Abstract
Silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) has a higher nonlinear threshold compared to silicon, which reduces the effect of two-photon absorption. However, the low thermo-optic coefficient and the reduced refractive index contrast of thin Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> waveguides lead to a low thermal tuning speed and low thermal efficiency. This paper demonstrates a widely tunable III-V/Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> hybrid-integrated external cavity laser with a relatively faster switching time. The Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> external feedback circuit is based on 800-nm-thick Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> waveguides with an optical confinement factor of 87%. It allows the reduction of the oxide under-cladding layer thickness to 4 μm and the oxide upper-cladding layer to 1.7 μm without additional loss. The switching time between two non-adjacent lasing wavelengths is 60.7 μs. The maximum output power is 34 mW under 500 mA injection current. The side mode suppression ratio is more than 70 dB over the tuning range of 58.5 nm. The laser intrinsic linewidth is 2.5 kHz.
Highlights
Silicon photonics has attracted immense research interests due to its complementary metal-oxidesemiconductor (CMOS) compatibility, low cost, and high refractive index contrast
Silicon nitride (Si3N4) has a higher nonlinear threshold compared to silicon, which reduces the effect of two-photon absorption
While germanium is mainly used for light detection, silicon is suitable for high-density photonic integration and high-speed modulation, and Si3N4 is attractive for passive devices because of its low propagation loss and high nonlinearity threshold
Summary
Silicon photonics has attracted immense research interests due to its complementary metal-oxidesemiconductor (CMOS) compatibility, low cost, and high refractive index contrast. While germanium is mainly used for light detection, silicon is suitable for high-density photonic integration and high-speed modulation, and Si3N4 is attractive for passive devices because of its low propagation loss and high nonlinearity threshold. The relatively low refractive index contrast of Si3N4 waveguides makes the waveguide propagation loss lower and coupling between the III-V gain chip and the Si3N4 PIC easier, thereby guaranteeing good laser performances. Though a Vernier-filter-based tunable hybrid laser used as the pump source for the Kerr soliton generation has been reported [48], to the best of our knowledge, there is no report on the improved thermal tuning performance benefiting from the thick Si3N4 platform. Compared with ECLs with a similar configuration, our laser shows improved output power and side mode suppression ratio (SMSR), while maintaining a comparable wavelength tuning range and a narrow linewidth
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