Coherent link between a Ti:sapphire comb and a 1.5 μm laser via nonlinear interaction in photonic crystal fiber

Optical clock networks have distinct advantages for the dissemination of time/frequency, geodesy, and fundamental research. To realize such a network, the telecom band and optical atomic clocks have to be coherently bridged. Since the telecom band and optical atomic clocks reside in a distinct spectral region, second-harmonic generation is usually introduced to bridge the large frequency gap. In this paper, we introduce a new method to coherently link a 1550 nm continuous wave laser with a Ti:sapphire mode-locked laser-based optical frequency comb. By coupling the 1550 nm continuous wave laser light and the Ti:sapphire comb light together into a photonic crystal fiber, nonlinear interaction takes place, and new comblike frequency components related to the 1550 nm laser frequency are generated in the visible region. Consequently, we can detect beat notes between two combs in the visible region with a signal-to-noise ratio of more than 40 dB in a resolution bandwidth of 300 kHz. With this signal, we realize an optical frequency divider for converting the frequency of optical clocks in the visible region to the telecom band at 1.55 μm. An out-of-loop measurement shows that the additional noise and uncertainty induced in optical frequency conversion are 5 × 10⁻¹⁸ at 1 s averaging time and 2.2 × 10⁻¹⁹, respectively, which are limited by the uncompensated light path fluctuation but fulfill precision measurement using state-of-the-art optical clocks.