Fiber Couplers

Fiber couplers belong to the basic components of many fiber-optic setups. Note that the term fiber coupler is used with two different meanings:

• It can be 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.
• It can also be a device for coupling (launching) light from free space into a fiber; see the article on fiber launch systems.

This article treats fiber couplers of the first type, coupling light from fibers to fibers. Such couplers can be fabricated in different ways:

• Two or more fibers can be thermally tapered and fused so that their cores come into intimate contact over some length of a few centimeters, for example. Such fused couplers can also be made 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.
• There are fiber-optic pump combiners and pump–signal combiners, which usually work with multimode pump fibers.
• There are planar lightwave circuits, containing things like branching waveguides, with fibers coupled to the inputs and outputs.
• Couplers can also be made from bulk optics, for example in the form of microlenses and beam splitters, which can be coupled to fibers (“fiber pig-tailed”).

One may omit one of the input ports of a 2-by-2 fiber coupler, obtaining a Y coupler, also called T coupler. It may also be called a tap coupler, particularly if only a small fraction of power is obtained at one output and used e.g. for power monitoring. Couplers with many inputs or outputs are called star couplers; they may be used, e.g., as fiber-optic splitters, e.g. for distributing cable-TV signals.

Figure 2 shows a numerical beam propagation simulation for a fiber coupler based of the first type as explained above. Here, the light distribution oscillates between the two fiber cores, and finally the larger part of the power remains in the original (upper) fiber. For light with other wavelengths, however, the coupling can be very different. Therefore, such couplers work only in a limited optical bandwidth. They can be used as dichroic couplers or beam combiners, for example for separating or combining two wavelength components (such as pump and signal light in a fiber amplifier).

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, and is usually quite large (many tens of decibels).

Limitations for Fiber Combiners

Coupling Loss

In many cases, all fibers involved are single-mode, i.e., they support only a single mode per polarization direction for a given wavelength. There are then 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 in fiber amplifiers to combine the signal input and the pump wave. Their insertion loss may be very small (e.g. far below 1 dB) for both inputs. Other wavelength-sensitive couplers are used as multiplexers (WDM couplers) in wavelength division multiplexing ({WDM=wavelength division multiplexing}) telecom systems to combine several input channels with different wavelengths, or to separate channels.

Multimode fiber combiners allow 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. As mentioned above, they can be used as dichroic couplers or beam combiners. They are sometimes also called WDM couplers (→ wavelength division multiplexing).

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-optic splitter, which distributes the power over a large number of output fibers for different customers.
• Fiber couplers can be used in fiber interferometers, for example for optical coherence tomography (OCT). Specially designed broadband couplers are often required for such purposes.
• 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 fiber amplifiers and lasers, dichroic couplers are often used for injecting pump light or eliminating residual pump light from the signal output.
• 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, fiber optics, fiber-optic pump combiners, dichroic mirrors, beam splitters, insertion loss, return loss