An optical coherence tomography system includes an optical source has an emission wavelength in the range of 1.6 .mu.m to 2.0 .mu.m, in particular having an infrared emission predominantly at a wavelength of 1.8 .mu.m associated with a transition between an upper energy level and a lower energy level and the optical source comprises an excitation system which generates stimulated emission from a pump level to the upper energy level. The optical source may include a Tm-doped fiber in an optical cavity.