S. N. Lea

High-resolution microwave frequency transfer over an 86-km-long optical fiber network using a mode-locked laser

We demonstrate the transfer of an ultrastable microwave frequency by transmitting a 30-nm-wide optical frequency comb from a mode-locked laser over 86 km of installed optical fiber. The pulse train is returned to the transmitter via the same fiber for compensation of environmentally induced optical path length changes. The fractional transfer stability measured at the remote end reaches 4×10(-17) after 1600 s, corresponding to a timing jitter of 64 fs.

Evaluation of the primary frequency standard NPL-CsF1

A new caesium fountain frequency standard (NPL-CsF1) at the National Physical Laboratory is described. Procedures for evaluation of the systematic frequency shifts are presented. The NPL-CsF1 has a short-term stability σy(τ) = 1.4 × 10−13τ−1/2, predominantly due to the local oscillator phase noise. The accuracy of 1 part in 1015 is limited by the uncertainty of the frequency shift due to collisions between cold atoms.

Absolute frequency measurement of a 1.5-µm acetylene standard by use of a combined frequency chain and femtosecond comb

We have developed and characterized a pair of Doppler-free acetylene-stabilized diode laser frequency standards as optical communications references. The Allan deviation sigma/f of an individual system reaches a minimum of 4 x 10(-14) at a sampling time of 5000 s, and the long-term lock-point repeatability is found to be 0.4 kHz (one standard uncertainty), corresponding to a fractional uncertainty of 2 x 10(-12). Using a combination of a frequency chain and a self-referenced femtosecond comb, we have measured the frequency of line P(16) of the v1 + v3 overtone band of 13C2H2 to be 194,369,569,385.9 (3.0) kHz. The uncertainty of this number is limited solely by the reproducibility of the standards.

Trapped ion optical frequency standards

Optical frequency standards based on narrow absorptions in laser-cooled single trapped ions have recently begun to demonstrate stabilities that are competitive with cold atom fountain microwave standards. This paper presents a short review of the wider state-of-the-art development of these single cold trapped ion frequency standards, coupled with a more detailed account of recent results achieved at National Physical Laboratory in respect of single ion systems based on 88Sr+, 87Sr+ and 171Yb+. Narrow linewidth data for the optical clock quadrupole and octupole transitions respectively at 674 nm in 88Sr+ and 467 nm in 171Yb+, are presented, together with a discussion of current systematics and future projections. The potential for optical clock operation is outlined.

An optical frequency standard based on the electric octupole transition in /sup 171/Yb/sup +/

The frequency of the /sup 2/S/sub 1/2/(F=0,m/sub F/=0)-/sup 2/F/sub 7/2/(F=3,m/sub F/=0) transition in a single, trapped, laser cooled ion of /sup 171/Yb/sup +/ has been measured with an improved narrow probe laser and a femtosecond laser frequency comb generator. Our best estimate of the frequency is 642 121 496 772.3 /spl plusmn/ 0.6 kHz by comparison with earlier measurements. The uncertainty is limited by measurement statistics and by the ac Stark shift.

Absolute frequency measurements of 633 nm iodine-stabilized helium–neon lasers

The frequency of a helium–neon laser stabilized to the hyperfine component f (alternatively denoted a16) of the 127I2 11-5 R(127) line at 633 nm has been measured with respect to the SI second using a femtosecond optical frequency comb generator based on a mode-locked Ti : sapphire laser and microstructure fibre. The standard uncertainty of this measurement is 0.6 kHz. The same laser was taken to BIPM mid-way through the measurements and its frequency compared to that of the BIPMs continuously maintained iodine-stabilized helium–neon laser BIPM-4. From this comparison, the frequency of BIPM-4 when stabilized to the same iodine hyperfine component f is 473 612 353 608.1 (0.7) kHz.

An evaluation of the frequency shift caused by collisions with background gas in the primary frequency standard NPL-CsF2

Collisions between cold cesium atoms and background gas atoms at ambient temperature reduce the cold atom signal in a fountain clock and at the same time produce a shift in the measured clock frequency. We evaluate the shift in the NPL-CsF2 cesium fountain primary frequency standard based on measurements of the fractional loss of cold atoms from the atomic cloud during the interrogation time combined with a model by Gibble that quantifies the relationship between the loss and the frequency shift.

Frequency measurement of the 2 S 1/2 _2 D 3/2 electric quadrupole transition in a single 171 yb+ ion

We report on precision laser spectroscopy of the ²S<inf>1/2</inf>(F = 0) −²D<inf>3/2</inf>(F = 2, m<inf>F</inf> = 0) clock transition in a single ion of ¹⁷¹Yb⁺. The absolute value of the transition frequency, determined using an optical frequency comb referenced to a hydrogen maser, is 688 358 979 309 310 ± 9 Hz. This corresponds to a fractional uncertainty of 1.3 × 10⁻¹⁴.

Progress towards 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.

Transportable cavity-stabilized laser system for optical carrier frequency transmission experiments

We report the design and performance of a transportable laser system at 1543 nm, together with its application as the source for a demonstration of optical carrier frequency transmission over 118 km of an installed dark fiber network. The laser system is based around an optical reference cavity featuring an elastic mounting that bonds the cavity to its support, enabling the cavity to be transported without additional clamping. The cavity exhibits passive fractional frequency insensitivity to vibration along the optical axis of 2.0×10(-11)  m(-1) s(2). With active fiber noise cancellation, the optical carrier frequency transmission achieves a fractional frequency instability, measured at the user end, of 2.6×10(-16) at 1 s, averaging down to below 3×10(-18) after 20,000 s. The fractional frequency accuracy of the transfer is better than 3×10(-18). This level of performance is sufficient for comparison of state-of-the-art optical frequency standards and is achieved in an urban fiber environment.