Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE
Progress towards optical frequency standards based on two-photon transitions in atomic silver
Mark D. Plimmer, Thomas Badr, Stéphane Guérandel, Yann Louyer, S. CHALLEMEL DU ROZIER, P. Juncar, M. Himbert, Joshua D. Silver, F. Biraben, F. Nez, G. Hagel
We report advances towards the development of frequency standards based on narrow two-photon transitions in the silver atom at 576 and 661 nm from the ground 4d105s2S1/2 state to the 2D3/2,5/2 levels of the configuration 4d95s2. We describe detection schemes for metastable atoms, localisation of the position of the metastable 2D5/2 level and diode-pumped Nd:YLF lasers around 1.3 μm destined to replace dye lasers for both cooling and spectroscopy of the clock transition. Our result for the position of the 2D5/2 level is more than a factor of two improvement over previous vales.
We report advances towards the development of frequency standards based on narrow two-photon transitions in the silver atom at 576 and 661 nm from the ground 4d105s2S1/2 state to the 2D3/2,5/2 levels of the configuration 4d95s2. We describe detection schemes for metastable atoms, localisation of the position of the metastable 2D5/2 level and diode-pumped Nd:YLF lasers around 1.3 μm destined to replace dye lasers for both cooling and spectroscopy of the clock transition. Our result for the position of the 2D5/2 level is more than a factor of two improvement over previous vales.
Over the past several decades, the field of atomic physics has seen outstanding developments, which have had an important impact on the development of atomic frequency standards. This is mainly due to improvements in laser technology, which has had a direct effect on the technique of optical pumping and made possible the cooling and trapping of ensembles of atoms to sub-Kelvin (sub-K) temperatures. The laser as such was developed in the 1960s. In early stages of its development, however, the use of the laser in the study of interactions between electromagnetic radiation and atoms was limited by the very restricted number of wavelengths available with the existing lasers and the rather narrow tunability of the instruments developed. The so-called dye laser extended the range of possible studies, but such an instrument is rather cumbersome and its size and complexity restricted its use to limited laboratory applications. The advent of room temperature solid-state diode lasers, which could be fabricated to emit wavelengths in resonance with many transitions in alkali atoms such as Cs and Rb, for example, as well as in ions such as Sr + and Yb + , among many others, changed the whole picture. The power emitted by such lasers may be several tens of milliwatts. The spectrum of the radiation emitted is relatively narrow and the frequency can be adjusted over a relatively broad range by means of doping the substrate with selected elements at the time of fabrication. Fine frequency tuning of the device can also be done by means of the temperature of the substrate and its driving current. This property allows the exciting of transitions in selected atoms from one single ground-state hyperfine level to an excited state. This makes possible nearly complete inversion of population in ground states through optical pumping. Furthermore, as will be seen below, the exchange of momentum between a photon and an atom makes possible alteration of atomic velocities leading to so-called laser cooling of atoms. These two possibilities by themselves have totally altered the landscape of research in the field of atomic frequency standards leading to new realizations in both the microwave and the optical ranges. We will review selected subjects and recent advances in atomic physics that have had a specific impact on the development of atomic frequency standards. For a broader overview of such advances in atomic physics, the reader is referred to the excellent review on the subject by Cohen-Tannoudji and Guéry-Odelin (2011) .
Over the past several decades, the field of atomic physics has seen outstanding developments, which have had an important impact on the development of atomic frequency standards. This is mainly due to improvements in laser technology, which has had a direct effect on the technique of optical pumping and made possible the cooling and trapping of ensembles of atoms to sub-Kelvin (sub-K) temperatures. The laser as such was developed in the 1960s. In early stages of its development, however, the use of the laser in the study of interactions between electromagnetic radiation and atoms was limited by the very restricted number of wavelengths available with the existing lasers and the rather narrow tunability of the instruments developed. The so-called dye laser extended the range of possible studies, but such an instrument is rather cumbersome and its size and complexity restricted its use to limited laboratory applications. The advent of room temperature solid-state diode lasers, which could be fabricated to emit wavelengths in resonance with many transitions in alkali atoms such as Cs and Rb, for example, as well as in ions such as Sr+ and Yb+, among many others, changed the whole picture. The power emitted by such lasers may be several tens of milliwatts. The spectrum of the radiation emitted is relatively narrow and the frequency can be adjusted over a relatively broad range by means of doping the substrate with selected elements at the time of fabrication. Fine frequency tuning of the device can also be done by means of the temperature of the substrate and its driving current. This property allows the exciting of transitions in selected atoms from one single ground-state hyperfine level to an excited state. This makes possible nearly complete inversion of population in ground states through optical pumping. Furthermore, as will be seen below, the exchange of momentum between a photon and an atom makes possible alteration of atomic velocities leading to so-called laser cooling of atoms. These two possibilities by themselves have totally altered the landscape of research in the field of atomic frequency standards leading to new realizations in both the microwave and the optical ranges. We will review selected subjects and recent advances in atomic physics that have had a specific impact on the development of atomic frequency standards. For a broader overview of such advances in atomic physics, the reader is referred to the excellent review on the subject by Cohen-Tannoudji and Guéry-Odelin (2011).
We demonstrate an optical frequency standard by locking the output frequency of an external cavity diode laser to the 780 nm D2 transition of diffuse laser cooled 87Rb atoms using frequency modulation spectroscopy (FMS). A 50-cm-long cold atomic cloud is obtained within a ϕ 2 cm ×l 50 cm vacuum glass tube, featuring greater simplicity and reliability. By analyzing the impact of power and detuning of cooling and repumping lasers, and power of frequency-modulated laser on FMS based on cold atoms, we choose the optimal parameter and lock the laser frequency to the 87Rb 52S1/2 (F = 2) → 52P3/2 (F′ = 3) cycling transition. The cold-atom-based optical frequency standard achieves an in-loop frequency stability of 3.3 × 10−15 at 1 s, with peak-to-peak frequency fluctuations being less than 50 Hz within 6000 s. This represents the integration of cold atoms into an optical frequency standard utilizing FMS for frequency stabilization. In contrast to the common cold-atom optical frequency standards, our approach requires fewer lasers and eliminates the need for a complex and expensive Pound–Drever–Hall (PDH) system. Its low complexity makes it a promising candidate for development into a space cold-atom clock, promoting advancements in space science and navigational positioning. Additionally, it possesses great potential to serve as an absolute wavelength standard.
We have developed frequency standards at 192.6 THz (1556.2 nm) based on the two-photon transitions in rubidium at 385.2 THz (778.1 nm). These standards use a high power DFB laser at 1556.2 nm and second harmonic generation (SHG) in a periodically poled lithium niobate (PPLN) crystal. The linewidth of the DFB is reduced to the kHz level using optical feedback from a confocal cavity. The SH light is used to injection-lock a 778.1 nm laser diode which allows to observe and lock the 1556.2 nm laser to the (5S/sub 1/2/,F/sub g/=2-5D/sub 5/2/,F/sub e/=4) two-photon transition /sup 87/Rb. Allan variance measurements between identical standards show a beat stability 2.5/spl times/10/sup -13///spl tau//sup 1/2/ for observation times between 100 ms and 10 s and a level of 5.8/spl times/10/sup -14/ for 100 s is obtained. These standards have been moved recently to the National Research Council, Ottawa, Canada in order to measure their absolute frequency.
This is a progress report on the realization of a compact and transportable frequency standard at 1556 nm based on a two-photon transition in rubidium at 778 nm. These hyperfine transitions present great metrological interest. They have a narrow theoretical linewidth of 150 kHz when observed with a 1556 nm laser, and their absolute frequency is known with an uncertainty of 5.2 X 10-12. In this experiment, we use a high power 1556 nm DFB laser and reduce its linewidth to the 10 kHz level using optical feedback from a confocal cavity. We generate its second harmonic in a periodically poled LiNbO3 crystal and use this signal to injection-lock a Fabry-Perot laser emitting 42 mW at 778 nm. The slave laser is used to observe the Doppler-free two- photon transitions: two counter-propagating beams excite rubidium atoms which emit a blue fluorescence on resonance. This 420 nm light is detected on the side of the Rb cell with a photomultiplier. Such an optical frequency standard at 1556 nm, standing in the multiwavelength telecommunications systems window, becomes an attractive source for absolute frequency calibration of WDM components, optical spectrum analyzers and wavemeters.
Active optical clocks, a new class of optical atomic clocks, work through stimulated emission lasing, a unique mechanism that is distinct from laser absorption spectroscopy of conventional optical clocks. With coherent weak optical feedback from a Fabry-Perot resonator operating in the bad-cavity regime, active optical clocks sustain a lasing oscillation at an atomic transition frequency. They are used directly as quantum optical frequency standards with expected stability better by about two orders of magnitude than that of the best clocks. This review starts with a simplified account of historical developments and the limitations of critical techniques in optical atomic clocks. After an introduction of the basic mechanism underlying active optical clocks, we discuss the most recent experimental results for several configurations and experimental set-ups of active optical clocks. These include those based on a 2-level atomic beam and 3-level laser-cooled trapped-atom system, those based on a 4-level atom scheme, and the Faraday active optical clock. Moreover, following the successful lasing and preliminary results on a Faraday active optical frequency standard with Faraday atomic filter using thermal Cs cell, we describe an experimental scheme of a Faraday active optical clock with a Faraday atomic filter using magic-lattice-trapped Sr atoms at the 689-nm transition, and Faraday optical clocks operating in the good-cavity regime. We conclude with a comprehensive comparison of the different active optical clocks, prospects for active Faraday optical clocks, and potential applications of active optical clocks.