建立了双平衡式外差探测IQ解调的数学模型，通过旋转λ/4波片来改变IQ信号间的相位差。从本振光强度过剩噪声、散粒噪声和热噪声三个方面对平衡式外差探测系统的信噪比(SNR)进行了仿真。仿真结果表明，当旋转λ/2波片使分束比在(0~0.272)和(0.728~1)范围内时，单源探测信噪比比平衡式探测的信噪比高；而分束比在(0.272~0.728)时平衡式探测的信噪比高于单源探测。搭建了2 μm双平衡式外差探测实验系统，当分束比为0.5时，平衡式外差探测的信噪比比单源外差探测的信噪比提高10 dB以上，从而证明了双平衡式外差探测系统在微弱信号检测中的可行性和优越性。

An IQ demodulation model for dual-balanced heterodyne detection is established, by rotating λ/4 wave plate to change the phase difference between the IQ signals. Signal-to-noise ratio (SNR) of balanced heterodyne detection is simulated with local excess intensity noise, shot noise and thermal noise. Simulation results show that signal-to-noise ratio of conventional heterodyne detection is higher than that of balanced heterodyne detection, when the beam splitter coefficient adjusted by rotating λ/2 wave plate is less than 0.272 or much than 0.728. Whereas, signal-to-noise ratio of conventional heterodyne detection system is less than that of balanced detection system. Finally, 2 μm dual-balanced heterodyne detection system is proposed, in which the signal-to-noise ratio is 10 dB higher than that of conventional heterodyne system with 0.5 beam-splitter coefficient. This result verify feasibility and advantage of the balanced heterodyne detection system for detecting weak signal.