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Nonoptical tipsample distance control for scanning near-field optical microscopy

when the laser output power is about 30 mJ/pdse. With so many round trips, such a large single pass wavefront distortion from the laser rod results in a higher lasing threshold and smaller slope efficiency for TEM,, oscillation. To further evaluate the corrective effects of the deformable mirror, a spatial filter was also put into the linear cavity before the output coupler (50%). The spatial filter consisted of two positive lenses and a ceramic pinhole plate. Because of these two lenses, the cavity was a stable one. Figure 2 shows the laser output power as a function of the flash-lamp voltage for the four configurations: with and without the pinhole plate, and with and without the deformable mirror. By inserting the pinhole plate into the cavity, we found the lasing threshold increased as expected. The obvious difference in the laser power output shown in Fig. 2 is the significantly increased slope efficiency for the two cases when the flat mirror (FM) was replaced with the deformable mirror (DM). The laser power does not exceed 30 mJ/pulse because the focused laser light near the pinhole position would break down the air. A 12-point adjustment on the deformable mirror was the simplest configuration to sufficiently correct the distortion. We chose mechanical adjustment with 00-120 screws rather than piezoelectric acturators, for the mirror surface control. With this arrangement, the shape of the interference pattern still did not have noticeable change after more than two months. Such a deformable mirror is inexpensive because of its simplicity. Although the deformable mirror significantly affects the stable cavity configuration, we did not observe a slope efficiency improvement in an unstable cavity with no intracavity focus. The deformable mirror alone does not ensure a Gaussian transverse profile; other optics such as a graded reflectivity mirror are required to achieve such a spatial distribution. When we used a graded reflectivity mirror as the output coupler, the improved laser spatial distribution was observed while using the deformable mirror for correction of the laser rod astigmatism.