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Case Study: Telecom Fiber with Parabolic Index Profile

We consider a telecom fiber of multimode type with a parabolic refractive index profile. It is known that this profile is suitable for reaching rather weak intermodal dispersion, i.e., weak differences in group index between the guided modes. This governs the differences in transit times between different modes, which typically limit the possible bit rate. With such a fiber design, one can exploit advantages of multimode fiber systems (e.g., cheaper transmitters) while still achieving a reasonably high bit rate.

With some numerical simulations, we will explore how strong the achieved reduction in intermodal dispersion is and how sensitive it is to slight changes in the doping profile.

We consider a germanosilicate fiber, having a fiber core with a mixture of GeO2 (germanate) and SiO2 (silica). Instead of directly entering the refractive index profile, we start with the GeO2 concentration profile, from which the index profile is then calculated. (That way, we have the index profile with its full frequency dependence.) We can use the Power FormMode Properties of a Germanosilicate Fiber”.

Analysis of the Nominal Fiber Design

We assume a core diameter of 50 μm and start with a parabolic concentration profile of the fiber core with a peak GeO2 concentration of 7%. The cladding is undoped, so that we got a constant refractive index there. The operation wavelength is 1550 nm (telecom C band). We enter all this into the form:

Power Form with definition of fiber parameters
Figure 1: Part of the Power Form with our definition of the fiber parameters, including a parabolic doping profile.

We see that the maximum doping concentration corresponds to a reasonable value of the numerical aperture of 0.174. (That quantity is really meaningful only for step-index fibers, but can still be used to get a feeling for the index contrast.)

We get 22 guided modes, or 39 when separately counting different mode orientations.

index profile and radial mode functions
Figure 2: The refractive index profile and the calculated radial intensity functions of all modes.

The table already shows that the group indices (n_g) are mostly quite similar, but for a better overview, we create a diagram:

group index vs. wavelength
Figure 3: Group index vs. wavelength for the fiber with a parabolic doping profile.

Most modes have quite similar values of the group index, but just a few of them have far lower values. Using the table in the form, we can easily identify them: the lowest two values are for LP05 and LP24 (at the bottom of the table, visible only after scrolling). The diagram also shows that these modes are relatively close to their cut-off wavelengths: they would cease to exist for slightly longer wavelengths. So this behavior is not surprising: such modes tend to have intensity profiles which reach substantially into the cladding, where of course we depart from the parabolic shape of the profile.

How problematic is that aspect for telecom applications? Potentially, it could really spoil the performance if at least a few of the modes have transit times differing a lot from those of all the others. However, one may launch the light such that highest-order modes do not have significant optical powers so that they become irrelevant.

We quickly compare this result with that for a step-index fiber with the same core diameter and a constant GeO2 concentration of 3.35% in the core, which leads to about the same number of guided modes. As expected, here we get a much larger variation of group indices:

group indices for step-index fiber
Figure 4: Group index vs. wavelength for the fiber with a step-index profile.

Here, not even the lowest-order modes have similar group indices. So we would definitely get far stronger intermodal dispersion. Let's quickly estimate how much that is in practice. For example, a difference in group index of <$\delta n_{\rm g}$> = 0.003 causes a change in transit time of <$\delta n_{\rm g} \: L / c$> = 10 ns through 1 km of fiber. That would already be fairly disturbing when trying to reach a bit rate of 1 Gbit/s, and is far worse than what group velocity dispersion in a single-mode fiber would do, e.g. for 20 nm difference in wavelength. Therefore, the improvement achieved with the parabolic doping profile is really important.

Modification of the Doping Profile

We now investigate the effects of some deviation from the parabolic doping profile – namely, adding a 4th order term. The form allows us to conveniently enter some additional definitions; we use the following:

c4 := 0.1
c2 := 1 - c4
c(r) := 7 * (1 - c2 * (r / r_co)^2 - c4 * (r / r_co)^4)

Then we can simply reference c(r) in the field for defining the doping profile. Our function has been defined such that for <$r = r_{\rm co}$> we get zero concentration, i.e., a continuous transition to the undoped cladding, and for <$r = 0$> the same peak concentration as before. The parameter c4 can be used for modifying the strength of deviation from the parabolic profile. We first look at the index profile and mode functions as before:

index profile and radial mode functions for modified profile
Figure 5: The refractive index profile and the calculated radial intensity functions of all modes for the modified doping profile with a 4th order term.

We see that the index profile got slightly modified. (Compare, for example, the refractive index for <$r$> = 10 μm with the situation of Figure 2.) Also, we got a few more guided modes, since the radial decay of refractive index is now a bit slower.

Now let's check the resulting group indices:

group indices for modified doping profile
Figure 6: Group index vs. wavelength for the fiber with a modified doping profile with 4th order term.

The variation of group indices is now substantially larger than for the parabolic profile, but still far smaller than for the step-index profile.

Conclusions

We can study in detail how the mode properties of a fiber change when its refractive index profile is modified in some way. That is useful, for example, to check how small fabrication tolerances need to be for reaching a given fiber performance.

You see that the calculation of fiber modes is straightforward with the RP Fiber Power software: simply enter its parameters, possibly involving an expression for the refractive index vs. radial position, and inspect the resulting numerical and graphical outputs. You can rapidly explore parameters and configurations. The Power Form gives you plenty of freedom, e.g. for defining complicated index profiles, besides offering various useful diagrams.