Recent Progress in Sub‐100 fs Mode‐Locked Bulk Lasers Near 2 µm

ABSTRACT Femtosecond lasers in the 2‐µm spectral range are essential for basic research and have widespread applications, such as the micromachining of nonmetallic materials, medical treatment, and national defense. In particular, mode‐locked solid‐state lasers based on Tm/Ho rare‐earth ions are the key drivers for the direct generation of ultrashort pulses in the 2‐µm spectral range. Since the first demonstration of <100 fs pulses produced using a Tm:MgWO 4 laser in 2017, significant progress has been made in achieving sub‐100 fs pulses at 2 µm using various Tm/Ho‐doped laser materials. To date, 26‐fs pulses (approximately 3.5 optical cycles, which represent the fewest cycles among all currently available lasers) have been demonstrated using the proposed thermal‐dependent electron‐phonon coupling effect. This paper comprehensively reviews state‐of‐the‐art mode‐locked thulium/holmium bulk lasers in the sub‐100 fs regime from the perspective of laser host materials. Herein, we summarize the optical properties and mode‐locked laser performance of several types of bulk laser materials (including crystals and ceramics). In addition, we highlight the future prospects and challenges of mode‐locked solid‐state lasers toward less than three optical cycle pulse generation in the 2‐µm spectral range.