Self‐Seeding Induced Bidirectional Mode‐Locking in Single‐Cavity Dual‐Comb Fiber Lasers

ABSTRACT

Bidirectional single‐cavity dual‐comb fiber lasers represent a promising laser architecture for dual‐comb metrology. However, establishing bidirectional mode‐locking typically requires sophisticated fine‐tuning with laser expertise owing to the strong gain competition between opposite directions, and the key mechanism for bidirectional mode‐locking remains obscured. Consequently, the practical applications of such lasers are so far limited. In this work, the critical role of reflection‐induced self‐seeding in bidirectional mode‐locking is discovered for the first time by the combination of real‐time spectral characterization and numerical modeling. Enlightened by the intriguing discovery, a robust bidirectional mode‐locking technique is invented, which instantaneously induces transitions from various unidirectional mode‐locked states to a stable bidirectional mode‐locked state in a turn‐key manner. Lastly, the impact of key parameters in this technique is explored, providing a general application guideline. Our study not only provides fundamental insights into the bidirectional mode‐locking mechanism in NPR‐based fiber lasers, but also offers a practical scheme for achieving bidirectional mode‐locking in such laser systems, which may promote their practical applications in various dual‐comb metrologies.