Margin Requirements
In a multinode WDM link, the main component of the system loss is not attenuation due to transmission link; but instead, it is the loss associated with the various subsystems. A typical link consists of multiple nodes, each equipped with a variety of components. The loss due to each component is high, which results in a severe penalty for system design. A typical WDM node might have a full optical multiplex section (OMS) that consists of arrayed waveguides (AWGs) and a switching matrix. A typical grating-based AWG has a 5 dB loss (insertion loss) associated with it. An optical signal that is passing through a node with two such AWGs (multiplexer and demultiplexer section) is typically subject to 10 dB loss in addition to the switching fabric loss. An estimate of the loss can be understood with the following argument.
Consider two nodes, each equipped with AWGs (loss = 5 dB) and switching fabric (loss = 3dB) in addition to connector loss (2 dB). If they are separated by 50 km of SMF (α = 0.2 dB/km), the total attenuation due to transmission is 10 dB (.2 × 50). However, at each node, the loss is 5 + 5 + 3 + 2, or 15 dB. In other words, the nodal losses can be higher as in comparison to transmission losses. This affects system designs and OSNR as well. The effect is indirect in the sense that output power from a node is affected due to such losses, which further affects OSNR due to Equation 4-21.
Table 4-1 shows the insertion loss due to typical elements. We have to quantize losses due to impairments in transmission. As mentioned in the introductory section to this chapter, dispersion can be quantified as a penalty in dB. Similar treatment can be done to other phenomena such as polarization and nonlinearities and so on.
Table 4-1 Insertion Loss and Other Losses for 1550 nm Operation
|
Component |
Insertion Loss |
Wavelength-Dependent Loss |
Polarization-Dependent Loss |
Cross-Talk NF |
|
Multiplexer Demultiplex (AWG) |
5 dB |
< 1 dB |
0.1 dB |
–40 dB |
|
Optical 2 × 2 add-drop switch |
1.2 dB |
< 0.2 dB |
0.1 dB |
–40 dBm |
|
Coupler (2 × 2) passive |
3 dB |
- |
- |
- |
|
Filter-Thin-film |
1 dB |
0.1 dB |
- |
–40 dBm |
|
Filter- AOTF/MZI |
1 dB |
0.1 dB |
- |
–35 dBm |
|
Interleaver |
2–3 dB |
- |
- |
- |
|
Optical cross-connect (OXC) Port to port |
3 dB typical without AWG loss |
< 0.4 dB |
0.1 dB |
–40 dBm |
Table 4-2 presents margin requirements for a good design. These margins adhere to variations in optical signal budgeting issues, especially on a dynamic level. The margins are generally chosen by evaluating a set of readings that represent the pseudo-population of a number of discrete events governing the entire sample space of optical signal design.
Table 4-2 Margin Requirements
|
Symptom |
Loss Margin |
|
Fiber dispersion |
1 dB |
|
SPM margin |
0.5 dB |
|
XPM margin |
0.5 dB |
|
DCU compensation |
6 dB |
|
FWM |
0.5 dB |
|
SRS/SBS |
0.5 dB |
|
PDL |
0.3 dB |
|
PMD |
0.5 dB |
|
Amplifier gain tilt (due to nonflat gain spectra) |
3.0 dB |
|
Receiver sensitivity tilt (wavelength dependence of PMD) |
0.5 dB |
|
Transmitter chirp |
0.5 dB |
|
AWG cross-talk |
0.2 dB |
|
Fiber connectors |
0.5 dB |