Agilent Technologies 83400 Series, User Manual, Page: 44 / 82

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General Information
Theory of Operation
Theory of Operation
Agilent 83402C and Agilent 83403C sources
The lightwave sources consist of a laser diode, bias circuitry, and control cir-
cuitry. An attenuator and impedance matching network blocks the DC compo-
nent from the RF input that modulates the laser light. The laser diode is made
from InGaAsP (Indium Gallium Arsenide Phosphide) and has a corresponding
back-face diode that is used to control or stabilize the lightwave output.
Notice that the laser creates light in both directions. The back-face diode
senses the laser output and sends a proportional current into the level control
circuit. This level control circuit sends more or less current through the coil to
adjust the bias current that controls the laser output.
The thermal control circuit uses a temperature sensor and a thermal electric
cooler to keep the laser at a steady ambient temperature. If the temperature
of the laser deviates from the present temperature by more than 5 degrees,
the thermal control circuit sends a signal to the level control circuit, which
then shuts down the laser. When this happens, the
TEMP
LED on the front
panel goes on.
The source also has a remote shut-down connector (BNC short) that, when
removed, turns off the laser. If properly used, this feature allows you to turn
the laser off from a distance. You will have to provide your own coaxial cable
with BNC connectors and, if desired, an appropriate switch or circuit. A typi-
cal example would be a laser setup in a room where you do not want anyone
entering while the laser is on. You could control the laser so that when the
door is opened, the remote shut-down connector is opened, and the laser
automatically turns off.
The
DC IN
connector provides power to the instrument from the
Agilent 8702A/B/D front-panel
PROBE POWER
connector or by a separate com-
patible power supply.