An optical fiber for use in fiber optic sensing of a test sample includes a first core and a second core. The second core is generally coaxially disposed within the first core and is sized smaller than the first core. The second core is capable of delivering pulsed laser energy from the laser for nonlinear optical excitation of the test sample. Nonlinear optical feedback signals can then be collected in both the first core and second core for improved detection efficiency relative to conventional single-mode and multi-mode fibers.