CA2016509A1 - Frequency conversion of optical radiation - Google Patents

Frequency conversion of optical radiation

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Publication number
CA2016509A1
CA2016509A1 CA002016509A CA2016509A CA2016509A1 CA 2016509 A1 CA2016509 A1 CA 2016509A1 CA 002016509 A CA002016509 A CA 002016509A CA 2016509 A CA2016509 A CA 2016509A CA 2016509 A1 CA2016509 A1 CA 2016509A1
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CA
Canada
Prior art keywords
optical
radiation
frequency
cavity
laser
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.)
Abandoned
Application number
CA002016509A
Other languages
French (fr)
Inventor
Douglas W. Anthon
Donald L. Sipes, Jr.
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BP Corp North America Inc
Original Assignee
BP Corp North America Inc
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Filing date
Publication date
Application filed by BP Corp North America Inc filed Critical BP Corp North America Inc
Publication of CA2016509A1 publication Critical patent/CA2016509A1/en
Abandoned legal-status Critical Current

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Classifications

    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES 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
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H01S3/109Frequency multiplication, e.g. harmonic generation
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES 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
    • H01S2301/00Functional characteristics
    • H01S2301/02ASE (amplified spontaneous emission), noise; Reduction thereof
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES 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
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/102Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
    • H01S3/1028Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation by controlling the temperature

Abstract

Abstract of the Disclosure An intracavity frequency-modified laser of improved amplitude stability is obtained by substantially eliminating spatial hole burning in the lasant material and maintaining the optical cavity of the laser at a temperature which results in substantially noise-free generation of output radiation.

Description

~lfi lng PATENT APPLICATION
FREQUI~N~Y CONVEP~ION QE QpTl~AL RA!21ATION

1~1~ of th~ Invention This invention relates to the convarsion of opticai radiation of one fr~quency into optical radiation of another frequency by int~raction with nonlinear optical material within an optical cavity. More particularly, it relates ~o a method for improving the amplitude stability of the frequency modified radiation from such a process.
Cross-R~f~r~n~ to R~!atecl A~j~

This application is a continuation-in-part of application Serial No.
207,666, filed June 16, 1988, which in turn is a continuation-in-part of application Serial No. 157,741, filed Februa~y 18,1988.
.

A laser is a cl0vice which has the ability to produce monochromatic, coherent ligh~ through tha stimulat~d emission of photons from atoms, molecules or ions of an active medium which have typically been excit~d from a ground state to a higher energy lev01 by an input of energy. Such a devic~ ;
contains an optical cavity or rasonator which is defin~d by highly refle~ing surfaces which forrn a ciosed round trip path for light, and the active m~dium is contained within the ~ptical cavity.
If a population inv~rsion is cr~ated by excitation of the active medium, the spontaneous omission of a photon from an excited atom, molecule or ion und~rgoinQ transition to a low0r anargy stats can stimulate the emission of photon~ ot substantially identical en3rgy from othar excited atoms, molecules or ions. As a consequ0ncs, the initial photon areates a cascade of photons bstwo~n tho reflscting surtac~s ot tha op~ical cavity which ara of substantiallyidentical ~ner~y and exac~ly in phase. A portiorl of this cascad~ of photons is then dischar~ed out of the op~ical cavity, for ~xample, by transmission through one or` mor~ of the rafl~cting surfaccs of the cavity. These discharged photons constitute the laser output.
Excitation of the~ active m~dium of a laser can be accomplishcd by a variety of m~hods. Howover, th~ most common m~thods ar~ op~ical pumping, ,. .

..
:

: . .

- ~ . - - , ~ . - .

'' ,. ' ' ` . I ' ., ' . . , ' ' . , . . ' .' ~ , 20~09 use of an electrical discharge, and the passage of an el~ctric current through the p-n junction of a ssmiconciuctor laser.
Semiconductor lasers eontain a p-n junction which forms a diode, and this junction functions as lhe active medium of ~he laser. Such devices, which 5 are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum ~allium arsenide alloys. Th~
~fficiency of such lasers in converting el~c~rical power to output radiation is relatively high and, for ~xample, c~n be in exoess of 40 percent.
The use of fl~shlamps, Iight-~mittin~ diodes ('as usad herein, this term 10 includes superluminescent diodes and superluminescent diode arrays) and laser diodes ~as used herein, this term includes laser diode arrays) to optically pump or excite a solid lasant material is well-known. Lasant materials oommonly used in such solid state lasers include crystalline or glassy host rnaterials into which an activ0 material, such as trivalent nsodymium ions, is 15 incorporated. Highly suitable solid lasant materials include substances wherein the active materiai is a stoichiometric component o~ the lasant material. Such stoichiometric materials include, for example, neoldymium pentaphosphate and lithium neodymium tetraphosphata. Detail0d surnmaries of conventional solid lasant mat~rials ara set forth in th0 ~ti~hQ~
20 Laser Sci~nce and I~chnolQgy, Vol. I, M. J. Weber, Ed., CRC Prsss, Inc., Boca Raton, Florida, 1982, pp. 72-135 and by A. A. Kaminskii in ~E2L~, \I~OI
14 ot the Sprin~er Series in Optical Sciences, D. L. MacAdam, Ed., Springer-Verlag, New York~ N.Y., 1981. Conventional host materials for neodymium ions include glass, yttrium aluminum garnet (Y3AI5012; ref0rred to as YAG), 25 YAI03 (ref0rred to as YALO), LiYF4 (r~err~d to as YLF), an~ gadelinium scandium gallium ~arnet (Gd3Sc2Ga3012) referred to as GSGG. By way of example, when neodymium-doped YAG is employad as the lasant material in an optically pumped ~olid state laser, it can be pumped by absorption of light having a wavelength of about 808 nm and can emit Ught havin~ a wav~length 30 ot 10~ nm.
U.S. Patent No. 3,624,545 issu~d to Ross on Nov~mber 30, 1971, describes an optically pumped solid stat~ las~r composcd of a YAG rod which is side-pumped by at least one semiconductor laser dlode. Similarly, U.S.
Patent No. 3,753,14~ issued to Chesler on August 14, 1973, discloses the us~
35 of one or mor~ light-smitting semiconductor diodes to ~nd pump a naodymium-dopsd YAG rod. The use of an array of pulsed laser diodes to end pump a solid lasant matenalsuch as neodymium-doped YA~is described in U.S. Patent No. 3,982,201 issu0d to Rosenkrantz et al. on Sept~mbar 1, 2~16~9 1976. Finally, D. L. Sipes, ~_~b~. Vol. 47, No. 2, 1985, pp. 74-75, has reported that the use of a tigh~ly focused semiconductor laser diode array to enci pump a neodymium-doped YAG results in a high efficiency conversion of pumping radiation having a wavelength of 810 nm to output radiation having a wavelength of 1064 nm.
Materials having nonlinear optical properties are well-known. For exampl~, U.S. Patent No. 3,949,323 issued to Bierlein et al. on April 6, 1976, discloses that nonlinear optical properties are possessed by mat0rials having tha formula MTiO~XO4) where M is at least one of K, Rb, Tl and NH4; and X is at least one of P or As, except when NH4 is preserlt, then X is only P. This generic formula includes potassium titanyl phosphate, KTiOP04, a particularly useful nonlinear material. Other known nonlinear optical materials include, but are not limited to, KH2P04, LiNbO3, KNbO3, 13-BaB204, Ba2Nal~lb5015, LilO3 Hl03, KB508 4H20, potassium lithium niobat0 and urea. A r~view of the nonlinear optical propertias of a number of different unia~(ial crystals hasbeen published in Sov~J. Q~antum Ele~rQ~, Vol. 7, No. 1, Janua~ 1977, pp.
1-13. Nonlinsar opticai materials have also been revi0wed by S. Singh in the CRC~Handbook of Las~r $~i~nc~ and T0~n~lQ~y, Vol. Ill, M. J. Weber, Ed.
CRC Press, Inc., Boca Raton, Florida, 1986, pp. 3-228.
The conversion of optical radiatlon of one fr~quency to optical radiation of another fr0quency through interac~ion with a nonlinear optical material is well-known and has been extensively studied. Examples of such conversion include harmonic genaration, optical mixing and parametric osciliation.
Second-harmonic ~neration or ~fr~quenoy doubling" is perhaps the most common and important ~xampl~ of nonlinear optics wherein part of the anergy of an optical wavc of angular frequency ~D propagating through a nonlinsar optical crystal is conYerted to ene~gy of a wave of angular frequency 2c~.
Second-harmonio ~eneration has been ~viewed by A. Ya~iv in Quantum ~, Seoond Ed., John Wiley & Sons, M~w York, 1975 at pages 407-434 and by W. Ko~chner in ~L~L~. Sprin~0r-Verlag, New York, 1976 at pa~es 491-524.
Electroma0netic waves having a frequency in the optical range anci propagatin~ throu~h a nonlin~ar crystal induc0 polarization wavas which have frequanciss equal to th~ sum and difference of those of tha 0xciting waves.
Such a polanization wave can transfer sn~r~y to an electromagnetic wava of the same fr~quency. The efficiency of snergy tran~fer from a polarization wave to th0 corresponding electromagnetic wave is a function of: (a) the magnitude of the second ordsr polarizability tensor, since this tensor alem~n~ determines "'; .~' `''.~, 2~6~
the amplitude of the polarization wave; and (b~ the distanc~ over which the polarization wava and the radiated 01ectromagnetic wave can remain suffici~ntly in phasa.
The coherence lertgth, Ic, is a measure of the phasa relationship 5 between the polarization wave and tha radiated wave which is çliven by the following relationship:
Ic =~ k 10 where ~k is the difference between the wavs vectors of the polarization and electroma~netio waves. More speoifically, the coherence length is the distance from the entrance surface of the nonlinear opticaJ crystal to the pointat which the power of the radiat~d ~lectromagnetic wave will be at its maximum value. Phase matching occurs when ~k = 0. The condition ak - O
15 can also bs expressed as n ~ = n co ~ n ~ where C3 = c~ ~ ~; co and ~ are thefrequencies of the input electromagnetic waves; ~3 iS the frequency of the radiated electromagnstic wave; and n1, n2 and n3 are ~he refractive indices of the respectiv~ wav~s in ~he nonlinaar optical crysta~. In th~ special cas~ of s~cond harmonic g~n~ration, ther~ is inci~nt radiation of only onl~ fr~quency, 20 ~I so that ~ 2 5 ~o and C~3 = 2~.
For àppr~ciable conversion of optical radiation of one fr0qu~ncy ~o optical radiation of another frequency In a nonlinear optical crystal, th~
interacting waves must stay substantially in phase throu~hout the crystal so that:
~k _ k3-k1-k2 ~ 2~
wh~re k1, k2 and k3 represent the wave number~ corr~spondin3 to radiation of fr~;u~ncics ~ 2 and a)3, rsspsctively, and I is the interaction 30 length in the nonlinear mat6rial. The term "substantially phase-matched," as us0d her0in, means that ~k ~ 21~/1 for a givsn nonlinaar optical crystal.
A convsntional rnethod for achieving phase-matching in a nonlinear optical material utilizas the fact that dispersion (tha chan~e of r~fractive inciex with frequency) can bé offset by using the na~ural birefrin~enca of uniaxial or 35 biaxial crystals. Such crystals havs two refractive indicas for a given di~ction o~ propa~ation which corr~spond to the two allowed or~hogonally polarizsd propagation modes. Accordingly, by an appropnate choiee of polariz~tion and dircction of propagation, it is olt~n possible to achieve phasa-matehing in a birefringent nonlin~ar opUcal crystal. Th~ term aphas~-match axisl" as used 40 har~in, r~fers to a line or direction throu~h a nonlin~ar optical crys~al alon~

.: . . .. ,, ... . . ... . . , .. ~ . ,... f ... .

~ 0 1 ~ 9 which th0 substantially phase-match0d conversion of a stated input radiation into a stated output radiation is permitted ~or at least certain polarizations of said input radiation.
Phase-matchin~ is generally of either Type I or Type ll. Type I phase-5 matching requires that the incident waves interacting in the nonlinear opticalmaterial have th~ sams polarization. Typs ll phass-matching requires that the incidsnt waves interacting in the nonlinear opticai material have or~hogonal polarizations.
Second harmonic generation within the ca~ity of a multilongitudinal 10 mode laser by an intracavity doubling crystal has recently baen analy~ed by T.
Baer, J. Opt. $oC! ~m, 1~, Vol. 3, No. 9, pp. 1175-1180 (1986). This report setsforth an experimental and thaoretical evaluation of the output of a Nd:YAG
laser which is pumped by a laser diode array and contains an intracavity doubling crystal. It is reported that large ampli~uds fluctuations and 15 longitudinal rnode instabilities result when the doubling crystal is inserted into the laser cavity. Howevar, it is also reported ~hat these instabiliti0s disappear whsn the laser is restrict~d to a singie oscillating mode by an intracavtty etalon. Further analysis ot amplitude instablllty in a multilongitudinal mode intracavi~y cloubled las~r has baen reported by X. G. Wu et al."l,~)~t~Soc. Am~
20 ~, Vol. 4, No. 11, pp. 1870-1877 (1987,~ and M. Oka et al., ~ç~, Vol.
13, No. 10, pp. 805-807 (1988).
U.S. Patent Nos. 4,656,635 (Apr. 7, 1987) and 4,701,929 (Oct. 20, 1987), both issu~d to Baer et al., disclose a laser diode-pumped, intracavity frequency-doubled, solid state laser. In these patents, it is stated that a 2~ problem with such deviees is the generation of amplitude noise, including large amplitude spikes, which prevent or limit use in applications requiring a highly stabl0 or constant output. It is further stated that this noise results from the combination of multiple longitudinal modes. Howev0r, it is discloscd that such noise can be rsduced or eliminatad by inserting an etalon into the laser 30 cavity and th6reby forcing the laser to operate in a stngls mode. It is also disclosed that it may be possible to reduce this noise by mode locking the ```
laser. It is further disclosed that amplitude fluctuations in such a device can be 01imina~ed by aiiminating spa~lal hol~ burning in the activ0 mfldium, f9r exampl~, by lnilizin~ a ring laser cavity geom01ry or placin~ the active medium 35 between quartar-wave plates.
:,.~, .

.,`',, ,",''' ', .,''',' '" ' ' ~: ' '. . ' 2 0 ~ 9 The present invention is dire~ed to a method for reducing or eliminating 5 the amplitude fluctuations ~noise) that can appear in thH output of a las~r UpOIl the insertion of a nonlinear optical crystal into ths laser cavity. Such nois~ is highly undasirabl~ if tha output radiation of such a laser is to ba used in applications such as optical storage of data, spectrosoopy, communications, projec~ion di~plays, laser pnnting and laser film readlwnte systems.
In a soiid state laser which is frequency modified by intracavity nonlinear optical means, we have found that th~ elimination of spatial hole burning in the lasant material is a partial but not sufficient maasure for elim-inating amplitud~ noise in the frequQncy modifisd output radiation from the device. More specifically, we have found that ~ha noise content of the output from such a device drops essentially to zero over csrtain well-detin~d temperature rangss or ~temperature windows." Accordingly, we have discovarad that substantially noise-fr~ op~ration can be achiev~d by maintaining tha temperatur3 of the laser cavity at a value within such a temperature window.
One embodiment of the invention is an apparatus for generating coharent optical radiation which comprises: (a) an optical cavity tor optical radiation of a first frequ~ncy; (b) solid lasant material dispos~d within said cavity for generating cohsrent optical radiation of said first frequency; (c) means for substantially eliminating spatial hole burnin~ in the lasant matcrial during aid gcneration of optical radiation of a first fr~qu~ncy; (d) nonlinear optical means within the optical cavity for convertin~ said optical radiation of a first fraquency ~nto coh0ront optical radiation of a soconai frequency; and (a) tcmporature control means for maintaining th~ ~mporatur~ of said optical cavity at a valu~ which results in substantially noise-fr~e ~ensration of said radiation of a second frequency.
Another embodiment of the invention is an optically pumped solid state laser comprisin~: (a) an optical cavity for optical radiation of a flrst frequ0ncy;
(b) optlcal pumpin~ msans for genoratin~ optical pumping radiation which comprTs0s a las~r diode; (c) solid lasant m~tarial which is disposed within saidoptical cavity, positionod to rcceive pumpin0 radiation from said optical pumping means, and eff~ctive for gon~rating coherent sptical radiation of said first frsqu~ncy upon recsiving said pumping radiation; ~d) means for substantialiy eliminatin~ spatial hole burnin~ in the lasant material during said ~ensration of optical radiation of a first fr~quency; (~ nonlin~ar optical m~ans .:
' ' ' .. .. : . .. . . .. . .. . .. . . .. . . . ....

~6~09 within the optical cavity for conver~ing said optical radiation of a firs~ frequency into coherent opticai radiation of a s~cond frequ~ncy; and (f) temperature control means for maintaining the t~mp2ratur~ of said optical savity at a value which results in substantially nois~-free generation of said radiation of a second frequency.
A further embodiment of the invention is a method for generating coherent optical radiation which comprises: (a) generating coherent optical radiation of a first ~requency from solid lasant material within an optical cavity for said radiation o~ a first frequ~ncy; (b) substantially preventing spatial hole burning in the lasant material during said generation of radiation of a first frequency with spatial hole buming suppression means; (o) eonver~ing said optical radiation of a first frequency into coherent op~ical radiation of a second frequency within said optical cavity with nonlin~ar optical means; (d) withdrawing said optical radiation of a second frequenoy from said optical cavity as output radiation; and (e) maintaining the temperature ot said optical cavity within a rangs over which said output radiation is substantially noise-free.
An object of the invention is to provide a las0r of improved amplitude stability which is fr~quency modified by intracavity nonlinear optical means.
Anoth~r object of the inv~ntion is to provida a multilongitudinal-mod0, optically-pumped, Intracavity-doubl~d solid state laser of improvQd amplitude s~ability. .
Another object of the invention is to provids a diode-pumped?
intracavity-doubled, multilongitudinal-mode, solid state laser of improved o~put stability.
A ~urther object of the invention is to provide a method fcr reducing the amplitude instabilities that ara obs~rvad in the output of a laser when a nonllnear optical material is insertsd into the laser cavity.
A still furthar obJect of the invention is ~o provide a method ~or improving ~he amplit~d~ stability of an intracavi~y fr~qu~ncy-do4bl~d las~r.
~Qn Q~ttlQ Drawin~
FIG. 1 of the drawings is a schematic representation of one embodiment ef th~ inv~ntion.
FIG. 2 of the drawings illustrates the optical output power of a frequency~oubl~d, lassr diode pump~d, solid state laser of ~he type illustrat~d by FIG. 1 and th~ root m0an squar0 (RMS) noise ~ntent o1 said output as a function of th~ tamperatur~ of the las~r oavity.

6 ~ ~ 9 FIG. 3 ol the drawings is a schematic represen~ation of a second embodiment of the invention.

While this invention is susceptible of embodiment in many forms, there are schematically shown in FIGS. 1 and 3 of the drawings two specific embodiments, with the understandir1g that the present disclosure is not intended ~o limit the inv~ntion to the embodiments ilhlstrat~d.
With r~ference to FIG. 1, optical pumping radiation 1 ~rom optical pumping means 2 and 3 is focused by focusin~ means 4 through quarter-wave plate 5 and into solid lasant material 6 which is capable of being pumped by the radiation from said pumping means (2 and 3~. Light emitted by ~he lasing of lasant matsriai 6 is contained within the lin~ar standing wav~ optical cavity15 defined by mirror 7 and a suitable r~flective coatin~ on surface 8 of quarter-wave plate 5, and such light is hereinafter r~ferr~d to as cavi~y radia~ion. Th~cavity radiation can be single longitudinal mode in character or it can be comprised of two or more longitudinal mod~s. Nonlinear optical c~ystal 9 is position~d within the optical cavity in such a mann0r that cavity radiation 20 circulating within the optical cavity can int~ract with tha crystal ah~ng a phase-match axis for th~ converslon of cavity radiation into radiation which is a harmonlc thereof. A sscond quart0r-wav0 plat~ 10 is positioned between nonlinear optical crystal 9 and mirror 7 7cr interaction with the cavity radiation.
The pair of quarter-wav0 plates 5 and 10 serves to suppr~ss spatial hol~
25 burning within lasant material 6. Harmonic radiation is generated upon interacUon of the cavity radiation with nonlin~ar optical crystal 9 and is passed throu~h mirror 7 as output radiaticn 11. i-inally, the las~r illustrated in FIG. 1 is provided with temperatura control m~ans (which is not shown in th~ tiç~ure) for adJusting and controlling the temperature of the laser's optical cavity and its 30 oontents.
Optical pumping mcans 2 andl 3 can compris0 any conv~ntional sourc~
of optical pumping radiation. Howovar, pr~f~rr~d sources of pumping rad~ation 1 include li~ht-am~tt~ng diodes and laser d~odes. Such diod~s are c~mmonly attached to a heat-resistant and thermally conductive heat sink and 35 are paokag0d ~n a m3tal housing. For ~fficient operation, the purnping radiation 1 is match~d with a suitable absorption band of ~he iasan~ mat~al ~.
Conwntional light-amitting diod0s and laser ~iodes are availabl~ which, as a function of composition, produc~ output radiation havin~ a wav01~ngth ov~r the range from about 630 nm to about 1600 nm, and any such d0vica ~' ' ' ~ ' ' ' ' i ' ' ' ' . ! ' : . ; , . . , i . . .

201~9 producing pumping radiation 1 of a wavelength effectiv~ to pump lasant material 6 can be used in the practice of this invention. For example, the wavelength of the output radiation from a GalnP based device can be varied from about 630 ~o about 700 nm by variation of the device eomposition.
Similarly, the wavelength of ~he outpu~ radiation frern a GaAlAs based device can be vari~d from about 750 to about 90û nm by variation of the devioe composition, and InGaAsP bas~d davices can b~ us~d to provide radia~ion in the wavel~ngth range from abeut 1000 to about 1600 nm.
A hi~hly suitable source of optical pumping radia~ion 1 consis~s of a gallium aluminum arsenide laser diode array 3, emitting light having a wavel~ngth of about 810 nm, which is attached to heat sink 2. Heat sink 2 can b~ passive in character. However, hcat sink 2 can also comprise a thermo~l0ctric cooler to h~ip maintain laser diode ~rray 3 at a constant temperature and thereby ensure optimal operation of laser diode array 3 at a constant wavelength. It will be appreciated, o~ course, that during operation the optical pumping means will be attach~d to a suitable pow3r supply.
El~ctrical l~ads from las~r diode array 3 which ar~ dir~ctsd to a power supply ar~ not illustratec in FIG. 1.
Focusing means 4 servas to focus pumping radiation 1 through quart~r-wave plate 5 and into lasant matcrial 6. This focusing results in a high pumping intensity anci an assoclat~d high photon to photon conversion efficicncy in lasant mat~rial 6. Focusing maans 4 can comprise any convontional means for focusing light such as a gradiant indsx l~nsl a ball lens, an aspheric l~ns or a combina~ion of lens~s.
Any convantional solid lasant mat~rial 6 can be utiliz~d provid~d tha~ it is capable of bein~ optically pumped by the optical purnping maans selected.
Suitabl~ lasant mat~rials include, bu~ ara not iimit~d to, solids selccted from the ~roup consistln~ ot glassy and crystalline host matorials which ar~ dop~d with an active material and substances wherein the active material is a stoichiom~tric component of thc lasant mat~rial. Highly suitabl~ active materials includo, but are not limited to, ions of chromium, titanium and the rar0 carth mctals. Hi~hly suitabls lasant mat~rials includc n~odymium-doped Y~G, neodymium-dop3d YALO, n~odymium-dopfld YLF, nsodymium-doped GSGG, nsodymium pentaphosphat~ and lithium neodymium t~traphosphat~.
By way ofsp~oific oxampl~,neodymium-dop~d YAGis a highly su~ablelasant mat~rial 6 for use in combination with an op~ioal pumping means which prociuces li~ht having a wavelength of about 810 nm. Wh~n pump~d with light 201~09 of this wavelength, neodymium-doped YAG can emit light having a wavelength of 1064 nm.
The pracis~ Qeometric shap~ of lasant ma~erial 6 can vary wW~ly. For example, lasant mat~rial 6 can bs rod-shaped, or rhomboh0dral in shape if 5 desirad, and l~ns-shaped surfaces can be us~d if desired. If desired, an end-pumped fiber of lasant material can be used. Highly suitable fibers for ghis purpose include, but are n~t limited ~o, glass optical fibers which are doped with ions of a rare ~arth metal such as n~odymium. The l~ngth of such a fib~r is easily adjusted to result in absorption of essentially all o~ th~ optical 10 pumping radiation 1. If a very long fiber is requireld, it oan b~ coil~d, on a spool for example, in order to minimize the overall llanyth of the laser of thisinvention.
The reflective coating on surface 8 of quar~er-wave plats 5 is selected in such a manner that it is substantially transparent to optical pumping radiation 1 15 but highly reflactive wlth respsct to th0 cavi~y radiation produced by thl3 lasing of lasant material 6. In a preferr~d embodim~nt, ~his coating will also be highly reflactive of the harmonic output radiation 11. High reflectivity of the coatingfor this harmonic radiation will serve to prevent the pump-sid~ loss of any harmonic radiation which is produced upon the refl0ction of cavity radiation 20 into nonlinear optical Grystal 9 by mirror 7. Such a coating is coriventional in character and can, for example, bo a dielectric coating which produces an appropriate high reflection and phass shift of the reflsct~d radiation.
Mirror 7 is selected in such a manner that it is highly reflective for th~
cavity radiation producad by the lasing of lasant material 6 but substantially 25 transparent to output radiation 11 which is gan~rat~d by ~he interaction of cavity radiation with nonlinear optical crystal g. Mirror 7 is conventional in character and, ~or example, can comprise any suitable conv~ntional ooating on any suitabie substrate.
Cavity radiation can be eithar single mode in character or it can be 30 comprised of two or rnore longituclinal modes of substantially the same frequancy. Unless th0 mod~ struc~ur~ is sxpr~ssly sp~clfi~d, r~fersnce h~r0in ~o the cavity radiation as having a sp0cific ~requ~ncy will ba undorstood to include all of the longitudinal modes of substantially the same fr~quancy which are ~nerat~d by the lasant material and support~d within tho optioal cavity.
35 Similarly, reference herein ~o frequancy-modified radia~ion r~sulting from int~raction of cavity radiation with the ncniinear optical material as having a sp~cific frcquenGy will b~ unders~ood to include th~ combina~ion of similar frsqu~ncies r0sulting from frsquency-modification of any plurality of ~0 , 2016~9 lon~itudinal modes in the cavity radiation. A preferred embodiment of the invention involves the use of multilongitudinal-mode cavity radiation in the laser which is illustrated in FIG.1.
The pair of quartsr-wave plates 5 and 10 are quarter-wave plat~s for 5 cavity radiation and s~rve as a means for substantially eliminating spatial hol~
burning in lasant material 6 by causing circular polarization of the cavity radiation and thereby creating a twisted mode optical cavity [the twisted mode technique for producing an axially uniform energy clensity in a laser cavity is i~
d~scribed by V. i ~uhov et al., Q~.~ , Vol. 4, No.1, pp. 142-143 ~1965)].
10 The pr~cise location of these two quarter-wavs plates within ~he optical cavity is not critical, provided tha~ lasant material 6 is placed b~twe~n them.
However, a prsferred embodimsnt of the invention involves placing both lasant rriaterial 6 and nonlinear optical material 9 between the pair of quarter-wave plates 5 and 10 as illustrat~d in FiG. 1.
15Any conventional msans for substantially eliminating spatiai hol~ -burning in the lasant material can be us~d in the practice of ~his invention. For example, spatial hole burning can be eliminated throu~h the use of a - -traveling-wave ring-like optical cavity (which is illustrat~d by the embodiment s~t forth in FIG. 3), by ~enerating circularly polarized light in the lasant material 20 (wh~ch is illustrated in FIG. 1 and is effected by the pair of quart~r-wavs plat~s S and 10), with m~chanical motion, or with ~lactro-optic phase rnodulation.
Cavity radiation circulating within the optical cavity dafin~d by mirror 7 and the reflectiva coating on surlace 8 of quarter-wave plate 5 is directed intononlinear optical crystal 9 along a phase-match a~tis of the crystal that permits 25 effici~nt conversion of cavity radiation to the desired harmonic.
Th0 ~ometric shape of the nonlin~ar op~ical crystal us2d in the practice of this inv~ntion can vary widely. For example, th~ crystal can be rod^shaped or rhomboh~dral in shape, and th0 crystal can have lens-shapad surfaces if de~ired. When possible, noncritical phase-matching is usually preferr~d in 30 order to minimks the effacts of b~am diver~ence and "walk-off."
Potassium titanyl phosphate, KTiOP04, is a hi~hly pr~ferr~d nonlinear optical matsrial. However, it will b~ appr0ciat0d that any nonlinear optical material can be utilizad in the practice of this inventioil. Suitable nonlinear opticat materials include, but are not limit~d ~o, KH3PO4, LiNbO3, KNbO3, 35 B-BaB204, Ba2NaNb5015, LilO3, H103, KB5~)8 4H20, potassium iithium niobate, urea and oompounds of ~he formula MTiO(XO") whers IUI is selsct~d from the group consisting of K, P~b and Tl, 3nd X is selectQd from the ~roup oonsisting of P and As.
~1 i, - , .

':, ' " ' .,, ' , . ~ ~ , ~ !; ,:, : : . . ' : :
:, . : ' : ' ' , ,.' .. . : ` ' , :
~.,' ' : ' ' ' ": ' -: .` . : . . , ' . . ~ ' '; . .

2~1g~09 Potassium titanyl phosphate belongs to the orthorhombic poin~ group mm2 (space group Pna21) which laoks a center of symmetry. This material can b~ phase-matched for tha conv~rsion of cavity radiation having a wavelength of 1064 nm to its second harmonic which has a wav~length of 532 5 nm. For this fraquency convsrsion, the potassium titanyl phosphate crystal canbe oriented for a Type 11 interaction wi~h cavity radiation propagating along a phass-match axis within the crystallographic xy-plane, 24 ~2 off the crystallographic x-axis and parpendicular to the orystallographic z-axis.
The laser illustrated by FIG. 1 is provid~d with tsmperature control 1 O means for adjusting and controlling th~ ~mperature of the lase~s optical caYi~y and its contents. This temperature control means can be of any conventional type, for example, an electrically powered resistance heater or lhermoel~ctric device, and is us~d to maintain the temperatur~ of tha optical cavity at a valuewhich results in substantially noise-free ~eneration of output radialion 11.
15 Through the use of such temperature control means, the temp3rature of the cavity and its contents are desirably maintained at i1C of th~ s~l~ct~d value, preferably at ~ 0.5C of the sel~cted value and more preferably at ~O.1 t:: of the selected value.
The und~sir~d fluctuations in the amplitude of output radiation 11, for 20 example, within a frequ~ncy range of about 1 kHz to about 50 MHz, are referr0d to as nois~ and ar~ conv~niently measured as perc0nt root mean square (% RMS) noise. Although an intracavity frequency-modified laser of improved amplitude stability is obtain~d through the subs~antial elimination of spatial hole burning in th~ lasant mat~nal 6, we have found that the nois~
25 contsnt of output radiation 11 drops essentially ~o zero over csrtain temp~rature ranges. More sp~cifically, if th~ noise contant of output radiation 11 is measured as a function of ~he tampe~ture of tha laser cavity, car~ain ran~es of temperature or "temperatur~ windows" are observed ov~r whioh th~
noise drops to ess~ntially zero. Th~ width of th0se windows and the pr0cise 30 temperalures at which they oocur are different for each individual laser. That is to say, 0v0n if a substantial sffort is made to conslruct two compl~tely idontical lasers, we hav~ found that they will be sufflc~ently diflarent that th~ noise cont0nt of output radiation 11 as a function of the t~mperatur3 of th~ las~r cavity will b0 a unique charactarist~c of ~ach d~vice. How0v~r, for a givan 35 laser, the noise content of the output radiation 11 as a ~nction of ~emperatur~
does not ehange si~nificantly ov0r lon~ periods of time (for axample, waeks or month~) or with repeated cycling over lar~0 tsmp~raturs ranges. Accordingly, a highly prQferred ~mbodimant of this inv~ntion compris0s locatin~ a window . . ~ .. . ... . ~ . . ~. . .... . . ....

20~6509 of substantially noise-free operation for a laser by measuring the noise contentof output radiation 11 as a func~ion of the temperature of ths optical cavity and maintaining th~ t~mparatur6 of the laser cavity at a valu~ within suoh a window during subsequent operation. These windows of substantially noisa-frae 5 operation are typically from about 1 to about 15C wide, and within such a window the % RMS noise will typically be less than about 0.2% and frequently less than about 0.1%. Tha windows of substantially noise-fres operation are easily identified by measuring the noise cont~nt of output radiation 11 over any range of temperatures which is conveni~nt from an operating point of 10 view, for example, from about 0 to about 100C, or mora conv~niently, from about 30 to about 65C. This measurement is d~sirably carried out over a range of at least about 5C and preferably over a ran!ge of at leas~ about 1 0Cor 20C; in order to giv0 a reasonabla sampling of the laser's temperature-related parformance.
In a spaoHic example of the embodiment illustrated in FIG. 1, neodymium-doped YAG is used as lasant rnat0rial 6, and tha nonlinear optioal crystal 9 is composed of potassium titanyl phosphate. The neodymium-doped YAG is optically pump3d by a multistripe laser diode array 3 which is attaehed to a thermoelectric cool0r 2 (th0 array and attach0d ~hermoelectric cooler is a 20 Model SDL 2422-HI d0vic~ manufactur~d by Sp~ctra Diod~ Labs of San Jose, Californla). Th~ laser dlode array 3 is a 10-strip~ array consisting of 3 mlcronstripes on 10 micron ccnters which can provids about 200 mW of pumping radiation 1 having a wav01sngth of about 810 nm. This pumping radiation 1 is focused by gradi~ni ind0x lens 4 whioh has a 0.29 pitch and is antirefl~ction 25 coated with respect to 81û nm wav~length rad~ation. The focused pumping radiation pass~s through quart~r-wave r~tardation platc 5 which is comprissd of quartz and is in the form of a circular plate havin~ a thickness of ~bout 1 mm and a 10 mm diamet~r. Input face 8 vf quarter wave plate 5 carries a multilayer di~lectric coating which is h~hly rsflective (R ~ 99.8%) at a 30 wavsl~ngth of 1064 nm and highly transparont (T ~ 80%) at a wavelongth of 810 nm. Output faco 12 of quart~r-waYs plata 5 carries an antir~fl~ction coatin~ (R ~ 0.2%) for light havin~ a wavfilength of 1064 nm and highly transparsnt (T ~ 80%) at a wavelength of 810 nm. The focused pumping radia~ion comes to a focus w~h~n lasant material 6 whioh conta~ns about 1%
35 naodymium and is in the form of a rod havin~ a 4 mm 10n~th and a 10 mrn diameter. The lasant material 6 is oriented for Icw thrashold operation at a wavel~ngth of 1064 nm and emits light (cavity radiation) havin~ a wavelength of 1064 nm in responsa to excitation by the pumping radiation. The suriac~s :
. -- ~ . . ..
. - .; , . .

~, , , " ~ , . . - , ... . . ..
- . . ..... .-,... : , ,. . . , :, .

of lasant material 6 are antirsflection coated (R ~ 0.2%) for light having a wavelength of 1064 nm and highly transparent (T > 80%) at a wavelength of 810 nm. Nonlinear optical crystal g is a rhombohedral prism of poSassium titanyl phosphate which has dimensions of 1 x 1 x 3 mm ~with a 3 mm 5 interaction length with th~ cavity radiation) and is cut for Type l-l phase-matched conversion of cavity radiation to its s0cond harmonic having a wav~l~ngth of 532 nm. Nonlinear optical crys~al 9 is an~ireflection coated with respect to both 532 nm and 1064 nm wavelength radiation. Nonlinear optiG~I
crys~al 9 and quarter-wave plate 5 are positioned in such a manner that the 10 crystallographic z-axis of crystal 9 is parallel to the optic axis of quarter-wave plate 5 about tha axis along whioh they encounter cavity radiation. Quartsr-wave plate 10 is id0ntical with plate 5 except that both of its suffaces are antireflection coated vith respect to 1064 nm wave-length radiation and i~ is desirably a half-wave pla~a or full-wave plate for 532 nm wavelangth radiation.
15 Ouarter-wave plate 5 and 10 are positioned in such a manner that thc optic axis of one makss a 45 angle with raspect to the optic axis of the oth0r about the axis along which they encounter cavity radiation. Oùtput radiation 11 having a frequency ot 532 nm is transmitted ~hrough mirror 7 which has a radius of curvature of 10 cm and carries a dielectric coating which is highly 20 re~lective (R > 99.8%) at a wavelength of 1064 nm and highly transparent at awav01ength of 532 nm. The optical cavity of this laser has a iength (distance from sufrace 8 to mirror 7) of about 20 mm. The optical cavity was wrapped with an elsctrically powersd resistance heater and fi~ed with a thermistor which could be used to: (a) measure the cavity temperature; and (b) control 25 the cavity temperature by providin~ a f0edbiack signal to tha power supply for the resistance h0ater.
The powar and parcent root mean square (~MS) noiss of Ihe 532 nm output radiation from the above-described laser wers measursd as a function of temperature over ~hs range from about 40 to about 60C. The r0sutts are 30 set forth in FIG. ~. With reference to FIG. 2, it will be noted that the P(MS noise in the output ractiation drops essGntially ~o zero ov0r certain ranges of temperature or "temperatur~ windows.R For exampl0, such a window appears at about 46.0~48.6C and anoth~r appears at about 49.0-56.4C. Ths precis~
location and width of these windows o~ subs~antially noise-free operation are 3S unique oharac~eristics of each individual laser and do not chang2 significantly with time (for exampl0, w~eks or months) or r0peated temperature cycling over several tens of d0grees centi~rada. Aocordin~iy, each laser can be made to operate in a substan~ially noise-free rnann~r by maintaining the optical cavity , ' .. ,. . ~ ,. , j .. , . . - , .. ~ .. , . ,. .. . ~ . . ... . . ... . . .

20~09 of the device at a temperature within such a window during operation.
Although the reason for the existenca ot these temperaturs windows is uncertain, it is balieved that th~ longitudinal mode oonfiguration within the optical oavity und~rgo~s chang~ or fluctuation whan significant ~MS nois~ is obs~rv~d--possibly as a rasult o~ t~mperatur~-induclsd chang~s in tha optical cavity len~th and component birefrin~nce together with assoeiated etalon affects.
It will a~so be notad from FIG. 2 that the output power is also a function of oavity temperature. Accordingly, a preferrsd embodim~nt ot the invention involves selecting a t~mp~rature window whioh yields: (a) substantially noise-free operation; and (b) relatively high output power.
FIG. 3 schematically illustrates a second embodiment ot the invention which involves the use of a traveling-wave ring-like optical cavity for the purpose of substantially eliminating spa~ial hole burning in the lasant material.
With referencs to FIG. 3, optical pumping radiation 21 from laser diode array 22 is focused by focusing rneans 23 into lasant material 24 which is capabl~ of being pumped by said pumping radiation. Lassr diode array 22 is attach0ci to h~at sink 25. Light emitted by the lasing of lasant material 24 is contained within the optical cavity detined by mirrors 26, 27 and 28 and by a sultabls reflectiv0 coating on surfaco 29 of lasant material 24, and such light is h~relna~ter r~f0rrad to as cavity radiation. A unidirsctional optical gata m~ansfor effecting unidirectional circulation of cavity radiation within th0 optical cavi~y is provided by the combination of polarizer 30, Faraday rotator 31, and half-wave plate 32. Nonlinear optical crystal 33 is positioned within the optical cavity in such a manner that cavity radiation circula~ing within th0 optioal cavity can intoract with the crystal along a phase-rnatch axis for tha conv~rsion of cavity radiation in~o radiation which is a harmonic th~reof. Harrnonic radiationis ~enerated upon interaction of the oavity radiation with nonlinear optical crystal 33 and is pass~cl through mirror 28 as output radiation 34. Finally, thelaser illustrated in FIG. 3 is providad with temperature control m~ans (which isnot shown in FIG. 3) for adjusting and controlling tha temp0ratur0 of th~ optical cavity and its cont~nts. This tamperatur~ control means is omployed to maintain the optical oavity of the laser at a vaiue which results in substantially nois~-frae generation of output radiation 34.
3S The refleclive coating on suflacQ 29 o~ lasant material 24 is selected in su~h a manner tha~ it is substantially transparent to optical pumpin~ radiation 21 but highly refloctive with r~sp~ct to the cavity radiation produc~d by tha lasing of lasant material 24. Mirrors 26 and 27 are hi~hly r~flective for the .

20i~09 cavity radiation producsd by the lasing of lasant material 24. Mirror 28 is highly re~lective for cavity radiation but substantially transparent to output radiation 34 which is gen~rated by thQ int~raction of cavity radiation with nonlinear optical crystal 33.
Any conven~ional polarization means can be utilized as polarizer 30, for example, a Brewster plate, suitable coatings on the mirrors o~ the optical cavity, a dielzc~ric polarizer, or a Brewster angl~ surfaca on the lasant material 24.
If neodymium-doped YAG is used as the lasiant material 24, ~he YAG
crystal itself can also serve as Faraday rotator ~1 if a magnetic field is .established along the axis of the crystal. In such an embodiment, a separate Faraday rotator 31 is not r~quired.
Conventional d~siyns for a travaling-wave opUcal cavity which can be ~mployad in th~ practice of this invention for the purpose of substantially :-15 ~liminating spatial hol~ burning in the lasant ma~erial are set forth in W.
Koechner, ~Lid-St~e Laser ~nginQ~rinq (Springer-Verlag, NeW York, Second Ed., 1988) at pp. 126-128 and in Siegman, L~Q~ (University . ~
Science Books, Mill Valley, California, 1 986) at pp. 532-538. .:

: . .

,, .. .... . ~ . . .. . . - ; . ~ , , . . . . , . ., . .

.. ,, ., , . ~ , ~. , . . .. . . .. .. . , . . . ~. .

Claims (4)

1. An optically pumped solid state laser comprising:
(a) an optical cavity for optical radiation of a first frequency;
(b) optical pumping means for generating optical pumping radiation which comprises a laser diode;
(c) solid lasant material which is disposed within said optical cavity, positioned to receive pumping radiation from said optical pumping means, and effective for generating coherent optical radiation of said first frequency upon receiving said pumping radiation;
(d) means for substantially eliminating spatial hole burning in the lasant material during said generation of optical radiation of a first frequency;
(e) nonlinear optical means within the optical cavity for converting said optical radiation of a first frequency into coherent optical radiation of a second frequency; and (f) temperature control means for maintaining the temperature of said optical cavity at a value within a range which results in substantially noise-free generation of said radiation of a second frequency wherein said temperature range is determined by measuring the noise content of said radiation of a second frequency as a function of the temperature of said optical cavity.
2. The laser of claim 1 wherein the optical cavity is a standing wave cavity for said radiation of a first frequency.
3. The laser of claim 2 wherein the optical cavity is a linear standing wave cavity for said radiation of a first frequency.
4. The laser of claim 2 wherein said means for eliminating spatial hole burning comprises a pair of quarter-wave plates within said optical cavity. 5. The laser of claim 4 wherein said lasant material and nonlinear optical means are positioned between said quarter-wave plates.
6. The laser of claim 1 wherein said optical cavity is of a ring-type and said means for eliminating spatial hole burning comprises unidirectional optical gate means for effecting unidirectional circulation of said radiation of a first frequency within the optical cavity.
7. The laser of claim 6 wherein said unidirectional optical gate means is comprised of a polarizer, a half-wave plate, and a Faraday rotator.
8. The laser of claim 1 wherein said second frequency is twice that of said first frequency.

9. The laser of claim 1 wherein said radiation of a first frequency is comprised of at least two longitudinal modes.
10. The laser of claim 1 wherein said nonlinear optical material is comprised of potassium titanyl phosphate.
11. The laser of claim 1 wherein said lasant material is comprised of neodymium-doped YAG.
12. A method for generating coherent optical radiation which comprises:
(a) generating coherent optical radiation of a first frequency from solid lasant material within an optical cavity for said radiation of a first frequency;
(b) substantially preventing spatial hole burning in the lasant material during said generation of radiation of a first frequency with spatial hole burning suppression means;
(c) converting said optical radiation of a first frequency into coherent optical radiation of a second frequency within said optical cavity with nonlinear optical means;
(d) withdrawing said optical radiation of a second frequency from said optical cavity as output radiation;
(e) measuring the noise in said output radiation as a function of the temperature of said optical cavity;
(f) determining a temperature range for said optical cavity over which said output radiation is substantially noise-free and (g) maintaining the temperature of said optical cavity at a value which is within said determined temperature range during subsequent production of said output radiation.
13. The method of claim 12 wherein said second frequency is twice that of said first frequency.
14. The method of claim 12 wherein said radiation of a first frequency Is comprised of at least two longitudinal modes.
15. The method of claim 12 wherein the optical cavity is a standing wave cavity for said radiation of a first frequency.
16. The method of claim 15 wherein the optical cavity is a linear standing wave cavity for said radiation of a first frequency.
17. The method of claim 15 wherein said spatial hole burning suppression means comprises a pair of quarter-wave plates within said optical cavity.

18. The method of claim 17 wherein said lasant material and nonlinear optical means are positioned between said quarter-wave plates.
19. The method of claim 12 wherein said optical cavity is of a ring-type and said means for eliminating spatial hole burning comprises unidirectional optical gate means for effecting unidirectional circulation of said radiation of a first frequency within the optical cavity.
20. The method of claim 19 wherein said unidirectional optical gate is comprised of a polarizer, a half-wave plate, and a Faraday rotator.
21. The method of claim 12 which additionally comprises optically pumping said lasant material with optical pumping means wherein said optical pumping means comprises a laser diode.
22. The method of claim 21 wherein said optical pumping means comprises a laser diode array.
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US4933947A (en) 1990-06-12
ATE105114T1 (en) 1994-05-15
EP0398570A3 (en) 1991-12-11
DE69008415D1 (en) 1994-06-01
EP0398570A2 (en) 1990-11-22
JPH0349278A (en) 1991-03-04
EP0398570B1 (en) 1994-04-27
DE69008415T2 (en) 1994-08-11

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