We studied theoretically the laser-plasma interaction, and performed experiments to investigate the mechanisms giving rise to optical damage in Borosilicate glass using nanosecond laser pulses at wavelength 1064 nm. Our experimental result shows that the optical damage process generated by nanosecond laser pulses is the result of an optically induced plasma. The plasma is initiated when the laser irradiance frees electrons from the glass. Although it may be debated, the electrons are likely freed by multi-photon absorption and the number density grows via impact ionization. Later when the electron gas density reaches the critical density, the electron gas resonantly absorbs the laser beam through collective excitation since the laser frequency is equal to the plasma frequency. The laser energy absorbed through the collective excitation is much larger than the energy absorbed by multi-photon ionization and impact ionization. Our experimental result also shows the plasma survives until the end of the laser pulse and the optical damage occurs after the laser pulse ceases. The plasma decay releases heat to the lattice. This heat causes the glass to be molten and soft. It is only as the glass cools and solidifies that stresses induced by this process cause the glass to fracture and damage. We also show the experimental evidence of the change of the refractive index of the focusing region as the density of the electron gas changes from sub-critical to overcritical, and the reflection of the over-critical plasma. This reflection limits the electron gas density to be not much larger than the critical density.
Hexanitrostilbene (HNS) is a secondary explosive widely used in a variety of commercial and military applications, due
in part to its high heat resistivity. Degradation of HNS is known to occur through exposure to a variety of sources
including heat, UV radiation, and certain chemical compounds, all of which may lead to reduced performance.
Detecting the degradation of HNS within a device, however, has required destructive analyses of the entire device while
probing the HNS in only an indirect fashion. Specifically, the common methods of investigating this degradation include
wet chemical, surface area and performance testing of the devices incorporating HNS rather than a direct interrogation of
the material itself. For example, chemical tests frequently utilized, such as volatility, conductivity, and contaminant
trapping, provide information on contaminants present in the system rather than the chemical stability of the HNS. To
instead probe the material directly, we have pursued the use of optical methods, in particular infrared (IR) spectroscopy,
in order to assess changes within the HNS itself. In addition, by successfully implementing miniature silicon (Si)
waveguides fabricated at Sandia National Laboratories to facilitate this spectroscopic approach, we have demonstrated
that HNS degradation monitoring may take place in a non-destructive, in-situ fashion. Furthermore, as these waveguides
may be manufactured in a variety of configurations, this direct, non-destructive, approach holds promise for
incorporation into a variety of devices.
Hexanitrostilbene (HNS) is a widely used explosive, due in part to its high thermal stability. Degradation of HNS is
known to occur through UV, chemical exposure, and heat exposure, which can lead to reduced performance of the
material. Common methods of testing for HNS degradation include wet chemical and surface area testing of the material
itself, and performance testing of devices that use HNS. The commonly used chemical tests, such as volatility,
conductivity and contaminant trapping provide information on contaminants rather than the chemical stability of the
HNS itself. Additionally, these tests are destructive in nature. As an alternative to these methods, we have been
exploring the use of vibrational spectroscopy as a means of monitoring HNS degradation non-destructively. In
particular, infrared (IR) spectroscopy lends itself well to non-destructive analysis. Molecular variations in the material
can be identified and compared to pure samples. The utility of IR spectroscopy was evaluated using pressed pellets of
HNS exposed to DETA (diethylaminetriamine). Amines are known to degrade HNS, with the proposed product being a
σ-adduct. We have followed these changes as a function of time using various IR sampling techniques including
photoacoustic and attenuated total reflectance (ATR).
Sandia National Laboratories' program in high-power fiber lasers has emphasized development of enabling technologies
for power scaling and gaining a quantitative understanding of fundamental limits, particularly for high-peak-power,
pulsed fiber sources. This paper provides an overview of the program, which includes: (1) power scaling of diffraction-limited
fiber amplifiers by bend-loss-induced mode filtering to produce >1 MW peak power and >1 mJ pulse energy
with a practical system architecture; (2) demonstration of a widely tunable repetition rate (7.1-27 kHz) while
maintaining constant pulse duration and pulse energy, linear output polarization, diffraction-limited beam quality, and
<1% pulse-energy fluctuations; (3) development of microlaser seed sources optimized for efficient energy extraction; (4)
high-fidelity, three-dimensional, time-dependent modeling of fiber amplifiers, including nonlinear processes; (5)
quantitative assessment of the limiting effects of four-wave mixing and self-focusing on fiber-amplifier performance; (6)
nonlinear frequency conversion to efficiently generate mid-infrared through deep-ultraviolet radiation; (7) direct diode-bar
pumping of a fiber laser using embedded-mirror side pumping, which provides 2.0x higher efficiency and much
more compact packaging than traditional approaches employing formatted, fiber-coupled diode bars; and (8)
fundamental studies of materials properties, including optical damage, photodarkening, and gamma-radiation-induced
We summarize the performance of mode-filtered, Yb-doped fiber amplifiers seeded by microchip lasers with
nanosecond-duration pulses. These systems offer the advantages of compactness, efficiency, high peak power,
diffraction-limited beam quality, and widely variable pulse energy and repetition rate. We review the fundamental limits
on pulsed fiber amplifiers imposed by nonlinear processes, with a focus on the specific regime of nanosecond pulses.
Different design options for the fiber and the seed laser are discussed, including the effects of pulse duration,
wavelength, and linewidth. We show an example of a microchip-seeded, single-stage, single-pass fiber amplifier that
produced pulses with 1.1 MW peak power, 0.76 mJ pulse energy, smooth temporal and spectral profiles, diffractionlimited
beam quality, and linear polarization.
Recent EPA regulations targeting mercury (Hg) emissions from utility coal boilers have prompted increased activity in
the development of reliable chemical sensors for monitoring Hg emissions with high sensitivity, high specificity, and
fast time response. We are developing a portable, laser-based instrument for real-time, stand-off detection of Hg
emissions that involves exciting the Hg (6 <sup>3</sup>P<sub>1</sub> ←6 <sup>1</sup>S<sub>0</sub>) transition at 253.7 nm and detecting the resulting resonant
emission from Hg (6 <sup>3</sup>P<sub>1</sub>). The laser for this approach must be tunable over the Hg absorption line at 253.7 nm, while
system performance modeling has indicated a desired output pulse energy ≥0.1 μJ and linewidth ≤5 GHz (full width at
half-maximum, FWHM). In addition, the laser must have the requisite physical characteristics for use in coal-fired
power plants. To meet these criteria, we are pursing a multistage frequency-conversion scheme involving an optical
parametric amplifier (OPA). The OPA is pumped by the frequency-doubled output of a passively Q-switched,
monolithic Nd:YAG micro-laser operating at 10-Hz repetition rate and is seeded by a 761-nm, cw distributed-feedback
diode laser. The resultant pulse-amplified seed beam is frequency tripled in two nonlinear frequency-conversion steps to
generate 253.7-nm light. The laser system is mounted on a 45.7 cm × 30.5 cm breadboard and can be further condensed
using custom optical mounts. Based on simulations of the nonlinear frequency-conversion processes and current results,
we expect this laser architecture to exceed the desired pulse energy. Moreover, this approach provides a compact, all-solid-
state source of tunable, narrow-linewidth visible and ultraviolet radiation, which is required for many chemical
We describe the design and performance of a high-repetition-rate single-frequency passively Q-switched Yb:YAG
microlaser operating near 1030 nm. By using short cavity length, an intracavity Brewster polarizer, and an etalon output
coupler, we are able to produce ~1-ns-long single-frequency pulses at repetition rates up to 19 kHz without shot-to-shot
mode hopping. The laser's output spatial mode is TEM<sub>00</sub> and its pulse energy varies between 31 μJ and 47 μJ depending
on repetition rate. Its peak optical-to-optical efficiency is 22%.
We report results from Yb-doped fiber amplifiers seeded with two microchip lasers having 0.38-ns and 2.3-ns pulse durations. The shorter duration seed resulted in output pulses with a peak power of >1.2 MW and pulse energy of 0.67 mJ. Peak power was limited by nonlinear processes that caused breakup and broadening of the pulse envelope as the pump power increased. The 2.3-ns duration seed laser resulted in output pulses with a peak power of >300 kW and pulse energy of >1.1 mJ. Pulse energies were limited by the onset of stimulated Brillouin scattering and ultimately by internal optical damage (fluences in excess of 400 J/cm<sup>2</sup> were generated). In both experiments, nearly diffraction-limited beam profiles were obtained, with M<sup>2</sup> values of <1.2. Preliminary results of a pulse-amplification model are in excellent agreement with the experimental results of the amplifiers operating in the low-to-moderate gain-depletion regime.
Optical firing sets need miniature, robust, reliable pulsed laser sources for a variety of triggering functions. In many cases, these lasers must withstand high transient radiation environments. In this paper we describe a monolithic passively Q-switched microlaser constructed using Cr:Nd:GSGG as the gain material and Cr<sup>4+</sup>:YAG as the saturable absorber, both of which are radiation hard crystals. This laser consists of a 1-mm-long piece of undoped YAG, a 7-mm-long piece of Cr:Nd:GSGG, and a 1.5-mm-long piece of Cr<sup>4+</sup>:YAG diffusion bonded together. The ends of the assembly are polished flat and parallel and dielectric mirrors are coated directly on the ends to form a compact, rugged, monolithic laser. When end pumped with a diode laser emitting at ~807.6 nm, this passively Q-switched laser produces ~1.5-ns-wide pulses. While the unpumped flat-flat cavity is geometrically unstable, thermal lensing and gain guiding produce a stable cavity with a TEM<sub>00</sub> gaussian output beam over a wide range of operating parameters. The output energy of the laser is scalable and dependent on the cross sectional area of the pump beam. This laser has produced Q-switched output energies from several μJ per pulse to several 100 μJ per pulse with excellent beam quality. Its short pulse length and good beam quality result in high peak power density required for many applications such as optically triggering sprytrons. In this paper we discuss the design, construction, and characterization of this monolithic laser as well as energy scaling of the laser up to several 100 μJ per pulse.
Environmental fate and transport studies of explosives in soil indicate that 2,4,6-trinitrotoluene (TNT) and similar products such as dinitrotoluene (DNT) are major contributors to the trace chemical signature emanating from buried landmines. Chemical analysis methods are under development that have great potential to detect mines, or to rapidly classify electromagnetically detected anomalies as mines vs. 'mine-like objects'. However, these chemical methods are currently confined to point sensors. In contrast, we have developed a method that can remotely determine the presence of nitroaromatic explosives in surface soil. This method utilizes a novel distributed granular sensor approach in combination with uv-visible fluorescence LIDAR (Light Detection and Ranging) technology. We have produced prototype sensor particles that combine sample preconcentration, explosives sensing, signal amplification, and optical signal output functions. These particles can be sprayed onto soil areas that are suspected of explosives contamination. By design, the fluorescence emission spectrum of the distributed particles is strongly affected by absorption of nitroaromatic explosives from the surrounding environment. Using ~1mg/cm2 coverage of the sensor particles on natural soil, we have observed significant spectral changes due to TNT concentrations in the ppm range (mg TNT/kg soil) on 2-inch diameter targets at a standoff distance of 0.5 km.
A prototype of an unattended ground sensor has been developed for detection of biological agent aerosols. This point sensor uses ultraviolet laser induced fluorescence (UV LIF) to detect aerosol biological microorganisms collected on filter media. The concept can be designed to be compact, low power, and hardened to survive harsh delivery environments such as airdrop. The prototype consists of an air sampling system, a filter exchange mechanism, an Nd:YAG microlaser that is frequency tripled and quadrupled to generate 355-nm and 266-nm excitation wavelengths, a spectrometer, an intensified CCD detector, and a data acquisition and control system. The analysis utilizes a spectral database of fluorescence signatures of biological organisms and common interferents measured by Sandia for the Army's Edgewood Research and Development Engineering Center (ERDEC) and the Department of Energy's Chemical and Biological Non-proliferation (DOE CBNP) program. The analysis algorithms are based on algorithms developed by Sandia for an airborne UV LIF lidar system.
The development of a mid-infrared cavity ringdown spectrometer for trace gas measurements is described. The device employs a novel light source based on periodically poled lithium niobate (PPLN). Narrow linewidth (<EQ 0.08 cm<SUP>-1</SUP> FWHM) mid-infrared radiation (at energies up to 15 (mu) J) is generated by three serial elements: a broadband optical parametric generator, a tunable spectral filter, and an optical parametric amplifier. Currently, spectral filtering is accomplished by an air-spaced Fabry-Perot etalon that allows 15 cm<SUP>-1</SUP> of narrowband continuous tuning anywhere between 6200 - 6780 cm<SUP>-1</SUP> and 3200 - 2620 cm<SUP>-1</SUP>. This can, in principle, be extended to the entire PPLN transparency window (2220 - 7690 cm<SUP>-1</SUP>) using multiple PPLN crystals and a suitable tuning element. The high gain of PPLN allows pumping by compact, high-repetition-rate solid-state laser sources, thereby minimizing the sensor size and allowing rapid spectral scans. Operation is demonstrated using both a 1 kHz Nd:YAG and a novel 120 Hz passively Q-switched Nd:YAG microlaser. Performance of the cavity ringdown sensor is characterized in terms of sensitivity, spectral coverage (segmented scans up to 350 cm<SUP>-1</SUP> long), measurement speed, and measurements in the presence of atmospheric background gases. Issues relevant to the ultimate portable implementation of the sensor are addressed, including the use of two alternative frequency filtering/tuning mechanisms (a fiber-optic etalon and an acousto-optically tunable filter plus an air-speed etalon) and implementation of frequency calibration.
Laser plasma sources convert 1 - 2% of the incident laser energy into soft x rays that can be used in multilayer-based reflective systems. These sources are useful in the laboratory for development of soft-x-ray projection lithography (SXPL). In the commercialization of SXPL technology, the laser plasma source offers the advantages of modularity and lower cost, when compared to the alternative synchrotron source. The characteristics of the source define requirements for other system components. The condensing system, which collects radiation from the plasma source and directs it onto the mask, must be designed to match the source size and the aperture of the imaging objective. The first surface of the condenser is subject to damage by unwanted debris from the plasma source. This paper discusses several of the major issues involved in using laser plasma sources in SXPL experiments and provides examples of experimental solutions. Simulated and actual soft-x-ray images are shown.
Thermally induced index of refraction gradients and their effect on the performance of diode-laser-pumped monolithic Nd:glass lasers are discussed. A simple technique for modeling thermal effects in these lasers is described. This model is useful in optimzing the cavity design and selecting the best glass for the monolithic ring laser.
Electronic imaging of laser induced fluorescence from a plane of laser light that intersects a reactive flow is becoming commonplace. Quite often, the fluorescence wavelength is longer than the laser excitation wavelength and hence the fluorescence is easily discriminated from the Rayleigh and Mie scattering, which is at the laser wavelength. In the case of resonance fluorescence, the fluorescence is sufficiently near the laser excitation wavelength that low fluorescent signals are obscured by Rayleigh and Mie scattering. However, recognizing that the fluorescence scattering is weakly polarized while the Rayleigh scattering light is strongly polarized suggests that a polaroid filter could improve the signal to noise by eliminating Rayleigh scattered light and passing half of the fluorescent scattered light. By rotating the polaroid filter, any amount of Rayleigh scattering and resonance fluorescence from CH as it occurs in the flame front of premixed methane flames.
The construction and operation of a diode-pumped Nd:YLF monolithic mini-laser operating at 1.053 μm is described along with its application of injection seeding a CW flashlamp pumped Nd:YLF Q-switched oscillator to produce single frequency pulses. Alignment of the mini-laser axis to the "c" axis of the Nd:YLF crystal allows lasing at 1.053 μm in a monolithic configuration. The Q-switched oscillator when seeded produces single frequency pulses 90% of the time.