Beyond the fundamental noise limit in coherent optical fiber links

It is well known that temperature variations and acoustic noise affect ultrastable frequency dissemination along optical fiber. Active stabilization techniques are in general adopted to compensate for the fiber-induced phase noise. However, despite this compensation, the ultimate link performances remain limited by the so called delay-unsuppressed fiber noise that is related to the propagation delay of the light in the fiber. In this paper, we demonstrate a data post-processing approach which enables us to overcome this limit. We implement a subtraction algorithm between the optical signal delivered at the remote link end and the round-trip signal. In this way, a 6 dB improvement beyond the fundamental limit imposed by delay-unsuppressed noise is obtained. This result enhances the resolution of possible comparisons between remote optical clocks by a factor of 2. We confirm the theoretical prediction with experimental data obtained on a 47 km metropolitan fiber link, and propose how to extend this method for frequency dissemination purposes as well

A pulsed-Laser Rb atomic frequency standard for GNSS applications

We present the results of 10 years of research related to the development of a Rubidium vapor cell clock based on the principle of pulsed optical pumping (POP). Since in the pulsed approach, the clock operation phases take place at different times, this technique demonstrated to be very effective in curing several issues affecting traditional Rb clocks working in a continuous regime, like light shift, with a consequent improvement of the frequency stability performances. We describe two laboratory prototypes of POP clock, both developed at INRIM. The first one achieved the best results in terms of frequency stability: an Allan deviation of σy(τ) = 1.7 × 10−13 τ−1/2, being τ the averaging time, has been measured. In the prospect of a space application, we show preliminary results obtained with a second more recent prototype based on a loaded cavity-cell arrangement. This clock has a reduced size and exhibited an Allan deviation of σy(τ) = 6 × 10−13 τ−1/2, still a remarkable result for a vapor cell device. In parallel, an ongoing activity performed in collaboration with Leonardo S.p.A. and aimed at developing an engineered space prototype of the POP clock is finally mentioned. Possible issues related to space implementation are also briefly discussed. On the basis of the achieved results, the POP clock represents a promising technology for future GNSSs

Metrological characterization of the pulsed Rb clock with optical detection

We report on the implementation and the metrological characterization of a vapor-cell Rb frequency standard working in pulsed regime. The three main parts that compose the clock, physics package, optics and electronics, are described in detail in the paper. The prototype is designed and optimized to detect the clock transition in the optical domain. Specifically, the reference atomic transition, excited with a Ramsey scheme, is detected by observing the interference pattern on a laser absorption signal. \ The metrological analysis includes the observation and characterization of the clock signal and the measurement of frequency stability and drift. In terms of Allan deviation, the measured frequency stability results as low as $1.7\times 10^{-13} \ \tau^{-1/2}$, $\tau$ being the averaging time, and reaches the value of few units of $10^{-15}$ for $\tau=10^{4}$ s, an unprecedent achievement for a vapor cell clock. We discuss in the paper the physical effects leading to this result with particular care to laser and microwave noises transferred to the clock signal. The frequency drift, probably related to the temperature, stays below $10^{-14}$ per day, and no evidence of flicker floor is observed. \ We also mention some possible improvements that in principle would lead to a clock stability below the $10^{-13}$ level at 1 s and to a drift of few units of $10^{-15}$ per day

Phase noise and amplitude noise in DDS

International audienceTheir article reports on the measurement of phase noise and amplitude noise of direct digital synthesizers (DDS), ultimately intended for precision time and frequency applications. The DDS noise Sφ(f) tends to scale down as 1/ν02, until the noise hits the limit due to the output stage. The spurs, however disturbing in general, sink power from the white noise. Voltage noise can be more critical in the digital power supply than in the analog supply. Temperature fluctuations are an issue at 10-3 ... 1 Hz Fourier frequency. Passive stabilization (thermal mass) proves to be useful. Other paramours affect the phase noise, like the clock frequency and power. The amplitude 1/f noise is of the order of -110 dB(V2/V2)/Hz in some reference (typical) conditions. Owing to the page and file size limitations, only a small part of the available data can be published here

Study and characterization of a microcalorimeter thermostat

A high-accuracy adiabatic microcalorimeter has been developed at Istituto Nazionale di Ricerca Metrologica (INRIM, Torino, Italy) for the realization of the radio frequency and microwave power primary power standard. The apparatus is intended to evaluate the efficiency of proper power sensors of thermoelectric or bolometric type by measuring their losses in terms of heat produced. These kind of measurements require a good thermal stabilization of the measurement chamber, therefore a dedicated thermostat has been designed, assembled and characterized at INRIM in order to obtain a thermal stability of +/-1 mK on a daily basis

Cell-related effects in the Pulsed Optically Pumped frequency standard

In this paper we describe two phenomena that may affect the medium-long term stability performance of the pulsed vapor cell Rb clock we implemented at INRIM. The first effect is an anomalously large temperature sensitivity of the clock frequency. This effect is related to the presence of large stems in the cell and the order of magnitude of the observed phenomenon may be explained in terms of a model based on the ideal gas law