Note: Descriptions are shown in the official language in which they were submitted.
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AUTO-FOCUS MICROLENS HOLDER
Field of the invention
The present invention relates to an auto-focus microlens holder,
particularly useful for collimation or conditioning of the light beams
radiated by
laser diodes or laser diode arrays.
Description of the prior art
The outstanding features of high-power semiconductor laser diodes as
compared to conventional, bulky laser sources give them a bright future for
use
in applications like optical pumping of solid-state lasers, light illuminator
systems, range finders, and direct coupling into optical fibres for convenient
delivery of laser light up to remote targets, as desired in various industrial
and
medical applications. High-power semiconductor laser devices are mostly
available in the form of elongated, thin laser diode bars comprising a
plurality of
individual laser emitters set along an axis parallel to the semiconductor PN
junction plane of the emitters. A schematic drawing of a laser diode bar is
shown
in FIG. 1.
Common laser diode bars typically comprise 10 to 50 individual equally
spaced emitters, which spread over a total width generally set to the 1-cm
standard width. Each individual emitter has typical dimensions of 50-200 Nm x
1
Nm, and they are represented by the small filled rectangles in FIG. 1. The
laser
cavity length, defined as the spacing between both front and rear cleaved
facets,
is on the order of 500-1000 Nm. Laser diode bars made from the AIGaAs
material system for emission of laser light in the 790-860 nm wavelength range
can routinely emit tens of Watts of CW optical power. When even higher optical
output powers are required from the laser source, several laser diode bars of
the
same geometry can be stacked one above the other using suitable mounting
means to give a two-dimensional laser diode array. The mounting means is
designed to hold the laser bars firmly in place while ensuring proper
electrical
biasing and
~
CA 02313357 2000-07-04
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cooling of each bar. The resulting total output power scales directly with the
number of stacked laser diode bars.
According to the well-known diffraction theory of coherent laser light, the
beams radiated out from the individual emitters of a laser bar spread
(diverge)
from the normal propagation direction perpendicular to the plane of the laser
front
facet. The rate of divergence of the beams along any given direction depends
critically upon the size of the individual emitters along the same direction.
As seen
in FIG. 1, the divergence angles of the beam escaping from a laser emitter of
the
size as given above are typically 40° FWHM (full width at half maximum)
along the
direction perpendicular to the junction plane, and 10° FWHM along the
direction
parallel to the junction plane. Unfortunately, the highly divergent character
of the
beams, particularly along the direction perpendicular to the junction plane,
makes
laser diode bars or arrays unsuited for most high-power applications unless
proper
optical elements are employed for reducing the divergence of the beams. It is
then said that the beams need to be properly collimated. To ensure efficient
collection of the emitted laser light, the collimation optics must present a
high
numerical aperture, and such optics are said to be fast. This explains why the
direction perpendicular to the junction plane is usually denoted as the fast
axis.
Accordingly, the direction parallel to the junction plane is commonly referred
to as
the slow axis.
A "collective" collimation of the entire set of laser beams escaping from the
plurality of identical laser emitters disposed linearly along the slow axis of
a laser
diode bar can be performed in a quite efficient manner along the most critical
axis
(fast axis) by using a single optical element shaped as an elongated glass
rod.
The glass rod is set in front of the laser diode bar in such a way that its
longitudinal
axis is parallel to the slow axis of the laser bar. The elongated rod acts as
a
cylindrical microlens that collimates along the fast axis the beams radiated
by all
emitters. Proper alignment of the cylindrical microlens is obtained by placing
it so
that its longitudinal axis, passing through the centre of the microlens, is
made
coincident with the plane formed by the optical axes of the individual
emitters of
the laser diode bar. The beams radiated from the plurality of laser emitters
CA 02313357 2000-07-04
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impinge on the curved side of the microlens, and are then refracted through it
while propagating perpendicularly to the longitudinal axis of the microlens.
Finally,
to ensure efficient collimation of the transmitted laser beams, that is,
minimising
their residual divergences after collimation, the front facet of the laser
diode bar
must lie within the back focal plane of the cylindrical microlens. In
practice,
several successful designs of cylindrical microlenses with high numerical
apertures have been developed for collimation along the fast axis of the beams
radiated by laser diode bars. These designs include for instance simple and
low-
cost cylindrical step-index fibre microlenses, cylindrical fibre microlenses
with
graded-index core (GRIN microlenses), and cylindrical microlenses with an
aspheric shape. Collimation of laser diode bars using step-index ordinary
optical
fibres and microlenses with an aspheric shape is detailed in U.S. Patents nos.
4,785,459 (Baer) and 5,081,639 (Snyder et al.), respectively.
The principle of collimating laser beams along the fast axis of a laser diode
bar by using properly positioned high numerical aperture microlenses can be
readily extended to the collimation of laser diode arrays by stacking, with
suitable
support means, several identical cylindrical microlenses, each of them being
individually registered to a laser diode bar of the array. Known in the art
are U.S.
Patents nos. 5,825,803 (Labranche et al.) and 5,875,058 (R. E. Grubman), which
describe beam collimation or conditioning of laser diode array assemblies by
means of GRIN microlenses. A schematic side view of a stacked laser diode
array
whose bars are collimated by cylindrical microlenses is depicted in FIG. 2.
The
collimated laser array assembly shown in this figure comprises three laser
diode
bars, and the figure is a side view of the assembly, so that the microlenses
are
seen only from their cross-section, which has a circular shape in this
example. As
it is readily seen, the longitudinal axis of each microlens is aligned with
the optical
axis of the laser diode bar that faces the microlens. Consequently, the
spacing P
(pitch) between the centres of the microlenses is equal to the spacing along
the
fast axis of the laser diode bars mounted in their support structure. The
positioning and fine alignment of the microlenses relative to the laser diode
array
require the use of a dedicated support structure in which the microlenses are
CA 02313357 2000-07-04
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mounted and held in place using proper adhesives or fixing means. A technique
now well known in the art for this purpose consists in inserting the
cylindrical
microlenses into grooves formed in a lens holder, thus resulting in an array
of
several correctly-spaced microlenses mounted parallel to each other and all
lying
in the same plane. After the microlenses have been mounted into the holder,
the
resulting microlens array then forms a firm, unitary assembly that can then be
placed in front of the laser diode array with correct positioning and
orientation so
that each laser diode bar is collimated by its own cylindrical microlens. As
shown
in FIG. 2, the spacing d between the plane of the front facets of the laser
diode
bars and the cylindrical microlens array must be fine tuned until the front
facets lie
within the back focal plane of the microlenses (having ideally all the same
focal
length ~ to ensure the lowest residual divergence for the collimated beams.
After
the microlens array has been correctly positioned, it is then attached to the
laser
diode array to form a collimated laser diode array assembly.
An example of a side view of a microlens holder placed in front of a three-
bar laser diode array is depicted schematically in FIG. 3. Details about the
design
of such a microlens holder are disclosed in U.S. Patents nos. 5,526,373 and
5,668,825 (Karpinsky). This design consists in forming properly spaced
parallel
grooves of rectangular cross-section in base substrate material. The size of
the
grooves must be adapted to the diameter of the microlenses to ensure adequate
alignment and firm attachment of the individual microlenses loaded into the
grooves. In the example illustrated in FIG. 3, the three cylindrical
microlenses
have the same diameter, so they are preferably inserted into grooves having
the
same size. In practice, however, the cylindrical microlenses often present
some
variations in their diameters which can be caused, for instance, by the
manufacturing tolerances of the fibre drawing process (typically ~2%), since
microlenses are generally fabricated in the same way as optical fibres.
A tight control over the outer diameter of the microlenses must be exercised
when they are intended to be loaded into microlens holders with grooves of
rectangular shape, otherwise severe degradation of the collimation efficiency
of
the microlens array will result. Because the diameter of the fibres needs to
be
CA 02313357 2006-03-29
tightly controlled, the additional quality control steps that must be carried
out are
expensive.
Some of the drawbacks that could be encountered when using
microlens holders with rectangularly shaped grooves are illustrated
schematically in FIG. 4. Apart from the obvious problem of inserting
microlenses
with oversized cross-section into precisely sized grooves, it is readily seen
that
the correct centering of a microlens having too small a diameter relative to
the
size of the grooves is nearly impossible. Due to the very short focal length
of the
microlenses used for collimation along the fast axis (on the order of a few
hundreds microns), even a minute misalignment of the microlenses results in a
collimated beam that will propagate off axis with a sizeable tilt angle,
resulting in
an increased overall beam divergence. In addition, since the whole microlens
holder must be precisely spaced from the front facet of the laser diode bars,
a
microlens of incorrect diameter and mounted into a rectangular groove will not
be properly spaced from its corresponding laser diode bar, thus degrading
further the collimation of the beam. This problem comes from the fact that the
focal length of most types of microlenses depends on the diameter of the
microlenses.
An example of microlens holder with V-shaped grooves whose
symmetrical side walls are tilted at an angle of 90° with respect to
each other is
taught in U.S. Patent no. 5,828,683 (B.L. Freitas). Unfortunately, V-shaped
grooves tilted at 90° do not allow the front facets of the laser diode
bars to
remain in the back focal plane of the cylindrical microlens independently of
the
diameter of the microlens.
Summary of the invention
An object of the present invention is to provide a microlens holder
comprising V-shaped grooves in which graded-index or step-index fibre
microlenses with circular cross-section are loaded in order to keep the front
facets of laser diode bars in the focal plane of the microlenses,
independently of
the exact diameter of the microlenses loaded into the grooves.
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In accordance with a first aspect of the invention, there is therefore
provided a microlens holder for supporting at least one microlens of circular
cross-section in alignment with a light source, each of the at least one
microlens
having a radius and a back focal length varying linearly with said radius
according to a gradient defining a positive constant factor k, the microlens
holder
comprising:
a base having at least one V-groove therein for receiving the at least
one microlens, each of said at least one V-groove having a pair of side walls,
both side walls being tilted by an angle 8 relative to the plane of the
emitting light
source, said value of A being defined by:
8 = cos
-'y+kj .
In accordance with another aspect of the invention, there is also
provided a microlens assembly comprising:
a microlens holder as described above;
at least one microlens mounted into said at least one V-groove of
the microlens holder; and
bonding means for bonding said at least one microlens to said V-groove.
In accordance with another aspect of the invention, there is also
provided a microlens holder for supporting at least one ball microlens in
alignment with a light source having an optical axis, each of the at least one
ball
microlens having a radius and a back focal length varying linearly with said
radius according to a gradient defining a positive constant factor k, the
microlens
holder comprising:
a base having at least one recess therein for receiving the at least
one microlens, each of said at least one recess having a conical side wall
symmetrical around the optical axis and tilted by an angle 8 relative to the
plane
of the emitting light source, said value of A being defined by:
B=cos' 1
- y+k).
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Preferably, the microlens assembly as described above may k>e used in
combination with a laser diode array for conditioning light emitted along
either
the slow or fast axes of the laser diode array. Alternatively, two microlens
assemblies as described above may be used with the microlenses of one holder
perpendicular to the microlenses of the other holder, in combination with a
laser
diode array for conditioning light along both its slow and fast axes.
Brief description of the drawin4s
The present invention and its advantages will be more easily understood
after reading the following non-restrictive description of preferred
embodiments
thereof, made with reference to the following drawings in which:
FIG. 1 (Prior Art) is a schematic perspective view of a laser diode bar
showing the typical divergence angles along both fast and slow axes of the
beam radiated by the plurality of emitters of the bar.
FIG. 2 (Prior Art) is a schematic side view illustrating cylindrical
microlenses for collimation along the fast axis of the beams emitted by a
stacked
laser diode array comprising three laser diode bars.
FIG. 3 (Prior Art) is a schematic side view of a collimated three-bar
stacked laser diode array showing the microlens holder with grooves of
rectangular shape adapted to the size of the microlenses.
FIG. 4 (Prior Art) illustrates some problems encountered when using
cylindrical microlenses of varying diameters with a microlens holder formed
with
grooves of rectangular shape.
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FIG. 5 is a schematic diagram illustrating the basic principle of the present
invention.
FIG. 6 is a schematic side view of a collimated stacked laser diode array
wherein collimation of the laser diode bars is achieved by cylindrical
microlenses
of different diameters loaded into an holder with V-shaped grooves having side
walls with suitable tilt angle.
FIG. 7A shows a top view of a microlens holder in accordance with a
preferred embodiment of the invention.
FIG. 7B shows a front view of a microlens holder in accordance with the
same preferred embodiment of the invention.
FIG. 7C shows a side view of a microlens holder in accordance with the
same preferred embodiment of the invention.
FIG. 8 depicts a side view of a holder for spherical ball lenses in
accordance with another embodiment of the invention.
Description of a preferred embodiment of the invention
As mentioned previously, one object of the present invention is to provide a
microlens holder for cylindrical microlenses with circular cross-section in
which
correct spacing of the microlenses relative to the front facet of the laser
diode bar
to collimate is obtained independently of the exact diameter of the
microlenses.
This appealing "auto-focus" property is made possible by forming into a
microlens
holder grooves with a V-shaped cross-section, in which the microlenses are
loaded therein.
The auto-focus property can be exploited using V-shaped grooves with side
walls tilted at special angles, and this is another object of the present
invention to
provide means for determining this angle, which depends on the specific design
of
the selected microlenses, but not on their diameter. Since this specially
determined tilt angle allows for mounting microlenses of varying diameter in a
same microlens holder, it is a further object of the present invention to
provide a
CA 02313357 2000-07-04
means for relaxing the manufacturing dimensional tolerances on the diameter of
fibre microlenses intended for collimation of laser diode bars or arrays.
It is another object of the present invention to provide a means to make
easier and much faster the optical alignment of the cylindrical microlenses
loaded
in the grooves formed in a holder, by taking advantage of the self-centering
property of microlenses with circular cross-section loaded into properly-
positioned
V-shaped grooves. This self centering property is independent of the diameter
of
the microlenses.
The optical coupling systems developed for coupling light between optical
fibres as well as for coupling light from single-emitter laser diodes into
optical fibre
ends may benefit from the present invention. These optical coupling systems
often make use of small lenses having a perfectly spherical shape with a
typical
diameter on the order of a few mm's. These lenses are generally known as ball
lenses. Mounting spherical ball lenses into a lens holder having a V-shaped
receptacle with side walls tilted at angles as prescribed by the invention
would
allow correct placement of the exit aperture of a light emitting device in the
back
focal plane of the ball lens, regardless of the exact lens diameter. It is
therefore
another object of the present invention to provide a holder for ball lenses of
spherical shape for which the exact placement of either the light emitter to
collimate or the focus plane for light to be focused is independent of the
diameter
of the ball lens mounted into the holder.
Although the principle of the present invention applies primarily to the
collimation of laser diode bars (both in the form of single devices or mounted
in
two-dimensional stacked arrays), lens holders designed according to the
present
invention could be of widespread use. For instance, the positioning of arrays
of
microlenses used for focusing the incoming light onto the photosensitive pixel
elements of array imagers (CCD image sensors, IR thermal imagers, arrays of
micro-bolometers or pyroelectric elements, etc...) may benefit from the use of
microlens holders comprising V-shaped grooves designed following the
principles
of the invention. Likewise, the principle can be found useful for collimation
or
conditioning of the light escaping from non-laser sources such as light-
emitting
CA 02313357 2000-07-04
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diodes or optical fibre outputs wherein correct spacing between the
collimating
lens and the output aperture of the light emitting device is critical.
The present invention will be described by first outlining its basic
principles
with reference to the schematic drawing presented in FIG. 5. In the preferred
embodiment of the invention, an elongated fibre microlens having a circular
cross-
section is seated into a V-shaped groove formed in a holder made of a suitable
material. A laser diode bar is positioned in such a way that its cleaved front
facet
from which the laser beam is radiated coincides with the trough of the V-
shaped
groove. In FIG. 5, the laser beam propagates upward and its outer edges are
outlined by the solid arrows.
The optical axis of the laser beam is perpendicular to the front facet of the
laser
diode bar and this optical axis passes through the centre of the microlens.
The
thickness T of the microlens holder, given by the spacing between the plane of
the
front facet of the laser diode bar and the crests of the groove, is not
critical to the
operation of the invention. However, the microlens holder must be thick enough
to
allow the microlens of radius r (half diameter) to be properly seated into the
groove. The plane of the front facet of the laser diode bar is separated from
the
vertex of the microlens by the working distance d, as illustrated in FIG. 5.
The
vertex is the point lying on the contour of the microlens which intersects the
optical
axis and which is nearest to the laser diode bar. The plane of the front facet
of the
laser diode bar must coincide with the back focal plane of the microlens in
order to
provide the best collimation of the beam radiated by the laser diode bar.
Stated
otherwise, the working distance d must be set equal to the back focal length
(bfn
of the microlens. The back focal length of any given microlens depends on the
microlens' radius as well as on the specific spatial refractive index
distribution of
the material from which the microlens is made. The V-shaped groove comprises
two symmetrical side walls tilted by the same angle 8 relative to the plane of
the
front facet of the laser diode bar.
One major aspect of the invention is to show that there exists a value of the
tilt angle 8 of the side walls that will make the working distance d equal to
the back
CA 02313357 2000-07-04
focal length of the microlens. Referring to FIG. 5, simple trigonometric
formulas
readily show that the angle 8 is related to the radius r of the microlens by:
r (1)
cos(B) _
r+d
The working distance d must be equal to the back focal length bfl of the
microlens,
5 so that the above equation can then be rearranged in the following manner:
1 (2)
cos(9) = 1 + bfl (r)
r
where the dependency of the back focal length upon the radius of the microlens
have been emphasised. Tilting the side walls at the angle 8 as given by Eq.
(2)
ensures therefore that the vertex of the microlens will be correctly spaced
from the
10 front facet of the laser diode bar. Moreover, it can be seen that the angle
8 of the
side walls which form the V-shaped groove does not depend on the radius of the
microlens, provided that the microlens design is such that the back focal
length
varies linearly with the microlens' radius, that is, btl(r~ = kr, where k is a
positive
constant factor depending on the specific design of the microlens. The
knowledge
of the constant factor k therefore permits calculation of the angle 8
according to
the following formula:
8 = cos-' ~ 1 + k ~ 0~ ~ a ~ 9~~ . (3)
Although the principle of the invention requires the use of cylindrical
microlenses whose back focal length varies linearly with the lens' radius, it
appears that this limitation is not a severe drawback because two of the most
appealing types of microlens for collimation of the fast axis of laser diode
bars fall
within this category.
The first type is quite simple and low cost since it consists in elongated
rods
(for instance step-index fibres with the jacket removed) in which the
refractive
index is homogeneous everywhere inside the microlens. As it is well known in
the
art, the back focal length of these step-index fibre microlenses is given by:
CA 02313357 2006-03-29
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b~STEP-INDEX 2 (Yl -1) 4
where n is the refractive index at the working wavelength of the glass
material
from which the microlenses are made. As a rule of thumb, one gets bfl(r) =
0.5r
for step-index microlenses made of fused silica (n ~ 1.5). The above equation
also reveals that step-index microlenses can be used for collimation of laser
diode bars provided that their refractive index n is smaller than 2.
Otherwise, the
focal plane of the microlens would be positioned inside the lens. It should be
noted that the back focal length of spherical ball lenses is calculated from
Eq. (4)
as well.
The second microlens design that falls within the category discussed
above is characterised by a gradual and smooth reduction of the refractive-
index
profile from the centre of the microlens up to the outer limits of the core.
These
graded-index (GRIN) microlenses are preferably of the non-full aperture
Luneberg type with a homogeneous cladding, which shows high collimation
efficiency along with low spherical aberration on the transmitted laser beam,
even at high numerical aperture. The design and fabrication of these GRIN
microlenses are detailed in U.S. Patent nos. 5,607,492 and 5,638,214 (Doric).
The back focal length of this type of GRIN microlens commercially available
from Doric Lenses Inc. (Ancienne-Lorette, Quebec, Canada) exhibits a linear
dependency with the microlens' radius since:
bflci,N = 0.37 r (5)
As compared to a step-index microlens of the same radius., a GRIN
microlens generally has a shorter back focal length. This makes these
microlenses very attractive for efficient gathering of the highly-diverging
beams
radiated by laser diode bars, since the front facet of the bar can be set
closer to
the vertex of the microlens. However, GRIN microlenses suitable for use in a
microlens holder fabricated according to the principle of the present
invention
are not limited to non-full aperture Luneberg-type lenses and could be, for
instance, full aperture Luneberg-type lenses with or without homogeneous
cladding, or non-full aperture GRIN lenses with a profile different from the
Luneberg type and with or without homogeneous cladding. In fact, it should be
emphasised that any microlens
CA 02313357 2000-07-04
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design of circular cross-section and leading to a linear dependency of the
back
focal length upon the radius of the microlens (with a known constant factor)
could
be suitable for collimation of the laser light emitted by laser diode bars
according
to the present invention.
Typically, the angle 0 at which the side walls of the V-shaped groove should
be tilted is obtained simply by reporting the values of the constant factor k
for both
step-index and GRIN microlens types in Eq. (3), thus giving:
eSTEP INDEX = 48.2° (for fused-silica fibers with n ~ 1.5)
ecRinr = 43.1 ° (k = 0.37)
Positioning the front facet of a laser diode bar at the trough of a V-shaped
groove with side walls tilted at angles as given above will then ensure proper
placement of the front facet in the back focal plane of the microlens loaded
into the
groove, and this, regardless of the exact diameter of the microlenses,
provided
that the tilt angle of the side walls is suited to the microlens type.
As another example, spherical ball lenses are often made of a glass
material with high refractive index to shorten their back focal length
(thereby
increasing their numerical aperture) without excessive curvature of the
lenses. For
instance, ball lenses made of a LaSF N9 Schott glass material having a
refractive
index around 1.83 for 830-nm light wavelength could be mounted in a holder
with
a V-shaped receptacle whose side walls are tilted at an angle given by
08A« = 24.9° (for LaSF N9 ball lenses with n = 1.83) .
The chief advantages offered by the present invention as compared to the
prior art are best exemplified by referring to the schematic drawing of FIG.
6. The
figure illustrates a stacked laser diode array whose individual laser beams
are
collimated by an array of microlenses mounted into a holder with V-shaped
grooves designed according to the present invention. The stacked laser diode
array assembly consists of four laser diode bars with identical
characteristics and
mounted in a suitable support structure not shown in the drawing. The laser
diode
bars are positioned in such a way that their individual front facets lie in
the same
plane, which is made coincident with the plane formed by the troughs of the V-
shaped grooves as well. Microlenses of the same type but having possibly
CA 02313357 2000-07-04
13
different diameters are inserted into the grooves. In this specific example,
they
serve for collimation of the laser beams along the fast axis of the laser
diode bars.
Even though the diameter of the various microlenses could vary significantly,
the
front facets of all laser diodes bars would remain in the back focal plane of
the
microlenses, therefore promoting efficient collimation of the laser beams
transmitted through the microlenses.
The possibility of using microlenses with varying diameters in the same
holder is particularly attractive for replacement of damaged microlenses by
spare
microlenses fabricated from another batch. In addition, those skilled in the
art will
recognise that the diameter of any given microlens to be loaded in a V-shaped
groove designed in the way as described above needs not be exactly the same
along the length of the microlens, provided that the diameter varies in a
linear
fashion along the length of the microlens.
Another significant advantage of the present invention relies on the
automatic self-alignment feature offered by the V-shaped grooves. This
advantage can be exploited by first ensuring adequate registration of the
laser
diode bars with the troughs of the V-shaped grooves, that is, the periodicity
of the
grooves must match the spacing between successive laser diode bars. Due to the
special shape of the grooves, loading of a microlens with circular cross-
section
into any of the grooves will provide self-alignment of the longitudinal axis
of the
microlens with the optical axis of the laser diode bar registered to the
groove. This
self alignment feature greatly facilitates the mounting of microlens arrays,
and it
leads to collimated output laser beams displaying minimum residual divergence
since all individual collimated laser beams are directed along the same
direction,
parallel to the optical axes of the laser diode bars. Although the laser diode
array
depicted in FIG. 6 comprises four laser diode bars along with their
corresponding
microlenses, it should be readily apparent that the principles of the present
invention could be implemented for collimation of laser diode arrays made up
of
any number of laser diode bars.
One preferred embodiment of a microlens holder fabricated following the
principle of the present invention is depicted in the three views presented in
FIGS.
CA 02313357 2000-07-04
14
7A through 7C. Referring first to the top view of FIG. 7A, it is shown that
the
microlens holder has preferably the shape of a rectangular mounting frame
having
a central part that is free of material. The microlens holder comprises two
parallel
support members having a top major surface in which V-shaped grooves have
been formed with side walls tilted at angles prescribed by the selected
microlens
type. A plurality of elongated fibre microlenses of suitable length are loaded
into
the V-shaped grooves to form an array of microlenses with their individual
longitudinal axes parallel to each other and aligned with the optical axes of
the
corresponding laser diode bars, the latter being not shown in FIG. 7. The
number
of microlenses is preferably equal to the number of bars forming the stacked
laser
diode array assembly, and the spacing between the troughs of the grooves is
preferably equal to the spacing between the laser diode bars. An anti-
reflection
coating can be deposited on the microlenses to minimise optical reflection
losses.
In this preferred embodiment of the microlens holder, the microlenses are
supported by the V-shaped grooves only over regions of limited length at both
ends of the microlenses. The length of the V-shaped grooves is given by the
width
of the support members of the microlens holder. This design permits the front
facets of the laser diode bars to be located in the back focal plane of the
microlenses while avoiding direct contact of the front facets with any part of
the
microlens holder. The risks of accidental damages to the laser front facets
are
therefore minimised. As a result, adequate positioning of the laser front
facets
relative to the microlens holder requires that the clearance between the inner
side
surfaces of the support members is made slightly larger than the width of the
laser
diode bars (typically 1 cm). Attachment of the microlenses to the holder via
grooves of reduced length and positioned at both ends of the microlenses is
possible by recognising that the typical diameter (about 600 p,m to 2-3 mm) of
the
microlenses used for collimation of laser diode arrays provides sufficient
mechanical rigidity to the microlenses. As a consequence, mechanical
deformation of the microlenses attached by their ends and having a length of
typically 1-1.5 cm is not a problem in real practice.
CA 02313357 2000-07-04
FIG. 7B is a front view of the microlens holder showing the microlenses
placed in the V-shaped grooves. The microlenses can be bonded to the side
walls
of the grooves by using proper adhesive well known in the art such as UV-
curing
epoxy cement or any other bonding technology compatible with the materials
from
5 which the microlenses and the holder are made. The microlens holder can be
fabricated from materials like various ceramics, silicon, or various metals.
It is
preferable that the selected material be easily machinable to allow higher
fabrication yields along with tight mechanical tolerances. According to the
preferred embodiment of the invention, it can be seen that the material from
which
10 the microlens holder is made needs not be optically transparent to the
beams
radiated by the laser diode bar or array because the beams do not travel
through
any part of the microlens holder after escaping from the laser diode bars.
Likewise, the adhesive used for bonding of the microlenses neither needs to be
optically transparent nor needs to be index matched to the material of the
15 microlenses. The V-shaped grooves formed on the top major surface of the
support members can be machined with side walls tilted at suitable angle using
a
dicing saw with a properly shaped saw blade, by electric-discharge machining,
or
by laser machining.
As it is shown from the side view of FIG. 7C, both support members of the
microlens holder have flat underside surfaces that permit attachment of the
whole
microlens holder to the mounting structure of the laser diode array. The
undersides of the support members are set in contact with proper flat surfaces
of
the mounting structure of the laser diode array. The thickness of the support
members is determined from the need to ensure that the plane of the front
facets
of the laser diode bars coincides with the back focal plane of the
microlenses, after
the microlens holder has been affixed to the laser diode array assembly.
The use of microlens holders fabricated according to the present invention
is not limited to the collimation along the fast axis of laser diode arrays.
Hence,
collimation of laser diode arrays along the slow axis, orthogonal to the fast
axis,
can be performed using such a microlens holder as well. In this case, the
spacing
between the V-shaped grooves would correspond to the spacing between the
CA 02313357 2000-07-04
16
individual laser emitters of the laser bars, while the length of the
microlenses
would be determined from the number of laser bars stacked in the array and
from
their pitch. As it is well known in the art, collimation along the slow axis
of laser
diode arrays using cylindrical microlens arrays requires that the laser diode
bars
be identical to each other and that the emitters of each laser diode bar be
aligned
relative to the corresponding emitters of the other stacked laser bars. The
lower
divergence of the laser diode beams along the slow axis promotes the use of
cylindrical microlenses with lower numerical aperture and longer focal length
as
compared to their counterparts designed for collimation along the highly-
diverging
fast axis. As a result, the microlens array used for collimation along the
slow axis
is positioned farther from the plane of the front facet of the laser emitters.
Although the various positioning and alignment tolerances are less demanding
for
efficient collimation along the slow axis, the present invention favours
faster and
easier alignment of the cylindrical microlenses and permits less stringent
tolerances on the diameter of the cylindrical microlenses as well.
Although the microlens holder depicted in the views of FIG. 7A-7C allows
mounting of a maximum of six microlenses, the number of microlenses to be used
can be varied according to the specific requirements dictated by the
characteristics
of the laser diode array to be collimated. For instance, the microlens holder
may
comprise only one microlens loaded into a V-shaped groove for collimation of a
single laser diode bar. In the same way, it will be obvious to those skilled
in the art
that various changes in the dimensions and structure of the embodiment of the
microlens holder described above can be carried out without departing from the
scope of the present invention.
As stated previously, the scope of the present invention can be extended to
the mounting of spherical ball lenses. The side view of an embodiment of a
holder
for mounting spherical ball lenses is depicted schematically in FIG. 8. Due to
the
perfect spherical shape of ball lenses, the lens holder has preferably a
rotational
symmetry around the optical axis, which passes through the centre of the
mounted
lens. The ball lens of radius r is seated in a recess formed on the front
major
surface of the lens holder, this recess having preferably a conical shape.
Suitable
CA 02313357 2000-07-04
17
adhesive of composition well known in the art can be used for attachment of
the
ball lens to the holder. However, attachment of the ball lens is not limited
to the
use of an adhesive. For instance, the ball lens can also be held in place
using a
circular plate affixed to the side wall of the conical recess, this plate
having a
clearance hole of suitable diameter to allow free transmission of the light
beam
refracted through the ball lens. The tilt angle A of the side wall of the
conical
recess is calculated from Eq. (3), and this angle thus depends upon the
refractive
index of the material from which the ball lens is made. Tilting the side wall
according to the principle of the present invention allows the mounting of
ball
lenses of various diameters while keeping the lens' back focal plane always in
the
same position, as indicated by the vertical dashed line in FIG. 8. The minimum
diameter for a ball lens to be mounted in such a holder is bounded by the
diameter
DA of the clear aperture of the holder, while the maximum diameter of the ball
lens
depends on the thickness T of the holder. The lens holder as depicted in FIG.
8
preferably comprises a ring protruding outwardly from its back major surface.
The
flat surface of the protruding ring lies within the back focal plane of the
lens
mounted into the holder, which coincides with the trough of the conical recess
as
well. This flat surface acts as a reference plane for proper placement of
either the
exit aperture of a light emitting device or the input end of an optical fibre
to be
coupled to the mounted ball lens. People skilled in the art will readily
recognise
that several modifications to the above-described embodiment of a holder for
spherical ball lenses can be made without departing from the scope of the
present
invention.
Referring to the basic principle of the invention, some intrinsic limitations
in the
use of a microlens holder fabricated according to this principle can be
readily
identified. As discussed above, the implementation of the invention requires
cylindrical microlenses having a circular cross-section. This restriction
therefore
precludes the use of microlenses with aspheric or piano-convex shapes. It
should
be apparent, however, that this limitation is of minor consequence, owing to
the
widespread use of microlenses with circular cross-section. Along with the
greater
ease of manufacture, a circularly-shaped microlens also provides easier
alignment
CA 02313357 2000-07-04
18
steps since its rotationally-symmetric structure obviates the need for fine
tuning of
the rotation of the microlens around its longitudinal axis. A further
limitation of the
present invention is the need for microlenses whose back focal length is
proportional to the radius (or diameter) of the microlenses. Likewise, this
limitation
does not restrict significantly the use of a holder fabricated according to
the
present invention because widely-used microlens designs for collimation of
laser
diode bars or arrays, that is, step-index and GRIN microlenses, exhibit such
linear
dependence of their back focal length.
The microlens array must be properly registered to the laser diode bar or
array
to be collimated in order to take full advantage of the appealing features
offered by
the present invention. As a result, the stringent manufacturing tolerances (in
the
pm range) which are of standard practice in this field must be maintained
during
the manufacturing and subsequent positioning of a microlens holder fabricated
according to the invention. As for any other microlens no~der to ae set in
rront or a
stacked laser diode array, the parallelism of the grooves formed in the holder
as
well as their relative spacing must be tightly controlled. Attention must be
paid to
the surface flatness of both sidewalls of the V-shaped grooves to ensure that
the
microlenses are correctly seated therein. The microlenses must exhibit
sufficient
stiffness when supported only by their ends to avoid mechanical deformations
that
could deviate the back focal plane of the microlens out of the plane of the
front
facet of the corresponding laser diode bar. Such mechanical deformations of
the
microlenses could lead to detrimental optical misalignment of the microlenses
as
well.
Although the present invention has been explained hereinabove by way of a
preferred embodiment thereof, it should be pointed out that any modifications
to
this preferred embodiment within the scope of the appended claims is not
deemed
to alter or change the nature and scope of the present invention.
