Polarization modulation of a submillimetre laser

A concept is described whereby simultaneous independent oscillation may be obtained at slightly different frequencies on two orthogonally linearly polarized modes of the same laser cavity. It involves simply replacing one of the end mirrors with a suitable diffraction grating, reflecting in zeroth order. Experiments on an HCN laser ( λ = 337 μm) show that the technique works successfully, though only on certain of the transverse modes; mode structure analysis has been performed to show which modes are involved. Polarization modulation at the difference frequency of 0.9 MHz, determined by the choice of grating, has been observed; its frequency stability is fully adequate for heterodyne interferometry or similar purposes. The technique should be applicable to other lasers.