Refractive Index Perturbation Research Articles

Numerical analysis of an advanced infrared-based metasurface surface plasmon resonance sensor for COVID-19 detection

The SARS-CoV-2 pandemic has highlighted the critical need for rapid and precise diagnostic methodologies. Conventional testing approaches often exhibit limitations in terms of speed, accessibility, and sensitivity. To address these constraints, we propose an advanced graphene-based biosensing platform for SARS-CoV-2 detection. This sensor integrates advanced nanophotonic principles, metamaterial physics, and two-dimensional material properties to achieve enhanced sensitivity through the detection of refractive index perturbations induced by SARS-CoV-2 biomolecules. Electromagnetic simulations were conducted to optimize the sensor design. The resulting configuration demonstrates a sensitivity of 1800 nmRIU−1 and a detection limit of 0.007 RIU. The sensor exhibits multi-wavelength sensing capabilities at 3.942 μm, 4.018 μm, 4.088 μm, and 4.201 μm, with a high figure of merit of 104.167 RIU−1 and quality factors ranging from 65.766 to 251. 750.The sensor's multilayer architecture, comprising tungsten, graphene, and strontium titanate synergistically enhances both sensitivity and selectivity. Its compact form factor and capacity for real-time analysis render it a promising candidate for point-of-care COVID-19 diagnostics.

Non-linear media in weakly curved spacetime: optical solitons and probe pulses for gravimetry

That light propagating in a gravitational field gets frequency-shifted is one of the basic consequences of any metric theory of gravity rooted in the equivalence principle. At the same time, also a time dependent material’s refractive index can frequency-shift light propagating in it. The mathematical analogy between the two effects is such that the latter has been used to study the optical analogue of a black-hole spacetime. Here, we combine these two effects by showing that light propagation in non-linear media in the presence of a moving refractive index perturbation can lead to a gravity-dependent blueshift. We find that the predicted blueshift surpasses the gravitational redshift even if the medium is considered to be perfectly stiff. In realistic scenarios, by far the strongest frequency shift arises due to the deformation of the dielectric medium and the corresponding photoelastic change of refractive index. This has the potential to facilitate optical sensing of small gravity gradients.

Turbulence-enhanced THz generation by multiple chaotically-distributed femtosecond filaments in air

Remote generation of electromagnetic waves in the terahertz (THz) by the volume of air medium gains increasing scientific interest due to the demand for this spectral range in solving various practical problems including those belonging to the nonlinear atmospheric optics. During self-focusing and filamentation of high-power femtosecond laser pulses in air, the source of THz radiation is the plasma of laser filaments. One of the main challenges of THz generation by a laser filament is increasing the value of optical energy conversion to the long-wavelength band. In this paper, we present the results of our studies on DC-biased THz generation during single-color filamentation in air of focused femtosecond Ti:Sapphire-laser pulses (800 nm, pulse energy up to 40 mJ). The distinctive feature of our study is that a femtosecond optical pulse propagates through a spatially-localized heated air layer containing randomly inhomogeneous refractive index perturbations, which mimics strong air turbulence. For the first time to our knowledge, we show, that the turbulent air layer formed at the beginning of the optical path allows increasing the yield of THz radiation up to 1.5 times due to the formation of multiple chaotically-arranged filaments resulting from random perturbations of the optical beam energy profile that reduces the subsequent THz (re)absorption in the filament plasma.

Mode coupling in FM-EDFAs caused by fiber bending: theory and experiments.

Refractive index perturbation caused by erbium-doped fiber (EDF) bending is inevitable in the fabrication of erbium-doped fiber amplifiers (EDFAs). The resulting mode coupling might bring about the deviation of theoretical results from experimental data. We present a theoretical model of FM-EDFAs with mode coupling due to fiber bending and carry out a proof-of-concept experiment by a 3.2-m-long EDF stretcher. Our experiments show that the fluctuation of modal gain due to fiber bending is about 1.5dB for L P 01 and L P 11e modes, and about 2.5dB for L P 11o mode, and the theoretical model is more useful for the FM-EDFA design in the presence of fiber bending.

Deeply-Tapered Ultrashort Long-Period Fiber Grating and Its Application to Ultrasensitive Transverse-Load Sensor

Deeply-tapered long-period fiber gratings (DT-LPFGs) have been demonstrated, which are fabricated by using the cylindrical-lens-assisted CO2 laser technique. Attributed to the effects of both the refractive-index perturbation and geometrical perturbation in the deeply-tapered structure, the proposed DT-LPFG is of the high coupling-efficiency and thus an ultrashort yet strong LPFG can be achieved. As a typical example, a LPFG with a length of only ∼1 mm and loss-dip of ∼ 33 dB in spectrum has been successfully obtained, which, based on our knowledge, is the shortest one among all those of the strong LPFGs reported to date. The fabricated DT-LPFG is also of the low polarization-dependence, which considerably benefits the practical application of grating itself. As an application of the proposed DT-LPFG, a transverse load sensor with sensitivity up to 1246.594 nm/(N/mm) has been successfully demonstrated, which represents the best result among all those of the fiber-based transverse load sensors.

Monolithic Quantum Dot Discrete Mode Laser on SOI

Single-longitudinal-mode light sources on a silicon platform exhibit tremendous potential in data transmission, integrated on-chip optical interconnects, and optical ranging. A unique InAs/GaAs quantum dot discrete mode (DM) laser directly grown on a silicon-on-insulator (SOI) substrate with embedded silicon gratings has been demonstrated here, which is regrowth-free and compatible with integrated silicon photonics. Attributed to refractive index perturbation and optical feedback from embedded silicon gratings prepatterned on the SOI substrate, this DM laser can provide stable single-mode operation with a threshold current of 45 mA and an output power of 9 mW. The maximum optical signal-to-noise ratio (OSNR) achieved is 43.6 dB with an optical linewidth of 157 kHz. The 3 dB direct modulation bandwidth is measured as 3.6 GHz. The DM laser can be externally modulated at 70 Gbit/s with non-return-to-zero (NRZ) modulation format and 80 Gbit/s under 4-level pulsed amplitude (PAM4) modulation format. Such SOI-based DM laser paves the way toward large-scale and high-power on-chip integrated lasers.

Robust single frequency index-patterned laser design using a Fourier design method.

We use a Fourier-transform based method to investigate the magnitude and robustness of mode selectivity in as-cleaved discrete-mode semiconductor lasers, where a small number of refractive index perturbations are introduced into a Fabry-Pérot laser cavity. Three exemplar index perturbation patterns are considered. Our results demonstrate the capability to significantly improve modal selectivity by choosing a perturbation distribution function that avoids placing perturbations near to the cavity centre. Our analysis also highlights the ability to select functions that can increase the yield despite facet phase errors introduced during device fabrication.

Optical fiber point sensors based on forward Brillouin scattering.

Forward Brillouin scattering interactions support the sensing and analysis of media outside the cladding boundaries of standard fibers, where light cannot reach. Quantitative point-sensing based on this principle has yet to be reported. In this work, we report a forward Brillouin scattering point-sensor in a commercially available, off-the-shelf multi-core fiber. Pump light at the inner, on-axis core of the fiber is used to stimulate a guided acoustic mode of the entire fiber cross-section. The acoustic wave, in turn, induces photoelastic perturbations to the reflectivity of a Bragg grating inscribed in an outer, off-axis core of the same fiber. The measurements successfully analyze refractive index perturbations on the tenth decimal point and distinguish between ethanol and water outside the centimeter-long grating. The measured forward Brillouin scattering linewidths agree with predictions. The acquired spectra are unaffected by forward Brillouin scattering outside the grating region. The results add point-analysis to the portfolio of forward Brillouin scattering optical fiber sensors.

Polarization-insensitive refractive index measurement using cascaded perpendicular tilted fiber Bragg gratings

This paper presents a possibility to improve the reliability of the measurement of the liquid solution refractive index using tilted fiber Bragg gratings (TFBGs) under various inputs of light polarization angle. The application of a specialized setup of cascaded TFBGs provides a great improvement in the stability of cladding resonances under various light polarization. In the present paper, the analysis of the amplitude of the cut-off cladding mode shows that the dependence of this parameter in the case of the proposed structure is reduced by 96.101% compared with a single TFBG with the same internal tilt angle of 7° and a depth of periodic refractive index perturbations. Stabilization of the spectra under various polarizations of inputted light could be observed in the case of all cladding modes, which could be used to improve the surrounding refractive index (SRI) measurement over a wider range.

Generalized Cross-Correlation Strain Demodulation Method Based on Local Similar Spectral Scanning.

Optical fiber measurement technology is widely used in the strength testing of buildings, the health testing of industrial equipment, and the minimally invasive surgery of modern medical treatment due to its characteristics of free calibration, high precision, and small size. This paper presents an algorithm that can improve the range and stability of strain measurements in order to solve the problems of the small range and measurement failure of optical fiber strain sensors based on optical frequency-domain reflectometry (OFDR). Firstly, a Rayleigh scattering model based on the refractive index perturbation of an optical fiber is proposed to study the characteristics of Rayleigh scattering and to guide the strain demodulation algorithm based on the spectral shift. Secondly, a local similar scanning method that can maintain a high similarity by monitoring local Rayleigh scattering signals (LSs) before and after strain is proposed. Thirdly, a generalized cross-correlation algorithm is proposed to detect spectral offset, solving the problem of demodulation failure in the case of a Rayleigh scattering signal with a low signal-to-noise ratio. Experiments show that the proposed method still has high stability when the spatial resolution is 3 mm. The measurement precision is 6.2 με, which proves that the multi-peaks or pseudo-peaks of the traditional algorithm in the case of a large strain, the high spatial resolution, and the poor signal-to-noise ratio are solved, and the stability of the strain measurement process is improved.

Temporal Evolution of Refractive Index Induced by Short Laser Pulses Accounting for Both Photoacoustic and Photothermal Effects

Materials such as silicon, copper, gold, and aluminum exhibit strong absorption and scattering characterization under short-pulsed laser irradiation. Due to the photoelastic effect and thermoelastic relaxation, the focal area may induce a local modulation in the refractive index, which can be detected with the intensity reflection coefficient perturbation. Normally, the thermal effect causes a weak refractive index change and is negligible, compared with the pressure-induced effect in most photoacoustic analytical systems. In this study, we present a theoretical model with the whole process of absorbed energy conversion analysis for the refractive index perturbation induced by both thermal effect and photoacoustic pressure. In this model, data analysis was carried out on the transformation of the energy absorbed by the sample into heat and stress. To prove the feasibility of this model, numerical simulation was performed for the photothermal and photoacoustic effects under different incident intensities using the finite element method. Experiment results on silicon and carbon fiber verified that the refractive index change induced by the photothermal effect can be detected and be incorporated with pressure-induced refractive index change. The simulation results showed very good agreement with the results of the experiments. The main aim of this study was to further understand the absorption and conversion process of short-pulsed light energy and the resulting photothermal and photoacoustic effects.

Modeling the propagation of a high-average-power train of ultrashort laser pulses.

We investigate the interpulse thermal interaction of a train of ultrashort laser pulses and develop a model to describe the isobaric heating of air by a train of pulses undergoing filamentation. We calculate the heating of air from a single laser pulse and the resulting refractive index perturbation encountered by subsequent pulses, and use this to simulate the propagation of a high-power pulse train. The simulations show deflection of laser filaments by the thermal refractive index consistent with previous experimental measurements.

Compact silicon-based polarization-independent 1.55/2 μm wavelength diplexer based on a multimode interference coupler with multiple shallow grooves

Faced with the rapid growth of internet traffic demand, which has posed a huge challenge to the current fiber-based communications systems, expanding the telecommunications bands from traditional near-infrared (NIR) region to mid-infrared (MIR) region is emerging as an attractive solution. And on-chip wavelength division multiplexing (WDM) application for NIR/MIR wavelengths is an essential element in future optical telecommunication systems. Here, we propose a compact silicon-based polarization-independent wavelength diplexer for the NIR/MIR wavelengths of 1.55/2 μm, where multiple rectangular grooves are shallowly etched on the silicon MMI waveguide for introducing the refractive-index perturbations on the MMI waveguide eigenmodes. And this refractive-index perturbation scheme provides a simple and scalable method to manipulate the four beat lengths of the two wavelengths at TE- or TM-polarization in a single waveguide via self-imaging effect, leading to a compact footprint. By optimizing the structural parameters, both NIR/MIR wavelength demultiplexing and polarization independence, for the first time, are obtained simultaneously in the present single device. From the results, the proposed diplexer is only 21.6 μm in length, and shows a wide bandwidth of ∼ 100/120 nm around the wavelength of 1.55/2 μm for insertion loss (IL) < 1 dB and extinction ratio (ER) > 15 dB. Moreover, we also realize a compact and polarization-independent wavelength diplexer for 1.31/2 μm by flexibly changing the dimensions, showing the flexibility and extensibility of our refractive-index perturbation scheme. Besides, fabrication tolerances are analyzed and mode propagation profiles are also demonstrated.

Enhanced SPR-Based Localized and Bulk Sensing Using a Plasmonic Nanopillar Array With Spacer

We report a substantially improved localized and bulk sensing response of a plasmonic sensor based on a plasmonic nanopillar array spacer-separated from a plasmonic thin film and operated using the conventional Kretschmann configuration. This paper describes theoretical investigations of the spectral and angular interrogation of the proposed sensors to determine the spectral or angular reflectivity minima. Shifts in the reflectivity minima are determined to obtain information on perturbations in localized (local binding analyte layer sensing) and bulk refractive index (bulk medium sensing) on the sensor surface. Optimization of the structural parameters of the proposed sensor yields a highly enhanced sensor performance, with an extremely high sensitivity (9920 nm RIU^&#x2212;1) and figure of merit (58.2 RIU^&#x2212;1) for bulk sensing response, as well as extremely high sensitivity (76 nm/nm) and figure of merit (0.5 nm^&#x2212;1) for localized sensing response upon spectral interrogation of the sensor. The localized surface sensitivity for the proposed sensor, which is evaluated to be 76 nm/nm, is higher than what has been reported till now. The proposed sensors do not require any sophisticated optical set-ups and can be implemented on the optical setups used for the conventional Kretschmann configuration based sensors. The proposed sensor design could easily be fabricated using the state-of-the-art top-down nanofabrication approaches and could be employed for developing highly sensitive biosensing platforms.

Coupling-Controlled Multiport Thermo-Optic Switch Using Polymer Waveguide Array

A multiport thermo-optic switch based on parallel polymer waveguides and microheater array is proposed. The basic unit is a 1×3 switch for Up Transfer, Isolation, and Down Transfer operations. By introducing refractive index perturbation along the selected waveguides through microheaters, the Transfer /Isolation condition can be switched locally, and light is then steered into different paths. A 1×5 switch is demonstrated experimentally as an example. Colorless ( 1.50 μm to 1.62 μm) and polarization independent operation is achieved with crosstalk suppression ratio larger than 10 dB, which can be further optimized. The design requires less waveguides and tuning sections than conventional Mach-Zehnder interferometer-based switches. 6 microheaters are placed but only 3 need to be powered on simultaneously to reach any of the 5 output ports. The maximal heater power is 33.8 mW over a coupling length of 1331.6 μm. This design can easily scale up to form an arbitrary cascaded M ×1 and 1× N switch network for high-density photonic integration.

Vector Magnetometer Based On Localized Scattering Between Optical Fiber Spectral Combs and Magnetic Nanoparticles

The apparent increase in magnetic field measurement has led to a growing demand for new sensing technologies. However, a long-lasting challenge unaddressed is how to achieve ultrahigh sensitive magnetic field measurement in both amplitude and orientation. Here, we demonstrate a compact-in-size and simple-to-implement fiber-optic magnetometer for highly sensitive vector magnetic field measurement. The sensor is based on magnetic nanoparticles coated tilted fiber Bragg grating inscribed in a commercial single mode fiber. The sensing mechanism stems from the backward coupled cladding mode resonances with narrow bandwidth (Q factor &gt; 10⁴), which is highly sensitive to slight surface refractive index perturbation. The switching of the magnetic field direction on the surface of fiber induces redistribution of coated magnetic nanoparticles with spatial anisotropy density and scattering by formation of magnetic chains hereafter the effective index is changed, which in turn modulates the cladding mode resonance with wavelength shift and amplitude attenuation. Through calculating the peak-to-peak intensity of target cladding modes of TFBG, the external magnetic field was precisely measured with an intensity sensitivity of 0.39 dB&#x002F;mT and a direction sensitivity of 0.038 dB&#x002F;rad. The inherent core mode can be used as a reference to get rid of temperature crosstalk and light source disturbance. The proposed sensor is compact in size and easy to be used in hard-to-reach place, showing good potentials for industrial applications.

Efficient and wideband acousto-optic modulation on thin-film lithium niobate for microwave-to-photonic conversion

Microwave photonics, a field that crosscuts microwave/millimeter-wave engineering with optoelectronics, has sparked great interest from research and commercial sectors. This multidisciplinary fusion can achieve ultrawide bandwidth and ultrafast speed that were considered impossible in conventional chip-scale microwave/millimeter-wave systems. Conventional microwave-to-photonic converters, based on resonant acousto-optic modulation, produce highly efficient modulation but sacrifice bandwidth and limit their applicability for most real-world microwave signal-processing applications. In this paper, we build highly efficient and wideband microwave-to-photonic modulators using the acousto-optic effect on suspended lithium niobate thin films. A wideband microwave signal is first piezoelectrically transduced using interdigitated electrodes into Lamb acoustic waves, which directly propagates across an optical waveguide and causes refractive index perturbation through the photoelastic effect. This approach is power-efficient, with phase shifts up to 0.0166 rad / √ mW over a 45 μm modulation length and with a bandwidth up to 140 MHz at a center frequency of 1.9 GHz. Compared to the state-of-the-art, a 9 × more efficient modulation has been achieved by optimizing the acoustic and optical modes and their interactions.

Localization of a Gaussian beam in an electrically tunable curved Airy channel in a paraelectric Mn:KLTN crystal.

An optical Airy channel is built in a paraelectric Mn: KLTN crystal via a photo-induced Airy beam based on the photo-refractive effect. A laser beam, incident with the main lobe of the Airy channel, propagates along the Airy channel with a bias field as the control parameter. We find that the light beam is nicely confined in the Airy channel and propagates along it, presenting Airy-like properties of non-diffraction and bending action under a certain voltage range; then a breathing soliton can be formed with bias electric field rising, originating from space-charge-field-induced nonlocal refractive index perturbation. The experiment is corroborated by simulation. This Letter opens up new possibilities for fabricating an electrical engineering functional device, such as optical routing.

Compact silicon three-mode multiplexer by refractive-index manipulation on a multi-mode interferometer.

Mode-division multiplexing can scale the capacity of optical communications and optical interconnects. We demonstrate an ultra-compact and fabrication-error tolerant silicon three-mode multiplexer by shallowly etching rectangular trenches on a multi-mode interferometer. Depending on the selected input port, the TE0 mode is converted to the eigenmodes of the bus waveguide. These modes are coupled to each other owing to the refractive-index perturbation induced by the shallow trenches and finally converted to a selected spatial mode at the output. A three-mode multiplexing device is experimentally demonstrated with a footprint of 2.00 × 17.05 µm2. The bandwidths for the three channels are >70 nm with crosstalk values below -10 dB.

Mode conversion from LP11 to LP02 based on in-line mode orientation control and antisymmetric refractive-index perturbation in a few-mode fibre

We present intra-fibre mode conversion from the antisymmetric LP11 mode to the circularly symmetric LP02 mode using a force-induced, long-period fibre grating accompanied by an in-line polarization controller. The field orientation of the incoming LP11 mode is aligned to the antisymmetric perturbation in the fibre grating to activate resonant mode coupling. Mode conversion is achieved by applying pressure with a spatial period of 430 μm along an interaction length of 50 mm. We verify the mode conversion process by observing the far-field patterns of the fibre output and comparing the transmission power with and without a high-numerical aperture, narrow-core fibre. The intra-fibre mode conversion system with erasable and reconfigurable natures exhibits an insertion loss of 0.48 dB and a 3-dB bandwidth of 52 nm.