A fiber optic coupler is a device used in fiber optic systems with single or more input fibers and single or several output fibers, which is different from WDM  devices. WDM multiplexer and demultiplexer divide the different wavelength fiber light into different channels, while fiber optic couplers divide the light power and send it to different channel.

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. 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.

Structure
A basic fiber optic coupler has N input ports and M output ports. N and M typically range from 1 to 64. M is the number of input ports (one or more). N is the number of output ports and is always equal to or greater than M. The number of input ports and output ports vary depending on the intended application for the coupler.

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:
Some couplers use side-polished fibers, providing access to the fiber core;
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”).

Types
Fiber optic couplers can either be passive or active devices. Passive fiber optic couplers are simple fiber optic components that are used to redirect light waves. Passive couplers either use micro-lenses, graded-refractive-index (GRIN) rods and beam splitters, optical mixers, or splice and fuse the core of the optical fibers together. Active fiber optic couplers require an external power source. They receive input signals, and then use a combination of fiber optic detectors, optical-to-electrical converters, and light sources to transmit fiber optic signals.

Types of fiber optic couplers include optical splitters, optical combiners, X couplers, star couplers, and tree couplers. The device allows the transmission of light waves through multiple paths.

Fused couplers are used to split optical signals between two fibers, or to combine optical signals from two fibers into one fiber. They are constructed by fusing and tapering two fibers together. This method provides a simple, rugged, and compact method of splitting and combining optical signals. Typical excess losses are as low as 0.2dB, while splitting ratios are accurate to within ±5 percent at the design wavelength. The devices are bi-directional, and offer low backreflection. The technique is best suited to singlemode and multimode couplers.

Choices for fiber optic coupler also include Single window narrow band, Single window Wide band, and Dual window Wide band fiber optic coupler. Single window fiber optic coupler is with one working wavelength. Dual window fiber optic coupler is with two working wavelength. For Single mode fiber, is optimized for 1310 nm and 1550 nm; For Multimode fiber, is optimized for 850 nm and 1310 nm.