Thermo-optic Phase Shifter Research Articles

UV-soft Imprinted Tunable Polymer Waveguide Ring Resonator for Microwave Photonic Filtering

A tunable polymer waveguide ring resonator is proposed and demonstrated for microwave photonic filtering. The ring resonator consists of a Mach–Zehnder interferometer (MZI) as the tunable coupler and two micro-heaters as the thermo-optic phase shifters. It was fabricated with inorganic–organic hybrid optical material polysiloxane-liquid series by a novel and simple UV-soft selective imprint technique. The coupling conditions, over-, critical- and under-coupling, were obtained with the micro-heater on the arm of MZI and the maximum notch depth of about 18 dB was achieved. The resonant wavelength was also tuned with the micro-heater on the ring waveguide. The dispersion induced power fading of radio over fiber link was overcome by the tunable ring resonator with its notch filtering function.

Integrated phased array for wide-angle beam steering.

We demonstrate an on-chip optical phased array fabricated in a CMOS compatible process with continuous, fast (100kHz), wide-angle (51°) beam-steering suitable for applications such as low-cost LIDAR systems. The device demonstrates the largest (51°) beam-steering and beam-spacing to date while providing the ability to steer continuously over the entire range. Continuous steering is enabled by a cascaded phase shifting architecture utilizing, low power and small footprint, thermo-optic phase shifters. We demonstrate these results in the telecom C-band, but the same design can easily be adjusted for any wavelength between 1.2 and 3.5μm.

Large-Scale Silicon Photonic Circuits for Optical Phased Arrays

We review recent advances in integrated large-scale optical phased arrays. The design and fabrication of large-scale optical phased arrays using silicon photonic circuits are discussed from device designs including the directional couplers, thermo-optic phase shifters, and optical nanoantennas, to system studies including phased array synthesis and noise analysis. By taking advantage of the well-developed silicon complementary metal-oxide-semiconductor (CMOS) fabrication technology, several large-scale integrated silicon photonic phased arrays are demonstrated, including two passive-phased arrays (64 × 64 and 32 × 32) with the ability to generate complex holographic images, an 8 × 8 active phased array for dynamic optical beamforming, and an 8 × 8 active antenna array with amplitude apodization. These optical phased array demonstrations, with up to 12 000 integrated optical elements, represent the largest and densest silicon photonic circuits demonstrated to date.

Numerical Modeling of the Optical Multiplexer on SOI Constructed by Multiple Coupled Waveguides

A new tunable optical multiplexer which utilized multiple coupled silicon wire waveguides on SOI structures is proposed and numerically studied. For the study, a modified effective index method, which utilizes the combined index profile with two spatial parameters, has been used. It correctly describes, in the 2-D case (with the 1% error), both the phase and the group indexes in a 3-D silicon wire waveguide and makes it possible to simulate complicated SOI structures by 2-D FDTD. The proposed multiplexer contains multiple silicon wire crossings. In order to provide a negligible scattering, the crossings are realized by means of a vertical up and down coupling through the silica buffer of tapered Si wires with the upper thick polymer waveguide. FDTD modeling proves a general conception of the multiplexer on SOI and demonstrates that a short 0.26 mm structure with 32 couplers has spectral resolutions 1.4 nm, internal loss -0.7 dB and sidelobes -20 dB. It provides the wavelength tuning (without Vernier principle) within FSR 35 nm at optical wavelength 1550 nm by the four sets of thermo-optical phase shifters with ΔT <;140 °C. A device of 0.7 cm in size will provide a 0.05 nm filter linewidth. It is CMOS compatible.

Efficient, compact and low loss thermo-optic phase shifter in silicon.

We design a resistive heater optimized for efficient and low-loss optical phase modulation in a silicon-on-insulator (SOI) waveguide and characterize the fabricated devices. Modulation is achieved by flowing current perpendicular to a new ridge waveguide geometry. The resistance profile is engineered using different dopant concentrations to obtain localized heat generation and maximize the overlap between the optical mode and the high temperature regions of the structure, while simultaneously minimizing optical loss due to free-carrier absorption. A 61.6 μm long phase shifter was fabricated in a CMOS process with oxide cladding and two metal layers. The device features a phase-shifting efficiency of 24.77 ± 0.43 mW/π and a -3 dB modulation bandwidth of 130.0 ± 5.59 kHz; the insertion loss measured for 21 devices across an 8-inch wafer was only 0.23 ± 0.13 dB. Considering the prospect of densely integrated photonic circuits, we also quantify the separation necessary to isolate thermo-optic devices in the standard 220 nm SOI platform.

Broadband nanoelectromechanical phase shifting of light on a chip

We demonstrate an optomechanical phase shifter. By electrostatically deflecting the nanofabricated mechanical structure, the effective index of a nearby waveguide is changed and the resulting phase shift is measured using an integrated Mach-Zehnder interferometer. Comparing to thermo-optical phase shifters, our device does not consume power in static operation and also it can operate over large frequency, wavelength, and power ranges. Operation in the MHz range and sub-μs pulses is demonstrated.

Large current MOSFET on photonic silicon-on-insulator wafers and its monolithic integration with a thermo–optic 2 × 2 Mach–Zehnder switch

n-channel body-tied partially depleted metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated for large current applications on a silicon-on-insulator wafer with photonics-oriented specifications. The MOSFET can drive an electrical current as large as 20 mA. We monolithically integrated this MOSFET with a 2 × 2 Mach-Zehnder interferometer optical switch having thermo-optic phase shifters. The static and dynamic performances of the integrated device are experimentally evaluated.

Electronically controlled optical beam-steering by an active phased array of metallic nanoantennas

An optical phased array of nanoantenna fabricated in a CMOS compatible silicon photonics process is presented. The optical phased array is fed by low loss silicon waveguides with integrated ohmic thermo-optic phase shifters capable of 2π phase shift with ∼ 15 mW of applied electrical power. By controlling the electrical power to the individual integrated phase shifters fixed wavelength steering of the beam emitted normal to the surface of the wafer of 8° is demonstrated for 1 × 8 phased arrays with periods of both 6 and 9 μm.

Reconfigurable silicon filter with continuous bandwidth tunability

We present the design and the fabrication of compact tunable silicon-on-insulator bandpass filters based on the integration of a Mach-Zehnder interferometer with ring resonators and activated via thermo-optic phase shifters. The proposed architecture provides wide filter bandwidth tunability from 10% to 90% of the free spectral range preserving the filter off-band rejection. Possible applications are channel subset selection in wavelength division multiplexing optical systems, adaptive filtering to signal bandwidth, and reconfigurable filters for gridless networking.

Ultrafast All-Optical Gating Operation Using Michelson Interferometer for Hybrid Integration of Intersubband Transition Switch on Si Platform

The direct butt-joint coupling between the Si-wire-based Michelson interferometer (MI) and the InGaAs/AlAsSb quantum-well waveguide was performed to realize the hybrid integrated ultrafast intersubband transition switch. By attaining the narrow gap between the waveguides, we have observed the Michelson interference spectra between the two signal lights: one is the phase modulated light from the InGaAs/AlAsSb quantum-well waveguide and the other is the light returned from the Si loop mirror of MI. Furthermore, by controlling the phases and powers of two signal lights with the thermooptic phase shifter and variable coupler in MI, we have demonstrated the ultrafast all-optical gating operation with a response time of 2 ps. The all-optical gating switch with a small size of 0.3 mm 2.35 mm was successfully fabricated by the hybrid integration technologies of III-V and Si materials.

Integrated NiSi waveguide heaters for CMOS-compatible silicon thermo-optic devices

We report the performance of NiSi-based heaters integrated with submicrometer silicon waveguides. The heaters were fabricated using a standard complementary metal-oxide-semiconductor (CMOS) silicidation process on a thin silicon slab laterally connected with a silicon rib waveguide. The intrinsic properties of such NiSi waveguide heaters were characterized by using them as thermo-optic phase shifters in a Mach-Zehnder interferometer. The power consumption P(pi) for obtaining a pi phase shift was measured to be as low as 20 mW, using CMOS-compatible drive voltages. The time constant of the thermo-optic response was less than 2.8 mus. Simulations suggest that a further reduction in the power consumption P(pi) is feasible.

Phase-controlled integrated photonic quantum circuits

Scalable photonic quantum technologies are based on multiple nested interferometers. To realize this architecture, integrated optical structures are needed to ensure stable, controllable, and repeatable operation. Here we show a key proof-of-principle demonstration of an externallycontrolled photonic quantum circuit based upon UV-written waveguide technology. In particular, we present non-classical interference of photon pairs in a Mach-Zehnder interferometer constructed with X couplers in an integrated optical circuit with a thermo-optic phase shifter in one of the interferometer arms.

Silica Waveguide DQPSK Demodulator With Wide Operation Range Enhanced by Using Stress Release Grooves

A 43-Gb/s differential quadrature phase-shift keying demodulator is demonstrated with a wide operation range enhanced by birefringence control achieved by using stress release grooves. The fabricated demodulator realizes a sufficiently low polarization-dependent frequency shift of less than 150 MHz over the C- and L-bands. In addition, we report an experimental result showing the reduction in the power consumption of a thermooptic effect-based phase shifter achieved by introducing the grooves.

Optical properties of new wide heterogeneous waveguides with thermo optical shifters

We present analysis and simulation of novel silicon-on-insulator (SOI) heterogeneous waveguides with thermo-optic phase shifters. New structure design contains a p-n junction on both sides of SOI ridge waveguide with 220 nm x 35 microm silicon core. Strongly mode-dependent optical losses (by additional free charge absorption) provide quasi-singe-mode behavior of wide waveguide with mode size approximately 10 microm. Local heater produces an efficient phase shifting by small temperature increase (DeltaT approximately 2K), switching power (< 40 mW) and switching time (< 10 micros). Mode optical losses are significantly decreased at high heating (DeltaT approximately 120 K).

Demonstration of a Fourth-Order Pole-Zero Optical Filter Integrated Using CMOS Processes

We demonstrate a compact fully tunable narrowband fourth-order pole-zero optical filter that is fabricated in a silicon complementary-metal-oxide-semiconductor foundry. The filter is implemented using silicon on oxide channel waveguides and consists of a Mach-Zehnder interferometer with two ring resonator all-pass filters (APFs) on each arm. The filter architecture is based on the sum and difference of the APFs responses. The ring resonators introduce a nonlinear phase response in each arm that allows carving narrow frequency bands out of a broad spectrum. In this paper, we demonstrate a 3-dB filter bandwidth of 1.0 GHz with a stopband rejection of better than 25 dB. The filter free spectral range is 16.5 GHz. Thermooptic phase shifters are used to tune the filter. As silicon has a large thermooptic coefficient compared to silica, the demonstrated filter requires a low tuning power of less than 300 mW. In addition, this filter is compact with dimensions 25 times smaller than the same filter would be if it were made using standard silica on silicon waveguides with a 0.8% step index contrast

Reset-free integrated polarization controller using phase shifters

An endless, reset-free polarization controller implemented with planar lightwave circuits using phase shifters for tuning is proposed and demonstrated. By avoiding the need for tunable polarization mode converters, simple fabrication processes and a large range of material systems can be used, since neither the electrooptic effect nor a rotatable birefringence axis are necessary for device operation. The proof of concept is demonstrated by implementing the equivalent of a rotating waveplate using Ge-doped silica-on-silicon waveguides and thermooptic phase shifters. The polarization controller is shown to have excellent tolerance to fabrication variations. Fast thermooptic control (on the order of 10 kHz) is achieved using high-index-contrast (4%) waveguides.

Investigation of thermo-optic effect and multi-reflector tunable filter/multiplexer in SOI waveguides

The paper presents an analysis of thermo-optic phase shifters in silicon-on-insulator (SOI) waveguide structures. It gives recommendations to provide high tuning characteristics at minimum power requirements. Then, this analysis is applied to the description of a novel type of reconfigurable optical add/drop multiplexer (ROADM) utilizing multi-reflector (MR) beam expanders and thermo-optic tuning in SOI structures. It is intended for use in high dense wavelength-division-multiplexing (HDWDM) flexible fiber-optic networks having multi-hundreds wavelength channels and advanced ITU grids (12.5 GHz, 25 GHz, 50 GHz).

Dynamic Gain Equalizer Using Proposed Adjustment Procedure for Thermooptic Phase Shifters Under the Influence of Thermal Crosstalk

A dynamic gain equalizer (DGE) is required to automatically compensate for gain variations related to fiber amplifiers. This paper proposes a DGE with thermooptic (TO) phase shifters based on a single-filtered-arm interferometer. A design procedure was newly developed for the control of the TO phase shifters, which could be used to accurately control the intended attenuation profile in one adjustment. In this procedure, the Taylor series method was used as an optimization algorithm, and thermal crosstalk was taken into account. From the point of view of thermal crosstalk, the behavior of phase shifters in an array and the effect of three-dimensional (3-D) trenches formed around the phase shifters were precisely investigated. A DGE module based on 1.5%-/spl Delta/ planar lightwave circuits and the 3-D trench structure are demonstrated. The measured spectra were found to be in good agreement with arbitrarily designed profiles under open-loop control, even when there was some thermal crosstalk between phase shifters. The results indicate that the design procedure and the estimation of the thermal crosstalk are effective for accurate control of the DGE.

Low Loss Fully Reconfigurable Wavelength-Selective Optical&amp;lt;tex&amp;gt;$1times N$&amp;lt;/tex&amp;gt;Switch Based on Transversal Filter Configuration Using Silica-Based Planar Lightwave Circuit

A low loss wavelength-selective switch is proposed and demonstrated. The switch has a 1/spl times/4 transversal filter configuration that includes arrayed waveguide gratings and thermooptic phase shifters, and can route arbitrary wavelength input light to arbitrary outputs with no loss variation. We confirmed its operating principle and obtained an average loss of 2.7 dB and a worst extinction ratio of more than 20 dB.

Polarization-Independent Thermooptic Phase Shifters in Silicon–Oxynitride Waveguides

The effective refractive index of dielectric waveguides can be tuned using the thermooptic effect. In general, the tuning efficiency is polarization-dependent owing to temperature-induced stress in the layers, which causes polarization-dependent loss in optical devices. These stress issues are analyzed and tested for a high-index-contrast waveguide structure based on a silicon-oxynitride core. Experimental results are in agreement with simulations. The relative difference in tuning efficiency for transverse electric and transverse magnetic polarized light can be tuned from -3% to +3% by appropriate waveguide technology control. The optimized thermooptic phase shifters show tuning efficiency differences below 0.25%, which are reproducible from wafer to wafer.