water using the linear taper geometry. The designed structure presents the merits of high sensitivity, which is 6 times higher than that of the thin-core fiber modal interferometer and makes it an excellent candidate for biochemical sensing applications.

Simultaneous Measurement of Refractive Index and Temperature Based on a Peanut-Shape Structure In-Line Fiber Mach–Zehnder Interferometer

We proposed a peanut-shape structure in-line fiber Mach-Zehnder interferometer (MZI) for simultaneous measurement of refractive index (RI) and temperature. Two kinds of demodulation methods were investigated and compared. The wavelength-related and phase-related character matrices of the in-line fiber MZI were determined for simultaneous measurement of RI and temperature, respectively. For the wavelength-related measurement, the highest temperature and RI sensitivities were 0.0709 nm/°C and -47.3620 nm/RIU, respectively. The phase-related measurement was with a lower measuring errors compared to the wavelength-related measurement, and the highest temperature and RI sensitivities were -0.0632rad/°C (0.0764 nm/°C) and 77.0995rad/RIU (-93.2429 nm/RIU), respectively. The minimum measuring errors of temperature and RI were 0.3800 °C and 0.0004 RIU, respectively.