Advances and Challenges of Ultrafast Fiber Lasers in 2–4 µm Mid‐Infrared Spectral Regions

Abstract Mid‐infrared (mid‐IR) ultrafast lasers are widely employed in biomedicine, molecular spectroscopy, material processing, and nonlinear optics. With the improvement of fiber gain media and other fiber optical elements, low‐cost, compact, and high‐efficiency fiber lasers open up new opportunities for 2–4 µm pulse generations, which calls for a comprehensive review of their mode‐locking mechanisms, gain media, and fiber laser system performance. This paper, beginning with an overview of pulse‐generation technologies, reviews recent progress on 2–4 µm mid‐IR ultrafast fiber lasers, including Tm 3+ ‐, Ho 3+ ‐doped, Tm 3+ /Ho 3+ codoped silicate fiber 2 µm lasers, and Er 3+ ‐, Dy 3+ ‐doped, and Ho 3+ /Pr 3+ codoped ZBLAN fiber 2.5–4 µm lasers. Among them, the status of 2–4 µm ultrafast fiber lasers based on 2D material passive mode‐locking is emphatically discussed. Meanwhile, the novel advances on mode‐locking and gain fibers of mid‐IR ultrafast fiber lasers are explored. Furthermore, current and prospective applications of such laser systems are also introduced in detail. This review finally summarizes challenges associated with future development of mid‐IR ultrafast fiber lasers, which provides an outlook on how to achieve more desirable laser performance (e.g., higher average power, higher pulse energy, and longer emission wavelength) that can lead to more practical uses of such lasers.