Measurement method for linewidth of single frequency laser

Measurement method for linewidth of single frequency laser
Several measurement methods for single frequency laser linewidth from coarse to fine, as well as their applicable scenarios and technical points, are summarized as follows:
1、 Direct measurement by spectrometer (rough measurement, linewidth ≥ GHz level)
Use a grating spectrometer or optical spectrometer (OSA) to determine whether it is a single longitudinal mode, measure the edge mode suppression ratio and longitudinal mode linewidth. The conventional optical spectrometer resolution is about 0.1-0.01 nm (corresponding to 0.1-10 GHz for near-infrared), and the high performance can reach 1 pm, which is only suitable for preliminary screening of wider linewidth laser.
2、 Scanning Fabry Perot (F-P) etalon (medium narrow linewidth)
Suitable for lasers with narrower linewidth or smaller longitudinal mode spacing than optical spectrometer resolution. When measuring, it is necessary to judge based on the ratio of the laser linewidth to the F-P cavity mode width: when the laser linewidth is much larger than the cavity mode width, it can be read directly; When similar, deconvolution is required; If it is much smaller than the time, it is necessary to replace it with a higher precision F-P or use edge modeling technology for estimation.
3、 Heterodyne/Delay Self Heterodyne and Self Homodyne (Narrow linewidth, kHz~MHz range)
When high coherence single frequency laser exceed optical spectrometer resolution, heterodyne method (with known extremely narrow linewidth reference laser) or time delayed self heterodyne/self homodyne method (without reference laser) are commonly used. The device is a fiber Mach Zehnder or Michelson interferometer, with a time delay of τ d_d (determined by the difference in fiber length) between the two arms; Connecting AOM Modulator frequency shift is self heterodyne, not connected is self homodyne. AOM Modulator can shift the beat frequency away from DC to reduce low-frequency interference.
4、 Precautions:
4.1 Long extension optical fibers are affected by vibration/temperature drift, and the stability of the interferometer needs to be improved;
4.2 The two power channels need to be balanced, and EDFA Optical Amplifier compensation or fiber ring multi cycle interferometer can be used;
4.3When the line width is extremely narrow, it can read a line width of 20 dB, and then deduce a line width of 3 dB based on the Lorentz line shape;
4.4 Use a polarization controller (PC) or Faraday rotating mirror (FRM) to suppress polarization fading.