Red, green, blue upconversion laser pumped by single wavelength infrared laser source

A full color upconversion laser pumped by a single wavelength infrared laser source is disclosed. The pump energy excites a rare earth doped crystal and can simultaneously lead to laser emission at several wavelengths. The laser includes a crystal of YLiF.sub.4 :Er 5% fabricated in a monolithic structure which incorporates the laser mirrors as dielectric coatings on spherical surfaces of the crysal rod; the mirrors are optically reflecting at one or more of the desired output wavelengths. The laser rod is mounted in a helium cryostat that permits the operating temperature to be varied between 15.degree. and 120.degree. K. The pump energy is supplied through one of the mirrors specifically designed to be simultaneously highly reflecting at the laser wavelength and highly transmitting at the pump wavelength. To achieve optimum efficiency a lens is used in the pump path to focus the pump beam in such a manner as to provide a match of the laser mode size and pumped region of the crystal.

BACKGROUND OF THE INVENTION 
The present invention relates to lasers, and more particularly to an 
upconversion laser which can provide red, green and blue light from a 
single wavelength pump infrared laser. 
In a conventional solid state laser, optical pumping is used to achieve a 
population inversion. Absorption of pump photons populates an excited 
state of the active ion which in general lies above the initial laser 
level. 
Upconversion mechanisms that convert infrared radiation to visible 
radiation have been known for many years in the field of phosphors. Recent 
reports of laser pumped laser operation have also appeared based on these 
processes. See, e.g. "Ion-pair upconversion laser emission of Er.sup.3+ 
ions in YAG, YLF.sub.4, SrF.sub.2 and CaF.sub.2 crystals," S. A. Pollack 
and D. B. Chang, J. Appl. Phys., Vol. 64, page 2885 (1988); and "An 
infrared pumped erbium upconversion laser," A. J. Silversmith, W. Lenth, 
and R. M. MacFarlane, Appl. Phys. Lett., Vol. 51, page 1977 (1987). Using 
infrared pumping, laser emission has been obtained in the green, red and 
infrared. Laser operation at shorter wavelengths has required either a 
single yellow pump or a combination of IR and either yellow or red for all 
lasers operating at a wavelength shorter than 550 nm. 
Energy addition using two pump wavelength excitation has been reported for 
UV/violet/blue laser output. LaF.sub.3 :Nd.sup.3+ pumped simultaneously 
at 591 nm and 788 nm has produced 380 nm laser output. "Violet CW 
neodymium upconversion laser," R. M. Macfarlane, F. Tong, A. J. 
Silversmith and W. Lenth, Appl.Phys. Lett., Vol. 52, page 1300 (1988). 
YLiF.sub.4 :Tm.sup.3+ pumped at 649 nm and 781 nm has produced a pulsed 
output at 450 nm. "Blue-green (450 nm) upconversion Tm.sup.3+ :YLF laser," 
D. C. Nguyen, G. E. Faulkner and M. Dullick, Appl. Optics, Vol. 28, page 
3553 (1989). The yellow wavelengths are not available from semiconductor 
diode sources, though the 780 pump wavelengths are. 
Single pump experiments have produced 551 nm laser action in YLiF.sub.4 
:Er.sup.3+ for 820 nm excitation. "An Infrared pumped erbium upconversion 
laser," id. Laser operation at 413, 730 and 1053 nm has been reported for 
YLiF.sub.4 :Nd.sup.3+ pumped at 604 nm. "Laser emission at 413 and 730 nm 
in upconversion-pumped YLiF.sub.4 :Nd.sup.3+," F. Tong, R. M. Macfarlane 
and W. Lenth, Technical Digest of Conference on Quantum Electronics and 
Laser Science (Optical Society of America, Washington DC 1989), Paper 
THKK4. For CW excitation in the vicinity of 800 nm, obtainable from 
semiconductor diodes, only one visible output wavelength, 551 nm, is known 
to have been reported by other workers. 
The kinetics involved in the upconversion process involving rare earth ions 
are very complex and remain to be completely characterized. 
Red-green-blue multicolored displays using efficient small size sources are 
needed for simulation purposes in training systems and for automotive 
and/or aircraft displays of all kinds. The improved efficiency of 
semiconductor laser pumped systems and the small size that they can 
achieve would greatly expand the number of application areas. 
SUMMARY OF THE INVENTION 
In accordance with the invention, laser operation in the visible (blue, 
green and red) is obtained by absorption of less energetic infrared pump 
photons and the use of upconversion processes that permit the excitation 
of ionic energy levels which lie above that being directly accessed by the 
pump photons. A pump source at a single wavelength obtainable from 
semiconductor diode lasers, exciting a rare earth doped crystal, can 
simultaneously lead to laser emission at several wavelengths. 
In a preferred embodiment, the laser crystal rod is fabricated from 
YLiF.sub.4 :Er.sup.3+ 5% disposed in a cooling system which permits 
cooling of the rod to 15 to 120 degrees Kelvin during laser operation. A 
first upconversion laser mirror is disposed adjacent a first end of the 
laser rod through which the pump energy passes into the laser rod. The 
first mirror is characterized by high transmissivity at the pump energy 
wavelength and high reflectivity at the upconversion wavelengths. A second 
upconversion laser mirror is mounted adjacent a second end of the laser 
rod and is characterized by a transmissivity in the range of 1% to 10% at 
the upconversion wavelengths. The laser rod is excited by a pump beam at 
about 800 nm for this example to generate visible light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the invention, a red green blue upconversion laser 
pumped by a single wavelength infrared laser source is provided. The laser 
operates by optically exciting with the laser pump source specific energy 
levels of the rare earth ion in the host material. 
Referring now to FIG. 1, a simplified schematic block diagram of an 
upconversion laser 20 embodying the invention is illustrated. The laser 20 
comprises a laser crystal rod 35 of YLiF.sub.4 :Er5% fabricated in a 
monolithic structure which incorporates the laser mirrors 40 and 45 as 
dielectric coatings on spherical surfaces 42 and 47 of the rod 35 which 
are optically reflecting at one or more of the desired output wavelengths. 
The laser rod 35 in this exemplary embodiment has a length of 5 
millimeters, a diameter of 5 millimeters, and the convex spherical end 
surfaces 42 and 47 have radiuses of 3 centimeters. 
The laser rod 35 is mounted in a helium cryostat 50 that permits the 
operating temperature to be varied between 15 and 120 degrees Kelvin (K). 
The pump energy is supplied by a semiconductor infrared laser 25, or a 
laser of a different kind such as a dye laser or Titanium Sapphire laser 
operating at about 800 nm through the laser mirror 40 which is 
specifically designed to be simultaneously highly reflecting at the laser 
wavelengths and highly transmitting at the pump wavelength. To achieve 
optimum efficiency, a lens 30 is used in the pump path to focus the pump 
beam in such a manner as to provide a match of the laser mode size and 
pumped region of the crystal 35. The internal laser mode is indicated by 
dashed lines 55 (FIG. 1). 
The dielectric coating 40 is fabricated to have high reflectivity at the 
upconversion wavelengths (669 nm, 551 nm and 470 nm), and high 
transmissivity at the pump wavelength (780-820 nm). The dielectric coating 
45 is fabricated to have a transmissivity in the range of 1% to 10% at the 
upconversion wavelengths to provide a means of outputting laser light from 
the laser 20. Such dielectric coatings are well known in the art; coatings 
of this type are commercially available, e.g., from Virgo Optics, Inc. 
Port Richey, Fla., and CVI Laser Corp., Albuquerque, N. Mex. 
The laser pump source 25 for exciting the upconversion laser can most 
efficiently be used if arrangement is made to configure the path of the 
pump radiation inside the active cavity of the rod 35 in a manner that it 
overlaps the active mode being generated by the upconversion laser. This 
is conventionally known as "mode matching". To achieve this overlap, the 
Gaussian mode parameters of the upconversion laser oscillator (beam waist 
size and location, mirror spot sizes and phase front curvatures) are 
computed based on the wavelength, mirror curvatures, mirror spacing (the 
rod length for a monolithic structure as in the laser of FIG. 1) and 
refractive index of the material forming the crystal rod 35. This 
information is combined with the known Gaussian beam parameters of the 
pump source from which a design can be established of the transfer optics 
that deliver the pump beam to the input mirror surface of the upconversion 
laser. This optical system can be as simple as a single lens or a 
combination of two or more lenses that achieve a beam magnification if 
required. For example, a simple plano-convex lens of focal length 300 
millimeters has been used for one particular application. It is 
occasionally useful to actually focus the pump beam somewhat more tightly 
than just described in order to control the transverse mode structure of 
the oscillation upconversion laser mode. This is a situation where so 
doing introduces a peaked distribution of optical pumping in a plane 
transverse to the propagation direction that enhances the on-axis gain in 
order to preferentially produce the TEM.sub.00 mode. It is this transverse 
mode that is desirable for the majority of applications. 
A semiconductor infrared laser operating at 797 nm is suitable for the 
purpose of the pump laser source 25. Such infrared lasers are commercially 
available; two examples are the Model SDL-3490-S and the Model SDL-5412-H1 
semiconductor lasers marketed by Spectral Diode Laboratories, San Jose, 
Calif. 
FIG. 2 shows the relevant levels of the Erbium ion in the YLiF.sub.4 host 
material. Emission at 470, 551, 618, 669 and 702 nm has been produced by 
exciting the .sup.4 I.sub.9/2 level near 800 nm. The latter four of the 
laser wavelengths can also be obtained by excitation of the .sup.4 
I.sub.11/2 energy level at 980 nm. Such a pump source is provided by a 
Titanium Sapphire laser, Model Titan CW, marketed by Schwartz 
Electro-Optics, Orlando, Fla.; this laser is tunable over the range 
700-1050 nm. 
Table 1 shows a set of operating characteristics for the several systems 
with a pump excitation at 797 nm. 
TABLE 1 
______________________________________ 
YLiF.sub.4 ER5% - LASER TRANSITIONS 
797 nm Excitation - 2.1 W Pump 
Wavelength Temp 
Transition (Angstroms) Power Output 
.degree.K. 
______________________________________ 
.sup.2 P.sub.3/2 - .sup.4 I.sub.11/2 
4700.4 0.l mW 20K 
.sup.4 S.sub.3/2 - .sup.4 I.sub.15/2 
5510.8, 5513.5 
430 mW 70K 
.sup.4 G.sub.11/2 - .sup.4 I.sub.11/2 
6184.6 0.5 mW 41K 
.sup.4 F.sub.9/2 - .sup.4 I.sub.15/2 
6685. (3 .ANG. Wide) 
26 mW 41K 
.sup.2 H.sub.9/2 - .sup.4 I.sub.11/2 
70l5.4, 7023.4 
50 mW 41K 
.sup.4 I.sub.13/2 - .sup.4 I.sub.15/2 
16160. 167 mW 100K 
______________________________________ 
It is possible, by varying the rod operating temperatures, to select the 
particular color output by the laser, as described, in "Dual wavelength 
visible upconversion laser," R. A. MacFarlane, Appl. Phys. Lett., Vol. 54, 
page 2301 (1989). 
Instead of using a helium cryostat to cool the laser rod, other means may 
be employed, such as a closed cycle cooler that does not consume cryogenic 
fluid. One such exemplary device is the split-Stirling linear drive cooler 
available from Hughes Aircraft Company, Electron Dynamics Group, Torrance, 
Calif. as the model 7020 H linear drive cooler. 
Wavelength selection is also possible by introducing a dispersive element 
such as a prism into the laser cavity and adjusting its orientation, in a 
manner well known to those skilled in the art. Other wavelength tuning 
elements include diffraction gratings and birefringent filters, also well 
known in the art. Use of such well known wavelength selection or tuning 
elements is generally preferred in a practical application to the use of 
temperature control to achieve wavelength selection. 
Attention to material purity is important in the development of efficient 
laser crystals, because it is important to eliminate all unwanted loss 
mechanisms that are associated with energy transfer from the active ion to 
residual impurities. One preferred technique for obtaining high purity 
host materials is known as reactive atmosphere processing, useful for the 
preparation and purification of the desired starting materials, and at 
various times in the crystal growth stage. By providing for removal of 
dissolved or precipitated anion impurities, the physical properties of the 
crystals, including quantum efficiency, IR transparency and mechanical 
strength, can be vastly improved. Reactive atmosphere processing is 
described in several U.S. Pat. Nos. assigned to the assignee of this 
application, including 3,826,817; 3,932,597; 3,935,302; 3,969,491; 
4,076,574; 4,128,589; 4,190,640; 4,190,487; and 4,315,832. 
Fluorides are the presently preferred materials of choice for fabrication 
of the laser crystal rods in accordance with this invention. Fluorides 
have the advantage of high melting points (1000 to 1600 degrees 
Centigrade), stability in the ambient environment, and well-characterized 
spectra of rare-earth dopants. In one technique for preparing the crystal, 
the laser crystals are prepared from the highest purity oxides 
commercially available, usually characterized by a 10- to 100-ppm 
contaminant level. The host oxide is doped by diffusion of the required 
oxide of the ion at an elevated temperature, (700 degrees C.) to give a 
mixture of dopant and host that can be converted easily to a fluoride 
using the exchange reaction of F.sup.- for O.sup.2-. Anhydrous HF can be 
used as the exchange initiator. Alternatively, it is possible to prepare 
the fluorides directly from the starting high purity oxides or carbonates 
by using HF as the source of F.sup.-. Both methods yield ultra-high-purity 
fluoride powders that can be subsequently melted in a high temperature 
furnace using a reactive atmosphere, such as CF.sub.4. 
A Czochralski growth scenario may be employed where the grown crystal 
represents only a very small volume fraction of the melt to yield 
uniformly doped specimens. The Czochralski method, allows continuous 
viewing of the growing crystal, and affords the opportunity of aborting 
the run and restarting should polycrystallinity or control problems 
develop. Furthermore, with the Czochralski method, laser quality crystals 
requiring no annealing are quickly produced in a matter of hours rather 
than days. 
For the Czochralski growth of the fluorides of interest, the reactive 
atmospheric processing (RAP) is based on anhydrous HF and CO, with helium 
as a carrier gas. The CO is a secondary RAP agent in that it efficiently 
eliminates a source of hydrogen impurity, H.sub.2 O produced by 
outgassing, by chemical reaction to produce volatile CO.sub.2 and H.sub.2. 
The fluoride crystals doped with rare-earth ions can be prepared as 
described above, starting with the host/dopant oxide mixture for 
subsequent conversion to fluorides. With the starting materials contained 
in a platinum crucible in a furnace, the furnace is evacuated to 10.sup.-3 
Torr at room temperature, then backfilled with 2 atm of helium. For 
crystal growth, the following gas flow conditions can be established and 
maintained throughout the run: helium flow of 3 liters per minute (STP), 
HF flow of 0.2 liters per minute (STP), and CO flow of 0.5 liters per 
minute (STP). The temperature of the charge is raised to the melting 
temperature over a period of four hours. The crystal pulling rate will be 
about 4 mm/h using a pull rod rotation rate of 15 rpm, to obtain 5-cm-long 
crystals having a uniform diameter of 0.5 cm. 
The YLiF.sub.4 laser rods doped with erbium (5%) are also available 
commercially, e.g., from the Litton Airtron Division of Litton, Inc. 
An alternative embodiment of the upconversion laser is shown in FIG. 3. 
This laser 100 is similar to the laser 20 of FIG. 1 except that the laser 
mirrors 110 and 115 are external to the laser rod 105. The end surfaces of 
the laser rod are coated with antireflection AR coatings. AR coating 120 
provides a low reflectivity (on the order of 0.1 percent) at both the pump 
wavelength and the upconversion laser wavelengths (red, blue, green). AR 
coating 125 has a low reflectivity at the upconversion wavelengths. As 
with the laser 20, the mirror 110 has high reflectivity at the 
upconversion wavelengths, and high transmissivity at the pump wavelength. 
The mirror 115 has a transmissivity in the range of 1% to 10% at the 
upconversion laser wavelengths. 
For some laser transitions it could be useful to arrange a cascade laser 
combination in order to provide a mechanism for depopulating the lower 
laser level which, due to its long radiative lifetime, might represent a 
bottleneck to CW operation at the visible wavelength. For example, with 
respect to FIG. 2, it could be useful to provide laser mirrors on the ends 
of the rod that permit laser oscillation at 2.8 microns on the .sup.4 
I.sub.11/2 -.sup.4 I.sub.13/2 transition in order to depopulate the .sup.4 
I.sub.11/2 level during visible laser operation on transitions that 
terminate on that level. 
One realization of a cascade system concerns the operation of the 
upconversion laser at two (or possibly more) wavelengths that are 
associated with a ladder relationship of the energy levels in the sequence 
of optical transitions giving rise to laser operation. This is exemplified 
in the sequence of the blue laser transition, .sup.2 P.sub.3/2 -.sup.4 
I.sub.11/2, followed by the IR laser transition .sup.4 I.sub.11/2 -.sup.4 
I.sub.13/2. The latter process represents a mechanism for depopulating the 
lower laser level of the first laser transition in a manner that could 
permit CW (non-self-terminating) operation that otherwise might be 
prohibited by a population bottleneck in .sup.4 I.sub.11/2. This result 
can be achieved in a single pump arrangement by providing mirrors at the 
ends of the laser rod that simultaneously provide the appropriate 
reflectivity at each of the desired wavelengths where laser cascade 
operation is to occur. For example, to operate the laser at 470 nm, using 
a pump near 800 nm, the mirrors 40 and 45 can be provided with the 
following characteristics: 
Mirror 40: 
High transmission at 800 nm. 
High reflectivity at 470 nm. 
High reflectivity at 2.8 microns. 
Mirror 45: Transmission for output at 470 nm (1-10%). 
At 2.8 microns, can choose high reflectivity for zero output or 1-10% 
transmission for some output. 
Again dual wavelength output can be accomplished by controlling the 
transmission of the output mirror. 
A quite different cascade can be identified that could significantly 
improve the operation of the upconversion laser pumped by a single IR pump 
wavelength by permitting higher temperature operation. Here the IR pump is 
used to drive laser emission from one crystal and both this one laser 
wavelength and the residual transmitted pump energy are combined to pump a 
second laser crystal that could be a different host/dopant combination. 
FIG. 4 illustrates such a cascaded upconversion laser arrangement. Here 
first and second laser rods 205 and 210 are arranged along the axis of the 
pump beam 235 from the pump laser 230. The pump beam 235 is focussed 
through lens 225. The first laser rod 205 may be, e.g., Nd:YAG, and the 
second laser rod 210 may be, e.g., Er:YLiF.sub.4. One end 207 of rod 205 
has a spherical end surface to which has been applied a mirror 207 which 
has high transmissivity at the pump wavelength and high reflectivity at 
the first laser wavelength. These wavelengths for an exemplary application 
are 800 nm and 1.06 microns. The other end 208 of the first laser rod 205 
has a flat surface to which is applied a mirror 209, characterized by a 
transmissivity of 1% to 10% at the first laser wavelength, e.g., 1.06 
microns for Nd:YAG. 
The output light from the first laser 205, with residual pump light from 
the pump laser 230, enters the second laser rod 210 through the flat 
mirror 211, applied to a flat end surface of the rod 210. The other end of 
the laser rod 210 has a spherical surface to which is applied a second 
mirror 213. The first mirror 207 is characterized by high transmissivity 
at the pump and first laser wavelengths, and high reflectivity at the 
second laser wavelengths. The mirror 213 is characterized by a 
transmissivity of 1% to 10% at the second laser wavelengths. 
As an example, an 800 nm pump beam from pump source 230 to give 1.06 micron 
emission from a monolithic YAG:Nd source laser comprising the first laser 
rod 205 could combine to pump the YLiF.sub.4 :Er rod 210 with direct 
excitation into the .sup.4 I.sub.9/2 level of Erbium, as discussed above. 
The 1.06 micron laser radiation from the YAG:Nd laser rod 205 could be 
absorbed on the long lived metastable level .sup.4 I.sub.13/2 to populate 
the .sup.4 F.sub.9/2 level of Erbium, and thus result in a laser operating 
in the red, without the necessity of cooling to achieve the necessary 
relative radiative and phonon de-excitation rates needed for laser 
operation. The additional population achieved in level .sup.4 F.sub.9/2 by 
this means can result in further upconversion pumping by pair processes 
out of this level to produce population in the .sup.2 P.sub.3/2 level, 
which is the upper laser level for the blue 470 nm transition. In general, 
the introduction of energy by whatever means into these higher levels of 
the ion energy system can contribute to improved operation for all three, 
red, green and blue wavelengths. In this instance, it is expected that 
operation would be facilitated by the use of separate laser crystals or 
crystal pairs, one for each of the desired colors. 
While the foregoing exemplary embodiments have employed erbium as the rare 
earth ion dopant, other rare earths may alternatively be used; examples 
include neodymium, ytterbium, terbium, thulium and holmium. These rare 
earths are characterized by the property that the energy levels are not 
very dependent on the host material. Host materials for the laser rod 
other than YLiF.sub.4 may be employed. One particularly attractive 
alternate host material is BaY.sub.2 F.sub.8. Other alternate host 
materials for the laser rod include oxide and halide glasses, and 
fluorozirconate and silica glass fibers. 
It is understood that the above-described embodiments are merely 
illustrative of the possible specific embodiments which may represent 
principles of the present invention. Other arrangements may readily be 
devised in accordance with these principles by those skilled in the art 
without departing from the scope and spirit of the invention.