Continuous variable bright squeezed state light field is a very important quantum resource. It can be used in various domains such as quantum metrology, quantum precision measurement, and quantum information. Examples include quantum-enhanced lidars, magnetometers, quantum-dense coding, quantum key distribution, and teleportation. Applications in these domains require continuous variable bright squeezed state light fields with relatively high power. In order to obtain a continuous variable high-power bright squeezed state light field, the main factors affecting the intensity of green light-induced infrared absorption in a nonlinear periodically poled KTiOPO₄ (PPKTP) crystal are studied in the process of parametric down conversion technique. Through the research on this manuscript, one of the main factors limiting the experimental preparation of continuous variable high-power bright squeezed state light field is found, which lays a foundation for overcoming the technical problem and developing continuous variable high-power bright squeezed state lights.

The experimental preparation system of continuous variable high-power bright squeezed state light field is shown in Fig. 1. The first part is the fundamental frequency light source of the experimental system, which is a high-power single-frequency Nd∶YVO₄ solid-state laser of 1064 nm. The second harmonic source is a flat-concave semi-monolithic standing cavity based on MgO∶LiNbO₃ crystal. The laser of 532 nm is obtained by a critical phase-matching technique in the nonlinear crystal. The second part is the core part of the experimental system, namely the optical parametric amplifier. It generates a continuous variable high-power bright squeezed state light field and is based on a semi-monolithic standing cavity composed of PPKTP crystal and a concave cavity mirror. The last part is the balanced homodyne detection part of the experimental system. The local oscillator and signal light are evenly divided and interfered on the 50/50 beam splitter and then injected into the balanced homodyne detector. The noise power spectrum of the bright squeezed optical field is measured by scanning the relative phase of the local oscillator and the signal light. In order to optimize the spatial mode distribution of the Gaussian beam in each part of the experimental system, improve the mode matching efficiency between the Gaussian beam and optical resonant cavity, and reduce the relative intensity noise and phase noise carried by the light field, we insert a three-mirror ring cavity as mode cleaner in the fundamental frequency optical path, the second harmonic optical path, and the local oscillator optical path of the balanced homodyne detection part of the experimental system. In addition, in the above preparation experiment system for continuous variable high-power bright squeezed state light field, all-optical resonant cavities and the relative phase of the light field are locked by Pound-Drever-Hall technology. The experimental preparation process of the continuous variable high-power bright squeezed state light field is as follows: the laser field output from the high-power single-frequency Nd∶YVO₄ solid-state laser of 1064 nm is divided into two parts by the 90/10 beam splitter. One part, as the local oscillator, is injected into the balanced homodyne detector to amplify the noise power of the squeezed state light field. The other part is divided into two beams with equal optical power by the 50/50 beam splitter. Specifically, one is injected into the flat-concave semi-monolithic standing cavity as the fundamental frequency light to generate the second harmonic light field for pumping the optical parametric amplifier, and the other is injected into the optical parametric amplifier as the seed light to generate the bright squeezed state light field. In order to obtain a high-power bright squeezed state light field, it is necessary to increase the power of seed light. However, the high-power seed light will cause the intense absorption of high-power bright squeezed state light field in PPKTP crystal, which leads to serious thermal effects. This will bring great challenges to the precise locking of the relative phase of the pump light and the seed light, as well as the stable control of the cavity length of the optical parametric amplifier. Therefore, we design an optical parametric amplifier, which is conducive to the generation of a wide-band squeezed light field.

Finally, at the analysis frequency of 3 MHz, the bright squeezed state light field with an optical power of 200 μW and squeezing degree of (-10.7±0.2) dB is directly measured (Fig. 2). According to the experimental data and theoretical calculation, the total optical loss during the transmission and detection of the high-power bright squeezed light field is (9±0.05)%. According to Eq. (2), the escape efficiency of the optical parametric amplifier can be calculated as (66.7±0.05)%. The optical parametric amplifier's intracavity loss due to the absorption of PPKTP crystal is estimated to be (5.8±0.05)% by removing the optical loss introduced by the optical devices and the detection process, which accounts for (64.4±0.05)% of the total optical loss. According to Beer-Lambert law, the absorption coefficient of PPKTP crystal under this condition is about 6.0×10^-2 cm^-1. By comparing with the experimental preparation system of the squeezed vacuum state, whose optical parametric amplifier's escape efficiency is 98.34%, and the corresponding absorption coefficient of PPKTP crystal is about 2.1×10^-4 cm^-1, it can be concluded that absorption of PPKTP crystal is the main reason for the increase in intracavity loss and the decrease in escape efficiency.

An experimental system for generating continuous variable bright squeezed light with high power is established. The bright squeezed light with the power of 200 μW and quantum noise reduction of (-10.7±0.2) dB is obtained by direct measurement at the analysis frequency of 3 MHz with a seed light power of 500 mW and light power of 145 mW pump (Fig. 2). According to the above experimental data and calculation, the total optical loss of the experimental system after the optical parametric amplifier is (9±0.05)%, and the intracavity loss introduced by the green light-induced infrared absorption effect is (5.8±0.05)%, accounting for (64.4±0.05)% of the total optical loss. Under these conditions, the absorption coefficient of PPKTP crystal to the high-power bright squeezed light is about 6.0×10^-2 cm^-1, while the absorption coefficient of PPKTP crystal to the squeezed vacuum state is about 2.1×10^-4 cm^-1. The above research results confirm that the optical parametric amplifier's intracavity loss introduced by the green light-induced infrared absorption effect becomes the main factor affecting the quantum noise of bright squeezed light with high power. The research conclusion of this manuscript points out the technical difficulties in the current experimental preparation of high-power bright squeezed state light field and the direction for developing a bright squeezed state light field with a higher power, stronger squeezing, and more stable index.