Photonic Chip-Based Soliton Microcomb Driven by a Compact Ultra-Low-Noise Laser

Abstract

Photonics chip-based soliton microcombs have been used in many applications including LIDAR, spectroscopy, coherent communication and astronomical spectrometer calibration [1]. Current-initiated soliton microcombs have been demonstrated [2, 3] recently, signifying improvements in the fabrication of high-Q Si 3 N 4 microresonators. However, both approaches suffer from limited input laser power, thus only demonstrated single-soliton at repetition rates above 149 GHz, which are challenging to detect with commercially available photodetectors. Here we demonstrate a single-soliton generation in 100-GHz-FSR Si 3 N 4 microresonators fabricated using the photonic Damascene reflow process [4], yielding the intrinsic Q-factor exceeding 15 million [5]. Using a compact hybrid laser with narrow linewidth, low relative intensity noise (−160 dBc/Hz at f offset =100 kHz) and high output power up to 100 mW [6], different comb states are observed by simply changing the current of the laser diode, without the need of complex tuning mechanism such as a single sideband modulator [7]. As the laser noise is directly transferred to the soliton comb line, this low-noise laser can be utilized in applications where the phase noise is a critical parameter, e.g. low-noise microwave generation or coherent communication. The experimental setup shown in Fig. 1 (a) consists of an ULN laser operated by a current source and temperature controllers to tune its frequency and power. After the light is coupled into the Si 3 N 4 photonic chip via double inverse nano tapers [8], the temperature of FBG/GC is changed to align the laser wavelength to the resonance of the microresonator. The laser diode current is increased (≈ 330 mA) until soliton existence range is sufficiently long. This is indicated by transmission signal directly observed after the chip on the photodetector (Fig. 1 c). Due to high-Q factor of the Si 3 N 4 , the soliton state can be accessed via simply frequency forward tuning [9], without the need of any complex soliton tuning mechanism. Further, different comb states are observed, i.e. modulation instability, multi-soliton state and single-soliton state, via laser diode current tuning. The coherence properties of the soliton-comb teeth is asserted by performing a heterodyne beatnote measurement using a reference laser with a short-time linewidth of 10 kHz (Fig. 1b). The soliton spectrum is fitted with a sech2 function corresponding to a 3 dB bandwidth of ≈19.3 nm and a ≈131.5 fs pulse.