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# RP Fiber Calculator – Documentation

Here we explain in detail how the RP Fiber Calculator software is used.
Each of the menu items explains one of the tabs.

## Launching a Beam

Here you can calculate how a Gaussian laser beam is coupled into a fiber. More precisely, it is calculated what optical powers get into the guided modes of the fiber – using the wavelength and corresponding modes as calculated in the “Guided modes” tab. The fiber end may be misaligned.

### Input Parameters

The following parameters can be entered:

• The position offset of the fiber end in <$x$>, <$y$> and <$z$> direction (where z corresponds to the longitudinal direction) against the beam focus position. For example, a positive <$z$> value means that the fiber end is placed behind the beam focus, and a positive <$x$> value means that the fiber is shifted to the right.
• The angular orientation error of the fiber end. If this is possible for the <$x$> direction, for example, the fiber runs to the right, and a centrally coupled beam initially runs to the left in the fiber.

It is always assumed that the fiber end is cut perpendicular to its axis.

You can also determine whether or not the Fresnel reflection of the (uncoated) fiber end is taken into account, and whether or not modes with very small power are displayed.

### Calculated Results

The results are obtained by calculating a complex overlap integral containing the transverse input beam profile and the mode profile.

The calculated results are displayed below when you press the button “Calculate launch efficiency”. For each guided mode of the fiber, the fraction of the incident power which gets into that mode is displayed numerically and graphically. (For modes with non-zero <$l$> value, the two possible orientations are listed separately: the one with the <$\sin l \varphi$> dependence is indicated with a negative <$l$> value.)

The calculation does not consider cladding modes. These, however, will usually exhibit much higher losses, so that after some propagation distance only the power in guided modes remains.