Lennart Robertsson

Delivery of high stability optical and microwave frequency standards over an optical fiber network

Optical and radio frequency standards located in JILA and National Institute of Standards and Technology (NIST) laboratories have been connected through a 3.45-km optical fiber link. An optical frequency standard based on an iodine-stabilized Nd:YAG laser at 1064 nm (with an instability of ∼4×10-14 at 1 s) has been transferred from JILA to NIST and simultaneously measured in both laboratories. In parallel, a hydrogen maser-based radio frequency standard (with an instability of ∼2.4×10-13 at 1 s) is transferred from NIST to JILA. Comparison between these frequency standards is made possible by the use of femtosecond frequency combs in both laboratories. The degradation of the optical and rf standards that are due to the instability in the transmission channel has been measured. Active noise cancellation is demonstrated to improve the transfer stability of the fiber link.

Absolute frequency atlas of molecular I/sub 2/ lines at 532 nm

With the aid of two iodine spectrometers, we report for the first time the measurement of the hyperfine splittings of the P(54)32-0 and R(57)32-0 transitions near 532 nm. Within the tuning range of the frequency-doubled Nd:YAG laser, modulation transfer spectroscopy recovers nine relatively strong ro-vibrational transitions of /sup 127/I/sub 2/ molecules with excellent SNR. These transitions are now linked together with their absolute frequencies determined by measuring directly the frequency gaps between each line and the R(56)32-0:/spl alpha//sub 10/ component. This provides an attractive frequency reference network in this wavelength region.

Femtosecond-laser-based optical clockwork with instability</= less than or equal to 6.3 X 10(-16) in 1 s.

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 </=6.3 chi 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 chi 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.

A new method to determine the absolute mode number of a mode-locked femtosecond-laser comb used for absolute optical frequency measurements

The generation of optical frequency combs, directly referenced to the SI second, can be used to make measurements of optical frequencies. This provides a supreme method for the realization of the meter. However, an approximate knowledge of the frequency of the radiation is normally needed for such measurements in order to determine the integral order of the comb component used. Such information is usually obtained by prior wavelength measurements of the radiation under study. This paper demonstrates a new method to determine the absolute mode number in optical frequency measurements using mode-locked femtosecond lasers, thus eliminating the need for complementary wavelength measurements. Measurements of the frequency of an iodine-stabilized He-Ne laser at /spl lambda/=633 nm and a Nd:YAG laser at /spl lambda/=532nm are given as examples.

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

The 8th International Comparison of Absolute Gravimeters 2009: the first Key Comparison (CCM.G-K1) in the field of absolute gravimetry

The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable.The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters (one of the 22 AGs was ultimately withdrawn) and nine relative gravimeters participated in the ICAG/RGC campaign.This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009.After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.

Absolute frequency measurement of molecular iodine lines at 514.7 nm, interrogated by a frequency-doubled Yb-doped fibre laser

The frequency of several hyperfine components of 127I2 has been measured at 514.67 nm using an optical frequency comb. Among them, the a3 component of the P(13)43-0 transition is recommended by the CIPM for the practical realization of the definition of the metre. The result is consistent with other measurements of the same line within 1.5 kHz of uncertainty, when the collisional shift is taken into account. A frequency-doubled distributed feedback fibre laser has been developed to carry out these measurements.

The BIPM laser standards at 633 nm and 532 nm simultaneously linked to the SI second using a femtosecond laser in an optical clock configuration

The study of an optical clock based on an iodine-stabilized Nd:YAG laser has been performed using a femtosecond laser. The optical frequency of the iodine-stabilized He-Ne laser at 633 nm and the Nd:YAG laser at 532 nm have been measured simultaneously with a relative uncertainty below 1/spl times/10/sup -12/.

Progress on the BIPM watt balance

Since the beginning of the development in spring 2005, considerable progress has been made on the BIPM watt balance. We have continued the development of a room temperature experiment to test the feasibility of simultaneous force and velocity measurements. We are now able to simultaneously acquire the velocity and voltage of the moving coil and to deduce their ratio. This article briefly recalls the main ideas of the BIPM watt balance and reports the progress to date and the preliminary results.

Fourth International Comparison of Absolute Gravimeters

In 1994, from 24 May to 8 June, the fourth International Comparison of Absolute Gravimeters was held at the Bureau International des Poids et Mesures, S?vres, France. Eleven absolute gravimeters were operated at five sites and then compared by means of a high-precision gravity network which was itself observed throughout the period. The results demonstrate that absolute gravimetry can be carried out to an accuracy of 3 ?Gal to 4 ?Gal, which represents a substantial improvement since the first international comparison (1981, 10 ?Gal). The relatively low rate of improvement, one third of an order of magnitude in fifteen years, is certainly limited by current technology for measuring small time and space intervals. A notable finding of the comparison was an unsuspected source of systematic error which can be eliminated by new hardware or corrected through a correction algorithm.