Special Report: Next Generation Internet Applications
2.2 Optical Processing Techniques As the design of all-optical networks progresses, the ability to modify or manipulate the optical signal will prove critical to the scalability of the network. Techniques for regenerating an optical signal, converting the wavelength to support optical routing, buffering for switching implementations, and encapsulating the optical signal all provide functionality critical to the successful proliferation of all-optical networks.
2.2.1 Optical Regeneration Optical signals degrade over long distances and due to various impairments. Causes for this degradation are dispersion, loss, crosstalk, and non-linearities of the optical components and the fiber.[11] In order to ensure signal integrity over long distances or high path losses, regeneration techniques are necessary. Historically, regeneration has occurred in the electrical domain, after an optical-electronic conversion. The electrical signal must then be converted back to the optical domain prior to re-transmission. For extremely long fiber runs, optical amplifiers and optical repeaters are necessary. Optical amplifiers boost the power of the optical signals in the fiber. Unfortunately, the noise on the line is also amplified. The answer to this noise problem is optical repeaters. Optical repeaters perform OEO regeneration. This process filters the noise and reconstructs the optical signal for retransmission to ensure signal integrity throughout each transmission segment. Therefore, it is standard practice to implement an optical repeater after every two optical amplifiers.
Optical regeneration can be accomplished in three different ways. The first level of regeneration is referred to as 1R regeneration. This regeneration is standard signal amplification and is performed by optical amplifiers. The second level of regeneration, or 2R regeneration, is performed by semiconductor optical filters, which reshape the signal by filtering the noise. The final regeneration level is 3R. This level provides amplification, reshaping, and re-timing on the optical signal. Recent advances have enabled network equipment to optically recover the pulse clock to, in effect, re-sync the optical signal. These advances are moving optical networks toward total transparency. Figure 8 depicts the three regeneration levels. Figure 8: Optical Regeneration Levels Optical regeneration in the optical domain would eliminate the need for the costly OEO conversions throughout the core network segments. With these conversions eliminated, scalability problems would be lessened due to the elimination of these transport bottlenecks. Further research is necessary to develop multiple simultaneous wavelength optical regeneration to fully integrate with DWDM technology.
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