Abstract
We present a CMOS-compatible, Q-switched mode-locked integrated laser operating at 1.9 µm with a compact footprint of 23.6 × 0.6 × 0.78mm. The Q-switching rate is 720 kHz, the mode-locking rate is 1.2 GHz, and the optical bandwidth is 17nm, which is sufficient to support pulses as short as 215 fs. The laser is fabricated using a silicon nitride on silicon dioxide 300-mm wafer platform, with thulium-doped Al2O3 glass as a gain material deposited over the silicon photonics chip. An integrated Kerr-nonlinearity-based artificial saturable absorber is implemented in silicon nitride. A broadband (over 100 nm) dispersion-compensating grating in silicon nitride provides sufficient anomalous dispersion to compensate for the normal dispersion of the other laser components, enabling femtosecond-level pulses. The laser has no off-chip components with the exception of the optical pump, allowing for easy co-integration of numerous other photonic devices such as supercontinuum generation and frequency doublers which together potentially enable fully on-chip frequency comb generation.
Highlights
High repetition-rate ultrafast mode-locked lasers (MLL) have unique advantages for applications such as photonic analog-to-digital converters, comb spectroscopy, optical arbitrary waveform generation and low-noise microwave synthesis
Repetition rates beyond 1 GHz were achieved by either active modulation techniques [1,2], which restricted the pulse duration to more than a few picoseconds, with nonlinearity-induced optical bistability where multimode noise suppression was necessary for a stable operation [3,4], or by introducing a semiconductor saturable absorber, in which case the pulse duration remained more than a few picoseconds [5,6,7]
Passive mode-locking techniques have been shown to generate femtosecond-level pulses at high repetition rates when used with some form of an external repetition-rate multiplier to bring the system into the GHz-level regime [8,9]
Summary
High repetition-rate ultrafast mode-locked lasers (MLL) have unique advantages for applications such as photonic analog-to-digital converters, comb spectroscopy, optical arbitrary waveform generation and low-noise microwave synthesis. The compact size of the gain cavity integrated on chip together with an on-chip passive mode-locking technique could provide GHz-level pulse repetition rates with pulse durations in the femtosecond regime without any external repetition rate multiplication [11,12]. Numerous passive photonic components necessary for integrated MLLs have already been demonstrated in this fabrication platform These devices include wavelength filters/couplers, mode-locking elements, and integrated diffraction gratings [17,18]. With a well-developed onchip gain platform and demonstrated passive components necessary for mode-locking, it is evident that on-chip high repetition rate MLLs will follow a similar development path to that of the conventional lasers, proceeding along the path from a continuous wavelength to Qswitched, to Q-switch-mode-locked, and to CW mode-locked devices. In this work we demonstrate a significantly improved laser architecture that allows for truly compact devices (23.5 × 0.78 mm) and achieves a cavity repetition rate of 1.2 GHz, with bandwidth that would support 215fs pulses
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