Fringe visibility and phase noise in superluminescent diodes

A Michelson interferometer has been used to characterize fringe visibility and noise due to phase fluctuations in a GaAlAs superluminescent diode. The superluminescent diode was chosen to have very little longitudinal mode structure. The fringe visibility decreases from unity at zero path-length difference (PLD) to 50 percent at 13 μm PLD and 10 percent for a PLD of 26 μm. Noise in the interferometer increases monotonically with PLD until a level 9 dB above that for zero PLD is reached at a total photodetector current of 60 μA. An expression relating this limiting noise level and the fringe visibility data to an amplitude correlation function is derived. A numerical value of 0.74 is determined for this correlation function in the limit of large PLD.