Coughlin was convicted in February for attacking a handcuffed prisoner causing bodily injury.
The attack took place in a jail cell.
The 20-year police veteran was also convicted of falsifying a report in an attempt to cover up the incident.
Plymouth Police Chief Michael Botieri says when he heard of the incident, the department followed established protocols.
The case was forwarded to the FBI for investigation.
Coughlin is set to report to prison in about six weeks.