Thermo-optical Characteristics Research Articles

Analysis of Silicon Channel Waveguide Thermo-Optic Switches by the Image Charge Method

Employing the “image charge” method, we theoretically study the influence of key physical parameters on the thermo-optic characteristics of a channel waveguide silicon-on-insulator (SOI) Mach–Zehnder Switch. The power consumption and spatial temperature profile of such structures are given as explicit functions of various structural, thermal, and optical parameters, offering physical insight not available in finite-element simulations. Agreement with finite-element simulations is demonstrated. The trends of switching power versus various parameters provide quantitative guide for further device optimization. This approach may also be useful for efficient modeling of an integrated optical circuit containing a large number of thermo-optic devices.

Thermo-optical properties of EuF2-based crystals

High-power solid state lasers of the mid-IR spectral range are extremely interesting for both basic research and technological applications and they are actively developing at present. However, the implementation of projects with the use of such lasers is hampered by the absence of Faraday isolators (FIs) for the radiation with the corresponding characteristics. The need for such devices is growing with laser power enhancement. They reduce the risk of self-excitation of the damage of amplifiers and optical elements and are a handy tool for organizing multipass schemes. Magneto-active crystals based on Eu2+ ions are promising media for the development of FIs for high-power lasers of the mid-IR spectral range because of the unique combination of their spectral and magneto-optical properties. Thermo-optical characteristics are also of principal importance when operating with high-power laser radiation. This letter presents the results of investigation of thermo-optical constants Q and P of the EuF2.11 solid solution crystal which determines the values of polarization and phase distortions of passing laser radiation. The measured values can be considered as the corresponding values for the EuF2 crystal, which is of greatest interest for the development of FIs for high-power mid-IR lasers. The obtained results can be used to determine the characteristics of FIs based on EuF2 crystals in various implementations of their optical schemes for various parameters of laser radiation. According to the estimates, for the radiation wavelength of 2 μm, the maximum permissible laser power of the Faraday isolator based on one magneto-optical element with [001] orientation will be more than 200 W.

Characterization of terbium containing cubic zirconia crystal for high power laser applications

A magneto-active terbium containing cubic zirconia crystal is an attractive material for creating Faraday isolators thanks to a high value of its Verdet constant and low absorption coefficient. The optical anisotropy parameter of this crystal which shows the distribution of orientations of the axes of thermally induced birefringence in a thermally loaded optical element has negative value. It means that there exists a dedicated orientation of crystallographic axes, at which the axes of thermally induced birefringence are arranged in the one direction and thermally induced depolarization may be reduced substantially. For assessing potential of this crystal for high-power applications we have investigated its temperature dependence of the Verdet constant that proved to correspond to the paramagnetic behavior. Thermo-optical characteristics are also of principal importance when operating with high-power laser radiation. We present results of measurements of thermo-optical characteristics Q and P which determines values of polarization and phase distortions of passing laser radiation. Measured values can be used to determine the characteristics of Faraday isolators based terbium containing cubic zirconia crystals in various implementations of their optical schemes for various parameters of laser radiation.

Low-power total internal reflection thermo-optic switch based on hybrid SiON-polymer X-junction waveguides.

We propose and demonstrate a low-power 2×2 total internal reflection thermo-optic switch based on an X-junction configuration formed with a silicon oxynitride (SiON) core and polymer cladding. Unlike X-junctions reported thus far, our proposed configuration features a slot formed on the center of the X-junction and filled with polymer cladding. With such a configuration, the opposite thermo-optic characteristics of SiON and polymer and, hence, heat utilization efficiency can be fully utilized. Our fabricated proof-of-principle switch shows extinction ratios of larger than 15.34dB and switching powers of less than ∼59.6 mW. The rise time and fall time of switching are 1.42 and 0.85ms, respectively. The insertion losses are less than 10.6dB for all channels, and the polarization-dependent loss is ∼0.3 dB.

EuF2-based crystals as media for high-power mid-infrared Faraday isolators

This letter presents the investigation of spectral, magneto-optical and thermo-optical characteristics of cubic EuF2+x solid solution crystals. The dispersion of the Verdet constant measured in the wavelength range of 532–1940 nm demonstrates that such crystals can be used for creating Faraday isolators for the wavelength of ~2 μm. The optical anisotropy parameter of EuF2+x crystals is negative, which means that there exists a dedicated orientation of the crystallographic axes at which thermally induced depolarization may be reduced substantially. The performed research shows that an EuF2 crystal is highly promising for the development of Faraday isolators for high-power mid-IR lasers.

Spectral, magneto-optical and thermo-optical properties of terbium containing cubic zirconia crystal

This letter presents the investigation of magneto-active media based on a cubic zirconia crystal. We studied spectral, magneto-optical and thermo-optical characteristics of a terbium containing cubic zirconia (TCZ) crystal aiming to estimate its applicability in magneto-optic applications. This crystal has significant advantages over the popular TGG crystal. The value of its Verdet constant for the radiation at the wavelength of 1075 nm is 31% higher than that of a TGG crystal. The value of the absorption coefficient is rather small and typical for commercially available TGG crystals. Its transmission spectrum is typical for terbium containing magneto-active media. The dispersion dependence of the Verdet constant had been measured for visible and near-IR ranges and presented in this paper. The optical anisotropy parameter which shows the distribution of orientations of the axes of thermally induced birefringence in a thermally loaded optical element had also been determined. Unlike a TGG crystal, the optical anisotropy parameter of a TCZ crystal is negative, which means that there exists a dedicated orientation of crystallographic axes, at which the axes of thermally induced birefringence are arranged in one direction and thermally induced depolarization may be reduced substantially. Such a combination of characteristics makes the TCZ crystal a very promising material for magneto-optical applications, for example, for creating Faraday isolators, including those with a high maximum permissible operating power.

Thermo-optic characteristics of hybrid polymer/silica microstructured optical fiber: An analytical approach

Microstructured optical fibers (MOFs) allow a variety of advanced materials to be infiltrated in their air-voids for obtaining the increased fiber functionality, and offering a new versatile platform for developing the compact sensors devices. We aim to investigate the thermal characteristics of high-index core triangular hybrid polymer/silica MOFs with circular air-voids infused with polymer by using the analytical field model [1]. We demonstrate that infiltration of air-voids with polymer, e.g., polydimethylsiloxane (PDMS) can facilitate to tune the fundamental modal properties of MOF such as effective index of the mode, near and the far-field profiles, effective mode area and the numerical aperture over the temperature ranging from 0 °C to 100 °C, for different values of relative air-void ratios. The evolution of the mode shape for a given temperature has been investigated in transition from near-field to far-field regime. We have studied the thermal dependence of splice losses between hybrid MOF and the standard step-index single-mode optical fiber in combination with Fresnel losses. For enhancing the evanescent field interactions, we have evaluated fraction of power associated with fundamental mode of hybrid MOF. We have compared the accuracy of our results with those based on full-vector finite-difference (FD) method, as available in the literature.

Thermo-optical and lasing characteristics of Cr^2+-doped CdSe single crystal as tunable coherent source in the mid-infrared

We report on a comprehensive characterization of Cr2+-doped CdSe single crystal as an efficient active material for tunable laser applications in the mid-infrared spectral region. Optical gain, thermo-optical behavior, power efficiency, scalability and wavelength tunability have been thoroughly investigated. Using an antireflection-coated crystal pumped by a Tm-fiber laser at at 1.94 μm, 1-W CW output power and 50% slope efficiency at 2.65 μm emission wavelength have been obtained in a diffraction-limited output beam. An output peak power of 2.5 W has been achieved without significant beam distortion in a quasi-CW regime. Exploiting an intra-cavity diffraction grating in a Littrow configuration, a maximum tuning range of 900 nm from 2.22 to 3.12 μm, limited by the finite bandwidth of resonator components, has been demonstrated with an uncoated crystal.

Definition of Thermo-Optical Characteristics of Uniaxial Crystals

Thermally induced distortions of laser radiation caused by the photoelastic effect during high-power beam propagation through optical elements cut along the optical axis of uniaxial crystals have been investigated. Thermo-optical characteristics for uniaxial crystals of all three syngony types have been introduced: the optical anisotropy parameter $\xi $ defining the distribution of the axes of thermally induced birefringence over the cross section of the optical element, and the thermo-optical constants $Q$ and $P$ specifying the value of thermally induced depolarization and thermal lens strength, respectively. The introduced thermo-optical characteristics may be used as applicability criteria of various uniaxial crystals for work with high-power laser radiation. The optical anisotropy parameter $\xi $ of trigonal and hexagonal crystals has been found to be equal to unity by virtue of the isotropy of their piezo-optical properties in a plane perpendicular to the optical axis. We have analyzed by way of example thermally induced polarization distortions in a tetragonal DKDP crystal and in a trigonal CeF3 crystal. The distributions and dependences observed in experiment fully agree with the theoretical considerations.

Design considerations for λ ∼ 3.0- to 3.5-μm-emitting quantum cascade lasers on metamorphic buffer layers

Quantum cascade lasers (QCLs) that employ metamorphic buffer layers as substrates of variable lattice constant have been designed for emission in the 3.0- to 3.5-μm wavelength range. Theoretical analysis of the active-region (AR) energy band structure, while using an 8-band k•p model, reveals that one can achieve both effective carrier-leakage suppression as well as fast carrier extraction in QCL structures of relatively low strain. Significantly lower indium-content quantum wells (QWs) can be employed for the AR compared to QWs employed for conventional short-wavelength QCL structures grown on InP, which, in turn, is expected to eliminate carrier leakage to indirect-gap valleys (X, L). An analysis of thermo-optical characteristics for the complete device design indicates that high-Al-content AlInAs cladding layers are more effective for both optical confinement and thermal dissipation than InGaP cladding layers. An electroluminescence-spectrum full-width half-maximum linewidth of 54.6 meV is estimated from interface roughness scattering and, by considering both inelastic and elastic scattering, the threshold-current density for 3.39-μm-emitting, 3-mm-long back-facet-coated QCLs is projected to be 1.40 kA/cm2.

Thermo-optic characteristic of DNA thin solid film and its application as a biocompatible optical fiber temperature sensor.

We report unique thermo-optical characteristics of DNA-Cetyl tri-methyl ammonium (DNA-CTMA) thin solid film with a large negative thermo-optical coefficient of -3.4×10-4/°C in the temperature range from 20°C to 70°C without any observable thermal hysteresis. By combining this thermo-optic DNA film and fiber optic multimode interference (MMI) device, we experimentally demonstrated a highly sensitive compact temperature sensor with a large spectral shift of 0.15 nm/°C. The fiber optic MMI device was a concatenated structure with single-mode fiber (SMF)-coreless silica fiber (CSF)-single mode fiber (SMF) and the DNA-CTMA film was deposited on the CSF. The spectral shifts of the device in experiments were compared with the beam propagation method, which showed a good agreement.

Assembly and Testing of a Thermal Control Component Developed in Brazil

The optical solar reflector is basically a mirror of second surface with low absorptivity/emissivity ratio and negligible degradation in the space environment, which makes it an excellent coating for thermal control of satellites. It works as a radiator and is used in particular parts of the external surfaces of the satellites in order to reject the undesirable heat to the deep space. In the Brazilian Space Programs, the radiators of the satellites are generally painted with special white paints in order to reject heat instead of the use of optical solar reflector. The problem of white-ink radiators is the high degradation of the thermo-optical properties that happen over the useful lives. Thus, a process of manufacturing and assembly of optical solar reflector was developed in Brazil. To validate this process in terms of mechanical and thermal properties, 3 types of optical solar reflector radiators were manufactured, and their absorptivity and emissivity properties at the temperature of 23 °C were measured. Optical solar reflector coupons were mounted on aluminum plates to perform vibration, thermal vacuum and thermal-shock tests. A study was also done to optimize the thickness of the glue to fix the structure of the satellite on the optical solar reflector. It showed an excellent environmental stability and maintained its thermo-optical characteristics after the tests.

Thermo-optical and magneto-optical characteristics of CeF3 crystal

Thermo-optical and magneto-optical characteristics of a uniaxial CeF3 crystal have been investigated. Its optical anisotropy parameter, magneto-optical figure of merit, and the ratio of the thermo-optical constants P and Q have been measured. The found characteristics have been compared with the corresponding values for the TGG crystal. Based on the obtained results it has been concluded that, despite its anisotropy, the CeF3 crystal is promising material for the development of Faraday isolators for high-power lasers.

Experimental study of photothermal specifications and stability of graphene oxide nanoplatelets nanofluid as working fluid for low-temperature Direct Absorption Solar Collectors (DASCs)

The use of nanofluids as the working fluid in systems for converting solar to thermal energy has led to remarkable progresses. This study has investigated the thermo-optical characteristics of nanofluids containing Graphene oxide nanoplatelets/deionized water as the working fluid for low-temperature direct absorption solar collectors (DASCs). The prepared samples of nanofluid contain graphene oxide nanoplatelets based on deionized water with weight percentages of 0.001, 0.005, 0.015 and 0.045. Results show that the applied nanofluid has suitable ability compared to the base fluid in absorbing solar energy, ranging from 200 to 2500nm. Also, results from calculation of absorbed energy's fraction suggest that the minimum height or thickness of the nanofluid layer having the ability of full sun's energy, is for the nanofluid with weight percentage of 0.045 and height of 3cm that has the ability of absorbing 99.6% of energy. The thermal conductivity coefficient of prepared samples was measured at temperature ranging 25–50. Results showed that increasing the weight percentage of nanofluid along with increase in temperature would improve nanofluid's thermal properties in comparison with the base fluid. Finally, by investigating the impact of weight percentage of the nanofluid's temperature on radiative properties and thermal conductivity, this nanofluid with strong absorption band in the range of 280–350nm (nanometers) was introduced and proposed as the appropriate environment for using direct absorption solar collectors.

Measurements of Thermo-Optical Characteristics of Cubic Crystals Using Samples of Arbitrary Orientation

Measurements of Thermo-Optical Characteristics of Cubic Crystals Using Samples of Arbitrary Orientation

Hybrid fiber resonator employing LRSPP waveguide coupler for gyroscope

Polarization error and temperature noise are two main limits to the performance of resonant fiber optic gyroscope (RFOG). To overcome these limits, we demonstrated a hybrid resonator consisting of a polymer-based long-range surface plasmon polariton (LRSPP) waveguide coupler and a silica fiber. Single-polarization property of LRSPP waveguide and the offsetting of the opposite thermo-optical characteristics between the polymer-based LRSPP waveguide and the silica fiber can effectively inhibit both the polarization error and the temperature noise of RFOG. The measured resonance spectrum of the hybrid resonator shows the absence of polarization noise. The temperature dependence of wavelength shift (TDWS) of resonator dropped to about 2 pm/°C, or even to 0 pm/°C with optimal structure, which dramatically improves the temperature stability of gyroscope system. In addition, the hybrid resonator also shows tremendous application potential in rate-grade and tactical-grade gyroscopes.

Thermo-optic Characteristics of Transparent Ceramic YAG in the Near Infrared Region

Thermo-optic Characteristics of Transparent Ceramic YAG in the Near Infrared Region

Analytical solution of the optimal steering law for non-ideal solar sail

This paper analyzes the problem of finding the optimal steering law for a flat solar sail whose propulsive acceleration is described by an optical force model. In particular, the problem amounts to looking for the optimal direction of the unit vector normal to the sail plane that maximizes the projection of the propulsive acceleration along a given direction. Starting from the known results from the literature, according to which a close form solution for the general case of not (fully) specularly reflecting sail cannot be retrieved, the propulsive acceleration is approximated by a mathematical model that closely resembles the classical optical force model. Using this new approach, the solution of the optimal steering law is shown to be written in an analytical form that is fully general and extremely accurate. In this sense, the proposed mathematical model may effectively be used to analyze the impact of the thermo-optical characteristics of the sail film on the optimal steering law within a wide range of different mission scenarios.

The study of thermochromic and thermo-optical properties of sol–gel V1−x−y WxSiyO2 films by response surface method

Vanadium dioxide (VO2) is an attractive thermochromic material for smart windows attributed to its reversible phase transition between semiconductors to metal phase at its phase transition temperature. In this paper, various quantities of tungsten and SiO2 were mixed into VO2 sols to fabricate V1−x−y WxSiyO2 gels. The gels were spin coated on a sodium-free glass slide and subsequently calcined at 500 °C in an atmospheric control furnace under reducing gas flow (H2–Ar, 5%–95%). The comprehensive investigation was conducted on the corresponding thermochromic and thermo-optical characteristics of VO2 composite films by response surface methodology and ANOVA analysis. Results show that the addition of SiO2 has greatly inhibited the agglomeration of particles. Additionally, the thermal induced infrared filtering effect is found to be strongly dependent on the mixing ratios of SiO2/W. However, the optical parameters such as refractive index and extinction coefficient were temperature-dependent and less influenced by the doping contents of SiO2 and W. The phase transition temperature of the doped VO2 films containing both SiO2 and W was dropped to the room temperature (31 °C). Furthermore, the temperature dependent electrical–optical properties of VO2 of the as-prepared films were strongly correlated with the dopant concentrations of W and SiO2.

Inorganic White Thermal-Control Coatings for Extreme Space Environments

White paints and clear anodizing are often used as thermal-control coatings. Inorganic white ceramic coating on AA2219 alloy prepared using novel Keronite plasma-electrolytic-oxidation technology was used in this study for the BepiColombo mission. The Keronite coating on a high copper-containing 2219 aluminum alloy was investigated in terms of its resistance to environmental exposures, such as corrosion, humidity, and high-temperature (400°C) UV/VUV radiation. The coating was characterized in terms of its microstructure, porosity, and adhesion, and their relations to the overall thermooptical performance and space suitability. Optical reflectivity, spectrophotometer color, solar absorptance, and hemispherical emittance have been measured and compared against that of currently used white silicone paint. The Keronite white coating has been shown to offer thermooptical characteristics acceptable as a space thermal-control material, while offering all the added benefits of 1) inorganic ceramic coating with extremely good adhesion, 2) environmentally friendly process with no toxic substances and heavy metals, 3) capability to coat shapes with complex geometries and non-line-of-sight areas, and 4) resistance to harsh space environment. The ceramic white coating has now been applied on Mercury Planetary Orbiter solar-array thermal shields.