Cladding-pumped fiber with helical rare-earth-doped core for fiber lasers and amplifiers

A high-power optical fiber laser/amplifier comprised of:

• a nominally multi-mode, rare-earth-doped helical-coiled core having an index of refraction n1 longitudinally disposed within an inner cylindrical cladding having an index of refraction n2, where n1>n2;
• an outer cladding of index of refraction n3<n2 enclosing said inner cylindrical cladding;
• an end plane including an end of said inner cylindrical cladding;
• an aperture in said end plane for connecting said inner cylindrical cladding to a source of pump radiation;
• said helical-coiled core terminating in said end plane, and being located outside of said aperture;
• and
• said helical-coiled core having a diameter for transmitting light in a single mode, with the diameter being greater than a maximum diameter for allowing a non-helical core to transmit light in a single mode, whereby
• said helical-coiled core effectively produces single-mode operation with a circularly polarized near-diffraction-limited beam quality output when said pump radiation source is used to inject pump radiation into said inner cylindrical cladding,
• said pump radiation does not impinge on said helical-coiled core where said helical-coiled core terminates in said end plane, and
• laser light emanating from said helical-coiled core is not transmitted to said pump radiation source.