Improvement on refractive index sensing by exploiting the tapered two-mode fibers
Schematic diagram of the tapered two-mode fiber (TTMF). The interference spectrum of the TTMF has been involved with continuous mode evolution from the HE11 and HE12 modes.
Micro fibers are known as optical waveguides with diameters close to the wavelength of transmitted light. Owing to its large fractional evanescent fields, a micro fiber can guide light with enhanced light-material interaction, which makes it a versatile platform for optical sensing on micro/nanometer scale with special advantages including small footprint, fast response, and high spectral sensitivity. Refractive index (RI) sensing with micro fibers has been engaged in a wide range of applications especially in biochemical analysis, environmental monitoring, and industry process monitoring. In fact, the sensitivity to the surrounding refractive index in such fields often falls in the range of 10-4-10-5 refractive index unit (RIU) for instruments, which is a challenge to the conventional micro fibers.
To overcome these limits, the research group including Dr. Bing Sun from Nanjing University of Posts and Telecommunications (NUPT) demonstrated a tapered two-mode fiber (TTMF) to be operated at the dispersion turning point (DTP). Near the DTP, ultra-high sensitivities sensing was numerically obtained and then verified experiment was carried out. This work has been published in Chinese Optics Letters, Volume 17, No. 11, 2019 (Fang Fang, et al., Improvement on refractive index sensing by exploiting the tapered two-mode fibers).
The dominating proportion of evanescent field of HE11 and HE12 modes and the involving periodic interference contribute to ultra-high sensitivities. Furthermore, the RI sensitivity of the resonance (i.e., S) can be calculated through the following equation: S=dλ/dnSRI=(1/G)×(∂Δneff/∂nSRI),G=Δneff-λ×∂Δneff/∂λ. The determinative parameters of S are the wavelength λ, the RI-induced variation of index difference ∂Δneff/∂nSRI, and the diameters of waist. And when G approaches zero, the RI sensitivity is dramatically become to ±∞. So, our simple motivation that the appearance of the DTP allowed for the wider waist diameter (∼4.0 μm) in two-mode fibers than that of other kinds of optical fibers was easily understood. Further tracking the resonant wavelength shift around the DTP, it is found that the sensor exhibits a sensitivity of 1.81×104 nm/RIU and a limit of detection down up to 3.29×10-5 RIU in a liquid glycerol solution.
Further work will be focused on the application in the detection of cancers biomarker in early-stage to help people understand biological processes.
折射率传感效率提升新手段
二模微纳光纤模式（HE11模和HE12模）运转过程及干涉光谱。
微纳光纤是一种直径接近传输光波长的波导。微纳米级直径使光纤表面存在大量的倏逝波，从而实现光与外界环境的相互作用。通过调控微纳光纤中传输光的模场分布，可以灵敏地感知外界多种物理量极其微弱的变化，因而，微纳光纤是一个理想的柔性高灵敏传感平台。
近年来，研究学者利用微纳光纤做了很多折射率传感方面的测试，结果表明，由于微纳光纤在形状、尺寸和结构上的特殊性，它们可以在化学分析和生物传感方面发挥重要作用。一般地，生物、化学等领域待测物折射率的变化是微小的（~10-5），这对常规意义上的微纳光纤提出了挑战。为克服上述方法存在的问题，南京邮电大学电子与光学工程学院、微电子学院先进光子技术研究团队采用二模微纳光纤，通过优化光纤拉制条件，使该二模微纳光纤在光学模式色散灵界点附近工作，获得了超高灵敏度传感，并进行了相关的验证实验。相关研究成果发表在Chinese Optics Letters 2019年第11期上(Fang Fang, Bing Sun, Zuxing Zhang, Jing Xu, and Lin Zhang, Improvement on refractive index sensing by exploiting the tapered two-mode fibers)。
该实验中使用的二模光纤具备独特折射率分布，能够保证微纳光纤中HE11模和HE12模占据绝大部分能量，同时兼具模式干涉和强倏逝场特性，进而可以利用干涉光谱的漂移监测外界折射率的变化。其干涉光谱的波长折射率灵敏度S=dλ/dnSRI=(1/G)×(∂Δneff/∂nSRI)，其中群有效折射率差G=Δneff-λ×∂Δneff/∂λ。根据上述公式，S与λ、nSRI及微纳光纤的直径有关，进一步，当G趋近0时，S趋近无穷大。数据表明，当微纳光纤的锥腰直径接近4 μm时，在波长1.32 μm附近出现了色散临界点，即G趋近0。实验测得折射率灵敏度为1.81×104 nm/RIU，其检测极限为3.29×10-5 RIU。通过优化锥腰直径将色散临界点转移到长波处，灵敏度得到进一步提高。
该项研究结果初步解决了常规折射率检测中检测极限不够的问题，有望应用于蛋白生物标志物检测，帮助人们了解生物学过程，进而运用于疾病的高危评估、早期诊断、检测定位、预后判断和治疗反应。