Anamorphic Prism

Due to the rectangular shape of the active region in diodes, most laser diodes produce elliptical beams with different divergence angles in the X and Y axes (Figure 1). This is often detrimental in many applications because elliptical beams will have a larger focused spot size compared to circular beams. The larger spot size results in lower irradiance (radiative flux per unit area) at the focal point, which may necessitate higher power input to the laser. Anamorphic prism pairs are particularly useful for circularizing elliptical beams.

Anamorphic prism pairs consist of two right-angle prisms used to reshape laser beams. They are most commonly used to transform elliptical beams into a circular profile, but they can also produce other elliptical beam profiles of various sizes. The optical principle behind shaping is refraction. Light bends in one direction or axis while the other axis remains unchanged (Figure 2). This compensates for the beam’s original divergence differences.

While a single prism can change the beam radius on one axis, it also alters the beam direction. A pair of prisms is needed to control the ellipticity of the beam while maintaining its original propagation direction (Figure 2). However, using anamorphic prism pairs requires precise angular alignment. Although not necessary, orienting one prism at Brewster’s angle is beneficial since it is the incident angle where p-polarized light is not reflected. The other surface of the prism should be perpendicular to the incident beam and should be coated with an anti-reflective (AR) coating for maximum throughput. This precise measurement is why customers most often purchase them as pre-aligned pairs.

High-end diodes often have anamorphic prism pairs built into the laser head for beam circularization. However, many lower-cost diodes do not. The cost of purchasing diodes without integrated prism pairs and separate anamorphic prism pairs is often lower than that of more expensive diodes.

Comparison with Cylindrical Lenses

Cylindrical lenses have optical power in only one direction and are commonly used to circularize elliptical beams as well (Figure 3). Cylinder lenses manipulate light by focusing or altering the wavefront curvature. As a result, anamorphic prisms have an advantage in applications with low wavefront distortion.

Cylindrical lenses offer more degrees of freedom than mounted anamorphic prism pairs, making them more difficult to align. Cylindrical lenses may tilt, making prisms more forgiving when aligning along independent axes. Close attention must also be paid to the focal length of the cylindrical lenses to ensure they are at the correct distance from the diode output to produce a collimated circular beam. Mounted anamorphic prism pairs are more user-friendly. They are pre-aligned with fixed expansion capabilities, so you don’t need to position and assemble them yourself as you would with cylindrical lenses. There is only one axis that the prisms must be independently aligned with, as you simply slide the prisms into the beam path. This eliminates additional alignment steps and saves users time and potential frustration. The physical position of the anamorphic prism pair relative to the incident laser beam is also less sensitive.

However, the additional degrees of freedom of cylindrical lenses give them greater flexibility, which can be useful in research applications and prototyping. They can also provide higher transmittance than anamorphic prism pairs, especially when AR coatings are used. Light passes through less material in cylindrical lenses, and p-polarized light will be lost if prisms are used at Brewster’s angle.

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