Patent ID: 7236672

Claim:
An optical system comprising: (i) an optical fiber with gain producing core with an index of refraction n 1 , surrounded by at least one cladding with an index of refraction n 2 , said cladding including at least one index reduced area with an index of refraction n 2 , such that n 1 >n 2 >n 2 , said core propagating a signal at a spatial fundamental mode at a signal wavelength λ 1 and at a power level sufficient to generate optical power at a wavelength λ 2 , where λ 2 >λ 1 , and said at least one index reduced area in combination with said core provides at least one cut-off fundamental spatial mode wavelength λ C , and λ 1 <λ C and λ 2 >λ C , wherein (i) said at least one index reduced area does not surround said core, and (ii) said core and said at least one index reduced area are situated in close proximity to one another such that the distance between adjacent edges of said core and said at least one index reduced area is less than the core diameter.