With two inputs and two outputs, this forms a differential amplifier stage (Fig. 2).

Unfortunately, the second laser doesn't emit enough light, so you need an amplifier stage, which is nearly as complicated as any of the lasers.

Where AC measurements are required, the rectifier can be placed after the amplifier stage, improving precision at low range.

Depending on on the manner in which the different amplifier stages are connected, one of the following amplifiers may result:

Also the loading effects on the driving amplifier stage are reduced.

Power handling capability is limited by the amplifier stages.

The resulting signal was fed to an amplifier stage.

The principle can be demonstrated mathematically; the receiver's noise figure is calculated by modularly assessing each amplifier stage.

Then the signal is applied to the amplifier stages.

The value of the grid resistor determines the gain of the amplifier stage.