A 90 degree Alternate-Phase (AP) on-off keying (OOK) transmission format
for high bit rate, long-haul optical transmission systems employing a
chirped or a chirp-free pulse stream generated by a pulse generator
(e.g., Mach Zehnder modulator) driven by mixing two electrical
signals--one for intensity modulation and another for pulse modulation.
These electrical signals may be two properly skewed sinusoidal electrical
signals at half the desired data rate thereby generating a pulse stream
in which the maximum optical phase modulation occurs at the intensity
peak of each pulse and is 90 degrees out of phase with its nearest
neighbors.