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Results in Optics

Quadrature squeezing in a degenerate optical parametric oscillator in the above-threshold regime

Kokeb B. Benti, Desalegn Ayehu

Published Mar 1, 2026• 0 citations
Publisher

Abstract

A degenerate optical parametric oscillator (DOPO) in the above threshold regime is a prominent source of squeezed field. In this study, a driven DOPO whose pump mode is coupled to a thermal bath while the signal mode is coupled to a squeezed vacuum reservoir is explored to understand the effect of the external squeezed light and the thermal fluctuations on the squeezing properties of both the intracavity and output pump and signal modes. Our findings showed that the injected squeezed vacuum reservoir enables the intracavity squeezing to surpass the maximum squeezing limit of 50% attainable in a DOPO coupled to an ordinary vacuum reservoir. We also found that the maximum squeezing of the output signal mode shifts from resonance to off-resonance in the presence of the injected squeezed vacuum field. Furthermore, the output pump mode exhibits nearly perfect squeezing for a squeezing parameter of r=2. Unlike the output signal mode, the maximum squeezing of the output pump mode occurs at resonance in the presence of the injected squeezed vacuum reservoir. Moreover, a considerable amount of squeezing is observed even in the presence of thermal fluctuations, as they are counterbalanced by the external squeezed light. Notably, relative damping rate between the pump and signal modes is observed to be a parameter that significantly influences squeezing, indicating the importance of optimizing the amount of damping depending on the intended purpose of the DOPO.

DOI
10.1016/j.rio.2026.101145

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The field of squeezed state is an important quantum resource in the study of quantum optics. In the application of quantum information, the spectrum bandwidth of the squeezed light field is an important index to limit the information transmission capacity. Currently, the optical parametric oscillator (OPO) is one of the most efficient ways to generate high squeezed non-classical optical fields. In this paper, the degenerate singly-resonant and doubly-resonant OPO structures are introduced. Both OPOs are composed of concave mirrors and periodically poled potassium titanyl phosphate crystals (PPKTP). The length of PPKTP crystal is 10 mm. The curvature radius of the curved surface is 12 mm, and it has high reflectivity at 1550 nm and 775 nm. The plane surface is coated with anti-reflection coating. The air gap length is 21 mm. The concave mirror is an output coupling mirror, and its radius of curvature is 25 mm. In the singly-resonant OPO, only the signal light resonates in the cavity, and the pump light passes through the nonlinear crystal twice and then outputs out of the cavity. The reflectivity of OPO output coupling mirror to the wavelength of 1550 nm is 88%. The linewidth of the corresponding fundamental frequency wave is 77.4 MHz. For doubly-resonant OPO, both the signal light and the pump light resonate simultaneously in the cavity. The reflectivity of OPO output coupling mirror to 1550 nm and 775 nm is 85% and 97.5%, respectively. The linewidth of the corresponding fundamental frequency wave and harmonic is 97.1 MHz and 15.6 MHz, respectively. Then the threshold of OPO is calculated. The threshold pump power of OPO increases with signal light transmittance increasing, but the threshold value of doubly-resonant OPO is obviously smaller than that of singly-resonant OPO. After that, the variation of the squeezing bandwidth of the squeezed light field generated by OPO with the transmittance of the signal is analyzed. Finally, we complete the design of quantum squeezer with low threshold (18 mW), broadband (84.2 MHz) and high stability (the standard deviation of locking baseline is 0.32 MHz) experimentally. The results show that compared with the singly-resonant optical parametric oscillator, the doubly-resonant cavity has the characteristics of low threshold and high stability, which is more suitable for the preparation and practical application of broadband squeezed light field.
We consider specific signatures of squeezing for time-modulated light fields and propose the scheme of an optical parametric oscillator driven by a continuously modulated pump field. The application of a periodically modulated driving field instead of a continuous wave field drastically improves the degree of quadrature integral squeezing. This quantity goes below the standard limit of 50% relative to the level of vacuum fluctuations. We develop semiclassical and quantum theories of an optical parametric oscillator under the influence of a pump field with harmonically modulated amplitude for all operational regimes, including numerical simulations at the threshold point. The results can be directly applied in time-resolved quantum communication protocols.
Environment-assisted approaches to nonclassical light offer a practical path to strong squeezing in imperfect, lossy platforms. In this paper, we study a degenerate parametric oscillator in which a coherently driven cavity is coupled to a broadband squeezed reservoir via a single-port mirror. At the same time, the intracavity dynamics include non-Hermitian (gain-loss-imbalanced) terms. Within input--output theory, we obtain closed-form expressions for the steady-state quadrature variances, the output squeezing spectrum, and the power spectrum, and map their dependence on the reservoir squeeze factor, the coherent drive amplitude, and the parametric gain. We find that the non-Hermitian contributions open operating windows in which the intracavity quadrature noise is markedly suppressed below the standard quantum limit and, depending on the parameter set, either sharpen or amplify spectral squeezing and power-spectral features at the output. The non-Hermitian coefficients are treated as effective, low-order drift parameters that describe calibrated imbalance between engineered source and sink channels after auxiliary degrees of freedom have been eliminated. The analysis is restricted to the stable Gaussian regime in which the drift matrix is stable, and the squeezed-reservoir diffusion matrix remains physical. The results demonstrate an environment-assisted approach in which reservoir engineering and coherent driving work together to enhance squeezing. The resulting parameter maps identify experimentally testable windows, rather than a unique device prescription, for combining reservoir squeezing, coherent driving, and controlled gain/loss imbalance in cavity-QED and nonlinear photonic settings.