If two laser beams with different optical frequencies are superimposed on a photodetector measuring the optical intensity, a beat note – i.e., a signal with the difference of the optical frequencies – can usually be observed, if some conditions are met:
- The spatial distributions of the two light fields must overlap and must not be orthogonal. (For example, there may be no beat note if a laser beam with TEM00 and TEM01 modes excited hits a detector.) Somewhat clipping the beams, or even just some non-uniformity of the detector surface, can solve that problem.
- The polarization states also must not be orthogonal.
- The optical frequency difference must be within the bandwidth of the photodetector.
- Obviously, the wavelengths must be within the range where the photodetector is sensitive.
As a fast photodetector can have a bandwidth of tens of gigahertz (or even higher), optical frequency differences of that order of magnitude can be measured e.g. by analyzing the photodetector output with an electronic frequency counter or an electronic spectrum analyzer. An important application of this is in frequency metrology. For example, the frequency of some laser can be measured by recording a beat note between that laser and a close-by optical signal with known optical frequency. Such measurements are greatly facilitated by an optical frequency comb which can cover a wide range of well-defined optical frequencies, so that a sufficiently nearby reference frequency for a beat measurement can be found for any optical frequency in that large range.
The linewidth of a beat note of two free-running lasers with uncorrelated laser noise (which is the usual situation) is larger than the linewidth of each laser separately. If one laser has a much larger linewidth than the other one, the beat note will have that linewidth. However, the beat linewidth can even be smaller than that of each laser if the phase noise of both lasers is at least partially correlated. In an extreme case, one of the lasers may be phase-stabilized so as to obtain a constant beat frequency, as defined e.g. by some electronic oscillator. The linewidth of the beat note, measured against a clock in synchronism with the electronic oscillator, can then be exactly zero, if the phase difference exhibits only small stationary fluctuations.
Optical beat notes are essential for the technique of optical heterodyne detection.
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