Highly precise thickness measurement of multilayer films based on the cross-correlation algorithm using a widely tunable MG-Y laser.

Inspired by the demodulation algorithm of Fabry-Perot composite sensors in the field of fiber-optic sensing, this paper proposes a method based on a widely tunable modulated grating Y-branch (MG-Y) laser combined with the cross-correlation algorithm to achieve a highly precise measurement of the optical thickness of each layer of a multilayer optical sample. A sample consisting of a double glass stack was selected, and the interference spectrum of the stacked sample was acquired using a widely tunable MG-Y laser. A fast Fourier transform (FFT) algorithm combined with a finite impulse response (FIR) bandpass filter was utilized to separate the different frequency components of the multilayer optical sample. The normalized spectra of each layer were reconstructed using the Hilbert transform. Subsequently, a cross-correlation algorithm was employed to process the normalized spectrum and determine the optical thickness of each layer with high precision. The samples were measured at predetermined locations, with 150 consecutive measurements performed to assess the repetition of the thickness. The standard deviation of these measurements was found to be lower than 1.5 nm. The results show that the cross-correlation algorithm is advantageous in the optical thickness measurement of multilayer films.

TWDM-assisted passive quadrature phase demodulation for a Fabry-Perot-based ultrasound localization detection.

We propose a time and wavelength division multiplexing (TWDM)-assisted passive quadrature phase demodulation mechanism in this Letter. Combining wavelength division multiplexers (WDMs) with a programmable modulated grating Y-branch (MG-Y) laser, this method realizes both fast switching of discrete wavelengths and fast activation of multiple sensing paths simultaneously. Deploying it on a fiber-optic dual-cavity Fabry-Perot (F-P) ultrasound sensor array, we achieve high-precision localization of partial discharge (PD) signal sources in a two-dimensional (2D) plane with a maximum distance error of 1.53 cm and a maximum angle error of about 3.02°. This demodulation scheme can balance the relationship between sensitivity, sampling rate, and time-delay-induced errors, and provides an innovative solution for high-frequency phase demodulation applications of sensor arrays, which is especially significant for high-frequency detection in specific environments like partial discharges, industrial nondestructive testing, and so on.

Fiber-optic integrated aerodynamic three-hole vector probe for high-velocity flow field measurement.

An integrated aerodynamic three-hole pressure probe (THP) based on a fiber-optic tip sensor array for high-velocity flow field vector measurement is developed and demonstrated in wind tunnel testing. The sensor array consisting of three miniature pressure fiber-tip sensors is integrated into three pressure conduits inside top area of the THP, which serves to mitigate pneumatic pressure loss and is expected for a more reliable analysis of flow characteristics. Fast real-time data acquisition is implemented by a compact self-developed multichannel white light interferometry (WLI) interrogator. Well-calibrated maps of the fiber-optic THP are developed in a subsonic free-jet wind tunnel to derive the velocity vectors in a yaw angular range of ±15° at Mach numbers of 0.2 Ma (∼70 m/s), 0.5 Ma (∼170 m/s), and 0.8 Ma (∼300 m/s) while related flow characteristics are analyzed. This work is desired to provide a potential candidate for turbomachinery experimental investigation in fluid mechanics community.

Compressed-sensing fiber-optic white light interferometry.

In this Letter, we propose a dynamic fiber-optic white light interferometry (WLI) based on the compressed-sensing (CS) principle. The time-varying interference spectra of a Fabry-Perot cavity under vibration are considered as a two-dimensional (2D) signal with respect to both laser wavelength and time, which can be compressively sampled using a programmable semiconductor laser source during the measurement process. After CS reconstruction, the spectrum acquisition rate is equal to the random wavelength modulation rate, up to 10 MHz in this Letter, providing an attractive alternative to laser-based dynamic interferometry. Numerical simulations and nanometer-scale vibration experiments verify the effectiveness of the scheme.

All-silica fiber-optic temperature-depth-salinity sensor based on cascaded EFPIs and FBG for deep sea exploration.

Using fusion splicing and hydroxide catalysis bonding (HCB) technology, an all-silica inline fiber-optic sensor with high-pressure survivability, high-resolution salinity measurement capability, and corrosion resistance for deep sea explorations is proposed and experimentally demonstrated. Two extrinsic Fabry-Perot interferometers (EFPIs) and a fiber Bragg grating (FBG) are cascaded in one single-mode fiber (SMF), enabling structural integration of single lead-in fiber and versatility of the sensing probe for temperature, depth, and salinity monitoring. The HCB technology offers a polymer adhesive-free assembly of one open-cavity EFPI for refractive index (RI) (salinity) sensing under normal pressure and temperature (NPT) conditions, showing obvious advantages of strong bonding strength, reliable effectiveness, and no corrosive chemicals requirements. The other EFPI formed by a fused structure is designed for pressure (depth) measurement. The cascading of EFPIs, especially the open-cavity EFPI immersed in water, will result in large light transmission loss and bring challenges to signal interrogation. Graded-index fiber (GIF) micro-collimators and reflective films are added to prevent dramatic degradations of signal intensity and fringe visibility underwater. Thereby, a Fabry-Perot (FP) cavity of several hundreds of microns in length and an open cavity of a thousand microns can be cascaded for underwater applications, effectively enhancing sensitivities and underwater signal readout simultaneously. Results show that the proposed sensor can well operate in the deep-sea pressure range of 0∼2039.43 mH2O, RI range of 1.33239∼1.36885 RIU, and temperature range of 23∼80 °C, with resolutions of 0.033 MPa, 4.16×10-7 RIU, and 0.54 °C, respectively. With the multi-parameter measurement capability, all-silica construction, and inline compact structure, the proposed sensor could be a potential candidate for deep sea exploration.

Noble Metallic Pyramidal Substrate for Surface-Enhanced Raman Scattering Detection of Plasmid DNA Based on Template Stripping Method.

In this paper, a new method for manufacturing flexible and repeatable sensors made of silicon solar cells is reported. The method involves depositing the noble metal film directly onto the Si template and stripping out the substrate with a pyramid morphology by using an adhesive polymer. In order to evaluate the enhancement ability of the substrate, Rhodamine 6G (R6G) were used as surface-enhanced Raman scattering (SERS) probe molecules, and the results showed a high sensitivity and stability. The limit of detection was down to 10-12 M for R6G. The finite-difference time domain (FDTD) was used to reflect the distribution of the electromagnetic field, and the electric field was greatly enhanced on the surface of the inverted pyramidal substrate, especially in pits. The mechanism of Raman enhancement of two types of pyramidal SERS substrate, before and after stripping of the noble metal film, is discussed. By detecting low concentrations of plasmid DNA, the identification of seven characteristic peaks was successfully realized using a noble metallic pyramidal substrate.

Rhombic optical path difference bias white light interferometric fiber-optic gyroscope.

We present a novel, to the best of our knowledge, white light interferometric fiber-optic gyroscope (IFOG) scheme using a fiber-optic rhombic optical path difference (OPD) bias structure to interrogate with a sensing coil to realize rotation rate measurement without a phase modulator. The OPD bias structure composed of four (2×1) 3 dB single-mode fiber couplers was constructed to implement non-reciprocal OPD bias. White light interferometric demodulation was utilized to acquire the change in OPD due to the Sagnac-phase shift. Absolute linear output can be obtained. We produced the principle prototype of rhombic OPD bias white light IFOG without utilizing a phase modulator. An experimental demonstration of the IFOG prototype system achieves linear output with respect to the OPD difference in detecting rotation rate.

Label-Free Near-Infrared Plasmonic Sensing Technique for DNA Detection at Ultralow Concentrations.

Biomolecular detection at a low concentration is usually the most important criterion for biological measurement and early stage disease diagnosis. In this paper, a highly sensitive nanoplasmonic biosensing approach is demonstrated by achieving near-infrared plasmonic excitation on a continuous gold-coated nanotriangular array. Near-infrared incident light at a small incident angle excites surface plasmon resonance with much higher spectral sensitivity compared with traditional configuration, due to its greater interactive volume and the stronger electric field intensity. By introducing sharp nanotriangular metallic tips, intense localization of plasmonic near-fields is realized to enhance the molecular perception ability on sensing surface. This approach with an enhanced sensitivity (42103.8 nm per RIU) and a high figure of merit (367.812) achieves a direct assay of ssDNA at nanomolar level, which is a further step in label-free ultrasensitive sensing technique. Considerable improvement is recorded in the detection limit of ssDNA as 1.2 × 10-18 m based on the coupling effect between nanotriangles and gold nanoparticles. This work combines high bulk- and surface-sensitivities, providing a simple way toward label-free ultralow-concentration biomolecular detection.

Linear-response and simple hot-wire fiber-optic anemometer using high-order cladding mode.

We present a single walled carbon nanotubes (SWCNTs)-coated tilted fiber Bragg grating (TFBG) hot-wire anemometer (HWA) with simple configuration, linear response, and high sensitivity. TFBG is utilized to effectively couple a pumping laser at 1550 nm to the cladding mode that is absorbed by the SWCNTs film immobilized on the fiber surface with good light-heat conversion efficiency. As a result, the TFBG is converted to a "hot wire", and the wind speed can be deduced from the output power of the laser, which is a function of both the wind-induced temperature change and the spectral profile of the cladding mode. The most significant aspect of the HWA system is that we use the Gaussian shape of the high-order TFBG cladding mode to compensate for the inherent nonlinear relationship between the heat loss and the wind speed that is an undesirable characteristic of existing HWA systems. The validity of this novel operating principle was verified theoretically and experimentally. Via careful control of the parameters, a good linear response of the HWA system was achieved, especially for the low wind speed range where nonlinearity was more conspicuous. It was demonstrated that, with a low input power of only 29.3 mW of the pump laser, an R2 value of 0.9927 was obtained in this fiber-optic HWA system with high sensitivity 7.425 dBm / (m/s) and resolution 0.0027 m/s in a small wind speed range (0-2m/s) considering the intensity resolution of OSA and the noise of the pump laser. Furthermore, the system also exhibits a simple and low-cost design with only one laser source and one low-cost power measurement component.

Differential-pressure fiber-optic airflow sensor for wind tunnel testing.

A differential-pressure fiber-optic airflow (DPFA) sensor based on Fabry-Perot (FP) interferometry for wind tunnel testing is proposed and demonstrated. The DPFA sensor can be well coupled with a Pitot tube, similar to the operation of the differential diaphragm capsule in the airspeed indicator on the aircraft. For differential pressure sensing between total pressure and static pressure in the airflow, an FP cavity is formed between the sensing diaphragm and a fiber end-face, and a tubule is inserted into the FP cavity. According to the principle of differential pressure derived from Bernoulli's equation, the airflow velocity can be determined by monitoring the change of the FP cavity length. The experimental results demonstrate that a DPFA sensor with 0∼11 kPa measurable range, 826.975 nm/kPa sensitivity, and 0.008% (0.89 Pa) resolution can be realized. Combined with a 100 Hz-sweep frequency self-developed white light interferometric (WLI) interrogator and a Pitot tube, the DPFA sensor can be used for measuring the airflow velocity of 2.0∼119.24 m/s with an accuracy of 0.61%. The system is applied to the analysis of the flat-plate boundary layer, a wind tunnel experimental model, where the results are consistent with those of the theoretical analysis and from the standard electronic pressure transducer. With the large measurable range, high sweep frequency, and high precision, the system has potential application value for wind tunnel experimental investigation and in-flight measurement of airspeed.

Miniature fiber-optic tip pressure sensor assembled by hydroxide catalysis bonding technology.

A miniature fiber-optic tip Fabry-Perot (FP) pressure sensor with excellent high-temperature survivability, assembled by hydroxide catalysis bonding (HCB) technology, is proposed and experimentally demonstrated. A standard single-mode fiber is fusion spliced to a fused silica hollow tube with an outer diameter (OD) of 125 µm, and a 1-µm-thick circular silicon diaphragm with a diameter slightly larger than the OD is bonded to the other endface of the hollow tube by HCB technology. The ultrathin silicon diaphragm is prepared on a silicon-on-insulator (SOI) wafer produced by microelectromechanical systems (MEMS), providing the capability of large-scale mass production. The HCB technology enables a polymer-free bonding between diaphragm and hollow tube on fiber tip with the obvious advantages of high alignment precision, normal pressure and temperature (NPT) operation, and reliable effectiveness. The static pressure and temperature response of the proposed sensor are discussed. Results show that the sensor has a measurable pressure range of 0∼100 kPa, which is well consistent with the measurement range of biological blood pressure. The pressure sensitivity is up to 2.13 nm/kPa with a resolution of 0.32% (0.32kPa). Besides, the sensor possesses a unique high-temperature resistant capability up to 600 °C, which can easily survive even in high-temperature sterilization processes, and it has a low temperature dependence of 0.09 kPa/°C due to the induced HCB bonding technology and the silicon-based diaphragm. Thus, the proposed fiber tip pressure sensor is desirable for invasive biomedical pressure diagnostics and pressure monitoring in related harsh environments.

Multiplexing fiber-optic Fabry-Perot acoustic sensors using self-calibrating wavelength shifting interferometry.

Flexible and stable demodulation techniques of large-scale fiber-optic Fabry-Perot (FP) acoustic sensors are highly desirable for accelerating their industrial applications. In this paper, we report a novel self-calibrating wavelength shifting interferometry (WSI) technique that enables simultaneous multi-point acoustic detection using diaphragm based fiber-optic FP acoustic sensors. A widely tunable modulated grating Y-branch (MG-Y) laser (1527∼1567 nm) performs high-speed wavelength switching, introducing phase-shifts in the wavelength domain for real-time phase retrieval. The proposed self-calibrating WSI is easily extended for multiplexing FP acoustic sensors by calibrating the corresponding phase-shift step of each sensor probe. Based on a modified Hariharan 5-step phase shifting algorithm, the phase-shift step for each channel can be calibrated in real-time, making the system robust in applications involving large environmental perturbations. An all-optical multi-point acoustic detection system based on WSI is proposed and experimentally demonstrated for the first time. Sound source localization experiments show that the multi-point acoustic detection system works stably and the positioning accuracy is about 2.42 cm.

Quadrature phase-stabilized three-wavelength interrogation of a fiber-optic Fabry-Perot acoustic sensor.

In this Letter, we propose a quadrature phase-stabilized three-wavelength demodulation technique for the interrogation of fiber-optic Fabry-Perot acoustic sensors. It is based on accurate and fast tuning of a monolithic modulated grating Y-branch laser. Three quadrature wavelengths are chosen to perform high-speed cavity length demodulation by wavelength switching, thereby avoiding imbalances and disturbances between the three optical paths in conventional three-wavelength quadrature phase-demodulation systems. A feedback-stabilization scheme for maintaining the quadrature phase condition is proposed for the first time, to the best of our knowledge, providing potential for long-term monitoring in harsh environments.

Common-path dual-wavelength quadrature phase demodulation of EFPI sensors using a broadly tunable MG-Y laser.

A common-path dual-wavelength phase demodulation technique for extrinsic Fabry-Perot interferometric (EFPI) sensors is proposed on the basis of a broadly tunable modulated grating Y-branch (MG-Y) laser. It can address the three main concerns of existing dual-wavelength phase interrogation methods: the imbalances and disturbances caused by two optical paths utilizing two lasers or two photodetectors, the restrictions between two operating wavelengths and the cavity length of EFPI, and the difficulty in eliminating the direct current (DC) component of the interferometric fringe. Dual-wavelength phase interrogation is achieved in a common optical path through high-speed wavelength switching. Taking advantage of the MG-Y laser's full spectrum scanning ability (1527 ∼ 1567 nm), initial cavity length and DC component can be directly measured by white light interferometry. Two quadrature wavelengths are then selected to perform high speed phase demodulation scheme. Three polyethylene terephthalate (PET) diaphragm based EFPI acoustic sensors with cavity lengths of 127.954 µm, 148.366 µm and 497.300 µm, are used to demonstrate the effectiveness.

Low-Cost Localized Surface Plasmon Resonance Biosensing Platform with a Response Enhancement for Protein Detection.

There are many potential applications for biosensors that can provide real-time analysis, such as environmental monitoring and disease prevention. In this study, we investigated a simple strategy for real-time protein detection, which had the advantages of affordability, fast response, portability, and ease of use. A robust quantification of protein interaction was achieved by combining capillary localized surface plasmon resonance (LSPR) sensors and complementary metal-oxide-semiconductor (CMOS) image sensors. Gold nanoparticles were modified on the inner wall of the capillary, which was used as a microfluidic channel and sensing surface. We functionalized one of the LSPR sensors using ligand bound to gold nanoparticle. Our proposed biosensing platform could be easily multiplexed to achieve high throughput screening of biomolecular interactions, and it has the potential for use in disposable sensors. Moreover, the sensing signal was enhanced by the extinction effect of gold nanoparticles. The experimental results showed that our device could achieve qualitative identification and quantitative measurement of transferrin and immunoglobulin G (IgG). As a field-portable and low-cost optical platform, the proposed LSPR biosensing device is broadly applicable to various protein binding tests via a similar self-assembly of organic ultrathin films.

Ultraviolet broadband plasmonic absorber with dual visible and near-infrared narrow bands.

We propose an ultraviolet broadband plasmonic absorber with dual narrow bands located separately in the visible and near-infrared regions. It employs a three-layer dielectric and metallic film structure based on a ring square nanodisk array. The interaction of surface plasmon resonance with a Fabry-Perot cavity resonance results in perfect absorption. The absorption efficiency is greater than 99.9% at wavelengths of 660 and 919 nm (visible and near-infrared), respectively, under normal incidence. In the ultraviolet region from 240 to 500 nm, absorption efficiency of over 90% can be achieved. The geometric symmetry of the ring square makes the perfect absorber polarization-independent and insensitive to large incident angle. This perfect absorber, which combines broadband and narrowband absorption, can be used as sensors, solar cells, or thermal emitters within one integrated device with further investigations.

Refractory Ultra-Broadband Perfect Absorber from Visible to Near-Infrared.

The spectral range of solar radiation observed on the earth is approximately 295 to 2500 nm. How to widen the absorption band of the plasmonic absorber in this range has become a hot issue in recent years. In this paper, we propose a highly applicable refractory perfect absorber with an elliptical titanium nanodisk array based on a silica⁻titanium⁻silica⁻titanium four-layer structure. Through theoretical design and numerical demonstration, the interaction of surface plasmon resonance with the Fabry⁻Perot cavity resonance results in high absorption characteristics. Our investigations illustrate that it can achieve ultra-broadband absorption above 90% from a visible 550-nm wavelength to a near-infrared 2200-nm wavelength continuously. In particular, a continuous 712-nm broadband perfect absorption of up to 99% is achieved from wavelengths from 1013 to 1725 nm. The air mass 1.5 solar simulation from a finite-difference time domain demonstrates that this absorber can provide an average absorption rate of 93.26% from wavelengths of 295 to 2500 nm, which can absorb solar radiation efficiently on the earth. Because of the high melting point of Ti material and the symmetrical structure of this device, this perfect absorber has excellent thermal stability, polarization independence, and large incident-angle insensitivity. Hence, it can be used for solar cells, thermal emitters, and infrared detection with further investigation.

Boronic Acid Functionalized Au Nanoparticles for Selective MicroRNA Signal Amplification in Fiber-Optic Surface Plasmon Resonance Sensing System.

MicroRNA (miRNA) regulates gene expression and plays a fundamental role in multiple biological processes. However, if both single-stranded RNA and DNA can bind with capture DNA on the sensing surface, selectively amplifying the complementary RNA signal is still challenging for researchers. Fiber-optic surface plasmon resonance (SPR) sensors are small, accurate, and convenient tools for monitoring biological interaction. In this paper, we present a high sensitivity microRNA detection technique using phenylboronic acid functionalized Au nanoparticles (PBA-AuNPs) in fiber-optic SPR sensing systems. Due to the inherent difficulty directly detecting the hybridized RNA on the sensing surface, the PBA-AuNPs were used to selectively amplify the signal of target miRNA. The result shows that the method has high selectivity and sensitivity for miRNA, with a detection limit at 2.7 × 10-13 M (0.27 pM). This PBA-AuNPs amplification strategy is universally applicable for RNA detection with various sensing technologies, such as surface-enhanced Raman spectroscopy and electrochemistry, among others.

Real-time biodetection using a smartphone-based dual-color surface plasmon resonance sensor.

We proposed a compact and cost-effective red-green dual-color fiber optic surface plasmon resonance (SPR) sensor based on the smartphone. Inherent color selectivity of phone cameras was utilized for real-time monitoring of red and green color channels simultaneously, which can reduce the chance of false detection and improve the sensitivity. Because there are no external prisms, complex optical lenses, or diffraction grating, simple optical configuration is realized. It has a linear response in a refractive index range of 1.326 to 1.351 (R2 = 0.991) with a resolution of 2.3 × 10 - 4 RIU. We apply it for immunoglobulin G (IgG) concentration measurement. Experimental results demonstrate that a linear SPR response was achieved for IgG concentrations varying from 0.02 to 0.30 mg / ml with good repeatability. It may find promising applications in the fields of public health and environment monitoring owing to its simple optics design and applicability in real-time, label-free biodetection.

A Novel Fiber Optic Surface Plasmon Resonance Biosensors with Special Boronic Acid Derivative to Detect Glycoprotein.

We proposed and demonstrated a novel tilted fiber Bragg grating (TFBG)-based surface plasmon resonance (SPR) label-free biosensor via a special boronic acid derivative to detect glycoprotein with high sensitivity and selectivity. TFBG, as an effective sensing element for optical sensing in near-infrared wavelengths, possess the unique capability of easily exciting the SPR effect on fiber surface which coated with a nano-scale metal layer. SPR properties can be accurately detected by measuring the variation of transmitted spectra at optical communication wavelengths. In our experiment, a 10° TFBG coated with a 50 nm gold film was manufactured to stimulate SPR on a sensor surface. To detect glycoprotein selectively, the sensor was immobilized using designed phenylboronic acid as the recognition molecule, which can covalently bond with 1,2- or 1,3-diols to form five- or six-membered cyclic complexes for attaching diol-containing biomolecules and proteins. The phenylboronic acid was synthetized with long alkyl groups offering more flexible space, which was able to improve the capability of binding glycoprotein. The proposed TFBG-SPR sensors exhibit good selectivity and repeatability with a protein concentration sensitivity up to 2.867 dB/ (mg/mL) and a limit of detection (LOD) of 15.56 nM.