Thermo-optic Effect Research Articles

Modelling of a miniature mid-IR thermo-optic spectrometer on chip based on a GaAs/In[formula omitted]Ga[formula omitted]P waveguide platform

Spectrometers based on integrated optical waveguides, as opposed to free space optics, provide many advantages including robustness, compactness, and low cost, combined with alignment-free and low-power operation. Interest in such a chip-based approach for integrated spectrometers is growing, but the integrated spectrometers demonstrated so far have largely been limited to operation at wavelengths up to about 5μm due to intrinsic material absorption. In this work, we design and model an on-chip thermo-optically tuneable spectrometer based on a novel waveguide material platform consisting of GaAs (core) on InGaP (cladding), with potential to operate in the wavelength range between 1 and 17μm to fully cover the Mid-IR molecular fingerprint region. The strong thermo-optic effect exhibited by both of these materials makes the platform a very promising candidate for on-chip spectroscopy. A series of simulations were performed to predict the optical and thermal properties of the device. We show that with the proper optical and thermal optimisation of the platform and by using an interferometer with spiralled waveguides of 60 mm length, the device can effectively achieve a resolution of 10 cm−1 with a maximum temperature excursion of about 83 K and at the cost of a few tens of Watts of power.

Temperature detection based on surface plasmon resonance at THz wave

A new terahertz (THz) temperature sensor, which is based on surface plasmon resonance (SPR) by prism coupling excitation at THz frequency, is proposed. A novel prism coupling structure is designed for the temperature sensor by using unique features of thermo-optic effect for silicon at 1 THz and the high THz emission effiency of InSb. The medium thickness for the THz temperature sensor structure is analysed to get the optimal value by the finite element method, so that the sensitivity can reach optimal value. The optimal thickness of each layer is given, and the sensitivity of THz temperature sensor can reach $$9.75\times 10^{{{-4}}^{\circ }}/{^\circ }{\mathrm{C}}$$ , and FWHM of the SPR spectrum can reach $$0.073{^\circ }$$ , within the detection range of temperature, $$0{-}80{^\circ }{\mathrm{C}}$$ , and the relationship between SPR angle and temperature is obtained within the detection range of temperature.

Scalable and Fast Optical Circuit Switch Based on Colorless Coherent Detection: Design Principle and Experimental Demonstration

We investigate a large-scale and fast optical circuit switch system that uses digital coherent technologies. The achievable port count is expanded by introducing colorless detection which eliminates channel selecting filters at receivers. Wavelength selection is performed using a shared local oscillator (LO) bank, which is configured by combining a multi-wavelength optical source with Silicon-photonic tunable filters (TFs). Microsecond switching times are realized with the thermo-optic effect available with the Silicon-photonic TF. Design optimization of the proposed system handles complex level of freedom, i.e., available wavelength number, space switch port count, optical amplifier location and gain, and device loss. In addition, colorless coherent detection induces penalties, which further complicates switch scale assessment. In this paper, we develop a simulator to clarify how those parameters affect switch performance; the switch throughput is maximized with consideration of the degradation stemming from colorless detection. The design performance accurately matches results measured in various system scenarios. A dual-carrier 256-Gb/s DP-QPSK experiment successfully demonstrates a 1,856 × 1,856 optical switch system with switching time under 3.52 μs. The resulting total throughput is 441 Tb/s assuming 7%-overhead HD-FEC. To the best of our knowledge, this is the first demonstration of 400-Tbps class large bandwidth optical switches that use TFs as wavelength selective devices for coherent detection.

Hybrid Integrated Silicon Nitride–Polymer Optical Phased Array For Efficient Light Detection and Ranging

Silicon nitride (SiN) optical phased arrays (OPAs) have emerged as a potential alternative to their silicon counterparts, owing to their low nonlinearity, easy scalability, and low propagation loss. However, the development of SiN OPAs for light detection and ranging (LiDAR) applications has attracted less attention due to the low thermo-optic (TO) effect of SiN. Moreover, SiN OPAs are considered to be undesirable for transmitters in LiDAR systems because of the limited longitudinal tuning efficiency of SiN grating antenna. We present a hybrid integrated SiN-polymer OPA for realizing an efficient LiDAR. Owing to the large TO effect of polymer, the proposed OPA with polymeric phase modulators resolves the power dissipation drawback of the SiN devices. The polymeric modulators are integrated with a SiN power splitter and grating antenna, and the coupling loss is alleviated via a spot-size converter. Additionally, a steering angle magnifying lens is used to enhance the tuning efficiency of the antenna, expanding the longitudinal beam steering range. The prepared OPA transmitter equipped with the lens exhibits decent two-dimensional beam steering, featuring stable emission characteristics over a 12° × 30° field of view. With the transmitter, efficient time-of-flight based LiDAR is demonstrated to provide a detection range of 10 m under a peak optical power of 55 dBm. The proposed SiN-polymer OPA is highly anticipated to circumvent the challenges of conventional SiN counterparts and potentially spearhead the development of a prominent OPA platform for advanced LiDAR schemes.

Thermally Induced Mode Amplification Characteristics of Large Mode Area Segmented Cladding Fiber

A model of amplifier based on large mode area (LMA) segmented cladding fiber (SCF) is proposed. High order modes (HOMs) filtering effect and thermal-induced modal evolution characteristics are theoretically investigated by the few-mode steady state rate equations and heat conduction equation. Simulation results show that an amplifier based on ytterbium doped SCF presents much better HOMs suppression and mode beam quality than the amplifier based on conventional step-index fiber when achieving single-mode operation by coiling fiber. Furthermore, heat load generating in the core of ytterbium doped SCF can change the pre-designed refractive index profile through the thermo-optic effect, which induces evolution of mode amplification characteristics, including mode loss decreasing and mode area shrinking. And the co-pumping amplifier ytterbium doped SCF is more advantageous to get large mode area of fundamental mode at the output end, while single-mode operation is achieved much more effectively in the counter-pumping amplifier. These results can provide guidance in the design of high-power single-mode fiber amplifier based on rare-earth doped LMA-SCF.

Thermal tuning of chirped SOI sidewall grating for tunable wavelength, delay, and bandwidth

In this work, we design a silicon-on-insulator (SOI) sidewall grating with tunable wavelength, delay, and bandwidth through thermal tuning. Incorporating uniform sidewall corrugations and a linearly chirped rib waveguide, the thermo-optic effect imposed on chirped rib waveguide changes the effective refractive index, due to the resultant of the temperature gradient. Consequently, tunable wavelength, delay, and bandwidth in SOI sidewall grating are achieved. In the numerical simulations, the designed SOI grating is demonstrated with a tunable wavelength from 1 560.7 nm to 1 561.9 nm, a tunable delay from 0 to 38 ps, and a tunable bandwidth from 1 nm to 1.5 nm.

Thermo-optically tunable spectral broadening in a nonlinear ultra-silicon-rich nitride Bragg grating

Spectral tunability methods used in optical communications and signal processing leveraging optical, electrical, and acousto-optic effects typically involve spectral truncation that results in energy loss. Here we demonstrate temperature tunable spectral broadening using a nonlinear ultra-silicon-rich nitride device consisting of a 3-mm-long cladding-modulated Bragg grating and a 7-mm-long nonlinear channel waveguide. By operating at frequencies close to the grating band edge, in an apodized Bragg grating, we access strong grating-induced dispersion while maintaining low losses and high transmissivity. We further exploit the redshift in the Bragg grating stopband due to the thermo-optic effect to achieve tunable dispersion, leading to varying degrees of soliton-effect compression and self-phase-modulation-induced spectral broadening. We observe an increase in the bandwidth of the output pulse spectrum from 69 to 106 nm as temperature decreases from 70°C to 25°C, in good agreement with simulated results using the generalized nonlinear Schrödinger equation. The demonstrated approach provides a new avenue to achieve on-chip laser spectral tuning without loss in pulse energy.

Numerical Simulation of Thermo-Optic Effects in an Nd: Glass Slab with Low Thermally Induced Wavefront Distortion

A gain slab configuration with a low thermally induced wavefront distortion, which is based on heating the edge by the cladding layer, is proposed. The gain slab will be applied to a helium-cooled Nd: glass multislab laser amplifier with an output of 100 J at a repetition rate of 10 Hz. Additionally, a 3D numerical simulation model is developed to analyze the thermo-optic effects in the gain slab. Some parameters, including the absorption coefficient (α) of the cladding layer, the shape of the pump beam, and the gap between the pump area and absorbing cladding layer, are optimized to eliminate the thermo-optic effects. The results indicate that the peak-to-valley (P-V) of the thermally induced wavefront distortion of the specific gain slab can be reduced by 61% if other parameters remain constant.

Multifunctional optoelectronic device based on graphene-coupled silicon photonic crystal cavities.

We present a hybrid device based on graphene-coupled silicon (Si) photonic crystal (PhC) cavities, featuring triple light detection, modulation, and switching. Through depositing single-layer graphene onto the PhC cavity, the light-graphene interaction can be enhanced greatly, which enables significant detection and modulation of the resonant wavelength. The device is designed to generate a photocurrent directly by the photovoltaic effect and has an external responsivity of ∼14 mA/W at 1530.8 nm (on resonance), which is about 10 times higher than that off-resonance. Based on the thermo-optical effect of silicon and graphene, the device is also demonstrated in electro-optical and all-optical modulation. Also, due to the high-quality (Q) factor of the resonate cavity, the device can implement low threshold optical bistable switching, and it promises a fast response speed, with a rise (fall) time of ∼0.4 μs (∼0.5 μs) in the all-optical switch and a rise (fall) time of ∼0.5 μs (∼0.5 μs) in the electro-optical hybrid switch. The multifunctional photodetector, modulator, and optical bistable switch are achieved in a single device, which greatly reduces the photonic overhead and provides potential applications for future integrated optoelectronics.

Photon emission and changes in fluorescent properties of bone after laser irradiation.

Laser scalpels used in medical surgery concentrate light energy, heating the tissues. Recently, we reported thermoluminescence emission from laser-treated soft tissues. Here we investigated the thermo-optical effects caused by a laser operating at 808 nm on animal bones (beef ribs) through luminescence and fluorescence imaging, thermal imaging and scanning electron microscopy. Laser-induced artificial lesions emitted luminescence peaking around 650 nm, with a half-life of almost 1hour. As concerns fluorescence, 24 hours after laser treatment we observed an increase of the emission and a shift from 500 (untreated) to 580 nm (treated). Recrystallization observed by SEM indicates that the temperature in the artificial lesions is over 600°C. We can conclude that laser treatment induces specific luminescent and fluorescent emissions due to heating of the bone and modification of its components. Monitoring these emissions could help prevent tissue overheating and its potential damages during laser-assisted medical procedures.

Compact resonant 2 × 2 crossbar switch using three coupled waveguides with a central nanobeam.

This theoretical simulation paper presents designs and projected performance of ∼1550-nm silicon-on-insulator (SOI) and ∼2000-nm Ge-on-Si-on-nitride and Ge-on-nitride 2×2 optical crossbar switches based upon a three-waveguide coupler in which the central waveguide is a nanobeam actuated by the thermo-optical (TO) effect. A TO heater stripe is located atop the central nanobeam. To implement accurate and realistic designs, the 3D finite difference time domain approach was employed. The metrics of crossbar switching, insertion loss (IL) and crosstalk (CT) were evaluated for choices of 3-waveguide structure parameters and TO-induced index changes. The predicted ILs and CTs were excellent, enabling the designed devices to be considered as fundamental building blocks in wavelength-division-multiplexed cross-connect (WXC) applications. Proposed here are compact, nonblocking space-and-wavelength routing switches to be constructed in a monolithic, industry-standard SOI chip (and in Ge-on-SON and GON chips). Specifics are given for realizing 16 × 16 × Mλ WXCs as well as reconfigurable, multi-resonant, programmable hexagonal and diamond meshes.

In-Fiber Mach–Zehnder Interferometer Sensor Based on Er Doped Fiber Peanut Structure in Fiber Ring Laser

A novel temperature sensor based on rare earth fiber peanut (REFP) shape structure in fiber ring laser (FRL) is proposed and demonstrated. A single-mode rare earth doped fiber is used to fabricate a peanut structure to produce Mach-Zehnder interferometer (MZI). Benefited from the strong thermo-optic effect of rare earth ions in fiber, high sensitivity measurement of temperature is exploited to realize. More importantly, there is no need for additional filters compared to traditional fiber laser sensors. The sensitivity of refractive index (RI) was measured to further verify the sensing performance of MZI. The experimental results show that compared with the external sensing unit, the Er-doped fiber peanut (EDFP) structure has higher thermo-optic effect coefficient and superior temperature sensitivity. The temperature and RI sensitivity of the EDFP is 0.158 nm/°C and -19.55 nm/RIU, respectively. The optical fiber MZI sensor based on the EDFP has the advantages of high sensitivity, good repeatability, simple fabrication, and compact structure. It has good application prospects in biomedicine, aerospace, and marine development.

Tunable broadband mode power dividers based on a wavelength-insensitive coupler using the thermo-optic effect for flexible modal power adjustment in a mode-division multiplexing network

We demonstrate tunable broadband two-mode power dividers based on a wavelength-insensitive coupler for a mode-division multiplexing system. Ultra-small T E 0 − T E 1 and T E 0 − T E 2 mode dividers based on Si-wire waveguides are designed based on the coupled-mode theory. Tunability is achieved by using the thermo-optic effect. In both dividers, arbitrary branching ratios can be realized over a wide wavelength range. We experimentally demonstrated for the first time, to the best of our knowledge, a tunable T E 0 − T E 1 mode divider based on Si-wire waveguides. From the experimental results, an arbitrary branching ratio can be achieved by adjusting the injection current to a TiN heater placed on top of one of the delay lines.

Gold-induced photothermal background in on-chip surface enhanced stimulated Raman spectroscopy.

Surface enhanced Raman spectroscopy (SERS) and stimulated Raman spectroscopy (SRS) are well established techniques capable of boosting the strength of Raman scattering. The combination of both techniques (surface enhanced stimulated Raman spectroscopy, or SE-SRS) has been reported using plasmonic nanoparticles. In parallel, waveguide enhanced Raman spectroscopy has been developed using nanophotonic and nanoplasmonic waveguides. Here, we explore SE-SRS in nanoplasmonic waveguides. We demonstrate that a combined photothermal and thermo-optic effect in the gold material induces a strong background signal that limits the detection limit for the analyte. The experimental results are in line with theoretical estimates. We propose several methods to reduce or counteract this background.

Towards electronic-photonic-converged thermo-optic feedback tuning

As Moore’s law approaching its end, electronics is hitting its power, bandwidth, and capacity limits. Photonics is able to overcome the performance limits of electronics but lacks practical photonic register and flexible control. Combining electronics and photonics provides the best of both worlds and is widely regarded as an important post-Moore’s direction. For stability and dynamic operations considerations, feedback tuning of photonic devices is required. For silicon photonics, the thermo-optic effect is the most frequently used tuning mechanism due to the advantages of high efficiency and low loss. However, it brings new design requirements, creating new design challenges. Emerging applications, such as optical phased array, optical switches, and optical neural networks, employ a large number of photonic devices, making PCB tuning solutions no longer suitable. Electronic-photonic-converged solutions with compact footprints will play an important role in system scalability. In this paper, we present a unified model for thermo-optic feedback tuning that can be specialized to different applications, review its recent advances, and discuss its future trends.

Optical Fiber Gas Pressure Sensor Based on Polydimethylsiloxane Microcavity

We experimentally demonstrated a high sensitive gas pressure sensor, which also has more concise processing steps and lower cost. The sensor was fabricated by splicing the single-mode fiber (SMF) and a short section of hollow-core fiber (HCF) which was filled with a Polydimethylsiloxane (PDMS) film to form an enclosed air microcavity with the length of 55 μm. The thin PDMS film also plays a role in reflecting the beam. The thermo-optical effect and the thickness of PDMS film can affect the cavity that is sensitive to external pressure. When the thickness of the PDMS membrane is 3 μm, the measured gas pressure sensitivity reaches to 52.143 nm/Mpa, the linearity is 99.86%, and the cross-sensitivity of the gas pressure over temperature is lower than 2.51 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> Mpa/°C. Moreover, the compact structure, high sensitivity, high mechanical strength, and convenient fabrication make it suitable for measuring gas pressure in harsh conditions.

Pump RIN coupling to frequency noise of a polarization-maintaining 2 µm single frequency fiber laser.

We investigated the frequency noise coupling mechanism of a 2 μm polarization-maintaining single frequency fiber laser (SFFL) theoretically and experimentally. The coupling of pump's relative intensity noise (RIN) to frequency noise of a single-frequency high-gain silica fiber laser is shown experimentally to be consistent with a theoretical model where thermal expansion and thermo-optic effect mediate the coupling. The measured and theoretical frequency noise of the 2 μm SFFL with three pump sources is compared. We find using a 1550 nm single frequency laser pump source produces the lowest frequency noise, less than 100 Hz/Hz at frequencies higher than 100 Hz.

Real-time observation of the thermo-optical and heat dissipation processes in microsphere resonators.

This work reports the real-time observation of the thermo-optical dynamics in silica microsphere resonators based on the dispersive time stretch technique. In general, the thermo-optical dynamics of silica microsphere resonators, including the thermal refraction and thermal expansion, can be characterized by the resonance wavelength shift, whose duration is at the millisecond timescale. However, this fast wavelength shift process cannot be directly captured by conventional spectroscopy, and only its transmission feature can be characterized by a fast-scanning laser and an intensity detector. With the advance of the time-stretch spectroscopy, whose temporal resolution is up to tens of nanoseconds, the thermo-optical dynamics can be observed in a more straight-forward way, by utilizing the pump-probe technology and mapping the resonance wavelength to the time domain. Here, the thermo-optical dynamics are explored as a function of the power and the scanning rate of the pump laser. Theoretical simulations reproduce the experimental results, revealing that the thermo-optical dynamics of silica microsphere resonators is dominated by the fast thermo-optical effect and the slow heat dissipation process to the surroundings, which leads to gradual regression of the resonance wavelength. This work provides an alternative solution for studying the thermo-optical dynamics in whispering gallery mode microresonators, which would be crucial for future applications of microresonator photonic systems.

On-chip silicon photonic controllable 2\u2009\xd7\u20092 four-mode waveguide switch

Multimode optical switch is a key component of mode division multiplexing in modern high-speed optical signal processing. In this paper, we introduce for the first time a novel 2 × 2 multimode switch design and demonstrate in the proof-of-concept. The device composes of four Y-multijunctions and 2 × 2 multimode interference coupler using silicon-on-insulator material with four controllable phase shifters. The shifters operate using thermo-optic effects utilizing Ti heaters enabling simultaneous switching of the optical signal between the output ports on four quasi-transverse electric modes with the electric power consumption is in order of 22.5 mW and the switching time is 5.4 µs. The multimode switch exhibits a low insertion loss and a low crosstalk below − 3 dB and − 19 dB, respectively, in 50 nm bandwidth in the third telecom window from 1525 to 1575 nm. With a compact footprint of 10 µm × 960 µm, this device exhibits a relatively large width tolerance of ± 20 nm and a height tolerance of ± 10 nm. Furthermore, the conceptual principle of the proposed multimode switch can be reconfigurable and scalable in multifunctional on-chip mode-division multiplexing optical interconnects.

Tunable WGM Laser Based on the Polymer Thermo-Optic Effect.

In this work, the thermo-optic effect in polymers was used to realize a temperature-tunable whispering-gallery-mode laser. The laser was fabricated using a capillary tube filled with a light-emitting conjugated polymer solution via the capillary effect. In the whispering-gallery-mode laser emission wavelength can be continuously tuned to about 19.5 nm using thermo-optic effect of polymer. The influence of different organic solvents on the tuning rate was studied. For a typical lasing mode with a bandwidth of 0.08 nm, a temperature-resolved tuning rate of ~1.55 nm/°C was obtained. The two-ring coupling effect is responsible for the suppression of the WGM in the micro-cavity laser. The proposed laser exhibited good reversibility and repeatability as well as a sensitive response to temperature, which could be applied to the design of photothermic and sensing devices.