Abstract
This paper investigates the experiment work of laser diode coherence properties which are the most fundamental property . In this experiment a scientific instrument designed which called Michelson interferometer to measure a beam of light thah splitting it into two equal halves by using a beam-splitter which one of the beams called reference beam and the second beam travels an extra distance , so it gets slightly out of the phase . the output power is measured by photo-detector as a function of the input current in order to determine threshold current , because of the laser diode is sensitive to temperature, which it set to 25ْC , the measured results was above the threshold current because laser diode