Passive phase and intensity noise suppression in optical combs via soliton self-frequency shift

Optical frequency combs have revolutionized numerous fields of modern science, particularly precision metrology and molecular spectroscopy. However, they inherently suffer from electronic and optical noise, which restricts their coherent spectral extension into critical mid-infrared and terahertz (THz) regimes, thereby demanding complex active stabilization systems. Here, we introduce a passive noise suppression technique based on the soliton self-frequency shift (SSFS) in optical fibers. Our method spectrally shifts a comb to a low-noise region, effectively isolating it from the initial parasitic optical noise and achieving simultaneous reductions of 15 dB in phase noise and 20 dB in intensity noise, as verified on a femtosecond electro-optic comb. This comb is then coherently extended into the 6 to 13 μm mid-infrared range via difference-frequency generation in a GaSe crystal and into the 0.1 to 5 THz range through optical-to-THz conversion in a photoconductive antenna. Our passive noise-suppression method is broadly applicable to other comb platforms, including mode-locked lasers and on-chip microcombs, whereas the broadband comb source is promising for various spectroscopic sensing applications.