U.S. patent application number 16/801857 was filed with the patent office on 2021-08-26 for second-harmonic generation crystal.
This patent application is currently assigned to Coherent LaserSystems GMBH & Co. KG. The applicant listed for this patent is Coherent LaserSystems GMBH & Co. KG. Invention is credited to Wolf SEELERT, Rudiger VON ELM.
Application Number | 20210265802 16/801857 |
Document ID | / |
Family ID | 1000005764834 |
Filed Date | 2021-08-26 |
United States Patent
Application |
20210265802 |
Kind Code |
A1 |
SEELERT; Wolf ; et
al. |
August 26, 2021 |
SECOND-HARMONIC GENERATION CRYSTAL
Abstract
An optic produces a beam of ultraviolet laser radiation from a
beam of visible laser radiation and spatially separates the
ultraviolet laser beam from the visible laser beam. The optic
includes two crystals made of the same optically-nonlinear material
that are contact bonded along a planar interface. One crystal has
principle crystal axes that are oriented for type-I second-harmonic
generation. The ultraviolet laser beam exits the optic through an
uncoated surface of the other crystal. The principle crystal axes
of the two crystals have different orientations and have reflection
symmetry about the planar interface.
Inventors: |
SEELERT; Wolf; (Lubeck,
DE) ; VON ELM; Rudiger; (Wielen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Coherent LaserSystems GMBH & Co. KG |
Gottingen |
|
DE |
|
|
Assignee: |
Coherent LaserSystems GMBH &
Co. KG
Gottingen
DE
|
Family ID: |
1000005764834 |
Appl. No.: |
16/801857 |
Filed: |
February 26, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/109 20130101 |
International
Class: |
H01S 3/109 20060101
H01S003/109 |
Claims
1. An optic for converting radiation having a fundamental
wavelength to radiation having a second-harmonic wavelength,
comprising: a first crystal made of an optically-nonlinear material
having principle crystal axes, the principle crystal axes of the
first crystal oriented to convert fundamental radiation to
second-harmonic radiation by second-harmonic generation; and a
second crystal made of the same optically-nonlinear material as the
first crystal, the first and second crystals bonded together along
a first planar interface, the first planar interface tilted with
respect to the fundamental radiation; wherein the principle crystal
axes of the first and second crystals have a mutual angular
separation and have reflection symmetry about the first planar
interface, the coefficients of thermal expansion of the first and
second crystals thereby matched along the first planar
interface.
2. The optic of claim 1, wherein the principle crystal axes of the
first and second crystals are oriented such that the fundamental
radiation is not refracted and the second-harmonic radiation is
refracted at the first planar interface.
3. The optic of claim 1, wherein the fundamental radiation enters
the optic through an uncoated input surface at Brewster angle with
respect to the fundamental radiation.
4. The optic of claim 1, wherein the fundamental radiation exits
the optic through an uncoated output surface at Brewster angle with
respect to the fundamental radiation.
5. The optic of claim 4, wherein a tilt angle of the first planar
interface with respect to the fundamental radiation and a distance
between the first planar interface and the output surface are
selected to separate the fundamental radiation from the
second-harmonic radiation on the output surface.
6. The optic of claim 4, wherein a tilt angle of the first planar
interface with respect to the fundamental radiation and a distance
between the first planar interface and the output surface are
selected to overlap the fundamental radiation and the
second-harmonic radiation on the output surface.
7. The optic of claim 4, wherein the second-harmonic radiation
exits the optic through another uncoated output surface at Brewster
angle with respect to the second-harmonic radiation.
8. The optic of claim 1, wherein the fundamental radiation enters
and exits the optic through parallel uncoated surfaces at Brewster
angle with respect to the fundamental radiation.
9. The optic of claim 1, wherein the first and second crystals are
bonded together by optical contact bonding.
10. The optic of claim 1, further including a third crystal made of
the same optically-nonlinear material as the first and second
crystals, the third crystal bonded to the first crystal along a
second planar interface, the second planar interface parallel to
the first planar interface, the principle crystal axes of the first
and third crystals having a mutual angular separation and having
reflection symmetry about the second planar interface, the
coefficients of thermal expansion of the first and third crystals
thereby matched along the second planar interface.
11. The optic of claim 10, wherein respective principle crystal
axes of the second and third crystal have the same
orientations.
12. The optic of claim 10, wherein the first and third crystals are
bonded together by optical contact bonding.
13. The optic of claim 1, wherein the principle crystal axes of the
first and second crystals are mutually separated by at least
5.degree..
14. The optic of claim 13, wherein the principle crystal axes of
the first and second crystals are mutually separated by at least
12.degree..
15. The optic of claim 1, wherein the second-harmonic generation is
type-I second-harmonic generation.
16. The optic of claim 1, wherein the optically-nonlinear material
is selected from the group consisting of beta barium borate (BBO),
lithium triborate (LBO), and cesium lithium borate (CLBO).
17. The optic of claim 1, wherein the fundamental radiation has a
wavelength of about 426 nanometers and the second-harmonic
radiation has a wavelength of about 213 nanometers.
18. A laser apparatus for producing a beam of laser radiation at a
second-harmonic wavelength, comprising; a laser delivering a beam
of laser radiation at a fundamental wavelength that is twice the
second-harmonic wavelength; an impedance-matched
resonant-enhancement cavity receiving the fundamental laser beam,
the resonant-enhancement cavity defined by a plurality of cavity
mirrors that are highly reflective at the fundamental wavelength
and are arranged to direct the fundamental laser beam along a
closed path within the resonant-enhancement cavity; and an optic
located in the closed beam path of the resonant-enhancement cavity,
the optic including a first crystal and a second crystal made of
the same optically-nonlinear material, the optically-nonlinear
material having principle crystal axes, the principle crystal axes
of the first crystal oriented to convert laser radiation at the
fundamental wavelength to laser radiation at the second-harmonic
wavelength by second-harmonic generation, the first and second
crystals bonded together along a planar interface that is tilted
with respect to the fundamental laser beam; wherein the principle
crystal axes of the first and second crystals are mutually
separated by an angle of at least 2.degree., the principle crystal
axes of the first and second crystals having reflection symmetry
about the planar interface, the coefficients of thermal expansion
of the first and second crystals thereby matched along the planar
interface.
19. The optic of claim 18, wherein the fundamental laser beam
enters the optic through an uncoated input surface and exits the
optic through a parallel uncoated output surface, the input and
output surfaces at Brewster angle with respect to the fundamental
laser beam.
20. The optic of claim 19, wherein a tilt angle of the planar
interface with respect to the fundamental laser beam and a distance
between the planar interface and the output surface are selected to
separate the fundamental laser beam from the second-harmonic laser
beam on the output surface.
21. The optic of claim 18, wherein the first and second crystals
are bonded together by optical contact bonding.
22. The optic of claim 18, wherein the second-harmonic generation
is type-I second-harmonic generation.
23. An optic for converting radiation having a first fundamental
wavelength and radiation having a second fundamental wavelength to
radiation having a sum-frequency wavelength, comprising: a first
crystal made of an optically-nonlinear material having principle
crystal axes, the principle crystal axes of the first crystal
oriented to convert first fundamental radiation and second
fundamental radiation to sum-frequency radiation by sum-frequency
generation; and a second crystal made of the same
optically-nonlinear material as the first crystal, the first and
second crystals bonded together along a planar interface, the
planar interface tilted with respect to the first and second
fundamental radiation; wherein the principle crystal axes of the
first and second crystals have a mutual angular separation and have
reflection symmetry about the planar interface, the coefficients of
thermal expansion of the first and second crystals thereby matched
along the planar interface.
24. The optic of claim 23, wherein the first and second fundamental
radiation exits the optic through an uncoated output surface at
Brewster angle with respect to the first and second fundamental
radiation.
25. The optic of claim 24, wherein a tilt angle of the planar
interface with respect to the first and second fundamental
radiation and a distance between the planar interface and the
output surface are selected to separate the first and second
fundamental radiation from the sum-frequency radiation on the
output surface.
26. The optic of claim 23, wherein the first and second crystals
are bonded together by optical contact bonding.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates in general to optical elements
for generating second-harmonic radiation. The invention relates in
particular to optically-nonlinear crystals for generating
ultraviolet wavelength radiation by harmonic conversion of visible
wavelength radiation and to separating the ultraviolet radiation
from residual visible radiation.
DISCUSSION OF BACKGROUND ART
[0002] In laser devices for providing ultraviolet wavelength
radiation, it is usual to produce the ultraviolet (UV) radiation by
harmonic conversion of visible wavelength radiation in an
optically-nonlinear crystal. Typically, the visible radiation is
produced by second-harmonic conversion of near-infrared (NIR)
wavelength radiation generated in a solid-state laser, such as an
optically-pumped semiconductor (OPS) laser.
[0003] By way of example, in one common arrangement for producing
continuous-wave UV radiation, the visible radiation is produced by
intra-cavity second-harmonic conversion of NIR radiation within a
solid-state laser. The visible radiation is coupled into an
impedance-matched resonant-enhancement cavity for the visible
radiation. An optically-nonlinear crystal within the resonant
cavity then converts the visible radiation to UV radiation by
type-I second-harmonic conversion. Using an OPS solid-state laser,
it is possible to produce UV radiation having a wavelength of 266
nanometers (nm) or less. A difficulty with type-I second-harmonic
conversion is that there is no inherent spatial separation between
the UV radiation produced and residual visible radiation.
[0004] Typically, the UV radiation is directed out of the resonant
cavity by a dichroic mirror having a thin-film dielectric coating.
This dichroic mirror may serve as one of the resonator mirrors,
reflecting the visible radiation and transmitting the UV radiation.
Alternatively, the dichroic mirror may be a separate intra-cavity
element, transmitting the visible radiation and reflecting the UV
radiation out of the resonant cavity.
[0005] A particular limitation of the laser arrangements described
above is damage to thin-film dielectric coatings caused by the UV
radiation. This limitation becomes more problematic for
shorter-wavelength UV radiation. Although intra-cavity elements can
be translated, shifting a damaged area out of the radiation and
exposing a virgin area, such shifting adds significant complexity
and cost to the laser device. Further, even a coating that has not
yet been damaged will typically have losses due to manufacturing
variances in layer-thickness or due to absorption by the materials
of the coating. Such losses reduce the efficiency of resonant
cavities using intra-cavity harmonic generation and ultimately the
useful lifetime of these resonant cavities.
[0006] An apparatus that overcomes these limitations is described
in U.S. Pat. No. 10,474,004, assigned to the assignee of the
present invention, the complete disclosure of which is incorporated
herein by reference. An uncoated birefringent prism receives
visible radiation UV radiation generated by an optically-nonlinear
crystal. The visible radiation and the UV radiation have orthogonal
linear polarizations. The birefringent crystal has an internal
surface oriented at Brewster angle for the visible radiation and
oriented for total internal reflection of the UV radiation. The
birefringent crystal is an additional element having optical
surfaces that must be oriented with relatively-high precision. Both
the visible radiation and the UV radiation must traverse two
surfaces of the birefringent crystal, with some unavoidable
reflection losses.
[0007] There is need for a laser device that generates UV radiation
and separates the UV radiation from visible radiation, which does
not expose any coatings to the UV radiation. Preferably, generation
and separation would be accomplished while adding minimal optical
elements, optical surfaces, complexity, and cost to the laser
device.
SUMMARY OF THE INVENTION
[0008] In one aspect, an optic for converting radiation having a
fundamental wavelength to radiation having a second-harmonic
wavelength in accordance with the present invention comprises a
first crystal made of an optically-nonlinear material. The first
crystal has principle crystal axes that are oriented to convert
fundamental radiation to second-harmonic radiation by
second-harmonic generation. A second crystal is provided that is
made of the same optically-nonlinear material as the first crystal.
The first and second crystals are bonded together along a first
planar interface. The first planar interface is tilted with respect
to the fundamental radiation. The principle crystal axes of the
first and second crystals have a mutual angular separation and have
reflection symmetry about the first planar interface. The
coefficients of thermal expansion of the first and second crystals
are thereby matched along the first planar interface.
[0009] In another aspect, a laser apparatus for producing a beam of
laser radiation at a second-harmonic wavelength in accordance with
the present invention comprises a laser delivering a beam of laser
radiation at a fundamental wavelength that is twice the
second-harmonic wavelength. An impedance-matched
resonant-enhancement cavity is provided and receives the
fundamental laser beam. The resonant-enhancement cavity is defined
by a plurality of cavity mirrors that are highly reflective at the
fundamental wavelength and are arranged to direct the fundamental
laser beam along a closed path within the resonant-enhancement
cavity. An optic is provided and located in the closed beam path of
the resonant-enhancement cavity. The optic includes a first crystal
and a second crystal made of the same optically-nonlinear material.
The optically-nonlinear material has principle crystal axes. The
principle crystal axes of the first crystal are oriented to convert
fundamental radiation to second-harmonic radiation by
second-harmonic generation. The first and second crystals are
bonded together along a planar interface that is tilted with
respect to the fundamental laser beam. The principle crystal axes
of the first and second crystals are mutually separated by an angle
of at least 2.degree.. The principle crystal axes of the first and
second crystals have reflection symmetry about the planar
interface. The coefficients of thermal expansion of the first and
second crystals are thereby matched along the planar interface.
[0010] In yet another aspect, an optic for converting radiation
having a first fundamental wavelength and radiation having a second
fundamental wavelength to radiation having a sum-frequency
wavelength in accordance with the present invention comprises a
first crystal made of an optically-nonlinear material. The first
crystal has principle crystal axes that are oriented to convert
first fundamental radiation and second fundamental radiation to
sum-frequency radiation by sum-frequency generation. A second
crystal is provided that is made of the same optically-nonlinear
material as the first crystal. The first and second crystals are
bonded together along a planar interface. The planar interface is
tilted with respect to the first and second fundamental radiation.
The principle crystal axes of the first and second crystals have a
mutual angular separation and have reflection symmetry about the
planar interface. The coefficients of thermal expansion of the
first and second crystals are thereby matched along the planar
interface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated in and
constitute a part of the specification, schematically illustrate a
preferred embodiment of the present invention, and together with
the general description given above and the detailed description of
the preferred embodiment given below, serve to explain principles
of the present invention.
[0012] FIG. 1A is a perspective view, FIG. 1B is a plan view, and
FIG. 1C is a side view schematically illustrating one preferred
embodiment of an optic in accordance with the present invention for
converting a beam of fundamental radiation into a beam of
second-harmonic radiation and separating the two beams, including a
first crystal bonded to a second crystal along a planar interface,
the separated beams exiting the optic through an uncoated output
surface.
[0013] FIG. 2A is a cross-sectional plan view and FIG. 2B is a
cross-sectional side view schematically illustrating further
details of the optic of FIGS. 1A-1C.
[0014] FIG. 3 is a magnified cross-sectional side view illustrating
further details of the optic of FIGS. 1A-1C.
[0015] FIG. 4A is a cross-sectional plan view and FIG. 4B is a
cross-sectional side view schematically illustrating another
preferred embodiment of an optic in accordance with the present
invention, similar to the optic of FIGS. 1A-1C, but including a
third crystal bonded to the first crystal along another planar
interface.
[0016] FIG. 5 is a side view schematically illustrating a method in
accordance with the present invention to fabricate the optic of
FIGS. 4A and 4B.
[0017] FIG. 6A is an example of the optic of FIGS. 1A-1C having the
principle crystal axes and the planar interface tilted at angles
selected to maximize separation between the beam of fundamental
radiation and the beam of second-harmonic radiation on the output
surface.
[0018] FIG. 6B is an example of the optic of FIGS. 1A-1C having the
principle crystal axes and the planar interface tilted at angles
selected to maximize overlap of the beam of fundamental radiation
and the beam of second-harmonic radiation on the output
surface.
[0019] FIG. 7A is a perspective view, FIG. 7B is a plan view, and
FIG. 7C is a side view schematically illustrating yet another
preferred embodiment of an optic in accordance with the present
invention, similar to the optic of FIGS. 1A-1C, but the beam of
fundamental radiation exiting the optic through one uncoated output
surface and the beam of second-harmonic radiation exiting through
another uncoated output surface.
[0020] FIG. 8 schematically illustrates one embodiment of a laser
system in accordance with the present invention, including a laser,
a resonant-enhancement cavity, and the optic of FIGS. 1A-1C.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Referring now to the drawings, wherein like components are
designated by like numerals, FIGS. 1A-1C schematically illustrate a
preferred embodiment of an optic 10 in accordance with the present
invention. FIG. 1A is a perspective view, FIG. 1B is a plan view,
and FIG. 1C is a side view of optic 10. The outside edges of optic
10 are emphasized by heavy lines in the drawings. Optic 10 includes
a first crystal 12 and a second crystal 14 that are made of the
same transparent optically-nonlinear material. First crystal 12 and
second crystal 14 are bonded together along a planar internal
interface 16.
[0022] First crystal 12 is oriented and arranged to convert a beam
of fundamental radiation 18 having a fundamental wavelength to a
beam of second-harmonic radiation 20 having a second-harmonic
wavelength by type-I second-harmonic generation. Two photons of
fundamental beam 18 are converted to each photon of second-harmonic
beam 20. The fundamental wavelength is twice the second-harmonic
wavelength for energy conservation. In type-I second-harmonic
generation, the fundamental radiation has ordinary linear
polarization and the second-harmonic radiation has extra-ordinary
linear polarization, as known in the art.
[0023] Here, "oriented" refers to orienting the principle crystal
axes with respect to the plane-of-polarization and the
direction-of-propagation of fundamental beam 18 to achieve momentum
conservation or "phase matching". Phase matching exploits the
birefringence of the optically-nonlinear material and is necessary
for efficient second-harmonic generation. The principle crystal
axes of second crystal 14 have a different orientation with respect
to fundamental beam 18. Therefore, crystal 14 is not phase matched
and does not produce significant second-harmonic radiation.
[0024] Here, fundamental beam 18 enters optic 10 through an
uncoated input surface 22, which is oriented at the internal
Brewster angle .beta..sub.F of the fundamental radiation to
minimize reflection losses. An alternative arrangement would have
input surface 22 at normal incidence to fundamental beam 18 and an
anti-reflection coating on input surface 22 to minimize reflection
losses. Fundamental beam 18 is partially converted to
second-harmonic beam 20 while propagating through first crystal 12.
The second-harmonic beam and a beam of residual fundamental
radiation 24 are incident on internal interface 16 at normal
incidence in the plane of FIG. 1B and at a non-normal
angle-of-incidence .gamma. in the plane of FIG. 1C. The
optically-nonlinear material has different refractive indices for
the orthogonally polarized beams. The ordinary-polarization
fundamental beam has the same refractive index in each crystal,
while the extra-ordinary-polarization second-harmonic beam has
different refractive indices, resulting in the second-harmonic beam
being refracted away from the residual fundamental beam at internal
interface 16.
[0025] Both residual fundamental beam 24 and second-harmonic beam
20 exit optic 10 through an uncoated output surface 26, which is
oriented at the internal Brewster angle .beta..sub.F with respect
to the residual fundamental beam to minimize reflection losses at
the fundamental wavelength. Input surface 22 and output surface 26
are parallel so that overall optic 10 does not cause an angular
displacement of the fundamental radiation; that is, fundamental
beam 18 and residual fundamental beam 24 propagate in the same
direction outside of optic 10. The different refractive indices of
the orthogonally polarized second-harmonic and residual fundamental
beams cause these beams to be refracted at different angles by
output surface 26 and creates an additional internally-reflected
beam 28 at the second-harmonic wavelength, depicted in FIG. 1B. The
refracted residual fundamental beam 24 and second-harmonic beam 20
diverge and become spatially separated as they propagate away from
output surface 26 of optic 10.
[0026] FIGS. 2A and 2B are cross-sectional views schematically
illustrating further details of optic 10. FIG. 2A is a plan view
and FIG. 2B is a side view. The internal beams are omitted in these
drawings for clarity of illustration. FIG. 2A indicates the
external Brewster angle .alpha..sub.F of fundamental beam 18 and
the external angle .alpha..sub.SH of second-harmonic beam 20
emerging from output surface 26. These beams diverge at an angle
.DELTA..alpha. in the plane of FIG. 2A and an angle .delta. in the
plane of FIG. 2B.
[0027] The principle crystal axes x, y, and z are oriented in first
crystal 12 and second crystal 14 as depicted in FIGS. 2A and 2B.
The principle crystal axes are symmetrically oriented with respect
to internal interface 16. Specifically, the principle crystal axes
have reflection symmetry about internal interface 16. One
particular advantage of this symmetrical arrangement is that the
coefficients of thermal expansion of the first and second crystals
are matched in all directions at the internal interface.
[0028] Matching the coefficients of thermal expansion is important
when bonding the crystals together. Crystals can be bonded using
commercial polymer adhesives. For example, one of the light-cured
optical adhesives available from Norland Products Inc. of Cranbury,
N.J. These adhesives are nominally transparent over relatively
broad wavelength ranges, can be cured at room temperature, and the
adhesive itself provides some compliance when a bond is stressed by
heating. However, such an adhesive layer is still weakly absorbing
and is degraded by high-power ultraviolet radiation. Sufficient
heating by optical radiation or otherwise will cause the bond to
fail if the crystals have different coefficients of thermal
expansion.
[0029] Where a bond is exposed to ultraviolet radiation, especially
for high-power applications, optical contact bonding is preferred.
Contact bonding forms direct chemical bonds between crystals,
eliminating intermediary adhesive layers, and is therefore
extremely reliable and durable. Contact bonding is achieved by
pressing together extremely-flat polished surfaces at a relatively
high temperature. For example, at temperatures exceeding
800.degree. C. when bonding beta barium borate (BBO). Therefore,
even small differences in the coefficients of thermal expansion can
cause significant stress during cooling, causing the crystals to
separate or crack. BBO has a factor of nine difference between
coefficients of thermal expansion along orthogonal crystal axes.
The inventive optic, having crystals made of the same material that
are oriented to match the coefficients of thermal expansion at
internal interface 16, can be contact bonded at high temperatures
without such failures due to thermal stress.
[0030] Another advantage of optic 10 is that there is minimal
reflection of fundamental beam 18 at internal interface 16, since
there is no change in refractive index when a beam having ordinary
polarization propagates therethrough. Minimizing power losses for
the fundamental beam is particularly important when optic 10 is
incorporated into a laser resonator or resonant-enhancement cavity.
Losses for the circulating fundamental beam significantly diminish
the efficiency of such a resonator and the impact of such losses is
enhanced by the nonlinearity of the second-harmonic generation.
[0031] FIG. 3 is a magnified view of internal interface 16 as it is
depicted in the cross-sectional side view of FIG. 2B. The principle
crystal axes of first crystal 12 and second crystal 14 are centered
on the internal interface in the drawing to emphasize their
symmetry about the internal interface. The z-axis of first crystal
12 is tilted at an angle .PHI. with respect to the internal
interface and the z-axis of second crystal 14 is tilted at an angle
.PHI.' in the opposite direction with respect to the internal
interface. These tilt angles .PHI. and .PHI.' are the same to match
the coefficients of thermal expansion. The z-axis of the first
crystal is at an angle .theta. with respect to fundamental beam 18
that is selected to achieve phase matching. The z-axis of the
second crystal is at a different angle .theta.' with respect to
fundamental beam 18. Second-harmonic beam 20 is refracted away from
residual fundamental beam 24 in the plane of the drawing by an
internal angle that is
.delta. n S .times. H .function. ( .theta. ' ) ##EQU00001##
in the paraxial approximation. Herein, n.sub.F is the refractive
index for the fundamental beam in both crystals, n.sub.F(') is the
refractive index for the second-harmonic beam in the first crystal,
and n.sub.SH(.theta.') is the refractive index for the
second-harmonic beam in the second crystal.
[0032] FIGS. 4A and 4B schematically illustrate another preferred
embodiment of an optic 30 in accordance with the present invention.
FIG. 4A is a cross-sectional plan view and FIG. 4B is a
cross-sectional side view of optic 30. Optic 30 is similar to optic
10, but has an additional third crystal 32, which is made of the
same optically-nonlinear transparent material as first crystal 12
and second crystal 14. First crystal 12 and third crystal 32 are
bonded together along a planar internal interface 34 that is
preferably parallel to internal interface 16. The principle crystal
axes of the third crystal preferably have the same orientations as
the respective principle crystal axes of the second crystal. An
uncoated input surface 36 is located on the third crystal of optic
30 and is oriented at the internal Brewster angle .beta..sub.F of
fundamental beam 18. Input surface 36 and output surface 26 are
preferably parallel.
[0033] An advantage of optic 30 over optic 10 is that fundamental
beam 18 propagates through a constant path length L in first
crystal 12, regardless of the location that the fundamental beam is
incident on input surface 36. This constant path length enables
translation of optic 30 without changing the overall efficiency of
second-harmonic generation. Parallel input surface 36 and output
surface 26 enable optic 30 to be translated without displacing any
of the external beams. Such translation can be used to extend the
useful life of optic 30, since an area or volume that has been
degraded or otherwise damaged by the UV radiation can be shifted
relative to the beams. Optic 30 can be translated in two
dimensions, as indicated by the double-headed arrows T in the
drawings, making a two-dimensional array of incident locations
available on the input surface. The maximum number of incident
locations will depend on the diameter of the fundamental beam
relative to the dimensions of the optic.
[0034] FIG. 5 is a side view schematically illustrating a method 40
in accordance with the present invention to efficiently and
precisely fabricate optic 30. A first preform 42 and a second
preform 44 are bonded together along a planar interface 46. First
preform 42 and a third preform 48 are bonded together along a
parallel planar interface 50. The principle crystal axes of first
preform 42 are oriented with respect to the parallel planar
interfaces to provide phase-matching for second-harmonic
generation. Second preform 44 and third preform 48 have principle
crystal axes symmetrically oriented with respect to the principle
crystal axes of first preform 42 and oriented to match the
coefficients of thermal expansion in all directions along each
parallel planar interface. Length L would be selected to optimize
the second-harmonic generation efficiency. Angle .gamma. is a
nominally free parameter that may be selected according to
considerations described below.
[0035] A plurality of optics 30 (two are depicted) can be
fabricated from the bonded preforms of FIG. 5 by cutting the bonded
preforms along cut lines 52. Material within the first, second, and
third preforms becomes respectively the first, second, and third
crystals of each optic 30. Planar interface 46 becomes internal
interface 16 and planar interface 50 becomes internal interface 34.
Input and output surfaces of the cut optics are then polished to a
desired optical quality. Method 40 minimizes the total volume of
waste material removed in the fabrication of optics 30 and
minimizes the number of discrete operations necessary to fabricate
a batch of optics. A similar method may be used to fabricate a
plurality of optics 10 by cutting two oriented and bonded
preforms.
[0036] A precise way to prepare the preforms is to cut them from
one larger block of the optically-nonlinear material. The first
preform would be cut at angle .gamma. along intended planar
interface 46. The cut surfaces are polished and the offcut becomes
the second preform. The second preform is rotated by 180.degree.
and planar interface 46 is formed by contact bonding the first
preform to the second preform. Similarly, the third preform is cut
from the first preform along intended planar interface 50, the cut
surfaces are polished, the third preform is rotated by 180.degree.,
and then the third preform is contact bonded to the first preform
to form planar interface 50.
[0037] FIG. 6A is a cross-sectional side view illustrating an
example of optic 10 having angle .gamma. selected to maximize the
separation between residual fundamental beam 24 and second-harmonic
beam 20 on output surface 26. For some optically-nonlinear
materials, it is known that the threshold for damage is lower
and/or the rate of degradation is higher when a surface is exposed
to fundamental radiation and UV radiation simultaneously; that is
respectively lower and higher than when the surface is exposed to
each radiation individually. Therefore, when using these
optically-nonlinear materials and particularly in high-power
applications, it is desirable to separate these beams. Another
advantage is that the fundamental radiation does not propagate
through an area on the output surface that has been degraded by the
UV radiation, which extends the useful life of the optic in an
intra-cavity application.
[0038] FIG. 6A depicts a specific example having optic 10 made of
BBO for operation at a temperature of about 200.degree. C. The
fundamental wavelength is 426 nm and the second-harmonic wavelength
is 213 nm. Residual fundamental beam 24 and second-harmonic beam 20
are depicted by boundary rays thereof. Fundamental beam 18 and
residual fundamental beam 24 have a diameter of approximately 330
micrometers (.mu.m) near internal interface 16. Angle .gamma. is
selected to be 29.25.degree.. Angle .theta. is 73.3.degree. and
angle .theta.' is 48.2.degree. to achieve both phase matching in
crystal 12 and symmetry of the principle crystal axes. The example
takes advantage of spatial walk-off of the second-harmonic beam
from the fundamental beam within first crystal 12, which is due to
birefringence. Boundary rays on one side of the second-harmonic
beam diverge from the fundamental beam at an angle of approximately
2.9.degree.. After refraction at internal interface 16, the
second-harmonic beam diverges from the residual fundamental beam at
an angle of approximately 6.4.degree.. A distance d between
internal interface 16 and output surface 26 of about 3 millimeters
(mm) is sufficient to separate the beams on the output surface.
[0039] FIG. 6B is a cross-sectional side view illustrating another
example of optic 10, but here angle .gamma. is selected to maximize
the overlap of residual fundamental beam 24 and second-harmonic
beam 20 on output surface 26. Again, in the specific example
depicted, optic 10 is made of BBO. The fundamental wavelength is
426 nm and the second-harmonic wavelength is 213 nm. Fundamental
beam 18 and residual fundamental beam 24 have a diameter of
approximately 330 .mu.m near internal interface 16. Angle .gamma.
is selected to be 12.5.degree.. Angle .theta. is 106.7.degree.,
which is equivalently an angle of 73.3.degree. with respect to
fundamental beam 18 to achieve phase matching, but having the
z-axis tilted in the opposite direction with respect to the
fundamental beam propagation. Angle .theta.' is 48.3.degree. to
achieve symmetry of the principle crystal axes. Changing the z-axis
tilt direction in first crystal 12 causes spatial walk-off of the
second-harmonic beam on the opposite side thereof. Boundary rays on
this opposite side of second-harmonic beam 20 diverge from
fundamental beam 18 at an angle of approximately 2.9.degree.. After
refraction at internal interface 16, the second-harmonic beam
converges towards the residual fundamental beam at an angle of
approximately 6.4.degree..
[0040] Together, FIGS. 6A and 6B demonstrate how nominally-free
angle .gamma. can be selected to manipulate the mutual separation
of the residual fundamental beam and the second-harmonic beam.
However, there will be a compromise between reflection of the
residual fundamental beam from the internal interface and
separating the beams on the output surface. Otherwise, angle
.gamma. can have any value between 0.degree. and a maximum angle
that corresponds to the largest acceptable dispersion imparted onto
the beams, while maintaining symmetry between the principle crystal
axes (tilt angle .PHI.=tilt angle .PHI.'). Preferably, the
principle crystal axes of the bonded crystals are mutually
separated by an angle 2.PHI. of at least 2.degree., to minimize
second-harmonic conversion in the second crystal. More preferably,
the principle crystal axes are mutually separated by an angle of at
least 5.degree., and most preferably by an angle of at least
12.degree.. For BBO, this is a mutual angular separation of the
z-axes (the crystallographic c-axes) of the two crystals, while the
x-axes (one of the crystallographic a-axes) is common to both
crystals.
[0041] The inventive optic can be made of other optically-nonlinear
materials suitable for second-harmonic generation, include lithium
triborate (LBO) and cesium lithium borate (CLBO). Angle .theta. in
first crystal 12 is derived from the requirements for phase
matching and energy conservation, respectively, which can be
expressed as:
k F + k F = k S .times. H ( 1 ) .lamda. F 2 = .lamda. S .times. H (
2 ) ##EQU00002##
where k are the wavevector magnitudes and .lamda. are the
wavelengths of the fundamental beam and the second-harmonic beam.
Referring to FIG. 3, refractive indices of the polarized
fundamental beam at wavelength .lamda..sub.F and the orthogonally
polarized second-harmonic beam at wavelength .DELTA..sub.SH are
respectively:
n F = n x .function. ( .lamda. F ) ( 3 ) n S .times. H .function. (
.theta. ) = 1 cos 2 .function. ( .theta. ) n y 2 .function. (
.lamda. SH ) + sin 2 .function. ( .theta. ) n z 2 .function. (
.lamda. SH ) . ( 4 ) ##EQU00003##
Equations (1) and (2) together require n.sub.F=n.sub.SH(.theta.),
so the phase matching angle .theta. in first crystal 12 can be
determined from Equations (3) and (4):
.theta. = cos - 1 .function. ( 1 n x 2 .function. ( .lamda. F ) - 1
n z 2 .function. ( .lamda. F ) 1 n y 2 .function. ( .lamda. SH ) +
1 n z 2 .function. ( .lamda. SH ) ) . ( 5 ) ##EQU00004##
[0042] Referring to FIG. 2A, by applying Snell's law at output
surface 26, the divergence angle .DELTA..alpha. and refractive
index of the second-harmonic beam can be calculated for a selected
angle .theta.':
.DELTA. .times. .alpha. = .alpha. S .times. H - .alpha. F = sin - 1
.function. ( n S .times. H .function. ( .theta. ' ) . sin
.function. ( .beta. F ) ) - .alpha. F ( 6 ) n S .times. H
.function. ( .theta. ' ) = sin .function. ( .alpha. F + .DELTA.
.times. .alpha. ) sin .function. ( .beta. F ) . ( 7 )
##EQU00005##
[0043] Referring to FIG. 2B, by applying Snell's law at internal
interface 16, the divergence angle .delta. can also be calculated
for a selected angle .theta.' and the corresponding angle
.gamma.:
.gamma. = 90 .times. .degree. - ( .theta. + .theta. ' ) 2 ( 8 )
.delta. = .gamma. .function. ( n S .times. H .function. ( .theta. '
) - n S .times. H .function. ( .theta. ) ) . ( 9 ) ##EQU00006##
[0044] Alternatively, angle .theta.' and the corresponding angle
.gamma. can be determined to achieve a desired divergence angle
.DELTA..alpha. between the residual fundamental beam and the
second-harmonic beam propagating away from the inventive optic. For
brevity, just the result for angle .gamma. is provided here, while
Equation 11 is simply a rearrangement of Equation 8:
.gamma. = ( 1 1 + n x .function. ( .lamda. F ) .times. sin .times.
.beta. F sin .function. ( .alpha. F + .DELTA. .times. .alpha. ) )
.times. ( 180 .times. .degree. - .theta. - cos - 1 .function. ( (
sin .function. ( .beta. F ) sin .function. ( .alpha. F + .DELTA.
.times. .alpha. ) ) 2 - 1 n z 2 .function. ( .lamda. S .times. H )
1 n y 2 .function. ( .lamda. s .times. H ) - 1 n z 2 .function. (
.lamda. s .times. H ) ) ) ( 10 ) .theta. ' = 180 .times. .degree. -
.theta. - 2 .times. .gamma. . ( 11 ) ##EQU00007##
[0045] Returning to the example above, where the fundamental
wavelength is 426 nm and the second-harmonic wavelength is 213 nm.
In BBO, at a temperature of 200.degree. C., the refractive indices
are calculated to be: n.sub.x(426)=n.sub.y(426)=1.686,
n.sub.z(426)=1.561, n.sub.x(213)=n.sub.y(213)=1.850, and
n.sub.z(213)=1.671. The internal Brewster angle is
.beta..sub.F=30.67.degree. and the external Brewster angle is
.alpha..sub.F=59.32.degree. at 426 nm. Angle .theta. is
73.3.degree. to achieve phase matching in the first crystal. For
the specific example above, having angle .gamma. selected to be
29.25.degree. and angle .theta.' of 48.2.degree. to achieve
symmetry of the principle crystal axes, the tilt angles are
.PHI.=.PHI.'=12.55.degree. and the divergence angle is
.DELTA..alpha.=2.3.degree.. The refractive index of the
second-harmonic beam in the first crystal is
n.sub.SH(.theta.)=1.684 and in the second crystal is
n.sub.SH(.theta.')=1.725.
[0046] FIGS. 7A-7C schematically illustrate yet another preferred
embodiment of an optic 80 in accordance with the present invention.
FIG. 7A is a perspective view, FIG. 7B is a plan view, and FIG. 7C
is a side view of optic 80. Optic 80 is similar to optic 10 of
FIGS. 1A-1C, but has a first output surface 82 and a second output
surface 84. First output surface 82 is oriented at the internal
Brewster angle .beta..sub.F of the fundamental radiation, to
minimize reflection losses for residual fundamental beam 24
emerging from second crystal 14. Second output surface 84 is
oriented at the internal Brewster angle .beta..sub.SH of the
second-harmonic radiation, to minimize reflection losses for
second-harmonic beam 20 emerging from second crystal 14. An
exemplary incident location 86 of the residual fundamental beam on
first surface 82 and a corresponding exemplary incident location 88
of the second-harmonic beam on second surface 84 are indicated in
FIG. 7A.
[0047] Second output surface 84 essentially eliminates internally
reflected beam 28 depicted in FIG. 1B and all of the
second-harmonic radiation generated in first crystal 12 emerges
from second surface 84. Another advantage of optic 80 is that the
second-harmonic beam is highly divergent from the residual
fundamental beam after emerging from second surface 84. Optic 30
could similarly be fabricated with a second output surface. The
incident locations available on the input surface would then lie
along a diagonal line when translating the optic.
[0048] FIG. 8 schematically illustrates a preferred embodiment of a
laser system 90 in accordance with the present invention for
producing a beam of second-harmonic laser radiation 20. Laser
system 90 includes a laser 92 delivering a beam of fundamental
laser radiation 18 to an impedance-matched resonant-enhancement
cavity 94 defined, here, by four cavity mirrors 96, 98, 100, and
102. The four cavity mirrors are highly reflective at the
fundamental wavelength and are arranged to direct the fundamental
laser beam along a closed path within resonant-enhancement cavity
94. Fundamental laser beam 18 couples into the resonant-enhancement
cavity through cavity mirror 96. Cavity mirror 98 is mounted on a
piezoelectric (PZT) transducer 104 for adjustment of the optical
length of the closed beam path. PZT transducer 104 requires an
electrical driver that is not depicted. Impedance matching is
achieved by continuous adjustment of the optical length of the
resonant-enhancement cavity.
[0049] Laser system 90 also includes optic 10, which is located in
the closed beam path of resonant-enhancement cavity 94 to partially
convert fundamental laser beam 18 to second-harmonic laser beam 20.
Cavity mirror 102 is located and arranged to reflect the
fundamental laser beam and allow the diverging second-harmonic
laser beam to propagate out of the resonant-enhancement cavity. The
inventive optic that spatially separates the laser beams eliminates
need for an additional intra-cavity dichroic mirror to direct the
second-harmonic laser beam out of the resonant-enhancement cavity.
Any of optic 10, optic 30, or optic 80 could be incorporated into
laser system 90.
[0050] The inventive optics described herein above are particularly
useful for type-I second-harmonic generation. However, one of skill
in the art would recognize that equivalent optics could be
fabricated for other optically nonlinear processes, such as type-II
second-harmonic generation and sum-frequency generation. In type-II
second-harmonic generation, two photons having the fundamental
wavelength and orthogonal linear polarizations are converted into
each photon having the second-harmonic wavelength and ordinary
linear polarization. In sum-frequency generation, two photons
having different fundamental wavelengths are converted into each
photon having a sum-frequency wavelength. That is, radiation having
a first fundamental wavelength .lamda..sub.F1 and radiation having
a second fundamental wavelength .lamda..sub.F2 is converted to
radiation having a sum-frequency wavelength .lamda..sub.SF. Here,
"fundamental wavelength" refers to a wavelength longer than the
sum-frequency wavelength. These wavelengths have the approximate
relation:
1 .lamda. F .times. 1 + 1 .lamda. F .times. 1 = 1 .lamda. SF . ( 12
) ##EQU00008##
Sum-frequency generation may also be a type-I process with a common
fundamental polarization or a type-II process with different
fundamental polarizations. It should be noted that second-harmonic
generation is a special case of sum-frequency generation, having
just one fundamental wavelength and usually just one fundamental
beam.
[0051] In summary, an optic is disclosed that partially converts a
fundamental beam to a second-harmonic beam and spatially separates
the second-harmonic beam from the residual fundamental beam. The
inventive optic comprises two or three crystals made of the same
optically-nonlinear material that are bonded along planar
interfaces. The principle axes of the crystals have reflection
symmetry about each planar interface to enable contact bonding of
the crystals for high-power applications. The output surfaces of
the inventive optic are at Brewster angle with respect to the
fundamental beam to minimize reflection losses and the output
surfaces are uncoated to minimize optical damage. The angular
separation between the second-harmonic beam and the residual
fundamental beam is determined by selecting the tilt angles of the
principle crystal axes and the planar interfaces with respect to
the fundamental beam. Importantly, these beams can be spatially
separated on the output surfaces to further minimize optical damage
and to extend the useful lifetime of the optic.
[0052] The present invention is described above in terms of a
preferred embodiment and other embodiments. The invention is not
limited, however, to the embodiments described and depicted herein.
Rather, the invention is limited only by the claims appended
hereto.
* * * * *