This Balanced Photo-Detector (BPD) combines large bandwidth (50 MHz) with low noise performance. At 1 MHz the typical dark noise density is only 1.7 pA/sqrt(Hz). At a wavelength of 800 nm this corresponds to an optical power noise density of 3.2 pW/sqrt(Hz). At an optical power of 8.3 μW (@800 nm) on both photodiodes, the shot-noise density reaches to the dark noise density. The device is equipped with two large-area (d = 0.8 mm) silicon PIN-photodiodes (Hamamatsu S5972). Those cover an optical wavelength range from 320 nm to 1 μm. At 800 nm these photodiodes have a quantum-efficiency of around 84%. Since the photodiodes are mounted on sockets they can be changed easily. They can also be replaced by other low-capacitance photodiodes, covering different wavelength ranges.
The BPD has a gain of 1E4 V/A and the broadband output signal drives up to ±3.5 V when terminated with 50 Ohm. Several monitor signals (I-PD+, I-PD-, I-PD Diff) are available for DC-readout or slow-control; they are also very handy when setting up and adjusting an optical experiment.
With the M4 mounting-thread at the bottom the BPD can be mounted directly onto an optical table. Since the mounting-adapter is made out of plastics the device remains electrically isolated from the optical table.In combination with the included floating power supply no ground-loops can occur.