Ultrafast laser propagation in transparent media can lead to long-range diffraction-free filamentation. The laser filament has numerous nonlinear optical effects such as the Kerr effect, self-phase modulation, four-wave mixing, and self-steepening. Supercontinuum covers the ultrabroad range from microwave to ultraviolet. Supercontinuum generation from femtosecond laser filaments has an ultrabroad band, high directionality, and tunability. Therefore, the supercontinuum can be used in absorption spectroscopy, remote gas detection, pulse compression, and few-cycle pulse generation. The supercontinuum is mainly induced by self-phase modulation, which considerably broadens the spectrum. The generation and regulation technology of the supercontinuum have matured gradually owing to the development of the commercial femtosecond laser. The supercontinuum is excellent broadband coherent radiation that can be extended to multiple octaves. The supercontinuum is applicable to absorption spectrum detection. The directionality of the supercontinuum benefits from the spatial directivity of the filament. Therefore, the supercontinuum can realize the remote sensing and detection. The spectrum-broadening and abundant nonlinear effects during supercontinuum generation can compensate for the dispersion to realize pulse self-compression. The conventional scheme generally undergoes two processes: spectrum broadening and pulse compression, which is inefficient and extremely limited by the compression elements for large energy pulses. The supercontinuum from the filaments provides a new way to solve this problem, simplifying the pulse compression process with great efficiency.

In specific application scenarios, the multi-dimensional modulation of the supercontinuum is essential for certain spectral distributions and conversion efficiency. In past decades, various schemes, including spatial shaping, pulse shaping, polarization modulation, focusing conditions modulation, and material modulation, were developed. Spatial shaping can directly adjust the spatial energy distribution of the input laser pulse. Supercontinuum conversion efficiency can be considerably improved by multiple filament generation, which overcomes the energy saturation effect compared to a single filament. Pulse shaping modulates the time-domain profiles of the laser pulse, which is useful for obtaining specific band supercontinuum generation. The polarization state of the laser field affects the nonlinear polarization process of the nonlinear medium, thereby changing the spectral range, spectral intensity, and polarization state of supercontinuum radiation. The focusing condition or physical properties and arrangement of the transmission medium are direct and convenient methods for controlling supercontinuum generation by changing the length and intensity of the filament.Recently, the supercontinuum has been widely studied and important progress has been achieved. The modulation schemes are summarized in Section 3. Researchers proposed the control of the energy distribution of the filament through beam spatial shaping to optimize the femtosecond laser filamentation and generation of a supercontinuum (Figs. 2-6). The time-domain characteristics of the femtosecond laser, such as pulse width, chirp, and initial spectrum, can directly affect the filament supercontinuum. Pulse shape regulation has important applications in the research of femtosecond laser filamentation and optimization of the supercontinuum (Figs. 7 and 8). Because the nonlinear coefficient, plasma density, and clamping light intensity are related to the ellipticity of the driving laser, incident laser polarization is also used for regulating the filament supercontinuum (Figs. 9-11). Additionally, it is demonstrated that the effective regulation of the supercontinuum in femtosecond laser filamentation can be achieved by controlling the focusing conditions and material (Figs. 12-14). This review focuses on physical mechanisms and modulation methods, and systematically summarizes the latest progress in the supercontinuum based on femtosecond laser filaments.

In summary, with the development of ultrafast laser technology and nonlinear optics, the physical mechanism of supercontinuum generation has been gradually clarified. Relevant experiments have been extended from solid and liquid to gas and applied to remote gas detection and sensing. This review summarizes the physical mechanism and multi-dimensional regulation scheme of filament-induced supercontinuum and briefly introduces the application scenarios of the supercontinuum, including transient absorption spectrum systems, remote sensing systems, and the seed of the mid-infrared and air lasers. At present, most of the experiments and simulations of the supercontinuum are limited to the laboratory; complex environmental conditions, such as turbulence, temperature, and humidity changes, should be considered in remote sensing. In addition, although the supercontinuum can also provide a direct seed source for an air laser, it must be optimized further and the generation distance, wavelength, and energy conversion efficiency of future air lasers must be controlled. In addition, the backscattering mechanism and spatial distribution of air lasers must be studied to develop related applications.