Jongchul Mun

Absolute frequency measurement of 1S0(F = 1/2)–3P0(F = 1/2) transition of 171Yb atoms in a one-dimensional optical lattice at KRISS

We measured the absolute frequency of the optical clock transition 1S0 (F = 1/2) - 3P0 (F = 1/2) of 171Yb atoms confined in a one-dimensional optical lattice and it was determined to be 518 295 836 590 863.5(8.1) Hz. The frequency was measured against Terrestrial Time (TT; the SI second on the geoid) by using an optical frequency comb of which the frequency was phase-locked to an H-maser as a flywheel oscillator traceable to TT. The magic wavelength was also measured as 394 798.48(79) GHz. The results are in good agreement with two previous measurements of other institutes within the specified uncertainty of this work.

The uncertainty associated with the weighted mean frequency of a phase-stabilized signal with white phase noise

The weighted mean is widely used in combining data sets of experimental measurements with a weight proportional to the value of the data number divided by the sample variance in a conventional method. However, this standard procedure is not appropriate for obtaining the weighted mean frequency of a phase-stabilized signal with white phase noise, since the data are autocorrelated. The autocorrelation is obtained in the case of white phase noise and a new weighting method is proposed. Using this, the uncertainty associated with the weighted mean frequency of a phase-stabilized signal with white phase noise is given. The effect of counter dead-time is also discussed.

Linewidth reduction of a distributed-feedback diode laser using an all-fiber interferometer with short path imbalance

The linewidth of a distributed-feedback (DFB) diode laser at 1156 nm, of which free-running linewidth was 3 MHz, was reduced to 15 kHz using an all-fiber interferometer with 5-m-long path imbalance. Optical power loss and bandwidth limitation were negligible with this short optical fiber patch cord. This result was achieved without acoustic and vibration isolations, and the frequency lock could be maintained over weeks. In addition to its simplicity, compactness, robustness, and cost-effectiveness, this technique can be applied at any wavelength owing to the availability of DFB diode lasers and fiber-optic components.

Cancellation of Phase Noise in 1.4 GHz RF Signal Transferred to a Remote Site through 13 km Fiber

A fiber-phase-noise compensating system was constructed for a 1.4 GHz reference frequency transferred through a 13-km-long fiber spool. The transfer instability was dependent on the temperature variation of the compensating system. With the room temperature variation stabilized within 0.3℃, the transfer instability was 4.6×10

Improvements of an 171 Yb optical lattice clock at KRISS

The development of an Yb optical lattice clock at the Korea Research Institute of Standards and Science (KRISS) is reported with current systematic uncertainty of 2.9×10-16. A highly stable clock laser at 578 nm was developed with a short-term linewidth of 3.5 Hz. The laser was locked to the clock transition and the fractional frequency stability was 2.0×10-15 at 1s. Collisional frequency shift for Rabi spectroscopy was theoretically and experimentally investigated and the uncertainty was greatly reduced to 2.9×10-17. We identified conditions to further reduce the collisional shift uncertainty to 10-18 level.

The statistical uncertainty associated with the weighted mean frequency in optical frequency comb comparison

The weighted mean is widely used in combining data sets of experimental measurements with a weight proportional to the value of the data number divided by the sample variance in a conventional method. However, this standard procedure is not appropriate for obtaining the weighted mean frequency of a phase-stabilized signal with white phase noise, since the data are autocorrelated. The autocorrelation is obtained in the case of white phase noise and a new weighting method is proposed. Using this, the uncertainty associated with the weighted mean frequency of a phase-stabilized signal with white phase noise is given. The effect of counter dead-time is also discussed. The result is significantly different from the conventional method, so that the weight is proportional to the square of data number divided by the sample variance, and as a result the uncertainty of the estimated mean is proportional to the sample standard deviation divided by the data number. In an optical frequency comb comparison, frequency combs are phase-locked to a common frequency reference and the frequency difference between optical frequency combs shows white phase noise properties. Thus, it is expected that this new weighting method can be utilized in frequency comparisons of optical frequency combs, settling existing controversy, and providing a way of achieving lower uncertainty.

Development of 171 Yb optical lattice clock at KRISS

We developed an ¹⁷¹Yb optical lattice clock and measured the absolute frequency of the optical clock transition ¹S₀ (F = 1/2) - ³P₀ (F = 1/2). The frequency was determined at 518 295 836 590 865.7 (9.2). The uncertainty is equivalent to 1.8×10^-14 relatively. The magic wavelength of lattice laser was founded at 394 798.18(79) Hz.

Linewidth Reduction of a Yellow Laser by a Super-cavity and the Measurement of the Cavity Finesse

Sum frequency generation was utilized to obtain a yellow laser with the wavelength of 578.4 nm for a probe laser of an Yb lattice clock. The output of an Nd:YAG laser with wavelength of 1319 nm and that of an Yb-fiber laser with wavelength of 1030 nm were passed through a waveguided periodically-poled lithium niobate (WG-PPLN) for sum frequency generation. It is required that the probe laser has a linewidth of the order of 1 Hz to fully resolve the Yb lattice clock transition. Thus, the linewidth of the probe laser was reduced by stabilizing the frequency to a super-cavity. This was made of ULE with a low thermal expansion coefficient, and was mounted on an active vibration-isolation table at the optimal point for the reduced sensitivity to vibration. Also, this was installed in a vacuum chamber, and the temperature was stabilized to 1 mK level. This system was installed in an acoustic enclosure to block acoustic noise. The finesse of the super-cavity was measured to be 380 000 from the photon life time of the cavity.