Thermo-optic Effect Research Articles

Temperature-Compensated Refractive Index Measurement Using a Dual Fabry–Perot Interferometer Based on C-Fiber Cavity

We demonstrate a simple-to-fabricate, temperature-compensated refractive index sensor using a dual Fabry-Perot optical fiber interferometer using C-fiber. The sensor device is formed by combining two types of in-fiber interferometers, including a C-fiber Fabry-Perot interferometer and a single mode fiber (SMF) Fabry-Perot (FP) interferometer. The C-fiber is a silica capillary with an open side, which allows external liquid to directly enter the internal hole. The C-fiber interferometer is sensitive to external refractive index (1704 nm/RIU) as well as temperature (-0.196 nm/°C) due to the thermo-optic effect and thermal expansion of the C-fiber cavity, while the SMF interferometer is only sensitive to ambient temperature (0.0118 nm/°C). Thus, temperature-compensated refractive index measurement can be achieved by examining the phase shift responses of the two FP interference peaks with transfer matrix approach, solving the problem of temperature sensitivity of RI sensors due to the relatively large thermo-optic coefficient of colloidal materials and aqueous samples.

Miniature Fizeau Interferometric Thermometer

Precise measurement of temperature is vital in a variety of industries and basic sciences. Numerous techniques have been proposed for temperature measurement, such as fiber Bragg gratings and Fabry–Perot interferometers. Despite several important accomplishments, such optical thermometers are still limited in their performances. Two major limitations are the low operational temperature and the paradox between a large dynamic range and a high resolution. Here, we propose a fiber-optic thermometer based on fiber-tip microspherical Fizeau interferometer made from high temperature-resistant material. The sensor is fabricated by inserting a section of single-mode fiber into a spherical microcavity to form a Fizeau interferometer. Thanks to the thermo-optic effect and thermal expansion effect, the variation of ambient temperature modifies the refractive index and the interferometric length of the microcavity, and then a corresponding change in the interferometric spectrum can be observed. The demodulation method is based on the combination measurement of interferometric fringe shift and free spectral range variation, which is capable of providing high resolution and wide dynamic range. Experimental results show that by using a simple wavelength tracking demodulation method, a sensitivity of 22 pm/°C with a high resolution and a dynamic range over 1000 °C can be achieved. Since the sensor is of small size, it exhibits fast response and recovery time which mean it can monitor ambient temperature variation in real time. Additionally, the structure is robust and compact which makes the technology an interesting alternative to conventional thermometers for temperature measurement.

On-chip scalable mode-selective converter based on asymmetrical micro-racetrack resonators

Abstract Mode division multiplexing (MDM) technology has been well known to researchers for its ability to increase the link capacity of photonic network. While various mode processing devices were demonstrated in recent years, the reconfigurability of multi-mode processing devices, which is vital for large-scale multi-functional networks, is rarely developed. In this paper, we first propose and experimentally demonstrate a scalable mode-selective converter using asymmetrical micro-racetrack resonators (MRRs) for optical network-on-chip. The proposed device, composed of cascaded MRRs, is able to convert the input monochromatic light to an arbitrary supported mode in the output waveguide as required. Thermo-optical effect of silicon waveguides is adopted to tune the working states of the device. To test the utility, a device for proof-of-concept is fabricated and experimentally demonstrated based on silicon-on-insulator substrate. The measured spectra of the device show that the extinction ratios of MRRs are larger than 18 dB, and modal crosstalk for selected modes are all less than −16.5 dB. The switching time of the fabricated device is in the level of about 40 μs. The proposed device is believed to have potential applications in multi-functional and intelligent network-on-chip, especially in reconfigurable MDM networks.

Micro-Nano Electro-Optic Modulator Structure Based on the Si/SiGe/Si Material

The modulation power consumption and the modulation efficiency are the key parameters of the electro-optic modulator, which directly affect the electro-optic modulator's photoelectric properties. Improving the performance of the electro-optic modulator, a micro-nano electro-optic modulator structure based on the Si/SiGe/Si material is proposed in this paper, which has low power consumption and high efficiency. After the plasma dispersion effects and the thermo-optic effects are analyzed, we can know that the performance of the electro-optic modulator could be affected by the carrier concentration and the temperature of modulator. Silicon Germanium (SiGe) material is attached to the common Silicon (Si) electro-optic modulator, and a large injection ratio is obtained from the Si/SiGe/Si double hetero-junction. With the modulation region's carrier concentration rise, and the working voltage and the power consumption of modulator all are reduced. The jugged active region structure is attached to the common Si electro-optic modulator, and the probability of inelastic collision among carriers is decreased, so the temperature rise of modulator can be reduced. The thermal-optic effects are weakened, and the modulation efficiency is increased. The simulation results show that the working voltage of the jugged SiGe modulator is less than that of the Silicon modulator at the same refractive index differences, and the jugged SiGe modulator has lower modulation power consumption; the jugged SiGe modulator's effective refractive index differences are more than the Silicon modulator's effective refractive index differences at the same working voltage, and the jugged SiGe modulator has higher modulation efficiency. Therefore, this jugged SiGe modulator is a micro-nano electro-optic modulator with lower power consumption and higher efficiency.

Low-loss nonlinear optical isolators in silicon

Asymmetric forward and backward transmission through photonic structures can be achieved via optical nonlinearities, but existing systems have typically used slow thermo-optic effects. A new resonator design has now enabled low-loss, non-reciprocal pulse routing based on the Kerr nonlinearity in integrated silicon waveguides.

Tunable multispectral near-infrared absorption with a phase transition of VO2 nanoparticles hybridized with 1D photonic crystals

We theoretically demonstrate a switchable multichannel near-infrared absorber in a composite structure based on vanadium dioxide nanoparticles embedded between two and one-dimensional photonic crystal mirrors. A switching of absorption behavior is induced through the reversible semiconductor-to-metal phase transition of vanadium dioxide nanoparticles via its temperature-dependent permittivity—thermo-optical effect. This behavior leads to a multi-wavelength reconfigurable optical response of the proposed structure from poorly absorbing to highly absorbing. For example, there is the possibility of enhancement of absorption from ∼0.14 to ∼0.75 at normal incidence of light by increasing the temperature beyond the critical value of ∼341 K when the vanadium dioxide nanoparticles transform from a semiconducting state into a metallic one. These properties make the considered structure applicable for use in multiband absorbers, light detectors, and optical switching devices.

Tunable nanophotonics enabled by chalcogenide phase-change materials

Abstract Nanophotonics has garnered intensive attention due to its unique capabilities in molding the flow of light in the subwavelength regime. Metasurfaces (MSs) and photonic integrated circuits (PICs) enable the realization of mass-producible, cost-effective, and efficient flat optical components for imaging, sensing, and communications. In order to enable nanophotonics with multipurpose functionalities, chalcogenide phase-change materials (PCMs) have been introduced as a promising platform for tunable and reconfigurable nanophotonic frameworks. Integration of non-volatile chalcogenide PCMs with unique properties such as drastic optical contrasts, fast switching speeds, and long-term stability grants substantial reconfiguration to the more conventional static nanophotonic platforms. In this review, we discuss state-of-the-art developments as well as emerging trends in tunable MSs and PICs using chalcogenide PCMs. We outline the unique material properties, structural transformation, and thermo-optic effects of well-established classes of chalcogenide PCMs. The emerging deep learning-based approaches for the optimization of reconfigurable MSs and the analysis of light-matter interactions are also discussed. The review is concluded by discussing existing challenges in the realization of adjustable nanophotonics and a perspective on the possible developments in this promising area.

Temperature distribution inside a double-cladding optical fiber laser or amplifier

The temperature distribution inside a double-cladding optical fiber laser or amplifier is examined in detail. Traditionally, the quantum defect in the core is taken to be the main source of heating in an active optical fiber. However, contributions from the parasitic absorption of the signal and the pump may also play an important role, especially for low quantum defect or radiation-balanced lasers and amplifiers. The contributions to the heating in both the core and the inner-cladding are considered and analyzed in general terms in this paper. In particular, it is shown that if the maximum tolerable surface temperature of the fiber relative to the ambient is taken to be 300°C to avoid damaging the fiber’s outer polymer cladding, the core temperature rises only in the range of 0°C–5°C relative to the inner-cladding for an air-cooled fiber. However, for a water-cooled fiber, the core temperature can be higher than the inner-cladding by as much as 50°C, potentially changing a single-mode core to multimode due to the thermo-optic effect.

Origin of the Kerr effect: investigation of solutions by polarization-dependent Z-scan

The nonlinear refractive index dependence on the incident light polarization state has been studied for pure chloroform and chloroform solutions of aminobenziliden-1,3-indandione derivatives. Measurements were done with linearly, elliptically, and circularly polarized light using 8 ns and 30 ps pulse duration 1064 nm lasers. This allows us to separate the electronic response, molecular reorientation, and thermo-optical components of the nonlinear refractive index. The refractive index variations with the change of laser pulse repetition rate were employed to identify the presence of the thermo-optical effect. Quantum chemical calculations of linear polarizability were used to estimate the magnitude of molecular-reorientation-induced refractive index changes for solvents and solutions. Overall, in this paper we have outlined various essential aspects that need to be taken into account to correctly interpret Z-scan measurement results for organic solvents and solutions.

Thermal vulnerability detection in integrated electronic and photonic circuits using infrared thermography.

Failure prediction of any electrical/optical component is crucial for estimating its operating life. Using high temperature operating life (HTOL) tests, it is possible to model the failure mechanisms for integrated circuits. Conventional HTOL standards are not suitable for operating life prediction of photonic components owing to their functional dependence on the thermo-optic effect. This work presents an infrared (IR)-assisted thermal vulnerability detection technique suitable for photonic as well as electronic components. By accurately mapping the thermal profile of an integrated circuit under a stress condition, it is possible to precisely locate the heat center for predicting the long-term operational failures within the device under test. For the first time, the reliability testing is extended to a fully functional microwave photonic system using conventional IR thermography. By applying image fusion using affine transformation on multimodal acquisition, it was demonstrated that by comparing the IR profile and GDSII layout, it is possible to accurately locate the heat centers along with spatial information on the type of component. Multiple IR profiles of optical as well as electrical components/circuits were acquired and mapped onto the layout files. In order to ascertain the degree of effectiveness of the proposed technique, IR profiles of complementary metal-oxide semiconductor RF and digital circuits were also analyzed. The presented technique offers a reliable automated identification of heat spots within a circuit/system.

Multilayer graphene improved interface thermal property for all-optical controlled fiber interferometer

As the oxide-induced disruption from silicon dioxide substrate would greatly decrease the thermal property of graphene, strategies to optimize the thermo-optical response through controlling laser spot size, layer number and covering length of graphene layer are experimentally studied and discussed. An all-optical controlled fiber Mach–Zehnder interferometer with multilayer and monolayer graphene achieved by thermo-optical effects is subsequently proposed to prove the improvements. Graphene layers with 6 mm length were transferred to the middle taper and heated by an external laser. The maximum switching extinction ratio reached 16.2 dB. The response time and modulation efficiency were approximately 620 μs and 0.22 dB mW−1, respectively. This work suggests an effective way to improve the thermal property of graphene-fiber hybrid structure for all-optical control in fiber based integrated system. Moreover, the cost and complexity of device manufacturing are greatly reduced.

Bioinspired "Skin" with Cooperative Thermo-Optical Effect for Daytime Radiative Cooling.

Energy-saving cooling materials with strong operability are desirable for sustainable thermal management. Inspired by the cooperative thermo-optical effect in the fur of a polar bear, we develop a flexible, superhydrophobic, and reusable cooling "skin" by laminating a poly(dimethylsiloxane) film with a highly scattering polyethylene aerogel. Owing to its high porosity (97.9%) and tailored pore size of 3.8 ± 1.4 μm, it can achieve superior solar reflectance (R̅sun ∼ 0.96) and high transparency to irradiated thermal energy (τ̅PE,MIR ∼ 0.8) at a thickness of 2.7 mm. Combined with the low thermal conductivity (0.032 W m-1 K-1) of the aerogel, the cooling skin exerts midday sub-ambient temperature drops of 5-6 °C in a metropolitan environment, with an estimated limit of 14 °C under ideal service conditions. Our generalized bilayer approach can be easily applied to different types of emitters, bridging the gap between night-time and daytime radiative cooling and paving the way for more cost-effective and scalable cooling materials.

Tellurium@Selenium core-shell hetero-junction: Facile synthesis, nonlinear optics, and ultrafast photonics applications towards mid-infrared regime

Tellurium (Te) is a promising material for photonics applications due to its intriguing advantages, such as excellent optical nonlinearity, larger saturable absorption, strong thermo-optic effect, and superior UV–Vis photo-response. However, the limited optical absorption and uncertain stability of Te has greatly limited its applications in mid-infrared and higher band optical regimes. In this contribution, Te@Se core-shell hetero-junction was constructed by epitaxial growth Se on Te nanotubes (NTs), which exhibits strong broadband linear and nonlinear optical response. Compared to pristine Te NTs and current mainstream nanomaterials, Te@Se shows stronger nonlinear absorption and lower saturation intensity at 800 nm. By employing Te@Se as a saturable absorber device of erbium-doped fiber and thulium-doped fiber lasers, the high-power mode-locked pulses with very stable operation in near- and mid- infrared regime can be acquired. These results suggest that Te@Se hetero-junction is a promising material with infinite potential for new generation ultrafast nano-photonics.

Hitless and gridless reconfigurable optical add drop (de)multiplexer based on looped waveguide sidewall Bragg gratings on silicon.

Reconfigurable optical add-drop filters in future intelligent and software controllable wavelength division multiplexing networks should support hitless wavelength switching and gridless bandwidth tuning. The hitless switching implies that the central wavelength of one channel can be shifted without disturbing data transmissions of other channels, while the gridless tuning means that the filter bandwidth can be adjusted continuously. Despite a lot of efforts, very few integrated optical filters simultaneously support the hitless switching of central wavelength and the gridless tuning of bandwidth. In this work, we demonstrate a hitless add-drop filter with gridless bandwidth tunability on the silicon-on-insulator (SOI) platform. The filter comprises the two identical multimode anti-symmetric waveguide Bragg gratings (MASWBG) which are connected to a loop. The phase apodization technique is utilized to weaken the intrinsic sidelobe interference of grating-based devices. By sequentially manipulating central wavelengths of the two MASWBGs with the thermo-optical effect, we can reconfigure the spectral response of the filter gridlessly and hitlessly. Specifically, the central wavelength of the device is shifted by 14.5 nm, while its 3 dB bandwidth is tuned from 0.2 nm to 2.4 nm. The dropping loss and the sidelobe suppression ratio (SLSR) are dependent on the bandwidth selected. Measured variation ranges of dropping loss and SLSR are from -1.2 dB to -2.5 dB and from 12.8 dB to 21.4 dB, respectively. The hitless wavelength switching is verified by a data transmission measurement at a bit rate of 25 Gbps.

Thermal shift of whispering gallery modes in tellurite glass microspheres

The tremendous progress made in recent years in the production of tellurite glasses has led to the development of different optical elements and devices on their basis such as microresonators and microlasers. The investigation of tellurite glass microresonators is of great importance for many potential applications. We studied theoretically and experimentally the nonlinear thermo-optical effects in tellurite glass microresonators under continuous-wave laser pumping. We calculated the temperature distributions for both, steady-state and transient non-stationary regimes and simulated shifts of the resonant wavelengths of whispering gallery modes. We produced Tm-doped tellurite glass microspheres and measured the resonant wavelength shifts as functions of the heat power. Our experimental results are in a good agreement with the theoretical predictions.

Self-induced thermo-optical effects in silicon and germanium dielectric nanoresonators

AbstractDielectric nanoresonators uniquely support both magnetic and electric resonances across a wide wavelength range. They are thus being exploited in a growing number of groundbreaking applications. In particular, they have been recently suggested as promising nanoheaters. However, while the thermo-optical properties of silicon and germanium resonators have been exploited to realize tunable metasurfaces based on external thermal inputs, the effect of self-induced optical heating onto their resonances has so far been neglected. In this study, we address the problem of self-heating of a thermo-optical resonator. In particular, employing a recursive procedure to account for the interdependence between the absorption cross section and the temperature of the resonator, we show that self-heating gives rise to a complex, nonlinear relationship between illumination intensity and temperature. Using both analytical and numerical models, we also observe that self-induced optical heating has nonnegligible effects on the spectral position of electric and magnetic resonances of spheres as well as anapole modes of nanodisks, even for moderate illumination intensities relevant for applications such as Raman scattering. Thus, our work demonstrates that self-induced optical heating must be properly accounted for when designing dielectric resonators for a wide range of devices.

Modeling Electrical Switching of Nonvolatile Phase-Change Integrated Nanophotonic Structures with Graphene Heaters.

Progress in integrated nanophotonics has enabled large-scale programmable photonic integrated circuits (PICs) for general-purpose electronic-photonic systems on a chip. Relying on the weak, volatile thermo-optic, or electro-optic effects, such systems usually exhibit limited reconfigurability along with high-energy consumption and large footprints. These challenges can be addressed by resorting to chalcogenide phase-change materials (PCMs) such as Ge2Sb2Te5 (GST) that provide a substantial optical contrast in a self-holding fashion upon phase transitions. However, current PCM-based integrated photonic applications are limited to single devices or simple PICs because of the poor scalability of the optical or electrical self-heating actuation approaches. Thermal-conduction heating via external electrical heaters, instead, allows large-scale integration and large-area switching, but fast and energy-efficient electrical control is yet to be achieved. Here, we model electrical switching of GST-clad-integrated nanophotonic structures with graphene heaters based on the programmable GST-on-silicon platform. Thanks to the ultra-low heat capacity and high in-plane thermal conductivity of graphene, the proposed structures exhibit a high switching speed of ∼80 MHz and a high energy efficiency of 19.2 aJ/nm3 (6.6 aJ/nm3) for crystallization (amorphization) while achieving complete phase transitions to ensure strong attenuation (∼6.46 dB/μm) and optical phase (∼0.28 π/μm at 1550 nm) modulation. Compared with indium tin oxide and silicon p-i-n heaters, the structures with graphene heaters display two orders of magnitude higher figure of merits for heating and overall performance. Our work facilitates the analysis and understanding of the thermal-conduction heating-enabled phase transitions on PICs and supports the development of future large-scale PCM-based electronic-photonic systems.

Packaging and precision testing of fiber-Bragg-grating and silicon ring-resonator thermometers: current status and challenges

In recent years photonic thermometers—temperature sensors based on optical frequency measurement which exploit the thermo-optic effect to translate thermal changes into frequency shifts—are gaining popularity as a possible alternative to their electrical counterparts: platinum resistance thermometers and thermocouples. In this work, we report our results of testing photonic thermometers based on silica fiber-Bragg-grating technology supplied by a commercial company, as well as preliminary testing results of a silicon ring-resonator thermometer developed at the National Research Council of Canada. The main purpose of showing these two examples is to highlight some of the challenges that need to be addressed if photonic thermometers are to replace thermocouples or platinum resistance thermometers in metrology laboratories and other environments where high accuracy and stability are required, namely the influence of packaging on the sensor’s performance and the need for rigorous testing to be done in a temperature metrology lab.

All-optical thermal control for second-harmonic generation in an integrated microcavity.

Nonlinear optical effects in integrated microcavities have been studied extensively with the advantages of strong light-matter interaction, great scalability, and stability due to the small mode volume. However, the pump lasers stimulating nonlinear effects impose obstacles for practical applications, since the material absorption causes thermal resonance drift and instability. Here we experimentally demonstrate an all-optical control of the thermal behavior in optical microcavities for tunable doubly-resonant second-harmonic (SH) generation on an integrated photonic chip. Through an auxiliary control laser, the temperature of a selected microring can be efficiently changed, thus allowing precise frequency tuning of the doubly-resonant wavelength while eliminating the distortion of the lineshape induced by the thermo-optic effect. Although the phase-matching conditions will limit the tuning range of 55GHz, the technique is still potential to achieve a larger tuning range in combination with temperature regulation. Additionally, this approach has the advantage of quick reconfiguration, showing a fast modulation rate up to about 256 kHz. The theoretical model behind our experimental scheme is universal and applicable to other microcavity-enhanced nonlinear optical processes, and our work paves the way for controlling and utilizing the thermal effect in the applications of microcavities.

Self-expanding fabricated optical millibubble resonators with strong thermo-optical effect

We develop a novel method for fabricating borosilicate millibubble resonators with strong thermo-optical effect. The method is based on self-expanding of air within a two ends sintered borosilicate capillary under the heating of a hydrogen–oxygen flame (HOF). The intrinsic thermo-optical effect of borosilicate millibubble resonator is experimentally demonstrated by a designed novel structure based on the principle of thermometer and the fabricated millibubble resonator has a Q factor exceeding ∼104. When filled with ethanol, borosilicate millibubble resonator shows the strong intrinsic thermo-optical effect by exhibiting a linear blue shift of nearly twice of its free spectral range (FSR) under 6.0 mW pump power change, which corresponds to a thermal enhanced tuning sensitivity as high as 0.124 nm/mW. The proposed borosilicate millibubble resonator is anticipated to find applications in optical filtering, all-optical switching, microfluidic sensing as well as all-optical micro-optics devices.