Abstract: A fiber optic system detects temperature transients in a superconducting magnet. A signal optical fiber is placed in function proximity with the superconducting magnet for responding to a temperature transient condition and a reference optical fiber is remote from the signal optical fiber. A coherent light outputs coherent light for input to the signal and reference optical fiber to provide a phase shift of about 90.degree. to the coherent light in the signal or reference optical fiber. In one embodiment, a low pass filter provides feed back to the phase modulator to stabilize the modulation while a high pass filter passes temperature transient signals to a suitable analyzer, such as a digital storage oscilloscope, or the like.