Since the discovery of the laser in the 1960s, a great amount
of research activity has led to an impressive increase of the overall
performances of the sources emitting in the visible or in the infrared
spectral regions. The most significant achievements for solid-state
lasers in the last 10 years are the increase in laser output power or
pulse energy by orders of magnitude due to the introduction of the
diode pumping of the gain media. This technology also led to a
remarkable improvement of the electrical to optical efficiency as
well as compactness and reliability of the sources. All these recent
technological breakthroughs have contributed to the fast evolution
of the field of photonics and a growing interest in solid-state lasers for many different
industrial and scientific applications. For example, in manufacturing, material processing,
or the medical areas, lasers are now routinely used to focus high-energy densities on a
surface. This ability also opens new opportunities in basic science interactions for
plasma physics or X-ray generation with sources delivering ultrashort pulses. Also
due to the directivity of optical antennas, lasers will undoubtly be applied in LIDAR
imaging systems, for ground or space communications or for monitoring of the
atmosphere. All these applications clearly require sources delivering high-quality
optical beams whose divergence must not exceed the diffraction limit during beam
propagation. In other terms, the wavefront emitted by a high-power laser must be
free of any aberrations or distortions which would degrade the brightness of the
source and thus would lead to a decrease of the system performances. Attaining
these operating conditions is an important challenge, since thermal loading due to
strong pumping of the gain media induces aberrated thermal lenses, which severely
affects the beam quality. It thus results in wavefront aberrations that reduce the
brightness of the source and that evolve when changing the operating conditions of
the source. Adaptive correction of phase aberrations in a laser cavity or in a master-
oscillator power-amplifier structure is thus a crucial problem that must be taken into
account in solid-state laser sources. An elegant approach offering a great potential to
solve this question involves nonlinear optical phase conjugation. This technique
permits the generation of a complex phase conjugate replica of a wavefront after
beam reflection on a nonlinear mirror, thus leading to a compensation of any
wavefront distorsions. This nonlinear reflection can be interpreted as the conjugate
wavefront generation due to a dynamic hologram in a material that exhibits a third-
order nonlinearity. Since the discovery of the effect in the early 1970s, optical phase
conjugation is now an established field of nonlinear optics, and it has opened very
important scientific and technological advances in laser physics over the last
decades. This book is devoted to the current development in the field of Phase
Conjugate Lasers with the objective of showing the impact of these innovative
concepts on the architectures and performances of a new class of solid-state lasers.
Phase conjugate lasers exhibit adaptive correction of their own aberrations whatever
their operating conditions, and they provide maximum brightness to the user for
a large diversity of scientific or industrial applications. The critical issue of this very
attractive approach is to identify the most efficient media and nonlinear mechanisms
that operate at the required wavelengths. In this perspective, the book presents the
basic physical phenomena and materials involved for efficient generation of the
conjugate waves for specific examples of laser sources. The book also develops in
detail an analysis of the laser architectures and nonlinear mirrors that are best suited
to operate in continuous-wave or pulsed regimes, respectively. The ability of phase
conjugate lasers to deliver beams with a high spatial and spectral quality is clearly
outlined in the different chapters.

After a brief overview of the basic principles of nonlinear optical phase
conjugation in Chapter 1, a large part of the book is devoted to lasers, including a
Brillouin phase conjugating mirror. In Chapter 2 the principles, the basic properties,
the materials (bulk and fiber geometry), and performances of stimulated Brillouin
scattering (SBS) mirrors are presented. Such nonlinear mirrors can be implemented
inside a laser resonator as shown in Chapter 3. Besides the demonstration of high
brightness operation, the authors analyze in detail the stability and the mode
structures of these unconventional nonlinear resonators. To achieve high power
with a near-diffraction-limited beam, master-oscillator power-amplifier (MOPA)
configurations are demonstrated in Chapter 4 in which both liquid and glass fiber
Brillouin conjugators are used. The fiber presents the advantages of compactness
and lower energy threshold due to the long interaction length of the fiber medium.
However, to achieve very high energy the use of SBS liquid cells is required as
presented in Chapter 5. Using the capability of phase conjugation to phase-lock
several beams issued from different amplifiers, the authors demonstrate up to 100 J
of output energy while keeping the beam quality close the diffraction limit. Some
applications may require solid-state SBS mirrors instead of liquid cells. For that
purpose, the authors of Chapter 6 investigate and characterize SBS properties of
bulk solid-state materials like organic crystals and glasses. The previous chapters
have concerned the ability of SBS mirrors to compensate for phase aberrations of
gain media. It is also important to highlight (as done in Chapter 7) that an SBS
nonlinear mirror can perform pulse compression in the time domain. This brings the
opportunity of controlling both spatial and temporal characteristics of laser pulses
with the same nonlinear mechanism. In the following chapters, alternative nonlinear
mechanisms are presented. In particular, infrared-sensitive photorefractive crystals
are used in Chapter 8. The authors detail the specific properties of this type of
nonlinear material and demonstrate dynamic correction of MOPA laser sources. It is
also shown in Chapter 9 that photorefractive crystals can be used to realize
a semiconductor laser diode cavity with phase conjugate feedback for spatial and
spectral filtering of the modes. In Chapter 10, a nematic liquid crystal cell is
implemented in a laser resonator to perform phase conjugation and correction of
intracavity distortions. This relies on the large anisotropy and nonlinear effects in
liquid crystals. Thermal gratings can also be used to build a self-adaptive phase
conjugate loop resonator as demonstrated in Chapter 11. In all these studies, two
distinct materials are employed for the gain medium and the phase conjugate mirror.
It is finally shown in Chapter 11 that laser gain media can perform phase conjugation
by using gain saturation as the nonlinear mechanism. Self-adaptive holographic loop
resonators are demonstrated using this interaction.

This book gives a complete review of the state of the art of phase conjugate
lasers, including laser demonstrators, performance, technology, and selection of the
most important and promising classes of nonlinear media.

We express our warm thanks to all our co-authors for their very valuable
contributions and for their fruitful discussions and cooperation during the
preparation of this book.

Jean-Pierre Huignard
Arnaud Brignon
Paris, 2003

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