Refractive index sensing based on photonic crystal fiber interferometer structure with up-tapered joints

Abstract

a b s t r a c tA simple, ultra compact and highly sensitive photonic crystal fiber interferometer (PCFI) for external refractive index (ERI) sensing was proposed and demonstrated in this paper. The PCFI was formed by splicing photonic crystal fiber (PCF) between two single mode fibers (SMFs) with a slight core-offset. The both joints were up-tapered joints which acted as mode splitter/combiner and were made by fusion tapering technique. The Mach-Zehnder interferometer (MZI) incorporated intermodal interference between core mode and cladding modes of the PCF. When the ERI changed, a RI variation of cladding modes would occur and the output interference spectrum would shift. By measuring the wavelength shift of the interference pattern, temperature-insensitive RI measurement could be achieved. In addition, the refractive index sensing properties with the different PCF diameters were also investigated experimentally. Experimental results showed that RI sensitivity could be up to 252 nm/RIU in the refractive index range of 1.333-1.379. And it could be anticipated that RI sensitivity could be improved if the PCF diameter continues to decrease. Meanwhile, the sensor had the advantages of simple structure, small size, high sensitivity, low cost and low temperature sensitivity.

High Sensitive Refractive Index Sensor Based on Cladding Etched Photonic Crystal Fiber Mach-Zehnder Interferometer

“…In the first collapsed region, both the core and cladding modes with different propagation constants are excited. The interference spectrum can be expressed using the following two-beam optical interference equation [14,22]…”

“…A comparison between the proposed MZI and related MZIs based on the splicing technology is listed in Table 1. Table 1 shows that the sensitivities of the proposed sensor have higher RI sensitivity than PCF with up-tapered joints (252 nm/RIU) [14], PCF-MZI with two HTCRS (181.96 nm/RIU) [16] [25]. Because the diameter of the corroded PCF decreases, the interaction between the evanescent wave of cladding mode and the surrounding environment of the fiber is enhanced.…”

“…In recent years, many novel in-line MZIs with the PCF structure have been proposed and studied. Y. Zhao et al [14] proposed a PCF interferometer structure with up-tapered joints for RI sensing, which achieved a high RI sensitivity of 252 nm/RIU in the RI range of 1.333 -1.379. D. Wu et al [15] proposed an RI sensor based on a tapered PCF MZI structure with an RI sensitivity of 51.902 nm/RIU in the range 1.3411 -1.3737.…”

Photonic Crystal Fiber–Based Interferometric Sensors

“…Highly sensitive refractometers have been demonstrated by means of tapering, either at just the splice points [26,27] or the entire PCF section [28] to expose the evanescent field, making the MZI more sensitive to external RI changes. By tapering at the splice point, when compared to direct splicing, Wang et al [27] was able to increase the sensor sensitivity from 224.2 to 260.8 nm/ RIU.…”

Recent Developments in Micro-Structured Fiber Optic Sensors

“…By using the temperature sensing properties of FBG, the temperature-compensated liquid level measurement was achieved by the sensor. The two splicing points of the SMF-PCF-SMF structure were also regularly tapered [288] and up-tapered [289]. The tapered splicing points further enhanced the sensitivity of the refractive index measurements.…”