Thanks for the tip, I didn't know we had an SR554!

I used a 20 Ohm dummy load for the current driver to figure out how to best use the transformer amplifier for the current noise measurement. I attached a sketch to make the terminology clear.

I made an adapter for the laser cable in which the return wire was cut to allow placing a resistor in series with the laser diode for sensing. The SR554 input (and before simpy the inductors I was checking out) is connected across this sense resistor, providing a path for the DC current so the current driver DC voltage budget is conserved. The transformer has a gain of x100 in passive mode, but also has an integrated optional pre-amp that raises this to x500 and lowers the output impedance.

To determine the current noise, I measured the transfer function from Current Mod In to Voltage across the Dummy Load (20 Ohm) and converted it to current with Ohm's law to know the current flow, then measured the transfer function Current Mod In -> Trafo Out so I can convert the voltage noise at the trafo output to current noise through the load. The result is ~3x better than the data I had previously taken using only the current monitor outputs.

The resistor R is optional, as the current can just as well flow through the primary inductor in the trafo. It helps keeping the trafo resonance under check, but as a consequence reduces peak gain. Anything over 1kOhm does not change the TF anymore. I'm not sure what the best choice is here, but I will check a few different configurations (passive vs active, different R or output loads for the passive case). In the attached figure the trafo resonance results in very low noise in a narrow band, but unfortunately this is not easily tunable.

The laser diode IV-curve is non-linear, which would make the calibration bias current dependent. Instead, I can use the calibration from current flowing to trafo output from the dummy measurement since the ohmic load gave me an exact measure for the current.