Fiber Optic Beam Delivery
Laser Beam Delivery Systems
material processing, the output of the laser must be focused onto the
material surface. Conventional beam delivery systems utilize lenses and
mirrors to accomplish this purpose. Specifically, the following
elements are used:
An upcollimator (beam expander) is used to increase
the size of the beam, and reduce its divergence (see below)
One or more mirrors are used to direct the beam
towards the material
An objective lens focuses the beam onto the sample
The difficulties with this type of system stem in
part from a basic characteristic of all lasers. As a laser beam travels
through space, it expands (or diverges). This expansion causes two
For delivery over long distances, the beam can
become very large, requiring commensurate increases in the diameters of
the optical elements1. In the case of the
objective lens, increasing the diameter limits the minimum focal
length, and may introduce aberrations in the optical performance. Both
of these factors increase the minimum focused spot size.
As the distance between the laser and the objective
lens changes, the focused spot size also changes. The only way to
maintain a constant spot size is to keep the optics fixed, and move the
material. For large objects, this may be difficult or impossible.
In addition to the problems caused by the laser
beam divergence, conventional beam delivery systems are rather
inflexible. Changing the relative positions of any of the elements can
cause misalignment problems, especially if any rotations are required
(such as welding or cutting contour surfaces). For these applications,
delivery of the laser radiation through a flexible optical system is
highly desirable. Ideal characteristics of this system include:
Constant beam diameter over a range of distances
Flexibility (position and orientation) in
positioning the focused spot
Complete enclosure of the beam, for safety reasons.
For these applications, optical fiber technology
holds high promise.
Optical Fiber Technology
An optical fiber (Figure 1) consists of two
concentric layers: a core surrounded by a cladding. The core and
cladding are typically both fused silica, but with slightly different
indices of refraction2. This construction allows
light traveling through the core at less than a critical angle3
to be totally reflected whenever it hits the core-clad interface. This
"total internal reflection" allows the beam to be propagated along the
length of the fiber, with all of the beam energy contained within the
core. A typical optical fiber used to deliver laser radiation has a
core diameter of 400 um to 1000 um, and a cladding
diameter of 1100 um. The fiber is typically enclosed in an
armor jacket (diameter 8 mm) to protect it from damage. Typical indices
of refraction are 1.457 for the core, 1.440 for the cladding. These
values result in a critical angle4 of about
81.2o. This in turn means that rays striking the end of the fiber at an
angle of 12.8o or less will be propagated. This angle is often referred
to as the acceptance half angle. The acceptance half-angle of the fiber
is often expressed in terms of numerical aperture (NA), which is the
sin of the angle5. For this fiber, the NA is sin
(12.8o), or 0.22. To avoid confusion, it should be noted that the
critical angle (which is referenced to the surface normal of the
core-clad interface) is a minimum angle for total internal reflection,
while the acceptance angle (which is referenced to the surface normal
of the fiber end face) is a maximum angle.
Figure 2. Optical Fiber
The above description refers to a particular type
of fiber, referred to as straight core, step index. Other types of
fibers (gradient index, dual clad, and tapered core) are also
available, and offer different combinations of parameters.
Once energy has entered the core (subject to the
angle constraints discussed above), it is propagated, with the only
losses due to absorption or scattering within the core material. These
losses, referred to as attenuation losses, are very low: the
attenuation factor is typically < 5 db/km, which corresponds to
a power loss of only 11% through a 100 meter long fiber.
As discussed above, as long as the angle of
incidence is greater than the critical angle, the beam will be
propagated within the core. Bends in the fiber may change the angle of
incidence, and potentially allow some of the energy to escape. However,
for bend radii as small as 150 mm, this effect is negligible.
In summary, optical fibers have the following
properties which make them appealing for delivery of high power laser
Optical fibers are thin and highly flexible
Optical fibers transmit radiation over long
distances with minimal energy loss.
The optical fiber completely contains the laser
beam within its core, keeping the beam diameter constant. It thus
eliminates the problems of beam divergence over long distances, and
reduces the risk of personnel exposure.
Fiber Optic Beam Delivery
A Fiber Optic Beam Delivery (FOBD) System includes
more than the optical fiber. Referring to Figure 2, the system includes
three additional subsystems:
Input Coupling Optics
Fiber End Connections
Output Coupling Optics
Figure 3. Fiber Optic Beam
Input Coupling Optics
The purpose of this optical assembly is to couple
the energy from the laser into the core of the fiber. The input
coupling optics generally include an upcollimator (which expands the
laser beam6), and a focusing lens assembly,
which focuses the beam into the fiber. To function properly, the system
must meet the following criteria:
All of the energy must be focused into the core of
the fiber. Energy that is focused into the cladding or outside of the
fiber can cause catastrophic failure near the end of the fiber,
especially at high power levels. Therefore, the diameter of the focused
spot must be smaller than the core diameter of the fiber, and the spot
must be aligned to the center of the core.
None of the energy can arrive at an angle greater
than the acceptance angle of the fiber. Any energy arriving at a
greater angle will not be completely reflected at the first core-clad
intersection; the energy escaping into the cladding will be lost, and
may also cause catastrophic failure. Therefore, the cone angle of the
input beam (determined by the size of the beam at the focusing lens,
and the focal length of the lens) must be less than the acceptance
angle of the fiber.
The fiber end connections serve several purposes:
Since the fiber core diameter and the size of the
focused spot are quite small (< 1 mm), alignment and stability
are critical, if catastrophic failure is to be avoided. At the same
time, easy replacement of fibers is required, ideally without the need
for realignment. A properly designed connector accomplishes both.
At a glass-to-air interface (such as the end of the
fiber), a percentage of the laser power can be reflected from the
surface (this reflection is also referred to as Fresnel losses).
Typically, the reflected power is about 4% of the incident power (for
2000 watts input, about 80 watts is reflected). The
connection system must be capable of dissipating the reflected energy
without either damaging the fiber or causing it to change position.
The ideal connection system will employ a method to
reduce the Fresnel losses at the surface. This increases the amount of
power delivered to the material to be processed, and it also reduces
the requirements to dissipate the reflected energy.
The fiber end connection typically consists of a
mechanical connector (with mating socket) which rigidly holds the
fiber. Possible methods to reduce the Fresnel losses include depositing
an anti-reflection (AR) coating on the fiber ends (this technique is
routinely used for fixed optics, but until recently has not been
feasible for optical fibers).
The purpose of the output coupling optics is to
collect the radiation leaving the fiber, and re-focus it onto the
material to be processed. The parameters of the focused beam, which
vary with the specific application, include spot size, beam profile,
depth of focus, and working distance.
The output coupling optics generally includes two
separate lens assemblies. The first assembly collimates the beam
leaving the fiber. Its f-number7 must be low
enough to collect all of the radiation leaving the fiber8.
The second lens assembly focuses the collimated beam onto the
workpiece. The final spot size is a function of the fiber core
diameter, the clear aperture of the focusing optics, the working
distance of the focusing lens assembly, and any optical aberrations.
1. Sterling, Donald J. Jr., Technician's
Guide to Fiber Optics, Second Edition, Delmar Publishers,
Marcuse, Dietrich, Theory of Dielectric Optical Waveguides,
Second Edition, Academic Press, 1991.
The divergence of a 2000 watt CW Nd:YAG laser is
roughly 25 mrad, or 1.4o. The beam from this laser would expand about
25 mm per meter of travel. An upcollimator (beam expander) reduces this
value by the expansion ratio, but it also increases the initial beam
diameter. For a beam path of 5 meters, the diameter of the raw beam
would be 130 mm; a 3:1 upcollimator would reduce this to about 60 mm.
In either case, standard laser optics (with diameters from 20 to 50 mm)
could not be used.
The index of refraction
(n) of a material is the ratio of the velocity of light in free space
to the velocity of light in the material.
The critical angle ac
is defined as arcsin (nc/nf),
where nc is the index of refraction of the core
material, and nf is the index of refraction of
half-angle Qc is calculated as arcsin [nf
N sin (90 - ac)].
The NA of a fiber can be calculated directly
from the indices of refraction of the core (nf)
and the cladding (nc): NA = [(nf)2
The upcollimator, by expanding the laser
beam, reduces its divergence. This, in turn, allows the beam to be
focused to a smaller spot.
The f-number of a lens (or optical system) is
the ratio of its focal length to the diameter of its clear aperture.
For a fiber with NA of 0.22, the f-number of
the collimating lens must be 2.2 or lower