2. 14. Subcarrier Multiplexing

Optical subcarrier multiplexing (SCM) is a scheme where multiple signals are multiplexed in the radiofrequency (RF) domain and transmitted by a single wavelength. A significant advantage of SCM is that microwave devices are more mature than optical devices; the stability of a microwave oscillator and the frequency selectivity of a microwave filter are much better than their optical counterparts. In addition, the low phase noise of RF oscillators makes coherent detection in the RF domain easier than optical coherent detection, and advanced modulation formats can be applied easily. A popular application of SCM technology in fiber optic systems is analog cable television (CATV) distribution. Because of the simple and low-cost implementation, SCM has also been proposed to transmit multichannel digital optical signals using direct detection for local area optical networks. A block diagram for a basic SCM system is shown in the figure. The modulated microwave subcarrier signals are obtained by frequency up conversion from the base band using a VCO. These sub carrier signals at frequency f1 are then summed in a microwave power combiner prior to the application of the composite signal to an injection laser which is d.c. biased to produce the desired intensity modulation. The intensity modulated optical signal is then transmitted over a single mode fiber and directly detected using a wide band photodiode before de-multiplexing and demodulation using a conventional microwave receiver. SCM has following disadvantages: 1.Source linearity: Distortion caused by this becomes noticeable when several sub carriers are transmitted from a single optical source 2.It operates at high frequency only 3.In digital systems SCM requires more bandwidth per channel than a TDM system.