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Laser beams and beat frequency.

by Bill E
Tags: beams, beat, frequency, laser
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Bill E
Oct14-11, 11:02 PM
P: 5
If I combine 2 laser beams of different wavelengths, would I get a beat frequency of the difference between the 2 lasers?

Thanks, Bill
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Andy Resnick
Oct15-11, 02:24 PM
Sci Advisor
P: 5,523
It is possible to measure both the sum and difference frequencies (heterodyne detection). IIRC, implementation requires the detector to be 'locked' to one of the oscillators- at least, the heterodyne optical systems I used have all set one of the oscillators as the 'reference'.
Oct15-11, 02:46 PM
P: 882
You can get even more: mixing two laser beams of close frequencies: f, f+Df (Df<<f) in a real medium (a little bit nonlinear) you may get harmonic waves: f+2Df, f-Df, etc.

This effect causes lots of problems to WDM telecommuniaction networks, as optical nonlinearity in the glass used for fibres is sufficient to generate cross-talks between lambda-channels: the data in channel f+2Df are noised by cross-talk generated by harmonic beat of signals transmitted in f and f+Df.
Special kind of optical fibres had to be developed to reduce this effect, they are installed nowadays, but it is impossible to use even half of channels of WDM over 30 years old long-distance fibres, which were not aware about such effects.

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