Dewar cooling chamber for semiconductor platelets

A Dewar cooling chamber having a mounting assembly therein capable of supporting a semiconductor platelet for translational movement in the x, y axes and tilting movement about the z axis. Cooling of the semiconductor platelet continually takes place even while the platelet is being moved in three dimensions. This cooling is accomplished by means of a flexible, conductive loop of material which interconnects a coolant source to a clamp surrounding the platelet. The clamp fixedly secures the semiconductor platelet to the mounting assembly. The cooling chamber is capable of maintaining the semiconductor platelet at liquid nitrogen temperatures and is therefore extremely useful within a semiconductor laser system.

BACKGROUND OF THE INVENTION 
This invention relates generally to cooling chambers, and, more 
particularly, to a Dewar cooling chamber which is capable of mounting 
semiconductor platelets therein for movement in three dimensions while 
maintaining the semiconductor crystal at a low temperature. 
In recent years the use of semiconductor devices has expanded greatly. An 
area of particular interest involving semiconductors is the optically 
pumped semiconductor laser. In fact, recent advances in laser research 
have led to the development by the inventors of optically pumped 
semiconductor lasers which incorporate therein an external resonant 
cavity. Of particular interest are such lasers as described in U.S. patent 
application Ser. No. 361,021 entitled "Tunable CW Semiconductor Platelet 
Laser" and U.S. patent application Ser. No. 361,019 entitled 
"Synchronously Pumped Mode-Locked Semiconductor Laser", both applications 
being filed together with this patent application by the present 
inventors. 
As clearly pointed out in the above-mentioned U.S. patent applications Ser. 
No. 361,021 and Ser. No. 361,019, in order to provide optimun outputs, the 
semiconductor platelets must be cooled to liquid nitrogen temperatures or 
below. In addition, a threshold of approximately 100 KW/cm.sup.2 requires 
an extremely tight beam focus for CW or quasi-CW lasing because the total 
power demanded by a larger spot size would be sufficient to destroy the 
semiconductor crystal. Furthermore, a small spot size is also required to 
eliminate amplified spontaneous emission. Therefore, it becomes essential 
to provide a mounting arrangement for the semiconductor crystal which not 
only allows for precise alignment of the crystal, but also provides 
sufficient cooling of the crystal to take place. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problems encountered in the past and as 
set forth in detail hereinabove by providing a Dewar cooling chamber which 
is readily adaptable for use with semiconductor devices such as, for 
example, a semiconductor laser. The Dewar cooling chamber of this 
invention not only sufficiently cools the semiconductor crystal without 
adversely affecting lasing, but also allows for appropriate movement in 
three dimensions of the semiconductor crystal with the stability necessary 
for laser operation. 
Making up the Dewar cooling chamber of this invention is a preferably 
tubular-shaped vacuum housing having a substantially square cross-section. 
A pair of end plates seal the tubular housing with one of the end plates 
having a centrally located opening therein covered by a transparent window 
to allow a beam of electromagnetic energy to pass therethrough. 
The semiconductor platelet crystal utilized with this invention is held 
securely in place within the cooling chamber, but is also capable of being 
moved in three dimensions; two translational directions along the x, y 
axes, respectively, and tilting movement about the z axis by a uniquely 
designed holder assembly. In order to accommodate the three dimensional 
movement of the semiconductor, the semiconductor crystal is thermally 
connected to a coolant reservoir by a flexible, conductive sheet of 
material. 
Since the major utilization of the Dewar cooling chamber of this invention 
is within an optically pumped semiconductor laser of the type described in 
U.S. patent applications Ser. No. 361,021 and Ser. No. 361,019 referred to 
hereinabove, the thin semiconductor platelet lasing medium is mounted 
directly on a dielectric mirror prior to mounting within the chamber. A 
10X microscope objective, capable of spot diameters less than 5 .mu.m is 
adjustably mounted within the cooling chamber of the present invention for 
focusing both the pump and semiconductor laser beams. 
It is therefore an object of this invention to provide a Dewar cooling 
chamber capable of adequately cooling a semiconductor platelet as well as 
mounting a semiconductor platelet therein for three dimensional movement 
in a vacuum. 
It is another object of this invention to provide a Dewar cooling chamber 
which is readily adaptable for use within a semiconductor laser. 
It is a further object of this invention to provide a Dewar cooling chamber 
which is economical to produce and which utilizes conventional, currently 
available components that lend themselves to standard, mass producing, 
manufacturing techniques. 
For a better understanding of the present invention, together with other 
and further objects thereof, reference is made to the following 
description taken in conjunction with the accompanying drawing and its 
scope will be pointed out in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference is now made to FIGS. 1 and 2 of the drawing which clearly 
illustrate the Dewar cooling chamber 10 of this invention. Cooling chamber 
10 is made up of a housing 12 preferably being of a tubular configuration 
having a substantially square cross-section as shown in FIG. 2 of the 
drawing. Although not limited to the following dimensions, optimum outputs 
can be obtained from laser systems of the type described in U.S. patent 
applications Ser. No. 361,021 and Ser. No. 361,019 referred to above and 
described more specifically hereinafter by utilizing cooling chamber 10 of 
the present invention having such dimensions. For example, chamber 10 can 
be formed of a stainless steel tube having a square cross-section 
approximately 15.times.15 centimeters by 11 centimeters in length having a 
1.0 centimeter wall thickness. 
A pair of end plates 16 and 18 seal the tube with one of the end plates 16 
having a centrally located opening 20 therein covered by a window 22. 
Window 22 is transparent to the wavelengths of interest so as to permit 
passage therethrough of both an optical pumping beam 24 and a laser beam 
26 in the manner illustrated more clearly in FIGS. 3 and 4 of the drawing. 
Any conventional coolant reservoir 28 is situated on top of housing 12 and 
preferably contains liquid nitrogen which is used for cooling purposes. 
Since the Dewar cooling chamber 10 of this invention finds particular use 
within a laser system, reference is now made to FIGS. 3 and 4 of the 
drawing which schematically illustrate typical semiconductor lasers 30 and 
32 which incorporate cooling chamber 10 therein. Additionally, for ease of 
understanding of this invention identical reference numerals will be used 
in all figures of the drawing to represent the same basic elements. In 
this manner a fuller understanding of the present invention along with its 
detailed description set forth below can be made. 
FIG. 3 is representative of a tunable CW semiconductor platelet laser 30 of 
the type more fully described in the above-mentioned U.S. patent 
application Ser. No. 361,021. Laser utilizes 30 utilities Dewar cooling 
chamber 10 in conjunction with a rotatable output mirror 34, a prism 36, a 
polarizing beamsplitter 38, a continuous wave pump source 40 providing 
pump beam 24, a microscope objective 42, a lasing medium in the form of a 
semiconductor platelet crystal 44, an end mirror 46 preferably made of 
sapphire, and laser beam 26. 
FIG. 4 is representative of a synchronously pumped mode-locked 
semiconductor platelet laser 32 of the type more fully described in the 
above-mentioned U.S. patent application Ser. No. 361,019. Laser 32 
utilizes Dewar cooling chamber 10 in conjunction with a translatable 
output mirror 48, polarizing beamsplitter 38, an actively mode-locked pump 
source 52 providing pump beam 24, microscope objective 42, a lasing medium 
in the form of a semiconductor platelet crystal 44, end mirror 46 and 
laser beam 26. 
Although two specific illustrative examples of the use of cooling chamber 
10 are given above, it should be noted that these examples are not to be 
construed as the only use for cooling chamber 10. These examples are only 
presented so that a complete understanding and appreciation of the 
components and make-up of cooling chamber 10 of this invention set forth 
in detail hereinbelow can be had. As shown in FIGS. 3 and 4, both 
microscope objective 42 and the crystal/mirror sandwich 47 are located 
within the confines of cooling chamber 10. 
Reference is once again made to FIGS. 1 and 2 of the drawing for a detailed 
description of the cooling and mounting arrangement for semiconductor 
crystal 44 within cooling chamber 10. Two translational stages 60 and 62, 
preferably in the form of Klinger model MRS 80 25 are secured to back 
plate 18 of chamber 10 and crystal/mirror sandwich 47 in a manner 
described below to allow the translational movement of the crystal/mirror 
sandwich 47 to take place along the x, y axes in the directions indicated 
by the arrows shown in FIG. 1. These translational stages 60 and 62 are 
controlled by conventional micrometer heads 64 located outside of chamber 
10 and which protrude through the walls of cooling chamber 10. The 
spindles 66 of the micrometer heads 64 are pushed directly against the 
respective sides of translational stages 60 and 62 so as to allow fine 
adjustment of crystal/mirror sandwich 47 with micron accuracy. 
Crystal 44 is mounted upon the reflective surface of sapphire mirror 46 to 
form the crystal/mirror sandwich 47 which is optically aligned with the 
pumping and laser beams 24 and 26, respectively. The crystal/mirror 
sandwich 47 is held in position within chamber 10 by a mounting assembly 
68 and a mounting plate 70 preferably made of steel. As clearly 
illustrated in FIGS. 1 and 2, mounting assembly 68 is in the form of a 
triangular-shaped structure secured by means of compression springs 69 to 
mounting plate 70. A plurality of alignment pins 71 maintain alignment 
between mounting assembly 68 and mounting plate 70. As a result of this 
arrangement, movement of translational stages 60 and 62 causes movement of 
mounting assembly 68 to take place. In this manner fine adjustment, in the 
x, y directions of crystal/mirror sandwich 47 can be performed by 
appropriate rotation of micrometer heads 64. 
The triangular structure of mounting assembly 68 includes a plurality of 
quartz tubing in order to form a frame 72. A quartz central support 74 is 
slidably mounted upon frame 72 for coarse adjustment of the crystal/mirror 
sandwich prior to fine adjustment thereof by micrometer heads 64. A 
plurality of set screws 73 fixedly secure central support 74 to frame 72 
once the coarse adjustment of crystal/mirror sandwich 47 has been 
accomplished. 
Mounting assembly 68 (along with the crystal/mirror sandwich) can be tilted 
about the z axis with respect to plate 70 by turning a pair of screws 78 
and 80 located at the corners of mounting assembly 68 as shown in FIG. 1. 
The force of adjustment screws 78 and 80 as they are rotated acts against 
the force of springs 69 thereby providing a stable relationship between 
mounting assembly 68 and mounting plate 70 while the tilting movement of 
mounting assembly 68 takes place. 
Screws 78 and 80 are connected to vacuum feedthroughs with electroformed 
nickel bellows (not shown) and can be adjusted while the laser associated 
therewith is in operation. Quartz tubing is used for the material of 
mounting assembly 68 because it exhibits low thermal conductivity and very 
low thermal expansion, minimizing stresses generated when crystal 44 is 
cooled down. As shown in FIG. 2, three pieces of quartz tubing make up 
frame 72. The tubing is interconnected by stainless steel connectors 82 to 
complete mounting assembly 68. 
Referring more specifically to the mounting of crystal/mirror sandwich 47, 
sapphire mirror 46 is clamped to the quartz crystal central support 74 by 
a stiff copper ring 84 and a plurality of screws 86. A thin sheet of 
indium (not shown) may be provided between sapphire mirror 46 and copper 
ring 84 in order to insure a good thermal connection therebetween. The 
stiff copper clamping ring 84 is soft soldered to a flexible copper loop 
86 which is made up of approximately 20 wraps of thin copper sheet. This 
loop 86 allows movement of mounting plate 70, mounting assembly 68 and 
crystal/mirror sandwich 47 to take place by more than 1.5 cm. 
More particularly, loop 86 is made up of a spiral of a single piece of 
copper 250 cm.times.2.5 cm.times.50 .mu.m brazed together at the top and 
bottom. The top of the loop 86 is connected to a hollow element 88 
operably connected to the liquid nitrogen reservoir 28. Therefore, by 
feeding the liquid nitrogen into the hollow element the conductive loop 86 
transfers this reduced temperature to clamp ring 84. Clamp ring 84 can 
provide an adequate cooling environment surrounding crystal/mirror 
sandwich 47 without adversely affecting the lasing ability of 
semiconductor platelet 44. Additionally, the flexibility of the conductive 
loop 86 allows adjustment of the crystal/mirror sandwich 47 to take place 
without disturbing the cooling thereof. It is possible, if so desired, to 
loosely surround the quartz triangular shaped mounting assembly 68 and 
copper loop 86 by three layers of "super-insulation" such as aluminized 
Mylar foil in order to reduce radiated heat losses. 
The cooling chamber 10 can be pumped to a pressure of 20 m torr when used 
in conjunction with a laser before lasing operation commences by any 
conventional vacuum pump 90. A charcoal dessicant further reduces 
convection losses. Temperature on mounting assembly 68 can be measured by 
three platinum RTD detectors (not shown) if desired. 
The microscope objective 42 (Leitz EF 10/0.25P) located within cooling 
chamber 10 is chosen for its relatively low reflection losses, roughly 
approximately 4% per pass. It is slidably connected by means of 
outstanding element 92 to front plate 16 of chamber 10. Objective 42 can 
be moved parallel to the beam 26 for appropriate focusing onto crystal 44 
by any conventional means (not shown). Typically, lasing can be 
accomplished over a range of 200 .mu.m in the focal distance for a cavity 
length 1.8 meters. 
The cooling chamber 10 of this invention is capable of maintaining crystal 
44 at a stable temperature of approximately 82 K. It is capable of cooling 
down from room temperature in approximately ten minutes, and the 380 ml 
capacity of the liquid nitrogen reservoir 28 is sufficient to hold the 
temperature substantially constant for over six hours without refilling. 
Although this invention has been described with reference to a particular 
embodiment, it will be understood that this invention is also capable of 
further and other embodiments within the spirit and scope of the appended 
claims.