Detection of ultrahigh resonance contrast in vapor-cell atomic clocks

Abstract

We propose and demonstrate a novel detection scheme of clock signals and obtain an ultrahigh resonance contrast up to 90%, which leads to the remarkable improvement of the precision of the signal-to-noise ratio. The frequency stability in terms of Allan deviation of the proposed detection scheme is improved by an order of magnitude under equivalent conditions. © 2012 Optical Society of America OCIS codes: 020.3690, 210.3810, 290.2558. Atomic clocks play a key role in current society life, especially in timekeeping, synchronization, and communication [1]. Active efforts have been made to improve performance of vapor-cell atomic clocks, particularly regarding contrast and signal-to-noise ratio (SNR). These two parameters play an important role in the achievement of good frequency stability. In the present day, people use optical pumping and the time domain Ramsey double-pulse approach [2,3] to interrogate an atomic ensemble contained in a closed cell. In that case the Allan deviation for characterizing frequency stability is [3,4] στwhere Q is the atomic line quality factor, SNR is the signal-to-noise ratio, and τ is the averaging time. Here T c is the cycle time of the atomic clock. In state-of-the-art devices, frequency stability is limited by shot noise, which results, for example, from residual light reaching the detector in optically pumped devices [5]. Over the past years, several techniques such as coherent population trapping [6] and N resonance [7] have been proposed to eliminate systematic effects that deteriorate clock performance and improve their short-term frequency stability. To date, contrasts up to 30% using absorption detection have been reported [5,8]. The frequency stability of the order of 10 −15 has been reported in a pulsed optically pumped (POP) vapor-cell clock [8]. However, the further improvement is limited by the difficulty of increasing signal contrast and atomic line quality factor.To address these problems, we develop a novel detection technique to make better performance for a vaporcell atomic clock, which is characterized by using dispersive detection, with the advantages of higher sensitivity and SNR over the absorptive counterpart [9,10]. In our modified scheme, probe light background is blocked by an analyzer in the measurement, and the shot noise is strongly suppressed. The forward-scattering signals, which result from magneto-optical rotation, has carried the Ramsey fringe information. We obtain the Ramsey fringe with an ultrahigh contrast up to 90%, under which the Allan deviation is improved by about an order of magnitude over the absorptive detection. On the other hand, the Allan deviation is defined by the atomic line quality factor. The magneto-optical rotation angle can exceed π∕2 when the microwave excitation increases and the forward-scattering signal diminishes quickly near the microwave resonance line. In this condition, a pseudoresonance with a narrower linewidth appears in the central zone of Ramsey fringe. The narrower linewidth leads to a higher atomic line q...

Ramsey-fringe shape in an alkali-metal vapor cell with buffer gas

“…Recently, also thanks to the development of laser diodes as optical pumping sources, Ramsey fringes in a hot atomic vapor have been studied for clock applications using an all-optical interaction (see, for example, Ref. [9]) or a more traditional double-pulse (microwave plus laser) approach [10][11][12][13]. In particular, ultrahigh contrast resonances leading to remarkable improvement in the signal-to-noise ratio have been detected in Ref.…”

High-contrast coherent population trapping based on crossed polarizers method

“…The square dot is the measured data, and the solid line is the fitting curve calculated from Eq. (8). The relative angle θ giving the minimum DC level is shifted from 0 • .…”

“…In contrast, dispersion detection methods have been reported to decrease the background signal level [7] [8]. This method regulates the light incident on a photo detector by selecting the light contributing to the resonance to reduce background signal.…”

Recent improvements on the pulsed optically pumped rubidium clock at SIOM

“…Since 2008, we focused on the POP atomic clock and fulfilled the optical detection [13,14] . In 2012, we realized a new dispersive detection method based on the Faraday effect [15] . With this new method, the contrast and signal-to-noise ratio (SNR) of the Ramsey fringe could be improved significantly, and the shot noise limit of the POP clock can be reduced further [16] .…”

“…With the use of a microwave switch (HMC-C019, Hittite Microwave Corp.), we could obtain the two microwave pulses for the Ramsey interrogation. Different from the traditional absorption detection, we use the dispersive detection, which is the total result of absorption and the magneto-optical rotation effect [15,17] . When the microwave pulse area exceeds π/4, the absorption effect becomes more obvious than the rotation effect, which leads to the reduction of the forward scattering signal, and the Ramsey fringe begins to split.…”