Since their 2017 demonstration of a passively mode-locked carbon-nanotube (CNT)-based thulium/holmium (Tm/Ho) fiber laser with the broadest tunability across 200 nm from 1860 to 2060 nm, researchers at Nanjing University (Nanjing, China) have now developed an actively mode-locked ultrafast 2 µm fiber laser that reaches communications-capable repetition rates of nearly 20 GHz. This is more than an order of magnitude better than the 1.6 GHz previously possible with this type of laser.
Active mode-locking is a relatively mature technology for pulse generation, but requires stable, finely tuned electro-optical components. To implement this technique, the laser was mode-locked by driving a lithium niobate phase modulator with a radio-frequency (RF) signal equal to integral multiples of the longitudinal mode spacing, enabling tunability from 14.6 MHz to 19 GHz with a 46 dB super-mode suppression ratio for the latter pulse rate. The team also observed rational harmonic mode-locking (RHML) at 2 μm and reached a 21 GHz repetition rate at a 7 GHz RF modulation frequency.
Unlike passive mode-locking, this source can be synchronized to an external clock for better stability and behaves similarly to a frequency comb, with the potential to reach 40 GHz repetition rates and extend further into the mid-infrared region beyond 2 µm. Reference: J. Qin et al., Opt. Express, 26, 20, 25769–25777 (2018).
