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Power scaling of diffusion-cooled lasers
Optics & Laser Technology 30 (1998) 331±336
Power scaling of diusion-cooled lasers H.J.J. Seguin * Department of Electrical Engineering, 238 Civil/Electrical Building, University of Alberta, Edmonton, Alta., Canada T6G 2G7 Received 29 June 1998; received in revised form 23 September 1998; accepted 24 September 1998
Abstract An excitation and optical extraction geometry suitable for compact high power gas lasers is described. Multiple slab discharges are established in a diusion-cooled radial electrode array. Each gain channel is independently driven from a common RF source via a resonant-cavity power distribution system. Radial excitation augmented with multi-channel selfinjection phase locking provides stable optical power extraction at good eciency. The concept is easily scalable to very high powers while dramatically reducing unit size and cost. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Radial array laser; High power slab CO2 laser; Multi-channel waveguide laser
1. Introduction Most lasers above 10 kW operate at relatively low wall-plug eciencies of 10±15%. Because of this fact, heat rejection considerations pervade their design. A prime example is the carbon dioxide system which is dominated by massive convective cooling apparatus. Fortunately, this age old restriction may now may be completely negated via a multi-channel, diusioncooled approach.
into each gain channel without discharge interaction and ecient beamlet extraction and compaction. These important operational features may be achieved by employing resonant-cavity, magnetic-loop coupling and injection-locking techniques in a physical geometry resembling the Zodiac astrological symbol. The device is scalable from very low, up to extremely high powers in a compact, light-weight, sealed-o enclosure.
3. Demonstration 2. The radial array This new methodology embodies a plurality of radially mounted slab discharges having a common optical extraction system [1±4]. Each narrow-gap gain channel generates an optical beamlet which is then combined into a single beam. An order of magnitude reduction in system size and weight may thus be achieved. When phase-locked, the focused intensity of the composite beam increases quadratically with the number of channels in the array, thereby yielding a device amenable to deep-penetration cutting and welding. Array performance is primarily a function of 2 parameters, namely: uniform deposition of RF energy * Tel.: +1-403-492-3335; fax: +1-403-492-1811 0030-3992/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 0 - 3 9 9 2 ( 9 8 ) 0 0 0 6 2 - 0
Proof-of-scaling was demonstrated by a Zodiac laser fabricated from 24 aluminum electrodes, depicted in Fig. 1. 5 cm wide extruded elements were assembled azimuthally with a 3 mm gap on insulating ceramic support rings. Symmetric double-sided discharge heating and ecient internal cooling inherent in the structure, negates instabilities in excitation and optical extraction. The radial array con®guration also provides enhanced beam symmetry and reduction in focal spot size due to a bene®cial `radial beam stacking' phenomenon [5]. This unit was designed for operation at elevated power levels. However, limited RF drive has so far restricted optical output to about 3.5 kW with 14% eciency. This data was derived with only 20% of drive capacity, thereby predicting optical extraction near 15 kW, when a suciently powerful RF generator
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H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Fig. 1. 24 channel Zodiac laser driven by resonant cavity power splitter system. 1 and 2 = toric resonator mirrors, 3 = water cooling manifold, 4 = Rf magnetic coupling loops, 5 = Rf resonant cavity, 6 = extruded electrodes, 7 = coaxial feeder line, 8 = RF generator, 9 = input Rf capacitive coupler.
becomes available. It is particularly noteworthy that the overall dimensions and weight of the structure are only 40 125 cm and 100 kG, respectively. 4. RP power division Since extended-area slab discharges exhibit low impedance, the array of Fig. 1 constitutes a heavy combined RF load. This feature, plus the fact that each electrode needs independent drive, ostensibly requires 48 RF matching networks to interface the array with the RF source. Fortunately, a simple, low cost solution to this formidable problem was devised by means a of resonant RF cavity, featuring multichannel magnetic-loop coupling. The technique embodies a thin-wall, metallic RF structure (cavity) formed from a half-wavelength section of short-circuited co-axial transmission line. When resonant at the typical 100 MHz operating frequency, the cavity functions as a multi-channel power splitter and impedance matcher for each electrode comprising the array [6]. The TEM00 electro-magnetic cavity mode supports a radial electric ®eld component, maximum at the mid-plane and decreasing to zero at the shorted ends and an azimuthal magnetic ®eld having the opposite spatial dependence. At resonance, the cavity exhibits a high impedance at midsection but low impedance at both ends. Consequently the device may be used as a transformer having an impedance transformation ratio equal to Qu/QL [7]. Qu and QL are, respectively, the `unloaded and loaded' Qs of the structure. Energy transformation
eciency is also a function of the loaded/unloaded Q ratio and can approach 100% under practical conditions [8]. Fig. 1 further reveals that the cavity fully encloses the radial array and so provides a compact RF driver for all discharge channels simultaneously. Excitation energy from an external RF source is supplied through co-axial lines at the cavity midsection. Alternate-polarity excitation for each electrode element is achieved via multiple magnetic-loops ®tted through the shortcircuited end faces. Since the low impedance magnetic coupling loops act as voltage sources, matching elements are not required to drive the multiple discharge channels. Loops and electrodes are cooled by re-circulating water from manifolds ®tted at each end.
Fig. 2. Toric resonator. 28 = radial multi-channel array gain media, 44 = output window, 48 and 50 = toric resonator mirrors, 46 = multiple beamlet output.
H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
5. Optical resonators Although the symmetrical radial construction provides wide ¯exibility in optical extraction, only the toric and unstable resonators of Figs 2 and 3 have thus far been implemented. As evident in Fig. 2, the toric con®guration provides beamlet extraction at the centerline [9]. Conversely, an unstable optical system extracts beamlets at the periphery [10], which are then combined into a single beam by a compactor. The axicon in Fig. 3 provides a simple solution to this task. 6. Phase-locking It is well known that brightness of a composite beam is maximized when all channels are phase-locked and several methodologies have been advanced to achieve this condition [11]. Although, phase-locking strategies usually are dicult to implement, the radial geometry is ideally suited to injection locking or MOPA operation. Since, all beamlets share the same optical extraction system, uni-phase-operation via external oscillator injection can be accomplished with either toric or unstable resonators. Alternatively, selfinjection phase-locking may be easily realized by employing the central region of the array as a core oscillator [12]. Phase-locking is enhanced by conditioning electrode surfaces to suppress any polarization preference [13]. Anodizing the extruded aluminum electrodes is a convenient way to achieve this condition.
Thus, the optical power extractable from such a structure may be increased by using more and/or wider electrodes. Fig. 4 illustrates the approach for a powerful Zodiac having 48, 10 cm wide electrodes. This 100 kW machine, only 45 cm in diameter, about 2 m long and weighing 150 kG would represent a signi®cant advance in very high power CO2 laser construction. 8. Super high power radial arrays It follows that still greater optical powers may be derived from compound structures, such as the 500slab, `triple radial array' of Fig. 5. This device proposed in Fig. 6, employs several ceramic vacuum tubes to pump RF energy directly into the power splitting resonant cavity. A lightweight and highly mobile unit only 1 m in diameter by 2 m long could generate an incredible optical output near 1 MW and thereby open new avenues for very-heavy-section material processing. 9. Beam characteristics Slab gain media intrinsically generate asymmetric beams. Consequently, orthogonal output beam divergence compensation is usually implemented to achieve maximal eectiveness in materials processing sequences. Composite beam nonuniformity is another
7. High power radial arrays In a radial array, each gain channel and corresponding beamlet is eectively in parallel with all others.
Fig. 3. Unstable resonator. 28 = radial multi-channel array gain media, 38 and 40 = unstable resonator mirrors, 42 = beamlet compacting axicon, 44 = output window, 46 = multiple beamlet output.
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Fig. 4. 48-slab, 100 kW Zodiac.
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H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Fig. 5. 500-slab `triple' Zodiac.
problem often encountered when combining conventional multiple slab laser beams to enhance overall power. Fortunately, the radial structure does not present such diculties due to an inherent radial beam stacking compensation eect. This useful phenomenon is simulated in the sequence of pro®les of Fig. 7. When 2 beamlets in Fig. 7(a) are stacked in parallel the composite intensity pro®le exhibits enormous asymmetry. However, Fig. 7(b) and (c) reveal that beam quality is rapidly improved when constituted from a multiplicity of stacked identical beamlets averaged around the periphery. Further insight on beam quality attainable from a radial array is provided by the sequence of diagrams in Fig. 8. These pro®les are computer predictions of near and far ®eld intensity distributions for both phase-locked and non-phase-locked operation [14]. It
Fig. 6. 500 channel 1 MW compound radial array laser.
Fig. 7. Computer simulation of radial beam stacking. A = output beam intensity pro®le for a single beamlet, B = output bean intensity pro®le for 2 stacked beamlets, C = output beam intensity pro®le for 4 stacked beamlets.
is again apparent that much smaller focal spots are obtained with phase-locking, such as by Zodiac selfinjection.
Fig. 8. Computer modeling of composite pro®les and beam quality. A and B = near ®eld and far ®eld output intensity pro®les for 8 nonphase locked beamlets, C and D = near ®eld and far ®eld output intensity pro®les for 8 phase-locked beamlets.
H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Fig. 9. Computed quality parameters of composite Zodiac beams. a = beamlet major axis halfwidth, b = beamlet minor axis halfwidth, r = output beamlet compaction radius, AR = a/b = beamlet aspect ratio.
The curves in Fig. 9 provide additional optical quality information in terms of the composite beam M2r achievable under both phase-locked and non-phaselocked conditions. The computer data reveals that best performance is obtained when a large number of closely spaced beamlets, each with an aspect ratio near unity, are combined. Thus, the design of a Zodiac laser should embody many channels with azimuthal beamlet compensation and near 100% ®ll factor radial compaction. Fig. 10 features an optical radial beam stacker [15], which accomplishes the tasks implicit in the computer simulations of Fig. 9. The device ®rst azimuthally expands and re-collimates each phase-locked beamlet,
335
extracted at the periphery of an unstable resonator (view C±C) into a unity aspect ratio. All beamlets are then stacked close-packed into a continuous annulus (view B±B) and ®nally radially compacted into a single solid pro®le (view A±A). In this manner, the beam stacker provides an aperture-®lling-factor near unity and a concomitant M2r of about 1.5. Further improvement in M2r , approaching 1, may be obtained through the addition of a spatial ®lter. Although the optical functions described above can be derived solely with the mechanical optical compaction apparatus of Fig. 10, the same process is achievable through diractive optic techniques [16]. This methodology featuring an amplitude-compensator followed by a phase-compensator, constructed via etched binary optical techniques, may ultimately be more ecient and cost-eective. 10. Conclusions Experiments suggest that power scaling of a compact laser from a few hundred watts up to several hundred kilowatts, is readily achievable using an RFdriven diusion-cooled radial slab array methodology. Stacking the many beamlets generated by such an array yields a clean and symmetric focal spot. Any resonator misalignment experienced during operation has a minimal eect on composite beam focal parameters and output power. Even a non-phase-locked Zodiac yields a symmetric non-polarized output which functions well in welding and cutting applications. Implementation of self-injection phase-locking can provide near diraction limited performance.
Fig. 10. Mechanical radial beam stacker. 1 and view C±C = beamlets entering stacker, 2 = resonator output mirror, 3 and 5 and view B±B = beamlet aspect ratio compensator, 6 and 7 = beamlet compacting axicon, 8 = output window, 9 and view A±A = stacked output beam, 10 = water cooled beam compensating and stacking diamond machined optics.
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H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Acknowledgements The author gratefully acknowledges the continuing ®nancial support of NSERC and assistance in the design, construction and testing of the scienti®c apparatus used during the course of this investigation, rendered by W. Bilida, J. Strohschein, H. Reshe, C. Rogozinski, V. Pohnert, H. Drexel G. Fye and D. Presakarchuk. References [1] Abramski R et al. 2-Dimensional wave-guide CO2 laser arrays and beam reformatting. In XI Int. Sym. Gas Flow and Chemlasers. Heriot-watt University, Edinburgh, Scotland, August 1996. p. 24. [2] Lapucci A, Cangioli G. Triple slab radio frequency discharge pumped CO2 laser. Appl Phys Lett 1993;62(1):7±9. [3] Abramski KM, Colley AD, Baker HJ, Hall R. IEEE J Quantum Electron 1996;32:340. [4] Yeldon EF, Seguin HJJ, Capjack CE, Nikumb SK. Multichannel slab discharge for CO2 laser excitation. Appl Phys Lett 1991;58(7):693±5. [5] Yelden EF, Scott SW, Strohschein JD, Seguin HJJ, Capjack CE. Symmetry enhancement and spot-size requction through radial beam stacking in a multichannel CO2 laser array. IEEE J Quantum Electron 1994;QE-30(8):1868±75.
[6] Bilida WD, Seguin HJJ, Capjack CE. Multi-channel CO2 laser eccitation with resonant cavities. Opt Laser Technol 1996;28(6):431±6. [7] Terman FE. Simple-resonant circuits, section 3. Engineers handbook. NY: McGraw Hill, 1943. p. 135. [8] Hamilton DR, Knipp JK, Horner Kuper JB. Klystrons and microwave triodes, MIT Rad. Lab. series, vol. 7. NY, McGraw Hill, 1948. p. 174. [9] Yeldon EF, Seguin HJJ, Capjack CE, Nikumb SK, Reshef H. Toric unstable CO2 laser resonator: an experimental study. Appl Opt 1992;31(12):1965±74. [10] Yeldon EF, Seguin HJJ, Capjack CE, Nikumb SK, Reshe R. Multichannel laser resonators: an experimental study. Opt Quantum Electron 1992;24:889±902. [11] Hagemeier HE, Robinson SR. Field properties of multiple coherently combined lasers. Appl Opt 1979;18:270. [12] Yeldon EF, Seguin HJJ, Capjack CE, Reshe H. Phase locking in a multichannel radial array CO2 laser. Appl Phys Lett 1992;62(12):1311±3. [13] Yeldon EF, Segnin HJJ, Capjack CE, Reshe H. Phase-locking phenomenon in a radial multislot CO2 laser array. J Opt Soc Am B 1993;10(8):1474±82. [14] Strohschein JD, Seguin HJJ, Capjack CE. Beam propagation constants for a radial laser array. Appl Opt 1998;37(6):1045± 1048. [15] Seguin HJJ. US patent #5,648,980, July 15, 1997. [16] Williams SW, Mansden PJ, Roberts NC, Venables MA. Excimer laser beam shaping and material processing using diffractive optics. In XI Int. Sym. Gas-¯ow and Chem-lasers. Heriot±Watt University, Edinburgh, Scotland, 1996. p. 58.
Power scaling of diusion-cooled lasers H.J.J. Seguin * Department of Electrical Engineering, 238 Civil/Electrical Building, University of Alberta, Edmonton, Alta., Canada T6G 2G7 Received 29 June 1998; received in revised form 23 September 1998; accepted 24 September 1998
Abstract An excitation and optical extraction geometry suitable for compact high power gas lasers is described. Multiple slab discharges are established in a diusion-cooled radial electrode array. Each gain channel is independently driven from a common RF source via a resonant-cavity power distribution system. Radial excitation augmented with multi-channel selfinjection phase locking provides stable optical power extraction at good eciency. The concept is easily scalable to very high powers while dramatically reducing unit size and cost. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Radial array laser; High power slab CO2 laser; Multi-channel waveguide laser
1. Introduction Most lasers above 10 kW operate at relatively low wall-plug eciencies of 10±15%. Because of this fact, heat rejection considerations pervade their design. A prime example is the carbon dioxide system which is dominated by massive convective cooling apparatus. Fortunately, this age old restriction may now may be completely negated via a multi-channel, diusioncooled approach.
into each gain channel without discharge interaction and ecient beamlet extraction and compaction. These important operational features may be achieved by employing resonant-cavity, magnetic-loop coupling and injection-locking techniques in a physical geometry resembling the Zodiac astrological symbol. The device is scalable from very low, up to extremely high powers in a compact, light-weight, sealed-o enclosure.
3. Demonstration 2. The radial array This new methodology embodies a plurality of radially mounted slab discharges having a common optical extraction system [1±4]. Each narrow-gap gain channel generates an optical beamlet which is then combined into a single beam. An order of magnitude reduction in system size and weight may thus be achieved. When phase-locked, the focused intensity of the composite beam increases quadratically with the number of channels in the array, thereby yielding a device amenable to deep-penetration cutting and welding. Array performance is primarily a function of 2 parameters, namely: uniform deposition of RF energy * Tel.: +1-403-492-3335; fax: +1-403-492-1811 0030-3992/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 0 - 3 9 9 2 ( 9 8 ) 0 0 0 6 2 - 0
Proof-of-scaling was demonstrated by a Zodiac laser fabricated from 24 aluminum electrodes, depicted in Fig. 1. 5 cm wide extruded elements were assembled azimuthally with a 3 mm gap on insulating ceramic support rings. Symmetric double-sided discharge heating and ecient internal cooling inherent in the structure, negates instabilities in excitation and optical extraction. The radial array con®guration also provides enhanced beam symmetry and reduction in focal spot size due to a bene®cial `radial beam stacking' phenomenon [5]. This unit was designed for operation at elevated power levels. However, limited RF drive has so far restricted optical output to about 3.5 kW with 14% eciency. This data was derived with only 20% of drive capacity, thereby predicting optical extraction near 15 kW, when a suciently powerful RF generator
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H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Fig. 1. 24 channel Zodiac laser driven by resonant cavity power splitter system. 1 and 2 = toric resonator mirrors, 3 = water cooling manifold, 4 = Rf magnetic coupling loops, 5 = Rf resonant cavity, 6 = extruded electrodes, 7 = coaxial feeder line, 8 = RF generator, 9 = input Rf capacitive coupler.
becomes available. It is particularly noteworthy that the overall dimensions and weight of the structure are only 40 125 cm and 100 kG, respectively. 4. RP power division Since extended-area slab discharges exhibit low impedance, the array of Fig. 1 constitutes a heavy combined RF load. This feature, plus the fact that each electrode needs independent drive, ostensibly requires 48 RF matching networks to interface the array with the RF source. Fortunately, a simple, low cost solution to this formidable problem was devised by means a of resonant RF cavity, featuring multichannel magnetic-loop coupling. The technique embodies a thin-wall, metallic RF structure (cavity) formed from a half-wavelength section of short-circuited co-axial transmission line. When resonant at the typical 100 MHz operating frequency, the cavity functions as a multi-channel power splitter and impedance matcher for each electrode comprising the array [6]. The TEM00 electro-magnetic cavity mode supports a radial electric ®eld component, maximum at the mid-plane and decreasing to zero at the shorted ends and an azimuthal magnetic ®eld having the opposite spatial dependence. At resonance, the cavity exhibits a high impedance at midsection but low impedance at both ends. Consequently the device may be used as a transformer having an impedance transformation ratio equal to Qu/QL [7]. Qu and QL are, respectively, the `unloaded and loaded' Qs of the structure. Energy transformation
eciency is also a function of the loaded/unloaded Q ratio and can approach 100% under practical conditions [8]. Fig. 1 further reveals that the cavity fully encloses the radial array and so provides a compact RF driver for all discharge channels simultaneously. Excitation energy from an external RF source is supplied through co-axial lines at the cavity midsection. Alternate-polarity excitation for each electrode element is achieved via multiple magnetic-loops ®tted through the shortcircuited end faces. Since the low impedance magnetic coupling loops act as voltage sources, matching elements are not required to drive the multiple discharge channels. Loops and electrodes are cooled by re-circulating water from manifolds ®tted at each end.
Fig. 2. Toric resonator. 28 = radial multi-channel array gain media, 44 = output window, 48 and 50 = toric resonator mirrors, 46 = multiple beamlet output.
H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
5. Optical resonators Although the symmetrical radial construction provides wide ¯exibility in optical extraction, only the toric and unstable resonators of Figs 2 and 3 have thus far been implemented. As evident in Fig. 2, the toric con®guration provides beamlet extraction at the centerline [9]. Conversely, an unstable optical system extracts beamlets at the periphery [10], which are then combined into a single beam by a compactor. The axicon in Fig. 3 provides a simple solution to this task. 6. Phase-locking It is well known that brightness of a composite beam is maximized when all channels are phase-locked and several methodologies have been advanced to achieve this condition [11]. Although, phase-locking strategies usually are dicult to implement, the radial geometry is ideally suited to injection locking or MOPA operation. Since, all beamlets share the same optical extraction system, uni-phase-operation via external oscillator injection can be accomplished with either toric or unstable resonators. Alternatively, selfinjection phase-locking may be easily realized by employing the central region of the array as a core oscillator [12]. Phase-locking is enhanced by conditioning electrode surfaces to suppress any polarization preference [13]. Anodizing the extruded aluminum electrodes is a convenient way to achieve this condition.
Thus, the optical power extractable from such a structure may be increased by using more and/or wider electrodes. Fig. 4 illustrates the approach for a powerful Zodiac having 48, 10 cm wide electrodes. This 100 kW machine, only 45 cm in diameter, about 2 m long and weighing 150 kG would represent a signi®cant advance in very high power CO2 laser construction. 8. Super high power radial arrays It follows that still greater optical powers may be derived from compound structures, such as the 500slab, `triple radial array' of Fig. 5. This device proposed in Fig. 6, employs several ceramic vacuum tubes to pump RF energy directly into the power splitting resonant cavity. A lightweight and highly mobile unit only 1 m in diameter by 2 m long could generate an incredible optical output near 1 MW and thereby open new avenues for very-heavy-section material processing. 9. Beam characteristics Slab gain media intrinsically generate asymmetric beams. Consequently, orthogonal output beam divergence compensation is usually implemented to achieve maximal eectiveness in materials processing sequences. Composite beam nonuniformity is another
7. High power radial arrays In a radial array, each gain channel and corresponding beamlet is eectively in parallel with all others.
Fig. 3. Unstable resonator. 28 = radial multi-channel array gain media, 38 and 40 = unstable resonator mirrors, 42 = beamlet compacting axicon, 44 = output window, 46 = multiple beamlet output.
333
Fig. 4. 48-slab, 100 kW Zodiac.
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H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Fig. 5. 500-slab `triple' Zodiac.
problem often encountered when combining conventional multiple slab laser beams to enhance overall power. Fortunately, the radial structure does not present such diculties due to an inherent radial beam stacking compensation eect. This useful phenomenon is simulated in the sequence of pro®les of Fig. 7. When 2 beamlets in Fig. 7(a) are stacked in parallel the composite intensity pro®le exhibits enormous asymmetry. However, Fig. 7(b) and (c) reveal that beam quality is rapidly improved when constituted from a multiplicity of stacked identical beamlets averaged around the periphery. Further insight on beam quality attainable from a radial array is provided by the sequence of diagrams in Fig. 8. These pro®les are computer predictions of near and far ®eld intensity distributions for both phase-locked and non-phase-locked operation [14]. It
Fig. 6. 500 channel 1 MW compound radial array laser.
Fig. 7. Computer simulation of radial beam stacking. A = output beam intensity pro®le for a single beamlet, B = output bean intensity pro®le for 2 stacked beamlets, C = output beam intensity pro®le for 4 stacked beamlets.
is again apparent that much smaller focal spots are obtained with phase-locking, such as by Zodiac selfinjection.
Fig. 8. Computer modeling of composite pro®les and beam quality. A and B = near ®eld and far ®eld output intensity pro®les for 8 nonphase locked beamlets, C and D = near ®eld and far ®eld output intensity pro®les for 8 phase-locked beamlets.
H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Fig. 9. Computed quality parameters of composite Zodiac beams. a = beamlet major axis halfwidth, b = beamlet minor axis halfwidth, r = output beamlet compaction radius, AR = a/b = beamlet aspect ratio.
The curves in Fig. 9 provide additional optical quality information in terms of the composite beam M2r achievable under both phase-locked and non-phaselocked conditions. The computer data reveals that best performance is obtained when a large number of closely spaced beamlets, each with an aspect ratio near unity, are combined. Thus, the design of a Zodiac laser should embody many channels with azimuthal beamlet compensation and near 100% ®ll factor radial compaction. Fig. 10 features an optical radial beam stacker [15], which accomplishes the tasks implicit in the computer simulations of Fig. 9. The device ®rst azimuthally expands and re-collimates each phase-locked beamlet,
335
extracted at the periphery of an unstable resonator (view C±C) into a unity aspect ratio. All beamlets are then stacked close-packed into a continuous annulus (view B±B) and ®nally radially compacted into a single solid pro®le (view A±A). In this manner, the beam stacker provides an aperture-®lling-factor near unity and a concomitant M2r of about 1.5. Further improvement in M2r , approaching 1, may be obtained through the addition of a spatial ®lter. Although the optical functions described above can be derived solely with the mechanical optical compaction apparatus of Fig. 10, the same process is achievable through diractive optic techniques [16]. This methodology featuring an amplitude-compensator followed by a phase-compensator, constructed via etched binary optical techniques, may ultimately be more ecient and cost-eective. 10. Conclusions Experiments suggest that power scaling of a compact laser from a few hundred watts up to several hundred kilowatts, is readily achievable using an RFdriven diusion-cooled radial slab array methodology. Stacking the many beamlets generated by such an array yields a clean and symmetric focal spot. Any resonator misalignment experienced during operation has a minimal eect on composite beam focal parameters and output power. Even a non-phase-locked Zodiac yields a symmetric non-polarized output which functions well in welding and cutting applications. Implementation of self-injection phase-locking can provide near diraction limited performance.
Fig. 10. Mechanical radial beam stacker. 1 and view C±C = beamlets entering stacker, 2 = resonator output mirror, 3 and 5 and view B±B = beamlet aspect ratio compensator, 6 and 7 = beamlet compacting axicon, 8 = output window, 9 and view A±A = stacked output beam, 10 = water cooled beam compensating and stacking diamond machined optics.
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H.J.J. Seguin / Optics & Laser Technology 30 (1998) 331±336
Acknowledgements The author gratefully acknowledges the continuing ®nancial support of NSERC and assistance in the design, construction and testing of the scienti®c apparatus used during the course of this investigation, rendered by W. Bilida, J. Strohschein, H. Reshe, C. Rogozinski, V. Pohnert, H. Drexel G. Fye and D. Presakarchuk. References [1] Abramski R et al. 2-Dimensional wave-guide CO2 laser arrays and beam reformatting. In XI Int. Sym. Gas Flow and Chemlasers. Heriot-watt University, Edinburgh, Scotland, August 1996. p. 24. [2] Lapucci A, Cangioli G. Triple slab radio frequency discharge pumped CO2 laser. Appl Phys Lett 1993;62(1):7±9. [3] Abramski KM, Colley AD, Baker HJ, Hall R. IEEE J Quantum Electron 1996;32:340. [4] Yeldon EF, Seguin HJJ, Capjack CE, Nikumb SK. Multichannel slab discharge for CO2 laser excitation. Appl Phys Lett 1991;58(7):693±5. [5] Yelden EF, Scott SW, Strohschein JD, Seguin HJJ, Capjack CE. Symmetry enhancement and spot-size requction through radial beam stacking in a multichannel CO2 laser array. IEEE J Quantum Electron 1994;QE-30(8):1868±75.
[6] Bilida WD, Seguin HJJ, Capjack CE. Multi-channel CO2 laser eccitation with resonant cavities. Opt Laser Technol 1996;28(6):431±6. [7] Terman FE. Simple-resonant circuits, section 3. Engineers handbook. NY: McGraw Hill, 1943. p. 135. [8] Hamilton DR, Knipp JK, Horner Kuper JB. Klystrons and microwave triodes, MIT Rad. Lab. series, vol. 7. NY, McGraw Hill, 1948. p. 174. [9] Yeldon EF, Seguin HJJ, Capjack CE, Nikumb SK, Reshef H. Toric unstable CO2 laser resonator: an experimental study. Appl Opt 1992;31(12):1965±74. [10] Yeldon EF, Seguin HJJ, Capjack CE, Nikumb SK, Reshe R. Multichannel laser resonators: an experimental study. Opt Quantum Electron 1992;24:889±902. [11] Hagemeier HE, Robinson SR. Field properties of multiple coherently combined lasers. Appl Opt 1979;18:270. [12] Yeldon EF, Seguin HJJ, Capjack CE, Reshe H. Phase locking in a multichannel radial array CO2 laser. Appl Phys Lett 1992;62(12):1311±3. [13] Yeldon EF, Segnin HJJ, Capjack CE, Reshe H. Phase-locking phenomenon in a radial multislot CO2 laser array. J Opt Soc Am B 1993;10(8):1474±82. [14] Strohschein JD, Seguin HJJ, Capjack CE. Beam propagation constants for a radial laser array. Appl Opt 1998;37(6):1045± 1048. [15] Seguin HJJ. US patent #5,648,980, July 15, 1997. [16] Williams SW, Mansden PJ, Roberts NC, Venables MA. Excimer laser beam shaping and material processing using diffractive optics. In XI Int. Sym. Gas-¯ow and Chem-lasers. Heriot±Watt University, Edinburgh, Scotland, 1996. p. 58.