Semi-monolithic cavity for external resonant frequency doubling and method of performing the same

The fabrication of an optical cavity for use in a laser, in a frequency doubling external cavity, or any other type of nonlinear optical device, can be simplified by providing the nonlinear crystal in combination with a surrounding glass having an index of refraction substantially equal to that of the nonlinear crystal. The closed optical path in this cavity is formed in the surrounding glass and through the nonlinear crystal which lies in one of the optical segments of the light path. The light is transmitted through interfaces between the surrounding glass in the nonlinear crystal through interfaces which are formed at the Brewster-angle to minimize or eliminate reflection.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to optical resonators and in particular to 
semi-monolithic cavities for doubling the frequency of the resonant beam 
and to the fabrication of semi-monolithic cavities for this purpose. 
2. Description of the Prior Art 
Compact and efficient, continuous wave lasers with wavelength below 600 nm 
are generally commercially unavailable. To generate coherent radiation 
with wavelength below 600 nm several approaches to double the available 
laser frequency using second harmonic generation in a nonlinear crystal 
have been used. Unfortunately, the peak output power of continuous lasers 
is generally low for any process that requires a high electric field 
strength in a laser beam which is incident on a nonlinear crystal. 
To enhance the second harmonic output power, one method is to place the 
nonlinear laser within the cavity of the laser. See, Bergqusit, H. 
Hemmati, and W. M. Itano, "High power Second Harmonic Generation of 257 nm 
Radiation in an External Resonant Cavity," Optics Communication, V.43, 
N.6, 437-442 (1982). In this approach, the high circulating power within 
the cavity passes repeatedly through the crystal. Since efficiency in the 
nonlinear process is quadractically dependent on the input power, this 
approach generates substantially higher output power than a single pass 
through the crystal. For some lasers, such as a semiconductor lasers, 
intra-cavity frequency doubling is either not possible or too cumbersome 
to implement. Also, in the standing wave cavities, spatial hole-burning of 
the waves within the cavity results in multiple mode operation and could 
lead to strong amplitude fluctuations in the second harmonic output. 
An alternative approach of the prior art is to double the fundamental 
frequency of the laser in a resonant external cavity. In this approach, a 
nonlinear crystal is located in a cavity external to the laser's own 
cavity. A portion of the laser's output is injected into the external 
cavity through a partially reflecting mirror. When the cavity is 
maintained on resonance with the input laser beam, constructive 
interference within the external cavity generates high circulating power. 
See, Ashkin, G. D. Boyd, and J. M. Dziedzic, "Resonant Optical Second 
Harmonic generation and Mixing," IEEE Journal of Quantum Electronics, V. 
QE-1, N. 6, 109-122 (1966). Depending on the losses in the external 
cavity, the circulating power could be tens of times that of the input 
beam power. A resonantly enhanced field at the fundamental wavelength then 
efficiently converts to the second harmonic. For this power to be 
efficient, the frequencies of the laser in the external cavity must 
coincide. An external cavity can resonate only in a single longitudinal 
mode. With a multimode laser incident on the external cavity, only one 
mode can resonate. Thus, the power content for the remaining modes is not 
used. Therefore, for the most efficient process, the input laser beam has 
to be a single mode laser. 
FIGS. 1a and 1b are simplified block diagrams that depict examples of a 
prior art discrete and monolithic resonant external frequency doubling 
cavities respectively. See, Godberg, M. K. Chun, I. N. Duling, and T. F. 
Carruthers, "Blue Light Generation by Nonlinear Mixing of Nd: YAG and 
GaAlAs Laser Emission in a KnbO3 Resonant Cavity, " V.56, N. 21, 2071-2073 
(1990); Koziovsky, C. D. Nabors, R. L. Byer, "Efficient Second Harmonic 
Generation of Diode-Laser-Pumped CW Nd:YAG Laser Using Monolithic MgO: 
LiNbO3 External Resonant Cavity," IEEE Journal of Quantum Electronics, 
V.24, N.6, 913-919 (1988); and Briger, H. Busener, A. Hese, F. Moers, and 
A. Renn, "Enhancement of Single Frequency SHG in a Passive Ring 
Resonator," Optics Communication, V.38, #5,6, 423-426 (1981). In FIG. 1a, 
the laser system, generally denoted by reference numeral 10, includes a 
nonlinear crystal 12 between a full reflection mirror 14 and a partially 
reflective mirror 16. Light from a separate laser 18 is incident on a beam 
splitter or partially silvered mirror 20. The beam 22 splits with a 
portion 36 going to a piezoelectric (PZT) driven mirror 24 and the other 
split portion is reflected into photodetector 26. The output of 
photodetector 26 is coupled to and amplified by a frequency lock-in 
amplifier 28, whose output in turn is coupled to a servocircuit 30. The 
output of servocircuit 30 is coupled in turn to a summing node 32. The 
other input to summing node 32 is coupled to an oscillator 34 operating at 
the resonant frequency, f, whose output is also coupled to and drives 
frequency lock-in amplifier 28. The output of summing node 32 in turn is 
then coupled to PZT driven mirror 24. The split portion 36 of light 
transmitted from laser 18 through mirror 20 is incident upon PZT driven 
mirror 24, is optoelectromechanically phase modulated and reflected as 
beam 38 to mirror 14. This phase modulated light is recirculated between 
opposing cavity mirrors 14 and 16 through crystal 12 with transmission 
occurring through mirror 16 as output beam 40. By this arrangement only 
single mode laser light at or near the fundamental resonant frequency, f, 
or its harmonics are able to constructively interfere in crystal 12. 
The monolithic external resonant cavity as shown in FIG. 1b similarly 
includes external laser 18, producing a laser beam 20 which is directed to 
a shaped nonlinear crystal 42. Crystal 42 has curved facets 44 and 46 at 
its opposing ends and a planar interlying facet 48. Beam 22 is partially 
reflective at facet 46 to reflect a portion of the incident beam into 
photodetector 50, which in turn is coupled to an electro-optical feedback 
control circuit 52. Control circuit 52 in turn is then coupled to crystal 
42 to drive crystal 42 on resonance. A portion of beam 54 within crystal 
42 is then transmitted through partially reflective facet 44 as an output 
beam 56. The example of the discrete mirror resonant cavity in FIG. 1a 
employs two or more cavity mirrors and works well and is a straightforward 
approach. In two mirror cavities feedback from the cavity to laser 
generally has a deleterious affect on the mode stability of the laser 
unless an isolator is placed between the laser and the cavity. Moreover, 
the second harmonic output is generated in two directions. This 
necessitates the use of at least one isolator between the laser and the 
resonant cavity. The addition of an isolator is undesirable due to the 
additional space required, the additional loss of power and its cost. If 
three or more mirrors are used in the cavity in a ring geometry as shown 
in FIG. 1b, the beam travels in one direction only and the reflective 
light from the input-coupler does not coincide with the incident laser 
beam. 
In the example of the monolithic external cavity in FIG. 1b, the cavity 
mirrors are polished and coated directly on facets 44, 46 and 48 of 
crystal 42. The facets of cavity 42 form the base of the mirrors. 
Monolithic cavities of this type have the advantage over the discrete 
cavity of a lower overall intra-cavity loss, no dispersion induced 
mismatch, mechanical stability, good frequency stability, compactness and 
lower overall cost. However, monolithic cavities such as shown in FIG. 1b 
suffer from certain disadvantages relative to the discrete cavities as 
shown in FIG. 1a. These disadvantages include the susceptibility for 
manufacturing errors, a loss of all degrees of freedom in cavity 
alignment, and difficulty or cost in obtaining nonlinear crystals with an 
area sufficiently large in size for the implementation of specific cavity 
designs. In addition, changes may be required in the cavity's length to 
accommodate locking at different resonant frequencies, and such 
implementations can be difficult. 
Therefore, what is needed is a design capable of frequency doubling in a 
manner which has the advantages of both a discrete mirror resonant cavity 
and monolithic external resonant cavity frequency doubling, but which is 
not susceptible to the disadvantages of either. 
BRIEF SUMMARY OF THE INVENTION 
The invention is an improvement in a nonlinear optical system having a 
resonant cavity. The improvement comprises a semi-monolithic resonant 
cavity having a first nonlinear portion and a second surrounding portion. 
The first nonlinear portion is characterized by a nonlinear optical 
response to light and the second surrounding portion is characterized by 
an index of refraction approximately equal to the first nonlinear portion. 
Light transmitted between first nonlinear portion and the second 
surrounding portion is transmitted through the adjacent interface surfaces 
of the first nonlinear portion and the second surrounding portion, which 
adjacent interface surfaces are defined at the Brewster-angle. As a 
result, ease of fabrication of the semi-monolithic cavity is facilitated. 
In one embodiment the first nonlinear portion and the second nonlinear 
portion are in contact at the adjacent interface surfaces. In a second 
embodiment the adjacent interface surfaces of first nonlinear portion and 
the second nonlinear portion formed at the Brewster-angle are spaced 
apart. 
The improvement further comprises a substrate to which the first nonlinear 
portion and the second surrounding portion of the semi-monolithic cavity 
are thermally coupled and physically supported. The temperature of the 
substrate may be controlled or at least thermally sinked. 
In one embodiment the second surrounding portion is provided with at least 
two mirrors for providing a closed optical path. The mirrors may be formed 
on curved surfaces provided on the second surrounding portion, may be 
total internal reflection mirrors, or graded index mirrors. 
In another embodiment the first nonlinear portion further comprises a 
nonlinear element and a pair of lenses. One of the pair of lenses is 
positioned at each end of the first nonlinear element. The adjacent 
interface surfaces at the Brewster-angle are defined between the second 
surrounding portion and the pair of lenses. One of the pair of lenses 
focuses light into the first nonlinear element and a second one of the 
pair of lenses collects light from the first nonlinear element for 
transmission from and into the second surrounding portion respectively. 
The improvement may further comprise a plurality of first nonlinear 
portions. The second surrounding portion is arranged and configured to 
form a closed optical path. The closed optical path is comprised of a 
plurality of optical segments. Each one of the plurality of first 
nonlinear portions is deposed in one of the optical segments. 
The semi-monolithic cavity may be employed in a laser cavity, a 
frequency-doubling external cavity, an optical parametric oscillator, an 
optical parametric amplifier, an optical parametric generator, a sum 
frequency mixer, or a difference frequency mixer. 
The invention is also defined as an improvement in the method of 
fabricating a semi-monolithic cavity comprising the steps of providing a 
first nonlinear portion having an index or refraction; and providing a 
second surrounding portion having an index of a refraction approximately 
equal to the index or refraction of the first nonlinear portion. 
Interfaces are formed for transmission of light between the first 
nonlinear portion and the second surrounding portion. The interfaces are 
formed at the Brewster-angle. At least two reflective surfaces are 
provided in the second surrounding portion to form a closed loop of light 
path segments in the semi-monolithic cavity so that the reflecting 
surfaces may be separately adjusted when the first nonlinear portion and 
the second surrounding portion are assembled to form the semi-monolithic 
cavity. 
The invention and its various embodiments having been briefly summarized 
can be better visualized in the following drawings where like elements are 
referenced by like numerals.

The invention and its various embodiments now having been illustrated in 
the foregoing drawings, it can be better understood by turning to the 
following detailed description. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The fabrication of an optical cavity for use in a laser, in a frequency 
doubling external cavity, or any other type of nonlinear optical device, 
can be simplified by providing the nonlinear crystal in combination with a 
surrounding glass having an index of refraction substantially equal to 
that of the nonlinear crystal. The closed optical path in this cavity is 
formed in the surrounding glass and through the nonlinear crystal which 
lies in one of the optical segments of the light path. The light is 
transmitted through interfaces between the surrounding glass in the 
nonlinear crystal through interfaces which are formed at the 
Brewster-angle or antireflection coated surfaces to minimize or eliminate 
reflection. 
What is described below is a semi-monolithic cavity structure used for 
frequency doubling of lasers and of other nonlinear processes, such as 
optical parametric oscillators and the like which are implemented 
externally to the laser cavity. The invention circumvents most of the 
problems associated with the monolithic cavities, while maintaining the 
majority of advantages of monolithic cavities over discrete cavities. The 
semi-monolithic cavity comprises a nonlinear optic frequency doubling 
crystal surrounded by a shaped block of glass or other material that has a 
similar index of refraction as the nonlinear crystal. The cavity's 
mirrors, which are both curved and flat, are fabricated on the glass block 
rather than on the nonlinear crystal material. This approach reduces cost 
significantly and eliminates deleterious affects of manufacturing errors. 
The invention is also characterized by the fact that nonlinear interaction 
within the cavity occurs only along a particular axis of the crystal. With 
a ring monolithic cavity therefore, typically only a single arm of the 
ring coincides with the active crystal axis that participates in the 
nonlinear interaction or process. 
The glass material on which the mirrors are fabricated is significantly 
easier to configure, polish and coat compared to nonlinear crystal 
materials. This fact reduces cost significantly and helps assure that the 
correct mirrors are applied to the external cavity and further provides a 
limited amount of freedom in alignment of the cavity mirrors prior to 
bonding. A major advantage of the semi-monolithic cavity invention is that 
it is free of design limitations or other considerations that arise from 
the limited availability of sizes of the nonlinear crystal. Although the 
semi-monolithic cavity contains more surfaces and therefore more loss 
elements within the monolithic cavity, the semi-monolithic cavity permits 
use of a Brewster-angled end facet as described below on the nonlinear 
crystal, thereby avoiding the need for an anti-reflection coating. The 
Brewster angle is that angle of incidence of a beam at which there is 
minimum reflection of the incident light, and in which the tangent of the 
angle is substantially equal to the common index of a fraction of the 
glass and the nonlinear crystal material. 
Disturbance of the cavity optical path length will give rise to an error 
signal useful for maintaining the cavity in resonance with a laser or the 
laser in resonance with the cavity. Intentional cavity length displacement 
may be implemented in the semi-monolithic architecture in a number of 
alternative embodiments. For example, in one method an electric field is 
applied to a nonlinear material where the proper electro-optical field of 
the beam is accessible. In another embodiment, the laser beam is bounced 
off a translation mirror, such as a piezo-electric mirror, prior to 
entering the resonant cavity. 
To maintain the cavity in resonance with the laser it will be required to 
dither the cavity length and use any of a number of established locking 
schemes to actively or passively lock the cavity in resonance with the 
laser. Alternatively, with some lasers such as diode lasers, it is 
possible to maintain the input laser beam in resonance with the cavity. 
Cavity length alterations are possible by adding a moving element to one 
of the reflecting surfaces of the cavity. Piezo-electric transducers 
(PZTs) are commonly used for moving a reflective surface of the cavity. 
Alternative approaches include application of pressure, acoustic, or an 
electric field to the nonlinear crystal. Conventional photodetectors are 
used to detect the light transmitted from the cavity, or light reflected 
from the cavity and interfered with light transmitted from the cavity to 
ascertain fluctuations of the power circulating within the cavity. The 
means for generating the error signal in a semi-monolithic cavity are 
conventional and hence will not be further elaborated. 
Turn now specifically to the embodiment of the invention diagramatically 
shown in side cross-sectional view in FIG. 2a. FIG. 2a and the additional 
embodiments of FIGS. 2b, 2c, and 2d each show examples of a multi-mirror 
semi-monolithic cavity. Two or more mirrors may be used to implement the 
external cavity structure. It is to be expressly understood that although 
four embodiments are illustrated, the invention specifically includes any 
implementation consistent with the teachings of the invention regardless 
of the number of mirrors utilized. Also, the polished ends of the 
nonlinear crystal may be perpendicular to the active optical axis of the 
crystal or can be Brewster-angled as described below. Total internal 
reflection may also be used internal to the semi-monolithic cavity for the 
mirrors, eliminating the need for a high quality, high reflective or 
partially reflective mirror coatings on one or all of the surfaces. As 
with any laser resonant cavity, the pertinent surfaces are coated, either 
with a dielectric, metallic or other type of coating for maximum or 
partial transmission and/or reflection at the fundamental and/or second 
harmonic wavelengths. FIGS. 2a-2d illustrate the nonlinear crystal only in 
combination with the surrounding or supporting glass block. However, it 
must be well understood that any one of the embodiments of FIGS. 2a-2d or 
others within the scope of the teaching invention is or can be employed in 
the laser system of the type as shown in FIG. 1b to obtain a frequency 
doubled output. Since the elements of the laser system 10 other than 
crystal 60 are incidental to the scope of the invention and are shared in 
common among all the embodiments, only that portion of laser system 10 
relating to nonlinear crystal 60 will be illustrated in FIGS. 2a-2d. 
In the first embodiment of FIG. 2a the crystal assembly, generally 
referenced by numeral 62, is thus comprised of a nonlinear crystal 60, a 
glass base 64, a first corner piece 66 and a second corner piece 68. As 
previously stated, base 64 and corner pieces 66 and 68 are made of glass 
or any other material having the same or an approximately equal index of a 
refraction as crystal 60. 
An input light beam 70 from laser 18 is incident upon mirrored surface 72 
of base 64. A portion of beam 70 is transmitted through mirrored surface 
72 as refracted beam 74. Refracted beam 74 propagates through base 64 into 
first corner piece 66 and impinges upon curved surface 76. Surface 76 is 
shape and coated to act as a mirrored to reflect beam 74 into crystal 60, 
preferably on or nearly parallel to an active axis. The interface surfaces 
78 between first corner piece 66 and crystal 60 are cut or formed in 
corner piece 66 and crystal 60 at the Brewster-angle for crystal 60 so 
that light beam 74 is totally or substantially transmitted through 
interface 78 without substantial reflection. Similarly, at the opposing 
end of crystal 60 are second interface surfaces 80 also cut or formed in 
corner piece 68 and crystal 60 at the Brewster-angle for the same purpose. 
Beam 74 continues then into second corner piece 68 and is partially 
transmitted through surface 82, which is provided with or formed as a 
partial mirror. A portion of beam 74 is then reflected as beam 84 while 
the remaining portion is output as a frequency-doubled output 86, which 
has been generated by the electro-optic feedback pumped into crystal 60 
and arising from its nonlinear properties. Interfaces 88 between first and 
second corner pieces 66 and 68 and base 64, respectively, may be provided 
with an antireflection coating as is interface 90 between crystal 60 and 
base 64. 
Thus, a comparison of FIG. 2a to FIG. 1b illustrates that nonlinear crystal 
42 of FIG. 1b has been replaced by the crystal assembly 62 of FIG. 2a, 
which is formed of a bonded composite assembled from nonlinear crystal 60 
with glass elements 64, 66 and 68 to provide the same or similar optical 
performance. The difference is that optical assembly 62 may now be more 
freely designed, since glass pieces 64, 66 and 68 can be readily 
manufactured, aligned, changed and fabricated to achieve the operational 
parameters desired in any given case than would otherwise be achieved if 
the entire assembly 62 had to be manufactured from a single nonlinear 
crystal material. 
FIG. 2b depicts a second embodiment of the invention wherein 
semi-monolithic cavity 62 is triangular in shape and is comprised of a 
shaped or faceted triangular base 92 combined with a first end piece 94, 
nonlinear crystal 60 and a second end piece 96. Laser beam 70 is once 
again incident upon a partially reflective surface 72 and is refracted to 
form beam 74. Beam 74 then passes through a pair of interface surfaces 98 
cut or formed at the Brewster angle between base 92 and first corner piece 
94. Again, interface 98 may be coated with anti-reflective material. As 
before, curved surface 100 of corner piece 94 is provided with a fully 
reflective mirror so that beam 74 is directed into crystal 60 along an 
optically active axis. The pair of Interface surfaces 102 between corner 
piece 94 and crystal 60 are as before cut or formed at the Brewster-angle 
for crystal 60 as are the opposing interface surfaces 104 at the opposing 
end of crystal 60. Beam 74 is therefore transmitted through interface 104 
to second corner piece 96 wherein it impinges upon curved surface 106, 
which is provided with a partial mirrored coating. Reflecting beam 84 is 
then returned to surface 72 to be recirculated. The remaining portion of 
beam 74 is transmitted through surface 106 as output beam 86. 
Semi-monolithic cavity 62 shown in disassembled form prior to bonding in 
assembly for the embodiment FIG. 2b is illustrated in side cross-sectional 
view in FIG. 3. Interface surfaces 102 and 104 through which inter-cavity 
beam 74 transverses are made or cut at the Brewster-angle and an 
anti-reflection coating may optionally be provided to minimize reflection 
losses. Interface surfaces 102 and 104 may come into contact with each 
other or a small gap may be left between them. Nonlinear crystal 60 may be 
bonded optically or with a bonding material to grasp block 92, 94 and 96. 
Elements, 92, 94, 96 and 60, may be severally or each bonded by adhesive, 
solder or other known or later discovered bonding methods to a flat, low 
thermal expansion surface or substrate 113, diagrammatically shown in FIG. 
2c. This substrate surface may be part of a thermoelectric cooler or other 
temperature controlled element to provide temperature control of nonlinear 
crystal 60 and surrounding optic elements 92, 94 and 96. 
Turn now to the third embodiment FIG. 2c, which illustrates in perspective 
view, semi-monolithic cavity 62. In this embodiment, laser beam 70 is 
incident upon a partially mirrored surface with 108. Refracted beam 84 
then impinges upon a mirrored surface 110. Surfaces 108 and 110 are 
defined on an integral glass block 112. Reflected beam 74 then impinges 
upon a curved mirrored surface 114 also defined on integral block 112. The 
beam is then reflected from curved mirrored surface 114 along an active 
optical axis of crystal 60 through interfaces 102 and 104 cut or formed at 
the Brewster-angle as before. The beam then internally impinges upon 
surface 108 and is partially reflected to recirculate within cavity 62 and 
partially transmitted as output beam 86. 
A fourth embodiment is shown in a cross-sectional view of FIG. 2d in which 
semi-monolithic cavity 52 is formed using a generally rectangular glass 
body 116 having corner cavity faceted mirrors 118a-d which are total 
internal reflection type mirrors. In this case, laser beam 70, such as a 
from a diode laser is incident upon one of the facet corners 118a to form 
refracted beam 74. Beam 74 propagates through body 116 to the opposing 
internal reflection mirror 118b, which reflects beam 74 to a third total 
internal reflection mirror 118c. The beam in turn is reflected from facet 
corner 118c through an interface 120 cut at the Brewster-angle and into a 
focusing lens 122. The light is focused by lens 122 into nonlinear crystal 
60 and then collected at the imposing end of crystal 60 by lens 124. 
Focused beam 74 then propagates through a second interface 126 again cut 
or formed at the Brewster-angle back into body 116 to impinge upon the 
fourth facet corner 118d. The beam is partially reflected and partially 
transmitted through facet corner 118d to form output beam 62 and 
recirculated reflective beam 84. 
Table 1 below lists illustrative examples of materials which may be used 
for nonlinear crystal 60 for second harmonic generation or for optical 
parametric oscillation in combination with various types of glasses or 
other easily polishable materials that have an index of refraction similar 
to the nonlinear material. The listed materials may be used in any one or 
all of the described embodiments. The glass block or cavity must be 
selected so that it is highly transparent at both the fundamental and the 
second harmonic wavelengths. 
TABLE 1 
______________________________________ 
approx. value of approx. value of 
Nonlinear crystal 
index of glass or other 
the index of 
material refraction material refraction 
______________________________________ 
lithium borate 
1.56 BK7 1.51 
(LBO) fused silica 
1.45 
beta barium 
1.65 SSK glass 1.65 
borate (BBO) 
potassium 1.75 sapphire 1.75 
titanyle SF11 glass 1.75 
phosphate (KTP) 
potassium 1.79 sapphire 1.75 
titanyle arvenate CsI 1.75 
(KTA) 
lithium iodine 
1.87 YAG 1.82 
trioxide (LiIO.sub.3) 
______________________________________ 
Returning to the embodiment of FIG. 3, the two individual mirrors formed on 
corner pieces 94 and 96 allow fine tuning of the reflection angle of the 
mirrors before they are firmly secured, bonded or affixed in place. In a 
monolithic cavity, if any error occurs in the fabrication of the curved 
surfaces of this portion of the cavity, such as failure to exactly 
co-align the apex of the curved surfaces, the entire cavity piece has to 
be discarded or completely reworked. In the semi-monolithic cavity 62 of 
the invention, minor corrections are possible since curved surfaces 76 and 
82 may be separated and formed on glass and materials that are easily 
worked, polished and coated. 
Alternatively, in the embodiment of FIG. 2c, curved mirror 114 is part of 
the integral glass block 112. Fabrication of glass block 112 that forms 
the principal part of semi-monolithic cavity 62 can be formed by a 
significantly more reliable and simpler procedure and is therefore less 
expensive than attempting to fabricate the entire structure 62 out of 
exotic nonlinear crystals. 
In the discrete mirror cavity as shown in FIGS. 1a and b provided with 
Brewster-angled surfaces on the nonlinear crystal, optical aberrations 
such as coma and astigmatism, occur when the curved cavity mirrors focus 
the intracavity beam 74 into crystal 60. Semi-monolithic cavity 62 is 
immune from this problem since the adjacent pair of facets on both the 
glass block or corner pieces and nonlinear crystal 60 are also at the 
Brewster's angle. Also, there is no significant change in the index of 
refraction when the intracavity beam enters or exits nonlinear crystal 60. 
Another alternative to using curved surfaces on the glass block that 
surround nonlinear crystal 60 is to use mirrors made from graded index 
rods as described by J. C. Vandrleeden "Resonant Cavities with Mirrors 
Made From Graded Index Rods" Journal of Applied Physics, Vol. 45, No. 1, 
201-208, (January 1974) incorporated herein by reference, where it is 
shown that optical cavities can be made with gradient index (GRIN) rod 
lenses. Such a lens has an index of refraction that is not uniform from 
one surface to the next, rather, it varies gradually. The spherical 
mirrors for the cavity are made by putting a high-reflectance dielectric 
coating on one of the flat ends of a GRIN rod segment. It is shown that 
one or two GRIN mirrors can be used to make all the usual resonator 
configurations. For example, two GRIN mirrors attached to a solid-state 
dielectric laser rod can be substituted for curved mirrors ground on the 
rod ends; the ends need only be reasonably flat. 
In this specification, the GRIN mirrors are attached to or slightly spaced 
away from the nonlinear crystal to forming a resonant external cavity. The 
graded index materials are constructed with flat/flat surfaces but coated 
on one side to act as a mirror. Such mirrors may replace the curved 
individual mirrors described above, thereby avoiding the need for 
fabrication of any curved surfaces. The graded index mirror does introduce 
a small amount of optical aberration. The aberrations would have to be 
considered when designing any particular system. 
The second harmonic output may be extracted from cavity 62, either through 
one of the cavity mirrors or through a dichroic beam-splitter that is 
placed immediately after nonlinear crystal 60. The dichroic beam-splitter 
efficiently transmits the fundamental wavelength and efficiently reflects 
the second-harmonic beam. Each technique for beam extraction has its 
advantages and disadvantages. Extraction of the second harmonic beam 
through one of the cavity mirrors is the generally practiced technique. 
However, if a dichroic beam-splitter with very high throughput at the 
fundamental frequency was available, it would result in a higher overall 
efficiency for the nonlinear conversion process. Both beam extraction 
methods are usable in semi-monolithic cavity 62. 
Many alterations and modifications may be made by those having ordinary 
skill in the art without departing from the spirit and scope of the 
invention. Therefore, it must be understood that the illustrated 
embodiment has been set forth only for the purposes of example and that it 
should not be taken as limiting the invention as defined by the following 
claims. 
For example, semi-monolithic cavity 72 as described above can be used in 
virtually any resonant optical or infrared cavity. In addition to 
frequency-doubling, the invention may be applied to all nonlinear optical 
processes and devices such as an optical parametric oscillators, optical 
parametric amplifiers, optical parametric generators, sum frequency 
mixing, difference frequency mixing and laser cavities. Thus, in addition 
to frequency-doubling, the technology described above may be applied to 
all optical cavities that utilize a solid or liquid phase medium. 
Although the foregoing embodiments have shown only a single nonlinear 
crystal in semi-monolithic cavity 62, it is to be expressly understood 
that two or more nonlinear crystals my be employed with one in each arm of 
the cavity as shown in FIG. 4. Both electro-optical and thermal tuning of 
cavity 62 may be applied either to nonlinear crystal 60 or to the matching 
optical materials surrounding nonlinear crystal 60. 
The words used in this specification to describe the invention and its 
various embodiments are to be understood not only in the sense of their 
commonly defined meanings, but to include by special definition in this 
specification structure, material or acts beyond the scope of the commonly 
defined meanings. Thus if an element can be understood in the context of 
this specification as including more than one meaning, then its use in a 
claim must be understood as being generic to all possible meanings 
supported by the specification and by the word itself. 
The definitions of the words or elements of the following claims are, 
therefore, defined in this specification to include not only the 
combination of elements which are literally set forth, but all equivalent 
structure, material or acts for performing substantially the same function 
in substantially the same way to obtain substantially the same result. In 
this sense it is therefore contemplated that an equivalent substitution of 
two or more elements may be made for any one of the elements in the claims 
below or that a single element may be substituted for two or more elements 
in a claim. 
Insubstantial changes from the claimed subject matter as viewed by a person 
with ordinary skill in the art, now known or later devised, are expressly 
contemplated as being equivalently within the scope of the claims. 
Therefore, obvious substitutions now or later known to one with ordinary 
skill in the art are defined to be within the scope of the defined 
elements. 
The claims are thus to be understood to include what is specifically 
illustrated and described above, what is conceptionally equivalent, what 
can be obviously substituted and also what essentially incorporates the 
essential idea of the invention.