Fiber Optic Beam
Delivery Systems
Conventional Laser Beam
Delivery Systems
For 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
difficulties:
-
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 µm to 1000 µm, and a
cladding diameter of 1100 µm. 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 81.2°. This in turn means that rays striking the end of
the fiber at an angle of 12.8° 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 (12.8°), 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 radiation:
-
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 Systems
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 Delivery System
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.
Fiber End Connections
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).
Output Coupling Optics
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.
References
1. Sterling, Donald J. Jr., Technician's Guide to Fiber
Optics, Second Edition, Delmar Publishers, 1993.
2. Marcuse, Dietrich, Theory of Dielectric Optical Waveguides, Second
Edition, Academic Press, 1991.
1 The
divergence of a 2000 watt CW Nd:YAG laser is roughly 25 mrad, or 1.4°. 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.
2
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.
3
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 the cladding
4
The
acceptance half-angle Qc is calculated as arcsin [nf N sin
(90 - ac)].
5
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 - (nc)2]´
6
The upcollimator, by
expanding the laser beam, reduces its divergence. This, in turn, allows the beam
to be focused to a smaller spot.
7
The f-number of a lens (or
optical system) is the ratio of its focal length to the diameter of its clear
aperture.
8
For a fiber with NA of 0.22,
the f-number of the collimating lens must be 2.2 or lower.
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