We have achieved a stability of 3/spl middot/10/sup -13/ /spl tau//sup -1/2/ for 3</spl tau/<30 s with a laser-pumped rubidium gas-cell frequency standard by reducing the effects due to noise in the microwave and laser sources. This result is one order of magnitude better than the best present performance of lamp-pumped devices.

https://doc.rero.ch/record/28622/files/Mileti_Gaetano_-_Laser-Pumped_Rubidium_Frequency_Standards_20120229.pdf

Laser-pumped rubidium frequency standards: new analysis and progress

Atomic frequency standards using square wave frequency modulation effectively interrogate the atomic line by switching back and forth between two frequencies with equal atomic absorption values. For a symmetric absorption line, the slope of the responses will also be equal. In the quasistatic limit, this would seem to be an ideal interrogation process: the sign reversal of frequency slope can be removed by detection electronics to give an essentially unvarying sensitivity to local oscillator frequency variations. Such an interrogation would seem to eliminate L.O. aliasing and relieve stringent requirements on L.O. phase noise. Nevertheless, sign changes in the interrogation and detection processes mean that the sensitivity is actually zero at some point in the cycle. We derive consequences of this fact by an analysis in terms of the sensitivity function g(t). For white phase noise, we derive an optimal form for g(t) and show that the aliased noise always diverges as g(t) approaches a constant. For flicker phase noise, we find a limiting form that could, in principle, eliminate the aliasing effect; in practice, however, the improvement is limited by a slow dependence on available bandwidth. Finally, we derive optimized forms for any phase noise spectrum.

Local oscillator limited frequency stability for passive atomic frequency standards using square wave frequency modulation

It is known that the local oscillator phase noise limits the frequency stability in passive frequency standards. When the probe signal is permanently applied to the atomic resonator, this limitation is due to an intermodulation effect. We have established that, with a square wave frequency modulation, a proper signal processing using a notch filter in the synthesis chain and a peculiar demodulation waveform significantly reduces the limiting effect. Experimental tests confirm the optimization of the performance by the theoretical demodulation waveforms. The previous State of the Art is improved by a factor of 4.5. At last, in order to determine the optimum signal processing for different modulation conditions, we have realized an automatic system. It compares the frequency stabilities obtained for various signal processing conditions, it increases the number of harmonics involved in the demodulation and changes their rates. This system makes it possible to determine the minimum limiting effect on frequency stability, in passive frequency standards where atoms have a permanent interaction with the probe signal.

Reducing the effect of interrogation phase noise on the frequency stability in passive frequency standards: experimental results and new investigation means

This paper studies the limitations due to the local oscillator on the short-term stability of a laser pumped rubidium frequency standard. Two effects have been considered: the increase of current noise on the resonance signal and the "aliasing" effect originated by the microwave phase noise. After reducing these two effects, the standard has demonstrated a short-term stability of /spl sigma//sub y/(/spl tau/)=3.10/sup -13/ /spl tau//sup -1/2/. We have measured the clock stability as a function of local oscillator phase modulation (PM) noise at harmonics of the sine wave modulation frequency. At present we are not aware of a complete theory for PM noise at Fourier frequencies that are very large compared to the line width. In some cases the PM noise at 100 times the nominal line width dominated the short term frequency stability. The details presented here should prove useful for testing a complete theory of this effect and give some insight into the design requirements of local oscillators for passive standards.

Improving the short-term stability of laser pumped Rb clocks by reducing the effects of the interrogation oscillator

Experimental measurements in our present diode-laser-pumped Rb buffer-gas-cell frequency standard show that the sensitivity to microwave PM noise is approximately a factor of 10 lower when square-wave modulation is used in place of the more traditional sine-wave modulation. This result virtually eliminates what had been a serious limitation to the frequency stability attainable in cell-type standards. It also has ramifications for low-cost, commercial standards.

http://ieeexplore.ieee.org/iel4/5822/15518/00717887.pdf

Frequency stability of cell-based passive frequency standards: reducing the effects of local-oscillator PM noise

Due to an intermodulation effect, the local oscillator phase noise may limit the medium term frequency stability of passive frequency standards. A new possibility of reducing this effect is presented. The mathematical approach is confirmed by a simulation of the frequency standard behavior. Optimal demodulation waveforms are given. >

R. Barillet

Papers

Reducing the effect of interrogation phase noise on the frequency stability in passive frequency standards: experimental results and new investigation means

A possibility of reducing the effect of interrogation phase noise on the frequency stability in passive frequency standards