2.1 Fiber loss mechanisms

 

Optical beam power traveling along the fiber decreases exponentially with distance.

There are different optical fiber losses mechanisms:

                 Rayleigh scattering,

                 Absorption,

                 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.

 

 

Figure 2.3 Illustration of Rayleigh scattering effect.

 

Rayleigh scattering strongly depends on a wavelength:

 

 

Rayleigh scattering losses can be expressed as:

 

 

Rayleigh 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 mechanism.

 

Absorption is a process of conversion of electromagnetic wave energy into other forms of energy (i. e. lattice vibration).

Intrinsic silica glass absorption occurs in both ultraviolet and infrared bands. Infrared absorption tail causes attenuation for the wavelengths longer than 1,6 mm.

Another 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 nm.

 

For silica 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

Macroscopic bands occur when installing fibers. When band diameter is not less than 50 mm the losses are negligible.

Microscopic band losses occur due to the local distortions of fiber geometry, or refractive index.

 

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:

 

 

The 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.