Yasuki Koga

Development of an optically pumped Cs frequency standard at the NRLM

Describes development of an optically pumped Cs frequency standard aiming at a primary frequency standard. Using this laboratory-type standard, a Ramsey resonance spectrum was observed. It has been shown that a laser power of about 1 mW is almost sufficient for two-frequency optical pumping. It has also been shown that the C-field strength can be measured using Zeeman coils by establishing a population difference among the Zeeman sublevel with sigma /sup +/ polarized laser light. >

Spectral width of saturated absorption spectra of Cs with a laser diode

The saturated absorption spectra and line widths of the Cs-D 2 line were observed as a function of the incident power. A minimum spectral width of 20 MHz was obtained when the incident power was under 0.1 mW and all of the hyperfine spectral lines and the cross-over resonance lines were resolved.

Phase locking of a continuous-wave optical parametric oscillator to an optical frequency comb for optical frequency synthesis

A continuous-wave optical parametric oscillator (OPO) was phase locked to an optical frequency comb in the 830-nm region. The optical frequency of the OPO was controlled by changing the cavity length of the pump laser. The residual phase noise under phase locking was 220 mradrms and the energy concentration to the carrier was 95%. Furthermore, the optical frequency fluctuations of a free-running OPO were measured by using an optical frequency comb that was phase locked to an atomic clock. The measured fluctuations were around 10 MHz in an hour.

Frequency measurements and hyperfine structure of the R(85)33–0 transition of molecular iodine with a femtosecond optical comb

Absolute frequency measurements of the R(85)33‐0 transition of molecular iodine at the blue end of the tuning range of a frequency-doubled Nd:YAG laser are implemented with a femtosecond optical comb based on a mode-locked Ti:sapphire laser. The hyperfine structure of the R(85)33‐0 transition is observed by use of high-resolution laser spectroscopy and is measured by the femtosecond optical comb. The observed hyperfine transitions are good frequency references for both frequency-doubled Nd:YAG and Nd:YVO4 lasers in the 532-nm region. High-accuracy hyperfine constants are obtained by our fitting the measured hyperfine splittings to a four-term Hamiltonian, which includes the electric quadrupole, spin‐rotation, tensor spin‐spin, and scalar spin‐spin interactions.

Light shifts in an optically pumped Cs beam frequency standard

Frequency shifts caused by light, which are called light shifts in an optically pumped Cs beam frequency standard, were estimated. Frequency shifts due to monolithic light were measured by introducing laser light along the Cs beam. The relative dependence of the shift on the laser frequency agreed very well with the theory, but the absolute shift was between one and two times that of the theory. The light shifts due to the optical pumping and optical detection in the standard are estimated to be less than 2*10/sup -15/ and 1*10/sup -16/, respectively, and both are negligible at the present state of development. >

Characteristics of an optically pumped Cs frequency standard at the NRLM

Characteristics of an optically pumped Cs frequency standard developed at the National Research Laboratory of Metrology (NRLM) are reported. The short-term frequency stability was estimated to be 1.1*10/sup -12// square root pi when the optical feedback technique was used for laser diode stabilization. Frequency shifts due to microwave power and pumping conditions were measured and their characteristics are described. >

Accuracy evaluation of the optically pumped Cs frequency standard at NRLM

The optically pumped cesium (Cs) frequency standard at the National Research Laboratory of Metrology, Ibaraki, Japan (NRLM), NRLM-4, was modified in various ways in order to improve frequency stability and accuracy, We determined the frequency stability by comparing the phase with that of a hydrogen maser: the Allan deviation was /spl sigma//sub y/(/spl tau/)=5/spl times/10/sup -13///spl radic//spl tau/. We estimated the frequency corrections and uncertainties due to various sources, and the total uncertainty was found to be 2.9/spl times/10/sup -14/. The frequency difference between NRLM-4 and the international atomic time (TAI) was also measured.

Recent Progress in Cs Beam Frequency Standards at the NRLM

Some causes and cures of frequency uncertainty in the cesium beam frequency standard, NRLM-II, are described. Accuracy reevaluation indicates about 2.2 × 10-13, and the standard frequency is about 8 × 10-14 below that of the International Atomic Time (TAI) scale. The frequency stability is estimated as better than 8 × 10-12 ¿-1/2 for short-term and 4 × 10-14 for 1-2 day averages.

A beam reversal experiment for the estimation of microwave power shifts in an optically pumped Cs beam frequency standard

The characteristics of microwave-power-dependent frequency shifts of an optically pumped Cs frequency standard are reported. The general features of the shifts are in good agreement with simple theoretical predictions. The shift is quite symmetrical in beam reversal, and the standard deviation of the difference between the observed and predicted shift is about 6*10-14. >

An ultrastable PLL mode-locked fiber laser with a hydrogen maser clock

We have greatly improved the stability of the repetition rate of a mode-locked fiber laser by using a phase-locked loop circuit with a hydrogen maser clock. The stability reached as high as 5.9 × 10-14for an averaging time of 100 s.