Tunable CW semiconductor platelet laser

A tunable CW semiconductor platelet laser having an optical pump source and an external resonant cavity. The resonant cavity is bounded by a pair of spaced-apart reflective elements and has a prism therein. One of the reflective elements is partially transmissive and rotatable about its vertical axis. The semiconductor platelet is mounted upon the other reflective element and both are mounted within a temperature controlled, vacuum environment while being capable of being moved in three dimensions. The prism located within the resonant cavity aids in laser tuning upon rotation of the partially transmissive reflective element.

BACKGROUND OF THE INVENTION 
This invention relates generally to semiconductor lasers and, more 
particularly, to a continuous wave optically pumped tunable semiconductor 
laser having an external resonant cavity. 
Lasers exist in many shapes and forms, yet the search for new types of 
lasers continues unabated. Lasers vary greatly in many aspects such as, 
for example, power, operating wavelength, cavity design, method of pumping 
and mode discipline (mode-locking, single-frequency, or chaotic 
operation). The single, most frequent means of laser identification is by 
the type of gain medium utilized within the laser, since the medium will 
strongly influence, if not dictate, the other considerations of laser 
design. 
Optically pumped semiconductor lasers are of especially great interest 
because of their potential for becoming a convenient, tunable, coherent 
source of electromagnetic radiation throughout the visible and near IR 
range of the spectrum. The most distinguishing feature of a semiconductor 
laser is that it does not deal with gain centers (atoms, ions, molecules, 
complexes) sparsely distributed in a passive medium or empty space, but 
rather with the phenomena of inverting the atoms in an entire block of 
solid, unlike any other kind of laser. Since the absorption and gain 
lengths in a semiconductor laser are very small compared to other lasers, 
this central fact greatly influences the choice of pumping scheme: active 
medium, heatsinking, cavity design, and sample geometry. Furthermore, 
semiconductors are crystals and their ordering implies spatial anisotropy 
(selection rules) and polarization, since polarization effects generally 
depend on crystal orientation. 
A single bulk semiconductor is the simplest amplifying medium since it is 
cheap, readily available, and requires far less processing than 
heterostructure. To date, most optically pumped semiconductor lasers have 
used either crystal faces or closely attached mirrors as the cavity 
reflectors; this, unfortunately, prevents the insertion of tuning elements 
into the cavity and lowers its optical quality. Furthermore, because of 
high threshold pump powers and severe heating problems, semiconductor 
lasers in use today involve short pump pulses. 
Additionally, most semiconductor crystals must be cooled to liquid nitrogen 
temperatures or below. Other problems arise with prior semiconductor 
lasers since a threshold of roughly 100 KW/cm requires a very tight beam 
focus for cw or quasi-cw lasing because the total power demanded by a 
larger spot size would destroy the crystal used therewith. The small spot 
size is also required to eliminate amplified spontaneous emission. 
As is clearly evident from the description above, semiconductor lasers, 
and, in particular, optically pumped semiconductor lasers although 
potentially highly desirable currently have numerous drawbacks which 
render them less than effective under certain circumstances. It would be 
extremely beneficial to produce an optically pumped semiconductor laser 
which does not fall victim to the above-mentioned shortcomings. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problems encountered in the past and as 
set forth in detail hereinabove by providing an optically pumped tunable 
semiconductor laser having an external resonant cavity. 
The optically pumped tunable semiconductor laser of this invention relies 
upon the use of an external resonant cavity. This is accomplished by 
utilizing a semiconductor crystal in the form of, for example, a cadmium 
sulfide platelet crystal which is mounted on one side thereof to a support 
in the form of, for example, a piece of sapphire. The same side of the 
sapphire to which the semiconductor platelet is secured is also 
dielectrically coated with a maximum reflectivity mirror. This mirror acts 
as one end of the external resonant cavity of the laser of this invention. 
Completing the other end of the resonant cavity is a rotatably mounted 
99.5% reflecting output mirror which is optically aligned with the 
sapphire mirror. 
Optically interposed between the two end reflective surfaces or mirrors of 
the external cavity is a polarizing beamsplitter which allows for the 
introduction of an optical pump beam from a suitable source, such as an 
AR.sup.+ laser. Also included within the external cavity is a prism so 
that the laser beam which passes through the prism can be tuned by 
rotating the output mirror about its vertical axis. The pump beam is 
focused onto the crystal platelet to a spot size of approximately 5 
micrometers by a 10.times. microscope objective which also serves to 
collimate the crystal fluorescence. 
It is essential in this invention that the sapphire end mirror and 
semiconductor platelet crystal lasing medium be located within a cooling 
chamber for appropriate lasing action to take place. In addition, since 
the distance between the microscope objective and crystal is small 
(.about.7 mm) the microscope objective must also be placed within the 
cooling chamber. 
The sapphire end mirror is held in place within the cooling chamber by a 
uniquely designed mount. Liquid nitrogen is thermally connected to the 
crystal/sapphire mount for cooling purposes. Also located within the 
vacuum/cooling chamber are other suitable mounts which permit movement of 
the sapphire end mirror and semiconductor platelet in three dimensions so 
that spots on different crystals can be utilized, the objective can be 
brought into focus, and proper alignment of the resonant cavity can be 
made. 
Proper utilization of the prism located within the cavity in conjunction 
with the 99.5% output mirror enables the laser of this invention to be 
tuned to three different Fabry-Perot modes, covering the range between 495 
and 501 nm. Within the strongest of these three modes the laser can be 
tuned to over 1.8 nm with a 0.1 nm bandwidth. The wavelengths between 
these modes can be reached by moving the crystal laterally so that the 
crystal Fabry-Perot length changes slightly. In addition, with a raise in 
temperature of the semiconductor crystal from 95 to 140 K., the laser 
wavelength can be increased from 497 to 504 nm. Further variance of the 
wavelength over this range can be accomplished by a combination of 
temperature and prism tuning. At temperatures above 140 K. the damage 
threshold is comparable to the laser threshold. 
It is therefore an object of this invention to provide an optically pumped 
semiconductor laser having an external cavity. 
It is also an object of this invention to provide a continuous wave 
optically pumped tunable semiconductor laser having an external cavity. 
It is another object of this invention to provide a semiconductor laser in 
which there are no jet fluctuations, therefore eliminating a very strong 
source of noise. 
It is still another object of this invention to provide a semiconductor 
laser which can be operated completely in a vacuum, eliminating 
atmospheric pressure fluctuations. 
It is a further object of this invention to provide a semiconductor laser 
which allows a stabilized signal-frequency laser to operate with fewer 
wavelength selecting elements than in the past while tuning can be 
accomplished by a variance of the temperature. 
It is still a further object of this invention to provide a semiconductor 
laser 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 FIG. 1 of the drawing which clearly illustrates in 
schematic fashion the various components which make up the optically 
pumped tunable semiconductor laser 10 of this invention. Laser 10 of the 
present invention incorporates therein an external resonant cavity 11. 
Resonant cavity 11 is bounded at one end thereof by a first reflective 
element 12 in the form of, for example, a piece of sapphire being 
dielectrically coated on one side thereof with a maximum reflectivity 
mirror. The other end of the resonant cavity 11 is formed by a partially 
reflective, rotatable output element in the form of, for example, mirror 
14 preferably having a 99.5% reflectivity. 
The lasing medium of this invention is made up of a very thin (&lt;5 .mu.m) 
semiconductor crystal 16 such as cadmium sulfide (CdS), CdS.sub.x 
Se.sub.1-x or In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y which is chosen for 
flatness and parallism by observing under a microscope the interference 
patterns created by a sodium lamp. The semiconductor crystal 16 is mounted 
upon the reflective surface of sapphire 12 using a thin film of low 
viscosity silicone oil applied on sapphire 12 next to crystal 16. 
Semiconductor crystal 16 held in place on sapphire 16 by surface tension. 
The oil layer is often less than 5 micrometers thick and does not crack 
when cooled. The crystal-mirror sandwich is held in place within a 
vacuum/cooling chamber 18 for movement in three dimensions. This movement 
includes translational movement along the x, y axes as well as two tilting 
directions about the z axis in a manner to be explained in detail 
hereinbelow. 
The lasing medium or semiconductor crystal 16 is optically pumped 
longitudinally by a laser beam 18 emanating from any suitable laser source 
20 such as a continuous wave Ar.sup.+ laser. Pump beam 18 is directed into 
the resonant cavity 11 by a conventional polarizing beamsplitter 22 
located therein. Focusing of pump beam 18 onto crystal 16 to a spot size 
of approximately 5 micrometers is accomplished by means of a conventional 
10.times. microscope objective 24 which is adjustably positioned within 
vacuum/cooling chamber 18. 
The laser beam 26 produced by semiconductor laser 10 of this invention is 
separated from pump beam 18 by polarizing beamsplitter 22 which transmits 
98% of the CdS emission. In order to make this possible, the c-axis of the 
CdS crystal 16 is vertically oriented. Then its fluorescense which 
primarily has ELc, is polarized perpendicularly to the vertically 
polarized optically pumping laser beam 18. 
Also located within the resonant cavity 11 is a prism 28 through which 
laser beam 26 is passed. The output beam can be tuned by appropriately 
rotating output mirror 14 about its vertical axis as shown by the arrow in 
FIG. 1. Rotational movement of mirror 14 can be accomplished either 
manually or by any conventional rotating device such as motor 30. 
Reference is now made to the vacuum/cooling chamber 18 shown in both FIGS. 
1 and 2 of the drawing. It is essential in the optically pumped tunable 
semiconductor laser 10 of this invention to locate the lasing medium, that 
is, semiconductor crystal 16, in a vacuum in which cooling thereof can 
take place. The vacuum/cooling chamber 18 forming part of this invention 
is not only capable of accommodating crystal 16 under the appropriate 
temperature conditions, but is also capable of moving crystal 16 along the 
x-y axis as well as tilting crystal 16 about the z axis. 
More specifically, vacuum/cooling chamber 18 is made up of a housing 32 
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 with laser 10 of 
this invention with chamber 18 being 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 34 and 36 seal the tube with one of the end plates 34 
having a centrally located opening 38 therein covered by a transparent 
material 40, transparent to the wavelengths of interest so as to permit 
passage therethrough of both the optical pumping beam 18 and laser beam 
26. Any conventional coolant reservoir 42 is situated on top of the 
housing 32 and preferably contains liquid nitrogen which is used for 
cooling purposes. Both the microscope objective 24 and the crystal/mirror 
sandwich are located within the confines of the vacuum/cooling chamber 18 
in a manner to be described in detail hereinbelow. 
Reference is once again made to FIGS. 1 and 2 of the drawing for a detailed 
description of the cooling and mounting arrangement for crystal 16. Two 
translational stages 44 and 46, preferably in the form of Klinger model 
MRS 80 25 are secured to back plate 36 of chamber 18 and crystal/sapphire 
sandwich described below to allow the translational movement of the 
crystal/mirror sandwich to take place in the directions indicated by the 
arrows shown in FIG. 1. These translational stages 44 and 46 are 
controlled by conventional micrometer heads 48 located outside of chamber 
18 and which protrude through the walls of vacuum/cooling chamber 18. The 
spindles 50 of the micrometer heads 48 are pushed directly against the 
respective sides of translational stages 44 and 46 so as to allow the 
appropriate movement of the crystal/mirror sandwich with micron accuracy. 
Crystal 16 and sapphire 12 are centrally located and optically aligned with 
the laser and pumping beams 18 and 26, respectively, and held in position 
by mounting assembly 52 and a mounting plate 53 preferably made of steel. 
As clearly illustrated in FIG. 2, mounting assembly 52 is in the form of a 
triangular-shaped structure supported by mounting plate 53 for slight 
movement therewith with respect to housing 32. The triangular structure 
includes a plurality of quartz tubing 54 and a quartz central support 55 
slidably mounted thereon for coarse adjustment. Mounting assembly 52 can 
be tilted in two directions with respect to plate 53 by turning a pair of 
screws 59 and 61 located at the corners of mounting assembly 52 as shown 
in FIG. 1. The screws are connected to vacuum feedthroughs with 
electroformed nickel bellows (not shown) and can be adjusted while the 
laser is in operation. Quartz tubing is used for the material of mounting 
assembly 52 because it exhibits low thermal conductivity and very low 
thermal expansion, minimizing stresses generated when crystal 16 is cooled 
down. As shown in FIG. 2, three pieces of quartz tubing 54 are 
interconnected by stainless steel connectors 63 to complete mounting 
assembly 52. 
Sapphire mirror 12 is clamped to the quartz crystal support 55 by a stiff 
copper ring 56 and a plurality of screws 65. A thin sheet of indium (not 
shown) may be provided between sapphire mirror 12 and copper ring 56 in 
order to insure a good thermal connection therebetween. The stiff copper 
support ring 56 is soft soldered to a flexible copper loop 58 which is 
made up of approximately 20 wraps of thin copper sheet. This loop 58 
allows transverse movement of mounting plate 55 and crystal 16 to take 
place by more than 1.5 cm in both directions. 
More specifically, loop 58 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 58 is connected to a hollow cold finger 60 
operably connected to the liquid nitrogen reservoir 42. It is possible, if 
so desired, to loosely surround the quartz triangular shaped mounting 
assembly 52 and copper loop 58 by three layers of "super-insulation" such 
as aluminized Mylar foil in order to reduce radiated heat losses. 
The vacuum-cooling chamber 18 can be pumped to a pressure of 20 m torr 
before lasing operation commences by any conventional vacuum pump 62. A 
charcoal dessicant further reduces convection losses. Temperature on 
mounting assembly 52 can be measured by 3 platinum RTD detectors (not 
shown) if desired. 
The microscope objective 24 (Leitz EF 10/.25P) located within 
vacuum/cooling chamber 18 is chosen for its relatively low reflection 
losses, roughly approximately 4% per pass. It is slidably connected by 
means of outstanding element 64 to front plate 34 of chamber 18. Objective 
24 can be moved parallel to the beam 26 for appropriate focusing onto 
crystal 16 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 vacuum/cooling chamber 18 maintains crystal 16 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 42 is sufficient to hold the temperature substantially 
constant for over six hours without refilling. 
With prism 28 inserted within resonant cavity 11 and with a 99.5% output 
mirror 14, semiconductor laser 10 of this invention can be tuned to three 
different Fabry-Perot modes, covering the range between 495 and 501 nm. 
With the strongest of these three modes, it can be tuned over 1.8 nm with 
a 0.1 nm bandwidth. The wavelength between these modes can be reached by 
moving crystal 16 laterally so that the Fabry-Perot length changes 
slightly. In addition, when the temperature of crystal 16 is raised from 
95 to 140 K., the laser wavelength increases from 497 to 504 nm. The 
wavelength can be varied continuously over this range by a combination of 
temperature and prism tuning and by the use of rotational output mirror 
14. 
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. For example, although an Ar.sup.+ laser 20 is generally utilized 
as the pumping laser, lasing could also be accomplished with the 488-, 
473-, and 458-nm lines. Thus, pumping even 200 meV above the band gap is 
possible without absorbion-depth problems. 
It is further possible with this invention to extend the use of laser 10 to 
a variety of other semiconductor elements such as, for example, CdSe CdSSe 
and InGaAsP crystals using a 514-nm Ar.sup.+ pump or other pump sources 
such as a Kr.sup.+ laser. Furthermore, several crystals can be mounted 
adjacent each other on the same sapphire mirror 12 as shown in phantom in 
FIG. 2, yielding a laser 10 which is easily tunable from 500 to 700 nm. 
Other crystals of ZnCdS can also be used. Heating problems in crystal 16 
might be further reduced by the use of epitaxial layers of CdS grown on 
sapphire 12. 
The semiconductor laser 10 of this invention eliminates jet fluctuations 
which are generally present in dye lasers and which are generally a very 
strong source of noise. In addition, laser 10 can be operated completely 
in a vacuum, eliminating atmospheric pressure fluctuations present in dye 
laser cavities. 
In addition, the spontaneous-emission spectrum is narrower with 
semiconductor laser 10 than those of dyes, allowing a stabilized 
signal-frequency laser to operate with fewer wavelength selecting 
elements, while tuning can be accomplished by varying the temperature 
within vacuum/cooling chamber 18. Furthermore, laser 10 of this invention 
has the capability for single-frequency operation tunable throughout most 
of the visible and the near IR region.