Abstract
Integrated semiconductor mode-locked lasers have shown promise in many applications and are readily fabricated using generic InP photonic integration platforms. However, the passive waveguides offered in such platforms have relatively high linear and nonlinear losses that limit the performance of these lasers. By extending such lasers with, for example, an external cavity, the performance can be increased considerably. In this paper, we demonstrate for the first time that a high-performance mode-locked laser can be achieved with a butt-coupling integration technique using chip scale silicon nitride waveguides. A platform-independent SiN/SU8 coupler design is used to couple between the silicon nitride external cavity and the III/V active chip. Mode-locked lasers at 2.18 GHz and 15.5 GHz repetition rates are demonstrated with Lorentzian RF linewidths several orders of magnitude smaller than what has been demonstrated on monolithic InP platforms. The RF linewidth was 31 Hz for the 2.18 GHz laser.
Highlights
Mode-Locked Lasers (MLL) are comb lasers: The optical spectrum of these lasers contains multiple laser lines with a constant frequency spacing between them
Fiber based MLLs are already commercially available, and used in several applications such as astronomical spectrograph calibration [1], supercontinuum generation [2] or high speed low jitter electrical analog to digital converters [3]. Integrating such a MLL on a chip would allow to apply the lasers in a number of high volume applications such as distributed sensing of greenhouse gasses [4, 5], and optical ranging in autonomous vehicles [6]
Generic InP platform are very attractive for the integration as they offer all building blocks to construct mode-locked lasers on chip [7]
Summary
Mode-Locked Lasers (MLL) are comb lasers: The optical spectrum of these lasers contains multiple laser lines with a constant frequency spacing between them. To improve on the waveguide losses in the monolithic extended cavity MLLs, III/V-on-Si and III/V-on-SiN mode-locked lasers have been demonstrated, where the different material systems are integrated using bonding or micro-transfer printing [12, 13]. In these types of lasers, the external waveguide cavity is made using a different material system with lower propagation losses than the passive waveguides in InP platforms, and the III/V gain medium is integrated by micro-transfer printing an amplifier on top of the waveguide circuit. A hybrid integration approach has been used to decrease the footprint and power usage of integrated Kerr comb sources In these devices, a III/V Reflective Semiconductor Optical Amplifier (RSOA) is coupled to an external cavity with a wavelength filter followed by a high Q ring resonator. Their repetition rates are 15.8 GHz and 2.18 GHz
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