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.
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].
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 Lipphardt | Phone: +49 (0)531 592 4428 Fax: +49 (0)531 592 69 4428 E-mail:  Burghard Lipphardt |