Wave measurement based on light refraction

Some authors have developed a few methods of measuring wave slopes based on light refraction, including the measurement method via the distribution of light intensity or color under water. A new method based on light refraction is specified for the measurement of wave surface elevation in wave flume via imaging technology. A plane painted with black and white stripes is put on the flume floor as an indication plane, which can be arranged easily and cheaply. Compared with the previous methods, the present method is less sensitive to the noise and nonlinear effects of optical process, which can be taken as a digital method. The CCD camera is fixed above the flume with its optical axis arranged vertically to grab the images of stripes modulated by the wave surface. The modulated value can be calculated from the Hilbert transform, and then the wave surface elevation can be obtained. The algorithm and experimental procedure are specified in detail, and some experimental results are provided to show the validity of the present method.

Light propagation and reflection-refraction event in absorbing media

The light propagation within an absorbing medium and the reflection and refraction at the interface of two absorbing media are studied. By using the unit vectors denoting the planes of constant field amplitude and constant phase respectively, the light propagation and attenuation are described by the effective refractive indices which depend on both the complex refractive index of the medium and the angle between the unit vectors. With the expression for the light propagation, the corresponding Snell＇s law and the expression of Fresnel coefficients are obtained, which can be applied to describe the reflection-refraction event at the interface between an arbitrary combination of transparent and absorbing media.

Negative refraction and reflection of Gaussian beam on two-dimensional photonic crystal slab

The refraction and reflection of Gaussian beam on two-dimensional photonic crystal （2DPC） slab are studied by the finite-difference time-domain （FDTD） method. It is shown that 2DPC slab enables negative refraction, which is demystified by the distribution of Poynting vector directions and values inside the 2DPC. A negative Goos-Haenchen shift is also found in the simulation.

Fourier analysis of elastic light scattering spectrum of epithelial cell nuclei

A new method for simultaneously determining the size and refractive index of epithelial cell nuclei is presented. The function of the modified elastic light scattering spectrum is regarded as a function of wave number factor, Q=2λ-1sin（θ/2）. The modified spectrum has a constant oscillation period with its frequency proportional to the average diameter of cell nuclei. To the same average diameter, the different relative refractive indexes of epithelial cell nuclei only induce the horizontal shift of the spectra. Both the oscillation frequency and the horizontal shift are quantified by the fast Fourier transform on the modified spectra. The average diameter can be figured out through the peak frequency divided by the value of the refractive index of the surrounding medium. The phase angle of the peak frequency has an approximate linear relationship with the relative refractive index of epithelial cell nuclei.

Light transmission in porous silicon dioxide filled with liquids of different refractive indices

Optical transmission at 532 nm from nonabsorbing disordered porous silicon dioxide has been studied experimentally. The transmission behaviors can be adjusted by filling the pores with liquids of different refractive indics, which are analyzed based on the theory of diffusion in a weak scattering regime. In our experiment, the transmission coefficient changes from a value less than 1% to one that is greater than 75%, that is, the opaque sample becomes transparent, which means that the transport mean free path of light within the material has been effectively adjusted. In addition, this method is a useful nondestructive method to derive the refractive index of an unknown bulk porous material.

Negative refraction and subwavelength imaging of a photonic-crystal slab for the frequencies in the third band

Negative refraction and subwavelength imaging properties of a two-dimensional （2D） photonic crystal （PC） slab are studied by the finite-difference time-domain method. The PC consists of a triangular lattice of air holes immersed in a dielectric. For a certain frequency range in the third photonic band, the directions of the group velocities and the phase velocities can be opposite, so the PC can work as a kind of negative refractive-index material. The light radiated from a point source can form a subwavelength image spot through the PC slab. Negative refraction and an effective refractive index of the PC slab n = -1 can be achieved for the incident wave with its incident angle within a certain range.

Multicolor coherence-induced negative refraction in three-level atomic system

Multicolor manipulation of negative refraction in a three-level A atomic system is theoretically investigated. Based on multicolor coherence, the negative refl-active index can be obtained with reduced absorption. The refractive index can also be controlled by changing the sun, of the phases of the sidebands in the trichromatic driving fields. By adjusting the sum phase, the refractive index can be varied between negative and positive in two different frequency bands. Furthermore, the frequency band corresponding to the negative refractive index is widened by increasing the intensity and the frequency difference of the trichromatic field.

Study of the characteristics of concentration sensing based on FBG Fabry-Perot cavity

Fiber Bragg grating Fabry-Perot (FBG F-P) cavity is used as the sensing model to measure the refractive index of the liquid solution. The cladding of the fiber, which is used as the F-P cavity, is etched by HF solution to enhance the sensitivity to the external refractive index. The experimental results show that with the concentration change of the external solution, the effective refractive index of etched fiber will change, thus the spectra of FBG F-P cavity will appear a spilt point. The relationship be...

Parameters optimization of π-phase-shifted long-period fiber grating for gas sensing

The gas-sensitive film coated π-phase-shifted long-period fiber grating(LPFG),consisting of two same long-period fiber gratings cascaded by a regular fiber with length of half of the grating period,is proposed for gas sensing.The sensitivity of the sensor is defined by the change rate of the transmission at specific wavelength with respect to the film refractive index.The qualitative and numerical analyses show that to get a high sensitivity,the optimal coupling strength of each long-period fiber grating is √2 π/4.With this coupling strength,the longer the grating is,the higher sensitivity the sensor has.By choosing appropriate grating period,a high sensitivity for the film with a certain refractive index and thickness can be attained.

Ultrathin atomic vapor film transmission spectroscopy: analysis of Dicke narrowing structure

Transmission sub-Doppler spectroscopy with confined atomic vapor film between two dielectric walls is theoretically studied. Because of atoms flying from wall to wall, where they get de-excited, the atomfield interaction time is anisotropic so that the contribution of slow atoms is enhanced, a sub-Doppler transmission spectroscopy （Dicke narrowing effect） can be obtained when the thickness of the film is much small or comparable with the wavelength even at small angle oblique incidence. It is feasible to get a sub-Doppler structure in a new region （L 〈 λ/4） in experiments.

Influences of Y_2O_3 dopant content on residual stress,structure,and optical properties of ZrO_2 thin films

Four kinds of Y2O3 stabilized ZrO2 （YSZ） thin films with different Y2O3 contents （from 0 to 12 mol%） are deposited on BK7 glass substrates by electron-beam evaporation method. The effects of different Y2O3 dopant contents on residual stress, structure, and optical properties of ZrO2 thin films are investigated. The results show that residual stress in YSZ thin films varies from tensile to compressive with the increase of Y2O3 molar content. The addition of Y2O3 is beneficial to the crystallization of YSZ thin film and transformation from amorphous to high temperature phase, and the refractive index decreases with the increase of Y2O3 molar content. Moreover, the variations of residual stress and the shifts of refractive index correspond to the evolution of structures induced by the addition of Y2O3.

Effect of crystalline lens surfaces and refractive index on image quality by model simulation analysis

The surfaces and refractive index of crystalline lens play an important role in the optical performance of human eye. On the basis of two eye models, which are widely applied at present, the effect of lens surfaces and its refractive index distribution on optical imaging is analyzed with the optical design software ZEMAX （Zemax Development Co., San Diego, USA）. The result shows that good image quality can be provided by the aspheric lens surfaces or （and） the gradient-index （GRIN） distribution. It has great potential in the design of intraocular lens （IOL）. The eye models with an intraocular implantation are presented.

Investigation of mode radiation loss for microdisk resonators with pedestals by FDTD technique

Mode radiation loss for microdisk resonators with pedestals is investigated by three-dimensional（3D） finite-difference time-domain（FDTD） technique.For the microdisk with a radius of 1μm,a thickness of 0.2μm,and a refractive index of 3.4,on a pedestal with a refractive index of 3.17,the mode quality（Q） factor of the whispering-gallery mode（WGM） quasi-TE_（7.1） first increases with the increase of the radius of the pedestal,and then quickly decreases as the radius is larger than 0.75μm.The mode radiation loss is mainly the vertical radiation loss induced by the mode coupling between the WGM and vertical radiation mode in the pedestal,instead of the scattering loss around the perimeter of the round pedestal.The WG M can keep the high Q factor when the mode coupling is forbidden.

Characteristics of a PLZT electro-optical deflector

A beam deflector was designed and fabricated by using a lead lanthanum zirconate titanate （PLZT） ceramic with high quadratic electro-optic （EO） effect, based on refraction at an interface between areas with and without applied voltage. Its EO coefficients of R33 = 2.1×10^-16 V^2/m^2 and R13 = -0.37×10^-16 V^2/m^2 were obtained. Moreover, the hysteresis characteristics of the EO deflection were observed, and the effects of temperature on the characteristics of beam deflector were also investigated in detail.

Negative refractive index in non-resonance spectrum area

A new concept to realize negative refractive index in non-resonance spectrum using chiral metamaterial is proposed.A low-index metamaterial is added to diminish the effective refractive index of the combined structure.Simulation and material parameter retrieval procedures are carried out to determine material performances.Results show evidence of negative refractive index and strong optical activity in the new chiral metamaterial.

Optical humidity-sensitive mechanism based on refractive index variation

A novel composite model is put forward for humidity-sensitive material based on Maxwell-Garnett and effective medium theory. The analytical expression of the relation between effective refractive index and relative humidity is shown with different absorption factors and porosities. The larger the absorption factor is, the higher the refractive index is. The refractive index of humidity-sensitive SiO2 film decreases with the increase of ceramic material porosity. The sensitivity of optical humidity sensor can reach the magnitude of 10-3. In comparison with the experimental humidity-sensing curve by the method of p-polarized reflectance and the analysis of mechanism, theoretical simulation is in agreement with experimental results. Therefore, this composite model is proved to be reasonable which lays new theoretical foundation in further research on optical humidity sensor.

Optical properties of highly photosensitive hybrid sol-gel films

A new organic-inorganic hybrid material,which shows photo-induced reduction of refractive index as well as volume contraction,is prepared using a sol-gel method.This material is coated on a Si substrate by spin-coating to manufacture film.After irradiated by ultraviolet（UV） light with a deuterium lamp for 5 h, the thickness of film decreases largely by 55%,and the refractive index of film changes from 1.484 to 1.445 at 550 nm.The film＇s optical thickness exhibits an exponential change with the increasing irradiation time.Futhermore.the photo-patternning of the organic-inorganic hybrid film without any further process （wet etching and thermal curing） is performed utilizing the volume contraction on UV-light irradiation. This film has potential applications for fabrication of patterned filter array and apodizing filter by direct light writing,and also demonstrates good temperature stability and immunity to visible light exposure.

A method of measuring the refractive index of extraordinary ray in uniaxial crystal with optic axis at an arbitrary orientation

A new equation to measure the refractive index of extraordinary ray in uniaxial crystal with the optic axis at an arbitrary orientation has been given in this letter, and the term in this equation makes the measurements to be relatively easy. The theoretical study shows that the accuracy achieved in the experiments attains to the order of magnitude in 10^-3. .

Reflex optical sensor based on dual metal gratings with high angular sensitivity

An interesting reflection phenomenon in a dual metal grating （DMG） structure is studied, which is related to the competition between Fabry-Prot （F-P） resonance effect and evanescent-field coupling effect inside the gap between the two composing single metal gratings. This competition leads to high angular sensitivity in response to the refractive index variation of the sample solution in the gap. A reflex optical sensor with high sensitivity based on DMG for detecting the change in refractive index is proposed and its performance theoretically is discussed.

Photosensitivity and photoluminescence of Sn/Yb codoped silica optical fiber preform

Sn/Yb codoped silica optical fiber preform is prepared by the modified chemical vapor deposition （MCVD） followed by the solution-doping method. Ultraviolet （UV） optical absorption, photoluminescence （PL） spectra under 978-nm laser diode （LD） pumping, and refractive index change after exposure to 266-nm laser pulses are obtained. There is only a little change in the PL spectra while a positive refractive index change up to 2×10^-4 is observed after 30-min exposure to 266-nm laser pulses. The results show that both of the peculiar photosensitivity of Smdoped silica and the gain property of Yb-doped silica fiber are preserved in the Sn/Yb codoped silica optical fiber preform. The experimental data suggest that the photosensitivity of the fiber preform under high energy density laser irradiation should be mainly due to the bond-breaking of oxygen deficient defects, while under relatively low energy density laser irradiation, the refractive index change probably originates from the photoconversion of optically active defects.