No abstract

Schlagwörter: Laserinterferometrie, absolute Distanzinterferometrie, Abstandsmessung, Phasenmessung, synthetische Wellenlänge, Beat-Frequenz Seit mehreren Jahren gibt es Versuche, konventionelle Laserinterferometer durch absolut messende Systeme abzulösen. Um die Mehrdeutigkeiten in der interferometrischen Längenmessung zu beseitigen, wird in der Regel der Einsatz von mehreren verschiedenen Laserwellenlän-gen vorgeschlagen. Mit der Entwicklung durchstimmbarer Laserquellen gewinnen Methoden an Bedeutung, die eine variable Wellenlänge nutzen. Sie bilden eine Alternative zu den bekannten Multi-Wellenlängen-Verfahren. Während der Laserdurchstimmung wird die von der Reflektorentfernung abhängige Gesamtphasenwinkelände-rung des Interferenzsignals gemessen. Phasenwinkelän-derungen werden jedoch auch durch Brechzahlschwankungen im durchstrahlten optischen Medium und Vibrationen des Retroreflektors hervorgerufen. Diese Störun-gen können beseitigt werden, wenn anstelle der kurzen Lichtwellenlänge eine mit zwei Laserstrahlen generierte variable synthetische Wellenlänge (VSW) genutzt wird. Das auf dieser Grundlage entwickelte VSW-Interferometer liefert daher auch unter industriellen Einsatzbedingungen gute Ergebnisse. Absolute distance interferometry using variable synthetic wavelengthSince many years there have been attempts to replace conventional laser interferometers by absolutely measuring systems. To remove the ambiguities in interferometrical length measurement in general, the application of several different laser wavelengths is offered. With the development of tunable lasers those methods become important which use a variable wavelength. They are an alternative to the well-known multi wavelength techniques. Düring the laser tuning the total change of the phase of the interferometric signal is measured, which depends of the reflector distance. However, alterations of the phase are also caused by changes of the refractive index of the optical medium and by vibrations of the retroreflector.These disturbances can be avoided if a variable synthetic wavelength (vsw) is generated by two laser beams instead of using the short wavelength of light. Therefore the vsw-interferometer, developed on this basis, supplies good results also under industrial conditions.

A mode-hop-free tunable external-cavity Littrow diode laser with intracavity acousto-optic modulators (AOMs) has been built. The modes of the red laser diode without a special antireflection coating are shifted by varying the injection current. The external resonator modes and the grating selectivity are independently electrically alterable by two AOMs. Thus, a tuning of the external resonator over up to 1900 GHz is possible. A precise computer control of laser diode and AOMs allowed a single-mode tuning of the whole laser with a tuning range of 225 GHz in 250 s. Additionally, we demonstrated fast tuning over 90 GHz in 190 micros and a repetition rate of 2.5 kHz.

No abstract

We developed a theory that describes fast mode-hop-free tuning of an external cavity diode laser in Littrow configuration with two acousto-optic modulators (AOMs) inside the laser cavity. The theory is based on synchronous shifting of the external cavity modes and the Littrow grating selectivity. It allows calculating the driving signals of both AOMs in order to reach a desired temporal variation of the laser frequency, including particularly fast tuning as well as an arbitrary shape of the tuning function. Furthermore, we present a laser setup for which the needed signals for both AOMs are generated by two direct digital synthesizer circuits. Thereby we were able to verify the theoretical predictions experimentally, achieving, e.g., sinusoidal single-mode tuning of the laser frequency over 40 GHz at a repetition rate of 10 kHz and over 12 GHz at 25 kHz. Finally, the limitations of the theory are discussed.