securing is one thing but encrypting is another. 1. Securing
you can have layers of security such as a good password and making sure the user you are using for the linkedserver has the proper permissions and not full on permission across databases.
TDE does not protect communications between client applications and SQL Server, so other encryption methods must be used to protect data flowing over the network....
Since a connection from one SQL Server to another is the same as connecting from a client to a server - you can enforce encrypted connections between the separate systems: