Christopher G. Brown

Ho-doped fiber for high energy laser applications

Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho3+ emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.1

Broadband measurements of the refractive indices of bulk gallium nitride

New measurements of the index of refraction for free-standing samples of gallium nitride are reported. A simple dispersive prism technique is used to obtain the birefringent indices from 500 to 5100 nm, covering most of the transparency range of this wide band gap semiconductor. Millimeter thick samples prepared by both ammonothermal growth and hydride vapor phase epitaxy are found to have nearly identical refractive indices. The observed dispersion fits well to a two-pole Sellmeier equation with an estimated overall accuracy of ± 0.002. Our results are found to be in good agreement with previous visible and near-IR measurements on thin-film GaN samples, however we observed significantly less dispersion in the mid-IR. Moderate heating of the samples also provided a new determination of dn/dT.

Atmosphere issues in detection of explosives and organic residues

This study makes a comparison of LIBS emission from molecular species in plasmas produced from organic residues on a non-metallic substrate by both a 5 ns Nd:YAG laser (1064 nm) and a 40 fs Ti:Sapphire laser (800 nm) in air and argon atmospheres. The organic samples analyzed had varying amounts of carbon, nitrogen, hydrogen, and oxygen in their molecular structure. The characterization was based on the atomic carbon, hydrogen, nitrogen, and oxygen lines as well as the diatomic species CN (B2Σ+ - X2Σ+) and the C2 (d3Πg - a3Πu). Principal Component Analysis (PCA) was used to identify similarities of the organic analyte via the emission spectra. The corresponding Receiver Operating Characteristics (ROC) curves show the limitations of the PCA model for the nanosecond regime in air.

Molecular signal as a signature for detection of energetic materials in filament-induced breakdown spectroscopy

Laser Induced Breakdown Spectroscopy (LIBS) by self-channeled femtosecond pulses is characterized for detection of energetic materials. Different polymers are spin coated on silicon wafers to provide a thin organic layer with controllable thickness ranging from 500 nm to 1 μm. Spectral analysis of atomic and molecular carbon emission shows CN molecular signal from samples that do not contain nitrogen. This can be explained by possible molecular recombination between native atomic carbon and atmospheric nitrogen. As a consequence, caution must be exercised when using spectral signatures based on CN emission for explosive detection by filament-induced LIBS.

Control of filamentation for enhancing remote detection with laser induced breakdown spectroscopy

We report on the use of a novel phase element to control the far-field intensity pattern generated by a high-peak-power, femtosecond laser. The pre-determined intensity pattern results in a well defined location of the filaments formed by the propagation of these beams through the atmosphere. This enhancement of the localization and repeatability of the intensity distribution can be extremely beneficial for laser induced breakdown spectroscopy (LIBS) of remote regions of interest.

Remote femtosecond laser induced breakdown spectroscopy (LIBS) in a standoff detection regime

The need for robust, versatile, and rapid analysis standoff detection systems has emerged in response to the increasing threat to homeland security. Laser Induced Breakdown Spectroscopy (LIBS) has emerged as a novel technique that not only resolves issues of versatility, and rapid analysis, but also allows detection in settings not currently possible with existing methods. Several studies have shown that femtosecond lasers may have advantages over nanosecond lasers for LIBS analysis in terms of SNR. Furthermore, since femtosecond pulses can travel through the atmosphere as a self-propagating transient waveguide, they may have advantages over conventional stand-off LIBS approaches1. Utilizing single and multiple femtosecond pulse laser regimes, we investigate the potential of femtosecond LIBS as a standoff detection technology. We examine the character of UV and visible LIBS from various targets of defense and homeland security interest created by channeled femtosecond laser beams over distances of 30m or more.

Detection and analysis of RF emission generated by laser-matter interactions

Plasmas produced by laser-matter interactions are a known source of electromagnetic radiation. However, little has been done to systematically study the electromagnetic radiation emitted from laser produced plasmas. It is our intent to provide detailed time and frequency domain measurements of such emitted radiation. An ultra-fast femtosecond high intensity laser and a superheterodyne receiver are employed to study laser-matter interactions for various materials in the frequency range 1-40GHz.

Discriminant analysis in the presence of interferences: Combined application of target factor analysis and a Bayesian soft-classifier

A method is described for performing discriminant analysis in the presence of interfering background signal. The method is based on performing target factor analysis on a data set comprised of contributions from analyte(s) and interfering components. A library of data from representative analyte classes is tested for possible contributing factors by performing oblique rotations of the principal factors to obtain the best match, in a least-squares sense, between test and predicted vectors. The degree of match between the test and predicted vectors is measured by the Pearson correlation coefficient, r, and the distribution of r for each class is determined. A Bayesian soft classifier is used to calculate the posterior probability based on the distributions of r for each class, which assist the analyst in assessing the presence of one or more analytes. The method is demonstrated by analyses performed on spectra obtained by laser induced breakdown spectroscopy (LIBS). Single and multiple bullet jacketing transfers to steel and porcelain substrates were analyzed to identify the jacketing materials. Additionally, the metal surrounding bullet holes was analyzed to identify the class of bullet jacketing that passed through a stainless steel plate. Of 36 single sample transfers, the copper jacketed (CJ) and non-jacketed (NJ) class on porcelain had an average posterior probability of the metal deposited on the substrate of 1.0. Metal jacketed (MJ) bullet transfers to steel and porcelain were not detected as successfully. Multiple transfers of CJ/NJ and CJ/MJ on the two substrates resulted in posterior probabilities that reflected the presence of both jacketing materials. The MJ/NJ transfers gave posterior probabilities that reflected the presence of the NJ material, but the MJ component was mistaken for CJ on steel, while non-zero probabilities were obtained for both CJ and MJ on porcelain. Jacketing transfer from a bullet to steel as the projectile passed through the steel also proved difficult to analyze. Over 50% of the samples left insufficient transfer to be identified. Transfer from NJ and CJ jacketing was successfully identified by posterior probabilities greater than 0.8.

Shock-wave generation in transparent media from ultra-fast lasers

Laser interactions with bulk transparent media have long been investigated for material processing applications involving ablation and shock wave generation in both the nanosecond and femtosecond pulse width regimes1. Shock waves have been studied in fused silica and other optical glasses but previously have been characterized by the morphology of the concurrent ablation. We perform ablation at distances of 30 meters using the non-linear self-channeling effect. Using silicon wafers as targets because of their clearly defined ablation zones, we examine the effect that the filament has on the thin SiO2 layer coating the wafers surface. It is observed that the surface layer experiences a shock wave resulting from the explosive forces produced by the plasma. The use of several laser pulses in burst mode operation leads to the observation of multiple shock fronts in the material, and the possibility of shock wave addition for higher damage. Optical interferometry will be used to characterize the shock wave dynamics, using both traditional means of focusing in the near field and at 30 meters using propagating self-channeled femtosecond pulses. The novelty of using self-channeling laser pulses for shock wave generation has many implications for military applications. These experiments are to be performed in our secure test range using intensities of 1014W/cm2 and higher incident on various transparent media. Interferometry is performed using a harmonic of the pump laser frequency. Experiments also include burst-mode operation, where a train of ultra-fast pulses, closely spaced in time, and novel new beam distributions, strike the sample.

Optical cooling in multi-level systems

A theoretical model for optical cooling is developed, which yields the overall efficiency of a single endpumped cooling system. This model includes the effects of background absorption and pump saturation, while in multi-level systems, the model accounts for the important energy transfer processes. Two-level efficiency is evaluated for the case of Yb:YAG and compared with a hypothetical three-level material with identical spectral properties. This model is readily modified to include more levels and different materials.