Evaluation of modal noise in multimode fiber-optic systems

Modal noise at fiber connecting points is studied theoretically and experimentally. Simple formulas for calculating modal-noise power, noise bandwidth, and nonlinear distortions are obtained by introducing a visibility concept. Modal noise for multimode operating LD is also analyzed. These theoretical results are applied to high-bit-rate digital systems and analog systems using direct IM. Then, it is shown that a 3-dB connection loss limits effective signal-to-noise ratio (SNR) to 22 dB in high-bit-rate digital systems, and that in analog systems, the SNR is improved more than 20 dB by using a superimposed pulse modulation with a frequency that is 100 times the baseband signal bandwidth.