Hi Arsalan,
Her father may be worried that he doesn’t know who you are and what your background is. He may be worried that you may corrupt his daughter.
If you come from a good family, you can try to use your good family name to help improve your situation.
The next time that he lets her out, perhaps to go to church or the mosque, you should find a way to send her a message and tell her to inform her father about the kind of family that you come from and that they are willing to talk to him and introduce themselves.
If you already love each other this much, you have both probably already considered marriage at some point in the future.
Her father may take this offer and decide to speak to a member of your family.
If he feels good about the interaction and the fact that you come from a good family, he may feel a lot more comfortable with the idea of letting his daughter out to meet you and may start letting it happen.