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Definition: fiber devices for coupling light from one or several input fibers to one or several output fibers
Figure 1: A 2-by-2 fiber coupler.
A fiber coupler is an optical fiber device with one or more input fibers and one or several output fibers. Light from an input fiber can appear at one or more outputs, with the power distribution potentially depending on the wavelength and polarization. Such couplers can be fabricated in different ways:
- Fibers can be thermally tapered and fused so that their cores come into intimate contact. This can also be done with polarization-maintaining fibers, leading to polarization-maintaining couplers (PM couplers) or splitters.
- Some couplers use side-polished fibers, providing access to the fiber core.
- Couplers can also be made from bulk optics, e.g. in the form of microlenses and beam splitters, which can be coupled to fibers (“fiber pig-tailed”).
Fiber couplers are usually directional couplers, which means that essentially no optical power sent into some input port can go back into one of the input ports. There is often a specification of return loss, which indicates how much weaker the back-reflected light is, compared with the input.
Limitations for Fiber Combiners
Coupling Loss
If all fibers involved are single-mode (i.e., support only a single mode per polarization direction for a given wavelength), there are certain physical restrictions on the performance of the coupler. In particular, it is not possible to combine two or more inputs of the same optical frequency into a single-polarization output without significant excess losses, except if the optical phases of the input beams are precisely adjusted and stabilized. That means that the two inputs to be combined would have to be mutually coherent.
However, such a restriction does not occur for different input wavelengths: there are couplers which can combine two inputs at different wavelengths into one output without exhibiting significant losses. Such dichroic couplers are used e.g. in fiber amplifiers to combine the signal input and the pump wave. Other wavelength-sensitive couplers are used as multiplexers in wavelength division multiplexing (WDM) telecom systems to combine several input channels with different wavelengths, or to separate channels.
Multimode fiber combiners allow e.g. the powers of two mutually incoherent beams to be combined without a power loss. However, this will cause some loss of brightness.
Bandwidth
Most types of couplers work only in a limited range of wavelength (a limited bandwidth), since the coupling strength is wavelength-dependent (and often also polarization-dependent). This is a typical property of those couplers where the coupling occurs over a certain length. Typical bandwidths of fused couplers are a few tens of nanometers.
Typical Applications
Some typical applications of fiber couplers are:
- In a cable TV system, the powerful signal from one transmitter is sent in to a fiber splitter, which distributes the power over a large number of output fibers for different customers.
- Fiber couplers can be used in fiber interferometers, e.g. for optical coherence tomography (OCT).
- Within the resonator of a fiber laser, a dichroic fiber coupler can be used to inject pump light, and another fiber coupler can be used as the output coupler. This technique is used particularly in fiber ring lasers, having no resonator ends where light could be injected.
- In high-power fiber lasers and amplifiers, multimode fiber couplers are often used for combining the radiation of several laser diodes and sending them into inner cladding of the active fiber (a double-clad fiber).
See also: fibers, dichroic mirrors, beam splitters
Categories: fibers and other waveguides, photonic devices
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