Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF T~E INVENTION
The field of the invention concerns a frequency-
doubled laser, and in particular a method and apparatus for
generating a frequency-doubled beam using Type I I phase-
matching in an intracavity second harmonic generation crystal.
DESCRIPTION OF THE PRIOR ART
Second Harmonic Generation (SHG) provides a means ofdoubling the frequency of a laser source. In this process, a
fundamental electromagnetic wave in a non-linear medium induces
a polarization wave with a frequency that is doubled that of
the fundamental wave. Because of dispersion in the refractive
index of the medium, the phase velocity of such a wave is a
function of its frequency, so the phase of the induced second
harmonic polarization wave is retarded from that of the
fundamental wave. Since the vector sum of all the generated
~econd harmonic polarizations yield the SHG intensity, the
intensity is limited by the phase retardation. A technique,
known as phase-matching, is designed to overcome this
difficulty by utilizing in uniaxial and biaxial crystals the
natural biref~ingence, i.e. the difference in the phase
velocity as a function of polarization, to offset the
dispersion effect so that the fundamental and second harmonic
wave can propagate in phase.
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There a~e two well known types of phase-matching,
which employ the polarization vectors of the incident
fundamental wave in different ways.
In Type I 2hase-matching, the fundamental wave i~
polarized perpendicular to the crystal's optic axis (an O or
ordinary ray) and the induced second harmonic wave is polarized
parallel to the optical axis (an E or extraordinary ray). (A
method utilizing Type I phase-matching i8 described in U.S.
Patent No. 4,413,342.) Since the fundamental wave i6 polarized
along the optic axes of the crystal, there is no change in its
linear polarization when it exits from the crystal. ~n
intracavity Type I SHG arrangement can easily be adopted to
take advantage of the higher power density available within the
laser cavity because the introduction of the SHG crystal will
not produce a significant polarization loss.
In Type II phase-matching, the linearly polarized
fundamental wave is equally divided into O and E rays by
requiring its polacization to be 45 with respect to the optic
axis of the crystal; the output second harmonic wave which
results is linearly polarized parallel to the optic axis (an E
ray). Here, the phase velocities of the O and E rays of the
incident fundamental wave are different due to the natural
birefringence of the crystal. In general, the linear
polarization of this input fundamental wave is turned into an
elliptical polarization as it propagates through the crystal.
The magnitude of the phase retardation between O and E rays is
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the product of the index difference in the material and the
effective optical path.
When such a Type II crystal is placed inside a laser
resonator, this phase retardation can cause serious power loss
because the laser's original linear polariza~ion will not in
general be properly maintained.
When the laser is randomly polarized, as is the case
in multimode lasers when the laser active medium is not
naturally birefringent and no polarizing elements are employed
intracavity, the Type II SHG c~ystal provides a phase
retardation between the polarization components resolved along
its O and E axes. This retardation, which doubles on the
return trip of the fundamental beam through the Type II SHG
crystal can affect the stability and output power of the laser
by affecting the laser's ability to optimize its polarization
relative to thermal or other induced birefringent effects in
the laser active medium. One can attempt to compensate this
phase retardation using a passive device such as a Babinet-
Soleil compensator. However, the retardation is usually
dependent upon temperature and variations in temperature can be
induced either by the ambient environment or by self-absorption
of the laser radiation (fundamental and/or second harmonic) in
the crystal itself. Such passive compensation thus becomes
difficult to maintain during standard laser operation. Due to
these problems, Type II SHG has typically been employed in an
extracavity arrangement in which the polarization of the
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exiting fundamental wave from the SHG crystal is unimportant.
Of course the advantage that the higher power density
intracavity fundamental wave within the laser cavity has in
generating second hacmonic, is lost.
Many lasers can have the temporal form of their output
power altered by a process known as Q-switching. Here, a
special device which alters the optical quality or Q of the
resonator is inserted into the beam within the resonator
cavity. This "Q-switch" can be activated to produce enough
optical loss to overcome the optical gain or amplification
supplied by the laser active medium, thereby inhibiting
oscillation. If the source exiting the laser active medium is
maintained on during the low Q-period, energy is stored in the
laser active medium in the form of an excess population
inver~ion. When the Q-switch is turned off (returning the
resonator quickly to its high Q state) this excess population
is utilized ~o produce a high-intensity, Q-switched pulse.
Since most Q-switches are electronically controlled, the
process is repeatable at high repetition rates making a
Q-switched laser a useful source of high intensity pulses.
Peak pulse intensities many thousands of times the la~er's
continuous wave output power level can be generated. Because
of the superior focusability and enhanced material interaction
cf shorter wavelengths, it is often of interest to frequency-
double the output of Q-switched lasers.
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SUMMARY OF TH~ INVENTION
It is a principal object of the invention to overcome
the disadvantages of a system using intracavity Type II phase
matching for SHG by having the effect of birefringence of the
SHG crystal be compensated for upon return passage of the
fundamental wave through the SHG crystal.
It is another object of the invention to provide laser
fcequency-doubling apparatus with a laser medium in which the
fundamental beam incident on the laser medium maintains its
original linear or random polarization.
A further object is to provide a system in which the
output, frequency-doubled beam, has a known polarization.
The system includes a laser harmonic generating means
for generating the second harmonic frequency of the fundamental
frequency emitted by the laser which may be a Q-switched laser,
means for dynamically compensating for any phase lags generated
in the fundamental beam passing through said harmonic
generating means, a first highly reflecting mirror at the
fundamental frequency, and a second mirror. The first and
second mirrors are positioned to form a cavity for the laser,
the harmonic generator and the compensating means.
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BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the
invention will become apparent upon reading the following
detailed description and upon referring to the drawings in
5 which:
Figures la, lb and lc illustrate second harmonic laser
generators according to this invention; and
Figures 2a, 2b and 2c show alternate embodiments of
the invention.
While the invention will be described in conjunction
with an example embodiment, it will be understood that it is
not intended to limit the invention to such embodiment. On the
contrary, it is intended to cover all alternatives,
modifications and equivalents as may be included within the
spirit and scope of the invention as defined by the aepended
claims.
DET~ILED DESCRIPTION OF THE INVENTION
In the following description, similar features in the
drawings have been given similar reference numerals.
Referring now to Figures la, lb, and lc, a frequency-
doubling laser system comprises the following elements aligned
along a common optical axis 8 as shown: a mirror 10, a
quarter-wave plate 12, an SHG crystal 14, a ~olarizer 16, and
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active laser medium 18 and a second mirror 20. Laser 18 is
adapted to generate a laser beam at a predetermined fundamental
frequency along common axis 8. For example the laser may be a
YAG laser which emits a beam at a wavelength of 1064 nm. The
active laser medium, a laser rod, may be included within a
pumping reflector with a pumping lamp. These latter laser
elements are well known in the art and therefore have not been
shown in Figure 1 for the sake of clarity.
Crystal 14 is a known second harmonic qenerator
crystal such as a KTP (potassium titanyl phosphate) crystal.
Crystal 14 is oriented with its optic axis, shown by arrow Z in
Figures la and lb, at an angle of 45 with respect to the angle
of polarization of the beam from laser 18. Thus, for example,
if the fundamental beam F from laser 18 is polarized
vertically, then as shown in these two figures, the 0 and E
axes of crystal 14 are oriented at an angle of 45 from the
vertical.
Plate 12 is selected to operate as a quarter-wave
plate at the fundamental frequency and simultaneously as a half
wave plate at the second harmonic frequency. The optical axis
of the plate (or its perpendicular indicated by arrow Q in
Figures la and lb) i8 oriented parallel to the polarization of
the laser beam.
Mirror 10 is highly reflective at the fundamental
frequency and highly transmissive at ~he second harmonic
frequency. Mirror 20 is highly reflective at the fundamental
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frequency. Mirrors 10 and 20 are positioned and arranged to
form a resonating optical cavity for the fundamental beam
generated by active laser medium 18, with the SHG crystal 14
and plate 12 disposed within the cavity.
Polarizer 16 is used to polarize the laser beam in the
vertical direction (V).
As this initial beam 22 propagates through the crystal
14, the crystal, in response to both the O and E components of
the beam 22 generates a beam 24 having double the frequency of
the fundamental beam oriented at 45 to the vertical (an E ray)
as shown. Beam 24 is transmitted through plate 12, and mirror
10 out of the cavity. Because plate 12 acts as a half-wave
plate, the beam at the doubled-frequency transmitted is rotated
by 90 and then through mirror 10. As shown by the arrow it i6
linearly polarized at 45 to the vertical direction.
As the fundamental beam 22 with its linear
polarization oriented at 45 to the Z axis propagates through
the SHG crystal, the birefringence causes a phase retardation
to occur between fundamental 0 and E components.
In Figures la and lb it is assumed that after passing
through crystal 14, The 0 component of the fundamental beam 22
lags behind the E component.
Without any phase lag compensatory means, the
fundamental beam reflected from mirror 10 and back through the
SHG crystal will exhibit twice the phase retardation shown
after one pass and the polarization of the beam reentering the
124~?~i5()
polarizer 16 (Figures la, lb) will not in general be linear and
vertical, resulting in significant and undesirable polarization
loss.
Therefore, in the present invention, beam 22 is passed
from SHG crystal 14 through plate 12 which is a quarter-wave
plate of the fundamental frequency. In Figure 1, as previously
mentioned, the plate 12 is shown with its optic axis parallel
(or perpendicular) to the polarization of the fundamental beam
incident on crystal 14. After reflection by mirror lO, the
beam 22' passes again through quarter-wave plate 12. As a
result of the two passes through plate 12, the polarization
components of beam 22 have been rotated by 9O so that, as
shown in Figure 1, the orientation of the E and O components of
beam 22' are reversed with respect to the orientation of the
components of beam 22. However component O still lags behind
E. The reflected beam 22' then passes through crystal 14 but
this second time, vertical component E is differentially phase
shifted by an amount identical to the first differential phase
shift with respect to O so that the components E and O of the
beam 22' as it leaves the crystal 14 are now in phase and
combine to yield linear polarization F'. Therefore by
interposing plate 12 between crystal 14 and mirror lO, the
birefringent effects of the SHG crystal are successfully
self-compensated and thereby eliminated.
As a result, the fundamental beam 22 incident on
crystal 14 and the fundamental beam 22' exiting from the
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crystal 14 have identical linear polarizations resulting in no
loss in the laser resonator.
Under the conditions described above, the harmonic
beam ge~erated within crystal 14 comprises a component ED at
45 to the vertical as shown. S~nce in many applications it is
desirable to obtain a freguency-doubled laser beam of a ~nown
linear polarization, plate 12 is constructed to act
simultaneously as a half-wave plate at the second harmonic
frequency thereby rotating beam 24 by 90. If plate 12 were
unspecified at the second harmonic frequency, the frequency-
doubled beam would have an arbitrary elloptical polarization.
As a result, beam 24 exiting from the optical cavity is
linearly polarized along the ordinary axis as shown.
In Figure lc, the beam emitted by laser active medium
18 has a random polarization and is shown in Figure 1 as being
resolved into two orthogonal components V and H.
In Figure lc, crystal 14 is oriented with its optic
axis, shown by arrow Z in Figure 1, parallel to one of the
components of the beam from laser active medium 18, for
example, component V. Thus, for example, component V from
laser 18 is oriented vertically along the Y axis and component
H horizontally along X. Then as shown in Figure lc, the E and
O axes of crystal 14 are oriented parallel and perpendicular to
the vertical.
Plate 12 is selected to operate as a quarter-wave
plate at the fundamental frequency. The optical axis of the
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plate indicated by arrow Q in Figure lc i8 oriented at 45 to
the V component of the fundamental beam.
Mirror 10 is highly reflective at the fundamental
frequency and highly transmissive at the second harmonic
frequency. Mirror 20 is highly reflective at the fundamental
frequency. Mirrors 10 and 20 are positioned and arranged to
form a resonating optical cavity for the fundamental beam
generated by active laser medium 18, with the SHG crystal 14
and plate 12 disposed within the cavity.
As the initial beam 22 propagates through the crystal
14, the crystal, in response to both the V and H components
(the Q and E rays) of the linearly polarized beam 22, generates
a beam 24 having double the frequency of the fundamental beam
oriented along the vertical (an E ray) as shown in Figure lc.
15 Beam 24 is transmitted through plate 12, and mirror 10 out of
the cavity.
As the fundamental beam 22 with its vertical and
horizontal polarizations oriented parallel and perpendicular to
the Z axis, propagates through the SHG crystal, the bire-
fringence causes a phase retardation to occur betweenfundamental V and H components (E and 0 rays respectively) of
fundmental beam 22.
In Figure lc it is assumed that after passing through
crystal 14, the 0 ray of the fundamental beam 22 lags behind
the E ray.
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124~9650
Without any phase lag compensatory means, the
fundamental beam reflected from mirror 10 and back through the
SHG crystal will exhibit twice the phase retardation shown
after one pass and the polarization of the beam reentering the
laser active medium 18 will not in general be the same as that
initially leaving 18 possibly resulting in significant and
undesirable losses or instability in laser 18.
Therefore, in the present invention, beam 22 is passed
from SHG crystal 14 through plate 12 which is a quarter-wave
plate of the fundamental frequency. In Figure 1, as previously
mentioned, the plate 12 îs shown with its optic axis at 45O to
the component V of the fundamental beam incident on crystal
14. After reflection by mirror 10, the beam 22' passes again
through quarter-wave plate 12. As a result of the two passes
through quarter-wave plate 12, the V and H rays of beam 22 have
been rotated by 90 so that, as shown in Figure 1, the
orientation of the E and O rays of beam 22' are reversed with
respect to the orienta~ion of the components of beam 22.
However ray O still lags behind E. The reflected beam 22' then
passes through crystal 14 but this second time, ray E is
differentially phase shifted by amount identical to the first
differential phase shift with respect to O so that the rays E
and O of the beam 22' as it leaves the crystal 14 are now in
phase and combined to yield fundamental beam components V and H
in the same phase as originally left crystal 12. Therefore by
interposing plate 12 between crystal 14 and mirror 10, the
1249~5(:~
birefringent effects of the SHG crystal are successfully
self-compensated and thereby eliminated.
As a result, the fundamental beam components V and H
incident on crystal 14 and the fundamental beam components V'
and H~ exiting from the crystal 14 have identical phase
relationships resulting in no loss or instability in the laser
resonator.
It should be appreciated that plate 12 and crystal 14
accomplish their intended purposes dynamically. In the present
invention, the phase lag is automatically and accurately
corrected regardless of the temperature of the crystal.
If necessary, a Q-switch 16 may be added between laser
18 and mirror 20 to Q-switch the laser beam in the normal
manner.
Another embodiment of the invention is shown in
Figures 2a, 2b, and 2c. In the embodiment of Figure 2a, the
frequency-doubler comprises a three-mirror cavity with a mirror
112, an SHG crystal 114, a quarter-wave plate 116, a second
mirror 118, a third mirror 120, a laser active medium 110 and a
polarizer 128. The laser 110, the crystal 114 and quarter-wave
plate 116 and polarizer 128 function in a manner identical to
their counterparts in the embodiment of Figure la. Mirror 120
is highly reflective at the fundamental frequency, mirror 112
is highly reflective at the fundamental frequency and highly
transmissive at the second harmonic frequency. In addition
mirror 112 is also positioned and arranged to focus the output
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of laser 110 on crystal 114 for effective second harmonic
generation. Mirror 118 is highly reflective at the fundamental
frequency and at the second harmonic frequency.
In ope~ation, a fundamental beam 122 produced by laser
active medium llO is reflected and focused by mircor 112 on
crystal 114. The crystal generates a linearly second harmonic
beam 124. After propagation through crystal 114, the O and E
components of fundamental beam 122 are phase shifted with
respect to each other as described in the embodiment of Figure
la. Also, as in this previous embodiment, the fundamental
frequency quarter-wave plate 116 and mirror 118 are used to
rotate the O and E component by 9O after reflection so that
passage of beam 122' back through crystal 114 puts all
components back in phase and restores the polarization to that
linear polarization which initially left laser active medium
llO. On the return trip through crystal 114, beam 122'
generates second harmonic beam 126, which is colinear with
reflected second harmonic beam 124'.
Thus, in this embodiment, the second harmonic
genecated on the return trip of the fundamental is not lost so
the potential exists for a second harmonic power gain of a
factor of two. Interference may occur between these beams
which will affect the stability of the SHG output intensity.
In order to overcome this undesirable effect, the polarizations
of the beams 124' and 126 are made orthogonal using a technique
similar to that described in U.S. Patent No. 4,413,342. Plate
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116, is simultaneously made a quarter-wave plate at the second
harmonic frequency. Beam 124 will, upon passage through 116.
reflection from 118 and return through 116, have its
polarization rotated by 90 and thereby be orthogonal and
non-interfering with beam 126. Beams 124~ and 126 are then
coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 is
reflected by mirror 112 toward laser 110. Mirror 120 completes
the optical cavity. Plate 116 compensates for the phase shift
in the O and E components of the fundamental beam as previously
described thereby insuring that beams 122 and 122' have the
same linear polarization.
Another embodiment of the invention is shown in Figure
lb. In this embodiment, the frequency-doubled laser comprises
15 a three-mirror cavity with a mirror 112, an SHG crystal 114, a
quarter-wave plate 116, a second mirror 118, a third mirror
120, a laser active medium 110, a Q-switch 119 and a polarizer
128. The laser 110, the crystal 114, quarter-wave plate 116,
Q-switch 119 and polarizer 128 function in a manner identical
to their counterparts in the embodiment of Figure lb. Mirror
120 is highly reflective at the fundamental frequency, mirror
112 is highly reflective at the fundamental frequency and
highly transmissive at the second harmonic frequency. In
addition mirror 112 is also positioned and arranged to focus if
25 necessary on the output of laser 110 on crystal 114 for
effective second harmonic generation. Mirror 118 is highly
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reflective at the fundamental frequency and at the second
harmonic frequency.
In operation, a fundamental beam 122 produced by laser
active medium 110 is reflected and focused by mirror 112 on
crystal 114. The crystal generates a linearly polarized second
harmonic beam 124. After propagation through crystal 114, O
and E components of fundamental beam 122 are phase shifted with
respect to each other as described in the embodiment of Figure
lb. Also, as in this previous embodiment, the fundamental
frequency quarter-wave plate 116 and mirror 118 are used to
rotate the O and E components by 90 after reflection so that
pa6sage of beam 122' back through crystal 114 puts all
components back in phase and restores the polarization to that
linear polarization which initially left laser active medium
llO. On the return trip through crystal 114, beam 122'
generates second harmonic beam 126, which is colinear with
reflected second harmonic beam 124~.
Thus, in this embodiment, the second harmonic
generated on the return trip of the fundamental through the SHG
crystal is not lost so the potential exi6ts for a second
harmonic power gain of a factor of two. Interference may occur
between these beams which will affect the stability of the SHG
output intensity. In order to overcome this undesirable
effect, the polarizations of the beams 124' and 126 are made
orthogonal using a technique similar to that described in U.S.
Patent No. 4,413,342. Plate 116, is simultaneously made a
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quarter-wave plate at the second harmonic frequency. Beam 124
will, upon passage through 116, reflection from 118 and return
through 116, have its polarization rotated by 90 and thereby
be orthogonal and non-interfering with beam 126. Beams 124~
and 126 are then coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 i~
reflected by mirror 112 toward laser medium 110. Mirror 120
completes the optical cavity while switch 119 Q-switches the
laser output in the conventional manner. Plate 116 compensate6
for the phase shift in the 0 and E components of the
fundamental beam as previously described thereby insuring that
beams 122 and 122' have the same linear polarization.
~ nother embodiment of the invention is shown in Figure
2c. In this embodiment, the frequency-doubled laser comprises
15 a three-mirror cavity with a mirror 112, an SHG crystal 114, a
quarter-wave plate 116, a second mirror 118, a third mirror
120, and a laser active medium 110. The laser 110, the crystal
114, and quarter-wave plate 116 function in a manner identical
to their counterparts in the embodiment of Figure lc. Mirror
120 is highly reflective at the fundamental frequency, mirror
112 is highly reflective at the fundamental frequency and
highly transmissive at the second harmonic frequency. In
addition mirror 112 can also be positioned and arranged to
focus the output of laser 110 on crystal 114 for effective
second harmonic generation. Mirror 118 is highly reflective at
the fundamental frequency and at the second harmonic frequency.
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In operation, a fundamental beam having random
polarization 122 produced by laser active medium llO is
reflected and focused by mirror 112 on crystal 114. The
crystal generates a second harmonic beam 124. After
propagation through crystal 114, the O and E rays of
fundamental beam 122 are phase shifted with respect to each
other as described in the embodiment of Figure lc. ~lso, as in
this previous embodiment, the fundamental frequency
quarter-wave plate 116 and mirror 118 are used to rotate the O
and E rays by 90 after reflection so that pasfiage of beam 122'
back through crystal 114 puts all components back in phase and
restores the polarization to that polarization which initially
left laser active medium llO. On the return trip through
crystal 114, beam 122' generates second harmonic beam 126,
which is colinear with reflected second harmonic beam 124'.
Thus, in this embodiment, the second harmonic
generated on the return trip of the fundamental is not lost so
the potential exists for a second harmonic power gain of a
factor of two. Interference may occur between these beams
which will affect the stability of the SHG output intensity.
In order to overcome this undesirable effect, the polarizations
of the beams 124' and 126 are made orthogonal using a technique
similar to that described in U.S. Patent No. 4,413,342. Plate
116, is simultaneously made a quarter-wave plate at the second
harmonic frequency. Beam 124 will, upon passage through 116,
reflection from 118 and return through 116, have its
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polarization rotated by 90 and thereby be orthogonal and
non-interfering with beam 126. Beams 124' and 126 are then
coupled out of highly transmissive mirror 112.
Beam 122', after passing through crystal 114 is
reflected by mirror 112 toward laser 110. Mirror 120 completes
the optical cavity. Plate 116 compensates for the phase shift
in the O and E rays of the fundamental beam as previously
described thereby insuring that beams 122 and 122' have the
same random polarization.
~ Q-switch 128 may be added to Q-switch the
fundamental beam as described above.
Obviously numerous other modifications may be made to
the invention without departing from its scope as defined in
the appended claims.
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