Key functions have been identified as requirements for the emerging optical network (see Figure 8). As component technologies advance, each of the functions required, such as tunable filters, space switches, and wavelength converters, will become more cost effective and practical.

Figure 8. Key Functional Blocks for WDM Transport Systems

One of the most promising technologies for optical networks is the semiconductor optical amplifier (SOA). By integrating the amplifier functionality into the semiconductor material, the same basic component can perform many different applications (see Figure 9). SOAs can provide integrated functionality of internal switching and routing functions that are required for a feature-rich network. Space switches, wavelength converters, and wavelength selectors all can be made from SOAs, which will lead to large cost reductions and improved performance in future optical-network equipment (see Figure 10).

Figure 9. Semiconductor Optical Amplifier Technology

Figure 10. All-Optical Semiconductor Wavelength Converter

Promising new gain-switching technology makes possible optical-space switches, selectable filters, and wavelength converters. Today's transmission systems employ NRZ at OC–48 (2.5 Gbps) and OC–192 (10 Gbps) data rates. However, new transmission technologies are being studied to open the way to OC–768 (40 Gbps). These new systems might be based on either electronic time division multiplexing (ETDM) or optical time division multiplexing (OTDM) 4X10–Gbps technologies. Advances are being made with integrated laser modulators that provide lower-cost narrowband transmitters (see Figure 11).

Figure 11. Integrated Laser Modulator

Soliton transmission, first deployed in submarine links, might find application in terrestrial networks to improve transmission performance or in some types of all-optical signal processing such as 3R regeneration. Research dealing with polarization-mode dispersion mitigation, phase-shaped binary transmission (PSBT), and fiber-grating technologies promise significant advances in the near future with regard to increasing system performance and network capacity.

All of these technologies aim to reduce the network cost and provide valuable new services to customers who are constantly demanding more bandwidth-intensive and flexible features from their network providers.