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We proposed a new primary calibration method of a laser Doppler vibrometer (LDV) using the optical modulated excitation of an electro-optical modulator and validated this approach by comparing mechanical modulated excitation with homodyne laser interferometer at the National Metrology Institute of Japan (NMIJ). The velocity sensitivity of the LDV was evaluated with the uncertainty budget at a high-frequency range up to 1 MHz, which is indicated as the upper limit of 50 kHz in ISO (International Organization for Standardization) 16063–41. As the results, we confirmed that velocity sensitivities between optical and mechanical modulated excitations are flat within their uncertainty from 100 Hz up to 1 MHz. Moreover, in order to evaluate the accurate phase shift of the accelerometer up to 20 kHz using optical modulated excitation, we also revealed the reliability of two time shifts between homodyne and heterodyne laser interferometers.

Angle sensors based on the laser autocollimation are often employed to evaluate surface profiles of a target of interest. The authors have developed a femtosecond laser angle sensor, in which a spectrometer or an optical spectrum analyzer with a single-mode fiber is employed as the photodetector for simultaneous capturing of the multiple optical modes. In this paper, the concept of the femtosecond laser angle sensor is applied to evaluate the surface profile of a target of interest. An optical setup is designed in such a way that each mode in the spectrum of the mode-locked femtosecond is utilized as the laser beam to measure the local slope of a measurement target at each different point to evaluate the surface profile. Some basic experiments are carried out by using the developed optical setup with a mode-locked femtosecond laser source to evaluate basic performances of the developed optical setup as an optical angle sensor.

We propose and demonstrate a novel technique for precisely and widely sweeping the frequency of a continuous-wave laser. One of the modulation sidebands of a slave laser generated with an electro-optic modulator is phase-locked to a master laser; in this situation, the slave carrier component can be swept by sweeping the modulation frequency. It does not require beat signal detection at varying and/or high frequency, thus providing a robust and reliable laser frequency sweep. Also, it requires neither a frequency comb for the sweep nor a large power loss. We successfully swept an 852 nm laser over 20 GHz; we confirmed that a second harmonic 426 nm laser could be continuously swept over 40 GHz.

Frequency-stabilized lasers at the telecom wavelengths are of great interest for a wide range of applications, including wavelength standards for optical communication systems [1], and an absolute frequency marker of an optical frequency comb. Especially in astro-comb application [2], it is considered to be a good candidate of an optical frequency reference laser since it has a good frequency connection to a fiber-based frequency comb.

We report an uncertainty evaluation of an $^{171}$Yb optical lattice clock with a total fractional uncertainty of $3.6\times10^{-16}$, 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 $\mathrm{6s^{2}\,^{1}S_{0}-6s6p\,^{3}P_{0}}$ 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 $a_{\mathrm{Z}}$ 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 $\mathrm{6s6p^{3}P_{0}-6s8p^{3}P_{0}}$, $\mathrm{6s8p^{3}P_{2}}$, and $\mathrm{6s5f^{3}F_{2}}$. The obtained values are $d=-1.1(4)$ $\mathrm{\mu}$Hz and $a_{\mathrm{Z}}=-6.6(3)$ Hz/mT$^{2}$. These improved coefficients should reduce the total systematic uncertainties of Yb lattice clocks at other institutes.

A terahertz (THz) frequency synthesizer based on photomixing of two near-infrared lasers with a sub-THz to THz frequency offset is a powerful tool for spectroscopy of polar gas molecules due to its broad spectral coverage; however, its frequency accuracy and resolution are relatively low. To tune the output frequency continuously and widely while maintaining its traceability to a frequency standard, we developed a photomixing THz synthesizer phase-locked to dual optical frequency combs (OFCs). While the phase-locking to dual OFCs ensured continuous tuning within a spectral range of 120 GHz, in addition to the traceability to the frequency standard, use of a broadband uni-traveling carrier photodiode for photomixing enabled the generation of CW-THz radiation within a frequency range from 0.2 to 1.5 THz. We demonstrated THz frequency-domain spectroscopy of gas-phase acetonitrile CH3CN and its isotope CH3 13CN in the frequency range of 0.600–0.720 THz using this THz synthesizer. Their rotational transitions were assigned with a frequency accuracy of 8.42 × 10−8 and a frequency resolution of 520 kHz. Furthermore, the concentration of the CH3CN gas at 20 Pa was determined to be (5.41 ± 0.05) × 1014 molecules/cm3 by curve fitting analysis of the measured absorbance spectrum, and the mixture ratio of the mixed CH3CN/CH3 13CN gas was determined to be 1:2.26 with a gas concentration of 1014–1015 molecules/cm3. The developed THz synthesizer is highly promising for high-precision THz-FDS of low-pressure molecular gases and will enable the qualitative and quantitative analyses of multiple gases.

We demonstrate the third harmonic generation of a 1542-nm laser using a dual-pitch periodically poled lithium niobate waveguide with a conversion efficiency of 66%/W2. The generated 514-nm light is used for saturation spectroscopy of molecular iodine and laser frequency stabilization. The achieved laser frequency stability is 1.1×10−12 at an average time of 1 s, which is approximately one order of magnitude better than the acetylene-stabilized laser at 1542 nm. Uncertainty evaluation and absolute frequency measurement are also performed. The developed frequency-stabilized laser can be used as a reliable frequency reference at the telecom wavelength for various applications including optical frequency combs and precision interferometric measurement.

This paper presents the development of a frequency comb using a mode-locked fiber laser as the light source for precision dimensional metrology. The mode-locked laser generates an optical pulse train with a central wavelength of approximately 1560 nm. The pulse train was amplified with an erbium-doped fiber amplifier, and its spectrum was broadened to the wavelength range from 1000 nm to 2000 nm. Furthermore, the carrier envelope offset beats were successfully detected by using a common-path f-2 f interferometer.

We have developed an 8-branch frequency comb using 3 stages of fiber couplers for frequency stabilization of multiple lasers. A Ti:Sapphire laser is frequency-stabilized to the comb and evaluated in regard to the frequency stability.

We demonstrate high-resolution spectroscopy of molecular iodine using the second-harmonic generation of a 1063 nm planar-waveguide external cavity diode laser. The hyperfine components of the R ( 38 ) 32 − 0 transition are observed with a signal-to-noise ratio of approximately 180 over a bandwidth of 1 kHz. The diode laser is frequency-stabilized to the observed hyperfine component a 10 and achieves a stability at a level of 10 − 13 for averaging time of 1 s. The absolute frequency of the a 10 hyperfine component of the R ( 38 ) 32 − 0 transition is determined as 564 010 092 161(6) kHz. The frequency-stabilized diode laser has various applications, including optical frequency combs and interferometric measurements.

We demonstrate an 8-branch Er:fiber frequency comb with seven application ports, which can be individually optimized for applications with different wavelengths. The beat between the comb and a cw laser has a signal-to-noise ratio exceeding 30 dB at a resolution bandwidth of 300 kHz. The 8-branch frequency comb is used to perform frequency locking for four repumping and lattice lasers, and the frequency measurement of two clock lasers of strontium and ytterbium optical lattice clocks. We have achieved reliable optical lattice clock operation, thanks to the stable frequency locking and measurement obtained by using the 8-branch frequency comb. The developed frequency comb is a powerful experimental tool for various applications, including not only optical lattice clocks, but also research on quantum optics that use many frequency-stabilized lasers.

We demonstrate a mode-locked Er:fiber laser with a compact fiber-pigtailed optical bench containing a free-space electro-optic modulator for high-speed feedback control. The achieved servo bandwidth of the beat frequency between a given comb component and a reference laser is approximately 850 kHz. The realization of a narrow-linewidth and highly stable frequency comb is confirmed by measuring both in-loop and out-of-loop beat frequencies. We also demonstrate the frequency measurement of two iodine-stabilized Nd:YAG lasers through the narrow-linewidth comb without stability degradation. The developed optical frequency comb can work continuously for weeks without maintenance.

The fine-structured spectrum of a terahertz (THz) frequency comb has been observed using asynchronous-optical-sampling THz time-domain spectroscopy with an extended time window covering multiple THz pulses. Fourier transformation of 10 consecutive THz pulses enables us to obtain the spectrum of THz comb mode having a linewidth of 25 MHz at intervals of 250 MHz. Further expansion of time window up to 100 consecutive THz pulses reduces the linewidth of THz comb mode down to 2.5 MHz while conserving interval of 250 MHz. The observed THz comb mode can be used as a precise and accurate frequency marker for broadband THz spectroscopy because it is phase-locked to a microwave frequency standard by laser control. The spectroscopy of pharmaceutical tablets and low-pressure molecular gas is used to demonstrate the utility of this approach, indicating a spectral resolution of 250 MHz. The proposed method enables frequency calibration of the THz spectrometer based on a microwave frequency standard.

We carry out sub-Doppler laser spectroscopy of hyperfine components of rovibrational transitions of molecular iodine at 578 nm using second harmonic generation of an 1156-nm external-cavity diode laser. The absolute frequencies of the hyperfine components can be determined by using an optical frequency comb to referenced to the Coordinated Universal Time of the National Metrology Institute of Japan. The hyperfine components at 578 nm can be good frequency references for the 1S o −3P o clock transition in atomic ytterbium.

We demonstrate a diode laser system operating at 399 nm that is stabilized to the ${\rm 6s^{2}\ {^1}S_{0} - 6s6p\ {^1}P_{1}}$ electric dipole transition in ytterbium (Yb) atoms in a hollow-cathode lamp. The frequency stability of the laser reached $1.1 \times 10^{-11}$ at an averaging time of $\tau = 1\ \mathrm{s}$. We performed an absolute frequency measurement using an optical frequency comb and determined that the absolute frequency of the laser stabilized to the ${\rm {^1}S_{0} - {^1}P_{1}}$ transition in $^{174}\mathrm{Yb}$ was 751 526 522.26(9) MHz. We also investigated several systematic frequency shifts while changing some of the light source parameters and measured several isotope shifts. The measured laser frequency will provide useful information regarding the practical use of the frequency-stabilized light source at 399 nm., Comment: 14 pages, 6 figures, 1 table

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.

We had reported in a previous paper the precision measurement of line profiles of a transition of 13 C acetylene in the near-infrared region, employing a tunable diode-laser spectrometer, the frequency of which is locked to an optical-comb signal [K.M.T. Yamada et al., J. Mol. Spectrosc. 249 (2008) 95–99]. In the present article we review the methodology for the experiment and the analysis in determining the Boltzmann constant from the Doppler width of a rovibrational transition on the basis of our experiences. Problems to be surmounted for further improvement in accuracy are discussed. To cite this article: K.M.T. Yamada et al., C. R. Physique 10 (2009).

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.

Line profiles of a rovibrational transition of 13C acetylene have been measured for various pressures in the near infrared region. In order to accomplish high precision in frequency, we have employed a diode-laser, the frequency of which is locked to an optical comb. By tuning the comb frequency we have achieved a continuous frequency tuning over 2 GHz for the measurement of Doppler broadened line profiles spread over 2 GHz. In addition we have stabilized the incident power of the laser on the sample cell by adjusting the gain of a fiber amplifier via a feed-back loop. Observed profile data have been analyzed by the generalized Voigt function to determine the Gaussian width precisely: at the present we realized a precision of 10 - 3 . The zero pressure line center position was determined with a precision of 10 - 9 .

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

The spectral linewidth of a 3.28 μm difference-frequency-generation source has been reduced to 3.5 kHz using a laser linewidth transfer technique [Opt. Express21, 7891 (2013)]. We use an optical frequency comb with a broad servo bandwidth to transfer a narrow linewidth of a pump laser, a 1.06 μm Nd:YAG laser, to a signal laser, a 1.57 μm external-cavity laser diode. This source enables us to record the Lamb dip of the ν3 band R(2) E transition of methane with a molecular spectral linewidth of 21 kHz while the frequency axis is absolutely calibrated.

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy. In this paper, we have demonstrated adaptive-sampling THz-DCS, allowing the use of unstabilized dual femtosecond lasers.

Absolute frequency of continuous-wave terahertz radiation was determined at an acuracy of 10−11 in real time by modulating a frequency spacing of photocarrier terahertz comb induced by a free-running femtosecond laser.

We constructed a THz frequency synthesizer based on photomixing of two continuous-wave lasers phase-locked to dual optical combs, and applied it for precision spectroscopy of molecular gas at low pressure.

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use. To overcome this limitation, here we have demonstrated adaptive-sampling THz-DCS, allowing the use of free-running femtosecond lasers. To correct the fluctuation of the time and frequency scales caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing clock signal in a data acquisition board. The results not only indicated the successful implementation of THz-DCS with free-running lasers but also showed that this configuration outperforms standard THz-DCS with stabilized lasers due to the slight jitter remained in the stabilized lasers.

A compact iodine-stabilized laser at 531 nm is developed with a frequency stability at the 10−12 level using a coin-sized diode laser module. This laser will be used for applications including interferometric measurements.

The adaptive sampling clock was extracted by dual THz-comb-referenced spectrum analyzers and used for THz dual comb spectroscopy (THz-DCS) with unstabilized dual lasers. The demonstrated results implied better spectroscopic performance than THz-DCS with stabilized lasers.

Frequency-control characteristics of a mode-locked erbium-fiber laser with an intracavity optically pumped ytterbium-fiber were investigated. The fixed frequency of the comb when the pump power to the ytterbium-fiber is changed varies with the power.

Real-time measurement of the absolute frequency of continuous-wave terahertz (CW-THz) waves is required for characterization and frequency calibration of practical CW-THz sources. We proposed a method for real-time monitoring of the absolute frequency of CW-THz waves involving temporally parallel, i.e., simultaneous, measurement of two pairs of beat frequencies and laser repetition frequencies based on dual THz combs of photocarriers (PC-THz combs) with different frequency spacings. To demonstrate the method, THz-comb-referenced spectrum analyzers were constructed with a dual configuration based on dual femtosecond lasers. Regardless of the presence or absence of frequency control in the PC-THz combs, a frequency precision of 10-11 was achieved at a measurement rate of 100 Hz. Furthermore, large fluctuation of the CW-THz frequencies, crossing several modes of the PC-THz combs, was correctly monitored in real time. The proposed method will be a powerful tool for the research and development of practical CW-THz sources, and other applications., Comment: 27 pages

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.

We have evaluated an ultra-stable laser system, which is based on the linewidth transfer method. In this system, a Nd:YAG laser at 1064 nm stabilised to a high finesse Fabry- Perot etalon is employed as a master laser. Beat signals between the high-speed controllable optical frequency comb locked to the master laser and another ultra-stable laser operated at 1.5 μm are observed to investigate the frequency stability. The frequency stability measured by the fractional Allan deviation is about 2 × 10−15 at 1 s averaging time, which almost coincides with the estimated thermal noise limit on the optical cavity. Furthermore, using the system, the 1S0-3P0 transitions in ytterbium and strontium atoms are observed. We have demonstrated that linewidth transfer technique based on the optical frequency comb combining with the narrow linewidth laser serves as a useful ultra-stable local oscillator for optical clocks.

Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low-pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5 MHz and enhanced spectral accuracy of 8.39 × 10(-7) in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.

An accurate, continuously tunable, terahertz synthesizer was proposed by photomixing of two continuous-wave lasers phase-locked to dual optical combs. This synthesizer enables us to perform the precise THz spectroscopy secured by microwave frequency standard.

We present experimental results of the frequency stability measurement of an external cavity blue diode laser stabilized with the saturated absorption signal of the 1S0–1P1 transition of Yb at 398.9 nm. Frequency stability and reproducibility of the stabilized laser were evaluated by using an optical frequency comb. Allan standard deviation of 5.4×10-12 with an averaging time of 256 s is achieved. The results show that our laser system has long-term stability and good performance for laser cooling and trapping experiments of Yb atom.

An Er-fiber laser comb with repetition frequency of 250 MHz made by the Center for Measurement Standards (CMS), Taiwan was shipped to the National Metrology Institute of Japan (NMIJ) to compare with the NMIJ fiber laser comb by simultaneously measuring the frequency of an optical cavity-stabilized Nd:YAG laser at 1064 nm. The measured average frequency difference is 32 mHz and corresponds to a relative difference of 1.1×10-16, which is limited by the measurement time. Furthermore, method for determining the comb mode number using two combs with large difference of repetition rate is proposed.

We propose employing an ultra-low thermal expansion ceramic for long cavity spacers that can reduce thermal noise effect on ultra-stable optical cavities relatively, since the high-rigidity ceramic should be insensitive to vibration. Thermal expansion properties are investigated using a prototype ceramic cavity. Simulation results show that responses of a long ceramic cavity to the vertical acceleration are similar to that of a short ultra-low expansion (ULE) glass cavity. Linewidth transfer technique based on an optical frequency comb combining with the narrow linewidth laser stabilized to the ceramic cavity would serve as a useful ultra-stable local oscillator for optical clocks.

We propose a novel, high-performance, and practical laser source system for optical clocks. The laser linewidth of a fiber-based frequency comb is reduced by phase locking a comb mode to an ultrastable master laser at 1064 nm with a broad servo bandwidth. A slave laser at 578 nm is successively phase locked to a comb mode at 578 nm with a broad servo bandwidth without any pre-stabilization. Laser frequency characteristics such as spectral linewidth and frequency stability are transferred to the 578-nm slave laser from the 1064-nm master laser. Using the slave laser, we have succeeded in observing the clock transition of (171)Yb atoms confined in an optical lattice with a 20-Hz spectral linewidth.

We report absolute frequency measurements of 81 hyperfine components of the rovibrational transitions of molecular iodine at 578 nm using the second harmonic generation of an 1156-nm external-cavity diode laser and a fiber-based optical frequency comb. The relative uncertainties of the measured absolute frequencies are typically $1.4\times10^{-11}$. Accurate hyperfine constants of four rovibrational transitions are obtained by fitting the measured hyperfine splittings to a four-term effective Hamiltonian including the electric quadrupole, spin-rotation, tensor spin-spin, and scalar spin-spin interactions. The observed transitions can be good frequency references at 578 nm, and are especially useful for research using atomic ytterbium since the transitions are close to the $^{1}S_{0}-^{3}P_{0}$ clock transition of ytterbium.

Laser frequency stabilization and atom manipulation are important techniques for optical frequency metrology. We report on the development of an ytterbium (171Yb) and a strontium (87Sr) optical lattice clock at the National Metrology Institute of Japan (NMIJ). The result of absolute frequency measurement of the 171Yb lattice clock is shown. We also report on narrow-linewidth fiber-based frequency combs that are used not only to measure the absolute frequency of the optical lattice clocks but also to servo control the frequency of the lasers for cooling and trapping Yb and Sr atoms.

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 report on the development of an ytterbium (171Yb) and a strontium (87Sr) optical lattice clock at the National Metrology Institute of Japan (NMIJ). The result of absolute frequency measurement of the 171Yb lattice clock is shown. We also report on narrow-linewidth fiber-based frequency combs that are used not only to measure the absolute frequency of the optical lattice clocks but also to servo control the frequency of the lasers for cooling and trapping Yb and Sr atoms.

A continuously tunable, phase-locked, THz synthesizer has been demonstrated. This THz synthesizer is based on photomixing of two optical frequency synthesizers composed of CW laser and optical comb, and its output frequency has the same uncertainty as that of a microwave frequency standard used.

The present status of the development of an 87Sr optical lattice clock at NMIJ is reported. We also describe our frequency doubling system with a periodically poled lithium niobate waveguide for the first stage cooling laser at 461 nm.

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.

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 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 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.

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 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.