WO2012082260A1

WO2012082260A1 - Method and system for pulse reformatting in optical amplifiers

Info

Publication number: WO2012082260A1
Application number: PCT/US2011/059688
Authority: WO
Inventors: Alvin Charles Erlandson; John Allyn Caird; Mark A. Henesian; Kenneth Rene Manes; Robert J. Deri; Andrew James Bayramian; Mary Louis Spaeth
Original assignee: Lawrence Livermore National Security, Llc
Priority date: 2010-11-08
Filing date: 2011-11-08
Publication date: 2012-06-21
Also published as: CA2816335A1; KR20130118337A; EP2619858A1; JP2014500979A; EP2619858A4; RU2013126418A

Abstract

A method for generating a frequency converted pulse includes providing a first pulse at a fundamental wavelength propagating in a first beamline and providing a second pulse at the fundamental wavelength propagating in a second beamline. The method also includes frequency converting the first pulse at the fundamental wavelength to provide a first frequency converted pulse at a frequency converted wavelength and frequency converting the second pulse at the fundamental wavelength to provide a second frequency converted pulse at the frequency converted wavelength. The method further includes optically combining the first frequency converted pulse at the frequency converted wavelength and the second frequency converted pulse at the frequency converted wavelength to provide the frequency converted pulse.

Description

METHOD AND SYSTEM FOR PULSE REFORMATTING IN OPTICAL

AMPLIFIERS

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No.

61/411,360, filed November 8, 2010, the disclosure of which is hereby incorporated by reference in its entirety for all purposes.

[0002] The following PCT applications (including this one) are being filed concurrently, and the entire disclosure of the other application is incorporated by reference into this application for all purposes:

• Application No. PCT/US2011__, filed November 8, 2011 entitled "MULTICRYSTAL FREQUENCY TRIPLER FOR THIRD HARMONIC CONVERSION" (Client Reference No. IL-12360; Attorney Docket No. 91920- 825120(0066 lOPC)); and

• Application No. PCT US2011_, filed November 8, 2011 , entitled "METHOD OF PULSE REFORMATTING FOR OPTICAL AMPLIFICATION AND FREQUENCY CONVERSION" (Client Reference No. IL-12359; Attorney Docket No. 91920-824881 (006010PC)).

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0003] The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

[0004] Projections by the Energy Information Agency and current Intergovernmental Panel on Climate Change (IPCC) scenarios expect worldwide electric power demand to double from its current level of about 2 terawatts electrical power (TWe) to 4 TWe by 2030, and could reach 8-10 TWe by 2100. They also expect that for the next 30 to 50 years, the bulk of the demand of electricity production will be provided by fossil fuels, typically coal and natural gas. Coal supplies 41% of the world's electric energy today, and is expected to supply 45% by 2030. In addition, the most recent report from the IPCC has placed the likelihood that man-made sources of C0₂ emissions into the atmosphere are having a significant effect on the climate of planet earth at 90%. "Business as usual" baseline scenarios show that C0₂ emissions could be almost two and a half times the current level by 2050. More than ever before, new technologies and alternative sources of energy are essential to meet the increasing energy demand in both the developed and the developing worlds, while attempting to stabilize and reduce the concentration of C0₂ in the atmosphere and mitigate the concomitant climate change.

[0005] Nuclear energy, a non-carbon emitting energy source, has been a key component of the world's energy production since the 1950's, and currently accounts for about 16% of the world's electricity production, a fraction that could - in principle - be increased. Several factors, however, make its long-term sustainability difficult. These concerns include the risk of proliferation of nuclear materials and technologies resulting from the nuclear fuel cycle; the generation of long-lived radioactive nuclear waste requiring burial in deep geological repositories; the current reliance on the once through, open nuclear fuel cycle; and the availability of low cost, low carbon footprint uranium ore. In the United States alone, nuclear reactors have already generated more than 55,000 metric tons (MT) of spent nuclear fuel

(SNF). In the near future, we will have enough spent nuclear fuel to fill the Yucca Mountain geological waste repository to its legislated limit of 70,000 MT.

[0006] Fusion is an attractive energy option for future power generation, with two main approaches to fusion power plants now being developed. In a first approach, Inertial Confinement Fusion (ICF) uses lasers, heavy ion beams, or pulsed power to rapidly compress capsules containing a mixture of deuterium (D) and tritium (T). As the capsule radius decreases and the DT gas density and temperature increase, DT fusion reactions are initiated in a small spot in the center of the compressed capsule. These DT fusion reactions generate both alpha particles and 14.1 MeV neutrons. A fusion burn front propagates from the spot, generating significant energy gain. A second approach, magnetic fusion energy (MFE) uses powerful magnetic fields to confine a DT plasma and to generate the conditions required to sustain a burning plasma and generate energy gain.

[0007] Important technology for ICF is being developed primarily at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), assignee of this invention, in Livermore, California. There, a laser-based ICF project designed to achieve thermonuclear fusion ignition and burn utilizes laser energies of 1 to 1.3 MJ. Fusion yields of the order of 10 to 20 MJ are expected. Fusion yields in excess of 200 MJ are expected to be required in a central hot spot fusion geometry if fusion technology, by itself, were to be used for cost effective power generation. Thus, significant technical challenges remain to achieve an economy powered by pure ICF energy.

SUMMARY OF THE INVENTION

[0008] The present invention relates generally to laser systems. More specifically, the present invention relates to methods and systems for producing high efficiency, harmonically converted laser power. Merely by way of example, the invention has been applied to methods and systems for harmonically converting laser light and combining the output of multiple beamlines. The methods and systems can be applied to a variety of other frequency conversion systems, laser amplifier architectures, and laser systems. [0009] According to an embodiment of the present invention, a multi-aperture

laser/amplifier system is provided that is characterized by high conversion efficiency. The multi-aperture system (also referred to as a multi-beamline system) utilizes a subset of the apertures or beamlines to provide for the low intensity portions of a high dynamic range pulse and the remaining apertures to provide for the high intensity portions of the high dynamic range pulse. In other embodiments, beamlines can be used to contribute to both low intensity and high intensity portions of the high dynamic range pulse.

[0010] According to another embodiment of the present invention, the number of beamlines that are active at a particular time during the pulse is adjusted in a proportion approximately equal to the peak power that is produced using the multi-aperture system. Thus, embodiments of the present invention provide for efficient production of harmonically converted, high dynamic range pulses using multiple beamline systems. Although some embodiments of the present invention are discussed in terms of pulsed laser and amplifier systems, embodiments of the present invention are applicable to CW laser and amplifier systems in which multiple beamlines are utilized. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0011] The present invention relates to methods for improving the efficiency of laser systems in which the combined output of several beamlines is used to produce temporally shaped optical pulses at harmonically-converted wavelengths. When the shaped pulse has a dynamic range that is greater than the efficient operating range of a harmonic converter, and each beamline produces the same (required) pulse shape, harmonic conversion efficiency tends to be low. In embodiments of the present invention, the desired output pulse shape is achieved not only by adjusting output power produced by individual beamlines, but also by adjusting the number of beamlines that are emitting. With this added degree of freedom, the dynamic range of the pulses produced by individual beamlines is reduced, instantaneous power and irradiance can be kept near the peak of the harmonic conversion efficiency vs. power (or intensity/irradiance) curve of the harmonic converters, and overall harmonic conversion efficiency is increased. [0012] According to an embodiment of the present invention, a method for generating a frequency converted pulse is provided. The method includes providing a first pulse at a fundamental wavelength propagating in a first beamline and providing a second pulse at the fundamental wavelength propagating in a second beamline. The method also includes frequency converting the first pulse at the fundamental wavelength to provide a first frequency converted pulse at a frequency converted wavelength and frequency converting the second pulse at the fundamental wavelength to provide a second frequency converted pulse at the frequency converted wavelength. The method further includes optically combining the first frequency converted pulse at the frequency converted wavelength and the second frequency converted pulse at the frequency converted wavelength to provide the frequency converted pulse.

[0013] According to another embodiment of the present invention, a method of providing a combined optical pulse is provided. The method includes providing a first pulse having a first temporal pulse shape characterized by a first profile during a first time period and a second profile during a second time period and providing a plurality of additional pulses. Each of the plurality of additional pulses has an associated temporal shape characterized by a profile and a pulse energy. The associated temporal shapes differ from the first temporal shape and a majority of the pulse energy is present during the second time period. The method also includes harmonically converting the first pulse and harmonically converting the plurality of additional pulses. The method further includes optically combining the harmonically converted first pulse and the plurality of harmonically converted additional pulses to form the combined optical pulse.

[0014] According to an alternative embodiment of the present invention, a laser system is provided. The laser system includes a first beamline operable to support a first laser beam and including first optical elements and a first harmonic converter optically coupled to the first beamline. The laser system also includes a second beamline operable to support a second laser beam and including second optical elements and a second harmonic converter optically coupled to the second beamline. The laser system further includes a processor operable to control one or more of the first beam's optical elements and one or more of the second beam's optical elements, a memory coupled to the processor, and optics operable to combine harmonically converted light from the first beamline and harmonically converted light from the second beamline.

[0015] Embodiments of the present invention are useful in a variety of optical amplifier systems to improve efficiency and to reduce costs of, for example, pulsed, harmonically- converted lasers. Exemplary systems in which embodiments of the present invention can be utilized include laser drivers for LIFE power plants, laser drivers for ICF experiments performed to improve understanding of nuclear- weapons physics and effects, lasers used to generate plasmas for high-energy-density studies, lasers to generate pressure pulses, lasers used to generate x-rays, or as components of x-ray lasers. [0016] Numerous benefits are achieved by way of the present invention over conventional techniques. For example, embodiments of the present invention provide pulsed,

harmonically-converted lasers useful in a variety of applications including lasers that pump high-energy titanium doped lasers, lasers used for material processing, lasers used to generate x-rays or x-ray lasers for photolithography, and the like. These and other embodiments of the invention along with many of its advantages and features are described in more detail in conjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a simplified plot illustrating harmonic conversion efficiency as a function of input intensity;

[0018] FIG. 2 is a simplified plot illustrating a high-dynamic-range pulse shape suitable for use with ICF lasers according to an embodiment of the present invention;

[0019] FIG. 3 is a simplified multi-dimensional plot illustrating output power as a function of time for six beamlines according to an embodiment of the present invention; [0020] FIG. 4 is a simplified plot illustrating harmonic conversion efficiency as a function of beamline number according to an embodiment of the present invention; [0021] FIG. 5A is a simplified schematic diagram of an amplifier architecture according to an embodiment of the present invention;

[0022] FIG. 5B is a simplified schematic diagram of an alternative frequency conversion architecture according to an embodiment of the present invention; [0023] FIG. 6 is a simplified flowchart illustrating a method of providing a frequency- converted pulse according to an embodiment of the present invention; and

[0024] FIG. 7 is a simplified flowchart illustrating a method of providing a combined optical pulse according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0025] Embodiments of the present invention relate to laser systems. More specifically, the present invention relates to methods and systems for producing high efficiency, harmonically converted laser power. Merely by way of example, the invention has been applied to methods and systems for harmonically converting laser light and combining the output of multiple beamlines. The methods and systems can be applied to a variety of other frequency conversion systems, laser amplifier architectures, and laser systems.

[0026] Harmonically-converted, pulsed, high-energy lasers have been developed for several applications such as studying ICF, pumping of titanium-doped sapphire lasers, processing of materials, and controlling chemical reactions. To meet needs for reliable and efficient harmonic conversion, teams of people have been involved in research and development of harmonic converter theory, materials, methods, and designs. Designs using various types, thicknesses, and arrangements of crystals, methods of phase matching, and operating intensities have been developed. Regardless, as harmonic conversion is a nonlinear process, harmonic conversion efficiency has a strong dependence on incident (or converted) laser intensity. Generally, harmonic conversion efficiency rises from zero at zero intensity to a high, peak value at high intensity, after which further increase in intensity causes harmonic- conversion efficiency to fall.

[0027] FIG. 1 is a simplified plot 100 illustrating harmonic conversion efficiency as a function of input intensity. In particular, FIG. 1 shows a representative curve for harmonic- conversion efficiency to a 3 ω (i.e., frequency tripled) output. The decrease in efficiency (i.e., peaking behavior) illustrated in FIG. 1 at approximately > 0.6 GW/cm is caused by conversion of harmonically-converted light back to the laser fundamental frequency, at high intensities.

[0028] The fact that harmonic conversion efficiency is high only over a limited intensity range has important implications, particularly for applications requiring optical pulses with high dynamic range. For high dynamic range pulses, portions of the pulse characterized by lower intensity will have low harmonic conversion efficiency, which will compromise the overall harmonic conversion efficiency of the pulse considered in its entirety.

[0029] FIG. 2 is a simplified plot illustrating a high-dynamic-range pulse shape suitable for use with ICF lasers according to an embodiment of the present invention. As an example of a pulse characterized by the high dynamic range illustrated in FIG. 2, ICF pulses can include an initial, long, low intensity "foot pulse" followed by a shorter, high intensity "drive pulse," with the power in the foot pulse being only a few percent of drive power. Although the majority of the energy in the pulse is provided in the second half of the pulse, this is not required by embodiments of the present invention and other implementations will utilize different temporal pulse shapes with the majority of the pulse energy in the first half of the pulse, the middle portion of the pulse, spread across multiple portions of the pulse, or the like.

[0030] When nonlinear optical crystals are used to convert both foot and drive portions of the pulse, conversion efficiency tends to be high only for one portion of the pulse (usually the drive pulse) but not for the other portion of the pulse. Typically, during harmonic

conversion, the beam size is adjusted so that peak intensity corresponds to the maximum conversion efficiency. For example, when a harmonic converter with the conversion efficiency profile illustrated in FIG. 1 is used to frequency convert the high dynamic range pulse shape illustrated in FIG. 2, the harmonic conversion efficiency for the foot pulse is -45% and the harmonic conversion efficiency for the drive pulse is -75%, resulting in an overall conversion efficiency for the entire pulse of -63%. According to embodiments of the present invention, the overall harmonic conversion efficiency can be increased, for example, by -20%, by increasing the harmonic conversion efficiency associated with the foot pulse. As a result, embodiments can achieve an overall harmonic conversion efficiency of -72% for the pulse shape illustrated in FIG. 2. As will be evident to one of skill in the art, high harmonic conversion efficiency is highly desirable as requirements for both the energy at the fundamental frequency and the input electrical power used to energize the laser are reduced. As illustrated in FIG. 2, the foot pulse has a peak (a maximum intensity of the pulse) at a time of -10 ns and the drive pulses have a peak at a time of -12 ns. Thus, the peak of the foot pulse occurs in this embodiment prior to the peak of the drive pulses. Additionally, it can be noted that the majority of the energy in the drive pulses occurs in the second half of the combined pulse (i.e., at time > 10 ns).

[0031] In the embodiment illustrated in FIG. 2, the combined pulse (also referred to as the main pulse) is subdivided into a foot pulse with about 25% of the combined pulse energy and a plurality of drive pulses with about 75% of the combined pulse energy. It should be noted that the combined pulse and the foot pulse overlap before the onset of the drive pulses at ~ 10 ns and the combined pulse is not illustrated prior to the onset of the drive pulses for purposes of clarity in the figure. However, embodiments of the present invention are not limited to this particular implementation and the combined pulse could be further subdivided, for example, into one or more foot pulses, one or more mid-range pulses, and one or more drive pulses. Thus, the pulses illustrated in FIG. 2 are merely provided by way of example to illustrate the use of multiple beamlines to increase harmonic conversion efficiency. In applications that utilize multiple beams impinging on a target, for example, ICF applications, it is possible to deliver the desired energy in pulses with differing temporal profiles delivered through multiple beamlines, enabling efficient frequency conversion utilizing embodiments of the present invention. In other implementations, the beams from multiple beamlines can be combined before delivery to the target.

[0032] According to embodiments of the present invention, the temporal shape of the pulses produced by each of the beamlines can be provided to increase conversion efficiency. Additionally, the number of beamlines can be operated in approximate proportion to the total power that is provided by each of the combined beamlines, enabling the system operator to even out the intensity produced by the individual beamlines and thereby increase the harmonic conversion efficiency. In contrast with some conventional designs, the various beamlines may be active during a portion of the combined pulse duration, for example, a beamline associated with a foot pulse may only be active during an initial portion of the combined pulse duration. In other implementations, as described below, the beamlines may be active during the entire pulse duration, with the temporal profile of the pulse in each beamline varying during the combined pulse duration.

[0033] One method for increasing harmonic conversion efficiency during the low-intensity foot pulse is a method that uses "quadrature frequency conversion" in which the intensity range over which the harmonic conversion is high is extended by using pairs of frequency conversion crystals rather than single crystals. In essence, one crystal efficiently converts at lower intensities while the other crystal efficiency converts at higher intensities. It has been shown that the improvements in conversion efficiency can be significant. This method can result in increased system costs due to doubling of the number of nonlinear optical crystals used for harmonic conversion.

[0034] Another method for increasing harmonic conversion efficiency uses small-diameter beams to produce foot pulses and large-diameter beams to produce drive pulses. Harmonic conversion efficiency is high overall since intensities are kept near the harmonic-converter optimum for both foot and drive beams. By propagating foot-pulse and drive pulse beamlines collinearly, illumination symmetry is preserved and the added cost and complexity of using separate beamlines is avoided. In these designs it is important to keep inner beams from overlapping with outer beams to avoid interference from generating high intensity features. This requirement can be met by separating the inner and outer beams with a donut- shaped apodizer, which reduces beam intensities gradually to prevent diffraction from causing high-intensity features. Since the donut-shaped apodizer occupies a significant fraction of the total beam area, energy extraction efficiency in the amplifiers is significantly reduced in some implementations. [0035] A third method for increasing harmonic conversion efficiency replaces the foot pulse with a functionally-equivalent train or "picket fence" of high-intensity, short pulses. By making picket-fence intensities comparable to drive-pulse intensities, high harmonic conversion efficiency is feasible. However, drawbacks associated with this method are increased complexity of the front-end laser system used to generate the train of short pulses, higher intensity and detrimental nonlinear phase shift within the laser system, and uncertainty with respect to target performance.

[0036] Embodiments of the present invention utilize a feature of large, indirect-drive ICF systems, namely that several beamlines are used within each laser port to produce the pulse energy appropriate for the ICF mission. As examples, four beams per port are used in the NIF and in the LMJ, an ICF system that is similar to the NIF and is being constructed in

France. As each beamline produces the full, high-dynamic range ICF laser pulse, low overall harmonic conversion efficiency ensues. According to the present invention, the desired pulse shape is achieved by varying both the number of beamlines that emit as well as the output power produced by the emitting beamlines. Since a significant portion of the dynamic range of the desired pulse is achieved by controlling the number of active beamlines, a smaller portion is achieved by varying the optical power within the active beamlines. With the power and intensity that individual beamlines produce falling within narrower ranges, harmonic converters can be designed such that intensity is close to the peak harmonic-conversion efficiency. As described throughout the present specification, the methods and systems provided by embodiments of the present invention achieve significant improvements in harmonic conversion efficiency in comparison with conventional systems.

[0037] According to an embodiment of the present invention, a small number of beamlines in each port produce the foot pulse while the remaining beamlines produce the drive pulse. For example, in a design with four beamlines per laser port, one beamline is used to generate the foot pulse while the remaining three beamlines are used produce the drive pulse.

[0038] In addition to increasing overall conversion efficiency, embodiments of the present invention can provide additional benefits not available using conventional systems. In some implementations, beamlines reserved for producing only foot pulses may not be available for producing power when the demand for peak power is greatest, that is, at the peak of the drive pulse. In this case, since only beamlines reserved for producing drive pulses contribute at the peak of the drive pulse, the peak power produced within each of the beamlines reserved for the drive pulse is greater than it would have been had all beamlines (i.e., both foot pulse and drive pulse beamlines) been available. The resulting higher peak power per beamline in the drive pulse beamlines is undesirable since optics experience greater peak intensities, greater damage risks, and greater nonlinear phase shift. Nonlinear phase shift correlates with the appearance of small-scale high-intensity features in the beam that increase damage risk and degrade beam focal-spot quality on target.

[0039] Embodiments of the present invention provide solutions to the issues discussed above by implementing an architecture in which the maximum power produced by individual beamlines is reduced or minimized by keeping all beamlines turned on during the peak of the drive pulse. In a particular embodiment, since keeping all the beamlines on at peak power extracts stored energy from the 1 ω amplifiers, no single beamline has sufficient remaining stored energy to produce the entire foot pulse. Thus, in this embodiment, the foot pulse is produced by several beamlines, which are turned on and off in sequence. Likewise, the drive pulse is produced by turning other beamlines on and off in sequence. At any instant in time, a sufficient number of beamlines is turned on to ensure that the peak power within any one beamline is within safe limits. By judiciously choosing the turn-on and turn-off times of the beamlines and the number of beamlines that are turned on at any one time, the overall required pulse shape can be produced while still keeping the intensity within each beamline close to the peak of the harmonic-conversion efficiency curve.

[0040] FIG. 3 is a simplified multi-dimensional plot illustrating output power as a function of time for six beamlines according to an embodiment of the present invention. The optical power from the six beamlines, when summed, will result in the pulse shape shown in FIG. 2. The plot shown in FIG. 3 is just one possible pattern for operating six beamlines within a laser port in order to produce the desired overall pulse shape, while maintaining high harmonic conversion efficiency. Again, using the harmonic conversion efficiency illustrated in FIG. 1 , the predicted overall harmonic conversion efficiency the overall pulse generated using the beamline profiles in FIG. 3 is 79% since a large fraction of the output power is produced near the peak harmonic-conversion efficiency.

[0041] Referring to FIG. 3, the first beamline is turned on during a first time period associated the foot pulse (i.e., between 0-6 ns) and then turned on again during a second time period associated with the drive pulse to provide a portion of the drive pulse (i.e., between 11-14 ns). The intensity in the beamline is selected to have a predetermined intensity profile as a function of time to provide the overall pulse after combination of the beamlines. As shown in FIG. 3, some beamlines can be operated only during the time period associated with the drive pulse, which is illustrated by beamline #6, which is turned on between 11 ns and 18 ns. Referring to beamline #1 in FIG. 3, embodiments of the present invention increase the energy extraction efficiency associated with the optical amplification process in comparison with conventional techniques. For beamline #1, there is residual energy left over in beamline #1 because the power utilized to produce the foot pulse is small in comparison with the capabilities of the beamline. Because the energy utilized to produce the foot pulse is small, there is extra stored energy left over in the beamline amplifiers, which can, therefore, be used to help contribute to the drive pulse portion of the combined pulse. Thus, embodiments of the present invention provide methods and systems to increase the energy extraction efficiency from the power amplifiers included in the beamlines.

[0042] Therefore, in some embodiments, although a particular beamline can be referred to as a foot beamline or a drive beamline, the particular beamlines can contribute to one or more portions of the combined pulse, for example, a foot beamline also contributing to a portion of the drive pulse energy. As illustrated in FIG. 3, programmable temporal pulse shaping of the individual beamlines opens up the opportunity to not only increase harmonic conversion efficiency, but to increase the power extraction efficiency of the various beamlines. [0043] An advantage of utilizing multiple beamlines to provide the combined pulse is that nonlinear phase shift in the amplifiers of the various beamlines can be reduced. In contrast with a system in which the foot pulse is provided by a first beamline and the drive pulses are provided by additional beamlines, production of a portion of the drive pulse using the first beamline can reduce the nonlinear phase shift by enabling a decrease in the intensity and power present in the additional beamlines. The reduction in intensity and power in the additional beamlines reduces the nonlinear phase shift, improves the beam quality, and reduces the risk of damage to the optical components.

[0044] Another advantage of utilizing pulse shaping illustrated in FIG. 3 relates to laser maintenance. Although the various beamlines produce different pulse shapes, all the beamlines can use equipment that is identical or at least similar enough to be interchangeable. Thus, only one type of spare is required for each laser component or subsystem in some embodiments. Additionally, when one beamline within a laser port fails, pulse shapes produced by the remaining beamlines can be reformatted so that the overall pulse shape produced within the laser port has the desired shape. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0045] Embodiments of the present invention have wide application in laser systems, and the methods and systems described herein are not limited to ICF applications. The methods and systems described herein can be used in other laser and amplifier architectures using several laser beamlines to produce shaped optical pulses at harmonically converted wavelengths. These other applications include, but are not limited to direct drive ICF and pumping of high-energy titanium-doped sapphire lasers (or other solid-state lasers) with harmonically converted light. Additionally, embodiments of the present invention are applicable to other laser and amplifier systems in which a nonlinear optical process other than harmonic conversion is used. As examples, such applications include, but are not limited to the use of pulsed laser light to generate x-rays and the use of pulsed laser light to pump x-ray lasers.

[0046] Although the beam profiles illustrated in FIG. 3 are associated with a system utilizing six beamlines, the present invention is not limited to this particular number of beamlines and other architectures are included within the scope of the present invention, for example, less than six beamlines, including two, three, four, or five beamlines, or more than six beamlines, for example, seven, eight, nine, ten, 11, 12, or more than 12 beamlines. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0047] Another benefit provided by embodiments of the present invention is that the temporal profiles of the various beamlines can be adjusted to compensate for degradation or failure of one or more of the beamlines, which can be referred to as a failure event. Since the system includes some excess capacity, if a beamline fails, the pulse shape originally provided by the failed beamline can be compensated for by adjusting the temporal profiles using the other beamlines to provide a combined pulse with the original temporal shape. As an example, if beamline #4 fails completely, the temporal shape produced by beamlines 1-3 and 5-6 can be adjusted to replace the contribution originally provided by beamline #4. If the beamline does not totally fail, but is only degraded, the difference between the original temporal shape for the degraded beamline and the degraded temporal shape can be

compensated for using the other beamlines. Thus, embodiments of the present invention utilize programmable pulse shapers as discussed in relation to FIG. 5A to adjust the temporal shapes of the various beamlines to compensate for failure or degradation of other beamlines.

[0048] FIG. 4 is a simplified plot illustrating harmonic conversion efficiency as a function of beamline number according to an embodiment of the present invention. The harmonic conversion efficiencies illustrated in FIG. 4 are associated with the beamlines illustrated in FIG. 3 and are time-integrated, energy conversion efficiencies. As illustrated in FIG. 4, conversion efficiency of beamline #1, which has a lower peak intensity than beamlines #2 through #6, as shown in FIG. 3, although lower than beamlines #2 through #6, is significantly higher than the conversion efficiency that would have been achieved had this beamline only included intensity during the foot pulse. By keeping the intensities in each beamline closer to the peak of the harmonic conversion efficiency curve, the overall time-integrated harmonic conversion efficiency for energy is 78.8% as shown by the dashed line in FIG. 4. In some embodiments, the use of intensities close to the conversion efficiency peak enables the use of a reduced number or the minimum number of beamlines needed to produce the required power while not exceeding the intensity at which the harmonic converter efficiency starts to decrease and while keeping the harmonically converted pulse energy evenly distributed between the various beamlines.

[0049] Embodiments of the present invention provide systems in which the harmonic conversion efficiency for harmonically converting all of the pulses in a set of pulses is close to peak conversion efficiency for one of the pulses. As illustrated in FIG. 4, the harmonic conversion efficiency for beamline #1 of 74.5% is close to and only minimally degraded from the harmonic conversion efficiency of beamline #5, with the highest harmonic conversion efficiency of 80.7%. In an embodiment, the harmonic conversion efficiency for harmonically converting the first pulse (e.g., the pulse associated with beamline #1) is within 20% of the harmonic conversion efficiencies for harmonically converting the other pulses associated with the other beamlines. In a particular embodiment, the harmonic conversion efficiency for harmonically converting the first pulse is within 10% of the harmonic conversion efficiencies for harmonically converting the other pulses. [0050] FIG. 5 A is a simplified schematic diagram of an amplifier architecture according to an embodiment of the present invention. Referring to FIG. 5 A, the system 500 includes a processor/controller 510 (referred to herein as a processor) that is used to perform

calculations related to system operation and provide control signals to the various system elements. A computer readable medium 512 (also referred to as a database or a memory) is coupled to the processor 510 in order to store data used by the processor and other system elements. The processor 510 interacts with the beamlines, harmonic converters, and optics as described more fully throughout the present specification. In an embodiment, the memory 512 can include a look up table that can be utilized to reprogram the pulse shapes of the outputs from the various beamlines upon failure or degradation of one or more of the beamlines.

[0051] The processor 510 can be a general purpose microprocessor configured to execute instructions and data, such as a Pentium processor manufactured by the Intel Corporation of Santa Clara, California. It can also be an Application Specific Integrated Circuit (ASIC) that embodies at least part of the instructions for performing the method in accordance with the present invention in software, firmware and/or hardware. As an example, such processors include dedicated circuitry, ASICs, combinatorial logic, other programmable processors, combinations thereof, and the like.

[0052] The memory 512 can be local or distributed as appropriate to the particular application. Memory 512 may include a number of memories including a main random access memory (RAM) for storage of instructions and data during program execution and a read only memory (ROM) in which fixed instructions are stored. Thus, memory 512 provides persistent (non- volatile) storage for program and data files, and may include a hard disk drive, flash memory, a floppy disk drive along with associated removable media, a Compact Disk Read Only Memory (CD-ROM) drive, an optical drive, removable media cartridges, and other like storage media.

[0053] The system 500 includes multiple beamlines including preamplifier modules (PAMs) #1 through #N (520-1 through 520-N). In some embodiments, there are six beamlines as illustrated in FIG. 3, although the present invention is not limited to this particular number of beamlines and the number of beamlines can range from two to more than two, for example, four, six, eight, ten, twelve, or the like. According to some embodiments of the present invention, the PAMs include a fiber laser seed source, a temporal pulse shaper, and preamplification stages operable to amplify pulses originating from the fiber laser seed source and modified by the temporal pulse shaper. The PAMs are not limited to this particular architecture. The inclusion of a PAM in each beamline as illustrated in FIG. 5 A enables the creation of pulses with differing temporal profiles in each beamline as described throughout the present specification.

[0054] The beamlines also include amplifiers #1 through #N (530-1 through 530-N). The amplifiers utilize optical gain to amplify pulses originating from their respective PAMs. According to some embodiments of the present invention, the amplifiers are high average power amplifiers including a plurality of amplifier slablets as suitable for production of high power pulses. In some implementations, control of the amplifiers is provided by a controller (not shown) in the PAMs, whereas in other implementations, amplifier control is provided by the processor 510 as illustrated in FIG. 5 A.

[0055] Each beamline is optically coupled to a harmonic converter (i.e., harmonic converters #1 through #N) (540-1 through 540-N). The outputs from the harmonic converters are combined using optics 550, which may be under control of processor 510, to focus to a single location on the target. Thus, the combined beam resulting from the various beamlines impinges on the target. In ICF applications, multiple implementations of the multiple beamline system illustrated in FIG. 5A will be utilized to provide combined beams impinging on multiple portions of the target.

[0056] Although each of the multiple beamlines utilize a harmonic converter in the embodiment illustrated in FIG. 5A, other embodiments can produce the foot and drive pulses utilizing other configurations. FIG. 5B is a simplified schematic diagrams of an alternative frequency conversion architecture according to an embodiment of the present invention. As illustrated in FIG. 5B, a combined pulse converter 590 can be used to convert the combined pulse (i.e., the high dynamic range pulse). A dichroic mirror 592 is used to split the frequency converted light toward mirror 594 and a foot pulse converter 596 is then used to frequency convert remaining light at the fundamental wavelength. In these embodiments, the combined and foot pulses can utilize different wavelengths, different directions of propagation, different polarizations. Additionally, as illustrated in FIG. 5A and 5B, different portions of the architecture can provide support for combined beams or individual beamlines as appropriate to the particular implementation. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0057] FIG. 6 is a simplified flowchart illustrating a method of providing a frequency- converted pulse according to an embodiment of the present invention. The method 600 includes providing a first pulse at a fundamental wavelength propagating in a first beamline (610) and providing a second pulse at the fundamental wavelength propagating in a second beamline (612). The method also includes frequency converting the first pulse at the fundamental wavelength to provide a first frequency converted pulse at a frequency converted wavelength (614) and frequency converting the second pulse at the fundamental wavelength to provide a second frequency converted pulse at the frequency converted wavelength (616).

[0058] The method further includes optically combining the first frequency converted pulse at the frequency converted wavelength and the second frequency converted pulse at the frequency converted wavelength to provide the frequency converted pulse (618). In some embodiments, the method additionally includes providing a third pulse at the fundamental wavelength propagating in a third beamline, frequency converting the third pulse at the fundamental wavelength to provide a third frequency converted pulse at the frequency converted wavelength, and optically combining the third frequency converted pulse at the frequency converted wavelength with the first frequency converted pulse and the second frequency converted pulse. A peak of the second pulse and a peak of the third pulse can occur after a peak of the first pulse, for example, if the first pulse is associated with a foot pulse and the second and third pulses are associated with drive pulses. Additionally, in some embodiments, the first pulse is characterized by a first energy during a first time period and the second pulse is characterized by a second energy during a second time period after the first time period. The present invention is not limited to the three beamlines discussed above, but is applicable to a larger number of beamlines as described throughout the present specification.

[0059] Embodiments of the present invention are applicable to a variety of frequency conversion processes, including frequency doubling, frequency tripling, optical parametric amplification, and the like.

[0060] It should be appreciated that the specific steps illustrated in FIG. 6 provide a particular method of providing a frequency-converted pulse according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in FIG. 6 may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. [0061] FIG. 7 is- a simplified flowchart illustrating a method of providing a combined optical pulse according to an embodiment of the present invention. The method 700 includes providing a first pulse having a first temporal pulse shape characterized by a first profile during a first time period and a second profile during a second time period (710) and providing a plurality of additional pulses, each of the plurality of additional pulses having an associated temporal shape characterized by a profile and a pulse energy (712). The associated temporal shapes differ from the first temporal shape and a majority of the pulse energy is present during the second time period. As examples, the first pulse can be similar to the pulse associated with beamline #1 in FIG. 3 and the plurality of additional pulses can be similar to the pulses associated with beamlines #2-#6 in FIG. 3.

[0062] The method also includes harmonically converting the first pulse (714) and harmonically converting the plurality of additional pulses (716). According to embodiments of the present invention, the individual beamlines are frequency converted independently in order to enable high harmonic conversion efficiencies. As an example, a harmonic conversion efficiency for harmonically converting the first pulse can be within 10% of harmonic conversion efficiencies for harmonically converting the plurality of additional pulses. In other embodiments, the harmonic conversion efficiency is within 20%, with 15%, with 5%, with 2.5%, or within 1%. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. [0063] Thus, the first pulse can be harmonically converted in a first beamline and the plurality of additional pulses can be harmonically converted in a plurality of additional beamlines. The method further includes optically combining the harmonically converted first pulse and the plurality of harmonically converted additional pulses to form the combined optical pulse (718). The combined optical pulse, which can be a high dynamic range pulse (as illustrated in FIG. 2) can be characterized by a peak intensity during the second time greater than a peak intensity during the first time.

[0064] According to a specific embodiment of the present invention, the method can include determining a failure event, which may be a degradation or failure related to at least one of the first pulses or one of the plurality of additional pulses. In some embodiments, the beamline will cease functioning, whereas in other embodiments, the output power and/or beam quality of the various beams can be degraded. The first temporal shape of at least one of the first pulse or the associated temporal shape associated with one of the plurality of additional pulses is modified to compensate for the failure event. [0065] It should be appreciated that the specific steps illustrated in FIG. 7 provide a particular method of providing a combined optical pulse according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in FIG. 7 may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. [0066] It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.

Claims

WHAT IS CLAIMED IS: 1. A method for generating a frequency converted pulse, the method comprising:

providing a first pulse at a fundamental wavelength propagating in a first beamline;

providing a second pulse at the fundamental wavelength propagating in a second beamline;

frequency converting the first pulse at the fundamental wavelength to provide a first frequency converted pulse at a frequency converted wavelength;

frequency converting the second pulse at the fundamental wavelength to provide a second frequency converted pulse at the frequency converted wavelength; and

optically combining the first frequency converted pulse at the frequency converted wavelength and the second frequency converted pulse at the frequency converted wavelength to provide the frequency converted pulse.

2. The method of claim 1 further comprising:

providing a third pulse at the fundamental wavelength propagating in a third beamline;

frequency converting the third pulse at the fundamental wavelength to provide a third frequency converted pulse at the frequency converted wavelength; and

optically combining the third frequency converted pulse at the frequency converted wavelength with the first frequency converted pulse and the second frequency converted pulse.

3. The method of claim 2 wherein a peak of the second pulse and a peak of the third pulse occur after a peak of the first pulse.

4. The method of claim 1 wherein the first pulse is characterized by a first energy during a first time period and the second pulse is characterized by a second energy during a second time period after the first time period.

5. The method of claim 1 wherein the frequency converted wavelength is half the fundamental wavelength.

6. The method of claim 1 wherein the frequency converted wavelength is one third the fundamental wavelength.

7. The method of claim 1 wherein the first beamline comprises a preamplifier module, an amplifier, and a harmonic converter.

8. The method of claim 7 wherein the second beamline comprises a second preamplifier module, a second amplifier, and a second harmonic converter.

9. A method of providing a combined optical pulse, the method comprising:

providing a first pulse having a first temporal pulse shape characterized by a first profile during a first time period and a second profile during a second time period;

providing a plurality of additional pulses, each of the plurality of additional pulses having an associated temporal shape characterized by a profile and a pulse energy, wherein the associated temporal shapes differ from the first temporal shape and a majority of the pulse energy is present during the second time period;

harmonically converting the first pulse;

harmonically converting the plurality of additional pulses; and optically combining the harmonically converted first pulse and the plurality of harmonically converted additional pulses to form the combined optical pulse.

10. The method of claim 9 wherein the first pulse is harmonically converted in a first beamline.

11. The method of claim 9 wherein the plurality of additional pulses are harmonically converted in a plurality of additional beamlines.

12. The method of claim 9 wherein the combined optical pulse is characterized by a peak intensity during the second time greater than a peak intensity during the first time.

13. The method of claim 9 wherein a harmonic conversion efficiency for harmonically converting the first pulse is within 20% of harmonic conversion efficiencies for harmonically converting the plurality of additional pulses.

14. The method of claim 13 wherein the harmonic conversion efficiency for harmonically converting the first pulse is within 10% of harmonic conversion efficiencies for harmonically converting the plurality of additional pulses.

15. The method of claim 9 further comprising: determining a failure event related to at least one of the first pulse or one of the plurality of additional pulses; and

modifying the first temporal shape of at least one of the first pulse or the associated temporal shape associated with one of the plurality of additional pulses.

16. A laser system comprising:

a first beamline operable to support a first laser beam and including first optical elements;

a first harmonic converter optically coupled to the first beamline; a second beamline operable to support a second laser beam and including second optical elements;

a second harmonic converter optically coupled to the second beamline;

a processor operable to control one or more of the first optical elements and one or more of the second optical elements;

a memory coupled to the processor; and

optics operable to combine harmonically converted light from the first beamline and harmonically converted light from the second beamline.

17. The laser system of claim 16 wherein the first laser beam comprises a first laser pulse and the second laser beam comprises a second laser pulse.

18. The laser system of claim 16 wherein the first beamline comprises a preamplifier module and a first amplifier.

19. The laser system of claim 18 wherein the preamplifier module is operable to produce a temporally shaped pulse.

20. The laser system of claim 18 wherein the second beamline comprises a second preamplifier module and a second amplifier.

21. The laser system of claim 16 wherein the memory includes a look up table defining a temporal profile associated with the first beamline and a temporal profile associated with the second beamline.