It is found experimentally that with no self pumped phase conjugation an intensive internal oscillation is observed inside a Cu doped KNSBN crystal pumped with an 8 mW laser beam of continous wave at 532 nm. But when the crystal is pumped with mode locked picosecond laser pulses with an average power of 1 mW at 532 nm, self pumped phase conjugation with hardly observed interal oscillation is easily formed. It suggests that our experimental results do not support the resonator mode of self-pumped phase conjugation.