Analysis of the phase-locking dynamics of a III-V-on-silicon frequency comb laser

We present the detailed characterization of the phase dynamics of a telecom hybrid III-V-on-silicon passively mode-locked laser with a ring cavity. We explore the various regimes of operation as a function of gain current and saturable absorber bias voltage. We use a steppedheterodyne measurement to quantify the spectral chirp and reconstruct the pulse envelope. With this technique, we are able to identify regimes of near-transform-limited operation, and we assess the degradation of mode-locking in the comb phase relationship when saturable absorber bias voltage is changed. Finally, we present a preliminary study of the phase-locking in hybrid mode-locked operation and demonstrate transform-limited pulses. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement Passive Mode-Locking (PML) is a well-known technique that uses a saturable absorber (SA) inside a laser cavity to generate a pulsed laser output with a broad optical frequency comb. This technique has been employed in a variety of laser devices and for a large number of applications including high resolution imaging [1], optical signal processing [2] and time-resolved measurements [3]. Nowadays, extensive work is dedicated to the generation of Optical Frequency Combs (OFCs) with integrated semiconductor laser technologies to satisfy the requirements for low energy consumption, small footprint and reproducibility. Integrated OFCs can also be obtained with injected Kerr microresonators which usually rely on power-consuming pumping lasers [4], even if battery-operated devices [5] and turnkey soliton microcomb operation [6] are now available. A challenge for integrated PML lasers technologies is to offer the possibility to reach low Free Spectral Range (FSR) values for optical cavities. Such a property is for example required for spectroscopic applications [7], where a ultra-dense set of comb lines allows to obtain a high resolution in the spectral analysis of gas. Former monolithic integrated technologies showed large bandwidth OFC emission but the FSR could not reach values lower than 10 GHz due to the losses imposed by the III-V material [8]. This limitation has been recently bypassed by the use of hybrid laser technologies, where the optical cavity is made of a silicon waveguide and light is evanescently coupled from the III-V active region to the underneath passive Si layer [9,10]. The low-loss Si waveguide enables the propagation of light in the cavity over several centimeters. OFC lasing in PML hybrid lasers has been demonstrated with multiple cavity geometries and laser designs [11,12], and a record 1 GHz FSR has been achieved, providing a ultra-dense comb spectrum containing more than 1400 lines [13]. The FSR of extended-cavity PMLs could be even further reduced by using ultra-low-loss-waveguide cavities based on for example the III-V-on-silicon-nitride technology [14]. The hybrid III-V-on-Si laser technology has proved mature enough for spectroscopic applications [15] and better understanding of the noise performance and pulse quality of these lasers is required. In PML semiconductor lasers, the overall performances are strongly affected by the SA properties, hence the ability to measure the influence of the SA during lasing is essential to control the laser operation. #412039 https://doi.org/10.1364/OSAC.412039 Journal © 2021 Received 8 Oct 2020; revised 11 Dec 2020; accepted 14 Dec 2020; published 8 Jan 2021 Research Article Vol. 4, No. 1 / 15 January 2021 / OSA Continuum 130 Fig. 1. Layout of the hybrid 8 GHz ring frequency comb laser. Semiconductor Optical Amplifier (SOA) and Saturable Absorber (SA) are integrated within the active Indium Phosphide (InP) material. The long spiral waveguide is etched in the underlying Si membrane that is bonded to InP. Light is evanescently coupled between active and passive