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

Optical fibers provide much more than a means to transport light between different locations. This article reviews how integration of functional fluid, solid, and gaseous materials in photonic crystal fibers enables control of their linear and nonlinear properties with applications in optoelectronics, sensing, and laser science.

/pdf/hybrid-photonic-crystal-fiber-40yznmw3kw.pdf

Hybrid photonic-crystal fiber

A surface plasmon resonance-based fiber-optic sensor for simultaneous measurement of refractive index and temperature of liquid samples is proposed and experimentally demonstrated. The sensor consists of a gold-coated MM-SM-MM optical fiber structure, whose sensitive section was partially covered with polydimethylsiloxane (PDMS) to generate two independent SPR resonance dips in the fiber transmission spectrum. One of the dips is generated by the bare gold-coated fiber section whose wavelength resonance is tuned by the refractive index and temperature of the surrounding medium. The other dip that is exclusively used to monitor the temperature variations of the liquid sample, whose central wavelength at 900 nm corresponds to PDMS refractive index at 20 °C, is produced by the polymerized gold-coated fiber section. The high refractive index and temperature sensitivity achieved, 2323.4 nm/RIU and −2.850 nm/°C respectively, the small size, the ease fabrication process, and the bio-compatibility of the proposed device are appealing characteristics that makes it ideal for practical bio-sensing applications.

Simultaneous measurement of refractive index and temperature using a SPR-based fiber optic sensor

We demonstrate a sub-micron silica diaphragm-based fiber-tip Fabry–Perot interferometer for pressure sensing applications. The thinnest silica diaphragm, with a thickness of ∼320  nm, has been achieved by use of an improved electrical arc discharge technique. Such a sub-micron silica diaphragm breaks the sensitivity limitation imposed by traditional all-silica Fabry–Perot interferometric pressure sensors and, as a result, a high pressure sensitivity of ∼1036  pm/MPa at 1550 nm and a low temperature cross-sensitivity of ∼960  Pa/°C are achieved when a silica diaphragm of ∼500  nm in thickness is used. Moreover, the all-silica spherical structure enhanced the mechanical strength of the micro-cavity sensor, making it suitable for high sensitivity pressure sensing in harsh environments.

Sub-micron silica diaphragm-based fiber-tip Fabry-Perot interferometer for pressure measurement.

We demonstrated a high-sensitivity strain sensor based on an in-fiber Fabry–Perot interferometer (FPI) with an air cavity, which was created by splicing together two sections of standard single-mode fibers. The sensitivity of this strain sensor was enhanced to 6.0  pm/μe by improving the cavity length of the FPI by means of repeating arc discharges for reshaping the air cavity. Moreover, such a strain sensor has a very low temperature sensitivity of 1.1  pm/°C, which reduces the cross sensitivity between tensile strain and temperature.

High-sensitivity strain sensor based on in-fiber improved Fabry–Perot interferometer

We investigated a novel and ultracompact polymer-capped Fabry-Perot interferometer, which is based on a polymer capped on the endface of a single mode fiber (SMF). The proposed Fabry-Perot interferometer has advantages of easy fabrication, low cost, and high sensitivity. The variation of the Fabry-Perot cavity length can be easily controlled by using the motors of a normal arc fusion splicer. Moreover, the enhanced mechanical strength of the Fabry-Perot interferometer makes it suitable for high sensitivity pressure and temperature sensing in harsh environments. The proposed interferometer exhibits a wavelength shift of the interference fringes that corresponds to a temperature sensitivity of 249 pm/°C and a pressure sensitivity of 1130 pm/MPa, respectively, around the wavelength of 1560 nm.

Simultaneous measurement of pressure and temperature by employing Fabry-Perot interferometer based on pendant polymer droplet

A novel intensity-modulated refractive index sensor based on an in-fiber Michelson interferometer is demonstrated by splicing a section of thin core fiber between two standard single mode fibers. Such a refractive index sensor exhibits an ultrahigh sensitivity of −208.24 and 125.44 dB/RIU at the refractive index of 1.440 and 1.500, respectively. The refractive index sensor is insensitive to temperature and thus solves the cross-sensitivity problem between temperature and surrounding refractive index. Moreover, the promising RI sensor has the advantages of short size (less than 2 mm) and easy fabrication.

Temperature-insensitive refractive index sensor based on in-fiber Michelson interferometer

We present a type of phase-shifted fiber Bragg gratings based on an in-grating bubble fabricated by femtosecond (fs) laser ablation together with a fusion-splicing technique. A microchannel vertically crossing the bubble is drilled by fs laser to allow liquid to flow in or out. By filling different refractive index (RI) liquid into the bubble, the phase-shift peak is found to experience a linear red shift with the increase of RI, while little contribution to the change of phase shift comes from the temperature and axial strain. Therefore, such a PS-FBG could be used to develop a promising tunable optical filter and sensor.

/pdf/tunable-phase-shifted-fiber-bragg-grating-based-on-4vjh7ixq64.pdf

Tunable phase-shifted fiber Bragg grating based on femtosecond laser fabricated in-grating bubble

We proposed and demonstrated a novel scheme for simultaneous measurement of temperature and refractive index (RI) by cascading a liquid-filled photonics crystal fiber (PCF) to a long period fiber grating (LPFG) written in a single-mode fiber. The liquid-filled PCF showed a high-temperature sensitivity of 1.695 nm/°C, which is two orders of magnitude higher than that of the LPFG (0.0642 nm/°C). The LPFG exhibited a blue shift with the increased RI. In contrast, liquid-filled PCF showed a total immunity to the surrounding RI. So, the temperature was solely measured by the liquid-filled PCF, and the RI information was extracted from the total response of the LPFG with the temperature effect having been compensated by the liquid-filled PCF.

Simultaneous Refractive Index and Temperature Measurement With LPFG and Liquid-Filled PCF

We demonstrated a compact tunable multibandpass filter with a short size of about 9 mm and a high wavelength-tuning sensitivity of up to −2.194  nm/°C by means of filling a liquid with a high refractive index of 1.700 into the air holes of a photonic crystal fiber (PCF). Such a PCF-based filter maintains an almost constant bandwidth and a large extinction ratio of more than 40 dB within the whole wavelength tuning range of more than 100 nm. Moreover, the transmission spectrum of the PCF-based filter is insensitive to the stretch force and the curvature of the fiber.

Yingjie Liu

Papers

Simultaneous measurement of pressure and temperature by employing Fabry-Perot interferometer based on pendant polymer droplet

Temperature-insensitive refractive index sensor based on in-fiber Michelson interferometer

Tunable phase-shifted fiber Bragg grating based on femtosecond laser fabricated in-grating bubble

Simultaneous Refractive Index and Temperature Measurement With LPFG and Liquid-Filled PCF

Compact tunable multibandpass filters based on liquid-filled photonic crystal fibers.