A Fiber coupler is a fiber optic equipment with at least three or more fiber optic ends. The main purpose of a fiber coupler is to either divide the input fiber optic light into several output fiber ends at a particular ratio or combine multiple input fiber optic beams at input end and produce lesser number (or just one) output beams at the output.

Light from an input fiber can come out of one or more outputs, and the power distribution among several outputs depends on the wavelength and polarization of the input optical signal.

For instance, a 1×2 30:70 ratio fiber coupler can split an optical beam of light into two parts at the ratio of 30:70 and transmitted separately into the other two fiber outputs on the other side of the fiber coupler.

Some types of Fiber couplers are limited to use in Single mode fiber while other fiber couplers also

On the basis of the handling capability of a Fiber coupler, a fiber coupler can be classified into two types:

1. Single Window Fiber Coupler
2. Dual Window Fiber Coupler

Single window fiber couplers can facilitate single wavelength with a narrow wavelength window whereas dual window fiber couplers are suitable for upto two wavelengths with a wide wavelength window for each wavelength.

There are wavelength-sensitive fiber couplers as well, which are used as multiplexers in wavelength division multiplexing (WDM) systems for combining several input channels with different wavelengths, or vice-versa.

In general, single-mode fibers are mostly put into use but there are specialized Multi-mode fiber combiners as well which allow the powers of two mutually incoherent beams to be combined without significant power loss.

One peculiar property of Fiber couplers is that Fiber couplers are directional in nature. In other words, the transmitted optical signal from an input port(s) can not get back to input ports again.

Some general applications of fiber couplers are as follows:

• Fiber couplers are used in fiber interferometers, e.g. for optical coherence tomography (OCT).
• Fiber couplers are typically put into use 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).
• Fiber couplers are also used within the resonator of a fiber laser