Uniform Refractive Index Research Articles

Femtosecond laser direct writing large-area fiber Bragg grating based on diaphragm shaping

We propose and demonstrate a new method of direct writing large-area fiber Bragg grating by femtosecond laser through the coating. By adding an adjustable diaphragm before the focusing objective, we can precisely control the length of the refractive index modulation line along the femtosecond laser incident direction up to 29.1 µm. In combination with femtosecond laser scanning fabrication technology, a uniform refractive index modulation plane can be inscribed in the fiber in a single scanning. Based on the plane-by-plane inscription method, we have fabricated a high-quality high-reflectivity fiber Bragg grating and a chirped fiber Bragg grating on 20/400 double-clad fiber core. The reflectivity of both gratings is greater than 99%, and the insertion loss is as low as 0.165 dB and 0.162 dB, respectively. The thermal slope of chirped fiber Bragg grating without any refrigeration is 0.088 °C/W and there is no obvious temperature increase when using the water cooling. Therefore, the fabrication method of large-area fiber Bragg grating based on diaphragm shaping can efficiently fabricate high-quality fiber Bragg grating in the large core diameter fiber, which has an important application prospect in high-power all-fiber oscillators, especially all-fiber oscillators in special wavebands.

Method for Refractive Index Uniformity Measurement Based on Two-flat and Three-flat Test

Method for Refractive Index Uniformity Measurement Based on Two-flat and Three-flat Test

X-Cut Lithium Niobate Optical Waveguide with High-Index Contrast and Low Loss Fabricated by Vapor Proton Exchange

Highly integrated and stable devices are appealing in optical communication and sensing. This appeal arises from the presence of high refractive index contrast and high-quality waveguides. In this study, we improved the vapor proton exchange (VPE) process, enabling large-scale waveguide fabrication and addressing the issue of liquid exchange during cooling. Additionally, we have prepared and characterized planar waveguides on X-cut lithium niobate (LN) crystals. The exchanged samples exhibit α and k1 phases, refractive index contrasts as high as 0.082, and exceptional refractive index uniformity. Furthermore, we utilized the same process to fabricate channel waveguides and Y-branch waveguides. We achieved low propagation losses in channel waveguides, accompanied by small mode sizes, and low-loss Y-branch waveguides with a highly uniform beam splitting ratio. All waveguides exhibited consistent performance across multiple preparations and tests, remaining free from aging effects for three months. Our results underscore the promising potential of VPE for creating Y-branch splitters and modulators in LN crystals.

Correlation between birefringence and absorption mapping in large-size Sapphire substrates for gravitational-wave interferometry

In high-sensitive laser interferometers, such as the gravitational-wave detector KAGRA, ultra-high-quality mirrors are essential. In the case of KAGRA, where cavity mirrors are cooled down to 20 K, large-size Sapphire crystals are used as the substrate for the main mirrors to achieve both a good optical quality (i.e., low absorption and uniform refractive index) and optimized thermal behavior under cryogenic temperatures. To implement the very tight optical specifications required by this demanding application, it is mandatory to test the optical homogeneity of different substrates. In order to characterize refractive-index inhomogeneities of large-size uniaxial samples such as the KAGRA Sapphire test masses, we developed a dedicated setup, allowing to resolve birefringence changes with a sensitivity in the order of Δn≈2×10-10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta n\\approx 2 \ imes 10^{-10}$$\\end{document} and a spatial resolution of 1mm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1\\,{{{\\hbox {mm}}}}^{2}$$\\end{document}. Moreover, the same setup allows us to simultaneously record residual absorption maps, thus allowing for a comparison between birefringence and absorption features. In this paper, we will present for the first time measurements on a KAGRA-sized Sapphire substrate which has been characterized in terms of absorption already in an earlier work. Both birefringence inhomogeneities and absorption distributions will be compared and correlations discussed.

Immersion laser separation: Enhancing efficiency and quality in cutting irregular lenses

Irregular lenses are commonly employed in various beam shaping, imaging, and other optical applications. They require secondary processing to meet specific design and assembly criteria. Bessel beam lasers have gained popularity for their ability to efficiently separate hard-brittle materials owing to their high processing quality and speed. However, the complex surface of irregular lenses creates challenges such as refraction and scattering, which impede the laser beam’s ability to form a laser center lobe inside the material. This paper proposes an innovative technique called immersion laser separation (ILS), where an irregular lens is immersed in a refractive index matching medium to create a compound with a uniform refractive index. This approach mitigates the impact of surface complexity on the optical path of the Bessel beam. The separation of a fast axis collimator (FAC) by ILS is used as an example to test the proposed technique. The distribution characteristics of the Bessel beam light field inside the lens material under different refractive index were investigated to find that the laser consistently forms a laser center lobe inside the material during cutting when the matching deviation is small. This results in a modified area that covers the entire surface. The maximum cutting speed can reach 50 mm/s, the cutting surface roughness is less than 700 nm, and the technique produces no chips or micro-cracks, ensuring uniformly high-quality separation. Experimental and simulation results are in close agreement, suggesting that the proposed technique is effective for the efficient, high-quality cutting of irregular lenses.

Absolute testing of two flats based on even and odd functions without testing refractive index uniformity

A novel absolute measurement method by using two plates based on the odd-even functions is proposed. Compared to existing methods, our method requires only two plates and the refractive index uniformity of the transmitted flat is not to be measured. The principle of the proposed method is discussed. The corresponding simulations are carried out to prove the feasibility of the method. Experiments on a 100 mm Zygo-interferometer yielded the PV value of the rest error is less than 3 nm. Error term affecting the calculation accuracy was analyzed including the rotation angle and pixel misalignment errors of the coordinate axis. The proposed method was proven to effectively obtain the accurate surface error.

Fabrication of a multilevel Fresnel axicon deep in fused silica by femtosecond laser machining.

This manuscript presents a simple approach to the manufacturing and optimization of a multilevel phase-only diffractive conical lens (Fresnel axicon or "fraxicon"). The method for recording deep type I modifications in fused silica was established and its ability proven. We showed the prospects and limitations of elements processed using this method. The fine and advanced parameters optimization allowed us to get a compensation mechanism for almost uniform refractive index change for each separate layer. The maximum diffraction efficiency of the fraxicon for a wavelength of 515 nm was ∼80%. The measured Bessel beam depth of field was compared with commercially available conical lens axicons and showed good agreement.

Particle filter algorithms for on-line prediction of transient heat flux of participating medium

The particle filter (PF) and corresponding improved algorithm (unscented particle filter (UPF)) are introduced to solve the on-line prediction of transient heat flux (q(t)) on the boundary of participating medium for the first time. The temperature-dependent thermophysical properties of the medium result in a highly nonlinear heat transfer system. The graded index medium (caused by the temperature-varying refractive index) is also taken into account, in which the radiation transfer is more complex than that in the uniform refractive index medium. The finite volume method is employed to address the transient coupled radiative and conductive heat transfer in medium. The impacts of particle number, process error covariance, measurement error covariance, sampling time interval, forms of heat flux and measurement errors on prediction results are researched in detail. The UPF algorithm, in which unscented Kalman filter technique is applied to pick an appropriate proposal density function to ameliorate the degradation phenomenon in original PF algorithm, outperforms the PF algorithm in terms of stability and accuracy of estimation results. All the prediction results demonstrate that UPF method is effective and reliable to solve the real-time reconstruction of q(t) even with measurement errors.

Active colloids in harmonic optical potentials (a) (a)Contribution to the Focus Issue Statistical Physics of Self-Propelled Colloids edited by Hartmut Löwen, Sabine Klapp and Holger Stark.

We study the motion of active Janus colloids in an optical trap using experiments, theory and numerical simulations. To achieve isotropic and harmonic confinement, we prototype microparticles with a nearly uniform refractive index and verify that, in the absence of activity, the confined motion is identical to that of optically homogeneous Brownian particles. If the activity is turned on by means of vertical AC fields, the density distributions are described by Boltzmann-like statistics (Gaussian with effective temperature) only for strongly confining traps, whereas weaker potentials give rise to non-Gaussian distributions with a bimodal shape. Our results showcase a simple way to study active soft matter in optical potential landscapes eliminating the optical torque.

Generation of orbital angular momentum light based on mechanically induced long-period fiber gratings using 3D printing technique

• An OAM beam generator was proposed with broadband and tunable operation wavelength. • OAM generation was achieved by employing 3D-printed mechanically induced few-mode fiber gratings. • By adjusting applied pressures and inclination angle of few-mode fibers, the operation wavelength can be tuned. • The operational bandwidth can be broadened by employing cascaded few-mode fiber gratings. We propose and experimentally demonstrate an orbital angular momentum (OAM) beam generation method based on mechanically induced few-mode fiber gratings fabricated using 3D printing technique. Experimental results indicate that the transmission depth at resonance wavelength of few-mode fiber gratings with axially uniform refractive index could be adjusted when different lateral pressures are applied onto the few-mode fibers, and hence mode coupling efficiency between the fundamental core mode and LP 11 mode could be controlled in this way. In addition, by changing the inclination angle of the few-mode fibers with respect to direction perpendicular to the mask grooves, the resonance wavelength of the few-mode grating could be changed as well. Besides uniform fiber gratings, we have also investigated the spectral broadening property of the few-mode cascaded fiber grating, and a 10-dB spectral bandwidth up to 114 nm is obtained experimentally. Our proposed OAM light generation method based on mechanically induced fiber gratings using 3D printing technique possesses several desirable merits such as low cost, ease of operation, flexibility in grating structure and no damage to fiber medium, which is anticipated to provide low lost and flexible solution to generation of broadband OAM light for future OAM-based communication systems.

Development and characteristics of infrared gradient refractive index chalcogenide glasses by hot pressing.

Compared with ordinary uniform lenses, the length and refractive index distribution of gradient refractive index (GRIN) lenses can effectively correct aberration and chromatic aberration. This advantage makes the miniaturization, integration, and lens lightweight possible. Although the visible GRIN lenses based on silicate glass are widely used, the infrared GRIN lenses based on chalcogenide glass are still elusive. This paper introduces a new method for preparing this kind of lens by hot pressing sintering diffusion of chalcogenide glasses. A series of chalcogenide glasses Ge10As22Se68-xSx (x = 4, 7, 10, 14, 24, 28, 34 mol%) with refractive index range from 2.37 to 2.57 (n@8 µm) and similar glass transition temperature (ΔTg < 10℃) were prepared by melt quenching. The relationship between Raman peaks and the refractive index of glasses was studied. Furthermore, the refractive index profile formed by elemental diffusion was characterized by Raman signals. The results show that the diffusion length reaches more than 290 µm, and larger diffusion distances can be achieved by stacking multiple layers. The obtained GRIN glass maintains good transmittance in the whole atmospheric window of 2 ∼ 12 µm.

Trajectory correction and position error analysis of underwater laser scanning

To improve the coordinate accuracy of underwater laser scanning point positions, the basic equation of the beam trajectory of the pyramidal polyhedron laser scanning system was deduced, based on the underwater vehicle. The influence of underwater nonuniform refractive index on the beam trajectory was analyzed. A beam trajectory correction model based on the time of flight was established. An experiment was performed to detect the offset positions of light spots under different incident angles and salinities. Compared with scanning results of the uniform refractive index, these results were in good agreement with the correction model. In the process of laser scanning position acquisition underwater, the influence of nonuniform refractive index on spot position cannot be ignored. The correction scheme of scanning point position offered significant reference for obtaining laser scanning point coordinates of underwater.

Lambertian illumination of dielectric scattering media with monochromatic light

We study the Lambertian illumination of dielectric media with monochromatic light by numerically solving Maxwell's equations in three dimensions. We compute the average energy density inside spherical and cubic objects with a uniform refractive index. The Lambertian illumination of objects with uniform refractive index reproduces the known enhancement of the average energy density equal to the third power of the refractive index if the surface is sufficiently textured. Furthermore, inhomogeneous refractive index distributions with scattering areas much smaller than the wavelength, of the order of the wavelength, and larger than the wavelength are studied. The average energy density inside these media is compared to effective medium theories. Our investigation shows that the average energy density enhancement inside scattering media with scatterers much smaller than the wavelength can be modeled accurately with effective medium approximations.

About copying a free-space electromagnetic field by an invisible object

The structure of a monochromatic electromagnetic field inside a spherical object that does not distort the wave field of external sources outside its surface has been studied using transformation optics methods. It is assumed that the object is made of isotropic materials with equal values of permittivity and permeability and consists of a spherical volume of material with a positive spatially uniform refractive index and an adjacent spherical layer of material with a negative inhomogeneous refractive index (i.e., it is a spherical superlens made of isotropic materials). It is shown that the field inside a sphere with a positive refractive index is the reduced-size copy of the field in which the object was located. This copy contains information about the radiation field of external sources in a spherical free-space volume whose radius depends on the object parameters and can be significantly larger than the radius of the object. It is shown that the copying properties of the object with given dimensions can be fully preserved if its outer layer is made of anisotropic materials.

Modeling of a sampled apodized fiber Bragg grating moisture sensor

This study presents a theoretical model that enables a fast and precise calculation of a sampled apodized fiber Bragg grating and the related sensor. Transfer matrix method was used since there is no analytical solution due to the complex refractive index profile. Errors brought by the uniform refractive index profile approximation were compensated, which allows a smaller number of divided segments and therefore a fast and memory efficient simulation. This compensation method realizes a high speed and high precision simulation. The theoretical spectra of both the grating and the sensor agrees well with those given by the experiments. According to the experiment, the sensitivity of the moisture sensor is −0.6165 pm/ppm in mineral oil. This theoretical model provides a new choice for the researchers and developers to design and optimize the sensing structure instead of a blind try and error process.

The Analysis of Light Intensity Transmission Coefficient Through Plastic Fibers for the Design of Closed Room Lighting without Electricity

The phenomenon of total internal reflection can be used to guide the light to be transmitted from one place to another, it is applied to the guiding principle of light in fiber optics. A fiber rope made of clear plastic material with a uniform refractive index, when one end fired a beam of light then the light will be forwarded along the strap so that out at the other end. This principle can be applied as a lighting source in a closed room by passing light from the outdoors by using plastic fiber, so it doesn’t need the electricity. In this study, theoretical analysis of the percentage of the intensity of light transmitted by the fiber plastic succeed as a function of the refractive index and determine the transmission coefficient for some plastic seeds are eligible to be used as a plastic fiber light successor. From the results of deriving the equation, the light transmission coefficient of plastic is and the terms of plastic seeds that can be used as a plastic fiber are having a refractive index . Based on refractive index data from profesionalplastics, the maximum transmission coefficient value for Ethylene Tetrafluoro Ethylene Copolymer is 82.23% and the minimum transmission value of Fluorinated Ethylene Propylene Copolymer is 55.99%.

Refractive index distribution of the ice-layer in ICF target from the interference method

In the inertial confinement fusion (ICF) system, the uniformity of the target is important to the successful ignition. A method based on the phase-shifting interferometry and ray tracing is proposed to obtain the 2D (two-dimensional) refractive index distributions of the ice-layer at three wavelengths (532 nm, 785 nm, 1064 nm), from which the 2D density distribution of the ice-layer is got. A differential method is utilized to get the reference refractive index, based on which the refractive index distribution of the layer is retrieved. For targets with uniform refractive index in the ice-layer, simulations show that the relative retrieval accuracies are better than 10 ppb. Experiments show that the refractive index distributions have good consistency at different wavelengths for the same target. The detectable normalized RMS (root mean square) of the refractive index distributions is at least 2.9E-4. The relative accuracies of the average refractive indexes of the shells are better than 1.5% for all three wavelengths. The optimal selection of the two rings on the wavefront map that yields minimal uncertainty to the reference refractive index is discussed. It is concluded that the proposed method and system poses great potential in the in-situ measurement of the refractive index and density distribution of the ICF target layers.

A COMPREHENSIVE REVIEW ON THIN FILM DEPOSITIONS ON PECVD REACTORS

The deposition of thin films by Plasma Enhanced Chemical Vapor Deposition (PECVD) method is a critical process in the fabrication of MEMS or semiconductor devices. The current paper presents an comprehensive overview of PECVD process. After a short description of the PECVD reactors main layers and their application such as silicon oxide, TEOS, silicon nitride, silicon oxynitride, silicon carbide, amorphous silicon, diamond like carbon are presented. The influence of the process parameters such as: chamber pressure, substrate temperature, mass flow rate, RF Power and RF Power mode on deposition rate, film thickness uniformity, refractive index uniformity and film stress were analysed. The main challenge of thin films PECVD deposition for Microelectromechanical Systems (MEMS)and semiconductor devices is to optimize the deposition parameters for high deposition rate with low film stress which and if is possible at low deposition temperature.

Investigation of Refractive Index Uniformity in Coatings on a Steep Lens

Investigation of Refractive Index Uniformity in Coatings on a Steep Lens

Structural and electrical properties of Al/BiFeO3/ZrO2/n-Si structure for non-volatile memory application

In this communication, the structural and electrical properties of metal–ferroelectric–insulator–silicon (MFIS) capacitor structure have been reported for non-volatile memory applications. Sol–gel with spin coating and rf sputtering process have been used for depositing BiFeO3 (BFO) and ZrO2 films, respectively. It has been observed that BFO film shows pure ferroelectric phase, uniform grain size and maximum refractive index at annealing temperature of 500 °C. Thermo-gravimetric analyzer and differential scanning calorimetry analysis indicate good agreement with X-ray diffraction of BFO film. In ZrO2 thin film, it has been observed that ZrO2 is in amorphous state at all annealing temperatures and the maximum refractive index has been found at annealing temperature of 400 °C. Al/ZrO2/n-Si (MIS), Al/BiFeO3/n-Si (MFS) and Al/BiFeO3/ZrO2/n-Si (MFIS) structures have been fabricated to investigate the electrical characteristics. The memory window has been observed by capacitance–voltage (C–V) characteristics and it improves from 1.9 V in MFS structure to 5.4 V in MFIS structure with 8 nm dielectric layer. Leakage current density has been observed by current density–gate voltage (J–V) characteristics and it is order of 10−5 A/cm2 in MF150 nmI8 nmS structure. No charge value degrades up to 1012 iteration cycles in MF150 nmI8 nmS structure.