Abstract: Coherent broadband light sources have been considered to be of significant value in optical metrology, molecular spectroscopy, ultrafast imaging, and broadband optical communications. Conventional supercontinuum sources have mainly relied on third-order and higher-order nonlinear optical processes, such as self-phase modulation, four-wave mixing, and Raman transitions. Due to the relatively low nonlinear coefficients of these processes, long interaction lengths and strict phase-matching conditions have been required, by which complex device configurations and bulky volumes have been introduced. A novel scheme for octave-spanning coherent broadband light generation was reported, which was based on the phase-matching-free second-order nonlinear optical process of difference-frequency generation in nanoscale two-dimensional crystals. In gallium selenide (GaSe) nanosheets with a thickness of approximately 88 nm and niobium oxyiodide (NbOI₂) nanosheets with thicknesses ranging from 100 nm to 187 nm, coherent broadband light covering a spectral range of 565 nm to 1906 nm (spanning more than one octave) was generated at the −40 dB level, using pump pulses with sub-nanojoule energy only. Compared with existing free-space bulk supercontinuum sources, the sample thickness was reduced by approximately five orders of magnitude, and the excitation power was decreased by about two to three orders of magnitude. A conversion efficiency of 0.12% was achieved for the coherent broadband light in NbOI₂, with a normalized efficiency of approximately 0.66%·μm⁻¹, which was improved by about one order of magnitude over previously reported results. An average fringe visibility of 0.91 was measured in interferometric fringe experiments, indicating excellent temporal coherence. Furthermore, the generated light source was applied to gas-sensing detection of O₂ absorption spectra (B-band at 687 nm and A-band at 760 nm) in ambient air, by which its practical utility was verified. The phase-matching-free coherent broadband light generation mechanism proposed in this work is considered to provide a novel technical route for the development of compact, highly integrated ultra-broadband coherent light sources.