U.S. patent application number 12/230523 was filed with the patent office on 2010-03-04 for solid-state laser.
This patent application is currently assigned to COBOLT AB. Invention is credited to Gunnar Elgcrona, Jonas Hellstrom, Kenneth Joelsson.
Application Number | 20100054289 12/230523 |
Document ID | / |
Family ID | 41350726 |
Filed Date | 2010-03-04 |
United States Patent
Application |
20100054289 |
Kind Code |
A1 |
Hellstrom; Jonas ; et
al. |
March 4, 2010 |
Solid-state laser
Abstract
A method for assembling an optically pumped solid-state laser
having an extended cavity is disclosed. The method comprises the
steps of providing a casing, mounting a TEC and a base plate in the
casing, and mounting a plurality of laser components on the base
plate using a UV and heat curing adhesive. Once the laser
components are correctly positioned and aligned on the base plate,
the adhesive is pre-cured using UV radiation. Final curing of the
adhesive is obtained by subjecting the entire laser package to an
ambient temperature of at least 100.degree. C. The base plate is
preferably selected to have a CTE similar to that of the laser
components in order to facilitate the high temperature curing. A
preferred material for the base plate is AlSiC.
Inventors: |
Hellstrom; Jonas; (Solna,
SE) ; Elgcrona; Gunnar; (Solna, SE) ;
Joelsson; Kenneth; (Solna, SE) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
COBOLT AB
Solna
SE
|
Family ID: |
41350726 |
Appl. No.: |
12/230523 |
Filed: |
August 29, 2008 |
Current U.S.
Class: |
372/34 ;
257/E21.002; 438/26 |
Current CPC
Class: |
H01S 3/0405 20130101;
H01L 2924/0002 20130101; H01S 3/0401 20130101; H01L 23/38 20130101;
H01S 5/02325 20210101; H01S 3/109 20130101; H01S 3/042 20130101;
H01S 5/02438 20130101; H01S 3/09415 20130101; H01S 3/025 20130101;
H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
372/34 ; 438/26;
257/E21.002 |
International
Class: |
H01S 3/04 20060101
H01S003/04; H01L 21/02 20060101 H01L021/02 |
Claims
1. A method for assembling an optically pumped solid-state laser,
comprising the steps of: providing a casing; mounting a
thermoelectric cooler (TEC) in said casing; mounting a base plate
for laser components on said TEC; mounting a plurality of laser
components on said base plate using a heat curing adhesive, at
least some of said laser components forming a resonant cavity for
the solid-state laser; and subjecting said casing, said TEC, said
base plate and said laser components to an ambient temperature of
at least 100.degree. C. in a heat curing step in order to cure said
adhesive.
2. The method of claim 1, wherein said laser components are mounted
on said base plate using an adhesive that is both UV and heat
curing, and wherein said heat curing step is preceded by a UV
pre-curing step for fixing position and orientation of said
components on said base plate.
3. The method of claim 1, wherein the heat curing step is performed
at an ambient temperature of at least 120.degree. C.
4. The method of claim 1, wherein a thermally conductive adhesive
is used for mounting said TEC in said casing and said base plate on
said TEC, and wherein said thermally conductive adhesive is cured
in a first heat curing step before said laser components are
mounted on said base plate, said first heat curing step being
performed by subjecting the casing, the TEC and the base plate to
an ambient temperature of at least 120.degree. C.
5. The method of claim 4, wherein the first heat curing step for
curing the thermally conductive adhesive is performed at an ambient
temperature of about 150.degree. C.
6. The method of claim 1, wherein said base plate has a coefficient
of thermal expansion between 5 and 12 ppm/K for temperatures up to
about 150.degree. C. and a thermal conductivity of at least 50
W/mK.
7. The method of claim 1; wherein the base plate is made from a
material having a density of less than 5 g/cm.sup.3.
8. The method of claim 1, wherein the base plate is made from
AlSiC, and has a thickness in the range from about 6 mm to about 15
mm.
9. The method of claim 1, wherein the base plate is made from
AlSiC, and has a thickness of about 8 mm.
10. The method of claim 1, wherein the resonant cavity has an
optical path length of 10-30 mm.
11. An optically pumped solid-state laser assembly comprising: a
casing; a primary thermo-electric cooler (TEC) attached to said
casing; a base plate for laser components attached to said TEC; a
plurality of laser components attached to said base plate, at least
some of said laser components forming a resonant cavity for the
solid-state laser; wherein the base plate is made from a material
having a coefficient of thermal expansion between 5 and 12 ppm/K
for temperatures up to about 150.degree. C. and a thermal
conductivity of at least 50 W/mK; and wherein said laser components
are attached to said base plate using an adhesive that is both UV
and heat curable.
12. The laser assembly of claim 11, wherein the base plate is made
from a material having a density of less than 5 g/cm.sup.3.
13. The laser assembly of claim 11, wherein the base plate is made
from AlSiC.
14. The laser assembly of claim 11, wherein at least one of said
laser components is attached to said base plate via a secondary
thermoelectric cooler (TEC) located between the base plate and said
at least one laser component.
15. The laser assembly of claim 13; wherein the base plate has a
thickness in the range from 6 mm to 15 mm.
16. The laser assembly of claim 13, wherein the base plate has a
thickness of about 8 mm.
17. The laser assembly of claim 11, wherein the resonant cavity has
an optical path length of 10-30 mm.
18. The laser assembly of claim 11, wherein said primary TEC has a
substantially quadratic shape.
19. The laser assembly of claim 11, wherein said base plate has a
substantially quadratic shape.
20. A method for assembling a diode pumped solid-state laser,
comprising the steps of: providing a casing; mounting a
thermoelectric cooler (TEC) in said casing; mounting a base plate
made from Aluminum Silicon Carbide (AlSiC) on said TEC; applying a
thermally curable adhesive to said base plate; positioning and
aligning a plurality of laser components on said base plate in
contact with said adhesive to form a diode pumped solid-state
laser; pre-curing said adhesive, once said laser components have
been positioned and aligned, by exposing said adhesive to
ultraviolet radiation; and, following said pre-curing, thermally
cure said adhesive by subjecting said casing, said TEC, said base
plate, said adhesive and said laser components to an ambient
temperature of at least 100.degree. C.
21. The method of 20, wherein the adhesive is thermally cured by
subjecting the casing, the TEC, the base plate, the adhesive and
the laser components to an ambient temperature of about 120.degree.
C.
Description
TECHNICAL FIELD
[0001] The present invention generally relates to improvements in
or relating to optically pumped solid-state lasers. In particular,
the present invention relates to improvements in or relating to
solid-state lasers having extended cavities and a plurality of
laser components and optical surfaces.
RELATED ART
[0002] U.S. Pat. No. 5,170,409 discloses a resonator assembly
suited for a diode pumped solid-state laser. The resonator assembly
comprises a transparent support plate, e.g. formed from Pyrex,
having a coefficient of thermal expansion significantly lower than
common metals. Mirror mounts are bonded to the support plate using
a UV curable adhesive. Before curing the adhesive using UV
radiation, the position of the parts can be adjusted until proper
alignment is achieved. By using a support plate formed from a
material with a low coefficient of thermal expansion, the stability
of the laser is said to be enhanced.
[0003] U.S. Pat. No. 6,178,188 discloses a laser assembly platform
with a silicon base. Features are etched in the silicon base in
order to simplify alignment of laser elements during assembly. In a
comparatively elaborate soldering scheme, layers of metal, alloys,
solder and/or other materials are applied to the silicon base
before placing the laser components at the desired positions. Such
soldering process is said to be preferred over using a thermal
epoxy because thermal epoxy would require heating to high levels
(e.g. 85.degree. C. for twelve hours).
[0004] U.S. Pat. No. 6,758,609 discloses joining of optically
coupled optoelectronic and fiber optic components using
electromagnetic radiation. It is explained therein, that thermal
cure adhesives are typically more stable post-cure than UV-curable
adhesives and typically result in less moisture pick-up and better
mechanical properties. Unfortunately, thermal cure adhesives may
require a long cure time. In contrast, UV-curable adhesives cure
much faster than thermal cure adhesives. UV-curable adhesives
requires a line of sight for UV radiation to reach the adhesive to
be cured. This is explained to be a drawback in that special design
configurations of optoelectronic component assemblies may obstruct
light paths and thus cause poor curing of the adhesive. To overcome
these limitations of UV-curable adhesives, U.S. Pat. No. 6,758,609
proposes the use of non-ionizing radiation in the form of RF and
microwave radiation, which are said to be interchangeable, for
rapid curing of the adhesive.
[0005] US 2008/0008217 discloses a heat sink for a laser module,
the heat sink being configured to provide a relatively low thermal
resistance for thermal management of the laser. The heat sink is
also configured to provide a coefficient of thermal expansion (CTE)
that is substantially matched to the CTE of the laser. The heat
sink comprises a substrate made out of a first material. The
substrate includes via holes filled with a second material distinct
from the first material of the substrate. By properly selecting the
first and second materials, configuring the overall mass of the
substrate with respect to the overall mass of the filled via holes,
and positioning and arranging the filled via holes with respect to
the laser device, a desired effective thermal resistance and CTE
for the heat sink may be achieved.
[0006] In a paper by Occhionero et al., "AlSiC for Optoelectronic
Thermal Management and Packaging Designs", SPIE Proceedings, Vol.
5288, pp. 495-499, November 2003, which is incorporated herein by
reference, there is disclosed AlSiC materials and their use in
optoelectronic packages. With respect to the mechanical design of
optoelectronic packages, it is concluded therein that by choosing
materials that have compatible CTE, mechanical stresses are reduced
and overall module reliability is improved. The appropriate AlSiC
material composition selection will best be determined by
evaluating the CTE values of all of the materials in the
optoelectronics module. Thermally induced stresses during module
assembly and during module operation can be mitigated by selection
of a compatible material set.
[0007] Despite the above efforts, there have been issues in the art
relating to the manufacture of optically pumped solid-state lasers
that limits production, long-term stability and high output powers,
particularly for such lasers having an extended cavity with a
plurality of laser components and optical surfaces.
SUMMARY
[0008] The inventive technology disclosed herein aims generally
towards improving manufacture, performance and long-term stability
of optically pumped solid-state lasers. In particular, the
inventive technology aims towards advancing manufacture as well as
operational performance and longevity of lasers having an extended
cavities, i.e. non-microchip lasers, comprising multiple elements
and optical surfaces. An exemplary laser targeted by the inventive
improvements disclosed herein may have a cavity length of about
10-30 mm. For such a laser, although comparatively compact,
mechanical tolerances for proper operation are very demanding.
Typically, components and elements in the laser should meet
tolerances of about 0.1 mrad in angle and about 10 .mu.m in
translation.
[0009] One common type of optically pumped lasers is the diode
pumped solid-state laser (DPSSL), which in this specification will
serve as the illustrative, non-limiting example. In a basic
configuration, a DPSSL comprises a pump diode laser, an optically
pumpable gain medium that emits optical radiation when pumped by
the diode, and a resonant cavity enclosing the gain medium.
[0010] An identified source of laser component dealignment is
mechanical fatigue and stress induced by operational cycling of the
laser. During the life of the laser, it undergoes a large number of
operational cycles, i.e. turning the laser on and off, or varying
the output power of the laser. Since operation of the laser causes
temperature gradients in the laser, stresses induced by thermal
expansion may eventually lead to mechanical fatigue and output
power degradation. Similar stress on the laser can also be caused
by thermal shock and mechanical movement during transport.
[0011] An identified source of optical surface contamination is
out-gassing from adhesives used for securing laser components to
the base plate of the laser. Due to manufacturing concerns, laser
components are typically secured to the base plate using UV-curable
adhesives. Such adhesives are convenient, because curing can be
effected once all laser components have been properly positioned
and aligned. Over time, however, these adhesives may out-gas
contaminants that adhere to optical surfaces in the laser. Such
contamination is particularly harmful for sensitive,
short-wavelength quasi three-level lasers.
[0012] The inventive improvements disclosed herein proposes a
combination of a thermally and UV curable adhesive and a base plate
having a coefficient of thermal expansion (CTE) that is generally
matched to the laser components in order to allow heat curing in an
elevated temperature environment. For proper heat curing of the
adhesive used for fixing the laser components, heating to a
temperature of more than 100.degree. C. must typically be effected.
It is indeed a challenge to devise a laser having an extended
cavity and multiple laser components and optical surfaces, which
can withstand being heated as a package to above 100.degree. C.
Even temperatures as high as 150.degree. C. may be required in some
instances.
[0013] Using high temperature curing adhesives would greatly
improve the robustness and ruggedness of the lasers. However, and
as indicated above, the laser must be designed specifically for the
purpose of being heated to such high temperatures, or else the
laser would typically not be operational following the harsh high
temperature treatment.
[0014] A number of measures, each of which brings an improvement
facilitating high temperature treatment, are therefore suggested
here. It should be noted that each of these measures can be
employed in isolation for facilitating high temperature treatment,
and each measure can also be combined with any other measure for
further improvement.
[0015] Improvements are obtained by mounting laser components, such
as optical crystals, mirrors, pump diodes, filters, thermoelectric
coolers etc. on a base plate made from a material having a CTE that
is generally similar to the CTE of the components mounted
thereon.
[0016] For further improvement, it is also suggested that the high
temperature curing of the adhesive be performed in more than one
step. For a multi-step high temperature curing scheme, it is
preferred to use the highest curing temperature for the first step,
and then use lower curing temperatures for the or each following
curing step.
[0017] Even if the CTE of the base plate and the laser components
mounted thereon is matched, there may still be issues relating to
temperature gradients within the package unless dimensions of for
example the base plate are properly selected.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In the detailed description below, reference is made to the
accompanying drawings, on which:
[0019] FIG. 1 schematically shows a simplified sectional side view
of an exemplary solid-state laser assembly;
[0020] FIG. 2 schematically shows a top view of an exemplary
solid-state laser assembly; and
[0021] FIG. 3 outlines the inventive method for assembling a
solid-state laser.
DETAILED DESCRIPTION
[0022] Embodiments provide a new scheme for attaching laser
components to a base plate using both pre-curing by actinic
radiation, such as ultraviolet radiation, and final curing by heat
treatment. This new scheme enables greatly improved specification
parameters to be met in terms of operation/storage temperatures,
shock and vibration, humidity, power stability, lifetime, and
dynamic pointing stability.
[0023] In the detailed description below, reference is made to
diode pumped solid-state lasers as an illustrative, non-limiting
example. However, it should be understood that the inventive
contributions disclosed herein can be applied to any solid-state
laser having an extended cavity and a plurality of elements and
optical surfaces.
[0024] Components and materials used in assembling a DPSSL are
selected such that the CTE thereof is substantially matched. This
alleviates many of the previous problems relating to tension
building up in glue joints during temperature changes. It has been
identified that high tension in joints could otherwise lead to
cracks, which in turn may lead to components moving or falling off
the base plate, ultimately causing laser output power failure.
[0025] The idea of CTE matching also allows the use of a new class
of adhesives for securing components to the base plate. The most
attractive adhesives available require thermal curing at high
temperatures. However, for traditional DPSSLs having a base plate
of aluminum, brass or similar, the thermal expansion that would
ensue from high temperature thermal curing would cause movement of
the laser component in relation to the base plate, and also high
tension in glue joints after the adhesive has cured and the device
has cooled down. By using CTE matching, an entire laser package can
be subjected to temperatures above 100.degree. C. and even up to
150.degree. C. without significant deterioration due to any of the
above-mentioned problems.
[0026] The process for securing the laser components to the base
plate can briefly be summarized as follows. First the adhesive is
partially cured using actinic radiation, such as ultraviolet (UV)
light. Although the joints are not entirely solid, such pre-curing
will fix the position and orientation of the components relative to
each other and the base plate. The pre-curing of the adhesive
provides a very precise positioning of the laser components on the
base plate. After the adhesive has been pre-cured using UV
radiation, a final curing step is performed in which the entire
package is subjected to an ambient temperature typically between
100 and 150.degree. C. to bake out and fully cure the adhesive.
[0027] The possibility to use high temperature curing adhesives
also opens up for another attractive advantage, since such
adhesives typically have a very low out-gassing of constituents.
Traditional adhesives used for DPSSLs have a tendency to out-gas
constituents during the life of the laser, which leads to
degradation, particularly for short wavelength lasers, due to
contamination of optical surfaces. Avoiding such out-gassing will
maintain high performance for a prolonged period of time compared
to traditional lasers.
[0028] An exemplary DPSSL comprises an outer casing for
environmental protection. Within the outer casing, the actual laser
components are mounted. The laser components are mounted on a base
plate, and the base plate is secured within the outer casing. In
order to provide overall temperature control, the base plate is
secured to the outer casing by an intermediate thermoelectric
cooler (TEC). The TEC is used for removing excess heat from the
base plate for dissipation through the casing.
[0029] A preferred adhesive for mounting laser components on the
base plate is EPO-TEK.RTM. OG198-50, available from EPOXY
TECHNOLOGY, Inc., Billerica, Mass. This adhesive is a UV/heat cure
epoxy for optical applications having excellent thermal cycling
properties and high shear strength. The adhesive can be pre-cured
using 100 mW/cm.sup.2 of light at 300-400 nm. To fully cure the
adhesive, it is subjected to an elevated ambient temperature of
150.degree. C. for 30 min., or 120.degree. C. for 60 min.
[0030] A preferred class of materials for the CTE matched base
plate is Aluminum Silicon Carbide (AlSiC). AlSiC has a CTE that can
be varied from about 6 ppm/K to about 12 ppm/K, depending on the Al
content. AlSiC also exhibits an attractive thermal conductivity of
more than 150 W/mK at typical operational temperatures, and a
density of about 3 g/cm.sup.3. Known or future materials of similar
characteristics can also be used.
[0031] The combination of high performance adhesives that are heat
curable, and a base plate made from such material brings about a
number of advantages. For example, the low density of the AlSiC
base plate leads to a comparatively low inertia of the base plate,
which in turn improves the shock resistance of the laser device.
Nevertheless, there is some trade off between shock resistance and
thermal management for the base plate. If the base plate is very
thin, then the shock resistance of the laser package is excellent
since only a small inertial mass will be present. However, a thin
base plate is not as mechanically stable as a thicker base plate.
As a guideline for the required shock resistance, the laser package
should be able to withstand an impact of 60 g during 8 ms. In
general, the density of the base plate material should preferably
be less than about 5 g/cm.sup.3.
[0032] The inventors of the present technology have made
experiments using a base plate of AlSiC that was 2 mm thick. This
base plate was attached to (the cold side of) a TEC that covered a
large portion of the face of the base plate. The TEC was, in turn,
attached to the bottom of an outer casing made from Kovar.RTM..
Kovar.RTM., which per se is well known in the art, is an
iron-nickel-cobalt alloy with a CTE of about 5 ppm/K for
temperatures up to about 400.degree. C. When this package was
subjected to the high temperature curing for fixing laser
components to the base plate, thermal gradients in the base plate
caused severe bending that resulted in a laser that was not
operational following the high temperature treatment. Tests were
made using gradually increasing thickness for the base plate, and
it was then found that a minimum thickness for the base plate is
about 6 mm. Most preferably, the thickness of the base plate is
about 8 mm. Thicker base plates can also be used, such as 10 mm,
but eventually inertia becomes significant such that the shock
resistance of the laser package is limited. Excess inertia in the
base plate may lead to failure as a result of shock wherein the TEC
connecting the base plate to the casing simply breaks due to the
inertial forces generated. For practical applications, the base
plate should not be thicker than about 20 mm, and preferably not
thicker than about 15 mm.
[0033] FIG. 1 schematically shows a sectional side view of an
exemplary embodiment of a DPSSL package 10. The DPSSL package
comprises an outer casing having a bottom portion 12, one or more
wall portions 13, and a lid portion 14. Although not shown
explicitly in FIG. 1, it is preferred to have the bottom portion 12
and the wall portions 13 made in one piece. As can be seen from the
Figure, the lid portion 14 is beveled around its upper
circumference. On one of the beveled sides of the lid portion 14,
there is provided an output aperture 15 for the generated laser
beam. In the exemplary embodiment, the outer casing is made from
Kovar.RTM.. Within the outer casing, there is provided a
thermoelectric cooler (TEC) 16, upon which a base plate 17 is
attached. On the base plate 17, the operational laser components
are mounted. In order not to overly complicate the drawing, such
laser components are not shown in FIG. 1. The DPSSL package further
comprises electrical connections, which are also not shown in FIG.
1 for reasons of illustrative clarity.
[0034] The TEC 16 is used, during operation of the laser, for
removing heat from the base plate 17 and to dissipate such heat
into the bottom portion 12 of the outer casing. As can be
understood when studying FIG. 1, the TEC will experience high
forces when the laser package is subjected to shock. If the inertia
of the base plate 17 is too high, then the TEC may break, leading
to complete laser failure. Therefore, the thickness of the base
plate is carefully selected in view of thermal management as well
as mechanical shock resistance. In the exemplary embodiment, the
thickness of the base plate 17 is 8 mm. The laser package would
typically still have satisfactory performance for a thinner base
plate of 6 mm thickness, or a thicker base plate of 10 or 15 mm
thickness, but experiments have shown that 8 mm is preferred for a
base plate material such as AlSiC. Generally, a large contact
surface area between the base plate 17 and the TEC 16 facilitates
the use of thicker base plates, and vice versa.
[0035] It has also been found that thermal expansion management is
improved when using quadratic, rather than rectangular, TECs and
base plates. Thus, it is generally preferred to use a base plate 17
and a TEC 16 that are generally quadratic in shape.
[0036] The preferred material for the base plate is AlSiC. From a
purely thermal management and thermal expansion point of view,
however, also other materials could a priori seem attractive, such
as CuW or Kovar.RTM.. However, experiments have shown that CuW and
Kovar.RTM. have a density that is simply too high in order to
achieve acceptable shock resistance.
[0037] As described above, the TEC 16 is operative to remove excess
heat from the base plate 17 and to dissipate this heat through the
bottom portion 12 of the outer casing. In order to prevent large
temperature gradients in the bottom portion 12, the thickness of
the bottom portion 12 should be limited to a few millimeters. In
the exemplary embodiment illustrated in FIG. 1, the bottom portion
12 of the outer casing has a thickness of 2 mm. For operation, the
entire laser package should be mounted to a structure, e.g. using
mounting feet 204 (see FIG. 2), for receiving the heat dissipated
through the bottom portion 12.
[0038] FIG. 2 schematically shows a top view of an embodiment of a
DPSSL package 20. The DPSSL package shown in FIG. 2 may correspond
to DPSSL package 10 shown in FIG. 1. The package 20 has an outer
casing with side walls 202, which may correspond to the wall
portions 13 shown in FIG. 1. Inside the package 20, there is
provided a base plate 201. The base plate is preferably made from
AlSiC, and may correspond to the base plate 17 shown in FIG. 1. The
package 20 also comprises connection pins 203 for connection of the
DPSSL package 20 to an external controller, as well as package
mounting feet 204 for securing the package 20 to a mounting
structure. Although not explicitly shown in FIG. 2 in order not to
overly complicate the drawing, it should be understood that a
plurality of the connection pins 203 are electrically connected to
corresponding components in the DPSSL package 20. On the base plate
201, laser components for the DPSSL are mounted. A pump diode unit
205 is provided for optical pumping. The pump diode unit 205 may
also, in some embodiments, be mounted on a TEC, which in turn is
attached to the base plate 201. Optical pumping radiation emitted
by the pump diode unit 205 is conditioned by pump beam optics 206
and then incident into a laser crystal 207. Directly upon the face
of the laser crystal 207, a first cavity mirror is provided, and
the pumping radiation enters the laser crystal 207 through this
first cavity mirror. Upon optical pumping, the laser crystal 207
emits fundamental laser radiation, which is directed by a second
cavity mirror 208 towards a non-linear element 209. The non-linear
element 209 is mounted on a dedicated TEC 210 in order to control
the temperature of non-linear element 209. In the non-linear
element 209, a portion of the fundamental laser radiation is
converted into frequency converted radiation, and remaining
fundamental laser radiation is directed by way of a third cavity
mirror 211 towards a fourth cavity mirror assembly 212. Frequency
converted radiation passes through the third cavity mirror 211
towards beam steering optics 213 and 214 to exit the DPSSL package
20 in the form of laser beam output at 216. Before the laser beam
is output at 216, it passes a telescope unit 215 for forming the
laser beam to a desired spatial profile.
[0039] The base plate 201 is mounted on a TEC in the same manner as
shown for base plate 17 and TEC 16 in FIG. 1, in order to control
the overall temperature of the base plate 201.
[0040] Typical dimensions of the DPSSL package 20 are a few tens of
millimeters. For example, the footprint of the outer casing may be
50 mm.times.50 mm. Compared to, for example, a standard diode
laser, the dimensions are rather large. This means that the DPSSL
package 20 is quite sensitive to relative movement between the
individual laser components. For this reason, it is here proposed
to use an inventive type of assembly in order to minimize such
relative movement between individual laser components.
[0041] The laser components mounted on the upper side of base plate
201 are secured by means of an adhesive that is both UV and heat
curable. The process for securing the laser components to the base
plate includes applying a UV/heat curable adhesive to the base
plate 201; positioning any desired laser components on the base
plate 201 in contact with the adhesive; aligning the laser
components to form an operational DPSSL; pre-curing the adhesive by
exposing the adhesive to UV radiation; and heat curing the adhesive
by placing the entire base plate (or even the entire package) with
the laser components in an oven at an elevated ambient temperature
to fully cure the adhesive. The final heat curing step is typically
performed at ambient temperatures of above 100.degree. C., such as
120.degree. C. or 150.degree. C. For any previously known DPSSL,
such temperatures would severely degrade laser performance due to
relative movement of the laser components caused by differential
thermal expansion of the individual parts, as well as temperature
gradients within the package. Therefore, it is here proposed to use
a material for the base plate 201 that has a coefficient of thermal
expansion (CTE) that is similar to the laser components mounted on
the base plate. A particularly suitable material for the base plate
is, as mentioned above, AlSiC. The combination of using a CTE
matched base plate and a UV/heat curable adhesive is a particularly
attractive feature of this invention.
[0042] It may be preferred to employ a multi-step curing process
during assembly of the laser package.
[0043] For example, referring again to FIG. 1, the base plate 17,
the TEC 16 and the outer casing 12 (typically having integral side
walls 13) are assembled in a first curing step. For attaching the
TEC 16 to the base plate 17 and to the bottom 12 of the outer
casing, it is preferred to use a thermally conductive adhesive. One
preferred adhesive for this purpose is EPO-TEK.RTM. H77, available
from EPOXY TECHNOLOGY, Inc., Billerica, Mass. For curing the
adhesive, the package is subjected to an ambient temperature of
150.degree. C. for about 1 h.
[0044] In a second step, laser components are mounted on the base
plate using an optical adhesive that is both UV and heat curing. As
mentioned above, a preferred adhesive for mounting laser components
to the base plate is EPO-TEK.RTM. OG198-50. Once the laser
components have been properly positioned and aligned on the base
plate and in contact with the adhesive, the position and
orientation of the laser components are fixed by pre-curing the
adhesive using UV radiation. Then the second heat curing step is
performed by subjecting the entire laser package to an ambient
temperature of 120.degree. C. for about 1 h.
[0045] For the exemplary embodiment shown in the drawings, there is
actually also a final curing step in connection with welding the
lid 14 to the side walls 13. In order to expel moisture from the
package before sealing, the entire package is heated to 105.degree.
C. Although the primary motivation for raising the temperature
during welding of the lid is to remove moisture from the laser
package, it has the beneficial side effect to bring about some
degree of post-curing of the adhesives used for assembling the
package. Although not required in order to achieve the advantages
of the present invention, it is preferred to seal the laser package
in a nitrogen atmosphere in order to avoid aggressive oxygen within
the sealed laser package and to prevent contamination e.g.
particles and gases from entering the package.
[0046] In the second step described above for mounting laser
components to the base plate, it is sometimes advantageous to
perform several sub-steps. For example, all cavity elements except
one cavity end mirror may be mounted in the first sub-step. The
final cavity mirror can then be carefully positioned and aligned,
and ultimately fixed in a second sub-step. Similar to the first
sub-step, the final cavity mirror is positioned and aligned before
pre-curing the adhesive using UV radiation, and then fully cured by
subjecting the entire laser package to an ambient temperature of
120.degree. C. for about 1 h.
[0047] In order to outline the method for assembling an optically
pumped solid-state laser according to the present invention,
reference is made to FIG. 3.
[0048] In a first step S1, an outer casing is provided. The outer
casing (cf. also FIG. 1) is preferably made from Kovar.RTM. and has
a bottom surface and side walls made from one piece. As described
above, the bottom of the outer casing preferably has a thickness of
about 2 mm, in order to be sufficiently thin to avoid excess
temperature gradients therein when heat is removed from the laser
package and into a substructure upon which the laser package is
installed. In order to improve mechanical integrity of the casing
under thermal stress induced by heating, the footprint of the
casing is preferably quadratic in shape.
[0049] In a second step S2, a primary TEC is mounted in the casing
and a base plate is mounted on said TEC. The primary TEC has the
purpose of keeping the base plate at a desired operational
temperature during use of the laser. In almost all situations, this
means that heat should be removed from the base plate, and
therefore the primary TEC is mounted with its cool side towards the
base plate and its hot side towards the bottom of the outer casing.
For fixing the base plate to the TEC and the TEC to the casing, a
thermally conductive adhesive is used. The base plate is made from
a material that has similar coefficient of thermal expansion (CTE)
as the laser components that should be mounted on the base plate.
In general, it is preferred to have a base plate made from a
material having a CTE between 5 and 12 ppm/K for temperatures up to
about 150.degree. C. (typical curing temperature for the thermally
conductive adhesive). For practical reasons, the thermal
conductivity of the base plate should preferably be at least 50
W/mK. Moreover, in order to provide good shock resistance for the
laser package, and in particular to avoid failure during shock due
to breakage of the primary TEC induced by inertial forces, the base
plate is preferably made from a material having a density of less
than about 5 g/cm.sup.3 and has preferably a thickness in the range
6-15 mm (preferably about 8 mm). A particularly suitable material
that fulfils the above preferred characteristics is AlSiC. One
example of a suitable thermally conductive adhesive is EPO-TEK.RTM.
H77 mentioned above.
[0050] The thermally conductive adhesive used for fixing the base
plate to the TEC and the TEC to the casing may now optionally be
cured in a first high temperature curing step S3. This optional
curing of the thermally conductive adhesive can be performed, for
example, by subjecting the casing, the primary TEC and the base
plate to an ambient temperature of about 150.degree. C. for about 1
h.
[0051] Once the base plate and the primary TEC have been mounted in
the outer casing, with or without the optional first curing step
S3, a plurality of laser components are mounted on the base plate
to form a resonant cavity for the laser. The laser components are
fixed to the base plate using a UV and heat curing adhesive. Once
the laser components are positioned and aligned on the base plate,
the UV and heat curing adhesive is pre-cured using ultraviolet
radiation in step S5 in order to fix the position and orientation
of the laser components. A suitable UV and heat curing adhesive for
fixing the laser components to the base plate is the
above-mentioned EPO-TEK.RTM. OG198-50.
[0052] Once the laser components have been fixed in position and
orientation by UV curing the adhesive, the entire laser package is
subjected to a second high temperature curing step S6 in order to
fully bake out the adhesive. If the optional first curing step S3
was used, then the second curing step can be performed by
subjecting the laser package to an ambient temperature of about
120.degree. C. for about 1 h. If the first, optional curing step S3
was not used, then this latter curing is preferably performed at a
slightly higher temperature (e.g. 150.degree. C.) and/or for a
longer period of time (e.g. 2 h.).
[0053] It is also possible to, in a first instance of step S4,
mount fewer than all required laser components on the base plate
and then iterate steps S4-S6 until all required laser components
have been attached to the base plate. For example, it may be
advantageous to position all required laser components except for
one cavity mirror during the first instance of step S4, and then
carefully position and align this final mirror in an iteration of
steps S4-S6.
[0054] Once all desired components and connections have been
provided in the laser package according to the above, the laser
package is typically sealed by welding a lid to the upper side of
the outer casing side walls Although the invention has been
described above with reference to drawings and preferred
embodiments, it should be understood that various modifications are
possible without departing from the spirit and the scope of the
invention as defined by the claims.
* * * * *