Thermo-optic Devices Research Articles

Thermally tunable silicon racetrack resonators with ultralow tuning power

We present thermally tunable silicon racetrack resonators with an ultralow tuning power of 2.4 mW per free spectral range. The use of free-standing silicon racetrack resonators with undercut structures significantly enhances the tuning efficiency, with one order of magnitude improvement of that for previously demonstrated thermo-optic devices without undercuts. The 10%-90% switching time is demonstrated to be ~170 µs. Such low-power tunable micro-resonators are particularly useful as multiplexing devices and wavelength-tunable silicon microcavity modulators.

Thermo-Optic Two-Mode Interference Optical Waveguide Device with Fast Response Time

Based on the two-mode interference principle, a thermo-optic two-mode interference waveguide structure with heat-insulating grooves in both sides of the two-mode interference core has been proposed for optical switching. The thermal analysis of the proposed structure with SiON core and SiO2 cladding has been performed by the finite difference method. The switching power of the device is estimated as ∼450 mW, and the response time is estimated as ∼98 μs, which is eight times faster than that of the existing thermo-optic two-mode interference device. The device length of the switch is 2.7 times less than that of the existing total internal reflection (TIR) thermo-optic switch.

Optimizing performance of plasmonic devices for photonic circuits

We demonstrate the feasibility of fabricating thermo-optic plasmonic devices for variable optical attenuation and/or low-frequency (kHz) signal modulation. Results of finite-element simulations and experimental characterization of prototype devices indicate that a plasmonic device can reach specifications similar to or better than commercially available thermo-optic integrated optical components. Specifically, we have considered the insertion loss, power consumption, footprint, polarization-dependent loss, extinction ratio, and frequency response of the plasmonic devices, in addition to fabrication and material-related issues. The most serious fabrication challenge is to realize metallic nanowire waveguides with a sufficiently accurate cross-section to ensure low polarization-dependent loss at high extinction ratios.

Variable optical attenuator using thermo-optic two-mode interference device with fast response time

A thermo-optic two-mode interference (TMI) waveguide structure with a silicon trench and heat-insulating grooves in both sides of the core has been proposed for a variable optical attenuator (VOA) with fast response time. Thermal analysis of the proposed thermo-optic TMI waveguide structure with a silicon oxinitride (SiON) core has been performed by using the implicit finite difference method. The heating power required to achieve the attenuated power of approximately -25.5 dB for a VOA with a silicon trench is 460 mW , which is approximately 1.8 times less than that of a VOA without a silicon trench. The response time is estimated as approximately 98 micros, which is faster than the response time of the existing VOA.

Monolithic Integration of a Multiplexer/Demultiplexer With a Thermo-Optic VOA Array on an SOI Platform

In this letter, a nine-channel 100-GHz arrayed waveguide grating multiplexer/demultiplexer is monolithically integrated with a Mach-Zehnder interferometer thermo-optic variable optical attenuators (VOAs) arrayed on a silicon-on-insulator platform. The on-chip transmission loss is ~ 6 dB and the crosstalk is less than -25 dB for the transverse-electric mode. The maximum modulation depths of different thermo-optic VOAs are similar, ~ 15 dB with 2.7-V bias. The frequency response of our device is fast (ges 100 kHz) for thermo-optic effect devices. The maximum power consumption of a single VOA is less than 35 mW.

Polymer dispersed liquid crystals based on poly(styrene-b-ethylene oxide), poly(bisphenol a carbonate) or poly(methylphenylsiloxane), and 4′-(hexyloxy)-4-biphenyl-carbonitrile: Analysis of phase diagrams and morphologies generated

Thermodynamic phase behavior was studied for polymer dispersed liquid crystal (PDLC) films consisting of blends of different thermoplastics such as poly (bisphenol A carbonate) (PC), poly(methylphenylsiloxane) (PMPLSi), poly(styrene-b-ethylene oxide) (PSEO) block copolymer, and low-molecular weight 4′-(hexyloxy)-4-biphenyl-carbonitrile (HOBC) liquid crystals. The phase diagrams of these PDLC systems were built up using thermal transitions determined by differential scanning calorimetry (DSC), transmission optical microscopy (TOM), and dynamic mechanical analysis (DMA). Final morphology of these PDLC films generated by temperature induced phase separation was monitored by TOM. The final morphology of HOBC blends with PC, PSEO, or PMPLSi could be modified by controlling the relative compositions resulting in PDLC systems with small and narrow size-distribution of nematic droplets, suitable for use in thermo-optical devices. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Beam expansion in thermo-optic-effect-induced total internal reflection and its applications in optical switches

The beam-propagation characteristics of the total internal reflection (TIR) induced by the thermo-optic effect are investigated. Based on the Fourier heat-transmission principle and the variable separation method, we derive an analytical transient expression of the thermal field for general thermo-optic devices. With the analytical expression, the time response and steady-state temperature distribution of thermo-optic devices are presented. The beam expansion rule of TIR in the thermal field is developed mathematically, and a quantitative calculation is given as well. To illustrate the application of the rule, an X-junction 2 x 2 TIR switch with high reflection efficiency is designed through theoretical calculation. The simulation shows that the structure exhibits a high reflection coefficient; the reflection loss is only -0.76 dB. The simulation results agree well with the theoretical calculation.

Influence of thermal isolating grooves on the performance of the Mach-Zehnder interferometer-type thermo-optic variable optical attenuator

Two types of silicon-on-insulator thermo-optic variable optical attenuators (VOAs) based on a Mach-Zehnder interferometer and a multimode-interference coupler are fabricated, one with thermal isolating grooves to improve heating efficiency and the other without Comparison of optical and electrical properties, such as insertion losses, the maximum attenuation levels and the corresponding power consumptions, and the response times, is carried out between the two types of VOAs. The comparison results Indicate that use of thermal isolating grooves leads to better values for most characteristics and is an effective way to improve the performance of Mach-Zehnder interferometer-type thermo-optic devices. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Tunable polymer double micro-ring filters

Based on the vernier effect, widely tunable polymer double micro-ring filters using the thermooptic effect or the electrooptic effect are demonstrated. A tuning enhancement factor of 40 was achieved at wavelengths near 1550 nm. The tuning rate for the thermooptic device is 120 GHz/mW and for the electrooptic device is 120 GHz/12 V. A tunable laser with a side-mode suppression ratio greater than 30 dB was demonstrated using this filter and erbium-doped fiber amplifier gain. Thermal tuning over 35 nm was achieved.

Vanadium Dioxide Thin Films for Thermo-Optical Switching Applications

ABSTRACTVanadium dioxide (VO2) thin films were fabricated by e-beam evaporation of vanadium thin films followed by thermal oxidation in oxygen ambient. The properties of the VO2 films were investigated for thermo-optical switching applications. Synthesized VO2 film displays a phase transition at 65 – 68 °C. It exhibits an abrupt change in optical reflectivity over the phase transition temperature range. Results for VO2 on a highly reflective metal layer are strongly dependent on the VO2 thickness. The optical switching has a major hysteresis of about 15 °C between the heating and cooling branches. The evolution of the surface morphology with the oxidation time was studied with a SEM. The VO2 film was patterned on microplatforms by metal lift-off technique. We conclude that the evaporation followed by oxidation is an effective method to produce active VO2 film for thermo-optical switching devices.

Enhancement of the Thermo-optical Properties in Dye-doped PDLCs

A detailed theimo-optical analysis of dye-doped Polymer Dispersed Liquid Crystal (D-PDLC) systems has been performed. A comparison of the optical properties between D-PDLCs and not doped PDLCs based on the same polymeric mafrix is addressed. For any D-PDLC composition investigated, the light intensity and the sample temperature are used to control or adjust the nonlinear part of its refractive index. We investigated the thermal responses of PDLC samples prepared with three different dyes at two dopant concentrations in order to evaluate the contribute of the dopants as a function also of its percentage in the polymeric matrix. Our experimental results demonstrate that self transparency effect, optical hysteresis and thermo-optical modulation are notably affected by the used dye. We show how the use of dye in preparing PDLC can be useful in order to design thermo-optical devices employing waveguides connected with the considered materials or to realise low cost thermal sensors.

Enhancement of the Thermo-optical Properties in Dye-doped PDLCs

A detailed theimo-optical analysis of dye-doped Polymer Dispersed Liquid Crystal (D-PDLC) systems has been performed. A comparison of the optical properties between D-PDLCs and not doped PDLCs based on the same polymeric mafrix is addressed. For any D-PDLC composition investigated, the light intensity and the sample temperature are used to control or adjust the nonlinear part of its refractive index. We investigated the thermal responses of PDLC samples prepared with three different dyes at two dopant concentrations in order to evaluate the contribute of the dopants as a function also of its percentage in the polymeric matrix. Our experimental results demonstrate that self transparency effect, optical hysteresis and thermo-optical modulation are notably affected by the used dye. We show how the use of dye in preparing PDLC can be useful in order to design thermo-optical devices employing waveguides connected with the considered materials or to realise low cost thermal sensors.

A thermo-optical device for in situ study of sintering : F.Raether et al. J. Therm. Anal.Calorimetry, vol 53, no 3, 1998, 717–735

A thermo-optical device for in situ study of sintering : F.Raether et al. J. Therm. Anal.Calorimetry, vol 53, no 3, 1998, 717–735

Far-Infrared Radiation Modulators Using High-Tc Superconductors

The potential of using high-Tc superconductors as intensity modulators for far-infrared radiation is investigated in this work. Reflectance and transmittance for several design structures are computed using the published optical constants of the superconductor YBa2Cu3O7 and substrate materials. Notable differences in the reflectance and transmittance between the superconducting state and the normal state are illustrated. The best results are obtained based on the reflectance of thin films (10 nm–50 nm thick) on thin substrates (less than 100 μm thick) and for radiation incident on the substrate. This study demonstrates that high-Tc superconductive thin films can be used to build far-infrared radiation modulators. Future experimental study is recommended in order to materialize this promising thermo-optical device.

Thermo-Optical Properties of a Polymer Dispersed Liquid Crystalline Polymer

Abstract A side-chain liquid crystalline polymer dispersed into an amorphous polymer matrix can be used as a thermo-optical device that exhibits scattering at low temperatures and becomes transparent when heated above the clearing temperature of the liquid crystalline polymer. The blend of the side-chain liquid crystalline polymer and the UV-polymerizable monomer is yet two-phasic before polymerization and exhibits some additional phase separation during polymerization. Especially the thermal stability, but also the mechanical stability, of the thermo-optical device is considerably improved when dispersing a liquid crystalline polymer instead of a low molar mass liquid crystal into a polymer matrix.

Thin-film thermo-optic GexSi_1−x Mach–Zehnder interferometer

We report on the design and fabrication of Ge(x)Si(1-x) Mach-Zehnder interferometers based on the thermo-optic effect. The modulators exhibit 8 dB of extinction with a pi-phase-shift power requirement, P(pi), of 245 mW. The 3-dB bandwidth of these thermo-optic devices is 88 kHz. Finally, we present calculations indicating the effect of various parameters on the modulation of the Mach-Zehnder interferometer.

Thermo-optical SEED devices: External control of nonlinearity, bistability and switching behaviour

We discuss novel fundamental properties of thermo-optical SEED devices and present experimental results performed on Si Schottky, diodes where the optoelectronically generated electrical power leads to optical bistability and photonic switching. In particular, it is shown that the external resistance yields an additional nonlinearity which leads to novel properties which result from the external control of the multistable and switching behaviour. As a result, bistable characteristics with negligible hysteresis widths but high contrast ratios, a novel multistability and tunable differential gain can be achieved. Additionally, the temperature range and also the total dissipated power which are necessary for switching the device can significantly be reduced when using the external resistance.

The theoretical and experimental behavior of a thermooptic bulk deflector fiber-optic circuit switch

The temperature-dependent refractive index change of the dielectric, glass, has been utilized in the fabrication of a one-dimensional 1 × 2 thermooptic bulk deflector. A two-dimensional 1 × 4 thermooptic device utilizing the optical isotropy of glass, achieving enhanced switching capabilities, has also been successfully fabricated and tested. The theoretical analysis of the physical property dT/dx of the glass is confirmed to correspond closely with the optical behavior of the switch in terms of its deflection angle and transient response.

Thermo-optic bistable devices: Theory of operation in freestanding films

This paper presents a theory for the operation of thin-film thermo-optic bistable devices. Special attention is paid to free-standing films. After a review of nonlinear Fabry—Perot resonators and how they work we solve the heat equation in the thin film subject to a heating effect. Switching speed and optical power consumption are functions of the rate of change of temperature in the presence of a heat source. The interplay of heat loss from the surface of the film with heat diffusion within the film complicate the dynamics considerably. Quantitative results are derived for the dependence of speed and power on the dimensions of the active region of the optical element. It is found that diffusion transients can effect more rapid switching at the expense of power but only for infrequent operation.