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In Minnesota, Grandparents rights to visitations are very limited. See Minnesota Revised Statutes Chapter 257C Section 257C.08. Grandparents can be granted visitations rights if they lived with their grandchild or if their son or daughter (the parent of the grandchild) is deceased. Nevertheless, since your son incarcerated presently and so not available to visit with his child, you may be able to use that to convince the Court to grant you visitation rights.
You do not write a letter to the Judge for visitation rights. You would file a Petition for Grandparent's Visitation Rights with the Court and send a copy of that Petition to the mother of your grandchild. A hearing would be scheduled on the matter and the Court would make a decision whether to grant your request.
You may retain an Attorney if you do not know how to navigate the system and how to prepare the Petition because I cannot tell you what to do. This site prohibits legal advice. You may also use the Court's self-help center for assistance. Click here for self-help center.