Composite laser array support

Nonmonolithic laser arrays having lasing elements mounted on a composite support that enables accurate positioning and separation of the lasing elements, and that enables low thermal, optical, and electrical cross-talk. The support includes a low thermal diffusivity region surrounded by high thermal diffusivity regions which have defined mounting surfaces onto which the lasing elements mount. Heat generated in the lasing elements is conducted away by the high thermal diffusivity regions, while the low thermal diffusivity region reduces thermal cross-talk between the lasing elements. Beneficially, the support assists current flow to (or from) the lasing elements.

The present invention relates to nonmonolithic laser arrays, their 
fabrication, and their assembly. 
BACKGROUND OF THE PRESENT INVENTION 
The performance of many devices, such as laser printers and optical 
memories, can be improved with laser arrays having independently 
controlled lasing elements. For example, laser printers which use an array 
of lasing elements can have higher printing speeds and better spot acuity 
than printers that use only a single lasing element. In many applications 
it is important that the array's lasing elements be accurately positioned 
and oriented. 
Monolithic laser arrays usually output light beams with the same 
wavelength. Typically, that wavelength can only be varied over a small 
range. However, in some applications, including color printing, it is 
desirable to output multiple wavelengths that span a wide range; for 
example, from the infrared through the visible. In color printing this 
enables one to match the laser output characteristics to photoreceptor 
response windows, or to separate overlapping laser beams by the use of 
dichroic filters. In some applications it may be desirable to emit 
multiple laser beams with different polarizations or spot profiles. 
Finally, it is almost always desirable to have low electrical, optical, 
and thermal crosstalk between lasing elements. 
As compared to present day monolithic laser arrays, nonmonolithic laser 
arrays can provide a greater range of laser beam characteristics (such as 
wavelength and polarization) and have lower electrical, optical, and 
thermal crosstalk. Thus, nonmonolithic laser arrays are frequently 
preferred. 
A nonmonolithic laser array typically consists of a plurality of individual 
laser diodes mounted on a support. Since in many applications the output 
laser beams must be accurately spaced, the supports should enable accurate 
positioning of the lasing elements, while not detracting from the 
advantages of nonmonolithic laser arrays. 
Prior art nonmonolithic semiconductor laser arrays usually use planar 
supports. These planar laser arrays have a major problem with how close 
the emitted laser beams can be spaced. This follows because a lasing 
eiement's laser stripe (the source of the laser beam) is generally placed 
at the center of the lasing element to avoid damage during the cutting of 
the wafer from which the lasing element is produced. Thus, in the prior 
art laser stripes could not easily be spaced any closer than the width of 
a lasing element if they are placed on a common planar substrate. 
Kato et al. in U.S. Pat. No. 4,901,325 teach a non-planar nonmonolithic 
laser array suitable for use with closely spaced lasing elements. A 
simplified view of that support in a nonmonolithic laser array is shown in 
FIG. 1. While the support 10 (with a spacer 12) enables the lasing 
elements 14 to be spaced within microns, absolute control of the lasing 
element spacing (how close the lasing elements are to their desired 
location) is not provided for. Further, the orientations of the lasing 
elements are not rigidly controlled. 
Thus, there exists a need for methods and devices that enable close, 
accurate spacing of lasing elements in a nonmonolithic laser array without 
excessive thermal, optical, and/or electrical cross-talk. Such methods and 
devices are even more desirable if they permit the accurate orientation of 
the lasing elements. 
SUMMARY OF THE INVENTION 
The present invention provides for nonmonolithic laser arrays comprised of 
lasing elements mounted on a support that enables accurate positioning and 
separation of the lasing elements, and that enables low thermal, optical, 
and electrical cross-talk. While the present invention is usable over a 
very large range of separations, it is particularly useful when separating 
lasing elements by less than about 250 microns. 
The support is a composite structure having a body from which a spacer 
protrudes. The support includes a low thermal diffusivity region 
surrounded by one or more high thermal diffusivity regions which have 
defined mounting surfaces onto which the lasing elements mount. Thus, the 
thickness of the spacer controls the horizontal separation of the lasing 
elements. 
If the body and/or the spacer are electrically insulative, an electrically 
conductive layer can be deposited over one or more of the external 
surfaces of the body and/or spacer, as required, to enable current flow to 
the lasing elements. Beneficially, the mounting surfaces of the lasing 
elements which connect to the support are electrical input terminals. The 
other terminals of the lasing elements connect via wires to their 
respective current sources. Thus, the support conducts current for both 
lasing elements. 
Heat generated by current flow through the lasing elements flows down the 
high thermal diffusivity regions of the spacer and into the body, which is 
beneficially heatsinked. The low thermal diffusivity region of the spacer 
provides a thermal barrier which reduces heat flow between the lasing 
elements. The spacer reduces or eliminates thermal cross-talk between the 
lasing elements, and also reduces the self-heating of the lasing elements 
(which causes laser droop). 
The support can be shaped to meet its particular application. In many cases 
the spacer will form a T-shape with the body. In another embodiment, the 
support has legs formed by a lengthwise slit through the spacer, while the 
legs have surfaces for receiving a lasing element. The lengthwise slit, 
which may be filled with a low thermal diffusivity material, forms the 
heat flow barrier. In another embodiment, the support is an integral unit 
of a low thermal diffusivity core sandwiched by a high thermal diffusivity 
material. Generally, the body will be in thermal connection with a 
heatsink.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
The following text makes reference to the characteristics of thermal 
diffusivity and thermal conductivity. Thermal diffusivity describes the 
ability of a material to transfer heat, while thermal conductivity is the 
product of thermal diffusivity and heat capacity. Since the object of the 
subsequently described composite structure is to optimize heat transfer 
away from heat generating lasing elements, thermal diffusivity is the 
characteristic of most interest. 
A typical lasing element used in the subsequently described embodiments is 
a semiconductor diode laser 20 as illustrated in FIG. 2. The diode laser 
20 is comprised of a substrate 22 doped to one electrical type (say 
n-type) having an overgrown multiple section epitaxial layer 24. The 
epitaxial layer 24 is comprised of 5 epilayers, 24a through 24e. The 
layers 24a and 24b are n-type; layers 24d and 24e are p-type; and layer 
24c is undoped. The various epilayers serve to confine the recombining 
carriers and the resulting emitted photons. An electrode layer 28 is 
formed over the layer 24. The electrode layer 28 can be patterned, or the 
material in layer 24 can be modified (for example by layer disordering or 
reverse doping), to confine the input current as required. A second 
electrode layer 30 is formed over the bottom of the substrate. The diode 
laser 20 is constructed such that current applied via the electrode layers 
28 and 30 causes the diode laser to emit light. 
As is well known, diode lasers require optical reflectors, which are 
usually implemented as cleaved facets, for operation. The optical 
reflector arrangement for the diode laser 20 is achieved using cleaved end 
faces 32 (only one of which is shown) which form a cavity for stimulated 
emission. During lasing, a laser beam 36 is emitted from the end face 32. 
To achieve the maximum spacing and positioning accuracy of the laser 
outputs the lasing elements 20 are mounted on the supports (described 
below) with the electrode layer 28 in electrical contact with the 
supports. This is advantageous because the thicknesses of the epilayers 
24a through 24e and the thickness of the electrode layer 28 are easier to 
control than the substrate thickness. Additionally, since the layers 24a 
through 24e and the layer 28 are very thin (about 2 .mu.m, inclusively), 
when mounted in this manner the laser beam outputs are very close to the 
supports. Thus, laser beam separation is essentially equal to the 
thickness of the support. Finally, mounting the lasing elements in this 
manner places the heat generating epilayers in good thermal contact with 
the support, thereby improving heat transfer. 
Referring now to FIG. 3, nonmonolithic laser arrays according to a first 
embodiment of the present invention have two lasing elements 20, 
designated 20a and 20b, mounted on a support 102. The support 102 is a 
T-shaped composite structure comprised of a low thermal diffusivity core 
104 surrounded by a high thermal diffusivity portion 106, which is 
electrically conductive (shown as portions 106a and 106b in FIG. 3). The 
lasing elements are mounted such that 1) they are in thermal contact with 
the support, and 2) their electrode layers 28 are in electrical contact 
with the high thermal diffusivity portion 106 (as described above). 
Illustratively, the core may be silicon while the high thermal diffusivity 
portion 106 may be copper, gold, or aluminum. 
In operation, electrical currents from current sources 108a and 108b are 
applied to the lasing elements 20a and 20b, respectively. The high thermal 
diffusivity portion 106 serves as a common conductor for both current 
sources. Considering now only lasing element 20a, current from the current 
source 108a is applied to the conductive layer 30 (see FIG. 2), passes 
through the lasing element 20a, causing it to emit light, flows from the 
lasing element 20a via the electrode layers 28 into and down the support 
102, flows through the support, and returns to the current source 108a. 
The lasing element 20b operates similarly with respect to current source 
108b. 
Current flow through the lasing elements generates heat. That heat flows 
down the high thermal diffusivity portion 106, cooling the lasing 
elements. This reduces laser droop and the chances of thermal run-away. 
Heat flowing through the high thermal diffusivity portion 106 is readily 
removed by proper heatsinking. The low thermal diffusivity core 104 
provides a thermal barrier which reduces heat flow between the lasing 
elements, and thus reduces thermal cross-talk. 
As all of the other described embodiments (below) electrically and 
thermally operate similarly, the above descriptions of electrical and 
thermal operations will not be repeated. 
The principles of the inventive nonmonolithic laser array may be practiced 
in many other embodiments than that shown in FIG. 3. For example, FIG. 4 
shows an alternative embodiment nonmonolithic laser array 150 which might 
be highly desirable in a specific application. The laser array 150 has two 
lasing elements 20, designated 20c and 20d mounted on a high thermal 
diffusivity support 152. The support 152 is comprised of a silicon inner 
core 154 surrounded by a diamond outer core 156. As diamond is 
electrically insulative, to provide a current path for the lasing elements 
20c and 20d, a conductive layer 158 is deposited over at least part of the 
outer core 156. Wires 160 connect the lasing elements and the conductive 
layer 158 to current sources (not shown in FIG. 4). The diamond/silicon 
composite structure makes a very effective support structure. 
An alternative nonmonolithic laser array 170, similar to the structure 
shown in FIG. 4, is shown in FIG. 5. The laser array 170 is comprised of 
two lasing elements 20, designated 20e and 20f mounted on a high thermal 
diffusivity composite sheet 172. The composite sheet is comprised of a 
silicon core 174 sandwiched between diamond sheets 176 which have a thin 
electrically conductive layer over their outside surfaces. As in FIG. 4, 
the lasing elements mount in electrical contact with the thin electrically 
conductive layer, and in thermal contact with the diamond sheets. A copper 
heatsink 178 provides a thermal sink for the composite sheet 172. 
Yet another nonmonolithic laser array 200 is shown in FIG. 6. The laser 
array 200 is comprised of two lasing elements 20, designated 20g and 20h, 
mounted on legs 202a and 202b, respectively, of a support 204. Between the 
legs is a gap 206 that may be filled with a low thermal diffusivity 
material. 
While the variously depicted embodiments work over a wide range of lasing 
element separations, they are particularly useful when the separation is 
less than about 250 .mu.m. This is because as the separation decreases, 
the need for thermal insulation between the lasing elements increases. 
The materials comprising the various elements of the specific embodiment 
being implemented should be selected to accomplish the goals of that 
specific element. 
A useful table for selecting materials via their thermal properties is: 
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Thermal Properties of Various Materials at 300 K. 
Heat Thermal 
Capacity Conductivity 
Diffusivity 
Material (J/cm.sup.3 /.degree.K.) 
(W/cm/.degree.K.) 
(cm.sup.2 /sec) 
______________________________________ 
Al 2.43 2.37 0.98 
Cu 3.43 3.98 1.16 
Au 2.48 3.15 1.27 
Pt 2.84 0.73 0.26 
Ag 2.48 4.27 1.72 
Si 1.63 1.41 0.86 
Diamond (CVD) 
1.82 12.00 6.59 
Diamond (Type IIa) 
1.82 20.00 10.99 
Ni 3.94 0.63 0.16 
Sn 1.25 0.73 0.59 
In 1.71 0.86 0.50 
______________________________________ 
The various embodiments can be fabricated in many ways. For example, a high 
thermal diffusivity material can be coated on a low thermal diffusivity 
material; a high thermal diffusivity material can be bonded (say with 
epoxy) onto a low thermal diffusivity material; or two high thermal 
diffusivity layers can be physically separated by air or by a low thermal 
diffusivity material (as in FIG. 6 ). 
Coatings of high thermal diffusivity materials on low thermal diffusivity 
materials can be accomplished in several ways, such as electroplating or 
electrodeposition, evaporation, or sputtering of metals, or by material 
growth, such as CVD diamond growth on silicon, nickel or platinum. When 
using electroplating, the process must be done under conditions which 
yield a compact film free of voids. Copper, silver and gold have been 
electroplated successfully onto silicon T shapes to thicknesses of 10 
.mu.m (deposition time of about 48 minutes). 
If electrodepositing metal onto a silicon T shaped submount (reference FIG. 
4), deposition is best performed after the submount is soldered to a 
copper heat sink using a solder such as Sn:Ag - 95:5. Presently, the 
bottom of the inverted T is prepared for soldering using evaporated Cr:Au. 
The sides of the silicon T are then coated with sputtered Ni:Pt. 
Electrodeposition is then performed, followed by sputtering a thin Ni:Pt 
layer on the inverted silicon T. Finally, In (Indium) is evaporated onto 
the silicon T. Electroless catalytic metal coatings might also be usable 
in fabricating the high thermal diffusivity elements. CVD diamond grown 
directly on silicon submounts is also possible. 
It is also possible to solder the various elements together. If solder is 
used, it is beneficial to use a high melting temperature solder, such as 
In-Sn, to avoid problems when the lasing elements are soldered in place 
(see below). 
Attachment of the lasing elements to the various supports is best performed 
using a low temperature solder, such as In. First, prior to soldering, the 
indium pellets used for soldering are immersed in a dilute hydrochloric 
acid solution for oxide removal. Then, the spacer's mounting surfaces are 
prepared for soldering by sputter deposition of a thin layer of nickel, 
followed by a thin layer of platinum. Solder is then deposited onto the 
spacers using thermal evaporation of the indium pellets from tungsten 
boats. The objective is to deposit a film that is thick enough for 
planarization and wetting, but thin enough to allow insignificant material 
flow. A good In film thickness is around 2-2.5 .mu.m. Next, the lasing 
elements are brought into close proximity with the indium layer on the 
spacer and aligned. Then, using visual observation, the temperature of the 
spacer is raised above the melting temperature of the indium solder and 
the lasing elements 20 are pressed into place using a vacuum collet. The 
vacuum is then released, but physical pressure with the collet is 
maintained. The heating source is then turned off and a cooling nitrogen 
gas stream is applied to the lasing element. When the solder has 
solidified the pressure on the lasing element is released. Cooling to room 
temperature then continues. 
The soldering procedure described above can be modified to fit the 
particular application and materials. However, in all cases surface 
preparation should be performed carefully to ensure good, reliable thermal 
and electrical connections. 
From the foregoing, numerous modifications and variations of the principles 
of the present invention will be obvious to those skilled in its art. 
Therefore the scope of the present invention is to be defined by the 
appended claims.