A singlemode-multimode-singlemode (SMS) fiber structure consists of a short section of multimode fiber fusion- spliced between two SMS fibers. The mechanism underpinning the operation of an SMS fiber structure is multimode interference and associated self-imaging. SMS structures can be used in a variety of optical fiber systems but are most commonly used as sensors for a variety of parameters, ranging from macro-world measurands such as temperature, strain, vibration, flow rate, RI and humidity to the micro-world with measurands such as proteins, pathogens, DNA and specific molecules. While traditional SMS structures employ a short section of standard multimode fiber, a large number of structures have been investigated and demonstrated over the last decade involving the replacement of the multimode fiber section with alternatives such as a hollow core fiber or a tapered fiber. The objective of replacing the multimode fiber has most often been to allow sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then summarizes and compares the performances of a variety of sensors which have been published under a number of headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their deployment in real world applications and discusses possible future developments of SMS fiber sensors.

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Singlemode-Multimode-Singlemode Fiber Structures for Sensing Applications—A Review

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adpr.202100044

Artificial Receptor based Optical Sensors (AROS): Ultrasensitive Detection of Urea

https://www.mdpi.com/2079-6374/10/11/179/pdf

State-of-the-Art Optical Microfiber Coupler Sensors for Physical and Biochemical Sensing Applications.

Optical microfiber-based refractometer, which relies on modal interference operating near the dispersion turning point (DTP), has attracted a high level of research interest. The ultrahigh sensitivity can be achieved by mitigating the group effective refractive index difference between the modes in the coupling. Through modifying the microfiber diameter, the group effective refractive index difference can be modified to zero which is defined as DTP. However, all the DTP-based sensors report low fabrication tolerance and narrow operation bandwidth of probing wavelength for the high-sensitivity region where the refractive index (RI) sensitivities higher than 104 nm/RIU can be achieved, which extremely limits the practical applications. To address the challenge, we manipulate and optimize the mode dispersion by using a tapered specially-designed single stress-applying fiber (SSAF). By introducing the stress-applying part (SAP) into the cladding, the wavelength sensitivity of the group effective RI difference can be significantly reduced through the flexibly engineerable control over the material index. In this way, the operation bandwidth is dramatically broadened for 500 nm in comparison with that of conventional microfiber with the same diameter based on DTP, indicating a broad range of optional probing wavelength and high fabrication tolerance. We experimentally verify the ultrahigh sensitivity and broadband operation around the DTP using an SSAF-based microfiber with a waist diameter of about 2.3 μm. The maximum RI sensitivity of 30563 nm/RIU is obtained.

Ultrasensitive Broadband Refractometer Based on Single Stress-Applying Fiber at Dispersion Turning Point

Lactic acid conversion into acrylic acid and other products over natural and synthetic zeolite catalysts: theoretical and experimental studies

Airborne molecular contaminants (AMCs) are critical issues for the performance of optics in high power lasers (HPLs). In this work we demonstrate an AMCs sensor based on an optical microfiber coupler (OMC) with mesoporous silica coating experimentally. AMCs adhered on the mesoporous silica coating modifies the surrounding refractive index (RI). Using Beam Propagation Method (BPM), the proposed structure was analyzed to demonstrate the feasibility of AMCs sensing. By coating a mesoporous silica layer on the surface of sensing area, the proposed structure was able to operate as an AMCs sensor. By monitoring the spectral shift in the wavelength domain, the sensor was shown to be capable of detecting concentration of AMCs. The experimental results show that the sensitivity of the sensor is concentration-dependent. The maximum sensitivity of 0.541nm/(mg/m3) is achieved. The detected signal is found to be proportional to AMCs present.

Sensing of airborne molecular contaminants based on microfiber coupler with mesoporous silica coating

A high temperature-sensitive long-period fiber grating (LPFG) sensor fabricated by the local fictive temperature modification is proposed and demonstrated. High-frequency CO2 laser pulses scan standard single-mode fiber (SMF), and the modification zones extended to the core of SMF. Experimental results demonstrate that the LPFG temperature sensors with 600 μm grating period and 32 period numbers offer the average sensitivity of 0.084 nm/C in the temperature range of room temperature (RM) to 875°C. The LPFGs fabricated here show exponential change in terms of the spectral wavelength shift versus changes in temperature. In addition, the maximum temperature sensitivity of 0.37 nm/C is achieved by employing long-period microfiber grating (LPMFG), fabricated by the microheater brushing technique and the local fictive temperature modification. LPMFG sensor exhibits better temperature characteristics due to a thinner diameter.

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Xiufang Xie

Papers

Sensing of airborne molecular contaminants based on microfiber coupler with mesoporous silica coating

A Low-Cost High-Temperature Sensor Based on Long-Period Fiber/Microfiber Gratings by Local Fictive Temperature Modification