High-sensitivity Mach-Zehnder interferometer for temperature and strain measurements based on an embedded microcavity

A refractive index insensitive temperature sensor based on coaxial dual-waveguide optical fiber was proposed and demonstrated. The coaxial fiber contains a central core along the fiber axis and an annular core between the inner/outer claddings. By inserting the coaxial fiber in between two single mode fibers through core-offset splicing, cladding modes are excited at the splice point and therefore a modal Mach-Zehnder interferometer is achieved. The effective refractive index of the inner cladding mode is independent of the external refractive index due to the existence of the annular core. Owing to the large thermo-optic coefficient difference between the coaxial fiber's core and cladding, the modal interferometer has high temperature sensitivity. Such an interferometer is extremely suitable for temperature measurement in wet or liquid environment.

A novel refractive-index (RI) Michelson interferometer based on a waist-enlarged taper is achieved. Such a device is fabricated by splicing a section of multimode fiber (MMF) at one end of single-mode fiber (SMF). Due to the fiber bitaper at the coupling point of SMF and MMF, the light is coupled into the MMF from lead in fiber core, and the intermodal interference will occur for the optical path difference between core mode and cladding mode. Then, the light will be reflected at the end of the fiber and recoupled back into the lead out fiber core by the fiber bitaper. When the lights return back to the lead out fiber, the intermodal interference will occur for the optical path difference between core mode and high order mode. The sensor has a linear response to RI with a sensitivity of −178.424 dB/RIU in the range of 1.351–1.4027 RIU. The temperature cross sensitivity is also analyzed, and the influence can be eliminated by simultaneous measurement of RI and temperature through the matrix equation. The proposed sensor features the advantages of easy fabrication, low cost, high mechanical strength, which make it a good candidate for bio-chemical measurement.

Multi-modes interferometer for magnetic field and temperature measurement using Photonic crystal fiber filled with magnetic fluid

An optical fiber sensor for the simultaneous measurement of microdisplacement and temperature based on balloon-shaped single-mode fibers cascaded with a fiber Bragg grating with two core-offset joints is proposed. The interference between the core mode and cladding mode is caused by the stimulation of the cladding mode by the core-offset joints' structure. The cladding of the core has a distinct refractive index, which causes optical path differences and interference. The balloon-shaped structure realizes mode selection by bending. As the displacement increases, the radius of the balloon-shaped interferometer changes, resulting in a change in the interference fringes of the interferometer, while the Bragg wavelength of the fiber grating remains unchanged. Temperature changes will cause the interference fringes of the interferometer and the Bragg wavelength of the fiber grating to shift. The proposed optical fiber sensor allows for the simultaneous measurement of microdisplacement and temperature. The results of the experiment indicate that the sensitivity of the interferometer to microdisplacement is 0.306 nm/µm in the sensing range of 0 to 200 μm and that the temperature sensitivity is 0.165 nm/°C, respectively. The proposed curvature sensor has the advantages of a compact structure, extensive spectrum of dynamic measurement, high sensitivity, and simple preparation, and has a wide range of potential applications in the fields of structural safety monitoring, aviation industry, and resource exploration.

Simultaneous measurement of displacement and temperature based on thin-core fiber modal interferometer

We report the formation of inner cladding modes in the optical fiber having an inner cladding structure. The inner cladding layer located between the core- and the cladding- layers of a conventional fiber might have, so called, inner cladding mode(s). The brief history of the inner cladding fiber and the spectral properties of the inner cladding mode are presented. By utilizing fiber gratings, the spectral properties of the inner cladding mode formed in Dispersion Compensating Fiber (DCF) are discussed. It was observed that one resonant peak of a long-period fiber grating was not sensitive to the variation on the cladding surface. With a fiber Bragg grating, a small group of unusual resonant Peaks was observed between the main Bragg Peak and the series of usual Peaks resulted from the mode coupling to counter-propagating cladding modes. Within the DCF by using fiber gratings, it is noted, at least one mode can be coupled to the inner cladding mode and a few outer cladding modes are severely affected by the inner cladding of the fiber.

A new type of evanescent field sensors based on a doubly cladding single mode fiber is proposed. The theoretical analysis shows that the measurable refractive index (RI) ranges for the sensors are limited in the single mode in the fiber becoming leaky and the range is less than the RI difference between the core and the cladding of the fibers. Two temperature sensors and a humidity sensor were fabricated. One temperature sensor showed a good linear response in a range of 60°C, the modulation depth of 40 dB, and the insertion loss of -1.4 dB. The humidity sensors fabricated by agarose gel had a measurable moisture range of 16 ~ 100% of RH. Its optical power variation with humidity is 8.5 dB and insertion loss is 3.9 dB. We confirmed that the DC single mode fiber sensors became a new type of intensity sensors characterized in good linear response, low insertion loss, high modulation depth, and low cost in fabrication.

We propose a novel all fiber Mach–Zehnder interferometer (MZI) based on photonic crystal fiber (PCF) filled with liquid crystal (LC). The interference between the core mode and the cladding modes of a PCF is utilized. To excite the cladding modes, a region is formed using fiber fusion splicer. Due to the fact that varying effective index difference between the core region and the LC-filled cladding region can cause different transmission spectra, we mainly study the MZIs with different LC-filled structures and different lengths of LC filling. The measured results demonstrate that quite clear interference spectra can be obtained. Through analysis spatial frequency spectrum and temperature spectrum of two MZIs with different LC-filled structures, we can obtain that the MZI with adjacent two LC-filled holes has clearer interference spectrum and higher temperature sensitivity. Thus we choose this MZI to measure the temperature sensitivity with different lengths of LC filling. When the length of LC filling is 2 cm, the temperature sensitivities can be enlarged to 1.59 nm/°C. The interferometer shows a good temperature tunability and sensitivity, which can be a good candidate for a highly tunable optical filtering and temperature sensing applications.

Directional torsion and strain discrimination based on Mach-Zehnder interferometer with off-axis twisted deformations

In this paper, we proposed and demonstrated a novel temperature sensor with high sensitivity based on special fibers. It works on the principle of strong resonance between core mode and cladding mode. Through the thin thickness of inner cladding, the cladding mode is strongly excited and the resonant center wavelength is very sensitive to the refractive index variation of coating material. By coating silicone around the special fiber, its temperature sensitivity was investigated experimentally. The results showed a high sensitivity and a good repeatability.

In this paper, a temperature sensor is designed and simulated using an interferometric configuration formed by inserting a photonic crystal fiber (PCF) between two single-mode fibers (SMFs). In this device, phase delay at the output is achieved by the index difference between core mode and cladding mode of PCF. In this type of interferometer, greater index difference is possible because of air-hole structure of PCF. It indicates that a small length of PCF can introduce large delay, i.e., large phase shift while keeping optical loss of the cladding mode at a relatively low level. With increase in temperature, effective refractive index difference between core and cladding of PCF decreases which results in decrease in peak output power of the interferometer. Variation of output power with temperature is observed to be linear; it is thus applicable as a temperature sensor.

A novel sensitivity-enhanced optical fiber sensor has been presented and demonstrated to measure liquid temperature and refractive index (RI) based on the reflective single-mode–multimode–single-mode fiber structure. The sensing part is packaged in a segment of capillary tube by hot glue. Due to the high coefficient of thermal expansion of hot glue, the increasing temperature bends the fiber in the tube. The measurement is achieved via monitoring the spectral shift of the modal interference between the core and the cladding modes in the ended single-mode fiber, which is well carved to provide a mirror interface. The Fresnel reflection occurred in the interface is utilized to measure liquid RI and temperature. The highest sensitivity of −92.6 pm/°C and −67.9 dB/RIU are achieved, respectively. This structure has many advantages, such as high sensitivity, expectation for simultaneous measurement of liquid temperature and RI, convenient operation, and low cost of fabrication.

The thermal dependence of the luminescent properties of Eu3+ doped Y2MoO6 phosphor and its potential application in temperature sensing

A thermally tunable multiwavelength Raman fiber laser based on a simple in-line interferometer is investigated. A novel in-line interferometer is realized by using a single long-period fiber grating incorporating a core misalignment method. Cladding modes from the guided core mode are excited in the splicing point between two optical fibers when two core regions are misaligned. By removing an external buffer coating layer in the fiber, the cladding modes can be propagated and recoupled to the core mode at a long-period fiber grating. Since the long-period fiber grating couples the core mode to the cladding modes, the interaction between the core mode and the cladding modes induces an interference pattern, which realizes a multichannel filter. Since the transmission characteristics of the long-period fiber grating are effectively controlled by changing the applied temperature, a thermally tunable multichannel filter based on the proposed in-line interferometer can be obtained. When the proposed multichannel filter is inserted within the ring cavity of a Raman fiber laser, a multiwavelength Raman fiber laser is realized. A high extinction ratio of more than 40 dB is achieved. The output power is stable, and the power fluctuation is measured to be less than 0.7 dB, and the proposed multiwavelength Raman laser has a high thermal tunablity of -0.48 nm/◦C.

We demonstrate a torsion sensor with rotation direction discrimination based on a pre-twisted core-cladding mode Mach–Zehnder interferometer (MZI) fabricated in single mode fiber by twisting the fiber during CO2 laser irradiation to induce a permanent screw-type distortion at the two points of coupling between the core and cladding. The proposed sensor can measure the twist rate with a sensitivity of up to 10.35 nm/(rad/cm) and determine its rotation direction (i.e., clockwise or counterclockwise) simultaneously. Furthermore, the sensitivity of the torsion sensor can be enhanced by increasing the initial shear strain.