Fiber Optic Beam Delivery Systems
Using Step Index Fiber
In
a fiber optic beam delivery system, the output optical assembly (end
effector) consists of a dual lens imaging system. The first lens
(collimator lens  L1) takes the rapidly diverging beam from the fiber
exit, and straightens, or collimates, it. This lens should be placed at
a distance exactly equal to its focal length from the fiber exit face.
If this is not done, the beam will not be collimated, and the imaging
convention described below is void. The second lens (L2) acts as an
objective, and focuses the beam to form an image of the fiber face.
If
the laser energy is delivered to the end effector through a step index
fiber, the beam, as it passes through the fiber, expands to fill the
fiber core. Therefore, the beam profile, at the exit face of the fiber,
is a top hat, with a diameter equal to the fiber core diameter. This
phenomenon is independent of the diameter of the beam when it enters
the fiber.
When
the fiber optic beam delivery system is configured as described above,
the diameter of the focused image is a function of the fiber core
diameter, and the ratio of the focal lengths of the two lenses:
If
lenses of identical focal length are used, the ratio will be 1:1, and
the focused spot diameter will equal the fiber core diameter. If a
longer focal length lens is used as the objective lens, then the ratio
will be >1, and the focused spot diameter will be greater than
the fiber core diameter.
For
example, if a 60 mm collimating lens and a 75 mm objective lens are
used with a 0.6mm (600 um) core fiber, then the diameter of the focused
spot is 0.75 mm (750 um, or ~.030"):
Placement
of the Objective lens in relation to the collimator lens is not
critical, but in general should be 6" or less.
In
selecting the collimating lens (L1) focal length, careful consideration
should also be given to the diameter of the lens, so as not to overfill
it. Step index fiber used by U.S. Laser Corp. generally has a numerical
aperture of 0.20. The numerical aperture (NA) of a fiber, which is
defined by the index of refraction of its core and cladding, determines
the maximum solid angle (2) at which a beam can be launched into it.
Using a 0.20 NA fiber, this acceptance angle is
~23E. This is the maximum angle that a beam can be safely launched
into, and propagated thru a fiber, and will also be the maximum angle
at the exit of the fiber. Knowing the maximum angle of the beam exiting
the fiber, we can then calculate the required clear aperture (A) of the
collimating lens to transmit the beam without loss.
For example, if we want to use a 60mm focal length
lens as a collimating lens with a 0.20 NA fiber, then:
Thus
the clear aperture of the lens must be at least
25.5mm to transmit the beam.
If
long working distances and/or small spot diameters are to be achieved,
it is then obvious from the above that longer focal length collimating
lenses are desirable. However, as the collimator focal length
increases, so does the clear aperture requirement, as shown in the
following table:
Lens Focal length (Fl_{L1})

50mm

60mm

75mm

90mm

100mm

Minimum Clear Aperture

22mm

25.5mm

32mm

39mm

42.5mm

Note:
Since the lens must be held in a mechanical mount, the actual lens
diameter should be at least 2mm larger than the clear aperture to allow
for mounting.
If
short focal length lenses are to be used, caution should be taken, as
optical distortions could be a significant factor. Where lenses are to
be used where the lens F# (the ratio of the lens focal length to the
lens diameter) is less than 2, optical aberrations can be significant.
These aberrations can be of sufficient magnitude that they become the
limiting factor in determining the ultimate focused spot size. In
general, short focal length lenses should be avoided. If they must be
used, aberration corrected multielement lenses should be used.
The
above discussions have focused on the maximum (or worst case) limits of
the fiber optic beam delivery system. Depending on the type of laser,
its operating level, and the optics used to couple the beam into the
fiber, performance better than described above may be possible. For
example, it may be possible to couple the beam into the fiber at angle
less than the maximum acceptance angle. Since the angle of the beam
exiting the fiber would also be less than the acceptance angle, this
would possibly allow the use of a longer focal length collimating lens
. This can only be done when the laser output characteristics have been
carefully measured (see U.S. Laser Technical Note on M^{2}
measurement). If done incorrectly, catastrophic damage to the fiber can
occur.
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