Unit of Length

Optical frequency standards typically display very low frequency noise. Similar to rf techniques, it can be investigated by analyzing the beat-note between two of those oscillators. In case of small spectral separations (
Δf ≤ 100 GHz) the difference frequency can be recorded by means of standard microwave techniques. However, in general the spectral offsets are in the Terahertz regime such that the straightforward approach is no longer applicable. In that case one utilizes the extremely broad comb-like spectra of mode-locked femtosecond lasers to bridge the spectral gap. The mode-locked laser acts as a phase coherent link thus tranferring the frequency of the 'virtual' beat-note between both oscillators to the rf regime for convenient analysis. This method allows to frequency-compare optical oscillators within the spectral range covered by the femtosecond laser and provides a key element for the characterization and operation of optical clocks [1]. In addition, the frequency comb can realize a vast range of rational division factors (1…10⁶), thus facilitating the comparison of the optical oscillators with the primary microwave standard, which are the base for the definition of the SI unit second [2].

In AG 4-35 we characterize ultra-stable optical and microwave oscillators by comparing them with each other and, moreover, we aim at improving the performance of the optical frequency dividers in terms of stability and uncerntainty [1].

[1] C. Hagemann, C. Grebing, T. Kessler, St. Falke, N. Lemke, Ch. Lisdat, H. Schnatz, F. Riehle, and U. Sterr, “Providing 10^‐16 short‐term stability of a 1.5 μm laser to op.cal clocks” IEEE Trans. Instrum. Meas. 62, 1556‐1562
(2013)

[2] B. Lipphardt, G. Grosche, U. Sterr, C. Tamm, S. Weyers, and H. Schnatz,
“The stability of an op.cal clock laser transferred to the interroga.on oscillator for a Cs fountain” IEEE Trans. Instrum. Meas. 58, 1258‐1262 (2009)

Dr. Christian Grebing
Phone: +49 (0)531-592 4351
Fax: +49 (0)531- 592 69 4351
Email: Christian Grebing