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We have developed preliminary conceptual laser system designs for the Laser ICF (Inertial Confinement Fusion) Fission Energy (LIFE) application. Our approach leverages experience in high-energy Nd:...

A tunable mid-infrared optical parametric oscillator has been demonstrated using a noncritically phase matched AgGaSe/sub 2/ crystal with pump wavelength tuning. Average power experiments have generated several hundred milliwatts of 2.63 and 3.71 /spl mu/m radiation using a high-repetition-rate, multiwatt 1.54 /spl mu/m pump source. Interferometric measurements of the AgGaSe/sub 2/ crystal during operation indicated only minor thermal lensing. >

A hybrid configuration of a heat-capacity laser (HCL) and a moving-slab laser (MSL) is proposed. Multiple volumes of solid-state laser material are sequentially diode-pumped and their energy extracted. When a volume reaches a maximum temperature after a “submagazine depth,” it is moved out of the pumping region into a cooling region, and a new volume is introduced. The total magazine depth equals the submagazine depth times the number of volumes. The design parameters are chosen to provide high duty factor operation, resulting in effective use of the diode arrays. The concept significantly reduces diode array cost and is considered enabling for many potential applications.

We have obtained nearly 1 J of blue-green radiation in 60-nsec pulses by Raman shifting an injection-locked XeCl laser in Pb vapor. The measured 50% Raman energy conversion has been observed in both oscillator and oscillator–amplifier experiments. These results indicate that an XeCl/Pb blue-green laser efficiency of >1.4% should be possible.

We report a new mode-locking technique, which greatly extends the number of dyes that can be used as saturable absorbers. When the absorber is enclosed in a high-Q transverse resonator its lifetime is reduced by stimulated emission to values that are sufficiently short to ensure complete recovery between passes. The technique has been demonstrated using rhodamine 6G for the amplifier and cresyl violet for the saturable absorber. In addition to its use for mode locking visible dye lasers, the technique also shows great promise for locking ultraviolet excimer lasers, such as krypton fluoride and xenon chloride lasers, using a visible dye for the slow saturable absorber.

Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National Ignition Facility (NIF) places stringent requirements on the control of each of the beam's pointing and overall wavefront quality. One quad of the NIF beams, 4 beam pairs, will be utilized for these experiments and hydrodynamic and particle-in-cell simulations indicate that for the fast ignition experiments, these beams will be required to deliver 50%(4.0 kJ) of their total energy(7.96 kJ) within a 40 μm diameter spot at the end of a fast ignition cone target. This requirement implies a stringent pointing and overall phase conjugation error budget on the adaptive optics system used to correct these beam lines. The overall encircled energy requirement is more readily met by phasing of the beams in pairs but still requires high Strehl ratios, Sr, and RMS tip/tilt errors of approximately one μrad. To accomplish this task we have designed an interferometric adaptive optics system capable of beam pointing, high Strehl ratio and beam phasing with a single pixilated MEMS deformable mirror and interferometric wave-front sensor. We present the design of a testbed used to evaluate the performance of this wave-front sensor below along with simulations of its expected performance level.

The fundamental power scaling limits for diffraction limited fiber lasers are reviewed. Relationships between the limits show there is an upper bound for single aperture power of conventional fiber lasers of 10-40 kW.

We report a differential synthetic aperture ladar (DSAL) concept that relaxes platform and laser requirements compared to conventional SAL. Line-of-sight translation/vibration constraints are reduced by several orders of magnitude, while laser frequency stability is typically relaxed by an order of magnitude. The technique is most advantageous for shorter laser wavelengths, ultraviolet to mid-infrared. Analytical and modeling results, including the effect of speckle and atmospheric turbulence, are presented. Synthetic aperture ladars are of growing interest, and several theoretical and experimental papers have been published on the subject. Compared to RF synthetic aperture radar (SAR), platform/ladar motion and transmitter bandwidth constraints are especially demanding at optical wavelengths. For mid-IR and shorter wavelengths, deviations from a linear trajectory along the synthetic aperture length have to be submicron, or their magnitude must be measured to that precision for compensation. The laser coherence time has to be the synthetic aperture transit time, or transmitter phase has to be recorded and a correction applied on detection.

Laboratory and field demonstration results obtained as part of the DARPA-sponsored coherent communications, imaging and targeting (CCIT) program are reviewed. The CCIT concept uses a phase conjugation engine based on a quadrature receiver array, a hologram processor and a spatial light modulator (SLM) for high-speed, digital beam control. Progress on the enabling MEMS SLM, being developed by a consortium consisting of LLNL, academic institutions and small businesses, is presented.

Laboratory breadboard results of a high-speed adaptive-optics system are presented. The wave-front sensor for the adaptive-optics system is based on a quadrature interferometer, which directly measures the turbulence-induced phase aberrations. The spatial light modulator used in the phase-conjugate engine was a microelectromechanical systems-based piston-only correction device with 1024 actuators. Laboratory experiments were conducted with this system utilizing Kolmogorov phase screens to simulate atmospheric phase distortions. The adaptive-optics system achieved correction speeds in excess of 800 Hz and Strehl ratios greater than 0.5 with the Kolmogorov phase screens.

Horizontal path correction of optical beam propagation presents a severe challenge to adaptive optics systems due to the short transverse coherence length and the high degree of scintillation incurred by propagation along these paths. The system presented operates with nearly monochromatic light. It does not require a global reconstruction of the phase, thereby eliminating issues with branch points and making its performance relatively unaffected by scintillation. The systems pixel count, 1024, and relatively high correction speed, in excess of 800 Hz, enable its use for correction of horizontal path beam propagation. We present results from laboratory and field tests of the system in which we have achieved Strehl ratios greater than 0.5.

Results of atmospheric propagation for a high-speed, large-actuator-number adaptive optics system are presented. The system uses a microelectromechanical system- (MEMS-) based spatial light modulator correction device with 1024 actuators. Tests over a 1.35-km path achieved correction speeds in excess of 800 Hz and Strehl ratios close to 0.5. The wave-front sensor was based on a quadrature interferometer that directly measures phase. This technique does not require global wave-front reconstruction, making it relatively insensitive to scintillation and phase residues. The results demonstrate the potential of large-actuator-number MEMS-based spatial light modulators to replace conventional deformable mirrors.

In conclusion, optical parametric oscillators (OPO)s constructed with KTiOPO/sub 4/, undoped KTiOAsO/sub 4/ (KTA), and AgGaSe/sub 2/ crystals were investigated for their use as mid-IR wavelength converters for a 1.047 pm Nd:YLF master oscillator power amplifier (MOPA). The two stage OPO conversion produced excellent beam quality and moderate average powers due to only minor crystal absorptions at the pump, signal, and idler wavelengths in AgGaSe/sub 2/.

Atmospheric induced aberrations can seriously degrade laser performance, greatly affecting the beam that finally reaches the target. Lasers propagated over any distance in the atmosphere suffer from a significant decrease in fluence at the target due to these aberrations. This is especially so for propagation over long distances. It is due primarily to fluctuations in the atmosphere over the propagation path, and from platform motion relative to the intended aimpoint. Also, delivery of high fluence to the target typically requires low beam divergence, thus, atmospheric turbulence, platform motion, or both results in a lack of fine aimpoint control to keep the beam directed at the target. To improve both the beam quality and amount of laser energy delivered to the target, Northrop Grumman has developed the Active Tracking System (ATS); a novel linear phase conjugation aberration compensation technique. Utilizing a silicon spatial light modulator (SLM) as a dynamic wavefront reversing element, ATS undoes aberrations induced by the atmosphere, platform motion or both. ATS continually tracks the target as well as compensates for atmospheric and platform motion induced aberrations. This results in a high fidelity, near-diffraction limited beam delivered to the target.

We demonstrate 3.80 µm idler generation with over 100% photon conversion efficiency from a pulsed 1.064 µm pump.

We have constructed a laser utilizing five 10 W cw diode bars to transverse-pump a Tm:YAG rod in a multiple-pass pump cavity. At an estimated rod temperature of 35 - 40 degree(s)C, 6.0 W of multi-mode and 3.8 W of single-mode output were obtained at 2.02 micrometers in a non- optimized resonator with 50 W incident pump power.

Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National Ignition Facility (NIF) places stringent requirements on the control of each of the beam's pointing, intra-beam phasing and overall wave-front quality. In this article experimental results are presented which were taken on an interferometric adaptive optics testbed that was designed and built to test the capabilities of such a system to control phasing, pointing and higher order beam aberrations. These measurements included quantification of the reduction in Strehl ratio incurred when using the MEMS device to correct for pointing errors in the system. The interferometric adaptive optics system achieved a Strehl ratio of 0.83 when correcting for a piston, tip/tilt error between two adjacent rectangular apertures, the geometry expected for the National ignition Facility. The interferometric adaptive optics system also achieved a Strehl ratio of 0.66 when used to correct for a phase plate aberration of similar magnitude as expected from simulations of the ARC beam line. All of these corrections included measuring both the upstream and downstream aberrations in the testbed and applying the sum of these two measurements in open-loop to the MEMS deformable mirror.

Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National Ignition Facility (NIF) places stringent requirements on the control of each of the beam's pointing and overall wavefront quality. One quad of the NIF beams, four beam pairs, will be utilized for these experiments and hydrodynamic and particle-in-cell simulations indicate that for the fast ignition experiments, these beams will be required to deliver 50% (4.0 kJ) of their total energy (7.96 kJ) within a 40-µm-diam spot at the end of a fast ignition cone target. This requirement implies a stringent pointing and overall phase conjugation error budget on the adaptive optics system used to correct these beam lines. The overall encircled energy requirement is more readily met by phasing of the beams in pairs but still requires high Strehl ratios and root-mean-square tip/tilt errors of approximately 1 µrad. To accomplish this task we have designed an interferometric adaptive optics system capable of beam pointing, high Strehl ratio, and beam phasing with a single pixilated microelectromechanical systems deformable mirror and interferometric wavefront sensor. We present the design of a testbed used to evaluate the performance of this wavefront sensor along with simulations of its expected performance level.

A new optical Doppler-velocimetry technique for seeded flow fields illuminated by a monochromatic laser light sheet has been developed. In contrast to conventional laser Doppler velocimetry based on scanned, point-by-point measurement, this technique yields global, simultaneous, quantitative visualization of the flow velocities over the illuminated plane. This technique is applied to a free-expansion jet at near sonic velocities in a small wind tunnel. 8 refs.

We analyze the scalability of diffraction-limited fiber lasers considering thermal, non-linear, damage and pump coupling limits as well as fiber mode field diameter (MFD) restrictions. We derive new general relationships based upon practical considerations. Our analysis shows that if the fiber's MFD could be increased arbitrarily, 36 kW of power could be obtained with diffraction-limited quality from a fiber laser or amplifier. This power limit is determined by thermal and non-linear limits that combine to prevent further power scaling, irrespective of increases in mode size. However, limits to the scaling of the MFD may restrict fiber lasers to lower output powers.

We introduce a novel phase-shifting pixellated interferometer based on a liquid-crystal spatial light modulator and simulate the expected performance. The phase-shifted frames are captured simultaneously, which reduces problems arising from vibrations and air turbulence. The liquid-crystal spatial light modulator is flexible and can be configured to provide a large number of phase-shift levels and geometries to reduce measurement error.

Experimental results are presented for an adaptive optics system based on a quadrature Twyman-Green interferometric wave-front sensor. The system uses a circularly polarized reference beam to form two interferograms with a pi/2 phase shift. The experiments conducted used Kolmogorov phase screens to simulate atmospheric phase distortions. Strehl ratio improvements by a factor of 8, to an absolute value of 0.45, are demonstrated.

The CdHg excimer has received attention as a possible efficient visible laser medium. In the experiments described here, a pulsed tunable laser was used to excite the Cd(5 3P1) atomic state, which forms CdHg excimers in three‐body collisions with Hg and Ar. Gain and absorption measurements were performed by using an argon‐ion laser to probe several wavelengths within the 470‐nm excimer emission band. Net absorption was observed at all wavelengths investigated. Acoustic and thermal disturbances which might mimic absorption have been carefully considered and it is determined that these are not important. We conclude that the CdHg excimer exhibits net absorption within its 470‐nm emission band. These results cast considerable doubt on the possibility of efficient laser action in CdHg.

A laser efficiency of 4.2% at 4.2‐J output has been obtained from a self‐sustained discharge‐pumped XeCl laser using a novel high‐voltage prepulse technique to provide efficient energy transfer from a low‐impedance transmission line to the load. High‐pressure discharge‐pumped lasers now offer a reliable, efficient, and scalable alternative to e beams for many applications in the visible and ultraviolet regions of the spectrum.

Narrowband ultraviolet radiation from a frequency‐doubled dye laser has been used to control the linewidth and polarization of a long pulse length, electron‐beam‐excited XeF laser. Linewidths of 0.004 nm have been achieved in the 353‐nm band of the XeF B→X laser transition. Over 90% of the energy in the free‐running laser pulse has been extracted in the narrowband injection‐locked pulse. Ratios of injection signal power to output laser power on the order of 1:5000 are adequate for efficient injection‐locking to occur.

A railgap has been developed which is triggerable by either corona or spark discharges. Trigger voltage and energy were 15 kV and 20 mJ, respectively. The switch was successfully tested over the range 15–35 kV, providing a channel spacing of about 1/cm.

Efficient Raman conversion of microsecond pulse XeF laser radiation into the blue‐green region via the second Stokes shift in hydrogen has been demonstrated using a Raman oscillator‐amplifier scheme. Strong depletion of the pump and the first Stokes radiation accompanied by a dominant second Stokes output was observed for the first time.

During the past few years major advances have been made, both in performance and theoretical understanding, of frequency converters based on the Raman effect. The impetus for the effort has come largely from the NAVY/DARPA interest in advanced blue-green communication systems, but other applications such as isotope separation and remote sensing have contributed as well. Parallel efforts exist in development of backward Raman pulse compressors for ICF applications, but these will not be discussed in this paper.

The property of Raman amplifiers for producing good beam quality Stokes output is investigated experiemntally. The theoretical basis of this effect is discussed.

We have carried out experimental and theoretical investigations of beam quality improvement in forward Raman amplifiers. In appropriate conditions, efficient conversion of laser beams of poor spatial quality into Stokes beams with nearly diffraction-limited wave fronts can be realized. The results were obtained using a configuration with parallel copropagating pump and Stokes beams. A discharge-pumped XeCl laser with an output of 0.5 J in a 60-ns pulse was used to carry out the experiments. In the present experiments, a near diffraction-limited low-energy Stokes beam was generated in a superfluorescent oscillator. The first Stokes order output of this oscillator was injected with the pump beam in a Raman amplifier with a Fresnel number as high as 150. The wave front of the pump was distorted using aberrators of variable strength and spatial frequency content. In conditions of amplifier Fresnel number and pump beam distortion where phase distortions on the pump did not lead to significant intensity distortions in the amplifier, the Stokes beam quality was not affected by the saturated amplification process. Pulse shapes and phase front interferograms were obtained for the pump and Stokes. The results are in good agreement with 3-D wave-optics code simulations.

We describe a new technique for studying thermally induced chemical transformations in cellulose. The apparatus consists of a carbon dioxide laser for heating, an IR thermometer, and an optical reflectance spectrometer for tracking the progressive discoloration of the sample. To illustrate the technique, we present measurements from a single piece of sample linen along five isotherms in the 200-290 degrees C range. We derive an algebraic expression for the reflectivity of the sample as a function of the areal concentrations of the chromophoric states produced at temperature. The results are then explained in terms of first-order chemical rate theory and a four-step model. From the measurements we derive the activation energies, Arrhenius constants, and reflectivities of the chromophoric states.

We report the results of a theoretical and experimental investigation of two important properties of Raman converters. The first property is Raman beam cleanup, which refers to the generation of high-quality Stokes beams in a Raman amplifier pumped by laser beams of poor spatial quality. The second is nearly distortion-free amplification of aberrated Stokes beams in a Raman amplifier pumped by good-quality laser beams. A parallel beam geometry was used with collinear pump and Stokes beams. In both cases, excellent energy-conversion efficiencies into the first Stokes order, of the order of 60%, were demonstrated at high amplifier gains of 100 to 1000. The spatial characteristics of the amplified beams were found to be in good agreement with model predictions.