 Software … powerful tools for your research & development!

# RP Fiber Power – Simulation and Design Software for Fiber Optics, Amplifiers and Fiber Lasers

## Example Case: Bending of a Large Mode Area Fiber

Here we show how RP Fiber Power can be used to analyze bend effects in fibers.

### Description of the Model

We consider a fiber with a simple step-index design: a core radius of 20 μm and a low numerical aperture of 0.05. At a wavelength of 1 μm, that design supports several guided modes, but one can try to operate it with the fundamental mode only.

For the beam propagation, we use a relatively large grid and simulate losses for cladding modes as an absorption which increases steeply with increasing distance from the core.

The script code to set up the beam propagation is shown here:

```; Grid parameters for beam propagation:
r_max := 100 um
N := 2^7
dr := 2 * r_max / N
z_max := 200 mm
dz := 100 um
N_z := z_max / dz
N_s := 5

loss(x, y) := 1e2 * ((x^2 + y^2) / (40 um)^2)^4

calc { set up the beam propagation }
begin
bp_set_grid(r_max, N, r_max, N, z_max, N_z, N_s);
bp_define_channel(lambda);
bp_set_n('n_f(sqrt(x^2 + y^2))');  { index profile }
bp_set_R('R(z)', '');
bp_set_loss('loss(x, y)');  { simulate losses for cladding modes }
bp_set_A0('A_lm_xy(0, 1, lambda, x, y)');  { initial amplitude }
end
```

### Results

Initially, we assume that the bending becomes tighter and tighter along the length of the fiber. The launched light is entirely in the fundamental mode. We then let the inverse curvature radius increase linearly with the distance. Figure 1 shows the amplitude distribution in the y-z plane.