Ion Optical Frequency Standard Research Articles

Investigation on the charge-state distribution diagnosis oflaser-produced Al plasmas

Laser-produced plasmas (LPP) have been known to possess great application potential under extreme ultraviolet (EUV) lithography, soft X-ray micrography, and heavy ion accelerator, highly charged ion optical frequency standards, pulsed laser deposition, astrophysics and nuclear fusion. The applications in these fields require a clearer understanding of the evolutionary behavior of the LPP. However, LPP is neither homogeneous nor static during their expanding and cooling process, which involves complex physical processes. For a deep understanding of the LPP, many diagnostic methods and techniques have been developed, such as spectroscopy, fast imaging, time-of-flight (TOF), laser interferometry and Thomson scattering. Among these methods, spectroscopy has been the most widely used diagnostic method, which can be used to obtain relatively more state information. However, the diagnosis results of charge-state distribution obtained by spatio-temporally resolved spectroscopy are local and transient, and cannot therefore obtain the overall distribution. In order to have a more comprehensive understanding of charge-state distribution and its evolutionary behavior of the LPP, other diagnostic methods are needed. The TOF method is used for the measurement of highly charged ions of the LPP, which can be used to obtain the overall charge-state distribution information. Therefore, in this work, spectrometry and TOF method were used for the diagnosis of charge-state distribution of the LPP in a vacuum. The EUV emission spectra of laser-produced Al plasmas were measured by using spatio-temporally resolved laser-produced plasmas spectroscopy technique. The charge-state distributions with different delay times at the distance of 2.0 mm from target surface were diagnosed by spectral simulation and their evolution behavior were also analyzed. The TOF spectra of laser-produced Al plasmas were measured by the TOF method, and the overall charge-state distribution of ions in laser-produced Al plasmas was obtained. The results revealed that spectroscopy can diagnose the local and transient charge-state distribution of ions in the laser-produced plasmas, and the TOF method can be used to diagnose its overall distribution, and the two diagnostic methods therefore complement each other. This work aims to clarify the complementarity of the two methods, and to provide a method support for the comprehensive and accurate understanding of the charge-state distribution and evolution evolutionary behavior during the expansion process of the LPP, and also provides a reference for further optimization and modification of the radiation hydrodynamics model of the LPP.

The progress of the precision spectroscopy with single trapped and laser cooled <sup>40</sup>Ca<sup>+</sup> ion

The experimental study at Wuhan Institute of Physics and Mathematics (WIPM), Chinese Academy of Scienses (CAS) on the precision spetroscopy based on single trapped Ca+ ion is described in this paper. A single Ca+ ion is trapped in a miniature Paul trap and laser cooled to Lamb-Dicke regime, and the ion can be trapped more than 2 months. The linewidth of the 729 nm laser for probing the Ca+ 4 s 2 S 1/2-3 d 2 D 5/2 clock transition is reduced to lessthan 1 Hz by locked to a super cavity, and the 729 nm laser can be locked more than 1 month continuously. Two similar Ca + ion optical frequency standards are set up for frequency comparison. The systematic uncertainty is evaluated to be ´ 10−17 for both ftrquency standards. The frequency difference of two frequency standards is measured to be 3.0 (5.5) ´ 10−17 within 42 d. The Allan deviation for a single clock is measured to be 1×10−14 τ −1/2 with a 3-d-continuous comparison, the stability for a reaches 7 ´ 10−17 at an averaging time of 20000 s. Meanwhile, the absolute frequency of the Ca+ 4 s 2 S 1/2-3 d 2 D 5/2 clock transition is measured to be 411042129776401.7(1.1) Hz with respect to the SI second through the Global Positioning System (GPSx). The precision spectroscopy experiments are carried out based on the single Ca+ ion system. The magic wavelengths for the Ca+ optical frequency standardare measured to be λ | mj |=1/2=395.7992(7) nm and λ | mj |=3/2=395.7990(7) nm, which agrees with theoretical calculation made by Tang et al. The lifetime of the 3 d 2 D 5/2 is measured to be 1174(10) ms, which is agree with the experimental measurement made by University of Oxford and University of Innsbruck and the recent theoretical calculation made by Sahoo.

Generation of Continuous-Wave 194nm Laser for Mercury Ion Optical Frequency Standard**Supported by the National Natural Science Foundation of China under Grant Nos 91436103 and 11204374.

A 194-nm cw laser is an essential part in the mercury ion optical frequency standard. We report the generation of over 2 mW continuous-wave radiation at 194 nm in a beta barium borate crystal using a simple sum frequency mixing (SFM) system. One source beams at 718 nm is resonantly enhanced with a cavity and the other at 266 nm makes a single pass. Considering the walk-off effect in SFM, the source beam waists are designed to be elliptical, thus the conversion efficiency can be promoted. The 266-nm beam produced by frequency doubling of 532-nm laser is shaped close to the diffraction limit to achieve better mode matching.

Coherent excitation of the6S1/2to5D3/2electric-quadrupole transition in138Ba+

The electric dipole-forbidden, quadrupole 6S1/2 <-> 5D3/2 transition in Ba+ near 2051 nm, with a natural linewidth of 13 mHz, is attractive for potential observation of parity non-conservation, and also as a clock transition for a barium ion optical frequency standard. This transition also offers a direct means of populating the metastable 5D3/2 state to measure the nuclear magnetic octupole moment in the odd barium isotopes. Light from a diode-pumped, solid state Tm,Ho:YLF laser operating at 2051 nm is used to coherently drive this transition between resolved Zeeman levels in a single trapped 138Ba+ ion. The frequency of the laser is stabilized to a high finesse Fabry Perot cavity at 1025 nm after being frequency doubled. Rabi oscillations on this transition indicate a laser-ion coherence time of 3 ms, most likely limited by ambient magnetic field fluctuations.

Ultrastable Frequency Dissemination via Optical Fiber at NICT

We have developed a radio-frequency (RF) dissemination system using optical fibers. The phase noise induced during the transmission is actively cancelled by the compensation system with a voltage-controlled crystal oscillator. A first proving test was conducted on an urban telecom fiber link with a length of 10 km, and a frequency stability of 1 X 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> was achieved at an averaging time of one day. As an application of ultrastable frequency dissemination, a 1-GHz signal based on a cryogenic sapphire oscillator was transferred through a 25-km fiber and used as a microwave reference for an optical frequency comb. A fractional frequency stability of an ultranarrow clock laser for a Ca <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion optical frequency standard was measured by the comb as 9 X 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> at 1 s, which included both laser stability and transferred reference stability.

A narrow linewidth and frequency-stable probe laser source for the 88Sr+ single ion optical frequency standard

In this paper, we describe in detail a narrow linewidth and frequency-stable laser source used to probe the 5s2S1/2–4d2D5/2 clock transition of the 88Sr+ optical frequency standard. The performance of the laser system is investigated with studies of its frequency drift rates and with high resolution spectra of the 88Sr+ clock transition. The observed short-term drift rates are typically in the range of 10 to 23 mHz/s, and the current long-term drift rate is 13.9(3) mHz/s. The laser stability, after subtraction of linear drifts, reaches 5×10−16 at an averaging time of 3000 s. This high level of stability is attributed for the most part to stabilization of the reference cavity at the temperature where the coefficient of linear thermal expansion crosses zero. An upper bound for the laser linewidth is given by the observation of a Fourier-transform limited resonance of 4.3 Hz (Δν/ν=1×10−14) on the 88Sr+ clock transition. The effective averaging time during the linewidth measurements was about 100 s.

Narrow-Line and Frequency Tunable Diode Laser System for S–D Transition of Ca+ Ions

A new type of diode laser is used for the development of an extremely narrow linewidth and frequency-tunable clock laser system for a Ca+ ion optical frequency standard. The required reduction of linewidth was achieved by locking the laser to an ultrahigh-finesse ultralow-expansion glass (ULE) reference cavity. The long-term frequency drift is reduced by stabilizing a cavity temperature at the point that the thermal expansion coefficient of the ULE is zero. As a result, the laser linewidth is decreased to lower than 25 Hz and the long-term drift is less than 0.025 Hz/s. The other laser offset locked to the stabilized laser is continuously frequency tunable in the entire free spectral range of the reference cavity without degradation of stability and linewidth. This laser system is actually applied to optical frequency standard development and used for the observation of the 2S1/2–2D5/2 transition of Ca+ ions.

Optical frequency standards

The evolution of atomic frequency standards since Essen's atomic clock fifty years ago has been considerable both in respect of microwave and optical standards. In particular, the development of trapping and laser cooling techniques for both atoms and ions has played a major role. This paper reviews the status of the development of single cold trapped ion and cold trapped atom optical frequency standards. Recent results show comb measurements of trapped ion optical frequency standards with accuracies close to Cs fountain limited operation. The factors affecting future stability and reproducibility are discussed. The opportunities for future standards capable of approaching reproducibility at the 10−18 level are outlined, together with the likely limitations arising.

Optimum measurement strategies for trapped ion optical frequency standards

The experimental parameters required in order to maximize the stability of a single-ion based optical clock are analysed. This stability is a trade-off between on the one hand the steepness of the discriminant signal derived from the laser excitation of the ion and on the other the binomial noise associated with the excitation of a single particle. Optimum conditions are found for both Rabi and Ramsey excitation of the clock transition.

Trapped Ion Optical Frequency Standards

Weak transitions in single cold trapped ions are highly reproducible as optical frequency standards on account of the high quality factors and stabilities achievable. In recent years, there has been a quickening pace in the development of these optical frequency standards, particularly due to the emergence of femtosecond laser combs for relating the optical frequency directly to the Cs standard. This paper reports progress on optical frequency standards based on the quadrupole transition in a single cold ion of 88Sr+ and the octupole transition in a single cold ion of 171Yb+.

Trapped ion optical frequency standards

Optical frequency standards based on narrow absorptions in laser-cooledsingle trapped ions have recently begun to demonstrate stabilities that arecompetitive with cold atom fountain microwave standards. This paperpresents a short review of the wider state-of-the-art development of thesesingle cold trapped ion frequency standards, coupled with a more detailedaccount of recent results achieved at National Physical Laboratory inrespect of single ion systems based on 88Sr+, 87Sr+ and 171Yb+.Narrow linewidth data for the optical clock quadrupole and octupoletransitions respectively at 674 nm in 88Sr+and 467 nm in 171Yb+, arepresented, together with a discussion of current systematics and futureprojections. The potential for optical clock operation is outlined.