The Characteristics And Applications Of Fiber Sensors Based On Hybrid Plasmonic Microfiber Knot Resonators

Optical microfiber has a diameter ranging from several hundred nanometers to several microns,which approximates the light wavelength scale.It is a typical micro/nano optical waveguide.Microfiber is often fabricated by heating and drawing the conventional commercial optical fiber.It has many excellent optical and mechanical properties,including large evanescent field,high nonlinearity,strong confinement,high mechanical strength,great configurability,and low insertion loss.Due to the large evanescent field outside the microfiber,the interaction between optical field and ambient is enhanced,which can be utilized as sensors.Microfiber knot resonator(MKR)is an optical coupling devices based on evanescent field coupling.The MKR has plenty of advantages such as low insertion loss,high finesse,easy fabrication,and compatibility with fiber systems.It can be utilized as an add-drop filter,current sensor,and magnetic-filed sensor.Surface plasmon-polaritons(SPP)have great capacity of field intensity enhancement and light confinement beyond the diffraction limit.However,there is a balance between light confinement and propagation loss for the SPP waveguides.One way to address this issue is to explore the hybrid plasmonic waveguide.In this paper,we combine an MKR with a metal substrate to form a hybrid plasmonic microfiber knot resonator(HPMKR),and then we use polydimethylsiloxane(PDMS)to encapsulate the HPMKR.With the polarization characteristics of HPMKR and the characteristics of PDMS,we can use this device to obtain a versatile sensor that can be used in pressure sensing,vibration sensing,and wearable sensing.1.We have successfully fabricated a microfiber with "flame-brushing method".The microfiber produced by this method has a uniform waist and a small insertion loss.The MKR is fabricated by tying a microfiber by hand with the help of high-precision translation stages.This method ensures a no-cutting and compact MKR structure made from a double-ended tapered fiber.The MKR has a high quality factor,a stable structure,and a small insertion loss.2.We have proposed a hybrid plasmonic microfiber knot resonator(HPMKR)and demonstrated its sensor applications.By functionalizing the substrate surface with a nanoscale gold film and tuning the structure of the MKR,the HPMKR shows an extinction ratio of more than 15 dB over broadband spectrum as a polarizer,and a Q-factor of more than 52,000 at the same time.We also demonstrate a robust multifunctional sensor by packaging the HPMKR with PDMS.For the weight sensing,it shows sensitivity of 18.28 pm/g(51.2 pm/kPa).And the device can response the vibration fairly well with a frequency range of 60-3000 Hz.3.We have demonstrated an ultrasensitive flexible wearable and cost-effective photonics sensor consisting of PDMS-HPMKR-PDMS sandwich structure.Our experimental results have proved that our sensor is a multi-functional wearable device which can response to a planar strain as low as 1‰.In addition,our pressure sensor has a high sensitivity of 0.83 kPa-1 to detect a pressing force as low as 30 Pa with an ascending time less than 20 ms.When attached to human skins,our sensor has been proved the ability of real-time monitoring the human clinical and physiological signals such as wrist pulse,respiration,and finger pulse.