The polarization characteristics of light are a description of the vibration direction of the electric field vector of light. There are five polarization states in total: completely unpolarized light, partially polarized light, linearly polarized light, elliptically polarized light, and circularly polarized light. However, these five polarization states do not exist absolutely in reality. In most cases, it is a mixture of one or several of them. Natural light is generally considered to be unpolarized light. Due to its spontaneous radiation working mechanism, the ASE light source emits light close to completely unpolarized light. This light has advantages in applications such as fiber optic sensing and optical device testing. For example, when testing the insertion loss of a single-axis polarization-maintaining device, the ASE light source can be directly used as the test light source. The measured loss minus 3dB is the effective insertion loss of the device. However, due to the influence of fiber optic transmission and fiber optic devices, the final output light is not absolutely completely unpolarized light. Some applications require a highly polarized ASE light source, so it is necessary to quantify its polarization degree to obtain a suitable light source.
Definition of Degree of Polarization DOP and Polarization Extinction Ratio PER
A beam of light hits the slowly rotating analyzer. The optical power is measured with a power meter behind the analyzer. The maximum value of the optical power I is recorded as Imax, and the minimum value is recorded as Imin. Imax and Imin appear twice for every 360° rotation of the analyzer.
Degree of Polarization: DOP=(Imax-Imin)÷(Imax+Imin), unit: percentage;
Extinction Ratio: PER=10log(Imax÷Imin), unit: dB;