G Wilpers

Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references

We use femtosecond laser frequency combs to convert optical frequency references to the microwave domain, where we demonstrate the synthesis of 10-GHz signals having a fractional frequency instability of < or =3.5 x 10(-15) at a 1-s averaging time, limited by the optical reference. The residual instability and phase noise of the femtosecond-laser-based frequency synthesizers are 6.5 x 10(-16) at 1 s and -98 dBc/Hz at a 1-Hz offset from the 10-GHz carrier, respectively. The timing jitter of the microwave signals is 3.3 fs.

Frequency Uncertainty for Optically Referenced Femtosecond Laser Frequency Combs

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

Stabilized frequency comb with a self-referenced femtosecond Cr:forsterite laser

The frequency comb of a Cr:forsterite femtosecond laser is stabilized using the f-to-2f self-referencing technique. The frequency noise of the comb components at 1064, 1314, and 1550 nm differs significantly from the noise of f/sub 0/.

Optical frequency/wavelength references

For more than 100 years, optical atomic/molecular frequency references have played important roles in science and technology, and provide standards enabling precision measurements. Frequency-stable optical sources have been central to experimental tests of Einsteins relativity, and also serve to realize our base unit of length. The technology has evolved from atomic discharge lamps and interferometry, to narrow atomic resonances in laser-cooled atoms that are probed by frequency-stabilized cw lasers that in turn control optical frequency synthesizers (combs) based on ultra-fast mode-locked lasers. Recent technological advances have improved the performance of optical frequency references by almost four orders of magnitude in the last eight years. This has stimulated new enthusiasm for the development of optical atomic clocks, and allows new probes into nature, such as searches for time variation of fundamental constants and precision spectroscopy.

Absolute frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser

A Cr:forsterite laser-based frequency comb is stabilized simultaneously to two NIST frequency references. Several optical frequency reference frequencies are then measured from 1315 nm - 1620 nm, including methane lines near 1330 nm.

International comparisons of femtosecond laser frequency combs

Two types of international comparisons of femtosecond laser frequency combs have been performed in France and the USA. Five combs were involved in the comparisons. Three combs, of which two are transportable, employ nonlinear photonic crystal fiber (PCF) to obtain a wide spectrum covering a full optical octave. The other two are based on broadband femtosecond lasers and require no PCF. The comparisons were performed by counting the optical heterodyne beats between pairs of combs. The frequency agreement among three combs was at the subhertz level in the 563 THz part of the comb spectrum when the combs were referenced to a hydrogen maser. When the combs were referenced to an optical standard, the frequency agreement among four combs was much improved and found to be at the /spl sim/10/sup -19/ level in the spectral range of 333-473 THz. The fact that this result is obtained by five independent measurement systems (combs) strengthens the conclusion that no systematic effects are present at this level.

Observation of large atomic-recoil-induced asymmetries in cold atom spectroscopy

The atomic-recoil effect leads to large (25%) asymmetries in a nearly ideal form of saturation spectroscopy based on Ca atoms that have been laser-cooled to 10 {mu}K. Starting with spectra from the more familiar Doppler-broadened domain, we show how the fundamental asymmetry between absorption and stimulated emission of light manifests itself when shorter spectroscopic pulses lead to the Fourier transform regime. These effects occur on frequency scales much larger than the size of the recoil shift itself, and have not been described before in saturation spectroscopy. These results directly impact precision spectroscopic measurements.

Ultra-high stability optical frequency standard based on laser-cooled neutral calcium

A beatnote between the Ca and Hg/sup +/ optical frequency standards via a mode-locked fs-laser frequency comb demonstrates the highest frequency stability measured to date. The high stability accelerates evaluation of the Ca standards systematic shifts.

Optical clocks with cold atoms

This paper discusses an optical atomic clock that uses a narrow atomic resonance to control the frequency of a spectrally narrow laser source. The atomically controlled frequency of the laser gives the clock oscillation frequency as an optical output and provides the stable reference for frequency and timing.

Optical and microwave frequency stability: some constraints

Optical frequency references achieve the best frequency stability of any oscillators by taking advantage of high Q = v/sub 0///spl Delta/v optical resonances. These systems are beginning to run into fundamental and technical limitations which are discussed.