Fiber optical current transformer（FOCT） is easy to be affected by all kinds of noise

which will reduce the measurement accuracy

especially in the case of small current. Therefore

how to improve the signal-to-noise ratio（SNR）is one of the key technologies of FOCT. Consequently

the noise sources of FOCT were comprehensively analyzed

which included the optical signal noise

signal detection noise

and the introduction of noise sources by the external environment

the calculation of FOCT SNR was also given

and the influence of different noise sources on SNR was simulated by the theory. Thereby

a transformer prototype was designed and tested. The results indicate that the SNR can be effectively improved by improving the receiving optical power of the detector

increasing the number of sensing optical fiber ring and selecting appropriate closed-loop feedback phase modulation bias operating point. The prototype can meet the precision demands of 0.2S at a rated current of 600 A. The SNR of the prototype is 30.2 d B while the operating point of system is 1% of the rated current. Meanwhile

the ratio error is less than 0.61%

and phase error is less than 12.39’. The optimization scheme for improving SNR is of great practical value

and it is instructive for the design of FOCT.