IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007 473
2-kW Average Power CW Phase-Conjugate
Solid-State Laser
Yuri A. Zakharenkov, Todd O. Clatterbuck, Vladimir V. Shkunov, Alexander A. Betin, David M. Filgas,
Eric P. Ostby, Friedrich P. Strohkendl, David A. Rockwell, Member, IEEE, and Robert S. Baltimore
Abstract—We have demonstrated stable operation of a 2-kW
Yb:YAG phase-conjugate master oscillator power amplifier (PC-
MOPA) laser system with a loop phase-conjugate mirror (LPCM).
This is the first demonstration of a continuous wave (CW)-input
LPCM MOPA operating at a power greater than 1 kW with a
nearly diffraction-limited output beam. The single-pass beam qual-
ity incident on the LPCM varied with the specific operating condi-
tions,butitwastypically∼20 times diffraction-limited (XDL). The
measured beam quality with an MOPA output power of 1.65 kW
was 1.3 XDL.
Index Terms—Beam quality, continuous wave (CW) high-energy
laser, laser amplifiers, phase-conjugate mirror (PCM), power in the
bucket, solid-state laser.
I. INTRODUCTION
O
VER THE PAST 25 years, it has been clearly established
that fairly simple devices based on nonlinear optical phase
conjugation (NOPC) are capable of eliminating many problems
arising from thermal loads in high-peak power pulsed solid-state
lasers [1]–[3]. Present purposes can be served by simply stating
that reciprocal phase aberrations induced in an optical beam by
any medium can, in principle, be compensated by reflecting the
aberrated beam off a phase-conjugate mirror (PCM) and passing
the beam back through the aberrating medium. The output beam
will have the same beam quality as the initial unaberrated beam.
More recently, the interest in high-power solid-state lasers
has broadened to include new applications requiring continuous
wave (CW) or quasi-CW operation. Single-rod [4] and multiple-
disk [5] configurations have been reported with output powers
>1 kW (but with significantly reduced power for operation with
M
2
< 10). Two-rod [6] and slab [7] lasers operating at greater
than 400 W have also been described. Finally, the “heat capacity
laser” [8] reached record power of 31.3 kW, but only for 1–2 s of
continuous operation, while operation for much longer times [9]
was reported at 19 kW.
These CW or quasi-CW systems are not very compatible with
stimulated Brillouin scattering (SBS) phase conjugation (which
Manuscript received December 5, 2006; revised March 21, 2007. This work
was supported in part by High Energy Laser Joint Technology Office and in part
by Space and Airborne Systems, Raytheon, El Segundo, CA 90245.
Y. A. Zakharenkov, T. O. Clatterbuck, V. V. Shkunov, D. M. Filgas,
F. P. Strohkendl, D. A. Rockwell, and R. S. Baltimore are with Raytheon
Space and Airborne Systems, El Segundo, CA 90245 USA (e-mail:
yuri_a_zakharenkov@raytheon.com).
E. P. Ostby is with Raytheon Space and Airborne Systems, El Segundo, CA
90245 USA and also with California Institute of Technology, Pasadena, CA
91125 USA.
A. A. Betin was with Raytheon Space and Airborne Systems, Raytheon, El
Segundo, CA 90245 USA. He is now with the General Atomics, San Diego, CA
92121 USA.
Digital Object Identifier 10.1109/JSTQE.2007.896565
has been utilized in the vast majority of pulsed phase-conjugate
(PC) solid-state lasers) for two main reasons. First, their output
powers are much lower than the peak powers achieved by even
modest pulsed solid-state lasers, making it difficult to reach the
SBS threshold. Second, even very low absorption coefficients in
the nonlinear medium can produce unacceptably high thermal
loads at required operational power levels.
In view of these limitations to the applicability of SBS phase
conjugation, we began developing an alternative NOPC archi-
tecture approximately 10 years ago. This alternative architec-
ture is called a loop phase-conjugate mirror (LPCM), and this
name is derived from the fact that the nonlinear medium is in-
corporated into a loop resonator [10]. Our LPCM exploits a
thermal nonlinearity, whereby four-wave mixing (FWM) oc-
curs in an absorbing medium. The physical basis of the FWM
is the fact that the presence of two interfering high-power laser
beams (a signal beam and a reference beam) in the absorbing
medium produces a spatially varying temperature distribution,
and consequently, a spatially varying refractive index or holo-
gram in the nonlinear medium. This hologram can be “read” by
a third beam, thereby producing PC replica of the input signal
beam.
This LPCM offers several important advantages over SBS
in high-average power laser applications. First, an LPCM can
function over a wide peak- and average-power dynamic range.
This allows operation at power levels ranging from low values of
a few watts to high values of many kilowatts. Second, an LPCM
offers broad wavelength coverage, and finally, it is compatible
with lasers producing short coherence lengths (∼1 cm or less).
Several low-power experimental LPCM demonstrations have
been reported [10], [11]; these demonstrations are consistent
with the advantages asserted earlier. However, to our knowl-
edge, there have been no prior reports of LPCMs that might
be used in high-power CW phase-conjugate master oscillator
power amplifier (PC-MOPA) laser systems.
This paper represents the first successful demonstration of
a quasi-CW solid state PC-MOPA laser system producing a
nearly diffraction-limited beam with power >1 kW. Section II
describes the overall system architecture, while Section III de-
scribes the LPCM, and Section IV summarizes our experimental
results.
II. S
YSTEM ARCHITECTURE
Fig. 1 shows a schematic diagram of our double-pass PC-
MOPA. The signal beam originates from a commercial Yb:YAG
master oscillator [12]. The master oscillator (MO) output passes
through a Faraday isolator and an outcoupler into the amplifier
1077-260X/$25.00 © 2007 IEEE
474 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007
Fig. 1. Schematic diagram of the PC MOPA system.
beamline that comprises one or more Yb:YAG amplifier slabs
(A1–A3).
Relay-imaging telescopes are an important element of the
MOPA system, and they are used throughout the amplifier chain
and also within the LPCM. In addition, high-power variable
apertures are located at the foci of the relay optics to function as
spatial filters. Each of these imaging telescopes is schematically
shown as a single pair of lenses in the figure, but in some cases,
a telescope can also contain cylindrical optics (not shown) that
might be used to change the aspect ratio of the beam.
The imaging optics and their spatial-filtering capability play
an important function in the MOPA system. First, the optics
reimage the pupil from one stage to the next, ensuring that
the beam exiting one amplifier is delivered to the next amplifier
with high efficiency. Consequently, during the initial signal pass
through the amplifiers, the imaging telescopes avoid any signif-
icant loss of aberration information at the hard apertures that
are distributed along the beam path. Such loss of information
would degrade the possible phase conjugation fidelity.
Second, during both passes through the amplifier chain, the
spatial-filtering capability prevents a large fraction of the highly
divergent amplified spontaneous emission (ASE) from entering
successive amplification stages and extracting power from the
amplifiers or causing a parasitic oscillation.
The amplifier transverse dimensions correspond to an aspect
ratio of 18:1, which approximately matches the far-field diver-
gence (10
◦
×0.6
◦
) of typical fast-axis-collimated laser diode
arrays. This allows coupling the output from a 2-D laser diode
array into the slab with a single lens. At the required power
levels, the thermal lensing in the thin (fast) axis of a slab is very
high if light would be traveling along the axis of the slab. The
amplifiers employ zigzag signal propagation, which has long
been known to mitigate the effects of fast-axis thermal lensing
in face-cooled laser slabs.
The high aspect ratio amplifier slabs induce only slight de-
polarization during operation. Hence, we can achieve the out-
coupling function by inserting a Faraday rotator (FR) into the
amplifier/LPCM path (for example, between A1 and A2, as
shown in the figure). This rotator is double-passed by the sig-
nal beam. Consequently, the second-pass output polarization is
orthogonal to that of the first-pass beam. A common polarizing
beam splitter then separates the MOPA output beam from the
orthogonally polarized oscillator input beam.
The LPCM is also schematically shown in Fig. 1. The aber-
rated first-pass beam, denoted as beam 1, enters the LPCM
and passes through the cell that contains the nonlinear medium.
Fig. 2. Schematic diagram of nonlinear cell (NC) indicating labels on the four
interacting beams.
Following the nonlinear cell (NC) is a Faraday isolator favor-
ing propagation in the counterclockwise direction, but that is
slightly detuned to allow a few percent of the initial beam to
pass through in the clockwise direction toward the LPCM am-
plifier A4. This amplifier raises the power of the beam, now
denoted as beam 3, to the same level as beam 1. This attenu-
ation of beam 1 is important to ensure that beam 1 does not
extract too much power from A4—system efficiency is greatly
enhanced if the counterpropagating beam extracts most of the
power from the amplifiers. Beams 1 and 3 are made to over-
lap in the nonlinear medium, where the interference between
the beams produces a spatially varying refractive index pattern
(i.e., a hologram) in the nonlinear medium.
This hologram provides the feedback necessary to initiate
oscillation in the counterclockwise direction for the loop laser
resonator, which is formed by the amplifier A4, the hologram
as a resonator mirror, and several folding mirrors, as shown in
Fig. 1. The beam generated by this laser oscillation is denoted
as beam 2 in the vicinity of the NC and as beam 4 just after it
is diffracted back into the loop by the dynamic hologram of the
NC (see Fig. 2). Under a wide range of operating conditions,
the loop output beam 2 is the conjugate of the loop input beam
1. Most of beam 2 passes through the NC to form the LPCM
output, thereby functioning as the conjugate seed that initiates
the second pass through the amplifier chain.
III. LPCM
The basic operation of an LPCM has been described ear-
lier [10], [11], and a detailed description of the present LPCM
is also available [13]. The key components of the LPCM are
the NC, the LPCM amplifier A4, and the isolator (see Fig. 1).
Diffraction by the hologram grating in the NC allows it to func-
tion simultaneously as a resonator mirror, a transverse-mode
selector, and an outcoupler. For typical operating conditions,
the hologram diffraction efficiency is designed to be low, ap-
proximately a few percent, so, the Q-factor of the ring-laser
resonator is low. The LPCM amplifier provides the gain re-
quired to compensate the resonator losses at the outcoupler and
the other optical elements. It also provides power for the PC
output beam. The isolator prohibits ring-laser oscillations in the
forward (clockwise) direction, preserving the optical power for
the backward (counter-clockwise) PC beam.
A variety of nonlinear processes can be used in the LPCM NC.
In this paper, we employ a thermal nonlinearity, and the nonlin-
ear medium is an absorbing liquid. The hologram arises from
ZAKHARENKOV et al.: 2-KW AVERAGE POWER CW PHASE-CONJUGATE SOLID-STATE LASER 475
the fact that the spatially varying intensity distribution produced
by the interference of beams 1 and 3 (the two beams incident
from the right-hand side in Fig. 2) induces a spatially varying
temperature distribution in the absorbing liquid, which leads to
a spatially varying index distribution through the temperature
dependence of the refractive index.
The choice of an optimum liquid is essential and is based on
the trade study reported in [14]. Consider the situation in which
a pair of plane waves propagating in an absorbing medium
forms a refractive index grating having a spatial period Λ.The
steady-state diffraction efficiency η for such a thermal grating
follows from the model presented in [14], [15], and for the
approximation of weak diffraction, the diffraction efficiency is
given by
η =(αL)
2
Λ
2
2πλ
2
2
dn
dT
/χ
2
I
1
I
3
(1)
where α is the liquid absorption coefficient at the signal wave-
length, L is the thickness of the absorbing layer in the direction
of the beam propagation, λ is the signal wavelength, and I
1
and I
3
are intensities of the writing beams. The first term in (1)
combined with the final terms (the beam intensities) represent
the total power absorbed in the medium, while the second term
(in square brackets) implicitly embodies the system geometry
(beam-crossing angle) within the grating spatial period. The
third term indicates how the refractive index responds to the
intensity grating formed by the two writing beams, including
the time dependence of the thermal gratings that is governed by
the thermal diffusivity of the nonlinear medium, χ. Equation (1)
shows that, for a given absorption, the two key material param-
eters are (dn/dT ) and χ. These parameters drive the material
selection. Additional medium-selection parameters include a
sufficiently high boiling temperature, a low chemical reactiv-
ity, and preferably a minimal health hazard. Typical candidate
nonlinear media include acetone, toluene, carbon tetrachloride,
benzene, and carbon disulfide.
We have developed a model to predict the LPCM per-
formance under typical operating conditions. The model as-
sumes that, for a broad range of extracted optical powers
P
ext
, the loop amplifier gain G, follows exponential saturation
G = G
0
exp(−P
ext
/P
sat
), characterized by a saturation power
P
sat
. The model further assumes that the grating reflectivity η
obeys (1), specifically that it is proportional to the product of
the powers of two writing beams as η = νP
1
P
3
. The effective
parameters for the loop amplifier—the small signal gain G
0
and the saturation parameter P
sat
—depend on the diode pump
power and the input beam alignment. The slope ν for the grating
efficiency is controlled by the sizes of the writing beams as well
as the cell geometry and operating conditions.
By simply invoking power balance per round-trip for both
propagation directions, one can derive explicit relations for the
output power P
2
and a threshold P
thr
with respect to the input
power P
1
; for the limit of interest, corresponding to a low grating
efficiency η 1, these expressions can be simplified to
P
2
≈ P
sat
T ln
P
1
P
thr
,P
thr
=
exp
τ
F
/νP
2
sat
G
0
T
√
τ
F
ν
(2)
where T is the loop round-trip transmission and τ
F
is the Fara-
day transmission of beam 1 in the forward (clockwise) direc-
tion. The loop reflectivity reaches a maximum for an input sig-
nal power P
1
= eP
thr
≈ 2.71P
thr
, yielding a reflected power
P
max
2
= TP
sat
, which is determined by only two parameters—
the amplifier saturation power and the linear round-trip loss.
Experiments demonstrate excellent agreement between the pre-
dictions of (2) and measured data [13].
So far, this discussion has focused on the energetics of the
LPCM. However, given that the function of the LPCM is to
achieve phase conjugation, it is appropriate to consider how
the loop resonator can be optimized such that the dominant
transverse mode of the resonator is the PC replica of the input
signal beam. Detailed studies indicate that the grating in the
NC possesses a fine-structured fringe pattern that preferentially
reflects the PC portion of beam 2 back into the loop resonator,
thereby allowing the PC mode to have the lowest threshold.
Several measures can be taken to discriminate in favor of the PC
mode against noise components [16]–[20] including reducing
the size of beam 3 relative to beam 1 and inserting a spatial filter
that efficiently transmits the input signal beam but blocks much
of the spatial noise.
IV. E
XPERIMENTAL RESULTS
The fully integrated PC-MOPA system was operated for ex-
tended time periods ∼30 min or more, exhibiting stable output-
beam parameters and excellent beam quality. Fig. 3 shows the
calculated and measured far-field intensity distributions in the
focal plane of a 530-mm lens while the PC-MOPA system was
producing ∼1.65-kW output power. The corresponding gains
and extracted powers of the individual power amplifiers are
summarized in Table I.
During these measurements, the near-field intensity distri-
bution approximately filled a rectangular aperture having di-
mensions of 5.5 ×4.5 mm
2
. The left-hand side of Fig. 3 in-
dicates the calculated far-field intensity distribution for such
a near-field aperture, assuming a uniform near-field intensity
and planar phase over the entire aperture. The measured far-
field intensity distribution is also shown for comparison, and
it is obvious that the two distributions are very similar. This
is quantified in the power-in-the-bucket (PIB) curves shown in
Fig. 3. The blue curve represents the calculated PIB curve for
the uniform intensity/phase distribution, while the red curve
represents the experimental measurements. At the 50% power
level, the experimental curve is ∼1.3 XDL. Note that far-field
power measurements through a series of hard apertures show
that >90% of the total PC-MOPA output power is contained
within the field of view of the video camera and analysis system
that was used to generate the far-field data in Fig. 3.
476 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007
Fig. 3. Calculated and measured far-field intensity profiles at the focal plane of
a lens with an effective focal length of 530 mm. Calculation assumes a uniform
intensity and phase over an entire 5.5 ×4.5 mm
2
aperture. Power-in-the-bucket
curve shows that the measured PC MOPA output beam quality is ∼1.3 XDL at
the 50% power level. The beam size, or D value, used in the calculated PIB is
5 mm.
TABL E I
G
AIN AND TOTAL DOUBLE-PASS EXTRACTED POWER FROM THE
NONOPTIMIZED YB:YAG POWER AMPLIFIERS WHILE OPERATING THE
OVERALL SYSTEM AT 1.65-KWOUTPUT POWER
Subsequent data were taken at higher powers. Fig. 4 shows a
temporal record of the MOPA output power ∼2 kW over a time
period of approximately 10 min.
In addition to the overall system, we quantified the perfor-
mance of the LPCM [13]. It suffices here to note that the ex-
perimental characterization of the two nonlinear components
of the operating LPCM—the thermal cell and the saturated
amplifier—both performed as expected, based on the aforemen-
tioned model.
We compared the performance of the overall PC-MOPA sys-
tem with the predictions of a model that uses measured charac-
teristics for each element and the transmission losses between
elements to calculate the power for both the first and second
pass, between each pair of amplifiers, and at the final output
beam. Each power amplifier was represented by an exponential
dependence of the gain G on extracted power P
ex
as given by
G = G
0
exp(−P
ex
/P
sat
), using measured values for the small-
Fig. 4. PC MOPA operation with output power exceeding 2 kW.
Fig. 5. Experimental points and model (curve) results for MOPA output power
versus master oscillator input power.
signal gain G
0
and the saturation power P
sat
. Although this
gain-saturation equation is identical to the one discussed earlier
in connection with the LPCM amplifier, the quantitative values
of the parameters G
0
and P
sat
are different, as they are also
for the various power amplifiers. This approximation provides
a good match with the empirical data for limited intervals of
extraction, and it is consistent with a more rigorous standard
saturation model for the gain media. The functional dependence
of the reflected power for the LPCM on its operating condi-
tions was derived from a separate model that matched all of our
LPCM data rather well.
Fig. 5 compares the PC-MOPA model with experimental re-
sults, where the overall output power is plotted against the input
power E
0
of the master oscillator light incident on the outcou-
pler. For these experiments, E
0
was varied by a factor of 5
with relatively minor changes in overall system performance.
For these calculations, the polarization outcoupler transmission
is taken as 95% both ways, and the transmission between the
amplifiers is taken as 80% for the first pass and 90% for the
second pass; the first and second pass spatial filter transmis-
sions between A3 and the LPCM were taken as 75% and 100%,
respectively.
V. C
ONCLUSION
We have generated up to 2 kW of quasi-CW average laser
power with a nearly diffraction-limited beam quality using a
double-pass PC-MOPA architecture incorporating an LPCM.
The output beam quality was quite good, and we present data
ZAKHARENKOV et al.: 2-KW AVERAGE POWER CW PHASE-CONJUGATE SOLID-STATE LASER 477
showing a beam quality of ∼1.3 XDL. The output power
achieved in this research was primarily limited by the perfor-
mance of the power amplifiers and not by the LPCM. Hence, we
believe scaling to significantly higher powers ∼25 kW or more
will be possible with improved amplifiers.
A
CKNOWLEDGMENT
The authors would like to thank N. P. Davis, J. J. Ichkhan,
R. G. Hegg, T. Matsuoka, S. McGanty, R. A. Reeder, D. Reinard,
S. Sorbel, R. Zamudio, and R. Zhou for their excellent support.
R
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Yuri A. Zakharenkov was born in Moscow, Russia,
on April 9, 1948. He received the M.Sc. degree in
physics from Moscow State University, Moscow, in
1972, and the Ph.D. degree in quantum radiophysics
from Lebedev Physical Institute, Moscow, in 1978.
During 1972–1991, he was a Senior Research As-
sociate and a Group Leader at the Lebedev Physi-
cal Institute, Moscow. During 1991–1992, he investi-
gated laser-produced plasma at Laser Facility Vulcan,
Rutherford Appleton Laboratory, Oxford, U.K. From
1993 to 1999, he was an Associate Researcher at
Short Pulse Laser Facility, Lawrence Livermore National Laboratory, Univer-
sity of California, where he was engaged in laser beam characterization. He has
designed and assembled focusing optics and plasma diagnostics for 100-TW
laser and investigated laser beam propagation through powerful high-aperture
laser amplifier. During 1999–2002, he was a Team Leader in telecom indus-
try, developing novel high-speed all-optical transceiver module for fiber optical
communication network. He joined Raytheon Space and Airborne Systems, El
Segundo, CA, as a Senior Principal Physics Engineer in November 2002 to take
part in high-power laser programs. He is the holder of two U.S. patents. He has
Authored/Coauthored two monographs on laser-produced plasma diagnostics,
65 publications in the field of lasers and nonlinear optics, and over 30 reports
in published conference proceedings.
Dr. Zakharenkov is a member of the Optical Society of America
Todd O. Clatterbuck was born in Columbus, OH,
in 1973. He received the B.A. degree in physics from
Wittenberg University, Springfield, OH, in 1996, and
the Ph.D. degree in atomic physics from the State Uni-
versity of New York (SUNY), Stony Brook, in 2003.
He is currently with the Raytheon Space and Air-
borne Systems, El Segundo, CA, where he performs
advanced development work in diode-pumped solid-
state laser systems. He has developed a number of
continuous-wave kilowatt-class laser systems based
mostly on diode-pumped Yb:YAG. His current re-
search interests include defense applications of diode-pumped ultrashort laser
systems. He has Authored/Coauthored numerous papers published in journal
and conference proceedings.
Vladimir V. Shkunov was born in Moscow, Russia,
on July 11, 1953. He received the M.S. and Ph.D.
degrees in physics from Moscow Institute of Physics
and Technology, Dolgoprudny, Russia, in 1977 and
1979, respectively.
During 1979–1997, he was a Junior Research As-
sociate, and then a Team Leader in the Institute for
Problems in Mechanics, Russian Academy of Sci-
ences, Moscow, studying basic aspects of nonlin-
ear optical phase conjugation, speckles, and dynamic
holographic gratings. During 1997–2000, he worked
on photorefractive optics and RF photonics as a Visiting Fellow, and then as
a Research Associate at JILA, University of Colorado, Boulder. From 2000 to
2003, he was a Senior Physicist in Trans Photonics, Chicago, IL, where he con-
tributed to developing direct writing techniques for polymer-based integrated
optics. In 2003, he joined the High-Energy Laser Team, Raytheon Space and
Airborne Systems, El Segundo, CA, as a Senior Principal Physics Engineer. He
has coauthored three monographs on phase conjugation and laser speckles, and
has published over 120 papers in the field of nonlinear optics and lasers.
Dr. Shkunov is a member of the Optical Society of America.