Defocusing effects

Another problem is due to fringing fields. To reduce these defocusing effects at the entrance and exit of the field, the pole pieces of the yoke of the electromagnet generally have specially shaped edges (Fig. 4.20). 

When a positive ion enters the filter, maintained under vacuum, at the origin O, its velocity vector in xyz space will determine its trajectory. The central portion of the quadrupole behaves like a tunnel in the O-z axis. The walls of the tunnel can either attract or repel an ion depending on the ion s position. The two positively charged electrodes will focus the ion in the O-z axis, corresponding to a potential valley (stability zone) while the two negatively charged electrodes will have a defocusing effect (potential maximum, unstable y-O-z plane). 

The two positively charged electrodes focus it along the z-axis, which corresponds to the bottom of a potential valley (stability zone), while the two negatively charged bars, in contrast, have a defocusing effect (appearance of an mound of potential in the yz-plane). 

Independently proposed hy Mack and Bernard, this model accounts for the effect of a changing aerial image focus (i.e., defocus effect) as it propagates... 

A quantitative study of the role played by the laser wavefront curvature and other geometrical/optical parameters on the transmitted far-field intensity profile [3] demonstration of self-defocusing effect for optical limiting application [4]. 

Figure 11.14. An experimental arrangement for aehieving the optical limiting effect using the self-phase modulation or self-focusing/self-defocusing effect. See chapter 12, however, for a fiber array limiting device that does not suffers from small field of view associated with this configuration involving pin-hole.

Figure 12.1. Experimental set up for optieal limiting action using external self-defocusing effect. Insert are photographs of the transmitted laser intensity distrihution at input power (a) helow 70 nano Watt and (h) above 100 nano Watt.

A limitation common to such self-defocusing or self-phase modulation effect on a Gaussian beam is that the nonhnearity in the central region could reach a saturation point as the laser intensity increases. In that case, the reorientation profile and the laser self-induced defocusing effect will diminish, thereby restricting the dynamic range of the limiting operation. 

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