In-fiber Modal Interferometer Research Articles

Low Refractive-Index and Temperature Crosstalk Fiber-Optic Liquid Level Sensor Based on Cascaded Quasi-Microfiber and Double-Cladding Fiber Structure

In this paper, a novel in-fiber modal interferometer for liquid level sensing is proposed and experimentally demonstrated based on the cascaded quasi-microfiber and double-cladding fiber (QM-DCF) structure. The QM is utilized to monitor the change of liquid level with intensity modulation and the part of DCF is always immersed into the liquid and serves to sense the variation of ambient temperature. Then the comprehensive tests are performed and the experimental results show that, due to the massive loss of evanescent field in the area of taper waist, a competitive liquid-level response up to -0.972 dB/mm with high linearity is gained and the calculated detection limit reaches ~ 10μm. Moreover, this cascaded structure is almost immune to the varied refractive index (RI) of liquid especially in the range of 1.33-1.357 RIU and its temperature sensitivity is 0.164 nm/°C but with a fluctuation of <; 0.1 dB. Thus, high intensity-sensitive liquid-level measurement is achieved with near-zero crosstalk in terms of RI and temperature. Besides our sensor with the merits of practicality, low-cost, ease of fabrication is very promising and potential for simultaneous high-discrimination measurement of multi-parameter.

Dual-Resonance-Coupling and Sensing Characteristics in an All-Solid Photonic Bandgap Fiber Interferometer Based on LP01 Cladding Supermodes

A dual-resonance-coupling phenomenon and the sensing characteristics of an in-fiber modal interferometer based on cascaded long period gratings inscribed in an all-solid photonic bandgap fiber are demonstrated. Theoretical and experimental investigations reveal that the dual-resonance dips result from the coupling between fundamental core mode and two different kinds of LP01 cladding supermodes. Due to the deep grooves and asymmetric index distribution along the long period grating caused by side illumination, this dual-resonance interferometer is sensitive to polarization and twist, and the dual-resonance dips according to different LP01 cladding supermodes exhibit different responses to twist. Furthermore, the transmission loss and the resonance wavelength of each interference dip show discrimination in sensitivities of temperature and twist, making it a good candidate for multiple physics parameters measurements.

All-Fiber Laser Curvature Sensor Using an In-Fiber Modal Interferometer Based on a Double Clad Fiber and a Multimode Fiber Structure.

An all-fiber curvature laser sensor by using a novel modal interference in-fiber structure is proposed and experimentally demonstrated. The in-fiber device, fabricated by fusion splicing of multimode fiber and double-clad fiber segments, is used as wavelength filter as well as the sensing element. By including a multimode fiber in an ordinary modal interference structure based on a double-clad fiber, the fringe visibility of the filter transmission spectrum is significantly increased. By using the modal interferometer as a curvature sensitive wavelength filter within a ring cavity erbium-doped fiber laser, the spectral quality factor Q is considerably increased. The results demonstrate the reliability of the proposed curvature laser sensor with advantages of robustness, ease of fabrication, low cost, repeatability on the fabrication process and simple operation.

In-fiber modal interferometer for simultaneous measurement of curvature and temperature based on hollow core fiber

An in-fiber modal interferometer is presented and experimentally demonstrated for simultaneous measurement of curvature and temperature. The sensing part is fabricated by splicing single mode fiber (SMF) and hollow core fiber (HCF) via two abrupt tapered joints. Light couples to the wall of HCF due to the collapse in abrupt taper region and then modal interference occurs among multiple modes. Not only intensity but also wavelength of the dip around 1556nm linearly responses to the curvature and temperature change thus a sensitivity matrix could be built to demodulate both curvature and temperature simultaneously. In addition, the interference dip around 1540nm performs a decrease trend and the highest curvature sensitivity of 5.05dB/m−1 is achieved in a wide range from 0.765m−1 to 3.423m−1.

Hollow-Core Microstructured Optical Fiber Gas Sensors

Recent progress in gas detection with hollow-core microstructured optical fibers (HC-MOFs) and direct absorption/photothermal interferometry spectroscopy are reported. For direct-absorption sensors, the issue of mode interference noise is addressed and techniques to minimize such a noise are experimentally demonstrated. Large-scale drilling of hundreds of low-loss micro-channels along a single HC-MOF is performed, and reduction of diffusion-limited response time from hours to ∼40 s is demonstrated with a 2.3-m-long HC-MOF. For photothermal inteferometry sensors, novel detection configurations based on respectively a Sagnac interferometer and an in-fiber modal interferometer are experimentally demonstrated. The Sagnac configuration avoids the need for complex servo-control for interferometer stabilization while the in-fiber configuration simplifies the detection, reducing the size and cost of the sensor system. Sub ppm gas detection can be achieved easily with photothermal interferometry HC-MOF sensors but is difficult to achieve for direct-absorption sensors with the current commercial HC-MOFs.

Experimental demonstration of fractional order differentiation using a long-period grating-based in-fiber modal interferometer

In this work we demonstrate both, experimentally and theoretically, that a long-period grating-based in-fiber modal interferometer can perform an all-optical arbitrary-order fractional differentiation. Experimentally, we fractionally differentiated to the 0.5th order a secant hyperbolic-like pulse of 23ps time width provided by a 1039.5nm emission wavelength modelocked fiber laser, with a chirp parameter of −30. An analytical expression relating the fractional order of differentiation n with the characteristics of the modal interferometer was also derived, with the purpose to simplify the design procedure. The proposal was corroborated also numerically. This device may find applications in real time phase recovery.

In-Fiber Modal Interferometer for Simultaneous Measurement of Refractive Index and Temperature

A compact and simple fiber in-line modal interferometer with an embedded fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of refractive index and temperature. The in-fiber modal interferometer is formed by cascading two offset-splicing joints in a section of fiber with an inscribed FBG. The interference pattern of the modal interferometer shifts with the ambient refractive index and temperature variation, while the Bragg wavelength of the FBG is only sensitive to temperature, so the proposed device can solve the cross-sensitivity of refractive index and temperature and measure dual parameters simultaneously. The refractive index and temperature sensitivities of −26.22 nm/RI and 46.5 pm/°C are obtained, respectively. The sensor is simple and compact, and shows good prospect in the application fields of chemical sensing, biochemical sensing, and environment monitoring for simultaneous measurement of refractive index and temperature.

In-fiber modal interferometer formed by offset-splicing in air-core photonic bandgap fiber

We present a compact in-fiber Mach–Zehnder interferometer (MZI) formed by an offset splicing technique in air-core photonic bandgap fiber (PBF). The MZI is implemented by using two cascaded offset-splicing joints which act as beam splitter and combiner. The influences of lateral offset amount and interferometer length on the performance of MZIs were investigated experimentally. Several in-fiber MZIs with different interferometer lengths were built and it is found that the interference dip wavelength spacing is inversely proportional to the interferometer length. The MZI relies on the interference occurring between the fundamental core mode (FCM) and a surface mode (SM) of the PBF. The potential applications of the proposed in-fiber MZI were further investigated as temperature and strain sensors. The temperature and longitudinal strain sensitivities of the MZI were measured to be 211.89pm/(°Cm) and −0.6pm/με, respectively.

In-Fiber Modal Interferometer Based on Coaxial Dual-Waveguide Fiber for Temperature Sensing

An in-fiber modal interferometer based on coaxial dual-waveguide optical fiber was demonstrated for temperature sensing. The modal interferometer is composed of a coaxial fiber, which is sandwiched in between two single mode fibers with small lateral core-offsets. Such a modal interferometer is insensitive to external refractive index (RI) and can work when the surrounding RI is larger than the RI of the fiber cladding. It has high temperature sensitivity of 78 pm/°C and low cross-sensitivity to strain of 0.004 °C/μe.

Volume Strain Sensor Based on Spectra Analysis of In-Fiber Modal Interferometer

An optical in-fiber modal interferometer-based volume strain sensor for earthquake prediction is proposed and experimentally demonstrated. The sensing element is formed by wrapping a multimode-singlemode-multimode fiber structure onto a polyurethane hollow column. Due to the modal interference between the excited guided modes in the fiber, strong interference pattern could be observed in the transmission spectrum. Theoretical analysis verifies that the resonant wavelength shifts as a result of the volume strain variation caused by the column deformation with square root relationship. Sensitivity > 3.93 p, / με within the volume strain ranging from 0 to 1300 με is also experimentally demonstrated. By taking the response of bidirectional change of volume strain and the sluggish character of the employed sensing material into consideration, the sensing system presents good repeatability and stability.

Magnetic Field Measurements Based on Terfenol Coated Photonic Crystal Fibers

A magnetic field sensor based on the integration of a high birefringence photonic crystal fiber and a composite material made of Terfenol particles and an epoxy resin is proposed. An in-fiber modal interferometer is assembled by evenly exciting both eigenemodes of the HiBi fiber. Changes in the cavity length as well as the effective refractive index are induced by exposing the sensor head to magnetic fields. The magnetic field sensor has a sensitivity of 0.006 (nm/mT) over a range from 0 to 300 mT with a resolution about ±1 mT. A fiber Bragg grating magnetic field sensor is also fabricated and employed to characterize the response of Terfenol composite to the magnetic field.

Long period grating assistant photonic crystal fiber modal interferometer

A novel in-fiber modal interferometer based on a long period grating (LPG) inscribed in a two-mode all-solid photonic bandgap fiber (AS-PBGF) is presented. After inserting a small piece of the AS-PBGF into two sections of standard single-mode fiber (SMF) via being spliced slight core offset, LPG is inscribed in the AS-PBGF. The LPG is especially designed to realize the coupling between two core modes of LP01 and LP11 in the AS-PBGF. Two core modes LP01 and LP11 of the AS-PBGF are excited firstly at the input spliced point and actualized energy exchange when they pass through the LPG. Then the two beams will interfere at the output spliced point to form a high-contrast in-fiber modal interferometer. The proposed interferometer has some advantages such as configuration compact, high interference contrast and the wavelength spacing well controlled by changing the position of the LPG without changing the total length of AS-PBGF.

In-fiber modal interferometer based on dual-concentric-core photonic crystal fiber and its strain, temperature and refractive index characteristics

An in-fiber modal interferometer is demonstrated by splicing a section of homemade dual-concentric-core photonic crystal fiber (DCCPCF) and two segments of single-mode fibers (SMFs) with collapsing the air holes in the splice regions. By analyzing the transmission spectra and the spatial frequency spectra of the interferometers, it's observed that the main interfering modes are the high-order ring modes, which are different from the previously reported interferometers using the single-, few- or multi-mode fibers. The influences of the lengths of DCCPCF and the first collapsed region on the interferometers were investigated experimentally. The strain, temperature and refractive index characteristics of the interferometer were analyzed.

In-fiber modal Mach-Zehnder interferometer based on the locally post-processed core of a photonic crystal fiber

We demonstrate a novel, compact and low-loss photonic crystal fiber modal Mach-Zehnder interferometer with potential applications to sensing and WDM telecommunications. By selectively collapsing a ~1-mm-long section of a hole next to the solid core, a pair of modes of the post-processed structure are excited and interfere at its exit. A modulation depth of up to ~13 dB and an insertion loss as low as 2.8 dB were achieved. A temperature sensitivity of -53.4 pm/°C was measured, making the device suitable for temperature sensing.

基于薄芯光纤模态干涉技术的折射率传感特性实验研究

A refractive-index sensor with micro-structured defect was proposed and demonstrated,and its refractive-index characteristics were studied experimentally.After inserting a segment of thin-core fiber into a standard single-mode fiber,high-order cladding modes were excited and interfered with the core mode to form an in-fiber modal interferometer because of the mismatch between thin-core fiber and standard single-mode fiber.By decreasing of the cladding-diameter of the thin-core fiber,the evanescent field of the cladding modes was enhanced,which improved the sensitivity of refractive-index.Experimental results show that the proposed device has advantages of low loss,low cost,high sensitivity and good linearity.

Hydrostatic pressure sensing with high birefringence photonic crystal fibers.

The effect of hydrostatic pressure on the waveguiding properties of high birefringence photonic crystal fibers (HiBi PCF) is evaluated both numerically and experimentally. A fiber design presenting form birefringence induced by two enlarged holes in the innermost ring defining the fiber core is investigated. Numerical results show that modal sensitivity to the applied pressure depends on the diameters of the holes, and can be tailored by independently varying the sizes of the large or small holes. Numerical and experimental results are compared showing excellent agreement. A hydrostatic pressure sensor is proposed and demonstrated using an in-fiber modal interferometer where the two orthogonally polarized modes of a HiBi PCF generate fringes over the optical spectrum of a broad band source. From the analysis of experimental results, it is concluded that, in principle, an operating limit of 92 MPa in pressure could be achieved with 0.0003% of full scale resolution.

Fiber-Optic High-Temperature Sensor Based on Thin-Core Fiber Modal Interferometer

We present a new fiber-optic high-temperature sensor based on a thin-core fiber (TCF) modal interferometer. A thin-core fiber, whose core radius is about half of the radius of a standard single-mode fiber (SMF), is inserted between standard SMFs to form an extremely simple in-fiber modal interferometer. The wavelength of the transmission dip increases linearly with the temperature. Experimental demonstration shows that it can be used to sense temperature up to 850 °C with a sensitivity of about 18.3 pm/ °C.

Low-cost high-performance fiber-optic pH sensor based on thin-core fiber modal interferometer

A new fiber-optic pH sensor based on a thin-core fiber modal interferometer with electrostatic self-assembled nanocoating is presented. After inserting a segment of thin-core fiber into a standard single-mode fiber, high-order cladding modes are excited and interfere with the core mode to form an in-fiber modal interferometer. The side surface of the sensor is then deposited with poly(allylamine hydrochloride) and poly(acrylic acid) nanocoating by electrostatic self-assembly technique. A fast and linear response is obtained in either acid or alkali solution (in the pH range 2.5 to 10) with resolution of 0.013 pH unit.

Optical flowmeter using a modal interferometer based on a single nonadiabatic fiber taper

A novel in-fiber modal interferometer is presented that is based on a nonadiabatic biconical fused taper that couples light between the cladding and the core, combined with the Fresnel reflection at the fiber end. It is observed that the returned light from this fiber structure shows a channeled spectrum similar to that of a two-wave Michelson interferometer. The application of this device as a fiber optic flowmeter sensor is demonstrated.