To better quantify the pulse-to-pulse amplitude stability of the two lasers we recorded the electrical radio-frequency (RF) spectrum of the envelope signal, collected at the output, using a fast photodetector. Unstable oscillation (in the pulse amplitude) was always observed for the long cavity design for the EYDFA () for both continuous wave (CW) and pulsed regimes, owing to the presence of a large number of cavity modes oscillating in the ring resonance. In complete contrast to this, the short-cavity configuration for the EDFA () could easily be stabilized to give the very clean result of , by simply adjusting the main cavity length to centre an, ideally, single cavity mode with respect to the ring resonance, thereby completely eliminating any main cavity low-frequency beating. Several stable oscillation conditions were found through tuning the delay by over 2 cm. For the same gain, the optical bandwidth (insets in ) for unstable laser operation was wider (that is, leading to shorter output pulses) because the instability resulted in a strong amplitude modulation of the optical pulse train in the main cavity, thus increasing the statistical peak power and enhancing the nonlinear interactions. This is a common occurrence in mode-locked lasers29
Although we expect that significant external temperature changes would eventually detune the external cavity length, once the thermal stability was reached, standard environmental conditioning (21 °C) was sufficient to maintain perfectly stable operation within the time-framework of our experimental sessions (several hours), highlighting the robustness of the stability condition.
We confirmed, by numerical simulations (Methods), the dependence of the stability on the relative phase between the main cavity modes and the ring resonator modes (). We found that instability could indeed be either induced or suppressed simply by optimizing the phase of the main cavity modes, that is, the spectral position of the main cavity modes relative to the ring resonances.
Theoretical dependence of laser stability on amplifier saturation energy and phase ΦMC of the main cavity modes.
We concluded our experiments by characterizing the phase noise7
. We found that all of the emitted lines had a bandwidth well below 130 kHz (full-width at half-maximum (FWHM)). This bandwidth has been found to be dramatically increased by the contribution of the environmental acoustic noise that we could minimize but not fully eliminate. Following the approach of ref. 24
, we discriminated the linewidth associated with the inherent bandwidth-quantum-limit, estimated to be below 13 kHz, which suggests that, ultimately, proper packaging and acoustic isolation would reduce the optical spectral linewidth even further. We also investigated the source timing jitter, that is, the stochastic deviation of the temporal pulse position in the train. Following refs. 7
, we estimate from the phase noise that the jitter contribution to the RF spectral lines is <10 KHz. This is a remarkable result for a 200 GHz mode-locked source that does not use any auxiliary stabilization mechanisms.
Our ultimate objective is a versatile, fully monolithically integrated laser source. Because waveguide amplifiers have been demonstrated in silica glass platforms, we do not envisage any fundamental issues preventing the full integration of our scheme. This proof-of-concept device represents a key step in realizing a fully integrated, stable, high–performance, laser source operating at flexible and very high repetition rates.
In summary, we propose and demonstrate a novel mode-locked laser based on a monolithic high-Q resonator, capable of generating both picosecond and sub-picosecond transform-limited pulses at a repetition rate of 200.8 GHz and beyond. Our device operates via a new mechanism that enables stable mode-locked lasing with negligible amplitude noise, and extrinsically limited phase noise, not constrained by the repetition rate. We believe this work represents a key milestone in the generation of ultra-stable, high repetition rate, optical pulse sources, particularly because of its CMOS-compatible monolithic platform.