Micro-combs: A novel generation of optical sources

Lithium niobate on insulator (LNOI) technology is revolutionizing the lithium niobate industry, enabling higher performance, lower cost and entirely new devices and applications. The availability of LNOI wafers has sparked significant interest in the platform for integrated optical applications, as LNOI offers the attractive material properties of lithium niobate, while also offering the stronger optical confinement and a high optical element integration density that has driven the success of more mature silicon and silicon nitride (SiN) photonics platforms. Due to some similarities between LNOI and SiN, established techniques and standards can readily be adapted to the LNOI platform including a significant array of interface approaches, device designs and also heterogeneous integration techniques for laser sources and photodetectors. In this contribution, we review the latest developments in this platform, examine where further development is necessary to achieve more functionalities in LNOI integrated optical circuits and make a few suggestions of interesting applications that could be realized in this platform. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits

We report on the experimental demonstration and analysis of a new waveguide principle using subwavelength gratings. Unlike other periodic waveguides such as line-defects in a 2D photonic crystal lattice, a subwavelength grating waveguide confines the light as a conventional index-guided structure and does not exhibit optically resonant behaviour. Subwavelength grating waveguides in silicon-on-insulator are fabricated with a single etch step and allow for flexible control of the effective refractive index of the waveguide core simply by lithographic patterning. Experimental measurements indicate a propagation loss as low as 2.1 dB/cm for subwavelength grating waveguides with negligible polarization and wavelength dependent loss, which compares favourably to conventional microphotonic silicon waveguides. The measured group index is nearly constant n(g) ~1.5 over a wavelength range exceeding the telecom C-band.

Subwavelength grating periodic structures in silicon-on-insulator: a new type of microphotonic waveguide.

We demonstrate low loss silicon waveguides fabricated without any silicon etching. We define the waveguides by selective oxidation which produces ultra-smooth sidewalls with width variations of 0.3 nm. The waveguides have a propagation loss of 0.3 dB/cm at 1.55 μm. The waveguide geometry enables low bending loss of approximately 0.007 dB/bend for a 90° bend with a 50 μm bending radius.

Low loss etchless silicon photonic waveguides

We have integrated lithographically patterned VO2 thin films grown by pulsed laser deposition with silicon-on-insulator photonic waveguides to demonstrate a compact in-line absorption modulator for use in photonic circuits. Using single-mode waveguides at λ = 1550 nm, we show optical modulation of the guided transverse-electric mode of more than 6.5 dB with 2 dB insertion loss over a 2-µm active device length. Loss is determined for devices fabricated on waveguide ring resonators by measuring the resonator spectral response, and a sharp decrease in resonator quality factor is observed above 70 °C, consistent with switching of VO_2 to its metallic phase. A computational study of device geometry is also presented, and we show that it is possible to more than double the modulation depth with modified device structures.

/pdf/compact-silicon-photonic-waveguide-modulator-based-on-the-2iqldlytlr.pdf

Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition

We report the first experimental observation in the optical domain of a dramatic width-dependent lateral leakage loss behavior for the TM-like mode of tight vertical confinement ridge waveguides formed in silicon-on-insulator. The lateral leakage loss displays a series of sharp cyclic minima at precise waveguide widths, and appears to be inherent to waveguide geometries of central importance to a wide variety of active devices in silicon photonics requiring lateral electrical access. This behavior is not predicted by the often-used effective-index-based methods, but is understood phenomenologically and also compared to prior numerical analysis and predictions of leaky mode behavior. It is shown that TM-like mode operation, critical to the operation of some active component designs, will require precision control of waveguide dimensions to achieve high performance

/pdf/width-dependence-of-inherent-tm-mode-lateral-leakage-loss-in-4gao9bah4k.pdf

Width Dependence of Inherent TM-Mode Lateral Leakage Loss in Silicon-On-Insulator Ridge Waveguides

A thermal oxidation fabrication technique is employed to form low-loss high-index-contrast silicon shallow-ridge waveguides in silicon-on-insulator (SOI) with maximally tight vertical confinement. Drop-port responses from weakly coupled ring resonators demonstrate propagation losses below 0.36 dB/cm for TE modes. This technique is also combined with “magic width” designs mitigating severe lateral radiation leakage for TM modes to achieve propagation loss values of 0.94 dB/cm. We discuss the fabrication process utilized to form these low-loss waveguides and implications for sensor devices in particular.

Low-loss silicon-on-insulator shallow-ridge TE and TM waveguides formed using thermal oxidation

Low-loss, quasi-planar ridge waveguide structures have been designed and fabricated in silicon-on-insulator material with waveguide propagation losses of <0.7dB∕cm at 1550nm, and a Q∼106 measured in a ring resonator configuration. These structures offer tight vertical field confinement with scalable bending losses, and should be of interest for integration with both active and passive components. Waveguides were fabricated using optical lithography and processing compatible with silicon electronics.

Low-loss quasi-planar ridge waveguides formed on thin silicon-on-insulator

Disclosed is a method of fabricating an optical waveguide device including the steps of forming a mask over a waveguide core material layer so as to leave a portion of the layer exposed, and exposing the structure to an oxidizing environment to form an oxide layer on the waveguide core material layer at least in the exposed portion thereby defining the lateral dimension of the waveguide core. The resulting waveguide core has extremely smooth surfaces for low optical losses.

Fabrication of optical waveguide devices

We report quasi-planar ridge waveguides with losses of < 0.7 dB/cm in thin silicon-on-insulator material including waveguide-based ring resonators with quality factors of Q ~ 106.

Mark A. Webster

Papers

Width Dependence of Inherent TM-Mode Lateral Leakage Loss in Silicon-On-Insulator Ridge Waveguides

Low-loss silicon-on-insulator shallow-ridge TE and TM waveguides formed using thermal oxidation

Low-loss quasi-planar ridge waveguides formed on thin silicon-on-insulator

Fabrication of optical waveguide devices

Low-loss thin SOI waveguides and high-Q ring resonators