The Degree Of Polarization (DOP) of a light source describes what portion of the total emitted light power P_total = P_pol + P_unpol is polarized and is defined as

Natural sunlight and light from a LED is completely unpolarized (DOP=0%) whereas light from a DFB laser is completely polarized light (DOP=100%).

The light emitted by an SLED is a combination of unpolarized spontaneous emission and amplified spontaneous emission, generally resulting in a DOP considerably less than 100%.