14.3 Optical Power Measurement Methods
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14.3.2 Optical Power Meters
The function of an optical power meter is to measure total power over a selected
wavelength band. Some form of optical power detection is in almost every piece of
lightwave test equipment. Handheld instruments come in a wide variety of types with
different levels of capability. Multiwavelength optical power meters using several
photodetectors are the most common instrument for measuring optical signal power
levels. Usually the meter outputs are given in dBm (where 0 dBm = 1 mW) or dBμ
(where 0 dBμ = l μW).
For example, using a Ge photodetector typically allows a measuring range of
+18 to −60 dBm in the 780-to-1600-nm wavelength band, whereas an InGaAs
photodetector allows a measuring range of +3 to −73 dBm in the 840-to-1650-nm
wavelength band. In each case, the power measurements can be made at a number
of calibrated wavelengths (for example, 11 calibrated wavelengths). User-selectable
threshold settings can let the instrument show a pass/fail on a built-in display. Connections can be made to a smart app via a USB port or a Bluetooth interface for data
reporting from the field, cloud storage, and workflow management. Interfaces for
email or messaging are other connection options.
14.4 Characterization of Optical Fibers
Many millions of kilometers of optical fibers have been fabricated and installed
worldwide. Various types of equipment for factory use have been developed to characterize the physical and performance parameters of these fibers. Whereas early
equipment tended to specialize on measuring only one or two parameters, modern
sophisticated instruments require only one simple fiber preparation to characterize
optical fibers accurately during the manufacturing process. These parameters include
mode-field diameter, attenuation, cut-off wavelength, chromatic dispersion versus
wavelength, refractive-index profile, effective area, and geometric properties such as
core and cladding diameters, core-to-cladding concentricity error, and fiber circularity. The two basic measurement methods used by this specialized equipment are
the refracted near-field method and the transmitted near-field techniques. This section
first describes these two techniques and then looks at some standard ways to measure
attenuation.
14.4.1 Refracted Near-Field Method
The refracted near-field measurement method is recommended by the ITU-T and
TIA for determining the refractive-index profile (RIP) [4]. This method determines
the index profile by moving a focused laser across the fiber end face and examining
587
14.3.2 Optical Power Meters
The function of an optical power meter is to measure total power over a selected
wavelength band. Some form of optical power detection is in almost every piece of
lightwave test equipment. Handheld instruments come in a wide variety of types with
different levels of capability. Multiwavelength optical power meters using several
photodetectors are the most common instrument for measuring optical signal power
levels. Usually the meter outputs are given in dBm (where 0 dBm = 1 mW) or dBμ
(where 0 dBμ = l μW).
For example, using a Ge photodetector typically allows a measuring range of
+18 to −60 dBm in the 780-to-1600-nm wavelength band, whereas an InGaAs
photodetector allows a measuring range of +3 to −73 dBm in the 840-to-1650-nm
wavelength band. In each case, the power measurements can be made at a number
of calibrated wavelengths (for example, 11 calibrated wavelengths). User-selectable
threshold settings can let the instrument show a pass/fail on a built-in display. Connections can be made to a smart app via a USB port or a Bluetooth interface for data
reporting from the field, cloud storage, and workflow management. Interfaces for
email or messaging are other connection options.
14.4 Characterization of Optical Fibers
Many millions of kilometers of optical fibers have been fabricated and installed
worldwide. Various types of equipment for factory use have been developed to characterize the physical and performance parameters of these fibers. Whereas early
equipment tended to specialize on measuring only one or two parameters, modern
sophisticated instruments require only one simple fiber preparation to characterize
optical fibers accurately during the manufacturing process. These parameters include
mode-field diameter, attenuation, cut-off wavelength, chromatic dispersion versus
wavelength, refractive-index profile, effective area, and geometric properties such as
core and cladding diameters, core-to-cladding concentricity error, and fiber circularity. The two basic measurement methods used by this specialized equipment are
the refracted near-field method and the transmitted near-field techniques. This section
first describes these two techniques and then looks at some standard ways to measure
attenuation.
14.4.1 Refracted Near-Field Method
The refracted near-field measurement method is recommended by the ITU-T and
TIA for determining the refractive-index profile (RIP) [4]. This method determines
the index profile by moving a focused laser across the fiber end face and examining