Ultrafast thulium fiber laser reaches communications-ready 20 GHz

Nominal 2 µm thulium fiber lasers have only reached repetition rates up to 1.6 GHz. However, active mode locking extends this rate more than an order of magnitude, making communications and data processing possible.

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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.

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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).

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