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

A novel curvature sensor based on optical fiber modal interferometer is demonstrated. It consists of two peanut-shape structures that are formed only by single mode fibers. The two peanut-shape structures can split and recombine the core and cladding modes, consequently, it produces modal interference. The experimental results show that the shift of the peak wavelength is almost linearly proportional to the change of curvature, and the sensitivities of the sensors with the lengths of 21, 26, and 30 mm are -18.46, -21.87, and 13.68 nm/m-1, respectively. The proposed curvature sensor is simple, high sensitive, and inexpensive.

An Optical Fiber Curvature Sensor Based on Two Peanut-Shape Structures Modal Interferometer

This paper presents a highly-sensitive, miniature, all-silica, dual parameter fiber-optic Fabry-Perot sensor, which is suitable for independent measurement of the refractive index and the temperature of the fluid surrounding the sensor. The experimental sensor was produced by a micromachining process based on the selective etching of doped silica glass and a simple assembly procedure that included fiber cleaving, splicing and etching of optical fibers. The presented sensor also allows for direct compensation of the temperature’s effect on the fluid’s refractive index change and consequently provides opportunities for the detection of very small changes in the surrounding fluid’s composition. A measurement resolution of 2x10−7RIU was demonstrated experimentally for a component of the refractive index that is related purely to the fluid’s composition. This resolution was achieved under non-stabilized temperature conditions. The temperature resolution of the sensor proved to be about 10−3°C. These high resolution measurements were obtained by phase-tracking of characteristic components in a Fourier transform of sensor’s optical spectrum.

High resolution, all-fiber, micro-machined sensor for simultaneous measurement of refractive index and temperature

A fiber-optic humidity sensor composed of a chitosan-coated, waist-enlarged bitaper-based Michelson interferometer (WEBMI) is proposed and experimentally demonstrated. Humidity changes interference pattern of the sensor head through refractive index of the chitosan coating thus relative humidity level can be determined from corresponding wavelength shift. The achieved sensitivity is further enhanced by more than 5 times, from 26 pm/%RH to 135 pm/%RH, by reducing the fiber cladding thickness and oxidizing the chitosan film. Experimentations show that the measurement is fully reversible and repeatable with fast response and recovery time of ∼5 s and 3 s, respectively.

Sensitivity-enhanced Michelson interferometric humidity sensor with waist-enlarged fiber bitaper

Fiber optic humidity sensor based on the graphene oxide/PVA composite film

A curvature sensor based on a polarization-dependent in-fiber Mach-Zehnder interferometer (MZI) is proposed. The MZI is fabricated by core-offset fusion splicing one section of polarization maintaining fiber (PMF) between two single mode fibers (SMFs). Two independent interference patterns corresponding to the two orthogonal polarization modes for the PMF are obtained. The couple efficiency between the core mode and the cladding mode decreased with the increasing of the bending on the MZI part. The curvature variation on the MZI part can be obtained by detecting the fringe visibility of the interference patterns. A difference arithmetic demodulation method is used to reduce the effects of the light source power fluctuations and temperature cross-sensitivity. Experimental results show that maximal sensitivity of −0.882 dB/m−1 is obtained under a measurement range of 0.1 to 0.35 m−1 for the curvature sensor. With the use of difference arithmetic demodulation method, the temperature-curvature cross-sensitivity and light source power fluctuations effects on the proposed sensor are decreased by 94% and 91%, respectively.

Polarization-dependent curvature sensor based on an in-fiber Mach-Zehnder interferometer with a difference arithmetic demodulation method

An in-line Mach–Zehnder-interferometer-based two-dimensional (2-D) micrometric displacement sensor was fabricated by inserting a polarization maintaining fiber (PMF) with a length of 1 cm between two conventional single-mode fibers (SMFs). The left end of the PMF was mismatch fusion spliced with an SMF. The right end of the PMF was connected to another SMF without fusion splicing. Two independent interference patterns corresponding to the two orthogonal polarization modes in the PMF were obtained. The visibility of the interference patterns depends on the 2-D micrometric displacement of the SMF along the right end of the PMF on the slow axis and fast axis directions. Experimental results show that the 2-D micrometric displacement sensor with sensitivity of −0.669 dB/μm on the slow axis direction and −0.301 dB/μm on the fast axis direction was obtained. With the use of the intensity demodulation method, the proposed sensor can overcome temperature and displacement cross-sensitivity effects.

Temperature-insensitive optical fiber two-dimensional micrometric displacement sensor based on an in-line Mach-Zehnder interferometer

A fiber sensor based on polarization-maintaining fiber loop mirror (FLM) for liquid refractive index absolute measurement is described. Two sections of polarization maintaining fibers (PMFs) with different length were inserted into the FLM. One section of PMF was chemical etched for liquid refractive index measurement and the other one for reference. Liquid refractive index absolute measurement can be obtained by detecting the interference wavelength difference corresponding to the two sections of PMFs. High sensitivity of 63.82 ± 0.04 nm/RIU (refractive index unit) and a resolution of 3.14 × 10−4 RIU were obtained for the proposed liquid refractive index sensor.

Yang You

Papers

Polarization-dependent curvature sensor based on an in-fiber Mach-Zehnder interferometer with a difference arithmetic demodulation method

Temperature-insensitive optical fiber two-dimensional micrometric displacement sensor based on an in-line Mach-Zehnder interferometer

A polarization-maintaining fiber loop mirror based sensor for liquid refractive index absolute measurement