A stabilized laser light
source is used together with an optical power meter to measure the composite
attenuation of an installation i.e. from connector to connector. The light
source comprises a laser diode module, an automatic power control circuit
(APC), a thermo-control circuit and an oscillator (see Figure).
The light from the laser
module is sent into the fiber via a system of lenses. Light from the rear face
of the laser diode is monitored and the value fed back via the APC. This gives
a constant output power during measurement. The temperature of the laser is
monitored and controlled by a Peltier element and the thermo control circuit,
which means that frequency variations are counteracted. Intensity modulation is
achieved by means of a built in oscillator. It should be noted that all light
sources require a heating time of around 10 minutes before the laser reaches
its operating temperature and a stable output power is obtained.
Block
diagram of a stabilized light source with laser diode module
The light sources used
today allow both internal and external modulation of the emitted light.
Continuous wave (CW) light should always be used for measurement of composite
attenuation in installations.
The
light from a stabilized light source is continuous (upper graph) while light
from OTDR is Pulsed (lower graph)
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