Hi,
There are many ways to implement the QPSK modulator, which depend on the level of detail you want. If you want a black box type component use the Optical QPSK Transmitter, which can be found in the “Default/Transmitters Library/Optical Transmitters” folder.
Also be sure to check out the examples we have in the “OptiSystem 12 Samples/Advanced modulation systems/PSK systems” folder.
Offset QPSK is a modulation scheme very similar to regular QPSK, where the in-phase and quadrature signals are not changed at the same time, so that the total signal never has sharp changes in phase. Wikipedia has a great article on the differences:
http://en.wikipedia.org/wiki/Phase-shift_keying
Regards