Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02602997 2007-09-19
LASER-BASED ABLATION METHOD AND OPTICAL SYSTEM
FIELD OF THE INVENTION
The present invention relates to the field of micro-machining and more
particularly
concerns an ablation method and optical system based on a low-cost laser,
which
can for example be used for cleaving or striping optical fibers.
BACKGROUND OF THE INVENTION
Laser micro-machining is an advantageous technology for the precision shaping
of
lo a variety of small objects, especially for optical fibers and other
waveguides or
optical components. In the particular case of optical fibers, micro-machining
techniques are often required to cleave the fiber (remove an end section) or
stripe
it (remove a portion of the cladding). CO2 lasers or the like are often used
in this
context.
One drawback of laser-based methods for cleaving or striping fibers is that a
portion of the laser energy absorbed at the fiber surface is diffused within
the fiber
through thermal conduction, resulting in a greater volume of material being
heated.
The volume elements at the surface are vaporised, but a significant amount of
the
2o remaining material is transformed into a liquid phase or has a low
viscosity which
results in deformations in the fiber. Under these conditions, the extremity of
the
fiber tends to take a rounded form under the effect of surface tensions.
For example, it is known in the art to cleave optical fibers using a laser
lathe, in
which the fiber is rotated while exposed to a high power laser beam. Systems
of
this type are shown in U.S. patent no. 4,710,605 (PRESBY) and European patents
no. EP0391598B1 and EP0558230B1. As mentioned above, one drawback of this
approach is that the fiber tends to be overheated, which has the negative
effects of
rounding the edges of the fiber, causing its end to "flare", i.e. enlarge its
diameter
3o beyond its nominal value, and cause a diffusion of the dopants which define
the
core of the fiber.
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Also known in the art is U.S. patent application no. US 2004/0047587 Al
(OSBORNE). Osborne teaches a cutting method and apparatus for optical fibers
and waveguides, using a stationary laser beam. Side and top schematized views
of the interaction of the laser beam 22 with the fiber 20 for this technique
are
respectively shown in FIGs. 1A and 1 B (PRIOR ART). The spatial intensity
profile
of the laser beam is optimized so as to obtain a sharp cutting edge of
sufficient
intensity to vaporise the matter to be cut through ablation. In order for this
method
to be efficient, it is required for the laser to have a significantly high
peak power, as
to the laser energy is spread over a relatively large area. As can be seen in
FIG. 1 B,
the laser peak power can be maximized by a good focalisation of the beam in
the
horizontal plane (in the plane of the page).
U.S. patent application no. US 2005/0284852 Al (VERGEEST) also teaches of a
laser-based technique for cutting optical fibers and the like. In the
disclosed
method, a laser beam is produced, either in continuous wave or forming very
short
pulses with steep edges, with sufficient peak energy to ablate matter from an
optical fiber or waveguide to be cut. The laser beam and fiber are moved
relative
to each other to operate the cut. FIGs. 2A and 2B (PRIOR ART) schematically
illustrate this method, respectively showing a side view and a top view and
the
interaction of the laser beam 22 with the fiber 20 for a technique of this
type. As
with the method of OSBORNE, the beam can be focalised in the horizontal plane
to maximise its peak power. However, it is here also focalised in the vertical
plane
as the beam is moved over the section of the fiber as opposed to being spread
over it.
Although the techniques of the two last prior art documents mentioned above
may
provide good quality cuts where thermal effects are reduced, they both
necessitate
the use of expensive high power cutting lasers in order to achieve those
results.
3o There is therefore a need for a less expensive method and apparatus which
allow
similar results to be obtained.
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SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a
laser-based
method for the ablation of volume elements across a section of a target
object.
The method includes the following steps of:
a) generating a light beam using a CO2 laser. The light beam forms long
pulses, each having a temporal shape defined by at least a rise time and a
plateau following the rise time, the light beam having a generally constant
peak power during the plateau;
b) moving the light beam across the section of the target object, this moving
being synchronized with the long pulses so that the light beam intersects
each volume elements of the section of the target object in synchronization
with the plateau of one of the long pulses of the light beam, thereby at least
partially ablating these volume elements through exposition to the peak
power; and
c) repeating step b) until the ablation is completed.
In accordance with another aspect of the present invention, there is also
provided
an optical system for the ablation of volume elements across a section of a
target
object.
The system first includes a CO2 laser for generating a light beam, this light
beam
forming long pulses, each having a temporal shape defined by at least a rise
time
and a plateau following the rise time. The light beam has a generally constant
peak power during the plateau. The system further includes moving means for
moving the light beam across the section of the target object. There are also
provided synchronizing means for synchronizing this moving with the long
pulses
so that the light beam intersects each volume elements of the section of the
target
object in synchronization with the plateau of one of the long pulses of the
light
3o beam. Thereby, the volume elements are at least partially ablated through
exposition to the peak power.
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The present invention may advantageously be used to cleave or stripe optical
fibers or the like, with minimal thermal effects, while using components of
lower
cost than for prior art equivalent systems.
Other features and advantages of the present invention will be better
understood
upon reading of preferred embodiments thereof with reference to the appended
drawings.
io BRIEF DESCRIPTION OF THE DRAWINGS
FIGs. 1A and 1 B(PRIOR ART) are respectively a side and a top schematic view
of the cleaving of an optical fiber using a first prior art method.
FIGs. 2A and 2B (PRIOR ART) are respectively a side and a top schematic view
of the cleaving of an optical fiber using a second prior art method.
FIG. 3 is a graph illustrating the relative intensity as a function of time
for laser
beams defining short and long pulses or in continuous wave mode.
2o FIG. 4 schematically illustrates the moving of a light beam according to
one aspect
of the present invention.
FIGs. 5A, 5B and 5C are respectively a side, a top and a front schematic view
of
the cleaving of an optical fiber using a method according to an embodiment of
the
present invention.
FIG. 6 is a diagram showing a system according to an embodiment of the
invention.
3o FIGs. 7A, 7B and 7C are schematic representations of variants of rotating
mirrors
for use in a system according to embodiments of the present invention.
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FIG. 8 is a schematic side view illustrating a method for cutting through an
optical
fiber according to one embodiment of the invention.
5 FIGs. 9A and 9B are schematic side views of the striping of an optical fiber
according to another embodiment of the present invention.
FIG. 10A schematically shows a non-symmetrical spatial profile of the light
beam
according to one embodiment of the invention; FIG. 10B shows the corresponding
io local temporal shape of the light beam intersecting each volume element of
the
optical fiber.
FIGs. 11A and 11B are side and front views, respectively, of a rotating disk
bearing a focussing lens according to an embodiment of the invention.
FIGs. 12A and 12B are front views of a rotating disk on which a plurality of
lenses
is mounted, respectively equidistant from the center of rotation of the disk
and at
different distances therefrom.
2o FIG. 13 is a schematic side view illustrating a method for striping an
optical fiber
according to one embodiment of the invention.
FIGs. 14A and 14B are side and front views, respectively, of a rotating disk
bearing a mirror according to an embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
In accordance with an aspect of the present invention, a CO2 laser, preferably
of
the type known as sealed RF-excited waveguide CO2 lasers, is used for the
ablation of volume elements across a section of a target object. Although the
present description will refer to the cleaving or striping of optical fibers
as
examples of applications of the present invention, it will be readily
understood by
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one skilled in the art that the invention could be used in a variety of
different
contexts such as removing paint or another coating from a small object,
removing
acrylic from a LED package, making grooves in a glass piece, polishing glass,
etc.
CO2 lasers are advantageous tools for micro-machining applications in
consideration of their cost, durability and ease of use. However, one
disadvantage
of the use of such devices in this context is that in order to attain their
maximum
available peak power, they require a substantial rise time, of the order of 50
to 100
ps. In addition, it is only possible to benefit from the maximum peak power
for a
io relatively short time, between about 10 ps and 1000 Ns.
This characteristic of CO2 lasers is best understood with reference to FIG. 3.
As
can be seen, to maximize the power of the laser, a long pulse 24 has to be
produced with a significant rise time, shown here it to be of about 100 ps. In
order
to produce a short pulse 26 using the same laser, the rise time has to be cut
short,
resulting in a much smaller peak power of the short pulse 26 produced.
Alternatively, the same laser can be used in CW (Continuous Wave) mode,
producing a beam of constant power 28 which is still less than the available
peak
power.
In the prior art discussed above, such as the OSBORNE and VERGEEST patent
applications, it is known to use such lasers either in short pulse or CW mode.
Accordingly, the selected lasers need to be powerful enough so that the peak
power obtained under such conditions is sufficient to ablate the fiber
material while
avoiding or limiting heat diffusion. By contrast, the present invention
provides a
method and apparatus allowing the use of a CO2 laser in long pulse mode,
therefore requiring a less powerful laser to obtain a similar usable peak
power.
The maximum available power of the laser in long pulse mode can be anywhere
between about 25 W and 1000 W.
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With reference to FIG. 4, the method of the present invention includes a first
step
of generating a light beam 22 using a CO2 laser. The light beam 22 forms long
pulses 24. In the illustrated embodiment, each long pulse has a substantially
rectangular temporal shape defined by a rise time 30, a plateau 32 following
the
rise time 30, and a fall time 34. It will however be understood by one skilled
in the
art that the long pulses 24 need not have such a straightforward shape but
could
include various power variations, as long as their temporal shape includes a
significant rise time 30 followed by a plateau 32, the light beam having a
generally
constant peak power during this plateau. The peak power of the light beam 22
io during the plateau 32 preferably corresponds to a maximum available power
ImaX of
the CO2 laser.
The method then includes a step of moving the light beam 22 across the section
of
the target object to be ablated, which is embodied by the extremity 21 of an
optical
fiber 20 in the embodiment of FIG. 4. The moving of the light beam 22 is
synchronized with the long pulses 24 so that the light beam 22 intersects each
volume element of the optical fiber 20 in synchronization with the plateau of
one of
the long pulses of the light beam 22. This is best understood by comparing the
position of the light beam 22 shown at the bottom of FIG. 4 with the intensity
of the
long pulse in each case. At point A in time, the rise time 30 of the long
pulse 24
begins and the light beam 22 is projected away from the extremity 21 of the
fiber
20. It remains so until at least point B where the rise time 30 ends and the
plateau
32 begins. Some time during this plateau 32, between points B and D, the light
beam 22 makes a passage across the extremity 21 of the fiber 20. This is
illustrated at point C. During this passage, each volume element of the
extremity of
the fiber "sees" a short effective pulse 36 having a peak power equal to that
of the
long pulse 24, and a pulse width corresponding to the interaction time between
the
light beam 22 and the corresponding volume element. The peak power is selected
to be sufficient to at least partially ablate these volume elements. By the
time point
3o D is reached, the light beam 22 is again directed away from the extremity
21 of the
fiber 20, and remains so for the entire duration of the fall time 34 and
beyond, as
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illustrated with respect to point E. This step can be repeated with subsequent
long
pulses until the required ablation is completed.
For a same laser, the above approach provides a power gain of a factor of
about 2
to 5 when compared to using the laser in CW mode and of about 3 to 10 in short
pulse mode.
Referring to FIGs. 5A to 5C, a preferred geometry for the light beam 22 used
in the
method above will now be discussed. To assist in this description, a xyz
i o coordinate system has been provided on FIGs. 1A, 1B, 2A and 2B (all PRIOR
ART) as well as on FIGs. 5A to 5C wherein the z axis represents the
propagation
axis of the light beam 22, and the light beam's cross-section is in an xy
plane
wherein the x and y axes are respectively perpendicular and parallel to the
endmost surface of the extremity 21 of the optical fiber 20. It will of course
be
understood that this coordinate system is presented for ease of reference only
and
is in no way considered to be limitative to the scope of the invention.
In the prior art, the cross-section of the light beam used for micro-machining
is
either circular as in the VERGEEST patent application (see FIG. 2A), or
elliptical
2o as in the OSBORNE patent application (see FIG. 1A). OSBORNE uses an
elliptically-shaped light beam in order for the beam to be large enough to
cover the
entire section of the fiber without any relative movement between the two. The
elliptical profile of the beam in the OSBORNE application therefore has a
short
axis perpendicular to the fiber extremity (x axis in FIG. 1A) and a long axis
parallel
to the fiber extremity (y axis).
In the preferred embodiment of the invention, the light beam 22 also has an
elliptical profile, but the long and short axes defining this profile are
inverted with
respect to the prior art of FIG. 1A. This is best seen in FIG. 5A. The short
axis is
therefore aligned collinearly with the movement of the light beam 22 as
described
above (both along the y axis), and the long axis is aligned perpendicularly to
this
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movement (along the x axis). The generation of a light beam having different
focalisation parameters along its two axes is well known in the art and can be
obtained through the use of appropriate focusing optics.
The level of focalisation of the light beam 22 along its long and short axes
is
dictated by the practical requirements of the targeted micro-machining
application.
In the current example of the cleaving of an optical fiber, it will be
understood that
the focalisation along the long axis must be sufficient to concentrate the
laser
intensity as much as possible, while not so strong as to result in a beam
io divergence which would preclude a straight cut. An appropriate compromise
should be sought, as for example shown in FIG. 5B. Along the short axis,
however, as can be seen in FIG. 5C, no compromise is necessary to ensure a
straight cut as this is accomplished by the movement of the light beam 22. The
beam can therefore be compressed as much as allowed by the focussing optics.
1s This particular approach allows an intensity gain at the fiber surface by a
factor of
about 2 to 5 when compared to a circular light beam, and by a factor of 5 to
20
when compared to an elliptical beam aligned along the other direction as for
example shown in FIG. 1A.
20 In accordance with alternative embodiments, the spatial profile of the
light beam
can be given a different shape, which need not be symmetrical. As will be
readily
understood by one skilled in the art, the spatial profile of the light beam
will directly
determine the temporal shape of the impulsion "seen" at each volume element of
the target object. An example of a non-symmetrical spatial profile 60 is shown
in
25 FIG. 10A, and the resulting local temporal shape 62 of the light beam
intersecting
each volume element is shown in FIG. 10B. In this particular case, the spatial
profile 60 of the light beam 22 has been designed to generate a low intensity
tail
64 in the corresponding local temporal shape 62, which can be useful to reduce
the thermal shock sometimes produced by exposure to a brief and intense pulse.
30 Of course, other spatial profiles, symmetrical or otherwise, could be used
depending on the circumstances and on the desired result.
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in
Referring now to FIG. 6, and according to another aspect of the present
invention,
there is provided an optical system 40 for the ablation of volume elements
across
a section of a target object such as an optical fiber 20.
The system 40 first includes a CO2 laser 42, which is preferably of the type
known
as sealed RF-excited waveguide CO2 lasers. The laser 42 generates a light beam
22. As explained above, the light beam 22 forms long pulses, each long pulse
having a temporal shape which includes a rise time, preferably of about 50 ps
to
io 100 ps, followed by a plateau, preferably of about 10 ps to 1000 ps. The
light
beam 22 has a generally constant peak power during the plateau, which can for
example be of the order of 25 W to 1000 W. The laser 42 is preferably
controlled
by a!aser control circuit 43.
The system 40 also includes moving means for moving the light beam 22 across
the section of the optical fiber 20 to be ablated. In the embodiment of FIG.
6, a
rotating mirror 44 is positioned in the path of the light beam 22 for this
purpose.
Preferably, the mirror 44 is rotated at a relatively constant speed in order
to avoid
having to fight its inertia. For example, a rotational speed of the order of
1000
2o RPM would be appropriate for a 2 inches (about 5 cm) mirror. Attainable
angular
speeds are advantageously greater with this approach than with a galvanometer
of
similar dimensions, although such a moving means could still be considered
within
the scope of the present invention. An appropriate support (not shown) is
provided
for rotating the mirror 44.
Several variants of a rotating mirror 44 are shown in F!Gs. 7A to 7C.
Referring
particularly to FIG. 7A, it is shown how the clockwise rotation of the mirror
44 has
the consequence of moving the resulting light beam 22 downward (within the
plane of the page). The mirror 44 can have a single or several usable mirror
faces
3o 46a, 46b, (...), and by way of example, FIGs. 7B and 7C respectively show
rotating mirrors having four and six such mirror faces 46. Increasing the
number of
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usable mirror faces 46 has the advantage of increasing the efficiency of the
ablation process using the system of the present invention. In accordance with
a
variant of this embodiment of the invention, different faces of a multi-face
mirror
could be "tilted" with respect to one another so that consecutive passages of
the
light beam 22 at the fiber 20 are along different optical paths intersecting
different
volume elements of the fiber 20. This is for example schematically illustrated
in
FIG. 8. This particular approach could be useful for cleaving fibers of a
large size,
as the light beam cuts a larger path in the fiber and can penetrate deeper
within
the fiber material. This approach also has the advantage of avoiding a too
intense
io local heating of a given volume element.
In accordance with alternative embodiments, the moving means may be embodied
by moving one or several optical elements across the path of the light beam.
The
optical elements may be reflective, refractive or diffractive or combinations
thereof.
Referring to FIGs. 11A and 11 B, there is shown such an embodiment where the
optical element is embodied by a focussing lens 66 mounted on the surface of a
rotating disk 68. The rotation of the disk 68 will bring the light beam 22 in
and out
of alignment with the lens 66. It will be noted that the use of such a device
will give
the resulting light beam projected towards the target object a slightly curved
trajectory, but that for most application this curvature may be disregarded. A
similar device where the lens 66 is replaced by a mirror 72 is shown in FIGs.
14A
and 14B.
A plurality of lenses 68 or other optical elements may be mounted on a single
rotating disk 68, increasing the number of passes the light beam 22 can make
along the target object for each full rotation of the disk 68. Referring to
FIG. 12A,
there is shown such a disk where 8 lenses are mounted. It will be noted than
for a
large number of optical elements, such as for example 8 and up, the rotating
disk
68 and lenses 66 of FIG. 12 A will generally be easier to manufacture than a
multi-
facet mirror according to the embodiment of FIG. 7C. Referring to FIG. 12B, I
a
variant of the embodiment of FIG. 12A, the lenses 66 may be mounted on the
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rotating disk 68 at different specific distances from the center of rotation
70 of the
rotating disk 68. In this manner, the light beam may be directed along
multiple
trajectories so as to intersect the target object at different locations. This
approach
may be particularly advantageous for some ablation operations, such as for
example for the striping of an optical fiber. FIG. 13 illustrates how a
rotating disk of
the type shown in FIG. 12B may be used to increase considerably the striping
speed of an optical fiber 20 by projecting the light beam 22 along multiple
trajectories.
io Referring back to FIG. 6, the system 40 according to the present embodiment
of
the invention further includes synchronizing means for synchronizing the
movement of the light beam 22 with the temporal shape of its long pulses. This
synchronization is done in such a manner that the light beam 22 intersects
each
volume element of the section of the optical fiber 20 in synchronization with
the
ts plateau of one of the long pulses of the light beam 22, as explained above.
In this
manner, each volume element of the optical fiber is exposed to the peak power
of
the laser 42 for a short time and at least partially ablated by this exposure,
while
minimizing heat diffusion within the fiber. The synchronizing means preferably
include an encoder 48 receiving signals from the mechanism rotating the mirror
44
20 or rotating disk, if provided, and a processor such as computer 50 in
communication with both the laser control circuit 43 and the encoder 48. In
this
manner, the processor can provide control signals to synchronize the laser
pulses
with the rotation of the mirror 44 or other optical element and to adjust the
rotation
speed according to the desired processing parameters.
As will be well understood by one skilled in the art, the optical system 40
may
further include any appropriate beam shaping optics 52 in the path of the
optical
fiber 22 as deemed required by the characteristics and geometry of a given
practical embodiment of this system. In the embodiment of FIG. 6, the beam
shaping optics 52 is shown to include components 52 between the laser 42 and
the rotating mirror 44, as well as a lens 54 downstrearn the rotating mirror
44.
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Preferably, the beam shaping optics is selected to shape the light beam 22 at
the
optical fiber 20 according to an elliptical profile defining a short axis and
a long
axis. As explained above, it can be advantageous to align the short axis
collinearly
to the direction of the moving of the light beam and the long axis
perpendicularly
thereto, as shown in FIG. 5A. In this configuration, the cylindrical lens 54
can focus
the light beam to the diffraction limit allowed thereby without any
consequence on
the straightness of the cut.
It will be understood by one skilled in the art that the system and method of
the
io present invention are not limited to making cuts at a right angle. By
changing the
relative angle of the light beam and the optical fiber, different cutting
planes can be
obtained. It is also possible to shape the extremity of the fiber along
multiple
planes, so as to form a two-face roof of a pyramidal shape, for example. By
slowly
turning the fiber on itself during the passage of the beam, a conical form can
also
be obtained.
Referring to FIGs. 9A and 9B, there is shown the use of a method and system
for
stripping an optical fiber, that is, removing a jacket 56 thereof, according
to
another embodiment. This is simply accomplished by sweeping the light beam
2o across the fiber as with the method explained above. The fiber can be move
longitudinally during this operation to remove the desired portion of the
jacket
therealong. It will be noted that mid-span stripping were experimentally
performed
using the technique on SMF28 fibers and tensile strength of 400 kPSI on
average
were obtained.
Of course, numerous modifications could be made to the embodiments described
above without departing from the scope of the present invention as defined in
the
appended claims.
