relative delays between each of the waveforms will be identical. However, the MSE
utilized three different types of links with many different lengths of fiber so accurate
time delay measurements are critical. The total delay of a fiber-optic link is determined
by two factors; electronic delays and fiber delays. Electronic delays constitute any
delays that arise from the transmitter and receiver electronics themselves. Fiber delays
are due to the propagation of light in the fiber-optic cable at a finite speed, which is
slower than the speed of light. In general, the time delay of a link can be modeled as
a constant delay (due to the electronics) plus a delay that is a linear function of fiber
length.
Each individual fiber (2001) or armored cable (2002) was cut and terminated
in the field, so the length of each fiber or cable could not be measured directly.
Therefore, the optical length of each fiber was measured with an Agilent E6000C
Optical Time-Domain Reflectometer (OTDR). The OTDR measures the length of the
cable by sending a pulse of light down the fiber and measuring the time it takes for
the pulse to be reflected back to the light source. The distance resolution of the OTDR
is inversely proportional to the pulse width of the light source (since the OTDR is
actually measuring time); hence a narrow pulse will yield a more precise measure of
fiber length. In addition, when used with multimode fiber, the distance resolution of the
OTDR will be limited by modal dispersion in the fiber, whereby the width of a pulse of
light is widened due to multi-modal propagation in the fiber.
The optical length in meters of a fiber, /, is given by
1 lmeasuredC
I= teasuredc (3-141)
2 N
The quantity c is the speed of light in a vacuum, equal to 3 x 108 m s 1 and
measured is the measured round-time of the light pulse emitted by the OTDR. The
quantity N is supplied by the manufacturer of the fiber and is typically referred to as
the group index, and is defined as the ratio of the speed of light in a vacuum to the