Active optical vortex fiber

An apparatus, comprising:

• a section of an active optical fiber, comprising:
• a central part having a first refractive index n1, wherein a diameter of the central part changes gradually along a length of the section of the active optical fiber forming a tapered longitudinal profile;
• an annular core radially surrounding the central part, the annular core being doped with at least one rare-earth element and having a second refractive index n2, wherein n2>n1 and wherein a birefringence of the annular core is less than 10−5;
• a first cladding layer radially surrounding the annular core and having a third refractive index n3, wherein n3<n2;
• and
• a second cladding layer radially surrounding the first cladding layer and having a fourth refractive index n4, wherein n4<n3, wherein a first portion of the section of the active optical fiber is configured to support a single-mode operation of an optical signal and a second portion of the section of the active optical fiber is configured to support a multimode operation of the optical signal, wherein the section of the active optical fiber comprises a spun active optical fiber, and
• the apparatus further comprising:
• a dichroic mirror for providing an output optical signal from the annular core of the second portion of the section of active optical fiber to an output of the apparatus.