Temperature-independent strain sensor based on simplified hollow-core photonic crystal fibers

Abstract

Fiber sensors based on Mach-Zehnder interference have attracted considerable attention over the past few decades. All kinds of fibers, including standard single mode fiber (SMFs), multimode fiber (MMFs), photonic crystal fibers (PCFs) and other special fibers, have been explored to fabricate fiber-based Mach-Zehnder interferometers (MZIs) for various applications. Recently, a new type of PCF, so-called simplified hollow core photonic crystal fiber (HCPCF), is proposed by Frdric Grme et.al in Ref. [l]. In Ref.[2], another simplified HCPCF with a similar structure but different dimensions is employed to fabricate LPGs in it.In this paper, we experimentally demonstrate an MZI based on an intermodal interference in the simplified HCPCF by simply splicing the HCPCF to SMFs with an appropriate core-offset [3]. Thanks to the special structure of the simplified HCPCF and air-guided property, the MZI shows almost insensitive to temperature, as low as 0±0.5 pmrC, but relatively high sensitivity to axial strain (-2.29 pmll-l£). This device has a promising application as a temperature-independent strain sensor. Fig. 1. Cross-section of the simplified HCPCF. Fig. 2. Transmission spectrum of the MZIF.The simplified HCPCF used in our experiments is same as that in Ref. [2]. As shown in Fig. 1, it has a hexagonal hollow core and only one layer air-holes cladding. The diameters of the core, air-holes cladding and outer cladding are about 22 I-lm, 70 I-lm and 140 I-lm, respectively. The thickness of silica walls separating holes is no more than 370 nm.According to the mode investigation reported in [2], this simplified HCPCF can guide light with different modes. By means of appropriate core-offset, not only fundamental core mode but also some higher-order modes will be excited simultaneously. Since the fundamental core mode and higher-order modes have different effective refractive indices, the phase difference will be accumulated after propagating through a certain length of HCPCF. Finally, the interference is generated when they are coupled from the PCF to SMF. Fig. 2 illustrates the transmission spectrum of an interferometer containing a ,,-,29.65 mm length of the simplified HCPCF. As shown, the insertion loss of the MZI is about 7.5 dB, the extinction ratio ranges from "-' 10 dB to 17 dB, and the average fringe spacing is about 38 nm.Then, the sensing experiments are performed. When the interferometer is tensioned by increasing force, all the dips undergo similar wavelength-shift. Here, we characterize the strain-sensing performance of the MZI by monitoring the

Sensitivity-enhanced fiber interferometric high temperature sensor based on Vernier effect

“…By tracing the shift of the envelope in the reflection spectrum, the temperature sensitivity is improved significantly. The maximum sensitivity, 15.68 pm/•C, with a linearity of 99.85% in the high temperature range from 300•C to 1200•C, is 14 to 57 times higher than with other configurations using similar HC-PCFs without the Vernier effect [18]- [21]. The sensitivities obtained in our experiments are also higher than those obtained with other compact types of optical fiber temperature sensors [6], [7], [22], [23].…”

“…The proposed novel miniaturized intrinsic fiber Fabry-Perot interferometric sensor (IFFPIs) made by pure silica is capable of withstanding high temperature up to 1200℃ with stable shape of the interference spectra and good linearity between wavelength shift and temperature change. The sensitivity can be enhanced significantly via the modified Vernier effect, comparing to other configurations using similar HC-PCFs [18]- [21]. The amplification factor can be further increased by reducing the length of the pure silica tip and increasing the length of the HC-PCF cavity.…”

“…The relation between wavelength shift and temperature keeps good linearity of 99.85% as depicted in Fig.4. By tracing the resonant peak shift of the envelope, the maximum high temperature sensitivity of the proposed sensor is approximately 15.68pm/℃, which is from 14 to 57 times higher than that of other configuration using similar HC-PCFs but absence of Vernier effect (sensitivity of 0.2 to 1.1pm/•C reported in [18]- [21]), and our result is better than those of compact temperature probes reported in [6], [7], [22], and [23]. Therefore, the proposed novel sensor structure with modified Vernier effect can increase the temperature sensitivity significantly, has a promising prospects in high temperature sensing application.…”