UV-green converting fiber laser using active tapers

A nonlinear conversion system, comprising:

• a light source for producing an input seed beam;
• a multimode fiber amplifier, comprising an active taper situated to receive the input seed beam and comprising a rare earth doped, variable mode core surrounded by at least an inner cladding, wherein at least a portion of the variable mode core has a varying core size, and wherein the active taper is coiled in a spiral-shape so that a rate of increase of a coiling radius corresponds to a rate of increase of the varying core size;
• an optical pump situated so as to direct pump optical radiation to at least a rare earth doped portion of the variable mode core;
• and
• a nonlinear crystal situated so as to receive an amplified beam produced by optical gain in the active taper and configured to produce a converted beam having a wavelength different than a wavelength of the input seed beam.

A method, comprising:

• directing a seed optical beam to an active taper comprising a rare earth doped, variable mode core surrounded by at least an inner cladding, wherein the variable mode core of the active taper is a variable diameter core doped with an active rare earth dopant and extending along a taper axis, wherein the variable diameter core has a core diameter dcore that is a function of position along the taper axis, and the dopant in the variable diameter core is confined to a diameter that is less than 0.9, 0.8, 0.6, or 0.5 dcore;
• pumping the active taper to produce optical gain by directing pump optical radiation to at least a rare earth doped portion of the variable mode core;
• amplifying the seed optical beam in the pumped, active taper so as to produce an amplified optical beam propagating in a fundamental mode of the active taper;
• directing the amplified optical beam to a nonlinear crystal;
• and
• frequency shifting the amplified optical beam so as to produce a frequency converted optical beam having a wavelength different from a wavelength of the seed optical beam.

A nonlinear conversion system, comprising:

• an active taper situated to receive an input seed beam and comprising a rare earth doped, variable diameter core extending along a taper axis and surrounded by at least an inner cladding, wherein the variable diameter core includes an input section in which the variable diameter core defines a first mode field diameter for fundamental mode propagation and an output section in which the variable diameter core defines a second mode field diameter for fundamental mode propagation, and wherein the variable diameter core has a core diameter dcore that is a function of position along the taper axis and the core diameter increases adiabatically as a function of position along the taper axis from the input section to the output section so that an amplified beam produced by optical gain in the active taper propagates substantially in a fundamental mode along the taper axis and a mode field diameter of the amplified beam increases from the first mode field diameter to the second mode field diameter between the input section and the output section;
• an optical pump situated so as to direct pump optical radiation to at least a rare earth doped portion of the variable diameter core;
• and
• a nonlinear crystal situated so as to receive the amplified beam from the output section of the active taper and configured to produce a converted beam having a wavelength different than a wavelength of the input seed beam.