Compact Rb optical frequency standard with 10−15 stability

Abstract

We achieved a low-cost and small-sized Rb optical frequency standard based on Rb 5S → 6P transition with 10 stability, which is comparable with that of the best 532 nm I optical frequency standards. In this system, we directly lock the 420 nm diode laser on the 5S F = 3 → 6P F = 4 hyperfine transition line without an additional Pound-Drever-Hall pre-locking system. The signal-to-noise-ratio reaches as high as 350 000 when the averaging time is at 1 s. Eventually by the fluctuation of the residual error signal after locking, the preliminary stability of the optical frequency standard reaches 1.2×10/τ, decreasing to 2.1 × 10 at 80 s. It shows potential in stability performance, experimental cost, and system volume compared with the 532 nm I optical frequency standard as a wavelength standard. It also opens a door for the achievement of wavelength standards by using higher excited states of alkalies.

Optical clock technologies for global navigation satellite systems

“…They typically employ modulation transfer spectroscopy (MTS) or frequency modulation spectroscopy (FMS) of optical transitions with linewidths of the order of MHz. With a compact Rubidium-based frequency reference using MTS near 420 nm, frequency instabilities of 2.1 × 10 -15 at an integration time of 80 s have been claimed, deduced from the error signal (Zhang et al 2017;Chang et al 2019). Iodine-based frequency references near 532 nm have been realized for many decades, resulting in compact and ruggedized setups, also with respect to applications in space (Nyholm et al 2003;Leonhard and Camp 2006;Zang et al 2007;Argence et al 2010;Schuldt et al 2017;Döringshoff et al 2017), showing frequency instabilities at the 10 -15 level for integration times between 1 and 1000 s. A very compact setup has been successfully flown on a sounding rocket, together with a frequency comb (Schkolnik et al 2017;Döringshoff et al 2019).…”

“…Summary of the key figures of the different optical clock technologies, together with the corresponding figures of the Galileo RAFS and PHM aValues on frequency instabilities deduced from the error signal(Zhang et al 2017).…”

Stabilizing diode laser to 1 Hz-level Allan deviation with atomic spectroscopy for Rb four-level active optical frequency standard

“…In particular, with more and more theoretical and experimental studies on Rb 420 nm 5S 1∕2 to 6P 3∕2 transition [37][38][39][40][41][42], it is well known that the natural linewidth of Rb 420 nm transition is narrow. Therefore, frequencystabilized laser working on Rb 420 nm transition based on MTS method is also a promising alternative for compact frequency standard for various applications [43].…”

“…In this paper, based on our preliminary experiment with a Allan deviation of 1.2 × 10 −14 ∕ √ [43], a Rb-referenced 420 nm blue diode laser system integrated in a 15-l box with a Allan deviation of 1.4 × 10 −15 ∕ √ and a signalto-noise ratio of 3,000,000 is achieved by optimizing the frequencies and gains of the servo loop circuit and the performance of diode laser. The laser source employed here is a 420 nm interference filter external cavity diode laser (ECDL) of which the Lorentz-fit linewidth is less than 30 kHz and the tuning range is more than 15 GHz [44].…”

Modulation transfer spectroscopy based on acousto-optic modulator with zero frequency shift

“…[10] Several MTS schemes have been reported in the literature. [11][12][13] The external frequency modulation required in MTS can be generated by using electro-optic modulators (EOMs), [12,14,15] acousto-optic modulators (AOMs), [16][17][18] or a combination of EOMs and AOMs. [13,19] Normally, a resonant circuit is used to drive an EOM, which makes it difficult to tune the modulation frequency flexibly.…”