Cooperatively frequency-locked multimode operation in proton implanted VCSELs

It is known that birefringence and astigmatism are responsible for lifting the frequency and polarization degeneracy of transverse modes in Vertical-Cavity Surface-Emitting Lasers (VCSELs). We report on observation of "cooperative frequency locking" of nearly degenerate TEM/sub 01/ and TEM/sub 10/ modes of a VCSEL. For a certain range of injection currents two modes become cooperatively frequency locked in phase quadrature to form a steady-state TEM*/sub 01/ hybrid mode (doughnut). This mode exhibits a helical wavefront structure with the phase singularity on the axis, thus degrading the beam quality.