A wavelength locker for use with tunable optical devices may include an etalon, a polarization beam splitter, and a first and second detector. The etalon may be positioned to receive a first beam and may include a first birefringent crystal having a first optical axis and a second birefringent crystal having a second optical axis. The first birefringent crystal may be coupled to the second birefringent crystal such that the first optical axis and the second optical axis are orthogonal to one another.