Frequency Noise
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: noise of the instantaneous frequency of an oscillating signal
Category: 
DOI: 10.61835/ipe Cite the article: BibTex plain textHTML Link to this page! LinkedIn
The term frequency noise refers to random fluctuations in the instantaneous frequency of an oscillating signal. The instantaneous frequency is defined as
$$\nu (t) = \frac{1}{{2\pi }}\frac{{{\rm{d}}\varphi }}{{{\rm{d}}t}}$$i.e. essentially as the temporal derivative of the oscillation phase <$\varphi$>. Any random deviation from a purely linear phase evolution is seen as frequency noise.
Specifically in the context of optical fields, frequency noise is the noise of the optical frequency, which is essentially the time derivative of the optical phase.
The power spectral density of frequency noise (with units of Hz2/Hz) is directly related to that of the phase noise:
$${S_\nu }(f) = {f^2}\;{S_\varphi }(f)$$where <$f$> is the noise frequency. For example, white frequency noise (<$S_{\nu }(f)$> = const) corresponds to phase noise with a power spectral density proportional to <$f^{-2}$>. In that case, the linewidth is <$\pi$> times the one-sided power spectral density of the frequency noise (or 2<$\pi$> times the two-sided power spectral density). Such a situation occurs e.g. in a single-frequency laser which is only subject to quantum noise and exhibits the Schawlow–Townes linewidth.
Some data sheets exhibit plots with units of Hz/sqrt(Hz) on the vertical axis; this is just the square root of the power spectral density of frequency noise.
Phase noise or frequency noise are just different ways of describing the same phenomenon. However, numerical processing of frequency noise rather than phase noise can have technical advantages in certain situations.
More to Learn
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Questions and Comments from Users
2021-06-21
Based on the definition of the instantaneous frequency in the first equation. To be more accurate, should the first sentence be something like this: The term frequency noise refers to random fluctuations in the frequency (or the instantaneous frequency) of an oscillating signal? Because the instantaneous frequency inherently includes the fluctuations in the frequency per definition.
The author's answer:
No, I would not state it that way, since it is not clear what exactly you mean with frequency. By saying instantaneous frequency, it becomes clear what exactly is meant.
2020-09-20
I think there is an error in this sentence: “The power spectral density of frequency noise (with units of Hz2/Hz)...”
The unit should probably be rad2/Hz?
The author's answer:
No, the instantaneous frequency has units of Hertz, not radians. However, you often find units of rad2/Hz in the literature; these refer to <$S_{\varphi}(f)$>, the noise PSD of the optical phase, which is directly related to frequency noise as explained in the article.