Photoelastic Effect Research Articles

Measurement of the three-dimensional distribution of uniaxial stress by terahertz time domain spectroscopy.

This paper presents a method to measure the three-dimensional distribution of uniaxial stress based on Terahertz Time Domain Spectroscopy (THz-TDS). The measurement principle was first established, which combines the computed tomography (CT) method and the photo-elastic effects. A classic filtered back-projection algorithm is adopted to calculate the three-dimensional stress fields from THz-TDS scanning images. Then, in verification experiment, the uniaxial stress distribution in the compressed cylinder and the stretched screw is obtained based on the measurement principle. Finally, the reliability of the proposed method has been verified by comparing the experiment results with the finite element simulation. A reasonable agreement is obtained.

A sandwich structure on Grapefruit microstructured fiber for measurement of non-coaxial torsion

A sandwich structure on Grapefruit microstructured fiber(GMF) for measurement of non-coaxial torsion is proposed and fabricated. The GMF and the single-mode fiber (SMF) are fused to make the SMF-GMF-SMF structure. The non-coaxial system is used to twist the GMF sensor. Because the two ends of the GMF are not at the same height, a tensile strain is introduced into the fiber. At the same time, the photoelastic effect and the Poisson effect also cause the phase difference to change, resulting in a transmission spectrum shift. The experimental results show that in the range of 0°∼360°, with the increase of the twist angle, the intensity of the transmission mode gradually increases, and the coupling efficiency of the higher-order mode increases. The torsion sensitivity can reaches 0.02315 dB/°. The sensor is simple in fabrication, high in sensitivity, and has a good application prospect in torsion measurement of various devices.

Studying the effect of quantum dots and parity-time symmetry on the magnification of topological edge state peak as a pressure sensor

In this paper, a novel pressure sensor employing parity-time symmetry for amplifying the sensing signal is proposed. The sensor design consists of a multi-layer dual–ternary photonic crystal with quantum dots embedded inside silicon dioxide and porous silicon layers as the unit cell of the structure. A topological edge state peak appears inside the photonic bandgap due to the sensor's symmetric design. The properties of the topological edge state peak are changing due to the photoelastic effect of the selected materials. Quantitive analysis of the sensor performance was conducted to optimize the periodicity number and the incidence angle. Moreover, the influence of silicon porosity and macroscopic Lorentz oscillation on the performance was examined.

Corneal elastic property investigated by terahertz technology

Terahertz (THz) spectroscopy technique has been applied in ex vivo biomechanical properties analysis of human corneas. Upon the application of light pressure on the cornea, the photo elastic birefringent effect, anisotropic deformation, thickness changes and hydration levels will contribute to the sudden phase changes of terahertz time domain signal. The shelf lifetime study shows that the phase shift is reduced and cornea loose the biomechanical properties with the increase of hydration level. Mechanical behaviors have been further studied based on the “fresh” cut corneas with the similar hydration levels. THz signal was collected by focusing inside of the cornea to avoid the phase shift due to light stress caused movement of the corneal surface. By this way, the amount of THz signal refractive index variation is correlated to the elastic property of the corneas. The correlation between the THz signal phase shift and refractive index shift due to the corneal strain can be used to derive the elastic Young’s modulus. Our results demonstrated the THz spectroscopy, as a non-contact and non-invasive detection method, could be potential for understanding the mechanism of corneal deformation under the action of intraocular pressure in the physiological environment in future.

Stress-induced birefringence in 3D direct laser written micro-optics.

3D direct laser writing is a widely used technology to create different nano- and micro-optical devices for various purposes. However, one big issue is the shrinking of the structures during polymerization, which results in deviations from the design and in internal stress. While the deviations can be compensated by adapting the design, the internal stress remains and induces birefringence. In this Letter, we successfully demonstrate the quantitative analysis of stress-induced birefringence in 3D direct laser written structures. After presenting the measurement setup based on a rotating polarizer and an elliptical analyzer, we characterize the birefringence of different structures and writing modes. We further investigate different photoresists and the implications for 3D direct laser written optics.

PhotoElasticFinger: Robot Tactile Fingertip Based on Photoelastic Effect.

The sense of touch is fundamental for a one-to-one mapping between the environment and a robot that physically interacts with the environment. Herein, we describe a tactile fingertip design that can robustly detect interaction forces given data collected from a camera. This design is based on the photoelastic effect observed in silicone matter. Under the force applied to the silicone rubber, owing to the stress-induced birefringence, the light propagating within the silicone rubber is subjected to the angular phase shift, where the latter is proportional to the increase in the image brightness in the camera frames. We present the calibration and test results of the photoelastic sensor design on a bench using a robot arm and with a certified industrial force torque sensor. We also discuss the applications of this sensor design and its potential relationship with human mechano-transduction receptors. We achieved a force sensing range of up to 8 N with a force resolution of around 0.5 N. The photoelastic tactile fingertip is suitable for robot grasping and might lead to further progress in robust tactile sensing.

Photoelastic Organogel with Multiple Stimuli Responses.

The photoelastic effect has many uses in mechanics today, but it is usually disregarded in flexible materials. Using 2-phenoxyethyl acrylate as a monomer and 4-cyano-4'-pentylbiphenyl (5CB) as a solvent, a multiple responsive photoelastic organogel (PO) with strong birefringence but low modulus is created. 5CB is a liquid crystal molecule that does not participate in the polymerization process and is always present as tiny molecules in the polymer. It endows the PO low modulus and high birefringence, as well as the ability to drive the birefringence using an electric field. This PO not only has high sensitivity and fast response as a photoelastic strain sensor, but also has a very sensitive response to heat, especially in the range of human body temperature. It also has a high dielectric constant and a strong correlation between the interference color and the applied electric field, allowing for easy writing and erasure of encrypted data. This unique multisignal response feature and low modulus that mimics human skin bring up new opportunities in the potential applications such as multiple information encryption, anticounterfeiting, and multifunctional wearable sensors.

Intermodal Four-Wave Mixing Process in Strain-Induced Birefringent Multimode Optical Fibers.

Our study investigated the partially degenerate intermodal four-wave mixing (IM-FWM) process in nonlinear multimode optical fibers with strain-induced birefringence. The difference in the refractive index along the two orthogonal directions was due to the photoelastic effect that occurred when the fiber under test (FUT) was subjected to uniformly applied diameter stress caused by winding on a cylinder of a given diameter. Our work analyzed how the nonlinear frequency conversion and the output modal field profiles depended on the degree of birefringence in FUT. The experimental results significantly affected the order of the excited moduli in fiber sections characterized by different amounts of birefringence. We also checked the efficiency of the FWM process for different polarizations of the pump beam to determine those for which the FWM process was most effective for the 532 nm sub-nanosecond pulses. More than 30% conversion efficiency was obtained for the FUTs with a length of tens of centimeters.

Radially polarized 33 W emission from a double-pass Ho:YAG thin-slab amplifier

The generation of a 33.7 W radially polarized output is demonstrated by way of double-pass amplification in a Ho:YAG thin-slab crystal. The amplifier system was seeded with the radially polarized output beam from a continuous-wave 13.2 W, 2.1 µm Ho:YAG rod laser, which exploited gain medium bifocusing and a ring-shaped pump spot to promote preferential lasing on the radially polarized Laguerre–Gaussian mode. By compensating for both the thermally induced stress birefringence in the thin-slab crystal and the Gouy phase shift resulting from astigmatic focusing of the 13.2 W seed, only marginal beam degradation was observed across the 95 W range of pump power into the amplifier.

Quantitative Characterization of the Anisotropy of the Stress-Optical Properties of Polyethylene Terephthalate Films Based on the Photoelastic Method.

Polyethylene terephthalate (PET) is one of the most commonly used substrate materials in the field of flexible electronics, and its stress-induced birefringence often has a detrimental effect on the optical properties of the device. Therefore, a deep and systematic understanding of the stress-optical properties of PET films is crucial for device design and manufacture. The photoelastic method is a direct optical measurement technique based on the stress-induced birefringence effect of materials, which has the advantages of being nondestructive and noncontact. In this work, the photoelastic method was used to quantitatively characterize the anisotropy of the stress-optical properties of PET films under the uniaxial stress state. First, a self-built reflection-transmission coaxial bidirectional photoelasticity measurement system was developed by means of a combination of transmission and reflection photoelasticity. Then, the stress-optical coefficients and isoclinic angles of PET films with different stretching angles were measured. Finally, the linear combinations of the photoelastic tensor components and refractive-index-related parameters were determined by fitting the analytical relationship between the stress-optical coefficients and isoclinic angles.

Giant and Controllable Photoplasticity and Photoelasticity in Compound Semiconductors.

We show that the wide-band gap compound semiconductors ZnO, ZnS, and CdS feature large photoplastic and photoelastic effects that are mediated by point defects. We measure the mechanical properties of ceramics and single crystals using nanoindentation, and we find that elasticity and plasticity vary strongly with moderate illumination. For instance, the elastic stiffness of ZnO can increase by greater than 40% due to blue illumination of intensity 1.4 mW/cm^{2}. Above-band-gap illumination (e.g., uv light) has the strongest effect, and the relative effect of subband gap illumination varies between samples-a clear sign of defect-mediated processes. We show giant optomechanical effects can be tuned by materials processing, and that processing dependence can be understood within a framework of point defect equilibrium. The photoplastic effect can be understood by a long-established theory of charged dislocation motion. The photoelastic effect requires a new theoretical framework which we present using density functional theory to study the effect of point defect ionization on local lattice structure and elastic tensors. Our results update the longstanding but lesser-studied field of semiconductor optomechanics, and suggest interesting applications.

Photo-Elastic Enhanced Optomechanic One Dimensional Phoxonic Fishbone Nanobeam

We investigated the strength of acousto-optical (AO) interaction in one-dimensional fishbone silicon nanobeam computationally. The structure can generate phononic and photonic band gaps simultaneously. We use defect cavity optical mode and slow light mode to interact with acoustic defect modes. The AO coupling rates are obtained by adding the optical frequency shifts, which result from photo-elastic effect and moving-boundary effect disturbances. The AO coupling rates are strongly dependent on the overlap of acoustic and optical mode distribution. The strength of AO interaction can be enhanced by choosing certain acoustic defect modes that are formed by the stretching of wings and that overlap significantly with optical fields.

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.

Coatings Sensitivity to the Quench Marks

During tempering process, the non-homogenous heating or rapid cooling can induce localized strain in the glass leading to birefringence (or optical anisotropy) phenomenon, a result of the photoelastic effect. Since transmission and reflection coefficients of interfaces at high angles can be quite different with the polarization, inhomogeneous birefringence may manifest as peculiar geometric patterns of bright or darkish shadows or iridescence effects in given polarized observation conditions. The patterns appearance may be at the origin of dispute between the client and the glass manufacturer. Each party may have a different perception, how strong the anisotropies are and what is permissible. With the use of an in-line scanner for the optical retardation, it is possible to control and optimise the tempering process homogeneity and thus reduce the visibility of the patterns. However, the presence of low emissivity coatings on the façades windows can alter the visibility of the quench marks: depending on the coating nature, the quench pattern visibility can be magnified or reduced. Here, we show the calculation of σQM, as a parameter representing the coating sensitivity to quench marks, i.e., the capability of a coating to reveal or hinder the iridescence pattern of tempered glass. Thanks to the angular measurements of the transmission and reflection in s and p polarization we compute the quench mark sensitivity by estimating a color contrast gradient with regard to the phase delay.

ZnS, CdSe and ZnSe magneto-optical and thermo-optical characteristics

In the presented work, the prospects for using pure ZnS and CdSe semiconductors for the development of Faraday isolators are investigated. Both materials have advantages over conventional magnetoactive materials. Their use can make it possible to expand the spectral range, in which isolators for high-power laser radiation can be implemented, both in the long-wavelength and short-wavelength regions. Measurements of the Verdet constant of crystals and the dependence of thermally induced depolarization caused by stress-induced birefringence on laser power were carried out. Based on the obtained experimental data, the parameters of the magneto-optical figure of merit were determined. The studied samples showed promising results, while the potential for increasing the magneto-optical figure of merit by optimizing the parameters of the production process is assessed as high. This possibility is illustrated by the example of ZnSe crystal samples obtained in various technological processes.

Measurement of stress-induced birefringence in a thin-disk laser with full-Stokes polarimetry.

Stress-induced birefringence leads to losses in solid-state laser resonators and amplifiers with polarized output beams. A model of stress-induced birefringence in thin disks is presented, as well as measurements of stress-induced birefringence in a thin disk in a multi-kilowatt oscillator. A full-Stokes imaging polarimeter was developed to enable fast and accurate polarimetric measurements. Experimental and simulated results are in good agreement qualitatively and quantitatively and show that the polarization loss due to stress-induced birefringence is negligible for ytterbium-doped thin disks with a thickness around 100 µm but becomes relevant in thicker disks. It is concluded that stress-induced birefringence should be taken into consideration when designing a thin-disk laser system.

On the behavior of uniaxial static stress loaded micro-scale fused silica beams at room temperature

The question of whether static stress in fused silica relaxes at room temperature is still under debate. Here, we report experimental data investigating the behavior of fused silica at room temperature when subjected to static tensile stress up to 2 GPa stress levels under different environmental conditions. Our measurements are performed using an innovative combination of methods; a femtosecond laser is used to accurately load a monolithic microscale test beam in a non-contact manner, fabricated using femtosecond laser processing and chemical etching, while the stress is continuously monitored using photoelasticity over an extended period spanning several months, in both dry and normal atmospheric conditions. The results are of practical importance, not only for photonics devices making use of stress-induced birefringence but also for flexures subjected to static loads.

INFLUENCE OF INVERSE PIEZOELECTRIC AND PHOTOELASTIC EFFECTS ON THE INDEX SURFACE OF THE NORMAL COMPONENT INVERSE TENSOR DIELECTRIC PERMEABILITY OF THE Bi12TiO20 CRYSTAL

The index surface of the normal component of variation of the inverse tensor dielectric constant of the photorefractive Bi12TiO20 crystal of 23 symmetry class is constructed and analyzed. The extremal values of the index surface of the normal component for Bi12TiO20 crystal with formed holographic grating are determined. It is shown that the combined action of the photoelastic and inverse piezoelectric effects leads to a change in the maximum and minimum values of the normal component of variation of the inverse tensor dielectric constant of the crystal with the wave vector of the holographic grating oriented along the crystallographic direction.

Transverse light-shift in a spin-exchange relaxation-free co-magnetometer: measurement, decoupling, and suppression.

The transverse light-shift can induce non-negligible polarization error in the output signal of spin-exchange relaxation-free (SERF) co-magnetometer. In this paper, a novel method for rapid measurement of transverse light-shift based on the error of steady-state response of co-magnetometer is proposed firstly, then the sources of transverse light-shift in a compact SERF co-magnetometer is modeled and analyzed from three aspects: the non-ideal linear polarization of probe laser, the circular dichroism of the atomic spin ensembles, and the stress-induced birefringence effect of the cell wall. Furthermore, the decoupling and suppression methods of transverse light-shift based on a degree of circular polarization (DOCP) regulation scheme is presented, to realize the decoupling measurement of the transverse light-shift introduced by the whole co-magnetometer cell, and cancel it out with the non-ideal linear polarization of the probe laser. Eventually, the DOCP regulation scheme suggested in this paper achieves more than a 67% suppression ratio in transverse light-shift, and the short- and long-term performance of SERF co-magnetometer are improved due to the reduction of the coupling effect between the probe laser power and transverse field. Moreover, the measurement, decoupling and suppression methods provided in this paper also have the potential to be applied to other atomic sensors, such as the SERF magnetometers and nuclear spin co-magnetometers.

Degree of Polarization of High-Power Laser Diodes: Modeling and Statistical Experimental Investigation

A statistical experimental investigation of the characteristic changes associated with the degree-of-polarization reduction of high-power laser diodes is reported. A simple model accounting for the stress-induced degree-of-polarization changes through the photoelastic effect is introduced to qualitatively support the experimental results. Functional characteristics addressed in the investigation are the threshold current, the slope efficiency, the polarization-resolved far field and near field, and the beam parameter product. Model outcomes and measured parameters related to different degree-of-polarization values have proven very useful for device optimization aimed to polarization multiplexing applications.