One application of ultrashort pulse filamentation is the coupling of external electric fields to filament plasmas and guiding of high-voltage discharges. However, the full physics of the guiding mechanism is still in question. Several models have been presented and explanations have been suggested to capture the full physics of the discharge event. For the first time, measurements of the electric field dynamics between two electrodes during filament-guided discharges are presented here, to the best of our knowledge. The electric field dynamics show an exponential growth region, a plateau, followed by a sharp drop off coinciding with the discharge event. We believe these results will ultimately answer the questions regarding the guiding mechanism.
Cryogenic solid-state laser materials offer many improvements in thermal, optical, structural, and lasing properties over
their room temperature counterparts. As the temperature of Yb:YAG decreases from room to 80K it transitions from
quasi-three-level lasing to a 4-level laser. In this study, we compare Yb:YAG thin-disk laser performance at room 293K
and 80K. To achieve this direct comparison we have built two cooling systems based on R134A refrigerant and also on
liquid nitrogen (LN2). We have made an analytical calculation of the small signal laser gain that takes into account the
spurious amplified spontaneous emission and photon re-absorption. The cold thin-disk laser clearly outperforms room
temperature operation, and the theoretical results shows room temperature gain flattening.
A ceramic ytterbium:yttrium aluminum garnet (Yb:YAG) thin-disk laser is investigated at 15°C (288 K) and also at 80 K, where it behaves as a four-level laser. We introduce a new two-phase spray cooling method to cool the Yb:YAG. One system relies on R134a refrigerant while the other uses liquid nitrogen (LN2). The use of two systems allows the same disk to be tested at the two temperatures. When the Yb:YAG is cooled from room to cryogenic temperatures, the lasing threshold drops from 155 W to near 10 W, while the slope efficiency increases from 54% to a 63%. A 277 W laser with 520 W of pump is demonstrated. We also model the thermal and structural properties at these two temperatures and estimate the beam quality.
At cryogenic temperatures, Yb:YAG behaves as a 4-level laser. Its absorption and emission cross-sections increase, and
its thermal conductivity improves. Yb:YAG thin disk laser performance at room and cryogenic (80°K) temperatures will
be presented. The Yb:YAG gain media is cooled using either a pressurized R134A refrigerant system or by a two-phase
liquid nitrogen spray boiler. Interchangeable mounting caps allow the same Yb:YAG media to be switched between the
two systems. This allows direct comparison of lasing, amplified spontaneous emission, and temperature performance
between 20°C and -200°C.
Yb:YAG thin-disk laser performance at room and cryogenic (80K) temperatures is presented. The Yb:YAG gain media,
which is Indium soldered to specialized CuW mounting caps, is cooled using either a pressurized R134A refrigerant
system or by a two-phase liquid nitrogen spray boiler. At cryogenic temperatures spontaneous emission measurements
reveal sharper transition lines and a decrease in the fluorescence lifetime. Lasing reflects that a true four-level laser.
Interchangeable mounting caps allow the same Yb:YAG media to be switched between the two systems. This allows
direct comparison of lasing, amplified spontaneous emission, and temperature performance at 15 °C and at -200 oC.
Operational performance of kilowatt-class thin-disk ceramic and single crystal Yb:Yag lasers is presented. High pump
power is applied to various thin-disk assemblies on two different test beds. The assemblies are composed of ASE caps,
200μm gain media, and heat sinks made of SiC, sapphire, or diamond. A novel mounting and cooling process is
described.
FEA modeling of the assemblies is performed using COMSOL stress and thermal computations to understand and
quantify thermal and stress effects on beam quality and laser output power. Under increased pump power, the thin-disk
can deform 5-10 μm in the center, destroying cavity stability. This is observed experimentally. The results of this work
indicate that a single thin-disk laser could simultaneously produce high beam quality and high power if novel thermal
management techniques are employed.
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.
An annular beam provides a new laser drilling mechanism, which we refer to as optical trepanning. A refractive axicon system has been designed to transform an input Gaussian laser beam into a collimated annular beam. The diffractive effects of the axicon system and a convex lens focusing the collimated annular beam have been studied using the Fresnel diffraction integral. The theoretical diffraction patterns are compared with the patterns measured with a laser-beam analyzer. The results show that the refractive axicon system can produce Gaussian-like annular beams with the capability of easily adjusting the size of the annular beam.
Percussion drilling and trepanning are two laser drilling methods. Circular laser spots are generally used in conventional
laser drilling. Annular laser beams provide a new laser drilling mechanism. When an annular beam is focused on the
workpiece surface, the material around the annulus laser spot is heated, melted, vaporized and removed, leading to the
formation of a hole. This process, which we refer to as optical trepanning, does not involve any rotating optics or rotating
workpiece. Based on the ray tracing technique, an optical lens system using axicon lenses has been designed to transform
a Gaussian circular laser beam into an annular laser beam. Both theoretical and experimental results of irradiances of the
annular beam along the optical system were analyzed. Optical trepanning experiments were conducted on stainless Steel-
316. Some geometrical and metallurgical characteristics during optical trepanning were investigated.
Laser drilling is very important in many industries such as automotive, aerospace, electronics, and materials processing. It can be used to produce critical components with novel hole geometry for advanced systems. Percussion drilling and trepanning are two laser drilling methods. In the conventional trepanning method, a laser beam is scanned along a circular or spiral orbit to remove material to achieve a desired hole shape. These orbits generally trace a circular path at the inner wall of the holes. This suggests that an annular beam can be used to accomplish trepanning, a technique we refer to as optical trepanning. The ray-tracing technique of geometrical optics is employed in this paper to design the necessary optics to transform a Gaussian laser beam into an annular beam of different intensity profiles. Such profiles include uniform intensity within the annulus, full Gaussian with maximum intensity, and half Gaussian with maximum intensities at the inner and outer surfaces of the annulus.
Optical trepanning is a new laser drilling method using an annular beam. The annular beams allow numerous irradiance profiles to supply laser energy to the workpiece and thus provide more flexibility in affecting the hole quality than a traditional circular laser beam. The refractive axicon system has been designed to generating a collimated annular beam. In this article, calculations of intensity distributions produced by this refractive system are made by evaluating the Kirchhoff-Fresnel diffraction. It is shown that the refractive system is able to transform a Gaussian beam into a full
Gaussian annular beam. The base angle of the axicon lens, input laser beam diameter and intensity profiles are found to be important factors for the axcion refractive system. Their effects on the annular beam profiles are analyzed based on the numerical solutions of the diffraction patterns.
An annular beam provides a new laser drilling mechanism, that we refer to as "optical trepanning". Based on ray tracing techniques, a refractive axicon system has been designed to transform an input Gaussian laser beam into an annular beam with an appropriate irradiance profile. The properties of the resulting annular beam are investigated in this paper. The diffraction patterns were measured, showing that a collimated annular beam can be obtained using a refractive axicon system. However, due to spherical aberration effects, the focused annular beams possess altered irradiance profiles with different focusing lenses. The effects of spherical aberration on the focused annular beam are also investigated.
Laser drilling is very important in many industries such as automotive, aerospace, electronics and materials processing. It can be used to produce critical components with novel hole geometry for advanced systems. Percussion drilling and trepanning are two laser drilling methods. In the conventional trepanning method, a laser beam in scanned along a circular or spiral orbit to remove material to achieve a desired hole shape. These orbits generally trace a circular path at the inner wall of the holes. This suggests that an annular beam can be used to accomplish trepanning, which we referred to as optical trepanning. The ray tracing technique of geometrical optics will be employed in this paper to design the necessary optics to transform a Gaussian laser beam into an annular beam of different intensity profiles. Such profiles include half Gaussian with maximum intensities at the inner and outer surfaces of the annulus, respectively, and full Gaussian with maximum intensity within the annulus.
Both government and industry have a long-standing interest in developing low-cost high-power semiconductor lasers for a variety of applications. These include material processing, long range sensing, and long-range communications. Key development goals are high brightness and high efficiency. This paper will review some of the applications, key laser performance features desired, and some of the past accomplishments in high power diode lasers form the High Power Semiconductor Laser Technology program. One of the primary outgrowths of HPSLT was the Grating Coupled Surface Emitting Laser (GCSEL). GCSEL progress will be highlighted.
KEYWORDS: Laser applications, Data modeling, Temperature metrology, Laser systems engineering, Thermal modeling, Systems modeling, Metals, Airborne laser technology, Pulsed laser operation, Laser development
This paper discuses the DELE lethality assessment methodology and explains requirements for experimental data and algorithm development within the laser lethality and vulnerability area. current applications and data sets cover a broad range of laser parameters and materials properties. In addition, new lasers, new materials, and new applications continue to be rapidly added to the area. The purpose of this paper is to introduce the audience to the formalized process/discipline of formulating vulnerability criteria for different High Energy Laser systems by emphasizing the laser/materials phenomenology. Some of the DELE laser facilities and diagnostic capabilities are reviewed.
Aspherical (graded-phase) mirrors are potentially useful in stable resonators to obtain a desired intensity distribution at the output of a low-loss or moderate-loss resonator. The mode volume for single-mode operation can also be increased over that which can be obtained using a stable resonator. Much of the work on this concept to date has concentrated on resonators that have a super-gaussian mode at the output mirror. Here we explore some concepts involving other amplitude distributions that may be used to further enlarge the mode volume, although the output distribution may not be as desirable as the super gaussian.
The United States government and industry have a long standing interest in developing high power in-plane semiconductor lasers for a variety of applications. These include material processing, long range sensing, and long range communications. The wavelength depends on the application, and sometimes on eye safety considerations. Key development goals are high brightness and high efficiency. This paper will review some of the applications, key laser performance features desired, and some of the accomplishments in high power diode lasers from the High Power Semiconductor Laser Technology program.
The United States government and industry have a long standing interest in developing high power semiconductor lasers for a variety of applications. These include material processing, long range sensing, and long range communications. The wavelength depends on the application, and sometimes on eye safety considerations. Key development goals are high brightness and high efficiency. This paper will review some of the applications, key laser performance features desired, and some of the accomplishments in high power diode lasers from the High Power Semiconductor Laser Technology program.
KEYWORDS: Chemical oxygen iodine lasers, Laser cutting, Chemical lasers, Laser applications, Fiber optics, Metals, High power lasers, Materials processing, Laser drilling, Analytical research
The Chemical Oxygen-Iodine Laser (COIL) has been developed at the Air Force Research Laboratory for military applications. For example, the COIL is to be use as the laser device for the ABL. A high power laser is useful for applications that require the delivery of a substantial amount of energy to a very small focused laser spot. The COIL is a member of the class of high power lasers that are also useful for industrial applications, including the materials processing task of high speed cutting and drilling. COIL technology has received considerable interest over the last several years due to its short, fiber- deliverable wavelength, scalability to very high powers, and demonstrated nearly diffraction-limited optical quality. These unique abilities make it an ideal candidate for nuclear reactor decommissioning and nuclear warhead dismantlement. Japanese researchers envision using a COIL for disaster cleanup and survivor rescue. It is also being studied by the oil and gas industry for well drilling. Any commercial or industrial application that requires very rapid, precise, and noninvasive cutting or drilling, could be readily accomplished with a COIL. Because of the substantial power levels available with a COIL, the laser could also be used for broad area applications such as paint stripping. This paper includes a collection of experiments accomplished at the Air Force Research Laboratory Chemical Laser Facility, including metal cutting, hole drilling, high power fiber optic transmission, and rock crushing.
A chemical oxygen-iodine laser (COIL) was used for cutting aluminum, titanium, inconel and copper plates. The laser was operated with a stable resonator having an intracavity aperture to produce a circular COIL beam with very few transverse modes. The multimode focal spot diameter was calculated to be 0.24 mm. The new aluminum cut was of particularly high kerf edge quality. These COIL cutting data are compared with an existing theoretical laser cutting model. Using thermophysical data for aluminum, titanium,l inconel and copper, this theory agrees very well with the data. To test the versatility of the model, the effects of different assumptions are examined; different assumptions produced very little effect on model predictions at high cutting speeds and a small difference at very slow cutting speeds. Overall, the theoretical model provides good agreement with experiments for a wide variety of metals.
The COIL has drawn considerable attention owing to its short, fiber-deliverable wavelength, scalability to very high powers, and nearly diffraction-limited optical quality. These unique abilities make it an ideal candidate for rapid cutting of thick metals and other laser materials processing applications. Cutting experiments have been conducted at the Air Force Research Laboratory Chemical Laser Facility for a number of metals including stainless steel, nickel, copper, titanium and aluminum. A simple model for the cutting performance has been very successful in predicting the cutting sped and depth for most of these materials. Different assumptions of this model produce very little effect on the model predictions at high cutting speeds an a small difference at very slow cutting speeds. Additional physical effects such as the effects of vaporization, which can be significant at slow cutting speeds when cutting very thick sections, is incorporated into the model. This paper demonstrates how various assumptions within the derivation of the model can lead to different forms of the scaling law.
Chemical Oxygen-Iodine Laser (COIL) is a member of the class of high power lasers for industrial applications, including the materials processing tasks of high speed cutting and drilling. COIL technology has received considerable interest over the last several years due to its short, fiber- deliverable wavelength (1.315 microns), scalability to very high powers (tens of kilowatts, cw), and demonstrated nearly diffraction-limited optical quality. Its material interaction characteristics are similar to the Nd:YAG laser since the wavelengths of these lasers are comparable. This paper documents the cutting performance of a high power, good optical quality COIL. In this experiment different types of materials were cut with the COIL, and the cut depth and width are presented here as a function of various process parameters.
KEYWORDS: Laser cutting, YAG lasers, Chemical oxygen iodine lasers, Metals, Carbon dioxide lasers, Information operations, High power lasers, Carbon dioxide, Absorption, Laser energy
The most widely used high power industrial lasers are Nd:YAG and carbon-dioxide lasers. Chemical oxygen iodine laser (COIL), whose wavelength (1.315 micrometer) is between that of YAG (1.06 micrometer) and carbon-dioxide (10.6 micrometer) lasers, is another high power laser for industrial applications. The cutting capability of these lasers is investigated. The cut depth depends strongly on the absorptivity of materials, kerf width and cutting speed. Absorptivity is an unknown parameter for which experimental data at high temperatures are unavailable. Theoretical values of the absorptivities of various metals are obtained using Hagen-Ruben's relation. It is found that the absorptivity of metals is linearly proportional to the square root of resistivity and inversely proportional to the square root of the wavelength. The absorptivities of COIL ad YAG lasers are 2.84 and 3.16 times larger than that of carbon-dioxide laser, respectively. Based on the theoretical values of absorptivity, the cut depth of metals are analyzed for various laser powers, cutting speeds for these lasers. Due to the wavelength dependence of absorptivity, the cut depths for COIL and YAG lasers are expected to be 2.84 and 3.16 times deeper than that for carbon-dioxide laser.
This paper examines the use of chemical oxygen-iodine laser (COIL) for depositing ceramic coating on a substrate. The cladding experiments were conducted in the environment of nitrogen gas, and the laser beam was focused to a rectangular spot with large aspect (length to width) ratio. The beam was scanned by keeping the length of the spot aligned to the transverse direction. Optical micrographs are presented for the morphology of the clad surface and the cross section of the cladding zone.
KEYWORDS: Laser cutting, Chemical oxygen iodine lasers, Chemical lasers, Metals, High power lasers, Laser development, Materials processing, Laser processing, Data modeling, Oxygen
A high power chemical oxygen-iodine laser (COIL) at the Air Force's Phillips Laboratory was evaluated to determine its cutting capability in thick section stainless steel. The results of this experiment were used to validate a new model that describes the effects of using rectangular- shaped beams for thick section materials processing. That model, while derived in a way similar to a model developed by researchers in Japan, predicts a completely different behavior when cutting extremely thick metal sections. A detailed understanding of the thick section metal cutting is necessary for the development of new commercial applications of very high power lasers, including nuclear power plant dismantlement, shipbuilding, and heavy equipment manufacture. Such applications depend on the demonstration of high power delivery of a laser beam through fiber optics. A case is made for the capability of delivering a COIL beam over 10 kilowatts, and an outline for a possible demonstration using off-the-shelf components is described. Areas requiring further research in high power materials processing technique are described, including advanced assist gas nozzle development, laser focal spot shaping to optimize processing rates and large area surface modification. A description is included of the cooperative research and development agreement program to describe how laser application developers can gain access to testing at the Phillips Laboratory COIL test facility.
The following paper discusses the area of technology transfer of Dual-Use Technologies between government and university laboratories and industry. Variuos mechanisms for technological cooperation are summarized. The status of technology transfer activities at the Alliance for Photomc Technology and the USAF Phillips Laboratory, Lasers and Imaging Directorate, Cooperative Mfairs Office are presented as examples of technological cooperation. See report # 2214-47 for more information.
A compact, repetitively pulsed HF chemical laser, built with a gas recirculator loop to cool and process the gas mixture is described. A simple corona phototriggered discharge is used to produce vibrationally excited HF molecules at typical pressures of 90-150 Torr. This laser produced over 5 J per pulse in single- shot operation, and a maximum average power of 500 W. at a repetition rate of 110 Hz. The electrical efficiency is 3%.
The propagation equations for the mode of a laser with significant variation of the index of refraction in one transverse dimension are simplified by a transformation of the coordinate system. The new z axis is parallel to the resonator optical axis, the curved ray that reproduces itself after a round trip propagation. The apertures of the resonator are offset by the transformation, and the linear variation of the index of refraction is accounted for by the transformation rather than by the addition of an explicit tilt to the phase of the field. The resonator optical axis is well approximated by a quadratic curve. An analytic expression is given for the optimum tilt of the end mirrors. Expressions are also derived for the optical axes of confocal unstable and half-symmetric stable resonators with mirror alignment that is uncorrected for the variation of the index of refraction. Numerical results for a laser with this type of medium-induced aberration are included.
The Krylov matrix method is a powerful numerical algorithm for efficiently and accurately calculating several of the lowest loss transverse bare cavity eigenmodes of unstable optical resonators. In current laser models, loaded cavity modes are calculated by accomplishing a functional expansion in bare cavity eigenmodes. By accomplishing the Krylov analysis, both the bare cavity design parameters and the eigenmode expansion set are calculated simultaneously. This provides a convenient resonator candidate screening process as an intermediate step in the full laser design process and is followed by a loaded cavity analysis when the bare cavity parameters are suitable. This paper reviews the Krylov procedure and discusses a convergence algorithm for it. Examples are presented to demonstrate the method.
A simple model for predicting the small signal gain as a function of flow direction is presented. It basically couples simplified kinetics and fundamental gas dynamics and allows for the heat release produced by the water deactivation of excited oxygen and iodine. The numeric results lead to a temperature rise in the gas, which causes a substantial decay of the small signal gain in flow direction. An analytic solution is also derived and results of both approaches compare favourably with experimental data.
An output beam with a nearly ideal focal spot can be obtained by combining an unstable ring resonator with 90 deg beam rotation with a lasing medium which has gain that would be too low for the effective use of a more conventional unstable resonator. As an example, results are presented of a simulation of an oxygen-iodine transfer laser with moderate gain. The simulation included a fairly detailed representation of the laser kinetics, including the effect of transverse gas flow. Results of the simulation include laser mode profiles and far-field patterns.
The purpose of the study is to present the basic concepts of coupled-laser devices as well as a historical summary of some of the experimental studies of laser coupling. Focus is concentrated on the experimental efforts with gas and chemical lasers. Divided output single and injection locked lasers, master oscillator and power amplifiers, coupled and multiple output resonators, nonlinear optics coupled devices, and hybrid coupled optical systems are discussed among laser-coupling techniques. Early experiments involving the coupling of carbon dioxide lasers and coupled photolytic iodine lasers are outlined, and works employing nonlinearly coupled devices, intracavity mixing, and gain medium are covered.
A simple model for predicting the small signal gain as a function of flow direction will be presented. The small signal gain was measured on the Weapons Laboratory Rotocoil 02/I* gain medium. The characteristics observed in the experiment show a decrease in the small signal gain as a function of distance from the nozzle exit plane. Further results indicated that the small signal gain decreased with time and that the gain increased when the cold trap was turned on. All of these effects suggest a temperature dependence of the small signal gain. The approach presented in this paper is to develop a simple model which includes a simplified kinetics model and the gas dynamics for the flowing medium. An analytic solution to the model equations is also derived. These models account for the reduction in small signal gain in the flow direction due to heat release into the cavity when compared to the Rotocoil small signal gain data. The results show that the rise in gas temperature in the flowing 02/I* medium is primarily due to water deactivation of the I* and the O2(1Δ) plus I* pooling leading to water deactivation of 1Σ. Such temperature rise in the flowing medium causes the small signal gain to decay substantially in the flow direction due to the square root of the temperature dependence in the stimulated emission cross section, the shift in the equilibrium constant with temperature and the decrease in density which is inversely proportional to the temperature.
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