Initial Demonstration of Mercury Wavefront Correction System

We present a high peak and average power laser system with ultrashort pulses at high repetition rates. Pulse shortening and peak power enhancement of a state-of-the-art fiber laser system is achieved by utilizing nonlinearity, namely selfphase modulation and subsequent compression in a chirped mirror compressor. The nonlinear interaction is achieved by propagation in a noble gas filled hollow core fiber with an inner diameter of 200 μm and a length of 0.5 m. A total second dispersion of -7000 fs² is applied by a chirped mirror compressor resulting in ultrashort pulses of 71 fs duration. This is achieved by coupling 400 MW, 800 fs pulses from the CPA system to the Xenon filled hollow core fiber. The average power at the output of the compressor is measured to be 10 W at 50 kHz repetition rate resulting in 200 μJ pulse energy. Hence, the compressed pulses have a peak power of more than 2 GW. Consequently, the pulses of the CPA system are shortened by a factor of ten and the peak power is enhanced by a factor of 5. In addition this approach offers further peak and average power scalability.

In the last year there have been reports from various research groups around the globe about the onset of modal instabilities (or dramatic degradations of the beam quality) in high average power laser systems. Many of these reports describe how the usual output Gaussian beam of the fiber laser system transforms into a higher order mode (most typically a LP 11 -like mode). This effect is commonly attributed to tr ansversal hole burning. However, this theory alone cannot really explain reports of this effect being observed in fibers in which only the central part of the fiber core has been doped (such as the Rod-Type fibers). As far as we know, up to date no theoretical work has been published in the subject to investigate the true physical origin of this effect. In this paper we present such study and the conclusions obtained from it. It has been found that conventional transversally-resolved rate equation models, able to take transversal hole burning into account, cannot explain this eff ect when preferential ga in designs are investigated. Acco rding to our investigations the inclusion of modal interference along the fibe r is crucial to explain this effect in all type of fibers. Unfortunately current BPM models (able to account for modal interference along the fiber) are not able to take into account transversal hole burning. Thus, we have developed an advanced active fiber model that combines BPM and the transversally-resolved rate equations. This model reveals, for the first time, the im portant role played by modal interference along the fiber. Keywords: Mode Instabilities, Beam Propagation, Fiber Laser

Nearly every new large-scale laser system application at LLNL has requirements for beam control which exceed the current level of available technology. For applications such as inertial confinement fusion, laser isotope separation, laser machining, and laser the ability to transport significant power to a target while maintaining good beam quality is critical. There are many ways that laser wavefront quality can be degraded. Thermal effects due to the interaction of high-power laser or pump light with the internal optical components or with the ambient gas are common causes of wavefront degradation. For many years, adaptive optics based on thing deformable glass mirrors with piezoelectric or electrostrictive actuators have be used to remove the low-order wavefront errors from high-power laser systems. These adaptive optics systems have successfully improved laser beam quality, but have also generally revealed additional high-spatial-frequency errors, both because the low-order errors have been reduced and because deformable mirrors have often introduced some high-spatial-frequency components due to manufacturing errors. Many current and emerging laser applications fall into the high-resolution category where there is an increased need for the correction of high spatial frequency aberrations which requires correctors with thousands of degrees of freedom. The largest Deformable Mirrors currently availablemore » have less than one thousand degrees of freedom at a cost of approximately $1M. A deformable mirror capable of meeting these high spatial resolution requirements would be cost prohibitive. Therefore a new approach using a different wavefront control technology is needed. One new wavefront control approach is the use of liquid-crystal (LC) spatial light modulator (SLM) technology for the controlling the phase of linearly polarized light. Current LC SLM technology provides high-spatial-resolution wavefront control, with hundreds of thousands of degrees of freedom, more than two orders of magnitude greater than the best Deformable Mirrors currently made. Even with the increased spatial resolution, the cost of these devices is nearly two orders of magnitude less than the cost of the largest deformable mirror.« less

Optical component specifications for high average power laser systems must address performance, efficiency, maintainability, and reliability. The requirements for high performance optics goes beyond conventional methods of specification. Methods of specification for this application are discussed in addition to several areas that need attention within the optics community. The specifications must be based upon the available methods of testing. The ability to test the optical components is crucial to both successful manufacturing and system operation. This means that both the manufacturer and user must have testing capability. For large system applications, the test facility must be designed to meet the demand of a high volume of optics as well as measurement precision. The facility for testing these parts is described for an application requiring thousands of optical components. This test capability is also an important part of system design and development when the performance of high performance optics can be defined and incorporated during planning for a large facility.

Using a stretched hollow core fiber and an acousto-optic modulator based pulse shaper, we demonstrate compression, characterization and shaping of ultrashort pulses from a high average power laser system.

In this article we describe a laser plasma source for Extreme Ultraviolet Lithography (EUVL) based on a liquid water jet target. Although jet targets are known for some time now, no attempts have been made to prove the functionality of the target under conditions similar to an EUVL production-line facility, that means illumination with high average power laser systems (in the multi-kW regime) at repetition rates in the kHz region. Such systems are currently under development. We used the MBI-burst laser to simulate these extreme illumination conditions. We examined the hydrodynamic stability of the target as a function of the laser repetition rate at different average laser powers (0.6kW and 5kW per burst). Additionally, the dependence of the conversion efficiency on pulse duration in the range from 30ps to 3ns was investigated. From our results one can conclude parameters for future design of driver lasers for EUVL systems.

The development of laser technique and various applications of laser radiation requires the use of devices that can modify as well as correct for the wavefront of laser beams. In these cases, the use of deformable mirrors is simply very important. In the high-power laser setups, the task of correction of laser aberrations assumes the use of thermally stabilized optical elements. We consider that two types of wavefront correctors are the most well-suitable for such an application: bimorph deformable mirrors and stacked-actuator (SADM) ones. Bimorph deformable mirrors proved their reliability and could be used in high-power industrial lasers. They can perfectly correct for low-order and are simple in manufacturing. On the contrary stacked-actuator mirror can reproduce high-order wavefront aberrations and thus have high spatial resolution of control elements and might be applied in fast adaptive optical systems to correct for laser radiation distributed through turbulent media.

A 30 kW, continuous wave (CW), Photo-initiated, Impulse-enhanced, Electrically-excited (PIE) CO2 laser is presented. This high power laser system is unique in that it is also capable of modulated optical output over a wide range of frequencies featuring multi-kilowatt peak power with high average power. The primary focus of this paper is documentation of the enhanced performance achievable from this laser when operated under a burst mode excitation technique. Excitation of the system, using a burst mode process, results in a dramatic change in the output characteristics of the laser. Control of burst frequency and duty cycle provides a convenient means to alter the time-varying nature of the output beam. Low burst frequencies result in clearly defined pulse profiles, falling to zero in between pulses. Increasing burst frequency results in a significant offset or quasi-CW time-varying output. At low burst durations, optical output has a triangular profile. Increasing burst duty cycles produce square optical pulse shape profiles. High frequency, large duty cycle operation produces an optical output approaching that observed in CW. The significance of these results are manifest in a major enhancement in deep penetration welding at reduced average laser powers. Welding performance, featuring a 400% increase in penetration as compared to CW welding, has been demonstrated at the same average power level. Reductions in average laser power and plasma suppression gas requirements could translate into major savings in high power laser welding applications utilizing this unique excitation process. Another attractive aspect of burst mode welding is its potential for deep penetration alloying of inexpensive base materials through incorporation of a computer controlled wire-feed/MIG system.A 30 kW, continuous wave (CW), Photo-initiated, Impulse-enhanced, Electrically-excited (PIE) CO2 laser is presented. This high power laser system is unique in that it is also capable of modulated optical output over a wide range of frequencies featuring multi-kilowatt peak power with high average power. The primary focus of this paper is documentation of the enhanced performance achievable from this laser when operated under a burst mode excitation technique. Excitation of the system, using a burst mode process, results in a dramatic change in the output characteristics of the laser. Control of burst frequency and duty cycle provides a convenient means to alter the time-varying nature of the output beam. Low burst frequencies result in clearly defined pulse profiles, falling to zero in between pulses. Increasing burst frequency results in a significant offset or quasi-CW time-varying output. At low burst durations, optical output has a triangular profile. Increasing burst duty cycles produce square optica...

To compensate for the wavefront distortions in high-power laser setups we developed stacked-actuator deformable mirror with aperture of 120 mm and 121 control actuators and bimorph deformable mirror with size of 410×468 mm. Design of the stacked-actuator mirror provides substitution of broken piezoactuators. Also, we can provide thermostabilization of the mirror surface through actuator body. Bimorph mirror has high surface amplitude deformation (about 20 um) and good flatness of the initial surface of the mirror (0.03 um RMS). Also, there is a possibility of preliminary adjustment by the piezostacks, which situated on the edge of the mirror.

There is growing interest at synchrotron light and X-ray free electron laser facilities to explore and improve the dynamic performance of piezoelectric bimorph deformable X-ray mirrors. Many beamlines, especially those dedicated to Macromolecular Crystallography, need to measure hundreds of samples per day. Shorter acquisition time requires rapid changes in the focus of the X-ray beam to condense the maximum photon density onto the sample. This is necessary to match the X-ray beam to the dimensions of the sample, or to probe variable sized regions of larger samples. Fine control of the X-ray beam becomes crucial for ensuring the highest quality of scientific data and increased throughput. Previous work at Diamond Light Source successfully changed the X-ray beam focus and stabilised it in under 10 seconds using piezoelectric bimorph deformable mirrors. Further updates to the controls software of the programmable HV-ADAPTOS high-voltage power supply (from CAEN / S.RI. Tech) now make it possible to control individual electrodes at 1 Hz using custom voltage profiles. This allows localized compensation of piezo creep, thus improving X-ray beam shape, significantly reducing stabilisation time, and eliminating curvature drift. For ex-situ validation, dynamic changes in the surface of the bimorph mirror need to monitored in real-time with sufficient spatial sensitivity. In this paper, we show that the active optical surface of a bimorph mirror (from Thales-SESO) can be accurately changed with sub-nanometre height sensitivity by dynamically monitoring the mirror's surface using an array of high-speed (up to 200 kHz) Zygo ZPS™ absolute interferometric displacement sensors mounted in an independent metrology frame.

The bimorph deformable mirror with a diameter of 320 mm, including 127 control electrodes, has been developed and tested. The flatness of the initial mirror surface of about 1 μm (P-V) was achieved by mechanically adjusting the mirror substrate fixed in the metal mount. To correct for the aberrations and improve the beam focusing in the petawatt Ti:Sa laser, the wide-aperture adaptive optical system with the deformable mirror and Shack–Hartmann wavefront sensor was developed. Correction of the wavefront aberrations in the 4.2 PW Ti:Sa laser using the adaptive system provided increases the intensity in the focusing plane to a value of 1.1 × 1023 W/cm2

High-power laser plays an important role in many fields, such as directed energy weapon, optoelectronic contermeasures, inertial confinement fusion, industrial processing and scientific research. The uniform nearfield and wavefront are the important part of the beam quality for high power lasers, which is conducive to maintaining the high spatial beam quality in propagation. We demonstrate experimentally that the spatial intensity and wavefront distribution at the output is well compensated simultaneously in the complex high-power solid-state laser system by using the small-aperture spatial light modulator (SLM) and deformable mirror (DM) in the front stage. The experimental setup is a hundred-Joule-level Nd:glass laser system operating at three wavelengths at 1053 nm (1ω), 527 nm (2ω) and 351 nm (3ω) with 3 ns pulse duration with the final output beam aperture of 60 mm. While the clear arperture of the electrically addressable SLM is less than 20 mm and the effective diameter of the 52-actuators DM is about 15 mm. In the beam shaping system, the key point is that the two front-stage beam shaping devices needs to precompensate the gain nonuniform and wavefront distortion of the laser system. The details of the iterative algorithm for improving the beam quality and the strategy of achiving high beam quality on spatial intensity and wavefront simultaneously are presented. Experimental results show that the output wavefront RMS value is 0.06, and simultaneously the output near-field modulation is 1.38:1 and the fluence contrast is 10.5% at 3 ns at 1053nm with 40-Joule-level energy.

The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.

Fast flow can be used to create the population inversion required for lasing action, or can be used to improve laser operation, for example by the removal of waste heat. It is pointed out that at the present time all lasers which are capable of continuous high-average power employ flow as an indispensable aspect of operation. High power laser systems are discussed, taking into account the gasdynamic laser, the HF supersonic diffusion laser, and electric discharge lasers. Aerodynamics and high power lasers are considered, giving attention to flow effects in high-power gas lasers, aerodynamic windows and beam manipulation, and the Venus machine. Applications of high-power laser technology reported are related to laser material working, the employment of the laser in controlled fusion machines, laser isotope separation and photochemistry, and laser power transmission.

Novel architectures of Petawatt-class, high peak power laser systems that allow operating at high repetition rates are opening a new arena of commercial applications of secondary sources and discovery science. The natural path to higher average power is the reduction of the total heat load induced and generated in the laser gain medium and eliminating other inefficiencies with the goal to turn more energy into laser photons while maintaining good beam quality. However, the laser architecture must be tailored to the specific application and laser parameters such as wavelength, peak power and intensity, pulse length, and shot rate must be optimized. We have developed a number of different concepts tailored to secondary source generation that minimize inefficiencies and maximize the average power. The Scalable Highaverage- power Advanced Radiographic Capability (SHARC) and the Big Aperture Thulium (BAT) laser are examples of two such high average power laser concepts; SHARC is designed for production of ion beams and x-rays, and exploration of high energy density physics at 1.5 kW average power, and BAT is envisioned for driving laser-based electron accelerators at 300 kW average power.