Experimental and numerical investigation on hollow core photonic crystal fiber based bending sensor

Optical curvature sensor with high resolution based on in-line fiber Mach-Zehnder interferometer and microwave photonic filter

Curvature measurement plays an important role in structural health monitoring, robot-pose measuring, etc. High-resolution curvature measurement is highly demanded. In this paper, an optical curvature sensor with high resolution based on in-fiber Mach-Zehnder interferometer (MZI) and microwave photonic filter (MPF) is proposed and experimentally demonstrated. The in-fiber MZI is constructed with a ring-core fiber (RCF) which is fusion spliced between two coreless fibers (CLFs). The structure of CLF-RCF-CLF is then sandwiched between two segments of single-mode fiber (SMF), making the whole interferometer structure of SMF-CLF-RCF-CLF-SMF. The operating principle is that different curvatures will cause the variations of the interference spectrum of MZI due to elastic-optic effect, and then the variations are converted into the frequency-shift of the MPF. The factors affecting the visibility of the interference spectrum are researched. A preliminary exploration of the multiplexing demodulation for the in-fiber-MZIs is also investigated and discussed, which is for the first time to the best of our knowledge, holding great potential to pave the way for constructing the sensing network composed of interferometric sensors. The curvature measurement sensitivity is -147.634 MHz/m^-1, and the resolution is 6.774 × 10^-6 m^-1 which is the highest value up to now.

Ultra-broadband 3 dB power splitter from 1.55 to 2 µm wave band

Extending the optical communication wavelengths to 2 µm can significantly increase data capacity. Silicon photonics, which is a proven device integration technology, has made rapid progress at 2 µm recently. As a fundamental functional element in the photonic design kit, the 3 dB power splitter has been extensively studied in both the 1.55 µm and 2 µm regime. While the device is highly desirable to operate over both wave bands, the large waveguide dispersion in silicon makes it challenging. In this work, we demonstrate an ultra-broadband power splitter on silicon, which has a 0.2 dB bandwidth exceeding 520 nm from 1500 to 2020 nm according to simulations. The beam splitter is realized by a triple tapered Y-junction, and its operational bandwidth is greatly increased by subwavelength grating structure. The device has an ultra-compact footprint of only 3µm×2µm. Due to the limitations on the setup and coupling technique, we measure the device bandwidth in 1.55 µm and 2 µm wave bands. The device insertion loss is measured to be below 0.4 dB from 1500 to 1620 nm and from 1960 to 2020 nm, respectively. According to these results, the proposed device is believed to be capable of operating over a broadband from 1.55 µm and 2 µm wavelengths.

Negative Curvature Hollow Core Fiber Based All-Fiber Interferometer and Its Sensing Applications to Temperature and Strain

Negative curvature hollow core fiber (NCHCF) is a promising candidate for sensing applications; however, research on NCHCF based fiber sensors starts only in the recent two years. In this work, an all-fiber interferometer based on an NCHCF structure is proposed for the first time. The interferometer was fabricated by simple fusion splicing of a short section of an NCHCF between two singlemode fibers (SMFs). Both simulation and experimental results show that multiple modes and modal interferences are excited within the NCHCF structure. Periodic transmission dips with high spectral extinction ratio (up to 30 dB) and wide free spectral range (FSR) are produced, which is mainly introduced by the modes coupling between HE₁₁ and HE₁₂. A small portion of light guiding by means of Anti-resonant reflecting optical waveguide (ARROW) mechanism is also observed. The transmission dips, resulting from multimode interferences (MMI) and ARROW effect have a big difference in sensitivities to strain and temperature, thus making it possible to monitor these two parameters with a single sensor head by using a characteristic matrix approach. In addition, the proposed sensor structure is experimentally proven to have a good reproducibility.

Strain sensitivity enhancement based on periodic deformation in hollow core fiber

Recent progress in optical fiber Mach-Zehnder interferometers (MZIs) has gained many achievements in sensing application. However, the strain sensitivity of optical fiber MZIs is low due to the low elasto-optical coefficient of silica. In this Letter, we propose and demonstrate a method to modulate the guided modes in an MZI based on a special hollow core microstructured optical fiber (HCMOF) by fabricating periodical deformations. Specifically, periodical deformations reduce the extinction ratio of the transmission spectrum. Furthermore, the axial tension modulates these periodical deformations, leading to the enhanced strain sensitivity in comparison to the configuration without deformations. In our experiment, the strain response from 0 to 1000µε is obtained with a sensitivity of 0.00359dB/µε corresponding to an improvement of 13 times compared with a sensor based on same HCMOF without deformations.

Axial strain applied in-fiber Mach-Zehnder interferometer for acceleration measurement

We present an axial strain applied in-fiber Mach-Zehnder interferometer (MZI) for acceleration measurement. A thin core fiber is sandwiched between two single-mode fibers with core offset to form the MZI. A controlled high fringe visibility in the transmission spectrum is obtained by applying an axial strain, leading to a large slope at the quadrature point. The MZI is then clamped to work as an accelerometer. Experimental results show that the resolution achieves 86 ng/√Hz (g is gravity of 9.8 m/s²), the dynamic range reaches as large as 104.1 dB and the linearity of acceleration response is as high as 99.994%. Moreover, the resonance frequency can be tailored by the clamped fiber length and applied axial strain. The proposed sensor is attractive for practical applications due to low temperature crosstalk, compact size and high sensitivity.

High-resolution, large-dynamic-range multimode interferometer sensor based on a suspended-core microstructured optical fiber

The performance of sensors, including optical fiber sensors, is commonly limited by the tradeoff between a large dynamic range and a high resolution. In this Letter, in order to optimize both, we propose an inline multimode interferometer sensor based on a suspended-core microstructured optical fiber. Due to the existence of multiple pairs of mode interferences, the transmission spectrum of the interferometer consists of dense fringes modulated by a lower envelope. Since these mode interferences take place in the uniform material with the same length, the dense fringes and the lower envelope have an identical sensing response without crosstalk. Hence, the sensor integrates the large dynamic range of the lower envelope and the high resolution of the dense fringes. Strain-sensing performance is investigated to validate the characteristic of the large dynamic range and the high resolution of the proposed sensor. The dynamic range, theoretically 0-9200 µɛ, is 12 times larger than for the dense fringes, and the resolution is 17.5 times higher than for the lower envelope.