Generation of multi-gigahertz trains of phase-coherent femtosecond laser pulses in Ti:sapphire waveguides

Miniature lasers producing ultrashort phase-coherent pulses at high repetition rates by stable mode-locking in ambient conditions can offer unique capabilities in various applications, spanning from microwave photonics to telecom and biological imaging techniques. Here, the operation of graphene mode-locked lasers based on channel waveguides written by femtosecond and picosecond laser pulses in Ti:sapphire crystals is demonstrated. Trains of pulses of 41.4-fs duration at a 21.25-GHz repetition rate were generated by capitalizing on the formation of solitons in their monolithic resonators through Gires–Tournois interferometers. The latter, allow for effective pulse shaping via tuning of the intracavity group delay dispersion while simultaneously enabling ultralow laser operating thresholds. A number of features of these sources, including their high-brightness and broad bandwidth, are essential ingredients for achieving high longitudinal resolution and sensitivity, which are the primary performance metrics of the Fourier domain/spectral domain variant of optical coherence tomography systems. A further doubling of the laser repetition rate to 42.5 GHz was achieved by coherent pulse interleaving in optical fiber technology, thereby underlining the potential of the Ti:sapphire waveguide lasers to produce highly stable, wide-spaced combs of phase-coherent optical frequency lines