Fiber Ends

Preparing Clean Fiber Ends: Stripping, Cleaving, Polishing

In most cases when a fiber is used, it is essential to prepare clean ends. A first step is usually to strip the polymer coating on the last centimeters, using a mechanical stripper. In in problematic cases, one may have to use a solvent (chemical stripping). The mantle of the glass fiber will then usually be quite clean, but the fiber end, if it simply has been broken, will still have an irregular shape. We thus need some method to obtain a nice surface – normally, a flat surface, which is perpendicular to the fiber axis, or sometimes with some other angle.

The most common method for preparing clean ends is cleaving. Essentially, this means controlled breaking of the glass of the bare fiber. One begins with making a tiny scratch on the side of the fiber, e.g. with a sharp diamond, carbide or ceramic blade, before or while some defined tension or bending is applied to the fiber. This causes the fiber to break, starting at the mentioned fracture point. Often, the resulting surface is quite smooth.

Cleaving is often done with a simple diamond blade. One slightly scratches the fiber and then breaks it e.g. by tipping an end with a finger. This procedure requires some practicing, and the results are somewhat variable. For more consistent results, one needs to cleave under more controlled conditions, using a precision fiber cleaver apparatus. Some of these devices can also be used to prepare angle cleaves (see Figure 2), with a relatively well controlled angle between the cleaved surface and the fiber axis.

Cleaving gets more difficult in non-standard situations, such as large fiber diameters or non-standard glass compositions. When cleaving fluoride fibers, for example, one at least needs to use adapted parameters for a precision cleaver.

Recleaving a fiber can be a substitute for cleaning, as it is hard to reliably clean fiber ends.

For very high-quality fiber surfaces, or when using fibers with large diameter, or when attaching a fiber connector, it may be necessary to apply some polishing procedure after cleaving. One may, for example, insert the fiber end into a ferrule (a hollow ceramic, glass or metal tube) and fix it there with a glue. The fiber is then polished down together with the glass tube, using a special polishing machine. This procedure allows one to produce a high-quality surface with an arbitrary well-defined orientation of the fiber surface. However, it takes substantially more time than simple cleaving, and of course it is essential to have all details of a polishing machine (e.g., the load force, speed and time) and the polishing agent well adapted to the ferrule and fiber material and size. Hand polishing is also possible, but usually leads to inferior results.

Polished fiber ends, other than cleaved ends, may have some convex curvature, resulting from the use of a flexible polishing pad. Such a “domed surface” facilitates a good contact e.g. between two single-mode fibers in a connector set.

Relevance of Cleave Angles

In some cases, it is important to have a cleaved fiber surface just perpendicular to the fiber axis. For example, this is often the case when a fiber is inserted into a fiber connector (see part 6), although some connectors require angle cleaves. Mechanical splices also don’t work well with non-perpendicular ends (see Figure 1).

joining two fibers with imperfect cleave

Figure 1: When a fiber cleave is not perpendicular, a fiber joint will not work well: an air gap will be formed, or alternatively a kink.

Note that due to refraction at the fiber end, a non-normal cleave causes a deviation of the output beam direction from the fiber axis (see Figure 2). Also, one then requires an appropriately tilted input beam for efficient launching. This makes the use of angle cleaves somewhat inconvenient.

angle-cleaved fiber end

Figure 2: When light comes out of a fiber with angled cleave, it will be somewhat deflected. The direction for reflected light is also shown; it will not go back to the fiber core.

The cleave angle also has an important influence on back-reflected light. If it is small, light reflected at the output surface (Fresnel reflection due to the index difference to air) will essentially travel backward in the fiber core. For large enough cleave angles, however, the light will entirely get into the cladding and will be lost there. This means that there is a very large return loss (e.g. 60 dB) despite a significant reflection, which for a normal cleave would cause a return loss of only 14 dB.

It depends on the fiber details how large the cleave angle needs to be for a high feedback suppression. For a usual single-mode fiber, for example, the mode has a beam divergence of several degrees. One may then require a cleave angle as large as 8°, for example. For a fiber with high numerical aperture, it may be even more. For large mode area fibers, however, rather small cleave angles are sufficient to suppress feedback.

In some cases, the Fresnel reflection from a fiber end is utilized, e.g. for an effective output coupler of a fiber laser, or in optical time-domain reflectometry (OTDR).

Other End Shapes

In most cases, fiber ends are just flat – either perpendicularly cut or at some angle against the fiber axis as discussed above. In some cases, however, one uses different geometrical shapes of fiber ends:

  • Lensed fiber ends are equipped with a strong curvature which leads to collimation or at least a reduction in beam divergence of the beam exiting the fiber. Due to the typically rather small core sizes, rather small curvature radii are required to obtain a substantial lensing effect. A particular implementation is the fiber ball lens, where a tiny glass sphere is fused to a fiber end. Special fusion splicers can be used for that purpose. The natural surface tension of the glass facilitates the fabrication of high quality fiber ball lenses.
  • The above mentioned glass sphere can also be processed further; for example, it can be equipped with a reflecting flat surface which reflects the outgoing beam to the side. This is useful, for example, for some medical applications where a fiber is embedded in an endoscope.
  • There are fiber axicon lenses, where near a fiber end the fiber diameter is rapidly reduced down essentially to zero. This can be achieved either by polishing (leading to a kind of pencil shape) or with the tapering technique. Only in the latter case, the core size is also reduced towards the end; this aspect, however, may not be important for the performance of the device. Light coming from the fiber and going through such an axicon end is focused down to a rather small diameter, so that it can be launched into a very small waveguide of a photonic integrated circuit, for example. Conversely, light from such a waveguide can be efficiently transferred into a single-mode fiber.
  • A fiber end may be tapered down (→ tapered fibers) and then cleaved in the region with reduced fiber diameter. Such a piece can be used for a mode field converter, if the reduced mode size at the smaller end fits to that of a different kind of fiber.
  • core-less end cap is a homogeneous glass part spliced to the end of a fiber. (In case of a photonic crystal fiber, one may simply collapse the holes in the end region using a fusion splicer.) Light coming from the fiber core will expand within the core-less end cap, so that its beam radius is substantially increased (at the intensity decreased accordingly) once it reaches the glass/air interface. Such devices allow the transfer of light at very high power levels from a fiber into air or vice versa.

Fiber Joints

Types of Fiber Joints

Optical fibers can be joined together, such that light is efficiently transferred from one fiber to another. There are various possibilities:

  • Mechanical splicing means that two fiber ends are tightly hold together with some mechanical means. That is usually done for permanent connections, but it may be possible to dismantle a splice without spoiling the fiber ends.
  • Another technique is fusion splicing, where the fibers are fused together, e.g. using an electrical arc. This leads to particularly low insertion loss and high return loss, if the two fiber cores are similar.
  • For non-permanent connections, one can also use fiber connectors (see below).

Coupling Losses of Imperfect Fiber Joints

A frequently asked question is how large will be coupling losses e.g. at a mechanical splice, when there is some kind of imperfection, for example

  • a parallel offset of the fiber cores,
  • a deviation between the fiber axis directions,
  • a mismatch of core sizes, or
  • an air gap between the fiber ends.

It turns out that some of the answers are quite different for single-mode and multimode fibers.

Single-mode Fibers

It is relatively easy to calculate coupling losses for single-mode fibers. Essentially, the guided mode from the first fiber (the input) creates some amplitude profile in the second fiber, which may be somewhat displaced, for example, due to an imperfect splice. One can now calculate the coupling efficiency as an overlap integral between that amplitude profile and that of the guided mode of the second fiber. (Numerical beam propagation is not required.)

单模光纤的耦合效率

A similar equation can be used for an angular mismatch:

角度不对准的单模光纤的耦合效率

This shows that the angular alignment is more critical for single-mode fibers with large mode area. For standard mode areas, the angular alignment is usually easier to achieve than the position alignment.

The following figures are based on the equations above.

insertion loss due to mode size mismatch

Figure 3: Insertion loss at a mechanical splice for single-mode fibers due to a mismatch of mode radii.
insertion loss due to a parallel core offsetFigure 4: 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 5: 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.
Concerning angle-cleaved fiber ends, it is often of interest how large the cleave angle needs to be to avoid significant reflection into the core mode. The equation can well be used for that; one simply has to keep in mind that the angular deviation of the reflected beam is twice the cleave angle. As an example, a fiber with a standard mode area of 100 μm2, having w = 5.64 μm, needs a cleave angle of at least 7.4° in order to have a backreflection below 10−4, i.e., at least 40 dB return loss, at 1.5 μm wavelength. For a large mode area fiber with 1000 μm2, 2.3° would be sufficient. Note that longer wavelengths require larger cleave angles, as they lead to large beam divergence.
Note, however, that the above equations are valid only for modes with Gaussian profiles. In the regime of high suppression of the back-reflection from an angled fiber end, however, the results are sensitive to deviations from a Gaussian mode profile.
It is interesting to consider some more details. For example, does it matter for the losses from which fiber the input light comes, if the mode sizes are different? According to the equation above, it doesn’t. This is true, although it might be surprising: one could imagine that going from a smaller core to a larger one causes lower losses than in the other direction. Note, however, that the smaller mode has a larger beam divergence, i.e., a wider field distribution in spatial Fourier space, and that is too much for the other fiber with larger mode. So the fraction of power lost at the splice really doesn’t depend on the direction; only the lost light is differently distributed over the cladding modes.One could imagine that when going from a smaller-mode fiber to one with a larger mode, one could avoid the coupling losses if the second fiber has the same NA despite the larger core. (It might still be single-mode if the V number is low enough.) After all, the angular range should be limited only by the NA. However, this expectation is wrong; a mismatch of mode sizes inevitably causes coupling losses, if both fibers are single-mode.Multimode FibersFor multimode fibers, the losses cannot be specified as a single number: they are generally mode-dependent. This means that for arbitrary input light fields, the resulting overall loss will depend on how the power is distributed over the modes. One can imagine, for example, that light is launched into low-order modes only with a laser, and that this leads to low splice losses. If one then strongly bends the fiber before the splice, the light might be redistributed into higher-order modes, and the splice losses get larger.As an example for coupling losses, consider a perfect mechanical splice between two step-index multimode fibers with equal NA of 0.2 (calculated from the maximum index difference), but the first fiber has a core diameter of 62.5 μm and the second one only 50 μm. We can calculate each mode of the first fiber, sum up the modulus squared of its overlap integral with all modes of the second fiber, and in that way obtain its coupling loss. (Alternatively, one may use a numerically simulated beam propagation, but this takes more computation time and is tentatively less precise.) Figure 2 shows the losses versus the m value of the modes. These losses are highest for low m but high l values.

多模光纤接头处的损耗

Figure 6: Mode-dependent coupling losses at a multimode fiber splice. The horizontal coordinate reflects the m value of each mode, while the color depends on l.

One may be surprised that the coupling loss for the LP14,3 mode is so high – about 10 dB, far higher than according to the ratio of mode areas (1.94 dB). However, that mode has a high portion of its power outside the radius of 25 μm, and in addition its intensity distribution in Fourier space reaches quite far out. The calculated result has been confirmed with a calculation based on numerical beam propagation, which is a quite independent check.

If one exchanges the two fibers, i.e., has an input coming from the smaller core, the coupling losses for all modes get much smaller:

多模光纤接头处的损耗

Figure 7: Same as Figure 3, but with the light input to the fiber with the smaller core.

So for multimode fibers, other than for single-mode fibers (see above), the coupling losses are substantially smaller when coming from the fiber with the smaller core. However, for some modes these losses are still substantial – for example, 2.8 dB for the LP55 mode. Numerical beam propagation confirmed that result. It shows that the field, when it gets into the fiber with larger core, starts to expand, and that expansion is later on not fully stopped at the new core/cladding boundary. This shows that not every field distribution within the core and with limited angular content can be well matched by the guided modes.

Such effects are less pronounced for fibers having many modes. Basically, one has to be aware that the modes of the smaller core span a mathematical space which is not a subspace of that for a larger core.

Fiber connectors are often used at the ends of fiber cables in order to provide non-permanent connections between fiber-coupled devices. In principle, they are used in a similar manner as electrical connectors. However, their use typically requires some more care, because fiber ends are relatively sensitive, and because fiber connectors are not always easy to attach to a fiber end.

Note that even a tiny particle of dust on the fiber core can cause substantial losses. (Therefore, one often protects a fiber connector with a dust cap during times when it is not plugged in.) Also, small imperfections of a fiber end may lead to a small air gap between the fiber ends, which causes reflection losses.

A great variety of fiber connectors has been developed, e.g. for applications in optical fiber communications. Some common types are ST, FC, SC and LC connectors. The different connectors types differ in various aspects, e.g. in terms of cost, size, ease of use, insertion loss and return loss, suitable fiber size, allowed number of mating cycles, suitability for multimode, single-mode and polarization-maintaining fibers and various other details.

Various optical components such as fiber couplers and laser diodes are often sold with fiber “pigtails”. This means that some fiber hangs out of the device, and the user may splice that to some other fiber, or attach a fiber connector to it.

One can also buy pure fiber pigtails, i.e., without an optical component. In that case, one obtains a fiber connector on one end of a (typically short) fiber, and nothing on the other end. One may, for example, integrate the open end into some device, and avoid the work of assembling a connector oneself. Of course, one might also take some jumper cable and cut it into two pigtails.

Some fiber pigtails have only some polymer buffer, but not a thick jacket like a fiber patch cord. There are also jacketed pigtails.

 

Source: rp-photonics