CA1262956A - Ultra-narrow bandwidth optical thin film interference coatings for single wave-length lasers - Google Patents
Ultra-narrow bandwidth optical thin film interference coatings for single wave-length lasersInfo
- Publication number
- CA1262956A CA1262956A CA000477480A CA477480A CA1262956A CA 1262956 A CA1262956 A CA 1262956A CA 000477480 A CA000477480 A CA 000477480A CA 477480 A CA477480 A CA 477480A CA 1262956 A CA1262956 A CA 1262956A
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- Prior art keywords
- laser
- coating
- wavelengths
- optical
- oscillation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 238000000576 coating method Methods 0.000 title claims abstract description 71
- 230000003287 optical effect Effects 0.000 title claims abstract description 45
- 239000010409 thin film Substances 0.000 title description 6
- 239000011248 coating agent Substances 0.000 claims abstract description 65
- 230000010355 oscillation Effects 0.000 claims abstract description 35
- XKRFYHLGVUSROY-UHFFFAOYSA-N argon Substances [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 claims abstract description 20
- 229910052786 argon Inorganic materials 0.000 claims abstract description 19
- 239000007789 gas Substances 0.000 claims abstract description 16
- -1 Argon ion Chemical class 0.000 claims description 18
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 16
- 230000003595 spectral effect Effects 0.000 claims description 13
- 238000000034 method Methods 0.000 claims description 12
- 239000003989 dielectric material Substances 0.000 claims description 10
- 235000012239 silicon dioxide Nutrition 0.000 claims description 8
- 239000000377 silicon dioxide Substances 0.000 claims description 8
- 238000002834 transmittance Methods 0.000 claims description 8
- TWNQGVIAIRXVLR-UHFFFAOYSA-N oxo(oxoalumanyloxy)alumane Chemical compound O=[Al]O[Al]=O TWNQGVIAIRXVLR-UHFFFAOYSA-N 0.000 claims description 6
- 230000005855 radiation Effects 0.000 claims description 5
- 230000015556 catabolic process Effects 0.000 claims description 4
- 238000006731 degradation reaction Methods 0.000 claims description 4
- 229910052743 krypton Inorganic materials 0.000 claims description 3
- 238000007789 sealing Methods 0.000 claims description 3
- 239000000758 substrate Substances 0.000 claims description 3
- 238000010891 electric arc Methods 0.000 claims description 2
- 238000001704 evaporation Methods 0.000 claims description 2
- 230000002459 sustained effect Effects 0.000 claims description 2
- 238000001228 spectrum Methods 0.000 abstract description 10
- 239000010410 layer Substances 0.000 description 23
- 230000005540 biological transmission Effects 0.000 description 5
- 150000002500 ions Chemical class 0.000 description 5
- 239000000463 material Substances 0.000 description 4
- 239000005304 optical glass Substances 0.000 description 4
- 238000013459 approach Methods 0.000 description 3
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 description 2
- 239000011247 coating layer Substances 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- DNNSSWSSYDEUBZ-UHFFFAOYSA-N krypton atom Chemical compound [Kr] DNNSSWSSYDEUBZ-UHFFFAOYSA-N 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- BSFODEXXVBBYOC-UHFFFAOYSA-N 8-[4-(dimethylamino)butan-2-ylamino]quinolin-6-ol Chemical compound C1=CN=C2C(NC(CCN(C)C)C)=CC(O)=CC2=C1 BSFODEXXVBBYOC-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 230000004075 alteration Effects 0.000 description 1
- 229910052681 coesite Inorganic materials 0.000 description 1
- 229910052906 cristobalite Inorganic materials 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 238000009501 film coating Methods 0.000 description 1
- 150000002222 fluorine compounds Chemical class 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052754 neon Inorganic materials 0.000 description 1
- 239000002674 ointment Substances 0.000 description 1
- 238000001579 optical reflectometry Methods 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 238000002310 reflectometry Methods 0.000 description 1
- 229910052682 stishovite Inorganic materials 0.000 description 1
- 230000001629 suppression Effects 0.000 description 1
- 239000004408 titanium dioxide Substances 0.000 description 1
- 229910052905 tridymite Inorganic materials 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08059—Constructional details of the reflector, e.g. shape
Abstract
ABSTRACT:
A wavelength selective mirror provides single wavelength operation of a gas laser. In a specific embodiment an output coupler optical interference coating allows oscillation of the 488.0 nanometer line of the Argon ion laser blue/green spectrum while suppressing all other lines in that spectrum, particularly adjacent lines at 476.5 nanometers and 496.5 nanometers.
A wavelength selective mirror provides single wavelength operation of a gas laser. In a specific embodiment an output coupler optical interference coating allows oscillation of the 488.0 nanometer line of the Argon ion laser blue/green spectrum while suppressing all other lines in that spectrum, particularly adjacent lines at 476.5 nanometers and 496.5 nanometers.
Description
~2~9S6 Docket No. F-~6-P
ULTRA-NARROW BANDWIDTH OPTICAL THIN FILM INTERFERENCE
_ _ __ __ _ _ _ _ COATINGS FOR SINGLE WAVELENGTH LASERS
B~CKGROUND OF THE INVENTION:
_ This invention relates generally to lasers and optical thin film inter~erence filters for providing narrow wavelength bandwidth operation of a laser.
This invention has specific application to wavelength selective optical elements for narrow wavelength bandwidth or single wavelength operation of a gas laser.
Many applications of lasers require single wavelength operation of the laser device. The Argon ion laser, which is used in laser reprographic systems for example, can oscillate on a number of lines in the 450 nanometer to 520 nanometer spectral region simultaneously. Traditionally, a single wavelength is selected in the laser by introducing a wavelenyth selective optical element, such as a prism or birefringent filter, into the optical cavity.
Alternatively, the laser device can be run at a low input power level such that only a single line is above threshold. -The above described techniques are deemed unacceptablein many instances.
Laser cavities with prisms or birefringent filters are significantly more dificult to align and to maintain in alignment than cavities with mirrors only. Also, complex methods to compensate for thermal changes in refractive index must be employed to stabilize lasers that utilize prisms and operate over a wide range of temperatures.
9S~
rrhe second method referred to above, running the laser at low input power level to obtain a single line, unnecessarily limits the single wavelength laser output power. Also, it allows virtually no selection o~ the line which oscillates, as nominally the highest gain line comes to threshold first.
It is a primary object of the present invention to obtain single wavelength or selected narrow wavelength bandwidth operation of a laser by apparatus and methods which avoid the problems of the prior art.
It is a related object to obtain single wavelength operation of a laser by using a laser mirror having an optical coating constructed to reflect over a sufficiently narrow enough bandwidth as to be effective to select the preferred operating wavelength.
SUMMARY _ THE INVENTION
In accordance with the present invention single wavelength or narrow wavelength bandwidth operation of a laser is obtained by using a laser mirror with sufficiently narrow bandwidth optical coatings to select the preferred operating wavelength or wavelength range.
Mirrors with ultra-narrow bandwidth coatings have several advantages as compared to prior art, single wavelength selecting apparatus and methods.
Said mirrors can be less expensive to fabricate than prisms or birefringent plates~
Said mirrors are much easier to mount mechanically than either birefringent plates or prisms.
Said mirrors will undergo significantly less shifting in wavelength selectivity with changes in temperatures and thus will not require bimetallic compensation.
~Z~i~9~6 Said mirrors are also significantly easier to aliyn and to adjust in lasers with internal optics than are other wavelength selective components.
One of the severest situations for wavelength selective mirrors in practical Argon ion lasers exists for the three laser lines at 476.5 nanometers, 4~8.0 nanometers and 496.5 nanometers. The 4~8O0 nanometer line has the highest gain and for low power (20mW) versions of the Argon ion laser, the highest output power. Wavelength selective output mirrors designed to facilitate oscillation only on this line in accordance with one specific embodiment of the present invention provide approximately 2% transmission at 488.0 nanomaters and sufficient transmission at 476.5 nanometers and 496.5 nanometers to suppress oscillation on these lines. For typical 20 milliwatt Argon ion lasers, transmissions greater than 15% at 476.5 nanometers and 10~ a-t 496.5 nanometers are adequate.
In the prior art, laser mirrors have been made of dielectric materials of alternating quarter wave optical thickness high and low refractive index layers, but the common materials used for these layers have yielded bandwidth zones which were too wide to support laser oscillation in the Argon ion blue/green spectrum at one given spectral line while completely suppressing all adjacent lines. A typical high index material used ln the prior art was Titanium Dioxide with a refractive index of 2.35 (@ 500.0 nanometers). A typical low i1 index material was Silicon Dioxide with a refractive index of l.45 (@ S00.0 nanometers). Reflective multilayer stacks fabricated from these dielectric materials were not effective to function as selective optical elements for single line operation of the laser.
~:6~9~
The present invention embodies one or more mirrors which are effective to accomplish single wavelength or narrow bandwidth selection.
The method and apparatus of the present invention comprise a conventional laser gain medium, such as (but not limited to) an Argon, Krypton, ~elium-Neon or other gas~
discharge tube coupled with an optical ca~ity comprising two or more mirrors. At least one of the mirrors has su~iciently narrow spectral characteristics to support laser oscillation on one line and to suppress laser oscillation on all other lines.
Any one or all mirrors may be wavelength selective including reflectors, high reflectors and output couplers. A wavelength selective ouput coupler may also be placed at both ends of a two mirror cavity or comprise several mirrors in a multi-mirror cavity. Many combinations exist with wavelength selective coatings and standard bandwidth coatings as either high reflectors or output couplers.
In one specific embodiment of the present invention, multiple alternating layers of Aluminum Oxide (having a reractive index o~ appro~imately 1.676 at 488.0 nanometers) and Silicon Dioxide (having a refractive index of approximately 1.448 at 488.0 nanometers) are used on BK~l optical glass to provide single line operation of an Argon ion laser at 488.0 nanometers while suppressing oscillation at the adjacent 476.5 nanometers and 496.5 nanometers lines and other lines in the Argon ion blue/green spectrum. In this embodiment the coating allows oscillation at the 488.0 nanometers line by providing high optical reflectivity and low optical transmission through the mirror at that wavelength and suppresses oscillation at the 476.5 nanometers line and 496.5 nanometers line and other lines in the Argon ion blue/green spectrum by providing the necessary optical transmission through the mirror in order to suppress ~2~2~
oscillation at those wavelengths.
The coating of the present invention thus reflects over a fairly narrow bandwidth to allow oscillation at the selected line while being effective to suppress oscillation over all other lines and in particular over closely spaced adjacent lines.
The materials used for the coating of the present invention are dielectric materials which can survive the sustained high temperatures required for hard sealing the mirror to the end of the cavity of the gas laser without fracture, craze, peeling, re-evaporation from the substrate or other degradation to the optical characteristics of the coating. The hard sealing capability of this mirror is very unique. In general, mirrors with thick coatings or many layers tend to craze, crack, or peel under high temperature stresses produced by the mirror frit cycles. Other dielectric materials such as other oxides or fluorides may also be utilized.
However, choices are llmited due to the requirements as discussed elsewhere in this specification.
The dielectric materials of the coating o~ the present invention can also survive the vacuum environment and the hard ultra-violet radiation coming from the arc discharge in plasma tubes of gas ion lasers wi-thout change of the stoichiometry (crystal structure or chemistry) of the coating in part due to the low u]traviolet wavelength absorption edge.
The method and apparatus of the present invention may also be utilized in an "open cavity" configuration where the mirrors are separated from the direct plasma discharge environment by ~rewster windows or the like. This invention may also be utilized in dye lasers, ring dye lasers, or solid state lasers wherever limited wavelength ranges or individual spectral lines are desired.
.
~62~ 69407-17 The coating stack comprising indlvidual layer thicknesses of the coating design may be periodic or aperlodlc.
Indlvidual layer thlcknesses depend upon the required theoretical structure and the bandwidth required to suppress oscillation of adjacen~ spectral lines. These concept~ are de~ailed in the description of the preferred embodiments.
Wavelength sele~tlve mirror apparatus and methods which incorporate the structures and techniques described above and which are effective to function as described above con~ti~ute further, specific ob~ects of this invention.
The laser of the present invention may be summarlzed as laser comprising, a single resonant optical cavity, an optical galn medlum, at least two reflective mirrors delimiting t.he optical cavity, and coating means on at least one mirror surface for providing operation of said laser at any selected single or band of wavelengths, including any weak or low gain wavelengths, independent of their optical gains and at substantially any current or power level while suppressing non-selected wavelengths such that, said coatin~ ad-justs thresholds of all selected and non~selected wavelengths, including any dominant or high gain wavelengths, sald coating having a coating reflectance to support laser oscillation for said selected ~ingle or ~and of wavelengths such that said selected single wavelength or band of wavelengths are above threshold, said coating further having a coating reflectance to ~uppre~s laser oscillatlon at said non-selected wavelengths, including dominant or high gain waveleng~hs, su~h tha~ said non-selected wavelengths are below ~hreshold.
The method of the invention may be summarized as coa~ing a mirror surface wi~h a coating having optical characteristics which provide for operation of said laser at B
~;2g56 any selected single or band o~ wavelengths, including any weak or low gain wavelengths, independent of their optiaal gains and at subs~antially any current or power level while suppresslng non-selected wavelengths such that, said coating adjusts thresholds of all selected and non-selected wavelengths, including any dominant or high gain wavelengths, said coating having a coating reflectance to support laser osclllation ~or said salected single or band of wavelengths such tha~ said selected single wavelength or band of wavelengths are above threshold, said coa~lng further having a coating reflectance to suppress laser oscillation at said non-selected wavelengths, including dominant or high gain wavelengths, such that said non-selected wavelengths are below threshold, moun~ing the mirror with respect to an optical cavlty of the laser in a positlon whera ~he mlrror coating is effective to allow oscillation for said selected single or band of wavelength~, and operating the laser at a high input power level, above a threshold at whi~h the laser would otherwise begin to oscillate a number of llnes simultaneously.
Other and further objects of the present invention will be apparent ~rom the following description and clalmY and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments o the present invention and the principles thereof and what are now considered to be the best modes contemplated for applying these principles. Other embodiments of the invention embodying the same or equivalent principles may be used and s~ructural changes may be made as desired by those skilled in the art without departing ~rom the presen~ invention and the purview of the appended claim~.
6a B
~L2~j2~3~6 BRIEF DESCRtPTION OF THE _ AW~NGS:
Figure 1 is a side elevation view of a mirror for a laser constructed in accordance with one embodiment of the present invention. The mirror shown in Figure 1 has an output coupler optical interference coating which will allow oscillation of the 488.0 nanometer line of the Argon ion laser blue/greeen spectrum while suppressing all other lines in that spectrum, particularly the adjacent lines at 476.5 nanometers and 496.5 nanometers. The coating layers are shown much exaggerated in Figure 1 for purposes of illustration, and the major part of the central portion of the coating stack has been broken away to reduce the overall vertical height of the drawing Figure 1.
Figure 2 is a graphical representation of an equation (equation 2 referred to in the specification) plotting the bandwidth of the reflectance zone of a quarter wave optical thickness stack versus the ratio of the refractive index of the high index layer to the refractive index of the low,index layer. The numerals at the top of Fig. 2 indicate the primary lines at which an Argon ion laser can be made to lase in the blue/green spectrum.
Figure 3 is a graph of the spectral reflectivity of a high reflector stack (H~)NH illustrating higher order reflectance zones. The fundamental zone occurs at lambda sub zero/lambda equals 1 (~0/~
Figure 4 is a graph showing the computed spectral curve of the wavelength selective output coupler coating for oscillation of the Argon ion laser 488.0 nanometer spectral line only.
~i2~
DhSCR.lP'rION OF THE PRE~FERRED EMBODIMENTS:
The present invention is directed to a reflecting mirror for a laser of the kind which can oscillate on a nurnber of lines simultaneously.
The reflecting mirror of the present invention has an output coupler optical interference coating ~7hich reflects over a fairly narrow bandwidth and which provides sufficiently narrow spectral characteristics to support laser oscillation on one line and to suppress laser oscillation on all other lines.
The wavelength selective mirror of the present invention is used with a conventional laser gain medium and has particular application to use as an internal, end mirror for the optical cavity of an Argon ion gas laser in which the reflecting mirror surface is exposed to the plasma environment within the cavity of the ~rgon ion gas laser.
Figure 1 shows one embodiment of an end mirror constructed in accordance with the present invention and used as one of the reflecting mirrors for an Argon ion laser constructed for single line operation on the ~88.0 nanometer line.
The wavelength selective mirror shown in Fig. 1 is indicated generally by the reference numeral 11.
The mirrGr comprises an optical glass 13, which in the specific embodiment illustrated is BK-l optical glass.
The mirror 11 shown in Figure 1 is a mirror having a surface 15. The surface 15 is coated with layers 17 and 19 of dielectric materials having alternating high and low refractive index values.
The thickness of the coating layers has been greatly exaggerated for purposes of illustration, and the central part of the stack has been broken away, as indicated in Figure 1 t to g5~
simp]ify the showing.
The mlrror shown in Figure 1 provides single wavelength operation by providing hi~h optical reflectance and low op-tical transmittance of light over a fairly narrow bandwidth at the desired operating line and suppresses oscillation at the other closely spaced adjacent lines by providing the necessary optical transmittance and reflectance of light in order to suppress oscillation at the wavelengths corresponding to the other closely spaced adjacent lines -- as will now be described in more detail below.
The thin film coating of the present invention utilizes two basic concepts of optical interference filters.
The following basic descriptions and concepts deal with multilayer stacks of alternating high and low refractive index, non-absorbing dielectric layers. Each layer being a quarter-wave in optical thickness where the quarter wave optical thickness equals the number 4 multiplied by the index of refraction multiplied by the physical thickness of the layer (QWOT~~nt).
First, it is known that multilayer stacks of alternating layers of high and low refractive index values, (nh and nl, respectively) where nh is approximately equal to n reflect over a fairly narrow bandwidth.
Secondly, the wavelength bandwidth of such stacks is a function of the order (fundamental or higher order) Oe the reflectance ~one, and to a lesser extent the optical thickness of each indivldual layer.
From basic thin film theory it is known that if the stack has high refractive index outer layers, and if the reflectance (R) i5 high, then to a first approximation the transmittance (T) of such stack is given by:
T = l~R ~4(nl/nh)2N(ns/nh2). (1) ~ JL2~9 whe~e nl = index of refraction of the low index layer nh = index of refraction of the high index layer ns = index of refraction of the substrate 2N+l = total number of layers in the stack R = reflectance of the coating T = transmittance of the coating From the above equation it follows that as the difference between nh and nl approaches zero, more layer pairs are required to achieve a given low transmittance. The bandwidth (BW) of the reflectance zone is given by:
BW = (4/~)arcsin [(nh/nl-l)/(nh/nl+l)]- (2) From this e~uation we note that the bandwidth o~ a high reflectance coating is only a ~unction of the two indices of refraction of the high and low index layers, and as the quantity (nh/nl) approaches unity, the bandwidth approaches zero. A graphical representation of this equation is illustrated in Figure 2.
Also from thin film theory it is known that reElectance zones of such stacks exist at wavelengths where the layers are odd integral multiples (N~/4 where N=1,3,5,...) of a quarter-wave optical thickness (~/4). Hence higher order reElectance zones occur at ~0/3, ~0/5,...for a structure with the fundamental reflectance zone centered at ~. The wavelength bandwidths of these higher order reflectance zones go as BW/3, BW/5,...and so on. Hence, successively higher order zones produce successively narrower wavelength bandwidths. A coating stack designed with all layers (2N~ /4 thick, where N=0,1,2,3,..., wil~ produce a wavelength bandwidth of 1/(2N~
times a design with all quarter-waves (~/4), e.g. the first higher order reflectance zone will have a wavelength bandwidth 9~6 of 1/3 of the Fundamental reflectance zone.
The technical requirements of the wavelength selective output coupler for a nominal 20 milliwatt 488.0 internal mirror Aryon ion gas laser are as follows. The primary spectral requirements are:
T ~ 2.0% at 488.0 nm, normal incidence T > 15.0% at 476.5 nm, normal incidence T ~ 10.0% at 496.5 nm, normal incidence A < 0.05~ at 488.0 nm, S ~ 0.1~ at 488.0 nm, where A = absorptance loss and S- scatter loss, with secondary requirements of sufficiently high transmittance for all other Aryon ion laser lines in the blue/green spectrum in order to suppress oscillation. Environmental and durability requirements are also somewhat stringent. The coating must possess high ultraviolet radiation damage resistance in order to survive bore radiation generated by the plasma discharge.
It must be capable of surviving lifetime requiremen~s on the or~er of thousands of hours due to the fact this application is for internal mirror ion lasers and the frit seal prevents replacement of failed mirrors. Air/Vacuum and frit cycle shift of the center wavelength of the coating must be minimal in cycles from atmosphere to vacuum and from high temperature excursions of the frit cycle in order to predict repeatability.
The coating must also be capable of surviving mirror seal frit cycles on the order of ~65 degrees Centigrade. After these cycles the coating must not degrade, crack, craze or peel and must continue to meet all technical specifications.
We have found that a coating consisting of 33 3~/4 thick layers of the format S,(HL)NH,Vacuum where N=16 of Aluminum Oxide (H) and Silicon Dioxide (L) meets the above ~L2~;~9~6 requirements~ The refractive index of A12O3 at ~88.0 nanometers is approximately 1.676 and the refractive index of SiO2 is approximately 1.448. From calculations using Equation 1 we find the transmittance of the coa-ting on BK-l (ns=1.522) optical glass ~o be 2.0%. The bandwidth (BW) calculation (Equation 2) yields 3.1% or lS.0 nanometers at 488.0 nanometers. This bandwidth is adequate for the simultaneous suppression of 476.5 nanometers and 496.5 nanometers as well as all other closely spaced adjacent lines in the blue/green spectrum. See Fig. 4 for the computed spectral curve. In laser testing to date the coating has not degraded, peeled or crazed in frit cycles to 500 degrees Centigrade. The coating shifts an average of 0.7 to 1.0 nanometers shorter as the coating is subjected to the frit cycles and the partial vacuum environment of the laser tube. This shift has been highly consistent and repeatable so the coating manufacturer can center the coating accordingly. Also in testing, no degradation of the coating due to the plasma discharge or ultraviolet radiation has been observed over 5000 hours o~
continuous operation of laser tubes.
While this invention has been disclosed in detail immediately above for operation of an Argon ion gas laser at the 488.0 nm line, the invention can also be used for operation of the Argon ion gas laser at other spectral lines (e.g., the 457.9, 476.5, 514.5l 351.1 and 363.8 nm lines) and can also be used for narrow bandwidth wavelength selection operation of other lasers, such as, for example, Krypton lasers, YAG rod or slab lasers, solid state lasers and ring or dye lasers.
While we have illustrated and described the preferred embodiments of our invention, it is to be understood that these are capable of variation and modification, and we therefore do not wish to be limited to the precise details set orth, but 95~
desire to ava:il ourselves o~ such changes and alterations as all within the purview of the following claims.
~ !
I
~ I
`
: :
ULTRA-NARROW BANDWIDTH OPTICAL THIN FILM INTERFERENCE
_ _ __ __ _ _ _ _ COATINGS FOR SINGLE WAVELENGTH LASERS
B~CKGROUND OF THE INVENTION:
_ This invention relates generally to lasers and optical thin film inter~erence filters for providing narrow wavelength bandwidth operation of a laser.
This invention has specific application to wavelength selective optical elements for narrow wavelength bandwidth or single wavelength operation of a gas laser.
Many applications of lasers require single wavelength operation of the laser device. The Argon ion laser, which is used in laser reprographic systems for example, can oscillate on a number of lines in the 450 nanometer to 520 nanometer spectral region simultaneously. Traditionally, a single wavelength is selected in the laser by introducing a wavelenyth selective optical element, such as a prism or birefringent filter, into the optical cavity.
Alternatively, the laser device can be run at a low input power level such that only a single line is above threshold. -The above described techniques are deemed unacceptablein many instances.
Laser cavities with prisms or birefringent filters are significantly more dificult to align and to maintain in alignment than cavities with mirrors only. Also, complex methods to compensate for thermal changes in refractive index must be employed to stabilize lasers that utilize prisms and operate over a wide range of temperatures.
9S~
rrhe second method referred to above, running the laser at low input power level to obtain a single line, unnecessarily limits the single wavelength laser output power. Also, it allows virtually no selection o~ the line which oscillates, as nominally the highest gain line comes to threshold first.
It is a primary object of the present invention to obtain single wavelength or selected narrow wavelength bandwidth operation of a laser by apparatus and methods which avoid the problems of the prior art.
It is a related object to obtain single wavelength operation of a laser by using a laser mirror having an optical coating constructed to reflect over a sufficiently narrow enough bandwidth as to be effective to select the preferred operating wavelength.
SUMMARY _ THE INVENTION
In accordance with the present invention single wavelength or narrow wavelength bandwidth operation of a laser is obtained by using a laser mirror with sufficiently narrow bandwidth optical coatings to select the preferred operating wavelength or wavelength range.
Mirrors with ultra-narrow bandwidth coatings have several advantages as compared to prior art, single wavelength selecting apparatus and methods.
Said mirrors can be less expensive to fabricate than prisms or birefringent plates~
Said mirrors are much easier to mount mechanically than either birefringent plates or prisms.
Said mirrors will undergo significantly less shifting in wavelength selectivity with changes in temperatures and thus will not require bimetallic compensation.
~Z~i~9~6 Said mirrors are also significantly easier to aliyn and to adjust in lasers with internal optics than are other wavelength selective components.
One of the severest situations for wavelength selective mirrors in practical Argon ion lasers exists for the three laser lines at 476.5 nanometers, 4~8.0 nanometers and 496.5 nanometers. The 4~8O0 nanometer line has the highest gain and for low power (20mW) versions of the Argon ion laser, the highest output power. Wavelength selective output mirrors designed to facilitate oscillation only on this line in accordance with one specific embodiment of the present invention provide approximately 2% transmission at 488.0 nanomaters and sufficient transmission at 476.5 nanometers and 496.5 nanometers to suppress oscillation on these lines. For typical 20 milliwatt Argon ion lasers, transmissions greater than 15% at 476.5 nanometers and 10~ a-t 496.5 nanometers are adequate.
In the prior art, laser mirrors have been made of dielectric materials of alternating quarter wave optical thickness high and low refractive index layers, but the common materials used for these layers have yielded bandwidth zones which were too wide to support laser oscillation in the Argon ion blue/green spectrum at one given spectral line while completely suppressing all adjacent lines. A typical high index material used ln the prior art was Titanium Dioxide with a refractive index of 2.35 (@ 500.0 nanometers). A typical low i1 index material was Silicon Dioxide with a refractive index of l.45 (@ S00.0 nanometers). Reflective multilayer stacks fabricated from these dielectric materials were not effective to function as selective optical elements for single line operation of the laser.
~:6~9~
The present invention embodies one or more mirrors which are effective to accomplish single wavelength or narrow bandwidth selection.
The method and apparatus of the present invention comprise a conventional laser gain medium, such as (but not limited to) an Argon, Krypton, ~elium-Neon or other gas~
discharge tube coupled with an optical ca~ity comprising two or more mirrors. At least one of the mirrors has su~iciently narrow spectral characteristics to support laser oscillation on one line and to suppress laser oscillation on all other lines.
Any one or all mirrors may be wavelength selective including reflectors, high reflectors and output couplers. A wavelength selective ouput coupler may also be placed at both ends of a two mirror cavity or comprise several mirrors in a multi-mirror cavity. Many combinations exist with wavelength selective coatings and standard bandwidth coatings as either high reflectors or output couplers.
In one specific embodiment of the present invention, multiple alternating layers of Aluminum Oxide (having a reractive index o~ appro~imately 1.676 at 488.0 nanometers) and Silicon Dioxide (having a refractive index of approximately 1.448 at 488.0 nanometers) are used on BK~l optical glass to provide single line operation of an Argon ion laser at 488.0 nanometers while suppressing oscillation at the adjacent 476.5 nanometers and 496.5 nanometers lines and other lines in the Argon ion blue/green spectrum. In this embodiment the coating allows oscillation at the 488.0 nanometers line by providing high optical reflectivity and low optical transmission through the mirror at that wavelength and suppresses oscillation at the 476.5 nanometers line and 496.5 nanometers line and other lines in the Argon ion blue/green spectrum by providing the necessary optical transmission through the mirror in order to suppress ~2~2~
oscillation at those wavelengths.
The coating of the present invention thus reflects over a fairly narrow bandwidth to allow oscillation at the selected line while being effective to suppress oscillation over all other lines and in particular over closely spaced adjacent lines.
The materials used for the coating of the present invention are dielectric materials which can survive the sustained high temperatures required for hard sealing the mirror to the end of the cavity of the gas laser without fracture, craze, peeling, re-evaporation from the substrate or other degradation to the optical characteristics of the coating. The hard sealing capability of this mirror is very unique. In general, mirrors with thick coatings or many layers tend to craze, crack, or peel under high temperature stresses produced by the mirror frit cycles. Other dielectric materials such as other oxides or fluorides may also be utilized.
However, choices are llmited due to the requirements as discussed elsewhere in this specification.
The dielectric materials of the coating o~ the present invention can also survive the vacuum environment and the hard ultra-violet radiation coming from the arc discharge in plasma tubes of gas ion lasers wi-thout change of the stoichiometry (crystal structure or chemistry) of the coating in part due to the low u]traviolet wavelength absorption edge.
The method and apparatus of the present invention may also be utilized in an "open cavity" configuration where the mirrors are separated from the direct plasma discharge environment by ~rewster windows or the like. This invention may also be utilized in dye lasers, ring dye lasers, or solid state lasers wherever limited wavelength ranges or individual spectral lines are desired.
.
~62~ 69407-17 The coating stack comprising indlvidual layer thicknesses of the coating design may be periodic or aperlodlc.
Indlvidual layer thlcknesses depend upon the required theoretical structure and the bandwidth required to suppress oscillation of adjacen~ spectral lines. These concept~ are de~ailed in the description of the preferred embodiments.
Wavelength sele~tlve mirror apparatus and methods which incorporate the structures and techniques described above and which are effective to function as described above con~ti~ute further, specific ob~ects of this invention.
The laser of the present invention may be summarlzed as laser comprising, a single resonant optical cavity, an optical galn medlum, at least two reflective mirrors delimiting t.he optical cavity, and coating means on at least one mirror surface for providing operation of said laser at any selected single or band of wavelengths, including any weak or low gain wavelengths, independent of their optical gains and at substantially any current or power level while suppressing non-selected wavelengths such that, said coatin~ ad-justs thresholds of all selected and non~selected wavelengths, including any dominant or high gain wavelengths, sald coating having a coating reflectance to support laser oscillation for said selected ~ingle or ~and of wavelengths such that said selected single wavelength or band of wavelengths are above threshold, said coating further having a coating reflectance to ~uppre~s laser oscillatlon at said non-selected wavelengths, including dominant or high gain waveleng~hs, su~h tha~ said non-selected wavelengths are below ~hreshold.
The method of the invention may be summarized as coa~ing a mirror surface wi~h a coating having optical characteristics which provide for operation of said laser at B
~;2g56 any selected single or band o~ wavelengths, including any weak or low gain wavelengths, independent of their optiaal gains and at subs~antially any current or power level while suppresslng non-selected wavelengths such that, said coating adjusts thresholds of all selected and non-selected wavelengths, including any dominant or high gain wavelengths, said coating having a coating reflectance to support laser osclllation ~or said salected single or band of wavelengths such tha~ said selected single wavelength or band of wavelengths are above threshold, said coa~lng further having a coating reflectance to suppress laser oscillation at said non-selected wavelengths, including dominant or high gain wavelengths, such that said non-selected wavelengths are below threshold, moun~ing the mirror with respect to an optical cavlty of the laser in a positlon whera ~he mlrror coating is effective to allow oscillation for said selected single or band of wavelength~, and operating the laser at a high input power level, above a threshold at whi~h the laser would otherwise begin to oscillate a number of llnes simultaneously.
Other and further objects of the present invention will be apparent ~rom the following description and clalmY and are illustrated in the accompanying drawings which, by way of illustration, show preferred embodiments o the present invention and the principles thereof and what are now considered to be the best modes contemplated for applying these principles. Other embodiments of the invention embodying the same or equivalent principles may be used and s~ructural changes may be made as desired by those skilled in the art without departing ~rom the presen~ invention and the purview of the appended claim~.
6a B
~L2~j2~3~6 BRIEF DESCRtPTION OF THE _ AW~NGS:
Figure 1 is a side elevation view of a mirror for a laser constructed in accordance with one embodiment of the present invention. The mirror shown in Figure 1 has an output coupler optical interference coating which will allow oscillation of the 488.0 nanometer line of the Argon ion laser blue/greeen spectrum while suppressing all other lines in that spectrum, particularly the adjacent lines at 476.5 nanometers and 496.5 nanometers. The coating layers are shown much exaggerated in Figure 1 for purposes of illustration, and the major part of the central portion of the coating stack has been broken away to reduce the overall vertical height of the drawing Figure 1.
Figure 2 is a graphical representation of an equation (equation 2 referred to in the specification) plotting the bandwidth of the reflectance zone of a quarter wave optical thickness stack versus the ratio of the refractive index of the high index layer to the refractive index of the low,index layer. The numerals at the top of Fig. 2 indicate the primary lines at which an Argon ion laser can be made to lase in the blue/green spectrum.
Figure 3 is a graph of the spectral reflectivity of a high reflector stack (H~)NH illustrating higher order reflectance zones. The fundamental zone occurs at lambda sub zero/lambda equals 1 (~0/~
Figure 4 is a graph showing the computed spectral curve of the wavelength selective output coupler coating for oscillation of the Argon ion laser 488.0 nanometer spectral line only.
~i2~
DhSCR.lP'rION OF THE PRE~FERRED EMBODIMENTS:
The present invention is directed to a reflecting mirror for a laser of the kind which can oscillate on a nurnber of lines simultaneously.
The reflecting mirror of the present invention has an output coupler optical interference coating ~7hich reflects over a fairly narrow bandwidth and which provides sufficiently narrow spectral characteristics to support laser oscillation on one line and to suppress laser oscillation on all other lines.
The wavelength selective mirror of the present invention is used with a conventional laser gain medium and has particular application to use as an internal, end mirror for the optical cavity of an Argon ion gas laser in which the reflecting mirror surface is exposed to the plasma environment within the cavity of the ~rgon ion gas laser.
Figure 1 shows one embodiment of an end mirror constructed in accordance with the present invention and used as one of the reflecting mirrors for an Argon ion laser constructed for single line operation on the ~88.0 nanometer line.
The wavelength selective mirror shown in Fig. 1 is indicated generally by the reference numeral 11.
The mirrGr comprises an optical glass 13, which in the specific embodiment illustrated is BK-l optical glass.
The mirror 11 shown in Figure 1 is a mirror having a surface 15. The surface 15 is coated with layers 17 and 19 of dielectric materials having alternating high and low refractive index values.
The thickness of the coating layers has been greatly exaggerated for purposes of illustration, and the central part of the stack has been broken away, as indicated in Figure 1 t to g5~
simp]ify the showing.
The mlrror shown in Figure 1 provides single wavelength operation by providing hi~h optical reflectance and low op-tical transmittance of light over a fairly narrow bandwidth at the desired operating line and suppresses oscillation at the other closely spaced adjacent lines by providing the necessary optical transmittance and reflectance of light in order to suppress oscillation at the wavelengths corresponding to the other closely spaced adjacent lines -- as will now be described in more detail below.
The thin film coating of the present invention utilizes two basic concepts of optical interference filters.
The following basic descriptions and concepts deal with multilayer stacks of alternating high and low refractive index, non-absorbing dielectric layers. Each layer being a quarter-wave in optical thickness where the quarter wave optical thickness equals the number 4 multiplied by the index of refraction multiplied by the physical thickness of the layer (QWOT~~nt).
First, it is known that multilayer stacks of alternating layers of high and low refractive index values, (nh and nl, respectively) where nh is approximately equal to n reflect over a fairly narrow bandwidth.
Secondly, the wavelength bandwidth of such stacks is a function of the order (fundamental or higher order) Oe the reflectance ~one, and to a lesser extent the optical thickness of each indivldual layer.
From basic thin film theory it is known that if the stack has high refractive index outer layers, and if the reflectance (R) i5 high, then to a first approximation the transmittance (T) of such stack is given by:
T = l~R ~4(nl/nh)2N(ns/nh2). (1) ~ JL2~9 whe~e nl = index of refraction of the low index layer nh = index of refraction of the high index layer ns = index of refraction of the substrate 2N+l = total number of layers in the stack R = reflectance of the coating T = transmittance of the coating From the above equation it follows that as the difference between nh and nl approaches zero, more layer pairs are required to achieve a given low transmittance. The bandwidth (BW) of the reflectance zone is given by:
BW = (4/~)arcsin [(nh/nl-l)/(nh/nl+l)]- (2) From this e~uation we note that the bandwidth o~ a high reflectance coating is only a ~unction of the two indices of refraction of the high and low index layers, and as the quantity (nh/nl) approaches unity, the bandwidth approaches zero. A graphical representation of this equation is illustrated in Figure 2.
Also from thin film theory it is known that reElectance zones of such stacks exist at wavelengths where the layers are odd integral multiples (N~/4 where N=1,3,5,...) of a quarter-wave optical thickness (~/4). Hence higher order reElectance zones occur at ~0/3, ~0/5,...for a structure with the fundamental reflectance zone centered at ~. The wavelength bandwidths of these higher order reflectance zones go as BW/3, BW/5,...and so on. Hence, successively higher order zones produce successively narrower wavelength bandwidths. A coating stack designed with all layers (2N~ /4 thick, where N=0,1,2,3,..., wil~ produce a wavelength bandwidth of 1/(2N~
times a design with all quarter-waves (~/4), e.g. the first higher order reflectance zone will have a wavelength bandwidth 9~6 of 1/3 of the Fundamental reflectance zone.
The technical requirements of the wavelength selective output coupler for a nominal 20 milliwatt 488.0 internal mirror Aryon ion gas laser are as follows. The primary spectral requirements are:
T ~ 2.0% at 488.0 nm, normal incidence T > 15.0% at 476.5 nm, normal incidence T ~ 10.0% at 496.5 nm, normal incidence A < 0.05~ at 488.0 nm, S ~ 0.1~ at 488.0 nm, where A = absorptance loss and S- scatter loss, with secondary requirements of sufficiently high transmittance for all other Aryon ion laser lines in the blue/green spectrum in order to suppress oscillation. Environmental and durability requirements are also somewhat stringent. The coating must possess high ultraviolet radiation damage resistance in order to survive bore radiation generated by the plasma discharge.
It must be capable of surviving lifetime requiremen~s on the or~er of thousands of hours due to the fact this application is for internal mirror ion lasers and the frit seal prevents replacement of failed mirrors. Air/Vacuum and frit cycle shift of the center wavelength of the coating must be minimal in cycles from atmosphere to vacuum and from high temperature excursions of the frit cycle in order to predict repeatability.
The coating must also be capable of surviving mirror seal frit cycles on the order of ~65 degrees Centigrade. After these cycles the coating must not degrade, crack, craze or peel and must continue to meet all technical specifications.
We have found that a coating consisting of 33 3~/4 thick layers of the format S,(HL)NH,Vacuum where N=16 of Aluminum Oxide (H) and Silicon Dioxide (L) meets the above ~L2~;~9~6 requirements~ The refractive index of A12O3 at ~88.0 nanometers is approximately 1.676 and the refractive index of SiO2 is approximately 1.448. From calculations using Equation 1 we find the transmittance of the coa-ting on BK-l (ns=1.522) optical glass ~o be 2.0%. The bandwidth (BW) calculation (Equation 2) yields 3.1% or lS.0 nanometers at 488.0 nanometers. This bandwidth is adequate for the simultaneous suppression of 476.5 nanometers and 496.5 nanometers as well as all other closely spaced adjacent lines in the blue/green spectrum. See Fig. 4 for the computed spectral curve. In laser testing to date the coating has not degraded, peeled or crazed in frit cycles to 500 degrees Centigrade. The coating shifts an average of 0.7 to 1.0 nanometers shorter as the coating is subjected to the frit cycles and the partial vacuum environment of the laser tube. This shift has been highly consistent and repeatable so the coating manufacturer can center the coating accordingly. Also in testing, no degradation of the coating due to the plasma discharge or ultraviolet radiation has been observed over 5000 hours o~
continuous operation of laser tubes.
While this invention has been disclosed in detail immediately above for operation of an Argon ion gas laser at the 488.0 nm line, the invention can also be used for operation of the Argon ion gas laser at other spectral lines (e.g., the 457.9, 476.5, 514.5l 351.1 and 363.8 nm lines) and can also be used for narrow bandwidth wavelength selection operation of other lasers, such as, for example, Krypton lasers, YAG rod or slab lasers, solid state lasers and ring or dye lasers.
While we have illustrated and described the preferred embodiments of our invention, it is to be understood that these are capable of variation and modification, and we therefore do not wish to be limited to the precise details set orth, but 95~
desire to ava:il ourselves o~ such changes and alterations as all within the purview of the following claims.
~ !
I
~ I
`
: :
Claims (15)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A laser of the kind which can oscillate on more than one wavelength simultaneously, said laser comprising, a single resonant optical cavity, an optical gain medium, at least two reflective mirrors delimiting the optical cavity, and coating means on at least one mirror surface for providing operation of said laser at any selected single or band of wavelengths, including any weak or low gain wavelengths, independent of their optical gains and at substantially any current or power level while suppressing non-selected wavelengths such that, said coating adjusts thresholds of all selected and non-selected wavelengths, including any dominant or high gain wavelengths, said coating having a coating reflectance to support laser oscillation for said selected single or band of wavelengths such that said selected single wavelength or band of wavelengths are above threshold, said coating further having a coating reflectance to suppress laser oscillation at said non-selected wavelengths, including dominant or high gain wavelengths, such that said non-selected wavelengths are below threshold.
2. The invention defined in claim 1 wherein the coating means provide single wavelength operation of the laser by allowing oscillation of one line while suppressing oscillation of closely spaced adjacent lines.
3. The invention defined in claim 2 wherein the coating means have spectral characteristics sufficiently narrow to allow oscillation of said one line by providing the appropriate optical reflectance for oscillation of said one line and to suppress oscillation of said closely spaced adjacent lines by providing the necessary optical transmittance through the mirror in order to suppress oscillation at the wavelengths corresponding to said closely spaced adjacent lines.
4. The invention defined in claim 1 wherein the laser is an internal mirror gas laser and at least one mirror has an internal surface exposed to a plasma environment within the cavity of the gas laser.
5. The invention defined in claim 1 wherein at least one laser mirror is external to a plasma environment within the laser.
6. The invention defined in claim 1 wherein the laser is an Argon ion gas laser and the coating means provide appropriate reflectance for oscillation of one of the following:
a. 457.9 nanometers;
b. 476.5 nanometers;
c. 488.0 nanometers;
d. 514.5 nanometers;
e. 351.1 nanometers;
f. 363.8 nanometers.
a. 457.9 nanometers;
b. 476.5 nanometers;
c. 488.0 nanometers;
d. 514.5 nanometers;
e. 351.1 nanometers;
f. 363.8 nanometers.
7. The invention defined in claim 1 where the laser is a Krypton ion gas laser.
8. The invention defined in claim 1 wherein the coating means comprise multilayer stacks of dielectric materials of alternating high and low refractive index values which reflect over a fairly narrow bandwidth.
9. The invention defined in claim 8 wherein the wavelength bandwidth of the coating stack is a function of the order of the reflectance zone and the optical thickness of each individual layer.
10. The invention defined in claim 9 wherein the laser is an Argon ion gas laser, the coating means are constructed to provide single wavelength operation at 488.0 nanometers, and the coating means comprise alternating layers of aluminum oxide (Al2O3) and silicon dioxide (SiO2).
11. The invention defined in claim 1 wherein the coating means comprise dielectric materials having differences in optical indices of refraction effective to determine the narrow width of the reflectance band.
12. The invention defined in claim 11 wherein said dielectric materials can survive the sustained high temperatures required for hard sealing the mirror to the associated cavity structure without fracture, re-evaporation from the substrate or other degradation to the optical or physical characteristics of the coating.
13. The invention defined in claim 12 wherein said dielectric materials can survive the vacuum environment and ultraviolet radiation coming from the arc discharge in plasma tubes of gas lasers without degradation to the optical or physical characteristics of the coating.
14. The invention defined in claim 13 wherein the coating means comprise layers of aluminum oxide (Al2O3) and silicon dioxide (SiO2).
15. A method of producing single wavelength operation of a laser of the kind which can oscillate on a number of lines simultaneously, at substantially any input power level r said method comprising, coating a mirror surface with a coaling having optical characteristics which provide for operation of said laser at any selected single or band of wavelengths, including any weak or low gain wavelengths, independent of their optical gains and at substantially any current or power level while suppressing non-selected wavelengths such that, said coating adjusts thresholds of all selected and non-selected wavelengths, including any dominant or high gain wavelengths, said coating having a coating reflectance to support laser oscillation for said selected single or band of wavelengths such that said selected single wavelength or band of wavelengths are above threshold, said coating further having a coating reflectance to suppress laser oscillation at said non-selected wavelengths, including dominant or high gain wavelengths, such that said non-selected wavelengths are below threshold, mounting the mirror with respect to an optical cavity of the laser in a position where the mirror coating is effective to allow oscillation for said selected single or band of wavelengths, and operating the laser at a high input power level, above a threshold at which the laser would otherwise begin to oscillate a number of lines simultaneously.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/595,296 | 1984-03-30 | ||
| US06/595,296 US4615034A (en) | 1984-03-30 | 1984-03-30 | Ultra-narrow bandwidth optical thin film interference coatings for single wavelength lasers |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA1262956A true CA1262956A (en) | 1989-11-14 |
| CA1262956C CA1262956C (en) | 1989-11-14 |
Family
ID=24382651
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA477480A Expired CA1262956C (en) | 1984-03-30 | 1985-03-26 | Ultra-narrow bandwidth optical thin film interference coatings for single wave-length lasers |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4615034A (en) |
| JP (1) | JPS60218601A (en) |
| CA (1) | CA1262956C (en) |
| DE (1) | DE3510554A1 (en) |
| FR (1) | FR2562269B1 (en) |
| GB (1) | GB2156577B (en) |
| IE (1) | IE56360B1 (en) |
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1984
- 1984-03-30 US US06/595,296 patent/US4615034A/en not_active Expired - Lifetime
-
1985
- 1985-03-08 IE IE613/85A patent/IE56360B1/en not_active IP Right Cessation
- 1985-03-21 DE DE19853510554 patent/DE3510554A1/en not_active Ceased
- 1985-03-26 JP JP60059740A patent/JPS60218601A/en active Pending
- 1985-03-26 CA CA477480A patent/CA1262956C/en not_active Expired
- 1985-03-29 FR FR858504813A patent/FR2562269B1/en not_active Expired - Lifetime
- 1985-04-01 GB GB08508440A patent/GB2156577B/en not_active Expired
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| DE3510554A1 (en) | 1985-10-10 |
| FR2562269B1 (en) | 1990-01-19 |
| IE56360B1 (en) | 1991-07-03 |
| GB2156577B (en) | 1988-12-21 |
| GB8508440D0 (en) | 1985-05-09 |
| GB2156577A (en) | 1985-10-09 |
| JPS60218601A (en) | 1985-11-01 |
| IE850613L (en) | 1985-09-30 |
| US4615034B1 (en) | 1990-05-29 |
| US4615034A (en) | 1986-09-30 |
| CA1262956C (en) | 1989-11-14 |
| FR2562269A1 (en) | 1985-10-04 |
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