Theoretical calculation of relative intensity noise of multimode vertical-cavity surface-emitting lasers

An analytical calculation of relative intensity noise spectra of weakly index-guided vertical-cavity surface-emitting lasers (VCSELs) in a multimode regime is performed. We obtain analytical expressions that incorporate the spatial dependence of electrical field and carrier density profiles. Expressions are derived for single-mode and two-transverse-mode operation. Our analysis shows that single-mode noise spectrum depends on the spatial mode profile through a modal effective volume. Two-mode noise spectra also incorporate the dependence on the degree of spatial overlapping between the modes. Two resonance peaks appear in the noise spectra of the individual modes and total power of the two-mode VCSEL. We analytically show that those peaks appear at frequencies that correspond to the relaxation oscillation frequencies of the multimode laser. Simple analytical expressions are also derived for noise spectra at zero frequency.