Cladding Mode Resonance Based Fiber for Temperature Measurement

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.

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.

Compared with the fiber Bragg grating (FBG), the tilted fiber grating (TFBG) has the coupling between core mode and cladding mode or the radiation mode because of the tilt angle, which improves the sensitivity to the external environment. B ased on the coupled mode theory and the simulation software OptiGrating , the spectral characterist ics of the TFBG with different tilt angles were studied. When the tilt angle is small, the resonant peaks on the transmission spectrum move in the long wave direction with the increase of the tilt angle. What ’ s more, the transmission loss and transmission width decrease and the group delay decreases. W hen the tilt angle is large enough, the TFBG has strong bifurcation effect, the Bragg resonant peak disappears while the cladding mode resonant peak appears in pairs, and the high order cladding mode birefringence effect is stronger than the low order clad ding mode. In the end of the paper, different angles of sensing applications were analyzed.

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.

Here we report on the simultaneous measurement of bending and temperature, carried out using a long-period fiber grating (LPFG) inscribed on polarization-maintaining fiber (PMF) with a CO₂ laser at ~10.6 μm. An LPFG written on PMF, referred to as a PM-LPFG, has an input-polarizationdependent resonance dip, and two separated resonance dips, designated as Dips A and B, are obtained with respect to orthogonal input polarization. At the resonance wavelengths of Dips A and B, the core mode is coupled into two different cladding modes that have different bending and temperature sensitivities. The fabricated PM-LPFG whose grating period and length are ~505 μm and ~14.65 mm, respectively, has two resonance wavelengths, i.e., λA= ~1479.98 nm and λB = ~1568.78 nm, measured with respect to two orthogonal input polarization states. The bending sensitivities of this PM-LPFG were measured as ~22.23 and ~33.38 nm/m-1 (adjusted R² values: ~0.9916 and ~0.9810) at λA and λB, respectively, in a curvature range of 1.41-2.30 m-1. The temperature sensitivities of the PM-LPFG were measured as ~0.132 and ~0.039 nm/°C (adjusted R² values: ~0.9929 and ~0.9980) at λA and λB, respectively, in a temperature range of 30-90 °C. These linear bending and temperature responses of the PM-LPFG at two different resonance wavelengths enable simultaneous measurement of bending and temperature variations applied to the PM-LPFG.

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.

A novel all-fiber Mach-Zehnder modal interferometer (MZMI) for temperature and strain measurements is demonstrated. The interference between the core and the cladding modes in the single mode fiber (SMF) is utilized. To excite the cladding modes, a microcavity is embedded in the SMF. After propagating a distance in the cladding, the cladding modes will couple back to the core mode at the other point fabricated by core-offset splicing. The MZMI exhibits a high temperature sensitivity (<0.1nm/ °C) in the range of 200-600 (°C). The strain response of the MZMI is also investigated. This compact, simple and cost-effective MZMI owns potential applications in temperature and strain measurements.

Long-period fiber gratings (LPFG’s) find applications in optical fiber communication systems and fiber sensor systems. Among others, it can be used as gain flattening filters (GFF’s) in the communication systems. Depend on the amplifier design, the GFF’s need to be either athermal, or have specific temperature sensitivities. The temperature sensitivity requiement sometimes can be very stringent. It has been known that the temperature and strain sensitivity are dependent on the fiber parameters and the order of the cladding modes it is used. In this paper we will describe the general method for finding suitable cladding mode in a specific fiber for specific requirements. We found that the polarization dependent losses (PDL) in high sensitivity modes are remarkably higher than the ones in common LPFG’s. In those high sensitivity filters achieved by the UV-beam side illuminating, the birefringence-related resonant wavelength separation (RWS), which is the central wavelength separation corresponding the slow and fast axis state of polarizations (SOP), can be in the range of 5 nm, which is remarkably larger than the reported values in other LPFG’s. There are two sources of birefringence which lead to PDL: fiber core ovality induced birefringence, which is intrinsic, and the anisotropic UV-beam exposure induced birefringence. We proposed methods to deal with those birefringence sources. The leads to almost complete remove of the RWS in the high sensitivity LPFG’s.

Tilted fibre Bragg grating (TFBG) with grating planes tilted at an angle of 3° corresponding to the fibre axis shows core mode and a large number of cladding-mode resonances in its transmission. Discriminating temperature and strain simultaneously using a single tilted fibre Bragg grating is proposed and demonstrated experimentally. The wavelength shifts of the TFBG in response to temperature and strain are investigated. The results show that the cladding modes and core mode have different strain sensitivities while the temperature sensitivities are equal. So, it is possible to monitor the core-mode resonance and the cladding-mode resonances of the TFBG spectrum, allowing the separation of the temperature and strain induced wavelength shifts.

Multi-tapered polarization-maintaining fiber-optic sensor for refractive index and temperature measurements

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.

A novel fiber curvature sensor based on Mach–Zehnder interferometer using up-taper cascaded long-period grating is proposed. The variation of the sensor's curvature will change the phase difference of core mode and cladding mode in the sensor, and will finally induce a wavelength shift in the transmission spectrum. By monitoring the wavelength shift, a sensor for bending measurement is achieved. Experimental results show that the curvature sensitivity of the proposed sensor is as high as −12.555 nm/m−1 in the curvature range of 0.706–3.444 m−1. As the curvature increases, the dip wavelength has a blue shift. The curvature resolution of the proposed sensor is about 1.59 × 10−3 m−1. And the temperature effect is also investigated. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:246–248, 2016

A temperature-insensitive fiber bending sensor is demonstrated by using a special optical fiber with cladding-mode resonance. The special fiber with a pure silica core and a fluorine-doped silica inner cladding shows strong cladding-mode resonance. The single-mode fiber (SMF)-special fiber-SMF is proposed as a sensor head to monitor the cladding-mode resonance. The response of the resonant spectrum exhibits high sensitivity to bending curvature and inherent insensitivity to temperature. The proposed special fiber bending sensor is simple and inexpensive.

A novel micro-displacement sensor based on the special fiber cladding mode resonance is proposed and demonstrated experimentally. The special fiber with double cladding was spliced into single mode fiber which presented very strong resonant spectrum. Based on the high sensitivity to fiber curvature, the micro-displacement sensor was researched. As indicated by experimental results, the sensitivity is achieved to be about 0.1 nm/um. With the high sensitivity, the micro-displacement sensor can be found wide applications in fields of structure health monitor, aerospace, MEMS and so on.

By using a finite element method with matched boundary conditions, we analyze an interaction between a central core mode (x- and y- polarizations), which shows as a supermode and a plasmonic mode by involvement of another (lateral) core mode in a biosensor based on a birefringent solid-core microstructured optical fiber. The transmission losses of the lateral core modes are larger than those of the corresponding central core modes for the x- and y- polarization components. A very interesting result is that the maximum value of the amplitude sensitivity for the second (lateral) core (II, x) mode (x- polarization) is at a wavelength close to the resonant wavelength of the first (central) core (I, x) mode. Also, the maximum value of the amplitude sensitivity for the second core (II, y) mode (y- polarization) is at a wavelength that is close to the resonant wavelength of the first core (I, y) mode.