Fig. 1. Different pickups of the output signal from an SMI: from rear PD2 and from front mirrors PD1 and PD1′.

Fig. 2. Simplified scheme of an SMI for the evaluation of front and rear output signals and noise.

Fig. 3. Top: the laser diode cavity has mirrors with (power) reflectivity R₁ and R₂, and the optical oscillating field E₀ is assumed constant inside the cavity; bottom: the second-quantization model, in which field E₀ is accompanied by the coherent state fluctuation ΔE_coh, and the vacuum state fluctuations ΔE_vac1,2 enter in the unused port of the mirrors, described as a BS because they have non-unitary transmission.

Fig. 4. (top) Pickup of the front SMI signal by means of a BS, deviating a fraction R_BS of power P₁ to PD1; (bottom) equivalent circuit for the evaluation of noise, showing the added fluctuation ΔE_vacBS entering in the unused port of the BS.

Fig. 5. Portion a′ of the beam outgoing from mirror M1 by means of a reflecting prism.

Fig. 6. Balancing of the two SMI signals detected by PD1 and PD2: a triangular waveform is applied to the bias and generates detected responses brought to be nearly identical (top trace), that is, with a residually small difference (bottom trace).

Fig. 7. Signals detected by PD1 and PD2 (red and blue) and their difference (yellow), exhibiting a noise 2.5 ± 1 dB smaller.

Fig. 8. Peaks of EMI superposed to the SMI of channels PD1and PD2 (yellow and yellow–green) and the difference channel (red), exhibiting a disturbance reduction of 25…30 dB.