For years, laser manufacturers have promised the ability to produce very high-power, cost-effective lasers that are stable outside of a laboratory environment. Recent requirements in aerospace and defense applications, such as LIDAR (Light Detection and Ranging), IR countermeasures, and laser targeting, are now calling on these manufacturers to deliver.

Figure 1. A beamshaper (top) redistributes the energy in the wavefront of a highly coherent beam to generate the desired, speckle-free profile over a limited depth of focus. In contrast, a controlled angle diffuser (bottom) overlaps multiple copies of the input beam to form the desired profile in the far-field. The diffuser is much more tolerant to changes in the input beam shape and modal structure, but the level of speckle in the output will be a function of the input beam coherence and quality.
Although some companies have stepped up to the challenge and successfully developed these devices, pushing the current limits of manufacturing and design does not come without a price. Generally, beam quality is sacrificed for higher output, either because multiple laser cavities are being used or because many modes are generated within a single cavity. Though occasionally the output will combine to form a beam that appears Gaussian-like in profile, more often than not these lasers have a structure that is neither Gaussian nor the frequently desired Top-Hat intensity profile. In addition, these intensity profiles often vary during operation.

Figure 2. An input beam with high, and possibly changing, structure can be remapped by a controlled angled diffuser to both homogenize and reshape the basic angular profile. In this image, an input beam with a Gaussian-like intensity profile has been transformed by a diffuser to a rectangular profile with uniform intensity in the far-field.
If the structure of the output beam intensity profile remains stable, and if the application only needs one particular profile at a limited projection range, customized refractive or diffractive beam-shaping solutions can provide high-quality and high-efficiency results. Essentially, beam shaping is the process of remapping the output angle for sections of the beam in such a way that the energy will overlap favorably at a specific distance. From Figure 1 (top), it is apparent that the distribution of energy is dependent upon the propagation length of the beam. As the projection length varies from that of the intended design, the device no longer functions properly. However, any variation in laser-to-laser output may require a unique design for each system. Furthermore, if the structure changes over time or during operation, these beam-shaping solutions may no longer provide adequate homogenization or profile redistribution.

Figure 3. Unlike a more traditional refractive element, which continuously reshapes the entire incident wavefront, diffractive elements typically break up each wavefront and interfere it with successive wavefronts to form the desired output pattern. This SEM image of the surface structures of a diffractive element shows small regions imparting discrete phase delays to sections of the wavefront. These structures are generally of micron, or even sub-micron, scale.
Fortunately, a solution exists that is much less dependent on the actual structure of the beam profile and more dependent on the nature of the structure. The solution comes in the form of diffractive diffusers and homogenizers that can produce very precise and controlled outputs, which change little from laser to laser. Basically, diffractive diffusers function by forming multiple replications of the input beam that overlap and are propagated forward in an array of defined angles to create a specific output geometry (Figure 1 bottom).


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