Light Sci. Appl. 6, e16260 (2017)

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A dense frequency comb is usually generated using bulky and costly femtosecond lasers, whereas low-cost integrated semiconductor mode-locked lasers can typically only generate a comb with a small number of spectral lines with a larger spacing and broader optical linewidth. Now, combining the best of both worlds, Zhechao Wang and co-workers have demonstrated an ultra-dense comb laser based on a compact, low-cost, frequency-stabilized comb generator. They present a III–V-on-silicon mode-locked laser that is passively mode-locked and operates at a record low repetition rate of 1 GHz. The use of a low-loss (∼0.7 dB cm−1) passive silicon spiral waveguide allows a long laser cavity with a length of 37.4 mm. The cavity features two optical amplifiers that are separated by a saturable absorber, and two distributed Bragg reflectors that form the mirrors of the cavity. The device emits over a wide optical bandwidth of 12 nm, generating a dense optical comb consisting of over 1,400 equally spaced optical lines with narrow linewidth of <400 kHz. The team points out that several features provide evidence for stable mode-locking including a sub-kHz 10-dB radio-frequency linewidth and a narrow longitudinal mode linewidth. The researchers envisage that their integrated dense comb lasers will be useful for high-resolution real-time spectroscopy applications.