A 15-GHz mode spacing optical frequency comb based on a Kerr-lens mode-locked Yb:Y₂O₃ ceramic laser has been developed. Individual modes were clearly resolved by a commercial spectrometer. To demonstrate the long-term operation of the optical frequency comb, a single longitudinal mode was phase-locked to a frequency-stabilized continuous wave laser and the repetition rate to a radio frequency standard. To the best of our knowledge, 15 GHz is the largest reported mode spacing (repetition rate) for both a Kerr-lens mode-locked laser and a direct femtosecond laser based-optical frequency comb.

We demonstrate a passively offset-frequency stabilized optical frequency comb centered at 1060 nm. The offset-free comb was achieved through difference frequency generation (DFG) between two portions of a supercontinuum based on a Yb:fiber laser. As the DFG comb had only one degree of freedom, repetition frequency, full stabilization was achieved via locking one of the modes to an ultra-stable continuous wave (CW) laser. The DFG comb provided sufficient average power to enable further amplification, using Yb-doped fiber amplifier, and spectral broadening. The spectrum spanned from 690 nm to 1300 nm and the average power was of several hundred mW, which could be ideal for the comparison of optical clocks, such as optical lattice clocks operated with Sr (698 nm) and Hg (1063 nm) reference atoms.

Sum frequency mixing has been demonstrated below 150 nm in KBeBO₃F₂ by using the fundamental with its fourth harmonic of a 6 kHz Ti: sapphire laser system. The wavelength of 149.8 nm is the shortest ever obtained to our knowledge by phase matching in nonlinear crystals. The output powers were 3.6 μW at 149.8 nm and 110 μW at 154.0 nm, respectively. The phase matching angles measured from 149.8 to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The measured transmission spectra of KBeBO₃F₂ crystals support the generation of coherent radiation below 150 nm.

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We propose a novel magneto-optical approach for the repetition frequency stabilization of optical frequency combs. We developed a Yb:fiber mode-locked laser with a fiber-based magneto-optic modulator used to stabilize one of the longitudinal modes to an optical reference with sub-hundred mrad residual phase noise. This modulator does not induce mechanical resonances and as such has the potential to achieve much broader feedback bandwidths than conventional modulators used for cavity length stabilization.