CA2358201A1 - Method and apparatus for illuminating a spatial light modulator - Google Patents

Abstract

Methods are disclosed for combining the radiation from two or more multiple emitter laser diode arrays in such a way that brightness is conserved. The methods have particular application in constructing a radiation line source of high power and good beam quality suitable for illuminating a spatial light modulator.
Apparatus for illuminating spatial light modulators is also disclosed.

Description

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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, 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.

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.

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 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;

~ 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;

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.

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 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 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 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 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 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.

Claims (4)

WHAT IS CLAIMED IS:

1. A method for combining the radiation from two or more laser diode arrays, each of said arrays having a front surface, said front surface having a plurality of emitting areas thereon, said method comprising;
a) mounting two or more laser diode arrays on a base, each said laser diode array having a front surface comprising a plurality of emitting areas thereon, with said front surfaces substantially in a common emitting plane and adjacent to each other, said emitting areas of said two or more laser diode arrays forming a substantially straight line in said common emitting plane;
b) collecting radiation in the fast axis and directing said radiation towards a spatial light modulator, said spatial light modulator spaced apart from said common emitting plane;
c) collecting and steering said radiation in the slow axis towards said spatial light modulator;
whereby an illumination line is formed at said spatial light modulator.

The method of claim 1 wherein said mounting comprises bonding said laser diode arrays to a common base.

3. The method of claim 1 wherein said mounting comprises:
a) bonding each of said laser diode arrays to a first base;
b) mounting each said first base on a second base.

4. An optical apparatus for illuminating a spatial light modulator comprising:
a) two or more laser diode arrays, each of said arrays having a front surface, said front surface having a plurality of emitting areas thereon, said laser diode arrays mounted adjacent to each other, said front surfaces substantially in a common plane, said emitting areas of said two or more laser diode arrays forming a substantially straight line in said common plane;
b) at least one optical element disposed to collect and direct said beams of light to at least partially overlap at said spatial light modulator.