have been a Lawrence Livermore specialty almost since the first
laser flashed in 1960. Dealing with the challenges that arise as
these lasers get bigger and more powerful is, of necessity, a specialty
The process of creating an
intense laser beam requires that the beam traverse many pieces of
optical glass. Oftentimes, the glass requires highly reflective
mirror coatings so that the lasers energy is not lost as it
passes through. But during the coating process, miniscule defectscalled
nodulesinevitably occur. As lasers get larger, they require
larger pieces of glass, and more defects occur. On a laser mirror
a half-meter across, there may be as many as a million defects.
While more than 99.99 percent of defects have no influence on optical
performance, a few of the defects will limit the laser fluence (a
measure of the energy passing through) that the mirror will survive;
finding those few among a million is like trying to find the proverbial
needle in a haystack.
In the mid-1990s, Livermore
began to explore the best means for locating coating defects. Optical
microscopy and atomic force microscopy can reveal nodules but cannot
distinguish the thermal defects in thin films that arise during
the coating process. Engineer Chris Stolz and others at Livermore
began working with scientists at Eastern Michigan University who
were experts in photothermal microscopy, an imaging technique that
can locate and characterize both nodules and thermal defects in
laser mirror coatings.
absorb light from the pump laser and cause a surface bump to
form. The probe laser detects the bump, and the photodetector
records changes in the probes optical diffraction caused
by the deformation. The resulting signal indicates the amount
of optical thermal absorption at the specified wavelength.
photothermal microscopy, a pump laser set at a specific wavelength
heats a surface. Surface and subsurface defects that absorb the
light at that wavelength cause a bump to form on the surface, as
shown in the figure above. A second laser beam, known as the probe
laser, detects the change in the surface, and a photodetector records
changes in the probes optical diffraction caused by the bump,
or deformation. The pump beam is chopped, or interrupted
periodically. The photodetector locks into the chopping frequency,
and the resulting photothermal signal is an indicator of how much
heat at the specified wavelength was absorbed.
The benefit of photothermal
microscopy was made clear in its earliest tests, which examined
a 9-millimeter by 9-millimeter area of coating. One defect, which
had the highest photothermal signal, was not visible optically.
Later, during laser damage testing, the defects with the highest
photothermal signal proved to have the lowest damage threshold.
(The damage threshold is the laser energy level that the material
is designed to endure but beyond which damage will likely occur.)
A high-energy laser needs glass with the highest possible damage
Studying various kinds of
defects in glass coatings allowed the researchers to zero in on
the few that reduce the damage threshold. Photothermal microscopy
images also validated the use of laser conditioningtreatment
of defects with a laseras a way to reduce the absorption of
defect fluences, as shown in the figure below.
A photothermal microscopy image and (b) the diffraction signal
of a nodule defect before laser conditioning. (c) The same defect
after laser conditioning. (d) Its diffraction signal has been
reduced by a factor of 125, which in turn reduces the likelihood
that this defect would cause damage at the National Ignition
Scanning to Imaging
The photothermal microscopy
system worked well but, because it used a raster-scanning technique,
was extremely slow. The results of raster scanning are what you
see when the graphics on a Web site or other computer graphics gradually
improve line by line or pixel by pixel. In photothermal microscopy,
the pump and probe beams are raster-scanned while the detector collects
data a single pixel at a time. Together, they generate a photothermal
microscopic map of a given inspection area at a rate of 1 second
per pixel. That speed is impractical for inspecting large surfaces
Recently, engineers Diane
Chinn and Stolz, working with others at Livermore and Wayne State
University, modified the scanning technology to create photothermal
imaging microscopy, which is 10,000 times faster than the raster-scanning
method. Using the imaging mode, photothermal microscopy can inspect
a 1-square-centimeter area in just 2 seconds.
For photothermal imaging,
Chinn and the others expanded the pump and probe beams to about
5 millimeters. They used a 1,024- by 1,024-pixel charge-coupled-device
camera to detect the diffracted pump beam. In scanning mode, the
detector was locked in electronically to the chopping frequency
of the pump beam, but in the imaging mode, the collected images
are phase-delayed relative to the pump beam to achieve optical lock-in.
The figure below compares
images of a glass sample with an antireflective coating using both
the raster-scanning and imaging modes. Aluminum dots were sputtered
onto the glass substrate before coating. The two images showing
aluminum absorption are comparable, but the time it took to produce
them is not. The raster-scanned image took 35 minutes to obtain,
while the one generated through the imaging mode took just 40 seconds.
A map of a sputtered aluminum dot under an antireflective coating
using photothermal microscopy in the raster-scanning mode. It
took 35 minutes to obtain this image. (b) The same dot imaged
using photothermal microscopy in the imaging mode. Producing
this image took just 40 seconds.
The teams proof-of-principle
work was funded by Laboratory Directed Research and Development.
Now, with project funding, the team is beginning to apply the process
to optics for the National Ignition Facility (NIF).
When NIF comes on line in
the next few years, it will be the largest, most energetic laser
in the world as well as the largest optical instrument ever built.
NIF will require lots of optical glass7,500 large optics (as
large as a meter along the diagonal) and more than 30,000 small
optics. The primary task of these optics will be to separate and
steer 192 laser beams through a 250-meter-long building and to amplify
the laser energy before that energy is focused onto a fusion target
the size of a dime.
Electron beam deposition
lays down multilayer coatings of hafnia and silica on NIF optics.
With the raster-scanning technique, imaging a 1-centimeter-square
area of NIF optical coating at 10-micrometer resolution would take
a full 278 hours. Inspecting the acres of coatings on NIF optics
would have taken decades at that rate. The faster photothermal microscopy
imaging technique, however, is a viable method for inspecting NIFs
of a Kind
We have proved that
this new system can produce the fast, high-quality images we need
to inspect NIF coatings, says Chinn. And it is a capability
that doesnt exist anywhere else in the world. Photothermal
imaging microscopy will have other uses as well. Chinn sees it helping
microtechnology engineers to assess computer chip lithographic techniques.
It may also be useful for studying hard coatings and thermal barriers
in the automotive and aerospace industries.
If this system works
in-house as we hope it will, continues Chinn, we plan
to move the technology into the coating vendors shops for
their use. This is a much faster method for identifying problem
areas than anything else out there.
Key Words: laser
glass, multilayer hafniasilica coatings, National Ignition
Facility (NIF), photothermal microscopy.
information, contact Diane Chinn (925) 423-5134 (firstname.lastname@example.org).