Various laboratory measurements are routinely performed on telecommunication fibers to test their performance as components of fiberoptics communication systems. Some of these measurements are listed below.
Attenuation is the loss of optical power as a result of absorption, scattering, bending, and other loss
mechanisms as the light travels
through the fiber. The total attenuation is a function of the wavelength
λ
of the light.
The total attenuation A between two arbitrary points X and Y on
the fiber is A(dB) = 10 log_{10} (P_{x}/P_{y}). P_{x}
is the power output at point X. P_{y} is the
power output at point Y. Point X is assumed to be closer to the
optical source than point Y. The attenuation coefficient or
attenuation rate α is given by α(dB/km)
= A/L. Here L is the distance between points X and Y. The cutback method is often used for measuring the total attenuation of an optical fiber. The cutback method involves comparing the optical power transmitted through a long piece of fiber to the power transmitted through a very short piece of the fiber. The cutback method requires that a test fiber of known length L be cut back to a length of approximate 2 m. It requires access to both ends of the fiber. The cutback method begins by measuring the output power P_{y} of the test fiber of known length L. Without disturbing the input conditions, the test fiber is cut back to a length of approximate 2 m. The output power P_{x} of the short test fiber is then measured and the fiber attenuation A and the attenuation coefficient a are calculated. Different launch conditions can lead to different results. For multimode fiber, the distribution of power among the modes of the fiber must be controlled. This is accomplished by controlling the launch spot size, i.e. the area of the fiber face illuminated by the light beam, and the angular distribution of the light beam. When the launch spot size is smaller than the area of the fiber face and the numerical aperture NA of the input radiation is smaller than the NA of the fiber, the fiber is said to be underfilled. Most of the optical power is concentrated in the center of the fiber and mainly loworder modes are excited. underfilled launch conditions When the launch spot size is larger than the area of the fiber face and the numerical aperture NA of the input radiation is larger than the NA of the fiber, the fiber is said to be overfilled. Light that falls outside the fiber core and light incident at angles greater than the angle of acceptance of the fiber core is lost. Overfilling the fiber excites both loworder and highorder modes. overfilled launch conditions Launch conditions affect the results of multimode fiber attenuation measurements. If too much power is launched into highorder modes, the highordermode power loss will dominate the attenuation results. Generally, fiber attenuation measurements are performed using underfilled launch conditions.
 
The cutoff wavelength of a single mode fiber is the wavelength above
which the fiber propagates only the fundamental mode. We need V = k_{f} a
NA = 2π a NA / λ_{0} < 2.405.
The cutoff wavelength of a
single mode fiber is a function of the fiber's radius of curvature.
Measuring the cutoff wavelength involves comparing the transmitted power
from a test fiber with that of a reference fiber as a function of
wavelengths.
The test fiber is loosely supported in a singleturn
with a constant radius of 140 mm. The transmitted signal power P_{s}(λ)
is recorded while scanning the wavelength in increments of 10 nm
or less over the expected cutoff wavelength. The launch and detection conditions are not changed while scanning
over the range of wavelengths. For the reference power measurement the launch and detection conditions are not
changed, but the fiber
is bent to a radius of 30 mm or less to suppress the secondorder mode at
all the scanned wavelengths. The
transmitted signal power P_{r}(λ) is
recorded while scanning over the same wavelength range as before.
The attenuation at each wavelength is calculated.
 
The modal bandwidth of
a multimode optical fiber can be measured by measuring the power transfer function
H(f) of the fiber at the band frequency
(f). Signals of varying frequencies (f) are launched into the test fiber and
the power exiting the fiber at the launched fundamental frequency is measured. This optical output power is denoted as P_{out}(f).
The test fiber is then cut back or replaced with a short length of fiber of
the same type. Signals of the same frequency are launched into the cutback
fiber and the power exiting the cutback fiber at the launched fundamental
frequency is measured. The optical power exiting the cutback or replacement
fiber is denoted as P_{in}(f). The magnitude of the optical fiber
frequency response is defined as H(f) = 10 log_{10}(P_{out}(f)/P_{in}(f)), or, if the launch conditions for the two experiments are not exactly the same, H(f) = 10 log_{10}(P_{out}(f)P_{in}(0)/P_{in}(f)P_{out}(0)). The fiber bandwidth is defined as the frequency at which the magnitude of the fiber frequency response has decreased to onehalf its zerofrequency value and H(f) = 3. This frequency is called the 3 decibel (dB) optical power frequency (f_{3dB}) and referred to as the fiber bandwidth. Bandwidth is normally given in units of megahertzkilometers (MHzkm). Converting to a unit length assists in the analysis and comparison of optical fiber performance.
 
Chromatic dispersion occurs because the index of refraction is a
function of wavelength and different wavelengths of light travel
through the fiber at different speeds. The chromatic dispersion of
multimode gradedindex and single mode fibers is obtained by measuring the
time it takes pulses of light with different wavelengths to travel through a
long piece of fiber. These measurements are made using
multiwavelengths sources such as wavelengthselectable lasers or multiple
sources of different wavelengths.
 
To make fiber geometry measurements, the input end of the fiber is
overfilled and mode filtered. The output end of the
fiber is viewed with a video camera. The image from the video camera is sent
to a computer for digital analysis. The computer analyzes the image to
identify the edges of the core and cladding. The cladding diameter is defined as the average diameter of the
cladding. The core diameter is defined as the average diameter
of the core. Cladding noncircularity, or ellipticity, is the
difference between the smallest radius of the cladding and the largest
radius of the cladding divided by the average cladding radius. Core noncircularity
is the difference between the smallest core radius and the largest
core radius divided by the average core radius. Core noncircularity is
measured on multimode fibers only. The corecladding concentricity error
for multimode fibers is the distance between the core and cladding centers
divided by the core diameter. The corecladding concentricity error
for single mode fibers is defined as the distance between the core and
cladding centers.

The core diameter is measured by measuring the power distribution in the nearfield
region close to the fiberend face, when the distance
between the fiberend face and detector is in the micrometers range.
The core diameter (D) is defined as the diameter at which the intensity is
2.5 percent of the maximum intensity.
 
The numerical aperture (NA) is a measurement of the ability of an
optical fiber to capture light. It is determined by measuring the farfield power distribution
in the region far
from the fiberend face. The emitted power per unit area is recorded as a function of
the angle θ some distance away
from the fiberend face. The distance between the fiberend face and
detector in the farfield region is in the centimeters (cm) range for
multimode fibers and millimeters (mm) range for single mode fibers.
The fiber NA is defined by the 5 percent or 0.05
intensity level, This 0.05 intensity level intersects the normalized
curve at scan angles θ_{A} and θ_{B}.
The fiber NA is defined as NA = sin((θ_{A}
θ_{B})/2).
 
The mode field diameter (MFD) of a single mode fiber is related to
the spot size of the fundamental mode. This spot has a mode field radius
r_{0}. The mode field diameter is equal to 2r_{0}.
Single mode fibers with large mode field diameters are more sensitive to
fiber bending. Single mode fibers with small mode field diameters show
higher coupling losses at connections. The mode field diameter of a
single mode fiber can be measured by measuring the output farfield power
distribution of the single mode fiber using a set of apertures of various
sizes.
 
Insertion loss is composed of the connection coupling loss and additional
losses in the fiber following a connection. In multimode fiber,
fiber joints can increase fiber attenuation following the joint by
disturbing the mode power distribution. In single mode
fibers, fiber joints can cause the secondorder mode to propagate in the
fiber following the joint. To measure insertion loss, power measurements are
made on an optical fiber before the joint is inserted
and after the joint is inserted. Initial power measurements at the detector (P_{0}) and at the
source monitoring equipment (P_{M0}) are taken before inserting the
interconnecting device into the test setup. The test fiber is then cut
and the device is inserted. After insertion the power at the detector (P_{1}) and at the source
monitoring equipment (P_{M1}) is measured again. The insertion loss is
calculated as Insertion loss (dB)= 10 log_{10}((P_{1}/P_{0})(P_{M0}/P_{M1}))
 
Reflection occurs at optical fiber connections. The
reflectance
R is the fraction of the incident intensity that is
reflected back into the source fiber at the point of the connection.
The return loss is defined as Return loss = 10 Log R. The reflectance R is measured using an optical source connected to one input of a 2 X 2 fiber optic coupler. Light is launched into the component under test through the fiber optic coupler. The light reflected from the component under test is transmitted back through the fiber optic coupler to a detector connected to the other input port. 