DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

An ultracompact, cost-effective, and highly accurate fiber optic temperature sensor is proposed and demonstrated. The sensing head consists of Fabry-Perot microcavity formed by an internal mirror made of a thin titanium dioxide (TiO2) film and a microscopic segment of single-mode fiber covered with Poly(dimethylsiloxane) (PDMS). Due to the high thermo-optic coefficient of PDMS the reflectance of the fiber-PDMS interface varies strongly with temperature which in turn modifies the amplitude of the interference pattern. To quantify the changes of the latter we monitored the visibility of the interference pattern and analyzed it by means of the fast Fourier transform. Our sensor exhibits linear response, high sensitivity, and response time of 14 seconds. We believe that the microscopic dimensions along with the performance of the sensor here presented makes it appealing for sensing temperature in PDMS microfluidic circuits or in biological applications.

Optical fiber temperature sensor based on a microcavity with polymer overlay.

A novel parallel structured fiber-optic Fabry-Perot interferometer (FPI) based on Vernier-effect is theoretically proposed and experimentally demonstrated for ultrasensitive strain measurement. This proposed sensor consists of open-cavity and closed-cavity fiber-optic FPI, both of which are connected in parallel via a 3 dB coupler. The open-cavity is implemented for sensing, while the closed-cavity for reference. Experimental results show that the proposed parallel structured fiber-optic FPI can provide an ultra-high strain sensitivity of −43.2 pm/μe, which is 4.6 times higher than that of a single open-cavity FPI. Furthermore, the sensor is simple in fabrication, robust in structure, and stable in measurement. Finally, the parallel structured fiber-optic FPI scheme proposed in this paper can also be applied to other sensing field, and provide a new perspective idea for high sensitivity sensing.

Ultrasensitive strain sensor based on Vernier- effect improved parallel structured fiber-optic Fabry-Perot interferometer

Needs for sensor miniaturization, versatile sensing solutions, and improved measurements’ performances in difficult operating environments have recently driven considerable research in optical fiber sensor for multiparameter measurements. Multiparameter sensors not only enable new sensors’ functionalities, but can also improve achievable measurement performances for some frequently measured parameters considerably. This study provides a review of work in the field of miniature fiber-optic sensors that allows independent and simultaneous measurements of two or more different physical or chemical parameters. Sensor designs and corresponding signal processing schemes are reviewed and compared.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-58/issue-7/072009/Multiparameter-fiber-optic-sensors-a-review/10.1117/1.OE.58.7.072009.pdf

Multiparameter fiber-optic sensors: a review

We demonstrated a compact and highly-sensitive curvature sensor based on a Mach-Zehnder interferometer created in a photonic crystal fiber. Such a Mach-Zehnder interferometer consisted of a peanut-like section and an abrupt taper achieved by use of an optimized electrical arc discharge technique, where only one dominating cladding mode was excited and interfered with the fundamental mode. The unique structure exhibited a high curvature sensitivity of 50.5 nm/m-1 within a range from 0 to 2.8 m-1, which made it suitable for high-sensitivity curvature sensing in harsh environments. Moreover, it also exhibited a temperature sensitivity of 11.7 pm/°C.

Asymmetrical in-fiber Mach-Zehnder interferometer for curvature measurement.

We fabricate and experimentally demonstrate a hybrid structured Fabry-Perot interferometer (FPI) embedded in the middle of a fiber line for simultaneous measurement of axial strain and temperature. The FPI is composed of a silica-cavity cascaded to a spheroidal air-cavity, both of which are formed in a hollow annular core fiber (HACF). The fabrication process of the FPI includes only a fusion splice between a single-mode fiber and a HACF and several electrical arc discharges at the HACF near the splice point. Experimental results show that the strain and temperature sensitivities of the air-cavity can be 5.2 pm/μe and 1.3 pm/C°, respectively, and those of the silica-cavity can be 1.1 pm/μe and 13 pm/C°, respectively. The different sensitivities of silica-cavity and air-cavity to strain and temperature enable us to implement simultaneous sensing in strain and temperature.

Hybrid structured fiber-optic Fabry-Perot interferometer for simultaneous measurement of strain and temperature

We propose a simultaneous temperature and curvature sensor based on a hollow annular core fiber (HACF) Mach-Zehnder interferometer (MZI). The MZI is fabricated by inserting a short section of HACF between two short pieces of multimode fibers, and interference occurs between the light beams transmitting along the air hole and annular core of the HACF. Curvature and temperature can be measured through fringe visibility variation and wavelength shift of the interference spectrum, respectively. The experimental results show that the temperature sensitivity is 30 pm/°C in the range of 30°C-100°C, and the maximum visibility variation is 32.4 dB from 0.555 to 5.405 m-1. In addition, the device is insensitive to external refractive index.

Simultaneous Measurement of Temperature and Curvature Based on Hollow Annular Core Fiber

We present a theoretical and experimental investigation on refractive index (RI) sensing characteristics of single mode fiber (SMF) based modal interferometers. Theoretical analysis reveals that interference between different modes in an SMF has a quite different response to the RI variation of the external medium. The interference between the core and lower order cladding modes has negative RI sensitivity whereas that between the core and higher order modes, or between two different order cladding modes have positive sensitivity. A single-mode-multimode-single-mode (SMS) fiber Michelson interferometer with a large-core step-index multimode fiber (MMF) is employed for experimental verification. In the SMS-based Michelson interferometer, the MMF acts as a mode coupler to excite cladding modes in the SMF. The RI response of the SMS-based structures with two different lengths of MMF are respectively tested in sodium-chloride water solutions. Experimental results show excellent agreements with the theoretical analysis.

Refractive Index Sensing Characteristics of Single-Mode Fiber-Based Modal Interferometers

The invention provides a micro toroidal cavity resonator based on a hollow inner-wall waveguide optical fiber and a manufacturing method of the micro toroidal cavity resonator. The micro toroidal cavity resonator comprises an input single-mode optical fiber, the hollow inner-wall waveguide optical fiber and an output single-mode optical fiber and is characterized in that the two ends of the hollow inner-wall waveguide optical fiber are welded to the input single-mode optical fiber and the output single-mode optical fiber respectively in a shaft staggered mode, the staggered distance of the axes of the optical fibers is equal to the difference value of the outer radius of the hollow inner-wall waveguide optical fiber and the radius of the core of each single-mode optical fiber, and the axis of the input single-mode optical fiber and the axis of the output single-mode optical fiber are located on the same straight line. According to the micro toroidal cavity resonator, the micro toroidal resonant cavity is integrated in one optical fiber, manufacturing is easy, the structure is compact, the size is small, and stability is high.

Micro toroidal cavity resonator based on hollow inner-wall waveguide optical fiber and manufacturing method of micro toroidal cavity resonator

We propose a refractive index insensitive temperature sensor based on hollow annular core fiber (HACF) Mach-Zehnder interferometer (MZI). The HACF is composed of a large-diameter air hole, an annular core around the air hole and a cladding. The MZI is fabricated by inserting a short section of the HACF between two short multimode fibers, and the interference occurs between the light beams transmitting along the air hole and the annular core of the HACF. Experimental results show that the MZI is insensitive to external refractive index and has temperature sensitivity of 30 pm/°C.

Boyang Qin

Papers

Hybrid structured fiber-optic Fabry-Perot interferometer for simultaneous measurement of strain and temperature

Simultaneous Measurement of Temperature and Curvature Based on Hollow Annular Core Fiber

Refractive Index Sensing Characteristics of Single-Mode Fiber-Based Modal Interferometers

Micro toroidal cavity resonator based on hollow inner-wall waveguide optical fiber and manufacturing method of micro toroidal cavity resonator

Refractive index insensitive temperature sensor based on hollow annular core fiber Mach-Zehnder interferometer