In an optical data transmission system, high frequency pulsed optical signals are transmitted along an optical fiber. These pulses are attenuated and distorted by transmission over long optical fibers making detection of data difficult at the receiving end. The allowable pulse profile is determined by an eye diagram which is non-symmetrical with respect to logic 1 and 0 levels. By positioning the eye diagram closer to the logic 0 level, detection in the presence of noise introduced during transmission of the optical pulses, can be significantly improved.