1981 HELMHOLTZ PRIZE (award ceremony on 16 March 1981)

Dr. Frank Spieweck for his work “Entwicklung eines Laser-Frequenznormals hoher Präzision im sichtbaren Spektralbereich bei 582 THz ” (Development of a laser frequency standard of high precision in the visible spectral range at 582 THz)

1981 Prizewinner: High-precision laser frequency standard in the visible range

[Translate to Englisch:] Prof. Dr.-Ing. Dieter Kind (li.), Dr. Frank Spieweck (re.)

Frank Spieweck, who was born in Fichtenau (near Berlin) in 1937, began studying physics in Braunschweig in 1958 and completed his studies in 1963 with a German "Diplom" thesis on "Untersuchungen an zwei Bandensystemen des Stickstoffes" (Investigations of two nitrogen band systems).

Starting in 1966, he worked at the Unit of Length laboratory at PTB Braunschweig. Using his research from there, he obtained a doctoral degree in 1974 from the TH Hannover under Herbert Welling for his work on argon ion lasers. Later, he became the scientific laboratory head at PTB and worked on – among other things – projects to define the Avogadro constant to prepare for the redefinition of the kilogram. He has been in retirement since 1999.

Like the fourth Helmholtz Prize, the fifth Helmholtz Prize was also awarded for precision measurements in the field of laser physics. The prize, which was endowed with 5,000 German marks, was awarded to Dr. Frank Spieweck of PTB Braunschweig in 1981 for his work “Entwicklung eines Laser-Frequenznormals hoher Präzision im sichtbaren Spektralbereich bei 582 THz” (Development of a laser frequency standard of high precision in the visible spectral range at 582 THz).

Frequency- or wavelength-stabilized lasers are of outstanding importance for the measurement of lengths by interference. As several different light wavelengths are required for a length measurement by light interference, PTB had, since 1968, been working on the stabilization of not only a red laser line as provided by the iodine-stabilized He-Ne laser, but also of a green laser line. From 1972 on, argon ion lasers were also stabilized with the aid of iodine molecule absorption lines. The green line of these lasers could be determined at 582 THz with high precision in 1975. Following comparative measurements with PTB lasers in England and France, the wavelength value of the line, which is at 514.5 nm, was internationally recommended in 1979. However, independent frequency measurements carried out at the MIT and at the Laboratoire de Physique des Lasers (LPL) in Paris had shown a surprisingly large frequency difference of 43.5 kHz for a certain hyperfine component of the line. One of the aims of Frank Spieweck's work was to explain this discrepancy.

For this purpose, he built two transportable frequency-stabilized argon ion lasers. He eliminated the short-term variations of the laser frequency by stabilizing it with the resonant frequency of a compact Fabry-Perot resonator. For long-term stabilization, he tuned the laser frequency to a molecular reference line of iodine. After that, the frequencies of the two lasers were compared by superimposing their radiation and measuring the resulting beat frequency. While one laser was tuned to a certain hyperfine component, the other laser was successively stabilized to the components to be measured.

It turned out that the beat frequencies only varied by 200 Hz. Over a period of four months, the frequencies of the two lasers deviated by a maximum of 1.6 kHz. The frequencies of the hyperfine components measured at PTB agreed with the frequencies determined at the LPL in Paris to approximately 1 kHz so that the discrepancy mentioned could be eliminated. Spieweck estimated the frequency uncertainty of his two portable lasers to be 5.5 kHz, which corresponded to a relative uncertainty of less than 10-11. The argon ion laser, together with an iodine-stabilized He-Ne laser, makes it possible to measure lengths very precisely and in a simple way by interference. In addition, the frequency of the iodine-stabilized argon ion laser is an important reference point for the calibration of spectral lines due to its large distance from the frequency of the iodine-stabilized He-Ne laser.

Literature

F. Spieweck: Entwicklung eines Laser-Frequenznormals hoher Präzision im sichtbaren Spektralbereich bei 582 THz (Development of a laser frequency. PTB-Mitteilungen 91, (1981), 336