The Solid-State, Heat-Capacity Laser (SSHCL) program at Lawrence Livermore National Laboratory is a multigeneration laser development effort scalable to the megawatt power levels. Wavefront quality is a driving metric of its performance. A deformable mirror with over 100 degrees of freedom situated within the cavity is used to correct both the static and dynamic aberrations sensed with a Shack-Hartmann wavefront sensor. The laser geometry is an unstable, confocal resonator with a clear aperture of 10 cm x 10 cm. It operates in a pulsed mode at a high repetition rate (up to 200 Hz) with a correction being applied before each pulse. Wavefront information is gathered in real-time from a low-power pick-off of the high-power beam. It is combined with historical trends of aberration growth to calculate a correction that is both feedback and feed-forward driven. The overall system design, measurement techniques and correction algorithms are discussed. Experimental results are presented.

A diode-pumped Yb:YAG laser has been demonstrated. A V-shape unstable resonator with a Super Gaussian coupling mirror was chosen. We describe the model that permits to choose the parameters of the cavity and predict the laser performances. A diode pumping architecture is used in which 941 nm radiation is homogenously delivered to the laser crystal. We present here the Pumping Delivery Optics and the laser performances.

A servomechanism for identification of the CO₂ laser lines, and searching desired laser signatures is elaborated. The laser signature is used as a standard for calibration of the servomechanism. Algorithms for process automation are find. The system can be used in servo-loop mechanisms for stabilization of the laser operation to a chosen emission line. The method can be expanded using different isotopes of carbon, and/or oxygen molecules in the laser medium. Some applications of the method are suggested.

The transverse output profile and mode competition in CO₂ lasers are significantly affected by the discharge current, as was reported by Witteman [IEEE J. Quantum Electron. QE-4, 786-8 (1968)]. He found that in a sealed laser, with a stable resonator, a spatial mode switching is observed upon increasing the current; due to a modification in the radial profile of the small signal gain. Through an atypical gain profile the lowest-loss bare cavity mode, usually dominant in laser dynamics, may have lower net cavity gain than a mode with higher diffraction losses. Through this work a dynamic differential equation for the homogeneously saturating gain is included in the original dynamic coupled modes method [Appl. Opt. 29, 3905-15 (1990)] and applied to a CO₂ unstable resonator, with suitable high current small signal gain profiles. By expanding the gain loaded cavity field into the bare cavity oscillation eigenstates, this new model provides a realistic temporal evolution of mode competition, output power and gain saturation within the resonator. We have found that although unstable resonators have excellent transverse mode discrimination the spatial mode switching may also occur, resulting in a significant modification in the output intensity profile. Thus, under certain design parameters, the common assumption of the small signal gain to be constant through the lasing medium may incur in serious inaccuracies for determining the transverse intensity profile and output power. The application of the method is fully described, and the results and their connection to relevant physical properties of gas lasers are discussed.

Ince-Gaussian modes form a complete family of exact and orthogonal solutions of the paraxial wave equation for elliptical coordinates. The transverse distribution of these fields is described by the Ince polynomials and have an inherent elliptical symmetry. These modes constitute a smooth transition from Hermite-Gaussian modes to Laguerre-Gaussian modes. We report the experimental observation of Ince-Gaussian modes directly generated in a stable resonator. By slightly breaking the symmetry of the cavity of a diode pumped Nd:YVO₄ laser and its pump beam configuration we were able to generate single high order Ince Gaussian modes with very high quality. The observed transverse modes and nodal patterns have the proposed elliptic structure and exhibit remarkable agreement with the theoretical predictions.

High-Q whispering-gallery modes with unidirectional emission are present in spiral - shaped microresonators despite completely chaotic ray dynamics. We demonstrate that formation of such modes is due to dynamical localization, and develop the theory of this effect.

Propagation of high-power femtosecond laser pulse through random media is accompanied by transverse spatial distortions of the laser beam. Occurrence of high-intensity small-scale perturbations due to atmospheric disturbance may result in beam breakup and filament generation. Coherent scattering on random ensemble of aerosol particles result in these disturbance. On the other hand, plasma formation, owing to focusing of light into aerosol microdroplet, may hinder filamentation. The purpose of this paper is to numerical study the propagation of a femtosecond laser pulse through water aerosol. In particular, we will find the transverse intensity distribution resulting from coherent scattering of a 800 nm 45 fs pulse with 10 - 60 GW peak power on the ensemble of water droplets with the atmospheric size distribution and the density 100 cm^-3. The forward scattering on aerosol particles takes into account the phase of the scattered radiation. The transverse distribution of the laser field behind the aerosol layer is calculated as the result of the interference of light fields formed by each particular particle. In the numerical simulations the input radius of a Gaussian beam was a = 2.5 mm. The average size of an aerosol particle was 4 μ. The length of a propagation path was set to the half of the diffraction length: 0.5^.ka². As the result we have shown that in aerosol medium it is possible forming of several hot spots containing approximately 1 critical power for self-focusing.

We propose a versatile set-up dedicated to programmable beam shaping of femtosecond pulses in a focal plane. A non-pixelated liquid-crystal light valve is used as the phase-front modulator. We demonstrate active and adaptive wavefront correction of a 4-μJ, 100-kHz amplified laser chain, where residual wavefront distorsions are decreased down to λ/15 peak-valley and λ/100 rms. The subsequent improvement for micromachining applications is investigated, and diffraction-limited holes are demonstrated on various materials. Moreover, beam patterning in the focal plane is also presented. Theoretical calculations of the required phase modulation are proposed, and experimental shapes are demonstrated, like square and circular top-hats, as well as triangle or doughnut shapes.

Typical applications of ultra-high-power femtosecond lasers include precision drilling and surface micro-machining of metals, and micro-structuring of transparent materials. However, high peak-power pulsed lasers are difficult to focus close to the diffraction limit because of aberrations that induce deviations from a perfect spatial wave-front. The sources of these aberrations include thermally induced and nonlinear optical distortions, as well as static distortions such as those introduced by gratings used in chirped-pulse amplification (CPA). A spatially clean beam is desirable to achieve the highest possible intensity on-target, and to minimize the energy deposited outside the central focus. One way to achieve this is to correct the wave-front using an adaptive optical element such as a deformable mirror, a more cost-effective solution than increasing peak intensity by providing further pulse amplification. The wave-front of the femtosecond system is measured using a Hartmann-Shack wave-front sensor, and corrected with a 37-channel deformable membrane mirror used slightly off-axis. The deformable mirror has been tested with a FISBA OPTIK μPhase HR digital interferometer, which is also used to calibrate the performance of the wave-front sensor. The influence of fluctuations of the laser on the measurement is minimised by averaging the centroid positions obtained from several consecutive frames. The distorted wave-front is compared to a reference flat wave-front which is obtained from a collimated laser diode operating at the same wavelength as the femtosecond system. The voltages on the deformable mirror actuators are then set to minimise the difference between the measured and reference wave-fronts using a simple least squares approach. Wave-front sensor and correction software is implemented in Matlab.

We propose a novel and simple technique to determine the temporal profile of ultra-short laser pulses uniquely from a measured auto-correlation spectrum. It involves forming a sequence of two laser pulses spaced by a time delay τ, the first one being the pulse to be characterized and the second one a reference pulse. This sequence is sent through any device that measures the Fourier transform (FT) magnitude of the sequence's temporal profile, such as a classical optical auto-correlator. This FT magnitude is then processed analytically using a novel algorithm to retrieve the temporal profile of the sample pulse unambiguously. The reference pulse can be either an unchirped symmetric pulse or any pulse with a known profile. This requirement does not constitute a limitation because once the temporal profile of a given pulse has been characterized by this technique, even though it may not be an unchirped symmetric pulse, this pulse can be used as the reference pulse to determine the profile of any other ultra-short pulse. Compared to other measurement techniques, such as frequency-resolved optical gating, our technique is much faster and simpler, in terms of both experimental and computational complexity. Simulations also show that the profile recovery is quite accurate even in the presence of strong noise on the measured FT magnitude.

Direct spatial profiling of high power near IR lasers can be extremely useful in determining the applicability of these lasers in materials processing. Because of the construction of these lasers, direct imaging in real time has been nearly impossible due to the high energy density at the focus. It is not possible to image a raw beam, because there is no raw beam in the conventional sense. We discuss the construction of a new imaging target that allows direct spatial imaging at the focus of these lasers, and give examples of its applicability.

In lasers with diffraction-limited beam quality, aberrations cause diffraction losses which reduce power output. Diffraction limit may not be attained due to phase distribution and amplitude modulation. Adaptive aberration correction has been principally achieved by either Deformable Mirrors or Phase Conjugate Mirrors. A new approach to adaptive optics based on wavelet-based phase extraction will be applied to distortion correction in diode array lasers.

A resonator configuration is proposed in which HR mirror is used in a folded configuration employing a Corner cube prism. Variable output is obtained by rotation of Quarter wave Plate. Two Resonator configurations are presented. Round trip Jones Matrix is calculated & loss factor is plotted against angle of rotation of Quarter wave plate for both configurations. Nd:YAG laser built in configuration 1 is described. Effect of rotation of corner cube prism on the loss factor & effect of misalignment of HR mirror on output energy is investigated.

We present a detailed study of the unstable Bessel resonator. The cavity of this laser consists of a reflective axicon and a convex spherical output coupler. A matrix method analysis for the bare resonator is employed to extract the eigenmodes and eigenvalues of the cavity, which allows us to obtain the fundamental and higher-order modes. Diffractive losses and relative phase shift behavior, in terms of both the varying radius of curvature of the output coupler and the cavity length, are studied with the matrix method and the Fox-Li algorithm. Direct comparison of the transverse mode profiles with a similar resonator employing a concave output coupler is performed, showing excellent agreement for large values of the radius of curvature. We also considered the effects of varying the aperture of the output coupler and the wedge angle of the axicon on the transverse mode profiles and diffractive losses.

We report the generation of Bessel-Gauss beams using a CO₂ laser resonator. The cavity is composed by a plane output mirror and a total reflective axicon, this configuration had been studied previously by Gutierrez-Vega et al [J.Opt.Soc.Am.A 20, 2113-22 (2003)]. Bessel-Gauss beams are produced directly from the cavity. The use of a reflective axicon instead of a refractive one results in reduction of surface-induced aberrations, minimizing absorption and increasing the non-diffracting distance. This results in a higher power non-diffracting laser beam with potential scientific and industrial applications. In order to characterize the resonator, we have obtained its output transverse intensity distribution. Additionally, we have numerically and experimentally studied the effects of mirror tilt on the output transverse mode structure. We have made numerical simulations of the misaligned resonator modes based on Bowers’s method [Appl. Opt. 31, 1185-98 (1992)]. Direct comparison of numerical and experimental results allow us to estimate the diffractive losses of the modes on the misaligned cavity and their dependence on the aligned bare cavity eigenmodes, thus providing valuable information of the output power dependence on mirror misalignment. Relevant experimental parameters and numerical procedure are fully described.

A theoretical time-dependent analysis of a high-average power copper HyBrID laser is proposed, pointing out the time-varying properties of beam quality parameters and brightness. Numerical data are compared to experimental measurements performed on a 80 W average power copper HyBrID laser. A significant improvement of the beam quality with time is put in evidence.

The efficient manufacturing of light-weighted mirror substrates is an important technology for the optical industry. The presentation deals with the prerequisites for the production of such substrates with respect to material properties, as well as manufacturing technology. Different materials are being presented with special emphasis on low- and zero thermal expansion materials. Their material properties of importance for the production of light weighted mirror substrates are being compared. Further more possible light weighting strategies are compared regarding light weighting success, manufacturing effort, necessary manufacturing technology and therefore price impact. For the successful implementation of these manufacturing processes a demonstrator part is shown including the flatness results achieved.

For the past 28 years, the Laser Hardened Materials Evaluation Laboratory (LHMEL) at the Wright-Patterson Air Force Base, OH, has worked with CO₂ lasers capable of producing continuous energy up to 150 kW. These lasers are used in a number of advanced materials processing applications that require accurate spatial energy measurements of the laser. Conventional non-electronic methods are not satisfactory for determining the spatial energy profile. This paper describes continuing efforts in qualifying the new method in which a continuous, real-time electronic spatial energy profile can be obtained for very high power, (VHP) CO₂ lasers.

We have shown that control of stochastic multiple filamentation may be performed with either large - scale spatial modifications of the beam, such as squeezing the whole beam, or relatively small-scale periodic light field perturbations introduced into the transverse beam distribution. We have found that the average conversion efficiency to the supercontinuum grows according to the similar law in both small beam and large beam cases, starting from the point of the parent filament formation. Stability of the supercontinuum signal grows essentially with decreasing initial beam size. Periodic intensity and phase perturbations are used to control stochastic filamentation arising in atmospheric turbulence. Regular phase fluctuations are introduced into the beam in the form of a lens array. With decreasing array period the spatial arrangement is attained earlier in the propagation distance. In addition, the amplitude of multiple filaments has smaller fluctuations relatively to the propagation in the regular medium. In the case of periodic intensity perturbations, control of stochastic filaments is more pronounced as compared with the phase perturbations of the same period. However, introduction of amplitude perturbations leads to the energy loss from the initial pulse.

We study the effect of boundary roughness on the resonant states broadening of the optical whispering-gallery-mode microdisk lasing cavities. We develop a new, computationally effective, and numerically stable approach based on the scattering matrix (S-matrix) technique that is capable to deal with both arbitrary complex geometry and inhomogeneous refraction index inside the two-dimensional cavity. The method presented has been applied to study the effect of surface roughness and inhomogeneity of the refraction index on Q-values of microdisk cavities for lasing applications. We demonstrate that even small surface roughness (Δr⟨ λ/50) can lead to an extreme degradation of high-Q cavity modes by many orders of magnitude. The results of numerical simulation are analyzed and explained in terms of wave reflection at a curved dielectric interface combined with the examination of Poincare surfaces of section as well as Husimi distributions.

Motivated by the nonlinear phase measurements in rare-earth-doped fibers reported by Digonnet, et al (J. Lightwave Technol. 15, 299 (1997)), I provide theoretical evidence of passive synchronous phasing in ytterbium-doped multi-core fiber laser arrays of up-to 7 cores due to the existence of a negative resonant nonlinear index to which transitions at UV frequencies contribute. The effect appears to become more elusive with increasing array size, and confirmation for a 19-core array, for example, is still sought.

We propose to generate few-fs or as X-ray laser pulses by beating of two or more X-ray laser lines with appropriate frequency separation. X-ray lasers operating on transitions in neon- or nickel-like ions typically have gain on several lines with difference frequencies of around 10¹⁵ Hz. Moreover, it is found in specific cases that a few almost equidistant lines may exhibit gain. Beating of these lines results in a series of pulses with durations down to the attosecond range. It is shown that phase locking can be achieved by means of a Langmuir wave in the X-ray laser medium itself, which is resonant with the difference frequency.

Radially polarized radiation shows some very interesting properties and has therefore gained interest in recent years. An overview of the advantages and the various applications where radially polarized modes are beneficial is given. In addition the different known methods to generate radial polarization are reviewed. In our work we developed a method to generate radially polarized laser beams by means of a polarization selective resonant grating mirror. The undesired polarization is coupled to a mode of the dielectric multilayer of the resonator end mirror and experiences severe losses while the radial polarization is not affected and oscillates in the laser resonator. Fundamental and higher-order radially polarized modes of high polarization purity and powers of more than 100W have been demonstrated.

We investigated lasing characteristics of resonance modes in a thin dielectric coated square cavity with round corners. After coating dye-doped PMMA on a square capillary, a thin polystyrene layer with a thickness of about 0.5 micron was overcoated. Lasing peaks corresponding to ring-type whispering gallery modes were observed in air environment. When the capillary was inserted into glycerine whose refractive index was slightly smaller than PMMA, the mode spacing of lasing peaks was severely reduced. We confirmed this reduced lasing peaks corresponded to waveguide-type modes distributing on the polystyrene layer and the laser gain was originated from evanescent-wave coupling mechanism.

We use the multi-mode lasing equations of Haken to analyze the stationary state lasing patterns of two-dimensional dielectric microcavity lasers of different shape, including the circle and various smooth deformations of the circle. We find a generic increase in the power output with deformation which is relatively insensitive to the specific form of the shape deformation. In addition we find strong mode selection in favor of librational modes (including but not solely the bow-tie modes) in the case when the pumping is concentrated near the center of the cavity. These results point towards an explanation of the dramatic results on power increase with deformation obtained by Gmachl et al. in quantum cascade micro-cylinder lasers. The sensitivity of the lasing solutions to the nature of the ray dynamics (chaotic, integrable and mixed) will also be analyzed.

We demonstrate that the phenomenon of Dynamical Anderson Localization of light leads to high-Q whispering-gallery modes in microcylinder and microdisk resonators with substantial surface roughness, and determines their lifetimes and emission patterns.

The recently suggested self-coupling microfiber coil optical resonator (COR) is a simplest functional element for the future microfiber-based photonics. It could be created by wrapping a microfiber on a dielectric rod with smaller refractive index. It is feasible that COR, which is produced from a drawn optical microfiber, will not suffer from surface roughness as e.g. the lithographically fabricated 2D microrings. Therefore, COR may have extremely small losses and generate high-Q resonances. In this paper, the theoretical study of the basic electromagnetic properties of the uniform self-coupling COR with N turns is presented. The eigenmodes of COR, which are qualitatively different from the modes of the known types of resonators, exist for the discrete values of the dimensionless coupling parameter K=κS ⟩ ½, where κ is the coupling coefficient between adjacent turns and S is the length of a turn. The spatial variation of the mode amplitudes does not have the wavelength scale oscillations and has no correlation with the period of COR, S. For certain series of K, the free spectrum range of COR is independent of the number of turns N and COR behaves similar to a single ring resonator. For N→∞, the microfiber coil optical waveguide (COW) has a simple dispersion relation implying the absence of stop bands. The value K = ½ corresponds to the crossover between two regimes of propagation: with and without zeroing of the group velocity. At the crossover, the dispersion relation of COW has inflexion points wherein the group velocity and the inversed group velocity dispersion simultaneously become zero.

Organic electro-optic materials offer exceptional processability (both from solution and the gas phase) that permit fabrication of flexible and conformal device structures and the integration of organic materials with a wide range of disparate materials. In addition, organic electro-optical materials have fundamental response times that are in the terahertz region, and useable electro-optic coefficients that are approaching 300 pm/V (at telecommunication wavelengths). In addition to fabrication by traditional lithographic methods, multiple devices on a single wafer have been fabricated by soft and nano-imprint lithography. In this presentation, we review the fabrication and performance evaluation of a number of all-organic and organic-silicon photonic ring microresonator devices. Both electrical and thermal tuning of devices, including both single and multiple ring micro-resonators, are demonstrated.

We theoretically show that achieving high Q-factors of small whispering gallery mode resonators, together with significant non-harmonism (non-equidistance) of its modes, allows for extensive analogy between properties of atoms and the resonator modes. The non-harmonism in very high-Q microresonators is caused by the material as well as geometrical dispersion. The established analogy leads to creating of a "photonic" media with properties similar to those of atomic media. For instance, effective "two-level" systems can be formed within the resonator. The "levels" of the systems, i.e. the resonator modes, can be coupled with microwave or optical fields. Strong confinement of the light in the resonator along with high Q-factors results in nonlinear processes at low light levels even with small nonlinearities typical of optically transparent materials.

We present a study of optical hyper-parametric oscillator based on a nonlinear high-Q whispering gallery mode resonator and demonstrate that the oscillator produces stable narrow band beat note of the pump, signal, and idler waves making an all-optical secondary frequency reference feasible. We discuss possibilities of tuning of the oscillator.

In a mode-locked laser, a wave packet of light of transverse dimension of the order of a mm, and longitudinal dimension of only a few micron, travels back and forth in a resonator of the order of one or two meter. It is difficult to conceive why a light bullet, six orders of magnitude shorter than the cavity, would care whether its central wavelength would fit as a sub-multiple of the cavity length. As the length of the resonator changes constantly because of vibrations, thermal drifts, the “central wavelength” of the intracavity fs pulse should also change constantly to follow the cavity resonances. We present evidence that this is indeed the case, and that the micron long wave packet traveling in the cavity does indeed keep record of cavity motion, with subwavelength accuracy. Applications range from distance measurements with a spatial resolution of 0.01 pm, and fs temporal resolution, to inertial navigation (measurement of acceleration and rotation). Stabilization of the mode-locked laser can enhance the resolution of these measurements by at least three orders of magnitude.

We investigate experimentally the self-compression behavior of high-power femtosecond pulses in normally dispersive solid bulk media with un-chirped laser pulses and negatively chirped laser pulses. It is demonstrated that high-power femtosecond laser pulses can be compressed by the nonlinear propagation in the transparent bulk media, and the temporal and spectral characteristics of resulted pulses were found to be significantly affected by the input laser intensity, with higher intensity corresponding to shorter compressed pulses. By the propagation in a piece of thin BK7 glass plate, a self-compression from 50fs to 20fs was achieved, with a compression factor of about 2.5. However, the output laser pulse was observed to be split into two peaks when the input laser intensity is high enough to generate supercontinuum and conical emission. When the input laser pulse is negatively chirped, the spectra of the pulse is reshaped and narrowed due to strong self-action effects, and the temporal pulse duration is found to be self-compressed, instead of broadening. With the increase in the input pulse intensity, the resulted self-compressed pulses became even shorter than the input laser pulse, and also shorter than sech² transform-limited pulse according to the corresponding spectra. The self-compression scheme is simple and robust, and it is promising as a new pulse compression method to achieve intense laser pulses of few cycles.

We discuss the new type of the closed loop adaptive optical system with bimorph corrector and M2 meter. The study of some multi-dither algorithms to be used in the system is presented. Multi-dither approach is applied to 40-TW TiS fs laser in JAERI, Japan, to improve the focusability of laser beam. We demonstrate that it is possible to obtain 75% of input power in first diffraction maximum with use both phase conjugate and multi-dither adaptive optical systems.

The HELEN laser is a three-beam, large aperture Nd:glass laser, used for plasma physics studies at the Atomic Weapons Establishment in the UK. Two of the beams nominally deliver 500 J each in 1 ns at the second harmonic (527 nm). The third beam, the “backlighter”, has recently been upgraded to operate as a chirped pulse amplification system and it now routinely delivers 70 J to target in 500 fs. Optimal focal spot performance is achieved using a closed-loop adaptive optics system, which ensures good wavefront characteristics, irrespective of whether previous firing of the amplifiers has induced refractive index variations in the laser glass. The system uses a 32 element bimorph mirror with 98 mm aperture, roughly half way through the laser chain. A Shack-Hartman wavefront sensor, positioned at the output of the laser is the diagnostic used to provide feedback to the deformable mirror. Correction of the static and slowly varying aberrations on the beam has been demonstrated. The fast aberrations induced during the flashlamp discharge have been evaluated. The improved focal spot characteristics result in an intensity on target of significantly greater than 10¹⁹ Wcm^-2.

We investigate the coherent cavity-field coupling in chains of polystyrene microspheres doped with CdSe nanocrystals. The coupled cavity emission is studied by imaging spectroscopy and polarization-sensitive mode mapping. The spatial dependence and polarization nature of coupled and uncoupled cavity states are discussed for applications as building blocks in coupled resonator optical waveguides. When coherent photon states are formed in the coupled cavities, the symmetric field distribution of a single microsphere evolves into a directional emission pattern with strongest modifications along the axes of the coupled cavity system. For weak coherent cavity coupling, the coupled modes show almost vanishing field intensity along the axis. In case of weak coupling still cavity modes can be observed for which the single sphere Q-factor is maintained which is promising for applications in coupled resonator optical waveguides.

Now optical reducing systems for extreme ultraviolet projection lithography are being actively developed. Optical elements of these systems are required to be of super-high optical quality. For systems operating in the 13-nm wavelength range, their optical distortions should not exceed 1 nm in magnitude. Manufacturing of such elements with super-high optical quality requires large financial injections. In this report, we consider how to use thermal deformation of an optical element exposed to light for improvement of optical quality of the element. It is shown, in particular, that residual quasi-static large-scale (20% of diameter of the element) optical distortions, about 15 nm in magnitude, can be compensated with the proposed technique down to 0.5 nm (i.e. ≈ λ₀/20 - λ₀/30 for EUV).