Efficient side pumping is produced by following the same general guidelines as for end pumping. For either geometry, the best efficiency is achieved by depositing the maximum amount of pump energy within the TEM00 mode volume. The pump and mode waist trade-offs are identical insofar as they affect the gain, threshold, slope efficiency, and thermal issues. However, one important difference in this regard is that with side pumping, a small pump waist can be produced over a larger axial range than with end pumping. This reduces thermal effects. To see this, imagine the classic side-pumped dye laser or dye amplifier. Recall that in this arrangement, a laser pump beam from a doubled Nd:YAG or other source is focused with a cylindrical lens onto the side of a dye cell. The focus forms a horizontal line near the cell wall as viewed from the pump laser propagation direction. However, from the direction of the dye laser optical axis, the active volume appears cylindrical; and, in fact, it forms a gain aperture. The dye concentration is adjusted to prevent penetration of the pump beam much beyond the diameter of the resonator mode volume within the cell.
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