Refractive Index Detection Limit Research Articles

Theoretical investigation of refractive index detection limit in the Surface Plasmon Resonance-triggered switch effect

Theoretical investigation of refractive index detection limit in the Surface Plasmon Resonance-triggered switch effect

Characterization and Sensing of Inert Gases with a High-Resolution SPR Sensor.

It is generally difficult to characterize inert gases through chemical reactions due to their inert chemical properties. The phase interference-sensing system based on high-resolution surface plasmon resonance (SPR) has an excellent refractive index detection limit. Based on this, this paper presents a simple and workable method for the characterization and detection of inert gases. The phase of light for the present SPR sensor is more sensitive to the change in the external dielectric environment than an amplitude SPR sensor. The limit of detection (LOD) is usually in the order of 10−6 to 10−7 RIU, which is superior to LSPR (Localized Surface Plasmon Resonance) sensors and traditional SPR sensors. The sensor parameters are simulated and optimized. Our simulation shows that a 36 nm-thick gold film is more suitable for the SPR sensing of inert gases. By periodically switching between the two inert gases, helium and argon, the resolution of the system is tested. The SPR sensing system can achieve distinguishable difference signals, enabling a clear distinction and characterization of helium and argon. The doping of argon in helium has a detection limit of 1098 ppm.

Dual-ring dual-wavelength fiber laser sensor for simultaneous measurement of refractive index and ambient temperature with improved discrimination and detection limit.

In this paper, an in-line Mach-Zehnder interferometer embedded in a fiber Bragg grating (FBG) is fabricated by a tiny offset-core splicing technique and inserted into a fiber ring laser to effectively improve the detection limit and discrimination of sensing. Through fine power adjustment, the stable operation of dual-wavelength lasing is completed in a single FBG for the first time to our knowledge, and simultaneous measurement of refractive index (RI) and ambient temperature is achieved with high discrimination due to the RI immunity of the FBG. The experimental results indicate that the sensitivities of RI and temperature are -30.57 nm/RIU and 131.1pm/°C, respectively, with very low intensity drift. Furthermore, the over 20 times improvement of the RI detection limit is obtained owing to the large extinction ratio (>30 dB) and small linewidth (≤0.13 nm).

Refractometer With Etched Chirped Fiber Bragg Grating Fabry–Perot Interferometer in Multicore Fiber

We report a sensitive and mechanically robust fiber-optic refractometer with temperature compensation based on a Fabry–Perot interferometer formed by two chirped fiber Bragg gratings on a seven-core multicore fiber. A small part of the fiber cladding at the sensing region is etched to expose the outer cores to the environment. While the optical modes supported by the outer cores are affected by both temperature and refractive index (RI) changes of the surrounding liquid, the optical mode in the central core of the fiber is affected by the temperature changes only. The wavelength shift due to the RI change is obtained by subtracting the wavelength shift of the central core from that of one of the outer cores so that the temperature effect is removed. The demodulation can also remove the errors caused by the wavelength drift of laser. Due to the narrow spectral features of the sensor, the sensor shows a low RI detection limit of ${2.4} \times {10}^{-{6}}$ at a background RI of 1.316 at a wavelength of ~1550 nm. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability.

Real-time compensation of errors in refractive index shift measurements of microring sensors using thermo-optic coefficients.

We report a method for compensation of errors caused by temperature fluctuations in refractive index measurements using Silicon photonic microring sensors. The method involves determination of resonance wavelength shifts caused by thermal fluctuations using real-time measurement of on-chip temperature variations and thermo-optic coefficient (TOC) of analyte liquids. Resistive metal lines patterned around Silicon microrings are used to track temperature variations and TOC of analyte is calculated by measuring wavelength shifts caused by controlled increments in device temperature. The TOC of de-ionized water is determined to be -1.12 × 10-4/°C, with an accuracy of ±8.26 × 10-6/°C. In our system, chip-surface temperature variations were measured with an instrument limited precision of 0.004 °C yielding a factor of 16 enhancement in tracking accuracy compared to conventional, bottom-of-chip temperature measurement. We show that refractive index detection limit of the microring sensor is also improved by the same factor.

Highly-sensitive optical biosensor based on equal FSR cascaded microring resonator with intensity interrogation for detection of progesterone molecules

We report a refractive index sensing system with a high sensitivity of 1579 dB/RIU and a low refractive index detection limit of 9.7 × 10−6 RIU. The sensing system was based on cascaded-microring resonators (CMRR) using intensity interrogation. The free spectrum range (FSR) for the reference ring was designed to be equal to the sensing ring. The sensing ring of the CMRR was modified with molecularly imprinted polymers (MIPs) for the detection of progesterone. The achieved limit of detection (LOD) was as low as 83.5 fg/mL.

Refractive index sensing based on semiconductor nanowire lasers

We demonstrate a refractive index (RI) sensor based on semiconductor nanowire lasers. It is shown that the TE01 mode is responsible for lasing and sensing with the lasing threshold of 3 μJ/mm2 and the lasing peak width as narrow as 0.22 nm. A RI sensitivity of 21.2 nm/RIU (refractive index units), a figure of merit of approximately 100, and an RI detection limit of 1.4×10−3 RIU are achieved.

Plasmonic crystal cavity on single-mode optical fiber end facet for label-free biosensing

Surface plasmon resonance (SPR) devices on single-mode optical fiber (SMF) end facets are desired for label-free biosensing, due to flexible light delivery, in vivo inspection capability, and seamless integration with fiber-optic communication techniques. We report a plasmonic crystal cavity structure that has a steep resonance near the plasmonic bandedge, a fabrication process to efficiently transfer and align the structure onto a bare SMF end facet, and characterization of its sensing performance. With a sensitivity of 571 nm RIU−1, a figure of merit of 68 RIU−1 and a real-time refractive index detection limit of 3.5 × 10−6 RIU, our sensors can be readily applied in common SPR biosensing experiments. They are over an order of magnitude more sensitive than reported modified-end multimode fiber SPR devices, while there are no reports on previous SMF end facet devices' detection limits which have very low figures of merit.

Phase error evaluation technique based on Fourier transform for refractive index detection limit of microfluidic differential refractometer

A novel phase error evaluation technique based on Fourier Transform Method is presented to estimate the phase error resolution from the spatial interference pattern. The highest phase error resolution is 3.4 × 10−8 × 2 π rad in simulation. The simulation has shown the validity of the proposed technique compared to the result in real experiment. The influence of bit depth, size of the pixel and the spatial period of the interference fringe on the phase error resolution is also discussed in detail. The results are helpful for choosing the suitable parameters of CCD camera and controlling the whole size of the integrated microfluidic differential refractometer.

Solution concentration and refractive index sensing based on polymer microfiber knot resonator

We report a simple and highly sensitive solution concentration and refractive index (RI) sensor based on a microfiber knot resonator (MKR). The sensor was assembled using a flexible poly(trimethylene terephthalate) microfiber of 1.3 µm diameter. The resonance peak shifts due to the variation of the concentration of the ambient medium surrounding the MKR were detected. In the mass concentration range of 0.6–1.2%, the concentration sensitivity and resolution of the sensor are 1.7 nm/% and 1.2 × 10−3%, respectively. The highest measured sensitivity of the sensor is about 95.5 nm/RIU (RI unit) and the RI detection limit is about 2.1 × 10−5 in the RI range from 1.39 to 1.41.

Ultracompact and high sensitive refractive index sensor based on Mach–Zehnder interferometer

We report an ultracompact and high sensitive refractive index (RI) sensor based on Mach–Zehnder interferometer (MZI) structure. The sensor was assembled using flexible poly(trimethylene terephthalate) nanofiber with a diameter of 700nm. The valley wavelength shifts of the output spectra of the sensor caused by the variation of ambient medium around the MZI structure region were measured. In the RI range from 1.34 to 1.39, the highest measured sensitivity of the sensor is about 1100nm/RIU (RI unit), which is 6 times higher than that of S-like abrupt taper based MZI sensors, and the RI detection limit is about 1.8×10−6. The sensor allows the construction of a low-cost, compact, simple, and highly sensitive sensing system.

Highly Sensitive Refractive Index Sensor Based on Adiabatically Tapered Microfiber Long Period Gratings

We demonstrate a refractive index sensor based on a long period grating (LPG) inscribed in a special photosensitive microfiber with double-clad profile. The fiber is tapered gradually enough to ensure the adiabaticity of the fiber taper. In other words, the resulting insertion loss is sufficiently small. The boron and germanium co-doped inner cladding makes it suitable for inscribing gratings into its tapered form. The manner of wavelength shift for refractive indices (RIs) differs from conventional LPG, and the refractive index detection limit is 1.67 × 10−5.

High-Throughput Label-Free Detection Using a Gold Nanoslit Array With 2-D Spectral Images and Spectral Integration Methods

We fabricated gold nanoslit arrays and experimentally compared the sensing capabilities using the single-polarization and multipolarization spectral integration methods. The signal-to-noise ratio and refractive index detection limit using the multipolarization method were increased to three times larger than the simple integration method. Combining 2-D spectral images and the spectral integration method, we demonstrated real time and multiple detections of 30-nm-gold nanoparticles. The detection limit of the system is down to 0.147 particles/μm2.

Chemical Vapor Sensors Based on Whispering Gallery Modes in an Optical Fiber Micro-Resonator Coated with a Polymer Layer

This study proposes a chemical vapor sensor using whispering gallery modes excited in an optical fiber-based resonator. A standard optical fiber with its plastic buffer removed is coated with a polymer layer to act as the vapor-sensitive optical resonator. Theoretical simulations of the sensor response, including the refractive index sensitivity and detection limit, are performed as a function of the polymer layer thickness. Calculations show that the sensor exhibits a very good linearity and a low detection limit on the order of 10−6 refractive index unit. The theoretical predications also provide guidelines for the design and fabrication of optical fiber-based whispering gallery mode resonators in chemical sensing.

Improving Surface Plasmon Detection in Gold Nanostructures Using a Multi‐Polarization Spectral Integration Method

A multi-polarization spectral integration method to increase the refractive index detection limit of gold nanostructures is presented. For a dual-period nanogrid structure, the proposed method increased signal-to-noise ratio and refractive index detection limit about 8 times larger than the simple intensity method. The nanogrid achieves a detection limit of 1.92 × 10−6 refractive index unit when the intensity stability is 0.2%.

Simulation and optimization of polymer-coated microsphere resonators in chemical vapor sensing

This study presents a chemical vapor sensor based on polymer-coated microsphere resonators. A theoretical simulation of the sensor response is performed, and optimization of the polymer layer thickness is investigated. Results show that the sensor exhibits a good linearity and a low detection limit of the refractive index change. Especially at the thermostable thickness of the polymer layer, the refractive index detection limit of the wavelength around 780 nm can be as low as ~2×10⁻⁸ refractive index unit for a spectral resolution of 10 fm, without any temperature control. Because of the good sensing performance and simple manipulation, the proposed sensor is a very promising platform for chemical vapor detections.

Nanoscale photonic crystal sensor arrays on monolithic substrates using side-coupled resonant cavity arrays

We present nanoscale photonic crystal sensor arrays (NPhCSAs) on monolithic substrates. The NPhCSAs can be used as an opto-fluidic architecture for performing highly parallel, label-free detection of biochemical interactions in aqueous environments. The architecture consists of arrays of lattice-shifted resonant cavities side-coupled to a single PhC waveguide. Each resonant cavity has slightly different cavity spacing and is shown to independently shift its resonant peak (a single and narrow drop) in response to the changes in refractive index. The extinction ratio of well-defined single drop exceeds 20 dB. With three-dimensional finite-difference time-domain (3D-FDTD) technique, we demonstrate that the refractive index sensitivity of 115.60 nm/RIU (refractive index unit) is achieved and a refractive index detection limit is approximately of 8.65×10-5 for this device. In addition, the sensitivity can be adjusted from 84.39 nm/RIU to 161.25 nm/RIU by changing the number of functionalized holes.

Design and optimization of liquid core optical ring resonator for refractive index sensing

This study performs a detailed theoretical analysis of refractive index (RI) sensors based on whispering gallery modes (WGMs) in liquid core optical ring resonators (LCORRs). Both TE- and TM-polarized WGMs of various orders are considered. The analysis shows that WGMs of higher orders need thicker walls to achieve a near-zero thermal drift, but WGMs of different orders exhibit a similar RI sensing performance at the thermostable wall thicknesses. The RI detection limit is very low at the thermostable thickness. The theoretical predications should provide a general guidance in the development of LCORR-based thermostable RI sensors.

Enhancing sensitivity of biconical tapered fiber sensors with multiple passes through the taper

A single biconical fiber taper is a simple and low-cost yet powerful sensor. With a distinct strength in refractive index RI sensing, biconical tapered fiber sensors can find their place in handheld sensor platforms, especially as biosensors that are greatly needed in health care, environmental protection, food safety, and biodefense. We report doubling of sensitivity for these sensors with two passes through the tapered region, which becomes possible through the use of sensitive and high-dynamic-range photodetectors. In a proof-of-principle experiment, we measured transmission through the taper when it was immersed in isopropyl alcohol-water mixtures of varying concentrations, in which a thin gold layer at the tip of the fiber acted as a mirror enabling two passes through the tapered region. This improved the sensitivity from 0.43 dB/vol % in the single-pass case to 0.78 dB/vol % with two passes through the taper. The refractive index detection limit was estimated to be 1.210 5 RI units RIU and 0.610 5 RIU in the single- and double-pass schemes, respectively. We predict that further enhance- ment of sensitivity may be achieved with a higher number of passes through the taper. © 2010 Society of Photo-Optical Instrumentation

In-fiber Fabry-Perot refractometer assisted by a long-period grating

We present an optical fiber refractometer based on a Fabry-Perot interferometer defined by two fiber Bragg gratings and an intracavity long-period grating that makes the light confined in the resonator interact with the surrounding medium. The external refractive index is monitored by the resonant frequencies of the Fabry-Perot interferometer, which can be measured either in transmission or in reflection. In this first experiment, wavelength shifts measured with a resolution of 0.1 pm have allowed one to establish a refractive index detection limit of 2.1x10(-5).