Ashutosh Rao

Heterogeneous microring and Mach-Zehnder modulators based on lithium niobate and chalcogenide glasses on silicon

Thin films of lithium niobate are wafer bonded onto silicon substrates and rib-loaded with a chalcogenide glass, Ge(23)Sb(7)S(70), to demonstrate strongly confined single-mode submicron waveguides, microring modulators, and Mach-Zehnder modulators in the telecom C band. The 200 μm radii microring modulators present 1.2 dB/cm waveguide propagation loss, 1.2 × 10(5) quality factor, 0.4 GHz/V tuning rate, and 13 dB extinction ratio. The 6 mm long Mach-Zehnder modulators have a half-wave voltage-length product of 3.8 V.cm and an extinction ratio of 15 dB. The demonstrated work is a key step towards enabling wafer scale dense on-chip integration of high performance lithium niobate electro-optical devices on silicon for short reach optical interconnects and higher order advanced modulation schemes.

Low-loss, submicron chalcogenide integrated photonics with chlorine plasma etching

A chlorine plasma etching-based method for the fabrication of high-performance chalcogenide-based integrated photonics on silicon substrates is presented. By optimizing the etching conditions, chlorine plasma is employed to produce extremely low-roughness etched sidewalls on waveguides with minimal penalty to propagation loss. Using this fabrication method, microring resonators with record-high intrinsic Q-factors as high as 450 000 and a corresponding propagation loss as low as 0.42 dB/cm are demonstrated in submicron chalcogenide waveguides. Furthermore, the developed chlorine plasma etching process is utilized to demonstrate fiber-to-waveguide grating couplers in chalcogenide photonics with high power coupling efficiency of 37% for transverse-electric polarized modes.

High-performance and linear thin-film lithium niobate Mach-Zehnder modulators on silicon up to 50 GHz

Compact electro-optical modulators are demonstrated on thin films of lithium niobate on silicon operating up to 50 GHz. The half-wave voltage length product of the high-performance devices is 3.1 V.cm at DC and less than 6.5 V.cm up to 50 GHz. The 3 dB electrical bandwidth is 33 GHz, with an 18 dB extinction ratio. The third-order intermodulation distortion spurious free dynamic range is 97.3  dBHz2/3 at 1 GHz and 92.6  dBHz2/3 at 10 GHz. The performance demonstrated by the thin-film modulators is on par with conventional lithium niobate modulators but with lower drive voltages, smaller device footprints, and potential compatibility for integration with large-scale silicon photonics.

Low-loss and high index-contrast tantalum pentoxide microring resonators and grating couplers on silicon substrates

A platform for high index-contrast integrated photonics based on tantalum pentoxide submicrometer waveguides on silicon substrates is introduced. The platform allows demonstration of microring resonators with loaded quality factor, Q, of 67,000 and waveguides with a propagation loss of 4.9 dB/cm. Grating couplers, with an insertion loss of ~6 dB per coupler and 3 dB bandwidth of ~50 nm, are also demonstrated and integrated with microring resonators.

Second-harmonic generation in periodically-poled thin film lithium niobate wafer-bonded on silicon

Second-order optical nonlinear effects (second-harmonic and sum-frequency generation) are demonstrated in the telecommunication band by periodic poling of thin films of lithium niobate wafer-bonded on silicon substrates and rib-loaded with silicon nitride channels to attain ridge waveguide with cross-sections of ~2 µm2. A nonlinear conversion of 8% is obtained with a pulsed input in 4 mm long waveguides. The choice of silicon substrate makes the platform potentially compatible with silicon photonics, and therefore may pave the path towards on-chip nonlinear and quantum-optic applications.

Second-harmonic generation in single-mode integrated waveguides based on mode-shape modulation

Second-harmonic generation is demonstrated using grating-assisted quasi-phase matching, based on waveguide-width modulation or mode-shape modulation. Applicable to any thin-film integrated second-order nonlinear waveguide, the technique is demonstrated in compact lithium niobate ridge waveguides. Fabricated devices are characterized with pulsed-pumping in the near-infrared, showing second-harmonic generation at a signal wavelength of 784 nm and propagation loss of 1 dB/cm.

Optical frequency comb generation by pulsed pumping

A synchronously pumped Kerr cavity is proposed and studied for power-efficient frequency comb generation in optical microring resonators. The system is modeled using the Lugiato-Lefever equation. Analytical solutions are provided for an ideal case and extended by numerical methods to account for optical loss and higher orders of dispersion. It is shown that the average power requirement is reduced by the duty cycle of the pulse with respect to the conventional continuous-wave-pumped microrings, and it is significantly lower than the pulsed pumping of straight waveguides.

Electro-optic and second-order nonlinear effects in thin film lithium niobate on silicon

Our recent progress on wafer-bonded thin-film lithium niobate on silicon electro-optic modulators operating up to 50 GHz, and thin-film nonlinear optical waveguide frequency converters, based on two approaches — periodic poling and poling-free grating-assisted quasi-phase matching — will be reviewed.

High-speed modeling of thin-film lithium-niobate-on-silicon electrooptic modulators

A model is developed for frequency-dependent response of compact high-speed thin-film lithium-niobate-on-silicon electrooptic modulators. The model predicts potential for > 100 GHz modulation bandwidth.

Damascene-patterned optical anisotropy in integrated photonics

We propose, simulate and experimentally demonstrate a method for realizing spatially-mapped birefringence onto integrated photonic devices and circuits. The fabrication method is based on applying a damascene-like process to dielectric film stacks to form anisotropic optical waveguides. An integrated polarizing beam-splitter (PBS) is realized with unprecedented performance: a record 0.52 octaves of fractional bandwidth (116 THz), maximum on-chip insertion loss of 1.4 ± 0.8 dB, and a minimum extinction ratio of 16 ± 3 dB, pushing it into the realm of wideband spectroscopy and imaging applications. Additionally, photonic structures such as polarization-selective beam-taps and polarization-selective microring resonators are demonstrated.