Abstract
Optical beat interference is a signal degradation mechanism in lightwave subcarrier multiplexed transmission that arises when different subcarrier channels are transmitted from separate lasers. For any two laser fields, optical mixing at the photodetector creates a beat signal in the rf domain at the difference of the two optical frequencies. If the spectrum of the beat signal overlaps with the subcarrier channel, there is a signal degradation called optical beat interference1,2 and when the interference is sufficiently high, it leads to a channel outage. This has been shown to be a significant limitation in subcarrier multiple access schemes.1 It is desirable to determine the conditions under which the optical beat interference power leads to outage. Recently, an optical interference measurement was reported for 1300-nm multimode lasers modulated at 800 MHz.2 It is anticipated that a high modulation index can lead to an improvement in the carrier to interference ratio (CIR) due to spectral broadening. In this paper, we report experimental measurements of optical beat interference for 1300 nm DFB lasers modulated at high frequencies at various modulation indices. It is shown that optical beat interference decreases as anticipated when the modulation index is increased. However, even at 100% modulation index, the optical beat interference can limit the subcarrier channel bandwidth to −10 MHz, for a 20-dB CNR requirement.
© 1992 Optical Society of America
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