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

Abstract

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.

Optical state selection process with optical pumping in a cesium atomic fountain clock*

“…The design and operation of the fountain clock used in our experiment are described elsewhere. [17,26,27] Here, only the essential part related to the optical state selection is introduced. As indicated in Fig.…”

Improvement of the short-term stability of atomic fountain clock with state preparation by two-laser optical pumping*

“…Then the OP is implemented experimentally to increase the atom population in the |F = 4, m F = 0 state, and improve the frequency stability of the fountain clock NIM6, which used an optically generated microwave as the local oscillator. [17][18][19][20][21] The improvements of signal-to-noise ratio (SNR) under different conditions are presented, and the influence of different dead times on the relative frequency stability due to the Dick effect is also shown at the end.…”