Fig. 1. Optical parametric conversation and QPM process of the pump, signal, and idler beams in the OPO. aj (j=0,1,2) and kj denote optical modes and wave vectors of the pump, signal, and idler, respectively. Superscripts y and z are corresponding to the y and z axes of the PPKTP crystal. KgII is the periodically poled vector for type-II QPM, and m is the QPM order with integer values.

Fig. 2. Experimental schematic of generating the intensity difference squeezed state. Laser, Nd:YVO4/LBO; DBS, dichroic beam splitter; HR, mirror with reflectivity higher than 99.95%; EOM, electro-optical modulator; MCR (MCI), mode-cleaner for red beam (infrared beam); OFR, optical Faraday rotator; HWP, half-wave plate; PBS, polarization beam splitter; BS, 50:50 beam splitter; OPO, optical parametric oscillator; PD, photoelectric detector; PZT, piezoelectric ceramic; SA, spectrum analyzer.

Fig. 3. Measurement for noise of the red laser. The red curve is measured when two photodiodes of the homodyne detector (PD1) are injected with a red laser, and the blue curve is measured by blocking one photodiode of PD1.

Fig. 4. Noise power of the generated intensity difference squeezed state versus different analysis frequencies. The blue curve is the SNL, and the red curve is the noise of the intensity difference squeezed state.