Abstract

Lightwave systems using radio frequency (rf) or microwave subcarrier multiplexing (SCM) have recently been suggested as a viable alternative method for video or even general broadband communications. If we can use subcarriers into tens of GHz, the information capacity of such a system would be comparable to other major multiplexing schemes, i.e., WDM and TDM. We call this system MDM (microwave division multiplexing). The MDM can be implemented using direct modulation (DM) of the laser diode or using an external interferometric modulator (EIM). Recently the DBM (double-beam modulation) was also suggested for MDM.¹ Different systems suffer from different nonlinearities and also fiber dispersions affect different systems differently. The dispersion-induced harmonic and intermodulation distortions are analyzed and compared for these systems. As an example, the DM suffers severely from harmonic distortions caused by fiber dispersion as well as nonlinearities of the light source. Therefore intermodulation distortions are worsened and information capacity and distance are limited. Our calculations show that for a fiber with a dispersion of 2 ps/km nm at 1.3-µm wavelength, the transmission distance for a 10-GHz subcarrier is <21 km for a modulation index m = 1 and 36 km for m = 0.5, if we want to keep the total harmonic distortions less than –40 dB.

© 1989 Optical Society of America