Frequency doubling a laser beam by using intracavity type II phase matching

A frequency doubler for a laser is disclosed in which a Type II SHG crystal is oriented to generate a second harmonic frequency beam in response to the orthogonal components of a fundamental beam. After the fundamental beam makes a round trip through the SHG crystal, any differential phase delays between the E and O rays of the fundamental beam due to birefringence are eliminated to improve the efficiency and stability of the cavity.

BACKGROUND OF THE INVENTION 
1. Field of the 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 II phase-matching in an intracavity second harmonic generation 
crystal. 
2. Description of the Prior Art 
Second Harmonic Generation (SHG) provides a means of doubling the frequency 
of a laser source. In this process, a fundamental electromagnetic wave in 
a nonlinear medium induces a polarization wave with a frequency that is 
double 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 second harmonic polarizations yields 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 
birefringence, 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. 
There are two well known types of phase matching, which empoly the 
polarization vectors of the incident fundamental wave in different ways. 
In Type I phase matching, the fundamental wave is 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 is described 
in U.S. Pat. No. 4,413,342.) Since the fundamental wave is polarized along 
the optic axes of the crystal, there is no change in its linear 
polarization when it exits from the crystal. An 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 polarization to be 
45.degree. 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 the product of the index dffference 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 when the laser is linearly 
polarized because the original linear polarization 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 crystal 
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 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 harmonic, 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 
exciting 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 inversion. When the Q-switch is turned off (returning the 
resonator quickly to its high Q state) this excess population is utilized 
to 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 laser's continuous wave 
output power level can be generated. Because of the superior focusability 
and enhanced material interaction of shorter wavelengths, it is often of 
interest to frequency-double the output of Q-switched lasers. 
OBJECTIVES AND SUMMARY OF THE 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 frequency doubling 
apparatus with a laser medium in which the fundamental beam incident on 
the laser medium maintains its original linear or random polarization. 
The system includes a laser harmonic generating means for generating the 
second harmonic frequency of the fundamental frequency emitted by the 
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.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to FIG. 1, 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, an active laser 
medium 18, an optional Q-switch 16, 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 FIG. 1 for the sake of clarity. 
The beam emitted by laser active medium 18 has a random polarization and is 
shown in FIG. 1 as being resolved into two orthogonal components V and H. 
Crystal 14 is a known second harmonic generator crystal such as a KTP 
(potassium titanyl phosphate) crystal. Crystal 14 is oriented with its 
optic axis, shown by arrow Z in FIG. 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 FIG. 1, 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 plate indicated by arrow Q in FIG. 1 is 
oriented at 45.degree. 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 O 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. 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 birefringence causes a phase retardation to occur between 
fundamental V and H components (E and O rays respectively) of fundamental 
beam 22. 
In FIG. 1 it is assumed that after passing through crystal 14, the O ray of 
the fundamental beam 22 lags behind the E ray. 
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 FIG. 1, as previously mentioned, the plate 12 is shown with 
its optic axis at 45.degree. 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.degree. so that, as shown in FIG. 1, the orientation of the E and O 
rays of beam 22' are reversed with respect to the orientation 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 combine 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 
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 FIG. 2. In this embodiment, 
the frequency doubled laser 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, 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 FIG. 1. 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. 
In operation, a fundamental beam having random polarization 122 produced by 
laser active medium 110 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 previous 
embodiment. Also, as in the previous embodiment, the fundamental frequency 
quarter-wave plate 116 and mirror 118 are used to rotate the O and E rays 
by 90.degree. after reflection so that passage 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 
110. 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. Pat. 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 
polarization rotated by 90.degree. 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. 
A 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.