Spectral Purity and Sources of Noise in Femtosecond-Demodulation Terahertz Sources Driven by Ti: Sapphire Mode-Locked Lasers

Abstract—Direct measurements of the spectral purity in tera-hertz femtosecond-demodulation sources are reported and com-pared to theory. Because these sources operate at very high har-monics ( 102 104) of the mode-lock frequency, a high spec-tral purity source is very dependent on a low-jitter femtosecond laser. Conversely, the spectral content of the terahertz sources pro-vides detailed information about timing jitter and stringent tests of models used to describe the jitter. We find that both the behavior of the central core, and the noise skirts of the power spectrum of our sources, can be quantitatively related to measured ripple and continuum amplitude noise on the Ar+ pump laser by use of mod-ulation theory. Index Terms—Laser noise, mode-locked lasers, modulation, spectral analysis, submillimeter wave generation, timing jitter. I