590
14 Basic Measurement and Monitoring Techniques
where L 1 is the length of the original fiber, L 2 is the length after cut off, and V 1
and V 2 are the voltage output readings from the long and short lengths, respectively.
Then the attenuation in decibels is
α =
10
4.950 − 0.002
log
6.58
2.21
= 0.95 dB/km
Drill Problem 14.1 A technician plans to use the cutback method to measure
the attenuation at 1550 nm of a 9.60-km long single-mode fiber. The measured
optical power at the far end is 410 μW. After cutting the fiber 2 m from the
source, the optical output measured is 680 μW. Show that the attenuation of
the fiber at this wavelength is 0.229 dB/km.
In carrying out this measurement technique, special attention must be paid to
how optical power is launched into the fiber because in a multimode fiber different
launch conditions can yield different loss values. The effects of modal distributions
in the multimode fiber that result from different numerical apertures and spot sizes
on the launch end of the fiber are shown in Fig. 14.5. If the spot size is small and
its NA is less than that of the fiber core, the optical power is concentrated in the
Fig. 14.5 The effects of launch numerical aperture and spot size on the modal distribution: a underfilling the fiber excites only lower-order modes; b an overfilled fiber has excess attenuation from
higher-order mode loss
14 Basic Measurement and Monitoring Techniques
where L 1 is the length of the original fiber, L 2 is the length after cut off, and V 1
and V 2 are the voltage output readings from the long and short lengths, respectively.
Then the attenuation in decibels is
α =
10
4.950 − 0.002
log
6.58
2.21
= 0.95 dB/km
Drill Problem 14.1 A technician plans to use the cutback method to measure
the attenuation at 1550 nm of a 9.60-km long single-mode fiber. The measured
optical power at the far end is 410 μW. After cutting the fiber 2 m from the
source, the optical output measured is 680 μW. Show that the attenuation of
the fiber at this wavelength is 0.229 dB/km.
In carrying out this measurement technique, special attention must be paid to
how optical power is launched into the fiber because in a multimode fiber different
launch conditions can yield different loss values. The effects of modal distributions
in the multimode fiber that result from different numerical apertures and spot sizes
on the launch end of the fiber are shown in Fig. 14.5. If the spot size is small and
its NA is less than that of the fiber core, the optical power is concentrated in the
Fig. 14.5 The effects of launch numerical aperture and spot size on the modal distribution: a underfilling the fiber excites only lower-order modes; b an overfilled fiber has excess attenuation from
higher-order mode loss