Coupling loss
Encyclopedia
Coupling loss also known as connection loss is the loss that occurs when energy
is transferred from one circuit, circuit element, or medium to another. Coupling loss is usually expressed in the same units
—such as watt
s or decibel
s—as in the originating circuit element or medium.
Coupling loss in fiber optics
refers to the power loss that occurs when coupling light from one optical device or medium to another. (See also Optical return loss.)
Coupling losses can result from a number of factors. In electronics (see Coupling (electronics)
), impedance mismatch between coupled components results in a reflection
of a portion of the energy at the interface. Likewise, in optical systems, where there is a change in index of refraction (most commonly at a fiber/air interface), a portion of the energy is reflected back into the source component.
Another major source of optical coupling loss is geometrical. As an example, two fibers coupled end-to-end may not be precisely aligned, with the result that the two cores overlap somewhat. Light exiting the source fiber at a portion of its core that is not aligned with the core of the receiving fiber will not (in general) be coupled into the second fiber. While some such light will be coupled into the second fiber, it is not likely to be efficiently coupled, nor will it generally travel in an appropriate mode in the second fiber.
Similarly, even for two perfectly aligned cores, where there is a gap of any significant distance between the two fibers, there will be some geometric loss due to spread of the beam. Some percentage of the light rays exiting the source fiber face will not intersect the second fiber within its entrance cone.
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
is transferred from one circuit, circuit element, or medium to another. Coupling loss is usually expressed in the same units
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
—such as watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s or decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s—as in the originating circuit element or medium.
Coupling loss in fiber optics
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
refers to the power loss that occurs when coupling light from one optical device or medium to another. (See also Optical return loss.)
Coupling losses can result from a number of factors. In electronics (see Coupling (electronics)
Coupling (electronics)
In electronics and telecommunication, coupling is the desirable or undesirable transfer of energy from one medium, such as a metallic wire or an optical fiber, to another medium, including fortuitous transfer....
), impedance mismatch between coupled components results in a reflection
Signal reflection
Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber, some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the...
of a portion of the energy at the interface. Likewise, in optical systems, where there is a change in index of refraction (most commonly at a fiber/air interface), a portion of the energy is reflected back into the source component.
Another major source of optical coupling loss is geometrical. As an example, two fibers coupled end-to-end may not be precisely aligned, with the result that the two cores overlap somewhat. Light exiting the source fiber at a portion of its core that is not aligned with the core of the receiving fiber will not (in general) be coupled into the second fiber. While some such light will be coupled into the second fiber, it is not likely to be efficiently coupled, nor will it generally travel in an appropriate mode in the second fiber.
Similarly, even for two perfectly aligned cores, where there is a gap of any significant distance between the two fibers, there will be some geometric loss due to spread of the beam. Some percentage of the light rays exiting the source fiber face will not intersect the second fiber within its entrance cone.