The light emitted by high-power semi-conductor lasers is geometrically asymmetric (figure 1). It is almost perfect in the direction perpendicular to the junction of the laser and typically 30 times worth (for a laser with a 200 µm wide emission region) in the other direction. On the other hand, the fiber has a symmetric geometrical acceptance cone which has to be bigger than the quality of the laser beam, to obtain an efficient coupling of the light.

When coupling only one laser in the fiber, much of the quality of the beam is lost: from 30 * 1 (µm*rad)², the quality goes to 40 * 40 (µm*rad)². To avoid such a loss, one can couple the light of several lasers in the same fiber by placing their beams next to each other in their "best" direction. This means that without increasing the size of the fiber, it is possible to multiply the power by 30 (figure 2).

The difficulty in combining tens of beams in one single fiber, lies in the accuracy of the alignment of the beams. Most of current systems of fiber-coupled semi-conductor lasers are based on a bigger laser ship, 10'000 µm * 1 µm in size, and constituted of several lasers as shown on Fig.1. Sophisticated optics is then used to combine the beams as on Fig. 2. The main difficulty in such a system lies in the fact that a laser cannot be linear to within 1µm over a 10'000 µm length. The coupling of such a laser in a fiber always suffers from this inaccuracy and light needs necessary to be coupled into a bigger fiber.

By using several individual smaller lasers such a problem is not encountered, but then it is necessary to align accurately many simple optical elements. The Optical SMD technique presented here is the ideal way of assembling accurately the numerous optical elements necessary to couple the light of 10 lasers (or more) into a single optical fiber.