Thermal Tuning Efficiency Research Articles

Ultra-Wideband Tunable Microwave Photonic Filter Based on Thin Film Lithium Niobate

In signal processing of the growing semaphore, a microwave photonic filter (MPF) is capable of dealing with high-frequency signals, offering the advantages of high bandwidth, easy tuning, and more. This paper presents an efficient tunable microwave photonic filter that features a wideband tuning capability and narrow-band filtering effect based on the lithium niobate on insulator (LNOI) material platform. A multi-mode waveguide race-track type microring resonator has been designed, along with a thermal electrode that utilizes the thermo-optical effect of lithium niobate to adjust the microring resonator. The packaged device has been tested, with a wideband tunable range of 4.7~38.2 GHz achieved. This allows for cross-band continuous tuning across the C-band to Ka-band range. When supplied with 29.1 mW of electric power, the thermal tuning efficiency reaches 9.2 pm/mW, enabling high-frequency tuning of up to 38.2 GHz. The filter possesses a high resolution, exhibiting a 3 dB bandwidth of 662 MHz.

A Silicon-Based On-Chip 64-Channel Hybrid Wavelength- and Mode-Division (de)Multiplexer

An on-chip 64-channel hybrid (de)multiplexer for wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) is designed and demonstrated on a 220 nm SOI platform for the demands of large capacity optical interconnections. The designed hybrid (de)multiplexer includes a 4-channel mode (de)multiplexer and 16-channel wavelength-division (de)multiplexers. The mode (de)multiplexer is comprised of cascaded asymmetric directional couplers supporting coupling between fundamental TE mode and higher-order modes with low crosstalks in a wide wavelength range. The wavelength-division (de)multiplexers consist of two bi-directional micro-ring resonator arrays for four 16-channel WDM signals. Micro-heaters are placed on the micro-resonators for thermal tuning. According to the experimental results, the excess loss is <3.9 dB in one free spectral range from 1522 nm to 1552 nm and <5.6 dB in three free spectral ranges from 1493 nm to 1583 nm. The intermode crosstalks are −23.2 dB to −33.2 dB, and the isolations between adjacent and nonadjacent wavelength channels are about −17.1 dB and −22.3 dB, respectively. The thermal tuning efficiency is ∼2.22 mW/nm over one free spectral range.

Fully Integrated Solid-State LiDAR Transmitter on a Multi-Layer Silicon-Nitride-on-Silicon Photonic Platform

We report a LiDAR transmitter incorporating both a hybrid-integrated tunable external cavity laser and a high-resolution 2-D optical phased array beam-steerer. Widely tunable single-mode lasing over a span of approximately 100 nm is achieved with >42 dB side-mode-suppression-ratio, 18 mW output power, and 2.8 kHz linewidth. Two-dimensional beam steering in a field-of-view of 140° × 16° exhibits a beam divergence of 0.051° × 0.016° measured in full width at half maximum. Leveraging the low propagation loss, negligible nonlinear loss, and low thermal sensitivity of silicon nitride, and combining the high mode confinement and efficient thermal tuning of silicon, the device affirms the feasibility of high-power on-chip lasing, power bottleneck elimination, and low on-chip insertion loss. It represents the first demonstration of a fully integrated LiDAR transmitter on the multi-layer silicon-nitride-on-silicon photonic platform, revealing the potential of complementary integration in the effort toward a LiDAR transmitter of sufficient optical power budget.

Thermo-Optic Phase Shifter with Interleaved Suspended Design for Power Efficiency and Speed Adjustment.

Conventional thermo-optic devices-which can be broadly categorized to that with and without a thermal isolation trench-typically come with a tradeoff between thermal tuning efficiency and tuning speed. Here, we propose a method that allows us to directly define the tradeoff using a specially designed thermo-optic phase shifter with an interleaved isolation trench. With the design, the tuning efficiency and speed can be precisely tailored simply by controlling the duty ratio (suspended length over total heater length) of the suspended design. Phase shifters are one of the main components in photonic-integrated circuits, and having phase shifters with a flexible design approach may enable the wide adoption of photonic applications such as an optical neural network and LiDAR.

Trapezoidal Angled Fiber Coupler for Microsphere Resonator With Enhancement of Fano Resonance

We propose and demonstrate a trapezoidal angled fiber coupler for excitation of whispering-gallery mode microsphere resonator, with enhanced Fano resonance. The reflection from the angled fiber is enhanced when the oblique angle is greater than critical angle, which is obtained as 30° theoretically for angled fiber. To enhance the endface reflection, the front tip of the angled fiber is polished flat to achieve the trapezoidal angled fiber. Compared with angled fiber, the proportion of Fano lineshape in spectra for trapezoidal angled fiber is nearly doubled and the slope is greatly improved, which reaches 304.55 dB/nm. Moreover, through the coupling of dual microspheres, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor is improved severalfold and the thermal tuning efficiency is increased by 50%. With the advantage of low cost, easy fabrication procedure and reflection mode operation, the proposed device shows a great potential for sensing, optical tuning and optical switching applications.

Efficient and compact sol-gel TiO2 thermo-optic microring resonator modulator

Thermo-optic (TO) modulators play an increasingly important role in wavelength routers, lidar, optical computing, and other reconfigurable photonic systems. Highly efficient TO tunable microring resonators (MRRs) were first demonstrated based on a sol-gel TiO2 platform in the 1310-nm waveband owing to the synergistic effect between the TiO2 core and SU-8 cladding with both the negative thermo-optical coefficients. The MRR modulator with SU-8 polymer as the top cladding layer exhibits a thermal tuning efficiency of 33.0 pm/mW, which is more than 14 times higher than that with silica top cladding. Its rise/fall times of 9.4 us/24 us and a Pπ power of 7.22 mW were achieved, indicating a relatively high TO modulator figure of merit among noncrystalline material platforms allowing monolithic integration on different substrates. These results yield a strong promise for applying the sol-gel TiO2 platform in photonic integrated circuits and suggest a new angle of view to design compact and efficient TO modulators in wearable devices, visible/infrared communication, and biophotonic applications.

Tunable narrow band add-drop filter design based on apodized long period waveguide grating assisted co-directional coupler.

Tunable add/drop filter based optical interconnects are an integral part of data centers as well as optical communications. Although add/drop filters based on ring resonators and waveguide Bragg gratings are well developed, long period waveguide grating (LPWG) based add/drop filters have little been investigated so far. In this article, we propose an apodized LPWG assisted co-directional coupler for narrow band add/drop filtering by combining silicon (Si) waveguide with titanium dioxide (TiO2) waveguide geometry. The proposed structure has been analyzed by combining the finite element method (FEM) and transfer matrix method (TMM), showing a good side lobe suppression ratio (SLSR) equal to 25.7 dB and an insertion loss of 0.6 dB. Owing to the high group index difference of Si and TiO2 waveguides, a narrow band response of 1.4 nm has been achieved with 800µm long LPWG. The opposite thermo-optic coefficients of Si and TiO2 ensures a good thermal tunability of the central wavelength. Considering a thin metallic heater of titanium nitride (TiN) the thermal tuning efficiency is found to be 0.07 nm/mW. Further, two LPWGs have been cascaded to realize a tunable dual channel filter with a minimum channel spacing of 185 GHz and a channel crosstalk better than 20 dB, showing its potential application towards dense wavelength division multiplexing.

Highly efficient thermo-optic tunable micro-ring resonator based on an LNOI platform.

We demonstrate a high-efficiency thermo-optic (TO) tunable micro-ring resonator in thin-film lithium niobate. Thermal insulation trenches around the heated micro-ring resonator and the underlying silicon substrate significantly reduce the heating power consumption and improve the tuning efficiency. Compared to conventional TO devices without thermal insulation trenches, the proposed device achieves a full free spectral range wavelength shift with a 14.9 mW heating power, corresponding to a thermal tuning efficiency of 53.7 pm/mW, a more than 20-fold improvement of tuning efficiency. The approach enables energy-efficient high-performance TO devices such as optical switches, wavelength routers, and other reconfigurable photonic devices.

Power-Efficient Thermal Optical Tunable Grating Coupler Based on Silicon Photonic Platform

A tunable grating coupler capable of dynamically changing the central wavelength with low power consumption is demonstrated and fabricated using a standard complementary metal–oxide–semiconductor process. The grating coupler is based on an efficient thermal tuning scheme employing free-standing silicon-on-insulator strip waveguides. The air gap provides thermal isolation between the heated grating structure and the underlying silicon substrate, which greatly enhance the heating efficiency. We achieve a thermal shift of 55 nm for the central wavelength with 19.5 mW and a record tuning efficiency of 2.82 nm/mW, which is more than one order of magnitude better than the previous demonstration. The measured rise time and fall time are 0.16 and 0.74 ms, respectively.

Low-Power-Consumption Integrated Tunable Filters for WDM Switching Applications in Silicon Photonics

We present the experimental results of thermally tunable double microring resonator devices for integrated wavelength division multiplexing applications. The devices demonstrated a free-spectral range up to 2.4 THz (19.2 nm) around 1550 nm and a channel rejection higher than 35 dB. Using Ti/TiN metal heaters, a power consumption of 20 mW per FSR per ring is achieved, which outperformed analogous devices fabricated using the standard silicon process. A thermal tuning efficiency of 0.95 nm/mW is accomplished, and a symmetric tuning is obtained with less than 1% of electrical compensation.

Efficient Thermal Tuning Employing Metallic Microheater With Slow-Light Effect

Thermal tuning acts as one of the most fundamental roles in integrated silicon photonics since it can provide flexibility and reconfigurability. Low tuning power and fast tuning speed are long-term pursuing goals in terms of the performance of the thermal tuning. Here, we propose and experimentally demonstrate an efficient thermal tuning scheme employing the metallic metal heater. The slow-light effect in the photonic crystal waveguide is employed to enhance the performance of the metal microheater. Meanwhile, the metal microheater is integrated on the side of the waveguide rather than on the top. Thanks to both the slow-light effect and the side-integrated microheater, the tuning efficiency is significantly enhanced, and the response time as fast as 2 $\mu \text{s}$ is obtained. Since the thermal tuning with metal heater has been widely applied in silicon photonics, the proposed scheme may provide a valuable solution towards the performance enhancement of the thermal tuning in silicon photonics.

Compact, submilliwatt, 2 × 2 silicon thermo-optic switch based on photonic crystal nanobeam cavities

We propose and experimentally demonstrate a 2×2 thermo-optic (TO) crossbar switch implemented by dual photonic crystal nanobeam (PCN) cavities within a silicon-on-insulator (SOI) platform. By thermally tuning the refractive index of silicon, the resonance wavelength of the PCN cavities can be red-shifted. With the help of the ultrasmall mode volumes of the PCN cavities, only ∼0.16 mW power is needed to change the switching state. With a spectral passband of 0.09 nm at the 1583.75 nm operation wavelength, the insertion loss (IL) and crosstalk (CT) performances were measured as IL(bar)=−0.2 dB, CT(bar)=−15 dB, IL(cross)=−1.5 dB, and CT(cross)=−15 dB. Furthermore, the thermal tuning efficiency of the fabricated device is as high as 1.23 nm/mW.

Optimization of a thermally tuned silicon-based reconfigurable optical power splitter with thermal isolations

In this paper, a thermally tuned silicon-based three-channel reconfigurable multimode interference (MMI) optical power splitter with four optimized thermal isolations is proposed. Specific and flexible reconfigurable functions (1×1, 1×2, and 1×3 MMI splitters) can be achieved by thermally tuning the heaters. By using a beam propagation method, the optimum geometry of the heaters, desired refractive index changes, and phase shifts of the MMI splitter are calculated at first. The temperature distributions of the devices without and with the thermal isolations are then analyzed by using the finite element method. Thermal crosstalk between adjacent heaters is observed and the influence of the thermal isolation geometry on the thermal crosstalk is examined subsequently. The geometry of the thermal isolations is optimized based on the trade-off between the thermal tuning efficiency and optical output characteristics. Finally, satisfactory improvements with the proposed structure are demonstrated with high-thermal efficiency (the maximum power dissipation decreases reach 43.7% and 55.2% for 1×1 and 1×2 MMI splitters, respectively) and good optical output quality (the maximum excess losses are as low as 0.365, 0.388, and 0.272 dB for 1×1, 1×2, and 1×3 MMI splitters, respectively; the crosstalk is less than −21.27 and −15.54 dB for 1×1 and 1×2 MMI splitters, respectively).

Efficiency Optimization of Silicon Photonic Links in 65-nm CMOS and 28-nm FDSOI Technology Nodes

Optical interconnects for system-in-package applications can be designed for various bit rates. In this paper, an optimization study is conducted to find the optimal parameters for electrooptical links, based on a silicon photonic technology. We focus on the bit rate to achieve highest possible power efficiencies. This paper takes all the elements of an electrooptical link into account: serialization stage, ring-resonator-based modulator, thermal stabilization, modulator driver, laser, receiver, deserialization stage, and clock-phase generation. The optimization is based on a simulation-supported database for single-ended transimpedance amplifiers (TIAs). For all the other, elements of the link simulation-based power consumption models are presented. Furthermore, an analytical solution for the TIA bandwidth and bit rate relationship is derived based on the system jitter, the TIA noise, transimpedance, bandwidth and minimal output swing, and the available input signal. Optimal bit rates are derived and discussed for a 65-nm CMOS and a 28-nm fully depleted silicon-on-insulator technology. We found that the optimal bit rates increase with more aggressive technology scaling and smaller photodiode capacitances, but decrease if lower static power consumptions can be achieved (e.g., by efficient thermal tuning of ring-resonator modulators). We conclude that further research should aim for lower tuning powers instead of higher speed.

Error-free operation of a polarization-insensitive 4λ x 25 Gbps silicon photonic WDM receiver with closed-loop thermal stabilization of Si microrings.

We report the first closed-loop operation of a 100 Gbps polarization-insensitive, 4-channel wavelength-tracking WDM receiver in silicon photonics platform. Error-free operation is achieved with input polarization scrambling over input wavelength change of 4.5 nm using efficient thermal tuning of Si microring demux, corresponding to greater than 60°C fluctuation in temperature.

Numerical Analysis on Thermal Tuning Efficiency and Thermal Stress of a Thermally Tunable SG-DBR Laser

Thermally tunable sampled grating distributed Bragg reflector (SG-DBR) lasers are very attractive for applications of optical coherent systems, where light sources with narrow linewidth are key requirements. In this paper, we focus on the thermal tuning efficiency and the thermal stress field of such lasers. The influence of a thermal isolation (air trench) design on the tuning efficiency of the laser is analyzed by using a thermal model based on the finite-element method (FEM) at first. The thermal tuning efficiency is found to be tenfold improved if the air trench is used. Influences of the air trench geometry configurations on temperature and thermal stress field are then examined in detail. Simulation results indicate that the most dangerous positions that can crack always appear at the output port and the corner of the air trench structure contact with the n-InP layer. The air trench geometry configuration of an SG-DBR laser is finally optimized (with a width of 70 $\mu\text{m}$ and a thickness of 1 $\mu\text{m}$ ) to ensure the reliability and the quasi-continuous wavelength tuning range of the device at the same time. With the optimized design, by using a developed temperature-dependent transfer matrix method-based optical model, optical output characteristics of the thermally tunable SG-DBR laser are demonstrated, including the output wavelength for the tuning range (1525–1585 nm covering C band) and the linewidth for all the channels (several tens to several hundreds of kilohertz). The simulated results have been found to be in good agreement with the experimental measurements previously reported, which are helpful to the thermal management and the design optimization of such devices.

Tunable Grating Couplers for Broadband Operation Using Thermo-Optic Effect in Silicon

We demonstrate a tunable grating coupler for broadband operation using an impurity-doped thermo-optic heater fabricated by complementary metal–oxide–semiconductor-compatible processes. As a thermo-optic heater, a p-n diode is employed to have a p-type region inside of an in-coupling grating connected with outside p+/n+ contacts and is heated directly by a current flow through the grating. Applying a forward-bias voltage of 7 V, we shift the central wavelength of the grating coupler from 1525 to 1574 nm with a coupling efficiency of close to −4 dB. Considering a 1-dB bandwidth at each central wavelength, we extend the operational bandwidth to nearly 90 nm. A thermal tuning efficiency of 0.23 nm/mW and transient responses of 32/ $22~\mu \text{s}$ for heating/cooling times are obtained.

Hitless tunable WDM transmitter using Si photonic crystal optical modulators.

We have developed compact Si Mach-Zehnder modulators that are assisted by wideband low-dispersion slow light in lattice-shifted photonic crystal waveguides. We have also developed Si triangular-shaped coupled-microring multiplexers that allow a box-like spectrum, a wide free spectral range, and an efficient thermal tuning. In this study, we integrated three sets of these devices in a small footprint of 2.0 × 0.7 mm(2) and achieved their 25 Gbps/ch operation as a wavelength division multiplexing transmitter. Moreover, we demonstrated hitless wavelength tuning using thermo-optic switches loaded in the bus waveguide.

Eight-Channel Optical Add-Drop Multiplexer With Cascaded Parent-Sub Microring Resonators

An eight-channel integrated optical add-drop multiplexer with cascaded parent-sub microring resonators is experimentally demonstrated on a silicon-on-insulator (SOI) substrate. Through thermal tuning, each channel can be independently switched between the adding and dropping states. The measured thermal tuning efficiency is 0.15 nm/mW. Typically, all eight channels can be multiplexed and demultiplexed with a 2.6-dB drop loss, a 0.36-nm bandwidth ( $>$ 40 GHz), and a $-$ 20-dB channel crosstalk. Moreover, the scheme to increase the number of operating channels is also discussed.

Integrated reconfigurable microring based silicon WDM receiver for on-chip optical interconnect

We demonstrate an integrated reconfigurable wavelength division multiplexing receiver on the silicon-on-insulator (SOI) platform. The receiver is composed of a 1 × 8 thermally tunable microring resonator filter and Ge–Si photodetectors. With low thermal tuning powers the channel allocation of the receiver can be reconfigured with high accuracy and flexibility. The thermal tuning efficiency is approximately 8 mW nm−1. We show eight-channel configurations with channel spacing of 100 GHz and 50 GHz and a configuration in which all eight channels cover an entire free spectral range of the ring with uniform channel spacing of 1.2 nm. Each channel can receive high-quality signals with a data rate of up to 13.5 Gb s−1; thus an aggregate data rate higher than 100 Gb s−1 can be achieved.