Coupled semiconductor ring lasers (Coupled lasers) can generate unique optical patterns, paving the way for advancements in spectroscopy and on-chip laser systems. Researchers demonstrated this by coupling two racetrack quantum cascade lasers (QCLs). Building on previous work with single racetrack QCLs exhibiting “dark” light pulses (Nozaki-Bekki solitons), the team investigated the effects of strong coupling between multiple lasers.
The coupled QCLs, shaped like racetrack resonators, allow light to circulate clockwise or counterclockwise, exchanging photons at their straight sections. Strong coupling causes the lasers’ resonant frequencies to split or hybridize. Unlike conventional coupling, both lasers were simultaneously biased above the threshold. This resulted in frequency locking, forming two distinct frequency combs, one for each set of hybridized modes.
Intriguingly, the two combs locked to the same repetition rate but repelled each other due to the strong coupling. One set of frequencies corresponded to “dark” light pulses, while the other produced “bright” pulses – a novel observation in free-running semiconductor lasers. The researchers generated ultrashort, bright, and dark pulses by biasing each laser.
These unique light patterns offer potential for various applications. The pulses could be extracted and amplified for nonlinear optical experiments. Furthermore, the system could enable advanced spectroscopy techniques like dual-comb spectroscopy. Here, the two frequency combs probe gas absorption lines, converting optical absorption into an electrical signal upon mixing.
This research highlights the potential of coupled lasers for generating diverse light patterns. Future studies will explore coupling more than two resonators, potentially leading to new coherent light states in multi-resonator systems and impacting optical technologies like communications, spectroscopy, and photonic chips.
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