This PDF file contains the front matter associated with SPIE Proceedings Volume 7836, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Protection of military aircraft from IR guided threats is paramount to ensure the survivability of aircrews, platforms, and to ensure mission success. At the foundation of all IRCM systems is the source; that component that provides the in-band radiant energy required for threat defeat. As such, source technology has evolved with IRCM technology to encompass the evolving systems architectures that encompass IRCM: 1) "Hot Brick" omni-directional sources; 2) arc lamps, and; 3) lasers. Lasers, as IRCM sources continue to evolve to meet the challenges of ever-evolving threats, superior techniques, economy of installation, and superior source technology. Lasers represent the single greatest advance in IRCM source technology and continue to evolve to meet ever more sophisticated threats. And have been used with great effect in all modern IRCM systems; evolving from frequency doubled CO2 lasers, to solid state lasers with OPOs, to semiconductor lasers including Quantum Cascade Lasers (QCLs); these last devices represent the latest advance in IRCM source technology offering all-band coverage, architectural simplicity, and economy of scale. While QCLs represent the latest advance in IRCM laser technology, fiber lasers show much promise in addressing multi-band operation as well as the ability to be coherently combined to achieve even greater output capability. Also, ultra-short pulse lasers are evolving to become practical for IRCM applications. Stay tuned ......

Performance requirements for laser sources, operating in the mid-IR, providing protection of airborne platforms from heat-seeking missiles are reviewed. The critical performance characteristic for a countermeasures laser is 'useful energy on target', which requires the laser to generate high brightness output in the appropriate spectral bands with rapid turn-on time. Integration with a compact beam director places an upper limit on the beam quality of the laser output. The key driver for the detailed laser design is to maximise the overall wallplug efficiency in order to minimise the complexity and volume, in turn maximising the reliability and reducing the cost. In particular routes to reduce the thermal management system for the laser produce the single largest improvement in overall wallplug efficiency, with the ultimate goal of realising a truly athermal laser. Candidate technologies for IR countermeasures lasers are briefly reviewed.

Among quasi-phase matching (QPM) materials, PPLN suffers from a limited transparency, strongly limiting both the output power and the beam quality above 4 μm. We are developing a new QPM technology based on Orientation- Patterned Gallium Arsenide (OP-GaAs) crystals, transparent up to 16 μm and showing excellent nonlinear and thermal properties and very low losses (<0.02 cm^-1). We demonstrated with such samples a high-repetition rate tunable OPO attractively pumped by a remote Thulium fiber laser and integrated in a 25×30×6 cm transportable head. A 3 W level output in the 3-5 μm range was obtained with a 53% efficiency and an unprecedented beam quality (M²=1.4), making this module most suited to study directed infrared countermeasures (DIRCM).

Improvement in hybrid vapour phase epitaxy growing techniques of quasi-phase-matched orientation-patterned GaAs (OP-GaAs) allows larger sample thickness and permits efficient operation as a mid-infrared optical parametric oscillator at Watt-level average output powers [1-3]. Especially its low absorption loss (- 0.01 cm^-1), its laser damage threshold comparable to ZGP (- 2 J/cm²) combined with a large nonlinear coefficient, a good thermal conductivity, excellent mechanical properties, and a wide transparency range (0.9-17 μm) are suitable properties for efficient non-critical phase matched OPOs. As there is no natural birefringence in GaAs, phase matching is independent of polarization and propagation direction, offering the ability to pump OP-GaAs with a variety of polarization states. Thus, even unpolarized or poorly polarized sources like simple fiber lasers have been efficiently used as pump sources [4-5]. The paper discuss the best OP-GaAs OPO results achieved, to our knowledge, using a Q-switched 2.09 μm Ho:YAG laser as pump source as well as results obtained with an OP-GaAs OPO directly pumped by a 2.09 μm Q-switched Tm,Ho:silica fiber laser. With a 2.09 μm Q-switched Ho:YAG fiber laser pump source up to 2.9 W of average output power was achieved at 20 kHz repetition rate, 3.9 W at 40 kHz and 4.9 W at 50 kHz. With a 2.09 μm Q-switched Tm³⁺,Ho³⁺:silica fiber laser pump source, up to 2.2 W of average output power was achieved at 40 kHz repetition rate, 1.9 W at 60 kHz and 1.3 W at 75 kHz in the mid-infrared range.

Although seen as nearly being impossible to realize, a quasi-three-level laser medium can be used in heat-capacity operation. In this operation mode, the laser medium is not cooled during lasing in order to avoid strong thermal lensing, which, in actively cooled operation, would result in a low beam quality or would even destabilize the laser cavity. Thus, in heat-capacity mode, the laser medium will substantially heat up during operation, which will cause an increase in re-absorption for a quasi-three-level laser medium, resulting in a general drop in output power over time. However, laser power, temperature rise, fluorescence and inversion are coupled by the temperaturedependent spectroscopic properties of the laser medium in a complex way. This paper presents an investigation on these thermal effects and upconversion in the resonantly pumped Er3+:YAG solid-state heat-capacity laser (SSHCL) system. These effects are important for the scaling properties on this laser towards medium- or high-energy systems, and to obtain a good beam quality from the laser itself. It is shown that the expected power drop of this quasi-three-level medium due to the rise in crystal temperature is very low, allowing for high-power operation on substantial time scales. The experimental results and the theoretical background will be explained in detail. The effect of fluorescence re-absorption on the laser properties, especially on threshold and laser efficiency will also be discussed. This fluorescence re-pumping, applicable in general to a large variety of lasers, can drastically increase the output power and thus laser efficiency at a given pump power. Up to 125 W and 89 J in 2 s are achieved using optimized doping levels for upconversion reduction.

For some medical fields in laser surgery and as a pump source for nonlinear materials to generate mid-IR radiation, e.g. for countermeasure applications, it is very useful to have a solid-state laser with high pulse energy at 2 μm. The rare earth ion Thulium offers a cross relaxation and can thus be directly diode pumped with common laser diodes around 800 nm for an efficient pumping. However, it was not considered for high pulse energy operation due to the high saturation fluence of around 62 J/cm² at 2 μm. A limiting factor has always been the damage threshold of the optical elements inside the cavity. One of the reasons is the strong thermal lens of YAG, which affects a change of the beam radius inside the resonator and additionally degrades the beam quality with increasing pump power. Using a new pump geometry of the Tm³⁺:YAG laser system, it is now possible to reach pulse energies > 13 mJ at a diffraction limited beam quality of M² < 1.1. The Q-switched Tm³⁺:YAG laser system uses an AOM operating at 100 Hz and will be described in detail. Due to the high pulse energy and very good beam quality, this laser is very interesting for nonlinear parametric frequency conversion.

Er³⁺:YAG eye-safe laser emitting at 1.6 μm is an interesting source for various applications such as remote sensing, ranging, designation and free-space communications for two main reasons: its emitting wavelength lies in a region of high atmospheric transmission and high sensor sensitivity and the resonant pumping into the ⁴I_13/2 upper laser manifold ensures highly efficient operation. The recent availability of internal grating stabilized narrow linewidth, high-power laser diodes in the 1.53 μm range, makes this laser even more appealing. The only shortcoming to be solved for a really efficient resonantly diode pumped Er³⁺:YAG laser is how to have a good overlap between the pump radiation and the laser cavity mode. Indeed, due to up-conversion processes among the Er³⁺ ions, to achieve efficient lasers it is necessary to use low doped samples. This requires the use of rods with lengths of several cm that are not compatible with the low beam quality of the diode lasers. In this work, we report on a resonantly diode pumped Q-switched Er³⁺:YAG laser with a crystalline fibre-like geometry emitting at 1.64 μm. In this scheme, the pump radiation is confined into a 60 mm long crystal with a diameter of 1.2 mm thanks to the multiple total internal reflections (TIR) that occur on the barrel surface, allowing efficiently pumping of such a long crystal. A maximum output power of more than 14 W in continuous wave mode and pulse energies of 8 mJ in Q-switching mode have been observed, when pumped with - 40 W of absorbed power. Even if these values are still far from the performances reported using hybrid fibre-bulk laser scheme, these results clearly show that TIR-based Er³⁺:YAG fibre-shaped crystalline rod laser is a promising technology for the development of efficient high-power and high-energy eye-safe laser. Finally, the effect of thermal lensing on such crystalline fibre geometry is discussed.

The paper gives an overview on pulsed 2 μm fiber laser technology that has strongly evolved during the last years. In the beginning, 2 μm fiber-amplifier systems have been considered to allow short-pulse generation at high repetition rates for pumping mid-IR conversion elements due to their possibility of generating short pulses by properly-driven laser diodes and successive amplification. However, this scheme turned out to be rather complex in architecture and needed low-phonon fluoride-glass fibers for efficient energy storage. Then a new technology has been developed to provide short pulses directly from a single, Q-switched fiber laser at very high repetition rates with silica fibers. Even taking into account the high phonon energies of these fibers, an efficient operation is possible. These fibers are also much more durable than fluoride fibers and thus better suited for military applications. Thulium-doped and thulium-holmium-codoped fibers are therefore the most promising candidates for pulse generation when some-ten-ns pulses at 100 kHz or more are needed. Average powers of over 32 W have been achieved with pulse durations down to 42 ns at > 100 kHz with Thulium-doped fibers. Tm-Ho codoping can provide longer emission wavelengths towards 2.1 μm, thus allowing direct conversion towards the mid-IR using ZGP crystals. On the Holmium emission, pulses of down to 58 ns have been achieved at average powers exceeding 14 W, being currently only limited by the available pump power. The theory of this short-pulse generation will also be explained. Pumping, e.g., quasi-phase-matched nonlinear GaAs, with a Tm-Ho-doped fiber laser, > 2 W of average power has been achieved in the mid-IR.

The key features and performances of a compact, lightweight, high power Er³⁺:Yb³⁺ glass laser transmitter are reported on. The theory employed to get an optimal design of the device is also described. In free running regime high energies of about 15mJ in 3ms long pulses were obtained, with an optical efficiency close to 85%. When q-switched by a Co: MALO crystal of carefully selected initial transmittivity, a high peak power in excess of 500 kW was obtained in about 9ns pulse duration, with an optical efficiency of 60%. The laser was successfully run with no significant power losses at repetition rates up to 5Hz due to a carefully designed heat sink which allowed an efficient conduction cooling of both the diode bars and the phosphate glass. The transmitter emits at a wavelength of 1535nm in the so-called "eyesafe" region of the light spectrum thus being highly attractive for any application involving the risk of human injury as is typically the case in remote sensing activities. Moreover, the spectral band around 1,5mm corresponds to a peak in the athmospheric transmittance thus being more effective in adverse weather conditions with respect to other wavelengths. Actually, the device has been successfully integrated into a rangefinder system allowing a reliable and precise detection of small targets at distances up to 20Km. Moreover, the transmitter capabilities were used into a state of the art infrared laser illuminator for night vision allowing even the recognition of a human being at distances in excess of 5Km.

μWe report on a high-power semi-ruggedized laser source, which can deliver radiation in all infrared transmission bands from 2 to 10 μm. In the source a 70 W Tm-doped fiber laser pumps a Q-switched Ho:YAG laser, which produces 37 W of output power at 2.1 μm. The Ho-laser pumps two ZnGeP₂ OPOs. The first generates a combined power of 14 W at 3.9 μm and 4.5 μm, and the second generates watt-level outputs at 2.8 μm and 8 μm. The OPOs have a novel V-shaped 3-mirror ring resonator design, which gives high efficiency and beam quality.

The protection of ships against infrared guided missiles is a concern for modern naval forces. The vulnerability of ships can be reduced by applying countermeasures such as infrared decoys and infrared signature reduction. This paper presents a set of simulation tools which can be used for assessing the effectiveness of these measures. The toolset consists of a chain of models which calculate the infrared signature of a ship (EOSM), generate an infrared image of the ship in a realistic sea and sky background (EOSTAR) and determine the behaviour of an infrared missile seeker against these images and simulate the complete missile fly-out including countermeasure deployment (EWM). All model components will be discussed. Typical simulation results will be shown.

We report on the development of a compact frequency conversion module (FCM) devoted to IR countermeasures. Several spectral lines are emitted simultaneously by the use of optical parametric oscillators (OPOs) pumped by a high repetition rate near-IR pulsed laser. A special attention has been paid on the optimization and the miniaturization of the FCM optomechanical design. The proposed compact design increases the alignment stability of the OPO cavities and in the same time facilitates its integration. Moreover, we have developed two-zone temperature controlled ovens enabling thermal management of the periodically poled nonlinear crystals. With proper adjustments of the applied temperature gradient, we have demonstrated that a significant improvement (more than 30 %) of the conversion efficiency can be obtained.

Directed energy applications for thin disk lasers demand improvements in materials, efficiency, thermal management, and most importantly beam quality. At the Air Force Research Laboratory's Directed Energy Directorate ceramic Yb:YAG materials are being investigated along with various cooling techniques. 10-14mm diameter 0.2mm thick disks are mounted on silicon carbide (SiC), sapphire, and diamond submounts. From a larger platform, more than 6kW power is obtained from unmounted and sub-mounted 35mm diameter disks. In conjunction with thermal modeling, we project a path towards high performance high power lasers.

The exhaust from jet engines introduces extreme turbulence levels in local environments around aircrafts. This may degrade the performance of electro-optical missile warning and laser-based DIRCM systems used to protect aircrafts against heat-seeking missiles. Full scale trials using real engines are expensive and difficult to perform motivating numerical simulations of the turbulence properties within the jet engine exhaust. Large Eddy Simulations (LES) is a computational fluid dynamics method that can be used to calculate spatial and temporal refractive index dynamics of the turbulent flow in the engine exhaust. From LES simulations the instantaneous refractive index in each grid point can be derived and interpolated to phase screens for numerical laser beam propagation or used to estimate aberration effects from optical path differences. The high computation load of LES limits the available data in terms of the computational volume and number of time steps. In addition the phase screen method used in laser beam propagation may also be too slow. For this reason extraction of statistical parameters from the turbulence field and statistical beam propagation methods are studied. The temporal variation of the refractive index is used to define a spatially varying structure constant. Ray-tracing through the mean refractive index field provides integrated static aberrations and the path integrated structure constant. These parameters can be used in classical statistical parameterised models describing propagation through turbulence. One disadvantage of using the structure constant description is that the temporal information is lost. Methods for studying the variation of optical aberrations based on models of Zernike coefficients are discussed. The results of the propagation calculations using the different methods are compared to each other and to available experimental data. Advantages and disadvantages of the different methods are briefly discussed.

The exhaust from engines introduces zones of extreme turbulence levels in local environments around aircraft. This may disturb the performance of aircraft mounted optical and laser systems. The turbulence distortion will be especially devastating for optical missile warning and laser based DIRCM systems used to protect manoeuvring aircraft against missile attacks, situations where the optical propagation path may come close to the engine exhaust. To study the extent of the turbulence zones caused by the engine exhaust and the strength of the effects on optical propagation through these zones a joint trial between Germany, the Netherlands, Sweden and the United Kingdom was performed using a medium sized military turboprop transport aircraft tethered to the ground at an airfield. This follows on earlier trials performed on a down-scaled jet-engine test rig. Laser beams were propagated along the axis of the aircraft at different distances relative to the engine exhaust and the spatial beam profiles and intensity scintillations were recorded with cameras and photodiodes. A second laser beam path was directed from underneath the loading ramp diagonally past one of the engines. The laser wavelengths used were 1.5 and 3.6 μm. In addition to spatial beam profile distortions temporal effects were investigated. Measurements were performed at different propeller speeds and at different distances from exhaust nozzle to the laser path. Significant increases in laser beam wander and long term beam radius were observed with the engine running. Corresponding increases were also registered in the scintillation index and the temporal fluctuations of the instantaneous power collected by the detector.

Military aircraft face a serious threat from early generation Man-Portable Air-Defence (MANPAD) systems. Robust countermeasures have to be used to counteract this threat. Most commonly these are used after the threat has been launched and detected. The ideal solution is to defeat the system pre-emptively before the missile is launched. One way to achieve this is to fire pre-emptive flares giving the MANPAD another hot source to track and lock-on to. However, use of pre-emptive flares can quickly deplete the flare magazines limiting the mission time and the area in which the aircraft will be protected. In this paper we discuss the use of CounterSim, a missile engagement and countermeasure simulation software tool, to investigate what effect the flare output and burn time may have on the effectiveness of preemptive countermeasures. The first set of simulations looks at a flare of full intensity and burn time pre-emptively released at the beginning of the simulations. Then, flares of reduced intensity and reduced burn time are used. In a second set of simulations the pre-emptive flare release time is investigated by delaying the firing up to one second from the beginning of the simulation.

Man-Portable Air-Defence (MANPAD) systems can employ a range of counter-countermeasures (CCM) to reject expendable IR decoys. Three hypothetical MANPAD models are based on reticle types and CCM features that may be found in 1st and 2nd generation MANPADs. These are used in simulations to estimate the probability of escaping hit (PEH) when no IR decoys are used, when IR decoys are deployed reactively and when decoys are deployed preemptively. These cases are simulated for seekers with no CCM and with a track angle bias CCM. The results confirm that the rise rate CCM significantly reduces the PEH when IR decoys are used reactively. The use of pre-emptive flares timed to deploy at or about the time when the seeker is uncaged increases the PEH significantly. A more detailed investigation of the effects of aircraft aspect angle and flare timing on miss distance was carried out to examine the effects of the CCM compared with no CCM. With the aircraft at an altitude of 1000m and a range of 2km there is a critical period in which a flare needs to be released in order to achieve a significant miss distance when the CCM is in use. The conical scan seeker used with the track angle bias CCM was the most effective combination requiring the shortest time during which the flare had to be deployed. Further simulations at longer ranges and different aircraft azimuth angles showed that there is a time window that is range dependant during which pre-emptive decoys are fully effective independently of the aircraft azimuth or threat direction.

We report on theoretical and numerical investigations of stimulated Brillouin scattering (SBS) in optical fibers. A theoretical framework is presented which involves a nonlinear triply-coupled time-dependent system of equations containing the optical, Stokes, and phonon fields. We examine short fibers where previous approximations for long fibers cannot be made. We consider modulation frequencies and linewidths starting at approximately the Brillouin gain bandwidth and all the way up to frequencies of the order of the Brillouin resonance frequency. We provide simulations showing the SBS suppression factor as a function of modulation amplitude and frequency for a single-sinusoidal modulator.

When a space object in spin stabilization, three-axis stabilization or roll-motion is running on orbit, heat transfer such as radiation and conduction takes place, which makes the temperature field on the object in an unsteady state. Besides, the methods of stabilization on orbit also affect the temperature distribution as well as projected area to a spaceborne sensor, thus influence IR signatures of the object. In this paper, by developing the attitude dynamics as well as the heat transfer models of an orbital object, the infrared characteristics are calculated and analyzed. Results indicate that temperature demonstrates periodical variation, and stabilization states of the space object have important impact on the temperature distribution and IR radiant intensity.

The recent break-through in semiconductor laser-module technology in the infrared region between 2 μm and 10 μm opens up new windows of opportunity for active sensing, imaging, and modulated power projection. Optically pumped semiconductor disk lasers and quantum cascade lasers cover the infrared spectral range continuously, either with tuneable small bandwidths for active spectral sensing or with broad bandwidth, high power (Watt level), and good diffraction properties (M2<2) for modulated power projection and (3D) imaging. This paper reviews the physics of infrared semiconductor disk lasers and quantum cascade lasers, explains the challenges of the module technology (the next higher integration level after chip technology) and outlines several security and defence related issues for future applications.

We report large beam steering effects, observed in the far-field pattern of InP-based mid-infrared quantum-cascade lasers along the slow axis. Changing the temperature by a few degrees around room temperature or varying the drive current strongly affects the lateral direction of the output beam. The position of maximum intensity in the far-field-distribution changes by more than 20°. This beam steering effect is correlated to changes in the lateral mode distribution, as revealed by time-resolved spectroscopy of the lasing spectrum.

Atmospheric density gradients bend light as can be seen with naturally occurring mirages. Shock waves also produce density changes that bend or refract light. While a single shock front from a supersonic projectile refracts light only on the order of hundreds of arcseconds, theoretical results indicate that beam deflections of thirty or more degrees are possible from periodic shock waves. Two types of idealized plane periodic shock waves are analyzed. A ballisticsderived periodic shock wave based on empirical data from projectile firings and a synthetic-derived periodic shock wave based on a tradeoff between peak pressure amplitude and shock wave period required to produce a target density gradient. Predicted laser beam refractions from both types of idealized plane periodic shock waves are presented. Predicted laser beam steering angle versus peak pressure for fixed period synthetic-derived periodic shock waves is presented. Predicted laser beam steering angle versus shock wave period for fixed peak pressure amplitude syntheticderived periodic shock wave is presented.

Quantum-cascade lasers (QCLs) operating in the 3-5 μm spectral region are increasingly finding application in a number of areas including gas sensing for both environmental and medical uses, communication, and infrared countermeasures. QCLs emitting at wavelengths near 4 μm and below have been especially challenging, requiring a very large conduction band discontinuity, a small electron effective mass, but also a relatively mature materials system. The focus of this contribution is on our own QCL designs based on the use of strain compensation with very high levels of strain in the individual layers; barriers based on AlAs, wells on In_0.73Ga_0.27As, and the entire structure on average lattice-matched to InP. For more flexibility to control both strain and conduction band potential, "composite barriers" are used, composed of AlAs and A_l0.5In_0.5As. Indirect valleys within the well material that can limit the photon energy to the energy difference between these valleys and the lower laser state are also pushed to higher energy by using strained In_0.73Ga_0.27As wells. Combining these design components, we have produced QCLs emitting at wavelengths covering the entire range down to 3 μm. These lasers have demonstrated high power in narrow stripes at cryogenic as well as room temperatures together with excellent beam quality.