the rotating E fields generated by g+ and T+ particles interact and give rise to the Transverse Modes in a laser.
An aperture that restrict the spread of this field in the plane perpendicular to OP, prevents such interactions and "destroys" the transverse modes. A conductive wire within the resonance cavity of the laser would also short the E fields and prevent their interaction in certain directions and so produce different interference patterns.
Have a nice day.
