A polarization maintaining optical fiber has a core extending along a longitudinal axis, where the core includes a rare earth and a first index of refraction. A cladding is disposed about the core and has a first thermal coefficient of expansion (TCE) and a second index of refraction that is less than the first index of refraction. The fiber can include a pair of longitudinally extending members, and each of the members can have a TCE that is different than the first TCE of the cladding. The pair can be spaced apart by a distance d.sub.i. A second cladding can be disposed about the first cladding, and the second cladding can have a third index of refraction that is less than the second index of refraction. The fiber can have a fundamental mode having a mode field diameter (MFD). The core can be a multimode core having a diameter d.sub.c that is greater than or equal to 15 microns, the numerical aperture of the core relative to the first cladding can be less than or equal to 0.12, the ratio d.sub.i/MFD can be greater than 3, and the fiber can provide a beat length of less than 3 mm, where the beat length is measured at a wavelength of 633 nm.