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Section 5.6: Axicons for
Generating Bessel Beams
Anamorphic prism pairs are other types of optics used for circularizing
elliptical beams. An anamorphic prism pair consists of two prisms used
to reshape a laser beam. 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. Similar to cylinder lenses, 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 5.10).
This compensates for the different divergences of the original beam.
While a single prism could change the beam radius in one axis, it would
also change the beam direction. A pair of prisms is required to manipulate
the ellipticity of the beam while maintaining the original direction
of propagation. While anamorphic prism pairs maintain parallelism to
the original direction, they do displace the beam perpendicular to this
direction. However, using anamorphic prism pairs requires precise angular
alignment. Although it is not required, it is useful for one prism to
be oriented at Brewster’s angle, which is the angle of incidence at which
there is no reflection of p-polarized light. The other surface of the prism
should be at normal incidence and should be anti-reflection (AR) coated
for maximum throughput. This precise measurement is why customers
most often buy them as a pre-aligned pair.
Higher-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 a diode without an integrated prism pair and a separate
anamorphic prism pair can be less than that of a more expensive diode.
Cylinder lenses have more degrees of freedom than mounted anamorphic
prism pairs, making them more difficult to align. Cylinder lenses
may tilt, making prisms more forgiving when aligning axes independently.
Close attention must also be paid to the focal length of cylinder
lenses so that they are the correct distance away from the diode output
to produce a collimated circularized beam. Mounted anamorphic prism
pairs are more user friendly. They are pre-aligned to have a fixed expansion
power, so you do not need to position and assemble them yourself
like you would with cylinder lenses. There is only one axis to which the
prisms must be independently aligned because you are merely sliding
the prism into the beam path. This removes an additional alignment
step, saving the user time and potential frustration. The physical position
of the anamorphic prism pair, relative to the location of the incident
laser beam, is also less sensitive.
However, the extra degrees of freedom of cylinder lenses give them
more flexibility, which may be useful in research applications and prototyping.
They can also provide a higher throughput than anamorphic
prism pairs, especially when using AR coatings. Light travels through
less material in cylinder lenses compared to anamorphic prism pairs,
and p-polarized light will be lost if prisms are used at Brewster’s angle.
Non-Power Direction
Power
Direction
Non-Power Direction
Power
Direction
Figure 5.8: Power and non-power directions in both rectangular and
circular cylinder lenses
f2
f1
Figure 5.9: Example of circularizing an elliptical beam using cylinder
lenses
Brewster’s Angle
Input Beam
Output Beam
Figure 5.10: An anamorphic prism pair acting as a beam expander in
one direction, which can circularize an elliptical beam
Cylinder Lenses Anamorphic Prism Pairs
Beam Displacement Not Displaced Displaced
Degrees of Freedom High Low
Alignment Sensitivity High Low
Throughput High Medium
Cost Low Low
Footprint Small Small
Table 5.2: Comparison of cylinder lenses and anamorphic prism pairs
for beam circularization
References
1. F. M. Dickey and S. C. Holswade, Laser Beam Shaping: Theory and
Techniques, Marcel Dekker, New York (2000).
2. Laskin, Alexander, and Vadim Laskin. "Refractive field mapping beam shaping
optics: important features for a right choice." Proc. ICALEO. Vol. 2010. (2010).
3. F.M. Dickey, S.C. Hoswade, D.L. Shealy, Laser Beam Shaping
Applications, Taylor & Francis, ISBN 0-8247-5941 (2005).
4. J. Durnin: J. Opt. Soc. Am. A 4 (1987) 651.
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