DeepLightSeekdefinition:
The measure of temporal coherence is the time required to characterize significant attenuation of coherence.
Coherent time can quantitatively represent the temporal coherence of light. In coherence theory, coherence time is defined as the time required for the decay of the field coherence equation. For a stable optical field (with stable statistical properties), the coherence of temporal coherence is defined as:
Where E (t) is the electric field at a certain location. At that time, τ=0, the coherence was 1, and it monotonically decreased with increasing time delay τ. For any shape of coherence equation, coherence time can be defined as:
When the coherence index decays (which occurs when the laser is only affected by quantum noise), the coherence time is equal to the exponential decay time.
Knowing the coherence time is crucial when the shape of the coherence equation (or Fourier spectrum) is known. However, knowing only the coherence time (or linewidth) cannot give all the properties of coherence.
The coherence time is closely related to the radiation linewidth, which is the width of the spectrum. In the case of coherent exponential decay as described above, the spectrum is of Lorentzian type, with a linewidth of (full width at half maximum):
The constant factors in the equation are different for coherent equations of different shapes (for example, Gaussian type is twice as large). On the other hand, knowing the linewidth can estimate the coherence time, but the conversion relationship between the two depends on the spectral shape.
If the frequency noise spectrum is not flat, but rises very high in the small noise frequency range, then even when the time delay is greater than the reciprocal of the linewidth, the coherence is not zero; This is very important in self heterodyne linewidth measurement.
The commonly used method for quantifying temporal coherence is not by using coherence time, but by using coherence length, which is the product of coherence time and the speed of light in vacuum.
Laser typically has a long coherence time, especially for single frequency solid-state lasers, which can reach several milliseconds. Long coherence time is required in many applications (see coherence).
