Mid-infrared ultrafast laser sources: Recent advances in generation mechanisms, key technologies, and difference frequency generation

Abstract

Mid-infrared (mid-IR) ultrafast laser sources, which cover the molecular vibrational fingerprint region and atmospheric transmission window, have garnered widespread attention due to their significant applications in spectroscopy, biomedicine, environmental monitoring, and precision machining. In recent years, rapid advances in ultrafast fiber lasers, nonlinear frequency conversion, and novel fiber technologies have greatly advanced mid- infrared ultrafast laser sources. Currently, the primary methods for generating mid-IR ultrafast sources include direct mode-locked lasers based on gain media and indirect schemes relying on nonlinear frequency conversion. Direct sources offer compactness and operational stability but are limited by gain bandwidth and material constraints, while indirect approaches provide broader wavelength tunability and spectral coverage through nonlinear optical processes, albeit with increased system complexity. This review focuses on the major advances in mid-infrared ultrafast sources over the past decade and systematically reviews their generation mechanisms and key technologies, with particular emphasis on nonlinear frequency conversion based on difference frequency generation. This approach employs a broadband femtosecond fiber laser as the pump source, enabling broadband, highly coherent mid-IR radiation output, and offer advantages such as a compact structure, high stability, and low cost. Furthermore, this review summarizes typical applications of mid-IR ultrafast sources in environmental monitoring, medical diagnostics, and precision machining, and discusses the potential of artificial intelligence optimization methods and novel fiber platforms, including hollow-core anti-resonant fibers, in pulse modulation, enhancement of nonlinear frequency conversion efficiency, and long-wavelength transmission. With the continuous development of high-energy ultrashort pulse fiber lasers, novel nonlinear materials, and advanced fiber structures, mid-infrared ultrafast laser sources are expected to achieve broader spectral coverage, higher pulse energy, and longer output wavelengths, thereby providing stronger support for future scientific research and engineering applications.

Keywords

mid-infrared ultrafast laser sources / mode-locked lasers / difference frequency generation / nonlinear frequency conversion

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