Optical Technologies
The WDM Technologies

With progresses in lasers and optical-electrical equipment technologies, it is possible to transmit one or more wavelengths on the same fiber. This is known as the wavelength-division multiplexing WDM. By adding of wavelengths into the same fiber, a transmission capacity and a bandwidth of the optical fiber are increasing and, therefore, a need for installation of additional optical fibers is decreased. In WDM systems, each used wavelength presents an independent channel. From a viewpoint of the WDM deployment in various network types and from a viewpoint of the channel spacing, three basic WDM network type are distinguished – long-haul, metropolitan, access [53].

Early broadband WDM (BWDM) systems operated with an overall broad channel spacing, because two wavelengths were separated and were located in two different transmission windows – at the 950 and 1300 nm or at the 1300 and 1550 nm. Latter wide WDM (WWDM) systems utilized more wavelengths in the transmission window and optical channels were usually separated a few of nm – the 1275.7, 1300.2, 1324.7 and 1349.2 nm. In present days, these systems can be employed in the PON networks with three wavelengths – the 1310, 1490 and 1550 nm. The most preferable recent dense WDM (DWDM) systems have a channel spacing usually not more than couple of nm in a range 1530 – 1625 nm of the erbium-fiber transmission window and use necessarily cooled lasers to prevent the wavelengths from drifting outside this window or from interfering with each other [54]. Latest coarse WDM (CWDM) systems utilize optical channels extended in a range 1270 – 1610 nm with a large channel spacing of 20 nm and therefore they use uncooled (non-thermally controlled) lasers.

Both – CWDM and DWDM – systems are the WDM types: the dense WDM is an implementation in long-haul networks distances and the coarse WDM is an implementation in metropolitan and access networks. Different demands for these two implementations require different architectures and determine performance demands for system components. The goal of DWDM systems is to maximize a bypassed distance without an electrical regeneration at amplifier costs spread over a maximum number of wavelengths. The goal of CWDM systems is minimize a components cost in the system, where a distance is shorter and amplifiers are not necessary.

DWDM systems – wavelengths in the 1530 – 1625 nm range; expensive cooled lasers to prevent wavelengths from drifting outside this region and from interfering with each other. Increasing of the channel number requires a narrowing of the channel spacing in filters, an extended spacing and a using of translators for reaching narrower channel spacing and an opening of the new spectral area – except a common C-band also a new L-band.

CWDM systems – wavelengths in the entire 1280 – 1625 nm band; less expensive uncooled lasers (cost savings are a direct reflection of the packaging differences between DWDM and CWDM lasers). Medium distances without amplifiers, an elimination of the EDFA amplifier bandwidth limit allows distributing of wavelengths over a broad region and locating of them sufficiently far between, a need for cheap multiplexers, demultiplexers, add/drop and switches (not simply retrofitted from DWDM systems) with a low loss, a high isolation and proper channel spacing.

The DWDM and CWDM systems have different sources operating at given wavelengths and have different filter for combining of wavelengths onto the same fiber at the transmitting end and for separating of wavelengths at the receiving end. However, technologies of used filter can be the same. Also, the adding and the dropping of wavelengths in midpoints of the system can be performed by the same technology for both systems. A main difference is in channel spacing – the DWDM channel spacing can be nearly 0.2 nm, the CWDM channel spacing is usually 20 nm. Therefore, it is possible to consider about a practical utilization of the CWDM/DWDM combination.

The CWDM is an alternative to expensive and complex DWDM-based architectures because it provides an occasion to continue in a direction given by the DWDM technology to all optical networks. An advantage of the DWDM in eliminating of expensive regenerators is not used in metropolitan networks, where optical amplifiers are not required or where block modules with cheap uncooled laser pumps easy satisfy demands on distances in a majority of metropolitan circuits and paths.

The CWDM varies from the DWDM in broader optical channel spacing between light sources that are multiplexed into the same fiber. As well, CWDM transmitters/receivers use an optical multiplexing technique for reaching serial equivalent data rates, whereas the DWDM multiplexes many serial data streams for reaching a bandwidth up to hundreds of Gbps. This is reached by using of a thermal control in the DWDM channel spacing. This exact spacing control allows combining a large number of separated channels. The typical CWDM system has spacing in order of some nm (some THz) and doesn’t require a thermal control. So, CWDM transmitters/receivers without a thermal control have directly modulated lasers and include lower-speed components for reaching of higher data rates.