We have successfully generated a new broadband coherent light source in the continuous wave (CW) regime which is an ensemble of multi-harmonic radiations (2403, 1201, 801, 600 and 480 nm) by implementing a frequency dividing technology. The system is uniquely designed that all the harmonics are generated and propagate coaxially which gives the advantage of robustly maintaining the phase coherence among the harmonics. The highlight is its huge potential for the arbitrary optical waveform synthesis in the CW regime which has not been performed yet due to the limitation of the existing light source.
KEYWORDS: Chemical species, Clocks, Laser stabilization, Ytterbium, Optical lattice clocks, Fiber lasers, Frequency combs, Signal to noise ratio, Strontium, Spectroscopy
We report the current status of our ytterbium optical lattice clock at the NMIJ, AIST. After the first measurement of the
clock transition frequency and the estimation of the uncertainty, we have been improving our clock. For an increased
signal to noise ratio of the observed spectrum, we employed an atom number normalization scheme. We stabilized the
frequency of the lattice laser using a fiber-based optical frequency comb. We also stabilized the intensity of the lattice
laser.
We have developed a one-dimensional optical lattice clock with ultracold 171Yb atoms. The absolute frequency of the
1S0(F = 1/2) - 3P0(F = 1/2) clock transition in 171Yb is determined to be 518 295 836 590 864(28) Hz with respect to the
SI second. Details of the experimental setups and atom trapping results are also described.
The recent development of optical frequency standards has been performed quite rapid and the better uncertainty than
that of microwave frequency standards will be realized in very near future. We are evaluating a one-dimensional 87Sr
optical lattice clock developed and located at the University of Tokyo, Tokyo, by using UTC(NMIJ) generated at NMIJ,
Tsukuba. The baseline length between those two sites is about 50 km. We constructed a time and frequency transfer link
using GPS carrier phase method for this link. We use GIPSY software and a newly developed one for data analysis. Our
developed one works in real time using carrier phase data and broadcast navigation data which are obtained from the
carrier phase receivers.
The present status of the development of the Yb optical lattice clock at NMIJ/AIST and future prospects are presented.
Experimental equipments such as vacuum systems and laser sources are explained in detail.
A phase-shifting interferometer (PSI) with equal phase steps, using a frequency-tunable diode laser and a Fabry-Perot cavity, is proposed for the Carre algorithm. The measurement accuracy of the Carre algorithm depends on the equality of the phase steps. Using the Fabry-Perot cavity as a highly stable optical frequency reference, a high degree of phase step equality can be realized in the PSI with an optical frequency shift. Our experimental scheme realizes an optical frequency step equality higher than 2.1×10-5 and a measurement repeatability of λ/850.
An Rb-stabilized diode laser has been developed for use in a high-precision interferometer. The light source is a commercially available external-cavity tunable diode laser. The laser frequency is stabilized to a Doppler-free absorption line of Rb by the third-harmonic technique. The laser emits an output beam with a high power (more than 7 mW) and fast frequency modulation (10 kHz). The relative optical frequency uncertainty of 4.3×10–10 is achieved for a 0.01-s averaging time.
We have developed a gauge block measurement system that uses three frequency-stabilized lasers. The stabilized lasers are as follows: an I2 stabilized offset locked He-Ne laser, an I2-stabilized Nd:YAG laser, and a Rb-stabilized diode laser. The I2-stabilized offset locked He-Ne laser is commercially available and its relative wavelength uncertainty is 2.5 X 10-11. An I2-stabilized Nd:YAG laser and a Rb-stabilized diode laser was developed in our institute and their relative wavelength uncertainties are 5 X 10-12 and 1 X 10-9, respectively. In the measurement system, laser beams were introduced to the interferometer using an optical multimode fiber. An interferometer fringe pattern was taken using a CCD camera and the excess fraction parts were calculated from the fringe pattern using the Fourier transform method. The excess fraction part obtained from the Rb-stabilized semiconductor laser was used only to determine the integer part of the fringe order, because the accuracy and stability of the wavelength were not sufficient for the long gauge block measurements. This interferometer can measure gauge blocks of up to 1000 nm long and the standard uncertainty of the interferometer is about 75 nm for a 1000 mm long gauge block.
The use of an optical frequency comb generated by an ultrafast mode-locked laser has been realized as a promising method of the direct comparison between microwave and optical frequencies. We are currently investigating frequency control of a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser. We stabilized the pulse repetition rate frep to a rf synthesizer locked to a cesium (Cs) clock to the Allan deviation of 4 X 10-12 in 1 s. We found that the position of the crystal, rotation of the chirped mirrors, and change of the pump-laser power can be used in controlling the carrier-envelope offset frequency fCEO. We extended the span of the comb to over one octave, i.e., from 530 nm to 1190 nm, at -20 dB using a photonic-crystal fiber made at the University of Bath. We are currently trying to measure the frequency of an iodine-stabilized Nd:YAG laser using a floating ruler of a f:2f frequency interval chain. We detected the self-referencing beat between the fundamental and second- harmonic frequencies of the comb, which will enable further precise comparison between microwave and optical frequencies through the control of the fCEO.
We have established four I2-stabilized Nd:YAG lasers to verify the frequency reproducibility of the lasers. The observed square root Allan variance of the four lasers was between 1 to approximately 4 X 10-14 depending on the obtained signal-to-noise ratio of the spectra, when the integration time is larger than 300 s. The observed frequency reproducibility of each laser was ranged from 9.1 X 10-14 approximately 1.5 X 10-13 (corresponding to frequency uncertainties of +/- 51 approximately 87 Hz). Frequency reproducibility of a group of lasers (four NRLM lasers) has been evaluated to be 8.2 X 10-13 (corresponding to a frequency uncertainty of +/- 640 Hz). One of the four NRLM lasers is a compact I2- stabilized Nd:YAG laser which is suitable to be transported to other laboratories for international frequency comparisons. Using this portable laser, we have accomplished frequency comparisons of Nd:YAG lasers between several metrological institutes in different countries. The absolute optical frequencies of the NRLM lasers were determined with an uncertainty of about 1.5 kHz by the frequency comparison between the NRLM and the JILA (formerly the Joint Institute for Laboratory of Astrophysics), Boulder, CO.
We have established a portable I2-stabilized Nd:YAG laser for the purpose of making wavelength standards at 532 nm and 1064 nm. All the optical parts of the laser systems were arranged on a 45 cm X 45 cm breadboard. The system was transported from NRLM to JILA for frequency comparison. The results of the comparison show that the Allan Variance of the portable laser reached < 3 X 10-13 when the integration time (tau) is larger than 100 s. The frequency differences between the NRLM and JILA lasers during 3-day measurements were consistent within +/- 35 Hz, but the matrix-averaged standard deviation of about 310 Hz, and offset are regarded as not yet fully satisfactory. The stability of the portable laser was further improved to about 3 X 10-14 by using a longer iodine cell and several frequency stabilization techniques.
The hot-band oscillation of CO2 laser with saturable absorber shows new features in passive Q-switching instability. A proposed model is applicable to the nonlinear dynamic process including vibration-to-vibration energy transfer which influence the passive Q- switching instability. The vibrational rate constant is determined for the first time from the analysis of the passive Q-switching instability.
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