CONFORMATEUR NUMERIQUE DE FIBRES A MICRO-LENTILLE

DIGITAL FIBRE MICROLENS SHAPER

Abrégé anglais

This invention relates to the microshaping of a tensed optical fibre such that
the coupling of
light from a source of laser light into the optical fibre, or between the
optical fibre and another
optical component, is optimized and, particularly, to the microshaping of such
tensed fibres in a
digital process whereby the coupling of light into or from the fibre is used
directly to control the
microshaping process.

Revendications

CLAIMS
1. ~A method for modifying the shape and finish of the lens of a lensed
optical fibre or
waveguide by precise and localized application of heat to the tip of the lens.
2. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claim 1, in
which localized heating is applied using an electrical discharge between two
or more
electrodes located in proximity to the lens.
3. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claim 1, in
which localized heating is applied using focussed laser radiation.
4. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claims 1 and
3, in which the laser providing localized heating is a carbon dioxide laser.
5. ~A method for modifying the lens of a leased optical fibre or waveguide as
in claims 1 to 4,
in which the electrical discharge or laser radiation is pulsed such that the
treatment of the
lens tip proceeds in controlled steps, with pulse durations of 1 to 100
microseconds and
intervals of 1 microsecond to several seconds between treatment pulses.
6. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claims 1, 2
and 5, in which the electrical discharge has an amplitude of between 1 and to
500
milliamperes.
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7. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claims 1 to 6,
in which light coupled to the optical fibre is monitored while the
modification is in process
and the monitored light is used to control the degree of lens modification.
8. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claim 7, in
which the near-field or the far-field distribution of light emerging from the
fibre is
monitored and modification of the lens is continued until such distribution
matches the
distribution of light from the source to which the leased fibre or waveguide
is to be
coupled.
9. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claim 7, in
which light reflected internally into the leased fibre or waveguide is
monitored as a
surrogate for the optical distribution of the lens, to allow control of the
degree of
modification of the lens.
10. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claims 1 to 6,
in which the focal length of the fibre lens is adjusted to a desired length.
11. ~A method for modifying the lens of a leased optical fibre or waveguide as
in claims 1 to 6,
in which the lens is smoothed or finished so as to reduce side lobes of
radiation and
background noise in the optical field.
12. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claims 1 to 6,
in which metallization of the fibre is removed in the first cycles of
treatment of the lensed
fibre or waveguide.
13. ~A method for modifying the lens of a lensed optical fibre or waveguide as
in claims 1 to 7,
in which the modification process is carried out in-situ to maximize coupling
between the
leased optical fibre or waveguide and the device to or from which light is
being coupled.
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14. ~An apparatus for modifying the lens of a lensed optical fibre or
waveguide in which heat is
applied precisely and locally using a controlled electrical discharge the
vicinity of the tip of
the lens such that the lens is modified in a controlled manner to optimize
coupling between
the lensed fibre or waveguide and a light source or other optical component or
device.
15. ~An apparatus for modifying the lens of a lensed optical fibre or
waveguide as in claim 14,
in which localized heating is applied using a controlled electrical discharge
in the vicinity
of the tip of the lens.
16. ~An apparatus for modifying the lens of a lensed optical fibre or
waveguide as in claim 14,
in which localized heating is applied using focussed laser radiation.
17. ~An apparatus for modifying the lens of a leased optical fibre or
waveguide as in claim 16,
in which the laser providing localized heating is a carbon dioxide laser.
18. ~An apparatus for modifying the lens of a lensed optical fibre or
waveguide as in claim 15 or
16, in which the electrical discharge or laser light is pulsed such that the
treatment of the
lens tip proceeds in controlled steps, with pulse durations of 1 to 100
microseconds and
intervals of 1 microsecond to several seconds between treatment pulses.
19. ~An apparatus for modifying the lens of a lensed optical fibre or
waveguide as in claims 15
and 18, in which the electrical discharge has an amplitude of between 1 and
500
milliamperes.
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Description

CA 02413050 2002-12-23
PHOTONOVA INC.
December 19, 2002
DIGITAL FIBRE MICROLENS SHAPER
Fedor Timofeev and Raman Kashyap
DISCLOSURE
BACKGROUND OF THE INVENTION
The efficient coupling of light to and from optical devices such as laser
diode sources, planar
optical waveguides, semiconductor optical gain devices, photodiodes or bulk
spatial optics into
cylindrical optical fibres is of great conunercial importance. The coupling is
often compromised
by differences in the spatial optical field shapes and spot sizes between the
devices being
connected, for example coupling of elliptical optical field shapes from a
semiconductor light-
emitting source to the circular geometry of an optical fibre.
Further, lenses manufactured on optical fibres or waveguides typically result
in non-
uniformities in the optical field of the lenses due to stresses, scratches,
deformations,
contaminants or other types of defects.
It is well known that the coupling of light from a source can be improved by
using either bulk
lenses or lenses integrated on the tips of optical fibres or waveguides, and
many designs and
mounting arrangements for such lenses have been described (see for example
U.S. Patent No.
5,764,838).
Practical methods of improving the coupling of laser light into optical fibres
have been
developed based on forming microlenses on the ends of the fibres into which
the light is to be
coupled. U.5. Patent No. 5,455,879 describes use of a wedge-shaped lens to
improve the
coupling of light from an elliptical shaped radiation source such as a laser
diode into a
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cylindrically symmetrical optical fibre. Such integrated lens designs have the
additional benefit
that reflectivity is reduced, further improving coupling. The degree of
coupling efficiency that
can be achieved in this approach is seriously limited, however, as the field
that would be
efficiently coupled into a wedge-shaped lens would be bi-modal owing to the
geometrical ray
paths, while the field of the source is typically elliptical.
Variants of the wedge-shaped design have been described, for example in U.S.
Patents Nos.
6,317,550, 6,301,406, 5,845,024, 5,256,851 and 5,101,457. In U.S. Patents Nos.
6,317,550 and
6,301,406, it is disclosed that a small flat section at the tip of a wedge-
shaped lens can sharply
improve the efficiency of coupling of light from an elliptical laser source
into a leased optical
fibre, with the improvement being most marked when this flat section has
dimensions on the
order of a few microns. With such a flattened lens tip, however, back-
reflection by the lens is
seriously increased and can be as much as 4% or higher. This can have a
deleterious effect on
the operation of a laser source. In U.S. Patent No. 5,256,851, a method is
disclosed for
micromachining of the tip of a leased fibre by laser ablation. However, in
this method the laser
energy penetrates deeply into the lens, and it is very difficult to control
introduction of defects
into the glass. U.5. Patent No. 5,845,024 discloses a method of mechanical
grinding of the tip of
a wedge-shaped lens integrated on an optical fibre, to provide a roughly
circular profile. This
has the disadvantage of being mechanically complex and diffcult to control,
while in addition
grinding leaves fine markings on the lens which reduce coupling efficiency,
increasing scattering
losses and back reflection. U.S. Patent No. 5,101,457 discloses a method of
microshaping of a
lens on the tip of an optical fibre by chemical etching. This process,
however, is complex and
difficult to control. Further, it and cannot be used in-situ and can result in
introduction of
contamination into the glass, adversely affecting coupling.
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While such approaches do improve coupling of laser light into a tensed fibre,
they are
severely restricted since no two semiconductor light sources are identical,
and forniing of the
microlens tip to optimize light coupling is difficult. Further, the methods of
the prior art do not
provide means for actively monitoring the optical field of a fibre microlens
while it is being
shaped, with the consequence that efficient fabrication of microlenses having
high coupling
efficiency is not possible. Also, most prior art methods are complex and
costly, while providing
limited coupling efficiency due to limitations of design or to contamination
or defects resulting
from the fabrication process.
SUMMARY OF THE INVENTION
The present invention provides a novel method of modifying a tensed fibre to
maximize the
coupling of laser light into the fibre. This is accomplished by applying
localized heating to the
tip of the lens. This may be applied, for example, by a series of weak
electrical discharges or,
alternatively, by pulses of light from a tightly focussed laser beam. Such
modification can be
carried out in a controlled manner so as to allow precise maximization of the
coupling of light
from the source into the fibre. The method can also be used to modify a Tensed
fibre to optimize
the coupling of light leaving the fibre to enter another fibre or device. An
apparatus is described
that can accomplish such microlens modifications.
The object of the invention may be achieved by applying a controlled
electrical discharge or
laser light to the tip of the Tensed fibre, while monitoring the coupling of
light from the light
source into the fibre. In a preferred embodiment of the invention, the
discharge or laser light
treatment is applied digitally, in short pulses that allow the change in
coupling efficiency to be
precisely controlled.
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CA 02413050 2002-12-23
In an alternative embodiment, the near- and/or the far-field distribution of
light from the light
source is mapped, and the fibre lens is then modified such that the
distribution of light leaving
the lensed fibre precisely matches this near- and/or far-field distribution
map.
In a further embodiment of the invention, the reflection of light launched
into the fibre from
the microlens is monitored during treatment, and this reflection is calibrated
against the change
in the optical field of the lens such that its value can be used to control
the microshaping of the
lens.
In another embodiment, the invention may be used to minimize or eliminate the
non-
uniformities in the optical field of lensed optical fibres due to stresses,
scratches, deformations,
contaminants or other types of defects.
DESCRIPTION OF THE DRAWINGS
Fig. la is a graphical representation of the far-field optical power intensity
of laser light at
1500 nm emitted from a fibre having an integrated wedge-shaped lens. The wedge
has a 63
degree angle and is polished to a 0.1 micrometer finish.
Fig. 1 b is a graphical representation of the angular distribution of optical
power emitted at
980 nm from a lensed fibre having a 51 degree single wedge, in which power is
measured as a
fimction of angle from the centreline of the fibre, perpendicular and parallel
to the flat of the
wedge.
Fig. 2a is a schematic representation of a typical arrangement used for
microlens shaping
using an electrical discharge by the method of this invention.
Fig. 2b is a schematic representation of a typical arrangement used for
microlens shaping
using a focussed laser beam by the method of this invention.
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CA 02413050 2002-12-23
Fig. 3 is a schematic representation of the tip of a Tensed fibre, indicating
for this
embodiment the area in which microlens shaping occurs.
Fig. 4a is a graphical representation of the far-field optical power intensity
of laser light at
1500 nm emitted from the fibre of Fig. la, following treatment of the
integrated wedge-shaped
lens by the method of this invention.
Fig. 4b is a graphical representation of the angular distribution of optical
power from a Tensed
fibre at 980 nm, in which power is measured as a function of angle from the
centreline of the
fibre, perpendicular and parallel to the flat of the wedge following treatment
with 76 discharge
pulses by the method of this invention.
Fig. 5 is a graphical representation of the angular distribution of optical
power from a
semiconductor laser source in comparison with the far-field distribution of
light emerging from a
Tensed fibre in which the lens has been modified by the method of this
invention to match the
two profiles.
Fig. 6 is a graphical representation of the variation of the reflectivity of
light launched into
the Tensed fibre of Fig. 5, and reflected from the internal surface of the
lens, as a function of the
number of discharges applied during modification of the lens.
Fig. 7 is a graphical representation of the variation of the losses in
coupling of light that has
been launched into a Tensed fibre, as it exits through the lens and is
reflected back to couple into
the fibre, as a function of the distance of the mirror from the tip of the
fibre lens.
Fig. 8 is a graphical representation of the logarithmic amplitude of the
optical field of light
emerging from a conical pulled lens on the tip of an optical fibre, before
treatment and following
two discharge treatments by the method of this invention.
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DETAILED DESCRIPTION OF THE INVENTION
Figs. 1 a and 1 b present measurements obtained with a single wedge-shaped
lens that has
been fabricated on the end of an optical fibre. Such lenses are produced, for
example, by
polishing as described in U.S. Patent No. 5,455,879. In Figs. la and lb, the
light-coupling
characteristics of such lenses are characterized by passing laser light
through the optical fibre
such that it exits through the wedge-shaped lens. Fig. la shows the far-field
distribution of
optical intensity of laser light having a wavelength of 1500 nm emerging from
such a lens; this
distribution shows a dark spot at its centre. The distribution clearly has two
lobes. Thus,
coupling of light into an optical fibre through such a lens would be
inherently inefficient, as no
practical laser light sources or other waveguiding devices have such a two-
lobed distribution of
output light intensity.
Fig. lb shows the optical power measured in a similar test using light having
a wavelength of
980 nm passed through a fibre having a single 51 degree wedge at the end.
Angular distribution
of power measured horizontal to the flat surfaces of the lens (curve 1) is
symmetrically
distributed about the centerline of the fibre. However, optical power measured
perpendicular to
the flat surfaces of the lens (curve 2) shows a strong minimum at the
centerline.
While it is known that modification of the tip of a fibre having a wedge-
shaped lens over a
dimension of a few microns from the centerline can improve coupling to a
semiconductor laser
light source, as disclosed for example in U.S. Patents Nos. 5,845,024 and
6,301,406, the
fabrication of such geometries requires precise and costly procedures and
optimization of
coupling of light from the source to the optical fibre is difficult.
The present invention discloses a method of modifying a Tensed fibre in a
controlled manner
such that it provides optimal coupling of light between an optical fibre and a
laser source. In one
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CA 02413050 2002-12-23
embodiment, the near- and/or far-field distribution of light from the
semiconductor laser source
is measured, and the Tensed fibre is then modified to have a matching near-
and/or far-field light
distribution. Alternatively, back reflection of light launched into the fibre
may be monitored as a
method of controlling the microshaping of the fibre lens. The invention allows
shaping to be
carried out on metallized fibres without previous demetallizing or cleaning,
as the fibre in the
immediate vicinity of the lens, within for example 5 to 20 microns, may be
demetallized and/or
cleaned in the first few cycles of the treatment.
Fig. 2a is a schematic representation of one embodiment of the arrangement
used to realize
the lens modification of this invention. An optical fibre (3) which may have a
core (4) has an
integral single wedge-shaped lens (5). The core (4) could for example have a
diameter of 2 to 10
microns, while the fibre could have an overall diameter on the order of 125
microns. The
cladding could be a single layer, or could be fabricated with two or more
layers and both the core
and the cladding could have refractive indices which are graded in the radial
direction. The
optical fibre (3) may be encapsulated in a protective glass or polymer or
other coating, and it
may be metallized for soldering or other purposes. The lens (5) by which the
fibre is terminated
could be fabricated by polishing, etching, drawing, or any other known method,
and it could be
wedge-shaped or of any other shape suited to the application for which it is
intended.
In the embodiment of the invention of Fig. 2a, an electrical discharge is
established between
two electrodes positioned near the lensed tip of the fibre ( 1 ). The
electrodes (6a and 6b) may be
of tungsten, graphite or any other suitable material capable of sustaining a
repeated electrical
discharge. Representative dimensions are shown in Fig. 2a, but these could be
adjusted by a
person skilled in the art, combined with selection of the electrical
parameters of the process, as
required to provide the required degree of processing. The electrical pulses
causing the electrical
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CA 02413050 2002-12-23
discharge between the electrodes (6a and 6b) may be of any suitable intensity
and duration, with
the geometry selected, for giving a stepwise change in the dimensions of the
tip of the lensed
fibre and thus of the coupling characteristics of the lens. For example,
pulses could be in the
form of a square wave or any other shape having typically an amplitude between
one and 500
milliamperes and duration on the order of 1 to 100 microseconds or more. Time
between pulses
is typically on the order of one tenth of a second but may be several seconds
or longer, and this
time may be controlled either automatically or by manually triggering the
treatment pulses.
Different types of materials used to make the optical fibre may require either
shorter or longer
duration discharges as well as greater or smaller discharge currents. It will
be evident to a person
skilled in the art that the precise geometrical and electrical parameters
necessary to achieve the
desired result will depend on humidity, atmospheric pressure, sharpness of the
lens shape, fibre
size, fibre type, ambient temperature and many other parameters. Any
combination of suitable
geometric and electrical parameters that achieves the objects of this
invention falls within its
scope.
Fig. 2b is a schematic representation of a second embodiment of the
arrangement used to
realize the lens modification of this invention. A laser beam (7) is focussed
by a lens or system
of lenses (8) such that the focussed beam (9) is incident on the tip of the
lens that is to be
modified. As for the embodiment of Fig. 2a, the laser light may be pulsed with
pulses of any
suitable intensity and duration suitable, with the geometry selected, for
giving a stepwise change
in the dimensions of the tip of the lensed fibre and thus of the coupling
characteristics of the lens.
Pulses could have a duration on the order of 1 to 100 microseconds or more,
and time between
pulses may be on the order of one tenth of a second or longer and may be
controlled either
automatically or by manually triggering the treatment pulses. Different types
of materials used
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CA 02413050 2002-12-23
to make the optical fibre may require either shorter or longer duration pulses
as well as greater or
smaller intensity of the treatment light. A carbon dioxide laser is well
suited to this application.
It will be evident to a person skilled in the art that the precise geometrical
and laser parameters
necessary to achieve the desired result will depend on humidity, atmospheric
pressure, sharpness
of the lens shape, fibre size, fibre type, ambient temperature and many other
parameters. Any
combination of suitable geometric and laser parameters that achieves the
objects of this invention
falls within its scope
Fig. 3 shows schematically the tip of a Tensed fibre at which shaping to be
carried out by the
method of this invention. The fibre may have a metallization coating (10).
This metallization
may for example be an electrolytically-deposited coating of a few microns of
nickel and a thin
flash of gold (less than 1 micron). Alternatively, it may be a vacuum
deposited coating such as,
for example, 50 nm of titanium, 100 nm of platinum and 200 nm of gold. All
such metallization
coatings in the region of the fibre tip can be removed precisely and locally
with application of a
single or a few electrical discharges or light pulses by the method of this
invention. The power
level is such that a first single or several discharges or light pulses do not
measurably affect the
glass of the fibre, but volatilize the thin metal coating. The first discharge
or light pulse, whether
or not the fibre is metallized, also serves to clean the surface of the lens
tip. Continuing
application of discharge or light pulses results in progressive modification
to the tip of the fibre
lens, for example in the region (11). This process of change is monitored and
controlled by
monitoring either the distribution of laser light exiting the Tensed fibre, or
the efficiency of
coupling of laser light into the fibre. The reflectivity of the microlens to
light launched into the
fibre may be used as a surrogate parameter to monitor the process.
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Fig. 4 illustrates the effect of this invention on the fibres used to obtain
the light distribution
results presented in Fig. 1. In Fig. 4a, the far-field distribution of 1500 nm
laser light is seen to
have changed after application of consecutive discharge pulses, eliminating
completely the two-
lobed distribution and giving an elliptical distribution which is typical of
the light distribution
from many commercial semiconductor laser sources.
In Fig. 4b, curve ( 12) is the angular distribution of optical power measured
horizontal to the
flat surfaces of the lens while curve (13) is the angular distribution of
optical power measured
perpendicular to the flat surfaces, both measured with laser light at a
wavelength of 980 nm
following treatment with 76 discharge pulses by the method of this invention.
Curve (13) may
be compared with curve (2) of Fig lb. The two-lobed distribution of far
optical field intensity
about the centerline of the fibre has been completely eliminated by the
treatment of the lens.
Fig. 5 illustrates graphically the application of the method of this invention
to matching the
coupling of a semiconductor laser to a tensed fibre. The curve ( 14) in this
figure is the
normalized far-field optical power distribution of a 980 nm semiconductor
laser source,
measured perpendicular to the long axis of the source. The curve (15) is the
far-field optical
power distribution of 980 nm wavelength light emerging from a Tensed fibre
which has been
modified by the method of this invention. The fibre in this case was a
Flexcore fibre having a
single wedge-shaped lens with a 51 degree interior half angle. Successive
discharges were
applied until the best match with the optical power distribution of the
semiconductor laser source
was obtained. The match is close, and the efficiency of coupling of light from
this laser into the
treated fibre is very high.
During modification of a tensed fibre by the method of this invention, it is
useful to monitor
the back reflection of light, which has been launched into the fibre,
reflecting from the internal
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CA 02413050 2002-12-23
surface of the lens. This reflectivity is a sensitive indicator of the degree
of modification of the
lens, and it changes continuously during the modification process, increasing
with the number of
discharges applied. Fig. 6 presents an example, for treatment of the Tensed
fibre of Fig. 5. The
reflectivity remains low, below 0.2%, during the treatment. It stays
relatively constant during the
initial discharge treatments, as fibre cleaning takes place and microscopic
distortions of the lens
tip are eliminated. Subsequently, the reflectivity increases monotonically
with the number of
discharges. The precise reflectivity values provide a useful and reproducible
indicator of the
degree of modification of the lens that has been achieved, and they can be
used for control
purposes in the application of the method of this invention.
The method of this invention can also be used to achieve a controlled
variation of the focal
length of a fibre microlens. Fig. 7 shows the variation of the coupling loss
for laser light
launched into a Tensed fibre and coupled back into the fibre following
reflection from a plane
mirror located in front of the lens. This coupling loss is plotted as a
function of the distance of
the reflecting mirror from the lens, so that the peak of the curve corresponds
to the focal length
of the microlens. The figure shows that the focal length of the lens can be
altered as the
microlens is treated by the method of this invention. Curve ( 16) shows the
focal length after 30
discharge treatments to be approximately 9 microns. After 38 discharge
treatments (curve 17)
the focal length is decreased to approximately 6.5 microns. In fact, the
method of this invention
may be used to modify a fibre microlens to have a desired focal length.
This invention may be used to clean and smooth the lenses of Tensed fibres or
wave guides, to
eliminate the non-uniformities in the optical field of a lens which are
inherently present in fibre
lenses manufactured by most practical processes. Such non-uniformities could
be due to
stresses, scratches, deformations, contaminants or other types of defects.
Fig. 8 is a graphical
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CA 02413050 2002-12-23
representation of the far optical field distribution of light emitted from a
conical lens which has
been formed by drawing the fibre to a point, for example as described in U.S.
Patent No.
4,589,897. Lenses of this type are widely available commercially. In this
case; the conical lens
was approximately 125 microns in diameter at its base and 300 microns in
length. Due to the
stresses and imperfections inherent in the drawing process, the optical
distribution prior to
treatment (curve 18 in Fig. 8) had marked side lobes together with (not shown)
a very noisy
background. Following treatment with two discharge pulses by the method of
this invention
(curve 19 in Fig. 8), the side lobes were effectively eliminated and the
optical distribution was
largely symmetrical. In addition, the noisiness of the background optical
signal was almost
entirely eliminated.
It should be understood that this invention is not limited to the specific
embodiments
described above but that various modifications obvious to those skilled in the
art, including the
use of the method with leased fibres fabricated from polymer or from different
glass
compositions, may be made therein without departing from the scope of the
following claims.
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