14.2 Survey of Test Equipment
585
14.2.6 Visual Fault Indicator
A visual fault locator (VFL) is a handheld pen-sized instrument that uses a visible
laser light source to locate events such as fiber breaks, overly tight bends in a fiber
(for example, in an equipment rack), or poorly mated connectors. The source emits
a bright beam of red light (e.g., 650 nm) into a fiber, thereby allowing the user to
see a fiber fault or a high-loss point as a glowing or blinking red light. In using such
a device, the events must occur where the fiber or connector is in the open so that
visual observation of the emitted red light is possible.
The nominal light output is 1 mW, so the light will be visible through a fiber jacket
at a fault point. This power level allows a user to detect a fiber fault visually. The
device generally is powered by one 1.5-V size AA battery and operates in either a
continuously on or a blinking mode.
14.3 Optical Power Measurement Methods
Optical power measurement is the most basic function in fiber optic metrology.
However, this parameter is not a fixed quantity and can vary as a function of other
parameters such as time, distance along a link, wavelength, phase, and polarization.
14.3.1 Physical Basis of Optical Power
To get an understanding of optical power, it is instructive to look at its physical basis
and how it relates to other optical quantities such as energy, intensity, and radiance.
• Light particles, called photons, have a certain energy associated with them, which
changes with wavelength. The relationship between the energy E of a photon and
its wavelength λ is given by the equation E = hc/λ, which is known as Planck’s
Law. In terms of wavelength (measured in units of μm), the energy in electron
volts is given by the expression E(eV) = 1.2406/λ (μm). Note that 1 eV = 1.60218
× 10
–19 J.
• Optical power P measures the rate at which photons arrive at a detector. Thus it is
a measure of energy transfer per time. Because the rate of energy transfer varies
with time, the optical power is a function of time. It is measured in watts or joules
per second (J/s).
• As noted in Chap. 4, radiance is a measure, in watts, of how much optical power
radiates into a designated solid angle per unit of emitting surface.
Because optical power can vary with time, its measurement also changes with time.
Figure 14.3 shows a plot of the power level in a signal pulse stream as a function
of time. Different instantaneous power level readings are obtained depending on the
585
14.2.6 Visual Fault Indicator
A visual fault locator (VFL) is a handheld pen-sized instrument that uses a visible
laser light source to locate events such as fiber breaks, overly tight bends in a fiber
(for example, in an equipment rack), or poorly mated connectors. The source emits
a bright beam of red light (e.g., 650 nm) into a fiber, thereby allowing the user to
see a fiber fault or a high-loss point as a glowing or blinking red light. In using such
a device, the events must occur where the fiber or connector is in the open so that
visual observation of the emitted red light is possible.
The nominal light output is 1 mW, so the light will be visible through a fiber jacket
at a fault point. This power level allows a user to detect a fiber fault visually. The
device generally is powered by one 1.5-V size AA battery and operates in either a
continuously on or a blinking mode.
14.3 Optical Power Measurement Methods
Optical power measurement is the most basic function in fiber optic metrology.
However, this parameter is not a fixed quantity and can vary as a function of other
parameters such as time, distance along a link, wavelength, phase, and polarization.
14.3.1 Physical Basis of Optical Power
To get an understanding of optical power, it is instructive to look at its physical basis
and how it relates to other optical quantities such as energy, intensity, and radiance.
• Light particles, called photons, have a certain energy associated with them, which
changes with wavelength. The relationship between the energy E of a photon and
its wavelength λ is given by the equation E = hc/λ, which is known as Planck’s
Law. In terms of wavelength (measured in units of μm), the energy in electron
volts is given by the expression E(eV) = 1.2406/λ (μm). Note that 1 eV = 1.60218
× 10
–19 J.
• Optical power P measures the rate at which photons arrive at a detector. Thus it is
a measure of energy transfer per time. Because the rate of energy transfer varies
with time, the optical power is a function of time. It is measured in watts or joules
per second (J/s).
• As noted in Chap. 4, radiance is a measure, in watts, of how much optical power
radiates into a designated solid angle per unit of emitting surface.
Because optical power can vary with time, its measurement also changes with time.
Figure 14.3 shows a plot of the power level in a signal pulse stream as a function
of time. Different instantaneous power level readings are obtained depending on the