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Fiber Joints

Definition: permanent or removable connections between fiber ends

German: Faserverbindungen

Category: fiber optics and waveguides

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In many applications of fiber optics, it is necessary to connect fiber ends (terminations) in some way such that light from one fiber can get into the other fiber without losing too much of its optical power. Examples are fiber lasers and systems for optical fiber communications.

There are different techniques for joining fiber ends:

fusion splice between PCF and conventional fiber
Figure 1: Microscope imagine of a fusion splice between a photonic crystal fiber (PCF, left side) and a conventional fiber (right side). The hole pattern of the PCF can be seen. The photograph has been kindly provided by NKT Photonics.

In any case, it is essential that the fiber ends are carefully prepared before joining them. In many cases, fiber ends with perpendicularly cut surfaces are needed. This has the advantage that the axis of the emitted light is in line with the fiber axis, and the alignment is comparatively simple. In other cases, the cleave angle has to have some deviation from 90°. Such angle cleaves have the advantage that light reflected at the air–glass interface will not get back into the fiber mode, provided that the cleave angle is sufficiently large.

It is also possible to use free-space optics for launching light from one fiber into the other. Typical configurations contain a single lens or two lenses. An advantage of that approach is that the fiber ends can be kept at a safe distance, making it less likely to damage them. Also, it is possible to efficiently couple light between fibers with different effective mode areas if the optics are designed accordingly. Furthermore, it is possible to insert other optical elements, such as an optical filter or a Faraday isolator. On the other hand, free-space optics have to be kept in stable alignment and are sensitive to dust. In addition, losses arising from the Fresnel reflections at the fiber ends cannot be avoided.

Tolerances for Low-loss Fiber Joints

Multimode Fibers

The alignment tolerances for connections between fibers are relatively low for multimode fibers, particularly for those with a large core area. A high coupling efficiency is possible if the parameters of the two fibers differ substantially. It is sufficient that the second fiber (receiving the light) has values of the core diameter and numerical aperture which are equal or higher than those of the first fiber. If these parameters are higher for the second fiber, the light in that fiber will in general have nearly the same power but a lower brightness, whereas a substantial coupling loss can occur for light coming from the second fiber.

If there is a transverse mismatch or an angular mismatch of the two fiber cores, this can lead to a coupling loss which may be estimated with geometrical considerations, if the light uniformly fills the whole core area and angular range. Note that the coupling loss generally depends on how the optical power is distributed over the fiber modes.

Single-mode Fibers

The situation is rather different for coupling of single-mode fibers. Here, the coupling loss can not depend on the direction of propagation. It can be low only if the following conditions are fulfilled:

  • The modal shapes and particularly the effective mode areas are similar.
  • The transverse and angular alignment of the modes is rather precise.

The effects of a transverse offset Δx and a mismatch of mode radii w1 and w2 on the coupling efficiency can be calculated with the following formula:

coupling efficiency for single-mode fibers

It is assumed that the mode fields can be well approximated by Gaussian functions. The equation ignores possible effects from Fresnel reflections at the fiber ends, which is perfectly reasonable for fusion splices, for example.

Calculator for Fiber Coupling Efficiency

Mode radius 1:
Mode radius 2:
Transverse offset:
Coupling efficiency: calc

Enter input values with units, where appropriate. After you have modified some inputs, click the "calc" button to recalculate the output.

If the fiber ends are not exactly perpendicularly cut, this can lead to an angular mismatch Δθ. Its effect on the coupling efficiency can also be calculated:

coupling efficiency for single-mode fibers with angular misalignment

This shows that the angular alignment becomes more critical for large mode area fibers.

Note that for large angles, where the coupling loss becomes very large, the results become quite sensitive to the mode shape. This means that the mentioned formula, which depends on the approximation of a Gaussian mode shape, may no longer produce accurate results.

Calculator for Fiber Coupling Efficiency

Mode radius:
Angle offset:
Refractive index:
Coupling efficiency: calc

Enter input values with units, where appropriate. After you have modified some inputs, click the "calc" button to recalculate the output.

The following figures are based on the equations above.

insertion loss due to mode size mismatch
Figure 2: Insertion loss at a mechanical splice for single-mode fibers due to a mismatch of mode radii.
insertion loss due to a parallel core offset
Figure 3: Insertion loss at a mechanical splice for single-mode fibers due to a parallel offset of the cores.
insertion loss due to an angular error
Figure 4: Insertion loss at a mechanical splice for single-mode fibers due to an error of the angle, as might result from a non-perpendicular cleave. This has been calculated for different ratios of the mode radius to the wavelength.


[1]R. Paschotta, tutorial on "Passive Fiber Optics", Part 6: Fiber Joints

(Suggest additional literature!)

See also: fibers, cleaving of fibers, mechanical fiber splices, fusion splicing of fibers, fiber connectors
and other articles in the category fiber optics and waveguides


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