Fiber Optics/Optical Fiber Losses
Optical fiber communication system works same as any other telecommunication system difference being instead of electrical signals travelling through metal cables, light carries information in glass fibers. Fig 2.1.a describes a simple telecommunication system consisting of information source, transmitter, channel and a receiver. Fig 2.1.b describes an optical fiber communication system.
Main components of an Optical Fiber Communication System are
- Information Source: Usually a person or computer or any entity which want to send information to the receiver.
- Transmitter : It consists of three stages. Firstly the information from the information source is converted into corresponding electrical signals. These electrical signals then drive an optical source(LED/LASER) modulate the light signal.
- Optical Fiber (Channel): Optical fibers are mainly glass or plastic fibers used for propagation of light over large distances. The propagation of light through the fiber is based on the principle of total internal reflection.
- Receiver: Receiver as transmitter also has three stages. The optical detector (LDR/Photodiode) detects the incoming optical signal and drives the electrical receive to convert optical signal into electrical signal and deliver it to the recipient using suitable transducer.
- Connectors: They are used for connection between two fibers or transmitter to fiber or fiber to receiver connections.
Also known as transmission loss. It can be defined as the loss of optical power when light travels long distances inside a transmission medium is known as attenuation. It is defined as the ratio of the optical input power (Pi) and optical output power (Po) .
Attenuation = (10/L)log10(Pi/Po)
Unit of attenuation is db/Km
One of the major forms of signal loss in optical fiber. When light travels in the glass/fiber medium impurities in the channel such as metal particles or moisture in the fiber can block some of the light, absorb it and dissipate light in form of heat resulting in loss. So absorption loss can be defined as the portion of signal loss due to its conversion into other forms of energy such as heat. It’s of two types namely: -
- Intrinsic Absorption: It is cause by the basic fiber-material properties. If we consider the fiber to free all the impurities and imperfections then all losses would be intrinsic. These can be overcome only by changing the fiber material.In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. So for silica in the wavelength regions of interest for optical communication infrared absorption tails makes negligible contributions..
Loss due to 1ppm of impurity (dB/km)
Absorption Peak Wavelength (um)
2. Extrinsic Absorption : It is caused by the presence of impurities in the fiber like metal ions such as Fe2+, Cu2+, Cr3+ or presence of hydroxyl ions (OH-) i.e. silicon-hydroxyl (Si-OH) bond inside the fiber. For lower losses (<1dB/km) the metal impurities should be below part per billion.Using dry fibers the OH ion concentration is reduced and we can see the peak at 1.39um disappears.
Light propagation inside the fiber is based on total internal reflection. Any irregularity even molecular-level irregularities in the surface of the fiber can cause the light to reflect light in random directions and results in scattering losses. It can be divided into two categories namely
- Linear scattering
- Non- Linear scattering.
- Linear scattering
1.1 Rayleigh Scattering: It is one of the main causes of fiber optic losses. In molten state the silica molecules move randomly and freeze at one place in solid state. This causes fluctuation in the density and consequently fluctuations in refractive index occur throughout the fiber. Scattering of light due to this is known as Rayleigh scattering and it result to 96% of the optical fiber losses.
1.2 Mie Scattering: Scattering of light can also occur if the inhomogeneities in the fiber are comparable to the guided wavelength. It is due to the imperfections in the cylindrical structure of the waveguide such as irregularities in the core–cladding interface, core–cladding refractive index differences along the ﬁber length, diameter ﬂuctuations, strains and bubbles.
Non-linear scattering losses
These type of losses occur due to the inelastic scattering of a photon to a lower energy photon i.e. the energy of the light signal is transferred to another wave of higher wavelength but lower energy. The energy difference results into a photon.
Non-Linear scattering is of two types
- Stimulated Raman scattering (SRS)
- Stimulated Brillouin scattering (SBS).
1.1 Stimulated Brillouin Scattering (SBS): Comes in effect at higher frequencies. When modulation of light happens due to thermal vibration inside the fiber it results in SBS. The scattering produces a acoustic photon which produces an optical frequency shift of around 10 GHz known as Stokes shift. Occurs only in backward direction.
1.2 Stimulated Raman scattering (SRS): It is similar to Brillouin scattering difference being that instead of an acoustic photon an optical photo is generated. Also it can occur in both forward and backward direction and can have an optical threshold of upto three times the magnitude compared to Brillouin scattering. It has a frequency shift of around 13 THz.
As the name suggests these losses occurs due to bending of fiber as it disrupts the path of light signal. It’s of two types namely
- Macro bends
- Micro bend losses.
1.1 Macro bends: The folding of optical fibers into tight bends is known as macrobends. This creates an angle too sharp that some of the light is not reflected back into the fiber and escapes the cladding resulting in loss of signal. The optical loss increases as the radius of the fiber is decreased. Different fibers have different specificatiosns on the amount of permissible bends without significant loss of signal. For G.657.B.3 fiber standard (International Telecommunication Union) the radius has been standardized as low as 5mm.
1.2 Micro bends: Small bends which are caused by pinching or squeezing of the fiber which leads to deformations in the fiber structure. This results in displacement of light and hence causes optical loss. [Halit Eren. "Optical Loss."]
Dispersion is described as the broadening of light pulses. It is the result of physical properties of the system. It’s of three types namely
- Model dispersion
- Material dispersion
- Wave-guide dispersion
1.1 Modal Dispersion: It occurs in multi-mode fibers only as in single mode fibers there is only one signal pulse. In multi-mode fibers as it has a larger core size it can guide several modes simultaneously. Each mode enters the fiber at different angles and travel different distance towards the receiver. During time the light waves spreads and this may lead to overlapping of the signals hence causing distortions in the signals.
1.2 Material dispersion: It occurs due to the fact that the spreading of light is dependent on the interaction of light with the refractive index of the fiber material i.e. Every wavelength travel with different speed inside the fiber and spreading of light depend on its wavelength.
1.3 Wave-guide dispersion: This type of dispersion mainly occurs in single mode fibers. This happened due to the reason that some part of light travel in cladding and most part of light travel inside the core. Due to the difference in the refractive index of both core and cladding. The light in each travel at different velocities hence causing spreading of light at receiver.
These type of losses occur at the interconnection of two fibers. It can happen if the connection between the fiber is misaligned or has air-gap or have unequal diameter or have tilted axis. The types of connector losses are depicted in the fig.