Spot profile and beam divergence in cholesteric liquid crystal lasers

The spot shapes of cholesteric liquid crystal lasers have been studied experimental and theoretically. A broad variety of profiles has been found depending on the oscillator architecture, thickness, and lasing conditions. Simple cells, lasing at the short-wavelength edge of the gap, give rise to simple spots, whereas if the laser wavelength is at the long-wavelength edge the spots are surrounded by rings. On the other hand, complex cells, which incorporate passive cholesteric mirrors connected with glass substrates, produce spots with many sharp concentric rings. In all cases we have demonstrated that the different intensity profiles can be explained as due to anomalous light scattering of the main laser propagating along the helical axis towards oblique directions. For simple cells, the beam divergence is essentially determined also by scattering, showing a decreasing tendency with the sample thickness. Complex cells have a much smaller beam divergence, and its value is limited by diffraction.