  A well-balanced detector with high sensitivity and low noise is presented in
this paper. The two-stage amplification structure is used to increase
electronic gain while keeping an effective bandwidth of about 70 MHz. In order
to further reduce electronic noise, a junction field-effect transistor(JFET) is
connected between photodiodes and transimpedance amplifier to reduce the impact
of amplifier leakage current. Benefit from these designs, the
root-mean-square(RMS) of noise voltage is about 6 mV with a gain of 3.2E5 V/W,
and it means an ultra-low noise equivalent power density of 2.2E-12 W/rtHz,
only half of common low-noise commercial detectors. In addition, two
photodiodes in similar frequency response are selected for detector and make
the common mode rejection ratio(CMRR) of detector reached 53 dB, about 13 dB
higher than commercial detectors. Further tests indicate that 16.8 dB
shot-noise to electronic-noise ratio is measured in our detector, which is
better than most high speed balanced detectors.
