You want to speed things up faster than light? (Just kidding.)
2.0 Mbps utilization may not be a bad thing, if that's an average use over a pretty long period of time. It's definitely nowhere near 100.0 Mbps top speed of the link if you're measuring over a short interval, though.
Is your laser equipment capable of running that link in full duplex mode? If yes, then set the two routers' interfaces to 100 Full.
Are your lasers properly aligned? With these it's usually either completely on or completely off. Intermittent throughput only happens due to weather (rain, fog, snow), environment (smoke or windy conditions), or UFOs (birds, aliens, people wearing jet packs, low flying aircraft).
Double-check the specifications on the 3660 routers. Can they handle Fast Ethernet at "wire speed" (that is, 148,880 64-byte frames per second)? I don't recall off-hand if they do. If not, and you don't need the routers for other reasons, you could substitute two less-expensive, L3-capable switches like Cat3550s instead. These will do "wire speed" switching and IP routing. (Or if you already have L3 switches available on both ends of the laser link, just patch the laser gear directly into them.)
Hope this helps.
