Theoretical Analysis and a New Optical Scheme for RIN Subtraction

Among the three main noises (shot noise, relative intensity noise (RIN), thermal noise) of high-precision fiber optic gyroscope (HFOG), RIN is dominant, which restricts the performance improvement of HFOG. Increasing modulation depth is an effective method for RIN subtraction, but it has various drawbacks. One of the main methods of RIN subtraction is to divide the light of ASE into two beams equally, one beam passes through the fiber coil to generate Sagnac effect signal, and the other beam records RIN information for reference. The signal and the RIN reference are separately converted into electrical signal and subtracted (electronic subtraction), or the signal and the RIN reference are added (optical addition), which can also suppress RIN. However, there are still several problems: the polarization angle is variable between 0° and 90° when the two beams of light are added together; the delay and optical power of the two beams are also different, which will affect the RIN subtraction effect. In this paper, the RIN attribute is analyzed theoretically when the beams of signal and reference are added, and it is found that when beams of reference and signal are orthogonal, the PSD of RIN will oscillate and be zero in some special positions. By redesigning the optical scheme and adjusting parameters, FOG setups are built to achieve zero PSD of RIN as far as possible for laboratory experiments. The experimental results show that the optimal modulation depth of FOG can be reduced from >7π/8 to 6π/11, and the ARW is reduced by 53%. RIN subtraction scheme designed in this paper can improve the ARW of FOG and avoid the negative effect caused by too large modulation depth.