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We have constructed a novel refractive index (RI) sensor based on a fiber optic Fabry-Perot interferometer (FPI) by splicing a section of hollow core fiber between a single-mode fiber and a photonic crystal fiber (PCF). Owing to the air holes in the cladding of the PCF, various substances, such as liquids and gases with different RI, can enter or leave the in-fiber air cavity, which makes the device usable as a refractometer. In this paper, the fiber optic FPI sensor has been used to monitor the RI changes of air with different pressures, and the experimental results show that such a sensor has an RI sensitivity of 805.1 microm/RIU, and hysteresis is not observed. Moreover, the easy fabrication method gives the in-fiber refractometer many potential applications in the sensing field.

A novel in-line all-fibre compact Fabry-Pérot interferometer (FPI) high temperature optical fibre tip sensor formed by fusion splicing two sections of singlemode fibres with a large intentional lateral offset is proposed and demonstrated, for the first time to the authors' knowledge. Simple fabrication steps include only cleaving and fusion splicing. Because no other processes are involved, the fabrication is easy, safe, and cost-effective. The sensor has been tested for high temperature measurements showing a sensitivity of 41 nm/8C with good linearity and repeatability, and is capable of operation at temperatures up to 10008C. It is expected that such a miniature FPI could find wide applications in mechanics, aeronautics, chemical sensing and metallurgy.Introduction: Fibre-optic Fabry-Pérot interferometer (FPI) sensors have been intensively studied in various physical and chemical sensing applications, such as temperature, strain, refractive index (RI), gas phase concentrations, owing to their simple configuration, small size and high resolution [1 -7]. The FPI is formed by two parallel separated mirrors partially reflecting the lead-in optical signals, and the two mirrors could be formed by air fibre end interface, coated film and refractiveindex mismatches between two fibres in the spliced joint, respectively [1][2][3][4][5][6]. For example, the two parallel separated mirrors of the extrinsic FPI and in-line fibre-optic etalon (ILFE) are mainly formed by the interface of air and fibre ends [1][2][3], and those of the intrinsic FPI are mainly formed by internal coated films or refractive-index mismatches between fibres in the spliced joints [4][5][6]. In this Letter, a novel geometry of a FPI based on larger lateral offset splicing induced by two parallel separated mirrors is presented and its application to high temperature sensing is demonstrated. The formation of the sensor is carried out with only two steps and uses only the inexpensive singlemode fibres (SMFs), making it extremely low cost.

We demonstrate low-loss splicing between a photonic crystal fiber (PCF) and a single-mode fiber (SMF) with a conventional electric-arc fusion splicer, where nitrogen gas (N₂) with a proper pressure is pumped into the air holes of the PCF to control the air-hole collapse ratio so as to optimize the mode-field match at the joint. The method is applicable to both solid-core and hollow-core PCFs. With this method, we achieve a splice loss (measured at 1550 nm) of ~0.40 dB for a solid-core PCF and ~1.05 dB for a hollow-core PCF. The method could find wide applications in the fabrication of PCF-based devices.

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