Abstract

A laser diode having high output power and good high-frequency characteristics is an attractive optical source not only for a digital system but also for an analog system. Although the nonlinearity of the laser diode limits its application to an analog system, modulation methods, such as PIM, PFM, and FM, can be used to overcome this drawback. But when a high C/N ratio is required, as in a long repeated line, the laser-diode noise cannot be neglected even if these modulation methods are used. For example, C/N of 50 dB at laser-diode output is necessary to obtain video S/N of 60 dB in an FM system, as is shown in Fig. 1. There are two kinds of laser-diode noise. They are the inherent noise that is observed even in an ideal laser diode and the excess noise caused by operational imperfections. Many discussions have been made on the inherent noise, mainly in the case of dc operation. The excess noise has been studied in relation to mode instability, including the effect of the reflected wave on noise level. But the dependence of the inherent noise on the operation condition including high frequency modulation and the effect of the reflected wave from a fiber line on the excess noise have not been examined sufficiently. We have studied both laser-diode noise under direct-current, sine-wave, and pulse-wave operation in various bias current levels. Further, the excess noise was measured with or without a fiber cable. Two methods for reducing the excess noise induced by reflection were examined.

© 1979 Optical Society of America