imaging the spatial mode emitted by the cavity. However, for higher spatial modes there is competition between the cylindrical and rectangular modes which are nearly degenerate as can be seen explicitly from Fig. \[pol\](c). Here again for the red detuned peak a single dominant spatial mode is observed while the blue detuned cavity emission peak shows multiple spatial modes of similar intensities. The purity of polarisation and the spatial mode is strong evidence for lasing in the case of the red detuned peak. The sharp threshold for the red detuned peak and the rapid increase in peak height as the atom number is increased is characteristic of lasing behaviour arising due to non-linear response with increase in an gain [@silfvast_laser_2004].

To measure the frequency at which the gain occurs, we pass a probe beam through the cavity and monitor the transmission. Fig. \[fano\](a) shows the transmitted intensity when the frequency of the probe beam is scanned while keeping the cavity detuning fixed on the red detuned peak of the last graph in Fig. \[atnogrid\](a). The blue trace represents the output with MOT atoms, and the red trace is the output when there are no atoms in MOT (the magnetic field 