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A light source to drive the (1)S0-(3)P0 transition in Yb atoms is generated by 2 solid state lasers: a Nd:YAG laser and an 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 zero-expansion temperature of a Fabry-Pérot etalon was determined by using a home-made fiber-based optical frequency comb running continuously for weeks. Frequency stabilization of the clock laser system was also evaluated by the optical frequency comb. more...

A fiber-based frequency comb has been developed to measure the frequency of a 633-nm iodine-stabilized laser at the National Metrology Institute of Japan (NMIJ). The measured frequency was consistent with the previous results measured with a Ti:sapphire-based comb. The NMIJ comb was shipped to Australia for the validation of the measurement capability of the National Measurement Institute, Australia (NMIA) and NMIJ combs using a common microwave reference and a common optical frequency. Consequently, the frequency consistency of the two combs was approximately 8 times10-17. Furthermore, we demonstrate that an absolute mode number of the comb can easily and clearly be determined by using the two combs. more...

The authors have demonstrated optical frequency synthesis using a fiber-based frequency comb system, referenced to an ultrastable microwave oscillator. The oscillator is based on a high Q-factor cryogenic sapphire resonator cooled with liquid helium. The 100-MHz signal synthesized from the 10.8-GHz oscillation frequency of the cryogenic sapphire oscillator (CSO) was used to stabilize the repetition frequency of the mode-locked fiber laser. When the synthesized optical frequency was compared with a rubidium two-photon stabilized laser at 778 nm, the measured fractional frequency stability was ~6 10-14 tau -12 for the averaging times tau between 1 and 100 s, and the best frequency stability was 3.0 10-15 for an averaging time of 1280 s more...

We are developing a highly stable optical-microwave frequency link system using an optical frequency comb. We consider the stability degradation factors in the frequency link system to be the synthesis, phase locking, and photo detection system of the repetition frequency. We evaluated these factors by undertaking phase noise measurements and we estimated the stability from these results. These estimations were consistent with experimental results. more...

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. more...

A comparison of two independent absolute optical frequency measurements has been carried out between the National Metrology Institute of Japan/National Institute of Advanced Industrial Science and Technology (NMIJ/AIST), Tsukuba, Japan and the JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, CO, using a portable iodine-stabilized Nd:YAG laser. The agreement between the two absolute measurements is better than the measurement uncertainty of 6.7/spl times/10/sup -13/ that can be attributed to the reproducibility limitations of the portable laser. This comparison is used to confirm the measured absolute frequency of an iodine-stabilized Nd:YAG laser at NMIJ/AIST (Y3), which is reported to the Consultative Committee for Length (CCL) for the determination of the absolute frequency value of iodine-stabilized Nd:YAG lasers. more...

We succeeded in observing one of the absorption lines of acetylene [P(16)] using an erbium-doped fiber-ring laser, which is the first such observation using a fiber-ring laser. The observed linewidth of the P(16) linear absorption was approximately 450 MHz. Mode competition and mode hopping were suppressed by using an ultra narrow-band fiber Bragg grating and a Fabry-Perot filter in the ring laser, respectively. Wavelength fine-tuning was carried out by changing the cavity length of the fiber-ring laser; the fine-tuning range was about 850 MHz (0.01 nm). Furthermore, wavelength tuning was achieved by controlling the temperature of the fiber Bragg grating, and the coarse timing range was about 0.2 mn. more...

A novel and compact Er-Yb:glass laser was developed with single-frequency output power >10 mW in a wavelength range from 1531 to 1547 nm. The laser output was efficiently frequency doubled at 770.1 nm in a single-pass periodically poled lithium niobate waveguide and a second harmonic power in excess of 15 /spl mu/W was achieved, with an internal conversion efficiency of /spl sim/220% W/sup -1/. This frequency-doubled beam was used for high-resolution saturated spectroscopy of the /sup 39/K 4S/sub 1/2 /-4P/sub 1/2 / absorption line. The best resolved crossover peak showed 16% line contrast and 10-MHz linewidth. In a second experiment, working at the fundamental frequency, saturation spectroscopy of /sup 13/C/sub 2/H/sub 2/ was achieved using a low-pressure gas cell inside a build-up cavity. The measured absorption dip has 3% line contrast and 1.2 MHz linewidth. Both the atomic and molecular saturated lines are very well suited for frequency locking in order to establish accurate frequency standards in the optical communication band. more...

We have established a compact and transportable I/sub 2/-stabilized Nd:YAG laser for international comparisons of laser frequency. The root Allan variance of the portable laser has reached 3.9/spl times/10/sup -14/ when the integration time is longer than 200 s. The results of an international comparison between the National Research Laboratory of Metrology (NRLM), Tsukuba, Japan and the JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, CO, USA show that the frequency difference of the portable laser NRLM-Y1 and the JILA laser JILA-W (f/sub NRLM-Y1/-f/sub JILA-W/) was -2.5 kHz, when the cold-finger temperatures of NRLM-Y1 and JILA-W were kept at -10/spl deg/C and -15/spl deg/C, respectively. The averaged frequency offset between two NRLM lasers (f/sub NRLM-Y1/-f/sub NRLM-Y2/) was -1.1 kHz. A frequency variation of about 1.2 kHz was found for the frequency offset between two NRLM lasers, after NRLM-Y1 was taken for a round trip to Sydney for a comparison organized by the National Measurement Laboratory, (NML), Australia. more...

We have made a frequency standard operating at the 1.5 /spl mu/m region using the saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/. To enhance the moderate power from a laser diode and to saturate a very weak molecular overtone transition, we used a Fabry-Perot (FP) cavity and inside of it we put an acetylene cell. In order to obtain a longer life time for the cell, we designed a new scheme consisting of a cavity with a ULE (ultralow expansion) glass spacer and a baked and sealed-off cell with Brewster windows. We observed the saturated spectrum having a 1 MHz spectral linewidth. The signal obtained using the same cell ten months later had the same signal-to-noise ratio (SNR) and spectral linewidth. We made two versions of the frequency standard and observed a beat note between these two lasers from which we derived the frequency stability, in terms of the square root of the Allan variance /spl sigma/(/spl tau/)=2.4/spl times/10/sup -13/ at an integration time /spl tau/=1000 s. more...