Patent Grant
10170883
This application claims priority to U.S. provisional patent application Ser. No. 62/498,674, titled “Method for Direct Compression of Laser Pulses with Large Temporal Ratios”, filed on Jan. 3, 2017, and hereby incorporated by reference.
For certain applications, such as Inertial Confinement Fusion (ICF), it is desirable to utilize laser light as part of an energy production process. The use of lasers today is widespread in industrial, scientific and engineering applications. However, for ICF a major drawback to large-scale adoption may have been the cost and time to field a decisive demonstration of ICF itself, as well as the lack of a clear path to an economically attractive commercial realization for an energy production cycle.
In “Optical Configurations for Fusion Laser,” by R. O. Hunter, Jr., Aspen, Colo., submitted Dec. 7, 2016, Patent No. 62/497,908, and incorporated by reference, and “Lookthrough Compression Arrangement,” by R. O. Hunter, Jr., Aspen, Colo., submitted Oct. 31, 2016, Patent No. 62/496,885, and incorporated by reference, Innoven has outlined new laser architectures and elements that may lead to such a cycle. This patent contains a description of another type of overall architecture that may further reduce the cost and complexity for this application. Even though formulated for ICF energy production, there are potentially many applications for the inventions described herein in other areas. “Optical Configurations for Fusion Laser,” supra, described how to take laser energy generated at a longer pulse length and relatively poor beam quality and then compress it in space and time to produce a high energy pulse (≳107 joules) in a short pulse length (˜10−9 seconds) to impinge over a small area (˜0.01-0.1 cm2) of an ICF target. Furthermore, it described how to do this at a reasonable efficiency.
The invented techniques enable the compression operation for the ICF application to occur in gaseous media and greatly reduced (near damage levels) the required area of material surfaces. This may result in laser systems with a 102:1, or greater, reduction in precision optical element area for a given energy compared to existing technology, such as exemplified in the National Ignition Facility (NIF) (see “The National Ignition Facility: Laser System, Beam Line Design and Construction,” by R. H. Sawicki, in M. A. Lane and C. R. Wuest (Eds.), Optical Engineering at the Lawrence Livermore National Laboratory II: The National Ignition Facility, Proceedings of SPIE, Vol. 5341, 2004, pp. 43-53, incorporated by reference herein for all purposes). Overall, the cost per unit of energy for the architecture and elements may be over 102-103 lower than such existing technology. In addition, the reduction in optical element number, size, and precision permits rapid demonstration and deployment of the technology. Herein, further reduction in complexity and cost from the architecture shown in “Optical Configurations for Fusion Laser,” supra, may be realized by more compact packaging and reduction in number of separate beam paths to produce such compression and optical quality improvement.
“Lookthrough Compression Arrangement,” supra, details how an optical arrangement with very high optical gain for the input seed extraction pulse, amplified by converting the energy from a pump pulse, may be realized while having an output pulse length shorter than the pump pulse and of better optical quality. To avoid optical damage, the extraction pulse output, increased by the optical gain relative to the input optical fluence is then transmitted to the target without impinging on material optical elements. Considering the assembly for the extraction pulse and pump pulse interacting in scattering medium to be described as a stage, a single or multi-section stage arrangement with different gaseous media and/or optical scattering properties is described that enables very high gains of the extraction pulse output relative to its seed input. Gains of 103-104 for particular applications and arrangements then may be realized, resulting in large reduction of costs relative to glass laser technology. In “Lookthrough Compression Arrangement,” supra, each separate channel of the Fast Compressor stage may be isolated from adjacent channels and may be driven by an associated pump pulse element, that may be compressed from the Primary Laser Source, such as that described in “Optical Configurations for Fusion Laser,” supra.
A copending patent application “Optical Configurations for Fusion Laser,” supra, described a general laser architecture that had a Primary Laser Source followed by a Compression Section and a Vacuum Transition that was directed towards irradiation of an Inertial Confinement Fusion (ICF) Target. This copending application described a method of forming the Compression Section that entailed an optically multiplexed Raman Aperture Combiner that addressed a multichannel array of Active Time Delay Mirrors (ATDMs) resulting in a large temporal compression ratio whose output then pumps a Fast Compressor.
There is a need for architecture and elements that may permit the temporal and spatial compression of low cost energy for the ICF application, leading to compression costs on the order of $1-$10/joule, a major improvement over the approximate $103/joule costs exemplified in NIF. Coupled with low cost Primary Laser Source energy generation, a cost reduction of over 102:1 may be desired, thereby leading to economics suitable for both ICF technology demonstration (target ignition) and commercial energy production.
The present architecture utilizes a Nonlinear Scattering Aperture Combiner that does not need to be optically multiplexed and then drives a Direct Compressor stage that produces a large temporal compression ratio of about 300:1 to pump a Fast Compressor. This eliminates the need for a separate array of ATDMs, multiplexing optical elements, and, at the approximate 107 joule energy output required for ICF, reduces the number of mechanical elements and gas interfaces from the order of 103 to about 10. In addition, this provides a large reduction of the volume of the gas containment region. In order to accomplish this, a technique for transversely segmenting by color and/or polarization of the optical extraction beams of the Direct Compressor has been invented. This technique may also be applied to the Fast Compressor as well.
The laser design presented is intended to produce low cost (˜$10.00/Joule) energy in a form suitable for use in the Innoven laser fusion technology. The key attributes are a short pulse (1 nanosecond is the baseline output pulse length), ultraviolet (0.25μ wavelength) light, large pulse energy (107+ joules), and the ability to produce small features (˜100 microns) on the target at about a 30 meter focal length. Furthermore, the technology should be amenable to a short, high confidence delivery cycle (a few years) to produce ignition, with commercial technology shortly following.
The fundamental concept is to generate the light at low cost, then optically compress it in time and space and then convert to a near diffraction limited output. The architecture chosen is exemplified by utilization of KrF laser technology to generate 107+ joules with a 3×10−6 second pulse length from multiple Large Pump Modules (LPMs), combine the output from multiple LPMs in a Nonlinear Scattering Aperture Combiner (NSAC), and then temporally and spatially compress the pulse in two additional stages, the Direct Compressor (DC) and the Fast Compressor (FC). The light, then passes through a Vacuum Transition (VT) and impinge on the Target (T). In addition, a Propagation Gas (P) that transmits the light beams without active compression may be used. The optic costs are low since, past the last surfaces to handle the long pulse LPM outputs to the NSAC, the laser light is coupled directly by high optical gain (˜104) amplifiers into the subsequent stage without material optical elements; consequently, no material surface sees the ˜103 joules/cm2 fluence output of the NSAC, DC, and FC. In essence, the stages act like very high gain mirrors with temporal compression, as the light may be redirected, focused and shaped as well. The stages may thus compress the light in time as well as space. The primary scattering process utilized in the compressors is Stimulated Molecular Scattering (STMS). The overall architecture is shown in
The Direct Compressor is a major innovation based on an extension of the near atmospheric pressure multisection backward compression arrangement described in “Lookthrough Compression Arrangement,” supra, extended longitudinally and transversely by techniques described herein to provide very high temporal compression ratios at large energy. The Preferred Embodiment presents detailed calculations of both a compact multisection FC and DC combination with based on color segmentation to produce very high output energies at low cost. The integration aspects are also considered in terms of producing various gas section embodiments. In addition, variations including the Direct Compressor functioning as both an aperture combiner and as a temporal compressor are shown. Overall, for a system of this complexity, there are many possible variants. The emphasis here is on the fundamental performance of very high fluence, high efficiency, low cost design for the ICF application.
Brillouin Scattering is generally a light scattering process whereby an acoustic wave in a gaseous, solid, or liquid medium is excited during a light scattering process. As with Raman scattering, the beam quality of the stimulated scattered beam may be decoupled from the pump beam and independent apertures combined into a diffraction-limited output under certain conditions. It has generally not been used at low gas pressure (˜1 atmosphere) due to the low gain coefficient and pump and extraction limitations due to optical element damage. In some embodiments herein, it is used in the Fast Compressor Stage [140], for example, where very high pump fluences may be realized for the arrangements described herein. The ASE constraints for Brillouin scattering may be similar to those associated with Raman scattering for geometries of interest. In a typical operating regime, the allowable gain length products are generally somewhat less than those characteristic of the Raman scattering process. The frequency shift for Brillouin scattering is small for short wavelengths, leading to very high quantum efficiencies (≥0.999) for certain cases given herein. The gains are given by the same formula shown in the Raman scattering discussion above and are describable in terms of a coupling coefficient to combine the effects of the parameters governing the gain. The nomenclature is generally the same as for the Raman coupling constant. Particularly for backward scattering at low gas pressures (−1 atmosphere) and short wavelengths, the scattered excitation is the velocity field of undivided atoms or molecules as opposed to collective acoustic oscillations. In the Russian literature, the overall process is called Stimulated Molecular Scattering (STMS) and has been described by kinetic theory (see, for example, V. S. Averbakh, A. I. Makarov, and V. I. Talanov, “Stimulated Molecular Scattering of Light in Gases at Different Pressures,” Soviet Journal of Quantum Electronics, Vol. 5, No. 10, pp. 1201-1206, 1976, incorporated by reference herein for all purposes).
Where the names Brillouin scattering, Raman scattering, and STMS are used, it is understood that the processes are interchangeable in the sense that other media scattering by a different mechanism may be substituted in that component.
Raman Scattering (include SVRS, SRRS) in this context generally refers to the scattering of light by an atom or molecule with internal degrees of freedom. In the scattering process, internal energy may be lost to or gained from the light field. Thus, the scattered light may be frequency shifted. For energy gained, the process is generally called anti-Stokes (higher scattered light frequency than the original light beam); for energy lost, the process is generally called Stokes scattering (lower frequency than the incident light beam). If Stokes scattering occurs multiple times, the first effect is called first Stokes; when the first Stokes downscatters, it is called second Stokes, etc. This convention also applies to Brillouin scattering and STMS. Stimulated Raman scattering may occur when there is net gain and is used extensively in the certain of the optical layouts described herein. A high intensity pump beam may pass through a Raman active medium and may be depleted; a second beam, often referred to as a seed beam, may be amplified in intensity. The pump beam energy may then be transferred to the seed beam. In gaseous media under certain circumstances, the beam quality of the seed beam may be decoupled from and made much better than that of the pump beam. Also, the quantum efficiency may be quite high since, for example, in nitrogen rotational Raman scattering the energy lost is generally ˜102 cm−1 out of 4×104 cm−1 incident energy at a 0.25 micron (krypton fluoride) pump laser wavelength, implying a quantum efficiency of approximately 0.9975. Since light can scatter off any of the internal degrees of freedom, the largest gain process is often chosen to work with. The gain may be a function of pump and scattered light wavelengths, pump and scattered light pulse lengths, type and strength of transition, scattering angle between pump and stimulating beam, polarization, and/or line widths of the pump and scattered beams. Large scale Raman scattering may be used to combine many independent pump beams and/or to produce diffraction limited output from poor beam quality pump beams. A coupling coefficient may be used to relate the Raman gain and the pump power per area (optical pump flux). The Greek letter γ is used herein and is usually in units of cm watt−1. The gain is generally given by γIp, with the pump flux, Ip, in watts/cm2. The gain is then in cm−1. Gain length products of γ Ip L, where L is a characteristic length, may describe the amplification properties (units=nepers).
Stimulated molecular scattering (STMS) See Brillouin scattering.
Amplified Spontaneous Emission (ASE). For the forward and backward scattering processes incorporated herein, the amplified spontaneous emission (ASE) generated and amplified by the light beams may be used as a design consideration. Characteristically, gain length products of 10-40 due to ASE limitations may be possible, depending on the geometry. At large enough ASE amplification, the desired light beam energy is converted to unwanted ASE and represents a loss. Typical calculations herein use 1% of the desired beam energy converted to ASE as a criterion.
Forward ASE (FASE). For a given beam of light, this is the amplified spontaneous emission within 90° angle from the propagation direction of the light.
Backward ASE (BASE). For a given beam of light, this is the amplified spontaneous emission within 90°-180° angle from the propagation direction of the light.
Pump Pulse. For a given section, or a stage, the energy coming into it generates gain for an extraction pulse impinging on the same geometric volume. The pump pulse energy may thereby be converted into an increase in the extraction pulse energy.
Compression. The energy in a pump pulse may be temporally and spatially compressed during conversion to the extraction pulse energy in addition to the normal optical compression that may be associated with linear optics. Stages may be cascaded to produce an overall compression whose performance may be the product of multiple stage independent performances.
Extraction Pulse. For a given section or stage, the extraction pulse converts pump pulse energy into amplified extraction pulse energy while passing through a common section and/or stage volume.
Color. For a given laser beam, it may be considered to have a centerline (average) frequency and a bandwidth often referred to as Full Width at Half Maximum (FWHM). If two beams are separated in frequency by more than a FWHM, they may be considered two different colors.
Section. Medium filled region where the Extraction Pulse is amplified as it intersects with the Pump Pulse. Often the section is filled with a uniform coupling medium. In certain designs, the coupling constant may vary as a function of position.
Stage. A stage is a set of sections performing a discrete optical function. For example, temporal compression and beam quality improvement may be found in a Direct Compression Stage with, say, 3 sections. A stage may also have continuously varying properties.
Color Segmentation. An element of an optical beam that is predominantly a single color. The segmentation may be longitudinal and/or transverse with respect to the beam propagation direction. One segment may be isolated from another in terms of ASE interaction by shifting the ASE that may be centered at a particular frequency from one segment out of the frequency band where it may be substantially amplified in the other segment by offsetting the two segment colors. A given color segment may be denoted CS with an associated C for color.
Polarization Segmentation. Discussed in terms of linear polarization, it may also be utilized with circular polarizations and/or mixture of polarizations. An optical beam segment may have a different polarization than another so the ASE generated in one is not substantially amplified in the other. A given polarization segment may be denoted PS with an associated PO for polarization.
Channel. Describes the envelope of the path a particular pulse length and color and/or polarization beam follows. Used herein to primarily describe the output of the Direct Compressor that may have different fluences and time delays as a function of the channel address as well as color, pulse length and polarization. Used to match the Direct Compressor output to crossing angle of Fast Compressor extraction pulse.
In this writing, the pulse temporal compression stage described as the Preferred Embodiment in “Optical Configurations for Fusion Laser,” supra, is replaced by an apparatus called a Direct Compressor that may be capable of compressing a long (in time) optical pulse in a much simpler arrangement than described in “Optical Configurations for Fusion Laser,” supra, and is then integrated with the type of Lookthrough Compression arrangement described in “Lookthrough Compression Arrangement,” supra, (called a Fast Compressor) in a compact fashion, thereby reducing the number of optical and mechanical elements from the exemplary case presented in “Optical Configurations for Fusion Laser,” supra. The number of major mechanical elements (transient openings, valves, environmental enclosures) separating the gaseous regions are markedly reduced. Production of direct compression of an optical beam by temporal ratios of over 102:1 at high efficiency may be obtained in a single stage by use of a multi-sectional Direct Compressor as described herein.
This patent application contains similar information to co-pending patent, “Integration of Direct Compressor with Primary Laser Source and Fast Compressor,” by R. O Hunter, Aspen, Colo., submitted Jan. 3, 2017, Patent No. 62/177,150, and incorporated by reference. This application is directed towards the Direct Compressor as it is unique in terms of high fluence, high temporal compression ratios, and high stage gain, leading to a very large reduction in laser costs. The Direct Compressor simplicity and uniqueness of design is emphasized herein. It may separately have many other applications than ICF. The detailed analysis of the DC coupled FC performance are the primary subject of this application.
Each active stage (LPM, NSAC, DC, FC) may have an associated front end to provide its seed laser beam. The controlling properties may be in the various stage discussions. The key amplification elements of the pulse compression stages (FC, DC) are referred to as sections. The LPM, VT, NSAC, T and P stages do not provide temporal pulse compression. The summary overall performance is shown in Table 1.
(1) ETARGET is laser energy through entrance holes to Hohlraum
(2) ηOVERALL is ETARGET/ ELPM
(3) τP is pulse length of energy delivered to target
(1)At VT Entrance.
(2)GLS is the large signal gain for that stage, defined as the optical output energy/input optical energy of the seed laser beam. For P and VT, the GLS is given nominally as 1 and is referenced to the total input energy as there are no seed beams for these stages. .
(3)εOUT is the output fluence (energy/area) of a given stage.
(4)ηS is the stage efficiency.
The 107 joules is energy deposited onto the Target including allowance for pointing error and fluctuations in the laser properties (noise). The three active combination/compressor stages have high efficiencies that for the FC includes some overall beam train losses. The detailed stage designs and integration for the FC and the DC are described in the following sections. The other elements are described in detail in applications to be filed.
The design approach eliminates large (>0.5 meter diagonal) optics and stores the pump energy in a long pipe. No optical quality laser energy storage glass is required. Previous ICF targets have entailed significant non-uniformities in the drive pressure applied and surface properties, leading to severe stability and symmetry problems. This laser is designed to have good noise properties in terms of the output of the FC and, in conjunction with the Targets described elsewhere, will produce a drive pressure uniformity sufficient for good implosions. In particular, for the Preferred Embodiment low target irradiation noise is produced by having multiple output segments (i.e., 12) from the FC project the same pattern on the Target to average the individual outputs. In addition, the projected patterns are imaged in a given segment from the injected signal. Thus, given the FC geometry, the different parts of the image on target have essentially common paths to produce a high degree of uniformity in a given segment projected pattern. Also, the amplification is further averaged by the fashion in which the pump and extraction beams in the FC are crossed and placing the Target near the focus. For a high quality optical beam, this smooths the Fast Compressor Optical Output Beam. The gain media produce relatively small nonlinear effects, leading to near diffraction limited imaging. The stages are operated far below breakdown limits (≲1/10) and are gas based; damage in the bulk media is not of major importance. The high stage gains result in input optics being subject to low fluence (˜0.1 joules/cm2 for a gain of around 104 in each stage). This design approach places a premium on being able to accurately model the laser media responses. The leading physical processes are the transient response time of the media, nonlinear index leading to intensity dependent imaging, and amplified spontaneous emission. The copending patent application, “Lookthrough Compression Arrangement,” supra, describes certain of the dominant phenomena in more detail.
The very low optical element volume and area enables a rapid development cycle and beam line demonstration before major manufacturing, which is minimal compared to, say, NIF. A major advance is the ability to achieve high stage gain at high efficiency for the three sequential active scattering stages. They need to be of high efficiency (0.6-0.9) in the aggregate. For this type of optical compression, a layout budget is that 0.7 of the LPM output energy is delivered into the desired target pattern. Since there are six major stages following the LPM, (NSAC, P, DC, FC, VT, T), this implies an average efficiency of about 0.94 per stage. The stages have nominally three main efficiencies; optical coupling, ASE, and extraction (ideal conversion of light from the previous stage). Some stages are simpler and do not convert the light from the previous stage (P, VT, T) and are expected to have higher efficiency. The design goal for an active stage (NSAC, DC, FC) is an extraction efficiency of 0.96 (excluding front ends), an ASE efficiency of 0.99 (1% loss in active regions) and an optical coupling efficiency of 0.95; leading to an overall efficiency of 0.94. Detailed cost and performance tradeoffs may be made for particular sections. These estimated efficiencies as are listed for a design based on the Preferred Embodiment in Table 2.
Low speed adaptive optics may be utilized to correct for linear medium disturbances for the FC. The stages have separate gas sections; some are separated mechanically (i.e., shutters) and in one variant, there are stable gas interfaces. Thus, around ten slow mechanical shutters of one meter aperture size are needed in total. The Direct Compressor (142) and Propagation Gas Regions (105) are in a large (1-2 meter) pipe. The major dynamic interface is the vacuum transition (110) in the VT stage.
The KrF LPMs are low cost, robust versions of previously demonstrated electron beam pumped technology. They represent a large fraction of the total cost and are built around known technology. The output windows or shutters with figured turning mirrors are the major optical elements.
The design approach relies on the ability to accurately model the gaseous media responses. The scattering processes are well understood and the large reduction in cost and complexity is achieved through detailed analysis of the compression and transition stages.
The earlier design (“Optical Configurations for Fusion Laser,” supra) was based on separate ATDMs sequentially addressed by a Raman Aperture Combiner (RAC) and resulted in some 36×8=288 separate time compression channels that were directed at eight separate Fast Compressor (FC) stages. The mechanical shutter count was around 1,000 with a typical 0.3-meter aperture. The design presented here may reduce the time compression channel count in the Direct Compressor (DC) stage to one. As described below, a few separate channels of the DC with the same temporal compression are staggered in time to give good matching to the FC. Also, the previous eight Fast Compressors have been reduced to one. One main aspect of such reductions in complexity is the ability to control the Amplified Spontaneous Emission (ASE). The Preferred Embodiment presented below is based on a level of segmentation at the 25 cm channel size with ASE isolation produced by color changes from segment to segment. This design delivers about 107 joules/m2 of optical area. Further improvement in the control of ASE could lead to up to about 108 joules/m2. This design illustrates the compaction achieved by integration of the various functions. The present design is based on a nominal 25 cm square color segment size due to two considerations:
The particular design shown includes calculations for the ASE losses in the various sections, the optical breakdown margin, and the nonlinear imaging parameter B.
There has not been an effort to optimize the entire design. The light generation region, the Large Pump Module (LPM), may be optimized separately for energy generated and delivered to the Nonlinear Scattering Aperture Combiner (NSAC). For the design approach chosen, the pulse length may vary somewhat from the nominal 3×10−6 seconds, however, it is unlikely that the LPM pump pulse will approach the approximately 10−8 seconds used to drive the Fast Compressor at an economic value (≲$5.00-$10.00/joule) in the near future. With the Direct Compressor in the chain of stages, the LPM may then be separately optimized. The NSAC and the DC have some detailed design interplay, yet it appears they may also be separately optimized. The drive for the preamplifier section of the FC has been packaged to be driven by the DC. This is a tradeoff with the complexity of the FC front end. Design changes may be anticipated in this area, particularly as the cost tradeoffs with the FC and the VT are explored in detail.
One area that may reduce complexity further are ASE suppression methods that enable single color operation with higher FC and/or DC output fluences and higher VT fluences.
The approach taken in the design is to consider the 25 cm segment size with commensurate ASE growth without any further suppression. To that end, the ASE losses are based on the estimate from the computations generated to date made by Innoven with a 3-D code.
The overall design method is to set the operating fluence based on a 1 nanosecond output pulse at 107 joules onto the target, set the ASE allowed fluence at the vacuum transition, set the sections for the FC stage, then set the sections for the DC stage. The design approach has been to make the stages at high efficiency (ηRAC ηP ηDC ηFC ηVT ηT≅0.7≥ηSTAGE˜0.94). For each stage, its efficiency is a product of three main parameters; ηOC, or optical coupling efficiency, ηE, or extraction efficiency, and ηASE, or efficiency due to spontaneous emission losses including amplification. The overall linear transmission efficiency is kept as a separate multiplier (due to impurity levels and low-level induced absorption in the gases), as it is not separated out by stage and sections. The Preferred Embodiment design is for a DC/FC crossing angle of 10°. The various factors will change as a function of the crossing angle, however, the choice permits a reasonable estimate of the geometric factors governing efficiency.
This patent application is aimed at the DC, as it provides very substantial advantages over previously published techniques (“Optical Pulse Compressor Systems for Laser Fusion,” by J. J. Ewing, R. A. Haas, J. C. Swingle, E. V. George, and W. F. Krupke, IEEE Journal of Quantum Electronics, Vol. QE-15, No. 5, May, 1979, pp. 368-379, “High-Efficiency Laser Pulse Compression by Stimulated Brillouin Scattering,” by M. J. Damzen and II. Hutchinson, Optics Letters, Vol. 8, No. 6, June 1983, pp. 313-315, “Raman Pulse Compression of Excimer Lasers for Application to Laser Fusion,” by J. R. Murray, J. Goldhar, D. Eimerl, and A. Szöke, IEEE Journal of Quantum Electronics, Vol. QE-15, No. 5, May 1979, pp. 342-368) and may provide simplifications to the compression technologies involving ATDMs and optical multiplexing of the combiner set forth in “Optical Configurations for Fusion Laser,” supra.
A second copending patent application, “Integration of Direct Compressor with Primary Laser Source and Fast Compressor,” supra, is aimed at the integration of the DC into the overall architecture, particularly as regards gas, mechanical and damage aspects of its operation.
Referring to
The seed laser source sizes may be estimated from Table 2 above; The EOUTPUT/GLS for the active stages are: LPM (1.6×102 joules), NSAC (1.5×103 joules), DC (1.7×103 joules), and FC (1.2×103 joules) in the Preferred Embodiment. The seed fluences and fluences are small enough to be handled by material surfaces (mirrors, lenses, etc.). As discussed below, avoidance of optical damage to the permanent optical elements in the NSAC (104) and the DC (142), given the very high operating fluence and inline optical geometry, is a major design consideration and various methods are shown in
The calculations performed to date indicate the FASE generated by the output of the NSAC (104) as it propagates through P (105), FC (110) and into the DC (142) is low enough that the full aperture of 1 meter may be utilized without having to expressly control the ASE (130) of the DC pump beam from the NSAC. In the DC, the extraction beam pulse length is now shorter by a factor of 1-3×102 than that of the pump beam. Consequently, the ASE for the DC extraction beam will produce very large losses unless controlled by some form of isolation.
One form is discussed below, called Color Segmentation, where adjacent segments of the DC extraction beam are driven by seed laser sources having offset frequencies with the offset being less than the effective DC gain bandwidth.
As shown in
Fast Compressor
The Preferred Embodiment is based on the optical architecture exemplified in
The analysis shown is based on constant area interaction regions in the scattering cells. The spatial variation produced by the focusing in the compression regions is a matter of specific design; compensating adjustments purposes by treating the interaction regions in a collimated fashion. Various Innoven codes for the stage performance and ASE calculations were utilized.
Direct Compressor
The Direct Compressor to drive the Fast Compressor in a matched fashion overall is shown in Table 3. A design for a 4-cell section plus preamplifier design is presented. The output is 5.37 εSAT. For example, with 1.5×107 joules incident over 1 m2, 1.5×103 joules/cm2=5.37 εSAT, or, εSAT=269 joules/cm2. This requires the following cell design layout:
(1)Gas at standard temperature and pressure. Mixtures constituents given by partial pressures.
(2)The Raman Cross section may interfere with the STMS. If the Raman rotational transition interferes with STMS at these compression ratios, the N2 may be isotopically mixed (N2(15), N(14) N(15), N2(14)) to lower the Raman cross section compared to the STMS, or Ar may be substituted for N2. The N2 is providing the coupling for the first ~50 meters. For the first section, the time compression ratio
For Raman compression with a forward and backward Raman cross section ratio of 1, ηC ≅10, so a value for ηC of ≲4, there is a large margin. Either the isotopic substitution in the N2 mixture or Ar substitution may be utilized if more margin is desired.
1θc = 2√{square root over ((Φη20))}
2Linear angle subtended by optical area. Using distance from NSAC to input of DC
3Using average section fluence
4Including losses
The B integral is fairly large for this design. As shown in the art, in the range on 2-3, the Bespalov Talanov instability may become significant. However, due to the difficult η20 in each section, the peak growth may be restricted to that of a single section. In addition, the gas mixtures may be adjusted. Imaging properties in the DC are less important since we need only to drive the FC. Replacing nitrogen with argon in Section 1 would reduce the B integral.
We assume 99.999% purity in all gases, which leads to a 10−5 impurity level. This means trace hydrocarbon density is low enough that with 25 eV for each molecule, giving 7.075×10−4 J/cm2 plus an estimated 3×10−4 J/cm2 for low level inverse bremsstrahlung effects. This results in a total of 60 J/cm2, or 4%, loss.
For forward ASE calculations, extraction pulses were run as 25 cm×25 cm squares. Backward ASE for extraction pulses, as well as pump pulses in both the forward and backward direction, were run as 25 cm diameter circles, except for color segmented pre-amp of the Fast Compressor array, which was run at a 10 cm diameter.
The layout and calculations shown were for a collimated FC output geometry. In fact, the focusing onto the target by the FC will produce a tapered DC/FC interaction region.
Color Coding
The fundamental isolation condition for FASE coupling from one transverse color segment to another is the color separation be large enough so that the FASE produced in one section is not amplified appreciably in the adjacent segment. The approximate condition is that the peak gain angle, and hence Brillouin shift from the color in a given segment for the FASE, is given by τph(θp)˜τpulse. Here, τph(θp) is the phonon lifetime as a function of the Brillouin angle (angle between the incident beam and the FASE Brillouin amplified first Stokes). Since
where ΔνB is the FWHM of the Brillouin line, the above approximate criterion may be utilized to set the adjacent color segment shift in color as a function of the pulse length, τ.
Detailed code computations should be performed prior to construction. The ASE values reported assume segment ASE decoupling and section decoupling. Polarization may also be utilized to isolate the FASE from one section to another. Consider two adjacent segments where the color offset is zero. Here, if linearly polarized, the extraction pulses in the two segments may have the electric field vectors orthogonal to one another (set at 90° offset) and then the growth ratio for FASE from one as amplified in the other may be substantially reduced and the two segments effectively isolated in terms of FASE.
Table 5 was generated from the Averbakh formulation (see, for example, V. S. Averbakh, A. I. Makarov, and V. I. Talanov, “Stimulated Molecular Scattering of Light in Gases at Different Pressures,” Soviet Journal of Quantum Electronics, Vol. 5, No. 10, pp. 1201-1206, 1976, incorporated by reference herein for all purposes) using viscosity data to provide an effective collision frequency.
Using the simple approximate criterion given above, for a 1 nanosecond pulse, the approximate peak angles for FASE are: He (˜4°), Ne (˜7.5°), N2 (˜11°), Ar (˜15°). One may estimate the needed color offset as roughly the Brillouin FWHM at that angle, for example, for a 1 nanosecond pulse, He (˜1.5×108 Hz), Ne(˜1.6×108 Hz), N2 (˜1.5×108 Hz), Ar (˜1.4×108 Hz) or, from the initial criterion,
Assuming the scattering medium is the same in adjacent color segments, once the colors are separated by Δνcolor>ΔνB, the adjacent cell coupling constant at that angle and frequency may be substantially less than in the originating segment. For the Preferred Embodiment, the Direct Compressor Extraction Pulse has almost a 30 nanosecond pulse length; hence the needed shifts may be smaller than the 1 nanosecond case given above. “Experimental Observation and Suppression of Transverse Stimulated Brillouin Scattering in Large Optical Components,” J. R. Murray, J. R. Smith, R. B. Ehrlich, D. T. Kyrazis, C. E. Thompson, T. L. Weiland and R. B. Wilcox, Journal of the Optical Society of America B, Vol. 6, No. 12, December 1989, has mentioned (p. 2405) gain reduction due to SBS by a factor of as much as 2 by circular polarization or segmentation into “smaller areas of perpendicular linear polarization,” as well as “One might also subdivide the aperture with absorbing barriers such that each individual sub-aperture has a growth time small enough to prevent SBS.” The use of color and/or polarization segmentation as outlined herein eliminates the need for absorbing barriers and, in the version herein, may reduce the SBS by much more than a factor of two mentioned by Murray as long as the segment color separation is large enough. This version of polarization segmentation may be utilized to provide more than a factor of two isolation as well. The two methods may be combined.
For such segmentation and certain operating conditions, the FASE may be reduced to essentially that for a single color segment. For a backward conversion process in such gases as shown, the linewidths (FWHM) are He (9.6×109 Hz), Ne (4.2×109 Hz), N2 (3.5×109 Hz), Ar (3×109 Hz) at a 180° scattering angle. This implies that a color segment shift of 1.50×108 Hz for the extraction pulse may work without any color shift across the pump beam(s). Thus, color segmentation would be of value in a compression section pumped by a single color, and, in addition, should be of efficacy in pumping a backward scattering converter with a single color extraction and/or offset segments in the converter extraction pulse. This is due to the forward/backward linewidth asymmetry, the bandwidth for an extraction pulse of such a converter may be much larger (as shown above for the particular exemplary cases, a factor of 30-100 in linewidth and for efficient extraction, a factor of some 10-30) then the color offset in the segments of the pumping compressor.
The Preferred Embodiment DC design is for a 4×4 array of colors with a total shift of −600 MHz. The array is analyzed as 25 cm rectangles with the ASE computed for each segment separately. A −600 MHz shift in Neon and the early DC sections will serve to amplify the ASE from a given color segment only in that section. As the FASE propagates at an angle with peak gain due to the polarization response time for Brillouin scattering, it enters another color segment adjacent to the gain segment. The frequency change is large enough that, at that angle, it is no longer amplified and may be absorbed or have a path gain near 0 from then on. In addition, in one axis transverse to the direction of the extraction pulse propagation, the extraction channel pulses are staggered in time. This would also reduce the gain; however, it should be a weaker effect and only reduce the FASE growth; it is not in the calculations.
In a given channel, the temporal characteristics are assumed the same in terms of the beginning and ending of the pulse. The full color modulation occurs throughout the pulse. From channel to channel, the beginning and ending times are different. Table 6 shows the differential shifts for a 10° crossing angle into the Fast Compressor. The channel shifts depend on the crossing geometry. By causality, channel 1 cannot affect channel 3 through small angle FASE. Thus, the vertical modulation may repeat across channels with sufficient transverse separation. Color sequencing in a given channel segment may also be utilized. The present design should have adequate modulation to ensure that the ASE component for a given segment will be isolated. The computations herein are based on this isolation technique. Others are possible, particularly given the ability to time vary polarization and color across all of the high fluence interfaces since specialized coatings and angles of optical elements are not present (polarization, chirping, etc.). The computational geometry utilized for the 3×4 array shown in
The front end delivers input pulses with aggregate energy of over 103 joules, with color separations as previously shown. They are tailored in time to give constant output power at the end of the DC. The time sequence is two 104 second pulses back-to-back for a two-deep array (2×4) or one 2×10−8 second pulse, and three 10−8 second pulses back to back to give one 3×10−8 second pulse/pump channel for the Preferred 3×4 array. The centerline frequency should be stable to within 50 MHz over the pulse and the bandwidth (FWHM) may be 150 MHz. The energy is injected on separate optical channels and is imaged as described in the next section.
One major aspect of this design is the need to prevent damage to the input optics for the NSAC (104) and the DC (142), as shown in
A second condition is met simultaneously by imaging a shaped pupil at the DC input optics onto the desired pump spatial shape at the FC region to ensure high optical coupling efficiency. Likewise, the NSAC output beam, though not color-segmented, should be shaped to provide high optical coupling efficiency to the DC extraction. For this design, the NSAC output beam is time stationary. The operating fluence over 1 m2 is 1.2-1.5×107 joules/m2. If nothing is converted, this full energy could impinge on the input optics of the D.C. Thus, the optical quality of the DC path needs to be good enough to support 102-103:1 contrast at the DC input optics plane, including diffractive effects. If conversion takes place, about 5% of the incident light is left, and the ratio falls to ˜10:1.
The propagation region is a region in which the optical input can spatially expand for the segments and overlap. Likewise, the NSAC region is displaced from the NSAC input optics plane. There are a variety of techniques. Disposable or grazing angle optics seems simplest. Other protection techniques include: high STMS or Raman gain sections in front of the input optic plan to extract the light optically in a perpendicular or ASE direction; active ASE amplifications in such sections; active absorbing regions (electrical breakdown, optically pumped); and switches based on transient mirror formation to inject the signals and decay away before the damaging light arrives back. In addition, a light guide geometry is shown later (
As an alternative to the above imaging arrangement is to utilize disposable flat segments to inject the signals into the FC and DC. For small aperture flats with relaxed flatness tolerances, the cost per shot would be acceptable (˜$1,000.00) and the optical train FC and DC imaging could be optimized for a high optical coupling efficiency, without constraints due to the damage to the input optics.
Another main alternative is to make the DC pipe into a light guide. See
For the mapped option, the DC high stage gain means that the injecting optics do not need to occupy a large area, as shown in
Fast Compressor
The Fast Compressor is based on a four-section stage with backward STMS conversion from the input pump beams out of the Direct Compressor. The optical quality of the output needs to be good. The target requirement is that it be able to project 100 μm features in a spot array with a relatively high degree of uniformity.
The preferred design has the target horizontally mounted with the DC pipe. One variant has the Target located above the DC in order to eliminate the shutter/flow boundaries for the FC section. Temperature and gradient control to within 1° C. is needed to reduce eddy mixing and maintain convective stability.
It should be noted that the calculations presented are for collimated FC and DC beams. The desired tailoring, due to the focusing in the interaction regions may result in a different geometry. The performance and design above and in Table 5 are for a collimated computational geometry.
For a crossing angle of 10°,
Fast Compressor Module
Just as in the DC design a FC module based on the 25 cm color segment size may be designed using a unit cell array with a preamplifier section. The design and performance parameters for such a design is shown in Table 10. These modules may then be stacked four high and three deep to give the full 107+ joule output energy.
23.33E−04
1θc = 2√(Φη20)
21 cm irradiance pattern at 30 m
3Linear angle subtended by optical coupling region (radians)
4Including losses
5Using average section fluence
For forward ASE calculations, extraction pulses were run as 25 cm×25 cm squares. Backward ASE for extraction pulses, as well as pump pulses in both the forward and backward direction, were run as 25 cm diameter circles, except the color segmented pre-amp of the fast compressor array, which was run at a 10 cm diameter.
The Fast Compressor Module and Array (3×4).
Table 11 gives the array performance. The primary coupling is the increase of pulse length needed for the three deep array. For the single module it is 10−8 seconds and for the array it is about 3×10−8 seconds. Thus, more fluence is needed in the pump beams coming from the DC.
23.33E−04
1θc = 2√(Φη20)
21 cm irradiance pattern at 30 m
3Linear angle subtended by optical coupling region (radians)
4Including losses
5Using average section fluence
For forward ASE calculations, extraction pulses were run as 25 cm×25 cm squares. Backward ASE for extracting pulses, as well as pump pulses in both the forward and backward direction, were run as 25 cm diameter circles, except the color segmented pre-amp of the fast compressor array, which was run at a 10 cm diameter. The ASE computations were with a 3-D code and presume segment and section isolation.
FC/DC Integration
The integration of the Fast Compressor and the Direct Compressor is outlined in the
A key aspect is that the modules are considered separately. Specifically, they are treated as not affecting one another by ASE, even when placed in proximity to one another. The pump fluence has to increase when the modules are placed together, and so the ASE from the pump increases. Table 11 (FC Array) gives the performance of the 3×4 array. Note the changes in pump properties (fluence, pulse length) as modules are added in the direction of the DC in the plane formed by FC and DC optical axes. As they are added in a direction vertical to that plane, the parameters per area remain the same.
In addition, it may be practicable to color-code the pump pulses from the DC in time sufficient to decouple them in terms of FASE for the pump beams. This also serves to identify a particular pump color with a particular extraction module, so if there is utility to separating the pumps of the modules, by color or other characteristic, this may be the means employed. For the particular baseline design described herein, it does not appear necessary.
One interesting aspect of the layout shown in
The basic section and channel allocation in these designs are set primarily by the FASE for the extraction pulse and the desire to pump with uniform fluence from the DC over the FC interaction region. However, the DC output beam fluence may be varied as well.
By employing the gases shown, the effective gain media in the FC (110) are N2 and Ne, and they have an overlapping spectral response for STMS. The above analysis does not include the slight gain peak differences, and the baseline design is for a single FC pump color set at the midpoint of their spectral peaks, i.e.,
The baseline design has the FC gases arranged in layers with the target horizontally removed from the interaction zone. One advantage to the variant shown in
For the DC (142) and the VT (112), physical segmentation between modules may be utilized in a straight forward fashion in place of color segmentation. However, in the FC interaction region, introduction of physical baffles become more difficult, and if the segmentation (module beam path) size is reduced to suppress ASE, at some point the separation apertures optically become too small to provide the desired target patterns. The array apertures may need to be optically phased across the segment size. Of course, optically having the FC segments is a straight forward use of the AO units.
In addition to the discrete section embodiments for compression stages, there are also continuous section embodiments. These allow the compression medium to be operated at a given value of the ratio of extraction fluence to saturation fluence, εS(φ), where φ is the crossing angle. The advantage of this arrangement is that a high extraction efficiency may be obtained without ASE that is sensitive to the exact value of the saturation fluence. In addition, the continuous variation of the coupling constant may entail and continuous variation in gas density, resulting in a shift of the Brillouin peak, thereby suppressing the ASE. A continuous variation in the coupling constant of the desired nature may be obtained, as shown in
One note is that if the FC is injected with two linear polarizations aligned ±45° to the direction of linear polarization for the pump beams, the coupling constants will decrease as ˜cos(45°), or, go to one-half the value compared to 0° rotation from the plane of the linear pump beam polarization. Given that the ASE for the extraction pulse is driven by the extraction pulse itself, at ±45° the two polarizations will act independently. Thus, for this particular case, the overall extraction coupling constant will be the same:
γ+45°=½γ0°=γ−45°,γTOT=γ+45°+γ−45°=γ0°.
The εSAT will be 2 εSAT0, and the ASE will scale as though one-half the fluence is driving each polarization. The results from the first case with one pump and one extraction polarization then go directly to this case except twice the output fluence will be obtained. Of course, the pump fluence will need to be doubled. The effect of convergence due to focusing at a 2:1 area ratio may be estimated to give the same results as the unfocused case for ASE and extraction efficiency yet twice the fluence. Thus, the Preferred Embodiment design may be directly extrapolated to twice the output fluence, assuming the ±45° FC input polarizations and twice the single linear polarization pump pulse fluence. A self-consistent, focused extraction and ASE code with the polarization dependence, including effect of pump area variation to give high coupling efficiency, should be utilized for a detailed design.
There are four main variants in terms of stage optical axis alignment: the first is the alignment of the NSAC with the DC as in
The FC crossing angle to the DC beams may be varied. The various parameters relating to the geometry and ASE vary considerably. The case shown for immediate application has a 10° crossing angle. For this condition, nominal 25 cm×25 cm optical extraction area module for both the DC and FC have been shown. Such a module is a building block in an array of modules. Tables 10 and 4 show such modules and combine them for a 10° crossing angle. For these designs, a 25 cm extraction module size has been chosen to provide acceptable imaging and to represent an ASE isolation size achievable with the techniques outlined herein. However, other sizes may be optimum as a function of crossing angle.
There are three primary considerations in the integration. First is optical performance in terms of delivering a large amount of energy into a relatively small target at high overall efficiency. Second, in order to minimize cost, is keeping the overall high optical quality area to a minimum. Third, since the Preferred Embodiment works at very high optical fluence, is to avoid damage to permanent optical elements.
The inline configuration shown appears to be the most straightforward; however, with this geometry the line of sight from the NSAC to the Direct Compressor may need to be changed to avoid the FC active sections. The coupling efficiency and reduced need to shape the beams of light have resulted in the inline configuration for NSAC to DC that is in the Preferred Embodiment. Both the LPM to NSAC and DC to FC coupling are at substantial offset angles, meaning that unconverted light from the pumping stage won't be directed back at the seed laser beam optics (see
In addition, any mechanical element the NSAC impinges on may vaporize and reduce the gas purity for subsequent shots. Thus, the means of controlling the gas interfaces may be constrained by having the NSAC-DC optical axis pass through the FC active region. Good optical interfaces between section gases is essential to a near diffraction limited FC output.
Various configurations are shown in
The length of the medium between the output of the FC and the VT may be set by both FASE and B integral buildups; a gas such as neon or helium is preferred for this section. The matching of the output of the DC to the FC efficiently was discussed in the DC section.
Another variant shown is the FC gas section being formed by a flowing gas stream.
In
This arrangement eliminates the need for an NSAC unless it finds use in generating the Primary Laser Source Beams (135); i.e. it is not needed to match the PLS to the DC. For the Direct Compressor (142), the analysis above for the Preferred Embodiment does not indicate the need for color and/or polarization segmentation in the Primary Laser Source Beam (135) as a means of suppressing the ASE for such beams, and the coupling constant for the Direct Compressor (142) extraction beams does not change substantially for typical offset angles from the direction of the Primary Laser Source Beams (135). Thus, the ability to operate at high fluence and high efficiency for such a Direct Compressor pumped directly by the Primary Laser Source may be realized.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6657188 | Hulet | Dec 2003 | B1 |
| 20100104058 | Birnbach | Apr 2010 | A1 |
| 20110286563 | Moses | Nov 2011 | A1 |
| 20140192829 | Henesian | Jul 2014 | A1 |
| 20140233687 | Jack | Aug 2014 | A1 |
| 20140286471 | Moses | Sep 2014 | A1 |
| 20140348283 | Perkins | Nov 2014 | A1 |
| 20180159290 | Hunter, Jr. | Jun 2018 | A1 |
| 20180191120 | Hunter, Jr. | Jul 2018 | A1 |
