A compact temperature sensor based on a fiber loop mirror (FLM) combined with an alcohol-filled high-birefringence photonic crystal fiber (PCF) is proposed and experimentally demonstrated. The output of the FLM is an interference spectrum with many resonant dips, of which the wavelengths are quite sensitive to the change of the refractive index of the filled alcohol for the interference of the FLM. Simulation analysis predicts a high temperature sensitivity, and experimental results show it reaches up to 6.6 nm/°C for the 6.1-cm-long PCF used in the FLM.

We introduce a novel photonic crystal fiber (PCF) temperature sensor that is based on intensity modulation and liquid ethanol filling of air holes with index-guiding PCF. The mode field, the effective refractive index and the confinement loss of PCF were all found to become highly temperature-dependent when the thermo-optic coefficient of the liquid ethanol used is higher than that of silicon dioxide and this temperature dependence is an increasing function of the d/Lambda ratio and the input wavelength. All the experiments and simulations are discussed in this paper and the temperature sensitivity of transmission power was experimentally determined to be 0.315 dB/ degrees C for a 10-cm long PCF.

A temperature-insensitive micro Fabry-Pérot (FP) cavity based on simplified hollow-core (SHC) photonic crystal fiber (PCF) is demonstrated. Such a device is fabricated by splicing a section of SHC PCF with single mode fibers at both cleaved ends. An extremely low temperature sensitivity of ∼0.273 pm∕°C is obtained between room temperature and 900°C. By drilling vertical micro-channels using a femtosecond laser, the micro FP cavity can be filled with liquids and functions as a sensitive refractometer and the refractive index sensitivity obtained is ∼851.3 nm∕RIU (refractive index unit), which indicates an ultra low temperature cross-sensitivity of ∼3. The FBG based RI sensor generally presents a low sensitivity on the order of ∼100 nm∕RIU (refractive index unit), and the FBG should be fabricated in an exposed-core fiber or a microfiber. It has been reported that LPFG can provide a sensitivity as high as 1500 nm∕RIU [3]. However, LPFG typically exhibits large temperature cross-sensitivity and nonlinear response to the surrounding RI. An ultra-high sensitivity, of up to 24; 373 nm∕RIU, is achieved by use of a highly birefringent microfiber loop [6]. By employing selective infiltration techniques of PCFs [7,12,13], embedded coupler, modal interferometer, and photonic band-gap structures can be fabricated, which exhibit even higher RI sensitivity such as 38;000 nm∕RIU [8]. However, such a sensor can only use the liquids with a RI higher than that of silica (∼1.46). To operate at around 1.33 of RI, a liquid-filled PCF sensor based on four-wave mixing has been demonstrated, with a high sensitivity of 8800 nm∕RIU, however, a large length of PCF (∼1 m) has to be used [11].A key issue that existed in the above mentioned configurations is temperature cross-sensitivity because it limits the sensor reliability. One of the solutions to this issue is the use of fiber-optical FP cavity as it exhibits very low temperature sensitivity of ∼1 pm∕°C, due to the small thermo-expansion coefficient of silica. Recently, the micro FP cavity has received increased research attention because of its low temperature cross-sensitivity, high RI and/or strain sensitivities, and convenient reflection mode of detection. The FP cavity fabricated by focused ion beam milling has been used to measure the RI around 1.30 with a high sensitivity of 1731 nm∕RIU [14] however; the temperature crosssensitivity of the device was not reported. By employing a femtosecond laser, micro FP cavity can be fabricated in single mode fiber (SMF) [15,16] and PCF [17], with a temperature sensitivity of larger than 2 pm∕°C, corresponding to a temperature cross-sensitivity of greater than 2 × 10 −6 RIU∕°C. By splicing a section of hollow-core PCF [18] or Er-doped fiber [19] with SMFs, strain sensors have been demonstrated, with further reduced temperature sensitivity of ∼0.81 and 0.65 pm∕°C, respectively.In this Letter, we demonstrate a micro FP cavity based on simplified hollow-core (SHC)-PCF for RI sensing with extremely low temperature cross-sensitivity. The device is ...

In this paper, a high-temperature fiber sensor based on an optical fiber Fabry-Perot interferometer is fabricated by splicing a section of simplified hollow-core fiber between two single-mode fibers (SMFs) and cleaving one of the two SMFs to a certain length. With the superposition of three beams of light reflected from two splicing joints and end face of the cleaved SMF, the modified Vernier effect will be generated in the proposed structure and improve the sensitivity of temperature measurement. The envelope of spectrum reflected from the proposed sensor head is modulated by the ambient temperature of the sensor head. By monitoring and measuring the shift of spectrum envelope, the measurement of environment temperature is carried out experimentally, and high temperature sensitivity of 1.019 nm= C for the envelope of the reflected spectrum was obtained. A temperature measurement as high as 1050 C has been achieved with excellent repeatability.

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