Giuseppe Marra

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.

Dissemination of an optical frequency comb over fiber with 3 × 10 −18 fractional accuracy

We demonstrate that the structure of an optical frequency comb transferred over several km of fiber can be preserved at a level compatible with the best optical frequency references currently available.

First accuracy evaluation of the NPL-CsF2 primary frequency standard

An accuracy evaluation of the caesium fountain NPL-CsF2 as a primary frequency standard is reported. The device operates with a simple one-stage magneto-optical trap as the source of cold atoms. Both the uncertainty in and magnitude of the cold collision frequency shift are reduced by taking advantage of the dependence of the cross section on the effective collision energy in an expanding atomic cloud. The combined type B uncertainty (typically 4 ? 10?16) is dominated by an estimate of the frequency shift due to the distributed cavity phase. When operated at single density, the short-term fractional frequency instability of NPL-CsF2 is 1.7 ? 10?13 at 1?s and limited by the noise of the room temperature quartz-based local oscillator. During a typical frequency measurement campaign, the fountain is operated in an alternating mode at high and low density in order to measure and correct for a residual collision shift. This increases the effective fractional frequency instability to 5.4 ? 10?13 at 1?s; consequently the averaging time required for the type A uncertainty level to match that of the type B is 20 days.

Accurate rubidium atomic fountain frequency standard

The design, operating parameters and the accuracy evaluation of the NPL Rb atomic fountain are described. The atomic fountain employs a double magneto-optical arrangement that allows a large number of 87Rb atoms to be trapped, a water-cooled temperature-stabilized interrogation region and a high quality factor interrogation cavity. From the uncertainties of measured and calculated systematic frequency shifts, the fractional frequency accuracy is estimated to be 3.7 ? 10?16. The fractional frequency stability, limited predominantly by noise in the local oscillator, is measured to be 7 ? 10?16 after one day of averaging. Based on the proposed quasi-continuous regime of operation of the fountain, the accuracy of the Rb standard of 5 ? 10?17 reachable in two days of averaging is predicted.

Test of special relativity using a fiber network of optical clocks

Phase compensated optical fiber links enable high accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution. By searching for a daily variation of the frequency difference between four strontium optical lattice clocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilation predicted by special relativity. We obtain a constraint on the Robertson-Mansouri-Sexl parameter |α|≲1.1×10^{-8}, quantifying a violation of time dilation, thus improving by a factor of around 2 the best known constraint obtained with Ives-Stilwell type experiments, and by 2 orders of magnitude the best constraint obtained by comparing atomic clocks. This work is the first of a new generation of tests of fundamental physics using optical clocks and fiber links. As clocks improve, and as fiber links are routinely operated, we expect that the tests initiated in this Letter will improve by orders of magnitude in the near future.

Suppression of amplitude-to-phase noise conversion in balanced optical-microwave phase detectors

We demonstrate an amplitude-to-phase (AM-PM) conversion coefficient for a balanced optical-microwave phase detector (BOM-PD) of 0.001 rad, corresponding to AM-PM induced phase noise 60 dB below the single-sideband relative intensity noise of the laser. This enables us to generate 8 GHz microwave signals from a commercial Er-fibre comb with a single-sideband residual phase noise of -131 dBc Hz(-1) at 1 Hz offset frequency and -148 dBc Hz(-1) at 1 kHz offset frequency.

Direct Selection and Amplification of Individual Narrowly Spaced Optical Comb Modes Via Injection Locking: Design and Characterization

Many applications of optical frequency combs (OFCs) require manipulation and amplification of individual comb modes, e.g., arbitrary waveform generation, terahertz generation and telecommunications. Extracting individual comb modes can be a challenging task for OFCs with narrow comb mode spacings (100 MHz to 10 GHz) due to the limitations of conventional optical filters. Optical injection locking can address this problem, but-due to the relatively large bandwidth (1 to 10 GHz) required for simple (i.e., without the need for additional feedback loops) and stable locking-can struggle when processing OFCs with sub-GHz comb mode spacings. Here, we present an approach to optical injection locking which incorporates a dither-free phase locked loop that allowed for long-term locking to OFCs with comb spacings below the high power injection locking bandwidth. As a result, we achieved robust injection locking directly to a sub-GHz OFC (250 MHz in our experiments). Optimization of the optical injection power is carried out using detailed phase noise characterization. We achieved an Allan deviation for the frequency variation of the slave laser with respect to the injected comb mode (1 s gate time) down to 9.7 × 10-17 and 4.4 × 10-19 at 1 s and 1000 s averaging times respectively, and a phase error variance of 0.02 rad2 (integration bandwidth of 100 Hz to 500 MHz).

Frequency stability and phase noise of a pair of X-band cryogenic sapphire oscillators

Oscillators based on cryogenic sapphire resonators can supply the levels of microwave phase noise and frequency stability required for advanced time-and-frequency applications. NPL has realized two such oscillators of identical design, which operate at 9.204 GHz and incorporate both Pound-loop and power stabilization. Running both simultaneously, and comparing their outputs both to each other and to H-maser-referenced frequency sources, the phase noise and absolute frequency stability of these oscillators have been measured for the first time, based on both FFT-spectrum analyser and frequency-counter data. We report a double-sided phase noise of −87.5 dBc at 1 Hz, scaling as 1/f3, and a modified Allan deviation of less than 5 × 10−15 at 1 s; the fractional frequency drifts of the two oscillators with respect to the H-maser were −2.9 × 10−11 and −6.0 × 10−12 per day, respectively. Scope for further improvement is assessed.

Common-path self-referencing interferometer for carrier-envelope offset frequency stabilization with enhanced noise immunity

A nonlinear interferometer design for stabilizing the carrier-envelope offset frequency of a Ti:sapphire frequency comb with superior immunity to air currents and acoustic noise is presented. The scheme uses a pair of Wollaston prisms for group-delay dispersion compensation, providing an all-common-path optical configuration. Out-of-loop phase noise measurements for an unshielded interferometer setup showed up to 15 dB improvement compared to a Michelson interferometer based system. Further simplification of the self-referencing scheme providing a compact single-Wollaston-prism design has been demonstrated.

ELISA: An ultra-stable oscillator for esa deep space antennas

The technology of the Sapphire Whispering Gallery Mode Resonator allows to surpass the frequency stability of traditional ultra-stable oscillators, and then can be exploited in number of applications requiring a high frequency stability as a frequency standard for the ESA ground station for which the requirement in term of frequency stability is σ<inf>y</inf>(τ) = 3 × 10⁻¹⁵ for integration times 1 ≤ τ ≤ 1000s.