Search

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. America OCIS codes: 120.6200, 120.3930.

A phase-shifting interferometry (PSI) with equal phase steps by use of 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 PSI with an optical frequency shift. Our experimental scheme realizes an optical frequency step equality higher than 5.1 x [10.sup.-5] and a measurement repeatability of [lambda]/800. OCIS codes: 120.3180, 050.2230, 120.5050.

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 new definition of the kilogram based on the Planck constant was implemented in May 2019. The National Metrology Institute of Japan (NMIJ) will realize the new kilogram by the X-ray crystal density method using a 28Si-enriched sphere. An international comparison of the realizations of the new kilogram by national metrology institutes was organized. As a participant in the comparison, the NMIJ realized the kilogram by the volume measurement and surface characterization of a 28Si-enriched sphere. Details of the measurement results and the uncertainty evaluation of the realization are described. The comparison of the realization results with previous realization results in 2016 is also provided. [ABSTRACT FROM AUTHOR]

The kilogram based on the Plank constant will be realized at the National Metrology Institute of Japan (NMIJ) by using the X-ray crystal density method. In this study, for the realization, the volume of a 28Si-enriched sphere, named AVO28-S5c, was measured. The volume was determined from diameter measurements taken in many directions using an optical interferometer. The apparent volume of the sphere, which does not consider the influence of the surface layers on the volume measurement, was determined with a relative standard uncertainty of $1.8\times 10^{-8}$. The reproducibility of the kilogram realization is important for the reliable dissemination of the kilogram at the NMIJ. Therefore, the volume determined in this study was compared with that determined in 2016. These volumes were shown to agree with the uncertainty. This article provides details on the measurements, improvements of the interferometer, data analysis, and uncertainty evaluation. [ABSTRACT FROM AUTHOR]

We describe frequency noise measurements for an optical frequency comb by using a delayed self-heterodyne method with a Michelson-type fiber-optic delay line interferometer without a low-noise reference laser. We measured the frequency noise power spectral densities (PSDs) for free-running and frequency-stabilized comb modes and estimated the uncertainties of the measurement results. For example, for the frequency-stabilized comb, the measured frequency noise PSD and its uncertainty were 23 dBHz2 Hz−2 and 0.66 dB, respectively, at a Fourier frequency of 10 kHz. We also measured the frequency noise PSDs of the comb modes by the conventional method using an ultrastable reference laser and compared the results with those that we measured with the delayed self-heterodyne method. The measurement results that we obtained with the two methods were consistent within their uncertainties, which shows that the delayed self-heterodyne method provides high reliability. This combined with its simplicity and user-friendliness suggest that the method has the potential to become a standard frequency noise measurement approach for frequency combs. [ABSTRACT FROM AUTHOR]

A continuously tunable, phase-locked, single-frequency, continuous-wave (cw) terahertz generator has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band unitraveling-carrier photodiode (UTC-PD) used as a photomixer in this study. This cw-terahertz generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser in the optical frequency region, phase locked to two independent optical combs. The continuous tuning range of the presented method was three orders of magnitude around 0.1 THz and two orders around 1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD. The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency metrology. [ABSTRACT FROM AUTHOR]

For a determination of the Avogadro constant by the X-ray crystal density method, the volume of a Si sphere manufactured from a Si crystal isotopically enriched in 28Si, named Si28-23Pr11, was measured at the National Metrology Institute of Japan (NMIJ). The volume was determined on the basis of diameter measurement in 2175 directions by an optical interferometer. The surface layers on the sphere were characterized by X-ray photoelectron spectroscopy and spectroscopic ellipsometry. Taking into account the effect of the surface layers on the diameter measurement, the volume of the Si core excluding the surface layers was determined with a relative uncertainty of $2.0\times 10^{\mathbf {-8}}$. The results were used to determine the Avogadro constant in 2017 by the International Avogadro Coordination project, which was used as an input datum to determine the CODATA 2017 adjusted value of the Planck constant $h$. This $h$ value will be used in the new definition of the kilogram. This paper provides details of the measurement apparatus and the uncertainty analysis of the Si core volume measurement. In addition, an approach to reduce the uncertainty in the Si core volume determination is proposed for the accurate realization of the kilogram based on the new definition at NMIJ. Details of the approach are also provided. [ABSTRACT FROM AUTHOR]

Phase noise induced by environmental disturbance was greatly reduced when the optical fiber was installed in an ultrastable environment. In this paper, the optical fiber length was 10 m, and the reduction in the phase noise power spectral density was approximately 70 and 30 dB at Fourier frequencies of 10 mHz and 10 Hz, respectively. To evaluate faint fiber noise, we developed a modified Mach–Zehnder heterodyne interferometer with reduced optical path length fluctuation. The Allan deviation calculated from the phase noise was $7 \times 10^{-20}$ at a 1-s averaging time and reached $2 \times 10^{-21}$ at a 10 000-s averaging time (measurement bandwidth: 500 Hz). In addition, we revealed the frequency bands in which fiber noise is reduced by selectively removing the environmental stabilization measures of evacuation, vibration isolation, acoustic shielding, and temperature stabilization from an ultrastable environment. [ABSTRACT FROM AUTHOR]

We report an uncertainty evaluation of an 171Yb optical lattice clock with a total fractional uncertainty of $3.6\times 10^{-16}$ , which is mainly limited by the lattice-induced light shift and the blackbody radiation shift. Our evaluation of the lattice-induced light shift, the density shift, and the second-order Zeeman shift is based on an interleaved measurement where we measure the frequency shift using the alternating stabilization of a clock laser to the 6s2 1S0–6s6p 3P0 clock transition with two different experimental parameters. In the present evaluation, the uncertainties of two sensitivity coefficients for the lattice-induced hyperpolarizability shift $d$ incorporated in a widely used light shift model by RIKEN and the second-order Zeeman shift $a_{Z}$ are improved compared with the uncertainties of previous coefficients. The hyperpolarizability coefficient $d$ is determined by investigating the trap potential depth and the light shifts at the lattice frequencies near the two-photon transitions 6s6p3P0–6s8p3P0, 6s8p3P2, and 6s5f3F2. The obtained values are $d=-1.1(4)\,\,{\mu }$ Hz and $a_{Z}=-6.6(3)$ Hz/mT2. These improved coefficients should reduce the total systematic uncertainties of Yb lattice clocks at other institutes. [ABSTRACT FROM AUTHOR]

In the determination of the Avogadro constant using the X-ray crystal density method with a 28Si-enriched crystal prepared by the International Avogadro Coordination (IAC) project, the volume measurement and surface analysis of 28Si spheres play a crucial role. For accurate volume determination, an optical interferometer has been improved taking account of the geometrical shape of the optical components. Furthermore, a new spectroscopic ellipsometer with a sphere rotation system has been developed for accurate surface analysis. The optical interferometer and the ellipsometer have been used for the volume determination and surface analysis of the 28Si spheres repolished by the IAC, respectively. On the basis of the preliminary uncertainty estimation, the relative standard uncertainty of the apparent volume measurement has been reduced from 4.4 \times 10^-8 to 2.7 \times 10^-8 by the improvement of the interferometer. The number of ellipsometric measurement points on the sphere surface has been increased from 20 to 7782 by the installation of the new ellipsometer, yielding more reliable information on the sphere surface. Details of the improvements and the preliminary results of the measurements of the 28Si spheres are given. [ABSTRACT FROM PUBLISHER]

Two techniques for terahertz (THz) frequency metrology based on frequency comb, namely, a THz-comb-referenced spectrum analyzer and a continuously tunable, single-frequency continuous-wave (CW)-THz generator, are reviewed. Since the frequency comb enables to coherently link the frequency among microwave, optical, and THz regions, it is possible to establish the THz frequency metrology traceable to time of the SI base units. Using a THz-comb-referenced spectrum analyzer based on a stable THz comb generated in a photoconductive antenna for THz detection, the absolute frequency of CW test sources in the sub-THz and THz regions was determined at a precision of 10^-11. Furthermore, a continuously tunable, single-frequency CW-THz generator was demonstrated around 120 GHz by photomixing of an accurately tunable CW laser and a tightly fixed CW laser in the optical frequency region, phase locked to two independent optical combs. The combination of the CW-THz generator with the THz-comb-referenced spectrum analyzer will open the door for establishment of frequency metrology in the THz region. [ABSTRACT FROM PUBLISHER]

A light source to drive the ¹S0-³P0 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. [ABSTRACT FROM AUTHOR]

Abstract: We report a method for optimizing the amplification of femtosecond optical pulses by using dispersion management. The amount of dispersion provided to the seed optical pulse of an erbium-doped fiber (EBF) has an optimal region that enhances the output power of an amplifier. The power enhancement is accompanied by spectral broadening, which originates from adiabatic narrowing in the erbium-doped fiber. The amplified optical pulses can be used to generate an octave-spanning optical frequency comb (OFC) by employing a highly nonlinear fiber (HNLF). [Copyright &y& Elsevier]

We 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 τ-1/2 for the averaging times τ between 1 and 100 s, and the best frequency stability was 3.0 × 10-15 for an averaging time of 1280 s. [ABSTRACT FROM AUTHOR]

Abstract: Doppler-broadened atomic and molecular spectra were observed with a one octave tunable, continuous-wave, doubly resonant, monolithic optical parametric oscillator (OPO) using 5% MgO-doped LiNbO3 as a non-linear crystal with a birefringent phase-matching configuration. By tuning the frequency of a pump laser, longitudinal mode selection over 20 successive modes, corresponding to a 60GHz span, was possible, owing to the simple structure of the monolithic OPO. Continuous frequency tuning was achieved using an external waveguide-type electrooptic phase modulator (EOM). By changing the modulation frequency of the EOM, frequency tuning of the optical sidebands over 12GHz was possible, which is larger than the one free spectral range of the monolithic cavity of 3GHz. We could observe the Cs-D1 (894nm), Cs-D2 (852nm), Rb-D1 (795nm), acetylene R9 (1520nm) and P9 (1530nm) transitions with the single monolithic OPO. [Copyright &y& Elsevier]

A low-threshold, single-frequency fiber laser was achieved without feedback control. This laser was a narrow linewidth fiber ring laser operating in the 1.55-μm regime and pumped by 1.48-μm InGaAsP laser diodes. Mode-hopping instabilities and multiwavelength oscillation due to spatial and polarization hole burning were eliminated by the use of an ultra-narrowband fiber Bragg grating and a polarization-dependent isolator. The laser exhibited stable single-frequency operation at all pump powers. The maximum output power was 1 mW, and the linewidth was less than 20 kHz. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 82(5): 21–29, 1999 [ABSTRACT FROM AUTHOR]