First: buy a power meter. They are really cheap and the only way to know for sure how much signal you got. It will also tell you how much launch power you have. The fiberstore modules are listed as 0 to +5 dBm launch power - if you got lucky it might be +5 and if you got a lower end module it might be close to 0. Obviously this makes a huge difference for how much power you get on the other end. Also it is said that the laser will lose power over time. Second you need to think in terms of power budget, not distance. So you got 68 km and the module is rated for 80 km - but not all fiber is not born equal. A power meter allows you to measure the true link loss. Third you did not tell what DWDM multiplexer you are using. A 44 channel DWDM multiplexer from Fiberstore can have up to 4.5 dB insertion loss. You might have two of those on your link for a total of 9 dB loss. Your 80 km module has a 23 dB link budget, so this leaves you with 23-9 = 14 dB budget. If your fiber has 0.25 dB loss per km, that is only 56 km. Regards, Baldur