Unfortunately you can't just increase the power into the fiber. It's complicated but I'll try to clarify (pun not intended) why you are OSNR challenged here.

Unfortunately you can't just increase the power into the fiber. It's complicated but I'll try to clarify (pun not intended) why you are OSNR challenged here. First, fiber and just about any optical material have a nonlinear index. So as you increase the power into the fiber, the light modifies the local index. This creates a kind of noise. A wavelength channel can do this to itself even. When you have lots of channels on the fiber, there is a lot of modulation of the refractive index by all those channels. This is call cross-phase modulation (XPM). You can think of XPM as a phase noise. As you increase the power in the fiber, there's a point where the XPM "noise" increases faster than the improvement in OSNR. So you have to keep the launch power low to keep the nonlinear penalty low. This limits our launch power and ultimately the OSNR we can achieve. Generally we try to operate near the peak of this curve.