A description referring now to fig1 an optical fiber 10 includes an inner multi core cladding section as shown comprising a single mode core 12 which in this particular embodiment is fabricated from silica glass having an index of refraction u03b7 core typically around of about 1 47 and a radius r 1 typically of around 2 5 u03bcm and doped with appropriate rare earth ions such rare earth ions constitute the active element for a fiber laser or fiber amplifier for example core 12 may be doped with erbium er ions typically denoted er 3 in which case optical fiber 10 exhibits gain at a wavelength typically around 1 5 u03bcm also the core may be doped with neodymium nd or ytterbium yb or combinations of er nd or yb such as for example er yb or nd yb the single inner mode core 12 is surrounded by a multi mode cladding region 14 having a circularly shaped cross section cladding region 14 encompasses the entire longitudinal extent of core 12 cladding region 14 may be provided for example from undoped silica having an index of refraction less than the index of refraction of the single mode core 12 in one particular embodiment for example the index of refraction u03b7 clad1 of cladding 14 may be about 1 44 and the outer radius r 2 of cladding 14 layer thickness is typically about 2 5 u03bcm in turn the first cladding layer 14 is surrounded by a second cladding layer 16 the second cladding layer is also provided having a circularly shaped cross section and is formed from a suitable polymer or from a conventional low index glass the index of refraction.