Patent Publication Number: US-7715459-B2

Title: Laser system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a Continuation-in-Part of U.S. patent application Ser. No. 11/787,180, filed on Apr. 13, 3007, entitled LASER SYSTEM, which was a Continuation-in-Part of U.S. patent application Ser. No. 11/584,792, filed on Oct. 20, 2006, entitled LASER SYSTEM, which claimed priority to U.S. Provisional Application Ser. No. 60/732,688, filed on Nov. 1, 2005, entitled 200 W GAS DISCHARGE EXCIMER OR MOLECULAR FLUORINE MULTICHAMBER LASER, and to Ser. No. 60/814,293 filed on Jun. 16, 2006, entitled 200 WATT DUV GAS DISCHARGE LASER SYSTEM, and to Ser. No. 60/814,424, filed on Jun. 16, 2006, entitled LONG LIVED MO IN MOPO CONFIGURED LASER SYSTEM, and was a Continuation-in-Part of U.S. patent application Ser. No. 11/521,904, filed on the Sep. 14, 2006, entitled LASER SYSTEM, and Ser. No. 11/522,052, filed on Sep. 14, 2006, entitled LASER SYSTEM, and Ser. No. 11/521,833, filed on Sep. 14, 2006, entitled LASER. SYSTEM, and Ser. No. 11/521,860, filed on Sep. 14, 2006, entitled LASER SYSTEM, and Ser. No. 11/521,834, filed on Sep. 14, 2006, entitled LASER SYSTEM, and Ser. No. 11/521,906, filed on Sep. 14, 2006, entitled LASER SYSTEM, and Ser. No. 11/521,858, filed on Sep. 14, 2006, entitled LASER SYSTEM, and Ser. No. 11/521,835, filed on Sep. 14, 2006, entitled LASER SYSTEM, and Ser. No. 11/521,905, entitled LASER SYSTEM, filed Sep. 14, 2006, the disclosures of each of which are hereby incorporated by reference. 
   The present application is related to U.S. patent application Ser. No. 11/447,380, entitled DEVICE AND METHOD TO STABILIZE BEAM SHAPE AND SYMMETRY FOR HIGH ENERGY PULSED LASER APPLICATIONS, filed on Jun. 5, 2006, and is related to U.S. patent application Ser. No. 10/881,533, entitled METHOD AND APPARATUS FOR GAS DISCHARGE LASER OUTPUT LIGHT COHERENCY REDUCTION, filed on Jun. 29, 2004, and published on Dec. 29, 2005, Pub. No. 20050286599, the disclosures of which are hereby incorporated by reference. The present application is also related to U.S. Pat. No. 6,549,551, issued on Apr. 15, 2003, to Ness et al, entitled INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL; and U.S. Pat. No. 6,567,450, issued on May 20, 2003, to Myers et al, entitled VERY NARROW BAND TWO CHAMBER HIGH REP RATE GAS DISCHARGE LASER SYSTEM; and U.S. Pat. No. 6,625,191, entitled VERY NARROW BAND TWO CHAMBER HIGH REP RATE GAS DISCHARGE LASER SYSTEM, issued on Sep. 23, 2003 to Knowles et al; and U.S. Pat. No. 6,865,210, issued on Mar. 8, 2005, to Ershov et al, entitled TIMING CONTROL FOR TWO CHAMBERED GAS DISCHARGE LASER SYSTEM; and U.S. Pat. No. 6,690,704, entitled CONTROL SYSTEM FOR TWO CHAMBER GAS DISCHARGE LASER SYSTEM, issued on Feb. 10, 2004 to Fallon et al; and U.S. Pat. No. 6,561,263, issued on May 6, 2003, to Morton et al., entitled DISCHARGE LASER HAVING ELECTRODES WITH SPUTTER CAVITIES AND DISCHARGE PEAKS; and U.S. Pat. No. 6,928,093, entitled LONG DELAY AND HIGH TIS PULSE STRETCHER, issued to Webb et al. on Aug. 9, 2005; the present application is also related to co-pending U.S. patent application Ser. No. 10/781,251, filed on Feb. 18, 2004, entitled VERY HIGH ENERGY, HIGH STABILITY GAS DISCHARGE LASER SURFACE TREATMENT SYSTEM, and Ser. No. 10/884,547, filed on Jul. 1, 2004, entitled LASER THIN FILM POLY-SILICON ANNEALING SYSTEM, published on Jun. 30, 2005, Pub. No. US-2005-0141580; and Ser. No. 11/173,988, entitled ACTIVE BANDWIDTH CONTROL FOR A LASER, filed on Jun. 30, 2005, and to Ser. No. 11/169,203, entitled HIGH PULSE REPETITION RATE GAS DISCHARGE LASER, filed on Jun. 27, 2005, and to Ser. No. 11/095,293, entitled GAS DISCHARGE LASER OUTPUT LIGHT BEAM PARAMETER CONTROL, filed on Mar. 31, 2005, and Ser. No. 11/095,976, entitled 6 KHZ AND ABOVE GAS DISCHARGE LASER SYSTEM, filed on Mar. 31, 2005; and Ser. No. 11/201,877, filed on Aug. 11, 2005, entitled LASER THIN FILM POLY-SILICON ANNEALING OPTICAL SYSTEM, Published on Dec. 8, 2005, Pub. No. US-2005-0269300; and Ser. No. 11/254,282, entitled METHOD AND APPARATUS FOR GAS DISCHARGE LASER BANDWIDTH AND CENTER WAVELENGTH CONTROL, and Ser. No. 11/346,519, filed on Feb. 1, 2006, entitled, VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, and Ser. No. 11/323,604, filed on Dec. 29, 2005, entitled MULTI-CHAMBER GAS DISCHARGE LASER BANDWIDTH CONTROL THROUGH DISCHARGE TIMING; and Ser. No. 11/363,116, entitled VERY HIGH REPETITION RATE NARROW BAND GAS DISCHARGE LASER SYSTEM, filed on Feb. 27, 2006, and Ser. No. 10/881,533, entitled METHOD AND APPARATUS FOR GAS DISCHARGE LASER OUTPUT LIGHT COHERENCY REDUCTION, filed on Jun. 30, 2004, and Ser. No. 10/847,799, entitled LASER OUTPUT LIGHT PULSE STRETCHER, filed on May 18, 2004, and U.S. patent application Ser. No. 11/394,512, entitled CONFOCAL PULSE STRETCHER, filed on Mar. 31, 2006, the disclosures of each of which are incorporated herein by reference. 

   FIELD OF THE DISCLOSED SUBJECT MATTER 
   The subject matter disclosed is related to high power gas discharge laser systems for DUV light sources, e.g., used in integrated circuit photolithography, e.g., in highly line narrowed versions, e.g., for immersion lithography and other lithography uses requiring high power and/or requiring longer component life in lower power applications, or broad band versions used for treatment of material on a workpiece substrate, e.g., laser annealing for low temperature poly-silicon processing (“LTPS”), such as thin beam sequential lateral solidification (“tbSLS”), and more particularly to a seed laser and amplification gain medium system with an improved power amplification stage providing higher gain and reduced ASE and coherency busting, e.g., for reduction in speckle. 
   BACKGROUND 
   Laser systems such as applicants&#39; assignee&#39;s laser systems, e.g., 7XXX and XLA-1XX, XLA-2XX and XLA-3XX model laser systems, with the 7XXX, being single chamber laser systems, e.g., ArF or KrF excimer laser systems, and the XLA models being multi-chamber laser systems, e.g., master oscillator-power amplifier (“MOPA”) configured laser systems, e.g., excimer MOPAs, may be used for such applications as above noted, e.g., laser annealing of materials on a workpiece substrate and as integrated circuit photolithography DUV light sources. The latter of which systems currently operating at 6 kHz and can produce about 60-90 watts delivered to, e.g., a photolithography tool in a burst of hundreds of pulses with about 10-15 mJ per pulse. This operation is relatively close to maximum due, e.g., to high pulse energy and concomitant optical damage problems. Also limitations exist such as in the form of fan size and speed needed to circulate the gas in the lasing chamber(s). 
   In order to preserve the endless march of Moore&#39;s law and with practical and economical EUV light source production perhaps not arriving quite on time, the photolithography tool makers have turned to a technology known as immersion lithography, whereby a fluid is interposed between the wafer and the mask/retical and projection lens assembly. 
   While much of the photolithography tool may still be utilized for immersion techniques, the DUV 193 nm light source will need to be significantly changed, in order to meet higher throughput requirements, and also for beam stability reasons (bandwidth, dose, center wavelength, etc.). 
   A further motivation for implementing a very high average power, e.g., 100 W, laser system, e.g., as an XLA laser system or other power amplification stage, is that further advances in lithographic resolution can be achieved through a “double exposure” technique. Scanner makers will no doubt want to produce a stepper that can provide double exposure with no loss in wafer throughput. Thus, wafer stage speed (scan speed) would also have to increase by approximately 2×. Thus, the laser average power must also increase by around 2× or perhaps even more to maintain throughput. 
   It is also possible that improved performance in the optics, e.g., with the application of optical coatings and/or angle of incidence changes could be used to increase the overall laser system output, although studies by applicants&#39; employer have indicated that there is not enough margin there to get to the required overall output energy levels and/or certain optics could not safely be modified in the effort. Thus this is not currently an available option because as currently perceived a relatively small percentage change in overall output energy is obtainable by this route alone, i.e., somewhere between ten to twenty percent. 
   Various assumptions and constraints may be applicable regarding illuminator component lifetimes and capabilities and the like which result in the conclusion that for a 6 kHz×33 mJ solution, pulse duration must increase by a factor of 4, and for a 12 kHz×17 mJ solution, pulse duration must increase by a factor of 2. Also, since the same degree of high polarization will be required from the laser light source, one can not use polarization coupling to combine separate laser beams to form a laser system output light pulse beam input to the scanner (though polarization coupling may be used elsewhere) and a 2× increase in power density through various components will cause increased depolarization. Whether or not the scanner (e.g., in the illuminator) can accommodate a change in laser beam size is also an issue. Some scanners may also not be able to accommodate a laser light source in the form of two input beams, e.g., as side-by-side laser beams with, e.g., interleaved pulsing. Applicants assume that laser spectral requirements will remain the same as for the equivalent lens used in single exposure systems. 
   One possible solution to the requirements noted above, a single XLA running at 6 kHz and with a 20-30 mJ pulse energy output from the power amplifier, has a number of problems in the effective implementation, not the least of which is the difficulties in getting to 6 kHz operation in an excimer seed to excimer amplifier gain medium multi-chamber laser system arrangement, for which applicants&#39; assignee has proposed certain design approaches discussed in one or more co-pending applications noted above. In addition, the most likely deterrent to such an approach is unacceptably high energy density on certain critical optical elements in the XLA system at 20-30 mJ output pulse energy. Alternatively one could try to implement a single MOPA XLA operating at 12 kHz with a 17 mJ output pulse energy from the power amplifier, however, getting to 12 kHz poses a number of problems, e.g., an approximately 8× increase in chamber blower power, significantly accentuated chamber acoustic impacts on output pulse parameters, and difficulties in maintaining chamber robustness at high voltage with such a high repetition rate. Similarly, a single MOPO XLA (with a power oscillator in place of the power amplifier) operating at 12 kHz with a 17 mJ output pulse energy from the amplifier would face much the same detrimental impacts to effective operation. A Single MOPA XLA tic-toc (e.g., a master oscillator—single aperture—seeding a plurality of amplifiers—multiple apertures—and recombined back to a single output—single aperture) with excimer seed operating the MO at 12 kHz and each amplifier gain medium operating at 6 kHz with each having 17 mJ output pulse energy would suffer from the same problems, however, only in the MO. A single MOPA XLA tic-toc with solid state seed operating at 12 kHz (tic-toc to 2 multi-pass PA&#39;s at 6 kHz each, 17 mJ output pulse energy from each) is a possibility, however, this would require a high average power solid state seed laser, e.g., with about a 12 W average power output, which is not currently available. Two “standard” six kHz XLAs could be used side by side to tic-toc a total of a 12 kHz of 17 mJ output pulse energy laser pulses, if acceptable from a cost standpoint for very high power (around 200 W) lithography laser light sources, e.g., for immersion lithography. Cost of consumables may be acceptable, e.g., for each individual laser system, but the overall cost of operation of the entire system essentially doubles. Other problems need also be addressed, however the above noted are what applicants currently believe to be the “show stoppers” to the various noted configurations meeting the requirements for performance and cost of operation for very high power laser operations, e.g., for immersion lithography laser light sources. 
   Applicants&#39; employer&#39;s competitor GigaPhoton has utilized multi-chamber seed laser/amplifier laser systems in a master oscillator power oscillator configuration, as shown, e.g., in U.S. Pat. No. 6,721,344, entitled INJECTION LOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued on Apr. 13, 2004 to Nakao et al; U.S. Pat. No. 6,741,627, entitled PHOTOLITHOGRAPHIC MOLECULAR FLUORINE LASER SYSTEM, issued on May 25, 2004 to Kitatochi et al, and U.S. Pat. No. 6,839,373, entitled ULTRA-NARROWBAND FLUORINE LASER APPARATUS, issued on Jan. 4, 2005 to Takehisha et al. However, not without certain problems not faced by a power amplifier (i.e., a fixed amplification path—one or more passes—through the amplification medium as opposed to laser oscillation). These may include, e.g., two critical challenges in the application of the injection locking method, e.g., to lithography. They are related to ASE and coherence. 
   Fork, et al. Amplification of femtosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) refers to a multipass power amplifier. U.S. Pat. Nos. 6,816,520, entitled SOLID STATE SYSTEM AND METHOD FOR GENERATING ULTRAVIOLET LIGHT, issued on Nov. 9, 2004 to Tolloch et al., relates to mixing schemes for 193 nm light generation with a solid state seed to an excimer laser; U.S. Pat. Nos. 6,373,869, entitled SYSTEM AND METHOD FOR GENERATING COHERENT RADIATION AT VACUUM ULTRAVIOLET WAVELENGTHS USING EFFICIENT FOUR WAVE MIXING, issued to Jacob on Apr. 16, 2002, relates to mixing schemes for 193 nm light generation. US Published Patent Application, Pub. No. US2005/0185683A1 relates to frequency shifting to get 193 nm light. U.S. Pat. No. 5,233,460, entitled METHOD AND MEANS FOR REDUCING SPECKLE IN COHERENT LASER PULSES, issued to Partlo et al. on Aug. 3, 1993 discusses misaligned optical delay paths for coherence busting on the output of gas discharge laser systems such as excimer laser systems. U.S. Pat. No. 6,191,887, entitled LASER ILLUMINATION WITH SPECKLE REDUCTION, issued to Michaloski et al. on Feb. 20, 2001, relates to coherence busting for speckle reduction in a multiple delay path pulse stretcher. U.S. Pat. No. 5,940,418, entitled SOLID-STATE LASER SYSTEM FOR ULTRA-VIOLET MICRO-LITHOGRAPHY, issued to Shields on Aug. 17, 1999 relates to MOPO/PA configurations where a solid sate laser is the MO for a solid state laser PO or PA but refers to an article as describing the production of 193 nm light using an excimer laser, a dye laser and a birefringent BBO crystal for frequency multiplication harmonic generation, Muckenheim et al., “Attaining the wavelength Range 189-197 by frequency mixing in B—BaB 2 O 4 ,” Appl. Phys. B 45 (1988), pp. 259-261. U.S. Pat. No. 6,031,854, entitled DIODE PUMPED CASCADE LASER FOR DEEP UV GENERATION, issued to Ming on Feb. 29, 2000 relates to a solid state cascade laser in which the output of a diode pumped solid state laser is used to pump another solid state laser to produce DUV light; U.S. Pat. No. 6,320,886, entitled LASER DEVICE, issued to Dawber on Nov. 20, 2001 relates to a solid state optical parametric generator (“OPG”) that is pumped by light produced by a pump source  4  that is disclosed also to be a solid state laser, and where the OPG is in a resonance cavity. U.S. Pat. No. 6,477,188, entitled LIGHT SOURCE, issued to Takaoka on Nov. 5, 2002, relates to solid state lasers seeding and/or pumping other solid state lasers or OPGs or OPOs. U.S. Pat. No. 6,590,698, entitled ULTRAVIOLET LASER APPARATUS AND EXPOSURE APPARATUS USING SAME, issued to Ohtsuki on Jul. 8, 2003, relates to a solid state feed of a seed into distributed fiber-optic amplifiers. U.S. Pat. No. 6,654,163, entitled OPTICAL AMPLIFIER ARRANGEMENT FOR SOLID STATE LASER, issued to Du on Nov. 25, 2003, relates to an amplifier gain medium that can be a gas discharge or solid state laser seeded from an undisclosed type of laser. U.S. Pat. No. 6,721,344, entitled INJECTION LOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued to Nakao et al. on Apr. 13, 2004 discloses an F 2  gas discharge laser in a MOPA or MOPO configuration with a gas discharge master oscillator seeding a gas discharge amplifier. U.S. Pat. No. 4,982,406, entitled SELF INJECTION-LOCKING LASER TECHNIQUE, issued to Facklam on Jan. 1, 1999, relates to a laser system that has so-called “self-injection locking” and appears to disclose a number of prior art systems, that inject a seed beam into an amplifier laser. U.S. Pat. No. 4,019,157, entitled METHOD AND APPARATUS FOR TUNING HIGH POWER LASERS, issued to Hutchinson on Apr. 19, 1977, relates to a pulsed gas laser (CO 2 ) seeded with a CW laser beam from a seed laser disclosed to be a CW CO 2  laser. U.S. Pat. No. 4,227,159, entitled COMMON-RESONATOR PRE-LOCKED LASER, issued to Barrett on Oct. 10, 1980 relates to a dye laser simultaneously pumped in a resonator cavity by an argon ion laser and a solid state Nd:YAG frequency doubled laser. U.S. Pat. No. 4,019,157, entitled METHOD AND APPARATUS FOR TUNING HIGH POWER LASERS, issued to Hutchinson on Apr. 19, 1977, relates to high power gas lasers which are seeded by a beam from a low power laser. U.S. Pat. No. 4,264,870, entitled AUTOMATIC LOCKING SYSTEM FOR AN INJECTION LOCKED LASER, issued to Avicola on Apr. 28, 1981, relates to an injection locked oscillator which is an optically pumped dye laser that is provided with a seed laser pulse from a “master oscillator” but this MO actually acts to create a population inversion in the ILO cavity at a wavelength selected by the wavelength of the master oscillator pulse prior to stimulated emission lasing in the ILO resulting from the pumping of the ILO flash lamp. U.S. Pat. No. 4,490,823, entitled INJECTION-LOCKED UNSTABLE LASER, issued to Komine on Dec. 25, 1984, relates to a laser system that has an optical switch to form the cavity first to include line narrowing in a stable resonator and thereafter to switch to an unstable resonator with the line narrowing package not in the cavity any longer. U.S. Pat. No. 4,606,034, entitled ENHANCED LASER POWER OUTPUT, issued to Eden et al. on Aug. 12, 1986, relates to population inversion created by a “seed” pulse before stimulated emission is caused in the amplifier by the amplifier being pumped. U.S. Pat. No. 4,689,794, entitled INJECTION-LOCKING A XENON CHLORIDE LASER AT 308.4 NM, issued to Brosman on Aug. 25, 1987, relates to an injection locked excimer gas discharge laser system, e.g., a XeCl laser which either uses line narrowing or an injection of a low level amount of radiation into the cavity to essentially do preionization so the gain achieved by the main pumping need not be so high. 
   Partlo et al, Diffuser speckle model: application to multiple moving diffusers, Appl. Opt. 32, 3009-3014 (1993), discusses speckle reduction techniques. 
   Ti:sapphire (Titanium-sapphire) lasers emit near-infrared light, tunable in the range from 650 to 1100 nanometers. These lasers are tunable and can generate ultrashort pulses. Titanium-sapphire refers to the lasing medium, a crystal of sapphire (Al 2 O 3 ) that is doped with titanium ions. A Ti:sapphire laser is usually pumped with another laser with a wavelength of 514 to 532 nm, for which argon lasers (514.5 nm) and frequency-doubled Nd:YAG, Nd:YLF, and Nd:YVO lasers (527-532 nm) may be used as discussed at http://en.wikipedia.org/wiki/Ti-sapphire_laser 
   Second harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process, in which photons, e.g., at a given wavelength, interacting with a nonlinear material are effectively “combined” to form new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons. Only under special circumstances, the rate of conversion of photons to the higher-energy photons is significant. The two fundamental requirements for efficient nonlinear power conversion are that the pump intensity is high over a certain propagation length, and that the involved beams preserve a certain phase relationship over that length. Under properly optimized conditions, it is possible to obtain more than 50% conversion efficiency (sometimes even more than 80%) by focussing an intense laser beam into a suitable nonlinear crystal. This is widely used, for example to generate green light at 532 nm from the near infrared output of a Nd:YAG laser at 1064 nm. Some common materials used for second harmonic generation are potassium titanyl phosphate (KTP), lithium triborate (LBO), cesium lithium borate (CLBO), lithium tantalate, and lithium niobate. 
   As mentioned above, a high conversion efficiency requires that the input light and the second harmonic light are kept in phase. This is not the case without special measures, because the speed of light in a material generally varies with wavelength due to dispersion of the index of refraction. In some nonlinear crystals, a particular combination of crystal orientation and crystal temperature can be found where, due to birefringence, the fundamental and second harmonic light both see the same index of refraction, and so remain in phase as they propagate. In other nonlinear materials, where this is not possible, periodic poling is used to keep the waves approximately in phase. This technique, called quasi-phase matching, is commonly used for lithium niobate and lithium tantalate, and greatly expands the options for efficient frequency doubling at various wavelengths and temperatures. http://en.wikipedia.org/wiki/Second_harmonic_generation. 
   Acousto-optic (“AO”) crystals are often used in optical systems to modulate, frequency shift, or diffract a laser beam. In the case of frequency shifting, the beam interacts with an acoustic wave that moves inside the crystal, Bragg-reflecting from the wave. The frequency of the reflected beam is the sum of the frequency of the original beam and the frequency of the acoustic wave. Depending on its direction of motion, the acoustic wave can contribute either a positive or negative frequency component. In an AO phase shifter, the beam to be phase shifted reflects first from one AO crystal that adds a radio-frequency (RF) component, then from a second AO crystal that subtracts an RF component of the same magnitude, restoring the beam&#39;s original frequency. The phase delays between the two RF signals can be varied, adding a controllable phase shift to the beam, as is discussed at http://lfw.pennnet.com/Articles/Article_Display.cfm?Section=ARTCL&amp;ARTICLE_ID=221417&amp;VERSION_NUM=3&amp;p=12 
   Published International Application WO 97/08792, published on Mar. 6, 1997 discloses an amplifier with an intracavity optical system that has an optical path that passes each pass of a sixteen pass path through the same intersection point at which is directed a pumping source to amplify the light passing through the intersection point. 
   R. Paschotta, Regenerative Amplifiers, found at http://www.rp-photonics.com/regenerative_amplifiers.html (2006) discusses the fact that a regenerative amplifier, may be considered to be an optical amplifier with a laser cavity in which pulses do a certain number of round trips, e.g., in order to achieve strong amplification of short optical pulses. Multiple passes through the gain medium, e.g., a solid state or gaseous lasing medium may be achieved, e.g., by placing the gain medium in an optical cavity, together with an optical switch, e.g., an electro-optic modulator and/or a polarizer. The gain medium may be pumped for some time, so that it accumulates some energy, after which an initial pulse may be injected into the cavity through a port which is opened for a short time (shorter than the round-trip time), e.g., with the electro-optic (or sometimes acousto-optic) switch. Thereafter the pulse can undergo many (possibly hundreds) of cavity round trips, being amplified to a high energy level, often referred to as oscillation. The electro-optic switch can then be used again to release the pulse from the cavity. 
   Alternatively, the number of oscillations may be determined by using a partially reflective output coupler that reflects some portion, e.g., around 10%-20% of the light generated in the cavity back into the cavity until the amount of light generated by stimulated emission in the lasing medium is such that a useful pulse of energy passes through the output coupler during each respective initiation and maintenance of an excited medium, e.g., in a pulsed laser system. 
   Uppal et al, Performance of a general asymmetric Nd: glass ring laser, Applied Optics, Vol. 25, No. 1 (January 1986) discusses an Nd: glass ring laser. Fork, et al. Amplification of femtosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) discloses a seed laser/power amplifier system with multiple passes through a gain medium in a ring configuration, which Fork et al. indicates can be “converted into a closed regenerative multi pass amplifier by small reorientations of two of the four mirrors that compose the resonator [and providing] additional means . . . for introducing and extracting the pulse from the closed regenerator”. This reference refers to an open-ended amplifier portion with fixed number of passes through the amplifier portion (fixed by the optics and, e.g., how long it takes for the beam to walk off of the lens and exit the amplifier portion) as a “resonator”. 
   Mitsubishi published Japanese Patent Application Ser. No. JP11-025890, filed on Feb. 3, 1999, published on Aug. 11, 2000, Publication No. 2000223408, entitled SEMICONDUCTOR MANUFACTURING DEVICE, AND MANUFACTURING OF SEMICONDUCTOR DEVICE, disclosed a solid state seed laser and an injection locked power amplifier with a phase delay homogenizer, e.g., a grism or grism-like optic, between the master oscillator and amplifier. United States Published application 20060171439, published on Aug. 3, 2006, entitled MASTER OSCILLATOR-POWER AMPLIFIER EXCIMER LASER SYSTEM, a divisional of an earlier published application 20040202220, discloses as master oscillator/power amplifier laser system with an optical delay path intermediate the master oscillator and power amplifier which creates extended pulses from the input pulses with overlapping daughter pulses. 
   As used herein the term resonator and other related terms, e.g., cavity, oscillation, output coupler are used to refer, specifically to either a master oscillator or amplifier portion, a power oscillator, as lasing that occurs by oscillation within the cavity until sufficient pulse intensity exists for a useful pulse to emerge from the partially reflective output coupler as a laser output pulse. This depends on the optical properties of the laser cavity, e.g., the size of the cavity and the reflectivity of the output coupler and not simply on the number of reflections that direct the seed laser input through the gain medium a fixed number of times, e.g., a one pass, two pass, etc. power amplifier, or six or so times in the embodiment disclosed in Fork, et al. Amplification of femtosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) and not on the operation of some optical switch in the cavity. In some of the literature an oscillator in which the round trip through the amplification gain medium, e.g., around a loop in a bow-tie or racetrack loop, is not an integer number of wavelengths, may be referred to as an amplifier, e.g., a power amplifier, while also constituting an oscillator laser. The term power amplification stage and more specifically ring power amplification stage is intended herein to cover both of these versions of a power oscillator, i.e., whether the path through the gain medium is an integer multiple of the laser system nominal center wavelength or not and whether the literature, or some of it, would refer to such an “oscillator” as a power amplifier or not. The closed loop path or oscillation loop as used herein refers to the path through the amplification gain medium, e.g., an excimer or similar gas discharge laser amplification stage, around which the seed laser pulse light oscillates in the amplification stage. 
   Yb3+ fiber lasers are inherently tunable, as discussed in J Nilsson et al “High-power wavelength-tunable cladding-pumped rare-earth-doped silica fiber lasers,” Opt. Fiber Technol. 10, pp 5-30 (2004). 
   SUMMARY 
   An apparatus and method is disclosed which may comprise: a very high power line narrowed lithography laser light source which may comprise: a solid state seed laser system which may comprise: a pre-seed laser providing a pre-seed laser output; a fiber amplifier receiving the pre-seed laser output and providing an amplified seed laser pulse which may comprise: a pulse having a nominal wavelength outside of the DUV range; a frequency converter modifying the nominal center wavelength of the output of the seed laser system to essentially the nominal center wavelength of the amplifier gain medium; a first and a second gas discharge laser amplifier gain medium each operating at a different pulse repetition rate from that of the seed laser output; a beam divider providing each respective first and second amplifier gain medium with output pulses from the seed laser; a beam combiner combining the outputs of each respective amplifier gain medium to provide a laser output light pulse beam having the pulse repetition rate of the solid state seed laser system. The seed laser may be selected from the group comprising Nd +3  doped fiber lasers and Yb +3  doped fiber lasers. The seed laser may comprise an Er:Yb predoped laser. The pre-seed laser may comprise a continuous wave laser or a pulsed laser. The amplifier laser may comprise a pulse amplifier laser. The solid state seed laser system may be tunable in nominal center wavelength and/or may have a plurality of nominal center wavelength operating points. The frequency converter may comprise a single or multiple non-linear frequency conversion stage. The amplifier gain medium may be selected from the group comprising: XeF, XeCl, KrF, ArF and F 2  gas discharge lasers. The amplifier gain medium may be selected from the group comprising: XeF, XeCl, KrF, ArF and F 2  gas discharge lasers. The apparatus and method may comprise a coherence busting mechanism sufficiently destroying the coherence of the output of the seed laser or the outputs of the amplifier gain mediums or both to reduce speckle effects in a processing tool using the light from the laser system. The coherence busting mechanism may comprise a first axis coherence busting mechanism and a second axis coherence busting mechanism. The coherence busting mechanism may comprise a beam sweeping mechanism. The beam sweeping mechanism may be driven in one axis by a first time varying actuation signal. The beam sweeping mechanism may be driven in another axis by a second time varying actuation signal. The first actuation signal may comprise a ramp signal and the second actuation signal comprising a sinusoid. The time varying signal may have a frequency such that at least one full cycle occurs within the time duration of a seed laser output pulse. The coherence busting mechanism may comprise an optical delay path with misaligned optics. The misaligned optics may produce a hall of mirrors effect. The coherence busting mechanism may comprise an optical delay path longer than the coherence length of the seed laser output pulse and a beam pointing angle offset mechanism. The coherence busting mechanism may comprise an active optical coherence busting mechanism and a passive optical coherency busting mechanism. The active coherence busting mechanism may comprise a beam sweeping device and the passive coherence busting mechanism comprising an optical delay path. The coherence busting mechanism may comprise: a first optical delay path with a delay longer than the coherence length of the seed laser output pulse and a second optical delay path in series with the first optical delay path and having a delay path longer than the coherence length of the seed laser output pulse and different from the delay path of the first optical delay path; a beam angular offset mechanism. The delay of the second optical delay path may be greater than the delay path of the first optical delay path. The coherence busting mechanism may comprise a pulse stretcher. The pulse stretcher may comprise a negative imaging optical delay path. The pulse stretcher may comprise a confocal mirror OPuS. The coherence busting mechanism may comprise beam flipping mechanism. The apparatus and method may comprise a very high power broad band lithography laser light source which may comprise: a solid state seed laser system which may comprise: a pre-seed laser providing a pre-seed laser output; a fiber amplifier receiving the pre-seed laser output and providing an amplified seed laser pulse which may comprise: a nominal wavelength outside of the DUV range; a frequency converter modifying the nominal center wavelength of the output of the seed laser system to essentially the nominal center wavelength of the amplifier gain medium; a first and a second gas discharge laser amplifier gain medium each operating at one half of the pulse repetition rate of the seed laser output; a beam divider providing each respective first and second amplifier gain medium with alternating output pulses from the seed laser; a beam combiner combining the outputs of each respective amplifier gain medium to provide a light source laser output light pulse beam having the pulse repetition rate of the solid state seed laser. The method and apparatus may comprise: a very high power lithography laser light source which may comprise: a solid state seed laser system having a nominal wavelength outside of the DUV range and a pulse repetition rate of at least 10 kHz; a first and a second gas discharge laser amplifier gain medium each operating at a different pulse repetition rate than the pulse repetition rate of the seed laser output; a beam divider providing each respective first and second amplifier gain medium with output pulses from the seed laser; a frequency converter modifying the nominal center wavelength of the output of the seed laser system to essentially the nominal center wavelength of the amplifier gain medium; a beam combiner combining the outputs of each respective amplifier gain medium to provide a laser output light pulse beam having the pulse repetition rate of the solid state seed laser. The solid state seed laser may be tunable in nominal center wavelength and/or has a plurality of nominal center wavelength operating points. The apparatus and method may comprise: a DUV pulsed lithography laser light source which may comprise: a solid state seed laser producing a pulsed solid state laser output beam; a DUV gas discharge laser amplifier gain medium; a converter wavelengths in which the gain medium is effective; a coherence busting mechanism intermediate the converter and the gain medium comprising an optical delay path separating the seed pulse into a main pulse and separate daughter pulses separated by at least the coherence and an angular beam displacement mechanism displacing the pointing angle of the main pulse and at least one of the daughter pulses. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows schematically and in block diagram form a very high average power laser light source according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 2  illustrates schematically and in block diagram form a very high average power laser light source according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 3  shows schematically in block diagram form an example of a very high average power laser light source according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 4  shows partly schematically and partly in block diagram form, by way of example an immersion laser lithography system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 5  shows schematically and in block diagram form a solid state seed laser to gas discharge amplifier laser system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 6  shows in block diagram form a solid state seed laser/amplifier laser system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 7  shows schematically and in block diagram form conversion of the output of a seed laser, e.g., with a frequency converter along with a beam divider, followed by coherency busting according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 8  shows schematically and in block diagram form a version of the embodiment of  FIG. 7  according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 9  shows schematically a coherency buster according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 10  shows schematically and partly in block diagram form an injection seeded DUV gas discharge master oscillator/amplifier gain medium laser system solid state master oscillator according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 11  shows schematically and partly in block diagram form an injection seeded DUV gas discharge master oscillator/amplifier gain medium laser system solid state master oscillator according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 12  shows schematically and partly in block diagram form an injection seeded DUV gas discharge master oscillator/amplifier gain medium laser system solid state master oscillator according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 13  shows schematically and partly in block diagram form an injection seeded DUV gas discharge master oscillator/amplifier gain medium laser system solid state master oscillator according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 14  illustrates in schematic and partly block diagram form an example of very high average output power laser system power amplification stage according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 15  shows partly schematically and partly in block diagram for an example of elements of a coherence busting scheme and the results of aspects of the scheme according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 16  illustrates in schematic and partly block diagram form an example of very high average output power laser system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 17  illustrates relative speckle intensity for a various E-O deflector voltages related to relative timing between the EO and the pulse generation in the seed laser according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 18  illustrates pointing shift relative to E-O voltage according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 19  illustrates an example of the timing of an E-O deflection voltage and a seed laser pulse spectrum according to aspects of an embodiment of the disclosed subject matter; 
       FIGS. 20A  and B illustrate the effect of beam combining according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 21  illustrates schematically and in partly block diagram form a beam combiner with divergence control according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 22  illustrates the effect of beam sweeping/painting on coherency according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 23  illustrates schematically and in partly block diagram form a very high power solid state seed laser and gain amplifier laser system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 24  shows schematically and in cartoon fashion the effects of multiple coherence busting schemes; 
       FIG. 25  illustrates schematically and partly in block diagram format a regenerative/recirculating power gain oscillator power amplification stage according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 26  illustrates schematically a coherency reduction scheme according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 27  illustrates schematically and partly in block diagram form a solid state seed laser/gain amplifier laser system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 28  illustrates schematically and partly in block diagram form a solid state seed laser/gain amplifier laser system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 29  illustrates normalized output pulse shapes from laser systems according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 30  represents schematically E-O cell laser steering input voltages according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 31  represents schematically in block diagram form a laser steering system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 32  represents schematically E-O cell laser steering voltages input signals according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 33  illustrates results of simulated beam pulse recombination results; 
       FIG. 34  illustrates exemplary coherency busting test results according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 35  illustrates schematically a coherency buster according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 36  illustrates exemplary coherency busting test results according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 37  illustrates exemplary coherency busting test results according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 38  illustrates a simulated E-O supply voltage with respect to a seed pulse intensity over time, according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 39  illustrates a test E-O supply voltage with respect to a seed pulse intensity over time, according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 40  illustrates a E-O cell drive circuit according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 41  illustrates schematically and partly in block diagram form a solid state seed laser with about 193 nm output light according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 42  illustrates schematically and partly in block diagram form a sold state seed laser with about 193 nm output light according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 43  illustrates various frequency up-conversion schemes; 
       FIG. 44  illustrates schematically and in block diagram form a broad band light source and laser surface treatment system using the DUV laser light according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 45  illustrates plots of ASE and output energy vs. seed pulse to amplifier delay time according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 46  shows schematically a seed injection mechanism and beam expander according to aspects of an embodiment of the subject matter disclosed; 
       FIG. 47  shows schematically a coherence buster optical delay path according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 48  shows schematically a coherence buster optical delay path according to aspects of an embodiment of the disclosed subject matter. 
       FIG. 49  shows schematically and in block diagram form a laser system according to aspects of an embodiment of the disclosed subject matter; 
       FIG. 50  shows schematically and in block diagram form a laser system according to aspects of an embodiment of the disclosed subject matter. 
   

   DETAILED DESCRIPTION 
   According to aspects of an embodiment of the disclosed subject matter there are certain performance requirements necessary of a very high power amplification stage cavity for, e.g., a 200 w laser system, e.g., with two amplifier gain medium chambers in parallel. They should produce linear polarization (&gt;98%). Each amplification stage should produce, and survive, ≧100 W average output energy, e.g., at 193 nm wavelength of ArF, or less stringently at longer wavelengths, e.g., 248 for KrF and 351 for XeF or 318 for XeCl, though even more stringent for F 2  at 157 nm. Each amplification stage in one embodiment may operate at about 6 kHz or above. The amplification stage optics must survive energy densities associated with 17 mJ/(losses)≅29 mJ per pulse near the amplification stage output. According to aspects of an embodiment of the disclosed subject matter, the amplification stage(s) can exhibit full seeding (at or near saturation) with relatively small seed laser energy. According to aspects of an embodiment of the disclosed subject matter seed laser energy may be no more than around 1 μJ, though the system overall output power in such cases may be less than 200 W. Applicants believe that the amplification stage should also support a moderately large angular distribution, e.g., to maintain the same angular spread of the seed laser, in order to avoid inadvertently improving coherence by, e.g., removing coherence cells, e.g., with a range of angles of within a few m Rad. Protection of the seed laser from reverse traveling radiation is also an important operational requirement. When properly seeded, ASE levels produced by the amplification stage, according to aspects of an embodiment of the disclosed subject matter, should be less than 3 xe-4 of total output. 
   According to aspects of an embodiment of the disclosed subject matter applicants expect that (1) the gain cross-section will be similar to existing ArF chambers, e.g., applicants&#39; assignee&#39;s XLA ArF laser system power amplifier (“PA”) chambers, (2) the gain length will also be similar to existing ArF chambers, (3) the gain duration will also be similar to existing ArF chambers. 
   According to aspects of an embodiment of the disclosed subject matter, applicants propose, e.g., a single MO/gain amplification medium XLA tic-toc with a solid state seed laser operating at 12 kHz with about a 1 mJ seed laser output light pulse energy and the two amplification stages each operating at around a 17 mJ output pulse energy. In addition, according to aspects of an embodiment of the disclosed subject matter, applicants propose the utilization of a regenerative gain media in which the oscillating laser generated light photons pass through the gain media at least twice per oscillation, e.g., a ring power amplification stage, which can enable the generation of up to ten times the output pulse energy in the ring power amplification stage compared, e.g., to a power amplifier (“PA”) in a MOPA configuration. With such a ring power amplification stage, in the tic-toc mode, the MO energy required may be significantly lower, e.g., in the μJ to tens of μJs range. For testing purposes applicants have simulated the input from a solid state 193 nm seed laser using a line-narrowed ArF laser. 
   Applicants have studied ASE vs. MO-PO timing difference for the different values of the above noted parameters with results as indicated in  FIG. 45 . Similarly a study of MOPO energy vs. MO-PO timing as a function of these same parameters also illustrated in  FIG. 45 . 
   In order to meet the requirements noted above, including, e.g., the constraints of known lithography laser light source technology, applicants propose, according to aspects of an embodiment of the disclosed subject matter, a number of overall architectures that are believed to provide workable ways to address the requirements and constraints noted above. The first may be to provide two multi-chamber laser systems along the lines of applicants&#39; assignee&#39;s XLA XXX laser system series, e.g., with two dual chamber laser oscillator/amplifier arrangements whereby each is configured to run at around 6 kHz producing output pulses at about 17 mJ with interleaved firing times to produce a single approximately 12 kHz system producing about 17 m per pulse. 
   Thus, e.g., according to aspects of an embodiment of the disclosed subject matter, illustrated schematically and in block diagram form in  FIG. 1 , a very high average power laser system, e.g., a 200 W immersion lithography laser light source  20  may comprise a plurality of oscillator/amplifier laser system output light pulse beam sources, e.g.,  22 ,  24 , each of which comprising, e.g., a master oscillator portion comprising master oscillator chambers  30 , such as those being sold by applicants&#39; assignee Cymer Inc. as part of an existing XLA XXX model multi-chamber laser system. Also included in each oscillator/amplifier laser system  22 ,  24  may be a power amplifier portion  32 , e.g., comprising an amplifier gain medium. Each of the two oscillator/amplifier laser systems  22 ,  24  provide an output light pulse beam to a beam combiner  40 , e.g., in an overleaving fashion. 
   Thus, e.g., with each laser system  22 ,  24  operating at 6 kHz and 17 mJ output laser light pulse beam pulse energy the combined output from the beam combiner  40  could be a 12 kHz 17 mJ output resulting in about a 200 W average power laser system. It will also be understood that the embodiment of  FIG. 1  may also be implemented with, e.g., a further plurality of identical oscillator amplifier laser systems  26 ,  28  to produce a 400 W average power laser system. Alternatively, each of the oscillator/amplifier systems  22 ,  24 ,  26 ,  28  could, e.g., operate at less than 6 kHz, e.g., each at 4 kHz and/or with a higher overall oscillator/amplifier system  22 ,  24 ,  26 ,  28  output laser light pulse beam pulse energy, e.g., up to around 33 mJ, to the extent that optical damage limits and cost of operation and other factors will allow, for various combinations of ultimate output  100  pulse repetition rate and pulse energy for a similar variety of average output power values from the system  20 . 
   Referring now to  FIG. 2  there is illustrated schematically and in block diagram form a very high average power tic-toc seed laser/amplifier system  50  according to aspects of an embodiment of the disclosed subject matter. The seed laser amplifier system  50  may include, e.g., a seed laser portion  30 , e.g., a solid state seed laser such as a Nd:YAG or a Nd:YLF or a Ti:Sapphire or a fiber laser or other solid state laser, e.g., operating at around 12 kHz with a 1-2 mJ output energy pulse and a pair of amplifier portions  32 , each being supplied with, e.g., the alternating output pulses from the seed laser portion  30 , e.g., through a beam splitter  52 , discussed in more detail elsewhere in the present application. The pulse could be supplied in other than an alternating fashion, depending on the repetition rates of the amplification stages. Each of the amplifier portions  32  can then be run at, e.g., around 6 kHz for a 200 W output with only a 17 mJ output from each of the amplifier portions  32 . 
   Referring to  FIG. 3  there is shown schematically in block diagram form an example of a very high average power multiple tic-toc seed laser/amplifier system  70  according to aspects of an embodiment of the disclosed subject matter. The system  70  may include, e.g., a first and a second seed laser  72  each supplying seed laser pulses to a pair of amplifier portions, e.g., amplifier gain media  74 , through a beam splitter  52  and with the output of each combined in a beam combiner  78  to provide a laser light source system output laser light pulse beam  100  with an average output power of at or above 200 W. The seed lasers could be, e.g., solid state lasers operating at, e.g., around 12 kHz and the amplifier portions could be, e.g., gas discharge lasers, e.g., excimer or molecular fluorine lasers operating at around 6 kHz. Alternatively, e.g., the seed lasers  72  could be excimer lasers, e.g, KrF, ArF, XeCl, XeF or molecular fluorine lasers operating at about 6 kHz with the respective pairs of tic-toc amplifier portions each operating at 3 kHz for a total of 12 kHz and 17 mJ per lithography or LTPS laser light source system output laser light pulses and a resultant average power of around 200 W. Frequency conversion, as discussed in more detail elsewhere in the present application may be needed to shift the wavelength of the seed laser(s)  72 , e.g., solid state lasers, up to the wavelength of the gas discharge laser amplifier portions  74 . The beam combiner  78  may be a single beam combiner as shown or cascading combiners as shown in the combiners  40 ,  42  in  FIG. 1 . 
   It will also be understood by those skilled in the art that various combinations and permutations of the arrangement illustrated in  FIG. 3  may be utilized. For example there may be a plurality of A seed lasers  72  operating at X kHz with each seeding a plurality of B amplifier portions  74 , each operating at X/B kHz and the combination providing AX system output laser light source output pulses in the output beam  100  of  FIG. 3 . Then, depending on the necessary average system output power, the pulse energy for the output of each of the plurality of amplifier portions  74  may be determined, e.g., with A=2 and B=2, as illustrated in  FIG. 3  and X=6 kHz the overall output beam  100  can have a 12 kHz output and with 17 mJ pulses out of the amplifier portions one gets around 200 w of average output power. The same may be said for the possible arrangements of  FIG. 1 . 
   It will be noted that a tic-toc amplifier LTPS or immersion lithography light source, e.g., seeded by a master oscillator running at, e.g., twice the repetition frequency of the, e.g., two amplifier chambers, could be two excimer laser chambers in a MO/amplification gain medium configuration. For example, each amplification medium could have a recirculating/regenerative ring power amplification stage, each of which is alternatively seeded by a master oscillator running at twice the repetition rate of either amplification stage excimer laser chamber. Such systems can be run at any of the desired wavelengths, e.g., DUV wavelengths, e.g., with the MO and PA/PO operating at 157 nm (F 2 ), 193 nm (ArF), 248 nm (KrF), 308 nm (XeCl) or 351 nm (XeF). Further, such systems could include solid state or excimer seed lasers operating at a higher pulse repetition rate seeding a plurality of power amplification stages, e.g., two, in tic-toc configuration, such as ring power amplification stages. 
   In  FIG. 4  there is shown partly schematically and partly in block diagram form, by way of example an immersion laser lithography system  80  according to aspects of an embodiment of the disclosed subject matter. The system  80  may include, e.g., a very high average power output laser light pulse beam source  20  such as shown in  FIG. 1  or  50  such as shown in  FIG. 2  or  70  such as shown in  FIG. 3 , supplying line narrowed pulses at 200 W or above average power to a scanner  90 , such as those made by ASML, Canon or Nicon. The scanner  90  may incorporate an illuminator  92 , a reticle  94  and a wafer stage  96  carrying a wafer  98  for exposure by the radiation from the light source  20 . On the wafer stage  96  may be a liquid source  102 , e.g., with the liquid being water having a different index of refraction than the ambient around the reticle  94  and stage  96 , and a liquid drain  104 , supplying the liquid  106  to cover the wafer  98  for immersion lithography. 
   It will also be understood that for purposes of coherence busting, either for excimer or other gas discharge seed lasers supplying excimer or other gas discharge laser amplifier portions or for solid state seed lasers, use of multiple amplifier portions with the beams combined as noted elsewhere in the present application may have beneficial effects in busting up the coherency and therefore, assisting in reducing the effects of the speckle, e.g., in integrated circuit photolithography or LTPS or tbSLS processing. It will also be understood that one or more of the various coherence busting techniques and/or combinations thereof disclosed herein may be utilized inside of the scanner  90 , whether that scanner  90  is an immersion scanner or not. 
   Turning now to  FIG. 5  there is shown schematically and in block diagram form a solid state seed laser to gas discharge amplifier laser system  120  according to aspects of an embodiment of the disclosed subject matter. The system  120  may include, e.g., a solid state pulsed seed laser  122 , e.g., an Nd:YAG or an ND:YLF tunable solid state laser  122 . The output of the laser  122  may pass through a coherence buster/frequency multiplier  126 , which may, e.g., be a single optical element, e.g., capable of both frequency shifting the output of the seed laser  122  and beam steering, as is explained in more detail elsewhere in this application with respect to coherency busting, or could be a frequency shifter along with a coherency buster in series, e.g., as shown in  FIG. 6  The system may also have, e.g., an amplifier gain medium such as a PA or PO  124 , or, e.g., a ring power amplification stage  124 , e.g., with the output  100  supplied to a scanner  90  (Shown in  FIG. 5 ). 
   It will be understood that with various tuning mechanisms may be used, e.g., operating temperature, as is know in the art, the solid state laser, e.g., a 1064 ns wavelength Nd:YAG (neodymium-doped yttrium aluminum garnet (Nd:Y 3 Al 5 O 12 )), or 1053 ns Nd:YLF (neodymium doped yttrium lithium fluoride) or a 1054 Ti:Sapphire laser (tunable from about 650 to 1100 nm, which can also produce ultra-short pulses of about_nm in length), and/or by line selection. This can take advantage of different transition spectra for the solid state laser  122  harmonic frequency generation and/or frequency addition/subtraction or other frequency shifting techniques, e.g., Raman shifting, which may be utilized to change the output frequency of the seed laser  122 . The desired frequency/wavelength for amplification in the amplifier portion  124  may be attained, e.g., within an acceptable Δλ from the nominal center wavelengths of around 351 for XeF, 248 for KrF, 193 for ArF and 157 for molecular fluorine to have acceptable amplifying lasing occur in the amplifier portion  124 , e.g., by amplified stimulated emission lasing, as is well understood in the art. As noted above, coherency busting of the type discussed elsewhere herein may be used inside the scanner  90  or other application tool, e.g., another micro-lithography tool or a thin beam laser annealing tool. 
   Turning to  FIG. 6  there is shown in block diagram form a solid state seed laser/amplifier laser system  120  according to aspects of an embodiment of the disclosed subject matter similar to that of  FIG. 6  wherein, e.g., a frequency multiplier  130  and a coherence buster  132  may be utilized to provide appropriate seed pulses to the amplifier laser portion  124  to accommodate, e.g., the high coherency of the seed laser output laser light pulse beam pulses and also their frequency shift to the desired frequency/wavelength for amplification, e.g., in the gas discharge amplification gain medium of the amplification stage  124 . 
   Turning to  FIG. 7  there is shown schematically and in block diagram form conversion of the output of a seed laser, e.g., with a frequency converter  130  along with a beam divider  140 , followed by coherency busting in one axis, e.g., the long axis of the laser beam or a first axis if the beam is not an elongated rectangle and the short axis or a second orthogonal axis if the beam is not an elongated rectangle, with a respective vertical axis coherency buster  142  and horizontal axis coherency buster  144 , as explained in more detail herein. The outputs of the coherency busters  142 ,  144  may be combined in a beam combiner  146 , which, as noted elsewhere, may also serve a coherency busting role, e.g., as shown in connection with  FIG. 35 , and/or  FIGS. 20  A and B, and provided as seed laser pulses to the amplifier gain medium portion  148 . 
   Turning to  FIG. 8  there is shown schematically and in block diagram form a version of the embodiment of  FIG. 7  in which, e.g., the frequency conversion in a frequency converter  130  occurs after the coherency busting, i.e., intermediate the beam combiner  146  and the amplifier portion  148 . 
   Turning now to  FIG. 9  there is shown in schematic form a pulse stretcher  160 , which can be, e.g., a version of the optical pulse stretcher (“OPuS”) sold with applicants&#39; assignee&#39;s laser systems however with, e.g., much shortened delay paths, as explained elsewhere (a so called mini-OPuS) not designed for pulse stretching per se, i.e., enough stretching for significant pulse elongation in the spatial and temporal domains, e.g., increasing the T is  by 4× or more and creating overlapping pulses, as in applicants&#39; assignee&#39;s currently sold OPuS pulse stretchers, as are more fully described in, e.g., U.S. Pat. No. 6,928,093, entitled LONG DELAY AND HIGH TIS PULSE STRETCHER, issued to Webb et al. on Aug. 9, 2005 referenced above. However, the same folding/inverse imaging effects on the beam for coherency busting purposes, or also as explained in regard to the beam mixer of  FIG. 35 , can be achieved. 
   The coherency buster  160  may have an input beam  162  incident on a beam splitter  164 , e.g., a partially reflective mirror  164  for the pertinent wavelength. Part of the beam  162  that is reflected into the delay path comprised of a plurality of mirrors, e.g., mirrors  166   a , is negatively imaged, e.g., twice, and on the final leg of the delay path passes through a pulse trimmer  170 . It will be understood that such optical coherence busters may have more or less than four mirrors, e.g., six mirrors, but are illustrated schematically with four for convenience and clarity. A portion of the light exiting the pulse trimmer  170  is reflected into the output beam  172  and a portion reenters the delay path. The delay path may be much shorter than the seven to ten meters or so of, e.g., a 4× OPuS, such that the second and third passes through the delay path do not overlap the pulses entering and leaving the coherency buster  160 , but rather do not even substantially stretch the pulses. The pulse trimmer  160  may be used, e.g., to shorten the ultimate output pulse  172 , e.g., by cutting off a portion of the pulse circulating in the coherency buster delay path using the pulse trimmer  170 , or much or all or substantially all of the second and subsequent passes through the delay path. The pulse trimmer  170  may be, e.g., a Pockels cell or other suitable fast acting light transmission switch, e.g., a light beam modulator/deflector, e.g., an electro-optic or acousto-optic device, e.g., a crystal that changes refractive index when excited by a field, e.g., an electric field, an acoustic field or a magnetic field. 
   In addition to traditionally used integrated circuit photolithography laser light sources, e.g., ArF and KrF, a fiber-laser-based solid-state 351 nm MO can also be realized according to aspects of an embodiment of the disclosed subject matter. Such a master oscillator architecture may be a simpler more robust solution than a bulk-solid-state laser. Such a laser may be utilized in the field of IC lithography as a very high power (200 w or greater) laser surface processing light source architecture that may, e.g., incorporate the fiber-laser-based master oscillator and also in a very low power MO output pulse laser system (e.g., with the MO operating at the μJ output pulse energy level for greater efficiency and lower cost of use and other advantages noted in the above referenced co-pending application filed on the same day as the present application), or in a very high power broad band application such as for LTPS or tbSLS use, e.g., as a 351 nm laser system. The use of pulse trimming with stimulated optical switches/modulators may be particularly useful between the master oscillator and amplification stage since the pulse energy may be lower there according to aspects of an embodiment of the disclosed subject matter. 
   According to aspects of an embodiment of the disclosed subject matter the generation of 351 nm radiation, e.g., coherent 351 nm radiation, can be done with a solid-state configuration having, e.g., a solid-state drive laser (or lasers) that drive linear or nonlinear frequency conversion stages. Generation of 351 nm laser radiation can be, as illustrated, attained by third harmonic conversion of the output of a Nd:YLF laser operating at 1053 nm. In order to use this approach as a seed laser for an XeF excimer amplifier/oscillator, however, one must ensure that the nominal center wavelength of the, e.g., Nd:YLF seed laser master oscillator matches the gain spectrum of XeF (two lines at 351.12 and 351.26 nm). An alternative approach could be to use an Yb-doped fiber laser as the fundamental drive laser seed pulse source. Yb3+ fiber lasers are inherently tunable, as discussed in J Nilsson et al “High-power wavelength-tunable cladding-pumped rare-earth-doped silica fiber lasers,” Opt. Fiber Technol. 10, pp 5-30 (2004), to allow operation between 1050 and 1065 nm. Fiber lasers offer some simplifications in design that may be of particular benefit in applications requiring ultra-reliability, such as LTPS and microlithography. Applicants propose using a pulsed fiber laser system as the source of moderate peak power (5-50 kW) high-repetition-rate (multi-kHz, e.g., up to about 12-15 kHz) 1054 nm narrowband pulsed radiation. Such a laser could be constructed using standard Yb 3+  pulsed fiber laser technology—either a q-switched fiber oscillator, a pulsed diode source that is fiber amplified, or a CW source (fiber oscillator or diode) that is modulated (internally or externally) and is fiber amplified. For example a CW solid state laser, e.g., a diode laser, with a very narrow bandwidth (very high spectral purity), e.g., a broadband laser, e.g., matched to the fiber laser, to provide a very narrow band seed to the pulsed solid state fiber laser for amplification and the production of a very narrow band pulsed solid state seed to the power amplification stage(s). Appropriate LMA (large-mode area) fiber technology may be used to minimize spectral degradation due to nonlinear effects in the fiber comprising the fiber laser amplification oscillator or any subsequent amplification stages. Using such approaches allows spatial beam quality to be maintained (there are techniques for ensuring single-mode operation in large mode area fibers) while reducing the peak power in the core of the fiber. After the 1054 nm radiation is generated, it can, e.g., be frequency upconverted directly to, e.g., about 351.2 nm, using two stages of nonlinear frequency conversion (second harmonic generation (“SHG”) of 1054 to 527 nm then sum frequency generation (“SFG”) with the residual fundamental to 351.2 nm (with ˜+/−0.1 nm bandwidth). 
   Turning now to  FIGS. 10-13  there are shown schematically and partly in block diagram form a plurality of injection seeded DUV gas discharge master oscillator/amplifier gain medium laser system solid state master oscillators  200  according to aspects of an embodiment of the disclosed subject matter. The master oscillator  200  may include, e.g., a Yb 3+  doped fiber oscillator amplifier  210 , e.g., with a diode pump  212  and a seed laser, e.g., a 1054 nm CW seed diode laser  214 , which may have a spectral purity of ≦100 MHz FWHM in the fundamental. 
   Referring to  FIG. 10  the master oscillator oscillation cavity may be formed by a rear cavity fully reflective mirror  220  and a partially reflective output coupler  222 , which may be 90% reflective at the nominal 1054 nm center wavelength of the fiber oscillator  210 . The master oscillator  200  may employ a Q switch  224  to allow for the output pulse energy of the master oscillator  210  to accumulate in the oscillation cavity until sufficiently high in energy before the Q-switch  224  is opened, as is well known in the art. The output of the master oscillator  200  may thus be pulsed by the frequency of operation of the Q-switch, e.g., at a rate of about 12 kHz. The output of the fiber oscillator laser  210  may be passed through a second harmonic generator  230 , followed by a frequency adder  232 , to add the original frequency to the second harmonic to generate a third harmonic, i.e., a wavelength of about 352 nm suitable for amplification, perhaps with some slight shifting to closer to 351, in, e.g., a XeF gas discharge laser power amplifier or power oscillator or ring power amplification stage amplifying gain medium (not shown in  FIGS. 10-13 ). 
   Turning to  FIG. 11  there is shown schematically and partly in block diagram form a solid state master oscillator  200  according to aspects of an embodiment of the disclosed subject matter. In this embodiment an external amplitude modulator  240 , e.g., an acousto-optic or electro-optic switch or other suitable mechanism, may be used to pulse the CW seed  214  into the fiber amplifier  210  to produce a pulsed output of the master oscillator  200 . 
   In the embodiment of  FIG. 12  the  1054  seed may utilize, e.g., a pulsed seed diode  250  to produce a pulsed output out of the master oscillator  210 , e.g., at around 12 kHz. In the embodiment of  FIG. 13  a tunable CW Yb 3+  master oscillator  260  may be switched into the fiber amplifier  210  with an external amplitude modulator, such as is discussed above, to get a pulsed seed laser output from the master oscillator  200 . The fiber amplifier  210  may utilize pump diodes  212  to pump the fiber amplifier  210 . 
   According to aspects of an embodiment of the disclosed subject matter applicants have determined certain characteristics desirably evidenced by a seed laser, e.g., a solid state seed laser, for a very high average power laser system, e.g., for photolithography or LTPS applications, including, e.g., pulse energy, pulse duration and timing jitter, which can drive the selection of a seed laser, e.g., a solid state seed laser to the choice(s) of Nd:YAG, Nd:YLF, Ti:Sapphire, and fiber lasers, as discussed elsewhere. 
   According to aspects of an embodiment of the disclosed subject matter applicants have also studied certain amplification stage resonator cavity properties. On the one hand may be a flat-flat cavity with simple beam splitter input/output coupling, which is simple of construction, though perhaps more wasteful of seed laser energy than is practical in a production system. On the other hand may be a recirculating or regenerative power oscillator, e.g., a ring power amplification stage, e.g., with a beam splitter/mirror input/output coupler and multiple passes through the gain medium per each oscillation within the oscillator cavity of the power amplification stage. It will be understood by those skilled in the art, as noted above, terms like oscillator, cavity and the like used in reference to, e.g., a MOPO configured laser system mean that the amplification portions of the laser system, seeded by a seed laser portion, lases due to stimulated emission from the seed beam pulse oscillating in the cavity. This is distinguished from what may be referred to as a power amplifier, such as the PA portions of applicants&#39; assignee&#39;s MOPA configured XLA XXX series laser systems. By contrast the amplification occurs in a power amplifier by stimulated emission during a gas discharge in the amplification gain medium of the amplifier portion of the laser system as the seed laser pulse is directed through the amplification gain medium in an excited state a fixed number of times by an optical arrangement, e.g., a two pass optical system as used in applicants&#39; assignee&#39;s current XLA XXX series laser systems. In some of the literature, however, an oscillating amplifier wherein the number of passes through the gain medium in the oscillation path, e.g., a bow-tie or racetrack loop path which is not an integer multiple of the nominal center wavelength of the laser output may be considered to be a “power amplifier” rather than a “power oscillator.” Therefore for purposes of this application and the appended claims the use of the term “ring power amplification stage” is intended to cover either type of power oscillator, regardless of the relation of the cavity size to the wavelength. 
   The flat-flat configuration may use a traditional polarization input/output coupling e.g., with a polarizing beam splitter and a quarter wave plate and partially reflective output coupler, e.g., as described in more detail below with respect to  FIGS. 23 and 28 . This may make more efficient use of the seed laser energy but could also be more susceptible to, e.g., thermal effects at high pulse energy and/or high average output power. Other input/output coupling could also be employed as explained in more detail in the above referenced co-pending provisional application filed concurrently with the present application. 
   Turning now to  FIGS. 14 and 16  there are illustrated in schematic and partly block diagram form examples of very high power, e.g., around 200 W or better average output power, laser systems,  280 , and  450 , respectively, according to aspects of an embodiment of the disclosed subject matter. These laser systems  280 ,  450  may be used, e.g., for immersion lithography use or for LTPS use, or the like, which may include, e.g., in the case of  FIG. 14  a ring power oscillator amplification stage configured laser system  280 . The system  280  may include a seed laser  286 , which may provide seed laser pulses at, e.g., around 0.1 mJ or less and a pulse repetition rate of, e.g., around 6 kHz, in a seed laser output light pulse beam  288  of laser output light pulses. The beam  288  from the seed laser  286  may pass through a seed injection coupling mechanism  300  into an amplifier gain medium portion  290  of the laser system  280 . 
   The amplifier gain portion  290  may comprise a ring power amplification stage chamber  292  containing a pair of gas discharge electrodes  294  one of which is seen in the view of  FIG. 14 . The chamber  292  may also comprise an input chamber section  296  and a beam reverser chamber section  298 , each of which may be formed with or attached to, e.g., by suitable leak proof means, the chamber  292 , such that, e.g., the optics in the input section  296  and in the beam reverser section  298  can be beneficially exposed to fluorine in the lasing gas mixture enclosed in the chamber sections  292 ,  296 ,  298 . 
   The seed injection mechanism may include, e.g., a beam splitter/input-output coupler  302  which may be coated with a coating or otherwise selected or made to be partially reflective to the seed laser light, e.g., at a nominal center wavelength of around 193 nm for ArF, 248 nm for KrF,  318  for XeCl or 351 for XeF laser systems, and a maximally reflective mirror  304  that is maximally reflective at the selected nominal center wavelength for the respective ArF, KrF, XeCl or XeF or the like gas discharge laser systems. 
   The beam reverser  310  may be similar to the power amplifier beam reversers, e.g., sold in applicants&#39; assignee&#39;s XLA MOPA configured laser systems, XLA XXX systems, or as discussed in more detail in the above referenced co-pending patent application Ser. No. 11,521,834 filed on the same date as the present application. In the input section  296  optically accessible through an input window  312  may be placed a beam expander  320 , which may be comprised of a prism  322  and a prism  324 , which together may narrow the beam  288  on its way into the chamber  292  and conversely expand it on its way out of the chamber  292 , the expansion on the way out serving to, e.g., protect the optical elements, e.g., the input/output coupler  300  and the narrowing of the beam  288  on the way into the chamber  292  serving to, e.g., narrow the beam  340  entering the amplification gain medium to approximately the width of the discharge between the electrodes  294  in a direction generally perpendicular to the separation of the electrodes  294 . 
   Baffles  330  may serve to, e.g., protect the optics in the input section  296  and the beam reverser section  298  of the chamber  292  from damage resulting from, e.g., debris circulating with the lasing gas mixture in the chamber  292 . 
   Inside the cavity of the ring power amplification stage  290  the beam  288  may take a first direction recirculating oscillation path  340  and return on a second direction recirculating oscillation path  342  to the seed injection mechanism  300  where the partially reflective input/output coupler acts as a traditional output coupler for an oscillator laser cavity and reflects part of the oscillating laser light photons to the Rmax mirror  304  and back along the path  340 . Thus the oscillation in the cavity formed by the seed injection mechanism  300  and the beam reverser  310  is a multi-pass oscillation path such that the oscillating photons pass through the gain medium between the electrodes  294  more than once, in the illustrated case twice, in different directions and on different paths for each oscillation loop. Such multi-pass oscillation, as noted, is distinct from the photons in a power amplifier making a fixed number of passes through the gain medium, e.g., two in applicants&#39; assignee&#39;s XLA XXX laser systems, without oscillating along such power amplifier light path. When the oscillation in the recirculating/regenerative path  340 ,  342  builds up enough pulse energy a laser system output laser light pulse beam  100  is produced from the seeded power oscillator laser system  280 . The seed laser  286  could be either a gas discharge, e.g., excimer or fluorine laser or a solid state laser. 
     FIG. 16  illustrates schematically and partly in block diagram form a ring power amplification stage laser system  490  configured similarly to applicants&#39; assignee&#39;s XLA XXX multi-chambered MOPA laser systems with the PA replaced by a ring power amplification stage  490  according to aspects of an embodiment of the disclosed subject matter. The laser system  450  may be comprised of an excimer gas discharge laser seed laser  452  which may comprise a master oscillator laser chamber  454 , with a line narrowing module  456  having a reflective element, e.g., a wavelength and bandwidth selective grating, forming a rear cavity mirror and a partially reflective output coupler  458  forming the other end of the master oscillator  452  oscillation cavity. The master oscillator  452  seed laser output laser light pulse beam of pulses leaving the output coupler  458  may pass through a metrology module (line center analysis module “LAM”)  470 , which may sample a portion of the output of the MO chamber  454 , using a beam splitter  472 , and also, in addition to a wavemeter (not shown) for measuring nominal center wavelength of the master oscillator seed laser output laser light pulse beam pulses may comprise an MO laser output light pulse beam pulse energy monitor  474  and an ASE monitor  476 , such as a fluorescence detector. The ASE detector, e.g., a broad band photodetector, may serve to detect the presence of a high enough intensity of broadband light to indicate the timing of the discharge in the amplification gain medium is off such that significant lasing in band is not occurring (the seed pulse is not timed to be in the cavity of the amplification stage during the discharge) and essentially only broad band lasing is occurring during the discharge in the amplification stage. 
   The master oscillator seed laser  452  output laser light pulse beam may then pass to a turning mirror  480  and from there to a seed injection mechanism  300  input to an amplifier gain medium portion  490 , which may comprise a ring power amplification stage chamber  492 , having a chamber input section  494  and a chamber beam reverser section  496 . It will be understood by those skilled in the art that this schematic view of the laser system  450  does not reflect various aspects of the optical path of the beam from the MO  452  to the PO chamber  442 , which are drawn schematically to conform to the plane of the paper and not the optical realities of the optical path between the two and into the amplification stage chamber  492 . 
   The seed injection mechanism  300  may include, e.g., a partially reflective input/output coupler  302 , e.g., a beam splitter similar to those sold with applicants&#39; assignee&#39;s laser systems, e.g., as part of an optical pulse stretcher (“OPuS”), and a maximally reflective mirror Rmax  304  for the given nominal center wavelength, with the partially reflective output coupler  302  serving as an input/output coupler as noted above and specifically as the output coupler for the ring power amplification stage  490  oscillation cavity (defined also by the beam reverser  310 ). The seed laser output laser light pulse beam from the MO  452  may pass into the ring power amplification stage chamber  492  through an input window  500  and also pass through a beam expander  510  as noted above with respect to  FIG. 14 . The input section  494  of the ring power amplification stage chamber  492  may also house the beam expander  510 , consisting of, e.g., a prism  512  and a prism  514 . Other forms of seed injection mechanisms may include those discussed in the above referenced co-pending provisional application filed on the same as the provisional application from which this application claims priority and the other co-pending applications claiming priority to that provisional application or the provisional application from which this present application claims priority. 
   The output of the ring power amplification stage oscillator  490  may be the overall system output laser light pulse beam of laser pulses, however, as illustrated in  FIG. 16 , this beam (eventually output beam  100  to the utilization tool, e.g., the scanner) passes also through a metrology unit (bandwidth analysis module “BAM”)  340 , where output laser light pulse beam bandwidth may be measured, e.g., for each pulse in the beam, and through a pulse stretcher, e.g., a 4× OPuS  520  which may include, e.g., a first delay path  522 , which the laser system output beam enters through beam splitter  526  and a second delay path  524  entered through beam splitter  528  (the delay paths formed by mirrors  530 ). Leaving the OPuS  520  the output beam  100  passes through a shutter  540  which may also have a beam splitter  542 , e.g., to take off a portion of the laser system output laser light pulse beam  100  to measure, e.g., pulse energy. 
   With the beam expander  320  in  FIG. 14 and 510  in  FIG. 16  placed inside the ring power amplification stage oscillation cavity there is, e.g., a reduction of the energy density on the maximum reflector  304  and partial reflector  302  that make up the input/output coupler  300  of the ring cavity of the amplification stage  290 ,  490  is achieved. With the beam reverser  310  moved to inside the cavity, the space vacated can house the BAM (or SAM). The use of protective optical coatings, e.g., with protectively coated calcium fluoride (“PCCF”) optics can be eliminated, e.g., on the beam reverser  310  and beam expander  320 ,  510 . There could also be no need for PCCF on the amplification stage chamber window  500  and the output window  500  could be at a 47 degree orientation. 
   A ring power amplification stage in certain applications can actually allow the use of much less energy from the MO, approximately 1-100 uJ instead of the present value of about 1-3 mJ. For example in regular photolithography where about one third to one half of the 200 W is actually required from, e.g., a non-immersion very high average power laser lithography light source system, a reduction in MO energy requirement of about 3 to 10 times could lead to a corresponding increase in LNM lifetime, based on current LNM lifetime models. In addition, such small MO energy could likely allow use of a low MO chamber pressure and partial pressure of fluorine, necessary for operation at greater than 1 mJ, e.g., at around 1.5-3 mJ, with beneficial results from a chamber lifetime perspective. And also, since the power amplification stage reaches strong saturation with 100 uJ of MO energy and below, e.g., down to about 5 μJ or so, output energy stability will be dominated by the good ring power amplification stage characteristics and not the less than ideal MO energy stability characteristics. The present Cymer XLA XXX MOPA systems are dominated by the MO energy instabilities. Other output laser beam parameters, e.g., pointing stability, profile stability, and ASE stability may also be beneficially impacted by a configuration according to aspects of an embodiment of the disclosed subject matter utilizing reduced MO energy output. 
   According to aspects of an embodiment of the disclosed subject matter applicants propose to use a 6 mirror coherency busting mechanism (for convenience herein optical pulse delay paths are indicated schematically as having four mirrors per delay path) which has been developed by applicants&#39; assignee for additional path delay inside either or both of the 1 st  or 2 nd  pulse stretchers in the OPuS used with applicants&#39; assignee&#39;s XLA model multi-chamber laser systems. Such a delay path can, e.g., produce-1 imaging for each sub-pulse. This is illustrated schematically and in cartoon fashion, e.g., in  FIG. 20A  wherein is illustrated the summation of these “flipped” sub-pulses. The flipped sub-pulses shown, e.g., in  FIG. 20B  can be used, e.g., for improved profile uniformity and symmetry. A 6 mirror design can convert pointing shifts into a divergence increase which may, e.g., be beneficial in a ring arrangement for ASE reduction. The standard 4 mirror design does not. It will be understood that the delay path for this coherency busting purpose need not be as long as the actual OPuS used for pulse stretching to get a much increased pulse T is , and overlapping pulses. Rather the coherency busting mechanism, a so-called “mini-OPuS”, among other characteristics can fold the pulses a certain number of times. This is illustrated by the pulse  580 , with the corner (pre-flip) designated  582  and the pulses  584 ,  586 ,  588 . In addition, due to misalignment of mirrors in the delay path, a “hall of mirrors” effect due to subportions of the beam being misaligned, may also reduce the coherency in the seed laser pulse, and, e.g., so long as the delay path exceeds the spatial coherency length of the beam. In this regard, a four mirror mini-OPuS, e.g., with confocal spherical mirrors for ease of alignment, may serve as a satisfactory coherency buster, even without beam flipping in both axis as explained elsewhere in this application. 
   According to aspects of an embodiment of the disclosed subject matter it may be necessary to combine two separate laser beams at various points within a system according to aspects of an embodiment of the disclosed subject matter. If only half of the entrance to a 6 mirror pulse stretcher is illuminated, the sub-pulses flip between top and bottom as shown, e.g., in  FIG. 20B . The summation of these “flipped” sub-pulses can lead to a filled in, full size profile, e.g., as illustrated in the pulse stretching simulation shown in  FIG. 33 , with the curve  562  showing the pulse before entering the delay path and curve  564  (black) after one delay path and  566  (red) after a second delay path. Laser divergence may then be used to fill in the center portion  568 , e.g., after some propagation, e.g., over about 1 m or so. 
   Turning now to  FIG. 26  there is shown a schematic representation of the effects of coherence busting according to aspects of an embodiment of the disclosed subject matter. Utilizing an imaging delay path, e.g., a pulse stretcher, e.g., a so-called optical pulse stretcher (“OPuS”), e.g., a 4× T is  six mirror OPuS sold with the above noted applicants&#39; assignee&#39;s laser systems, and illustrated in United States Patents and co-pending applications noted above, or a modified version thereof with a shorter delay path used, e.g., for folding the beam on itself and/or for delay exceeding the coherence length as discussed herein, the so-called mini-OPuS, one can achieve a degree of coherence busting, e.g., between the MO and amplifier gain medium, e.g., a PA or a PO or a ring power amplification stage. Other forms of coherence busting e.g., as illustrated in  FIG. 35  could be used alone or in combination with such a “mini-OPuS,” e.g., as illustrated in  FIG. 33  and elsewhere herein or as the mini-OPuS itself. 
   According to aspects of an embodiment of the disclosed subject matter, the pointing/divergence sensitivity of a pulse stretcher, e.g., a 4 mirror 6 mirror pulse stretcher, e.g., a regular OPuS such as a 4× T is  OPuS, or a so-called mini-OPuS, or the delay path discussed in more detail in regard to  FIG. 35 , can be put to advantage, e.g., by adding active mirror control with feedback from, e.g., a pointing/divergence sensor, illustrated, e.g., in  FIGS. 21 and 23 . Such advantages include creating or sustaining, e.g., a hall of mirrors effect whereby, e.g., the laser output light pulse beam being smoothed in the delay path actually becomes something like a plurality of beams of very slightly different pointing and thus angle of incidence on the various mirrors of the pulse stretcher and/or down stream of the delay path(s). Applicants assignee has observed this in pulse stretchers where it is very difficult to perfectly align the mirrors, e.g., of the currently used 4× T is  OPuS pulse stretcher, thus creating the hall of mirrors effect that reduces the coherence of the laser output light pulse beam exiting the pulse stretcher. Thus the beam  860  forms a plurality of separate beams  82 . In  FIG. 26  this is also illustrated schematically and as a result of a flat-flat cavity  850  with slightly misaligned mirrors forming the rear of the cavity  852  and an output coupler  854 , but the same effect has been observed in an OPuS by applicants employer with the coherence busting effect noted above. The cavity illustrated in  FIG. 26  may also have a polarizing input coupler  858  and a quarter wave plate  856 . 
     FIG. 26  illustrates a reduction in coherency, e.g., when using both the reflectivity of an OC and an Rmax, e.g., in a flat-flat cavity with, e.g., a polarizing input coupling from a seed laser source of seed laser pulses. The angles have been exaggerated for clarity of illustration. There are, e.g., multiple rays produced by a static fan out, i.e., “hall of mirrors” effect, e.g., created between the OC and the Rmax. The theoretical energy weighting of these rays, assuming no transmission losses through the cavity and perfect reflectivity is shown below. 
                                   Ray               Number   Fractional Energy   Normalized Energy                                            1   0.2 = 0.200   0.3125       2   0.8 * 0.8 = 0.640   1.000       3   0.8 * 0.2 * 0.8 = 0.128   0.2000       4   0.8 * 0.2 * 0.2 * 0.8 = 0.0256   0.0400       5   0.8 * 0.2 * 0.2 * 0.2 * 0.8 = 0.00512   0.0080       6   0.8 * 0.2 * 0.2 * 0.2 * 0.2 * 0.8 = 0.00102   0.0016                    
One may assume that each ray is incoherent from all others, e.g., where the path length between the OC and the Rmax is maintained to be longer than the temporal coherence length and, e.g., with non-overlapping stretching, i.e., of much less than the pulse length. Each ray may also be assumed, e.g., to be angled slightly different from all others since, e.g., perfect alignment is believed to be extremely difficult, especially in the vertical direction. Applicants believe that about 37 μrad of angle difference in the vertical direction is needed to create uncorrelated speckle. Summing the normalized energy weighting to give the equivalent number of independent pulses and taking the square root to give the reduction in standard deviation, the sum from the above is 1.56. The square root is 1.25 and thus the standard deviation when using both OC and Rmax reflections is predicted to be 0.551/1.25=0.440, which comports well with a value that applicants have measured, i.e., 0.427.
 
   Static fan out, otherwise referred to herein as a hall of mirrors effect, believed to be essentially unavoidable with manual alignment, produces a single pulse speckle contrast with amplification in an amplification gain medium that is 2.50× smaller than the seed laser alone. This reduction is the equivalent of 6.3 uncorrelated sub-pulses. Some of this contrast reduction is due to the weak line content from the XeF power oscillator used for testing the effects of the oscillation amplification stage, but most is believed to be due to the static fan out effect. Likely, many of the sub-pulses created by the OPuS-like static fan out characteristics of the OC-Rmax (OC-rear cavity mirror) reflections are all amplified to nearly equal intensities and thus create more equivalent independent pulses than shown in the above table. 
   Tilt angle required to produce uncorrelated speckle patterns may be significant. The first big jump in equivalent pulses, from 1.0 to 1.55, is believed by applicants to be mostly due to the poor pulse-to-pulse repeatability of the speckle patterns when running as a MOPO. Even without changing the mirror tilt at all, two pulses are correlated no better than 30-35%. With seed only, this pulse-to-pulse correlation has been found to be about 85-90%. The long slow rise in equivalent pulse number does not even reach a value of 2.0 until about 400 μrad of mirror tilt as illustrated, e.g., in  FIG. 37 . This result could mean, e.g., there may be a need for a large angular sweep, of about ±500-1000 μrad, e.g., to create several uncorrelated speckle patterns in a single pulse. 
   Through experimentation relating to coherence applicants&#39; employer has learned that, e.g., sub-pulses produced by a pulse stretcher are incoherent and lead to a different fringe pattern if their angles are slightly shifted. The pin hole fringe pattern shifts maximum to minimum when input angle is λ/2d. 
   A plot of pointing shift (inferred by applicants from speckle shift measurements) v. E-O cell applied voltage is shown in  FIG. 18 . According to aspects of an embodiment of the disclosed subject matter applicants propose to sweep the pointing of the seed laser within a single pulse in order to reduce the speckle contrast within. This may be done, e.g., with electro optical elements, e.g., elements  712  and  714  shown illustratively in the schematic and partly block diagram illustration of aspects of an embodiment of the disclosed subject matter found in  FIG. 23 . Using vertical expansion prior to input of a seed laser pulse into an excimer power oscillator, e.g., a XeF chamber, placed as close to an input coupler, e.g., a beam splitter, and with a clear aperture of the E-O deflector at around 3.2 mm in diameter, the deflector may have to be upstream of the vertical expansion (not shown in  FIG. 23 ). To minimize any translation in the oscillator cavity, e.g., the XeF cavity  730 , e.g., associated with the angular tilt from the E-O deflector, it may be desirable to place the E-O deflector as close to the amplifier cavity as possible. 
   Use of a solid state laser source for lithography has been proposed in the past and not pursued for two reasons. Solid state lasers are not considered capable of the high average power required for lithography and a solid state laser produces single mode output which is highly (perfectly) coherent. According to aspects of an embodiment of the disclosed subject matter applicants propose to address the low average power problem with, e.g., a hybrid solid state seed/excimer amplifier combination. The high coherence properties of the solid state seed can be addressed in a number of ways according to aspects of embodiments of the disclosed subject matter, e.g., by creating sub-pulses, e.g., that are separated in time longer than the coherence length along with changing the seed laser pointing, e.g., over very short time scales, e.g., within a single laser pulse, or a combination of both. Coherency busting has been found by applicants to be of benefit in dual chamber gas discharge (e.g. excimer) seed/gas discharge (e.g., excimer) amplifier portion lasers as well. 
   De-phasing of a speckle pattern can be seen from a diffuser  670  to occur with a λ/2d where d is the illumination length of a slit aperture or diameter of a circular aperture, e.g., as illustrated schematically and in cartoon fashion in  FIG. 22 . Incoherence of a speckle pattern can also be seen to occur from each sub-pulse produced by a pulse stretcher, which can, e.g., be further exploited by, e.g., intentionally misaligning each pulse stretcher, e.g., a mirror(s) in the pulse stretcher, by a very slight amount. In point of fact, applicants&#39; employer has discovered by testing that it is very hard to precisely align the mirrors in, e.g., an 4× T is  OPuS type of pulse stretcher, and they are slightly out of alignment almost all the time, without having to intentionally misalign them. This amount of “ordinary” misalignment has been found by applicants employer to be an amount sufficient to achieve a desired level of speckle reduction and is illustrated schematically in  FIG. 26 , as discussed above. 
   The effective number of equivalent independent laser pulses can be seen to be equal to the T is  magnification of the each pulse stretcher. Each OPuS pulse stretcher of the kind noted above may have a multiplication of around ˜2.4×. With, e.g., three stages of pulse stretching, the number of independent sub-pulses will be (2.4) 3 =13.8. Since speckle contrast scales with the number of independent sub-pulses, N, as 1/√N, pulse stretchers can provide an output speckle contrast of 1√13.8=26.9% with an input speckle contrast of 100%. Since this may still be too high a speckle contrast, according to aspects of an embodiment of the disclosed subject matter a mechanism(s) may be provided to reduce the speckle contrast into or out of the pulse stretcher(s). The same can be said for the so-called mini-OPuSs discussed elsewhere. 
   Pulse trimming has been demonstrated, e.g., with the utilization of electro-optics, e.g., at 193 nm. Rather than polarization rotation, used in some other forms of pulse trimming, electro-optics can be used for beam steering, e.g., steering a seed laser light pulse beam within a single pulse in the beam. Utilization of such, e.g., at the output of the seed laser, can result in, e.g., according to aspects of an embodiment of the disclosed subject matter, the electro-optic material(s) only needing to be subject to a low average power seed laser beam. By, e.g., randomly and/or continuously changing the beam steering, e.g., within a single laser pulse, the angular acceptance of the power amplification stage can be “painted” or filled in for each laser pulse. As a result, a main pulse can have a divergence set, e.g., by the PO/power amplification stage optical configuration and not, e.g., by the seed laser characteristics. A greatly reduced coherence for the laser system output laser light pulse can be the result. 
   According to aspects of an embodiment of the disclosed subject matter an injection controlled amplifier laser system, e.g., with a plane cavity and flat rear mirror, may have suitable energy stability, e.g., for seed pulse inject energies in the range of 0.0085 to 0.99 mJ. This energy of the beam may be, e.g., incident on the rear mirror of, e.g., a power amplification stage, which may form the input coupler from the seed laser. This reflector may have, e.g., about a 90% reflection and about 10% transmission. Therefore, the seeding energy entering the amplification stage cavity itself may be, e.g., about an order of magnitude smaller than what is incident onto the back reflector. With a ring cavity, especially with a partially reflecting seed injection mechanism according to aspects of an embodiment of the disclosed subject matter, discussed elsewhere herein, e.g., the input seed energy may be much less wasted, e.g., about 80% is injected to the amplification stage. An Rmax and OC can be in an F 2  containing environment, and thus more robust, though, e.g., if polarization coupling is used, coupling efficiency may still be less than optimum for certain applications. A suitable architecture, e.g., in a MOPA configuration may be a 2-channel (“tic-toc”) solid state seed laser, e.g., a 3 rd  harmonic Nd:YLF MO or Nd:YAG system (tuned, e.g., to 351 nm) along with a pair of two 3-pass XeF PA modules. Such a system in a MOPO, e.g., a master oscillator/power amplification stage (such as a ring power oscillator amplification stage) configuration is also considered as an effective alternative. Such a two channel MOPO approach may be similar to the MOPA configuration, i.e., with two seeded power oscillators. Various coupling techniques could be used, e.g., MO coupling using a polarization technique or a seed inject mechanism. Efficiency v. E mo  for differing PO/PA configurations has been found to be better for a MOPO or a three pass MOPA, though four pass MOPAs were not tested. Exemplary pulse width (FWHM) has been found to be for an MOPO about 17.3 ns, for a MOPA, single pass, about 13.9 ns and for a MOPA triple pass about 12.7 ns. 
   Applicants have examined speckle patterns for decorrelation with angular shift, e.g., in a MOPO output beam, e.g., with a Nd-YLF seed laser and a XeF power oscillator (e.g., a flat-flat polarization coupled arrangement). With the relative timing between the XeF discharge and the seed laser pulse adjusted and angular and spatial adjustment also made for maximum suppression of the weak line (353) produced by the XeF gain. 
   The maximum intensity of the seed pulse has been observed to occur during the initial, very low level, fluorescence of the amplification stage. This very low level fluorescence (and thus gain) is believed to be enhanced by this seed light, as observed in MOPO output. Adjustment of the timing of the seed earlier than or later than, e.g., about 20 or so ns before the amplification stage firing can, e.g., lead to an increase in weak line output. 
   According to aspects of an embodiment of the disclosed subject matter coherence busting may be accomplished by beam steering, e.g., with electro-optical elements, e.g., pointing of the seed beam during a single pulse using, e.g., a ConOptics E-O deflector assembly matched for the desired nominal center wavelength. Such E-O devices may be like those used in CD and DVD writers that use a doubled Ar-ion line near 351 nm having E-O deflectors used to modulate the beam. With a pointing coefficient of, e.g., about 0.6 μrad/volt and with a capacitance of 50 pF, even a full mrad of deflection requires only 1,700V. A drive circuit useful for pulse trimming, e.g., as illustrated schematically in  FIG. 40  (discussed in more detail elsewhere in the present application) can be used, e.g., with a resister in series to produce a controlled sweep rate, e.g., during a single pulse. The seed pulse duration can be around 15 ns, so the rate of rise is well within the capabilities of such a driver, for reasonable pointing changes, such as up to a m Rad. With a pumping diode current of around 30 A and 4 A to the oscillator pump diode, the seed laser output laser light pulse beam pulse energy was determined to be 1.2 mJ, sufficient for seeding a gas discharge laser, e.g., a XeF gas discharge laser. 
   A plot of speckle contrast (average speckle cross-correlation versus mirror tilt—input angle change) for a MOPO configuration is shown by way of example in  FIG. 36 . A similar plot for only a seed laser pulse passing through an amplifier gain medium in an oscillator configuration, but without excitation of the amplifier gain medium is illustrated by way of example as plot  590  in  FIG. 37 , which also shows by way of example a plot  592  of equivalent independent pulses. A similar plot is shown in  FIG. 34 , for the seed laser pulse only in the PO, with curve  596  being the equivalent independent pulses, curve  594  being the normalized standard deviation and curve  598  being the cross correlation. Similar to the MOPO case, it takes about 150-250 μrad of tilt to produce completely or essentially completely uncorrelated speckle patterns and about two equivalent independent pulses. But, as described above, the starting speckle contrast for no shift may be smaller than with the OC reflection only by a factor of about 1.25. Thus according to aspects of an embodiment of the disclosed subject matter applicants have discovered that, e.g., a MOPO single pulse speckle contrast may be significantly lower than a seed-only case, because, e.g., static fan out of the rays produced by the multiple OC-Rmax-OC-Rmax reflections, e.g., because each of these reflections exit at the illustrated separate slightly different angle, producing uncorrelated speckle patterns as shown by way of example in  FIG. 26 . 
   According to aspects of an embodiment of the present application applicants believe that this discovery may be utilized to greatly simplify the necessary coherence busting scheme. Instead of creating the electro-optic capability of, e.g., steering and/or more rapidly modulating (“hybrid painting,” in the case of using both), e.g., the entire divergence space, e.g., in one or both axes (e.g., requiring high frequency devices), one can slightly misalign the seed to the PO, e.g., in one axis or the other or both, to exploit this spreading static ray out effect, the so-called hall of mirrors effect. It may then also be possible to use, e.g., only a linear sweep of pointing along one axis or the other or both, e.g., where the one axis is the other axis in the case of spreading only in one axis, with, e.g., a greatly reduced requirements on the E-O drive electronics. In the simplest case, misalignment spreading (beam fan-out so-called hall of mirrors effect, may be employed in only one axis and “singly painting” in the other, e.g., with a saw tooth signed to a tilt mirror and without AC creating hybrid painting. More complex permutations and combinations of these coherency busting techniques may also be applicable. 
     FIG. 38  gives an example of an idealized high frequency painting E-O voltage signal superimposed on a ramped (time varying) E-O DC voltage signal in relation to the intensity of the seed pulse being “painted”, e.g., into a delay path or into the amplifying gain medium, e.g., a PA or PO or other power amplification stage. The ramp voltage may be created, e.g., by a fast R-C decay of an E-O cell capacitance as illustrated schematically in the circuit of  FIG. 40 . Due to certain constraints on a test circuit that applicants have so far built and tested, e.g., limited RF frequency, impedance mismatch, E-O load cell capacitance mismatch and the like, the actual voltages delivered by the “painting” circuit are shown in  FIG. 39 , as best as could be measured considering difficulties with probe loading, etc. These are approximately 25% of the needed RF frequency (e.g., about 100 MHz as opposed to 400 MHz) and 10% of the needed peak to peak voltage (e.g., around±200 kV as opposed to ±2000 kV). The painting voltages could, of course, be better optimized, however, the test circuit was used to demonstrate the effectiveness of “painting” the seed beam into the amplifier gain medium for coherency/speckle reduction, e.g., with hybrid painting using both time varying DC steering and AC modulation, e.g., one in one axis and the other in a second axis, e.g., orthogonally related to each other. 
   Applicants experimental measurements have determined that with no ramp and no AC voltage the 2D speckle contrast overall is 76.8% and varies from the horizontal to the vertical axis. With painting using the ramp alone the speckle contrast overall was 29.4%, again varying in the two axes. Painting with the AC alone gave a speckle contrast overall of 59.9%, again varying in the two axes. With the ramp and AC voltages applied the spectral contrast was 28.1% overall and varying in both axes. This was using a less optimized circuit than the one of FIG.  40 , which was not available for the testing and the actual tested circuit test results are shown in  FIG. 39 . 
   Applicants believe that a more optimized circuit, shown by way of example in  FIG. 40 , will even improve further the reduction in speckle contrast. The circuit  1100  of  FIG. 40  may include, e.g., an E-O cell, such as noted above, with an E-O cell capacitance  1104  and an impedance matching inductor  1110 , and an N:1 step-up transformer  1120 . Also included as illustrated may be, e.g., a DC power supply  1122  charging a capacitor  1126  through a large resistor  1130  and an RF frequency generator connected to a fast acting switch, e.g., a transistor  1140  (in reality a bank of transistors in parallel), through a  501  resistor. Also the capacitor  1126  discharges through a small resistor  1142  when the switch  1140  is closed. 
   According to aspects of an embodiment of the disclosed subject matter “painting” may also be done upstream of the amplifier gain medium, e.g., by tilting a mirror upstream of the amplification, e.g. a piezo-electrically adjustable mirror. The results with the seed only, both with OC only and with OC plus Rmax reflections, look very similar to those measured by applicants such as with tilting a mirror, e.g., through a diffuser as illustrated in  FIG. 22 . As with the previous measurements, the OPuS-like characteristics of the OC-Rmax reflections can be seen to lead to single-pulse speckle contrast values reduced by the equivalent number of sub-pulses produced. The angular tilt required to produce uncorrelated speckle patterns was determined to be about 200-250 μrad, again similar to the results with tilting the mirror, e.g., downstream from the power amplification stage. 
   Applicants have performed characterizations of a solid state MO./power amplification stage using an excimer seed laser, e.g., greatly attenuated to simulate the expected pulse energy of, e.g., a 193 nm solid state laser. The pulse duration produced, however, did not match that expected from a 193 nm solid state laser. Applicants believe that proper simulation of the seed pulse duration should further reduce the total seed laser energy required for MO/power amplification stage operation. Using a pulse trimmer, e.g., a Pockels cell to which was applied a step voltage, e.g., timed to trim the later portion of the excimer seed pulse shape (¼λvoltage=2.5 kV), and due to the rise time of the excimer seed laser pulse and the fall time of the Pockels cell, the shortest practical pulse shape attained was about 9 ns FWHM and ˜15 ns foot-to-foot. Trimming the later portion of the seed pulse was determined to have virtually no impact on the MO/power amplification stage output pulse characteristics, e.g., intensity, even with approximately 25% of the seed pulse energy eliminated. However, as noted elsewhere in the present application pulse trimming may further reduce speckle by eliminating a portion(s) of the output pulse with the greatest coherency (least speckle contrast). 
   Required limits on ASE as currently understood are believed to be attainable with around 5 uJ of seed laser energy and below, e.g., with a long seed pulse shape. Saturation test results have shown applicants that output energy can be attained and the same ASE upper limit levels can be achieved with only 3.75 uJ of seed laser energy when using a short duration seed pulse. Further reductions in seed pulse duration might be possible, resulting in even smaller seed energy requirements. However such further reductions in seed energy may be unnecessary since applicants envision using ˜10 uJ of solid state 193 nm seed energy. Shorter pulse durations may prove difficult since, e.g., two stages of mini-OPuS may be used, e.g., between seed laser and the power amplification stage, with a requirement that the delay length of each mini-OPuS be greater than the seed laser pulse duration, the resulting stretched pulse then being approximately 10 ns FWHM. 
   According to aspects of an embodiment of the disclosed subject matter it is contemplated to apply a time changing voltage on a timescale similar to the seed pulse duration, e.g., by applying a DC voltage level until triggered, at which point the high voltage may be shorted to ground, e.g., via a stack of fast MOSFETS, e.g., illustrated schematically in  FIG. 40  as a single transistor  1130 . A plot of the applied voltage and the seed laser pulse shape are shown in  FIG. 19 . Placing a series resister between the E-O cell terminal and voltage supply can be used to control, e.g., the voltage slope applied to the E-O cell. The 50 pF capacitance of the E-O cell in series with, e.g., a 200Ω resister gives an initial slope of about 10 11  μrad/s. The voltage across the E-O cell drops, e.g., as seen in  FIG. 19  from the DC level to nearly zero in a time similar to the seed pulse duration. By changing the relative timing between the E-O cell pulser and the seed laser one can, e.g., change the amount of pointing sweep that occurs during the seed pulse. In addition, one can change the value of the initial DC voltage to effect a greater or lesser pointing sweep during the seed pulse. Applicants have tested this fast pointing capability, e.g., with the seed laser only and reflecting from an OC only, therefore, with no OPuS effect from the multiple reflections from the OC and Rmax and no effects due to MOPO operation. Without optimizing for relative timing between the E-O cell and the seed pulse, applicants captured speckle patterns for a range of timing between the two. Applicants applied three difference levels of DC voltage to the E-O cell in order to change the maximum available pointing slope. The results showed a minimum speckle intensity normalized standard deviation at about 57 ns relative timing. Without any angular shift during the seed pulse, at both small and large relative timing values, below and above 57 ns the speckle contrast is high. This correlates with values found by applicants during static testing. When, e.g., the relative timing places the E-O Cell voltage slope coincident with the seed pulse, the speckle pattern of a single pulse is smeared in the vertical direction, in a dramatic and satisfactory way. 
   One can normalize these contrast values to the maximum value in order to evaluate the percentage reduction in contrast, e.g., brought about by the dynamic pointing shift. At the optimum relative timing point the speckle contrast was found to be reduced to about 40% of its peak. Using the 1/√{square root over (N)} assumption for equivalent number of independent pulses the data can be used to derive the number of pulses required to achieve this level of speckle contrast reduction. At the optimum relative timing, and with 3 kV applied to the E-O cell, the contrast reduction was found to be equivalent to 6 pulses. Even higher voltage levels (and thus even larger pointing shift during a single pulse) could improve this result. Applicants performed similar measurements with the seed laser pulse entering the power amplification stage cavity, but no discharges between the amplification stage electrodes and noted that reflections from the OC and the Rmax in the XeF cavity, from the OPuS effect, beam spreading alone, indicated that the maximum speckle contrast was reduced by the amount predicted by the OPuS effect (N=1.56 with a 20% OC, giving 1/√{square root over (n)}=0.80. Thus 70% contrast becomes 56%). The effect of smearing, even though the initial speckle contrast is lower, appears not to change when adding the secondary reflections from the full XeF cavity. The equivalent pulse for speckle reduction is still about 6. 
   Applicants performed similar measurements with amplification stage cavity electrodes discharging and thus implicating the effects of the amplification within the amplification stage cavity, which indicated as shown in  FIG. 17  the decrease in the impact on speckle reduction through seed beam sweeping. With such a configuration, the effect was found to be just over half of the equivalent number of pulses produced, i.e., about 3, when operating as a MO/amplification stage, also found was a rather large reduction in peak speckle contrast, with no smearing. Previous measurements of MO/amplification stage operation showed a reduction equivalent to about 6 pulses. These results show a reduction equivalent to about 8 pulses. Applicants suspect that the amplification stage cavity may discriminate against off-axis ray angles, e.g., in a flat-flat cavity, and thus the spray of angles sent into the cavity may not all be equally amplified (this could be corrected, e.g., with a true stable cavity, e.g., employing a curved OC and a curved Rmax). Another explanation may be that not all of the seed pulse takes part in controlling the amplification stage characteristics. Maybe only, e.g., the first 5 ns of the seed pulse&#39;s 10-15 ns pulse duration controls the amplification stage and thus the E-O sweep is not fast enough to occur within that smaller window. This may also be corrected, e.g., by using a smaller resister and a shorter sweep. 
   Referring to  FIG. 35 , a beam mixer  1050  is shown for operation on a beam  1052  (which for illustrative purposes has been shown as having an upper white half and a lower black half). As explained in greater detail below, the beam mixer  1050  can be used to alter the intensity profile of a beam, e.g. improving intensity symmetry along a selected axis of a beam, and can be used to reduce beam coherency, or both. For the embodiment shown, the beam mixer  1050  includes a beam splitter  1054  and mirrors  1056   a - c.    
   For the arrangement shown in  FIG. 35 , the beam can be initially incident upon the beam splitter  1054  whereupon a portion of the beam may directed, via reflection, toward mirror  1056   a  and the remainder  1066  is transmitted (, e.g., with substantially no change in direction) through the beam splitter  1054  and exits the beam mixer  1050  on an output beam path. In one setup, a beam splitter  1054  reflecting about forty to sixty percent of the incident light, e.g. fifty percent, may be used. For this setup, about fifty percent of the initial beam incident upon the beam splitter  1054  is directed toward the mirror  1056   a . For the beam mixer  1050 , mirrors  1056   a - c  may typically be flat, maximum reflectivity mirrors. As shown in  FIG. 35 , mirror  1056   a  may be positioned and oriented to receive light from the beam splitter  1054  at an angle of incidence of approximately thirty degrees. As further shown, mirror  1056   b  may be positioned and oriented to receive light reflected from mirror  1056   a  at an angle of incidence of approximately thirty degrees, and mirror  1056   c  may be positioned and oriented to receive light reflected from mirror  1056   b  at an angle of incidence of approximately thirty degrees. 
   Continuing with  FIG. 35 , light reflected from mirror  1056   c  can be made to be incident upon the beam splitter  1054  at an angle of incidence of about forty-five degrees. For a fifty percent reflectivity beam splitter, about half of the light from mirror  1056   c  is reflected onto the output beam path  1066  and about half of the light from mirror  1056   c  passes through the beam splitter  1054  on a beam path toward mirror  1056   a , as shown. Thus, the output beam path includes a combined beam containing the portion of the initial beam  1052  that passed through the beam splitter  1054  and the portion of light from mirror  1056   c  that is reflected from the beam splitter  1054 . Similarly, the light on the path from the beam splitter  1054  to mirror  1056   a  includes a combined beam containing the portion of the initial beam  1052  that is reflected by the beam splitter  1054  and the portion of light from mirror  1056   c  that is transmitted through the beam splitter  1054 . 
   The beam entering the beam mixer  1050  in  FIG. 35  is shown illustratively as having a rectangular cross-section that defines a long axis  1058 . This type of beam is typical of a laser beam produced by an excimer laser with the long axis corresponding to the direction from one discharge electrode to the other. A typical beam may have dimension of about 3 mm by 12 mm. Moreover, for the output of an excimer laser, the intensity profile in one axis, e.g., the long axis  1058  is typically unsymmetrical, whereas the intensity profile in the other axis, e.g., the short axis (i.e. the axis normal to the long axis  1058 ) is approximately Gaussian. Although the beam mixer  1050  shown is particularly suitable for improving symmetry of a high power excimer discharge laser, it is to be appreciated that it can be used in conjunction with other types of laser systems and for other applications, for example, the beam mixer may be used to reduce coherency in a beam generated by a solid state laser. 
     FIG. 35  shows that the beam extends along the axis  1058  from a first edge  1060  to a second edge  1062 .  FIG. 35  also shows that the mirrors  1056   a - c  establishing a spatially inverting path which has a beginning  1064  and an end  1066 . As  FIG. 35  illustrates, the inverting path may be characterized in that a part of the beam near the first beam edge  1060  at the beginning  1064  of the inverting path translates to the second beam edge at the end  1066  of the inverting path. More specifically, for the mixer  1050  shown, a photon at the ‘top’ of the beam which strikes mirror  1056   a  translates and leaves mirror  1056   c  at the ‘bottom’ of the beam. Since the inverting path constitutes a delay path, there will be some temporal stretching of the pulse, however, this can be minimized by minimizing the delay path, e.g., to a length of about a ns or so. with suitable delay path time, etc., as noted elsewhere, the beam mixer  1050  could form a coherence buster mini-OPuS, e.g., as discussed in regard to  FIG. 47 . 
   The beam mixer  1050  may be placed in between the seed beam laser portion and the amplifier laser portion of a MOPA or MOPO configured multi-chambered laser system, or other master oscillator amplification gain medium arrangements, e.g., with a power amplification stage, such as a ring power amplification stage, such as that shown in  FIGS. 15 ,  16 ,  23 ,  25   27  and  28 . Specifically it may be substituted for the mini-OPuS ( 376  and/or  380 ) of  FIG. 15 . As an example, either or both of the short delay path pulse stretchers  376 ,  378  inserted between the MO  372  as shown in  FIG. 15  and the PO amplifier portion  394  of the multi-chamber laser system of  FIG. 15  may have substituted for it the beam mixer  1050  of  FIG. 35 . 
     FIG. 15  shows partly schematically and partly in block diagram form an example of a coherence busting scheme  360  and the results of aspects of the scheme according to aspects of an embodiment of the disclosed subject matter, e.g., in terms of beam divergence and thus coherence busting. The illustrated system may incorporate, e.g., an oscillator/amplifier laser  370 , e.g., including a solid state or excimer seed laser  372 , and an oscillator amplifier laser  394 , or other power amplification stage, e.g., a ring power amplification stage. The amplifier gain medium  394  may be, e.g., an excimer laser arranged in a power oscillator configuration, e.g., with a fully reflective rear cavity mirror  396  and an input/output coupler, e.g., a partially reflective mirror  398 . It will be understood that other seed laser/amplification stage arrangements, some of which are discussed herein, may also be used with the schematically illustrated coherence busting scheme shown by way of example in  FIG. 15 . 
   At the output of the seed laser  372  is illustrated a representation of the seed laser output laser light pulse beam pulse coherency  374  containing a single dot indicative of relatively high coherency. The output of the seed laser  372  may be passed through one or more coherency busters, e.g.,  376 ,  378 , e.g., as shown by example in  FIG. 9 , or  1050  illustrated in  FIG. 35  (discussed in more detail in the co-pending application noted above, Ser. No. 11/471,383) or other optical arrangements such as disclosed in US2005/0286599, referenced above, or one or more mini-OPuS coherence busting mechanisms discussed above, or combinations thereof. A possible embodiment according to aspects of an embodiment of the disclosed subject matter may be the use of a confocal OPuS, e.g., one like that disclosed in the co-pending U.S. patent application Ser. No. 10/847,799, entitled LASER OUTPUT LIGHT PULSE STRETCHER, filed on May 18, 2004, referenced above, with, e.g., two confocal spherical mirrors and four passes of delay path, i.e., from the beam splitter to mirror No. 1 to mirror No. 2 back to mirror No. 1 and back to mirror No. 2 and then returned to the beam splitter, passing through, e.g., an offset correction optic, e.g., as discussed in the co-pending U.S. patent application Ser. No. 11/394,512, entitled CONFOCAL PULSE STRETCHER, filed on Mar. 31, 2006, referenced above. This version of a so-called “mini-OpuS” may comprise two pulse stretchers in series, e.g., with a delay path offset selected to interleave the high frequency peaks in the temporal pulse intensity curve of the output of the master oscillator, such that individual mini-peaks superimposed on the general humped or multi-humped shape of the output pulse from the MO become interleaved in the treated pulse, with advantages in reducing speckle. This may be achieved by, e.g., a delay offset of about 2 ns for a first one ns and then three ns delay line mini-OPuS pair or about a 1 ns delay between a 3 ns and 4 ns delay line mini-OPuS pair in series or for a 4 ns and 5 ns delay line mini-OPus pair in series. It will be understood that the pulse itself will not be stretched sufficiently to overlap other pulses, but rather sill essentially not be stretched at all, since the delay path is so much shorter than the ten or so meters of delay path in the normal pulse stretching OPuSs currently sold by applicants&#39; assignee. 
   The preferred embodiment uses a first delay something more than ins due to increased alignment problems with the shorter delay and increased aberrations in the pulse as stretched in a shorter delay path. Each of the delay paths is, however longer than the coherence length of the pulse and the second delay path is longer than the first, to achieve coherence busting effects discussed herein. 
   The mini-OPuS pulse stretchers may be selected and arranged to, e.g., fold the beam on itself or fan it out in first one axis, e.g., in a first mini-OPus  376 , resulting in the coherency representation  378  and then in another orthogonally related axis, e.g., in a second mini-OPuS  380 , resulting, e.g., in the coherency representation  390 . A pulse trimmer/pulse steerer  392 , e.g., and electro-optical (“E-O”) element  392  may sweep (paint) the seed beam into the input/output coupler  400  of the amplifier portion  394  resulting in the blurring in one axis as shown in the pulse coherency representation out of the power oscillator  410  (and also the coherence representation  410  into the amplification gain stage  394 ). The “regular” or “standard” OPuS, e.g., a 4× T is  OPuS (roughly ten meters of delay path), which may contain, e.g., 2 delay paths  412 ,  420  initiated by a first beam splitter  414  and a second beam splitter  422 , similarly may be arranged to fold the beam on itself in first one axis and then a second resulting, e.g., in the pulse coherency representations of, respectively,  414  and  424 . The final coherency representation  424  shows schematically that the coherency of the seed beam has been greatly reduced, i.e., the beam has been smeared in its passage from the seed laser  372  to the amplifier gain medium  394   a  and as amplified in the amplifier gain medium  394  and subsequently further having its coherency busted in the 4× regular OPuS  412 ,  420 . 
   It will be understood by those skilled in the art that depending on the initial coherency of the pulse, e.g., out of the seed laser, e.g., almost completely coherent in the case of solid state seed lasers to very little coherency, but still coherency that is desired to be even further reduced, e.g., with an excimer seed laser the type, number and arrangement of coherency busting elements may vary. For example, it may only be necessary to do active coherency busting, e.g., with one form or another of pulse steering/painting, for solid state seed lasers, and this may in some cases for some applications prove to need only a ramp or only AC pulse deflection, i.e., in one axis or the other, or may prove to need both DC and AC pulse painting (Hybrid painting) along with OPuS effect coherency busting both between the MO and amplifier gain medium, e.g., PO or PA or other amplification gain medium stage, e.g., a ring power amplification stage, and also may need to employ the effect of the regular OPuS pulse stretcher(s) on the output of the amplifier gain medium. With an excimer gas discharge laser MO, with relatively much lower coherency than from a solid state seed laser, only passive coherency busting, e.g., between the MO and gain amplifier medium may be needed, e.g., with one or both of the mini-OPuSs  376 ,  380  or other passive optical elements as noted above between the MO and amplifier gain medium. 
   One may still need, however, to do beam steering also, e.g., with an active beam steering mechanism for even more smearing of the pulse (more divergence), that may be less essential and need a smaller sweeping angle. Such a seed laser mini-OPuS is believed to need approximately only a 1 foot total path delay each and can also be conveniently built onto the seed laser optical table as is currently the practice for relay optics in applicants&#39; assignee&#39;s XLA series laser systems. 
     FIG. 17  illustrates an exemplary relative speckle intensity for a 1 kV E-O deflector voltage v. relative timing. The relative standard deviation curve  550  is for 1 kV and the equivalent pulse curve is curve  550 ′. A 2 kV E-O deflector voltage curve  552  and equivalent pulse curve  552 ′ are also shown as is a 3 kV E-O deflector voltage curve  554  and equivalent pulse curve  554 ′. An example of a point shift vs. E-O voltage curve  560  is shown by way of example in  FIG. 18 . 
   It will be understood by those skilled in the art that an apparatus and method is disclosed for reaching very high average output power, e.g., around 200 W or more with an excimer or molecular fluorine gas discharge laser system in the DUV range of wavelengths, e.g., 351 for XeF, 318 for XeCl, 248 for KrF, 193 for ArF and 157 for F 2 , utilizing, e.g., a power oscillator or other amplification gain stage, e.g., a ring power amplification stage, with little or no significant ASE interfering with the in-band desired radiation output of the laser system, e.g., with a ratio between the ASE and in-band radiation at or below about 5×10 −4 , e.g., with, e.g., a 100 uJ pulse energy input into the power amplification stage cavity per pulse. According to aspects of an embodiment of the disclosed subject matter unwanted ring power amplification stage light propagates backwards and can also be sampled for diagnostics and ASE feedback control. Adding a small amount of line-narrowing, e.g., with prism tuning, can also help suppress ASE from the power amplification stage. Also according to aspects of an embodiment of the disclosed subject matter a PA may be used, e.g., along with a solid state MO, e.g., a 4 pass amplifier with no oscillation but with acceptable amplification and perhaps even high enough saturation. With such a design it may be necessary, e.g., for the 4 passes to each traverse the entire gain cross-section in each of the 4 passes. The cavity may have 2 prisms on each side of the cavity, in order to, e.g., reduce the energy density on the coated cavity optics and also provide dispersion for ASE reduction. 
   In addition, it may not be that the ultimate ASE levels in a MOPO, or other master oscillator/power amplification stage configurations, necessarily increase with decreasing MO energy, such that according to aspects of an embodiment of the disclosed subject matter decreasing MO output energy even below 10 μJ may not result in unacceptable ASE, even without, e.g., a partially reflective off axis seed injection mechanism and/or a regenerative ring power amplification stage configuration. A cavity with beam expansion and crossing beams may be constructed that does not exceed the cavity length of today&#39;s XLA, e.g., with the beam expansion prisms far enough away from the chamber to allow lateral translation for beam crossing, e.g., at a distance of a few centimeters of the chamber window, dictated by, e.g., beam width and crossing angle. A separate vessel for the prisms and/or beam reverser optics could also allow the use of a direct F 2  supply, e.g., at a different concentration than in the lasing gas mixture, e.g., at around 1% concentration. This could also, e.g., avoid contamination from the optics holders. 
   The effect of inverse imaging, e.g., in an optical delay path, e.g., in a mini-Opus with a delay path of only about one foot, is illustrated in  FIG. 20A , e.g. for an input beam  580 , in which a beam corner  582  is designated by the square initially in the lower right hand corner of the beam  580 . For a first sub-pulse  584 , e.g., between an entrance beam splitter and a first mini-OPuS mirror, the beam corner  582  remains the same. In a second sub-pulse  586 , e.g., reflected from the first mirror, the beam has been, e.g., negatively imaged, e.g., to a second mini-OPuS mirror and the beam corner has moved to the upper left hand corner and then for a third sub-pulse  588 , reflected to a fourth mini-OPus mirror, where the beam corner has been negatively imaged back to the bottom right hand corner, as illustrated in the FIG. Combining all of these sub-pulses into an output pulse, with a relatively short optical pulse delay such that the pulse is not very significantly stretched from a T is  standpoint, can still substantially reduce coherency by this effect of folding the beam on itself a plurality of times, depending on the number of mirrors in the delay path. 
     FIG. 20  B illustrates this same effect, e.g., on half of the beam, e.g., is the beam had been split into two halves before entry into the delay path of, e.g., two separate sources, e.g., two solid state seed lasers operating at X kHz in, e.g., a 2× kHz system. As can be seen the two halves are similarly negatively imaged in each sub-pulse resulting in even further reduction in coherency in an overall output pulse formed, e.g., by the combination of the two half pulses into a single output pulse, e.g., of the shape shown by way of example in  FIG. 20A . 
   Turning now to  FIG. 21  there is shown schematically and partly in block diagram form a beam combiner system  600 , according to aspects of an embodiment of the disclosed subject matter. The beam combiner system  600  may include, e.g., a first amplifier gain medium portion  602  and a second amplifier gain medium portion  604 , each of which may be, e.g., a PA or PO of ring power amplification stage, as described elsewhere in the present application. The output of each of the amplifier portions  602 ,  604  may pass through a beam expander  608 , which may include a prism  610  and a prism  612 , e.g., magnifying the beam by, e.g., about 2×. A turning mirror  620  may steer a first laser system output light pulse beam  622  from the amplifier  602  to a second turning mirror  624  which may steer the pulse beam  622  to form a pulse beam  632  onto a beam splitter for a first pulse stretcher  640  and thence to a beam splitter  646  for a second pulse stretcher  644 . A turning mirror  630  may steer a second laser system output light pulse beam  632  from the second amplifier  604  to a second turning mirror  634 , which may steer the beam  632  to form a beam  634  to be incident on the beam splitter  642  and thence the beam splitter  646 . The output of the first OPuS and second OPuS, which may be “mini-OPuSs” as discussed elsewhere in the present application, may pass through another beam splitter  650 , where, e.g., a small portion of the laser system output laser light pulse beam may be diverted, e.g., for metrology purposes, e.g., focused by a focusing lens  652  into a divergence detector  654 , which may be part of a control system (not shown) providing feedback control signals  656 , e.g., to the beam splitters  642 ,  646  of the first and/or second OPuSs  640 ,  644  or the turning mirrors for each of the beams  632 ,  634  to, e.g., insure the pointing from both amplifiers remain overlapped in the far field so that the beam appears to be as one beam, and also, e.g., so that the two pulse stretchers maintain the pointing chirp introduces=d, e.g., due to the confocal nature of the OPuS(s). 
     FIG. 22  illustrates schematically the impact of changing the pointing of the beam (sweeping the beam) in terms of coherency/speckle reduction. A pulse stretcher  662  may receive a laser system output laser light pulse beam  100  on a beam splitter  664  and, e.g., through changing the angle of the beam splitter sweep the pointing of the beam  100  across a slit, e.g., with a slit diameter of d, onto a diffuser  670 . The resultant detected speckle pattern  680  indicates that the sweeping reduces the coherency contrast and thus speckle. 
   Turning now to  FIG. 23  there is illustrated by way of example in schematic and partly block diagram form a very high power solid state seeded immersion lithography laser light source  700 , which may include, e.g., a high pulse repetition rate, e.g. a 12 kHz, solid state seed laser  702 . The output of the seed laser  702  may pass through formatting optics  704 , which can include, e.g., a lens  706  and a lens  708 , which may be used to, e.g., to reformat the beam from a round beam to a shape concomitant with the shape of the gain medium in the amplifier portion. The output laser light pulse beam from the seed laser  702  may then be passed through an x axis electro-optical (“E-O”) steering mechanism  712 , and/or a y-axis E-O steering mechanism  714  or both, e.g., an E-O cell model referenced above, each providing, in a respective axis, e.g., orthogonal to each other, a sweep of the beam in order to paint a reasonable percentage of the utilization tool (e.g., scanner or annealing tool) aperture, e.g., about 1 mrad, along with a high frequency AC painting voltage, as explained elsewhere in the present application. The laser output light pulse beam pulses from the seed laser  702  may then be split in a beam divider to provide alternating (“tic-toc”) input pulses into a respective one of an amplifier gain medium, e.g., a first power oscillator  730  and a second power  730 . The power oscillators  730  may comprise a ring power oscillator. 
   The beam divider  720  may comprise, e.g., a beam splitter  722  that selectively transmits, e.g., 50% of the output beam from the seed laser  702  onto a turning mirror  724  and a turning mirror  726 , leading into the second amplifier gain medium  730  and reflects 50% to a turning mirror  728  leading to the second gain amplifier medium  730 , e.g., on each pulse the beam splitter  720  could also comprise, e.g., an electro-optical or acousto-optical beam deflector alternating actuated to rend light to folding mirror  728  or folding mirror  724  on alternate pulses. 
   Each respective gain amplifier medium  730  may include, e.g., a power amplification stage chamber  732 , an input coupler/rear cavity mirror  734 , e.g., a concave mirror with an aperture on the axis of revolution of the mirror surface admitting the seed laser beam into the cavity formed by the rear cavity mirror  734  and a front cavity mirror  736  as are known in the art of unstable oscillation cavities. It will be understood that the amplifier gain medium may be in other configurations mentioned in the present application, e.g., a stable resonator with, e.g., a seed injection mechanism, discussed in the co-pending and contemporaneously filed application referenced above, and e.g., a ring power amplification stage, or a power amplifier, without an oscillator cavity and with only a fixed traversal path for amplification while the gain medium is energized (e.g., a population inversion exists) as is known in the art, without laser oscillation occurring, i.e., without an output coupler as is known in the art of laser oscillation cavities. In oscillation cavity environments, e.g., the convex mirrors could be replaced, e.g., by an input coupler such as the seed injection mechanism, discussed in more detail elsewhere in the present application, and the convex mirror  736  replaced with an output coupler. Beam expanding, beam combining and coherency busting and divergence measuring of the respective output beams  766  from the first amplifier gain medium  730  and  764  from the second amplifier gain medium  730 , and feedback control may occur as discussed in regard to  FIG. 21  with respective beam expander  740 , comprising, e.g., prisms  742  and  744 , beam combiner comprising mirrors  750 ,  752  from the first amplifier gain medium  730  and mirrors  760 ,  762  from the second amplifier gain medium  730  and pulse stretchers  640  and  644  and metrology unit  654 . 
     FIG. 24  illustrates schematically the results of a coherency busting scheme on an output laser pulse, e.g., in relation to a scanner acceptance window, e.g., introducing horizontal and vertical (as illustrated in the plane of the page drawing of  FIG. 24 ) directions. The dot  780  illustrated schematically and by way of example an initial seed laser output pulse profile  780 . The pattern of pulses  782  illustrate a pattern of sub-pulse profiles  782  after beam folding in a perfectly aligned beam delay path, or through a misaligned pulse stretcher(s) or both, or a combination thereof, and the circles  784  around each represent the effect on the profile of electro-optical smearing. 
     FIG. 25  illustrates schematically and partly in block diagram form by way of example a ring power amplification stage oscillator laser system  800  and a seed injection mechanism  812 , as discussed in more detail in the co-pending and contemporaneously filed patent application discussed above. The laser system  800  may comprise, e.g., a with bow-tie ring power amplification stage  804  and a seed laser, e.g., a solid state or gas discharge seed oscillator  802 . The seed oscillator  802  may be isolated from the oscillator cavity of the power amplification stage  804  by an isolator to prevent unwanted lasing from feedback photons, which may be unnecessary, e.g., with a proper seed injection mechanism  812 . The power amplification stage section  804  may include, e.g., a power amplification stage chamber  810 , a seed injection mechanism  812 , which may include, e.g., an input/output coupler  814  and a maximally reflective (“Rmax”) mirror  816  beam reverser  820 , reflecting the output beam  806  from the seed oscillator  802  into the amplifier portion chamber  804 , and also include a beam reverser/returner  820 , which may include, e.g., a first maximally reflective mirror  822  and a second mirror  824 , e.g., made of a material, like the Rmax mirror  816 , selected to be maximally reflective for a suitable band around the nominal center wavelength of the laser system, e.g., 351 for XeF, 318 for XeCl, 248 for KrF, 193 for ArF and 157 for F 2 . The seed injection mechanism and beam returner, as explained in more detail in the co-pending and contemporaneously filed patent application referenced above, may be arranged so as to form the oscillation cavity of the power amplification stage  840  (whether technically speaking an oscillator or amplifier oscillator stage, i.e., depending on cavity length), such that on each oscillation along an oscillation path  826 ,  828  the output beam  806  from the seed laser  802  passes more than once through the gain medium formed between a pair of discharge electrodes (not shown in  FIG. 25 ) per oscillation, i.e., along the path  826  in a first direction and  828  in a second direction generally opposite to the first direction but through the gain medium per every oscillation during the formation of the output laser light pulse beam  100  which eventually leaves the oscillation cavity  804 , as is well know in the art of laser oscillation creating an output laser light pulse beam from a laser oscillator. It will be understood that the angle of offset of the beams  826 , 828  is greatly exaggerated for illustration purposes and could be around 1 μrad. 
     FIG. 27  illustrates schematically and partly in block diagram form a solid state seed/power amplifier laser system  880  according to aspects of an embodiment of the disclosed subject matter. The system may incorporate a solid state 12 kHz seed laser  882  and a pair of amplifier gain media, e.g., a pair of power amplifier chambers  888 . An optical interface module  884  may receive the output of the seed laser  882  and direct it in tic-toc fashion into the respective amplifier gain medium  888 , e.g., on alternating pulses. The optical interface module  884  may comprise, e.g., a pair of cylindrical telescopes  886 , which may serve to format the beam, e.g., because the output may be astigmatic with the telescope serving to remove the astigmatism, and may also include, e.g., an input optics module  890 , each including, e.g., a mirror  902 , a mirror  908  and a mirror  910 , which together with mirrors  904  and  906  may form, e.g., a fixed number of passes, e.g., three passes through the gain medium between electrodes (not shown in  FIG. 27 ) in an amplifier gain medium configured, e.g., as a three pass power amplifier (“PA”). that is, no laser oscillation occurs in the amplifier gain medium. The respective outputs of the respective power amplifier  888  may be steered by beam turning mirror  930 ,  932  on the one hand and  934 ,  936  on the other through a respective energy sensor. These output beams from the system  880  may be combined in a beam combiner as discussed elsewhere in the present application. 
   A coherency buster, e.g., an automated two axis angular adjustment mechanism  910 , e.g., modulating the tilt of the respective mirror  910  in the input optics module  890  may serve a similar purpose to that of the X and Y axis beam steering electro optic elements  712 ,  714  of the embodiment of  FIG. 23 , e.g., by sweeping the beam entering the amplifier gain medium from side to side and/or up and down for greater divergence and thus coherency busting as discussed elsewhere herein. 
   Turning now to  FIG. 28  there is illustrates schematically and in partly block diagram format a seed laser/amplifier gain medium laser system such as a solid state seed/power amplification stage laser system  950  according to aspects of an embodiment of the disclosed subject matter. The system  950  may include, e.g., a seed laser, e.g., a solid state 12 kHz seed laser  952  the output of which may enter into an optical interface modules  884 , e.g., into a respective one of a pair of cylindrical telescopes  886 , as in the embodiment of  FIG. 27 . Input coupling modules  960  may include, e.g., a polarizing beam splitter  962 , an Rmax  964 , a quarter wave plate  966 , and an input coupler Rmax mirror  968 , which together function to couple output of the seed laser  952 , respective seed beam  970 ,  972 , into the respective gain amplifier medium, e.g., a power amplification stage oscillator having an output coupler  982 , by e.g., using a polarization coupling. Turning mirrors  984 ,  986 ,  994 ,  996  serve the same purpose as the respective turning mirrors in the embodiment of  FIG. 27 . 
     FIG. 29  represents an illustrative normalized MOPO intensity  1000 , a normalized single pass PA intensity  1002  and a normalized two pass PA intensity  1004 . 
     FIG. 30  represents an illustrative macroscopic steering pulse  1010 , which may comprise a plurality of alternating high and low DC voltages  1010 ,  0102 , and  1014 , which may repeat in some pattern, e.g., of three different high voltages, as illustrated and a superimposed alternating current high frequency steering voltage  1016 , which may occur, e.g., both at the higher voltage and at the low voltage. As illustrated, e.g., the high voltages may have different pulse durations and different low voltage duration intervals as well. As shown in  FIG. 32 , these high voltages  1032  may be of the same value and same low voltage duration interval  1036  with superimposed AC  1034 . 
     FIG. 31  illustrates schematically and in block diagram form an optical switching and painting system  1020 , according to aspects of an embodiment of the disclosed subject matter, which may include, e.g., a solid state seed  1022 , a frequency converter  1024 , and an optical switch and painter  1026 , which may include an electro-optical beam director that, e.g., deflects the beam into a first one of an amplifier gain medium  1030  when the pulse, e.g., as shown in  FIG. 32  is high ( 1032  in  FIG. 32  and into the other amplifier gain medium  1032 , when the pulse is low ( 1036  in  FIG. 32 ) and also applies the AC beam steering  1034  into each amplifier  1030 ,  1032 . A second frequency shifter  1028  may be intermediate the beam splitter/painter  1026  and the respective amplifier gain medium  1032 , and may be in addition to the frequency shifting of the element  1024  or in lieu thereof. 
   According to aspects of an embodiment of the disclosed subject matter applicants propose to generate 193 nm laser light utilizing a solid-state seed laser, e.g., the generation of coherent 193 nm radiation in a solid-state configuration with a solid-state seed drive laser (or lasers) that drive linear or nonlinear frequency conversion stages. One potential seed laser is the pulsed Yb fiber laser, lasing at around 1060 nm, tunable in the 1050-1080 nm region. Such lasers constitute a mature and powerful fiber laser technology, which may, e.g., be configured to produce short temporal duration pulses (1-5 ns) at multi-kilohertz repetition frequencies. To generate 193 nm using 1060 nm as the longest wavelength mixing source, according to aspects of an embodiment of the disclosed subject matter, applicants propose to use, e.g., sum frequency generation (“SFG”) with a long wavelength and a moderately short wavelength to generate deep ultraviolet (“DUV”). Second harmonic generation (“SHG”) to reach 193 nm is not possible, due to the present lack of a 236.5 nm source as the other mixing wavelength. However, such a source could be derived by fourth harmonic generation, (“FHG”) of the 946 nm output of a q-switched diode-pumped Nd:YAG laser (946 nm being a lower efficiency transition in Nd:YAG. 
   The output of the Nd:YAG is essentially a fixed wavelength, and overall tunability could be provided by tuning the output wavelength of the Yb fiber laser, e.g., a Yb +3  fiber laser. Tunability of the Yb fiber laser output could be obtained via a CW diode seed laser, e.g., a New Focus Vortex TLB-6021. Such a diode laser seeders can provide fast wavelength control over limited wavelength ranges, e.g., via internal PZT control of reflectors, as desired for lithography source applications and have a high spectral purity. Nd:YAG lasers are operable at multi-kilohertz repetition frequencies, ensuring the overall system repetition rate can meet the repetition rate requirements for a practical excimer laser injection seeding source. 
   To achieve narrow bandwidth operation, both laser sources need individually to be narrowband. In Nd:YAG systems, this may be achieved, e.g., by injection seeding with a CW lower power Nd:YAG laser, e.g., in a non-planar ring oscillator architecture that is operating, e.g., with a single longitudinal mode output. In the Yb fiber laser case, the bandwidth could be assured via the CW diode laser seeder, which typically operates at very narrow linewidths, e.g., on the order of 100 MHz FWHM. Further, appropriate large-mode area (“LMA”) fiber technology could be used to minimize spectral degradation due, e.g., to nonlinear effects in the fiber comprising the fiber laser oscillator or any subsequent amplification stages. 
   To generate 193.4 nm radiation, e.g., as illustrated schematically and partly in block diagram form in  FIG. 41 , a system  1200  including, e.g., a pulsed 946 nm Nd:YAG laser  1204  seeded by a 946 nm seed laser, e.g., a 946 nm CW Nd:YAG seed laser  1202 , which the output of the Nd:YAG laser  1204  frequency doubled, in a frequency converter  1206 , which may include, e.g., a frequency doubler  1208 , e.g., a non-linear material such as an LBO or KTP crystal, followed by either another frequency doubler (not shown) or a third harmonic generator  1210  and a fourth harmonic generator  1212  (e.g., each done using sum-frequency generation with residual pump radiation, e.g., using the above noted crystals), either approach generating the fourth harmonic at 236.5 nm. The 236.5 nm radiation can then be mixed, e.g., in a sum frequency generation with the 1060 nm output of the a Yb fiber laser in a final nonlinear crystal mixing stage, sum frequency generator  1240 , e.g., a CLBO or a BBO. That is, e.g., 1/1040 (0.000943)+1/236.5 (0.00423)=1/193.3 (0.005173). The fiber laser  1222  may have a rear oscillation cavity mirror  1224  and a front window  1226 , with a Q-switch  1228 . 
   CLBO is cesium lithium borate, which is an effective 4 th  or 5 th  harmonic generator for Nd:YAG output light, can be phase matched up for 193 nm operation and has a damage threshold of &gt;26 GW/cm 2 . BBO is beta barium borate (b-BaB 2 O 4 ), which is one of the most versatile nonlinear optical crystal materials available and most commonly used for second- or higher-order harmonic generation of Nd:YAG, Ti:Sapphire, argon ion and alexandrite lasers. CLBO is preferred because of its higher transparency and high acceptance angle, which may, however, require cryogenic cooling for phase matching, also being problematic because CLBO is a hygroscopic material). The alternative is BBO, which can be phase matched but is being operated very close to its absorption band edge at ˜190 nm. BBO also has much a narrower acceptance angle than CLBO, but this can be managed through optical design, e.g., with anamorphic focusing. According to aspects of an embodiment of the disclosed subject matter both lasers  1024 ,  1022  can be made relatively powerful, e.g., with peal output power of greater than about 25 KW, helping to compensate for any inefficiencies in the nonlinear frequency conversion stages  1206 ,  1240 . 
   According to aspects of the disclosed subject matter, the generation of 193.3 nm with solid state laser(s) for seeding an excimer amplifier gain medium may also be done, e.g., by the use of mature drive laser technologies, which may be wavelength tunable in a similar fashion to current tuning of excimer lasers. A seed laser system  1200 ′, illustrated schematically and in partly block diagram form in  FIG. 42 , may comprise, e.g., an Er fiber laser  1260 , e.g., lasing at around 1550 nm but tunable in the 1540-1570 nm range. Er fiber lasers are available, and use similar generic technologies to Yb fiber lasers. Such an approach is attractive because of the maturity of fiber and pump diode laser technology for this wavelength range, applied, e.g., in fiber-based telecommunications, e.g., erbium-doped fiber amplifiers or EDFAs used as signal boosters in optical fiber communication. 
   According to aspects of an embodiment of the disclosed subject matter applicants propose to use a pulsed fiber laser oscillator  1260  as the source of moderate peak power (e.g., 5-50 kW) high-repetition-rate (multi-kHz, e.g., at least 12) 1546.5 nm narrowband pulsed radiation. That laser  1260  could be constructed using standard pulsed fiber laser technology, to use a single-mode CW tunable narrowband diode laser  1262  as an injection seeder for the fiber laser oscillator  1260  to ensure narrowband, single wavelength performance, and also to allow the fast wavelength tunability required for lithography light source applications. An example of the type of diode laser seeder  1262  is, e.g., a New Focus Vortex TLB-1647, which uses an external cavity diode configuration with PZT wavelength actuation for high-speed wavelength drive over a limited wavelength range, in parallel with mechanical drive for extended wavelength range operation. Further, appropriate large-mode area (“LMA”) fiber technology could be used to minimize spectral degradation, e.g., due to nonlinear effects in the fiber comprising the fiber laser oscillator or any subsequent amplification stages. Using such approaches can, e.g., allow spatial beam quality to be maintained, employing techniques for ensuring single-mode operation in large mode area fibers, while reducing the peak power in the core of the fiber. After the 1546.5 nm radiation is generated, it may then be frequency upconverted directly to 193.3 nm, e.g., using five stages of nonlinear frequency conversion, either second harmonic generation, or sum frequency generation. This can be achieved through the steps listed in  FIG. 43 , one of which is illustrated by way of example in  FIG. 42 , wherein ω refers to 1546.5 nm and 8 ω becomes 193.3 nm. In  FIG. 42  there is shown the generation of the second harmonic 2ω of 1546.5 nm in SHG  1208 , and the third harmonic generation, e.g., by adding the base frequency to the second harmonic to in SFG  1258  to get 3ω, and frequency doubling 3 W to get 6ω in frequency double  1258 , followed by similar such sum frequency generations as just noted in SFGs  1252  and  1254  to get, respectively, 7ω and 8ω. In addition, according to aspects of an embodiment of the disclosed subject matter relatively low-power pulsed fiber laser oscillator outputs, e.g., seeded by a diode laser for spectrum/wavelength control, could then be boosted in peak power via, e.g., a subsequent stage(s) of fiber amplification (not shown). Applicants propose also, the development of an all-fiber solid state drive laser based on this approach. 
   Turning to  FIG. 44  there is illustrated schematically and in block diagram form a laser treatment system, e.g., and LTPS or tbSLS laser annealing system, e.g., for melting and recrystallizing amorphous silicon on sheets of glass substrates at low temperature. The system  1070  may include, e.g., a laser system  20  such as described herein and a optical system  1272  to transform the laser  20  output light pulse beam from about 5×12 mm to 10 or so microns×390 mm or longer thin beams for treating a workpiece, e.g., held on a work piece handling stage  1274 . 
   MOPO energy vs. MO-PO timing has been examined at different values of seed laser energy, ArF chamber gas mixture, percentage reflectivity of output coupler (cavity Q) and seed laser pulse duration, with the results as explained in relation to  FIG. 45 . 
   ASE vs. MO-PO timing has been examined for different values of seed laser energy, ArF chamber gas mixture, percentage reflectivity of output coupler (cavity Q) and seed laser pulse duration with the results also explained in relation to  FIG. 45 . 
   Turning to  FIG. 45  there is shown a chart illustrating by way of example a timing and control algorithm according to aspects of an embodiment of the subject matter disclosed. The chart plots laser system output energy as a function of the differential timing of the discharge in the seed laser chamber and the amplification stage, e.g., the ring power amplification stage as curve  600   a , which is referred to herein as dtMOPO for convenience, recognizing that the amplification stage in some configurations may not strictly speaking be a PO but rather a PA though there is oscillation as opposed to the fixed number of passes through a gain medium in what applicants&#39; assignee has traditionally referred to as a power amplifier, i.e., a PA in applicants&#39; assignee&#39;s MOPA XLA-XXX model laser systems, due, e.g., to the ring path length&#39;s relation to the integer multiples of the nominal wavelengths. Also illustrated is a representative curve of the ASE generated in the amplification stage of the laser system as a function of dtMOPO, as curve  602   a . In addition there is shown an illustrative curve  604   a  representing the change in the bandwidth of the output of the laser system as a function of dtMOPO. Also illustrated is a selected limit for ASE shown as curve  606   a.    
   It will be understood that one can select an operating point on the ASE curve at or around the minimum extremum and operate there, e.g., by dithering the control selection of dtMOPA to, e.g., determine the point on the operating curve  602   a  at which the system is operating. It can be seen that there is quite a bit of leeway to operate around the minimum extremum of the ASE curve  602   a  while maintaining output pulse energy on the relatively flat top portion of the energy curve to, e.g., maintain laser system output pulse energy and energy σ, and the related dose and dose a constant, within acceptable tolerances. In addition as shown, there can be a concurrent use of dtMOPO to select bandwidth from a range of bandwidths while not interfering with the E control just noted. 
   This can be accomplished regardless of the nature of the seed laser being used, i.e., a solid state seed or a gas discharge laser seed laser system. Where using a solid state seed laser, however, one of a variety of techniques may be available to select (control) the bandwidth of the seed laser, e.g., by controlling, e.g., the degree of solid state seed laser pumping. Such pump power control may, e.g., put the pumping power at above the lasing threshold in order to select a bandwidth. This selection of bandwidth may shift or change the pertinent values of the curve  604   a , but the laser system will still be amenable to the type of E and BW control noted above using dtMOPO to select both a BW and concurrently an operating point that maintains the output energy of the laser system pulses at a stable and more or less constant value in the flat top region of the illustrated energy curve  600 . It is also possible to use a non-CW solid state seed laser and to adjust the output bandwidth. For example, selection of the output coupler reflectivity of the master oscillator cavity (cavity-Q) can adjust the output bandwidth of the seed laser system. Pulse trimming of the seed laser pulse may also be utilized to control the overall output bandwidth of the laser system. 
   It can be seen from  FIG. 45  that either the selected ASE upper limit or the extent of the portion of the energy curve that remains relatively flat with changes in dtMOPO may limit the range of available bandwidth for selection. The slope and position of the BW curve also can be seen to influence the available operating points on the ASE curve to maintain both a constant energy output and a minimum ASE while also selecting bandwidth from within an available range of bandwidths by use of the selection of a dtMOPO operating value. 
   It is similarly known that the pulse duration of discharge pulses in a gas discharge seed laser, among other things, e.g., wavefront control may be used to select a nominal bandwidth out of the seed laser and thus also influence the slope and/or position of the BW curve  604  as illustrated by way of example in  FIG. 45 . 
   According to aspects of an embodiment of the subject matter disclosed one may need to select an edge optic, that is an optic that may have to be used, and thus perhaps coated, all the way to its edge, which can be difficult. Such an optic could be required, e.g., between the output coupler, e.g.,  162  shown in  FIG. 2  and the maximum reflector, e.g.,  164 , shown in  FIG. 2 , together forming a version of a seed injection mechanism  160 , shown in  FIG. 2 , e.g., depending upon the separation between the two, since there may be too little room to avoid using an edge optic. If so, then the edge optic should be selected to be the Rmax, e.g., because of the ray path of the exiting beam as it passes through the OC portion  162 . From a coatings standpoint it would be preferable to have the OC be the edge optic because it has fewer layers. However, an alternative design, according to aspects of an embodiment of the subject matter disclosed has been chose by applicants and is illustrated schematically and by way of example in  FIG. 30 , e.g., wherein the use of an edge optic can be avoided, e.g., if a large enough spacing is provided between out-going and in-coming ring power amplification stage beams, e.g., as created by the beam expander,  142  shown in  FIG. 2 , e.g., prisms  146 ,  148 . For example, about a 5 mm spacing between the two beams has been determined to be satisfactory enough to, e.g., to avoid the use of any edge optics. 
   As illustrated by way of example in  FIG. 46  the laser system, e.g., system  110  illustrated by way of example in  FIG. 2 , may produce a laser system output pulse beam  100 , e.g., using a ring power amplification stage  144  to amplify the output beam  62  of a master oscillator  22  in a ring power amplification stage  144 . A beam expander/disperser  142 , shown in more detail by way of an example of aspects of an embodiment of the subject matter disclosed may be comprised of a first expansion/dispersion prism  146   a , and a second expansion/dispersion prism  146   b , and a third prism  148 . 
   The seed injection mechanism  160  may comprise a partially reflective input/output coupler  162 , and a maximally reflective (Rmax) mirror  164 , illustrated by way of example and partly schematically in  FIG. 30  in a plan view, i.e., looking down on the seed injection mechanism and m expansion/dispersion  160  and the ring power amplification stage chamber (not shown) into and out of which, respectively the beams  74  and  72  traverse, that is from the perspective of the axis of the output beam  62  traveling from the master oscillator chamber  22 , which in such an embodiment as being described may be positioned above the chamber  144  (the beam  62  having been folded into the generally horizontal longitudinal axis as shown (the beam also having been expanded in the MOPuS in its short axis, as described elsewhere, to make it generally a square in cross-sectional shape. 
   With regard to the configuration of the beam expansion prisms  146   a ,  146   b  and  148  inside the ring power amplification stage cavity a similar arrangement may be provided to that of the beam expansion on the output of the power amplifier (“PA”) stage in applicants&#39; assignee&#39;s XLA-XXX model laser systems, e.g., with a 4× expansion, e.g., provided by a 68.6° incident and 28.1° exit, e.g. on a single prism or on two prisms with the same incident and exit angles. This can serve to, e.g., balance and minimize the total Fresnel losses. Reflectivity coatings, e.g., anti-reflectivity coatings may be avoided on these surfaces since they will experience the highest energy densities in the system. According to aspects of an embodiment of the subject matter disclosed the beam expander/disperser  160  may be implemented with the first prism  146  split into to small prisms  146   a , and  146   b , which may be, e.g., 33 mm beam expander prisms, e.g., truncated, as shown by way of example in  FIG. 30 , to fit in the place where one similarly angled prism could fit, with the split prism having a number of advantages, e.g., lower cost and the ability to better align and/or steer the beams  72 ,  74  (in combination with the beam reverser (not shown in  FIG. 30 ) and the system output beam  100 . 
   The master oscillator seed beam  62  may enter the seed injection mechanism  160  through the beam splitter partially reflective optical element  162 , acting as an input/output coupler, to the Rmax  164  as beam  62   a , from which it is reflected as beam  74   a  to the first beam expander prism  146   a , which serves to de-magnify the beam in the horizontal axis by about ½× (it remains about 10-11 mm in the vertical axis into the plane of the paper as shown in  FIG. 30 ). The beam  74   b  is then directed to the second beam expansion prism  148 , e.g., a 40 mm beam expansion prism, where it is again de-magnified by about ½× so the total de-magnification is about ¼× to form the beam  74  entering the gain medium of the ring power amplification stage (not shown in  FIG. 30 . the beam is reversed by the beam reverser, e.g., a beam reverser of the type currently used in applicants&#39; assignee&#39;s XLA-XXX model laser system PAs and returns as beam  72  to the prism  148 , e.g., having crossed in the gain medium in a bow-tie arrangement or having traveled roughly parallel, perhaps overlapping to some degree in a version of a race-track arrangement. from prism  148  where the beam  72  is expanded by roughly 2× the beam  72   b  is directed to prism  142   b  and is expanded a further approximately 2× into beam  72   a . Beam  72   a  is partially reflected back to the Rmax as part of beam  62   a  and is partially transmitted as output beam  100 , which gradually increases in energy until an output beam pulse of sufficient energy is obtained by lasing oscillation in the ring power amplification stage. The narrowing of the beam entering the amplification gain medium, e.g., the ring power amplification stage has several advantageous results, e.g., confining the horizontal widths of the beam to about the width of the electrical gas discharge between the electrodes in the gain medium (for a bow-tie arrangement the displacement angle between the two beams is so small that they each essentially stay within the discharge width of a few mm even thought they are each about 2-3 mm in horizontal width and for the race track embodiment, the bean  72  or the bean  72  only passes through the gain medium on each round trip, or the beams may be further narrowed, or the discharge widened, so that both beams  72 , 74  pass through the discharge gain medium in each round trip of the seed beams  72 ,  74 . 
   The positioning and alignment of the prisms  146   a ,  146   b  and  148 , especially  146   a  and  146   b  can be utilized to insure proper alignment of the output beam  100  from the ring power amplification stage into the laser output light optical train towards the shutter. The beam leaving the input/output coupler  162  may be fixed in size, e.g., in the horizontal direction, e.g., by a horizontal size selection aperture  130 , forming a portion of the system aperture (in the horizontal axis) to about 10.5 mm. Another aperture, e.g., in the position roughly of the present PA WEB, e.g., in applicants&#39; assignee&#39;s XLA-XXX laser system products, can size the beam in the vertical dimension. since the beam has about a 1 mRad divergence, the sizing may be slightly smaller in each dimension than the actual beam dimensions wanted at the shutter, e.g., by about 1 mm. According to aspects of an embodiment of the subject matter disclosed applicants propose that a system limiting aperture be positioned just after the main system output OPuS, e.g., a 4× OPus. A ring power amplification stage aperture may be located about 500 mm further inside the laser system. This distance is too great to avoid pointing changes turning into position changes at the specified measurement plane (present system aperture). Instead the limiting system aperture can be located just after the OPuS, and may have a 193 nm reflecting dielectric coating instead of a stainless steel plate commonly used. This design can allow for easier optical alignment, while at the same time reduce heating of this aperture. 
   According to aspects of an embodiment of the subject matter disclosed, applicants propose to implement a relatively stress-free chamber window arrangement similar to or the same as that discussed in an above referenced co-pending U.S. patent application, e.g., at least on the bean reverser side of the chamber, because of the use of, e.g., a PCCF coated window a this location. 
   According to aspects of an embodiment of the subject matter disclosed, applicants propose to, e.g., place ASE detection, e.g., backward propagation ASE detection, in either the LAM or in an MO wavefront engineering box (“WEB”), or in a so-called MOPuS, which can, e.g., include elements of the MOWEB from applicants&#39; assignee&#39;s existing XLA-XXX model laser systems along with the mini-OPuSs discussed elsewhere in this application and in the co-pending application Ser. No. 11/521,834 referenced herein, as well as, e.g., beam expansion, e.g., using one or more beam expansion prisms to expand the output beam of the MO in its short axis, e.g., to form generally a square cross-sectional beam. The current MO WEB and its beam turning function is represented schematically as the turning mirror, e.g.,  44  shown in  FIG. 2 . As a preference, however, the backward propagation detector may be placed “in” the MO WEB/MOPuS, that is, e.g., by employing a folding mirror (fold # 2 ), e.g.,  44  in  FIG. 2 , with, e.g., a reflectivity of R=95% instead of R=100% and monitoring the leakage through this mirror  44 . Some drift and inaccuracy of this reading may be tolerated, e.g., since it may be utilized as a trip sensor (i.e. measurements in the vicinity of 0.001 mJ when conditions are acceptable—essentially no reverse ASE—as opposed to around 10 mJ when not acceptable—there is reverse ASE), e.g., when the ring power amplifier is not timed to amplify the seed pulse, but still creates broad band laser light. Existing controller, e.g., TEM controller, cabling and ports and the like for new detectors may be employed. The detector may, e.g., be the detector currently used by applicants&#39; assignee on existing XLA-XXX model laser systems to measure beam intensity, e.g., at the laser system output shutter. 
   According to aspects of an embodiment of the disclosed subject matter one or more mini-OPuS(s), which may be confocal, such that they are highly tolerant to misalignment and thus of potentially low aberration, e.g., for the off-axis rays needed in the proposed short OPuS(s), the so-called mini-OPuS, can have delay times of 4 ns and 5 ns respectively, where more than one is employed. These values were chosen so that both OPuSs exhibit low wavefront distortion with spherical optics in addition to appropriate delay paths for coherence busting. The low wavefront requirement may actually prevent significant speckle reduction from the mini-OPuS(s) unless an angular fan-out from the output of the mini-OPuS(s) is generated, e.g., by replacing a flat/flat compensating plate with a slightly wedged plate, so that the transmitted beam and the delayed beam in the mini-OPuS are slightly angularly offset from each other. The laser beam, e.g., from the master oscillator is partially coherent, which leads to speckle in the beam. Angularly offsetting the reflected beam(s) reentering the mini-OPuS output with the transmitted beam, along with the delay path separation of the main pulse into the main pulse and daughter pulses, can achieve very significant speckle reduction, e.g., at the wafer or at the annealing workpiece, arising from the reduction in the coherence of the laser light source pulse illuminating the workpiece (wafer or crystallization panel). This can be achieved, e.g., by intentionally misaligning the delay path mirrors, probably not possible with a confocal arrangement, but also with the addition of a slight wedge in the delay path prior to the beam splitter reflecting part of the delayed beam into the output with the transmitted beam and its parent pulse and preceding daughter pulses, if any. For example, a 1 milliradian wedge in the plate will produce an angular offset in the reflected daughter pulse beam of 0.86 milliradians. 
   The optical delay path(s) of the mini-OPuS(s) may have other beneficial results in terms of laser performance and efficiency. According to aspects of an embodiment of the disclosed subject matter, as illustrated schematically in  FIG. 47 , the laser beam, e.g., seed beam  500  from the seed source laser (not shown in  FIG. 47 , may be split into two beams  502 ,  504  using a partially reflective mirror (beam splitter)  510 . This mirror  510  transmits a percentage of the beam into the main beam  502  and reflects the rest of the beam  500  as beam  504  into an optical delay path  506 . The part  502  that is transmitted continues into the rest of the laser system (not shown in  FIG. 47 ). The part  504  that is reflected is directed along a delay path  506  including, e.g., mirrors  512 ,  514  and  516 , with mirror  514  being displaced perpendicularly to the plane of the paper in the schematic illustration, in order to allow the main beam  502  to reenter the rest of the laser system, e.g., to form a laser output beam or for amplification in a subsequent amplification stage. The beam  504  may then be recombined with the transmitted portion  502  of the original beam  500 . The delayed beam  504  may be passed through a wedge (compensator plate)  520  essentially perpendicularly arranged in the path of beam  504 . Thus, the daughter pulse beam(s)  504  from the delay path  506  are slightly angularly displaced from the main part of the beam in the transmitted portion  502  in the far field. the displacement may be, e.g., between about 50 and 500 μRad. 
   The length of the delay path  506  will delay the beam pulses so that there is a slight temporal shift between the part of the beam that is transmitted and the part that is reflected, e.g., more than the coherence length, but much less than the pulse length, e.g., about 1-5 ns. By selecting the appropriate path length, which determines the delay time, the addition of the two beams can be such that the energy in the pulse is spread into a slightly longer T is , which in combination with later pulse stretching in the main OPuS(s) can improve laser performance, as well as providing other beneficial laser performance benefits. 
   Two mini-OPuSs may be needed to achieve the desired effect. The offset time between the pulses from the two mini-OPuSs may be, e.g., one nanosecond. Based upon optical and mechanical considerations, the delays selected for the stretchers may be, e.g., a 3 ns delay path in the first mini-OPuS and a 4 ns delay path in the second. If the delay is shorter, the optical system, e.g., if it uses confocal or spherical mirrors can introduce unacceptable aberrations. If the delay is longer, it may be difficult to fit the system into the available space in the laser cabinet. The distance the beam must travel to achieve the 3 ns delay is 900 mm and to delay by 4 ns is 1200 mm. A confocal optical system  500 , minimizing the sensitivity to misalignment, illustrated schematically in  FIG. 48  may consist of two mirrors  522 ,  524 , whose focal points are located at the same position in space and whose center of curvatures are located at the opposite mirror, along with a beam splitter  526 . A compensator plate  530  (e.g., a wedge) can be added to insure that the reflected beam and the transmitted beam are slightly misaligned as noted above with respect to  FIG. 48 . In this case, the compensator plate is placed in the path of the delayed beam at an angle for proper functioning. 
   The delay path time(s) in the mini-OPuS(s) for coherence busting and other purposes may be as short as about the temporal coherence length and as long as practical due to the noted optical and space considerations, such as misalignment and aberration tolerance. If there are two or more mini-OPuSs then the delay path in each must be different in length, e.g., by more than the coherence length and selected such that there is no significant coherence reaction (increase) due to the interaction of daughter pulses from the separate OPuS(s). For example the delay path times could be separated by at least a coherence length and by not more than some amount, e.g., four or five coherence lengths, depending on the optical arrangement. 
   According to aspects of an embodiment of the subject matter disclosed applicants propose to employ a coherence-busting optical structure that, e.g., generates multiple sub-pulses delayed sequentially from a single input pulse, wherein also each sub-pulse is delayed from the following sub-pulse by more than the coherence length of the light, and in addition with the pointing of each sub-pulse intentionally chirped by an amount less than the divergence of the input pulse. In addition applicants propose to utilize a pair of coherence-busting optical delay structures, where the optical delay time difference between the pair of optical delay structures is more than the coherence length of the input light. Each of the two optical delay structures may also generate sub-pulses with controlled chirped pointing as noted in regard to the aspects of the previously described coherence busting optical delay structure. 
   According to aspects of an embodiment of the disclosed subject matter two imaging mini-OPuSs, which may be confocal, such that they are highly tolerant to misalignment and thus of potentially low aberration, e.g., for the off-axis rays needed in the proposed short OPuSs, the so-called mini-OPuSs, and can have delay times of 4 ns and 5 ns respectively. These values were chosen so that both OPuSs exhibit low wavefront distortion with spherical optics. The low wavefront requirement may prevent significant speckle reduction from the mini-OPuSs unless an angular fan-out from the mini-OPuSs is generated, e.g., by replacing a flat/flat compensating plate with the slightly wedged plate. 
   It will be understood by those skilled in the art that according to aspects of an embodiment of the disclosed subject matter, adequate coherence busting may be achieved sufficiently to significantly reduce the effects of speckle on the treatment of a workpiece being exposed to illumination from the laser system, such as in integrated circuit photolithography photoresist exposure (including the impact on line edge roughness and line width roughness) or laser heating, e.g., for laser annealing of amorphous silicon on a glass substrate for low temperature recrystallization processes. This may be accomplished by, e.g., passing the laser beam, either from a single chamber laser system or from the output of a multi-chamber laser system or from the seed laser in such a multi-chamber laser system before amplification in another chamber of the multi-chamber laser system, through an optical arrangement that splits the output beam into pulses and daughter pulses and recombines the pulses and daughter pulses into a single beam with the pulses and daughter pulses angularly displaced from each other by a slight amount, e.g., between, e.g., about 50 μRad and 500 μRad and with each of the daughter pulses having been delayed from the main pulse(s), e.g., by at least the temporal coherence length and preferably more than the temporal coherence length. 
   This may be done in an optical beam delay path having a beam splitter to transmit a main beam and inject a portion of the beam into a delay path and then recombining the main beam with the delayed beam. In the recombination, the two beams, main and delayed, may be very slightly angularly offset from each other (pointed differently) in the far field, referred to herein as imparting a pointing chirp. The delay path may be selected to be longer than the temporal coherence length of the pulses. 
   The angular displacement may be accomplished using a wedge in the optical delay path prior to the delayed beam returning to the beam splitter which wedge imparts a slightly different pointing to the delayed beam (a pointing chirp). The amount of pointing chirp, as noted above may be, e.g., between about 50 and 500 μRad. 
   The optical delay paths may comprise two delay paths in series, each with a respective beam splitter. In such an event each delay path can be different in length such that there is not created a coherence effect between the main and daughter pulses from the respective delay paths For example, if the delay in the first delay path is 1 ns the delay in the second delay path could be about 3 ns and if the delay in the first delay path is 3 ns the delay in the second could be about 4 ns. 
   The wedges in the two separate delay paths may be arranged generally orthogonally to each other with respect to the beam profile, such that the wedge in the first delay path can serve to reduce coherence (speckle) in one axis and the wedge in the other delay path can reduce coherence (speckle) in the other axis, generally orthogonal to the first. thus, the impact on speckle, e.g., contribution to line edge roughness (“LER”) and/or line width roughness (“LWR”), e.g., at the wafer in exposure of photoresist in an integrated circuit manufacturing process can be reduced along feature dimensions in two different axes on the wafer. 
   According to aspects of an embodiment of the subject matter disclosed, with, e.g., a 6 mrad cross of the bowtie in a bowtie ring power amplification stage, the magnification prisms inside the ring cavity may be slightly different for the in-going and outgoing beams, and could be arranged so that the beam grows slightly as it travels around the ring or shrinks slightly as it travels around the ring. Alternatively, and preferably according to aspects of an embodiment of the subject matter disclosed, a result of breaking the larger beam expansion prism into two separate pieces, e.g., enabled by larger spacing between out-going and in-coming beams, e.g., about 5-6 mm, as illustrated by way of example in  FIG. 30 , applicants propose to adjust the angles of the two prisms, e.g.,  146 ,  148  shown schematically in  FIG. 4 , such that they result in the same magnification for both out-going and in-coming beams, e.g., beams  100  and  62 , respectively, shown illustratively and schematically in  FIG. 30 . 
   According to aspects of an embodiment of the subject matter disclosed applicants propose to place the Rmax, e.g.,  164  and the OC, e.g.,  162  portions of the version of the seed injection mechanism containing an Rmax  164  and an OC  162 , e.g., along with the positioning of the system horizontal axis beam output aperture on that same stage. This enables, e.g., prior alignment of each as an entire unit and removes the need for field alignment of the individual components. This can allow, e.g., for the position of the Rmax/OC assembly, e.g.,  160 , shown in  FIG. 2  (a seed injection mechanism) to be fixed, just like the OC location in a applicants&#39; assignee&#39;s single chamber oscillator systems (e.g., XLS 7000 model laser systems) is fixed. Similarly, such an arrangement can allow for the achievement of tolerances such that the Rmax/OC are positioned relative to the system aperture properly without need for significant ongoing adjustment. The beam expansion prism may be moveable for alignment of the injection seed mechanism assembly with the chamber  144  of the amplification gain medium and the output beam  100  path with the laser system optical axis. 
   According to aspects of an embodiment of the subject matter disclosed applicants propose to employ a coherence-busting optical structure that generates multiple sub-pulses delayed sequentially from a single input pulse, wherein also each sub-pulse is delayed from the following sub-pulse by more than the coherence length of the light, and in addition with the pointing of each sub-pulse intentionally chirped by an amount less than the divergence of the input pulse. In addition applicants propose to utilize a pair of coherence-busting optical delay structures, where the optical delay time difference between the pair of optical delay structures is more than the coherence length of the input light. Each of the two optical delay structures may also generate sub-pulses with controlled chirped pointing as noted in regard to the aspects of the previously described coherence busting optical delay structure. 
   According to aspects of an embodiment of the subject matter disclosed applicants propose to position a mechanical shutter to block the MO output from entering the ring, when appropriate, similar to such as are utilized on applicants&#39; assignee&#39;s OPuSs, e.g., to block them during alignment and diagnosis. The exact location could be, e.g., just above the last folding mirror prior to the ring power amplification stage, where the mini-OPuSes are protected during unseeded ring power amplification stage alignment and operation. 
   Turning now to  FIG. 49  there is shown schematically and in block diagram a laser DUV light source according to aspects of an embodiment of the disclosed subject matter. The system  1300  may include, e.g., a plurality of seed laser systems, which may be solid state lasers,  1302 ,  1304 ,  1306 , for example as described elsewhere in the present application, with the seed laser  1306  being an nth seed laser in the system. for each seed laser there may be a corresponding amplification laser system, e.g.,  1310 ,  1320  and  1330 , with the amplification laser system  1306  being an nth amplification laser system. each amplification laser system  1310 ,  1320 ,  1330  may have a plurality of A, in the illustrative case A=2, amplification gain mediums  1312 ,  1314 , and  1322 ,  1324  and  1332 ,  1334 , with the amplification gain mediums  1332 ,  1334  comprising an exemplary nth amplification gain medium system  1330 . Each gain medium  1312 ,  1314 ,  1322 ,  1324 ,  1332 ,  1334  may comprise a gas discharge laser, such as an excimer or molecular fluorine laser, and more specifically may comprise a ring power amplification stage as described elsewhere in the present application and in above identified co-pending applications filed on the same day as the present application. Each of the respective A amplification gain mediums  1312 ,  1314  and  1322 ,  1324  and  1332 ,  1334  may be supplied with output pulses from the respective seed laser  1302 ,  1304  and  1306  by a beam divider  1308 . The respective amplifier gain mediums  1312 ,  1314 ,  1322 ,  1324  and  1332 ,  1334  may operate at a fraction of the pulse repetition rate X of the respective seed lasers, e.g., A/X. A beam combiner  1340  may combine the outputs of the amplifier gain mediums  1312 ,  1314 ,  1322 ,  1324 ,  1332 ,  1334  to form a laser system  1300  output laser light source beam  100  of pulses at a pulse repetition rate of nX. 
   Turning to  FIG. 50  there is illustrated schematically and in block diagram form a laser system  1350  according to aspects of an embodiment of the disclosed subject matter. which may comprise a plurality of seed lasers  1352   a ,  1352   b  and  1352   c  which may be solid state lasers,  1352   a ,  1352   b ,  1352   c , for example as described elsewhere in the present application, with the seed laser  1352   c  being an nth seed laser in the system  1450 . Each of the seed lasers may feed a pair of respective amplifier gain mediums  1356 ,  1358 ,  1360 ,  1362  and  1364 ,  1366 , with the amplifier gain mediums  1364 ,  1366  being the nth pair in the system  1350 , corresponding to the nth seed laser  1352   c , with a respective beam divider  1354 . Each amplification gain medium may be a gas discharge laser, such as an excimer or molecular fluorine laser, and more specifically may comprise a ring power amplification stage as described elsewhere in the present application and in above identified co-pending applications filed on the same day as the present application. Each of the pairs of amplification gain mediums  1356 ,  1358 ,  1360 ,  1362 , and  1364 ,  1366  may operate at ½ the pulse repetition rate X of the respective seed laser  1252   a ,  1352   b  and  1352   c , with the seed lasers  1352   a ,  1352   b  and  1352   c  all operating at the same pulse repetition rate X, to produce a laser light source output light beam of pulses  100  at nX, or each may operate at a respective pulse repetition rate X, X′, X″ . . . X n′  some but not all of which may be equal to others, such that the output pulse rate in the output pulse beam  100  is ΣX′+X″ . . . X n , through a beam combiner  1370 . 
   It will be understood by those skilled in the art that disclosed in the present application is a method and apparatus which may comprise: a very high power line narrowed lithography laser light source, e.g., a 200 w or above laser light source, which may comprise: a solid state seed laser system which may comprise: a pre-seed laser providing a pre-seed laser output; a fiber amplifier receiving the pre-seed laser output and providing an amplified seed laser pulse which may comprise: a pulse having a nominal wavelength outside of the DUV range; a frequency converter such as a non-linear optical device that creates a second harmonic or frequency shifts with the fundamental or other frequency converted harmonics, modifying the nominal center wavelength of the output of the seed laser system to essentially the nominal center wavelength of the amplifier gain medium; a first and a second gas discharge laser amplifier gain medium each operating at a different pulse repetition rate from that of the seed laser output; a beam divider providing each respective first and second amplifier gain medium with output pulses from the seed laser; a beam combiner combining the outputs of each respective amplifier gain medium to provide a laser output light pulse beam having the pulse repetition rate of the solid state seed laser system. The seed laser may be selected from the group comprising Nd +3  doped fiber lasers and Yb +3  doped fiber lasers. The seed laser may comprise an Er:Yb predoped laser. The pre-seed laser may comprise a continuous wave laser or a pulsed laser. The amplifier laser may comprise a pulse amplifier laser. The solid state seed laser system may be tunable in nominal center wavelength and/or may have a plurality of nominal center wavelength operating points. The frequency converter may comprise a single or multiple non-linear frequency conversion stage. The amplifier gain medium may be selected from the group comprising: XeF, XeCl, KrF, ArF and F 2  gas discharge lasers. The apparatus and method may comprise a coherence busting mechanism sufficiently destroying the coherence of the output of the seed laser or the outputs of the amplifier gain mediums or both to reduce speckle effects in a processing tool using the light from the laser system. The coherence busting mechanism may comprise a first axis coherence busting mechanism and a second axis coherence busting mechanism. The coherence busting mechanism may comprise a beam sweeping mechanism such as a stimulated beam modulator or a tiltable mirror. The beam sweeping mechanism may be driven in one axis by a first time varying actuation signal. The beam sweeping mechanism may be driven in another axis by a second time varying actuation signal. The first actuation signal may comprise a ramp signal and the second actuation signal comprising a sinusoid. The time varying signal may have a frequency such that at least one full cycle occurs within the time duration of a seed laser output pulse. The coherence busting mechanism may comprise an optical delay path with misaligned optics. The misaligned optics may produce a hall of mirrors effect. The coherence busting mechanism may comprise an optical delay path longer than the coherence length of the seed laser output pulse and a beam pointing angle offset mechanism. The coherence busting mechanism may comprise an active optical coherence busting mechanism and a passive optical coherency busting mechanism, such as a delay path and/or a beam pointing displacement mechanism. The active coherence busting mechanism may comprise a beam sweeping device and the passive coherence busting mechanism comprising, e.g., an optical delay path and/or a beam pointing angle displacement mechanism. The coherence busting mechanism may comprise: a first optical delay path with a delay longer than the coherence length of the seed laser output pulse and a second optical delay path in series with the first optical delay path and having a delay path longer than the coherence length of the seed laser output pulse and different from the delay path of the first optical delay path; a beam angular offset mechanism. The delay of the second optical delay path may be greater than the delay path of the first optical delay path. The coherence busting mechanism may comprise a pulse stretcher. The pulse stretcher may comprise a negative imaging optical delay path. The pulse stretcher may comprise a confocal mirror OPuS. The coherence busting mechanism may comprise beam flipping mechanism. The apparatus and method may comprise a very high power broad band lithography laser light source which may comprise: a solid state seed laser system which may comprise: a pre-seed laser providing a pre-seed laser output; a fiber amplifier receiving the pre-seed laser output and providing an amplified seed laser pulse which may comprise: a nominal wavelength outside of the DUV range; a frequency converter modifying the nominal center wavelength of the output of the seed laser system to essentially the nominal center wavelength of the amplifier gain medium; a first and a second gas discharge laser amplifier gain medium each operating at one half of the pulse repetition rate of the seed laser output; a beam divider providing each respective first and second amplifier gain medium with alternating output pulses from the seed laser; a beam combiner combining the outputs of each respective amplifier gain medium to provide a light source laser output light pulse beam having the pulse repetition rate of the solid state seed laser. The method and apparatus may comprise: a very high power lithography laser light source which may comprise: a solid state seed laser system having a nominal wavelength outside of the DUV range and a pulse repetition rate of at least 10 kHz; a first and a second gas discharge laser amplifier gain medium each operating at a different pulse repetition rate than the pulse repetition rate of the seed laser output; a beam divider providing each respective first and second amplifier gain medium with output pulses from the seed laser; a frequency converter modifying the nominal center wavelength of the output of the seed laser system to essentially the nominal center wavelength of the amplifier gain medium; a beam combiner combining the outputs of each respective amplifier gain medium to provide a laser output light pulse beam having the pulse repetition rate of the solid state seed laser. The solid state seed laser may be tunable in nominal center wavelength and/or has a plurality of nominal center wavelength operating points. The apparatus and method may comprise: a DUV pulsed lithography laser light source which may comprise: a solid state seed laser producing a pulsed solid state laser output beam; a DUV gas discharge laser amplifier gain medium; a converter converting the output of the solid state seed laser to a wavelength within a range of wavelengths in which the gain medium is effective; a coherence busting mechanism intermediate the converter and the gain medium comprising an optical delay path separating the seed pulse into a main pulse and separate daughter pulses separated by at least the coherence and an angular beam displacement mechanism displacing the pointing angle of the main pulse and at least one of the daughter pulses. 
   Applicants have simulated through calculations speckle reduction as relates to the location of coherence lengths within a single gas discharge (e.g., ArF or KrF excimer) laser system output pulse after such a pulse has passed through the two OPuS pulse stretchers sold on laser systems manufactured by applicants&#39; assignee Cymer, Inc., used for pulse stretching to increase the total integrated spectrum (T is ) to reduce the impact of peak intensity in the laser output pulse on the optics in the tool using the output light from the laser system, e.g., a lithography tool scanner illuminator. There are two OPuS in series, with the first having a delay path sufficient to stretch the T is  of the output pulse from about 18.6 ns to about 47.8 ns and the second to stretch the pulse further to about 83.5 ns, e.g., measured at E955 (the width of the spectrum within which is contained 95% of the energy of the pulse. 
   Starting with the unstretched pulse, applicants divided the pulse into portions equal to the approximate coherence length, assuming a FWHM bandwidth of 0.10 pm and a Gaussian shape for the coherence length function. The impact of the pulse stretching on the coherence length portions of the pulse after passing through the first OPuS was to show that a first intensity hump in the spectrum of the stretched pulse was made up of the coherence length portions of the main pulse, a second intensity hump was pad up of coherence length portions of the main pulse overlapped with coherence length portions of a first daughter pulse. A third hump in the intensity spectrum is the result of overlapping of the first and second daughter pulses. Looking at the individual coherence length portions of the two humps applicants observed that the multiple versions (including daughters) of the coherence length portions remained sufficiently separated to not interfere with each other. 
   After passage through the second OPuS the simulated spectra, again only looking at the content of the first three humps in the stretched pulse, in the simulation (under the second hump were contributions from the original undelayed pulse, as before, the first delayed pulse from the first OPuS, as before and the first delayed pulse from the second OPuS), applicants observed that in this second pulse the multiple versions of the coherence length portions were very close together. This is caused by the fact that the first OPuS has a delay of ˜18 ns and the second has a delay of ˜22 ns. Thus only ˜4 ns separates the versions of the coherence length portions, which is still not close enough for interference. 
   Under the third hump applicants observed contributions from the first delayed pulse from the first OPuS, the second delayed pulse from first OPuS, the first delayed pulse from the second OPuS, and the second delayed pulse from second OPuS. applicants observed that the separation between some related coherence portions is larger than for others in the third hump in the intensity spectrum of the pulse stretched by two OPuSs. This increase in separation is due to the fact that two round trips through each OPuS equal ˜36 ns=18*2 and ˜44 ns=22*2. Thus the separation between coherence lengths grows with each round trip. 
   Applicants concluded that for a mini-OPuS as described in this application a single mini-OPuS with delay equal to one coherence length will create a train of pulses that dies out after about 4 coherence length values. Thus, applicants determined that for a single mini-OPuS to be effective, the two main OPuSs should not bring any daughter coherence lengths to within 4 coherence lengths of each other. but, applicants have observed in the simulation that the main OPuSs do just that, though only marginally so. The separation between coherence lengths for the third and greater humps is sufficient. Applicants believe that the impact of a single mini-OPuS between MO and amplification gain medium will be nearly the full expected coherence busting effect. A second mini-OPuS between MO and PA may not adequately interact with the two main OPuss. The empty spaces, not filled with related coherence length portions of the spectra pulse humps get more scarce when one combines a single min-OPuS and two regular OPuSs, and the second may be too much. according to aspects of an embodiment of the present invention applicants propose the coordinated change of the regular OPuS delay lengths when the mini-OPuS(s) are installed, including whether they are part of the laser system or installed down stream of the regular main OPuSs, e.g., in the lithography tool itself. Applicants believe that such mini-OPuS(s) can fill in the valleys of the pulse duration somewhat, leading to an increase in T is , e.g., allowing a reduction in the delay lengths of one of the two main OPuSes for better overall coherence length separation. 
   It will be understood by those skilled in the art that the aspects of embodiments of the disclosed subject matter disclosed above are intended to be preferred embodiments only and not to limit the disclosure of the disclosed subject matter(s) in any way and particularly not to a specific preferred embodiment alone. Many changes and modification can be made to the disclosed aspects of embodiments of the disclosed invention(s) that will be understood and appreciated by those skilled in the art. The appended claims are intended in scope and meaning to cover not only the disclosed aspects of embodiments of the disclosed subject matter(s) but also such equivalents and other modifications and changes that would be apparent to those skilled in the art. In additions to changes and modifications to the disclosed and claimed aspects of embodiments of the disclosed subject matter(s) noted above others could be implemented. 
   While the particular aspects of embodiment(s) of the LASER SYSTEM described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. §112 is fully capable of attaining any above-described purposes for, problems to be solved by or any other reasons for or objects of the aspects of an embodiment(s) above described, it is to be understood by those skilled in the art that it is the presently described aspects of the described embodiment(s) of the disclosed subject matter are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the disclosed subject matter. The scope of the presently described and claimed aspects of embodiments fully encompasses other embodiments which may now be or may become obvious to those skilled in the art based on the teachings of the Specification. The scope of the present LASER SYSTEM is solely and completely limited by only the appended claims and nothing beyond the recitations of the appended claims. Reference to an element in such claims in the singular is not intended to mean nor shall it mean in interpreting such claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to any of the elements of the above-described aspects of an embodiment(s) that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Any term used in the specification and/or in the claims and expressly given a meaning in the Specification and/or claims in the present application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as any aspect of an embodiment to address each and every problem sought to be solved by the aspects of embodiments disclosed in this application, for it to be encompassed by the present claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element in the appended claims is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”. 
   It will be understood also be those skilled in the art that, in fulfillment of the patent statutes of the United States, applicant(s) has disclosed at least one enabling and working embodiment of each invention recited in any respective claim appended to the Specification in the present application and perhaps in some cases only one. For purposes of cutting down on patent application length and drafting time and making the present patent application more readable to the inventor(s) and others, applicant(s) has used from time to time or throughout the present application definitive verbs (e.g., “is”, “are”, “does”, “has”, “includes” or the like) and/or other definitive verbs (e.g., “produces,” “causes” “samples,” “reads,” “signals” or the like) and/or gerunds (e.g., “producing,” “using,” “taking,” “keeping,” “making,” “determining,” “measuring,” “calculating” or the like), in defining an aspect/feature/element of, an action of or functionality of, and/or describing any other definition of an aspect/feature/element of an embodiment of the subject matter being disclosed. Wherever any such definitive word or phrase or the like is used to describe an aspect/feature/element of any of the one or more embodiments disclosed herein, i.e., any feature, element, system, sub-system, component, sub-component, process or algorithm step, particular material, or the like, it should be read, for purposes of interpreting the scope of the subject matter of what applicant(s) has invented, and claimed, to be preceded by one or more, or all, of the following limiting phrases, “by way of example,” “for example,” “as an example,” “illustratively only,” “by way of illustration only,” etc., and/or to include any one or more, or all, of the phrases “may be,” “can be”, “might be,” “could be” and the like. All such features, elements, steps, materials and the like should be considered to be described only as a possible aspect of the one or more disclosed embodiments and not as the sole possible implementation of any one or more aspects/features/elements of any embodiments and/or the sole possible embodiment of the subject matter of what is claimed, even if, in fulfillment of the requirements of the patent statutes, applicant(s) has disclosed only a single enabling example of any such aspect/feature/element of an embodiment or of any embodiment of the subject matter of what is claimed. Unless expressly and specifically so stated in the present application or the prosecution of this application, that applicant(s) believes that a particular aspect/feature/element of any disclosed embodiment or any particular disclosed embodiment of the subject matter of what is claimed, amounts to the one an only way to implement the subject matter of what is claimed or any aspect/feature/element recited in any such claim, applicant(s) does not intend that any description of any disclosed aspect/feature/element of any disclosed embodiment of the subject matter of what is claimed in the present patent application or the entire embodiment shall be interpreted to be such one and only way to implement the subject matter of what is claimed or any aspect/feature/element thereof, and to thus limit any claim which is broad enough to cover any such disclosed implementation along with other possible implementations of the subject matter of what is claimed, to such disclosed aspect/feature/element of such disclosed embodiment or such disclosed embodiment. Applicant(s) specifically, expressly and unequivocally intends that any claim that has depending from it a dependent claim with any further detail of any aspect/feature/element, step, or the like of the subject matter of what is claimed recited in the parent claim or claims from which it directly or indirectly depends, shall be interpreted to mean that the recitation in the parent claim(s) was broad enough to cover the further detail in the dependent claim along with other implementations and that the further detail was not the only way to implement the aspect/feature/element claimed in any such parent claim(s), and thus be limited to the further detail of any such aspect/feature/element recited in any such dependent claim to in any way limit the scope of the broader aspect/feature/element of any such parent claim, including by incorporating the further detail of the dependent claim into the parent claim.