Anamorphic Prism Pairs

Most laser diodes produce elliptically-shaped beams with different divergence angles in the x and y axes because of the rectangular shape of the diode’s active region (Figure 1). This is detrimental in many applications, as elliptical beams will have a larger focused spot size than circular beams. Larger spot sizes lead to a lower irradiance (flux of radiant energy per unit area) at the point of focus, which in turn may require a higher power input to the laser. Anamorphic prism pairs are particularly useful for circularizing elliptical beams.

Figure 1: The geometry of laser diodes causes them to produce elliptical beams with two different divergence angles.

An anamorphic prism pair consists of two prisms used to reshape a laser beam.1 They are most often used to transform elliptical beams into circular profiles, but they can also produce other elliptical beam profiles in a range of sizes. The optical principle at play behind the reshaping is refraction; the light is bent in one direction, or one axis, while the other axis is kept constant (Figure 2). This compensates for the different divergences of the original beam.