The optically generated microwave oscillator with long-term stability locked at hydrogen maser

We present an oscillator of optically generated microwaves (OGMs) based on an optical frequency comb (OFC) and an ultrastable laser (USL). The USL provides short-term stability, and the OFC transfers the short-term stability to radio frequencies. Frequency synthesis is used to transfer the stability of the OGM oscillator to an oven-controlled crystal oscillator (OCXO). To improve the long-term stability of the OCXO, an additional compensating loop was applied to lock the long term stability of OGM at a hydrogen maser by compensating the frequency drift of the USL. The phase noise of the OCXO is maintained sufficiently low through these locked loops. The OCXO’s phase noise at 1 Hz is about −116 dBc/Hz. The short-term stability (1–100 s) of the OCXO with different compensating periods is better than 4 × 10−15. By replacing the H-maser (BIPM code: 1404850) with the OGM as the local oscillator, the stability of the NIM6 Cs fountain clock is improved from (1.2 × 10−13)τ−1/2 to (5.1 × 10−14)τ−1/2. This work should help optimize fountain clocks for better type A uncertainty or faster frequency offset evaluations.