Using a compact optical link transmission model, the degradation of signal degre e of polarization (DOP) induced by polarization mode dispersion (PMD), especiall y the second-order PMD, and the input signal state of polarization in high-speed optical fibre communication systems is studied in detail. It is pointed out tha t the signal DOP can be used as the feedback control signal in the optical adapt ive first-order PMD optimal compensation systems compatibly. However, if the sec ond-order PMD is considered, the control algorithm will be complicated, and ther e will be the possibility that the system is trapped in a local optimum which wo uld downgrade compensator the performance.