The influence of the anisotropic Zeeman effect on optical decoherence was studied for the 1.54 m telecom transition in Er 3+ :Y 2 SiO 5 using photon echo spectroscopy as a function of applied magnetic field orientation and strength. The decoherence strongly correlates with the Zeeman energy splittings described by the groundand excited-state g factor variations for all inequivalent Er 3+ sites, with the observed decoherence times arising from the combined effects of the magnetic dipole-dipole coupling strength and the ground-and excited-state spin-flip rates, along with the natural lifetime of the upper level. The decoherence time was maximized along a preferred magnetic field orientation that minimized the effects of spectral diffusion and that enabled the measurement of an exceptionally narrow optical resonance in a solid-demonstrating a homogeneous linewidth as narrow as 73 Hz.