A control system for thermo optical control of focus position of an energy beam in an additive manufacturing apparatus has a first doped medium and a second doped medium, each of which is optically transparent and doped with a dopant. The first doped medium has a positive thermo-optical coefficient (dn/dT) and the second doped medium has a negative thermo-optical coefficient (dn/dT) and is in series with the first doped medium. An energy beam input or coupling is configured to generate or receive an energy beam that is required to be controlled, the energy beam being within a first wavelength range and directed towards the first and second doped mediums. An absorbed beam input or coupling is configured to generate or receive at least one absorbed beam in a second wavelength range which is different from the first wavelength range, the absorbed beam being directed towards the first and second doped mediums. The first and second doped mediums have a higher beam absorption characteristic in the second wavelength range than in the first wavelength range, causing the absorbed beam to have a higher absorption than the energy beam in the first and second doped mediums and the first and second doped mediums each have a coating which allows transmission at both the first and the second wavelength ranges.