Using a laser that is frequency-locked to a Fabry-Perot etalon of high finesse and stability, we probed the 5d¹⁰6s ²S_1/2(F equals O, m_F equals O) $ARLR 5d⁹6s^{2 2}D_5/2 (F equals 2, m_F equals O) electric-quadrupole transition of a single laser-cooled ¹⁹⁹Hg⁺ ion stored in a cryogenic radio-frequency ion trap. We observed Fourier-transform limited linewidths as narrow as 6.7 Hz at 282 nm (1.06 X 10¹⁵ Hz). The functional form and estimated values of some of the frequency shifts of the ²S_1/2 $ARLR ²D_5/2 clock transition (including the quadrupole shift), which have been calculated using a combination of measured atomic parameters and ab initio calculations, are given.

A series of two searches of a frequency difference in the vibrational spectrum of the enantiomers of CHFClBr by using Fabry-Perot cavities is presented. No difference is observed within the present sensitivity of 10^-13. This experiment is limited by pressure shifts induced by uncontrolled impurities of the samples. We propose to use a two-photon Ramsey fringes scheme with a molecular beam to push the relative sensitivity much below 10^-14 in the range where the effect is expected.

Single laser-cooled ions in radiofrequency traps can serve as the basis for highly stable optical frequency standards. Here we present recent results on In⁺, using the ¹S₀ yields ³P₀ line at 236.5 nm wavelength as the clock transition. This resonance has a natural linewidth of only 0.82 Hz and systematic shifts should be controllable at the mHz level. A single indium ion is stored in a miniature Paul- Straubel trap and laser cooled to a temperature of about 100 (mu) K. The clock transition is excited using a frequency quadrupled 946 nm Nd:YAG laser. A fractional resolution of 1.3 (DOT) 10^-13 has been achieved so far (linewidth of 170 Hz at 1267 THz). The absolute optical frequency of the clock transition has been measured with an uncertainty being less than 3 (DOT) 10^-13 using a frequency chain and a methane-stabilized HeNe laser that was calibrated with a cesium clock.

We present the optical frequency measurements of the 2S-8S/D and 2S-12D two-photon transitions in hydrogen and deuterium. From an analysis taking into account these results and the very accurate measurements of the 1S-2S transition, we show that the optical frequency measurements have superseded the microwave determination of the 2S Lamb shift and we deduce optimized values for the Rydberg constant and for the 1S and 2S Lamb shifts. We report also the recent development of our 1S-3S experiment.

We constructed a widely tunable spectrometer with sub-Doppler resolution and high sensitivity in the 1.66-micrometer region using a Fabry-Perot cavity as an absorption cell and a low- power extended-cavity laser diode as a light source. The light electric field is enhanced at the antinodes of the standing wave in the cavity cell, which enabled observation of saturated absorption spectra of the molecular overtone bands even though their transition dipole moments are small. The spectrometer sensitivity was drastically enhanced using a frequency modulation technique. The attained sensitivity allowed us to reduce sample gas pressure, optical power, and modulation amplitude, which resulted in a resolution of 320 kHz. We applied the spectrometer to precise frequency measurements of the 2(nu) ₃-band transitions of methane, and determined 66 frequency differences between them and two absolute frequencies with an accuracy of 40 and 600 kHz, respectively. We also recorded hyperfine-resolved spectrum of the 2(nu) ₄ band of methyl iodide, which gave unambiguous assignments.

We report the recent progress of our work on the development of an optical frequency standard in the 1.5 micrometer region based on a Doppler-free acetylene absorption. We have developed compact and portable acetylene-stabilized diode lasers for practical use. We have realized the frequency stability and reproducibility of about 2 X 10^-13 and 2 X 10^-11 respectively using a third-harmonic signal detection. We have measured the absolute frequency of the acetylene transition with an uncertainty of about 10^-10 using a frequency chain based on a two-color mode-locked fiber laser. One of the important applications of the acetylene optical frequency standard is the WDM optical communication system, which requires precise absolute frequency references for a hundred of channel frequencies in the wavelength region of 1530 - 1565 nm. We have developed a wide-range heterodyne optical phase-locked diode laser using the acetylene-stabilized laser as a master in order to realize the WDM channel frequency standards.

A two-photon transition in cold Rb atoms will be probed with a phase-coherent wide-bandwidth femtosecond laser comb. Frequency domain analysis yields a high resolution picture where phase coherence among various transition pathways through different intermediate states produces interference effects on the resonantly-enhanced transition probability. This result is supported by the time domain Ramsey interference effect. The two-photon transition spectrum is analyzed in terms of the pulse repetition rate and carrier frequency offset, leading to a cold-atom-based frequency stabilization scheme for both degrees of freedom of the femtosecond laser.

We review here the historical development of an optical comb generation technology. In 1991, an optical comb as wide as 1 THz was generated for the first time for the purpose of the optical frequency measurement, and the difference frequency between two lasers was measured. The measured difference frequency was only 500 GHz at that time. However, measurable difference frequency has been increased by thousand times in nine years. Finally, absolute frequency measurement of laser has become possible.

The frequency of the intercombination line ³P₁ - ¹S₀ of Calcium at 657 nm is phase-coherently measured in terms of the output of a primary cesium frequency standard using an optical frequency comb generator comprising a < 10 fs Kerr-lens mode-locked Ti:Sapphire laser and an external microstructure fiber for self-phase-modulation. The measured frequency value of v_Ca equals 455 986 240 494 276 Hz agrees within its relative uncertainty of 4 X 10^-13 with the values previously measured with a traditional harmonic frequency chain and the value recommended for the realization of the SI unit of length. Furthermore, we investigate the coherence properties of the broadened frequency comb and find no indications for incoherent scattering processes in the microstructure fiber.

We describe in detail an optical clockwork based on a 1 GHz repetition rate femtosecond laser and silica microstructure optical fiber. This system has recently been used for the absolute frequency measurements of the Ca and Hg⁺ optical standards at the National Institute of Standards and Technology (NIST). The simplicity of the system makes it an ideal clockwork for dividing down high optical frequencies to the radio frequency domain where they can readily be counted and compared to the existing cesium frequency standard.

A single ion, trapped, laser cooled and probed on an ultra- narrow transition, provides what is widely considered the best approximation to an ideal, isolated optical frequency reference. Our group has been actively studying the 445-THz (674 nm), 5s²S_1/2-4d²D_5/2 transition in ⁸⁸Sr⁺. Individual Zeeman component linewidths of 250 Hz were observed and a probe laser system was locked to the center frequency of the Zeeman spectrum. A cesium-based frequency chain was used to measure the center frequency of the ⁸⁸Sr⁺ S-D spectrum to an accuracy of 200 Hz. Under our current experimental conditions, the magnitudes of the systematic shifts in the linecenter position are estimated to be less than 1 part in 10¹⁵. As part of our efforts in improving the trapped ion standard, we have studied the coherent excitation of a single ion via Rabi pulse and Ramsey fringe interrogation. Initial results yielding Ramsey fringe widths down to 840 Hz were obtained, and some decoherence and motional properties of the ion system were investigated. The Sr⁺ standard was applied recently to the well-known He- Ne/I₂ standard at 633 nm. This measurement provided an accuracy comparison between a traditional frequency chain and a femtosecond laser frequency comb. In addition, after several months and transportation over a significant portion of the globe, ⁸⁸Sr⁺ calibrated 633 nm lasers from NRC and BIPM have shown agreement to 1 kHz in their originally determined absolute optical frequencies. The results of these intercomparisons point to potential worldwide accuracy improvement of working 633 nm radiation standards.

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 f_rep 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 f_CEO. 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 f_CEO.

Two independent mode-locked femtosecond lasers are synchronized to an unprecedented precision. The rms timing jitter between the lasers is 4.3 fs observed within a 160 Hz bandwidth over tens of seconds, or 26 fs within a 50 kHz bandwidth. Novel multi-stage phase locked loops help to preserve this ultrahigh timing resolution throughout the entire delay range between pulses (10 ns). We also demonstrate that the same level of synchronization can be achieved with two lasers at different repetition frequencies. Under such synchronization, phase lock between the carrier frequencies of the two fs lasers has been achieved.

A laser that is frequency stabilized to the intercombination transition at (lambda) equals 657 nm of laser cooled ⁴⁰Ca atoms by means of time-domain atom interferences has been developed at PTB. The relative uncertainty of this optical frequency standard is 2.5 X 10^-13 and the frequency of the laser has been measured with both, a conventional frequency measurement chain and a femtosecond laser. To reduce the uncertainty of this standard the dependence of its frequency on various parameters was investigated by using phase sensitive and frequency sensitive atom interferometers. Upper bounds were derived for various contributions to the uncertainty as e.g. the influence of gravitational acceleration, collisions of the ballistic atoms, or curvature of the wave fronts of the interrogating laser beams. Further reduction of the uncertainty of the standard is expected from the application of sub-Doppler cooling techniques. A method was devised based on the narrow intercombination transition that enabled us to reduce the velocity spread of the Ca atoms in one dimension close to the recoil limit. The method uses the repeated selection and accumulation of slow atoms from the pre-cooled atomic cloud and the repeated rethermalization of the remaining atoms and results in an increased visibility of the interference structure.

Following a recommendation made by the Comite International des Poids et Mesures in 1983, the 17th Conference Generale des Poids et Mesures adopted a new definition of the metre based on the speed of light and introduced the 'mise en pratique,' a set of experimental procedure giving a list of recommended wavelengths and their associated uncertainties for the practical realization of the metre. At the time the most widely used radiation in the world for over ten years was that around 633 nm using an iodine-stabilized He-Ne laser. Results covering a 25-year period of international comparisons between lasers from forty-two national metrology institutes and those of the Bureau International des Poids et Mesures are presented. The information from these comparisons was used to establish the original mise en pratique of 1983 and their subsequent revisions in 1992 and 1997 incorporating the progress and improvements in techniques made in the interim. In 1997, all these comparisons were included in a program of key comparisons which were then assimilated into a key comparison database as required by the Mutual Recognition Arrangement (MRA) signed in Paris in 1999; the ongoing key comparison is designated BIPM.L-K10. Of seventy-two measured lasers from thirty-seven countries, only eleven lasers had their frequencies outside the (sigma) uncertainty given in the mises en pratique of 1992 and 1997. At a time when promising major developments are taking place in the field of optical frequency standards, it is an appropriate moment to summarize the present status of the practical realization of the metre in the international context.

An optical frequency standard, based on the 674 nm ²S_1/2-²D_5/2 'clock' transition in a laser cooled trapped strontium ion is currently being evaluated at the UK National Physical Laboratory. The probe laser is a narrowed AlGaInP diode laser locked to a highly-stable and ultra-low- expansion high-finesse cavity. Laser linewidths of less than 200 Hz have been observed, measured by scanning over a single Zeeman component at 674 nm. The development of this laser is described, together with factors currently limiting the observed linewidth. Three ⁸⁸Sr⁺ traps are now in operation, and the probe laser source can be simultaneously locked to the center of the ²S_1/2 - ²D_5/2 Zeeman multiplet in any two of the three traps, allowing a comparison between them. Recent results on this comparison are presented.

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.

Forbidden transitions in single trapped ion systems are being considered as references for future optical frequency standards. These standards are expected to have application as highly stable and reproducible optical clocks, realizations of the metre and as optical frequency standards in their own right. This paper prescribes the work carried out at the National Physical Laboratory over recent years to use a highly forbidden ²S_1/2 - ²F_7/2 467 nm electric octupole transition in a single ion of ¹⁷¹Yb⁺ as a frequency reference. A review of the previous measurements needed to the locate octupole transition in this 171-isotope is given and a more detailed discussion of recent work is presented. This includes spectroscopy of the octupole transition with kilohertz resolution and a direct measurement of its optical frequency. Measurements of the dynamic Stark and the quadratic Zeeman shifts on the octupole frequency are also discussed.

This paper explores the affect of phase modulation (PM), amplitude modulation (AM), and thermal noise on the rf spectrum, phase jitter, timing jitter, and frequency stability of femtosecond lasers and other precision sources. Using these concepts we can suggest how some noise aspects of femtosecond pulsed lasers should scale.

Clocks and oscillators have traditionally played an important role in space navigation and communication, as well as in space science and in tests of fundamental physical laws. Microwave clocks currently in use in space will be replaced by laser cooled clocks capable of significantly improved performance. Beyond this, lasers will play an important role as oscillators used for their spectral purity in interferometry based experiments, and will likely be used as the local oscillators for future optical clocks. In this paper a short review of the role of clocks and oscillators in space is presented, together with some future projections for laser based systems.

We review wavelength accuracy and calibration issues for wavelength division multiplexed (WDM) optical fiber communication and describe our work on wavelength calibration references. We have developed wavelength references for National Institute of Standards and Technology (NIST) internal calibration and transfer standards to help industry calibrate their instrumentation. The transfer standards are NIST Standard Reference Materials that utilize the absorption lines of acetylene and hydrogen cyanide in the 1500 nm region. Two higher accuracy references for NIST internal use are based on laser stabilization to absorption lines of rubidium (at 1560 nm) and methane (at 1314 nm). We are developing calibration references for the WDM L-band (approximately 1565 - 1625 nm) and are investigating references for the 1300 - 1400 nm region.

A tunable diode pumped micro-crystal solid-state laser for a miniaturized interferometer was developed. The interferometer is realized as an integrated optic chip. The measurement system is designed for absolute distance interferometry to measure at an operation distance of 50 m, with a maximum linear movement of 5 cm and an accuracy of 5 nm. To achieve this task the laser wavelength is stabilized in the infrared at two wavelengths with a fixed frequency distance of larger than 80 GHz. The reproducible scanning between these two wavelengths is carried out by frequency stabilization of the 2nd harmonic to the absorption lines of an iodine gas cell. The linewidth of the stabilized laser is smaller than 200 kHz. The laser emits the fundamental wavelength (lambda) ₁ equals 1064 nm and the second harmonic (lambda) ₂ equals 532 nm. The laser is continuously tunable without mode hoping over a tuning range of more than 210 GHz (at 532 nm). An optical output power of 150 mW is achieved in the fundamental wavelength and 0.5 mW in the second harmonic part.

We have developed a continuous-wave deep-ultraviolet single- mode coherent light source intended for laser cooling of silicon atoms, especially ²⁸Si atom. It has a laser cooling transition between 3s ³P₁ and 4s ³P₀ whose corresponding wavelength is 252.4 nm and linewidth is 28.8 MHz, respectively. There has been no report on such a coherent light source, and therefore laser cooling of silicon atoms has never been achieved. To develop this coherent light source, we employed two-stage highly efficient frequency conversions by use of binary external cavities. In the first stage, the fundamental light was a 1.3-W, 746-nm Ti:sapphire laser and the second harmonic light was generated in a LBO crystal placed in a bow-tie ring cavity. Acquired 373-nm radiation power was 530 mW and the conversion efficiency was 40%. In the second stage, 50 mW of 252-nm light was obtained through sum frequency mixing in a BBO crystal placed in another bow-tie ring cavity between the 480-mW, 373-nm light from the first stage and the 380-mW, 780-nm light from the laser diode. By use of a piezotransducer and frequency modulation of laser diode, each radiation was enhanced in the same cavity for doubly resonant sum frequency mixing. These performance characteristics of this system should be enough to achieve laser cooling of silicon atoms.

Of all the fundamental constants of nature, the Newtonian constant of gravitation, G, has been one of the most difficult to measure. The current CODATA value of G has an uncertainty of 1.5 parts in 1000. Although recent experiments have produced values with uncertainties smaller than this, the adopted CODATA uncertainty reflects the fact that there is still substantial disagreement between the values from these experiments. The majority of previous measurements have used torsion pendulums or balances to convert the small gravitational attraction of a laboratory source mass into a relatively large mechanical displacement. However, our approach is to use simple pendulums, which results in a small displacement that we measure very accurately. This means that the attraction of the source masses is measured against a restoring force provided by earth's gravity rather than the less well-understood torsion of a wire. Also, the shorter period of our pendulums allows us to make measurements much more rapidly than in most other experiments. In our apparatus, two mirrors, each suspended as a simple pendulum, form a Fabry-Perot cavity. A He-Ne laser locked to this cavity monitors the relative displacement of these two pendulums (through changes in its frequency) as laboratory source masses are moved, altering the gravitational pull on the mirrors.

Tuning and output characteristics of widely-tunable, continuous-wave, doubly-resonant optical parametric oscillators (cw-DROs) are reported from a practical viewpoint. Using 5% MgO-doped LiNbO₃ as a nonlinear crystal, we fabricated a 532 nm pumped, monolithic cw-DRO which is tunable over 1 octave from 777 nm to 1687 nm. The threshold of the DRO was 10 mW and the DRO produced the signal power of 4 mW and the idler power of 3 mW when it is pumped at 68 mW pump-power. It operated very stably in a single-longitudinal-mode pair of signal and idler without any mode-hopping over 1 day even under free-running condition, owing to the high mechanical stability of the monolithic cavity and the strong thermal self-locking mechanism. By stabilizing the crystal temperature so as to maintain the output power constant, the output power fluctuation was suppressed to 1.3% rms. The arbitrary longitudinal-mode-selection was possible by combining the crystal temperature and the pump-frequency tuning. By the simultaneous control of the crystal temperature and the pump- frequency, the continuous frequency tuning over 500 MHz was achieved.

The novel concept of the electro-optical parametric oscillator (EOPO) as a frequency stabilization scheme is presented. Effects that are limiting the performance of the basic EOPO scheme and approaches to overcome the dominant imperfections are discussed. A certain frequency noise cross talk that takes place between the pump and the EOPO output is suppressed by locking the frequency of the pumplight to the frequency of the EOPO output as an active reference. Fluctuations of the loop delay time also lead to a pulling of the frequency of the EOPO output. Therefore a novel loop pulling cancellation scheme based on amplitude modulation spectroscopy is presented and demonstrated in this paper.

A novel Er-Yb:glass quasi-monolithic diode-pumped laser has been developed to realize a high-accuracy frequency standard at 1.54 micrometer based on saturated absorption of isotopic acetylene. This compact oscillator shows low amplitude- and frequency-noise, wide wavelength tunability (approximately 20 nm), and continuous output power in excess of 20 mW with excellent linear polarization (approximately 30 dB extinction ratio). Employing this laser source sub-Doppler spectroscopy of the acetylene around 1.54 micrometer has been performed. To obtain the necessary saturation intensity (approximately 3.5 W/mm²), the absorbing sample is placed inside a Fabry- Perot cavity with a Finesse of approximately 150. The dispersion signal of the sub-Doppler resonance, useful to stabilize the laser frequency, has been obtained by dithering the Fabry-Perot piezo and employing a lock-in detection scheme.

A novel method is described for fast frequency modulation or frequency control of diode lasers which avoids problems associated with bias current modulation, namely amplitude modulation and thermal phase delays. The method is based on amplitude modulated, non-interfering control light with a wavelength near the transparency region of the laser diode, which specifically modifies the spectral gain profile yielding a constant gain but a controllable refractive index at the lasing wavelength. A phase-lock between the emission of two extended cavity diode lasers which could not be achieved using bias current modulation was realized using this method. Additionally this method allows for a realization of different special modulation types for laser diodes: amplitude- modulation-free frequency modulation, frequency-modulation- free amplitude modulation or single sideband modulation at modulation frequencies up to the relaxation oscillation frequency.

Three independent iodine stabilized laser systems have been set-up at the Physikalisch-Technische Bundesanstalt (PTB) for applications in frequency- and dimensional metrology. The lasers use saturated absorption of the P(54) 32 - 0 iodine transition in an external iodine cell as a frequency reference. The signals for stabilizing the lasers are obtained by using wavelength modulation spectroscopy and frequency modulation spectroscopy, respectively. With the different setups we have achieved instabilities of < 2 (DOT) 10^-13 for an integration time of 1 s and an agreement of the stabilized laser frequencies of the individual systems to better than 3 kHz. Together with absolute frequency measurements performed recently these lasers can compete with optical frequency standards based on atoms or ions. The drawbacks and advantages of the individual set-ups are discussed with respect to frequency stability, reproducibility and uncertainty. Besides the reproducibility, impurities of the iodine cells limit the uncertainty of the stabilized frequency. We ave used one of our systems to investigate a random sample of iodine cells and compared these results with the ones obtained with an iodine-stabilized HeNe laser.

We present an optical frequency standard at 1.54 micrometer based on the saturated absorption of the (nu) ₁ + (nu) ₃ band of acetylene ¹³C₂H₂. An external resonator containing an absorption cell at a pressure of 1 Pa was used to build up the power and to increase the absorption length. A linewidth-narrowed DFB diode laser was stabilized to this resonator using the Pound-Drever-Hall technique. The molecular absorption was detected by wavelength modulation spectroscopy and by cavity-enhanced frequency-modulation spectroscopy, using a modulation frequency equal to the free- spectral range of the resonator. We have achieved a frequency stability of 2 X 10^-12 for averaging times of 1000 s, as measured by interferometrically comparing the laser to an iodine-stabilized helium-neon laser.

A brief theoretical consideration showed that the impurities contained in cadmium droplets play a considerable role on red lines oscillation of the He-Cd II white-light lasers. Several procedures have been adopted to improve the purity of cadmium droplets in the laser tube, an excellent result of strengthening the output as well as lowering the threshold of red lines has been achieved.

CO₂ lasers emit laser lines at about 90 discrete wavelengths around 10 micrometer. A blazed grating is usually employed as the output mirror in the resonator. By adjusting the angle between the grating and the optical axis, a desired laser line can be selected since all other potential laser lines will suffer from high losses. For the purpose of increasing the laser lines so as to broaden its pumping ability for far infrared lasers, the dependence of lasing lines on the components as well as the total pressure has been investigated. The laser envelope used was 1200 mm long with a capillary diameter 12 mm. Lasing lines was in situ measured by using Wang's method. Experimental results will be reported in the manuscript.

In 1977 the 532 nm radiation obtained by frequency-double iodine-stabilized Nd:YAG lasers was included in the revised list of recommended radiations for the practical realization of the definition of the meter. In a planned group of reference lasers at this wavelength at the BIPM, the first two have been completed and a series of beat measurements has been made. The relative short-term stability was typically 5 (DOT) 10^-14. Over longer durations a typical minimum stability floor of about 6 (DOT) 10^-15 is reached. The frequency difference between the BIPM lasers measured over a 10-day period was found to be 63 Hz with a standard uncertainty of 161 Hz. The hyperfine spectrum of the R(56)32-0 line in iodine has been measured using beat frequency techniques and excellent agreement with theory is found, with a residual of the fit of only 400 Hz.

The new generation of transportable He-Ne/CH₄ ((lambda) equals 3.39 micrometer) optical frequency standards (TOFS) stabilized over resolved magnetic hyperfine structure (MHFS) and recoil doublet of F₂⁽²) methane line is considered. The main limitations of the performance of present devices are caused by the residual first order Doppler effect manifestation. Several physical and technological improvements are in progress for reaching the level of 10^-13 - 10^-14 in frequency reproducibility/repeatability, while a relative frequency stability may exceed the stability of H-maser at averaging times (10^-3 - 10⁴) s. In combination with 'fs-comb generators' the compact and precise He-Ne/CH₄ standards will be able to transfer their excellent short and middle term stability (10^-14 per sec now, 10^-15 per sec in near future) to the desired range of spectrum. In such a way they can play a role of narrow spectrum interrogative oscillators for super accurate frequency standards based on laser cooled atoms/ions or can be used as one of the optical references for fs-comb optical synthesizers.