As the number of interconnected layers on the front side of integrated circuits increase， it becomes increasingly difficult to detect the internal electrical signals from the front side of integrated circuits. A probing optical path based on the common-path interferometer is designed in this study to use a laser for detecting the internal electrical signal waveform from the back side of an integrated circuit without making contact. To this end， a signal processing method based on a lock-in amplifier is proposed to extract the weak electro-optical signal of the device carried in the reflected light， which exploits the strong noise rejection and high sensitivity of the lock-in amplifier technique. Considering that the electro-optical signal of the device varies periodically with the electrical signal， the electrical signal from the photodetector is first processed by the lock-in amplifier to eliminate most of the noise， and then the averaging technique is employed to further suppress the external noise and improve the signal-to-noise ratio of the electro-optical signal of the device. Finally， the electro-optical signal of a device drowned in noise is extracted and the electrical information inside the device is reconstructed. The electro-optical signal of the circuit node with a dynamic operating current in the order of μA inside the chip is successfully detected using the proposed method. The signal-to-noise ratio of the extracted signal reaches 4.99 dB， while the signal-to-noise ratio of the signal obtained solely via the averaging technique is only -44.29 dB. This paper presents a novel method for the optical detection of electrical signals inside integrated circuits， which can be applied to perform dynamic defect detection of integrated circuits in the future.