Optical Polarimeter 193 2.6.3 Optical Polarimeters An optical polarimeter is an instrument used to measure the Stokes para- meters of optical signals [20, 21, 22, 23]. According to the definition of Stokes parameters given by Equations 2.6.5­2.6.8, they can be obtained by measuring the optical powers after the optical signal passes through polarization-sensitive optical devices such as linear polarizer and retarders. If we define the following seven measurements: P ¼ total optical power P 0 ¼ optical power measured after a linear horizontal polarizer P 90 ¼ optical power measured after a linear vertical polarizer P þ45 ¼ optical power measured after a linear þ45 polarizer P À45 ¼ optical power measured after a linear ­45 polarizer P þ45 ,l/4 ¼ optical power measured after a l/4 retarder and a linear þ45 polarizer P À45 ,l/4 ¼ optical power measured after a l/4 retarder and a linear ­45 polarizer