Optical imaging system employing vortex fiber for multiple-mode illumination

An optical fiber for a STED microscopy system, comprising:

• a core region having a core radius rcore and a refractive index ncore, and an outer cladding region having an outer cladding radius rcladding and a refractive index ncladding, wherein Δncore=ncore−ncladding, and wherein Δncore in a first part of the core region is greater than 0;
• a trench region surrounding the core region, the trench region having an inner radius equal to rcore and an outer radius rtrench, and a refractive index ntrench less than ncore, wherein Δntrench=ntrench−ncladding;
• and
• a ring region surrounding the trench region, the ring region having an inner radius equal to rtrench and an outer radius rring, and a refractive index nring greater than ncladding, wherein Δnring=nring−ncladding,
• wherein the values of rcore, rring, rtrench, rcladding, Δncore, Δntrench and Δnring are selected such that the optical fiber has a refractive index profile that includes:
• (1) a steep refractive index proximate to a peak amplitude value of a mode intensity profile of a fundamental LP01 mode of a first light propagating through the optical fiber, and
• (2) a steep refractive index proximate to a peak amplitude value of a mode intensity profile of higher-order LP11 modes of a second light propagating through the optical fiber, wherein the higher-order LP11 modes comprise a linear combination of modes, including cylindrically polarized TM01 and TE01 modes and mixed HE21(even) and HE21(odd) modes, such that at least one of the cylindrically polarized modes has an effective refractive index sufficiently separated from effective refractive indices of the mixed modes to allow for coupling of the second light to the at least one cylindrically polarized mode with minimal coupling to the mixed modes or for coupling of the second light to the mixed modes with minimal coupling to the cylindrically polarized modes.