Dependence Of Refractive Index Change Research Articles

A Review of Sensitivity Enhancement in Interferometer-Based Fiber Sensors.

Optical fiber sensors based on an interferometer structure play a significant role in monitoring physical, chemical, and biological parameters in natural environments. However, sensors with high-sensitivity measurement still present their own challenges. This paper deduces and summarizes the methods of sensitivity enhancement in interferometer based fiber optical sensors, including the derivation of the sensing principles, key characteristics, and recently-reported applications.The modal coupling interferometer is taken as an example to derive the five terms related to the sensitivity: (1) the wavelength-dependent difference of phase between two modes/arms , (2) the sensor length , (3) refractive index difference between two modes/arms , (4) sensing parameter dependent length change , and (5) sensing parameter dependent refractive index change . The research papers in the literature that modulate these terms to enhance the sensing sensitivity are reviewed in the paper.

Trajectory manipulation of an Airy pulse near zero-dispersion wavelength under a free-carrier-generated linear potential

We investigate the dynamics of an Airy pulse that experiences free carrier generated optical linear potential in the vicinity of zero group velocity dispersion (GVD) wavelength inside a Si based waveguide. The optically induced potential can be realized by an inhomogeneous medium which possesses a time dependent refractive index. We propose a pump-probe scheme in Si-based waveguide where a strong continuous wave (CW) pump excites free carriers that leads to a linear potential through a time dependent refractive index change which is experienced by the finite energy Airy pulse (FEAP) (probe). The linear potential significantly manipulate the dynamics of a FEAP and leads to a monotonous spectral shift. We mathematically model the dynamics of the Airy pulse using linear dispersion equation containing an optical potential term and establish the general solution of the pulse for non-vanishing third order dispersion (TOD). We derive the expression of the trajectory of FEAP which deviates significantly from its usual ballistic nature and can be tailored with the strength of the linear potential. For positive TOD, the propagating Airy pulse experiences a singularity and flips in time domain. We theoretically derive that for a specific potential strength the flipping region is squeezed to a point and revives thereafter. We propose an exact analytical solution beyond flipping region for this specific case. Our theoretical analysis corroborates well with the numerical results. The present study may be useful in applications related to pulse reshaping and trajectory manipulation

Pulse energy dependence of refractive index change in lithium niobium silicate glass during femtosecond laser direct writing.

Femtosecond laser-induced refractive index changes in lithium niobium silicate glass were explored at high repetition rate (300 fs, 500 kHz) by polarized light microscopy, full-wave retardation plate, quantitative birefringence microscopy, and digital holographic microscopy. We found three regimes on energy increase. The first one corresponds to isotropic negative refractive index change (for pulse energy ranging 0.4-0.8 μJ/pulse, 0.6 NA, 5μm/s, 650μm focusing depth in the glass). The second one (0.8-1.2 μJ/pulse) corresponds to birefringence with well-defined slow axis orientation. The third one (above 1.2 μJ/pulse) is related to birefringence direction fluctuation. Interestingly, these regimes are consistent with crystallization ones. In addition, an asymmetric orientational writing effect has been detected on birefringence. These topics extend the possibility of controlling refractive index change in multi-component glasses.

Theoretical study of polarization dependence of carrier-induced refractive index change of quantum dot.

The influences of dot material component, barrier material component, aspect ratio and carrier density on the refractive index changes of TE mode and TM mode of columnar quantum dot are analyzed, and a multiparameter adjustment method is proposed to realize low polarization dependence of refractive index change. Then the quantum dots with low polarization dependence of refractive index change (<1.5%) within C-band (1530 nm - 1565 nm) are designed, and it shows that quantum dots with different material parameters are anticipated to have similar characteristics of low polarization dependence.

Compatibility of Silicon Mach-Zehnder Modulators for Advanced Modulation Formats

Silicon photonics which is compatible with mature complementary metal oxide semiconductor (CMOS) fabrication process has been extensively demonstrated for monolithic integration of photonic and electrical circuits. We show that an integrated silicon Mach-Zehnder modulator (MZM) may be used for advanced modulation formats despite the nonlinear dependence of refractive index change with applied voltage in the free-carrier depletion modulator. We experimentally demonstrated the use of a silicon MZM for direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) modulation with advanced data formats of quadrature phase-shift keying (QPSK), 8 phase-shift keying (8PSK) and 16-quadrature amplitude modulation (16-QAM). The measured bit error rate performance of the back-to-back and 50 km single mode fiber transmission of each format is well below the forward error correction limit.

Curing Methods Yield Multiple Refractive Index of Benzocyclobutene Polymer Film

Refractive index control of benzocyclobutene (BCB 4024-40) is achieved by facilitating different conditions during the thermal curing of BCB film. Refractive index (RI) change of 1.49% is obtained with curing of BCB film using an oven, while the RI change is 0.1% when the BCB is cured using a hotplate. The two different curing methods exhibit a temperature dependent refractive index change of the BCB photosensitive polymer. By carefully controlling the curing conditions, multiple layers of BCB with different RI can be fabricated, which can then be applied in the fabrication of optical waveguides.

Theoretical calculations of the carrier induced refractive index change in tensile-strained InGaAsP for use in 1550 nm semiconductor optical amplifiers

Nonlinear polarization rotation (NPR) in semiconductor optical amplifiers (SOAs) has applications in all-optical signal processing. Modeling NPR in a SOA requires knowledge of the carrier density induced refractive index change. The tensile-strained bulk SOA has attracted much interest due to its relative ease of fabrication and commercial devices are now available. In this letter we determine the polarization dependent refractive index change in such a SOA, with an InGaAsP active region, operating in the 1550 nm region and investigate its dependence on carrier density and wavelength.

Refractive Index Change in Nanoscale Thermosensitive Hydrogel for Optoelectronic and Biophotonic Applications

ABSTRACTThe familiar property responses of isopropylacrylamide hydrogel, cause the hydrogel to undergo a discontinuous volumetric phase across a critical level of stimuli. Poly(N-isopropylacrylamide) hydrogel is a recognized response to global temperature stimuli across the low critical solution temperature (LCST). The basic PNIPAM hydrogel undergoes its phase change at a LCST of 34°C, where water is expelled from the interior of the gel microsphere, as the temperature increases past LCST. The responsiveness of PNIPAM hydrogel offers the potential for controlling optical properties of a medium such as refractive index and scattering index. In the present work we present the refractive index of microsphere/microbeaded hydrogel structures necessary for optical application. The particle size of the hydrogels nanostructures at room temperature is observed to be 300-400 nm as estimated by dynamic light scattering and agree well with scanning electron microscopy measurements. The temperature dependent refractive index change of hydrogel microspheres have been measured using variable angle spectroscopic ellipsometry and shows a 10% change as the temperature changes from 33°C to 34°C.

The intensity dependent refractive index change of bacteriorhodopsin measured by the Z-scan and phase-modulated beams techniques

Abstract A comparative analysis of intensity dependent refractive index change of a thin polymer bacteriorhodopsin (bR) film by the Z -scan and phase-modulated beams techniques is given. The analysis is conducted at two excitation wavelengths, λ =543 and 633 nm, corresponding to the opposite slopes of the bR ground-state absorption band resulting in opposite signs of the refractive index change. A three-level model of the bR photo-cycle is proposed to describe the experimental data.

Application of the polyconjugated main chain polymer DPOP-PPV for ultrafast all-optical switching in a nonlinear directional coupler

The observation of ultrafast all-optical switching in a polyconjugated main chain polymer (DPOP-PPV), nonlinear directional coupler at a wavelength with an energy below two-photon resonances is reported. The effect responsible for the switching is clearly the nonresonant instantaneous intensity dependent refractive index change. To our knowledge these results represent the first demonstration of an ultrafast device using a polyconjugated polymer, where the switching is not drastically limited by absorption processes.

The intensity dependent refractive index change of photochromic proteins

We report the nonlinear refractive index behaviour of bacteriorhodopsin and photoactive yellow protein films. A Michelson interferometer with a probe and excitation beams was used in the measurements. By using this technique instead of the Z-scan we can easily separate the index change caused by photochromism and the thermal expansion. Sigmoidal index changes in the logarithmic intensity scale were observed.

Self-focusing in Terbium Gallium Garnet using Z-scan

When illuminated near its resonance with an Ar ion laser beam ( λ=488 nm), laser induced thermal self-focusing is observed in Terbium Gallium Garnet. The crystal exhibits a strong intensity dependent refractive index change Δ n. The Z-scan technique is used to study the beam waist change due to Δ n. The refractive index is found to be well described by a quadratic spatial distribution model. Both the sign and the distribution coefficient of Δ n are determined.

Secondary-ion-mass spectrometry and near-field studies of Ti:LiNbO3 optical waveguides

Secondary-ion-mass spectrometry (SIMS) and near-field (NF) methods have been applied to study Ti:LiNbO3 optical waveguides. Ti concentration as a function of diffusion process parameters has been studied by the SIMS method. The main determining factors that were found to affect the depth-diffusion behavior of titanium in LiNbO3 slab waveguides are the initial thickness of the dopant film and the diffusion temperature. Anisotropy in the diffusion rate for Xand has been applied to the refractive index profile reconstruction for single-mode optical channel waveguides. A sharp change in the index at the air–guide interface has been observed, as expected. The dependence of refractive index change on Ti concentration has been found to be nonlinear, such as quadratic, approximately.

Ruby laser‐induced frequency dependent refractive index change in KCl:Li+ FA crystal

Ruby laser‐induced frequency dependent refractive index change in KCl:Li⁺ F_A crystal

Two step excitation in LiTaO 3:Fe for optical data storage

A stepwise two-photon excitation is used to record elementary volume phase holograms in LiTaO 3:Fe crystals. We demonstrate readout without erasure, however, desired optical erasure via two-photon processes is still possible. Time, intensity, and Fe-concentration dependence of refractive index change are investigated and can be explained by the known photorefractive process in this material.

Dispersion characteristics of diffused channel waveguides

A variational method is used to calculate dispersion curves for channel waveguides fabricated by masked diffusion. Composition profiles for the waveguides are obtained by numerical solution of the diffusion equation, assuming that the diffusant concentration in the unmasked region of the substrate surface is constant during the diffusion process. A linear dependence of refractive index change on diffusant concentration is assumed. The dependence of the number of guided modes on mask gap width and diffusion depth is determined as an aid to the design of modulators and switches for integrated optics.