A photonic field generator was a device capable of generating and sustaining holographic projections over a large area.
The group of renegade holograms led by Iden utilized a photonic field generator in 2377 to create the illusion of a damaged ship, used as a decoy to lure a Hirogen vessel into close proximity, and then setting off an explosion to damage it. Iden later convinced B'Elanna Torres, chief engineer of the USS Voyager, to begin modifications on the generator to allow it to support the holomatrices of the entire holographic crew, so they could deploy a series of them on a Y-class world and establish a colony. Although initially reluctant, B'Elanna began working with Kejal, Iden's engineer, to make various adjustments, including increasing the optronic capacity, successfully adjusting it to support the holograms.
Later, when Iden had the generator beamed to the surface of the planet so he and his crew could hunt a Hirogen crew, B'Elanna and Kejal shut down the holograms and activated The Doctor's program, so he could stop Iden, who was using The Doctor's mobile emitter, from murdering a defenseless Hirogen. (VOY: "Flesh and Blood")