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Definition: optical devices for modifying the shapes of laser beams
In general, a beam shaper (or beam converter) is an optical device which somehow reshapes a light beam, i.e., it modifies its spatial properties. For example, there are refractive and microoptic 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, in order 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.
Figure 1: Setup of a two-mirror beam shaper [2]. The lower part shows how an array of beamlets from the emitters of a diode bar is rearranged so as to obtain a roughly equal beam quality in both directions.
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.
There are other devices performing essentially the same function with other means, e.g. with microoptics 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] or for homogenizing the beam, i.e., making its intensity distribution more uniform.
Bibliography
| [1] | J. R. Leger and W. C. Goltsos, “Geometrical transformation of linear diode-laser arrays for longitudinal pumping of solid-state lasers”, IEEE J. Quantum Electron. 28 (4), 1088 (1992) |
| [2] | W. A. Clarkson and D. C. Hanna, “Two-mirror beam-shaping technique for high-power diode bars” (reflective beam shaper for symmetrizing the beam quality and preserving the brightness), Opt. Lett. 21 (6), 375 (1996) |
| [3] | A. von Pfeil, “Beam shaping of broad area diode lasers: principles and benefits”, Proc. SPIE 4648, 82 (2002) |
| [4] | J. F. Monjardin et al., “Correction of beam errors in high-power laser diode bars and stacks”, Opt. Express 14 (18), 8178 (2006) |
| [5] | X. Gu et al., “All-fiber laser beam shaping using a long-period grating”, IEEE Photon. Technol. Lett. 20 (13), 1130 (2008) |
See also: beam quality, laser diodes, diode bars, diode stacks, brightness
Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
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