Microwave Frequency Standards Research Articles

Temperature of laser-cooled /sup 171/Yb/sup +/ ions and application to a microwave frequency standard

Microwave frequency standards based on buffer gas-cooled /sup 171/Yb/sup +/ ions have demonstrated high stability but are limited in accuracy by the second-order Doppler shift caused by thermal motion. We have previously obtained near shot noise-limited Ramsey fringes with a laser-cooled ion cloud. Here, we present measurements confirming that the ion temperature remains <1 K throughout the microwave interrogation period for a Ramsey pulse separation of up to 10 s and longer. The potential stability of the ions as a frequency standard is better than /spl sigma//sub y/ (/spl tau/)=5/spl times/10/sup -14/ /spl tau//sup -1/2/, and estimates of the systematic offsets to the clock frequency and their uncertainties indicate that a total uncertainty of four parts in 10/sup 15/ or better is achievable.

Optical-pumping noise in laser-pumped, all-optical microwave frequency references

We demonstrate that optical pumping plays a significant role in determining the noise in certain types of laser-pumped vapor-cell microwave frequency standards by changing the way in which the laser’s FM noise is converted to AM noise by the optical-absorption profile. When this FM–AM conversion is the dominant noise source, the noise spectrum of the transmitted intensity can be dramatically altered by the optical-pumping process. FM noise at Fourier frequencies larger than the optical-pumping time is converted to AM noise differently from noise at lower Fourier frequencies. This effect can modify the optimum design of vapor-cell frequency references and adds an additional FM–AM-related noise source that cannot be eliminated with laser tuning.

Progress toward an optical frequency chain at NPL

/sup A/n optical frequency chain is being constructed at the National Physical Laboratory (NPL), Teddington, U.K., with the object of making absolute frequency measurements of optical frequency standards in single, cold, trapped Sr/sup +/ and Yb/sup +/ ions. The initial frequency reference is a methane-stabilized He-Ne laser system, but our ultimate goal is an all-optical chain to the caesium primary microwave frequency standard. We have constructed a CW singly resonant PPLN optical parametric oscillator, pumped by a high-power single-frequency Nd:YLF ring laser, yielding 0.45 W of light at the methane frequency. By cascaded frequency doubling of this output, a near-IR reference can be established at four times the methane frequency. We are developing optical frequency divider stages, and an optical frequency comb generator, as tools for linkages to the ion trap frequencies.

Raman polarization-selective feedback schemes for all-optical microwave frequency standards

All-optical locking of the frequency difference between two laser diodes onto a Raman transition is demonstrated. The properties of Raman-type polarization spectroscopy are discussed and exemplified in the cases of the 3.036 GHz Raman resonance of the 85Rb D2 line. The generation of a polarization-selective Raman resonant optical feedback ensures the Raman optical locking of the slave diode laser onto the master oscillator.

Determination of the ground-state hyperfine splitting of trapped 113Cd+ ions.

The ground-state hyperfine splitting of trapped $^{113}\mathrm{Cd}^{+}$ ions has been measured to be 15 199 862 858(2) Hz. The transition was detected by a laser microwave double-resonance method in the presence of He buffer gas. The result was corrected for the second-order magnetic-field dependence. This paper also discusses the possibility of using ${\mathrm{Cd}}^{+}$ ions as a microwave-frequency standard. \textcopyright{} 1996 The American Physical Society.

Efficient quenching of population trapping in excited Yb+

We report on a study to determine the efficiency of various buffer gases in quenching the population trapping states of laser-excited ytterbium ions. The ions were confined in an rf-dc hybrid trap, and their $^{2}$${\mathit{S}}_{1/2}$${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$${\mathit{P}}_{3/2}$ transition was excited with 329-nm laser light. Buffer gases used include ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, ${\mathrm{CO}}_{2}$, Ar, and He at several different pressure regimes. We have identified ${\mathrm{N}}_{2}$ to be highly efficient in quenching the population of the trapping states and thus be the most suitable buffer gas to use with ytterbium for the development of a trapped-ion microwave frequency standard.

Laser cooling of 171Yb+ ions in a linear paul trap

We report laser cooling of trapped 171Yb+ ions. The ions are confined in a linear Paul trap. Temperatures below 1 K have been achieved, and evidence of a phase transition to a crystal-like state has been observed. The metastable 2 D 3/2 level is drained using a transition at 609.1 nm not previously used for this purpose. Laser cooling of 171Yb+ may be of importance to the development of new microwave frequency standards.

All-optical microwave frequency standard: a proposal

A simple method to produce a clock transition with purely optical means by modulated pumping is described. The field-independent ground state resonance of /sup 87/Rb atoms using sinusoidal modulation of the injection current of an AlGaAs laser diode emitting at 780 nm (FM modulation) is observed. The 6.835 GHz resonance with a subharmonic modulation frequency of 1.139 GHz is detected. A high-contrast resonance peak is observed and a condition for zero light shift is found. The linewidth is 3 kHz (at 6.835 GHz) in this preliminary experiment, due to the small size of the light beam ( approximately 2 mm diameter) and the low buffer gas pressure (680 Pa) that was used. A theoretical model that explains the main features of the experiment is described. >

Optical frequency counting from the UV to the near IR

A method for measuring optical frequencies from the UV to the near IR relative to a microwave frequency standard is proposed. The concept is to measure the frequency difference of two known ratios ((1/2) and ?) of an optical frequency f relative to the cesium clock. By employing optical parametric oscillators and wideband modulators to link the ((1/2))f and (?)f frequencies, a precise and accurate optical frequency comb can be provided in the ~1-2-microm wavelength region. By using this comb, most optical frequencies from the UV to the near IR can be measured or synthesized. A configuration for measuring the frequency of the hydrogen 1S-2S transition is described.

Optical high-order subharmonic excitation of free cyclotron electrons

Homogeneous laser radiation in the visible or infrared range can excite high-order subharmonics at the cyclotron frequency of free electrons in the millimeter or microwave range. This may provide coherent links between lasers and rf or microwave frequency standards. In order to divide the frequency of a CO(2) laser (lambda approximately 10 microm) by a factor of 100 down to lambda approximately 1 mm in one step, a cw laser power as low as 10(-6) W is sufficient.

Time and Frequency Standards Based on Charged Particle Trapping

Microwave or optical frequency standards based on internal resonance transitions of ions confined in electromagnetic traps have the fundamental advantages of long observation times and small perturbations. These advantages are somewhat offset by low signal to noise ratios. Work at NBS has concentrated on microwave hyperfine transitions of atomic ions stored in Penning-type ion traps. The use of narrowband, tunable light sources for state selection and detection and for reducing the average kinetic energy of the ions (laser cooling) is an important feature of this work. Results to date include the fluorescence detection and cooling to about 50 mK of a single Mg+ ion and the observation of a 0.012 Hz linewidth on a 300 MHz 25Mg+ hyperfine transition. A frequency standard based on 201Hg+ ions is under development. Related work, mostly based on RF-type ion traps, is underway at several other labs.

Measurement of the Absolute Frequencies of CO2 Laser Transitions by Multiplication of CO2 Laser Difference Frequencies

It is verified experimentally that frequency differences between CO2 laser lines and the harmonics of these differences can be used as intermediate frequencies to bridge the gap between microwave frequency standards and infrared frequencies. Criteria for the design of a microwave to 30-THz frequency chain are discussed, and experimental details of the measurement of five CO2 isotope laser frequencies to an accuracy of ±2 parts in 109 are presented. The values obtained for the CO2 isotope transitions are 26334.776614 GHz for P(40) of 13C16O2, 27979.469512 GHz for R(6) of 13C18O2, 28665.704581 GHz for P(6) of 12C16O2, 29442.483293 GHz for R(30) of 12C16O2 and 33328.991337 GHz for R(8) of 12C18O2.

Microwave time and frequency standards

Today's time and frequency standards which are in active use ranging from the definition of the second to spacecraft applications are all based on atomic resonances in the microwave region controlling high‐performance quartz crystal oscillators. The present status of these standards will be presented focusing in particular on cesium, hydrogen, and rubidium devices, as well as on new quartz crystal standards. A coherent picture of their physical principles and limitations is given based on the common attempt to reduce perturbations and Doppler effects. The sharpness of the resonance line is linked to the flicker noise stability limit of the various standards, and some speculation is made on where and how much further improvements may occur. It is demonstrated how new design concepts and new physical methods most likely will significantly improve the accuracy, stability, and practical usefulness of time and frequency standards.

Application of the light-shift effect to laser frequency stabilization with reference to a microwave frequency standard

A new method allowing laser frequency stabilization with reference to a microwave oscillator, independently of the laser intensity, is described. The method makes use of the dependence of the ground-state hfs transition frequency on the optical radiation frequency in alkali atoms irradiated by quasi-resonant light. Preliminary experimental investigations are reported in the case of a cw GaAs diode laser tuned to the D 2 absorption line in a cesium gas cell. The absolute laser frequency exhibited variations of 1.4 MHz r.m.s. around an average value determined to within 2 parts in 10 10 for a period of 5 minutes. The possibility of defining a cesium beam, reference wavelength connected with the time standard is discussed.

New Frequency Measurements and Techniques in the 30-THz Region

It is proposed that the frequency differences between CO2 laser ines and the harmonics of these differences might be used as intermediate frequencies to bridge the gap between microwave frequency standards and infrared frequencies to simplify absolute measurements of the latter. Tungsten-nickel point-contact diodes would be used as harmonic generators and mixers. The preliminary results of an experimental feasibility study are presented. A description of a novel two-frequency laser, built for this study, is given. It is shown that beat frequencies between differences up to 715 GHz and microwave frequency harmonics up to the twelfth can be observed; interactions with at least the third harmonic of a CO2 frequency difference are possible, producing signals with an S/N of 51 dB in a 2.2-s time constant receiver; attempts to observe a beat between a CO2 frequency and the seventh harmonic of a difference were not successful.

Current status and development prospects of quantum frequency standards (review)

The current status of quantum frequency standards is considered. A systematic comparison is made of the results achieved so far. The advantages and disadvantages of the principal types of microwave and optical quantum frequency standards are discussed. An analysis is given of the prospects of further progress which might result from improvements in the existing devices or from the development of new ones.

Frequency lock of the hydrogen cyanide laser to a microwave frequency standard

Frequency locking of HCN laser transition at 890 GHz to harmonic of crystal controlled oscillator

Phase Stabilized Klystron System and Its Application to Microwave Spectroscopy and Microwave Frequency Standards

A system is described for stabilizing the frequency and phase of a klystron where its frequency is to be varied continuously at any predetermined rate. This system, when applied to a microwave spectrograph and frequency standard, greatly improves the resolution, sensitivity, and accuracy of frequency measurements. The system also has application to superheterodyne detection systems in which the local oscillator is phase locked to the signal klystron.

Microwave Frequency Standard

Here described is a high stability UHF source which is used as a microwave frequency standard. Frequency measurement requires the use of a calibrated communication receiver to measure the difference between a known harmonic of the UHF source and the unknown microwave frequency. Frequencies as high as 27 kmc have been measured and the frequency stability is approximately 1 part per million.

A frequency meter for microwave spectroscopy.

A frequency meter using known absorption lines as microwave frequency standards has been developed. By mixing signals of unknown microwave frequency fx, standard microwave frequency f0, and known radiofrequency signals nfc from a crystal-controlled harmonic generator, it is possible to obtain a set of ``beat'' frequencies fr which can be detected by a calibrated radio receiver. From the relation fr=±m(fx−f0)±nfcit is possible to determine values for the unknown microwave frequency fx. Tests on methanol absorption lines with ammonia lines as microwave frequency standards show that microwave frequencies can be determined to ±0.03 mc/sec. Advantages of this device over other frequency meters are discussed; the accuracy is ultimately limited by the uncertainties in the value of the standard frequency f0.