Randal L. Schmitt

Diode-laser-pumped Nd:YAG laser injection seeding system

We have designed and tested a compact injection seeding system consisting of a diode-laser-pumped Nd:YAG master oscillator and a permanent-magnet Faraday isolator. With active resonator frequency stabilization, this system permits highly reliable single-axial-mode operation of a Q-switched Nd:YAG laser over a period of hours. The system is capable of injection seeding both stable and unstable resonator designs and is suitable for injection seeding commercial lasers with only minor modifications.

Instantaneous three-dimensional concentration measurements in turbulent jets and flames

An experiment is described in which the instantaneous three-dimensional gas-concentration distributions in turbulent jets and flames were recorded. A resonant scanning mirror was used to sweep a laser sheet through the volume of a flow field. During the 1.4-μsec duration of the laser pulse, an electronic framing camera imaged the scattered light from the gas in as many as 12 parallel planes within the flow. A two-dimensional charge-coupled-device array was used to digitize the images from the framing camera in real time. The measurement period was brief enough to freeze the motion of the gas.

Gain switching of a monolithic single-frequency laser-diode-excited Nd:YAG laser

We report the use of gain switching to obtain 60 mW of single-longitudinal-mode peak output power from a laser-diode-excited monolithic Nd:YAG laser. The device is demonstrated to operate at repetition rates in excess of 1 kHz and exhibits a spectral linewidth of less than 8 MHz. This oscillator provides an ideal source for injection seeding of laboratory Nd:YAG laser systems.

Microlaser-pumped periodically poled lithium niobate optical parametric generator–optical parametric amplifier

For what is believed to be the first time, a single-longitudinal-mode passively Q-switched Nd:YAG microlaser is used to pump a narrow-bandwidth periodically poled lithium niobate (PPLN) optical parametric generator-optical parametric amplifier (OPG-OPA). Before amplification in the OPA, the output of the OPG stage was spectrally filtered with an air-spaced etalon, resulting in spectroscopically useful radiation (bandwidth, ~0.05 cm(-1) FWHM) that was tunable in 15-cm(-1) segments anywhere in the signal range 6820-6220 cm(-1) and the idler range 2580-3180 cm(-1). The ability to pump an OPG-OPA with compact, high-repetition-rate, intrinsically narrow-bandwidth microlasers is made possible by the high gain of PPLN. The result is a tunable light source that is well suited for use in portable spectroscopic gas sensors.

Two-wavelength single laser CH and CH(4) imaging in a lifted turbulent diffusion flame.

A new technique has been developed which allows simultaneous 2-D mapping of CH and CH 4 in a turbulent methane flame. A flashlamp-pumped dye laser using two back mirrors produces output at 431.5 and 444 nm simultaneously. The 431.5-nm line is used to excite the (0, 0) band of the A(2)Delta-X(2)Pi system of CH, and the fluorescence of the (0, 1) transition is observed at 489 nm. Coincidentally, the spontaneous Raman scattering from CH(4) also occurs near 489 nm for a 431.5-nm excitation. To separate the CH(4) and CH contributions, the 444-nm line is used to produce a spontaneous Raman signal from CH(4) that is spectrally separated from the CH fluorescence. Subtraction of the signals generated by the 431.5- and 444-nm wavelength beams yields separate measurements of CH(4) and CH. Raman-scattered light records the instantaneous distribution of the fuel, and simultaneously the CH fluorescence indicates the location of the flame zone. The resulting composite images provide important insight on the interrelationship between fuel-air mixing and subsequent combustion.M. Namazian is with Altex Technologies Corporation, 109 Via De Tesoros, Los Gatos, California 95030; R. L. Schmitt is with Sandia National Laboratories, Combustion Research Facility, Livermore, California 94550; and M. B. Long is with Yale University, Department of Mechanical Engineering, New Haven, Connecticut 06520.

High-peak-power (>1.2 MW) pulsed fiber amplifier.

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/cm2 were generated). In both experiments, nearly diffraction-limited beam profiles were obtained, with M2 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.

Diode-laser-pumped monolithic Nd:YLF laser operating at 1.053 μm

We describe a monolithic diode-laser-pumped Nd:YLF minilaser that operates at 1.053 microm. By aligning the c axis of the Nd:YLF crystal parallel to the laser resonator axis we are able to suppress completely lasing on the (higher-gain) 1.047-microAtmra nsition of Nd:YLE We describe the design and fabrication of the laser and report its cw and gain-switched performance. Our measurements of the laser performance agree well with calculations.

Development of an unattended ground sensor for ultraviolet laser-induced fluorescence detection of biological agent aerosols

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 Armys Edgewood Research and Development Engineering Center (ERDEC) and the Department of Energys 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.

Frequency-locked, injection-seeded, pulsed narrowband optical parametric generator

A frequency-locked, injection-seeded, pulsed optical parametric generator (OPG) has been developed for short-range infrared differential absorption lidar (DIAL) applications. The periodically poled lithium niobate OPG is pumped by a passively Q-switched Nd:YAG microlaser and is seeded by a distributed feedback (DFB) diode laser. The OPG is designed for DIAL measurement of a narrow R-branch transition of methane at 3.2704 microm. The output of the OPG is a two-pulse sequence with a 100-micros temporal separation between the pulses, where the first pulse is absorbed by methane and the second pulse is not absorbed. The first pulse is actively locked to the methane absorption feature by use of the derivative of the transmission spectrum through a reference cell. Although the device was not optimized for output power, the 3.27-microm OPG output energies of the first and second pulses are 5.5 and 5.9 microJ, respectively, producing 21 mW when operated at 1818 Hz.

Power scaling of fiber-based amplifiers seeded with microchip lasers

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.