Assembly of cooled laser diode arrays

An assembly of N laser diode arrays. Each diode is fitted to the end of a base which forms the anode and is traversed by a hole. The holes of all the bases define a channel for the circulation of fluid for cooling the assembly of diode arrays. Each module includes an array, a base and a corresponding cathode. The modules are separated from each other by a flat joint or gasket.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the field of assemblies of laser diode arrays and 
their cooling systems. 
The fields of application of assemblies of laser diode arrays are numerous 
and can be grouped into two major families: 
applications where the laser radiation of the diodes is used directly 
(beams shaped by optics or carried by fibres) and essentially relating to 
the medical sector and the industrial sector (marking); 
applications where the laser radiation of the diodes is used for the 
optical pumping of laser media (energy storage media or nonlinear effect 
media), e.g. the optical pumping of crystalline materials, such as oxides 
or fluorides doped with ions of rare earths (Nd, Er, Tm, Ho, Yb, Pr, Ce, . 
. . ) or Cr, Co and similar ions, optical pumping of laser material, such 
as dyes and the interaction with nonlinear effect media (generation of 
harmonics, parametric amplifiers or oscillators). 
2. Discussion of the Background 
Laser diode arrays and in more general terms laser diodes, have an 
optical/electrical efficiency of approximately 25 to 33%. Consequently the 
power lost by heating represents 66 to 75% of the consumed electrical 
power. In a continuous 20 Watt array, the active volume (semiconductor) is 
generally very small (micrometric dimensions) and typically 0.5 to 1 
mm.sup.3, whilst the thermal power density is between 60 and 160 
kW/cm.sup.3. The array is generally welded to a base (made from brass or 
copper), which has several functions: 
a rigid, mechanical support function, 
an electrical function (anode), 
a thermal function, due to a high thermal conductivity. 
In all commercial laser diode arrays, heat conduction takes place from the 
bottom, through the substrate and in general the anode base. This base 
must be in contact with a cold source in order to remove the thermal power 
and limit the temperature rise in the semiconductor. The connection in the 
upper part (cathode) cannot generally ensure an adequate thermal 
conductivity and the heat exchange takes place by natural convection of 
the ambient air. 
In order to create a cold source, it is either possible to use a metal 
(brass or copper) cold box, or a Peltier effect thermocouple. 
These procedures assume a good contact between the base of the array and 
the cold source, in order to minimize the contact thermal resistivity. The 
Peltier effect technology is very widely used in low power applications 
and when there is only a limited number of diodes. There is no example of 
the use of this technology for continuous power arrays (typically 21 Watt 
on average). 
A so-called "microchannel" technology is described in U.S. Pat. No. 
5,105,429 and U.S. Pat. No. 5,105,430. This technology is based on the 
assembly of modules in order to form a bidimensional emitting structure, 
each module having a diode array placed on a substrate with microchannel 
cooling. 
Improvements or variants have been made, relating to the internal 
architecture of the structure, the construction of the stacks or the way 
in which the cooling fluid circulates. The principle of cooling by a 
microchannel structure remains the same, there being the benefit of a 
large exchange surface (radiator) by having a network of fins in a 
conductive material. The material can be the actual semiconductor or a 
material having a good heat conduction, such as silicon, copper or 
diamond. 
The microchannel radiator can be integrated into each array base in order 
to have an autonomous module, or can be common to assemblies of arrays 
(surface of bidimensional assembly). 
The field of application of microchannel technology is that of stacks of 
arrays. 1 cm wide or wider array stacks make it possible to obtain 
emissive areas of several square centimeters. This is the most widely used 
alternative in connection with surface emission diode networks. For the 
reasons indicated below, the development of the microchannel method is 
very delicate. 
Firstly, this microchannel method is only accessible by highly 
sophisticated etching technologies. The size of the channels is also 
approximately 100 microns, with a spacing of 50 to 150 microns. Therefore 
the cooling liquid must be filtered with great care so as not to clog the 
network of microchannels. This leads to a significant pressure loss which, 
for reasonable fluid flow rates of approximately one liter per minute, 
assume high pressures (exceeding 4 bars). 
Finally, in the case of optical pumping, the known stacks of microchannel 
arrays do not make it possible to pump media, e.g. having a cylindrical 
shape. A more conventional technology consists of assembling the arrays in 
stack form and cooling them from the rear in contact with a water box. 
This technology can only be envisaged for quasi-continuous emission 
arrays, i.e. for average powers of 1 to 4 Watts and remains the solution 
proposed on commercial products. 
U.S. Pat. No. 5,031,184 describes a semiconductor pumping diode cooling 
device. This device has a complex structure, because it involves an 
assembly on Peltier elements, which themselves rest on a base. 
No information is given concerning the assembly of the system and 
particularly the retention of the parts and joints with respect to the 
base. 
In such a device, it would appear to be difficult to be able to guarantee 
the sealing and mechanical strength of the system, particularly as a 
positioning from below and the two sides is difficult to implement. 
Moreover, thermal expansions may give rise to mechanical stresses which 
are difficult to contain within such an assembly. 
From the thermal standpoint, it would appear to be very difficult with the 
device of U.S. Pat. No. 5,031,184 to finally regulate the temperature of 
the semiconductors by means of Peltier elements, when the latter are 
sandwiched between two water cooling systems, whose function is to ensure 
a good heat removal due to a good heat exchange coefficient. Although this 
principle may be usable for a single element it cannot be used for two 
elements, because the effect of the first Peltier element would have a 
consequence on the second element, making the second Peltier element 
compensate the defect, giving rise to the same consequences on the third 
element and so on. 
SUMMARY OF THE INVENTION 
The object of the invention is to propose a linear, modular assembly of 
laser diode arrays integrating the cooling system necessary on the one 
hand for the removal of the thermal power dissipated by the arrays, and on 
the other for thermostatting the emitters, i.e. the stability of the 
emission wavelength of the diodes. 
Therefore, the invention relates to an assembly of N laser diode arrays, 
characterized in that: 
each diode is assembled on the end of a base forming an anode and traversed 
by a hole, all the holes defining a channel for the circulation of a fluid 
for cooling the linear assembly of diode arrays, each module, constituted 
by an array, the base and a cathode, being separated from each immediately 
adjacent module by a flat joint, 
each base being provided with a second hole, all the second holes being 
aligned when the arrays are juxtaposed in the operating position, 
two holding flanges are placed at the ends of the linear assembly of 
arrays, said flanges being tightened by a screw, which passes through the 
assembly through all the second holes. 
This device permits an optimum integration of the mechanical and thermal 
functions. 
Thus, according to the invention, it is possible to ensure a linear 
assembly of laser diodes and simultaneously a direct cooling of the base 
making it possible to reduce the thermal resistance of the assembly. The 
cooling on the one hand makes it possible to evacuate the thermal power 
dissipated by the arrays and on the other to stabilize the emission 
wavelength of the diodes. The presence of a flat joint or gasket between 
two immediately adjacent modules ensures the cooling fluid sealing, but 
also the electrical independence of the modules. 
Such a linear assembly or device permits the pumping of laser media having 
an elongated or cylindrical shape. 
Two holding or retaining flanges are placed at the ends of the linear 
assembly of arrays and fixing means (all the second holes and screw) make 
it possible to ensure the necessary mechanical strength, the screw 
permitting the tightening of the flanges. Said screw traverses the 
assembly and can be peripherally covered with an insulating sheath. 
Electrical connection means can be provided for supplying the diode arrays. 
A series supply can e.g. be provided, in which the anode associated with 
each array is connected to the cathode of a neighbouring array.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The single drawing illustrates and embodiment of a linear, modular assembly 
of laser diode arrays integrating the cooling system. The assembly is 
obtained from identical parts (unitary module), which are assembled in 
juxtaposed manner (linear assembly). 
Each laser diode array 4 (typical dimensions: width 1 cm, depth 0.5 to 0.6 
mm, height 0.1 to 0.2 mm) is welded to a base 6, which is in the present 
case an anode base, whose width slightly exceeds that of the array. The 
base is made from metal, e.g. copper or brass and is optionally covered by 
electrolytic deposition with a gold and nickel coating permitting welding 
by brazing. The emission from each array takes place at the junctions 
(between semiconductors) 5, assembled in groups and whose dimensions are a 
few micrometers or a few dozen micrometers. 
The array 4 is fitted to one of the ends 8 of the base 6, which has two 
holes 10, 12 completely passing through it in its width direction. The 
first hole 10 is positioned as close as possible to the location of the 
array 4, so as to ensure a good heat exchange between the array and the 
cooling fluid and is used for the circulation of the latter. Said first 
hole 10 e.g. has a diameter of 2 mm and is positioned at less than 2.5 mm 
from the array 4, which ensures a good heat exchange. The heat exchange is 
of the forced convection type, by turbulent mode fluid. The diameter of 
the hole 10 and the fluid flow rate are preferably determined in such a 
way as to optimize the pressure losses. Preferably, the fluid is 
circulated so as to optimize the pressure losses. 
The second hole 12, e.g. with a diameter of 5.2 mm and in a central 
position, permits the passage of a screw holding together the modules. 
With each anode 6 corresponds a cathode 14 and an insulator 16, positioned 
between the cathode and the anode. 
In the drawing identical unitary modules 20, 22, 24, 26 are linearly 
juxtaposed. Each module is separated from each neighbouring module by a 
joint 32, 34, 36, 38. In each joint, holes corresponding to the holes 10, 
12, made in each base, give a free passage for the circulation of a 
cooling fluid and for fixing means 42, which ensure the holding of the 
assembly. The electrical insulation between neighbouring cathodes is 
obtained in that the cathodes are narrower than the anodes. 
The joints ensure on the one hand the mutual electrical insulation of the 
arrays and on the other, the sealing of the assembly with respect to the 
cooling fluid, whose circulation is designated in the drawing by arrow 44. 
The fluid traverses all the unitary modules in the same way, the assembly 
automatically ensuring the circulation of the fluid through the succession 
of bases and joints. 
The assembly is held by its ends using two flanges 46, 48, which serve as 
an inlet and outlet connection for the cooling fluid. These two flanges 
are kept integral with the assembly by fixing means 42, which hold 
together the modules. In the embodiment given, the flanges are tightened 
by a screw 42, which traverses the assembly. Said screw is peripherally 
covered with an insulating sheath, in order to ensure the mutual 
electrical insulation of the modules. 
In the linear assembly according to the invention, the diode arrays can be 
supplied electrically independently of one another. 
Therefore different types of electrical connections can be envisaged, in 
particular a series-type supply where the anode of each array (base) is 
connected to the cathode of its neighbour. No matter which solution is 
retained, it can be implemented by means of a connector 50 fitted on the 
rear face of the assembly. This connector can be made from an insulating 
material (e.g. Delrin or Lucoflex), to which are connected metal 
connectors with dimensions and shape adapted to the desired assembly and 
to the current carried. The electrical connection takes place by means of 
metal screws 52 in the anode and cathode parts, through the insulating 
block. 
The device described ensures the effective cooling of the continuous power 
arrays, by means of a low thermal resistivity, e.g. approximately 
0.7.degree. C./W, for a water flow rate of approximately 1 liter/minute. 
It is also autonomous, because it ensures the mechanical assembly with one 
another of the arrays without any need for a supplementary support. 
Finally, said device has the advantage of being modular and permits the 
assembly of a number of unitary modules, e.g. between one and about ten.