Design of a Field-Mapped Beam Homogenizer
- Tuesday, 01 January 2008
Light beam homogenizer designs used for decades are largely based upon lenticular lens arrays that date back to the 1940s and 1950s. A more recent design is from the late 1980s that comprises a pair of crossed-cylindrical lenticular lens structures coupled with a condensing lens. The 1980s design has 10 optical surfaces which represents a significant source of optical loss, long beam path (typically ~ 1.5 meters) and high cost from having so many optical elements. Further, this design requires careful and precise alignment of the light beam in relation to the optics comprising the homogenizer, in particular angular rotation of the lenslets and their positioning in the X-Y plane of the optical axis. These designs are unable to produce a homogenized ring illumination, which is useful for ablating polymer insulation from metal bonding pads in microelectronic fabrication of multichip modules and memory chips.
Figure 1 illustrates a patent pending optical device called a “Field-Mapped Beam Homogenizer” that transforms a spatially, nonuniform light beam into a spatially homogeneous profile suitable for precision laser micromachining solar concentrators, or as an illumination source for photolithography and TV projection displays.
The unique optical device “field-maps” a homogenized illumination by using spherical, cylindrical, axicon or prism optical segments — or in some applications a combination of these various elements — and placing the optical segments in a “mapped” configuration whereby light passing through each optical segment directs the light to overlap at a homogenized plane with the desired shape (rectangle, square, rectangular or circular ring illumination). The geometry of the homogenized field is limited only by the fabrication techniques used in segmenting and how the segments are physically arranged.
The field-mapped homogenizer in its basic form is fabricated from spherical or cylindrical lenses. The lenses can be either negative or positive, depending on the type of illumination, size and numerical aperture required. The lenses are segmented from larger lens elements in a predetermined way and then specific segments are selected and repositioned in a specific order so that the light passing through each lens segment recombines at the desired homogenized plane. Figure 2 illustrates this process with 3 plano-concave lenses to form a 1 × 3 segment homogenizer for simplicity of explanation.