Sounds like you really need a 50Ω Laser driver circuit (first) which just happens to invert the input pulse (second), and which happens to have a high-input impedance so as not to load the 400uA signal source (third).
If you had specified this in the first post, we wouldn't have wasted twenty posts explaining about hex inverters which will probably not work...
If you had specified this in the first post, we wouldn't have wasted twenty posts explaining about hex inverters which will probably not work...