®
`
`®
`
`Europfiisches Patentamt
`European Patent Office
`Office européen des brevets
`
`® Publication number:
`
`0 1 46 509
`A2
`
`EUROPEAN PATENT APPLICKHON
`
`@ Application number: 848502954
`@ Date oifilingto-1.10.84
`
`@ '“’‘-C‘-‘3 H 01 S 3/03, H 01 3 3/097,
`H O1 S 3/04
`
`® Priority: 07.10.83 ussesesa
`
`® Applicant: MINNESOTA LASER CORPORATION,
`2452 North Prior Avenue, Roseville Minnesota 5511 3 (US)
`
`@ Date of publication oi‘ application: 26.06.85
`Bulletin 85/26
`
`inventor: Gruber, Carl L, Rt. 1, Box 364, St Michael
`Minnesota (US)
`inventor: Wlllenbring, Gerald R., 1834 Stanford Avenue,
`St Paul Minnesota (us)
`
`® Designated Contracting States: AT BE CH FR IT LI NL
`SE
`
`Representative: Strism, Tore et al, Strfim 8: Gulliirsson AB
`Studentgatan 1 P.O. Box 4188, S-203 13 Malmfi (SE)
`
`@ External electrode transverse high frequency gas discharge laser.
`
`@ A circular bore transversely excited gas discharge
`laser (10) is disclosed which may be constructed and op-
`erated with no physical contact between the active discharge
`and the metal excitation electrode structure (14). The dis-
`charge is excited by RF in the frequency range 10 MHz to
`1 GHz applied to a transverse metal electrode structure (14)
`designed to maintain a relatively uniform electric field in the
`discharge bore (12). The disclosed laser may be configured
`as a waveguide laser or a large bore laser operating in a
`N nonwaveguide mode.
`< A means for inductively coupling RF energy from a suit-
`able RF energy source to the electrode structure (14) and a
`0 means for attaching mirrors (15) to the discharge tube using
`no organic sealing material is disclosed. Without physical
`O contact between the active discharge and the metal excita-
`If’ tion electrode structure or organic sealants, long lifetime,
`superior laser performance, and capability for liquide cool-
`0 ing of the discharge tube is achieved.
`
`EP014
`
`mfi
`
`B &
`
`ACTORUM AG
`
`GILLETTE 1413
`
` ‘.1? 3
`
`5”‘
`
`'fi
`
`..
`
`GILLETTE 1413
`
`
`
`®
`
`Europfiisches Patentamt
`European Patent Office
`Office européen des brevets
`
`® Publication number:
`
`0 1 46 509
`A2
`
`EUROPEAN PATENT APPLICKHON
`
`@ Application number: 848502954
`@ Date oifilingto-1.10.84
`
`@ '“’‘-C‘-‘3 H 01 S 3/03, H 01 3 3/097,
`H O1 S 3/04
`
`® Priority: 07.10.83 ussesesa
`
`® Applicant: MINNESOTA LASER CORPORATION,
`2452 North Prior Avenue, Roseville Minnesota 5511 3 (US)
`
`@ Date of publication oi‘ application: 26.06.85
`Bulletin 85/26
`
`inventor: Gruber, Carl L, Rt. 1, Box 364, St Michael
`Minnesota (US)
`inventor: Wlllenbring, Gerald R., 1834 Stanford Avenue,
`St Paul Minnesota (us)
`
`® Designated Contracting States: AT BE CH FR IT LI NL
`SE
`
`Representative: Strism, Tore et al, Strfim 8: Gulliirsson AB
`Studentgatan 1 P.O. Box 4188, S-203 13 Malmfi (SE)
`
`@ External electrode transverse high frequency gas discharge laser.
`
`@ A circular bore transversely excited gas discharge
`laser (10) is disclosed which may be constructed and op-
`erated with no physical contact between the active discharge
`and the metal excitation electrode structure (14). The dis-
`charge is excited by RF in the frequency range 10 MHz to
`1 GHz applied to a transverse metal electrode structure (14)
`designed to maintain a relatively uniform electric field in the
`discharge bore (12). The disclosed laser may be configured
`as a waveguide laser or a large bore laser operating in a
`N nonwaveguide mode.
`< A means for inductively coupling RF energy from a suit-
`able RF energy source to the electrode structure (14) and a
`0 means for attaching mirrors (15) to the discharge tube using
`no organic sealing material is disclosed. Without physical
`O contact between the active discharge and the metal excita-
`If’ tion electrode structure or organic sealants, long lifetime,
`superior laser performance, and capability for liquide cool-
`0 ing of the discharge tube is achieved.
`
`EP014
`
`mfi
`
`B &
`
`ACTORUM AG
`
`GILLETTE 1413
`
` ‘.1? 3
`
`5”‘
`
`'fi
`
`..
`
`GILLETTE 1413
`
`
`
`0146509
`
`EXTERNAL ELECTRODE TRANSVERSE
`
`HIGH FREQUENCY GAS DISCHARGE LASER
`
`Technical Field
`
`The invention pertains to transverse RF exci-
`
`tation gas lasers with circular geometry having external
`electrode and to the attachment of mirrors to the laser
`
`discharge tube.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`Background of the Invention
`
`Since their earliest development, gas lasers
`
`have been constructed using hollow dielectric tubes.
`
`Excitation of the active laser gaseous medium has been
`
`traditionally accomplished by applying a relatively
`
`large DC voltage longitudinally along the length of the
`
`discharge tube via two or more metal electrodes places
`
`in contact with the gaseous medium at the ends of the
`
`discharge tube or at points intermediate to the ends.
`
`Early in the development of gas laser technology,
`
`the
`
`advantages of removing metal electrodes from contact
`
`with the active gaseous laser medium and using RF exci~
`
`interest in removing
`tation was recognized. However,
`the metal electrodes from contact with the active
`
`gaseous laser medium apparently did not reach the peak
`
`necessary to lead to the development of a laser
`
`utilizing the technique.
`
`In a similar manner, RF
`
`excited lasers were left to future development.
`
`Recent development of waveguide lasers has
`
`stimulated renewed interest in RF laser excitation, and
`
`particularly inductive RF coupling to the laser as
`
`disclosed in U.S. Patent No. 3,772,611 issued November
`
`13, 1973 to Peter W. smith.
`
`The inductive coupling
`
`mechanism disclosed by Smith was ineffective in pro-
`viding for high frequency excitation and results in a
`
`non—uniform discharge.
`
`
`
`0146509
`
`U.S. Patent No. 4,169,251 issued September 25,
`
`1979 to Katherine D. Laakmann discloses a method for
`
`obtaining transverse RF discharge excitation of a wave-
`
`guide laser. This method requires contact between the
`laser medium and the transverse metal electrode struc-
`
`ture. Problems with
`
`reactions of the excited gas with
`
`the metal electrodes
`
`inside the laser discharge tube
`
`will
`
`inevitably lead
`
`to reduced laser lifetime in a
`
`sealed laser and ultimate degradation of laser perfor-
`
`10
`
`mance.
`
`The invention disclosed in the Laakmann patent
`
`required a generally rectangular laser geometry rather
`
`than circular. This results in the probability of exci-
`
`tation of undesirable optical modes rather than the
`
`15
`
`axially symmetric modes characteristic of a circular
`geometry. Furthermore,
`the Laakmann lasing device spe-
`cifically is limited to transverse
`RF excited waveguide
`lasers.
`
`with the metal electrodes in contact with the
`
`discharge medium,
`
`random discharge instabilities can
`
`20
`
`occur in the discharge medium resulting in fluctuating
`
`25
`
`30
`
`laser output power as well as mode instability.
`
`the rectangular "slab" construction of the
`Further,
`waveguide geometry makes it virtually impossible to
`apply mirrors directly to the ends of the laser struc-
`ture.
`The application of mirrors directly to the ends
`
`of the-laser is highly desirable for modular construc-
`
`tion and long laser lifetime.
`
`Summary of the Invention
`
`The present
`
`invention is intended to circu-
`
`vent
`
`the undesirable feature of the prior art while
`
`retaining the inherent advantages of transverse RF
`discharge excitation such as reduction in required
`discharge voltage, reduced gas dissociation,
`increased
`
`35
`
`operating efficiency, and discharge stability.
`
`
`
`0146509
`
`I: particular,
`
`the present
`
`invention comprises
`
`construction of a transversely excited RF discharge
`
`laser of generally circular geometry from a single or
`
`monolithic and homogeneous piece of dielectric material.
`
`Electrodes are placed on the external surface of the
`
`discharge chamber and are therefore not
`
`in contact with
`
`the active discharge.
`
`The interposed layer of
`
`dielectric material serves not only to isolate the
`
`electrode material from the discharge, but to provide a
`
`10
`
`discharge stabilizing, purely reactive (lossless),
`
`series impedance between the electrodes and the active
`
`discharge volume. Furthermore,
`
`the invention is not
`
`limited by application to waveguide lasers but can also
`
`15
`
`be used with large bore laser structures which will be
`defined as lasers with bore areas greater than lOmm2.
`
`Design of the discharge tube allows for cir-
`
`cular symmetry to be maintained in the entire laser
`
`structure,
`
`including a circular cross-section laser
`
`discharge chamber.
`
`The electrode design maintains a
`
`-20
`
`nearly uniform electric field across the entire
`
`discharge volume,
`
`thereby providing more uniform laser
`
`pumping and promoting propagation of a single low order
`transverse laser mode.
`
`An inductive RF coupling mechanism is
`
`25
`
`30
`
`35
`
`disclosed that is efficient and readily tunable while
`
`still allowing for pi network coupling if desired.
`
`The circular geometry provides for convenient
`
`attachment of mirrors directly to the ends of the
`
`discharge tube with brazed flanges and a malleable metal
`
`compression seal.
`
`Long laser structure can be fabri-
`
`cated by simply coupling a series of short sections
`
`together with all metal sealed flange assemblies or
`
`direct glassed or brazed connections. Thus, single unit
`
`integrity can be maintained to meter—length laser bores.
`
`The circular geometry further provides for application
`
`
`
`0146509
`
`of integral concentric cooling and RF shielding enclo~
`
`sures,
`
`thus allowing use of dielectric fluid laser
`
`cooling and minimal EMI emissions from the complete
`
`operating laser assembly.
`
`The laser of the present
`
`invention has the
`
`advantages over the prior art of:
`
`(a)
`
`relatively long shelf life and operating
`
`life due to the sealed nature of the lasing tube and
`
`absence of metal electrodes in direct contact with the
`
`10
`
`gas discharge;
`
`(b)
`
`a high degree of mechanical ruggedness
`
`and stability as a result of the monolithic construction
`
`of the discharge tube and the mirror assemblies;
`
`(c)
`
`excellent beam quality and stability as a
`
`result of the circular bore and the uniform field main-
`
`tained by the capacitively coupled external electrodes;
`
`(d)
`
`the capability of modulating the laser
`
`beam output power;
`
`(e)
`
`relatively high efficiency because of the
`
`ability to use low power RF excitation to form a glow
`discharge or plasma; and
`(f)
`
`relatively small, compact packaging.
`
`
`T Brief Description of the Drawings
`
`Referring to the drawings, wherein like
`
`_
`
`numerals represent like parts throughout
`views:
`
`the several
`
`15
`
`20
`
`25
`
`FIGURE 1A is,a longitudinal sectional view of
`
`he waveguide laser embodiment of the present invention.
`
`30
`
`FIGURE 1A illustrates the single solid homogeneous block
`
`of dielectric material used to construct the laser wave-
`
`guide and electrode assembly.
`_
`FiGURE 1B is a longitudinal view of the wave-
`
`guide laser embodiment as viewed from an orientation
`
`35
`
`with is rotated 90° from the orientation shown in FIGURE
`lA.
`
`
`
`I'I'1'f\I'I1\l‘I
`L‘ LUUKL
`
`1C illustrates a cross—sectional view
`
`0146509
`
`of the laser of FIGURES 1A and 1B taken along section
`line A-A.
`
`FIGURE 1D illustrates an end View of the laser
`
`of FIGURES 1A, 1B, and 1C.
`
`FIGURE 2A is a longitudinal sectional view of
`
`the large bore, non—waveguide embodiment of the present
`
`invention. Again,
`is evident.
`
`the single solid block construction
`
`FIGURE 28 is a cross—sectional view of the
`
`laser of FIGURE 2A taken along section line C—C.
`
`FIGURE 3A is a longitudinal View of the wave-
`
`guide laser embodiment illustrating an exploded view of
`
`one of the mirror mounting assemblies and a cross-
`
`sectional View of the other mirror mounting assembly.
`
`FIGURE 3 also illustrates the cooling fluid and
`
`electromagnetic interference housing as well as the gas
`reservoir.
`
`FIGURE 3B is the end view of the body flange.
`
`FIGURE 4 is a schematic diagram of the network
`
`for coupling RF power to the electrodes by mutual
`tance.
`
`induc-
`
`V
`
`FIGURE 5 illustrates another cross—sectional
`
`view of the laser of FIGURES 1A and 1B taken along sec-
`
`tion line A—A with an x and y axis superimposed.
`
`FIGURE 6 illustrates a schematic of an equiva-
`
`lent electrical circuit for the laser tube.
`
`Detailed Description of the Invention
`
`Referring to Figure 1A and Figure 1B we have
`
`the waveguide gas laser embodiment according to the
`invention.
`The laser tube 10 is seen to consist of a
`
`body‘1l being composed of a single solid or monolithic
`
`homogeneous block of dielectric material such as Beo or
`
`A1203 or other suitable ceramic, glass, or dielectric
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`
`
`0146509
`
`material.
`
`The dielectric material is of a high degree
`
`of purity and of a high density.
`
`A circular hole, as shown in Figures 1C and 1D
`
`is formed near the center of the body 11 along the
`
`entire length,
`
`thus defining the active laser volume or
`
`chamber 12, within which the RF—excited discharge
`
`exists.
`
`The chamber 12 of the preferred embodiment is
`
`drilled, or otherwise formed,
`
`through the center axis.
`
`The shape of the body 11 is shown in Figures 1C and 1D
`
`as roughly circular in the preferred embodiment.
`
`However, any other external shape is also feasible such
`
`as a rectangular shape. Although the preferred embodi-
`
`ment shows one laser exciting discharge chamber 12, more
`
`than one chamber 12 may be formed in the same body ll.
`
`The chamber(s) 12 may be formed in the solid body 11
`
`material by drilling or other convenient forming method.
`
`The chamber 12 diameter should be suitable for guiding
`
`laser light according to conventional wisdom which is
`
`approximately
`
`1 to 3.5mm when used to guide 10.6 micro~
`
`10
`
`15
`
`20
`
`meter to 9.6 micrometer laser light.
`
`The use of a precise circular bore is advan-
`
`tageous because a very uniform and stable output beam
`
`can be generated as a result.
`
`A uniform and stable beam
`
`in most applications, especially when
`is very important
`the laser is used for such delicate and precise purposes
`
`25
`
`as surgery and industrial processing.
`
`In contrast,
`
`prior art lasers are often constructed by making a sand-
`
`wich out of slabs of metal and ceramic thereby forming a
`
`square waveguide as.taught in the Laakmann patent to
`
`which reference has previously been made.
`
`the design
`
`approach of the Laakmann patent can lead to lasers with
`
`unstable and/or non—uniform output beams.
`
`-
`
`.
`
`Electrode Grooves
`
`Electrode grooves 13 are formed on diametri-
`
`cally opposite sides of the outer wall of the body 11,
`
`The opposing grooves 13, having a generally "v" shape,
`
`30
`
`35
`
`
`
`0146509
`
`are formed accurately parallel to the formed discharge
`
`chamber 12.
`
`The exact shape and depth are determined in
`
`the manner set forth hereinafter in this specification.
`
`The grooves 13 are formed by the method of grinding or
`
`molding.
`
`The grooves 13 extend along the length of the
`
`body 11,
`
`tapering towards the ends, and terminating a
`
`small distance from the end of the body 11, allowing for
`
`attachment of body flanges 16 to the ends of the body 11
`
`with no disruption of the symmetric cross-section at the
`
`ends.
`
`The bottom of the groove 13 extends to within a
`
`small distance of the chamber 12 and has an approximate
`
`included angle, G<, as shown in Figure 1C.
`A cross-
`section
`of the grooves 13 is illustrated most clearly
`
`in Figure 1A.
`
`A frontal view of a groove is best
`
`illustrated in Figure 1B.
`Electrodes
`
`Continuous, external electrodes 14 are formed
`
`-in the grooves 13 by appropriate means for metallization
`
`such as the thick film screening technique or vacuum
`
`evaporation, or other commonly used methods.
`
`The shape
`
`of the electrode 14 grooves 13 is chosen in such a way
`
`as to maintain a nearly uniform electric field in the
`
`discharge chamber 12 when RF energy is applied to the
`
`electrodes 14 in grooves 13. This requires a specific
`
`relationship between the distance d, chamber diameter,
`
`2a, and the groove 13 angle‘°(as illustrated in Figure
`1C.
`
`inch)
`
`,
`
`A typical set of dimensions is d = O.l5cm (0.060
`O6 = 11° for Be0 body 11 material with a =
`0.1l4cm (0.045 inch).
`
`A variation of the electrode 14 structure uses
`
`a series of longitudinal electrically separated
`
`electrode segments to help achieve a uniform electric
`
`field in the chamber 12.
`
`Non-Waveguide Laser‘
`
`Referring to Figure 2, we have a preferred
`
`embodiment of the conventional non—waveguide bore gas
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`
`
`0146509
`
`laser according to the invention.
`
`The laser tube 10 is
`
`seen to consist of a body 11 being composed of a single
`
`solid homogeneous piece of dielectric material such as
`
`Beo or A1203 or other suitable ceramic or glass
`material.
`An electrode 14 configuration designed to
`
`5
`
`provide a nearly uniform electric field within the
`
`chamber 12 is applied to the outer wall of the tube 10
`
`rather than being placed in a groove 13 as used with the
`
`waveguide laser.
`
`10
`
`The discharge chamber 12 is filled with any
`
`desired gaseous laser medium. Typically, a C02 laser
`mixture comprising CO2, He, and CO and/or N2 is used,
`but
`the invention can be used with He and Ne or eximer
`
`or any other gas laser mixture at an appropriate total
`
`15
`
`pressure for optimum laser efficiency.
`
`Hermetic Sealing and Contamination Reduction
`
`The sealed life of many CO2 lasers is limited
`by several factors including (1)
`leakage of the internal
`
`gas or laser medium to the outside and/or seepage of
`contaminants from the environment into the interior of
`
`20
`
`the laser due to small leaks;
`
`(2) outgassing of contami-
`
`nants from the internal surfaces inside the laser tube
`
`10 and/or (3) reactions of the excited gas with internal
`metal electrodes.
`
`25
`
`30
`
`The laser of the preferred embodiment elimin-
`
`ates the major cause of shortened sealed laser life by
`
`placing the electrodes 14 external to the discharge
`chamber 12.
`
`In order to achieve long shelf life the laser
`
`must be hermetically sealed and free of internal con—
`
`tamination. Filling with research grade ultra-high
`
`purity gases such as CO2, N2 or He eliminates one major
`cause of internal contamination.
`
`In order to further eliminate contamination
`
`35
`
`from inside the laser, an ultra-clean laser assembly
`
`
`
`0146509
`
`procedure is used in the production of the laser of the
`
`preferred embodiment prior to filling the laser with the
`
`laser gas medium.
`
`As a final means of inhibiting the reduction
`
`of output performance of the laser due to contaminants,
`
`a gas reservoir 39 with a volume of about
`
`100cc
`
`(compared with the internal volume of the
`
`waveguide
`
`chamber 12 which is less than lcc) can be connected to
`
`the fill tube 18 to dilute the effects of
`
`contamination
`
`and/or the changes in gas composition.
`
`Tests of the laser of the preferred embodiment
`
`without any gas reservoir 39 have demonstrated more than
`
`500 hours of operation without a significant Change in
`
`power output. Therefore,
`
`the volume of the gas reser-
`
`voir 39 can be reduced significantly without affecting
`
`the laser lifetime.
`
`In fact,
`
`the demonstrated 500 hours
`
`of operation without any gas reservoir is commercially
`
`acceptable in today's marketplace.
`
`The present inven-
`
`tion is, therefore, being successfully produced with
`
`varying sizes of gas reservoirs 39 from none to 100cc
`
`depending upon whether the application requires assured
`
`long life without offsetting needs for weight reduction
`
`or requires the maximization of weight reduction without
`
`the necessity for assured life beyond what is commer-
`
`cially acceptable in today's marketplace.
`
`Since the body 11 is a single enclosed ceramic
`
`tube with external electrodes 14, seals are only
`
`required on the ends of the body 11 where the laser
`
`mirrors 15 are mounted. With the number of seals and
`
`total seal surface relatively small,
`
`the sealing process
`
`is simple and the seals more stable.
`
`The prior art
`
`square geometry laser tube is not amenable to use of the
`
`simply constructed mirror mounts of the present inven-
`
`tion.
`
`I
`
`To maintain the hermetically sealed condition
`
`of the preferred embodiment, a unique means of attaching
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`
`
`0146509
`
`the mirrors 15 to the body 11 is used.
`
`The mirrors 15
`
`are attached to the laser body 11 of Figures 1A, 1B or
`The
`
`2A as shown in the embodiment of Figures 3A and 3B.
`
`ends of the laser body 11 are metallized around the cir-
`
`cumference to form a narrow metallized ring 17 on both
`
`side as shown in Figures 1A, 1B, 2A and 3A. These
`
`metallized rings 17 are approximately 0.025mm (one mil.)
`thick.
`
`As illustrated in the Figures,
`
`they are not to
`
`scale.
`
`A formed metal body flange 16 is slipped over
`
`10
`
`the laser tube 10 and brazed to the metallized rings 17
`
`forming an ultra—high vacuum seal.
`
`One of the body flanges 16 (shown on the left
`
`side of Figure 3A) has a welded—on gas fill and eva~
`
`cuation aperture 18 formed in it to support a gas fill
`
`15
`
`and evacuation tube for ultimate connection to the laser
`
`gas reservoir 39.
`
`Cooling of the laser tube 10 is very important
`
`in order to obtain high output power from the laser,
`
`to
`
`~prevent overheating of any of the components, and to
`
`20
`
`enhance the stability of the laser output. The laser of
`
`the present invention provides for total immersion of
`
`the laser tube 10 in inert, non~toxic cooling fluid.
`
`The cooling fluid has a dielectric constant which makes
`
`the fluid electrically inert and prevents any shorting
`
`25
`
`of the electrodes or other active circuit elements even
`
`though the fluid is in direct contact with the electro-
`
`des and other active circuit elements.
`
`The total immer-
`
`sion of the laser tube 10 in direct contact with the
`
`cooling fluid provides very uniform and effective heat
`
`30
`
`exchange.
`
`To provide a means for containing the cooling
`
`fluid,
`
`the body flange 16 has recessed annular ring 19
`
`for the purpose of retaining a sealing O—ring 20 which
`
`provides a leakproof seal between a cooling fluid
`
`35
`
`housing 21 and the body flanges 16. Cooling fluid
`
`-10..
`
`
`
`0146509
`
`recess area 22 allows cooling fluid to surround the ends
`
`of the body ll.
`the chamber 12
`
`An active discharge also exists within
`
`at this point requiring cooling of this
`
`portion of the
`
`body 11.
`
`The cooling fluid housing 21
`
`supports three
`
`cooling fluid ports 23.
`
`Two of the ports
`
`23 are for the input of cooling fluid and the other is
`23 of the fluid.
`
`A conventional recir-
`
`for the output
`
`culating pump and source of cooling fluid is connected
`
`to the cooling fluid ports 23 to provide the interior of
`
`10
`
`the housing 21 with a continuously recirculating source
`
`of cooling fluid,
`A heat exchanger may be connected in
`line with the recirculating pump to shed the heat energy
`
`which the cooling fluid has absorbed from the laser tube
`
`10,
`
`thereby allowing the cooling fluid to be continu-
`
`15
`
`ously re—used.
`
`The cooling fluid housing 21 also supports an
`RF connector 44 for connection of the electrodes 14 to a
`
`source
`
`of RF power.
`
`The RF connector is a standard
`
`male and female coaxial connector set.
`
`In the preferred
`
`embodiment the female portion of the connector is
`
`attached to the cooling fluid housing with coaxial cable
`
`connecting between the electrodes 14 and the female por-
`
`tion of the RF connector 44.
`
`The high frequency discharge of the laser tube
`
`10 as a result of the RF excitation,
`
`is a source of
`
`electromagnetic interference which must be shielded.
`
`Effective electromagnetic shielding is accomplished by
`the combination of the stainless steel mirror assemblies
`
`and by the fabrication of the housing 21 using aluminum
`
`which provides effective shielding much as a conven-
`
`tional coaxial cable provides electromagnetic shielding.
`I
`Figure 33 illustrates the end View of a body
`
`flange 16 as viewed from the left side_as it is shown in
`
`Figure 3A.
`
`Figure 3B shows threaded mirror flange
`
`receiving holes 25. Annular laser medium communication
`
`20
`
`25
`
`30
`
`35
`
`_11_
`
`
`
`0146509
`
`port 26 allows the laser medium to flow from the laser
`
`medium fill tube assembly 45 to the chamber 12 opening.
`
`As seen in Figure 3B,
`
`the body flange 16 does not
`
`obstruct the line of sight of the chamber 12 allowing
`
`the laser beam to pass out of the chamber 12 and impinge
`
`upon mirror 15.
`
`Mirror flange 24 is mounted to body flange 16
`
`using mirror flange mounting screws 27 inserted through
`
`apertures in the mirror flange which extend entirely
`through the mirror flange 24 and which are axially
`
`10
`
`aligned with the threaded mirror flange receiving holes
`25.
`
`There are six of these mirror flange mounting
`
`15
`
`20
`
`25
`
`30
`
`screws located equidistant from one another around the
`
`periphery of the mirror flange 24.
`
`A first circular
`
`malleable metal ring 31 such as indium or its alloys
`
`with time and lead is placed between the body flange 16
`
`and the mirror flange 24. As the mounting screws 27 are
`
`tightened the malleable ring 31 cold flows in the space
`
`between the two flanges l6 and 24 forming a conformal
`
`vacuum seal between the two flanges 16 and 24.
`
`A circular laser mirror 15 is placed in mirror
`
`recess 29 formed in mirror flange 24. A second malleable
`metal ring 32 is placed between the mirror flange 24 and
`
`the mirror 15.
`
`In front of the laser mirror 15 is
`
`The combination of the
`placed a polymer washer 30.
`mirror sandwiched between the washer 30 and the metal
`
`ring 32 allows uniform pressure to be applied and to be
`
`maintained sufficient to seal without fracturing the
`
`mirror. Compression plunger 33 is inserted in the mirror
`
`recess 29 and is affixed to the mirror flange 24 using
`
`screws 36 through compression plunger apertures 37 which
`
`are axially aligned with threaded receiving holes 38 in
`the mirror flange 24.
`Mirror flange 24 has a centrally located laser
`
`35
`
`beam aperture 34 aligned with the chamber 12 of the
`
`-12-
`
`
`
`0146509
`uu
`““te 16 to allow the laser beam to Contact
`the
`
`laser
`
`laser mirror 15. One of the two compression plungers 33
`
`(the one on the left in Figure 3A)
`
`is fabricated with an
`
`aperture 35 in axial alignment with the laser beam aper-
`
`ture 34 of the mirror flange and the longitudinal axis
`12.
`
`This compression plunger 33 is
`
`of the chamber
`affixed to the
`
`laser tube 10 end having a partially
`
`transmissive output mirror 15 such as since selenide to
`
`allow the laser beam to exit the laser tube 10.
`
`The
`
`other compression plunger 33 forms a solid end cap and
`
`is used in conjunction with the highly reflective laser
`
`mirror 15 such as enhanced silicon.
`
`Alignment of the laser mirrors 15 with each
`
`other to achieve parallelism between the mirrors 15 to
`
`within micro—radians is accomplished by torqueing the
`There are four mirror
`
`mirror flange set screws 46.
`
`flange set screws 46 although for illustration purposes
`
`Figure_3A shows only one set screw 46 on the exploded
`
`portion of the drawing on the left side.
`
`The right
`
`mirror flange 24 also has four set
`
`screws 46 although
`the
`
`none are shown. As the set screws 46 are torqued,
`
`left circular fin 48 of the mirror
`
`flange 24 bends rela-
`
`tive to the right circular fin 48 if any set screw 46 is
`
`torqued more or less than another.
`
`By varying the
`
`torque of the set screws 46 and observing the output of
`
`the
`the laser tube during the torqueing procedure,
`alignment of the mirrors relative to one another can be
`affected.
`
`The variation of torque on the set screws 46
`
`actually bends the mirror recess 29 enough to affect
`
`the alignment.
`
`In this manner mirrors 15 can be tilted
`
`by up to 2° with respect to the laser axis while main-
`
`taining an ultra—high vacuum seal.
`
`No organic materials are needed to provide the
`required vacuum seal integrity of the end flange
`
`assemblies.
`
`The properties of malleable metals such as
`
`-13-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`
`
`0146509
`indium or it's alloys with lead and tin are such at no
`
`special requirements are placed on the material or pre—
`
`paration of the mirrors 15. For CO2 lasers, for example,
`Znse or GaAs or Ge mirrors may be used, as well as
`
`others for this and other gas laser mixtures.
`
`Sturdy materials such as stainless steel are
`
`used for the flanges 16 and 24 and the compression
`
`plunger 33 to provide structural rigidity, dimensional
`
`stability, and freedom from corrosion.
`
`10
`
`For the purposes of non—waveguide lasers as
`
`shown in Figures 2A and 2B,
`
`the first indium ring 31 can
`
`be replaced by a metal bellows or other seal brazed to
`
`the body flange 16 and the mirror flange 24 at their
`
`outer periphery. Mirror adjustment is then effected by
`external means.
`
`15
`
`Lasers longer than approximately 25—30cm
`
`(10-12 inches) can be provided by the coupling of laser
`
`tubes together by virtue of mating body flanges 16 with
`
`malleable metal seals between adjacent body flanges 16
`
`or by using long continuous formed ceramic tubes. These
`
`longer lasers can achieve up to 20 watts or greater of
`
`continuous output power as contrasted with approximately
`
`5 watts continuous output power for a single tube 10
`
`laser of 25—30cm (10-12 inches)
`
`in length.
`
`Inductive Coupling
`
`20
`
`25
`
`Figure 4 also illustrates a means for induc-
`
`tively coupling RF energy to the laser discharge tube 10
`
`electrodes 14.
`
`A secondary coil 40 of suitable metallic
`
`conductor such as copper or silver is wrapped around the
`
`30
`
`laser body 11, appropriately insulated from the
`
`electrode material 14.
`
`The ends of the secondary coil
`
`40 are soldered to opposite electrode surfaces at points
`I
`
`41 as shown,
`
`thus forming a resonant circuit with the
`
`coil inductance and the interelectrode_capacitance.
`
`For
`
`35
`
`frequencies at which the electrode structure greater
`
`than.A/8 (where A/2 is the characteristic wave length in
`
`-14..
`
`
`
`0146509
`
`the electrode structure), more than one secondary coil
`
`40 may e required to maintain a uniform voltage along
`
`the length of the laser tube 10.
`
`A primary coil 42 of larger diameter than the
`
`secondary coil
`
`is formed concentrically around the
`
`secondary coil 40 to mutually couple RF power or energy
`
`between the two;
`
`The ends of the primary coil 42 are
`
`connected to the RF power supply 44.
`
`Adjustment of the
`
`turns ratio and the position of the primary coil 42 with
`
`respect to the secondary coil 40 allows the impedance
`
`transformation to be adjusted. with the correct number
`
`of turns for each coil, excellent
`
`impedance matching is
`
`obtained with a voltage standing wave ratio approaching
`1.0.
`
`Trimmer capacitor 43 compensates the inductance of
`
`the RF transformer to yield a nearly pure resistance to
`
`be seen at
`
`the primary coil terminals.
`
`A source of RF
`
`energy 44 is connected to the primary coil terminals by
`
`a coaxial cable or other suitable means.
`
`The preferred
`
`embodiment of Figure 4 can be replaced by a simple pi
`
`coupling network, with solder connections to the
`
`electrode materials, as an alternative embodiment.
`
`RF power or energy can be coupled to the pri-
`
`mary coil 42 either through the laser mirror and body
`
`flanges 16 and 24, or through the cooling fluid housing
`
`21 by means of a coaxial connector as shown in Figure
`3A:
`
`So as to not further complicate Figure 3A,
`
`the pri-
`
`mary coil 42 and the secondary coil 40 has not been
`
`shown. Only a stub of coaxial cable is shown; it being
`
`understood that the coaxial cable 13 is to connect to
`
`the primary 42 coils which surround the laser body 11
`
`within the cooling fluid housing 21.
`
`The preferred embodiment of Figures 1A, 1B,
`
`and 3A have_been operated using the coupling network
`
`shown in Figure 4 as a C0
`
`2
`
`laser with 100 Torr gas
`
`pressure, yielding up to 5 watts output power from a
`
`tube 10 inches long.
`
`-15-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`
`
`Electrode Geometry
`
`O 1 4 6 S 0 9
`
`The electrode 14 geometry is important
`
`to
`
`achieve the end result of a uniform discharge along the
`
`entire length of the laser tube 10 at an electric field
`
`intensity suitable for the chamber 12 diameter of the
`
`particular laser tube 10 and laser gas mixture to be
`used.
`
`The electrode 14 cross—sectional curve as
`
`10
`
`15
`
`shown in Figure 5 is the cross-section shown in Figure
`
`1C reproduced with an x and y superimposed upon it to
`
`provide an x—y reference for the mathematically
`
`expressed cross—sectional shape functions of the
`The actual
`electrodes 14 for a symmetric laser tube 10.
`
`electrode grooves 13 used in the preferred embodiment
`
`closely approximate these mathematical expressions.
`
`The electrodes 14 are shaped to provide an
`
`essentially uniform electric field in the chamber 12 of
`
`the laser tube 10.
`
`Different electrode 14 shapes are
`
`required depending upon the chamber 12 diameter as well
`as the dielectric constant of the laser tube 10
`
`20
`
`material.
`
`It has been found that the distance from the
`
`center of the chamber 12 to the low point of the
`
`electrode groove 13 must be very close to the radius of
`
`the chamber to avoid the necessity of a large voltage
`
`'25
`
`source.
`
`For chamber 12
`
`diameters of l—3mm,
`
`the elec~
`
`trodes 14 are formed as a shaped groove 13 as shown in
`
`Figures 1C and 5 in the laser tube body 11.
`
`The shaped
`
`grooves 13 run parallel
`laser tube 10.
`
`For the large bore, non—waveguide,
`
`to the longitudinal axis of the
`
`30
`
`laser,
`
`similar shape function has been derived for
`
`application to the outer body 11 wall.
`In the case of the small bore chamber 12,
`
`the
`
`waveguide laser,
`
`the cross~sectiona1 shape of the
`
`electrode groove 13 providing a uniform field in the
`
`35
`
`unexcited chamber 12 is given by:
`
`-16..
`
`
`
`0146509
`
`Formula 1
`
`E = <1 + 1/Er)x + (1 — 1/Er)
`E0
`
`xaz
`X2 + Y2
`
`where
`
`VD
`
`is the potential measured between the
`electrodes 14.
`
`E
`
`E
`
`x
`
`a
`
`is the uniform electric field intensity in
`the chamber 12.
`
`is the relative dielectric constant of the
`
`laser tube 10 material.
`
`is the distance along the x axis as shown
`
`in Figure 5.
`ck.-. A4.-;.—..-.~(-.
`L-:§Al‘;3
`XJARZI Lo$l{\¢V;
`
`—_.1 Ana JUL‘.-\
`a.1.L\J::':§ Lint;
`
`-.9-us
`-u
`1 GAGA»)
`
`Flt!‘
`“.433
`
`''E) bV m
`
`in Figure 5.
`is the chamber 12 radius.
`
`In designing a particular electrode 14 geometry, E0 is a
`given since the field intensity necessary to achieve an
`
`output result with a given laser gas mixture is relati—
`
`vely fixed.
`
`The diameter of the chamber 12 is also a
`
`predetermined factor as well as the electrode 14 poten-
`
`tial, VD.
`the electrode grooves 13 can be easily computed.
`
`with these parameters,
`
`the exact geometry of
`
`To
`
`determine the electrode groove 13 geometry for various
`:
`and a is a relatively straight
`combinations of E0, V
`forward mathematical gomputation.
`Formula 1 was derived for the unexcited
`
`discharge chamber 12 ——that is a chamber 12 without a
`
`discharge medium.
`
`The following equation results from the
`
`generalized expression of Formula 1:
`
`Formula 2
`
`VD = E0
`
`{(1 + 1/E >d + (1 — 1/g >
`If
`1.’
`
`a2 1
`d
`
`10
`
`15
`
`20
`
`25
`
`30
`
`which expresses VD necessary to produce a uniform elec-
`tric field (as would be produced by a parallel plate
`
`35
`
`_1'7._
`
`
`
`0146509
`
`electrode structure) of E0 throughout a chamber 12 of
`radius "a" surrounded by a dielectric material having a
`
`dielectric constant of Er with electrodes spaced a
`distance d from the center of the chamber 12 at y = O.
`
`In the case of the large bor
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