Multiwavelength SCM Systems.

The combination of  WDM and SCM provides the potential of designing broadband passive optical networks capable of providing integrated services (audio , video , data , etc) to a large number of subscribers. In this scheme , multiple optical carriers are launched into the same optical fiber through the WDM technique. Each optical carrier carries multiple SCM channels using several microwave subcarriers. 

One can mix  analog and digital signals using different subcarriers or different optical carriers. such networks are extremely flexible and easy to upgrade as the demand grows.

The limiting factor for multi wavelength SCM networks is inter channel crosstalk resulting from both the liner and nonlinear processes.The nonlinear effects that produce interchannel crosstalk are as SRS and XPM (cross phase modulation). The probe power varies with time because of SRS and XPM , and the crosstalk is defined as the ratio of radio frequency powers in the two channels. The XPM induced crosstalk increases and the Raman induced crosstalk decreases with the modulation frequency but each has the same magnitude.

The linear crosstalk results from the phenomenon of optical beat interference. It occurs when two or more users transmit simultaneously on the same optical channel using differnt subcarrier frequencies.

As the optical carrier frequencies are then slightly different , their beating produces a beat note in the photocurrent. If the beat note frequency overlaps an active subcarrier channel, an interference signal would limit the detection process in a way similar to IMD.

Multiwavelength SCM systems are quite useful for LAN and MAN applications. They can provide multiple services with only one optical transmitter and one optical receiver per user because different services can use different microwave subcarriers. This approach lowers the cost of terminal equipment in access networks. Different services can be offered without requiring synchronization, and microwave subcarriers can be processed using commercial electronic components.

The main advantage of multiwavelength SCM is that the network can serve NM users , where N is the number of optical wavelengths and M is the number of microwave carriers by using only N distinct transmitter wavelengths. The optical wavelengths can be relatively far apart technology is used to provide broadband integrated services to the subscriber.