All-fiber Mach-Zehnder interferometers for sensing applications

Few-mode multicore fiber enabled integrated Mach-Zehnder interferometers for temperature and strain discrimination

We proposed and experimentally demonstrated paralleled Mach-Zehnder interferometers (MZIs) in few-mode multicore fiber (FM-MCF) for temperature and strain discriminative sensing. A section of FM-MCF is sandwich-spliced between two single-mode multicore fiber (SM-MCF) with a rotational offset. The arbitrarily controlled angular misalignment generates intentional intermodal interferences in outer cores of the FM-MCF thus multiple MZI structures are implemented. Experimental results show that the temperature sensitivities are 105.8 pm/°C and 223.6 pm/°C for two outer cores, strain sensitivity is 13.96 pm/με for the outer core 1 and 11.7 pm/με for the outer core 2, respectively. Due to the low condition number of the cross coefficient matrix dependent on the temperature and strain response indexes, the temperature-strain cross sensitivity can be efficiently eliminated. In addition, the structure's fabrication process is simple, cost effective, and repeatable. The sensing structure can be applied to a wide range of measurements and is expected to develop potentials by building a higher dimensional matrix with more cores.

A Novel Strain Sensor with Large Measurement Range Based on All Fiber Mach-Zehnder Interferometer

We have proposed a high sensitive photonic crystal fiber (PCF) strain sensor based on the Mach-Zehnder interferometer (MZI). The sensing head is formed by all-fiber in-line single mode-multimode-photonic-crystal-single mode fiber (SMPS) structure, using only the splicing method. Such a strain sensor exhibited a high sensitivity of −2.21 pm/με within a large measurement range of up to 5000 με and a large fringe visibility of up to 24 dB. Moreover, it was found that the strain sensitivity was weekly dependent of the length of PCF or MMF. In addition, the sensor exhibited the advantages of simplicity of fabrication, high sensitivity and larger fringe visibility.

Highly Sensitive Strain Sensor Based on a Novel Mach-Zehnder Interferometer with TCF-PCF Structure

A highly sensitive strain sensor based on a novel fiber in line Mach-Zehnder interferometer (MZI) was demonstrated experimentally. The MZI was realized by splicing a section of photonic crystal fiber (PCF) with the same length of thin core fiber (TCF) between two single mode fibers (SMFs). The fringe visibility of MZI can reach as high as 20 dB in air. In particular, the strain sensitivity of −1.95 pm/με was achieved within a range from 0 to 4000 με. Furthermore, the strain properties of different length of MZI was investigated. It was found that the sensitivity was weekly dependent on the length of MZI. The strain sensitivities corresponding to the MZI with 35 mm PCF, 40 mm PCF and 45 mm PCF at 1550 nm band were −1.78 pm/με, −1.73 pm/με and −1.63 pm/με, respectively. Additionally, the sensor has advantages of simple fabrication, compact size and high sensitivity as well as good fringe visibility.

Sensitive refractive index sensor based on an assembly-free fiber multi-mode interferometer fabricated by femtosecond laser

We propose and demonstrate a highly sensitive refractive index (RI) sensor based on a novel fiber-optic multi-mode interferometer (MMI), which is formed with a femtosecond-laser-induced in-core negative refractive index modified line in a standard single mode fiber. The proposed MMI structure is directly written with femtosecond laser in one step, which removes the splicing process needed in conventional MMI fabrication and also significantly improves the robustness. This device exhibits a high sensitivity to surrounding refractive index, with a maximum sensitivity up to 10675.9 nm/RIU at the RI range of 1.4484-1.4513. The distinct advantages of high sensitivity, compact, robust and assembly-free all-fiber structure make it attractive for real physical, chemical and biological sensing.

Controlled modulation of depolarization in laser speckle

A new technique based on superposition of two speckle patterns is proposed and demonstrated for controlled modulation of the spatial polarization distribution of the resultant speckle. It is theoretically and experimentally demonstrated that controlled modulation of the spatial polarization distribution of laser speckle can be achieved by proper choice of the polarization states as well as the average spatial intensity of the constituent speckles. It is shown that the proposed technique is useful to generate different speckle patterns with sinusoidal variation in their degree of polarization, which can be tuned from zero to unity. This technique can find application in sensing, biomedical studies, and in determining the rotation of an electric field vector after passing through a scattering medium.

Hybrid Fabry-Perot interferometer for simultaneous liquid refractive index and temperature measurement

A compact and high sensitivity sensor with a fiber-tip structure is proposed and demonstrated for simultaneously liquid refractive index (RI) and temperature sensing. The device is fabricated by inserting a tiny segment of capillary tube between single-mode fibers (SMFs) to form two cascaded Fabry-Perot interferometers (FPIs). The theoretical and experimental results demonstrate that the ambient liquid RI and temperature can be simultaneously determined by the intensity and shift of the resonant wavelength in the reflection spectrum. Our proposed device has the highest RI sensitivity of ~216.37 dB/RIU at the RI value of 1.30; a high spatial resolution owing to its compact size (with dimension <400 μm) makes it promising for high precision bio/chemical sensing applications.

Photonic crystal fiber modal interferometer based on thin-core-fiber mode exciter

A thin-core-fiber excited photonic crystal fiber modal interferometer has been proposed and experimentally demonstrated. By employing a thin-core fiber as the mode exciter, both of the core and cladding modes propagate in the photonic crystal fiber and interfere with each other. The experimental results show that the transmission dips corresponding to different-order modes have various strain responses with opposite shift directions. The strain sensitivity could be improved to 58.57  pm/με for the applied strain from 0 to 491 με by utilizing the wavelength interval between the dips with opposite shift directions. Moreover, due to the pure silica property of the employed photonic crystal fiber, the proposed fiber modal interferometer exhibits a low-temperature sensitivity of about 0.56  pm/°C within a temperature range from 26.4°C (room temperature) to 70°C. Additionally, the proposed fiber modal interferometer has several advantages, such as good stability, compact structure, and simple fabrication. Therefore, it is more applicable for strain measurement with reducing temperature cross-sensitivity.

Novel method of detecting movement of the interference fringes using one-dimensional PSD

In this paper, a method of using a one-dimensional position-sensitive detector (PSD) by replacing charge-coupled device (CCD) to measure the movement of the interference fringes is presented first, and its feasibility is demonstrated through an experimental setup based on the principle of centroid detection. Firstly, the centroid position of the interference fringes in a fiber Mach-Zehnder (M-Z) interferometer is solved in theory, showing it has a higher resolution and sensitivity. According to the physical characteristics and principles of PSD, a simulation of the interference fringe’s phase difference in fiber M-Z interferometers and PSD output is carried out. Comparing the simulation results with the relationship between phase differences and centroid positions in fiber M-Z interferometers, the conclusion that the output of interference fringes by PSD is still the centroid position is obtained. Based on massive measurements, the best resolution of the system is achieved with 5.15, 625 μm. Finally, the detection system is evaluated through setup error analysis and an ultra-narrow-band filter structure. The filter structure is configured with a one-dimensional photonic crystal containing positive and negative refraction material, which can eliminate background light in the PSD detection experiment. This detection system has a simple structure, good stability, high precision and easily performs remote measurements, which makes it potentially useful in material small deformation tests, refractivity measurements of optical media and optical wave front detection.

Bent-fiber intermodal interference based dual-channel fiber optic refractometer

We present a novel dual-channel fiber optic interferometer based on intermodal interference from single-mode fiber (SMF) bending. This dual-channel interferometer has simple structure, consisting of two bare fiber semicircular bending regions with different bending radiuses connected by a section of straight fiber. A dual-channel interferometer with bending radiuses of 4 mm and 4.3 mm is fabricated and refractive index (RI) sensing is realized by measuring the wavelength shift of the resonance dips in the transmission spectrum of the dual-channel interferometer. In the RI range of 1.3403 to 1.3726, the corresponding RI sensitivities for these two channels are 207 and 245 nm/RIU (refractive index unit) and the RI resolutions are about 6.57 × 10⁻⁵ RIU and 5.55 × 10⁻⁵ RIU, respectively.

A thermally annealed Mach-Zehnder interferometer for high temperature measurement

An in-fiber Mach-Zehnder interferometer (MZI) for high temperature measurement is proposed and experimentally demonstrated. The device is constructed of a piece of thin-core fiber (TCF) sandwiched between two short sections of multimode fiber (MMF), i.e., a MMF-TCF-MMF structure. A well-defined interference spectrum is obtained owing to the core-mismatch, and the interference dips are sensitive to the ambient temperature. The experimental results show that the proposed interferometer is capable of high temperature measurement up to 875 °C with a sensitivity of 92 pm/°C over repeated measurements. The explored wavelength drop point may limit the measurement range, which can be improved by repeated thermal annealing.

Liquid level measurement by applying the Mach-Zehnder interferometer based on up-tapers

A novel scheme for liquid level detection using an all-fiber Mach-Zehnder interferometer, based on two up-tapers, is proposed and experimentally investigated. The effective refractive indices of the axisymmetric modes LP(0n) are analyzed while it is partly immersed in liquid. The sensitivity of the sensor is greatly dependent on the fiber length between the two up-tapers, as well as the order of the cladding modes through the calculation. The experimental results show that the sensitivity of the Mach-Zehnder interferometer reaches a 22.5 nm wavelength shift per meter of level change with the middle fiber length of 14.3 cm. The proposal might be attractive since the up-tapered liquid level sensor could easily be fabricated and used for a large measuring range.

Anti-resonant reflecting guidance in alcohol-filled hollow core photonic crystal fiber for sensing applications

Mechanism and sensing applications of antiresonant reflecting guidance in an alcohol-filled simplified hollow-core (SHC) photonic crystal fiber (PCF) are demonstrated. By filling one air hole in the air cladding of the PCF with alcohol, anti-resonant reflecting guidance of light can be achieved and energy leakage of the core modes can be induced at resonant wavelengths of the Fabry-Pérot (F-P) resonator formed by the alcohol-filled layer combined with the silica cladding in the cross-section of the PCF. The proposed structure exhibits periodic lossy dips in the transmission spectrum, of which the visibilities are sensitive to the refractive index of surrounding medium due to the reflectivity variation of the F-P resonator. Water level sensing is experimentally realized with this principle and the lossy dip exhibits a linear decrease against water level with a sensitivity of 1.1 dB/mm. The sensor is also sensitive to environmental temperature and a temperature sensitivity of -0.48 nm/°C is obtained between room temperature and 60 °C.

Fiber in-line Mach-Zehnder interferometer based on near-elliptical core photonic crystal fiber for temperature and strain sensing

A fiber in-line Mach-Zehnder interferometer is fabricated by selectively filling liquid into one air hole of the innermost layer of a photonic crystal fiber (PCF). The refractive index of the liquid is so close to that of the background silica in the wavelength range of 1300-1600 nm that the two-mode PCF evolves into multimode PCF with an elliptically shaped core. Due to the different propagation constants, interference can occur between the fundamental mode and higher-order modes of the liquid-filled PCF. Such a device is applied in temperature and strain measurements with high sensitivities of 16.49 nm/°C and -14.595 nm/N, respectively.

Tapered fiber Mach-Zehnder interferometers for vibration and elasticity sensing applications

We demonstrate the optical measurements of heart-beat pulse rate and also elasticity of a polymeric tube, using a tapered fiber Mach-Zehnder interferometer. This device has two abrupt tapers in the Er/Yb codoped fiber and thus fractional amount of core mode is converted into cladding modes at the first abrupt taper. The core and cladding modes propagate through different optical paths and meet again at the second abrupt taper to produce interferences. The mechanical vibration signals generated by the blood vessels and by an inflated polymeric tube can perturb the optical paths of resonant modes to move around the resonant wavelengths. Thus, the cw laser signal is modulated to become pulses to reflect the heart-beat pulse rate and the elasticity of a polymeric tube, respectively.

In-fiber quasi-Michelson interferometer with a core-cladding-mode fiber end-face mirror

An in-fiber quasi-Michelson interferometer working on reflection is proposed and experimentally demonstrated. The device consists of a short section of multimode fiber (MMF) followed by a single-mode fiber (SMF) whose end face is terminated by a thick silver film. The MMF excites cladding modes into downstream SMF via the mismatched-core splicing interface. The core-cladding modes are reflected back by the silver film and recoupled to the core of lead-in SMF through the MMF. A well-defined interference pattern is obtained as the result of core-cladding mode interference. A configuration with a 40 mm pigtail SMF at a wavelength of 1528 nm exhibits a water level sensitivity of -49.8 pm/mm and a liquid refractive index sensitivity of -574.6 (pm/mm)/RIU (refractive index unit). In addition, the selected dip provides a considered temperature sensitivity of -61.26 pm/°C and a high displacement sensitivity of -1018.6 pm/mm.

Vibration sensing using a tapered bend-insensitive fiber based Mach-Zehnder interferometer

In this study, a novel fiber-optic sensor consisting of a tapered bend-insensitive fiber based Mach-Zehnder interferometer is presented to realize damped and continuous vibration measurement. The double cladding structure and the central coating region of the in-fiber interferometer ensure an enhanced mechanical strength, reduced external disturbance, and a more uniform spectrum. A damped vibration frequency range of 29-60 Hz as well as continuous vibration disturbances ranging from 1 Hz up to 500 kHz are successfully demonstrated.

High-visibility photonic crystal fiber interferometer as multifunctional sensor

A photonic crystal fiber (PCF) interferometer that exhibits record fringe contrast (∼40 dB) is demonstrated along with its sensing applications. The device operates in reflection mode and consists of a centimeter-long segment of properly selected PCF fusion spliced to single mode optical fibers. Two identical collapsed zones in the PCF combined with its modal properties allow high-visibility interference patterns. The interferometer is suitable for refractometric and liquid level sensing. The measuring refractive index range goes from 1.33 to 1.43 and the maximum resolution is ∼1.6 × 10⁻⁵.

Polarization-dependent curvature sensor based on an in-fiber Mach-Zehnder interferometer with a difference arithmetic demodulation method

A curvature sensor based on a polarization-dependent in-fiber Mach-Zehnder interferometer (MZI) is proposed. The MZI is fabricated by core-offset fusion splicing one section of polarization maintaining fiber (PMF) between two single mode fibers (SMFs). Two independent interference patterns corresponding to the two orthogonal polarization modes for the PMF are obtained. The couple efficiency between the core mode and the cladding mode decreased with the increasing of the bending on the MZI part. The curvature variation on the MZI part can be obtained by detecting the fringe visibility of the interference patterns. A difference arithmetic demodulation method is used to reduce the effects of the light source power fluctuations and temperature cross-sensitivity. Experimental results show that maximal sensitivity of -0.882 dB/m(-1) is obtained under a measurement range of 0.1 to 0.35 m(-1) for the curvature sensor. With the use of difference arithmetic demodulation method, the temperature-curvature cross-sensitivity and light source power fluctuations effects on the proposed sensor are decreased by 94% and 91%, respectively.