OptiFiber can find the modes for the fibre core with no cladding, the place of interest in your picture. That will include the evanescent field in the air. There is a high index contrast from the core to air, so you will probably need to use vector modes to find the modes accurately. If you need to use vector modes, it is better to use OptiMode, since it has better capacity to display vector mode fields. Try to find LP modes as well, and compare. If LP is appropriate, the vector mode effective indeces will be grouped near the LP mode indeces (There are more vector ones, so a few vector modes found close to each LP mode). If there are vector modes far from any LP one, you know that LP is not adequate. The core without cladding will have more modes than the fibre with cladding. It could be too many. Too many to make modal analysis practical, and you will need to use another method… FDTD?