# Circularize the laser diode beam using cylindrical lens

Urban and Civil

Nov 15, 2013 (4 years and 7 months ago)

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Circularize the laser diode beam using cylindrical lens

Cylindrical lenses focus or expand light in one axis only. They can be used to expand the
output of the laser diode into a symmetrical beam. The rectangular cross
-
section of the
laser diode cavity re
sult
s

in a beam shape
with
a wider angular output from the narrow
cavity dimension and a narrower angular output from the wide cavity dimension.
Laser
diodes have an asymmetric radiation pattern which, when collimated, becomes an
elliptical beam pattern. A
s a result, anamorphic prism pairs are often used to convert the
elliptical collimated beam to a circular shape.

One can also use a pair of cylindrical lenses to
achieve similar results to the anamorphic
prism pairs. One cylindrical lens can be used to
co
llimate the laser diode
’s

slow axis
and
a second cylindrical lens can be used to collimate the fast axis
.
By simply matching the
laser diode beam divergence ratio (vertical vs. horizontal) to the ratio of the focal length
s

of the two lenses, a nearly symme
trical cir
cular beam
can be created.

Since
a
laser diode is considered as a point source, the two lenses should be placed a
distance equal to their respective focal lengths from the laser diode emitter
orthogonally

to create a collimated beam output. Make

sure the beam size at each lens does not exceed
the clear aperture of the lens. The output beam width is equal to 2f tan
Θ (
where

Θ
is
beam divergence
).

Below is an example of how to match the laser diode beam divergence ratio with the
focal length ratio of two cylindrical lenses to create a nearly circular beam.

Specifications of the Thorlabs HL6320G, 635nm, 10mW Laser D
iode

G
1

= 11

Divergence x
-
axis FWHM (deg)

G
2

= 37

Divergence y
-
axis FWHM (deg)

Divergence ratio: 3.364

Specifications of cylindrical lenses

F
1

= 50.0mm

Thorlabs’ LJ1695L1

F
2

= 12.7mm

Thorlabs’ LJ1942L1

Focal length ratio: 3.937

Specifications of
output beam

d
1

= 2f
1

tan (G
1
) = 19.44mm

d
2

= 2f
2

tan (G
2
) = 19.14mm

As you can see, the output beam is slightly asymmetric but with a great improvement on
the
beam shape
quality.