Experimental research on low-noise and high-gain balanced detectors in mHz−MHz band

Balanced detector is a fundamental component for the accurately measuring quantum state fluctuations, especially quantum noise, which is crucial for future quantum-enhanced interferometric gravitational wave detectors utilizing squeezed light. By using a transimpedance amplifier (TIA) model core for balanced detection, a detailed theoretical and practical analysis is conducted on the electronic factors that affect the performance of the detector in the target ultra-low-frequency range. The TIA stage is meticulously designed using a high-performance integrated operational amplifier characterized by low offset voltage drift. In order to ensure the critical gain stability for ultra-low-frequency operation, this design adopts low temperature-drift metal foil resistors. Subsequent voltage amplification is achieved using a noninverting amplifier configuration to attain the necessary high electrical gain, while strictly managing overall electronic noise. By recognizing the criticality of common-mode noise rejection for quantum noise measurements, the photodiode (PD) nonlinear response compensation mechanism is analyzed and optimized. This is achieved through the innovative implementation of a differential fine-tuning circuit (DFTC) coupled with an adjustable bias voltage (ABV) compensation scheme. Experimental validation confirms the effectiveness of the optimized design. The compensation scheme utilizing DFTC and ABV successfully achieves a high common mode rejection ratio (CMRR) exceeding 75 dB@500 Hz. Crucially, the detector achieves an electronic noise spectral density of 3.5 × 10<sup>–5</sup> V/Hz<sup>1/2</sup> within the 1 mHz–1 Hz band, exceeding the requirements for laser intensity noise (1 × 10<sup>–4</sup> V/Hz<sup>1/2</sup>) in space-based gravitational wave detection. Furthermore, the detector demonstrates high gain capability and bandwidth: with an incident detection light power of 4 mW, the balanced detector achieves a gain of 20 dB maintained in a wide frequency range from 1 mHz to 1 MHz. This work presents the design, detailed analysis, and experimental realization of optimized balanced detectors specifically tailored for high-sensitivity measurements in the millihertz gravitational wave frequency band. The achieved low electronic noise base below 1 Hz and high CMRR meet the key requirements for future space-based gravitational wave detectors to detect squeezed states of light. This optimized balanced detector provides important components and technical support for the next-generation space-based gravitational wave detection and millihertz squeezed light characterization.