We opened up the little Thorlabs battery operated PD to see what was inside. Rana took some pictures, and I drew a schematic (attached). It's just a diode, biased with a battery (albeit a crazy 22.5V battery).
Comment by KA: PD is Hamamatsu S1223-01 Si PIN diode.
What a crazy battery. The main point is that it looks like this can be used for reasonable purposes: uses a load resistor on the BNC connector and you can use some pre-amp (e.g. Busby box or SR560) to have a low noise PD readout. You can also use the SR560 in its A-B mode as an 'opamp'. Ground the A input and the use a pole at 1 Hz and make the Output go into the B input through some large series resistor. The BNC from the PD gets Teed into the B input as well.
Then this becomes a transimpedance circuit readout of the diode, using the current noise of the SR560 as the limit.