Note: Descriptions are shown in the official language in which they were submitted.
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s METHOD AND APPARATUS FOR ILLUMINATING
A SPATIAL LIGHT MODULATOR
FIELD OF THE INVENTION
The invention disclosed herein relates generally to the field of semiconductor
laser diodes and more particularly to semiconductor laser diodes which have
linear
arrays of emitters. The invention relates particularly to methods and
apparatus in which
laser radiation output of two or more such diodes is combined to illuminate a
spatial
light modulator.
BACKGROUND OF THE INVENTION
i 5 Semiconductor laser diodes are used in many applications where compact
size
and/or high efficiency is important. Semiconductor laser diodes offer
relatively low
cost, high reliability and simplicity of use.
Single emitter multi-mode laser diodes are commonly available in various
wavelengths with radiation power output up to 2 Watts or more. These lasers
typically
2 o have a rectangular or stripe emitter around 1 ,um high and in the region
of 20 ~.m - 50 0
~,m long. Fundamental problems of heat removal and optical emitter facet
damage place
an upper limit on the power per unit length of emitter that can be extracted
without
significantly reducing the operating lifetime of such laser diodes.
To use diode lasers in applications that need more than a few Watts of
radiation
2s power it is common to use an array of single emitter diodes. It is possible
to form such
an array using single emitter diodes mounted in a mechanical support but it is
more
common to fabricate the array of emitters on a monolithic substrate. These
devices,
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known as laser diode bars, are available in many configurations with radiation
power of
up to 50 Watts. Laser diode bars have found application in machining, welding
&
soldering, medical, imaging, pumping for solid-state lasers and many other
applications
that require low cost, reliable, compact radiation sources.
A monolithic laser diode array 1 is shown in FIG. 1. It consists of a
io semiconductor substrate 2 upon which is formed an array of emitters 3.
Adjacent
emitters have a dead space between them that does not emit light. Due to
emitter
geometry, the radiation beam 4 is substantially asymmetrical while also having
differing
divergence rates in the x-axis and y-axis directions. The full width
divergence in the y-
axis is typically in the range of 40° to 100° and in the x-axis,
8° to 20°. Because of the
high divergence, the y-axis is often referred to as the "fast" axis while
correspondingly
the x-axis is referred to as the "slow" axis. The high beam divergence of
semiconductor diode lasers makes it necessary to collimate or focus the beams
emitted
by such lasers for most applications.
The beam quality in the y-axis can be very good, with an MZ value of close to
2 0 1Ø M2 is a dimensionless parameter that characterises the degree of
imperfection of a
laser beam. An ideal, diffraction-limited, Gaussian profile beam would have an
MZ of
1Ø Any departure from the ideal results in an MZ value of greater than 1Ø
The MZ
of the beam from a laser diode in the x-axis is very poor, signifying a
substantial
deviation from a perfect beam. This difference in the beam quality, along with
the
differing divergence rates for the x and y axes, make it necessary to treat
the axes
separately when designing a collimation scheme.
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Spatial light modulators offer an advantage in imaging in that they can be
fabricated as mufti-channel devices, thus reducing system complexity while
increasing
imaging speed. Spatial light modulators are optical modulators constructed to
spatially
modulate, according to prescribed input, a readout optical beam. Spatial light
modulators having a single line of modulating elements or areas are of
particular use in
1 o imaging tasks although in some applications mufti-line devices can also be
advantageous. Examples of spatial light modulators include a wide range of
electro-
optical, acousto-optical, and electromechanical devices.
While laser diode bars have several advantages for illuminating a spatial
light
modulator one must first overcome the challenges set the by format of the
laser diode
1 s beam. For optimal illumination of a line spatial light modulator, the
laser bar radiation
must be precisely transformed into a line of uniform illumination in a manner
that
maximizes brightness. Brightness is defined as the luminous flux emitted from
a
surface per unit solid angle per unit of area.
Commonly assigned patent US 5,517,359, to Gelbart discloses a method of
2 o formatting the output from a laser diode to form a line source
particularly useful for
illuminating a spatial light modulator. Radiation from each emitter is fully
overlapped
at the modulator in both the x and y axes. A cylindrical microlens collimates
the
radiation in the y-axis. In the x-axis an array of cylindrical microlens
elements
collimate and steer the radiation towards a common target point, some distance
from the
25 laser, overlapping the radiation profiles.
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The overlapping of emitter radiation profiles is advantageous should one or
more
emitters fail. Since the overall profile is the sum of a plurality of
emitters, an emitter
failure only reduces power and does not substantially change the profile. In
contrast, if
only the fast axis is collimated and the slow axis is allowed to diverge up to
a point
where the beams overlap only partially, an emitter failure will severely
compromise the
1 o profile. Another advantage of overlapping is that dead space between
emitters is
effectively removed, creating a high brightness illumination line.
A problem that occurs in using many laser diodes bars is that, as a result of
stress-induced bending of the device wafer, the emitters are not in a
perfectly straight
line; a characteristic known as "smile" . While bars have been manufactured
with sub-
micron smile, it is more common to have to deal with around 5 - 1 0 m of
smile. A
non-negligible smile prevents precisely aligning the beams in the y-axis and
thus
degrades line quality. In commonly assigned US patent No. 5,861,992 to Gelbart
an
individual microlens is mounted in front of each emitter. The microlens is
adjusted in
the y-axis direction to line up all emitter radiation profiles at a target
plane. In this case
2 o the microlenses also perform collimation of the emitters in both axes and
additionally
can be used to steer the emitter profiles to overlap in the x-axis direction.
The
microlenses are individually sliced from the centre of a moulded aspheric
lens, such that
each slice is substantially the same as the diode array pitch.
Advances in semiconductor materials have lead to the available power from
laser
2 s diodes bars more than doubling over the past few years. However, despite
these
advances, it is unlikely that there will be a further doubling of power levels
in the near
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s
s future unless there is a significant breakthrough in the art. On the other
hand,
applications continue to demand higher overall laser powers.
US patent No. 4,716,568 discloses a plurality of linear diode laser array
subassemblies stacked one above the other and simultaneously powered from a
single
source. In this configuration, power can easily be scaled by simply adding
more laser
1 o diode arrays. The downside is that it is very difficult to design
combination systems
that deal with the radiation asymmetry while simultaneously preserving
brightness for a
vertical stack. While this combination scheme is effective at increasing the
power
available, the loss of brightness counters much of the gain, particularly for
demanding
imaging applications.
1 s US patent No. 6,240,116 discloses a stepped reflector that can be used to
combine beams from multiple laser diodes, simultaneously correcting some of
the
asymmetry while conserving brightness. However the stepped reflector is a
complex
component to manufacture and will have a significant impact on system cost and
complexity. Additionally it is still necessary to individually microlens each
emitter to
2 o achieve a good profile.
Accordingly, there is a need for apparatus and methods for combining the beams
from two or more laser diode arrays to achieve higher power than is available
from a
single bar diode. There is a particular need for such methods and apparatus
which:
~ combine the radiation in such a way that brightness is maximized;
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~ minimise the additional cost and complexity involved in producing a
combined laser array source;
~ preserve the beam quality in the y-axis so that a substantially Gaussian
profile is maintained; and,
combine the beams in such a manner that the far field profiles are
to substantially uniform in the x-axis.
SUMMARY OF THE INVENTION
This invention provides methods for constructing high power, high quality, and
high brightness illumination sources for spatial light modulators from two or
more
multiple emitter laser diode arrays. The invention also provides apparatus for
illuminating spatial light modulators and systems which incorporate such
apparatus. By
mounting two laser diode arrays adjacent to each other and providing optics
operative to
collect and steer the radiation towards a target plane the radiation of two or
more laser
diodes can be combined while maintaining beam quality and brightness.
BRIEF DESCRIPTION OF THE DRAWINGS
2 o In drawings which illustrate non-limiting embodiments of the invention:
FIG. 1 depicts a generic prior art laser diode array;
FIG. 2 is a graphical depiction of the far field profile of an idealized line
source;
FIG. 3 depicts a particular embodiment that combines the radiation from two
individual laser diode bars to form a single high power line source;
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FIG. 4 depicts an embodiment of the invention that combines the radiation from
two laser diode bars using microlenses associated with each emitter;
FIG. 5 depicts an alternative embodiment of the invention advantageous in
reducing off axis aberration from the microlenses; and,
FIG. 6 depicts an embodiment of the invention that combines two laser diode
1 o arrays on a common base.
DESCRIPTION OF THE PREFERRED EMBODIMENT
This invention involves combining the radiation of two or more laser diode
bars.
More specifically the invention relates to combining the radiation of two or
more diode
bars where the bars are mounted side by side. Collecting optics are placed in
front of
the bars to format and direct the radiation to form a radiation profile.
In this disclosure the term "laser diode array" or "array" refers to an array
of
emitters on a monolithic semiconductor substrate. The term "laser diode bar"
or "bar"
refers to a "laser diode array", permanently mounted on a base. The base
provides for
mounting electrical connections and/or heat removal. The product sold by most
laser
2 o diode vendors is a "laser diode bar" as described above. Where the
distinction is
immaterial, the device will simply be referred to as a "laser diode" or just
"laser"
Furthermore, the term "optical element" refers to any element operative to
change the properties of a beam of light. A lens is an example of an optical
element. A
mirror is another example of an optical element. The term "microlens" is used
to refer
to an optical lens element of small size.
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Furthermore, the terms "collecting optics" or "collecting" are used to denote
the optics or the process of gathering diverging light from a source, such as
a set of
laser diode emitters and forming a collimated or converging beam of light
along a
unidirectional path towards a target plane. Although the light may be focussed
at the
target plane, this is not necessarily required in the aforegoing definition.
1 o FIG. 3 shows a pair of laser diode bars 15 each comprising a laser diode
array 2
mounted on a base 20. A common microlens 21 collects the y-axis radiation for
both
lasers. Microlens array 22, comprising microlenses 23, collects the x-axis
radiation
from each emitter. Microlenses 23 are also operative to steer beams 24 from
each
emitter in the x-z plane, forming an overlapped line profile 25 at a point
some distance
away from diode bars 15. Laser diode bars 15 and optical elements are mounted
on a
rigid support base (not shown).
FIG. 2 shows an idealized profile of an illumination line suitable for
illuminating a spatial light modulator. In the y-axis direction, the beam is
formed into
to a narrow substantially Gaussian profile 10. In the x-axis, all emitters
have been
2 0 overlapped to form a line with the characteristic top-hat shape 11. The
overlapped
profile will typically have less variation than individual emitter profiles
and is thus
effective in smoothing out random variations in emitter profiles.
The bars 15 shown in FIG. 3 are an example of a narrow package bar, which is
not much wider in the x-axis than the diode array chip, facilitating close
side-by-side
z 5 mounting. An example of such a bar is supplied by Coherent Inc of Santa
Clara,
California under part number B1-83-SOC-19-30-B. This laser diode bar is a
fluid
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s cooled SOW bar comprising 19 emitters with a 30% fill factor. Fill factor is
defined as
the percentage of the x-axis array dimension filled by radiation emitting
emitters. The
method of cooling of the diode bar could be convective, conductive or fluid
based and is
not directly material to the present invention.
The microlens element 21 is an optical element suitable for collimating the
fast
i o axis of a laser diode bar. It must be able to collect the high numerical
aperture beams
from the laser emitters in the y-axis without significant degradation in the
beam quality.
A specially designed spherical, aspherical or a graded index element may be
needed for
a specific set of design considerations. Microlenses for fast axis collimation
are
available from Blue Sky Research (Milpitas, CA), LIMO - Lissotschenko
Mikrooptik
15 GmbH (Dortmund, Germany) and NSG America, Inc (Somerset, NJ).
The microlens array 22 is an array of microlenses at a fixed pitch determined
by
the emitter geometry. The degree of overlap between the emitter radiation
profiles is
selected by choosing the pitch of the microlens array to be less than the
pitch of the
emitters on the laser diode array. A microlens pitch slightly less than the
emitter pitch
2 o will steer the radiation from outer emitters towards a central target
point causing the
overlap.
Regardless of how close together bar packages 15 are mounted, there will be
some dead space between them that must be taken into account. It is possible
to use
two individual microlens arrays but it is cheaper and simpler to use a single
array
25 element where a few microlenses in the centre are not used. For example, a
1 cm laser
diode array with 19 emitters spaced 500 m apart the spacing between adjacent
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to
s microlenses will be slightly less than 500 m. A 2 mm dead space between bars
would
result in not using four of the microlenses.
There are several options for aligning the bars and collimation optics. One
possibility is to fix the first bar and align the collimation optics to this
bar to achieve the
desired line profile 25 at a target plane. Once aligned, the collimation
optics are fixed
i o in place, and the second bar aligned to produce substantially the same
line profile at the
target plane. Another possibility is to fix the position of the collimation
optics and then
align both bars to the optics. Regardless of the method chosen there may be
the need
for iterative alignment where it is necessary to coarse align each element and
then more
finely align the elements in a second or even third pass.
1 s An advantage of the present embodiment is that the laser diode bars are
available
as standard items. Another advantage of this embodiment is that the beam
quality is
maintained in the fast axis, while doubling the available power and maximizing
brightness.
Another embodiment of the present invention shown in FIG. 4 is particularly
2 o advantageous in correcting misalignment between the bar diodes in the x-y
plane as well
as correcting deviations from straightness of the laser emitters. In FIG. 4,
laser diode
arrays 2 are each mounted on a base 20. Individual microlenses 31 are placed
in front
of each emitter of laser diode arrays 2. The microlenses are aligned in the y-
axis
direction to direct all emitter images towards line 25 on a target plane so
that they
2 5 averlap in the y-axis direction. At the same time the lines are overlapped
in the x-axis
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direction at the target plane, either partially or completely, by aligning
each microlens
31 in the x-axis.
An advantage of this embodiment is that the radiation from each emitter is
individually aligned allowing very precise overlapping at a target plane. With
care, an
extremely tight overlap can be achieved maximizing brightness.
i o Yet another embodiment shown in FIG. 5 is advantageous for a configuration
where the distance between the target plane 32 and microlenses 31 is reduced
thus
increasing the steering angle for outer emitter microlenses. These microlenses
have to
provide much more steering towards the target than central microlenses. This
means
that these outer microlenses end up aligned well off their optical axis
resulting in off
15 axis optical aberrations. The aberrations can degrade the uniformity of the
line profile,
which will likewise degrade the combined profile of all emitters.
In FIG. 5 an optical element 40 is introduced in front of the microlens array
that
has the effect of steering the radiation towards the centre of target plane 32
for outer
emitters while having lesser effect on inner emitters. In this embodiment the
microlens
2 o elements 31 are not required to steer the radiation in the x-z plane since
this steering is
now mostly provided by optical element 40. Microlens elements 31 can still
provide
minor corrections to steering but off-axis aberrations are reduced by the
inclusion of
optical element 40.
The optical element 40, as described, can also be added to the embodiment
25 depicted in FIG. 3 or any of the other embodiments detailed in this
disclosure. In each
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case, the addition of element 40 reduces the steering requirement on the
microlens
elements, thus reducing off-axis aberrations from outer emitter/microlens
combinations.
Yet another embodiment is depicted in FIG. 6, which has the advantage of
combining two laser diode arrays in a single package. This embodiment is
useful in
situations where space limitations are severe or where long-term stability of
the diode
1 o bar position is a critical issue. The dead space between adjacent bars can
also be
further reduced since the array positioning is now only dependent on array
placement
tolerances and not additional mechanical mounting tolerances. The common base
also
provides improved long-term stability of the relative bar positions since, in
general,
array bonding processes will result in lower long term drift than mechanically
mounting
15 two separate packages. The term "bonding" is used to indicate a process
whereby the
laser diode array is permanently fixed to a base. Improved stability is
important in
cases where the collimating optics are very sensitive to misalignment or when
the
absolutely highest line quality is sought.
In FIG 6 two laser diode arrays 2 are permanently bonded to a common base
2 0 50. The collimating elements are shown split into two pieces 21 and 21',
22 and 22' .
The need to split the optical elements for collimating each laser diode array
depends on
the optical sensitivity of the collimating elements and the mounting accuracy
of the
arrays. It is unlikely that laser diode array mounting tolerances can be
controlled to a
degree where a single element can be used as was shown in the previous
embodiment of
2 5 FIG. 3. Because the arrays are in fixed orientation after bonding any
alignment error
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between the two bars could not be eliminated if a one-piece collimation
element were
used.
The collimating schemes of the embodiments shown in FIG. 4 and FIG. 5 can
also be applied to the embodiment shown in FIG. 6. In this case individual
microlenses
are simply aligned to collect and direct the radiation from each emitter to a
target.
1 o It should be understood that the above descriptions of the preferred
embodiments
are intended for illustrative purposes only, and are not intended to limit the
scope of the
present invention in any way. Those skilled in the art will appreciate that
various
modifications can be made to the embodiments discussed above without departing
from
the spirit of the present invention.
