The AirFill channel was used to create the straight edge of the “D”. I overlapped an index of refraction of 1 with the fiber material, so that you could have different widths of the Nm material. If the width would be very narrow the circular profile underneath might appear again above the square Nm material.
To calculate the loss of the mode you need to do a simple calculation by hand. The mode solver will give you the modal index, which can have an imaginary component. If you are unfamiliar with the calculation I would suggest taking a look at Agrawal’s Optical Fiber Communication textbook, which explains the attenuation coefficient. In any case to convert the imaginary part of the modal index into a loss (dB/km) you can use the following equation:
Loss = (-2*(Nimag)*2*PI/lambda*4.34)
the length units will be the same as the wavelength units.