U.S. patent application number 11/261010 was filed with the patent office on 2007-05-03 for high power, end pumped laser with off-peak pumping.
This patent application is currently assigned to Laserscope. Invention is credited to Gerald Mitchell.
Application Number | 20070098024 11/261010 |
Document ID | / |
Family ID | 37968665 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070098024 |
Kind Code |
A1 |
Mitchell; Gerald |
May 3, 2007 |
High power, end pumped laser with off-peak pumping
Abstract
A laser configuration producing up to 100's of Watts of output
is provided, based on a solid-state gain medium, a source of pump
energy which is detuned from the maximum absorption wavelength for
the gain medium, and optics arranged to deliver the pump energy
through an end of the gain medium to propagate along the length of
the gain medium. The length of the gain medium and the doping
concentration in the gain medium are sufficient the absorption
length is on the order of 10's of millimeters, and more than 1/3 of
the length, and that 90 percent or more of the pump energy is
absorbed within two or fewer passes of the gain medium. A pump
energy source that supplies 100 Watts to 1000 Watts or more.
Inventors: |
Mitchell; Gerald; (Los
Altos, CA) |
Correspondence
Address: |
HAYNES BEFFEL & WOLFELD LLP
P O BOX 366
HALF MOON BAY
CA
94019
US
|
Assignee: |
Laserscope
San Jose
CA
|
Family ID: |
37968665 |
Appl. No.: |
11/261010 |
Filed: |
October 28, 2005 |
Current U.S.
Class: |
372/10 |
Current CPC
Class: |
H01S 3/094084 20130101;
H01S 3/0621 20130101; H01S 3/061 20130101; H01S 3/1611 20130101;
H01S 3/09415 20130101; H01S 5/405 20130101; H01S 3/005 20130101;
H01S 3/0809 20130101; H01S 3/11 20130101; H01S 3/08072 20130101;
H01S 3/109 20130101; H01S 3/0612 20130101; H01S 3/1643 20130101;
H01S 3/0817 20130101 |
Class at
Publication: |
372/010 |
International
Class: |
H01S 3/11 20060101
H01S003/11 |
Claims
1. A laser system, comprising: a gain medium having a doping
concentration and an absorption profile for absorption efficiency
over a range of pump energy wavelengths and having a maximum
absorption efficiency within the range, the gain medium having a
first end, a second end and a length of 50 millimeters or more
between the first and second ends; a source of pump energy having a
wavelength at which the absorption efficiency is less than the
maximum; and optics arranged to deliver the pump energy through the
first end of the gain medium to propagate along the length of the
gain medium; wherein the optics, pump energy wavelength, the length
of the gain medium and the doping concentration of the gain medium
establish absorption of greater than 80% of the pump energy
delivered to the gain medium, and a 1/e absorption length of
greater than one third the length of the gain medium.
2. The laser system of claim 1, wherein greater than 90% of the
pump energy delivered to the gain medium is absorbed in the gain
medium.
3. The laser system of claim 1, wherein greater than 95% of the
pump energy delivered to the gain medium is absorbed in the gain
medium.
4. The laser system of claim 1, wherein the optics arranged to
deliver the pump energy include a component to redirect pump energy
that exits the second end back through the gain medium, and greater
than 90 percent of the pump energy is absorbed within 2 passes
through the gain medium.
5. The laser system of claim 1, wherein the output wavelength is in
a range from about 200 to about 1100 nm.
6. The laser system of claim 1, wherein the 1/e absorption length
is less than the length of the gain medium.
7. The laser system of claim 1, wherein the gain medium comprises a
solid-state host with Nd doping, and the absorption efficiency of
the pump energy is about 20% or less of the absorption efficiency
at a peak near 808 nm in the profile.
8. The laser system of claim 1, wherein the gain medium comprises a
solid-state host with Nd doping, and the pump energy has a
wavelength in a range of about 799-803 nm.
9. The laser system of claim 1, wherein the gain medium comprises a
YAG host with Nd doping within a range of about 0.2 to about 0.4
atomic percent.
10. The laser system of claim 1, wherein the gain medium comprises
a doped solid-state host, and including an undoped end-cap on the
first end.
11. The laser system of claim 1, wherein the gain medium comprises
a doped solid-state host, and including an undoped end-cap on the
first end and an undoped endcap on the second end.
12. The laser system of claim 1, wherein the gain medium comprises
a doped solid-state host, and the optics arranged to deliver the
pump energy are adapted to focus the pump energy at a focal point
near the first end of the gain medium.
13. The laser system of claim 1, wherein the optics arranged to
deliver the pump energy are adapted to image the pump energy at an
image plane near the first end of the gain medium with a spot size,
including optical elements arranged to provide a resonant cavity to
provide a laser output, the resonant cavity being mode-matched with
the spot size of the pump energy at the image plane.
14. The laser system of claim 1, wherein the source of pump energy
comprises an array of laser diodes.
15. The laser system of claim 1, wherein the source of pump energy
supplies about 100 Watts or more.
16. The laser system of claim 1, wherein the source of pump energy
supplies about 500 Watts or more.
17. The laser system of claim 1, including optical elements
arranged to provide a resonant cavity to provide a laser output,
wherein the pump energy is sufficient to generate laser output at
greater than 100 Watts.
18. The laser system of claim 1, including optical elements
arranged to provide a resonant cavity with a component for
frequency conversion to provide a frequency converted laser output,
wherein the pump energy is sufficient to generate the laser output
at greater than 100 Watts.
19. The laser system of claim 1, including optical elements
arranged to provide a resonant cavity with a Q-switch and a
component for frequency conversion to provide a frequency converted
laser output, wherein the pump energy is sufficient to generate the
laser output at greater than 100 Watts.
20. The laser system of claim 1, including optical elements
arranged to provide a resonant cavity to provide a laser output,
wherein the pump energy is sufficient to generate laser output at
greater than 100 Watts with M.sup.2 of less than 30.
21. The laser system of claim 1, including a second source of pump
energy and optics arranged to deliver the pump energy from the
second source through the second end of the gain medium to
propagate along the length of the gain medium.
22. A laser system, comprising: a gain medium comprising an Nd
doped crystalline host having a doping concentration and having a
peak absorption efficiency near about 808 nm, the gain medium
having a first end, a second end and a length between the first and
second ends; a source of pump energy delivering more than 100 Watts
with a wavelength at which the absorption efficiency less than 20
percent the peak; and optics arranged to deliver the pump energy
through the first end of the gain medium to propagate along the
length of the gain medium.
23. The laser system of claim 22, wherein the absorption efficiency
of the pump energy is about 10% or less of the absorption
efficiency at the peak.
24. The laser system of claim 22, wherein the gain medium comprises
a YAG host with Nd doping within a range of about 0.05 to about 0.5
atomic percent.
25. The laser system of claim 22, wherein the gain medium comprises
a YAG host with Nd doping within a range of about 0.2 to about 0.4
atomic percent.
26. The laser system of claim 22, wherein the length of the gain
medium and the doping concentration of the gain medium establish a
1/e absorption length of 50 millimeters or more.
27. The laser system of claim 22, including an undoped end-cap on
the first end.
28. The laser system of claim 22, including an undoped end-cap on
the first end and an undoped endcap on the second end.
29. The laser system of claim 22, wherein the gain medium comprises
a doped solid-state host, and the optics arranged to deliver the
pump energy focus the pump energy at a focal point near the first
end of the gain medium.
30. The laser system of claim 22, wherein the optics arranged to
deliver the pump energy are adapted to image the pump energy at an
image plane near the first end of the gain medium with a spot size,
including optical elements arranged to provide a resonant cavity to
provide a laser output, the resonant cavity being mode-matched with
the spot size of the pump energy at the image plane.
31. The laser system of claim 22, wherein the source of pump energy
comprises an array of laser diodes.
32. The laser system of claim 22, wherein the source of pump energy
supplies about 500 Watts or more.
33. The laser system of claim 22, including optical elements
arranged to provide a resonant cavity to provide a laser output,
wherein the pump energy is sufficient to generate laser output at
greater than 100 Watts.
34. The laser system of claim 22, including optical elements
arranged to provide a resonant cavity with a component for
frequency conversion to provide a frequency converted laser output,
wherein the pump energy is sufficient to generate the laser output
at greater than 100 Watts.
35. The laser system of claim 22, including optical elements
arranged to provide a resonant cavity with a Q-switch and a
component for frequency conversion to provide a frequency converted
laser output, wherein the pump energy is sufficient to generate the
laser output at greater than 100 Watts.
36. The laser system of claim 22, including optical elements
arranged to provide a resonant cavity to provide a laser output,
wherein the pump energy is sufficient to generate laser output at
greater than 100 Watts with M.sup.2 of less than 30.
37. The laser system of claim 22, including a second source of pump
energy and optics arranged to deliver the pump energy from the
second source through the second end of the gain medium to
propagate along the length of the gain medium.
38. A laser system, comprising: a gain medium having a doping
concentration and an absorption profile for absorption efficiency
over a range of pump energy wavelengths and having a maximum
absorption efficiency within the range, the gain medium having a
first end, a second end and a length between the first and second
ends; a source of pump energy having a wavelength at which the
absorption efficiency substantially less than the maximum; and
optics arranged to deliver the pump energy through the first end of
the gain medium to propagate along the length of the gain medium
and adapted to image the pump energy at an image plane at or near
the first end of the gain medium with a spot size; optical elements
arranged to provide a resonant cavity to provide a laser output,
the resonant cavity being mode-matched with the spot size of the
pump energy at the image plane and including a Q-switch and a
component for frequency conversion to provide a frequency converted
laser output; wherein optics arranged to deliver the pump energy,
the length of the gain medium and the doping concentration of the
gain medium establish a 1/e absorption length of greater than one
third the length of the gain medium and 90% or more of the pump
energy is absorbed within 2 or less passes through the gain medium,
and wherein the pump energy is sufficient to generate the frequency
converted laser output at greater than 50 Watts.
39. The laser system of claim 38, wherein the pump energy is
sufficient to generate the frequency converted laser output at
greater than 100 Watts.
40. The laser system of claim 38, wherein the source of pump energy
supplies about 500 Watts or more.
41. The laser system of claim 38, wherein the source of pump energy
comprises an array of laser diodes delivering about 500 Watts or
more, and the optics arranged to deliver the pump energy establish
a substantially uniform intensity across the spot size in at least
one dimension and include fast axis collimation lenses slightly
defocused to improve uniformity of the intensity profile at the
focal point in a second dimension.
42. The laser system of claim 38, wherein the gain medium comprises
a solid-state host with Nd doping, and the absorption efficiency of
the pump energy is about 10% or less of the absorption efficiency
at a peak near 808 nm in the profile.
43. The laser system of claim 38, wherein the gain medium comprises
a YAG host with Nd doping within a range from 0.05 to 0.5 atomic
percent.
44. The laser system of claim 38, wherein the gain medium comprises
a YAG host with Nd doping within a range of about 0.2 to about 0.4
atomic percent.
45. The laser system of claim 38, wherein the gain medium comprises
a doped solid-state host, and including an undoped end-cap on the
first end, the first end being at an interface between the doped
solid-state host and the undoped end-cap.
46. The laser system of claim 38, wherein the gain medium comprises
a doped solid-state host, and including an undoped end-cap on the
first end and an undoped endcap on the second end.
47. A laser system, comprising: a gain medium comprising a
crystalline host with Nd doping at a concentration between about
0.05 and 0.5 atomic percent having a maximum absorption efficiency
near 808 nm, the gain medium having a first end, a second end and a
length between the first and second ends of 50 millimeters or more,
and including an undoped end-cap on the first end, the first end
being at an interface between the doped solid-state host and the
undoped end-cap; a array of laser diodes supplying pump energy
greater than 500 Watts, having a wavelength at which the absorption
efficiency substantially less than the maximum; and optics arranged
to deliver the pump energy through the first end of the gain medium
to propagate along the length of the gain medium and adapted to
image the pump energy at an image plane at or near the first end of
the gain medium with a spot size, with a substantially uniform
intensity across the spot size in at least one dimension; and
optical elements arranged to provide a resonant cavity to provide a
laser output, the resonant cavity being mode-matched with the spot
size of the pump energy at the image plane, and including a
Q-switch and a component for frequency conversion to produce a
frequency converted output, the frequency converted output greater
than 100 Watts.
48. The laser system of claim 47, including an undoped end-cap
having a first surface on the second end and having a second
surface that is at least partially reflective at wavelength of the
pump energy to redirect unabsorbed pump energy back into the host
toward the first end.
49. The laser system of claim 47, wherein the crystalline host
comprises YAG.
50. The laser system of claim 47, wherein the gain medium comprises
a YAG host with Nd doping within a range of about 0.2 to about 0.4
atomic percent.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to laser systems, and more
particularly to high-power, end-pumped laser systems with
solid-state gain media.
[0003] 2. Description of Related Art
[0004] High power laser output is desired over a broad range of
wavelengths and disciplines throughout the scientific, industrial
and medical fields. Many systems have been developed to generate
high-power. However, systems generating output power levels in
excess of several hundred Watts become very complex. Also, some
systems generate such high-powers only at the expense of beam
quality.
[0005] In solid-state systems, in order to generate a higher output
powers, the amount of energy used for pumping the gain medium is
increased. However, many solid-state media exhibit thermal lensing
or other problems causing aberrations in output beam. The pump
energy can be applied from the side of the gain medium, known as a
side-pumping, or from the end of the gain medium, known as the
end-pumping, in most systems. Other systems create complex optics
for filling a gain medium with pump energy. Side-pumping is
relatively inefficient; so that the conversion of pump energy into
laser output is low at relatively high pump energies. End-pumping
is more efficient. However, an upper limit is quickly reached for
end-pumped gain media, where rapid absorption of pump energy in a
small volume within the first few millimeters of the gain media
causes thermal fracture. Thermal damage to solid-state gain media
can be controlled by sophisticated cooling techniques, such as is
employed in so-called disk lasers. Also, thermal lensing which
occurs in some solid-state gain media can be managed by including
undoped endcaps. An undoped end-cap bonded on a gain medium
prevents deformation at the surface of the gain medium due to the
high absorption and heat generation on the surface.
[0006] Techniques have been investigated that increase the volume
of the gain medium in which the pump energy is absorbed, and in
which the resulting heat is generated, to prevent thermal damage
and manage thermal lensing. One way to distribute heat generation
within the gain medium which has been investigated includes
reducing the doping concentration of the active material. At lower
doping concentrations, less energy is absorbed within a given
volume. See, Honea et al., "Analysis of intracavity-doubled
diode-pumped Q-switched Nd:YAG laser producing more than 100 W of
power at 0.532 .mu.m," OPTICS LETTERS, Vol. 23, No. 15, Aug. 1,
1998, pages 1203-1205.
[0007] Butterworth, U.S. Pat. No. 6,898,231 B2 describes a laser
based on a gain medium comprising neodymium Nd doped yttrium
orthovanadate ("vanadate"), in which the pump energy is set at a
wavelength which is absorbed with an efficiency substantially less
than that of the peak absorption wavelengths, thereby allowing more
of the pump energy to penetrate a greater volume of the gain medium
before being absorbed and distributing the generated heat. In the
Butterworth patent for example, the vanadate crystal was on the
order of five millimeters long, and the doping concentration was
relatively high at about 0.5 atomic percent. Hardman et al.,
"Energy-Transfer up Conversion and Thermal Lensing and High-Power
End-Pumped Nd:YLF Laser Crystals," IEEE JOURNAL OF QUANTUM
ELECTRONICS, Volume 35, No. 4, April 1999, describes a
longitudinally pumped laser with the pump wavelength detuned in
order to increase the absorption length within the YLF host to
about three millimeters. Pollnau et al., "Up Conversion-Induced
Heat Generation and a Thermal Lensing in Nd:YLF and Nd:YAG,"
PHYSICAL REVIEW B, Volume 58, No. 24, 15 Dec. 1998, p. 16076-16092,
also describes off-peak pumping for a YLF host, while suggesting
that off-peak pumping is not necessary for a YAG host because "rod
fracture is not a problem." (See; Pollnau et al., page 16077). See
also, Wu et al., U.S. Pat. No. 6,347,101 B1; and Chang et al., U.S.
Pat. No. 6,504,858 B2. The Wu et al., Chang et al., Butterworth,
Hardman et al. and Pollnau et al. publications describe systems
that use relatively low pump powers from diode lasers, on the order
of 20 or 30 Watts. The resulting output powers of the lasers are
therefore relatively small, and not suitable for many
applications.
[0008] High power outputs have been achieved for Tm:YAG diode
pumped lasers producing 2 .mu.m wavelength outputs, and applying
off-peak pumping with pump powers over 300 Watts. Tm doped media
demonstrate two-for-one cross relaxation, improving pumping
efficiency, so long as the pump intensity is high enough. Honea et
al. report up to 115 W 2 .mu.m wavelength outputs in this
configuration, with off peak pumping. Honea et al., "15-W Tm:YAG
Diode-Pumped Solid-State Laser," IEEE JOURNAL OF QUANTUM
ELECTRONICS, Vol. 33, No. 9, September 1997, pages 1592-1600.
[0009] It is desirable to provide a laser system generating
high-quality, high-power outputs in a manufacturable
configuration.
SUMMARY OF THE INVENTION
[0010] A high quality laser easily producing over 100 Watts output
power is provided using a laser configuration, described herein,
based on a solid-state gain medium, a source of pump energy which
is detuned from the maximum absorption wavelength for the gain
medium, and optics arranged to deliver the pump energy through an
end of the gain medium to propagate along the length of the gain
medium.
[0011] The optics delivering the pump energy, the length of the
gain medium and the doping concentration in the gain medium are set
in described configurations so that that 80 percent or more, and
preferably more than 90 percent, of the pump energy is absorbed
within the gain medium. In configurations described, these
parameters are designed so that the absorption length, at which 1/e
of the pump energy is absorbed, is on the order of 10's of
millimeters, greater than 50 millimeters in some embodiments, and
preferably at least one third, greater than one half in some
embodiments, of the length of the gain medium.
[0012] Embodiments of the laser system described herein include a
pump energy source that supplies 500 Watts or more of energy at a
wavelength which has an absorption efficiency that is about 20
percent or less of a maximum absorption efficiency for the gain
medium and active element. For example, a laser system is described
with a gain medium comprising a rod shaped YAG host with a Nd
doping concentration between about 0.05 and 0.5 atomic percent,
where the length of the doped YAG host is substantially greater
than 50 millimeters, such as 100 millimeters, with a diameter on
the order of 10 millimeters or less, and with a source of pump
energy providing greater than 500 Watts in a wavelength between 799
and 803 nanometers. Intra-cavity doubled outputs of more that 100
Watts are produced in described configurations. More than 200 Watts
output power at 1.064 .mu.m is available in these configurations.
The configurations described herein are scalable to produce lower
powers, of for example 50 Watts, up to 1000 Watts or more, output
in the frequency converted wavelengths, and correspondingly high
power primary wavelength outputs. The output wavelength can be
configured to fall in a range from about 200 to about 1100 nm in
embodiments described herein.
[0013] In embodiments of the laser system described herein, the
gain medium comprises a long rod of the crystalline host such as
YAG, with an undoped end-cap on the first end through which the
pump radiation enters the crystal, and optionally with an undoped
end-cap on the second end as well.
[0014] The pump energy is delivered using optics in an embodiment
described herein, which focus the pump energy at a focal point near
one end of the gain medium, for propagation along the length of the
gain medium. Optical elements are included that are arranged to
provide a resonant cavity, which is mode-matched with the spot size
of the pump energy at the focal point. The result of this
configuration is a high quality output beam, with M.sup.2 less than
30, suitable for coupling into fiber optic delivery systems and
focusing on relatively small targets.
[0015] An embodiment is described in which the laser system is
configured to provide output beams that are a frequency converted
beam, such as a first, second or third harmonic of the primary
wavelength within the laser system. In such an embodiment, a
component for frequency conversion is included within the resonant
cavity. Also, an embodiment of the system includes a Q-switch
within the resonant cavity, for producing high energy, high
frequency pulses of output laser light.
[0016] The laser configuration described herein is suitable for
generation of more than 100 Watts of output at 532 nanometers using
an Nd:YAG gain medium and diode laser pumping source. The
configuration is stable, easily manufactured and low-cost.
[0017] In summary, an end-pumped high-power laser is described
which produces a low M.sup.2 beam with stable output power. Output
powers greater than 100 and even greater than 1000 Watts can be
produced using solid-state hosts and diode laser pump sources. The
laser configuration supports efficient intra-cavity frequency
conversion.
[0018] Other aspects and advantages of the present invention can be
seen on review of the drawings, the detailed description and the
claims, which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a simplified diagram of a diode pumped,
solid-state laser system producing over 100 Watts frequency
converted output power.
[0020] FIG. 2 illustrates one end of a gain medium in a system such
as described with reference to FIG. 1.
[0021] FIG. 3 is a schematic illustration of the distribution of
pump energy at one end of the gain medium for a system such as
described with reference to FIG. 1.
[0022] FIG. 4 illustrates in intensity profile on at least one
dimension of the pump energy delivered to one end of the gain
medium for a system such as described with reference to FIG. 1.
[0023] FIG. 5 is a graph of absorption efficiency versus wavelength
for pump energy sources in an Nd:YAG gain medium.
[0024] FIG. 6 is a simplified diagram of an alternative diode
pumped, solid-state laser system for producing high output
powers.
DETAILED DESCRIPTION
[0025] A detailed description of embodiments of the present
invention is provided with reference to the FIGS. 1-6.
[0026] FIG. 1 illustrates a high-power laser system comprising a
gain medium 10 that includes a doped crystalline host, having a
first end 11 and a second end 12. The gain medium 10 in a
representative embodiment comprises Nd:YAG having a length of about
100 millimeters and a diameter of about 4.5 millimeters. The gain
medium 10 is water cooled in exemplary embodiments, along the sides
of the host. Undoped endcap 13 about 10 millimeters long in this
example, is bonded on the first end 11 of the gain medium 10, and
undoped endcap 14 also about 10 millimeters long in this example,
is bonded on the second end 12 of the gain medium 10.
[0027] In the high-power end-pumped configuration shown, the
undoped endcap 13 can be diffusion bonded but preferably grown on
at least the first end 11. In embodiments where significant pump
energy reaches the second end of the host 10, another undoped
endcap 14 can be diffusion bonded but preferably grown on the
second end 12. The output end of the undoped endcap 14 is coated so
that it is reflective at the pump energy wavelength, while
transmitting at the resonant mode. In this manner, the pump energy
that is unabsorbed at the second end 12 is redirected back to the
rod to be absorbed. At the very high pump powers possible using the
configuration described herein, rod-end lens effects play a very
significant role in the stability of the resonator. Strong
absorption of the pump energy at the surface of the gain medium can
cause significant distortion to the end face and at high-power
levels rod fracture. Rod distortion leads to strong spherical
aberration of the beam which severely reduces the quality of the
beam. By bonding undoped endcaps onto the doped rod ends, the
distortion is avoided, because the absorption now takes place in
the bulk and not at a surface. Also, the fracture limit is higher
and end effects are substantially eliminated.
[0028] A source of pump energy in the illustrated embodiment
comprises a diode array 15. A representative embodiment employs a
seven bar stack of diode lasers, with each bar producing 100 Watts
for 700 Watts total pump energy, centered on 801 nanometers. The
wavelength of the bars changes plus or minus 1.5 nanometers in
normal operating conditions providing pump energy within a range of
about 799 to about 803 nanometers.
[0029] FIG. 5 shows the absorption efficiency versus pump energy
wavelength over practical range of wavelengths, for Nd:YAG. As
shown, a maximum in the range occurs at about 808 nanometers. The
pump energy range of 799 to 803 lies substantially off the peak at
808, at a level that is less that 20 percent of the maximum
absorption. For 801, plus or minus 1.5 nanometers, the absorption
is less than about 10% of the maximum absorption at the peak near
808 nanometers. Other pump energy ranges are suitable as well,
including wavelengths near 825 nanometers or beyond the illustrated
range. One specific advantage of pumping at wavelength with
absorption efficiencies that are substantially off peak is a
tolerance to wavelength shifts. When pumping at 801 nanometers in
the Nd:YAG in the described embodiment, wavelength shifts of plus
or minus 1.5 nanometers have essentially no effect on the laser
output.
[0030] Pump energy is delivered through optics, including a fast
axis collimation lens 16, a polarization multiplexer which acts as
a beam interleaver, brightness doubler 17, and a set of lenses 18
arranged as a telescope to focus the pump energy near the first end
11 of the gain medium 10. The pump energy is delivered at the
output of the fast access collimation lenses 16 on a path 20 to the
beam interleaver, brightness doubler 17. The pump energy is
concentrated to one half its width at the output of the beam
interleaver, brightness doubler 17 on path 21 and is delivered
through the lenses 18 on path 22 to a focal point at or near the
first end 11 of the gain medium 10.
[0031] In embodiments of the invention, the fast axis collimation
lens 16 can be deliberately defocused slightly to facilitate
homogenization of the pump beam at the focal point in the gain
medium 10. The beam interleaver, brightness doubler 17 reduces the
width of the pump energy output by one half, facilitating focusing
of the pump energy into a relatively small diameter rod shaped gain
medium 10, with a longer working distance. The lenses 18 can be
varied to adjust the spot size at an image plane in the gain medium
10 over a range of operating parameters as suits a particular
implementation. For example, the spot size at the focal point can
be varied over range about 10 percent to about 90 percent of the
diameter of the rod shaped gain medium 10.
[0032] The pump energy passes through a beam splitter 19 that is
used to turn the resonating energy to the optics defining resonant
cavity. The system includes optical elements including concave
mirror 25, that is highly reflective at the resonating energy of
1064 nanometers, beam splitter 19, which is reflective at 1064
nanometers and transmissive at the wavelength of the pump energy
source around 801 nanometers, concave mirror 26 that is highly
reflective at 1064 nanometers and transmissive at an output
wavelength of 532 nanometers, concave mirror 27 that is highly
reflective at both 1064 and 532 nanometers, and concave mirror 28
which is highly reflective at both 1064 and 532 nanometers. The
optical elements 25, 19, 26, 27, 28 define a resonant path 32 which
is essentially Z-shaped, with a tail between then beam splitter 19
and the highly reflective concave mirror 25.
[0033] In the illustrated embodiment, Q-switch 29 is placed in the
resonant cavity between the mirrors 26 and 27. Also, a nonlinear
crystal 30, such as LBO, is placed between the mirrors 27 and 28.
The Z-shaped resonant cavity can be configured as discussed in U.S.
Pat. No. 5,025,446 by Kuizenga, imaging the resonant mode at one
end of the gain medium 10 at the nonlinear crystal 30. The
configuration described is stable and highly efficient for
frequency conversion. The configuration shown in FIG. 1 produces a
frequency converted output (wavelength 532 nanometers in
illustrated embodiment) of greater than 100 Watts on line 31.
[0034] The pump spot size at the image plane near the first end 11
of the gain medium 10 affects in the mode quality of the laser
system, controls the gain, and the strength of the thermal lensing.
FIGS. 2 and 3 illustrate features of the pump spot size at the
focal point. FIG. 2 shows the gain medium 10, and the undoped
endcap 13 on the first end 11 of the gain medium 10. The pump
energy is focused on path 22 to the focal point near the first end
11. This establishes an aperture near the first end for the
resonant mode in the cavity. The gain is inversely proportional to
the area and divergence of the pump beam at the focal point near
the first end 11 of the gain medium 10 at the doped/undoped
interface of the rod. The smaller the spot size, the high the gain
for a given rod. The thermal lens is also inversely proportional to
the pump spot size at the image plane. As the pump spot gets
smaller, the thermal lens increases. Also, the distribution of
light across the pump spot has a strong effect on the thermal lens.
FIG. 3 illustrates the distribution light from the pump energy
source at the first end 11 on the rod, which results from imaging
the output of the laser diode source on the first end 11 of the
rod. As illustrated in FIG. 3, there are seven rows of diode laser
outputs, such as row 50. The result is a substantially uniform
intensity profile, as illustrated in FIG. 4 along the horizontal
dimension in the FIG. 4, which lies on an axis that is parallel to
the row 50 of laser diode spots. The rows are separated by a small
distance in the vertical dimension in an embodiment where the fast
axis collimation lenses 16 are focused. By slightly defocusing the
fast axis collimation lenses 16, the distribution of energy can be
made more uniform in the second, vertical dimension. The system is
designed therefore to homogenize and flatten the pump profile to
reduce the thermal lensing.
[0035] Also, the spot size at the image plane affects transverse
modes of the laser. The transverse modes of the laser are
controlled by the pump spot size and distribution of energy within
about the first 30 percent of the rod length in which a most of the
pump energy is absorbed. As the spot size at the image plane is
reduced, the mode quality improves. The optical elements 25, 19,
26, 27, 28 defining the resonant cavity are configured to mode
match with the aperture defined by the pump energy spot size at the
focal point.
[0036] The doping concentration in the gain medium 10 is chosen
based on the mode quality and output power required. The doping
level is relatively low to allow distribution of the thermal load
along the optical axis of the gain medium 10 (e.g., 1/e absorption
length of more than 50 millimeters in a rod less than 10
millimeters in diameter), thereby reducing the thermal stresses
induced at the input to the gain medium. In an embodiment
described, the doping concentration is about 0.27 atomic percent
for the rod shown in FIG. 1, that is about 100 millimeters long
between the first end 11 and the second end 12, and pumped
substantially off-peak at about 801 nanometers where the absorption
efficiency is less than 10 percent of the maximum absorption
efficiency at the peak near 808 nanometers for Nd:YAG. The 1/e
absorption length for this embodiment is about 66 millimeters, more
than half the length of the 100 millimeters rod.
[0037] Ranges of doping concentrations for embodiments of the
invention comprising an Nd:YAG rod can fall within about 0.05 and
about 0.5 atomic percent, and more preferably in a range between
about 0.2 and 0.4 atomic percent for readily and consistently
manufacturable commercial applications. The pump energy wavelength,
doping concentration and the length of the rod are adapted in a
preferred embodiment, so that the absorption length is over one
third the rod length, and more than 90 percent of the pump energy
is absorbed within two passes along the length of the rod, as the
unabsorbed pump energy which reaches the second end 12 of the rod
is reflected back towards the first end 11. The amount of
unabsorbed pump energy that reaches the first end 11 is very low,
and has insubstantial effects on the characteristics of the pump
energy at the focal point.
[0038] By establishing a suitable combination of parameters
including the length for the gain medium, the doping concentration,
the pump energy profile at the image plane, and the pump energy
wavelength, output powers greater than 100 Watts of frequency
converted output at 532 nanometers are readily generated with an
Nd:YAG rod about 100 millimeters long and about 4.5 millimeters in
diameter with reasonably high quality beam. The technology is
scalable to configurations supporting pump energy in the kilowatt
range for hundreds of Watts of output power in the primary and
harmonic wavelengths for the laser.
[0039] Beam quality can be characterized by the parameter M.sup.2.
The higher M.sup.2, the lower the beam quality, and the more
difficult it is to focus of the beam on a small spot and to couple
the beam into small numerical aperture delivery devices such as
fiber optics. M.sup.2 of less than 30 is readily achieved using the
technology described herein, allowing coupling into fiber optics on
the order 100 microns and up in diameter, which provides a beam
with low divergence suitable for many high-power applications of
laser light, including medical applications.
[0040] The technology described herein is adaptable to other
configurations of the resonant cavity, with or without frequency
conversion and with or without Q-switching, and adaptable to other
gain media and pump energy sources within the parameters described
herein.
[0041] FIG. 6 illustrates one alternative laser system
configuration, in which pump energy is provided at both ends of the
gain medium, for higher output powers. In the illustrated system, a
gain medium 100 having a length between a first end 101 and a
second end 102, on the order 50 to 100 millimeters or longer, is
provided. Undoped endcaps 103, 104 are grown on the first end 101
and second end 102 respectively. A first source of pump energy 105
directs more than 100 Watts of pump energy through a beam splitter
107 to an image plane near the first end 101 at a wavelength which
is detuned from the maximum absorption wavelength for the gain
medium. A second source of pump energy 106 directs more than 100
Watts of pump energy through a beam splitter 108 to an image plane
near the second end 102, at a wavelength which is detuned from the
maximum absorption wavelength. Optical components 109 and 110 are
arranged to provide resonant cavity for the primary wavelength. The
combination of parameters including the length for the gain medium,
the doping concentration, the pump energy profiles at the focal
points on the ends of the gain medium, and the pump energy
wavelengths substantially detuned from the maximum absorption
wavelengths, are established for absorption lengths of at least
one-third of the rod length, so that high output powers and high
quality beams are produced. The resonator components represented by
block 10 are adapted to a particular application of the laser
system, and may comprise of mirrors, polarizers, Q-switches,
non-linear crystals, apertures, filters, etalons, half wave plates,
and other devices.
[0042] Embodiments of the laser system may deploy one or more diode
stacks or other pump energy sources, may include one or more gain
media, and may include a variety of resonant cavity configurations.
Laser systems employing the technology described herein can be
implemented that operate in a continuous wave CW mode, a Q-switched
mode and mode-locked modes, depending on the preferred output
characteristics.
[0043] An end-pumped, high-power laser is described which produces
an output with low M.sup.2 and stable output power. The doping
level of the gain medium is adjusted, the pump wavelength is
detuned off major pump bands, and the length of the gain medium is
a selected to allow very high-power pumping while maintaining low
thermal stress and thermal lensing. The gain aperture for the
system can be determined by the spot size of the pump energy imaged
on an end of the gain medium. The laser system can be very
efficiently intra-cavity frequency converted to second, third and
higher harmonic frequencies.
[0044] While the present invention is disclosed by reference to the
preferred embodiments and examples detailed above, it is to be
understood that these examples are intended in an illustrative
rather than in a limiting sense. It is contemplated that
modifications and combinations will readily occur to those skilled
in the art, which modifications and combinations will be within the
spirit of the invention and the scope of the following claims.
* * * * *