Showing papers by "Leo W. Hollberg published in 2002"

Miniature vapor-cell atomic-frequency references

Abstract: We propose a sub-millimeter-scale vapor-cell atomic-frequency reference based on a micromachined vapor cell, all-optical excitation, and advanced diode-laser technology. We analyze theoretically the performance of such a device as a function of cell size. Initial measurements on small-scale vapor cells support the theoretical treatment.

Accuracy evaluation of NIST-F1

Abstract: The evaluation procedure of a new laser-cooled caesium fountain primary frequency standard developed at the National Institute of Standards and Technology (NIST) is described. The new standard, NIST-F1, is described in some detail and typical operational parameters are discussed. Systematic frequency biases for which corrections are made - second-order Zeeman shift, black-body radiation shift, gravitational red shift and spin-exchange shift - are discussed in detail. Numerous other frequency shifts are evaluated, but are so small in this type of standard that corrections are not made for their effects. We also discuss comparisons of this standard both with local frequency standards and with standards at other national laboratories.

Coherent population trapping resonances in thermal (85)Rb vapor: D(1) versus D(2) line excitation.

Abstract: We have compared coherent population trapping (CPT) resonances, both experimentally and theoretically, for excitation of the D1 and D2 transitions of thermal Rb85 vapor. Excitation of the D1 line results in greater resonance contrast than excitation of the D2 line and in a reduction in the resonance width, in agreement with theoretical expectations. These results translate into a nearly tenfold improvement in performance for the application of CPT resonances to a frequency standard or a sensitive magnetometer when the D1 line, rather than the D2 line, is used.

Efficient frequency up-conversion in resonant coherent media

Abstract: We demonstrate an efficient frequency up-conversion based on generation of large atomic coherence in a cascade system. Two infrared, low power laser fields tuned to the vicinity of the two-photon transition in Rb vapor were converted spontaneously into infrared and blue radiation. Extension of the technique into other spectral regions using highly excited states seems feasible.

Phase-coherent link from optical to microwave frequencies by means of the broadband continuum from a 1-GHz Ti:sapphire femtosecond oscillator

Abstract: An optical clockwork is created with a compact 1-GHz repetition-rate laser and three nonlinear crystals. The broadband continuum output of the laser covers sufficient bandwidth to provide direct access to its carrier-envelope offset frequency without the use of a microstructure fiber. We phase lock the femtosecond comb to a Ca optical standard and monitor the stability of the repetition rate, fr, at 1 GHz. We demonstrate that the short-term stability of the microwave output of the optical clock is at least as good as that of a high-performance hydrogen maser.

Femtosecond-laser-based optical clockwork with instability≤ less than or equal to 6.3 X 10-16 in 1 s

Abstract: Two octave-spanning optical-frequency combs (750-MHz comb spacing) are phase locked to a common continuous-wave laser diode The measured instability of the heterodyne beat between the two combs demonstrates that the intrinsic fractional frequency noise of a comb is ≤63×10-16 in 1 s of averaging across the ∼300‐THz bandwidth Furthermore, the average frequencies of the elements of the two combs are found to agree within an uncertainty of 4×10-17 across the entire octave We demonstrate the possibility of transfering the stability and accuracy of the best current optical standards to ∼500,000 individual oscillators across the visible and near-infrared spectrum

Miniature frequency standard based on all-optical excitation and a micro-machined containment vessel

Abstract: A microwave frequency standard is provided which allows for miniaturization down to length scales of order one micron, comprising a modulated light field originating from a laser that illuminates a collection of quantum absorbers contained in a micro-machined cell. The frequency standard of the present invention can be based on all-optical excitation techniques such as coherent population trapping (CPT) and stimulated Raman scattering or on conventional microwave-excited designs. In a CPT-based embodiment, a photodetector detects a change in transmitted power through the cell and that is used to stabilize an external oscillator to correspond to the absorber's transition frequency by locking the laser modulation frequency to the transition frequency. In a stimulated Raman scattering (SRS) embodiment, a high-speed photodetector detects a laser field transmitted through the cell beating with a second field originating in the cell. Both the locked laser modulation frequency and the beat frequency are very stable as they are referenced directly to the atomic transition.

Temperature dependence of coherent population trapping resonances

Abstract: We measure the properties of coherent population trapping (CPT) resonances in Cs vapor cells as a function of temperature. We expected the CPT signal to increase with higher vapor density, but instead the signal fades away above a certain density. Two possible density-dependent explanations are discussed: spin-exchange collisions, which are found to give no relevant contribution at the temperatures considered here, and increased absorption due to the optical thickness of the vapor. The dependence of the dark-line resonance amplitude as a function of cell temperature can be well represented by a simple model based on the optical thickness of the vapor as a function of temperature.

Wavelength references for 1300-nm wavelength-division multiplexing

Abstract: We have conducted a study of potential wavelength calibration references for use as both moderate-accuracy transfer standards and high-accuracy National Institute of Standards and Technology (NIST) internal references in the 1280-1320-nm wavelength-division-multiplexing region. We found that most atomic and molecular absorption lines in this region are not ideal for use as wavelength references owing to factors such as weak absorption, complex spectra, or special requirements (for example, frequency-doubling or excitation with an additional light or discharge source). We have demonstrated one of the simpler schemes consisting of a tunable diode laser stabilized to a Doppler-broadened methane absorption line. By conducting a beat-note comparison of this reference to a calcium-based optical frequency standard, we measured the methane line center with an expanded uncertainty (2/spl sigma/) of /spl plusmn/2.3 MHz. This methane-stabilized laser now serves as a NIST internal reference.

Method of minimizing the short-term frequency instability of laser-pumped atomic clocks

Abstract: A method is provided for optimizing the performance of laser-pumped atomic frequency references with respect to the laser detuning and other operating parameters. This method is based on the new understanding that the frequency references short-term instability is minimized when (a) the laser frequency is tuned nominally a few tens of MHz away from the center of the atomic absorption line, and (b) the external oscillator lock modulation frequency is set either far below or far above the inverse of the optical pumping time of the atoms.

Grating-tuned semiconductor MOPA lasers for precision spectroscopy

Abstract: A standard grating-tuned extended-cavity diode laser is used for injection seeding of a tapered semiconductor laser/amplifier. With sufficient injection power the output of the amplifier takes on the spectral characteristics of the master laser. The authors have constructed master-oscillator power-amplifier (MOPA) systems that operate near 657 nm, 675 nm, 795 nm, and 850 nm. Although the characteristics vary from system to system, the authors have demonstrated output powers of greater than 700 mW in a single spatial mode, linewidths less than 1 kHz, coarse tuning greater than 20 nm, and continuous single-frequency scanning greater than 150 GHz. The authors discuss the spectroscopic applications of these high power, highly coherent, tunable diode lasers as applied to Ca, Hg{sup +}, I{sub 2}, and two-photon transitions in Cs.

Role of spurious reflections in ring-down spectroscopy.

Abstract: Spurious coherent reflections from optical elements that re-enter an exit port of a two-mirror ring-down cavity can significantly change the effective reflectivity of the cavity mirrors, thus altering the cavity decay time. For a 25-cm-long Fabry-Perot cavity with a decay constant of 40 mus , we find that a specular reflection of only 10(-4) of the transmitted ring-down power that is mode matched back toward the cavity could change the decay time by as much as +/-0.4 mus , depending on the phase of the returning reflection. The perturbation of the decay time is proportional to the electric field, so a decrease in the spurious reflected power of 100 times will result in a perturbation that is only 10 times smaller. We demonstrate the effect with a cw system by purposely introducing a spurious reflection.

Performance of small-scale frequency references

Abstract: We consider theoretically the expected performance of ultra-small vapor-cell frequency references with cell volumes below 1 mm/sup 3/. The short-term stability is found to degrade as the size is reduced. The amount of degradation depends on whether a buffer gas or wall coating is used for the atomic confinement. Preliminary experimental results with all-optical excitation indicate that the atomic Q-factor behaves as expected.

Resonant enhancement of refractive index in a cascade scheme

Abstract: The refractive index of coherently driven rubidium vapour is experimentally investigated in a three-level cascade con® guration using a selective reection technique. The maximum measured resonant change in the refractive index is ¢n ' 0:1. The selective reection is accompanied by a four-wave-mixing process that can reach π90% eA ciency.

Analysis of noise mechanisms limiting frequency stability of microwave signals generated with a femtosecond laser

Abstract: The femtosecond laser is a key element of a coherent link between optical and microwave domains. Such a link has already enabled measurements of optical frequencies with an accuracy of a current time standard, the caesium beam microwave 'clock' (S.A. Diddams et al., Phys. Rev. Lett. vol. 84, pp. 5102-5105, 2000; T. Udem et al., ibid., vol. 86, p. 4996, 2001). Making use of a femtosecond laser, it is also possible to transfer the frequency stability of an optical 'clock' to radio frequencies. This may result in redefinition of a 'second' when optical frequency standards supersede their microwave counterparts in terms of accuracy (J.L. Hall et al., IEEE J. Quantum Electron., vol. 37, no. 12, pp. 1482-1492, 2001; L. Hollberg et al., ibid., vol. 37, no. 12, pp. 1502-1513, 2001). In this work we discuss the basic noise mechanisms affecting the frequency stability of a microwave signal produced with a femtosecond laser, including power-to-phase conversion in the photodetector and laser beam-pointing fluctuations.

Compact Microwave Frequency Reference Based on Coherent Population Trapping

Abstract: A simple, compact and low-power microwave frequency rcferencc bascd on CPT resonances in Cs vapor is descrihcd. The 14 cm’ physics package exhibits a rcsonancc width of 620 IIz at 4.6 Gllz, a short-tcrm fractional frequency instability of 1 . 3 ~ lO-’”/m, arid dissipalcs less than 30 niW, not including temperature control. Wc discuss the prospccts for exlrcme miniaturization to sub-millimctcr dimcnsions.

A femtosecond-laser-based optical clockwork

Abstract: In this article we summarize our progress in developing and testing a femtosecondlaser-based optical clockwork that provides a singlostep phasocoherent connection between emerging optical frequency standards and the cesium microwave standard on which the SI second is based. This clockwork enabled absolute optical frequency measurements with statistical uncertainty 2 x and it was used in the demonstration of an optical clock with instability 5 7 x in 1 s. Recent experiments demonstrate the intrinsic fractional instability and inaccuracy of thc clockwork are less than 6.3 x in 1 s and 4 x lo-”, respectively.

Miniaturized laser magnetometers and clocks

Abstract: It is with great pleasure that I contribute to this volume honoring Theodor Hansch on the occasion of his 60th birthday (or, in the present context of precision measurements, shouldn’t we call it his 61st, really?). As a Ph.D. student constructing the original Garching frequency chain in his lab I was infected with a “passion for precision” that has influenced my subsequent career in laser spectroscopy and atomic physics. Here I would like to report on the application of some of the techniques we developed in Garching for frequency measurements on atomic hydrogen to the spectroscopy of thermal alkali atomic vapors, leading to very sensitive optical magnetometers and to compact cesium atomic clocks.

Atomic clocks of the future: using the ultrafast and ultrastable

Abstract: The application of ultra-fast mode-locked lasers and nonlinear optics to optical frequency metrology is revolutionizing the field of atomic clocks. The basic concepts and applications will be reviewed using the latest results on optical atomic frequency standards.

Phase coherent link from optical to microwave frequencies via a 1 GHz octave-spanning Ti:sapphire femtosecond oscillator

Abstract: An optical clockwork is created with a compact octave-spanning 1 GHz repetition rate laser, avoiding use of a microstructure fiber. The short-term microwave output of the optical clock is as stable as a hydrogen maser.

A 40Ca OPTICAL FREQUENCY STANDARD AT 657 NM: FREQUENCY MEASUREMENTS AND FUTURE PROSPECTS

Abstract: Due to higher transition frequencies, optical frequency standards promise significantly improved stability over that of their microwave counterparts Our laser-cooled %a optical frequency standard is based on the 'So (m=O) --t 'PI (m=O) intercombination line at 657 nm, and has demonstrated a fractional frequency instability of 4 ~ 1 0 ' ~ at 1 s Using a modelocked femtosecond-laser-based frequency comb referenced to the primary Cs standard, we have made an absolute frequency measurement of this transition with an uncertainty of +26 Hz (lo) A discussion of the major systematic effects in our system is included In an effort to reduce our largest systematic uncertainty, we have investigated second-stage cooling in one dimension on the narrow "clock" transition through quenching of the excited state We have transferred 45 % (22 % net efficiency) of the atoms into a narrow distribution with a temperature of 4 pK, representing a reduction in temperature by a factor of 500

Femtosecond lasers in precision time and frequency metrology

Abstract: Summary form only given. As a recent test of the femtosecond-laser-based optical clockwork, we have compared two femtosecond lasers that have their spectra broadened to /spl sim/300 THz in microstructure fiber. The discrete frequency modes of each octave-spanning spectrum are phase-locked to a common continuous wave laser. The measured instability of the heterodyne beat between the two femtosecond systems demonstrates that the intrinsic noise of each system is /spl les/6.3 /spl times/ 10/sup -16/ in 1 s of averaging across the 300 THz bandwidth. Furthermore, the average frequencies of the frequency modes of the two systems are found to agree within an uncertainty of /spl les/4 /spl times/ 10/sup -17/ across the entire octave. This demonstrates the possibility to transfer the sub-Hertz stability and accuracy of the best current optical standards to /spl sim/500,000 individual oscillators across the visible and near-infrared spectrum.

Broadband noise on supercontinuum generated in microstructure fiber

Abstract: The broad supercontinuum generated by pumping highly nonlinear microstructure fiber with a femtosecond laser exhibits broadband amplitude noise. We experimentally investigate this amplitude noise as a function of wavelength, input pump power, and chirp.

Frequency metrology with optical clocks: comparison of the Ca and Hg/sup +/ clock transitions

Abstract: We present results on optical atomic clocks based on lasers stabilized to optical transitions (in Ca and Hg/sup +/) and whose frequencies are phase-coherently divided down to the microwave domain with a spectrally-broadened mode-locked fs-laser.