Edges coupling operator

If we now invoke the A centering (i.e., point 2 is translated by b/2 + c/2), point 2 becomes point 3. But clearly, point 1 will go directly to 3 if a 2X operation is carried out about a 2, axis cutting the b edge of the cell at bl4. In a similar way, every twofold rotation can be coupled with the -centering translation to generate one of the 2, axes. 

Fig. 11. Schematic representations of the commutator sequences starting from /, for (A) effective POO and lOO, (B) effective ILL, (C) effective PPP, and (D) effective III coupling topologies. The following rules and abbreviations are used. Terms in the density operator appear as nodes in the diagram Circles contain polarization operators which are represented by the symbols z,. Rounded boxes contain zero-quantum operators (ZQ), = which are represented by the symbols A,. Straight-edge boxes contain an-...

For m = 1, first-order DFB operation is obtained. The laser radiation is observed from the sample edge in this case. For a second-order DFB resonator (m = 2), laser radiation is also emitted perpendicular to the sample surface. This part of the emission does not participate in the feedback of the laser process and acts as radiation loss. Therefore lower coupling coefficients k and higher threshold energy values are expected for second- and higher-order DFB lasers compared to first-order DFB structures. 

---