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Optical setup for the experiment on the thorium nuclear clock.

Optical Clocks with Trapped Ions

Working Group 4.43

Photonically generated microwave signals with high frequency stability

Most conventional ultra low noise microwave sources are based on quartz oscillator references at 5 MHz because of their superior low phase noise at Fourier frequencies < 1 kHz. Relative frequency instabilities of 1⋅10-13 can be realized for averaging times up to a few seconds. Even lower relative frequency instabilities below 10-15 can be achieved with sapphire loaded cavity oscillators (SLCO). However this approach is expensive and requires cryogenic cooling.

A team of AG 4.43 and AG 4.31 has developed a microwave source based on an optical reference which provides a relative frequency instability of < 10-14 in 1 s. Here, the outstanding short term stability of a highly stable laser is converted into the microwave region using a frequency comb and applying the so called transfer-technique in order to stabilize the frequency of a dielectric resonator oscillator (DRO). It has been demonstrated by several groups that photonically generated microwave signals can meet the short-term performance of the best cryogenic sapphire loaded cavity oscillators.

A photonically generated microwave signal at 9.6 GHz is used at PTB to interrogate the atoms in a caesium fountain clock. As a result of its extremely low phase noise level (< - 110 dBc/Hz at 10 Hz offset (see fig. 1)) the frequency stability of the caesium fountain clocks is no longer limited by frequency fluctuations of the interrogation oscillator [1, 2].

Bild Fourier frequency f (Hz)

Fig. 1: Single-sideband phase noise power density L as a function of the Fourier frequency. Red: measured phase noise of the stabilized 9.6 GHz DRO; black specified phase noise of the free-running DRO; green: 5MHz quartz at 9.6 GHz; blue: cryocooled SLCO.

Literature

[1] B. Lipphardt, G. Grosche, U. Sterr, C. Tamm, S. Weyers and H. Schnatz: The stability of an optical clock laser transferred to the interrogation oscillator for a Cs fountain [pdf]4), IEEE Trans. Instrum. Meas. 58, 1258‐1262 (2009)

[2] S. Weyers, B. Lipphardt, H. Schnatz: Reaching the quantum limit in a fountain clock using a microwave oscillator phase locked to an ultrastable laser [pdf] 2), Phys. Rev. A 79, 031803(R) (2009)

[3] Chr. Tamm, N. Huntemann, B. Lipphardt, V. Gerginov, N. Nemitz, M. Kazda, S. Weyers,  E. Peik: Cs-Based Optical Frequency Measurement Using Cross-Linked Optical and Microwave Oscillators [pdf]2), Phys. Rev. A  89, 023820 (2014)

Contact

Dipl-Ing. Burghard LipphardtPhone: +49 (0)531 592 4428
Fax: +49 (0)531 592 69 4428
E-mail: Burghard Lipphardt