- Functional Coatings Broaden Laser Applications
High-power coatings make possible the construction of even higher power laser systems.
Coated surfaces have played an important role in commercial laser technology for about 30 years.
However, it was not until recently that development and production of functional
optics faced changes, including increases in reflection and transmission behavior
in certain spectral ranges. Coatings are used in three primary laser application
areas: dielectric high-power lasers, adaptive optics and gradient mirrors for
unstable resonators. Because they can boost the performance of the final product,
the method of coating and the choice of substrate are important considerations for
manufacturers of optical components.
The constraints of several process parameters
limit the performance of coatings produced conventionally by high-vacuum vapor deposition.
The overall pressure, as well as the partial pressure, is decisive because the average
free path length is responsible for the stoichiometry — the proportions of
elements and compounds in a substance. When heating the material at the bottom of
the vacuum chamber, it is important that the same material composition (i.e., with
no breaking of molecules) condenses at the top of the amorphous coating layers on
the substrate surface. The trick is to optimize those pressures according to the
requirements of the final product.
On the other hand, surface adhesion
of the vaporized particles should always be at maximum. The adhesion is determined
by the kinetic energy of the vaporized molecules and single atoms during condensation.
By heating the substrate, the molecules
receive additional energy during condensation, which has the same physical effect
as a higher-speed, kinetic energy toward the substrate surface. The mobility of
the vaporized particles on the substrate is closely associated with the substrate
temperature during the condensation phase and has crucial influence on the microstructure
of the resulting coatings.
The three-zone model established by
Movchan and Demchishin distinguishes three characteristic temperature ranges with
regard to the relationship between the substrate temperature and the melting temperature
of the dielectric. This yields the optimum temperature, where the vapor particles
can mobilize themselves in the best way possible on the substrate surface to eliminate
any residual humidity. Substrates must be heated to their optimal temperature of
about 300 °C because conventional drying doesn’t remove humidity. Heating
also contributes to better adhesion of the layers.
With conventional coatings manufactured
in this fashion, it has long been possible to satisfy the needs of most laser users.
However, the market demand for high-power coatings with lower thermal drift has
increased. Decreasing thermal drift was possible only by compacting the coatings.
This minimized the number of embedded H2O molecules without embedding other dopant
molecules and changing the stoichiometry of the vapor particles. Dopant molecules
are primarily metallic pollutants from the vapor source or from the walls of the
coating chamber. At high target excitation, they also come into the vapor deposition
stream and are added to the substrate surface. This can result in an unwanted increase
of absorption in the coating layers, which drastically reduces the damage threshold.
Figure 1. In the typical ion-beam-supported
vapor deposition equipment setup, the relatively low thermal particle energy of
about 0.1 eV dissipates quickly.
The compacting is achieved by the use
of ion-assisted deposition sources, plasma sources that support the vapor deposition
process by energetically favoring the self-arranging process of the particles in
the forming layer. The electron cyclotron resonance source (Figures 1 and 2) is
also an ion source for the production of high-power laser coatings.
During the conventional thermal vapor
deposition process, the relatively low thermal particle energy of only about 0.1
eV dissipates quickly, creating porous coatings with the typical column structures.
A molecule hitting the substrate surface binds on the spot. Because no further thermal
excitation processes take place, a spatial rearrangement is no longer possible.
Figure 2. With ion-assisted
deposition and electron cyclotron resonance, particles move at a much higher average
speed on their way to the substrate.
With ion-assisted deposition and electron
cyclotron resonance, particles move at a much higher average speed on their way
to the substrate. With energies in the range of 5 to 10 eV, the energetic potential
of the particles has reached that of surface binding energies, enabling processes
like diffusion, particle swaps and momentum transfer. The higher mobility of the
molecules after hitting the substrate surface produces a higher package density.
Ion deposition sources formerly used
a plasma for the energy transfer that required a DC discharge between an anode and
a cathode for excitation. The erosion of the cathode filament by interaction with
the reactive gas oxygen and the resulting pollution of the equipment and the layers
with filament material are important disadvantages of such an arrangement.
On the other hand, electron cyclotron
resonance sources of the second generation produce an alternating electromagnetic
field for plasma excitation, which is generated by a magnetron and coupled into
the process chamber by a hollow waveguide system and a quartz half-dome. Only with
such a nonfilament and metal-free arrangement is it possible to produce coatings
with few dopant molecules. This is a great advantage over comparable ion-assisted
With electron cyclotron resonance sources,
we get an alternating field of the frequency f by the cyclotron resonance
ω = 2π x f = e × B/m
where e is the elementary charge, m
the electron mass and B magnetic flux density. The magnetic flux density typically
has an order of magnitude of about 1000 Gauss with coil currents of more than 100
A. The resulting resonant frequency usually is adjusted at exactly 2.45 GHz because
this is a radio-frequency range released by telecommunications authorities for industrial
use according to interference. This microwave radiation with a wavelength of about
12 cm powers up to about 600 W. With adjustment, such sources work constantly and
reliably for a longer time.
It is also possible to manufacture
hard coatings without heating the substrates, so that even polycarbonate or plastic-coated
glass and sapphire fibers can be coated. Also, very thick packages can be stacked
up with a large number of layers; e.g., high-reflective alternating coatings (high-reflective
mirrors) for the near- to mid-infrared range. The damage thresholds, measured at Hannover Laser Zentrum in Hannover, Germany, reach more than 70 J/cm2 at 1064 nm and a 10-ns pulse duration.
Adaptive laser optics
With adaptive optics, a new technology improved
the imaging and beam quality in time-variant optical systems by real-time correction
of time-dependent aberrations that arise primarily from temperature drift. This
is done using mirrors that are deformed locally by means of mechanical actuator
elements to equalize the measured wavefront deviations. This method of wavefront
compensation prevents a substantial deterioration of the beam quality and increases
gain. It could enable a high-power laser system to be built that would produce beam
quality very close to that of a single-mode laser.
The substrate material of deformable
mirrors typically consists of an etched silicon foil with an area of 1 cm2 and only
a few microns thick, onto which a metal coating and several dielectric coatings
are deposited (Figure 3). The dielectric coating raises the damage threshold as
well as the reflectivity. Piezoelectric actuators, working with frequencies of
some kilohertz, deform the mirrors
Figure 3. A piezoelectrically
deformable foil with a coating of silver and six layers of dielectric coating is
optimized for 1064 nm.
Adaptive optics did not arise from
a single pioneering invention, but evolved from many individual developments over
a long period. It is an interdisciplinary area, because know-how and techniques
were put together from many fields, including optics, electronics and materials
technology, and were used for manufacturing optical systems.
The resonator essentially determines the beam
quality of a laser. With stable resonators, good beam quality can be accomplished
by limiting the number of possible transverse modes using diaphragms, which also
limits the output power with the mode volume. Therefore, the beam quality can be
optimized only for a low input power range.
With unstable resonators, the mode
volume can be determined by the size of the output mirror. The beam quality does
not depend on the pump power anymore because, with varied internal lenses, the focus
radius and remote field divergence change in opposite directions at the same time.
The beam is coupled out around a higher
reflective circular spot that has been coated on a nonreflecting substrate. One
would suspect diffraction circular fringes at first. However, the so-called dot
also can be accomplished with a nonconstant degree of reflection that continuously
varies with the spot radius. Because of the transition, one also speaks of soft
Gauss reflection diaphragms or gradient mirrors that avoid the occurrence of diffraction
To deposit such a gradient mirror coating,
the substrate would have to be positioned in the center of the coating chamber directly
above the active evaporation source. However, this is not possible because the objective
is to coat several substrates in a single batch.
Therefore, the gradient mirrors must
execute a double rotation during the coating process. This “planetary motion”
consists of the rotation of the substrate around the center of the vacuum chamber
to distribute mass uniformly, and around its own axis for a radially symmetric
thickness distribution behind the aperture (Figure 4).
Figure 4. This illustrates the radial symmetric thickness distribution
of a coating at rotation of the substrate during the coating process. The characteristics
of an area evaporization source: a is the angle between the viewing direction and
the symmetry axis of the evaporization source; m/m0 is the under angle a emitted
mass, normalized to the under a = 0° emitted mass. The exponent n determines
the shape of the vapor clouds: The bigger the n, the more directed is the cloud
and the less material is emitted at greater angles.
The reflection profile R is calculated
for a Gauss-shaped course with the following:
R(r) = Ro × exp (-2r/w)n
where R(r) is the reflection with regard
to the radius, Ro the center reflection, w equals 1/e2 the radius of intensity reflection,
r the radius and n the Gauss order exponent.
An Nd:YAG laser at a wavelength of
1064 nm measured a reflection profile. The beam was split into measurement and reference
beams. A short-focal-length lens focused the measurement beam. The mirror to be
measured, mounted on an adjustable table, moved through the focus, and a large-area
PIN photodiode detected the light that it transmitted.
High-power coatings that are manufactured with
electron cyclotron resonance sources in resonator optics and beam guidance make
possible the construction of laser systems with even higher power, opening up the
possibility of many new application areas. For example, adaptive optics produced
as micromirror elements in single-chip technology in connection with fiber optics
have the potential to revolutionize electric/optical control technology. The unstable
resonator has been ignored for a long time, but with gradient mirrors, it has achieved
The author would like to thank H. Weber and H.
Albrecht, formerly of LMTB Berlin, and D. Ristau, all at Hannover Laser Zentrum.
Meet the author
Martin Schacht is production and research and
development manager of optical thin-film coatings, fiber optics and filter configuration
at Laser Components GmbH in Olching, Germany.
- adaptive optics
- Optical components or assemblies whose performance is monitored and controlled so as to compensate for aberrations, static or dynamic perturbations such as thermal, mechanical and acoustical disturbances, or to adapt to changing conditions, needs or missions. The most familiar example is the "rubber mirror,'' whose surface shape, and thus reflective qualities, can be controlled by electromechanical means. See also active optics; phase conjugation.
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