Refractive Index Measurement Range Research Articles

Optical refractometry using lensless holography and autofocusing.

Conventional optical refractometry methods are often limited by a narrow measurement range, complex hardware, or relatively high cost. Here, we present a novel refractometry method to measure the bulk refractive index (RI) of materials (including solids and liquids) using lensless holographic on-chip imaging and autofocusing, which is simple, cost-effective, and has a large RI measurement range. As a proof of concept, two compact prototypes were built to measure the RIs of solid materials and liquids, respectively, and they were tested by measuring the RIs of a ZnSe plate and a microscopy immersion oil. Experimental results show that our devices have an average accuracy of ~3 × 10-4 RI unit (RIU) with an estimated precision of ~3 × 10-3 RIU for solids; and an average accuracy of ~1 × 10-4 RIU with an estimated precision of ~3 × 10-4 RIU for liquids. We believe that this cost-effective and portable RI measurement platform holds promise to be used in laboratory and industrial settings.

Simple method to measure the refractive index of liquid with graduated cylinder and beaker.

A simple method is introduced to measure the refractive index (RI) of a liquid with an experimental device composed of a graduated cylinder and a beaker which are coaxial. A magnified image of the graduated cylinder is formed as the liquid is poured into the beaker. Optical path analysis indicates that the RI of the liquid is equal to the product of the image's diameter magnification and the RI of air, irrelevant to the beaker. Theoretically, the RI measurement range is unlimited and the liquid dosage could be small as well. The device is used to carry out experiments by means of both the photographic method and telescope method to measure RIs of three kinds of liquids. The results show that the measured RIs all fit their published values well.

A Highly Sensitive Fiber-Optic Fabry-Perot Interferometer Based on Internal Reflection Mirrors for Refractive Index Measurement.

In this study, a new type of highly sensitive fiber-optic Fabry–Perot interferometer (FFPI) is proposed with a high sensitivity on a wide refractive index (RI) measurement range based on internal reflection mirrors of micro-cavity. The sensor head consists of a single-mode fiber (SMF) with an open micro-cavity. Since light reflections of gold thin films are not affected by the RI of different measuring mediums, the sensor is designed to improve the fringe visibility of optical interference through sputtering the gold films of various thicknesses on the inner surfaces of the micro-cavity, as a semi-transparent mirror (STM) and a total-reflection mirror (TRM). Experiments have been carried out to verify the feasibility of the sensor’s design. It is shown that the fabricated sensor has strong interference visibility exceeding 15 dB over a wide measurement range of RI, and the sensor sensitivity is higher than 1160 nm/RIU, and RI resolution is better than 1.0 × 10−6 RIU.

Experimental Comparison of Fiber-Optic Surface Plasmon Resonance Sensors with Multi Metal Layers and Single Silver or Gold Layer

Fiber-optic surface plasmon resonance sensors with multi metal layers (Ti/Ag/Au) and single silver or gold layer are presented. In comparison with widely used single-layer sensors, the multi-layer sensor shows a wider refractive index measurement range (in a range of 1.3344 to 1.4111 RIU) and a better resolution in the high refractive index region (refractive index over 1.3697 RIU). In addition, the sensitivity of multi-layer sensor is between those of sensors with silver and gold layers, the chemical stability, and the film adhesion capability of multi-layer sensor are much better than those of silver layer sensor.

Liquid refractive index sensing independent of opacity using an optofluidic diffraction sensor.

We have implemented a multifunctional optofluidic sensor that can monitor changes in the refractive index and pressure of biofluid simultaneously and can detect free-solution molecular interaction in situ. In this Letter, we demonstrate two major improvements of this sensor proven by both simulation and experiments. One improvement is the broader measurement range of refractive index by making the diffraction grating with high-index polymer. The other improvement is the separation of refractive index sensing from opacity sensing by using the relative power ratio of diffraction orders. This simple, compact and low-cost multifunctional optofluidic sensor has the potential to be used for in situ biofluid monitoring.

Ultrasensitive Refractive Index Sensor Based on Microfiber-Assisted U-Shape Cavity

A novel in-fiber Mach-Zehnder interferometer formed by a microfiber-assisted U-shape cavity (MFA-USC) with ultrahigh refractive index (RI) sensitivity and an ultra-wide RI measurement range is demonstrated. The MFA-USC is fabricated by splicing a section of micro-fiber (diameter is ~ 63 μm, length is 688 μm) tapered from single-mode fiber (SMF) between two sections of SMF with designed fiber geometry relationship. An ultrahigh sensitivity (higher than 126 000 nm/RIU at a RI of 1.445) within a RI range 1.320-1.446 and 1.462-1.738 has been experimentally achieved. This sensitivity is the highest ever reported to date to the best of our knowledge.

TiO_2 nanoparticle thin film-coated optical fiber Fabry-Perot sensor

In this paper, a novel TiO(2) nanoparticle thin film coated optical fiber Fabry-Perot (F-P) sensor had been developed for refractive index (RI) sensing by monitoring the shifts of the fringe contrast in the reflectance spectra. Using in situ liquid phase deposition approach, the TiO(2) nanoparticle thin film could be formed on the fiber surface in a controlled fashion. The optical properties of as-prepared F-P sensors were investigated both theoretically and experimentally. The results indicated that the RI sensitivity of F-P sensors could be effectively improved after the deposition of nanoparticle thin-films. It was about 69.38 dB/RIU, which was 2.6 times higher than that of uncoated one. The linear RI measurement range was also extended from 1.333~1.457 to 1.333~1.8423. More importantly, its optical properties exhibited the unique temperature-independent performance. Therefore, owing to these special optical properties, the TiO(2) nanoparticle thin film coated F-P sensors have great potentials in medical diagnostics, food quality testing, environmental monitoring, biohazard detection and homeland security, even at elevated temperature.

Compact refractometer based on extrinsic-phase-shift fiber Bragg grating

A novel method to measure refractive index of liquids by using an extrinsic-phase-shift fiber Bragg grating (EPS-FBG) is proposed and experimentally demonstrated. The EPS-FBG is obtained by separating two identical FBG reflectors with a micrometer air gap as a phase-shift. This gap forms a phase-shift peak in the transmission spectrum which is dependent upon the refractive index (RI) of the liquid filled in the gap. A large RI measurement range from 1.33 to 1.47 u.r.i. (unit of refractive index) was obtained in the experiment. Experimental results agreed well with the numerical analysis, showing a linear response with the RI sensitivity of ∼1.69 nm/u.r.i. (RI resolution of ∼5.92 × 10 −3 u.r.i.). Moreover, the micrometer interaction length in the proposed design makes it potential for microfluidic application.

A refractometer based on fiber-to-liquid planar waveguide coupler

A fiber-optic liquid refractometer based on fiber-to-planar waveguide coupler is proposed and demonstrated. The liquid plays a role of highly multimode planar waveguide (PWG), which is evanescently coupled to a side-polished single mode fiber. A simple method, which acquire the refractive index (RI) of a liquid using two adjacent resonance wavelengths of the coupler, is presented. Furthermore, it is experimentally shown that the measurement range of RI can be expanded by series connection of two different fiber-to-PWG couplers. The resolution of the proposed and fabricated refractometer is of the order of 10 −5 to 10 −4.