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| United States Patent |
6,304,584
|
|
Krupke
, et al.
|
October 16, 2001
|
Blue diode-pumped solid-state-laser based on ytterbium doped laser crystals
operating on the resonance zero-phonon transition
Abstract
The invention provides an efficient, compact means of generating blue laser
light at a wavelength near .about.493+/-3 nm, based on the use of a laser
diode-pumped Yb-doped laser crystal emitting on its zero phonon line (ZPL)
resonance transition at a wavelength near .about.986+/-6 nm, whose
fundamental infrared output radiation is harmonically doubled into the
blue spectral region. The invention is applied to the excitation of
biofluorescent dyes (in the .about.490-496 nm spectral region) utilized in
flow cytometry, immunoassay, DNA sequencing, and other biofluorescence
instruments. The preferred host crystals have strong ZPL fluorecence
(laser) transitions lying in the spectral range from .about.980 to
.about.992 nm (so that when frequency-doubled, they produce output
radiation in the spectral range from 490 to 496 nm). Alternate preferred
Yb doped tungstate crystals, such as Yb:KY(WO.sub.4).sub.2, may be
configured to lase on the resonant ZPL transition near 981 nm (in lieu of
the normal 1025 nm transition). The laser light is then doubled in the
blue at 490.5 nm.
| Inventors:
|
Krupke; William F. (Pleasanton, CA);
Payne; Stephen A. (Castro Valley, CA);
Marshall; Christopher D. (Livermore, CA)
|
| Assignee:
|
The Regents of the University of California (Oakland, CA)
|
| Appl. No.:
|
187327 |
| Filed:
|
November 6, 1998 |
| Current U.S. Class: |
372/22; 372/20; 372/21; 372/36; 372/41; 372/75 |
| Intern'l Class: |
H01S 003/10; H01S 003/04; H01S 003/16; H01S 003/091 |
| Field of Search: |
372/75,22,36,41,20,21
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Font; Frank G.
Assistant Examiner: Rodriguez; Armando
Attorney, Agent or Firm: Wooldridge; John P., Thompson; Alan H.
Goverment Interests
The United States Government has rights in this invention pursuant to
Contract No. W-7405-ENG-48 between the United States Department of Energy
and the University of California for the operation of Lawrence Livermore
National Laboratory.
Claims
What is claimed is:
1. A ytterbium-doped solid state laser with an output wavelength in the
.about.490-496 nm region, comprising:
a resonant optical cavity;
a ytterbium-doped solid state laser gain medium within said resonant
optical cavity;
means for optically pumping said laser gain medium;
an inter-cavity polarizer for forcing said solid state laser gain medium to
generate laser light on the zero-phonon-line (ZPL) transition of the
2F5/2-2F7/2 resonance emission band of said gain medium, wherein said
inter-cavity polarizer is oriented to allow transmission of polarized
light matching the predominant polarization of said ZPL transition of said
ytterbium-doped solid state laser gain medium, while providing loss for
non-ZPL transitions emitting predominantly in the orthogonal polarization
with respect to the polarization of said ZPL of said ytterbium-doped solid
state laser gain medium; and
a nonlinear harmonic doubler crystal for doubling the frequency of said
zero-phonon-line (ZPL) transition of the 2F5/2-2F7/2 resonance emission
band of said gain medium.
2. The ytterbium-doped solid state laser of claim 1, wherein said means for
optically pumping said laser gain medium comprise at least one laser
diode.
3. The ytterbium-doped solid state laser of claim 1, further comprising
dichroic optics to provide wavelength discrimination for said laser light
within said resonant cavity on said ZPL.
4. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a crystal having
an apatite structure.
5. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a crystal having
an apatite structure, doped with trivalent ytterbium (Yb.sup.3+).
6. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a crystal having
an apatite structure, doped with trivalent ytterbium (Yb.sup.3+) and is
selected from a group consisting of Sr.sub.5 (VO.sub.4).sub.3 F (S-VAP),
Ca.sub.5 (PO.sub.4).sub.3 F (C-FAP), Sr.sub.5 (PO.sub.4).sub.3 F (S-FAP),
Ba.sub.5 (PO.sub.4).sub.3 F (B-FAP) and M.sub.5 (AO.sub.4 ).sub.3 F where
A is selected from a group consisting of P and V and M is selected from a
group consisting of Ca, Sr, Br and Pb.
7. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a double tungstate
crystal of the form (Ha)(RE)(WO.sub.4).sub.2, doped with trivalent
ytterbium (Yb.sup.3+), and wherein Ha is a monovalent alkali ion selected
from a group consisting of Li, Cs, Na, Rb.sup.+ and K, and wherein RE is a
trivalent rare earth ion selected from a group consisting of La, Gd and Lu
ions.
8. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a crystal having a
double tungstate composition, doped with trivalent ytterbium (Yb.sup.3+)
and is selected from a group consisting of KY(WO.sub.4).sub.2,
KGd(WO.sub.4).sub.2, and KLu(WO.sub.4).sub.2.
9. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a C-FAP (Ca.sub.5
(PO.sub.4).sub.3 F) crystal having an apatite structure that is doped with
trivalent ytterbium (Yb.sup.3+).
10. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a S-FAP (Sr.sub.5
(PO.sub.4).sub.3 F) crystal having an apatite structure that is doped with
trivalent ytterbium (Yb.sup.3+).
11. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises M.sub.5
(PO.sub.4).sub.3 F, where M is selected from a group consisting of Ca, Sr,
Ba and Pb, has an apatite structure, and is doped with trivalent ytterbium
(Yb.sup.3+).
12. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a S-VAP (Sr.sub.5
(VO.sub.4).sub.3 F) crystal having an apatite structure that is doped with
trivalent ytterbium (yb.sup.3+).
13. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a
KY(WO.sub.4).sub.2 crystal having a double tungstate composition that is
doped with trivalent ytterbium (Yb.sup.3+).
14. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a
KGd(WO.sub.4).sub.2 crystal having a double tungstate composition that is
doped with trivalent ytterbium (Yb.sup.3+).
15. The ytterbium-doped solid state laser of claim 1, wherein said
ytterbium-doped solid state laser gain medium comprises a
KLu(WO.sub.4).sub.2 crystal having a double tungstate composition that is
doped with trivalent ytterbium (Yb.sup.3+).
16. The ytterbium-doped solid state laser of claim 1, wherein said
nonlinear harmonic doubler crystal is selected from a group consisting of
LBO, KNbO.sub.3 and KTP.
17. The ytterbium-doped solid state laser of claim 1, wherein said
nonlinear harmonic doubler crystal comprises KTP doped with tantalum.
18. The ytterbium-doped solid state laser of claim 1, wherein said
nonlinear harmonic doubler crystal comprises KTP doped with niobium.
19. The ytterbium-doped solid state laser of claim 1, wherein said
nonlinear harmonic doubler crystal comprises KTP doped with sodium.
20. A method for optically pumping a dye solution, comprising:
providing a ytterbium-doped solid state laser gain medium within an optical
cavity;
optically pumping said laser gain medium;
forcing said solid state laser gain medium to emit laser light on the
zero-phonon-line (ZPL) transition of the 2F5/2-2F7/2 resonance emission
band of said gain medium, wherein said step of forcing said solid state
laser gain medium to emit laser light on the zero-phonon-line (ZPL)
transition is carried out with an inter-cavity polarizer, wherein said
inter-cavity polarizer is oriented to allow transmission of polarized
light matching the predominant polarization of said ZPL transition of said
ytterbium-doped solid state laser gain medium, while increasing the
effective losses for nonZPL transitions emitting predominantly in the
orthogonal polarization with respect to the polarization of said ZPL of
said ytterbium-doped solid state laser gain medium;
doubling the frequency of said zero-phonon-line (ZPL) transition of said
2F5/2-2F7/2 resonance emission band of said gain medium to produce second
harmonic light within the wavelength range of 490 nm to 496 nm; and
directing said second harmonic light onto a dye solution having an
excitation band within the wavelength range of 490 nm to 496 nm.
21. The method of claim 20, wherein said dye solution comprises dye
selected from a group consisting of fluorescein isothiocyanate dye and
phycoerthrin dye.
22. The method of claim 20, further comprising:
selectively labeling the bases of DNA fragments with dye selected from a
group consisting of fluorescein isothiocyanate dye and phycoerthrin dye to
produce dye labeled DNA fragments;
passing said dye labeled DNA fragments through multiple channels of a
multichannel electrophoresis sequencing apparatus;
illuminating said muliple channels with said second harmonic light, wherein
one fluorescent color will fluoresce for each of the labeled bases of DNA;
and
detecting the position of each said fluorescent color to determine the base
sequence of said DNA fragments.
23. The method of claim 22, wherein the step of forcing said solid state
laser gain medium to emit laser light on the zero-phonon-line (ZPL)
transition includes forming said optical cavity with dichroic optics
having a reflection bandwidth narrow enough to allow laser oscillation
only on said zero-phonon-line (ZPL) transition.
24. The method of claim 22, wherein the step of producing laser radiation
includes providing an ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure.
25. The method of claim 22, wherein the step of producing laser radiation
includes providing an ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure, doped with trivalent
ytterbium (Yb.sup.3+).
26. The method of claim 22, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure, doped with trivalent
ytterbium (Yb.sup.3+), wherein said laser gain medium is selected from a
group consisting of C-FAP, S-FAP, S-VAP, B-FAP and M.sub.5
(AO.sub.4).sub.3 F where A is selected from a group consisting of P and V
and M is selected from a group consisting of Ca, Sr, Br and Pb.
27. The method of claim 22, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprising a double tungstate crystal of the form
(Ha)(RE)(WO.sub.4).sub.2, doped with trivalent ytterbium (Yb.sup.3+), and
wherein Ha is a monovalent alkali ion selected from a group consisting of
Li, Cs, Na, Rb and K, and wherein RE is a trivalent rare earth ion
selected from a group consisting of La, Gd and Lu ions.
28. The method of claim 22, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprises a crystal selected from a group consisting of a C-FAP crystal
and an S-FAP crystal, wherein said crystal has an apatite structure that
is doped with trivalent ytterbium (Yb.sup.3+).
29. The method of claim 20, further comprising:
selectively staining chromosomal DNA fragments with fluorescent dye
selected from a group consisting of fluorescein isothiocyanate dye and
phycoerthrin dye to produce dye stained chromosomal DNA fragments;
passing said dye stained chromosomal DNA fragments through multiple
channels of a multi-channel electrophoresis sequencing apparatus, wherein
said second harmonic light is directed onto said dye solution by
illuminating said muliple channels with said second harmonic light,
wherein one fluorescent color will fluoresce for each of the four genetic
letters A, G, T, C of said chromosomal DNA; and
detecting the position of each said fluorescent color to read the genetic
letter sequence of said chromosomal DNA fragments.
30. The method of claim 29, wherein the step of forcing said solid state
laser gain medium to emit laser light on the zero-phonon-line (ZPL)
transition includes forming said optical cavity with dichroic optics
having a reflection bandwidth narrow enough to allow laser oscillation
only on said zero-phonon-line (ZPL) transition.
31. The method of claim 29, wherein the step of producing laser radiation
includes providing an ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure.
32. The method of claim 29, wherein the step of producing laser radiation
includes providing an ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure, doped with trivalent
ytterbium (Yb.sup.3+).
33. The method of claim 29, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure, doped with trivalent
ytterbium (Yb.sup.3+), wherein said laser gain medium is selected from a
group consisting of C-FAP, S-FAP, S-VAP, B-FAP and M.sub.5
(AO.sub.4).sub.3 F where A is selected from a group consisting of P and V
and M is selected from a group consisting of Ca, Sr, Br and Pb.
34. The method of claim 29, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprising a double tungstate crystal of the form
(Ha)(RE)(WO.sub.4).sub.2, doped with trivalent ytterbium (Yb.sup.3+), and
wherein Ha is a monovalent alkali ion selected from a group consisting of
Li, Cs, Na and K, and wherein RE is a trivalent rare earth ion selected
from a group consisting of La, Gd and Lu ions.
35. The method of claim 29, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprises a C-FAP crystal having an apatite structure that is doped with
trivalent ytterbium (Yb.sup.3+).
36. The method of claim 20, further comprising:
selectively staining blood or bone marrow cells with antibodies tagged with
at least one fluorescent dye, wherein said at least one fluorescent dye is
selected from a group consisting of fluorescein isothiocyanate dye and
phycoerthrin dye to produce dye stained cells;
passing said dye stained cells in liquid suspension single file in a cell
flow stream through a flow cytometer apparatus, wherein said second
harmonic light is directed onto said dye solution by illuminating said
cell flow stream with said second harmonic light, wherein each fluorescent
dye excited will fluoresce at its characteristic wavelength; and
detecting said characteristic wavelength to immunotype cells labeled with
specific antibodies.
37. The method of claim 36, wherein the step of forcing said solid state
laser gain medium to emit laser light on the zero-phonon-line (ZPL)
transition includes forming said optical cavity with dichroic optics
having a reflection bandwidth narrow enough to allow laser oscillation
only on said zero-phonon-line (ZPL) transition.
38. The method of claim 36, wherein the step of producing laser radiation
includes providing an ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure.
39. The method of claim 36, wherein the step of producing laser radiation
includes providing an ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure, doped with trivalent
ytterbium (Yb.sup.3+).
40. The method of claim 36, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprising a crystal having an apatite structure, doped with trivalent
ytterbium (Yb.sup.3+), wherein said laser gain medium is selected from a
group consisting of C-FAP, S-FAP, B-FAP, S-VAP and M.sub.5
(AO.sub.4).sub.3 F where A is selected from a group consisting of P and V
and M is selected from a group consisting of Ca, Sr, Br and Pb.
41. The method of claim 36, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprising a double tungstate crystal of the form
(Ha)(RE)(WO.sub.4).sub.2, doped with trivalent ytterbium (Yb.sup.3+), and
wherein Ha is a monovalent alkali ion selected from a group consisting of
Li, Cs, Na, Rb and K, and wherein RE is a trivalent rare earth ion
selected from a group consisting of La, Gd and Lu ions.
42. The method of claim 36, wherein the step of producing laser radiation
includes providing a ytterbium-doped solid state laser gain medium
comprises a C-FAP crystal having an apatite structure that is doped with
trivalent ytterbium (Yb.sup.3+).
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to means of generating blue laser light, and
more specifically, it relates to the excitation of biofluorescent dyes (in
the .about.490-496 nm spectral region) utilized in flow cytometry,
immunoassay, DNA sequencing, and other biofluorescence instruments.
2. Description of Related Art
Fluorescent probe techniques are widely utilized in a variety of biomedical
research and diagnostic applications, owing largely to inherently high
detection sensitivity and selectivity. Since the early 1950s, fluorescent
techniques have been used to probe molecular interactions between small
ligands and biological macromolecules, as well as interactions among
components in biological assemblies (e.g., cells). Applications soon
extended to the areas of nucleic add and cell membrane research.
Fluorescent probes have increasingly been used by cell biologists to probe
cellular and subcellular function structures, as in fluorescence
microscopy imaging and immunofluorescence microscopy techniques.
A flow cytometer [1] is often used to implement biofluorescence techniques.
In a flow cytometer, cells in a fluid suspension are stained with
fluorescent dyes that bind to specific molecules on the surface of the
cell or to the nucleic acids within the cell nucleus. These cells then
pass in single file through a focused laser beam which excites the
fluorescent dye molecules. The wavelength of the laser is chosen to match
the wavelength of a fluorescence excitation band of the dye molecule.
Appropriately placed detectors capture a portion of the scattered and
fluorescent light emitted by the cell as it passes through the laser beam.
Several different fluorescent dyes may be employed, each emitting at a
wavelength differing from that of the others. A corresponding number of
appropriately filtered photomultiplier detectors measure the fluorescent
intensities from their corresponding dye emitters. Each data stream
represents a distinct measurement of information about immunological
marker molecules on the cell surface or in the nucleic acids of the
nucleus. The use of antibodies for cell identification is possible because
the external membranes of different kinds of cells express different
moieties called cell surface antigens. Immunotyping is used to describe
the science of identifying cell using antibodies. For example, antibodies
are commonly used to identify leukocytes in human blood. The development
of multiparameter flow cytometry [1] for immunotyping clinical material
has provided new insight on diagnosis and management of disease. Using
this approach, considerable information can be rapidly obtained for the
treatment of immuno-deficiency diseases, such as AIDs, to evaluate
immunosuppressive therapy associated with organ transplantation, and to
manage cancer therapy. Clinical immunotyping is mostly applied to cells
from the blood or bone marrow using antibodies labeled with fluoscein
and/or phycoerthrin dyes, excited in an excitation band peaked at a
wavelength near 493 nm. Using different antibodies labeled with
fluorescein and other fluorochromes such as phycoerthrin, multiple cell
types can be identified simultaneously using a single cell suspension in a
flow cytometer.
In the past decade there has been a precipitous increase in interest in,
and the prospects for, genetic medicine based on a complete mapping of the
human genome. To sequence the human genome [2], chromosomal DNA fragments
are selectively stained chemically with one of four different fluorescent
dyes (one fluorescent color for each of the four genetic letters A, G, T,
C), and passed through a multi-channel electrophoresis sequencing
apparatus). The multiple channels are illuminated with laser radiation
matching the excitation bands of the fluorescent dyes, and the genetic
letter sequences are readout by detecting the positions of the four
fluorescence colors. Four color fluorescence DNA sequencing [2] has
enabled the onset of an international campaign to sequence the complete
human genome in a few years time.
In spite of the vast number of fluorescent probes that are available for
research purposes, only a handful of fluorescent dyes have gained
wide-spread use in commercial biofluorescence instrumentation (such as the
Model 373A DNA Sequencer produced commercially by Applied Biosystems Inc.
(ABI), Foster City, Calif.). The dyes developed specifically for
wide-spread commercial use in most flow cytometers [1], immunoassay [3]
and four color fluorescence DNA sequencers [4] were selected (in part) for
their ability to be efficiently excited by the 488 nm or 514 nm radiation
from an argon ion gas laser (the only visible laser source then deemed
practical for use in commercial bio-fluorescence instruments. These
standard biofluorescence dyes, and their excitation (fluorescence) maxima
in nm, are as follows [8]: FITC (fluorescein isothiocyanate) 494 (518),
NBD-HA (aminohexanoic acid) 492 (548), tetramethylrhodamine 548 (578), and
Texas Red 580 (604). The ABI dye primer product names are FAM, JOE, TAMRA,
and ROX. Another widely used commercial dye in flow cytometers for
immunotyping is phycoerythrin. Although the 488 nm laser line of the argon
ion laser is a few nm to the blue side of the FAM excitation peak, the
argon ion laser as been found (until now) to be the best practical source
for most commercial biofluorescence instruments utilizing FAM and other
dyes, such as phycoerythrin. Typical laser excitation powers of a few
milliwatts to a few tens of milliwatts are employed in commercial
biofluorescence instruments.
New four color dye staining techniques have been recently developed [9] to
increase DNA sequencing throughput and accuracy. The dye (FAM), with an
excitation peak wavelength near 494 nm, is used to absorb 488 nm
excitation radiation from an argon ion laser, and the excitation energy is
transferred (nonradiatively) to energy transfer primers (designated F10F,
F10J, F3T, and F3R) that fluoresce at peak wavelengths wavelengths of 525,
555, 580, 605 nm, respectively.
As biofluorescent research, diagnostic, and clinical instrumentation
applications expand and proliferate, there is a pressing need to provide a
more compact, efficient, reliable, and long-lived dye excitation laser
source than the argon ion laser, for use in these instruments, especially
in clinically used flow cytometers and in DNA sequencers. A large
preponderance of these instruments utilize the fluorescein dye primer FAM
that is excited optimumly in the blue spectral region peaked at .about.493
nm [8], or the fluorochrome phycoerythrin (peaked at the same wavelength).
Two leading approaches to producing generally blue laser sources are by (i)
harmonic doubling of near infrared radiation from either AlGaAs
(.about.860 nm) or InGaAs (.about.980 nm) laser diodes in
quasi-phase-matched nonlinear optical waveguides [11], producing generally
blue radiation near .about.430 nm and .about.490 nm, respectively; and
(ii) harmonic doubling in bulk nonlinear optical crystals (such as KNbO3)
of the near infrared radiation emitted from a diode-pumped,
neodymium-doped solid state crystal laser operating on the
quasi-three-level laser transition [12].
Direct harmonic doubling of AlGaAs laser diode radiation at .about.860 nm
in quasi-phase-matched waveguides is the baseline approach of two national
consortia devoted to developing short-wavelength (.about.430 nm) blue
laser sources for the low power optical data storage application. This
approach requires the use of relatively expensive frequency-stabilized
laser diodes, matched to the conversion peak of the nonlinear doubling
waveguide, and very close control of operating temperature [11]. Operating
lifetimes of the nonlinear quasi-phased matched waveguides have proved too
limited for commercial use [13]. Blue output powers have been generally
limited to a few to tens of milliwatts, and prospects for the practical
scaling of blue output power beyond tens of milliwatts is problematic.
The practical prospects of providing >tens of milliwatts of generally
blue light by harmonic doubling of the fundamental infrared radiation from
a diode-pumped rare-earth-doped solid state laser emitting in the near
infrared spectral region are relatively greater than the approach just
described. However, as discussed below, the use of the trivalent neodymium
(Nd) rare-earth-ion as the active laser ion cannot produce the desired
color of blue radiation preferred for exciting commercial biofluorescent
probe dyes, especially the FAM and phycoerythrin dyes near .about.493 nm.
To overcome the deficiencies of these two approaches, it is proposed to
construct blue-radiation emitting diode-pumped solid state lasers (DPSSLS)
using ytterbium-doped dielectric laser crystals, in particular preferred
Yb doped apatite and tungstate crystals, operating on their resonance
(true 3-level) ZPL transitions generally falling within in the spectral
region from .about.965-992 nm. The generated fundamental radiation at a
wavelength within the .about.965-992 nm spectral region is then
harmonically doubled to the blue spectral region near .about.493 nm using
a suitable nonlinear harmonic doubler.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an efficient, compact
means of generating blue laser light at a wavelength near .about.493+/-3
nm, based on the use of a laser diode-pumped Yb-doped laser crystal
emitting on its ZPL resonance transition at a wavelength near
.about.986+/-6 nm, whose fundamental infrared output radiation is
harmonically doubled into the blue spectral region.
It is another object of the invention to provide a means for exciting
biofluorescent dyes (in the .about.490-496 nm spectral region) utilized in
flow cytometry, immunoassay, DNA sequencing, and other biofluorescence
instruments, generated by harmonically doubling infrared laser radiation
at a wavelength near .about.980-992 nm emitted from a Yb-doped crystal
laser configured to lase on the resonance ZPL transition.
The present invention discloses a compact, efficient, diode-pumped solid
state laser emitting at a blue wavelength with the spectral region from
490 to 496 (.about.493+/-3 nm), based on ytterbium doped laser crystals.
As taught in the present invention, such a laser source can be provided by
harmonically doubling the near infrared output radiation from a ytterbium
doped crystal laser forced to operate on its zero-phonon-line (ZPL)
transition. Preferred host crystals for this purpose will have strong ZPL
fluorecence (laser) transitions lying in the spectral range from
.about.980 to .about.992 nm (so that when frequency-doubled, they produce
output radiation in the spectral range from 490 to 496 nm).
Such blue solid state lasers provided by this invention may also find
competitive economic use in other large commercial applications that
utilize blue laser sources, and that also could benefit from such an
efficient, compact, reliable, and longlived blue laser source, such a
optical data storage, consumer home HDTVs, and commercial theater image
projection systems.
The invention involves the use of preferred Yb-doped apatite crystals such
as Yb:Sr.sub.5 (PO.sub.4).sub.3 F, configured to lase on the resonant ZPL
transition near 985 nm (in lieu of the normal 1047 nm transition). The
laser light is then doubled into the blue at 492.5 nm.
Yb:KY(WO.sub.4).sub.2 The invention involves the use of alternate
preferred Yb doped tungstate crystals, such as Yb:KY(WO4)2, configured to
lase on the resonant ZPL transition near 981 nm (in lieu of the normal
1025 nm transition). The laser light is then doubled in the blue at 490.5
nm.
The use of the trivalent neodymium (Nd) rare-earth-ion as the active laser
ion cannot produce the desired color of blue radiation preferred for
exciting commercial biofluorescent probe dyes, especially the FAM and
phycoerthrin dyes near .about.493 nm, whereas this can be accomplished
using the trivalent ytterbium (Yb) rare-earth-ion as the active laser ion.
The invention is useful in a number of applications, including determining
the base sequence of said DNA fragments, reading the genetic letter
sequence of chromosomal DNA fragments and immunotyping, immunoassaying and
immunophenotyping cells labeled with specific antibodies. Examples of
technology and methods for DNA sequencing and preparing
fluorescent-labeled DNA are shown in U.S. Pat. Nos. 5,755,943, 5,639,874,
5,674,743 and 5,571,388, which are all incorporated herein by reference.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the 4f electronic manifolds (and their crystal field Stark
splittings) of the trivalent Yb and Nd rare earth ions in dielectric
crystals (schematic), lying the energy range from 0 to 12,500 wavenumbers.
FIG. 2 shows the spectral ranges of known fluorescence transitions of
trivalent Yb and Nd laser ions in dielectric crystals.
FIG. 3 shows ZPL laser transition figures-of-merit (FOM) values for known
Yb doped dielectric crystals.
FIGS. 4A and 4B show the polarized absorption and emission spectra,
respectively, of Yb:S-FAP for the electric field polarized parallel to the
c axis of the crystal [18].
FIGS. 5A and 5B show the polarized absorption and emission spectra,
respectively, of Yb:S-FAP for the electric field polarized perpendicular
to the c axis of the crystal [16].
FIG. 6 shows the polarized absorption and emission spectra of Yb doped
KY(WO4)2 crystal [17], for the electric field polarized parallel to the a,
b, and c crystal axes.
FIG. 7 shows a schematic layout of the essential components of the
.about.493 nm blue laser device of the present invention.
FIG. 8 shows the calculated output power efficiencies of end-pumped
Yb:S-FAP and Yb:KY(WO.sub.4).sub.2 ZPL transition lasers, for an assumed
value of the product of Yb doping concentration timed gain crystal length
equal to 2.times.10.sup.19 ions/cm.sup.2.
FIG. 9 shows the calculated output power efficiencies of end-pumped
Yb:S-FAP and Yb:KY(WO.sub.4).sub.2 ZPL transition lasers, for an assumed
value of the product of Yb doping concentration timed gain crystal length
equal to 4.times.10.sup.19 ions/cm2.
BRIEF DESCRIPTION OF THE TABLES
Table 1 lists known ZPL transition wavelengths of Yb doped dielectric
crystals [15, 17].
Table 2 lists assumed laser model input values for calculating the laser
performance of end-pumped Yb:S-FAP and Yb:KY(WO.sub.4).sub.2 ZPL
transition lasers emitting near .about.980 nm.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows the low-lying electronic energy levels for trivalent Nd and Yb
laser ions. Each ion possesses electronic manifolds whose number and
energy are characteristic of the number of optically active 4f electrons
each ion possesses (3 for Nd and 13 for Yb). The 13 f-electrons of the Yb
ion result in only two electronic manifolds, the ground .sup.2 F.sub.7/2
manifold, and the excited .sup.2 F.sub.5/2 manifold lying near
.about.11,000 wavenumbers in all dielectric crystals. Each of these
manifolds is split into a number of Stark levels by the local crystal
field surrounding the lattice site in which the Yb ion resides. The
optical transition between the lowest Stark level of the ground manifold
and the lowest Stark level of the excited manifold is called the
zero-phonon-line (ZPL) transition, and is designated in FIG. 1. The
ensemble of optical transitions between other pairs of Stark levels of the
two manifolds give rise to an absorption band and an emission band
generally lying spectrally to the blue and red of the ZPL transition,
respectively.
The more complex energy level structure of the trivalent Nd ion is also
shown in FIG. 1. The three f-electrons of the trivalent Nd ion result in
several tens of electronic manifolds, the relevant lowest five of which
are shown in FIG. 1. Each of these manifolds is also split into a number
of Stark levels by the local crystalline electric field. The excited
.sup.4 F.sub.3/2 manifold serves as the upper laser manifold for all
observed Nd ion laser transitions. All four of the lower lying .sup.4 I
electronic manifolds serve as terminal manifolds for laser transitions,
giving rise to known [14] Nd laser emissions at wavelengths centered near
.about.940 nm (.sup.4 I.sub.9/2), .about.1060 nm (.sup.4 I.sub.11/2),
.about.1340 nm (.sup.4 I.sub.13/2), and .about.1820 nm (.sup.4
I.sub.15/2). In FIG. 1, only the two shortest wavelength fluorescence
bands are shown. Directing attention to the resonance .sup.4 F.sub.3/2
-.sup.4 I.sub.9/2 band near .about.940 nm (analogous to the .sup.2
F.sub.5/2 -.sup.2 F.sub.7/2 resonance band of Yb), this band will be
characterized by a ZPL transition between the pair of lowest lying Stark
levels of the .sup.4 F.sub.3/2 and .sup.4 I.sub.9/2 manifolds, and
absorption and emission bands lying spectrally to the blue and red of the
ZPL transition, respectively.
The specific wavelengths and intensities of fluorescence transitions
occurring in the fluorescence bands of Yb and Nd ions are characteristic
of the crystalline host material into which the Yb and Nd ions are doped,
and can vary substantially from one host material to another. Several
hundred absorption and emission spectra of Nd doped crystals have been
reported in the literature and have been summarized by Kaminskii [14]. Yb
doped crystals have not been studied nearly as much. However, prompted by
the recent development of efficient InGaAs laser diodes emitting in the
.about.900-980 nm spectral region (overlapping the .sup.2 F.sub.5/2 pump
absorption bands of Yb) DeLoach, et. al. [15], measured and reported the
polarized absorption and emission spectra of a number of Yb doped
dielectric crystals. FIG. 2 shows the known [14,15] spectral regions of
fluorescence transitions in Yb and Nd doped crystals. It also shows the
band of infrared wavelengths from .about.980 to .about.992 nm, which when
harmonically doubled, corresponds to blue emission wavelengths from
.about.490 to .about.496 nm of interest for biofluorescence excitation
laser sources, especially for the dyes FAM and phycoerthrin.
As can be seen from FIG. 2, the Nd ion characteristically fluoresces in the
spectral range from .about.900 to .about.960 nm on the .sup.4 F.sub.3/2
-.sup.4 I.sub.9/2 resonance band, and from .about.1040 to .about.1080 nm
in the .sup.4 F.sub.3/2 -.sup.4 I.sub.11/2 band. We see from FIG. 2 that
the Nd laser ion will only be able to produce blue harmonically doubled
radiation in the blue spectral region from .about.450 to 475 nm (and in
the green spectral region from .about.520-540 nm). As examples, the
well-known Nd:YAG laser crystal emits in the resonance band at a
wavelength of .about.946 nm [12] and produces harmonically doubled
radiation at a wavelength of .about.473 nm; the Nd:YVO4 laser crystal
emits in the resonance band at a wavelength of .about.915 nm and produces
harmonically doubled radiation at a wavelength of .about.456 nm. In
general, harmonically-doubled resonance Nd doped crystal lasers can only
generate blue light at wavelengths well outside the spectral region for
optimally exciting fluorescein-based biofluorescent dyes, such as FAM and
phycoerthrin.
In contrast, again as seen from FIG. 2, the Yb ion fluoresces
characteristically in the spectral range from .about.965 to .about.1060 nm
on the singular .sup.2 F.sub.5/2 -.sup.2 F.sub.7/2 resonance band. By
forcing the Yb ion to emit laser radiation on the shorter wavelength
transitions of this band (generally on the ZPL transitions in the
.about.965 to 992 nm region), such Yb lasers can be harmonically doubled
to produce the desired .about.490-496 nm bue radiation, and especially
.about.493 nm radiation, optimum for exciting fluorescein based dyes such
as FAM and phycoerthrin. To achieve practical laser sources of
.about.490-496 nm blue radiation, Yb doped crystals with superior
spectroscopic properties in the spectral region from .about.980 to 992 nm
will be preferred. Payne, et al. [15, 16] have developed an analysis that
identifies three spectroscopic figures-of-merit (FOM) for Yb lasers
operating on the .sup.2 F.sub.5/2 -.sup.2 F.sub.7/2 resonance manifold
(including the ZPL transition): 1) a large laser (ZPL) transition
cross-section, 2) a small value of the pump intensity, I-min, that renders
the laser crystal transparent at the laser wavelength (i.e. sufficient to
overcome resonance absorption at the laser transition wavelength). This
minimum pump intensity ("I-min") varies inversely with the pump transition
cross-section, so a large pump transition is desired, and 3) large Stark
splitting, E.sub.p, between pump terminal Stark level and the initial
laser transition Stark level.
As mentioned above, DeLoach, et al. [15] reported quantitative polarized
absorption and emission spectra of most known Yb doped dielectric
crystals. Additional data by Mikhailov [17] is also relevant. From these
data one can determine the ZPL wavelengths, ZPL transition cross-sections
of candidate Yb crystal laser materials, pump transition cross-sections
(and thereby I-min values), and the excited manifold Stark splitting FOM
for these materials. FIG. 3 shows the results of this analysis, plotted on
a log-log scale to better contrast the FOMs of candidate Yb laser
materials. FIG. 3 plots pairs of values (for each Yb doped crystal) of the
ZPL transition cross-section and I-min. Yb doped materials with superior
laser spectroscopic characteristics for achieving relatively low laser
threshold and high power conversion efficiency will lie to the upper left
quadrant of FIG. 3. Prospective Yb doped ZPL transition gain materials
lying to the lower right quadrant of FIG. 3 will generally exhibit higher
laser threshold pump flux and lower power conversion efficiencies, for a
given available pump flux.
Among all presently researched Yb doped dielectric crystals with measured
absorption and emission spectra, the class of Yb doped apatite crystals
are seen from FIG. 3 to exhibit the highest FOMs for ZPL transition
lasers, and accordingly are preferred Yb-doped crystals for use in the
present invention. According to Table 1, the apatite Yb doped crystals
also all exhibit ZPL transition wavelengths lying in the desired spectral
range (983-986 nm). FIGS. 4A and 4B show the polarized absorption and
emission spectra, respectively, of Yb:S-FAP [18] for the electric field
polarized parallel to the c axis of the crystal. FIGS. 5A and 5B show the
polarized absorption and emission spectra, respectively, of Yb:S-FAP [16]
for the electric field polarized perpendicular to the c axis of the
crystal. Laser gain media and laser systems comprising Yb:FAP and related
materials are disclosed in U.S. Pat. No. 5,280,492, which is incorporated
herein by reference [19]. Ytterbium doped vanadate laser host crystals
having the apatite crystal structure (and Yb absorption and emission
spectra similar to Yb:S-FAP) are disclosed in U.S. Pat. No. 5,341,389,
which is incorporated herein by reference [20]. From FIGS. 4 and 5, we see
that the Yb doped apatite crystal exhibits a strong narrow, highly
polarized (electric field perpendicular to the crystal c-axis) ZPL
transition, and broader, strong polarized (electric field parallel to the
crystal c-axis) pump absorption transition lying near 900 nm, and well to
the blue of the ZPL transitions (i.e., manifests a large excited manifold
Stark splitting). FIGS. 4 and 5 also show that the apatite also possesses
a strong, highly polarized (electric field perpendicular to the crystal
c-axis) emission feature at the longer wavelength of .about.1050 nm. The
latter transition terminates on a Stark level of the .sup.2 F.sub.7/2
ground manifold lying near 600 wavenumbers, and accordingly has a
relatively low thermal population at room temperature (compared to the
population in the ground Stark level). Thus, when the excited .sup.2
F.sub.5/2 manifold of Yb:S-FAP is pumped to produce a population inversion
and gain in the crystal in the ZPL transition, a higher gain will also be
produced in the longer wavelength .about.1050 nm transition. Thus, to
force laser oscillation in the desired shorter-wavelength ZPL transition,
optical elements must be incorporated into the laser resonator that
provide selective loss at the longer wavelength .about.1050 nm transition.
In doing so, the net round-trip gain on the ZPL transition can be made to
exceed that on the longer wavelength transition, thus suppressing
oscillation at the longer wavelength. Optical elements that can provide
the required selective loss include dichroic coatings on the laser
resonator mirrors, and (if the gain transitions are appropriately
polarized) a polarizer placed in the laser resonator with an appropriate
orientation relative to the gain crystal axes. Inspection of the relative
polarizations of the pump, ZPL, and parasitic longer wavelength
transitions in Yb:S-FAP, indicate that either or both dichroic and
polarization methods for long wavelength transition suppression may be
employed. Overall then, the quantitative spectroscopic features of
Yb:S-FAP (and, similarly, other Yb doped apatites) are essentially ideal
for Yb doped crystal lasers operating on their ZPL transitions, and enable
the design and realization of efficient and practical near infrared lasers
with outputs in the 983-986 nm spectral region.
TABLE 1
Zero-Phonon-Line Transition Wavelengths of Yb:Host Crystal [15].
Host Crystals ZPL Transition Wavelength (nm)
LiYF4 (YLF) 972.0
LaF3 974.7
SrF2 966.5
BaF2 966.7
KCaF3 972.0
KY3F10 974.2
Rb2NaYF6 968
BaY2F8 972.6
Y2SiO5 (YOS) 979
Y3Al5O12 (YAG) 968.3
YA1O3 (YAP) 978.5
Ca5(PO4)3F (FAP) 983.5
Sr5(PO4)3F (S-FAP) 985.3
Sr5(VO4)3F (S-VAP) 986
LuPO4 976.1
LiYO2 972.6
ScBO3 974.6
KY(WO4)2 981.4*
KGd(WO4)2 981.3*
*values taken from ref [17].
FIG. 6 shows the polarized absorption and emission spectra of Yb doped
Yb:KY(WO.sub.4).sub.2 crystal [17], with electric fields polarized
parallel to the a, b, and c crystal axes. This crystal exhibits a strong
polarized pump absorption transition near .about.933 nm and a very strong
polarized ZPL transition at a wavelength of 981 nm. The excited manifold
Stark splitting between these two transitions is less than that found for
the apatites, but appears to be sufficiently large to be for practical
laser design. Similarly, although the pump transition cross-section is
smaller than those found in the apatites (and therefore results in a
larger value for I-min for the tungstates than found in the apatites), Yb
doped double tungststates lie sufficiently within the upper left quadrant
of FIG. 3 to be a preferred Yb doped ZPL transition laser crystal in the
present invention.
The Yb doped crystals lying in the lower right quadrant of FIG. 3 possess
relatively weak ZPL transition cross-sections and relatively high values
of I-min, and accordingly do not possess spectroscopic features that
easily lend themselves to the realization of practical ZPL transition
lasers emitting in the 965-992 nm spectral region.
FIG. 7 shows a schematic layout of the essential functional components of a
.about.493 nm blue laser device of the present invention. The laser
comprises of six main elements: a laser diode pump source 20, a laser
resonator cavity resonant at two wavelengths (.about.986 and .about.493
nm) formed by mirrors, 22 and 25, a Yb-host laser gain crystal 19, a
harmonic doubling crystal 23, a polarizer 24, and pump coupling lenses 21.
The Yb-host gain element, harmonic doubling crystal, and polarizer are
placed within the laser resonator cavity. The laser cavity mirror 22 is
coated on its inner surface with a dichroic dielectric coating that allows
high transmission of pump light at a wavelength matching the pump
transition wavelength (nominally in the 900-950 nm region), and that
highly reflects at the ZPL transition wavelength of the Yb-host gain
material, and at half the ZPL transition wavelength (i.e, at the
harmonically doubled wavelength, .about.493 nm). This dichoric should also
possess a relatively low reflectivity at wavelengths to the spectral red
of the ZPL transition, where optical gain in the pumped Yb gain medium may
be present. The laser output coupling mirror 25 also has a dichroic
coating placed on its inner surface that provides high reflectivity at
both the pump wavelength (i.e., in the 900-950 nm resion) and at the
Yb-host ZPL transition laser wavelength (.about.986 nm), and provides an
intermediate value of reflectivity at half the ZPL transition wavelength
(i.e., at .about.493 nm). The value of reflectivity at .about.493 nm is
selected to optimize the power conversion efficiency from pump light to
blue output light. This dichroic coating is also designed to provide
significant transmission at wavelengths lying to the spectral red of the
ZPL transition where the Yb-host gain element may provide amplification.
Pump coupling lenses 21 collect, shape, and transport pump light through
the resonator mirror 22 and its dichroic coating, and focuses it in the
Yb-host gain crystal element 19. The polarizer element 24 is oriented to
allow a maximum transmission of polarized light at the ZPL transition
wavelength of the Yb-host gain element, while increasing the effective
loss at longer wavelengths emitted by the Yb-host gain element. In
operation, the pump light deposited in the Yb-host gain element creates a
population inversion on the ZPL transition and other longer wavelength
transitions. However, the relatively higher effective losses at these
longer wavelength transitions, provided in the laser cavity by the
oriented polarizer and by the dichroic resonator mirrors 22 and 23, causes
the threshold pump flux for the longer wavelength transitions to be larger
than that of the ZPL transition, resulting in laser oscillation at the ZPL
transition wavelength near .about.985 nm. Efficient operation of a Yb-FAP
laser on its ZPL transition has been demonstrated [25] using cavity
dichroics alone to suppress oscillation at longer wavelength transitions.
Desired blue light near .about.493 nm is achieved through inclusion of a
nonlinear harmonic doubling crystal 23 within the laser cavity. The
.about.986 nm radiation generated within the laser cavity at the ZPL
transition interacts with the doubler crystal and generates harmonic
radiation within the cavity at a wavelength of .about.493 nm. Laser output
at the desired wavelength of .about.493 nm is emitted through the
partially transmitting (at .about.493 nm) dichroic mirror on the output
coupler mirror 25.
At the particular wavelength range of interest to this invention (near
.about.986), preferred nonlinear harmonic materials are available which
can be configured to double under highly favorable non-critically
phase-matched (NCPM) conditions. It is well known in the field of harmonic
conversion that it is highly advantageous to utilize a nonlinear harmonic
doubler that is non-critically phased matched (NCPM), that is, a doubler
for which the phase-matching direction of the fundamental and harmonically
doubled radiation is along a principal optical axis of the crystal. In
this case, there is no spatial "walk-off" of the fundamental and frequency
doubled laser beams. In the context of the present invention, the
fundamental wavelength lies near .about.986 nm. A review of the nonlinear
optics literature indicates that there are two nonlinear crystals that may
be used to generate .about.493 nm blue light in the non-critical phase
matched (NCPM) condition, at or near room temperature: 1) so-called B-cut
KNbO.sub.3 [21], and 2) KTP crystal doped with a few percent niobium,
tantalum [22] or sodium [23]. At the fundamental wavelength of .about.986
nm, an alternate NCPM harmonic doubler material is LBO [24], operated at a
temperature near 240.degree..
To estimate the performance of a Yb:host laser operating at the ZPL
transition wavelength, we utilize the theory of Beach [26] that gives a
means for calculating the optimized laser output of a 3-level laser in
terms of the pump flux, Yb doping concentration (n.sub.0), crystal gain
length (l.sub.s), resonator single-pass transmission (T), and output
coupling fraction at .about.986 nm (R.sub.out). FIGS. 8 and 9 present
calculated output power conversion efficiencies of optimized Yb-S-FAP
(.about.986 nm) and an Yb:KY(WO.sub.4).sub.2 (.about.981 nm) ZPL lasers,
as a function of the pump flux incident on the Yb:host gain element (and
for the assumed Yb crystal and resonator values, listed in Table 2). In
these calculations, the harmonic doubler is removed from the laser cavity,
and the reflectivity of the output coupling mirror 25 at the ZPL
transition wavelength nm is adjusted for maximum laser slope efficiency at
the ZPL transition wavelength (e.g. .about.986 and 981 nm, for S-FAP and
KY(WO.sub.4).sub.2, respectively). FIGS. 8 and 9, respectively, are
calculated for assumed values of the product of the Yb doping
concentration (n.sub.0) and crystal gain element length (ls), equal to 2
and 4.times.10.sup.19 ions/cm.sup.2, respectively. From FIGS. 8 and 9, we
see that the Yb-S-FAP laser has a lower threshold flux and a higher output
power conversion efficiency than the Yb:KY(WO.sub.4).sub.2 laser, for all
of the pump flux values shown. For this reason, Yb-apatite laser crystals
are preferred for use in the present invention. However, similar
calculations for other Yb:host crystals shown in FIG. 3 indicate that the
ZPL transition laser performance of Yb:KY(WO.sub.4).sub.2 greatly exceeds
that of the other host materials, and therefore Yb:KY(WO.sub.4).sub.2 and
Yb:KGd(WO.sub.4).sub.2 are also preferred Yb doped crystal host materials
for use in the present invention.
TABLE 2
Parameter Values for Modeling [26] the Laser Performance of
Yb:S-FAP and Yb:KY(WO4)2 ZPL Transition Lasers.
Parameter Unit S-FAP KY(WO4)2
f,a,p Initial Pump Stark Level 0.7262 0.6055
Boltzman Fraction
f,b,p Terminal Pump Stark Level 0.0127 0.0653
Boltzman Fraction
f,a,l Terminal Laser Stark Level 0.7262 0.6055
Boltzman Fraction
f,b,l Initial Laser Stark Level 0.8117 0.7479
Boltzman Fraction
no*ls = Yb concentration*crystal E19 ion/cm2 2,4 2,4
length (two cases)
Spectroscopic pump transition E-20 cm2 13.8 5
cross-section
Spectroscopic laser transition E-20 cm2 15.4 1.8
cross-section
Pump transition wavelength nm 905 933
Laser transition wavelength nm 985 981
2F/52 fluorescence decay time msec 1.1 0.6
Crystal temperature Kelvins 300 300
Cavity transmission 0.98 0.98
Pump reflectivity 1 1
Mode fill factor 0.9 0.9
Pump delivery efficiency 1 1
Changes and modifications in the specifically described embodiments can be
carried out without departing from the scope of the invention, which is
intended to be limited by the scope of the appended claims.
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TABLE 1
Zero-Phonon-Line Transition Wavelengths of Yb:Host Crystals [15]
Host Crystals ZPL Transition Wavelength (nm)
LiYF4 (YLF) 972.0
LaF3 974.7
SrF2 966.5
BaF2 966.7
KCaF3 972.0
KY3F10 974.2
Rb2NaYF6 968
BaY2F8 972.6
Y2SiO5 (YOS) 979
Y3Al5O12 (YAG) 968.3
YA1O3 (YAP) 978.5
Ca5(PO4)3F (FAP) 983.5
Sr5(PO4)3F (S-FAP) 985.3
Sr5(VO4)3F (S-VAP) 986
LuPO4 976.1
LiYO2 972.6
ScBO3 974.6
KY(WO4)2 981.4*
KGd(WO4)2 981.3*
*values taken from ref [17].
TABLE 2
Parameter Values for Modeling [26] the Laser Performance of
Yb:S-FAP and Yb:KY(WO4)2 ZPL Transition Lasers
Parameter Unit S-FAP KY(WO4)2
f,a,p Initial Pump Stark Level 0.7262 0.6055
Boltzman Fraction
f,b,p Terminal Pump Stark Level 0.0127 0.0653
Boltzman Fraction
f,a,l Terminal Laser Stark Level 0.7262 0.6055
Boltzman Fraction
f,b,l Initial Laser Stark Level 0.8117 0.7479
Boltzman Fraction
no*ls = Yb concentration*crystal E19 ion/cm2 2,4 2,4
length (two cases)
Spectroscopic pump transition E-20 cm2 13.8 5
cross-section
Spectroscopic laser transition E-20 cm2 15.4 1.8
cross-section
Pump transition wavelength nm 905 933
Laser transition wavelength nm 985 981
2F/52 fluorescence decay time msec 1.1 0.6
Crystal temperature Kelvins 300 300
Cavity transmission 0.98 0.98
Pump reflectivity 1 1
Mode fill factor 0.9 0.9
Pump delivery efficiency 1 1
* * * * *
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