fiber couplers (original) (raw)

Definition: fiber devices for coupling light from one or several input fibers to one or several output fibers, or from free space into a fiber

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Related: Tutorial on Passive Fiber Optics Part 8: Fiber Couplers and Splittersfibersfiber opticsfiber-optic pump combinersfiber shufflesinsertion lossreturn loss

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Contents

What is a Fiber Coupler?

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 more 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:

Figure 1: A 2-by-2 fiber coupler.

• 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 splitter, also called Y coupler or 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 on 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).

Figure 2: Amplitude distribution in a fiber coupler, obtained with a numerical simulation of beam propagation, done with the software RP Fiber Power.

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) 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 into 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).

Frequently Asked Questions

What is a fiber coupler?

A fiber coupler is an optical fiber device that connects multiple fibers, allowing light from an input fiber to be distributed to one or more output fibers. The term can also refer to a fiber launch system for coupling light from free space into a fiber.

How are fiber couplers fabricated?

Common methods include thermally tapering and fusing two fibers together (fused couplers), using side-polished fibers, or building planar lightwave circuits. They can also be made from bulk optics like microlenses and beam splitters connected to fibers.

What is a directional coupler?

A directional coupler is designed so that light entering an input port is transferred only to the output ports, with very little light reflecting back into any of the input ports. This performance is quantified by the return loss, which is typically very high.

Is it possible to combine light from two fibers into one without loss?

Combining two mutually coherent beams into a single-mode fiber can be done with low loss if their optical phases are stabilized. However, combining incoherent beams of the same wavelength into a single-mode fiber results in significant loss. Beams with different wavelengths can be combined with low loss using a dichroic coupler.

What is a dichroic fiber coupler?

A dichroic fiber coupler is a wavelength-sensitive device designed to combine or separate light at different wavelengths. A common application is in fiber amplifiers to combine pump light and signal light into a single fiber.

What are typical applications of fiber couplers?

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 101 suppliers for fiber couplers. Among them:

⚙ hardware

DK Photonics uses a unique fusing technique and polarization-maintaining fibers to fabricate the polarization maintaining fused coupler (PMC). The coupling ratio can be selected according to the customer’s request. It features low excess loss, small size and high polarization extinction ratio. PMC is widely used for optical sensors and optical gyros.

⚙ hardware

TOPTICA´s COOL patent pending fiber coupling concept is new, because it does not use any mechanical micro adjustable parts, which normally are the first to move due to thermal or mechanical influences.All major optical components are solidly mounted and the optical micro alignment is unsusceptible to mechanical and thermal distortions or translations.

⚙ hardware

The G&H line of HI REL fused fiber optic components are available in PM and SM form. They are deployed in environments such as undersea and space where the costs of component replacement are prohibitive and reliability is of premier concern.

G&H is established as a preferred supplier of these components to most major undersea telecommunications equipment manufacturers. Our HI REL capability is built upon the foundation of an extended history of also manufacturing very reliable components for land-based (or terrestrial) systems in volume.

⚙ hardware

CSRAYZER’s polarization-maintaining filter or fused coupler series products are used to split inputs from a polarization-maintaining optical fiber according to the given coupling ratio. They are widely used in fiber lasers, optical fiber amplifiers, optical fiber communications and fiber sensors, having compact dimensions, low insertion loss, low polarization dependent loss and high stability, and the ability to work under different temperature conditions.

⚙ hardware

Schäfter+Kirchhoff's high precision fiber couplers (fiber port) are optimized for high pointing stability and long-term stability. They provide efficient coupling of ­collimated laser radiation into ­single-mode and PM fiber cables. They are available as 60SMS and 60SMF version with a large variety of focal lengths and different optics including aspheres, achromats and apochromats. Well-proven in the lab and in industrial fiber coupling sets.

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