3D Meta-Optics are optical components that are based on the engineering of the electromagnetic fields in 3D dielectric
structures. The results of which will provide a class of transformational optical components that can be integrated at all
levels throughout a High Energy Laser system. This paper will address a number of optical components based on 2D
and 3D micro and nano-scale structures and their performance when exposed to high power lasers. Specifically, results
will be presented for 1550 nm and 2000 nm spectral bands and power densities greater than100 kW/cm2.
Laser damage of optical materials, first reported in 1964, continues to limit the output energy and power of pulsed and continuous-wave laser systems. In spite of some 48 years of research in this area, interest from the international laser community to laser damage issues remains at a very high level and does not show any sign of decreasing. Moreover, it grows with the development of novel laser systems, for example, ultrafast and short-wavelength lasers that involve new damage effects and specific mechanisms not studied before. This interest is evident from the high level of attendance and presentations at the annual SPIE Laser Damage Symposium (aka, Boulder Damage Symposium) that has been held in Boulder, Colorado, since 1969.
We discuss the physical and optical properties of Sc2O3 single layers deposited by the dual ion beam sputtering technique
at oxygen partial pressures ranging from 1.7×10-5 to 5.1×10-5 Torr. The films are amorphous with crystallite size ~10
nm and have surface roughness RMS values of 1.2±0.3 nm. The refractive index at 1 μm is 1.95. Absorption loss is shown
to be sensitive to the oxygen partial pressure during growth. Multiple-pulse damage experiments suggest that the scandia
film deposited at the higher oxygen partial pressure accumulates laser-induced trap defects more slowly than the scandia
film deposited in a lower oxygen partial pressure atmosphere.
The single pulse femtosecond laser induced damage threshold (LIDT) of hafnia and silica films is not affected by the
ambient gas pressure. In vacuum, the multiple pulse LIDT drops to ~10% (~10%) of its atmospheric value for hafnia
(silica). The water vapor content of the ambient gas was found to control the change in the LIDT. The LIDT of bulk
fused silica surfaces did now show any dependence on the ambient gas pressure. Hydrocarbons (toluene) did not change
the multiple pulse LIDT for Hafnia films
In this work we use electron spin resonance (ESR) spectroscopy to investigate defects in dual ion beam sputtered HfO2
and SiO2 films. "As-grown" SiO2 films exhibit an ESR feature consistent with an E' center associated with an oxygen
vacancy previously reported. A similar feature with axial symmetry is seen in HfO2 films. The defect giving rise to the
HfO2 ESR feature is distributed throughout the film. In addition, post process annealing of HfO2 and SiO2 films greatly
reduces these defects.
The cavity optics within high power free-electron lasers based on energy-recovering accelerators are subjected to
extreme conditions associated with illumination from a broad spectrum of radiation, often at high irradiances. This is
especially true for the output coupler, where absorption of radiation by both the mirror substrate and coating places
significant design restrictions to properly manage heat load and prevent mirror distortion. Besides the fundamental lasing
wavelength, the mirrors are irradiated with light at harmonics of the fundamental, THz radiation generated by the
bending magnets downstream of the wiggler, and x-rays produced when the electron beam strikes accelerator diagnostic
components (e.g., wire scanners and view screens) or from inadvertent beam loss. The optics must reside within high
vacuum at ~ 10-8 Torr and this requirement introduces its own set of complications. This talk discusses the performance
of numerous high reflector and output coupler optics assemblies and provides a detailed list of lessons learned gleaned
from years of experience operating the Upgrade IR FEL, a 10 kW-class, sub-ps laser with output wavelength from 1 to 6
The IR Upgrade FEL at the Thomas Jefferson National Accelerator Facility (JLab) was used to make measurements of the absorption in laser cavity mirrors, both high reflectors and outcouplers. Measurements were made at 10, 6, and 3 um, by determining the temperature rise of the cooling water of the FEL cavity mirrors while operating at high average power, and by using a laser vacuum calorimeter and interpreting the data using the ISO 11551 standard.
Noncontact optical profilometry was used to characterize the surface flaws on cavity mirrors used in the IR Upgrade FEL at the Tomas Jefferson National Accelerator Facility (JLab). The FEL exposes the cavity mirrors, which have multilayer dielectric coatings, to a unique pulse format. To date, when lasing at 6 mirons at a PRF of 37.4 MHz, the circulating cw power is in excess of 100 kW, the peak cw irradiance exceeds 30 kW/cm2, and the peak irradiance of each pulse is of order 1 GW/cm2. While state-of-the-art, these coatings are far from defect-free, yet have survived those operating conditions without damage after hours of use. The use of noncontact profilometry and the latest software allows us to characterize the size, depth, and distribution of defects in the area covered by the beam footprint in a way that is far more useful than a scratch-dig value. These data provide benchmarks for what defects can be tolerated for lasers having similar irradiances.
With the availability of terawatt laser systems with subpicosecond pulses, laser damage to optical components has become the limiting factor for further increases in the output peak power. Evaluation of different material structures in accordance to their suitability for high-power laser systems is essential. Multi-shot damage experiments, using 110 fs laser pulses at 800 nm, on polycrystalline single layer gold films and multi-layer (gold-vanadium, and gold-titanium) films were conducted. The laser incident fluence was varied, in both cases, from 0.1 to 0.6 J/cm2. No evidence of surface damage was apparent in the gold sample up to a fluence of 0.3 J/cm2. The multilayer sample experienced the onset of surface damage at the lowest fluence value used of 0.1 J/cm2. Damage results are in contrast with the time resolved ultrafast thermoreflectivity measurements that revealed a reduction of the thermoreflectivity signal for the multilayer films. This decrease in the thermoreflectivity signal signifies a reduction in the surface electron temperature that should translate in a lower lattice temperature at the later stage. Hence, one should expect a higher damage threshold for the multilayer samples. Comparison of the experimental results with the predictions of the Two-Temperature Model (TTM) is presented. The damage threshold of the single layer gold film corresponds to the melting threshold predicted by the model. In contrast to the single layer gold film, the multi-layer sample damaged at almost one third the damage threshold predicted by the TTM model. Possible damage mechanisms leading to the early onset of damage for the multilayer films are discussed.
The IR Demo FEL User Facility at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) provided users with a unique source of laser radiation from 1999 to 2001. Utilizing superconducting RF linacs with electron recirculation and energy recovery, the machine lased with up to 2100 W of average power output at 3.1 microns with very high beam quality. It was capable of output wavelengths in the 1 to 6 micron range and produced ~0.7 ps (and shorter) pulses in a continuous train at ~75 MHz. This subjected the cavity optics to a unique combination of high average and peak irradiances. In addition, cavity optics were subjected to high energy X-rays in a high vacuum environment. In this talk I will summarize how the optics (cavity and transport) survived these conditions. Upgrades that are underway will extend operation beyond 10 kW average power in the near IR and kilowatt levels of power at wavelengths from 0.3 to 14 microns. Drawing from our experience and from research presented at these symposia, I will present the design of these new lasers.
Lasing at high power in FELs has been achieved so far only with a near-concentric resonator . Though this design can scale up to quite high power, it is ultimately limited by the mirror steering stability as the resonator design approaches con-centricity. This constraint may be avoided by using a near-confocal resonator operated in a ring configuration. It is found that, if a small amount of gain focusing is present, the near-confocal resonator eigenmodes are modified such that the lowest order mode collapses around the electron beam and is large in the return (non-focusing) direction. This eigenmode is stable and is relatively insensitive to changes in the mirror radii of curvature and the strength of the electron beam focusing. This paper will present the theory of this new concept.
Material processing with lasers has grown greatly in the previous decade, with annual sales in excess of $1 B (US). In general, the processing consists of material removal steps such as drilling, cutting, as well as joining. Here lasers that are either cw or pulsed with pulsewidths in the microsecond(s) time regime have done well. Some applications, such as the surface processing of polymers to improve look and feel, or treating metals to improve corrosion resistance, require the economical production of laser powers of the tens of kilowatts, and therefore are not yet commercial processes. The development of FELs based on superconducting RF (SRF) linac technology provides a scaleable path to laser outputs above 50 kW, rendering these applications economically viable, since the cost/photon drops as the output power increases. Such FELs will provide quasi-cw (PRFs in the tens of MHz), of ultrafast (pulsewidth approximately 1 ps) output with very high beam quality. The first example of such an FEL is the IR Demo FEL at the Thomas Jefferson National Accelerator Facility (Jefferson Lab), which produces nearly 2 kW of high average power on a routine basis. Housed in a multilaboratory user facility, we as well as members of our user community have started materials process studies in the areas mentioned earlier. I will present some of the first results of these studies. I will also briefly discuss the status of our DOD-funded project to upgrade the FEL to 10 kW in the mid IR.
Mirror heating in a high power FEL can alter the optical mode and affect the gain of the laser. This can lead to a large reduction of the laser power from ideal values. Measurements of the power and mode size in the Jefferson Lab IR Demo laser have shown clear evidence of mirror distortion at high average power loading. The measurements and comparisons with modeling will be presented. Both steady state and transient analyses and measurements are considered.
A versatile free electron laser (FEL) user facility has recently come on line at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) providing high average (kilowatt-level) power laser light in the infrared. A planned upgrade of the FEL in this facility will extend the wavelength range through the visible to the deep UV and provide the photobiology community with a unique light source for a variety of studies. Planned and potential applications of this FEL include: IR studies of energy flow in biomolecules, IR and visible imaging of biomedical systems, IR and visible studies of photodynamic effects and UV and near visible studies of DNA photodamage.
The performance of laser pulses in the sub-picosecond range for materials processing is substantially enhanced over similar fluences delivered in longer pulses. Recent advances in the development of solid state lasers have progressed significantly toward the higher average powers potentially useful for many applications. Nonetheless, prospects remain distant for multi-kilowatt sub-picosecond solid state systems such as would be required for industrial scale surface processing of metals and polymers. We present operation results from the world's first kilowatt scale ultra-fast materials processing laser. A Free Electron Laser (FEL) called the IR Demo is operational as a User Facility at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, USA. In its initial operation at high average power it is capable of wavelengths in the 2 to 6 micron range and can produce approximately 0.7 ps pulses in a continuous train at approximately 75 MHz. This pulse length has been shown to be nearly optimal for deposition of energy in materials at the surface. Upgrades in the near future will extend operation beyond 10 kW CW average power in the near IR and kilowatt levels of power at wavelengths from 0.3 to 60 microns. This paper will cover the design and performance of this groundbreaking laser and operational aspects of the User Facility.
Jefferson Lab's IR Demo FEL Facility includes an associated 600 m2 user facility containing six separate laboratory areas. In the summer of 1999 we began delivery of beam int two of these labs as part of our commissioning of the FEL optical transport and laser safety systems. The high average power capability in the mid-IR, along with an ultrafast high PRF temporal structure makes this laser a unique source for both applied and basic research. While commissioning, we conducted several test, primarily of laser-materials interactions that take advantage of the unique characteristics of this FEL. An overview of the FEL facility and its current performance, along with a synopsis of current and future experiments, will be presented.
The Thomas Jefferson National Accelerator Facility (formerly known as CEBAF) has embarked on the construction of a 1 kW free-electron laser operating initially at 5 microns that is designed for laser-material interaction experiments and to explore the feasibility of scaling the system in power for Navy defense and industrial applications. The accelerator system for this IR demo includes a 10 MeV photocathode-based injector, a 32 MeV CEBAF-style superconducting radio-frequency linac, and single-pass transport that accelerates the beam from injector to wiggler, followed by energy-recovery deceleration to a dump. The initial optical configuration is a conventional near-concentric resonator with transmissive outcoupling. Following commissioning, the laser output will be extended to an operating range of 3-to-6.6 microns, and distributed to six labs in a user facility built with funds from the Commonwealth of Virginia. A description of the machine and facility and the project status are presented.
The Thomas Jefferson National Accelerator Facility (formerly known as CEBAF) has embarked on the construction of a 1 kW free-electron laser operating initially at 3 microns that is designed for laser-material interaction experiments and to explore the feasibility of scaling the system in power and wavelength for industrial and Navy defense applications. The accelerator system for this IR demo includes a 10 MeV photocathode-based injector, a 32 MeV CEBAF-style superconducting radio-frequency linac, and single-pass transport which accelerates the beam from injector to wiggler, followed by energy-recovery deceleration to a dump. The electron and optical beam time structure in the design consists of a train of picosecond pulses at 37.425 MHz pulse repetition rate. The initial optical configuration is a conventional near-concentric resonator with transmissive outcoupling. Future upgrades of the system will increase the power and shorten the operating wavelength, and utilize a more advanced resonator system capable of scaling to high powers. The optical system of the laser has been modeled using the GLADR code by using a Beer's-law region to mimic the FEL interaction. Effects such as mirror heating have been calculated and compared with analytical treatments. The magnitude of the distortion for several materials and wavelengths has been estimated. The advantages as well as the limitations of this approach are discussed.
The Laser Processing Consortium, a collaboration of industries, universities, and the Continuous Electron Accelerator Facility in Newport News, Virginia, has proposed building a demonstration industrial processing laser for surface treatment and micro-machining The laser is a free-electron laser with average power output exceeding 1 kW in the ultraviolet. The design calls for a novel driver accelerator that recovers most of the energy of the exhaust electron beam to produce laser light with good wall-lug efficiency. The laser and accelerator design use technologies that are scalable to much higher power. We will describe the critical design issues in the laser such as the stability, power handling, and losses of the optical resonator, and the quality, power, and reliability of the electron beam. We will also describe the calculated laser performance. Finally progress to date on accelerator development and resonator modeling will be reported.
The laser properties of the LiCaAlF6:Cr3+ (Cr:LiCAF) material are reviewed. The impact of the absorption and emission spectra on the laser performance is discussed. Laser-pumping and flashlamp-pumping experiments have shown that Cr:LiCAF has potential for high efficiency, although the presence of scattering losses remains a significant problem. The "gradient freeze" growth technique has been found to generate lower-loss material compared to Czochralski growth. The thermal lensing of Cr:LiCAF has been measured to be small and is in rough agreement with the magnitude expected on the basis of the intrinsic thermo-optical properties.