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PV: Plenarvorträge

PV IV

PV IV: Plenarvortrag

Freitag, 6. April 2001, 09:00–09:45, H105

Counting the heart beat of a hydrogen atom — •Theodor W. Hänsch — Max-Planck–Institut für Quantenoptik, Garching, Germany; Ludwig–Maximilians–Universität, München, Germany

Precision spectroscopy of the simple hydrogen atom permits unique confrontations between experiment and fundamental theory. This pursuit has inspired dramatic advances in the art of measuring the frequency of light, so that we have recently been able to measure the absolute frequency of the narrow 1S–2S two–photon resonance in atomic hydrogen to 1.9 parts in 10¹⁴. It has now become possible to measure virtually any laser frequency with unprecedented precision, using just a single small mode–locked femtosecond laser emitting a periodic train of ultrashort pulses. In the frequency domain, such a laser provides a vast comb of evenly spaced spectral lines or laser modes. The spacing of the comb lines precisely equals the pulse repetition rate, and the absolute frequencies of all these lines can be determined by measuring or controlling the rate at which the carrier phase slips relative to the envelope from pulse to pulse. This task is highly simplified if the comb spectrum is broadened in a nonlinear optical fiber via self–phase–modulation so that it spans more than an optical octave. The recent dramatic advances in optical frequency synthesis will be reviewed and future prospects discussed.