Beam Shapers

In general, a beam shaper (or beam converter) is an optical device which modifies its spatial profile. For example, there are refractive and micro-optic beam shapers which create a flat-top (top-hat) beam from a Gaussian input beam.

Another kind of beam shaper is often used in conjunction with a high-power laser diode, for example with a diode bar, to make both its beam radius and beam quality more symmetric with respect to two orthogonal directions. This facilitates e.g. launching the beam into an optical fiber with a circular cross-section, or pumping a solid-state gain medium. A popular type of device, which is based on two highly reflective mirrors and largely preserves the brightness, is described in Ref. [2]. Other devices are based on micro-optical structures, for example containing arrays of small prisms, to perform a similar function with a smaller device. Originally, such brightness-preserving beam shapers were applied to diode bars, but they can equally well be used with diode stacks.

Note that it would not be sufficient simply to use a combination of cylindrical lenses so as to achieve equal spot sizes in both directions; the beam qualities and thus the beam divergence angles in both directions would then remain very different. If a circular focus is formed in that way, the beam divergence at least in one direction is much larger than achievable with a beam shaper.

When selecting beam shapers for generating uniform intensity profiles (e.g., flat-top beams), it is useful to distinguish between two fundamental design approaches:

• Field mapping: These devices (often using aspheric lenses or DOEs) redistribute the energy of a coherent single-mode input beam in a deterministic way. They preserve the spatial coherence and wavefront quality, allowing for a large depth of field and no speckle. However, they are highly sensitive to the alignment, size, and shape of the input beam.
• Beam integration (homogenizers): These devices (e.g., microlens arrays or multifaceted pipes) break the beam into many sub-beams and superimpose them. They effectively scramble the wavefront, which reduces spatial coherence and works well with multi-mode or unstable sources. The downside is often the generation of interference patterns (speckle) if the source has some coherence, and typically a strictly defined working plane.

There are other devices performing essentially the same function with other means, e.g. with diffractive micro-optics or with fiber bundles. In the latter case, the outputs of individual emitters of a diode bar are coupled into separate fibers of a bundle, which are arranged in a linear array on the side of the diode bar, but as a circular array on the output end.

Other beam shapers perform different functions. For example, there are devices for correcting the “smile” of diode bars [4].

Finally, there are beam homogenizers for making the intensity distribution of a beam more uniform.