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我们理论上研究了基于热铷原子系综中的四波混频效应实现信噪比优化的系统。我们分析了非相敏放大器与相敏放大器中信号光信噪比的变化情况。在非相敏放大器中,信号光为相干态注入,闲置光为真空态注入,此时信号光在被放大的同时,信噪比降低,其噪声系数大于1。在相敏放大器中,信号光和闲置光均为相干态注入,此时在特定的条件下,信号光在被放大的同时信噪比得到提高,其噪声系数小于1。在注入光束间的相对相位=2 kπ(k为整数)时,信号光的有效增益最大且噪声系数最小。在这样的相对相位下,只要注入闲置光光强高于注入信号光光强的17.2%时,信号光就能够在被放大的同时信噪比得到提高。特别地,当注入闲置光光强同时低于注入信号光光强时,就实现了弱光对强光信噪比的优化作用。
In theory, we have studied a system for optimizing the signal-to-noise ratio based on the four-wave mixing effect in the thermal rubidium ensemble. We analyze the change of signal-to-noise ratio of signal light in non-phase-sensitive amplifier and phase-sensitive amplifier. In the non-phase-sensitive amplifier, the signal light is injected into the coherent state, and the idle light is injected into the vacuum state. At this time, the signal light is amplified and the signal-noise ratio decreases, and the noise figure is greater than one. In the phase-sensitive amplifier, the signal light and the idle light are all injected in a coherent state. Under certain conditions, the signal light is amplified while the signal-noise ratio is improved, and the noise figure is less than one. The effective gain of the signal light is maximized and the noise figure is minimized when the relative phase between injected beams = 2 kπ (k is an integer). In such relative phase, as long as the idle light intensity is higher than 17.2% of the intensity of the injected signal light, the signal light can be amplified while the signal-noise ratio is improved. In particular, when the idle light intensity is injected and the light intensity is lower than the injected signal light intensity, the optimization effect of weak light on the strong light signal-noise ratio is achieved.