| DP-LIBS: Double-pulse
Breakdown Spectroscopy INTRODUCTION
Laser-Induced Breakdown Spectroscopy (LIBS) is an extremely powerful
and flexible analysis technique that utilizes the energy in a
short, intense laser pulse to vaporize or “ablate” a
small volume of sample material. The ablated target material
(whether solid, liquid, or gas) absorbs enough energy to vaporize
and then ionize the constituent atoms, creating a small cloud
of plasma (free ions and electrons) that expands rapidly. As
the cloud expands and cools, a significant fraction of the ions
recombine
to form excited atoms, which eventually decay via spontaneous emission
to the atomic ground state (the state of lowest
energy in the atom). The photons given off during spontaneous emission
can be collected and spectrally analyzed which
provides a “spectral fingerprint” of all the constituent
elements of the target and the plasma. Since each element has a unique
spectral fingerprint, relative and absolute
elemental concentrations within the target material can be determined.
Because the target material is interrogated by a laser beam and the
plasma is analyzed by “all–optical” techniques,
the analysis can be performed remotely and on hazardous materials.
PROGRAM
The Army Research Laboratory (ARL) at Aberdeen Proving Ground has
been actively engaged in the development of a stand-off LIBS capability
to counter the threat of terrorist attacks using unidentified explosives
for several years. In late 2005, ARL recognized that the desired
stand-off mode of operation would require a double pulse to generate
adequate plasma (in addition to short pulse duration and high beam
quality) for successful target atomic elements identification. Since
only a small solid angle is available when detecting an optical signal
from plasma at a distance, enhancing the optical signal would be
a must for stand-off LIBS. Research had shown that the double pulse
would enhance the laser produced plasma intensity by a factor of
100 times. Consequent efforts to employ dual lasers (to provide the
double pulse) showed the time interval between pulses could be adequately
controlled, but the precise pointing accuracy required could not
be satisfied. Therefore, in early 2006 the Army solicited Phase I
SBIR feasibility studies from industry on using a single laser to
produce the double pulse. JMAR responded, leveraging its proprietary
BriteLight™ technology
for producing intense laser plasma, and in September 2006 was the
only contractor awarded a Phase II contract to build and demonstrate
such a laser.
In May 2007, JMAR was able to demonstrate a prototype
double pulse laser which exceeded performance specifications.
TECHNOLOGY
In order to meet the Army’s requirements, JMAR developed a
new laser concept and redesigned the BriteLight’s Master Oscillator
(patent pending) resulting in a demonstrated double pulse operation
using two Pockels cells. The Pockels cells are used as fast optical
switches. Only a fraction of the energy stored in the laser’s
Nd:YAG rod is allowed to generate the first pulse by fast switching
(on and off). At a delayed time (1 -10 μs) the second Pockels
cell allows the remaining stored energy to generate the second laser
pulse. This technique permits full control over the ratio between
the first and second pulse and the delay time between the two pulses.
The laser pulse energy is then amplified using a similar MOPA configuration
to the BriteLight. JMAR demonstrated its double pulse prototype achieving
laser induced breakdown at 30 feet from the target (twice the specified
contractual goal) with 50 mJ per pulse.
SUMMARY
It is widely accepted that double-pulse LIBS gives significant optical
signal enhancement for detection. A single laser is preferred for
generating the double-pulse because of advantages in size, complexity,
and beam alignment over the dual laser approach. JMAR has developed
a single resonator double pulse laser under contract from the ARL,
demonstrating full control over the pulse separation, relative intensities
of first to second pulse, and pulse duration. Such a laser provides
a unique tool for LIBS characterization and optimization, and provides
the prototype for stand-off explosives and drug detection operations
at distances of 30 feet or more.
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