Ultra-Low-Noise Balanced Detectors for Optical Time-Domain Measurements

Two ultra-low-noise balanced detectors used for optical time-domain measurements are presented in this paper. Both simultaneously achieve high bandwidth and low noise that is extremely difficult to realize in commercial detectors. Two-stage amplification circuits based on transimpedance amplifiers (TIAs) were used for these detectors, with special modifications implemented for different applications. To further suppress noise, a low-noise junction field-effect transistor is connected between the photodiode and TIA to reduce the impact of amplifier leakage current in one of the detectors. Benefiting from this design, a 70-MHz sensitive detector with a gain of 3.2E5 V/W and low noise-equivalent power (NEP) density of 2.2 pW/rtHz was implemented. For another balanced detector used in higher bandwidth applications, the differential TIA circuit is used, with the detector achieving a 250-MHz bandwidth with a gain of 5E4 V/W, equivalent to NEP of 6.2 pW/rtHz. Due to the difficulty of achieving high bandwidth with low noise, we perform theoretical analysis and simulations for our designs to ensure that these two design goals are realized simultaneously. The performance of the detectors is consistent with our analysis. In addition, a simplified optical testing system was built to test and calculate the performance of the detectors in their respective applications. The results show that both detectors are well-balanced and achieve a common mode rejection ratio greater than 50 dB.