Temperature-insensitive refractive index sensing by use of micro Fabry–Pérot cavity based on simplified hollow-core photonic crystal fiber

Abstract

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 ...

Photonic Crystal Fiber–Based Interferometric Sensors

“…A RI sensor was realized by drilling micro-holes into a simplified hollow core (SHC) PCF micro cavity using a femtosecond laser to allow the analyte to enter the cavity [10]. A RI sensitivity of 851 nm/RIU and a low cross sensitivity with temperature of~3.2 Â 10 À7 RIU/ C was obtained.…”

Pressure, temperature and refractive index determination of fluids using a single fibre optic point sensor

“…For instance, to determine temperature and RI simultaneously several different systems such as a partially cone-shaped Fibre Bragg Grating (FBG) [7], an in-line Mach-Zehnder Interferometer (MZI) embedded in a FBG [8], a core-offset MZI combined with a FBG [9] or a Long Period Grating (LPG) based MZI [10] have been reported. Also a fibre optic sensor based on a SU-8 sensor tip [11], a dual-cavity Fabry-Perot Interferometer (FPI) [12][13] or a temperature-insensitive FPI relying on a photonic-crystal fibre [14] have been proposed. To measure both pressure and RI a combination of an open path and pressure sensitive cavity FPI has been described [15].…”

Compressible fiber optic micro-Fabry-Pérot cavity with ultra-high pressure sensitivity

“…Various types of fiber optic pressure sensors have been demonstrated to fulfill specific pressure applications, including fiber Bragg gratings (FBGs) [1], photonic crystal fiber (PCF) devices [2] and interferometers [3,4], and the pressure sensitivity achieved is typically on the order of −10 pm/Mpa. Among above mentioned configurations, the pressure sensors that are based on Fabry-Pérot (FP) interferometer have shown promising results for static and dynamic pressure measurements [5,6], as well as other physical and chemical parameter detections such as temperature [7], strain [8,9] and refractive index [10]. The FP interferometers can achieve extremely high sensitivity and flexibly demodulated in the wavelength domain [3,4] and frequency domain [6,8], or simply use light intensity detection [11].…”