Theory of intensity to phase noise conversion of fiber optic sensors using phase generated carrier scheme

The photodetection intensity noise to demodulated phase noise conversion process of fiber optic sensors using phase generated carrier (PGC) scheme is investigated through theoretical calculation and experimental verification. Several categories of intensity noise are calculated, according to their relation between intensity noise power spectral density and photodetection current. The results revealed that demodulated phase noise power level increases by several decibels over the relative intensity noise power level of detected light power signal. Phase noise power level could also fluctuate with demodulated phase signal. Increase of phase noise power level varies according to the noise type, as well as the fluctuation amplitude. For noises that intensity noise power level unrelated to detected light power, such as electronic noise of detector circuit, phase noise power level increases by about 3.7 dB and do not fluctuate with demodulated phase signal. For noises that intensity noise power level proportional to detected light power, such as signal-amplified spontaneous emission(ASE) beat noise of optical amplifiers, phase noise power level fluctuates with demodulated phase signal by about 5.7 decibels and averagely increases about 3.7 decibels over a 2 pi period of demodulated phase signal. For noises that intensity noise power level proportional to square of detected light power, such as light source relative intensity noise, phase noise power level fluctuates with demodulated phase signal by about 9.0 decibels and averagely increases about 4.8 decibels over a 2 pi period of demodulated phase signal. Verification experiments are demonstrated on electronic noise, ASE-signal beat noise and light source relative intensity noise separately.