Luna 4600 - Page 189

Optical Backscatter Reflectometer 4600 183
User Guide
i + 1 i
8
Frequency Domain Return Loss
Return loss is the ratio of optical power output by the device to the optical power
input to the device, and is expressed in dB. In terms of the transfer function, it is
defined as
RL
i
=
20Log( E
r
⁄ E
in
) .
where the subscript “i” corresponds to the “i
th
” data point, E
r
is the reflected field
and E
in
is the input field.
Figure 8-8. Return Loss data of a fiber Bragg grating.
Frequency Domain Group Delay
Group delay (GD) is defined to be the rate of change of phase as a function of
frequency. Physically, this property corresponds to the delay experienced by the
sinusoidal envelope of a modulated optical signal as it propagates through the device.
In terms of the transfer function, the group delay is given by
GD
i
=
arg ( H H
∗
)
--------------------------------
,
∆ω