We report an all-fiber passively Q-switched laser using a large mode area (LMA) Yb3+ -doped fiber claddingpumped at 915 nm and an unpumped single-mode (SM) Yb3+-doped fiber as the saturable absorber (SA). The saturable absorber SM fiber and LMA gain fiber were coupled with a fiber taper designed to match the fundamental spatial mode of the LMA fiber and the expanded LP01 mode of the single mode fiber. The amplified spontaneous (ASE) intensity propagating in the single mode SA saturates the absorption before the onset of gain depletion in the pumped fiber, switching the fiber cavity to a high Q-state and producing a pulse. Using this scheme we demonstrate a Q-switched all-fiber oscillator with 32 μJ 93 ns pulses at 1030 nm. The associated peak power is nearly two orders of magnitude larger than that reported in previous experimental studies using a single Yb+3 saturable absorber fiber. The pulse energy was amplified to 0.230 mJ using an Yb3+-doped cladding pumped fiber amplifier fusion spliced to the fiber oscillator, increasing the energy by eight fold while preserving the all-fiber architecture.
We report a passively Q-switched all-fiber laser using a large mode area (LMA) Yb3+-doped fiber cladding-pumped at 915 nm and an unpumped single-mode Yb3+-doped fiber as the saturable absorber (SA). The saturable absorber and gain fibers were first coupled with a free-space telescope to better study the composite system, and then fusion spliced with fiber tapers to match the mode field diameters. ASE generated in the LMA gain fiber preferentially bleaches the SA fiber before depleting the gain, thereby causing the SA fiber to act as a passive saturable absorber. Using this scheme we first demonstrate a Q-switched oscillator with 40 μJ 79 ns pulses at 1026 nm using a free-space taper, and show that pulses can be generated from 1020 nm to 1040 nm. We scale the pulse energy to 0.40 mJ using an Yb3+-doped cladding pumped fiber amplifier. Experimental studies in which the saturable absorber length, pump times, and wavelengths are independently varied reveal the impact of these parameters on laser performance. Finally, we demonstrate 60 μJ 81 ns pulses at 1030 nm in an all fiber architecture using tapered mode field adaptors to match the mode filed diameters of the gain and SA fibers.
We describe a photofragment laser-induced fluorescence (PF-LIF) method that can be applied to the short-range-standoff
detection of low-volatility organophosphonate chemical warfare agents (OP-CWAs) on surfaces. It operates by
photofragmenting a surface-bound analyte and then actively interrogating a released phosphorous monoxide (PO)
fragment using LIF. We demonstrate a single-pulse-pair (pump = 500 μJ @ 266 nm; probe = 20 μJ @ 248 nm) surface
detection sensitivity of 30 μg/cm2 for the organophosphonate diisopropyl isothiocyanate phosphonate (DIPP) on
aluminum and 210 μg/cm2 for the same analyte on a more porous concrete surface. By detecting the PO photofragment,
the method indicates the presence of organophosphonates; however, we show that it also responds to other phosphorouscontaining
compounds. Because of its limited specificity, we believe that the method may have most immediate use as a
mapping tool to rapidly identify "hotspots" of OP-CWAs. These would then be confirmed using a more specific tool. As
one method of confirming the presence of OP-CWAs (and identifying the agent), we demonstrate that the probe beam
can be used to acquire Raman-scattering spectra of the target area.
In situ spatial and temporal surface temperature profiles of CO2 laser-heated silica were obtained using a
long wave infrared (LWIR) HgCdTe camera. Solutions to the linear diffusion equation with volumetric and surface
heating are shown to describe the temperature evolution for a range of beam powers, over which the peak surface
temperature scales linearly with power. These solutions were used with on-axis steady state and transient
experimental temperatures to extract thermal diffusivity and conductivity for a variety of materials, including silica,
spinel, sapphire, and lithium fluoride. Experimentally-derived thermal properties agreed well with reported values
and, for silica, thermal conductivity and diffusivity are shown to be approximately independent of temperature
between 300 and 2800K. While for silica our analysis based on a temperature independent thermal conductivity is
shown to be accurate, for other materials studied this treatment yields effective thermal properties that represent
reasonable approximations for laser heating. Implementation of a single-wavelength radiation measurement in the
semi-transparent regime is generally discussed, and estimates of the apparent temperature deviation from the actual
outer surface temperature are also presented. The experimental approach and the simple analysis presented yield
surface temperature measurements that can be used to validate more complex physical models, help discriminate
dominant heat transport mechanisms, and to predict temperature distribution and evolution during laser-based
material processing.
As part of the U.S. Department of Homeland Security Detect-to-Protect (DTP) program, a multilab [Sandia National
Laboratories (SNL), Lawrence Livermore National Laboratories (LLNL), Pacific Northwest National Laboratory
(PNNL), Oak Ridge National Laboratory (ORNL), and Los Alamos National Laboratory (LANL)] effort is addressing
the need for useable detect-to-warn bioaerosol sensors for public facility protection. Towards this end, the SNL team is
investigating the use of rapid fluorogenic staining to infer the protein content of bioaerosols. This is being implemented
in a flow cytometer wherein each particle detected generates coincident signals of correlated forward scatter, side
scatter, and fluorescence. Several thousand such coincident signal sets are typically collected to generate a distribution
describing the probability of observing a particle with certain scattering and fluorescence values. These data are
collected for sample particles in both a stained and unstained state. A linear unmixing analysis is performed to
differentiate components in the mixture. In this paper, we discuss the implementation of the staining process and the
cytometric measurement, the results of their application to the analysis of known and blind samples, and a potential
instrumental implementations that would use staining.
As part of the U.S. Department of Homeland Security Detect-to-Protect program, a multilab [Sandia National
Laboratories (SNL), Lawrence Livermore National Laboratories (LLNL), Pacific Northwest National Laboratory
(PNNL), Oak Ridge National Laboratory (ORNL), and Los Alamos National Laboratory (LANL)] effort is addressing
the need for useable detect-to-warn bioaerosol sensors for public facility protection. Towards this end, the SNL team is
employing rapid fluorogenic staining to infer the protein content of bioaerosols. This is being implemented in a flow
cytometry platform wherein each particle detected generates coincident signals of forward scatter, side scatter, and
fluorescence. Several thousand such coincident signal sets are typically collected to generate a probability distribution
over the scattering and fluorescence values. A linear unmixing analysis is performed to differentiate components in the
mixture. After forming a library of pure component distributions from measured pure material samples, the distribution
of an unknown mixture of particles is treated as a linear combination of the pure component distributions. The
scattering/fluorescence probability distribution data vector a is considered the product of two vectors, the fractional
profile f and the scattering/fluorescence distributions from pure components P. A least squares procedure minimizes the
magnitude of the residual vector e in the expression a = fPT + e. The profile f designates a weighting fraction for each
particle type included in the set of pure components, providing the composition of the unknown mixture. We discuss
testing of this analysis approach and steps we have taken to evaluate the effect of interferents, both known and unknown.
The advent of novel quasi-phase matched materials based on patterned growth gallium arsenide offer the possibility of
broadly tunable IR sources covering the long- (5-12μm) and mid-wave (3-5μm) infrared spectral regions. From the
standpoint of chemical sensing, the long-wave infrared region between 8-12μm is attractive since it is an atmospheric
window, many functional groups absorb in this region and absorptions tend to be strong compared to the mid-IR. We
are employing orientation patterned GaAs as part of cw difference frequency spectrometer. In this system, light from
two, tunable external cavity diode lasers covering the 1.3μm and 1.5μm telecom bands was amplified then mixed in an
orientation- patterned GaAs crystal, producing radiation in the 7-9μm region. The system serves as a source for a cw
cavity ring-down spectrometer for ultra-trace gas detection applications. The combined tunability of the source,
coupled with the sensitivity of cavity ringdown spectroscopy will allow both detection and identification of a wide
range of species with unprecedented performance.
The design and successful fielding of sensors and detectors vital for homeland security can benefit greatly by the use of advanced signal and image processing techniques. The intent is to extract as much reliable information as possible despite noisy and hostile environments where the signals and images are gathered. In addition, the ability to perform fast analysis and response necessitate significant compression of the raw data so that they may be efficiently transmitted, remotely accumulated from different sources, and processed. Proper decompositions into compact representations allow fast pattern detection and pattern matching in real time, in situ or otherwise. Wavelets for signals and curvelets for images or hyperspectral data promise to be of paramount utility in the implementation of these goals. Together with statistical modeling and iterative thresholding techniques, wavelets, curvelets and multiresolution analysis can alleviate the severity of the requirements which today’s hardware designs can not meet in order to measure trace levels of toxins and hazardous substances. Photonic or electrooptic sensor and detector designs of the future, for example, must take into account the end game strategies made available by advanced signal and image processing techniques. The promise is the successful operation at lower signal to noise ratios, with less data mass and with deeper statistical inferences made possible than with boxcar or running averaging techniques (low pass filtering) much too commonly used to deal with noisy data at present. SPREE diagrams (spectroscopic peak reconstruction error estimation) are introduced in this paper to facilitate the decision of which wavelet filter and which denoising scheme to use with a given noisy data set.
A new Optical Parametric Oscillator for the mid infrared wavelength region of 3-3.8 micrometer is developed with an idler output power of more than 1 Watt. The OPO is pumped with a 10 Watt, cw, Nd:YAG Laser and consists of a bowtie ring cavity (FSR 320 MHz) resonating at the signal wavelength. The wavelength is controlled with a fan-out PPLN crystal and a low finesse intracavity Fabry-Perot. The idler output could be tuned over more than 24 Ghz modehop-free, by tuning the pump laser and keep the OPO cavity fixed. Mode hop tuning over 100 Ghz could be obtained changing the length of the intracavity Fabry-Perot. This high power OPO is combined with photoacoustic spectroscopy in order to develop a sensitive trace gas detector for LifeScience applications.
Lightsources employing quasiphasematched (QPM) nonlinear materials have demonstrated unique attributes for chemical sensing in the near- to mid-infrare spectral range (1 - 5 micrometers ). The advent of patterned-growth GaAs allows the first practical extension of QPM materials to operation in the long-wave IR (5 - 12 micrometers ). That wavelength range is particularly attractive for chemical sensing because it contains an atmospheric window, many molecular groups absorb there at distinct frequencies, and their absorptions tend to be strong relative to those in the near- and mid-IR. Here, the application of orientation-patterned GaAs (OPGaAs) for use in a continuous wave (cw) difference frequency spectrometer is described. The outputs of two external- cavity diode lasers operating in the 1.3 and 1.5 micrometers telecom bands are mixed in a OPGaAs crystal, producing tunable radiation at wavelengths near 8 micrometers . The application of the source to the measurement of a water vapor rovibrational absorption line is presented.
Photoacoustic spectroscopy is a sensitive, on-line and non- invasive tool to monitor concentrations of trace gases in ambient air. With the appropriate high power lasers in the mid-IR wavelength region gas mixtures can be analyzed, at and below the part per billion level. Within the development of novel IR laser sources, a continuous wave optical parametric oscillator based on periodically poled lithium niobate in combination with photoacoustic detection has been applied to detect traces of several hydrocarbons in nitrogen. At an idler wavelength of around 3.3 micrometers , the cw OPO produced approximately 300 mW of single mode radiation. Preliminary results show detection limits on methane, ethane, butane and pentane of around 1 ppb. This trace gas detector will be used within medical applications. E.g., the trace gas composition of exhaled air is able to give information about a wide variety of processes in human body. In addition, such analysis has the potential to monitor processes non-invasive, on-line and fast for diagnostic purposes related to acute or chronic diseases.
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-1 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-1 of narrowband continuous tuning anywhere between 6200 - 6780 cm-1 and 3200 - 2620 cm-1. This can, in principle, be extended to the entire PPLN transparency window (2220 - 7690 cm-1) 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-1 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.
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