Fiber based optical coherence tomography (OCT), which is combined by OCT and fiber probe, presents advantages of high longitudinal resolution, flexibility and miniaturization structure. However, the depth of focus (DOF) and transverse resolution depended on OCT fiber probes are two constraining parameters. The realization of high transverse resolution imaging could result in the reduction in the effective imaging range. To address this issue, a focal length controllable fiber probe based on microcavity structure is proposed in this paper. A segment of alcohol-filled silica capillary is inserted between a single-mode fiber (SMF) and a section of gradient-index (GRIN) fiber and works as a beam expander. By adjusting the surrounding temperature, the expanding condition changes due to the alcohol refractive index is different, eventually the focal length of the probe can be tuned. Experimental results prove that the focal length can be tuned freely from ~1835 μm to ~1650 μm while the temperature of the alcohol is changed from 20°C to 60°C.
A Mach-Zehnder interferometer (MZI) based on core-offset splicing technique for sensing applications is demonstrated. The interferometer is composed of a section of single-mode fiber sandwiched by two core-offset spliced standard optical fibers. The refractive index (RI) of the external media around the middle section of the MZI could be detected by investigating the peak wavelength of the MZI spectrum. Experimental results show that the proposed sensor exhibits a sensitivity to external refractive index of 17361 nm per refractive index unit and the temperature sensitivity of only 33 pm/°C. The present device has great potential in biochemical and medical sensing due to the advantages including easy fabrication, high sensitivity and excellent compactness.
In this paper, an abnormal grating evolution was recorded during microfiber Bragg grating (mFBG) inscription under 193nm excimer laser. Within 20 minutes exposing, a Type IIa FBG could be obtained with above 20dB strength in 8.5 μm microfiber. This regenerated mFBG had good survival ability against high temperature up to 800 °C. Moreover, the strain response of the regenerated grating was enlarged by the microfiber structure. Thus, highly sensitive strain sensor with considerable temperature resistance could be obtained, which had potential applications in gas/oil and aerospace territory.
We demonstrate an ultrasensitive temperature sensor by sealing a highly-birefringent microfiber into an alcoholinfiltrated copper capillary. With a Sagnac loop configuration, the interferometric spectrum is strongly dependent on the external refractive index (RI) with sensitivity of 36800nm/RIU around RI=1.356. As mainly derived from the ultrahigh RI sensitivity, the temperature response can reach as high as −14.72 nm/°C in the range of 30.9-36.9 °C. The measured response time is ~8s, as determined by the heat-conducting characteristic of the device and the diameter of the copper capillary. Our sensor is featured with low cost, easy fabrication and robustness.
We demonstrated a simple method for temperature-independent refractive index measurement by use of two cascaded fiber Bragg gratings fabricated in single-mode fiber and microfiber, respectively. The reflective peaks of the two FBGs exhibit almost identical temperature sensitivity of 10.1 pm/°C and different responses to ambient refractive index. Based on the differential measurement method, of the issue of temperature cross-sensitivity for FBG sensors is solved. The refractive-index sensitivity of the sensor is 17.22 nm/RIU when the diameter of microfiber is 6.5 μm.
A compact microfiber sensor is implemented with the twist of a continuous rectangular microfiber. The structure can exhibit extremely-high sensitivity of around 24,373nm per refractive-index unit and temperature stability of better than 0.005nm/oC, implying a great suppression of cross-sensitivity. Thia sensor is featured with compact size, high sensitivity, easy fabrication, robustness, and low connection loss with all-fiber system.
Orientation-recognized two-dimensional vibration sensor based on a polarization-controlled cladding-to-core recoupling is demonstrated experimentally. A compact structure in which a short section of multi-mode fiber stub containing a weakly tilted fiber Bragg grating (TFBG) is spliced to another single-mode fiber without any lateral offset. Several well defined lower-order cladding resonances in reflection show different polarization dependence due to the tilted grating vector excitation. Both orientation and amplitude of the vibration can be determined unambiguously via dual-path power detection of the orthogonal-polarimetric odd-cladding-modes. Meanwhile, the unwanted power fluctuations and temperature perturbations can be definitely removed via core mode monitoring.
In this paper, temperature compensated microfiber Bragg grating (mFBG) is realized by use of a liquid with a negative
thermo-optic coefficient. The effects of grating elongation and the index change of silica glass are compensated by the
liquid through evanescent-field interaction. As a result, the reflective wavelength shifts by only 30 pm when the
temperature varies from 15 to 60°C. The proposed method is promising due to the compactness and high flexibility of
the device.
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