Enhanced longitudinal mode spacing in blue-violet InGaN semiconductor   laser
  A novel explanation of observed enhanced longitudinal mode spacing in InGaN semiconductor lasers has been proposed. It has been demonstrated that e-h plasma oscillations, which can exist in the laser active layer at certain driving conditions, are responsible for mode clustering effect. The resonant excitation of the plasma oscillations occurs due to longitudinal mode beating. The separation of mode clusters is typically by an order of magnitude larger that the individual mode spacing. 
Igor V. Smetanin, Peter P. Vasil'ev
physics.optics