If you operate a diode detector in the "square law" region, the voltage output
is proportional to the incident power. You don't get a huge dynamic range
where this is true (10-20 dB, perhaps?), but on the other hand, people have
been building diode detector based power meters for decades. Different diodes
have different curves, so changing diode type would affect the calibration.

I am not sure whether, confining operation to the square law region, it would be
possible to build a power meter that has no DC amplifier (like the Struthers
wattmeter).

73

Tony I0JX