356 OPTICS LETTERS / Vol. 25, No. 5 / March 1, 2000
Dense plasma diagnostics with an amplitude-division
soft-x-ray laser
interferometer based on diffraction gratings
J. Filevich,* K. Kanizay, M. C. Marconi,* J. L. A. Chilla, and J. J. Rocca
Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado 80523
Received November 2, 1999
We report the demonstration of an amplitude-division soft-x-ray interferometer that can be used to generate
high-contrast interferograms at the wavelength of any of the saturated soft-x-ray lasers (5.6– 46.9 nm) that
are available at present. The interferometer, which utilizes grazing-incidence diffraction gratings as beam
splitters in a modif ied Mach–Zehnder conf iguration, was used in combination with a tabletop 46.9-nm laser to
probe a large-scale
共⬃2.7-mm-long兲 laser-created plasma. 2000 Optical Society of America
OCIS codes: 340.7450, 140.7240.
Interferometry with visible and ultraviolet lasers has
been used for more than three decades to obtain mul-
tidimensional maps of the electron density in a large
variety of dense plasmas.
1,2
However, free–free
absorption and refraction of the probe beam limit the
maximum electron density, plasma size, and plasma
density gradient that can be probed with optical
lasers.
3
The development of gain-saturated soft-x-ray
lasers has opened the possibility of extending laser
interferometry to a significantly broader range of
plasma parameters. In pioneering interferometry
work conducted at Lawrence Livermore National
Laboratory, a 15.5-nm Ne-like Y laser pumped by the
Nova laser has successfully probed dense laser-created
plasmas.
3,4
The resent advent of gain-saturated
tabletop soft-x-ray lasers has created the opportunity
to develop portable soft-x-ray tools that will allow
detailed maps of the electron-density evolution in a
great variety of dense plasmas.
Recently, our group demonstrated plasma inter-
ferometry with a tabletop soft-x-ray laser combining
a 46.9-nm capillary-discharge laser with a wave-front-
division interferometer based on the Lloyd’s mirror.
5,6
The Lloyd’s mirror has the advantage of constituting
the simplest possible soft-x-ray interferometer. How-
ever, it also presents several limitations. First, the
probe and the reference beams are very closely spaced.
This limits the types of plasma that can be probed.
Second, the maximum size of the plasma region that
can be probed and the interfringe spacing are not
independent, as both depend on the grazing-incidence
angle of the Lloyd’s mirror. Finally, since this is a
wave-front-division interferometer in which the fringe
visibility relays the spatial coherence of the laser, it
is difficult to maintain good fringe visibility over a
large area. This last limitation also applies to Fresnel
bimirror interferometers.
7
These limitations can be
overcome by use of an amplitude-division interfero-
meter scheme in which the interference pattern is
obtained by superposition of two beams with identical
transverse coherence characteristics, such as the
skewed Mach–Zehnder interferometer developed by
Da Silva et al. that uses thin strained multilayer films
to split and recombine the beam.
3,4
However, this
solution cannot be implemented at all wavelengths
(e.g., 46.9 nm, the wavelength of our Ne-like Ar laser),
owing to the lack of materials with adequate optical
constants for the manufacturing of multilayer beam
splitters in some regions of the spectrum. To over-
come this problem we have developed a novel soft-x-ray
amplitude-division interferometer in which diffraction
gratings are used as beam splitters. By proper tai-
loring of the gratings, the interferometer can be made
to operate at any selected soft-x-ray wavelength. An
additional advantage of the use of diffraction gratings
is their increased resistance to catastrophic damage
produced by fast particle debris. A phase-shifting
point diffraction soft-x-ray interferometer that uses a
grating to improve the efficiency was recently demon-
strated by Medecki et al.
8
The soft-x-ray interferometer, which consists of a
modified Mach–Zehnder configuration,
9
is schemati-
cally illustrated in Fig. 1. The soft-x-ray laser beam
Fig. 1. Schematic diagram of the soft-x-ray amplitude-
division grating interferometer and plasma diagnostics
setup.
0146-9592/00/050356-03$15.00/0 2000 Optical Society of America
March 1, 2000 / Vol. 25, No. 5 / OPTICS LETTERS 357
is diffracted off the first grating into the two arms of
the interferometer. The incidence angle and the blaze
of the grating are selected to allow for most of the laser
radiation to be evenly split between the zero and the
first diffraction orders. This even split is essential for
obtaining good fringe visibility. Elongated grazing-
incidence mirrors are placed on the zero- and the first-
order arms to redirect the light toward the second
grating, which recombines the beams to generate the
interference pattern. We can adjust the fringe spacing
and the orientation by changing the tilt on the grazing-
incidence mirrors. The plasma that is to be studied is
introduced into one of the arms of the interferometer.
ASi
兾Sc multilayer mirror
10
of 30-cm focal length and
艐40% ref lectivity images the plasma with a magnifi-
cation of ⬃253 onto a microchannel plate (MCP) CCD
detector. A flat mirror (not shown in Fig. 1) was used
to relay the image.
The interferometer was prealigned with an infrared
(824-nm) semiconductor laser diode selected to have
a temporal coherence length of ⬃250 mm, similar to
that of the soft-x-ray laser. To allow the alignment
of the interferometer with the diode laser we designed
the gratings with two vertically separated rulings
on the same substrate. We ruled one part of the
substrate with 300 lines兾mm to diffract the 46.9-nm
laser beam and the other with 17.06 lines兾mm to
diffract the beam of the alignment diode. With this
ratio of groove spacing we obtained the same disper-
sion and therefore the same path for the beams of
the soft-x-ray laser and the alignment laser diode.
We motorized all the relevant movements in the
system to fine tune the interferometer once the
system was in vacuum. Interferograms with very
good visibility 共⬃0.5兲 were obtained over the entire
field of view. The interferometer showed excellent
stability.
The interferometer was used in combination with
a 46.9-nm capillary-discharge tabletop laser
11
to
map the electron-density evolution of a line-focused
laser-created plasma generated on a Cu slab target.
We generated the plasma by focusing a
Q-switched
Nd
:
YAG laser operating at its fundamental wavelength
of 1.06 mmintoan⬃30-mm-wide line focus ⬃2.7 mm in
length. The results discussed below correspond to the
study of a plasma created with 0.36-J Nd
:
YAG laser
pulses of ⬃13-ns FWHM duration. The probe-laser
pulses were generated by amplification of the 46.9-nm
line of Ne-like Ar in an Ar capillary plasma excited
by a fast-discharge current pulse. This tabletop laser
produces pulses with an energy of ⬃0.13 mJ and 1.2-ns
FWHM duration.
11
The far-field beam profile has
an annular shape and a peak-to-peak divergence of
⬃4.6 mrad.
Figure 2 shows a sequence of interferograms that
describe the evolution of the laser-created plasma.
The fringes are observed to shift with time owing
to the presence of the plasma. At the later times
the plasma cools, and absorption of the probe beam
by the plasma becomes noticeable. Electron-density
maps were computed from the measured number
of fringe shifts at each location, assuming that the
plasma is uniform along the line focus. Figure 3
illustrates a sequence of two-dimensional electron-
density distributions corresponding to the sequence
of interferograms shown in Fig. 2. The formation,
expansion, and decay of the plasma at the end of
the pump-laser pulse can be observed. We have
been able to probe the plasma at locations as close
as 25
30 mm from the target, where the electron
density is 艐5 3 10
19
cm
23
and the density gradient is
steep. Another series of measurements for 1.8-mm-
long line-focused plasmas created by 0.6-J Nd
:
YAG
laser pulses probed electron densities as great as
1 3 10
20
cm
23
. Ray-tracing computations show that
these measurements would be difficult to realize with
an ultraviolet laser, owing to strong refraction caused
by the large density gradients. This is illustrated in
Fig. 4, which shows that, although refraction of the
soft-x-ray laser probe is small, a hypothetical 265-nm
probe beam would be totally def lected out of the
plasma.
In conclusion, we have demonstrated a new soft-x-
ray interferometer that one can combine with any of the
currently available saturated soft-x-ray lasers to study
many high-density laboratory plasmas that are of sig-
nificant interest. The use of this interferometer with
Fig. 2. Sequence of soft-x-ray interferograms depicting
the evolution of a 2.7-mm-long line-focused plasma gen-
erated by a Nd
:
YAG laser (l 苷 1.06 mm, 0.36 mJ, 13 ns,
FWHM). The time delay with respect to the beginning of
the Nd
:
YAG laser pulse is shown in each interferogram.
358 OPTICS LETTERS / Vol. 25, No. 5 / March 1, 2000
Fig. 3. Electron-density distributions corresponding to
the interferograms of Fig. 2.
Fig. 4. Refraction of 46.9- and 265-nm probe beams propa-
gating through a plasma with the density gradient cor-
responding to the electron-density distribution measured
12.5 ns after initiation of the laser pulse. The probe
beams are assumed to enter the plasma parallel to the tar-
get at a distance of
30 mm from the surface.
shorter-wavelength soft-x-ray lasers of short pulse
duration will allow the probing of near-solid-density
plasmas.
This work was supported by U.S. Department of En-
ergy grant DE-FG03-98DP00208. The development
of the soft-x-ray laser was supported by the Na-
tional Science Foundation. M. Marconi acknowledges
the support of the Universidad de Buenos Aires and
the Consejo Nacional de Investigaciones Cient
´
ıficas y
T
´
ecnicas. We thank R. J. Berklund and O. E. Mar-
tinez for their contributions and A. Vinogradov, Yu.
Unspenskii, and their groups for the development of
the multilayer mirrors. J. J. Rocca’s e-mail address is
rocca@engr.colostate.edu.
*Permanent address, Departamento de Fisica, Fac-
ultad de Ciencias Exactas y Naturales, Universidad de
Buenos Aries, Buenos Aires, Argentina.
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