Showing papers by "Leo W. Hollberg published in 2007"

Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb

Abstract: The control of the broadband frequency comb emitted from a mode-locked femtosecond laser has permitted a wide range of scientific and technological advances--ranging from the counting of optical cycles for next-generation atomic clocks to measurements of phase-sensitive high-field processes. A unique advantage of the stabilized frequency comb is that it provides, in a single laser beam, about a million optical modes with very narrow linewidths and absolute frequency positions known to better than one part in 10(15) (ref. 5). One important application of this vast array of highly coherent optical fields is precision spectroscopy, in which a large number of modes can be used to map internal atomic energy structure and dynamics. However, an efficient means of simultaneously identifying, addressing and measuring the amplitude or relative phase of individual modes has not existed. Here we use a high-resolution disperser to separate the individual modes of a stabilized frequency comb into a two-dimensional array in the image plane of the spectrometer. We illustrate the power of this technique for high-resolution spectral fingerprinting of molecular iodine vapour, acquiring in a few milliseconds absorption images covering over 6 THz of bandwidth with high frequency resolution. Our technique for direct and parallel accessing of stabilized frequency comb modes could find application in high-bandwidth spread-spectrum communications with increased security, high-resolution coherent quantum control, and arbitrary optical waveform synthesis with control at the optical radian level.

Precision atomic spectroscopy for improved limits on variation of the fine structure constant and local position invariance.

Abstract: We report tests of local position invariance and the variation of fundamental constants from measurements of the frequency ratio of the 282-nm 199Hg+ optical clock transition to the ground state hyperfine splitting in 133Cs. Analysis of the frequency ratio of the two clocks, extending over 6 yr at NIST, is used to place a limit on its fractional variation of <5.8x10(-6) per change in normalized solar gravitational potential. The same frequency ratio is also used to obtain 20-fold improvement over previous limits on the fractional variation of the fine structure constant of |alpha/alpha|<1.3x10(-16) yr-1, assuming invariance of other fundamental constants. Comparisons of our results with those previously reported for the absolute optical frequency measurements in H and 171Yb+ vs other 133Cs standards yield a coupled constraint of -1.5x10(-15)

Chip scale atomic devices

Abstract: Chip-scale atomic devices combine elements of precision atomic spectroscopy, silicon micromachining, and advanced diode laser technology to create compact, low-power, and manufacturable instruments with high precision and stability. Microfabricated alkali vapor cells are at the heart of most of these technologies, and the fabrication of these cells is discussed in detail. We review the design, fabrication, and performance of chip-scale atomic clocks, magnetometers, and gyroscopes and discuss many applications in which these novel instruments are being used. Finally, we present prospects for future generations of miniaturized devices, such as photonically integrated systems and manufacturable devices, which may enable embedded absolute measurement of a broad range of physical quantities.

Microfabricated saturated absorption laser spectrometer.

Abstract: We demonstrate a miniature microfabricated saturated absorption laser spectrometer. The system consists of miniature optics, a microfabricated Rb vapor cell, heaters, and a photodetector, all contained within a volume of 0.1 cm3. Saturated absorption spectra were measured with a diode laser at 795 nm. They are comparable to signals obtained with standard table-top setups, although the rubidium vapor cell has an interior volume of only 1 mm3. We discuss the performance and prospects for using such systems as a miniature optical wavelength reference, compatible with transportable instruments.

Frequency Uncertainty for Optically Referenced Femtosecond Laser Frequency Combs

Abstract: We present measurements and analysis of the currently known relative frequency uncertainty of femtosecond laser frequency combs (FLFCs) based on Kerr-lens mode-locked Ti:sapphire lasers. Broadband frequency combs generated directly from the laser oscillator, as well as octave-spanning combs generated with nonlinear optical fiber are compared. The relative frequency uncertainty introduced by an optically referenced FLFC is measured for both its optical and microwave outputs. We find that the relative frequency uncertainty of the optical and microwave outputs of the FLFC can be as low as 8times10-20 and 1.7times10-18, with a confidence level of 95%, respectively. Photo-detection of the optical pulse train introduces a small amount of excess noise, which degrades the stability and subsequent relative frequency uncertainty limit of the microwave output to 2.6times10-17

Absolute frequency measurement of the neutral 40Ca optical frequency standard at 657 nm based on microkelvin atoms

Abstract: We report an absolute frequency measurement of the optical clock transition at 657 nm in 40 Ca with a relative uncertainty of 7.5 × 10 −15 , one of the most accurate frequency measurements of a neutral atom optical transition to date. The frequency (455 986 240 494 135.8 ± 3.4) Hz was measured by stabilizing a diode laser system to a spectroscopic signal derived from an ensemble of 10 6 atoms cooled in two stages to a temperature of 10 µK. The measurement used a femtosecond-laser-based frequency comb to compare the Ca transition frequency with that of the single-ion 199 Hg + optical frequency standard at NIST. The Hg + frequency was simultaneously calibrated relative to the NIST Cs fountain via the NIST time scale to yield an absolute value for the Ca transition frequency. The relative fractional instability between the two optical standards was 2 × 10 −15 for 10 s of averaging time and 2 × 10 −16 for 2000 s. (Some figures in this article are in colour only in the electronic version)

Optical-to-microwave frequency comparison with fractional uncertainty of 10- 15

Abstract: We report the technical aspects of the optical-to-microwave comparison for our recent measurements of the optical frequency of the mercury single-ion frequency standard in terms of the SI second as realized by the NIST-F1 cesium fountain clock. Over the course of six years, these measurements have resulted in a determination of the mercury single-ion frequency with a fractional uncertainty of less than 7×10-16, making it the most accurately measured optical frequency to date. In this paper, we focus on the details of the comparison techniques used in the experiment and discuss the uncertainties associated with the optical-to-microwave synthesis based on a femtosecond laser frequency comb. We also present our most recent results in the context of the previous measurements of the mercury single-ion frequency and arrive at a final determination of the mercury single-ion optical frequency: f(Hg+)=1 064 721 609 899 145.30(69) Hz.

Demonstration of high-performance compact magnetic shields for chip-scale atomic devices

Abstract: We have designed and tested a set of five miniature nested magnetic shields constructed of high-permeability material, with external volumes for the individual shielding layers ranging from 0.01to2.5cm3. We present measurements of the longitudinal and transverse shielding factors (the ratio of external to internal magnetic field) of both individual shields and combinations of up to three layers. The largest shielding factor measured was 6×106 for a nested set of three shields, and from our results we predict a shielding factor of up to 1×1013 when all five shields are used. Two different techniques were used to measure the internal field: a chip-scale atomic magnetometer and a commercially available magnetoresistive sensor. Measurements with the two methods were in good agreement.

A proposed laser frequency comb-based wavelength reference for high-resolution spectroscopy

Abstract: High resolution spectroscopy is the foundation for many of the most challenging and productive of all astronomical observations. A highly precise, repeatable and stable wavelength calibration is especially essential for long term RV observations. The two wavelength references in wide use for visible wavelengths, iodine absorption cells and thorium/argon lamps, each have fundamental limitations which restrict their ultimate utility. We are exploring the possibility of adapting emerging laser frequency comb technology in development at the National Institute of Standards and Technology in Boulder, Colorado, to the needs of high resolution, high stability astronomical spectroscopy. This technology has the potential to extend the two current wavelength standards both in terms of spectral coverage and in terms of long term precision, ultimately enabling better than 10 cm/s astronomical radial velocity determination.

High-contrast coherent population trapping resonances using four-wave mixing in 87 Rb

Abstract: We demonstrate very high-contrast coherent population trapping1 (CPT) resonances by using four-wave mixing in 87Rb atoms. In the experiment, we take advantage of the spectral overlap between F=2-->F' and F=3-->F' optical resonances on the D1 line of 87Rb and 85Rb atoms, respectively, to eliminate the DC-light background from the CPT resonance signal. We observe a CPT resonance with a contrast in the range of 90%, compared with a few percent achieved by alternative methods.

Stable Laser System for Probing the Clock Transition at 578 nm in Neutral Ytterbium

Abstract: We describe a new laser system we have developed to probe the ultra-narrow 1S0 harr 3P0 clock transition at 578 nm in neutral ytterbium. The yellow light is produced by sum frequency generation in a periodically poled waveguide. With approximately 100 mW each from a fiber laser and Nd:YAG laser, we produce 10 mW of visible light. Stabilization of the laser to a resonance of a high finesse, environmentally isolated cavity has enabled resolution of spectroscopic features as narrow as 5 Hz.

Optical frequency standards based on mercury and aluminum ions

Abstract: Single-trapped-ion frequency standards based on a 282 nm transition in 199 Hg + and on a 267 nm transition in 27 Al + have been developed at NIST over the past several years. Their frequencies are measured relative to each other and to the NIST primary frequency standard, the NIST-F1 cesium fountain, by means of a self-referenced femtosecond laser frequency comb. Both ion standards have demonstrated instabilities and inaccuracies of less than 1 × 10 −16 .

Advances in Chip-Scale Atomic Frequency References at NIST

Abstract: Coherent population trapping (CPT) resonances usually exhibit contrasts below 10 % when interrogated with frequency modulated lasers. We discuss a relatively simple way to increase the resonance contrast to nearly 100 % generating an additional light field through a nonlinear four-wave mixing interaction in the atomic vapor 1 . A similar method can also be used to create a beat signal at the CPT resonance frequency that can injection-lock a low-power microwave oscillator at 3.4 GHz directly to the atomic resonance 2 . This could lead to chip-scale atomic clocks (CSACs) with improved performance. Furthermore, we introduce a miniature microfabricated saturated absorption spectrometer 3 that produces a signal for locking a laser frequency to optical transitions in alkali atoms. The Rb absorption spectra are comparable to signals obtained with standard table-top setups, although the rubidium vapor cell has an interior volume of only 1 mm 3 and the volume of the entire spectrometer is around 0.1 cm 3 .

Low-noise optical injection locking of a resonant tunneling diode to a stable optical frequency comb

Abstract: Optical injection locking of a resonant tunneling diode (RTD) oscillator has been demonstrated using ultrashort pulses from a mode-locked Ti:sapphire laser operating at a 1GHz pulse rate. The source of the optical signal is a mode-locked femtosecond laser whose optical frequency comb is phase locked to a H-maser stabilized frequency synthesizer. An exceptionally large capture range of more than 5MHz is observed. The system produces stable microwave signals with low phase noise, which at 1GHz is less than −74dBc∕Hz for a 10Hz offset. The noise of the microwave injection-locked RTD signal matches that of the input optical pulses.

Measurement of excited-state transitions in cold calcium atoms by direct femtosecond frequency-comb spectroscopy

Abstract: We apply direct frequency-comb spectroscopy, in combination with precision cw spectroscopy, to measure the $4s4p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}\ensuremath{\rightarrow}4s5s\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$ transition frequency in cold calcium atoms. A $657\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ ultrastable cw laser was used to excite atoms on the narrow $(\ensuremath{\gamma}\ensuremath{\sim}400\phantom{\rule{0.3em}{0ex}}\mathrm{Hz})$ $4{s}^{2}\phantom{\rule{0.2em}{0ex}}^{1}S_{0}\ensuremath{\rightarrow}4s4p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}$ clock transition, and the direct output of the frequency comb was used to excite those atoms from the $4s4p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}$ state to the $4s5s\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$ state. The resonance of this second stage was detected by observing a decrease in population of the ground state as a result of atoms being optically pumped to the metastable $4s4p\phantom{\rule{0.2em}{0ex}}^{3}P_{0,2}$ states. The $4s4p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}\ensuremath{\rightarrow}4s5s\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$ transition frequency is measured to be $\ensuremath{ u}=489\phantom{\rule{0.2em}{0ex}}544\phantom{\rule{0.2em}{0ex}}285\phantom{\rule{0.2em}{0ex}}713(56)\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$; an improvement by almost four orders of magnitude over the previously measured value. In addition, we demonstrate spectroscopy on magnetically trapped atoms in the $4s4p\phantom{\rule{0.2em}{0ex}}^{3}P_{2}$ state.

Self-Injection Locking of a Microwave Oscillator by Use of Four-Wave Mixing in an Atomic Vapor

Abstract: We demonstrate self-injection locking of a low-power, compact microwave oscillator by use of feedback generated from four-wave mixing in an atomic vapor. The four-wave mixing process creates an effective microwave filter with extremely high off-resonant signal suppression. This type of locking results in a shorter locking time, increased short-term stability, improved close-in phase noise, and reduced spurious signals as compared to current techniques used in miniature atomic clocks. Other possible benefits and drawbacks are mentioned.

Long-term stability of NIST chip-scale atomic clock physics packages

Abstract: We discuss the long-term stability of the NIST chip-scale atomic clock (CSAC) physics packages. We identify the major factors that currently limit the frequency stability of our CSAC packages after 100 s. The requirements for the stability of the vapor cell and laser temperature, local magnetic field, and local oscillator output power are evaluated. Due to the small size of CSAC physics package assemblies, advances MEMS packaging techniques for vacuum sealing and thermal isolation can be used to achieve the temperature stability goals. We discuss various ideas on how to aid temperature control solutions over wide variations in ambient temperature by implementing atom-based stabilization schemes. Control of environment-related frequency instabilities will be critical for successful insertion of CSACs into portable instruments in the areas of navigation and communication.

Chip-scale atomic devices at NIST

Abstract: We provide an overview of our research on chip-scale atomic devices By miniaturizing optical setups based on precision spectroscopy, we have developed small atomic sensors and atomic references such as atomic clocks, atomic magnetometers, and optical wavelength references We have integrated microfabricated alkali vapor cells with small low-power lasers, micro-optics, and low-power microwave oscillators As a result, we anticipate that atomic stability can be achieved with small size, low cost, battery-operated devices Advances in fabrication methods and performance are presented

Generation of coherent population trapping resonances with nearly 100% transmission contrast

Abstract: We demonstrate a simple setup for observing very high contrast coherent population trapping resonances in rubidium - 87. The high contrast is achieved by eliminating the 52 P 1/2 DC-light background through polarization and spectral filtering.

Improved Limits on Variation of the Fine Structure Constant and Violation of Local Position Invariance

Abstract: We report tests of local position invariance (LPI) and constancy of fundamental constants from measurements of the frequency ratio of the 282-nm 199Hg+ optical clock transition to the ground-state hyperfine splitting in 133Cs. Analysis of the frequency ratio, extending over six years at NIST, is used to place a limit on the fractional variation of the two clocks of less than 5.8times10-6 per change in normalized solar gravitational potential, and a limit on fractional variation of the fine structure constant at alpha dot/alpha < 1.3x10-16 yr-1, assuming invariance of other fundamental constants. Comparison of our results with those previously reported for the absolute optical frequency measurements of coupled 171Yb+ versus other constraint 133Cs standard yields a coupled constraint of -0.04times10-15 < alpha dot/alpha < 0.46times10-15 yr-1 and -2.39times10-15 < d/dt In muCs/muB < 0.47times10-15 yr-1.

The Yb and Ca Standards: Approaches to High Stability, High Accuracy, and Transportable Optical Atomic Clocks

Abstract: This talk presents two different types of neutral atom optical clocks and emphasizes the systems' differences in potential performance and complexity. Approaches in attaining high stability, high accuracy, and transportable optical atomic clocks are discussed in detail.

Lattice-based optical clock using an even isotope of Yb

Abstract: We describe progress toward an optical lattice clock based on an even isotope of Yb. The 1 S0 → 3 P0 clock resonance in 174 Yb is accessed through a magnetically induced spectroscopic technique. Using ≈ 1 mT static magnetic fields and ≈ 10 µW of probe light power we generate Rabi frequencies of several hertz. The narrow spectroscopic features that result (< 10 Hz FWHM) require a highly stabilized laser at the clock transition wavelength of 578 nm. We describe a new all solid-state laser system that shows hertz level stability. In order to cancel the slow drift of the cavity, spectroscopy is performed on the clock transition to provide feedback to the laser. Using a Ca neutral atom frequency standard as a reference oscillator,we show high stability and an effective method for investigating clock frequency shift systematics.

Wide-Band Suppression of Laser Intensity Noise

Abstract: A wide-band power control system based on an electro-optical modulator and transimpedance amplifier has been constructed. It is capable of reducing laser intensity fluctuations to the shot noise limit over the range of Fourier frequencies from a few tens of Hz to a few MHz with the low boundary set by the light handling capacity of a photodetector and the upper boundary imposed by the spurious resonances of a given electro-optical modulator. We discuss a general approach to the design of laser intensity control system and its noise properties.

Chip-Scale Atomic Devices Based on Microfabricated Alkali Vapor Cells

Abstract: We describe recent progress in the development of millimeter-scale instruments based on alkali atom vapor cells implemented with microfabrication techniques. Because of their small size and correspondingly low power requirements, these "chip-scale" atomic clocks and magnetometers have the potential to bring atomically precise instrumentation to portable, battery-operated systems such as GPS receivers, remote sensors and wireless communication devices. In addition, wafer-level processing and assembly potentially allows for very low cost per instrument if high volumes are produced.

Injection-locked femtosecond Ti:sapphire lasers

Abstract: We demonstrate injection locking of 1 and 1.5 GHz femtosecond Ti:sapphire lasers by a second, physically separate and independent 1 GHz laser. In some cases, both the pulse envelopes and optical carriers are locked.

Reduction of optical field noise by differential detection in atomic clocks based on coherent population trapping

Abstract: We present preliminary results showing that some noise sources in vapor cell atomic clocks based on coherent population trapping (CPT) can be suppressed with differential detection. The scheme we propose differs from more conventional differential detection in that both optical fields pass through the alkali vapor cell but have different polarizations, one circular and one linear. Because CPT resonances are only excited by the circularly polarized beam, the linearly polarized beam can be used to reduce several important sources of noise. With this technique, we demonstrate reduction of the short-term frequency instability of a CPT atomic frequency reference by a factor of about 1.5.

Increasing the Mode-Spacing of Stabilized Frequency Combs with Optical Filter Cavities

Abstract: We explore methods and limitations for increasing the mode spacing of a 1 GHz optical frequency comb using a Fabry-Perot filter cavity. Applications to optical and microwave waveform generation are highlighted.