Multimode fiber coupling

Coupling into multimode fibers

Please note: The following is only true for coupling into multimode fibers. More information concerning single-mode, PM fiber-coupling can be found here

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Unlike for single-mode fiber coupling, the formulae for calculating the coupling/collimating focal length is different for coupling compared to collimating. Information on collimating multimode fibers is found here.


For multimode fiber coupling, either the fiber couples type 60FC-A19.5 or the collimators of type 60FC can be used. If a collimator is selected then it can be used for fiber-coupling by using it in reverse mode and placing it in an adjustable mirror mount (or other mechanics providing the same degrees of freedom). This gives all degrees of freedom to achieve a high coupling efficiency.

Selection of coupling focal length for a multimode fiber and for a circular beam emitted by a point source

Assuming a Gaussian beam with M2=1 , the coupling focal length for coupling into a multimode fiber is calculated from the beam diameter (1/e2-level) and the nominal fiber NA NAnom (usually defined on the 1%-level)

{!{!{f\prime_{min}=\frac{1.5\cdot\emptyset_{beam}}{2\cdot NA_{nom}}}!}!}


The factor 1.5 is the correction factor for the different beam levels used for the beam diameter and the fiber NA.


When coupling into multimode fibers the core diameter usually is much larger than the spots produced by the coupling lens. Here, the acceptance angle of the fiber (the nom. fiber NAnom) is more crucial.

  • Using a focal length too short (f'<fmin) will result in a lower coupling efficiency due to the limited fiber acceptance angle. 
  • A coupling focal length too long can cause insufficient mode mixing, resulting in unwanted beam characteristics.

The optimum focal length should be chosen to be fmin or slightly larger. Please make sure that the NA of the coupling optics needs to be larger than the fiber NA!

Please note: When working with multimode fibers with unusually small core diameters (<<50µm), a check if the resulting focus spot is still smaller than the core diameter might be necessary. This is unneccesary for standard core diameters e.g. 50µm, 100µm or 400µm.


Considerations concerning mode mixing

Multimode beam profiles can be unstable and asymmetric depending on how modes are excited. To achieve a stable, symmetric "super-Gaussian" distribution, you must increase mode mixing using one or more of these strategies:

  •     Increase fiber length: Allows more distance for modes to mix.
  •     Coil the fiber: Uses a smaller bend radius to force mode mixing.
  •     Optimize coupling: Use a shorter focal length (closer to the min. focal length from above) to excite more modes upon entry.
More information can be found here.

Coupling an Extended Source Into a Multimode Fiber

The Beam Parameter Product (BPM) of an extended light source

When coupling the radiation of an extended source into a multimode fiber, there are principle limitations.


An extended source, e.g. a LED source or a plasma typically emits a beam from a large area and with a large angular spectrum. This beam characteristics (area x angle) is given by the so-called beam parameter product BPM or, more precisely, the étendue.


The multimode fiber however, (as every fiber type), only has a limited acceptance angle and core area (defined by the numerical aperture and the core diameter, respectively). Only light that is focused with the right angle (-> numerical aperture) and focus size (-> core diameter) is coupled into the fiber, everything else is not transmitted. The multimode fiber accepts beams with the corresponding BPM.


Every components has different BPMs it emits or accepts (source, optics, as well as fiber). In classical optics, this BPM is an invariant, which means that it is not possible to change the BPM of an optical system by means of optical imaging.


An extended source , e.g. a LED source or a plasma emits a beam that automatically has a large BPM as both the emitter area as well as the angular spectrum is large. This also means that you need to choose a fiber that accepts a large BPM (large NA, large core diameter) in order to couple as much light as possible into the fiber.

So how do I choose the right coupling optic?

It is unimportant, if you use an optics with a small focal length (smaller spot, wide angle) or a longer focal length (larger spot, smaller angle), as the main limitation is the non-changeable BPM and the ratio of BPM to the BPM accepted by the fiber.

This is as long as the optics fulfills the following criteria:
• It is coated with an AR-coating for the appropriate wavelength
• It has a numerical aperture larger than the one of the multimode fiber (otherwise more light is lost)
In other words: there is no optimum focal length when choosing the right coupling optics, the NA of the optics needs to be large enough.

Note: Single-mode fibers have a smaller NA and a smaller MFD. Maximum coupling efficiency is achieved for an ideal Gaussian beam (M2 = 1, no astigmatism) when the convergence of the focused, circular beam equals the effective NAe2 of the fiber. Then the laser spot on the fiber end face equals the mode field diameter MFD of the single mode fiber. Thus single-mode fibers accept a much smaller BPM compared to multimode fibers. 

It is impossible to couple a significant amount of light emitted by an extended source into a single-mode fiber.