Crystal Selection for Medium in Laser Ultrasonic Imaging Applications
Abstract:
This paper aims to find which lasing crystal would be best for use in ultrasonic
generation to produce images of a specimen. Many different papers on the subject are presented
and interpreted in relation to the application of laser ultrasonic generation and this review will
help in acquiring basic knowledge of laser ultrasonics, in finding a specific crystal for an
experiment, or in quickly determining a suitable crystal.
Introduction:
Ultrasonics is the use of ultrasonic waves for inspection, characterization, imaging and
modification of a medium. In this practice, physical waves are sent through media with a desired
intensity, frequency, duration and orientation to affect the medium as desired. These waves are
then observed and data may be collected and interpreted to diagnose, measure or image the
medium.
There are many methods of generation and detection of ultrasound. Generation methods
include transducers (devices that transfer energy from one form to another, including
ferroelectrics, electromagnetic systems and piezoelectrics, which convert electrical energy into
physical displacements), physical excitation (simply striking a material will cause a wave to
propagate) and laser stimulation (which will be discussed in detail later). Methods of detection
include transducers and optical devices, such as lasers.
Within ultrasonics, laser ultrasonics is a technique that involves using laser light to
generate and detect ultrasonic waves in materials. These ultrasonic waves allow the user to
determine a variety of characteristics and properties of a material such as the Young’s modulus,
density, dimensions, integrity of the material, porosity, and from these measurements virtually
every material property can be calculated. This makes laser ultrasonics a valuable technique in
materials characterization and imaging.
Laser ultrasonics has many advantages over conventional ultrasonic techniques including
non-contact application, speed, ability to automate ultrasonic systems and accuracy of data.
These advantages make laser ultrasonics a valuable tool in flaw detection in aircrafts. The
complex geometry and large size of aircrafts make using contact piezoelectric transducers slow
and difficult but laser ultrasonics can quickly and easily scan over the large area of aircrafts and
reliably detect flaws in the aircraft body. Laser ultrasonics is also convenient in applications such
as extrusion in metals that involves molten metal and extremely high temperatures. Here again
conventional techniques cannot be used because they cannot contact the extremely hot surface.
However, laser ultrasonics can inspect the molten metal and its data can be used to evaluate the
material with relative ease.
This paper aims to find a crystal, of the many different lasing crystals, that would be best
used in ultrasonic imaging applications. Many papers and writings are presented that have
extensively studied and commented on the many different solid state lasers. This information is
used to determine which crystal will be best used for the purposes of ultrasonic generation,
which is accomplished by assessing various parameters of the crystals including functionality,
lifetime, frequency, availability, cost, power and ease of use.
Finding the proper crystal is important for many reasons. The reasons may include cost,
application, economic and environmental concerns, laser training and experience. By presenting
all of the solid state options and finding the best overall choice, readers can obtain a better
knowledge of laser ultrasonics and be able to confidently begin experiments using a solid state
laser.
Literature Review:
Laser Ultrasonics and the crystals involved in laser ultrasonics have been well
documented in literature. Although lasers are a relatively new tool used in science and
engineering, there is a wealth of sources to reference and draw knowledge from. Kiss
summarizes the birth and development of lasers in his paper in Applied Optics, “The conditions
for maser action at optical frequencies were first described by Schawlow and Townes in 1958.
The first demonstration of laser action by Maiman two years later was achieved using ruby (Al2
03:Cr3+), a crystalline solid system. In the following years, much effort was expended on the
search for new laser transitions in various media: crystalline solids, gases, liquids, glasses,
plastics, and semiconductors.” (Kiss, 1966) These efforts to search for new laser transitions in
various media have grown the wealth of knowledge of lasers we have available today, which
may be referenced to study crystals used in laser generation.
The acronym “laser” stands for “Light Amplification by Stimulated Emission of
Radiation”. This electromagnetic radiation is collimated (meaning the photons are parallel in
direction of propagation), temporally consistent (meaning the photons are in perfect phase and
are the same frequency) and intense (due to the focused energy of many photons). Lasers are
used in many applications and there are many different mediums to generate laser light for a
given application. In considering solid state lasers, the lasers are made with crystals and glasses
that are doped with divalent and trivalent rare earth ions such as Nd3+, Yb3+, Er3+, Tm3+, Pr3+
and Ho3+ or with transition metal ions such as Ti3+, Cr2+, Cr3+ and Cr4+ (Koechner, 2006).
These dopants in combination with different crystals correspond to a myriad of parameters to be
utilized in laser ultrasonics.
Let us first consider the laser crystals, or hosts, in laser generation. The host crystal is
very important in producing a desired laser beam. In addition to hosting the dopant atoms, the
crystals dictate: the energy levels the dopants can be excited to and what ground state they will
return to (which produces electromagnetic radiation); the amount of laser light transmitted by the
laser (the more optically transparent the glass or crystal, the more optical energy that will be
transmitted); the ability of the laser to be operated for long periods of time or be used in
continuous wave applications (the thermal conductivity of the crystals is important in dissipating
energy so consistent and reliable operation of the laser can be maintained); and the intensity of
the laser (more sites for dopant materials per volume will generate more radiation) (Paschotta,
2007).
Paschotta has made a summarized list of different properties for different crystal mediums
that is a good reference to begin crystal selection. This list is reproduced here:

Garnets such as Y3Al5O12 (YAG), Gd3Ga5O12 (GGG), and Gd3Sc2Al3O12 (GSGG):
hard and chemically inert materials, optical isotropic, with high thermal conductivity

Sapphire (Al2O3) (e.g. for titanium–sapphire lasers) and aluminates such as YAlO3
(YALO, YAP) for neodymium doping: high hardness and thermal conductivity,
anisotropic

Sesquioxides such as Y2O3, Sc2O3: isotropic, high hardness and thermal conductivity

Vanadates such as YVO4 and GdVO4: very high laser cross sections of Nd3+,
anisotropic

Fluorides, e.g. YLiF4 (YLF): good UV transparency, birefringence, large energy storage
capability of Nd:YLF; also LiCAF, LiLuF, LiSAF as chromium-doped broadband gain
media

Silicates, e.g. MgSiO4 (forsterite): broad gain bandwidth

Monoclinic double tungstates such as KGd(WO4)2 (KGW) and KY(WO4)2 (KYW):
combination of relatively high Yb3+ laser cross sections, large gain bandwidth, and high
thermal conductivity

Disordered tetragonal double tungstates such as NaGd(WO4)2 (NGW) and NaY(WO4)2
(NYW): particularly large gain bandwidth of ytterbium

Chalcogenides such as ZnS or ZnSe for mid-infrared lasers
(Paschotta, 2007)
Kiss also has made a table on host crystals that may also be referenced and is listed
below. Kiss makes comments on the crystal’s properties and what dopants can fit within the
crystal lattices, which is very informative. (Kiss, 1966)
Figure 2 (Kiss, 1966)
Now that we have reviewed the basics on the crystal hosts, we can now study the influences
of the dopants. The dopant atoms in the crystal hosts are the atoms that take in light or electrical
energy and have their valence electrons excited to a higher electron state. When the electrons
return to their ground state they emit photons and these photons are trapped in a resonant cavity
to collimate the light into a laser beam. These dopants include divalent and trivalent rare earth
ions such as Nd3+, Yb3+, Er3+, Tm3+, Pr3+, Ho3+ and transition metal ions such as Ti3+,
Cr2+, Cr3+, Cr4+. Kiss has also made a table of these dopants and their properties which is
located below as figure 2.
It is also important to devote specific attention to one property in particular, the
characteristic wavelength of a laser system. This very important parameter is dictated by both
the crystal and dopant choice. The wavelength determines the amount of energy transferred to
the material under experimentation, which is integral in laser ultrasonics. The amount of energy
is paramount because if the energy absorbed is too high, the sample under investigation may be
damaged, but if the energy absorbed is too low, inadequate data may result. The wavelength also
determines what optics and environment the experiment can be performed in as different
wavelengths interact differently to various optics, gases and liquids. These implications of the
value of the wavelength make determining the proper wavelength for a given experiment
extremely important, in fact, proper choice of wavelength may be the most important decision is
choosing a crystal for a given application (Powell, 1998). Figure 3 is taken from
LexelLaser.com and can be quickly referenced to find a suitable wavelength for a given
experiment.
Although there are many solid state laser possibilities and literature regarding their
application and theory, there are still many possibilities yet to be discovered and more
applications and properties not yet explored. More crystal and dopant combinations need to be
discovered and experimented with to find even more ways to incorporate lasers into science and
engineering industries. With even more combinations to choose from, the already expansive field
of laser technology can find new problems in industry and research to solve.
It is clear that there are many different properties and parameters that can be manifested
with selection of a lasing crystal and its dopants. One can choose a crystal based on its
wavelength, material properties (such as thermal expansion and toughness) and what dopants can
be added. By using this information, a crystal can be chosen for a given experiment in laser
ultrasonics that best meets the experiments needs.
Figure 2 (Kiss, 1966)
Figure 3 : http://www.lexellaser.com/techinfo_wavelengths.htm
Annotated Bibliography:
Fan, T.Y., Edward L., and Byer, R.L. (1988). Diode laser-pumped solid-state lasers
IEEE Journal of Quantum Electronics, 24(6), 895-912.
This article discusses the differences between flashlamp-pumped lasers and solid-state
lasers. It will help in rounding out the basics of solid state lasers and will be instrumental in
choosing a specific crystal for a given application. A highlight of this article was the then recent
research in wavelength extension which may be vital in a given application.
Kiss, Z. J., & Pressley, R. J. (1966). Crystalline solid lasers. Applied Optics, 5(10), 14741486.
This is a classic paper on solid state lasers. It will provide a backbone for my overview of
crystals and its contents will help the reader get a basic understanding of how crystals are used in
solid state laser systems. The article surveys crystalline solid lasers, discusses impurities and
dopants in crystals and then finishes with the characteristics of operating continuous wave
lasers.
Koechner,W. (2006) Solid-state laser engineering. New York, Springer.
This book is a very extensive reference on almost every aspect of solid-state lasers. I will
use this book extensively in choosing a specific crystal and in demonstrating the basics in lasers.
In particular, this book has 64 pages dedicated to solid-state laser materials and their properties. I
will most likely use these pages to ultimately conclude my thesis.
Krupke, W.F. (2000) Ytterbium solid-state lasers. The first decade. IEEE Journal of Quantum
Electronics, 6(6), 1287 – 1296.
This article reviews many differences in Ytterbium lasers. Ytterbium is an element in
YAG (Ytterbium Aluminum Garnet) crystals that is substituted for other elements (typically rare
earth elements or transitional metal elements) to produce desired lasing effects. This article looks
at the different properties obtained by different changes in these crystals. This source may prove
to be one of the most important sources in choosing a specific crystal.
Laser wavelengths chart. Retrieved September 30, 2012, from Lexel Laser website :
http://www.lexellaser.com/techinfo_wavelengths.htm
This is a simple website that is only a chart of different wavelengths obtained from
different lasers. It will serve as a quick reference in writing and determining the proper lasing
medium. It does not provide any qualitative information and should not be considered for the
reader to inspect.
NATO Advanced Research Workshop on Physics of Laser Crystals. (2002). Physics of laser
crystal. Boston: Kluwer Academic Publishers
This book gives an in-depth look into laser crystals and their engineering and physical
specifications. I will use this reference extensively for general and specific information. It will be
one of my primary sources.
Paschotta, R. (2007). Laser Crystals. Retrieved September 30, 2012, from The Encyclopedia of
Laser Physics and Technology website: http://www.rp-photonics.com/laser_crystals.html
This article provides a good overview of crystals used as a lasing medium. I will
generally reference this article and use it to present the many different options for lasing
mediums and what properties can be acquired.
Paschotta, R. (2007). Laser crystals versus glasses. Retrieved September 30, 2012, from The
Encyclopedia of Laser Physics and Technology website: http://www.rpphotonics.com/laser_crystals.html
This article specifically focuses on the differences between crystals and glasses as lasing
mediums. This will be used in my review and will primarily be used to pick a specific crystal for
a given application.
Paschotta, R. (2007). Neodymium-doped gain media. Retrieved September 30, 2012, from The
Encyclopedia of Laser Physics and Technology website: http://www.rpphotonics.com/neodymium_doped_gain_media.html
Neodymium is one of, if not the most widely used dopant in solid-state lasers. This article
provides a look at the many different crystals and glass media that incorporate Neodymium as a
laser-active dopant. If this thesis ultimately chooses a Neodymium doped solid-state medium,
this article will be important in that decision.
Powell, R. C. (1998) Physics of solid-state laser Materials. New York: Springer.
This may be my best and most extensive reference. The book contains 415 pages
dedicated to solid-state laser materials. It will without a doubt be used in every aspect of my
thesis and will prove to be a great reference. When I am in the process of finding a specific solid,
these sections on each different material will be consulted and will allow for a more complete
and decisive conclusion.