Ultra-efficient and fully isotropic monolithic microring modulators in a thin-film lithium niobate photonics platform.

The large electro-optic coefficient, r33, of thin-film lithium niobate (LN) on insulator makes it an excellent material platform for high-efficiency optical modulators. Using the fundamental transverse magnetic optical mode in Z-cut LN enables isotropic in-plane devices; however, realizing a strong vertical electric field to capitalize on r33 has been challenging. Here we present a symmetric electrode configuration to boost the vertical field strength inside a fully-etched single-mode LN waveguide. We use this design paradigm to demonstrate an ultra-compact fully isotropic microring modulator with a high electro-optic tuning efficiency of 9 pm/V, extinction ratio of 20 dB, and modulation bandwidth beyond 28 GHz. Under quasi-static operation, the tuning efficiency of the modulator reaches 20 pm/V. Fast, efficient, high-contrast modulation will be critical in future optical communication systems while large quasi-static efficiency will enable post-fabrication trimming, thermal compensation, and even complete reconfiguration of microring-based sensor arrays and photonic integrated circuits.

Fundamental electro-optic limitations of thin-film lithium niobate microring modulators.

We investigate the impact of waveguide curvature on the electro-optic efficiency of microring resonators in thin-film X-cut or Y-cut lithium niobate (in-plane extraordinary axis) and derive explicit relations on the response. It is shown that such microring modulators have a fundamental upper bound on their electro-optic performance (∼50% filling factor) which corresponds to a specific arrangement of metal electrodes surrounding the microring and yields nearly identical results for X-cut and Y-cut designs. We further show that this limitation does not exist (i.e., 100% filling factor is possible) with Z-cut microring modulators or can be circumvented (i.e., ∼100% filling factor is possible) in X-cut and Y-cut modulators that use a race-track configuration with segmented electrodes. Comparison of our analytical results with multiphysics simulations and measured electro-optic efficiencies of microring resonators in the literature demonstrates the validity and accuracy of our approach.

High performance fully etched isotropic microring resonators in thin-film lithium niobate on insulator platform.

We present our design, fabrication, and experimental results for very high-performance isotropic microring resonators with small radii (∼ 30 µm) based on single-mode strip waveguides and transverse magnetic (TM) polarization in a fully etched lithium niobate (Z-cut) thin-film on insulator. The loss of the devices is predicted to be < 10 dB/cm, and is measured to be ∼ 7 dB/cm. The measured optical responses of microring resonators exhibit an extinction of ∼ 25 dB (close to critical coupling), a 3 dB optical bandwidth of 49 pm (∼ 6 GHz) for all-pass structures, an extinction of ∼ 10 dB for add-drop structures, and a free spectral range of ∼ 5.26 nm, all of which are in excellent agreement with the design. This work is the first step towards ultra-compact and fully isotropic optical modulators in thin-film lithium niobate on insulator.

Realization of alignment-tolerant grating couplers for z-cut thin-film lithium niobate.

We present the design, modeling, fabrication, and characterization of grating coupler devices for z-cut lithium niobate near 1550 nm. We first experimentally measure the sensitivity of the insertion loss of a conventional grating coupler to translational misalignment through a three-factor full factorial design of experiment. Next, we design grating couplers that are significantly less sensitive to misalignment. The fabricated devices experienced less than 7 dB of excess insertion loss for combined misalignments of up to ± 5 µm in plane and up to -2 µm or + 10 µm out of plane.

Photonic switching in high performance datacenters [Invited].

Photonic switches are increasingly considered for insertion in high performance datacenter architectures to meet the growing performance demands of interconnection networks. We provide an overview of photonic switching technologies and develop an evaluation methodology for assessing their potential impact on datacenter performance. We begin with a review of three categories of optical switches, namely, free-space switches, III-V integrated switches and silicon integrated switches. The state-of-the-art of MEMS, LCOS, SOA, MZI and MRR switching technologies are covered, together with insights on their performance limitations and scalability considerations. The performance metrics that are required for optical switches to truly emerge in datacenters are discussed and summarized, with special focus on the switching time, cost, power consumption, scalability and optical power penalty. Furthermore, the Pareto front of the switch metric space is analyzed. Finally, we propose a hybrid integrated switch fabric design using the III-V/Si wafer bonding technique and investigate its potential impact on realizing reduced cost and power penalty.

Tapless and topology agnostic calibration solution for silicon photonic switches.

We leverage the photo-conductance (PC) effect in doped phase-shifter heaters for both controlling and calibrating Mach-Zehnder interferometer (MZI) switch elements. Both the steady-state and the transient response are experimentally characterized, and compact models for the PC current are developed. Utilizing the PC effect, a topology-agnostic algorithm is then outlined. The calibration procedure is experimentally verified against calibration with external photo-detectors using a non-blocking 4×4 Benes switch consisting of six 2×2 MZIs. It is shown that our PC-based approach agrees with the PD-based procedure within less than 2.5% of difference between the obtained calibrated values. Based on the calibrated PC values, all possible routing configurations are measured for extinction ratio (9.92-21.51dB), insertion loss (0.88-4.59dB), and exhibiting performances far below the 7% FEC limit (bit error rate of 3.8 × 10- 3) using 25 Gbps 4-level pulse-amplitude-modulation signals (PAM4).

Software-defined control-plane for wavelength selective unicast and multicast of optical data in a silicon photonic platform.

We demonstrate a programmable control-plane based on field programmable gate array (FPGA) with a power-efficient algorithm for optical unicast, multicast, and broadcast functionalities in a silicon photonic platform. The platform includes a silicon photonic 1×8 microring array chip which in conjunction with a fast tunable laser over the C-band is capable of delivering software controlled wavelength selective functionality on top of spatial switching. We characterize the thermo-optic response of microring resonators and extract key parameters necessary for the development of the control-plane. The performance of the proposed architecture is tested with 10 Gb/s on-off keying (OOK) optical data and error-free operation is verified for various wavelength and spatial switching scenarios. Lastly, we evaluate electrical power and energy consumption required to reconfigure the silicon photonic device for all possible wavelength operations and output ports combinations and show that unicast, multicast of two, three, four, five, six, seven, and broadcast functions are achieved with energy overheads of 0.02, 0.07, 0.18, 0.49, 0.76, 1.01, 1.3, and 1.55 pJ/bit, respectively.