The coherence length of a single-frequency monolithic Nd:YAG ring laser is determined by using a highly unbalanced all-fiber Mach-Zehnder interferometer. A coherence length larger than ⋍ 1500km and a 3 dB linewidth less than ⋍ 60 Hz is estimated. Furthermore, an easy method to measure the temperature dependence on several laser parameters is described. This method is based on the laser emission in multiple axial modes, which is enforced by coherent optical feedback. A model explaining the multimode behaviour is presented.