Used with 8702B or 8703A lightwave component analyzer. See specification and user guide at 1st pdf link below.

Description on Agilent 83410C, 83411C/D, and 83412B receivers :

The lightwave receivers consist of an optical input connector and fiber, launch objects, a PIN diode detector, a thin-film hybrid amplifier, and one DC circuit board. The receiver’s input is a keyed HMS 10 connector with a graded index fiber (9 um for the Agilent 83411C/D receivers, 62.5 um for the Agilent 83410C and 83412B receivers) that is terminated into a graded index lens. The design provides good environmental sealing and allows a low loss and low reflection signal coupling to the diode. The PIN diode has high linearity, wide bandwidth, and low dark current. Notice that it is directly coupled into the first amplifier stage. This physically small low impedance connection limits the effect of standing waves and diode capacitance on the frequency response of the receiver. Three AC (capacitor) coupled 50 ohm amplifiers follow the first amplifier stage.

Active bias circuitry is used in the first amplifier stage to hold its DC bias fixed, while the DC photocurrent from the PIN diode varies. A combination threshold/hysteresis function on the DC board monitors the output of the active bias current and uses the signal to light the OVER ILLUMINATION LED if the average optical input power exceeds 3.5 mW. If the input power continues to increase, the receiver goes into an unspecified state and measurement data will be invalid. If average power exceeds 5 mW, permanent damage could result.