Untersuchung der Differenzfrequenzen zwischen zwei Lasern
The frequency stability of He-Ne-Lasers was investigated by heterodyning two free running laser oscillators. Measurements were performed in a laser device of high mechanical stability protected against acoustical noise. For single mode operation the stability and spectral purity of the laserfrequency was found to be well defined by the constancy and bandwidth of the beat frequencies.In multimode operation beat frequencies were found which were nearly independent from the oscillator cavity length. They were produced by inevitable nonlinearities in the electronic equipment. For pointing out this effect more clearly, an additional nonlinear element was introduced into the electronic circuit. The additional nonlinearity was found to produce the same beatfrequencies as observed before, but with higher amplitudes. Presence of these second order beats may lead to misinterpretation of the frequency stability.From the minimum obtained beat frequency of 200 c/s in single mode operation it was concluded that a minimum laser bandwidth of smaller than 200 c/s was realized.