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We have been developing a new clock laser system for Yb and Sr optical lattice clocks. The system is based on an infrared laser with a narrow linewidth, and a fiber-based frequency comb transferring the linewidth to required plural wavelengths. We demonstrate that fiber-based frequency combs with a multi-branch configuration can transfer laser linewidth to another wavelength at the millihertz level. An intra-cavity electro-optic modulator is employed to obtain a broad servo bandwidth for repetition rate control.

Experimental efforts to reduce the uncertainty of the ytterbium (171Yb) optical lattice clock at NMIJ, AIST are shown. Especially, atom number normalization scheme is presented.

We have been developing a new clock laser system for an Yb optical lattice clock. The system does not include a high finesse cavity at 578 nm, but a fibre-based frequency comb, which is tightly phase-locked to a narrow linewidth Nd:YAG laser at 1064 nm. The Nd:YAG laser is phase locked to a high finesse cavity at 1064 nm made from ultra low expansion glass.

A frequency-stabilized light source emitting at 556 nm is realized by frequency-doubling a 1112-nm laser, which is phase-locked to a fiber-based optical frequency comb. The 1112-nm laser is either an ytterbium (Yb)-doped distributed feedback fiber laser or a master-slave laser system that uses an external cavity diode laser as a master laser. We have achieved the continuous frequency stabilization of the light source over a five-day period. With the light source, we have completed the second-stage magneto-optical trapping (MOT) of Yb atoms using the 1S0 - 3P1 intercombination transition. The temperature of the ultracold atoms in the MOT was 40 uK when measured using the time-of-flight method, and this is sufficient for loading the atoms into an optical lattice. The fiber-based frequency comb is shown to be a useful tool for controlling the laser frequency in cold-atom experiments., 18 pages, 6 figures, submitted to and accepted by J. Opt. Soc. Am. B (ID 125081)

We demonstrate that fiber-based frequency combs with multi-branch configurations can transfer both linewidth and frequency stability to another wavelength at the millihertz level. An intra-cavity electro-optic modulator is employed to obtain a broad servo bandwidth for repetition rate control. We investigate the relative linewidths between two combs using a stable continuous-wave laser as a common reference to stabilize the repetition rate frequencies in both combs. The achieved energy concentration to the carrier of the out-of-loop beat between the two combs was 99% and 30% at a bandwidth of 1 kHz and 7.6 mHz, respectively. The frequency instability of the comb was 3.7x10(-16) for a 1 s averaging time, improving to 5-8x10(-19) for 10000 s. We show that the frequency noise in the out-of-loop beat originates mainly from phase noise in branched optical fibers.

We have developed a one-dimensional optical lattice clock using a fermionic isotope of ytterbium (171Yb). The absolute frequency of the 1S0 – 3P0 clock transition in 171Yb is determined with respect to the SI second.

We measure the absolute frequency of 12 sub-Doppler lines of the $P(6)$ and $P(7)$ transitions in the ${\ensuremath{\nu}}_{3}$ band of ${^{12}\text{C}\text{H}}_{4}$ using difference-frequency-generation (DFG) of $1.064\text{\ensuremath{-}}\ensuremath{\mu}\text{m}$ Nd:YAG and $1.55\text{\ensuremath{-}}\ensuremath{\mu}\text{m}$ DFB lasers and a fiber-based frequency comb referenced to International Atomic Time. The relative uncertainties are typically $9.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$, and the determined value for the $P(7)$ ${F}_{2}^{(2)}$ line agrees with the International Committee for Weights and Measures recommendation with a discrepancy of $(1.2\ifmmode\pm\else\textpm\fi{}8.4)\text{ }\text{kHz}$. The combination of the DFG spectrometer and the frequency comb enables determining energy levels as accurately as microwave spectroscopy.

We propose a fiber-based, terahertz-comb-referenced spectrum analyzer which has the advantages of being a portable, alignment-free, robust, and flexible apparatus suitable for practical use. To this end, we constructed a 1550-nm mode-locked Er-doped fiber laser whose mode-locked frequency was stabilized precisely by referring to a rubidium frequency standard, and used it to generate a highly stable terahertz (THz) frequency comb in a photoconductive antenna or an electro-optic crystal. By standardizing the THz comb, we determined the frequency accuracy of an active-frequency-multiplier-chain (AFMC) source to be 2.4 x 10(-11). Furthermore, the potential of the THz spectrum analyzer was effectively demonstrated by real-time monitoring of the spectral behavior of the AFMC source and a photomixing source of two free-running CW lasers at adjacent wavelengths.

We proposed a fiber-based, asynchronous optical sampling THz time-domain spectroscopy system by combination of mode-locked Er-doped fiber lasers and photoconductive antennas. The proposed system will become a fast, compact, alignment-free, robust, and flexible apparatus without any mechanical moving parts.

We present arbitrary single-frequency continuous-wave (CW) terahertz (THz) generator based on photomixing of a scanned CW laser and a fixed CW laser, which are locked to two independent optical combs, respectively. Behavior of tight locking and continuous tuning of its output frequency is monitored at real time by a THz-comb-referenced spectrum analyzer.

We have developed a one-dimensional optical lattice clock with ultracold 171 Yb atoms. The absolute frequency of the 1 S 0 ( F = 1/2) - 3 P 0 ( F = 1/2) clock transition in 171 Yb 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.

We have developed a fiber-based frequency comb system consisting of a simple mode-locked fiber laser and a backward pumping amplifier combined with a highly nonlinear fiber with a short zerodispersion wavelength. As a result, the signal to noise ratio of the obtained carrier-envelope-offset frequency beat is larger than 45 dB at a bandwidth of 100 kHz. Furthermore, we have succeeded in measuring the optical frequencies of a 1542-nm acetylene-stabilized laser and a 532-nm iodinestabilized Nd:YAG laser continuously for more than one week using the fiber-based comb system. The long-term measurement revealed that the frequency stability of the iodine-stabilized laser was 5.7 x 10(-15) with 100 000 s averaging.

A light source to drive the 1S 0 - 3P 0 transition in Yb atoms is generated by two solid state lasers: a Nd:YAG laser and a Yb:YAG laser, using a sum-frequency generation (SFG) scheme. With a ridge waveguide (WG) periodically poled lithium niobate (PPLN) device, SFG power of about 150 mW is obtained at the required frequency. The performance of the clock laser system is evaluated with studies of frequency stability measured by an optical frequency comb and with atomic absorption spectra of the 171Yb clock transition at 578 nm.

A stable light source obtained using sum-frequency generation (SFG) is developed for high-resolution spectroscopy at 578 nm. Hyperfine transitions of molecular iodine are observed by using the SFG light source with saturation spectroscopy. The light source is frequency stabilized to the observed hyperfine transition and achieves a stability of 2*10-12 for a 1-s averaging time. The absolute frequency of the light source stabilized on the a1 component of the R(37)16-1 transition is determined as 518304551833 (2) kHz. This transition serves as a frequency reference for the 1S0 - 3P0 optical clock transition in neutral ytterbium (Yb)., Comment: 8 pages

We demonstrate a precision frequency measurement using a phase-stabilized 120-km optical fiber link over a physical distance of 50 km. The transition frequency of the 87Sr optical lattice clock at the University of Tokyo is measured to be 429228004229874.1(2.4) Hz referenced to international atomic time (TAI). The measured frequency agrees with results obtained in Boulder and Paris at a 6*10^-16 fractional level, which matches the current best evaluations of Cs primary frequency standards. The results demonstrate the excellent functions of the intercity optical fibre link, and the great potential of optical lattice clocks for use in the redefinition of the second., 14 pages, 3 figures

A light source to drive the 1S0-3Po transition in Yb atoms is generated by two solid state lasers: a Nd:YAG laser and a Yb:YAG laser, using a sum-frequency generation (SFG) scheme. SFG power of about 22 mW is obtained at the required frequency.

We have magneto-optically trapped cold ytterbium atoms using an intercombination transition for realizing an optical lattice clock. For this purpose, we have developed a laser-diode-based light source, which is phase-locked to an optical frequency comb, thus attaining both laser frequency stabilization and laser linewidth narrowing.

A monolithic-ring-cavity Yb:YAG laser is frequency doubled and used to perform Doppler-free spectroscopy of molecular iodine at 515 nm. The Allan standard deviation of the iodine-stabilized Yb:YAG laser is better than 6.2times10-14 for a 10-s averaging time, which is limited by the stability of the H-maser involved in the frequency measurement using an optical comb.

We have been developing a spectrometer to record an accurate absorption line profile of a rovibrational transition of acetylene in the 1.54 mum region using an extended cavity laser diode (ECLD) and a mode-locked fiber frequency comb. With the spectrometer, we have recorded the absorption line profiles for various pressures and have observed the collisional narrowing effect on the inhomogeneous Gaussian widths. Information of the Gaussian width which reflects the Maxwell-Boltzmann distribution can be used to estimate accurate temperature of the sample gas and also have possibility to provide a way to determine the Boltzmann constant.

An optimized method for amplifying femtosecond pulses by using dispersion management is reported. An amplifier with an optimal amplification region enhances the output power with spectral broadening by adiabatic narrowing in an EDF.

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.

We report a compact optical distance meter using intermode beats of a fiber-delivered frequency comb. We realized micrometer agreement over hundreds of meters to a length-measuring laser interferometer which is the reference standard at NMIJ/AIST.

We performed broadband dual-frequency-comb spectroscopy in the near-infrared region with a much higher resolution than the Fourier limit by using discrete Fourier transforms and spectral interleaving. We observed the resonant spectrum of a Fabry-Perot cavity over a spectral range of 187 to 218 THz using this technique, and measured its free spectral ranges and finesses. The recorded spectrum includes cavity resonance lines with widths of about 2 MHz, which is much narrower than the resolution of 48 MHz determined by the observation time window.

We present an all-optically stabilized, erbium-doped mode-locked fiber laser with an optically pumped ytterbium-doped fiber. The mode-locked fiber laser has two frequency-control actuators that are pump laser powers for erbium-doped and ytterbium-doped fibers. We investigate the frequency-control characteristics of the mode-locked laser and find that the fixed points for the two actuators are sufficiently apart from each other, realizing the simultaneous phase locking of the repetition and carrier envelope offset frequencies. We describe a long-term frequency measurement of an acetylene-stabilized laser at 1542 nm using an all-optically stabilized frequency comb.

We propose simple schemes to phase-lock the carrier envelope offset frequency (fceo) referring to the repetition rate (frep) of an optical frequency comb. We demonstrate the locking of fceo such that fceo = (1/2)frep, (1/3)frep, and (2/3)frep. The Allan deviation and signal-to-noise ratio of the coherent δ-function peak for the in-loop beat signal are 5.3 × 10−17/τ and 80–85 dBHz, respectively, where τ is the averaging time of the frequency measurement. These new locking schemes simplify the sign and mode-number determination in frequency measurements.

We demonstrate a compact iodine-stabilized laser operating at 531 nm using a coin-sized light source consisting of a 1062-nm distributed-feedback diode laser and a frequency-doubling element. A hyperfine transition of molecular iodine is observed using the light source with saturated absorption spectroscopy. The light source is frequency stabilized to the observed iodine transition and achieves frequency stability at the 10(-12) level. The absolute frequency of the compact laser stabilized to the a(1) hyperfine component of the R(36)32 - 0 transition is determined as 564074632419(8) kHz with a relative uncertainty of 1.4×10(-11). The iodine-stabilized laser can be used for various applications including interferometric measurements.

We have carried out dual-comb spectroscopy and observed in a simultaneous acquisition a 140-THz-wide spectrum from 1.0 to 1.9 {\mu}m using two fiber-based frequency combs phase-locked to each other. This ultra-broad wavelength bandwidth is realized by setting the difference between the repetition rates of the two combs to 7.6 Hz using the sub-Hz-linewidth fiber combs. The recorded spectrum contains five vibration-rotation bands of C${_2}$H${_2}$, CH${_4}$, and H${_2}$O at different wavelengths across the whole spectrum. The determined transition frequencies of C${_2}$H${_2}$ agree with those from the previous sub-Doppler resolution measurement of individual lines using CW lasers within 2 MHz., Comment: 13 pages, 4 figures, 1 table

A continuous-wave (cw) optical frequency synthesizer is demonstrated by using a monolithic-type cw optical parametric oscillator (cw-OPO) and an optical frequency comb. The cw-OPO is phase locked to an optical frequency comb that is phase locked to an atomic clock. The output frequency of the cw-OPO is frequency shifted with an electro-optic modulator, which makes it possible to tune the frequency continuously over 10 GHz. Furthermore, Doppler-free spectroscopy is performed using the optical frequency synthesizer for a cesium D1 line at 895 nm. The observed linewidth of 5 MHz is the natural linewidth of cesium. The center frequency of the line is consistent with a previous report.

We present an arbitrary optical single-frequency generator based on a femtosecond optical frequency comb. The absolute frequency accuracy is around 1kHz at 282THz, the stability is 2·10-14at 100s and the tuning speed exceeds 30GHz/s.

We present a displacement metrology setup based on a Fabry-Perot cavity and a tunable optical single-wavelength generator based on a femtosecond optical frequency comb. The setup provides sub-pm (sub-10 pm) stability over 1(30) minute(s), respectively.

We demonstrated the phase-locking and continuous tuning capabilities of a continuous-wave, monolithic optical parametric oscillator (cw-OPO). At first, the cw-OPO was phase-locked to an optical frequency comb in the 830 nm region. The residual phase noise under phase-locking was 220 mrad in rms and the energy concentration to the carrier was 95%. Next, the signal light from the cw-OPO was phase-modulated by an external electrooptic modulator (EOM) in the microwave frequency. The single sideband was extracted by using a diffraction grating and a spatial filter. We could observe the Doppler broadened absorption and fluorescence profile of the Cs-D/sub 2/ line by tuning the signal wavelength to 852 nm and by scanning the microwave modulation frequency. We are trying the sub-Doppler spectroscopy of several atoms and molecules with the phase-locked cw-OPO and these results will also be presented.

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.

The carrier envelope phase of a polarization-maintaining fiber frequency comb laser is stabilized for long periods of time using near orthogonal fast and slow controls of carrier envelope phase and repetition rate.

The repetition rate and carrier-envelope phase offset frequencies of a turnkey, all-fiber-based continuum generator were phase locked to a hydrogen maser. The frequency of the hydrogen maser was calibrated with a highly stable cesium atomic clock, and therefore a fully phase-locked optical frequency comb with well-defined absolute frequencies was obtained. In contrast with the commonly used Ti:sapphire-laser-based systems, we have accomplished a fully turnkey system with no user-adjustable parts. To evaluate the performance of this novel system, we performed absolute frequency measurements in the telecommunications region and at 1064 nm and compared them with our traditional Ti:sapphire-based comb.

Absolute frequencies of an acetylene-stabilized laser at 1542 nm and an iodine-stabilized Nd:YAG at 1319 nm are measured at their second harmonics by use of a femtosecond optical comb based on a mode-locked Ti:sapphire laser. A frequency measurement system based on a mode-locked fiber laser (1.56mum) is also being developed for metrological applications in the telecommunication bands

We are developing a highly stable optical-microwave frequency link system. We consider the stability-degradation factors in the frequency link system to be the synthesis, the phase lock and the photo detection system of the repetition frequency. We evaluated these factors by undertaking phase noise measurements

The absolute frequency of an acetylene-stabilized laser at 1542 nm is measured at its second harmonic (771 nm) by use of a femtosecond optical comb based on a mode-locked Ti:sapphire laser. Frequency stability and reproducibility of the acetylene-stabilized laser are evaluated by the femtosecond comb with a H maser as a frequency reference. The absolute frequency of a laser diode stabilized on the P(16) transition of 13C2H2 is determined to be 194 369 569 383.6(1.3) kHz. The acetylene-stabilized laser serves as an important optical frequency standard for telecommunication applications.

A simple rubidium stabilized diode laser has been developed for a gauge block interferometer. The laser light source is a commercially available external-cavity tunable diode laser (New Focus Inc.). Laser frequency stabilization is realized by the third-harmonic technique, where a fast frequency modulation (10 kHz) is applied on the current. The absolute laser frequency was calibrated by an absolute optical frequency measurement system using a femtosecond mode-locked laser. The relative uncertainty of the laser frequency reached 4.3×10 -10 for an averaging time of 0.01 s. The phase error in the interferometeric measurement due to the optical frequency modulation is theoretically indicated to be small enough to measure long gauge blocks of up to 1000 mm. Long gauge block measurement of up to 1000 mm was successfully demonstrated using the developed rubidium stabilized diode laser and the I 2 -stabilized offset locked He-Ne laser (633 nm).

We correct the errors in the uncertainty budget. The determined ratio (νYb/νSr) is corrected to be 1.207 507 039 343 341 2(17) with a fractional standard uncertainty of 1.4 × 10−15.

An external cavity diode laser near 633 nm with Littrow configuration is constructed. With this laser and third order derivative technique, we have observed the iodine saturation absorption spectrum of P33 (6-3). The effect of different modulation width has been given.

In this paper, a preliminary result for an iodine frequency stabilized tunable diode lasers at 633 nm is reported. The frequency stability of the diode laser was 2 X 10 -11 , the tune ability was 10 nm and the line-width was 100 kHz. In addition, five groups of stronger hyperfine transitions of iodine molecule, which are available to be reference lines for diode laser frequency standard, beside the well-known transition R(127)11-5 at 633 nm region were observed by means of saturated absorption in the configuration of external iodine cell and their absolute frequencies were measured by a spectrum analyzer and a scanning Fabry-Perot cavity. The program for calculating the hyperfine structures and strength of transitions for rotational vibrational bands of iodine molecule was developed and the predictions for possible transitions around 633 nm including the transition strength are given.

We perform spectroscopic observations of the 698-nm clock transition in $^{87}$Sr confined in an optical lattice using a laser linewidth transfer technique. A narrow-linewidth laser interrogating the clock transition is prepared by transferring the linewidth of a master laser (1064 nm) to that of a slave laser (698 nm) with a high-speed controllable fiber-based frequency comb. The Fourier-limited spectrum is observed for an 80-ms interrogating pulse. We determine that the absolute frequency of the 5s$^{2}$ $^{1}$S$_{0}$ - 5s5p $^{3}$P$_{0}$ clock transition in $^{87}$Sr is 429 228 004 229 872.0 (1.6) Hz referenced to the SI second., Comment: 5 pages, 4 figures

A synthetic-wavelength interferometry of optical frequency combs is proposed for the pulse-to-pulse alignment in absolute distance measurement. The synthetic wavelength derived from the virtual second harmonic and the real second harmonic is used to bridge the interference intensity peak-finding method and the heterodyne interferometric phase measurement, so that the pulse-to-pulse alignment can be linked directly to single-wavelength heterodyne interferometry. The experimental results demonstrate that the distance measured by the peak-finding method with micrometer accuracy can be improved to the nanometer level by applying the method proposed.

We have developed a Fabry–Pérot etalon with an ultralow expansion ceramic spacer and investigated the thermal properties of the ceramic. We found that the ceramic cavity has a “zero crossing temperature” at room temperature where the thermal expansion coefficient crosses zero. The sensitivity of the thermal expansion coefficient of the ceramic to temperature change is about five times greater than that of ultralow expansion (ULE) glass at around the zero cross temperature. Nevertheless, owing to the high specific rigidity and relatively low thermal expansion at room temperature, the ceramic should be an attractive material for long optical cavities that can reduce the effect of the thermal noise relatively.