Tapered fiber laser

An optical fiber microlens comprising:

• an optical fiber having a core doped with an ion optically excitable and having a three-level optical transition when pumped at a first end by a multi-mode pump source at a pump wavelength for lasing at a signal wavelength at a second end wherein the core is mode transformed from the first end to the second end thereof such that the optically-active fiber is multi-moded at the pump wavelength proximate to said first end, and is single-moded at the signal wavelength proximate to the second end, and a cladding, a longitudinal fiber axis extending through the core, the cross-section of the core in a plane perpendicular to the axis being a rectangle having a major and a minor axis;
• a wedge-shaped microlens on one end of the fiber, the microlens including surfaces that intersect at a line that substantially bisect the fiber core and lies on the major axis.

An optically-active fiber comprising:

• a core doped with an ion optically excitable and having an optical transition at a signal wavelength, said core having a refractive index, wherein said core is tapered from a first end to a second end thereof such that the optically active fiber is multi-moded at said signal wavelength proximate to said first end, is single-moded at said signal wavelength proximate to said second end, and has overlap factors ΓS and ΓP of the signal mode and pump mode such that the overlap ratio ΓS/ΓP is between 0.1 and 10;
• and
• a cladding surrounding said core and having a lower refractive index than said refractive index of said core.

An optically-active fiber comprising:

• a core doped with an ion optically excitable and having an optical transition at a signal wavelength, said core having a refractive index, wherein said core is tapered from a first end to a second end thereof such that the optically-active fiber is multi-moded at said signal wavelength proximate to said first end, is single-moded at said signal wavelength proximate to said second end, and has overlap factors ΓS and ΓP of the signal mode and pump mode with the dopant profile such that the overlap ratio ΓS/ΓP is between 0.2 and 5;
• and
• a cladding surrounding said core and having a lower refractive index than said refractive index of said core.