Abstract
Optical frequency combs emerge as a promising technology that enables highly sensitive, near-real-time spectroscopy with a high resolution. The currently available comb generators are mostly based on bulky and high-cost femtosecond lasers for dense comb generation (line spacing in the range of 100 MHz to 1 GHz). However, their integrated and low-cost counterparts, which are integrated semiconductor mode-locked lasers, are limited by their large comb spacing, small number of lines and broad optical linewidth. In this study, we report a demonstration of a III-V-on-Si comb laser that can function as a compact, low-cost frequency comb generator after frequency stabilization. The use of low-loss passive silicon waveguides enables the integration of a long laser cavity, which enables the laser to be locked in the passive mode at a record-low 1 GHz repetition rate. The 12-nm 10-dB output optical spectrum and the notably small optical mode spacing results in a dense optical comb that consists of over 1400 equally spaced optical lines. The sub-kHz 10-dB radio frequency linewidth and the narrow longitudinal mode linewidth (<400 kHz) indicate notably stable mode-locking. Such integrated dense comb lasers are very promising, for example, for high-resolution and real-time spectroscopy applications.
Highlights
Optical frequency combs provide the long sought link between the radio frequency domain and the optical domain
Traditional means of comb generation rely on mode-locked fiber lasers[4] and mode-locked titanium-sapphire lasers[5]
By leveraging the low optical loss of silicon waveguides, we present a III-V-on-silicon modelocked lasers (MLLs) that passively modelocks at a record-low repetition rate of 1 GHz
Summary
Optical frequency combs provide the long sought link between the radio frequency domain and the optical domain. Using the so-called dual-comb spectroscopic technique[3], broadband absorption spectra can be measured with superior resolution and acquisition time compared with other techniques such as standard Fourier transform spectroscopy (FTIR) In this technique, one of the combs is used as an array of local oscillators to down-convert the lines of the probe comb to the RF domain. Traditional means of comb generation rely on mode-locked fiber lasers[4] and mode-locked titanium-sapphire lasers[5] These bulky and costly lasers combine a broad comb bandwidth with a small line spacing of approximately 100 MHz. Reducing the size, cost and power consumption of such optical comb generators is of paramount importance to extend the application range of optical frequency combs. A chip-scale, low-power consumption and low-cost comb source that can be electrically pumped and generates a broad comb with a small line spacing is desired
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