Heterodyne optical beat interference limitations on wavelength division multiplexed networks

In Wavelength Division Multiplexed (WDM) / Subcarrier Multiplexed (SCM) networks, information is transmitted by modulating a radio frequency subcarrier, which in turn is used to intensity modulate an optical signal. When the signals from two or more lasers operating in a WDM/SCM system are combined at a detector, optical beat interference occurs at a frequency equal to the difference in optical frequencies of the lasers. If this beat signal occurs at a frequency near a subcarriers frequency, it will interfere with the information being transmitted. In this paper we report an experimental results of beat interference from two different optical carriers and show that the results agree with an analytical model we developed. By changing the temperature of a Laser we were able to tune its wavelength resulting in different beat interference over the dc to 12 GHz region we measured, since the beat interference center frequency is the difference in wavelength of the two lasers. When designing a WDM network, one has to take into account the optical beat interference of different optical carriers to achieve an acceptable bit error rate (BER) or SIR. In our case, to achieve 30 dl3 SIR we need to keep the optical carriers at least 9 GHz apart which corresponds to 0.72 Angstrum, indicating a rather poor spectral efficiency.