In this paper, we report record nanosecond output energies of gain-switched Cr:ZnSe lasers pumped by Q-switched
Cr:Tm:Ho:YAG (100 ns @ 2.096 μm) and Raman shifted Nd:YAG lasers (7 ns @ 1.906 μm). In these experiments we
used Brewster cut Cr:ZnSe gain elements with a chromium concentration of 8x1018 cm-3. Under Cr:Tm:Ho:YAG
pumping, the first Cr:ZnSe laser demonstrated 3.1 mJ of output energy, 52% slope efficiency and 110 nm linewidth
centered at a wavelength of 2.47 μm. Maximum output energy of the second Cr:ZnSe laser reached 10.1 mJ under H2 Raman shifted Nd:YAG laser pumping. The slope efficiency estimated from the input-output data was 47%.
We developed a cryogenically cooled Yb:YAG laser as the pump beam for a pulsed Raman laser based on CVD grown
diamond crystals. The Q-switched cryogenic Yb-doped YAG 1030 nm pump laser delivered 340 W at 40 kHz with
diffraction-limited beam quality, with an optical efficiency of 80%. The record average power of 24.5 W was generated
from the Raman laser at 1193 nm. Modeling of the performance confirmed the corresponding Raman gain coefficient,
13.5 cm/GW. The laser was operated at room temperature and under cryogenic cooling at 77 K, with equal performance.
The full potential of terahertz imaging systems for nondestructive aerospace imaging applications has not been realized
due to the lack of data linking damage and defects to terahertz signatures coupled with the complexity of modeling the
signatures. Terahertz systems (0.1 - 2.0 THz) may be ideally suited for NDI applications because of the ability of THz
radiation to penetrate through substances commonly found on the surfaces of aircraft structures while maintaining the
optical resolution required to detect defects. We will discuss several systems that we have used to study the signatures of
a set of target samples with known defects.
This study quantifies terahertz (THz) or sub-millimeter imaging performance during simulated rotary-wing brownout or
whiteout environments based on geographic location and recent/current atmospheric weather conditions. The
atmospheric conditions are defined through the Air Force Institute of Technology Center for Directed Energy
(AFIT/CDE) Laser Environmental Effects Definition and Reference or LEEDR model. This model enables the creation
of vertical profiles of temperature, pressure, water vapor content, optical turbulence, and atmospheric particulates and
hydrometeors as they relate to line-by-line layer extinction coefficient magnitude at wavelengths from the UV to the RF.
Optical properties and realistic particle size distributions for the brownout and whiteout particulates have been developed
for and incorporated into LEEDR for this study. The expected imaging performance is assessed primarily at a
wavelength of 454 μm (0.66 THz) in brownout conditions at selected geographically diverse land sites throughout the
world. Seasonal and boundary layer variations (summer and winter) and time of day variations for a range of relative
humidity percentile conditions are considered to determine optimum employment techniques to exploit or defeat the
environmental conditions. Each atmospheric particulate/hydrometeor is evaluated based on its wavelength-dependent
forward and off-axis scattering characteristics and absorption effects on the imaging environment. In addition to realistic
vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic
(percentile) format are used. Most evaluated scenarios are brownout environments over ranges up to 50 meters. At submillimeter
wavelengths and the short ranges studied, preliminary results indicate the main source of image degradation
in brownout conditions is water vapor content, even with visibility less than 10 m and strong optical turbulence.
Frequency Domain (FD) fluorimetry, capitalizes on the frequency response function of a fluorophore and offers independence from light scatter and excitation/emission intensity variations in order to extract the sample's fluorescent lifetime. Mercury vapor lamps, a common source of industrial facility lighting, emit radiation that overlaps the UV/blue absorption spectrum of many fluorophores and may be used as an efficient and portable excitation source. The AC power modulation of mercury vapor lamps modulates the lamp's intensity at 120 Hz (in the United States) and higher harmonics. The fluorescent lifetimes for 3 different materials (willemite, uranium doped glass and U3O8) are measured with conventional techniques and compared with the FD technique using the power harmonics from a mercury vapor lamp. The mercury lamp measurements agree to within 25% of the conventional methods.
Phase Fluorimetry, or Frequency Domain (FD) Fluorimetry, capitalizes on the phase delay from excitation modulation of fluorescent media and offers independence from light scatter and excitation/emission intensity variations in order to extract the sample's fluorescent lifetime. Samples which fluoresce in the UV are commonly excited with UV laser sources, which are not necessarily high power, portable devices. Mercury vapor lamps, a common source of industrial facility lighting, emit wavelengths (365 nm, 405 nm, and 436 nm) that overlap the UV/blue spectrum and may be used as an efficient and portable excitation source. Mercury vapor lamps show strong peak intensities at 120 Hz and higher harmonics, due to the modulation of facility power at 60 Hz in the United States. For this research effort, single exponential decay will be assumed and lifetime calculation will be performed by least squares analysis with corrections made for lamp intensity variations at the harmonics of facility power.
We investigate experimentally the threshold and slope efficiency of a vertical cavity surface- emitting semiconductor laser with half-wave spaced quantum wells as functions of the pump wavelength in the 700 - 900 nm. While most of these devices demonstrated to date have employed pump wavelengths in the range 700 - 760 nm for optimum absorption in the spacers between the quantum wells (and minimal absorption in the epitaxial mirror structure), we show that equally good performance be obtained using longer wavelengths appropriate for diode laser pumping, provided that the pump wavelength is chosen to match a resonant cavity mode. For maximum pumping efficiency using the latter technique a stabilized single-mode pump laser should be used.
Bismuth fluoride, BiF, a possible energy acceptor molecule in a chemically-pumped donor- acceptor electronic transition laser has been produced in a flow tube reactor. A high voltage discharge was used to produce bismuth and fluorine atoms which combined to form ground state BiF(X). Laser induced fluorescence studies confirmed that BiF was formed following the discharge. A vibrational temperature of 581 +/- 35 K was measured.