Patent ID: 8520299

Claim:
A system comprising: (A) an optical fiber comprising: (A1) a gain-doped region for guiding a higher-order mode, comprising: (A1a) a core having a first index of refraction, the core being rare-earth (RE) doped: and (A1b) an inner cladding located radially exterior to the core, the inner cladding having a second index of refraction, the second index of refraction being less than the first index of refraction, the inner cladding being RE doped; and (A2) a trench located radially exterior to the inner cladding, the trench comprising air holes, the trench having a third index of refraction, the third index of refraction being less than the second index of refraction; (B) a signal source optically coupled to the optical fiber, the signal source for launching a signal into the gain-doped region of the optical fiber; and (C) a cascaded Raman resonator (CRR) optically coupled to the optical fiber, the CRR for directly pumping the gain-doped region of the optical fiber.