Hello friends i have one query that we can connect routers and switches directly on fiber and with high capacity why we uses transmission system like sonet,sdh and dwdm?
SONET and SDH are better explained by others, but DWDM allows you to stuff up to 96+ connections into a fibre pair, by separating the connections into different wavelengths. This means that instead of a single 100Gbps link over a single pair, you could have 96+ 100Gbps links. DWDM (Dense Wavelength Division Multiplexing) is an improvement over CWDM (Course Wavelength Division Multiplexing, which allows for 18 channels over single pair), mostly to move the active signals into the L and C-bands, where EDFAs (eridium doped fibre amplifiers) are effective, so they can be used to regenerate the signal when pushing it long distance.
The reason you don't see these types of systems generally is because they are insanely expensive (relative to other network gear), but they are very cost-effective compared to example laying new transoceanic cables. Submarine cables are also usually very low in fibre count, for example the new SeaLion cable from Finland to Germany has only 6 strands of fibre if memory serves. I'm not really familiar with the reason for this, but it probably has something to do with the EDFAs mentioned earlier.
So, DWDM is only used to add capacity over existing fibre links, if all you need is a single link then connecting it directly to the equipment would be preferable from a cost perspective.