An ultra-high power single mode, SM, laser system comprising: a booster stage, comprising: a fiber to fiber rod amplifier extending through free space and configured with a multimode, MM, non-uniformly dimensioned core and a cladding coextending with and surrounding the core; a MM pump light output fiber extending through free space and including a terminal region coextending with a terminal region of the amplifier, said system comprising furthermore: a laser head receiving the terminal regions of respective amplifier and pump light output fibers; and a mirror provided in the laser head and having a central opening, which is dimensioned to be traversed by the amplified signal light in the propagating direction, and the mirror being configured to redirect the pump light incident thereon in a counter-propagating direction so that the light is coupled into the core of the amplifier, and wherein the booster stage further comprises an unconfined delivery cable traversed by the amplifier and pump light output fiber, the terminal regions (40, 48) of respective pump light output fiber and amplifier extending substantially parallel to one another and projecting over a downstream end of the delivery cable into the laser head; and wherein the MM core is doped with light emitters and structured with: a small diameter uniform input region configured to guide a single mode, SM, signal light along a fiber part of the amplifier in a propagating direction, and a mode transforming region bridging the input and output portions; and wherein the MM core further includes a large diameter uniform output region extending from the mode transforming region and guiding the SM signal light along the terminal region of the amplifier. a booster stage, comprising: a fiber to fiber rod amplifier extending through free space and configured with a multimode, MM, non-uniformly dimensioned core and a cladding coextending with and surrounding the core; a MM pump light output fiber extending through free space and including a terminal region coextending with a terminal region of the amplifier, a fiber to fiber rod amplifier extending through free space and configured with a multimode, MM, non-uniformly dimensioned core and a cladding coextending with and surrounding the core; a MM pump light output fiber extending through free space and including a terminal region coextending with a terminal region of the amplifier, said system comprising furthermore: a laser head receiving the terminal regions of respective amplifier and pump light output fibers; and a mirror provided in the laser head and having a central opening, which is dimensioned to be traversed by the amplified signal light in the propagating direction, and the mirror being configured to redirect the pump light incident thereon in a counter-propagating direction so that the light is coupled into the core of the amplifier, and wherein the booster stage further comprises an unconfined delivery cable traversed by the amplifier and pump light output fiber, the terminal regions (40, 48) of respective pump light output fiber and amplifier extending substantially parallel to one another and projecting over a downstream end of the delivery cable into the laser head; and wherein the MM core is doped with light emitters and structured with: a small diameter uniform input region configured to guide a single mode, SM, signal light along a fiber part of the amplifier in a propagating direction, and a mode transforming region bridging the input and output portions; and a laser head receiving the terminal regions of respective amplifier and pump light output fibers; and a mirror provided in the laser head and having a central opening, which is dimensioned to be traversed by the amplified signal light in the propagating direction, and the mirror being configured to redirect the pump light incident thereon in a counter-propagating direction so that the light is coupled into the core of the amplifier, and wherein the booster stage further comprises an unconfined delivery cable traversed by the amplifier and pump light output fiber, the terminal regions (40, 48) of respective pump light output fiber and amplifier extending substantially parallel to one another and projecting over a downstream end of the delivery cable into the laser head; and wherein the MM core is doped with light emitters and structured with: a small diameter uniform input region configured to guide a single mode, SM, signal light along a fiber part of the amplifier in a propagating direction, and a mode transforming region bridging the input and output portions; and wherein the MM core further includes a large diameter uniform output region extending from the mode transforming region and guiding the SM signal light along the terminal region of the amplifier.