Table of Contents
Advertisement
Operation
Theory of Operation
Theory of Operation
The Agilent 11980A has optical couplers and fiber whose wavelength charac-
teristics allow the Agilent 11980A to be used throughout the 1250 nm to
1600 nm telecommunications band. Operation below 1250 nm will cause
amplitude loss.
The input signal is split into a long path and a short path. The long path (about
760 meters for the standard Agilent 11980A, and 5.2 kilometers for Option
005) causes the two signals to lose phase coherency above a certain fre-
quency. This frequency is determined by the delay time in the long length of
fiber. When these two signals are mixed at the photodetector (termed a homo-
dyne process), the phase fluctuations are heterodyned, or shifted, to a 0 Hz IF
and displayed on an electrical spectrum analyzer.
The short path contains the polarization adjustment. The polarization adjust-
ment operates on the principle of rotating an optical birefringence in the plane
of propagation of the entering light energy. This birefringence is realized by
rotating a looped optical fiber. The physical stress in the fiber induces a bire-
fringence in the refractive index properties of the fiber, causing a polarization
shift. This effect is similar to the quarterwave plate used in bulk optics.
Rotation of the optical birefringence causes the polarization of the fiber's elec-
tric field to change state. Thus, you can more closely match the polarization
states of the interfering beams. The adjustment will not rotate a given polar-
ization state to any other desired state. It will, however, change the polariza-
tion state sufficiently to avoid an interference null due to orthogonal
polarization states of the interfering beams.
After adjusting polarization to achieve maximum signal amplitude (as seen on
the electrical spectrum analyzer), the combined beams are cabled to the out-
put connector on the Agilent 11980A. This signal can then be analyzed by a
lightwave signal analyzer.
4-2
Advertisement
Table of Contents
