Potassium Titanyl Phosphate (KTiOPO4 or KTP) is an excellent NLO crystal, widely used in both commercial and military lasers. However conventional KTP suffer a significant drawbacks. The gray track phenomena in conventional KTP limit its application in high repetition and high power laser system. The occurrence of gray-track can be measured by an increase of bulk absorption by a strong CW 532nm green laser within several minutes. This measurement can be performed with Photo-thermal Common-path interferometer.