Editing Diode-pumped solid-state laser (section)

=== Some numbers ===

High power laser diodes are fabricated as bars with multiple single strip laser diodes next to each other.

Each single strip diode typically has an active volume of:
{| class="wikitable"
|-
|1&nbsp;μm || 2&nbsp;mm || 100&nbsp;μm
|-
|Height || Depth || Width
|-
|fast axis || optical axis || slow axis
|}

and depending on the cooling technique for the whole bar (100 to 200) μm distance to the next laser diode.

The end face of the diode along the fast axis can be imaged onto strip of 1&nbsp;μm height. But the end face along the slow axis can be imaged onto a smaller area than 100&nbsp;μm. This is due to the small divergence (hence the name: 'slow axis') which is given by the ratio of depth to width. Using the above numbers the fast axis could be imaged onto a 5&nbsp;μm wide spot.

So to get a beam which is equal divergence in both axis, the end faces of a bar composed of 5 laser diodes, can be imaged by means of 4 (acylindrical) cylinder lenses onto an image plane with 5 spots each with a size of 5&nbsp;mm x 1&nbsp;mm. This large size is needed for low divergence beams. Low divergence allows paraxial optics, which is cheaper, and which is used to not only generate a spot, but a long beam waist inside the laser crystal (length = 50&nbsp;mm), which is to be pumped through its end faces.

Also in the paraxial case it is much easier to use gold or copper mirrors or glass prisms to stack the spots on top of each other, and get a 5 x 5&nbsp;mm beam profile. A second pair of (spherical) lenses image this square beam profile inside the laser crystal.

A volume of 0.001&nbsp;mm<sup>3</sup> active volume in the laser diode is able to saturate 1250&nbsp;mm<sup>3</sup> in a Nd:YVO<sub>4</sub> crystal.