ABSTRACT
Conventional beam delivery system has
the problems caused by the laser divergence. We can say that this system is
inflexible because any change in the relative position of any of the element
causes misalignment problems. For these applications, delivery of the laser
radiation through a flexible optical system is highly desirable which includes
some ideal characteristics like constant beam diameter over a range of
distance, flexibility in positioning the focused point ,complete enclosure of
the beam for safety reasons. In short by
using optical beam delivery system we can avoid bulky design, complex
manufacturing, commissioning and servicing.
INTRODUCTION
The word
laser, actually an acronym for Light Amplification by Stimulated
Emission of Radiation. Laser machining is a material removing
process that is achieved through interactions between the laser and target
material. The process of laser machining
is actually a few different processes grouped into one; those being one, two
and three dimensional laser machining, which are, drilling, cutting and shaping
respectively. Additionally, laser machining includes the subcategories of laser
grooving, marking and scribing. In conventional laser beam delivery system
laser beams travel in a straight line in which mirrors or prisms are used to
change the direction of propagation, other optical elements can shape the beam
or split it into the multiple beams as required. But most of the today’s solid
state lasers use a fiber beam delivery system which provides a safe and
flexible beam delivery path to the work piece.
PROBLEMS
WITH CONVENTIONAL LASER DELIVERY SYSTEM
For material
processing, the output of the laser must be focused onto the material surface.
Conventional beam delivery system utilizes lenses and mirrors to accomplish
this purpose. Conventional beam delivery system consists of following elements:
1)
An upcollimater is used to increase the
size of the beam and reduce it divergence.
2)
One or mirrors are used to direct the
beam towards the material.
3)
An objective lens is used to focus the
beam on to the work piece.
The
difficulties with this type of system are in part from a basic characteristic
of all lasers. As a laser beam travels through space it expands or diverges.
This causes
two difficulties:
1)
When the laser beam is to be delivered
over a long distance the beam can become very large, requiring proportionate
increase in the diameters of the optical elements. In the case of the objective
lens, increasing the diameter limits the minimum focal length, and may
introduce aberrations in the optical performance both these factors increase
the minimum focus size.
2)
As the distance between the laser and
the objective lens changes, the focus spot size also changes. The only way to
maintain spot size is to keep the optics fix, and move the material. For large
objects this may be difficult or impossible. All of these makes the
conventional beam delivery system a difficult one.
3)
To overcome these difficulties laser
beam delivery through optical fiber gains the importance, which has ideal characteristics as given below:
§
Constant beam diameter over a range of
distance
§
Flexibility in positioning the focused
point
§
Complete enclosure of the beam, for
safety reasons
WHAT
IS OPTICAL FIBER?
An optical
fiber 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 refraction. This construction allows light traveling through the
core at less than a critical angle( where the critical angle ac is
defined as arcsine (nc/nf), where nc is the
index of refraction of the core material, and nf is the index of
refraction of the cladding )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
angle 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( 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]´) which is the sin of angle. For this fiber NA
is sin (12.8°), or 0.22.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.
§
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.
TYPES
OF OPTICAL FIBERS:
There are mainly
two types of optical fibers:
1)
Singlemode fiber:
This fiber has a small core diameter typically 5/125µm core/cladding. The light follows a path straight down the middle of the fibers core. It is typically used for long distance telecommunications.
This fiber has a small core diameter typically 5/125µm core/cladding. The light follows a path straight down the middle of the fibers core. It is typically used for long distance telecommunications.
This fiber has
a large core diameter typically 50µm. This fiber will propagate hundreds of
different modes at the same time. Functionally this fiber can be thought of as
a light pipe and is used for high power delivery and light gathering
applications.
FIBER OPTIC BEAM DELIVERY SYSTEM:
A Fiber Optic
Beam Delivery (FOBD) System includes more than the optical fiber.
The system
includes three additional subsystems:
1)
Input Coupling Optics
2)
Fiber End Connections
3)
Output 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 beam), 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-number (where f-number of a lens or optical system
is the ratio of its focal length to the diameter of its clear aperture) must be
low enough to collect all of the radiation leaving the fiber. 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.
ADVANTAGES
1)
A fiber delivery system enables
instrument designers the ability to place the laser in a practical location and
deliver the beam of light via optical fiber.
2)
Optical fiber beam delivery system is a
flexible system in which any surface of the workpiece can be machine without
moving the workpiece.
3)
Single-mode spatially filters the laser
beam, suppresses the higher-order transverse modes and allows only the lowest
order mode of transmission. In this way,
it can be used to improve the quality of a laser beam without the need for
spatial filters.
4)
In fiber optical laser beam delivery
system complex arrangements of bulk optics and spatial filters are not needed
to produce the right beam quality at the right point. Hence bulky instrument
design and complex manufacturing, commissioning and servicing is avoided.
5)
Fiber optic beam delivery system can be
automated or combined with robot technology.
6)
The high brightness and high beam
quality required for micro-machining can be easily achieved by using large core
optical beam delivery system.
APPLICATIONS
1)
Optical fiber delivery system has wider
applications in advanced micro-machining system due to its flexibility.
2)
Optical fiber delivery system is widely
used in telecommunication field.
3)
This system is also applicable in
medical field like medical digonestics, photodynamic theory.
4)
Optical fiber delivery system is used
in recent laser lightning shows.
CONCLUSION
It can clearly
be seen that laser beam delivery system has particularly attractive advantages
over conventional beam delivery system in most respects.. The greatest advantages of it are that there
is no laser divergence and flexibility in positioning the laser beam. This ability to be so flexible is desirable
in today’s world, especially in the telecommunication and medical field where
flexibility is a desirable quality to have.
The future of laser beam delivery through optical fiber holds great promise,
especially the advancing fields of science, such as micro-machining, where
lasers might be useful in creating cuts in materials to greater accuracies than
could even possibly be imagined with conventional methods.
REFERENCES:
1.
Sterling
2.
Marcuse, Dietrich, Theory of
Dielectric Optical Waveguides, Second Edition, Academic Press, 1991.
3.
www.fiber_optics.org
4.
www.laserist.org
5.
www.motionnet.com
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