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

A wideband signal generator according to one embodiment of the invention includes a variable frequency source and a direct digital synthesizer. Local oscillators, signal analyzers, modulators, demodulators, and other equipment including one or more such generators are also disclosed.

Wideband signal generators, measurement devices, methods of signal generation, and methods of signal analysis

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

In passive frequency standards where atoms have a permanent interaction with the probe signal, the local oscillator phase noise limits the short and medium term frequency stability. It has been shown that this spurious effect cannot be suppressed whenever there is any truncation in the spectrum of the resonator response. However, a simultaneous processing of the probe signal, similar to that of the NIST, and of the resonator response-by means of an appropriate demodulation waveform-makes it possible to significantly reduce the limiting effect. First achieved with a square wave frequency modulation, this result is now extended to various modulation waveforms. It is shown that, for various modulation waveforms, the use of an optimized demodulation provides a moderate additional improvement to a moderate 2f/sub M/ rejection: this result confirms that, for a cesium clock using a slow frequency modulation, a high 2f/sub M/ rejection and a proper demodulation are necessary to strongly reduce the limiting effect. It is also shown that a non controlled distortion in the demodulation waveform may degrade the performance. At last, for a square wave frequency modulation and in the case of a flicker phase noise, a convenient demodulation reduces the spurious effect by a few dBs without any rejection in the microwave spectrum.

Limitation of the frequency stability by local oscillator phase noise: new investigations

In frequency standards in which the atoms have a continuous interaction with the probe signal, local oscillator phase noise may limit medium term frequency stability. This spurious effect cannot be suppressed whenever there Is any truncation in the spectrum of the resonator response. Nevertheless, a simultaneous processing of the probe signal, similar to that of the NIST, and of the resonator response (by means of an appropriate demodulation) makes it possible to reduce this limiting effect. Previously achieved with a square wave frequency modulation, this result is now extended to various frequency modulations. An uncontrolled distortion in the demodulation waveform may significantly degrade the performance. For the case of a square wave phase modulation, the limiting effect also exists, but it is smaller than for a frequency modulation. When the phase noise of the local oscillator is naturally "not flat", it is possible to easily reduce the spurious effect: using the quasi-static approximation, one can calculate various optimized demodulation waveforms and the corresponding improvements. For the simplest optimized demodulation (f/sub M/, 3f/sub M/), theoretical predictions are experimentally confirmed for flicker phase noise and flicker frequency noise.

Limitation of the frequency stability by local oscillator phase noise: new investigations and natural improvements

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. >

D. Venot

Papers

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

Limitation of the frequency stability by local oscillator phase noise: new investigations

Limitation of the frequency stability by local oscillator phase noise: new investigations and natural improvements

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