), electronic noise ( \(σ_e\) ), laser fluctuation noise ( \(σ_l\) ) and shot noise ( \(σ_s\) ). Since the total noise is the geometric mean of all these independent noises, the largest noise source dominates. The target movement noise is due to the body movement during speech and usually has a smaller magnitude in the audio frequency band compared to \(σ_l\) and \(σ_s\) (see Methods for SNR analysis of REAL). Electronic noises including the Johnson–Nyquist noise depend on temperature and fabrication of the chips. They are independent of the laser power. Laser fluctuation can play a role and in our case. Our laser fluctuation is also an important factor to consider. In our case, laser power fluctuation within the audio band is measured to be ~0.005%. Since the APD signal is proportional to the collected laser power, this noise power is approximately proportional to square of laser power (