Properties of intensity noises of laser diodes due to reflected waves from single-mode optical fibers and its reduction

The intensity noises due to reflected waves from the single-mode fibers were found to be about 10 dB larger than those of the multimode fibers due to the large coupling coefficient to a specific mode between the laser and the coupled optical fibers. The noise enhancement of about 20 dB was observed in the both types of fibers when the direct modulation was applied for the laser diode with reflected waves, and the lower cutoff modulation frequency related to noise enhancement was found to be about 10 kHz. It was found that the noise of the narrow stripe laser is about 10 dB smaller than that of the wide stripe laser, and it was estimated that the optical isolator with a reverse isolation loss of 30 dB provides noise reduction of 40 dB.