def METHOD_NAME(collection_name: str, kms_key_arn: Optional[str] = None) -> str:
    policy: Dict[str, Any] = {
        "Rules": [
            {
                "ResourceType": "collection",
                "Resource": [
                    f"collection/{collection_name}",
                ],
            }
        ],
    }
    if kms_key_arn:
        policy["KmsARN"] = kms_key_arn
    else:
        policy["AWSOwnedKey"] = True
    return json.dumps(policy)