Fiber Optic Couplers Selection GuideFiber Optic Couplers Selection Guide

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Fiber optic couplers are optical devices that connect three or more fiber ends, dividing one input between two or more outputs, or combining two or more inputs into one output. The device allows the transmission of light waves through multiple paths. 

 

Passive and Active Couplers

Fiber optic couplers can either be passive or active devices.

 

Passive fiber optic couplers are said to be passive as no power is required for operation. They 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 signal(s), 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

Types of fiber optic couplers include splitters, combiners, X-couplers, trees, and stars, which all include single window, dual window, or wideband transmissions. 

 

Fiber optic splitters take an optical signal and supply two outputs. They can further be described as either Y-couplers or T-couplers.

  • Y-couplers have equal power distribution, meaning that the two output signal each receive half of the transmitted power.
  • T-couplers have an uneven power distribution. The signal outputs still carrier the same signal, however the power of one output is greater than the second output. 

Fiber Optic Couplers Selection Guide

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Fiber optic combiners receive two signals and provide a single output. The output signal is typically comprised of multiple wavelengths, due to the amount of interference that occurs when attempting to combine two signals that share the same wavelength.

 

 Fiber Optic Couplers Selection Guide

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X-couplers carry out the function of a splitter and combiner in one package. They are a 2x2 coupler that combines the power of two signals and then divides the signal between two outputs.

 

Star couplers have M inputs and N outputs (MxN). They are used to distribute the power from all of the inputs to all outputs.

 

Fiber Optic Couplers Selection Guide

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Tee couplers either have 1 input and M outputs (1xM) or N inputs and 1 output (Nx1).

 

 Fiber Optic Couplers Selection Guide

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Mechanical Specifications

When specifying optical couplers you should consider the fiber optic cable, the coupler type, signal wavelength, number of inputs and outputs, as well as insertion loss, splitting ratio, and polarization dependent loss (PDL).

 

Fiber Optic Cable

Optical couplers support one of two cable types, single mode or multimode, which will allow either single or multiple paths for light to travel through the fiber respectively. Multimode cables limit the distance that the signal can travel as multiple paths of transmission cause modes of light to disperse limiting the transmission bandwidth. Multimode may be further classified as being step-index or graded-index. Graded-index fibers have improved operational characteristics; the refractive index gradient refracts separate modes of light propagation towards the central core, limiting the dispersion rate.

 

Fiber Optic Couplers Selection Guide

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Number of Inputs and Outputs

Optical couplers can be specified by the number of ports used for signal transmissions going in as well as out. The number and type of ports will be used to describe whether the device is in a splitter, combiner, X-coupler, star or tree coupler.

 

Coupler Type

Optical couplers should be selected based on the bandwidth or window. Regardless of the port types used, fiber optic couplers can be designed for single window, dual wavelength or wideband transmissions.

  • Single window couplers are designed for a single wavelength with a narrow wavelength window.
  • Dual wavelength couplers are designed for two wavelengths with a wide wavelength window for each.
  • Wideband couplers are designed for a single wavelength with a wider wavelength window.   

Wavelength

The electromagnetic spectrum encompasses all frequencies and wavelengths of light. The visible spectrum is a finite range of wavelengths that can be detected by the human eye. Fiber optic couplers transmit light waves from the far visible region, red (630nm), to the near infrared region (1700nm). Within this region specific frequency bands are used as to avoid absorption bands within the carrier medium, the optical fiber. Wavelength choices for fiber optic couplers include 633 nm, 830 nm, 1060 nm, 1300 nm, and 1550 nm.

 

 

Fiber Optic Couplers Selection Guide

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Insertion Loss 

Fiber optic components disrupt signal transmissions as the continuous core of the optical fiber carrying the signal mates with the component in a manner that causes some amount of scattering and reflection of light waves. The degree to which the signal has been depleted is described as insertion loss, which is the undesired attenuation of the signal measured in decibels (dB).

 

Splitting Ratio

The splitting ratio is the distribution of power among the output fibers of a coupler; it is also referred to as the coupling ratio. A splitting ratio of 50/50 means that there is an equal distribution of optical power; a 60/40 ratio means 60% of the power is transmitted to a primary output and 40% to the secondary output.

 

Polarization Dependent Loss (PDL)

The polarization dependent losses are defined as the attenuation caused by polarization. 

 

 

References:

 

Fiber Optic Couplers

  

Fiber Optics Glossary

 

Introduction to Optical Fibers, dB, Attenuation and Measurements