A control system for thermo optical control of focus position of an energy beam in an additive manufacturing apparatus, the control system including: a first doped medium and a second doped medium, each of which is doped with a dopant, wherein: the first doped medium has a positive thermo-optical coefficient (dn/dT), thus configured to act like a controllable positive (converging) lens; and the second doped medium has a negative thermo-optical coefficient (dn/dT), thus configured to act like a controllable negative (diverging) lens, and is in series with the first doped medium; an energy beam input configured to generate, or an energy beam coupling configured to 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; and a plurality of absorbed beam inputs (104.1, 104.2) configured to generate, or a plurality of absorbed beam couplings configured to receive, respective absorbed beams (106.1, 106.2) in a second wavelength range which is different from the first wavelength range, the absorbed beams being parallel to one another and being directed towards the first and second doped mediums; and a control unit configured to control (i) the energy beam input or coupling and (ii) the absorbed beam inputs or couplings, thereby to control an intensity of the energy beam and the absorbed beams, wherein 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 beams to have a higher absorption than the energy beam in the first and second doped mediums and wherein the first and second doped mediums are optically transparent at the first wavelength range, and wherein the first and second doped mediums each have a coating which allows transmission at both the first and the second wavelength ranges. a first doped medium and a second doped medium, each of which is doped with a dopant, wherein: the first doped medium has a positive thermo-optical coefficient (dn/dT), thus configured to act like a controllable positive (converging) lens; and the second doped medium has a negative thermo-optical coefficient (dn/dT), thus configured to act like a controllable negative (diverging) lens, and is in series with the first doped medium; the first doped medium has a positive thermo-optical coefficient (dn/dT), thus configured to act like a controllable positive (converging) lens; and the second doped medium has a negative thermo-optical coefficient (dn/dT), thus configured to act like a controllable negative (diverging) lens, and is in series with the first doped medium; an energy beam input configured to generate, or an energy beam coupling configured to 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; and a plurality of absorbed beam inputs (104.1, 104.2) configured to generate, or a plurality of absorbed beam couplings configured to receive, respective absorbed beams (106.1, 106.2) in a second wavelength range which is different from the first wavelength range, the absorbed beams being parallel to one another and being directed towards the first and second doped mediums; and a control unit configured to control (i) the energy beam input or coupling and (ii) the absorbed beam inputs or couplings, thereby to control an intensity of the energy beam and the absorbed beams, wherein 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 beams to have a higher absorption than the energy beam in the first and second doped mediums and wherein the first and second doped mediums are optically transparent at the first wavelength range, and wherein the first and second doped mediums each have a coating which allows transmission at both the first and the second wavelength ranges.