Basic Features of Optical Fiber

Basic Features of Optical Fiber : This article will cover the basic features of optical fiber used exclusively in Telecom's SDH and DWDM networks. We all know very well that optical fiber used in communication systems consists of a cylindrical glass core and a glass cladding. The outermost layer is a plastic wear-resisting coating. The whole fiber is cylindrical.

We also know that thickness of the core and refractive indexes of the core material and cladding material are critical to the properties of the fiber. We have been studying till now that optical fibers divide into two parts - single mode optical fiber and multimode optical fiber but single mode optical fiber has the advantages of low internal attenuation, large bandwidth, easy upgrades and capacity expansion and low cost. Now, I am going to explain below some important points about the basic features of optical fiber.

Features of Optical Fiber

Basic Features of Optical Fiber

Physical Dimension (Mode Field Diameter)

The single mode fiber core diameter is 8 ~ 9 μm in the same magnitude as the operating wavelength 1.3 ~ 1.6 μm. Due to the optical diffraction effect, it is not easy to measure the exact value of the fiber cord diameter.

Furthermore, since the field intensity distribution of the fundamental mode LP01 is not limited within the fiber core, the concept of single mode fiber core diameter is physically meaningless and actually it should be replaced with the concept of mode field diameter. Mode field diameter measures the level of attention of the fundamental mode field spatial intensity distribution within the fiber.

The nominal mode diameter of the G.652 fiber in the 1310nm wavelength region should be 8.6 ~ 9.5μm with a deviation of less than 10%, and the nominal mode of the G.655 fiber in the 1550nm wavelength region should be filed 8 ~ 11 μm with deviations less than 10%.

Mode Field Concentricity Error

Mode field concentricity error refers to the distance between the mode field center and the cladding of the interconnected fiber. The fiber connector loss is proportional to the square of the mode field concentricity error.

So reducing the mode field concentricity error is one of the major factors to reduce the loss of fiber area and it should be strictly controlled in the process. Mode field concentricity error of two types of single mode optical fiber G.652 and G.655 should not exceed than 1. Generally, it should be less than 0.5.

Bend Loss

Diversion of optical fibers will cause radiation loss. Indeed, the bend arises for an optical fiber in two cases. One is that the curvature radius of the bend is much larger than the diameter of the fiber (such a bend can occur when fiber cable is laid). The second case is microbend. There are many reasons for microbends. Microbends not widely found it is limited to process conditions, can occur during the process of producing fiber and cable. Microbends of different curvature radii are randomly distributed along the fiber.

The larger curvature radius bent fiber transmit fewer modes than the straight fiber, some part of modes also radiated out from the fiber to cause loss. Randomly distributed fiber microbands will result in mode coupling in the fiber and cause energy radiation losses. The twist loss of the fiber is unavoidable because it cannot be guaranteed that there will be no turning of the fiber and cable in any form during the production or use process.

The twist loss or bend loss mode is related to the field diameter. The twist loss or bend loss of the G.652 fiber should not be larger than 1dB at the 1550nm wavelength region, and the twist loss or bend loss of the G.655 fiber should not be larger than 0.5dB at the 1550nm region.

Attenuation Constant

The attenuation in optical fiber is mainly determined by three types of losses: absorption loss, scattering loss and twist loss or bend loss. Bend loss, as described above, has no major effect on the attenuation constants in the fiber. So, it is the absorption loss and scattering loss that mainly determine the attenuation constants in the fiber.The loss of absorption is due to the fiber content where excessive metal impurities and OH-ions absorb light causing damage.Often scattering losses occur in the case that a portion of optical power is scattered outside the fiber core when uneven refractive index distribution emerges within the local field fiber and light scattering due to subtle changes in fiber material density and uneven density of compositions. Causes As SiO2, GeO2 and P2O5. Or, if some defect occurs or some bubbles and gas scabs persist at the core – cladding boundary, scattering losses may occur.

The physical amplitude of these structural defects is much larger than that of lightwave, due to which the wavelengths scatter independently and shift the entire curve of the upward fiber loss spectrum. However, such scattering loss found in optical fiber is much lower than the former.

If we combine the above losses, the attenuation constant of single mode fiber at 1310nm and 1550nm wavelength areas is 0.3~0.4dB/km (1310nm) and 0.17~0.25dB/km (1550nm), respectively. As per ITU-T Recommendation G.652, the attenuation constant at 1310nm and 1550nm should be less than 0.5dB/km and 0.4dB/km, respectively.

Dispersion Coefficient

Dispersion in optical fibers refers to a physical phenomenon of signal distortion, when different modes carrying signal energy or differing in different frequencies of a signal have different group velocities and dispersions from each other during propagation. Typically, three types of dispersion exist in optical fibers.

Modal dispersion: This type of dispersion occurs when the fiber carries multiple modes of signal at the same frequency signal energy and different modes have different time delays during transmission.

Material dispersion: Because the refractive index of a fiber core material is a function of frequency, the signal components of different frequency propagate along the fiber at different velocities. This causes dispersion.

Waveguide dispersion: In the optical fiber, a signal travel different at frequencies in the same mode, This type of dispersion occur because of different group velocities during propagation.

These three types of dispersion are called chromatic dispersion. It is defines by the ITU-T G.652, a zero dispersion wavelength range of 1300nm~1324nm and a maximum dispersion slope of 0.093ps/(nm2.km). It is considered that at the wavelength range of 1525~1575nm, the dispersion coefficient is approximately 20ps/(nm.km). ITU-T G.653 defines a zero dispersion wavelength 1550nm and a dispersion slope of 0.085ps/(nm2.km) in the wavelength range of 1525~1575nm where the maximum dispersion coefficient is 3.5ps/(nm.km). The absolute value of the dispersion coefficient of G.655 fiber should be within 0.1~6.0 ps/(nm2.km) in the range of 1530~1565nm.

Dispersion characteristics of several types of fiber.

Dispersion characteristics

Cutoff Wavelength

To avoid modal noise and dispersion penalty, the cutoff wavelength of the shortest optical fiber cable in the system must be less than the shortest operating wavelength of the system. The position of the cutoff wavelength can at least guarantee single mode transmission in the cable and suppress the occurrence of higher order modes or reduce the penalty of the generated high order mode noise power to a negligible degree. There are three types of cutoff wavelengths defined by ITU-T.

➤It is considered the cutoff wavelength of primary coating fiber in jumper cable shorter than 2m.
➤It is considered the cutoff wavelength of 22m cable optical fiber.
➤It is considered the cutoff wavelength of 2~20m jumper cable.

For G.652 fiber, the cutoff wavelength is 1260nm in 22m cable, 1260nm in 2~20m jumper cable, and 1250nm in jumper cable shorter than 2m. For G.655 fiber, the cutoff wavelength is 1480nm in 22m cable, 1470nm in primary coating fiber of jumper cable shorter than 2m, and 1480nm in 2~20m jumper cable.