Analog AM fiber optic systems have begun to replace coax cable for local distribution within a CATV network, while digital systems are being used for headend or hub site elimination and for transmitting various data services. In the past, these analog and digital transmissions systems are operated separately from each other over separate optical fibers. However, as these CATV systems grow and expand, the current trend in CATV system design incorporates wavelength-division multiplexing to combine both the analog and digital signals for transmission using the same fiber. This allows system expansion by increasing the number of signals transmitted on fiber currently installed. As these systems grow, the forward path transmission ceases to be the only required path. Today's CATV system may also require a return path network to handle data from the Internet via cable modems. This article will focus on both two fiber and single fiber two-signal WDM CATV system design. For additional information on WDM systems using more than two signals, see articles on CWDM and DWDM. Unidirectional CATV Transmission (Forward Path) Before 1980, most CATV systems were coax based, but by the early 1980's the CATV industry began using direct modulated 1310 nm VSB/AM links for distribution super trunks. Figure 1 illustrates a typical system architecture including a super trunk. By transporting a high quality replica of the headend signals, this system reduced the number of cascaded amplifiers required.
Figure 1 - Typical Super Trunk CATV Architecture
By the early 1990's, CATV providers began using multichannel digital systems to transport large numbers of uncompressed, broadcast-quality, digitized video channels between the headends. Still operating in the 1310 nm wavelength window, in this configuration, a previous separate headend is replaced by very high quality signals that are transported by a multichannel digital system from a "master" headend. Figure 2 illustrates this configuration. The advent of high performance externally modulated 1550 nm VSB/AM transmitters and erbium-doped fiber amplifiers (EDFAs) changed the architecture of CATV system design once again. These 1550 nm links are used to carry signals between headend sites over long distances, using the EDFA as an in-line amplifier.
Figure 2 - Hybrid Analog/Digital CATV Architecture
The high performance 1550 nm systems vary slightly in that a few additional optical components are required. Illustrated in Figure 3, this system also incorporates optical splitters in addition to the EDFA. In this configuration, the transmitter is assumed to have dual outputs, a common feature for these new transmitters. The first optical output of the 1550 nm transmitter feeds a secondary headend 1310 nm transmitter. The second optical output goes into a 1 x 2 optical splitter. The first output feeds directly into a 1550 nm receiver for distribution from the main headend to a 1310 nm transmitter. The second output of the optical splitter feeds an EDFA. The signal is amplified optically and forwarded to the optical receiver which supplies a third headend located many miles away in the system.
Hybrid 1310 nm & 1550 nm VSB/AM CATV Architecture
The first three architectures use no WDM components and represent completely analog architectures. As CATV systems grow, the need to expand each fiber's transmission capacity grows with it. Wavelength-division multiplexing allows both analog and digital signals to co-exist on a single fiber. Figure 4 illustrates a unidirectional WDM AM CATV/Digital transport system.
Figure 4 - Unidirectional Analog/Digital CATV Transport using WDM
In the configuration shown in Figure 4, the signal from the 1310 nm CATV AM transmitter and the 1550 digital transmitter are wavelength-division multiplexed onto one fiber. At the receive, the signals are demultiplexed and output to the correct receivers. In order to maintain system quality, the WDM must be a high isolation type that prevents interference between the 1310 nm analog signal and the 1550 nm digital signal. A bidirectional configuration of this analog/digital CATV transport system is illustrated in Figure 5.
Figure 5 - Bidirectional Analog/Digital CATV Transport Using WDM
