Thermo-optic Switch Research Articles

Demonstration of ultra-compact and low-loss 2 × 2 silicon thermo-optic switches based on a mode-conversion nanobeam cavity

Optical switch is an essential component in integrated photonic circuits. A mode-conversion nanobeam cavity (MCNC) coupled with two waveguides has been employed to realize ultra-compact and low-loss 2 × 2 thermo-optic switches in silicon-on-insulator. This system can exhibit either Fano or Lorentzian lineshape in transmission spectra, dependent on the coupling structure. It has a low dropping loss, and two outputs are in the same direction, owing to the unidirectional coupling between the resonant mode and bus waveguides. Here, we have demonstrated a high-performance 2 × 2 Fano switch with a bandwidth of 5.2 nm and a footprint of only 35.5 × 1 µm2. The insertion loss (IL) and crosstalk (CT) are 0.7 dB and −54.1 dB in the bar state, respectively, while the IL and CT are 0.9 dB and −17.4 dB in the cross-state, respectively. In addition, 2 × 2 optical switches with a Lorentzian transmission lineshape have also been realized and then applied to construct a four-channel reconfigurable optical add-drop multiplexer (ROADM). Through thermal tuning, the ROADM has achieved a channel spacing of 200 GHz or 400 GHz, with an inter-channel CT below −12.3 dB or −17.2 dB, respectively. To our knowledge, we have reported the first demonstrations of 2 × 2 Fano switch and ROADM based on MCNCs. The proposed 2 × 2 switches will find potential applications in advanced photonic integrated circuits.

Efficient Thermo-Optic Switching and Modulating in Nonlocal Metasurfaces.

High-Q optical resonances in nonlocal metasurfaces, benefiting from significantly enhanced light/matter interactions, feature strong responses even under a weak external stimulus. In this work, we leverage the high-Q resonances of quasi-guided modes (QGMs) supported by a photonic crystal slab (PCS) structure to achieve efficient optical switching/modulation. The QGMs with an experimentally measured Q-factor of ∼2200 are realized by shifting every second column of air holes in a rectangular lattice within a silicon slab. At a weak illumination intensity of less than 4.0 W/cm2 from a 532 nm continuous-wave pump laser, the QGM resonance around 1550 nm experiences a pronounced spectral shift, with modulation depth exceeding 55%. This is attributed to the thermo-optic response caused by photothermal heating of the metasurface triggered by the absorption of the pump laser in silicon, which is further verified by the electrical heating approach. Our reported results showcase a simple yet effective way of tailoring light propagation in nonlocal metasurfaces.

Thermo-Optic Switch with High Tuning Efficiency Based on Nanobeam Cavity and Hydrogen-Doped Indium Oxide Microheater

We propose and experimentally demonstrate an efficient on-chip thermo-optic (TO) switch based on a photonic crystal nanobeam cavity (PCNC) and a hydrogen-doped indium oxide (IHO) microheater. The small mode volume of the PCNC and the close-range heating through the transparent conductive oxide IHO greatly enhance the coupling between the thermal field and the optical field, increasing the TO tuning efficiency. The experimental results show that the TO tuning efficiency can reach 1.326 nm/mW. And the rise time and fall time are measured to be 3.90 and 2.65 μs, respectively. In addition, compared with the conventional metal microheater, the measured extinction ratios of the switches are close (25.8 dB and 27.6 dB, respectively), indicating that the IHO microheater does not introduce obvious insertion loss. Our demonstration showcases the immense potential of this TO switch as a unit device for on-chip large-scale integrated arrays.

Low-loss silica waveguide 1×8 thermo-optic switch based on large-scale multimode interference couplers

Optical switches play key role in signal crossing and interconnection. Low-loss and compact optical switches are highly demanded in on-chip optical routing. A silica waveguide 1 × 8 thermo-optic switch based on cascaded 1 × 8 multimode interference (MMI) and 8 × 8 MMI couplers is experimentally demonstrated. Beam propagation method is adopted in the theoretical design and optimization. Standard CMOS technology fabrication is used in switch preparation. Air trenches are introduced to enhance the thermal modulation. At wavelength 1550 nm, the measured insertion loss and crosstalk of this 1 × 8 switch is less than 3.69 dB and −16.29 dB, respectively. And the extinction ratio is larger than 16.68 dB for all routing states. The rise time and fall time are 1.0 ms and 1.24 ms respectively. Compared with the 8-channel switch based on cascaded stages structure, the size reduces by 30 %. With future bandwidth improvement, this demonstrated switch has good potentials in the application of all-optical network routing.

Ultrahigh extinction ratio and a low power silicon thermo-optic switch.

The silicon thermo-optic switch (TOS) is one of the most fundamental and crucial blocks in large-scale silicon photonic integrated circuits (PICs). An energy-efficient silicon TOS with ultrahigh extinction ratio can effectively mitigate cross talk and reduce power consumption in optical systems. In this Letter, we demonstrate a silicon TOS based on cascading Mach-Zehnder interferometers (MZIs) with spiral thermo-optic phase shifters. The experimental results show that an ultrahigh extinction ratio of 58.8 dB is obtained, and the switching power consumption is as low as 2.32 mW/π without silicon air trench. The rise time and fall time of the silicon TOS are about 10.8 and 11.2 µs, respectively. Particularly, the figure of merit (FOM) has been improved compared with previously reported silicon TOS. The proposed silicon TOS may find potential applications in optical switch arrays, on-chip optical delay lines, etc.

Angularly offset multiline dispersive optical phased array enabling large field of view and plateau envelope

We propose and demonstrate an angularly offset multiline (AOML) dispersive silicon nitride optical phased array (OPA) that enables efficient line beam scanning with an expanded field of view (FOV) and plateau envelope. The suggested AOML OPA incorporates multiline OPA units, which were seamlessly integrated with a 45° angular offset through a thermo-optic switch based on a multimode interference coupler, resulting in a wide FOV that combines three consecutive scanning ranges. Simultaneously, a periodic diffraction envelope rendered by the multiline OPA units contributes to reduced peak intensity fluctuation of the main lobe across the large FOV. An expedient polishing enabling the angled facet was diligently accomplished through the implementation of oblique polishing techniques applied to the 90° angle of the chip. For each dispersive OPA unit, we engineered an array of delay lines with progressively adjustable delay lengths, enabling a passive wavelength-tunable beam scanning. Experimental validation of the proposed OPA revealed efficient beam scanning, achieved by wavelength tuning from 1530 to 1600 nm and seamless switching between multiline OPAs, yielding an FOV of 152° with a main lobe intensity fluctuation of 2.8 dB. The measured efficiency of dispersive scanning was estimated at 0.97°/nm, as intended.

Low thermal crosstalk silicon MZI optical switch with high speed and low power consumption

Abstract We developed a compact thermo-optic Mach–Zehnder interferometer switch with a direct heating heater using multimode interference and achieved a sufficiently low thermal crosstalk performance. Large-scale switch systems, such as optical neural networks, require thermo-optical switches with low power consumption, fast switching speed, compact size, and low thermal crosstalk. This switch is equipped with a heater that directly heats the Si core waveguide, which is a structure that connects non-doped Si wires between phase shifters and a heatsink. As a result, a significant miniaturization with a phase shifter length of approximately 7 μm, low π-phase shift power consumption of less than 20 mW, and fast switching in sub-microseconds were achieved. The improved phase shifter showed a very small figure of merit of 8.89 mW∙μs. Simultaneously, transmission spectrum measurements of nearby ring resonators show that the thermal crosstalk is significantly reduced even at a distance of only 30 μm. This device can contribute to the overall circuit performance and footprint reduction in large-scale optical integrated circuits and optical neural network configurations.

Four-Mode Thermo-Optic Switch Based on Y-Junctions Integrated With Multimode Interferometers

Four-Mode Thermo-Optic Switch Based on Y-Junctions Integrated With Multimode Interferometers

A High Extinction Ratio Silicon Thermo-Optic Switch With Variable Splitter and Spiral Phase Shifters

A High Extinction Ratio Silicon Thermo-Optic Switch With Variable Splitter and Spiral Phase Shifters

Frequency tunable switching between transmission and absorption modes based on graphene and VO2 layers at the THz range

In this paper, a tunable multilayer structure is proposed, which can provide reversible switching between transmission and absorption modes at the terahertz region without reconfiguration. The layered structure consists of graphene, Si, and VO2 layers. Transfer matrix method is employed to describe the optical properties of the layered structure. The switching phenomenon occurs based on the VO2 transition from insulator to metal phase by varying temperature. Furthermore, active frequency tuning by graphene chemical potential for thermally switching transmission and absorption with near-perfect intensity is observed. Incidence angular stability for TE and TM is up to 20° and 40°, respectively. Finally, the Si layer thickness effect demonstrates that mainly defines the peak number and frequency for both functions. Theoretical implementation confirms the potential usefulness of the proposed configuration in applications such as reversibly tunable smart absorbers, filters, or thermo-optical switches for high-speed optical systems.

Low-power and wide-band 1 × 8 silica waveguide optical switch

A silica waveguide 1 × 8 thermo-optic switch consisting of cascaded three-stage Mach-Zehnder interferometers (MZIs) is demonstrated. The multimode interference (MMI) coupler is adopted in the MMI-MZI 1 × 2 switch unit due to its insensitivity to wavelength and favorable fabrication tolerance. Beam propagation method (BPM) is used in the switch design and optimization. Standard CMOS technologies have been adopted in the preparation of switch fabric. At 1550 nm, the measurement results of 1 × 8 switch show that the fiber-to-fiber insertion loss ranges from 3.4 to 3.6 dB for all routing states. Meanwhile, the extinction ratio is no less than 15.63 dB, and the crosstalk is restrained to be lower than −15.65 dB. Due the adoption of MZI structure, the average driving power for the channel switching is 315.8 mW. Within the wavelength range from 1500 to 1620 nm, the extinction ratio is larger than 15.06 dB, the insertion loss is less than 5.8 dB, and PDL is smaller than 1.18 dB for all routing states. The experimental results fit the simulations well, which proves the proposed switch good potentials in wideband applications.

Programmable optical switching integrated chip for 4-bit binary true/inverse/complement code conversions based on fluorinated photopolymers.

In this work, programmable optical switching integrated chips for 4-bit binary true/inverse/complement optical code conversions (OCCs) are proposed based on fluorinated photopolymers. Fluorinated bis-phenol-A novolac resin (FAR) with low absorption loss and fluorinated polyacrylate (FPA) with high thermal stability are self-synthesized as core and cladding layer, respectively. The basic architecture of operating unit for the photonic chip designed is composed of directional coupler Mach-Zehnder interferometer (DC-MZI) thermo-optic (TO) switching, X-junction, and Y-bunching waveguide structures. The waveguide module by cascading 16 operating units could realize OCCs function through optical transmission matrix. The response time of the 4-bit binary OCCs is measured as about 300 µs. The insertion loss and extinction ratio of the actual chip are obtained as about 10.5 dB and 15.2 dB, respectively. The electric driving power consumption for OCCs is less than 6 mW. The true/inverse/complement OCCs are achieved by the programmable modulation circuit. The proposed technique is suitable for achieving optical digital computing system with high-speed signal processing and low power consumption.

Polarization-insensitive silicon optic switch based on mode manipulated power splitters and phase shifters

A polarization-insensitive thermo-optic switch is proposed and demonstrated on the silicon-on-insulator platform with a 220 nm silicon core layer. The present device is based on the Mach–Zehnder interferometer structure, consisting of polarization-insensitive power splitters and polarization-insensitive phase shifters (PIPSs). The polarization-insensitive power splitter has been realized by employing an adiabatic directional coupler, which utilizes the fast adiabatic mode evolution by introducing cubic Bézier curves on outer contours, providing broadband 3-dB power splitting for TE and TM polarization modes with only 70 µm coupling length. For the novel PIPSs, the ridge waveguide with large aspect ratio, based on the mode hybridness property, could obtain the same power consumption (Pπ) for an optical switch working at TE and TM polarizations. Experimental results indicate that the measured insertion losses are less than 2 dB and the extinction ratios are larger than 15 dB over a 40 nm wavelength band (covering the C-band).

Multimode optical switch based on cascaded Mach-Zehnder interferometer waveguides.

We present a 1 × 1 multimode optical switch for E11, E21, E12, and E22 modes based on cascaded Mach-Zehnder interferometer (MZI) waveguides, where the primary MZI is used to split E11, E21, E12, and E22 modes into E11 or E12 mode and then couple back to the original mode at the output, and the secondary MZIs are the modulation arms of the primary MZI. In addition, the secondary MZIs are designed to be mode-insensitive for switching E11 and E12 modes simultaneously. As a proof of concept, we fabricate the device with polymer material to achieve thermo-optic switching for the four modes. Our experimental device exhibits the extinction ratios of larger than 10.2 dB with a power consumption of 5.5 mW and response times of less than 1.28 ms for each mode. The presented device can be widely applied in mode-division multiplexing (MDM) systems where multimode switching is needed.

Electrically tunable thermoresponsive optic switch for smart window application based on dye-doped cholesteric liquid crystal

We demonstrate a cholesteric liquid crystal (CLC) smart window that functions as a reversible thermo-optic switch between a transparent state at a low temperature and a low-transmission state at a higher temperature. This smart window is based on the addition of a thermoresponsive handedness-reversible chiral dopant in a mixture of a typical chiral dopant, a dichroic dye and nematic liquid crystal, yielding a thermosensitized CLC. In such a homeotropically anchored dye-doped CLC, the thickness-to-pitch ratio increases as the temperature rises, causing the device to switch from the homeotropic texture to the fingerprint texture. The absorption characteristic of the dichroic dye enables the homeotropic and fingerprint configurations in the dye-doped CLC to create two distinct transmission levels—the transparent and opaque optically stable states, respectively. Such a switching mechanism entails no additional electric power and, thus, is energy-saving. Moreover, the switching temperature at which the texture transition takes place can be actively increased by increasing voltage for the smart window. This allows the CLC device to be perfectly adaptable to the need of a user and, in turn, applicable to a wider variety of environments.

Mode-selective switch on silica-based PLC platform

Optical mode switch is a crucial component for the mode-division multiplexing (MDM) system. In this paper, we demonstrate a thermo-optical (TO) mode-selective switch on silica-based planar lightwave circuit (PLC) platform. The 2 × 1 thermo-optical (TO) mode-selective switch based on Mach–Zehnder interferometer consists of an adiabatic coupler and a symmetric Y-branch. The input optical mode E00 (E10) is converted to E10 (E00) mode while the light is coupled from different input ports by changing the working state of heater. With 2% index-contrast, a compact mode switch on silica-based PLC platform is fabricated. According to experimental results, the fabricated mode-selective switch shows an insertion loss lower than 5 dB and a mode extinction ratio higher than 24 dB for two modes at 1550 nm. The crosstalk is lower than −10.54 dB from 1500 nm to 1600 nm. The power consumption for two modes are 433.24 mW and 440.15 mW, respectively. The rise time and fall time of the switch are 560.9μs and 794.8μs.

Mode-independent thermo-optic switch based on the total-internal-reflection effect.

A broadband mode-independent thermo-optic (TO) switch using the total-internal-reflection (TIR) effect is proposed and experimentally demonstrated on a polymer waveguide platform. By optimizing geometric parameters of the TIR switch, a mode-independent TO switching function with a large bandwidth and extinction ratio can be realized for E11, E12, and E21 modes. The measurement results show an extinction ratio larger than 18.1 dB with a driving power of 160 mW for each mode over the wavelength range of 1500-1620 nm. The designed structure can also be cascaded to form a 1 × N switch network for mode-division multiplexing (MDM) systems, which greatly improves the network flexibility.

Parabolic MMI Coupler for 2 × 2 Silicon Optical Switch With Robustly High Extinction Ratio for Four Paths

We propose and experimentally demonstrate a Mach-Zehnder silicon thermo-optic switch based on a parabolic multi-mode interference coupler. By judiciously designing the parabolic structure, the 2×2 MMI can exhibit extremely small imbalance of output power around 0.1 dB over the C band with good fabrication tolerance according to simulation. As such, for all four possible switching paths, high average extinction ratios over 35 dB can be achieved from 1520 nm to 1580 nm in experiment, with maximum values concurrently over 40 dB. The proposed parabolic MMI occupies a small area of merely 2.4 μm × 9.5 μm.

Flexible waveguide integrated thermo-optic switch based on TiO2 platform.

Mechanically flexible photonic devices are critical components of novel bio-integrated optoelectronic and high-end wearable systems, in which thermo-optic switches (TOSs) as optical signal control devices are crucial. In this paper, flexible titanium oxide (TiO2) TOSs based on a Mach-Zehnder interferometer (MZI) structure were demonstrated around 1310 nm for, it is believed, the first time. The insertion loss of flexible passive TiO2 2 × 2 multi-mode interferometers (MMIs) is -3.1 dB per MMI. The demonstrated flexible TOS achieves power consumption (Pπ) of 0.83 mW, compared with its rigid counterpart, for which Pπ is decreased by a factor of 18. The proposed device could withstand 100 consecutive bending operations without noticeable degradation in TOS performance, indicating excellent mechanical stability. These results provide a new perspective for designing and fabricating flexible TOSs for flexible optoelectronic systems in future emerging applications.

Review of 2 × 2 Silicon Photonic Switches

With the advent of 5G, artificial intelligence (AI), Internet of Things (IoT), cloud computing, Internet plus, and so on, data traffic is exploding and higher requirements are put forward for information transmission and switching. Traditional switching requires optical/electrical/optical conversions, which brings additional power consumption and requires the deployment of large amounts of cooling equipment. This increases the cost and complexity of the system. Moreover, limited by the electronic bottleneck, electrical switching will suffer from many problems such as bandwidth, delay, crosstalk, and so on, with the continuous reduction in device footprint. Optical switching does not require optical/electrical/optical conversions and has lower power consumption, larger capacity, and lower cost. Silicon photonic switches received much attention because of their compatibility with the complementary metal-oxide-semiconductor (CMOS) process and are anticipated to be potential candidates to replace electrical switches in many applications such as data center and telecommunication networks. 2 × 2 silicon photonic switches are the basic components to build the large-scale optical switching matrices. Thus, this review article mainly focuses on the principle and state of the art of 2 × 2 silicon photonic switches, including electro-optic switches, thermo-optic switches, and nonvolatile silicon photonic switches assisted by phase-change materials.