Phase noise of single frequency fiber laser

The phase noise of single frequency fiber laser refers to the random phase fluctuation of single frequency laser field vector. Generally, it is not mainly expressed in time domain, but in frequency domain. In the frequency domain, there are two definitions of phase noise.
1. Phase fluctuation power spectral density (PSD)
2. Single sideband phase noise
The meaning of phase fluctuation power spectral density (PSD) is the mean square phase fluctuation within the unit bandwidth (1Hz) FM away from the center frequency (also known as carrier frequency meter F0). The phase units of fluctuation are rad (radian) and Urad (micro radian), so the unit of phase fluctuation power spectral density is Rad2 / Hz. Sometimes we will choose the form of root mean square, and the unit will change to rad / √ Hz or Urad / √ Hz.
The phase fluctuation power spectral density (PSD) is defined as single sideband, but the influence of noise on the signal (single frequency laser) is mainly reflected in the modulation of carrier frequency, and the spectrum is symmetrical with carrier frequency as the center, resulting in sideband. We generally do not accept the practice of negative frequency, so we "fold" the part of negative frequency to the part of positive frequency.
The meaning of single sideband phase noise refers to the power spectral density of one phase modulation sideband of the signal off the center frequency (carrier frequency as F0) FM, that is, the ratio of signal power in 1Hz bandwidth to carrier power. The unit is DBC / Hz. Where DBC is the ratio of power at this frequency to total power in dB. The single sideband phase noise of an oscillator at an offset frequency is defined as the ratio of the signal power within the 1Hz bandwidth at that frequency to the total power of the signal. Therefore, the unit DBC / Hz has no dimension.
The phase noise is indicated by the ratio of the area of the rectangle within the 1Hz bandwidth at the offset frequency FM to the area included under the overall power spectrum curve. The approximate value is the difference between the height of the curve at the center frequency and the height of the curve at FM.
In essence, single sideband phase noise is the main manifestation of short-term frequency stability in the frequency domain. If the single frequency signal is very stable, its sideband will gradually decrease away from the main frequency from the spectrum. Generally, we are very concerned about the sideband at 10Hz, 100Hz, 1kHz and 10kHz away from the main frequency. If it is in logarithmic coordinates, the amplitude of the sideband is subtracted from the amplitude of the main frequency, and the unit is DBC, Then converted into unit bandwidth, in DBC / Hz.
Phase noise mainly shows the frequency stability of single frequency signal. In frequency domain, it is noise sideband, that is, phase noise. The corresponding in the time domain is called signal jitter.
It should be understood that single sideband noise is half of the power spectral density of phase fluctuation! Obviously, the latter includes the sideband power spectral density (PSD) corresponding to the negative frequency.