https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwjmg5-T07rMAhWWc44KHW8ZCYUQFgg0MAM&url=http%3A%2F%2Fwww.fiber-optic-tutorial.com%2Fcomparison-of-different-optical-amplifiers.html&usg=AFQjCNHBrJgZ-9B5TuvtOKmvzzpZCalqMw&sig2=kdQXr_wULGMTMO2ggPMpZw
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