Comprehensive intensity and phase noise analysis of a femtosecond Cr:ZnSe Kerr-lens mode-locked laser

We present a comprehensive characterization of a Kerr-lens mode-locked ultrafast Cr:ZnSe laser that generates nearly transform-limited pulse trains with a duration of 42 fs and an optical bandwidth of 8.2 THz, at an emission wavelength of 2.4 μm and 213 MHz repetition frequency. Our detailed investigation of intensity noise and pulse repetition-rate phase noise underscores the exceptional spectral purity of the Cr:ZnSe laser, even when pumped by a multi-longitudinal mode Er-fiber laser. This remarkable performance is attributed to the efficient random second harmonic generation mechanism within the ZnSe polycrystal, which significantly mitigates the transfer of pump laser intensity noise to the Cr:ZnSe laser, achieving in the mode-locking regime more than 13 dB pump noise attenuation in the Fourier bandwidth from 10 Hz to 10 MHz, also resulting in the complete suppression of the Cr:ZnSe resonance relaxation oscillations. Additionally, we report on the frequency stabilization of the pulse repetition frequency against an RF rubidium clock, down to a fractional frequency stability of 2×10−11 for an integration time of 10 s, demonstrating a pulse jitter of less than 1.6 ps over an integration bandwidth from 100 Hz to 100 MHz.