High-sensitive Mach-Zehnder interferometric temperature fiber-optic sensor based on core-offset splicing technique

This paper provides the design, fabrication, and experimental demonstration of a novel, compact, and simple-to-construct optical fiber-tip sensor for the simultaneous measurement of the refractive index (RI), temperature, and level of liquid samples by means of surface plasmon resonance (SPR) phenomenon. The proposed fiber-tip SPR sensor consists of a multimode fiber spliced to a sensing element, it comprises a gold-coated single-mode fiber tip, whose end face is coated with a metallic mirror. By covering part of the sensing element with a thermo-responsive material, whose thermo-optic coefficient is high, it is possible to measure temperature; whereas the metal-bare section is used to measure the changes in the RI and level of a liquid sample. As the RI and temperature of the liquid sample increase, the position of the resonance wavelengths ( ${\boldsymbol {\lambda }_{\boldsymbol {bare}}}$ and ${\boldsymbol {\lambda }_{\boldsymbol {Covered}}}$ ) shifts toward larger and shorter wavelengths, respectively. On the other hand, as the level of the sensed sample increases, the deepness of ${\boldsymbol {\lambda }_{\boldsymbol {bare}}}$ increases too, but the position of ${\boldsymbol {\lambda }_{\boldsymbol {bare}}}$ and ${\boldsymbol {\lambda }_{\boldsymbol {Covered}}}$ remains constant for a particular liquid and temperature. We expect that this novel sensor becoming a promising candidate for biomedical, food, and/or industrial applications.

SPR Fiber Tip Sensor for the Simultaneous Measurement of Refractive Index, Temperature, and Level of a Liquid

A novel high sensitivity temperature sensor based on polydimethylsiloxane (PDMS) coated the Multi-Thin-Multi mode Fiber was presented. The reliable material PDMS with higher negative thermal optical coefficient and thermal expansion coefficient was selected to improve the sensitivity of the optical fiber’s temperature sensing. A section of thin-mode fiber was connected in the middle of two multi-mode fibers, so as to form the Multi-Thin-Multi mode Fiber structure, which can make the light in the cladding better interact with the external PDMS. For the two dips of the interference spectra, the temperature sensitivities of the original fiber sensor were 47.14 pm/°C and 0.093 dB/°C. After the sensing part of the fiber was coated, its temperature sensitivities were become as 75.04 pm/°C and 0.231 dB/°C, which 1.6 and 2.5 times improved, respectively. In the temperature stability experiment, a standard deviation of the wavelength variation was established to demonstrate the good stability of the sensor.

The Polydimethylsiloxane Coated Fiber Optic for All Fiber Temperature Sensing Based on the Multithin–Multifiber Structure

Healthcare and biosensing have attracted wide attention worldwide, with the development of chip integration technology in recent decades. In terms of compact sensor design with high performance and high accuracy, photonic crystal structures based on Fano resonance offer superior solutions. Here, we design a photonic crystal structure for sensing applications by proposing modeling for a three-cavity-coupling system and derive the transmission expression based on temporal coupled-mode theory (TCMT). The correlations between the structural parameters and the transmission are discussed. Ultimately, the geometry, composed of an air mode cavity, a dielectric mode cavity and a cavity of wide linewidth, is proved to be feasible for simultaneous sensing of refractive index (RI) and temperature (T). For the air mode cavity, the RI and T sensitivities are 523 nm/RIU and 2.5 pm/K, respectively. For the dielectric mode cavity, the RI and T sensitivities are 145 nm/RIU and 60.0 pm/K, respectively. The total footprint of the geometry is only 14 × 2.6 (length × width) µm2. Moreover, the deviation ratios of the proposed sensor are approximately 0.6% and 0.4% for RI and T, respectively. Compared with the researches lately published, the sensor exhibits compact footprint and high accuracy. Therefore, we believe the proposed sensor will contribute to the future compact lab-on-chip detection system design.

Simultaneous sensing of refractive index and temperature based on a three-cavity-coupling photonic crystal sensor.

We propose and experimentally demonstrate a spurious level and phase noise improved Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) based on a self-injection-locking (SIL) technique. The scheme applies a dual-loop FDML-OEO structure, in which a long optical fiber delay loop is used to injection-lock the OEO with a short oscillating optical fiber delay loop. SIL is achieved so long as the delay of the long loop is tuned at the integral multiple of the oscillation loop. The spur suppression ratio of the wideband linear frequency modulated (LFM) signal generated by the FDML-OEO can be improved by 14 dB under SIL. Furthermore, the modification of the spur suppression ratio depending on the injection power is also demonstrated. The phase noise of the proposed OEO is -127.5 dBc/Hz at 10 kHz offset, which is much improved comparing with a free-running OEO.

Spurious level and phase noise improved Fourier domain mode-locked optoelectronic oscillator based on a self-injection-locking technique.

A novel differential intensity-measurement high-sensitivity refractive index (RI) sensor based on cascaded dual-wavelength fiber laser and single-mode-no-core-hollow-core-no-core-single-mode (SNHNS) structure is proposed and demonstrated. The sensing unit consists of one uniform fiber Bragg grating (FBG) and an SNHNS structure as all-fiber interferometer filter. The dual-wavelength fiber laser has a ring cavity composed of two FBGs with central wavelengths of 1550.10nm and 1553.61nm. Through monitoring the wavelength shift and the output power difference of the dual-wavelength fiber laser, the simultaneous measurement for RI and temperature is realized. In our experiment, the proposed fiber laser sensor exhibits high RI sensitivities of −193.1dB/RIU and 174.8dB/RIU in the range of 1.334-1.384. The relative variation of output power at the two FBG wavelengths shows a higher RI sensitivity of −367.9dB/RIU with better stability, which is greater than the traditional modal interferometer structure. Meanwhile, the temperature sensitivity of the proposed sensor is 8.53 × 10−3nm/°C, and the changes of laser output power caused by temperature are −0.223dB/°C and 0.215dB/°C.

High-sensitivity refractive index and temperature sensor based on cascaded dual-wavelength fiber laser and SNHNS interferometer.

We report a dual-wavelength fiber laser sensor based on a uniform fiber Bragg grating (UFBG) and a Polyvinyl alcohol (PVA) film-coated long-period grating (LPG) as sensor probe for simultaneous detection of relative humidity (RH) and temperature. Two UFBGs and the LPG were inscribed in three single mode fibers utilizing phase/amplitude mask technology and mobile scanning technique via the 248 nm UV laser. In our experiment, the RH function based on the differential intensity measurement at dual-wavelength output from 30% to 85% is in a quadratic equation, and the RH sensitivity coefficient of 0.358 dB/% shows good linear relationship and stability from 55% to 85%. Meanwhile, the temperature sensitivity coefficients based on wavelength detection and differential intensity detection are 9.1 pm/°C and 0.21 dB/°C, respectively. The structure of dual-wavelength fiber laser sensor with high signal to noise ratio (SNR), narrow spectral width and good stability enables simultaneous measurement of the RH and temperature with high accuracy and good repeatability.

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Wavelength- and Intensity-Demodulated Dual-Wavelength Fiber Laser Sensor for Simultaneous RH and Temperature Detection

The multiobject fiber-fed spectrograph is the core scientific instrument for large-scale spectroscopic surveys. For closed-loop control of fiber positioning, fiber metrology systems are implemented in numerous fiber-fed spectrographs. The position accuracy of the fiducial fiber in the focal plate directly affects the performance of the fiber metrology system. However, there are currently no suitable methods and devices for measuring the fiducial fibers with high accuracy in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). To solve this problem, this study proposed a novel online scanning measurement method for fiducial fibers in which a scanning camera was set up in front of the focal plate, and the rotation and translation movements of the focal plate were combined to set a polar coordinate measurement system. First, the pole and polar axis of the polar coordinate frame were determined, and the compensation values of the polar radius and angle of the polar coordinate of the fiducial fibers were solved in the field of view of the scanning camera. A prototype measurement platform was set up to verify the feasibility and scientific validity of the method. Experiment results show that the polar radius accuracy of the proposed method met the measurement requirements. The X–Y measurement accuracy can be further improved when a higher-precision rotary stage is adopted. Thus, the difficulties in online accurate measurement for fiducial fibers can be tackled by the proposed method with good operability in LAMOST.

Wenqi Liu

Papers

High-sensitivity refractive index and temperature sensor based on cascaded dual-wavelength fiber laser and SNHNS interferometer.

Wavelength- and Intensity-Demodulated Dual-Wavelength Fiber Laser Sensor for Simultaneous RH and Temperature Detection

Novel Fiducial Fiber Scanning Measurement Method Based on a Polar Coordinate System for LAMOST