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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

End-resonance clock and all-photonic clock

Yuan‐Yu Jau, W. Happer, Fei Gong, Alan M. Braun, Martin H. Kwakernaak

Published Jan 11, 2008• 1 citation
Publisher

Abstract

The end-resonance clock uses strong hyperfine end transition to stabilize the frequency of the local oscillator. Comparing to the conventional 0-0 atomic clock, end resonance has very small spin-exchange broadening effect. The spin-exchange rate is proportional to the number density of the alkali-metal atoms. By using the end resonance, we are able to use very high dense vapor to obtain a much better signal to noise ratio. On the other hand, the end resonance suffers from the first-order magnetic field dependence. This problem, however, can be solved by simultaneously using a Zeeman end resonance to stabilize the magnetic field. Here, we report the most recent result of the end-resonance clock. In addition, we report a whole new technique, push-pull laser-atomic oscillator, which can be thought as all-photonic clock. This new clock requires no local oscillator. It acts like a photonic version of maser, which spontaneously generates modulated laser light and modulated voltage signals. The modulation serves as the clock signal, which is automatically locked to the ground-state hyperfine frequency of alkali-metal atoms.

DOI
10.1117/12.773803

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The end-resonance clock uses strong hyperfine end transition to stabilize the frequency of the local oscillator. Comparing to the conventional 0-0 atomic clock, end resonance has very small spin-exchange broadening effect. The spin-exchange rate is proportional to the number density of the alkali-metal atoms. By using the end resonance, we are able to use very high dense vapor to obtain a much better signal to noise ratio. On the other hand, the end resonance suffers from the first-order magnetic field dependence. This problem, however, can be solved by simultaneously using a Zeeman end resonance to stabilize the magnetic field. Here, we report the most recent result of the end-resonance clock. In addition, we report a whole new technique, push-pull laser-atomic oscillator, which can be thought as all-photonic clock. This new clock requires no local oscillator. It acts like a photonic version of maser, which spontaneously generates modulated laser light and modulated voltage signals. The modulation serves as the clock signal, which is automatically locked to the ground-state hyperfine frequency of alkali-metal atoms.
Laser-atomic oscillator is a newly developed method for simultaneously generating stable optical and electrical modulations with only a few components. It requires a semiconductor laser, a vapor cell, and maybe a few optical components. No photodetector and electronic feedback are needed. In this new system, the ground-state hyperfine coherence of alkali-metal atoms is spontaneously generated. The modulated laser light with a spectrum of a small optical comb is automatically produced, and the spacing between the comb teeth is photonically locked to the hyperfine frequency. The charge carriers in the semiconductor laser are also modulated at the hyperfine frequency. A well-designed laser-atomic oscillator can be thought as an “all-photonic clock,” a new kind of active atomic clock. The system is like an active maser in photonic version. It does not need a local oscillator and the assistance of microwave circuitry, and it consumes less power for the operation compared to passive, vapor-cell atomic clocks. Besides, laser-atomic oscillator can be very compact due to its simple structure. Recently, small or miniature atomic clocks have been considered to have many applications, and passive atomic clock is the main approach of making miniature atomic clocks. Complexity and the power consumption are still the major challenges. We believe that the technique of laser-atomic oscillator can bring some advantages in manufacturing small atomic clocks.
A vapor of alkali-metal atoms in the external cavity of a semiconductor laser, pumped with a time-independent injection current, can cause the laser to self-modulate at the "field-independent 0-0 frequency" of the atoms. Push-pull optical pumping by the modulated light drives most of the atoms into a coherent superposition of the two atomic sublevels with an azimuthal quantum number m=0. The atoms modulate the optical loss of the cavity at the sharply defined 0-0 hyperfine frequency. As in a maser, the system is not driven by an external source of microwaves, but a very stable microwave signal can be recovered from the modulated light or from the modulated voltage drop across the laser diode. Potential applications for this new phenomenon include atomic clocks, the production of long-lived coherent atomic states, and the generation of coherent optical combs.
In this thesis, we present new studies of alkali-hyperfine resonances, new optical pumping of alkali-metal atoms, and the new measurements of binary spin-exchange cross-section between alkali-metal atoms and xenon atoms. We report a large light narrowing effect of the hyperfine end-resonance signals, which was predicted from our theory and observed in our experiments. By increasing the intensity of the circularly polarized pumping beam, alkali-metal atoms are optically pumped into a state of static polarization, and trapped into the hyperfine end-state. Spin exchange between alkali-metal atoms has minimal effect on the end-resonance of the highly spinpolarized atoms. This new result will possibly benefit the design of atomic clocks and magnetometer. We also studied the pressure dependence of the atomic-clock resonance linewidth and pointed out that the linewidth was overestimated by people in the community of atomic clock. Next, we present a series study of coherent population trapping (CPT), which is a promising technique with the same or better performance compared to the traditional microwave spectroscopy. For miniature atomic clocks, CPT method is thought to be particularly advantages. From our studies, we invented a new optical-pumping method, push-pull optical pumping, which can pump atoms into nearly pure 0-0 superposition state, the superposition state of the two ground-state hyperfine sublevels with azimuthal quantum number m = 0. We believe this new invention will bring a big advantage to CPT frequency standards, the quantum state preparation for cold atoms or hot vapor, etc. We also investigated the pressure dependence of CPT excitation and the line shape of the CPT resonance theo-
We consider advantages of the "end" resonances for the miniaturized alkali vapor-cell atomic clock, namely a higher signal-to-noise ratio (SNR) and a narrower linewidth at high vapor densities. We report our preliminary observations of the unusual phenomenon of light narrowing, in which the rubidium end-resonance linewidth is substantially reduced and the SNR greatly improved as the laser light intensity is increased. Contributions to the total linewidth from other broadening mechanisms are discussed.
We report the first demonstration of a laser-atomic oscillator with cesium (Cs) atoms. A laser-atomic oscillator is analogous to an active mode-locked laser with a self-excited modulator, i.e., atoms, at a ground-state hyperfine transition frequency. Therefore, a laser-atomic oscillator can be configured as the simplest active atomic clock or a self-oscillating, earth-field atomic magnetometer that delivers oscillation signals both optically and electrically. With the current experimental Cs laser-atomic oscillator (Cs-LAO) setup, when it is configured as an atomic clock using the 0–0 hyperfine transition, the short-term fractional frequency instability is around the 10−10 level. When it is configured as a self-oscillating magnetometer using a magnetically sensitive 3–3 hyperfine transition, the magnetic field sensitivity is around 100 fT/Hz at 60 Hz. The presented Cs-LAO uses a cavity length from ∼6.5 to ∼11.4 cm. Ultimately, the minimal length of a Cs-LAO device can be ≤1.63 cm. Our new efforts unlock the potential of building truly chip-scale atomic clocks and magnetometers.
The alkali-vapor-cell atomic clock, of either the conventional or the coherent population trapping type, offers one of the most viable approaches to making ultraminiature and chip-scale devices. Unfortunately, this atomic clock suffers from two laser-induced noise processes: conversion of laser phase noise (PM) to amplitude noise (AM) and ac-Stark-shift fluctuations. Here we demonstrate a method for circumventing these problems in a passive and miniaturizable manner based on pressure broadening of the relevant optical transitions. For this purpose we have constructed a conventional atomic clock employing Rb^87 vapor confined with 100 Torr of N2. In this relatively high-pressure environment, both PM-to-AM conversion efficiency and the ac-Stark shift are reduced. Though we employ a phase-noisy, single-mode diode laser and lock the laser frequency to the pressure-broadened 1.6-GHz D_1 absorption line of Rb, we obtain excellent short-term frequency stability [i.e., sigma_y(tau)=1.8 × 10^−12/tau^½]. Moreover, as a single resonance cell generates locking signals for both the laser wavelength and the crystal oscillator, the atomic clock has real potential for miniaturization.