Non-imaging Laser Diode Array Beam Shaper

Abstract

Laser diode arrays are inexpensive, compact, efficient, and reliable light sources. However, the output beam, which is highly elliptical, is difficult to both efficiently collect and concentrate. This has limited continuous wave (CW) diode sources to power levels under 100 Watts. The fast axis of the laser diode array output tends to have good beam quality, M2~ 2, and a source size of ~ 1 um. The slow axis of laser diode broad stripes and of array bars is highly multimode, typically having an angular distribution of 10 degrees at the 1/e2 power points. The slow axis beam quality for a 500 um wide stripe is thus M2~ 70, and M2~ 1400 for a one centimeter array. One means of improving the brightness is to individually fiber couple broad stripe diodes, and then bundle the fiber ends together. Another is to collimate the fast axis of one centimeter arrays using a fiber lens1, and then fiber couple into a linear array of fibers. The output ends of the fibers are also bundled together. Both techniques are compact and relatively simple, and power is scaled by increasing the number of fibers in the bundle. M2 ~ 350 are available at 60 Watts output using the latter technique. It is difficult to scale beyond 100 Watts without increasing M2 to values greater than 500. This is due to loss of brightness in fiber coupling, as an essentially linear source is being coupled into a round fiber, or in mode mixing as light propagates through the fiber. Brightness is further reduced in fiber bundling by the presence of the fiber cladding, and by the less than unity packing factor of round fibers.