Abstract

Most applications of frequency combs require locking of the carrier-envelope offset frequency f_CEO and the repetition rate f_rep [1]. Recently, we established passive stabilization of the carrier-envelope phase (CEP) of an Er:fiber laser via difference frequency generation (DFG) between two components derived by spectral broadening in a highly nonlinear fiber (HNF) [2,3]. Here we demonstrate a frequency-domain characterization and various attractive features provided by this scheme. The CEP is passively stabilized at the full f_rep of 100 MHz. The broadening of the optical linewidth of the comb is investigated and narrowed by active stabilization of f_rep at high bandwidth. To this end, the current of the pump diodes and an extra-cavity electro-optic modulator (EOM) are used. This scheme enables a completely polarization-maintaining design for the fiber oscillator that is mode-locked without any free-space components by a saturable absorber mirror. The output of the oscillator passes a fiber-coupled EOM and pumps a HNF after amplification to pulse energies of 6 nJ. A CEP-stable pulse train centered at a wavelength of 1550 nm is obtained after DFG in periodically poled lithium niobate which then seeds a two-branch multi-stage amplifier. One branch is used for heterodyning with various single-frequency reference lasers in order to investigate the phase noise properties of the comb. These measurements yield a passive optical linewidth of the CEP-stable system of less than 100 kHz at 1550 nm. A correlation between the spectral bandwidth of the oscillator and the phase-stable linewidth as function of oscillator pump power discloses competitive interplay between the reduction of Gordon-Haus jitter and the onset of solitonic fission (Fig. 1a,b). The supercontinuum linewidth before DFG is investigated at 860 nm, 1064 nm 1550 nm and 2128 nm, respectively. A quadratic increase with frequency detuning from the origin of the comb at 1550 nm is found (Fig. 1 c). The second amplifier branch is frequency doubled to an average output power beyond 130 mW. We lock the repetition rate via controlling the pump current. Deviations of f_rep below 3.5 mHz are achieved at a gating time of 300 ms via the feedback from fluorescence emitted after sequential two-photon 5S – ⁸⁵ 1/2 5D_5/2 excitation of Rb (Fig. 1d) [4]. Detailed analysis of the phase noise spectrum reveals that a higher bandwidth is needed for efficient reduction of the optical linewidth. Therefore, high-frequency fluctuations of the repetition rate are corrected with an extra-cavity EOM, leaving the passive phase-lock of f_CEP unaffected and preserving the excellent stability of the system in general. With this scheme, active stabiliziation with a sub-Hz linewidth is demonstrated when locking to an external narrow-linewidth laser (Fig. 1e).

© 2015 IEEE