Intensity Noise Dynamics in Terahertz Quantum Cascade Lasers

Terahertz (THz) quantum cascade lasers (QCLs) provide compact sources of coherent radiation in a spectral region that remains difficult to access with conventional semiconductor lasers. Yet, despite extensive studies of their frequency‐fluctuations, the frequency‐dependent signatures and microscopic origins of their intensity‐fluctuations are largely unexplored. Here, we measure the intensity‐noise power spectral density of single‐mode, frequency‐comb and harmonic‐frequency‐comb THz QCLs using ultrafast graphene thermoelectric nanodetectors integrated on optical platforms. Beyond the pronounced low‐frequency (1/f) noise associated with carrier transport through the heterostructure, we uncover distinct high‐frequency power‐law regimes that depend strongly on both comb state and active‐region design. Harmonic frequency combs exhibit, on average, up to ∼10% lower relative intensity noise than conventional frequency combs, despite their enhanced nonlinear modal dynamics. The relative intensity noise (RIN) decreases with the optical power P 0 following the power law , with , revealing a crossover from spontaneous‐emission‐dominated fluctuations to regimes increasingly governed by non‐radiative carrier relaxation and electron–phonon scattering. These results establish a unified picture of intensity‐noise dynamics in THz QCLs and show that active‐region engineering and comb‐state control offer direct routes to tailoring fluctuations in intersubband lasers. Our findings provide a foundation for low‐noise THz sources for precision metrology, coherent spectroscopy, and quantum photonics.