2.1 Fiber loss mechanisms
Optical beam power traveling along
the fiber decreases exponentially with distance.
There are different optical fiber losses
Macroscopic and microscopic bends,
Simulated nonlinear scattering.
Figure 2.1 Spectral
attenuation of a silica optical fiber.
Figure 2.2 Attenuation
of the singlemode step index (SM SI) and multimode graded index (MM GI) silica
glass fibers as a function of a wavelength.
Rayleigh scattering is a wavelength dependent process that depends
on material inhomogeneities smaller than wavelength.
Illustration of Rayleigh scattering effect.
scattering strongly depends on a wavelength:
scattering losses can be expressed as:
attenuation restricts the use of fibers at short wavelength. The Rayleigh
scattering defines the fundamental limit of reachable fiber attenuation. Within
optical windows the Rayleigh scattering is the most significant attenuation
Absorption is a process of conversion of electromagnetic
wave energy into other forms of energy (i. e. lattice vibration).
silica glass absorption occurs in both ultraviolet and infrared bands. Infrared
absorption tail causes attenuation for the wavelengths longer than 1,6 mm.
problem is connected with impurity absorption losses. Today, the most important
impurity in optical fibers is water in the form of hydroxyl ions (OH-). The most significant OH- losses occur at 950, 1250 and 1380
fiber, the lowest losses of about 0,18 dB/km can be obtained in the region of
1550 nm. They are very close to the fundamental scattering limit. For longer
wavelengths, silica attenuation is increasing. In order to obtain fiber with
lower losses than silica waveguides, it is necessary to implement different
materials with low intrinsic absorption in a more far infrared region.
Figure 2.4 Spectral
attenuation of different material fibers.
Macroscopic and microscopic bends
bands occur when installing fibers. When band diameter is not less than 50 mm
the losses are negligible.
band losses occur due to the local distortions of fiber geometry, or refractive
Stimulated Brillouin backscattering (SBS) occurs when an
optical signal beam creates periodic traveling optical grating. SBS spectrum is
very narrow and shifted of couple GHz. The threshold power for SBS effect is:
where: d –
fiber core diameter, adB – fiber attenuation, n – optical beam bandwidth, l – beam wavelength.
Stimulated Raman scattering (SRS) is a nonlinear process with a
high frequency photon generation. SRS propagates in forward and backward
directions. The threshold optical power is given by:
threshold optical power is up to three orders of magnitude higher than the
Brillouin threshold. The scattered beam in silica fiber has a spectral width of
about 40 THz.