Type of optical fiber losses

Type of optical fiber losses : In this article I will explain that how many types of losses are found in optical fiber. I will try to discuss in detail the optical losses found in practical or field. We all know that the OTDR is an instrument that practically checks the status of optical fiber it means OTDR used for the detect, locate and measure events at any location in the fiber link.

First of all, we will discuss on what kind of loss of optical fiber is encountered through the OTDR. Practically we also use Sources, Power Meters and Attenuators in the field. Before using optical fiber for transmission, we have to have some essential information of optical fiber.

Loss in fiber optic

Type of optical fiber losses

Optical fiber test for transmission

Before using optical fiber for transmission, we have to get information related to losses. Before using optical fiber for transmission, we have to get information related to losses. Below are the main measurements implemented on optical fibers.

➤End-to-End Optical Link Loss
➤Rate of attenuation per unit length
➤Attenuation contribution to splices, connectors, couplers (events)
➤Length of fiber or distance to an event
➤Linearity of fiber loss per unit length (Attenuation discontinuities)
➤Reflectance or Optical Return Loss

Installation and Maintenance Testing

During installation and maintenance we require different testing which is related to optical losses. During installation we test the length, attenuation, reflectance, end-to-end loss, optical return loss.

During maintenance of optical fiber network required different testing like cable attenuation, attenuation and reflection of splices and terminations. maintenance team periodically check the degradation and also losses of transmitter, receiver, cable, connector etc. All these problems are related to optical fiber losses. Losses are found in optical fiber at different wavelengths, which I am mentoring below-

➤0.2 dB/km for single-mode fiber loss at 1550 nm;
➤0.35 dB/km for single-mode fiber loss at 1310 nm;
➤1 dB/km for multi-mode fiber loss at 1300 nm;
➤3 dB/km for multi-mode fiber loss at 850 nm;
➤0.05 dB for a fusion splice
➤0.1 dB for a mechanical splice;
➤0.2 - 0.5 dB for a connector pair;
➤3.5 dB for a 1 to 2 splitter (3 dB splitting loss plus 0.5 dB excess loss).

Type of losses occur in Connectors, patchcords and pigtails

Return Losses : Return loss occur due to light reflected back to the source at the junction. It also measured in decibels (dB). Return losses value measured between 35 and 50 dB for PC ( (Physical Contact), 60 to 90 dB for APC (Angled Physical Contact).

PC and APC contact surface

Losses due to contact of angle

Return loss due to reflection : Mode of light are diffused at the coupling point (red arrow). Cleaned PC connector exhibits approx. 14.7 dB return loss against air and 45-50 dB when plugged in. On the other hand APC connector exhibits at least 55 dB RL against air and 60-90 dB when plugged in.

Insertion loss : When different core radii fiber/ connector,patchcord etc.are used with different refractive indexes or eccentricities of the core insertion loss occur due to various reasons including reflections and roughness on the end faces, pointing errors or radial misalignment. Insertion loss value occur the range from 0.1 to 0.5dB.

Extrinsic losses : If the fiber cores are more precisely coupled, less light energy is lost. For this reason, high precision fibers are glued together in precision ceramic ferrules. Extrinsic losses occur due to reflections, roughness on end faces, angular errors (angular pointing error) or radial misalignment (concentrations). Reflection and roughness play a subordinate role in loss.

Losses Occur in Optical Fiber 

If we study all the losses found in optical fiber, we find that there are five types of losses in optical fiber.

➤Absorption loss,
➤Scattering loss,
➤Dispersion loss,
➤Radiation loss,
➤Coupling loss.

Absorption loss

The absorption loss is related to the manufacturing of the optical fiber cable like material composition and fabrication process. Absorption loss due to some dissipation of optical power as listening in a fiber cable.

During optical fiber manufacturing, it is especially noted that optical glass is very pure, no any type of impurities. But sometimes even if not wanting, the impurities remain. we can say that glass fibers are extremely pure. Absorption loss due to impurities present in the optical fiber, it means absorption in optical fiber depend upon concentration and light wavelength.

There are two types of absorption found in optical fiber-
1. Intrinsic absorption
2. Extrinsic absorption

1. Intrinsic absorption : The intrinsic absorption occur in the ultraviolet region, this absorption is not very common in practical but it happen due to the electronic absorption band. Basically in this absorption light particle interacts with an electron and stimulates it to a higher energy level.

The main reason for intrinsic absorption is that vibration frequency of atomic bonds in the infrared region. We all must have read in the basic concept of optical fiber that optical fiber is made of silica glass.Vibration of silicon-oxygen (Si-O) bonds create the absorption in the optical fiber.

The interplay between the vibrating bond and the electromagnetic field of the optical signal causes internal absorption. Due to the interaction between electromagnetic field and optical signal light energy is transferred from the electromagnetic field to the bond.

2. Extrinsic absorption : Extrinsic absorption  is much more important than intrinsic due to impurities introduced into the fiber material during manufacture - iron, nickel and chromium. In practice, we cannot say that extrinsic abortion or intrinsic is one of the more important. Practically, both abstraction is important.

Modern manufacturing techniques can reduce impurity levels below 1 part in 1010 due to the transition of metal ions to higher energy levels. For some of the more common metallic impurities in silica fibers the table shows attenuation due to peak attenuation wavelength and impurity concentrations of 1 in 109.

Radiation losses

The loss of radiation is also called bending loss,bending in optical fiber more than bending radius, when the fiber is curved occur the radiation loss at that point. There are two types of radiation losses: micro-bending losses and macro bending.

Radiation loss occur in optical fiber due to bend, bend occurs in optical fiber only for two reasons. First reason is that curvature radius of the bend is much larger than the diameter of the fiber. Second reason is the micro bend. The bend 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.

Scattering loss

Basically, scattering losses occur due to the interaction of light with density fluctuations within a fiber. Density changes when optical fiber is produced when optical fibers are manufactured.

Linear Scattering Losses : Linear scattering occurs when optical energy is transferred from the dominant mode of operation to the adjacent mode. This is proportional to the injected input optical power in the dominant mode. o Linear scattering is divided into two categories: Mie scattering and Rayleigh scattering.

Non- Linear Scattering Losses : The scatter loss in the fiber is also due to non-linearity, that is, if the optical power at the output of the fiber does not change proportionally with the power change at the fiber's input, the optical fiber is said to work in operating in the non-linear mode. Non-linear scattering is divided into two categories: Stimulated Raman Scattering and Stimulated Brillouin Scattering.

Dispersion loss

Dispersion is a measure of temporal propagation that occurs when a light pulse propagates through an optical fiber. Dispersion is sometimes referred to as delay distortion in the sense that the delay of diffusion time makes the pulse wider.

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