4,901,330
`[11] Patent Number:
`[191
`United States Patent
`
`Wolfram et 3],
`[45] Date of Patent:
`Feb. 13, 1990
`
`[54] OPTICALLY PUMPED LASER
`
`[75]
`
`Inventors: Thomas Wolfram, Wheaten; Bruce A.
`Vajak, Naperville; Edward T. Mass,
`Jr.’ Batavia: Robert D.
`Wheatfln. 31-1 Of 111-
`
`[731
`
`A3518?‘-‘"31’-3 A3309“ C0l'l30|'|lfi0lls C]1iC3S0- m»
`
`[21] Appl. No.: 221,670
`
`[22] Filed:
`
`Jul. 20, 1988
`
`Int. Cl.‘ ............................................ .. Hols 3/091
`[51]
`[52] US. Cl. ...................................... .. 312/75; 372/46;
`372/5{); 372/71
`[58] Field of Search ..................... .. 372/43, 45. 46, 50,
`372/69, T0, 71, 75
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`372/46
`“$1 _m 6/1988 W 1 h
`1
`*
`*
`3 ° 9‘ 3 '
`...................... 372/75
`4,791,631. 12/1933
`Bfll.lIl1Cl"E Ct til.
`Primary Exam1'ner—William L. Sikes
`Assistant Exam:’ner—B. R. R. Holloway
`Attorney, Agent, or Firm—-Gary C. Cunningham;
`William H. Magidson; Ralph C. Medhurst
`[5,]
`ABSTRAC1.
`
`An optically pumped laser which includes_a laser dic_)de
`array for generating optically pumped radlation havmg
`:1 uniform intensity distributed over a broad band-width,
`and a lasant material with an absorption band for receiv-
`ing radiation within such bandwidth.
`
`27 Claims, 6 Drawing Sheets
`
`
`
`ASML 1117
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`

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`US. Patent
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`L Feb. 13,1990
`
`Sheet 1 of6
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`4,901,330
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`U.S. Patent
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`Feb. 13,1990
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`Feb. 13, 1990
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`Feb. 13, 1990
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`
`1
`
`OPTICALLY PUMPED LASER
`
`4,901,330
`
`BACKGROUND OF THE INVENTION
`1. Field of the invention
`This invention relates to an improved optically
`pumped laser. In particular, the optically pumped laser
`includes a laser diode array for generating optically
`pumped radiation having a uniform intensity distributed
`over a broad bandwidth and a lasant material with an
`absorption band for absorbing radiation within the
`above bandwidth.
`2. Description of the Prior Art
`A laser is a device which has the ability to produce
`monochromatic, coherent light through the stimulated
`emission of photons from atoms or molecules of an
`active medium or lasant material which have typically
`been excited from ground state to a higher energy level
`by an input of energy. Such a device contains an optical
`cavity or resonator which is defined by highly reflec-
`tive surfaces which form a closed round trip path for
`light, and the active medium is contained within the
`optical cavity.
`If a population inversion is created by excitation of
`the lasant material, the spontaneous emission of a pho-
`ton from an excited atom or molecule returning to its
`ground state can stimulate the emission of photons of
`identical energy from other excited atoms or molecules.
`As a consequence, the initial photon creates a cascade
`of photons between the mirrors of the optical cavity
`which are of identical energy and exactly in phase. A
`portion ofthis cascade of photons is then discharged out
`of the optical cavity, for example, by transmission
`through one or more of the reflecting surfaces of the
`cavity.
`Excitation of the lasant material of a laser can be
`accomplished by a variety of methods, such as. by opti-
`cal pumping, current injection or the use of an electrical
`discharge. Optical pumping involves the creation of a
`population inversion through the absorption of light by
`a lasant material. The use of light from noble gas are
`lamps, tungstenghalogen lamps,
`light—emitting diodes,
`laser diodes and laser diode arrays to optically pump or
`excite the lasant material of a laser is well known.
`In order to effect optical pumping, the photons deliv-
`ered to the Iasant material from a radiant source must be
`of a very precise character as within the absorption
`band of the lasant material. In particular, the pumping
`radiation must be of a wavelength which is absorbed by
`the lasant material to produce the required population
`inversion.
`
`U.S. Pat. No. 3,624,545 issued to Ross describes an
`optically pumped solid state laser composed of a
`neodymium-doped
`yttrium
`aluminum
`garnet
`(Nd:YAG) rod which is side-pumped by at least one
`semiconductor laser diode. Similarly, U.S. Pat. No.
`3,753,145 issued to Chesler, discloses the use of one or
`more light-emitting semiconductor diodes to end pump
`a Nd:YAG rod. The use of an array of pulsed laser
`diodes to end pump a solid lasant material such as
`neodymium-doped YAG is described in U.S. Pat. No.
`3,982,201 issued to Rosenkrantz et al.
`Lasers, such as semiconductor diode lasers, are acti-
`vated by the application of an electrical current. Laser
`diodes are efficient pumps for optically pumped lasers
`since the output radiation from the laser pump is a single
`wavelength (or a very narrow band of wavelengths)
`which is selected in such a manner that matches the
`
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`absorption band or peak of the lasant materiallto be
`optically pumped. Unfortunately, it is frequently diffi-
`cult to match the output radiation of the laser pump '
`with the appropriate absorption band ofthe Iasant mate-
`rial, since the output radiation from the laser pump
`source is generally of a single wavelength which is
`selected in such a manner so as to precisely match the
`absorption band peak of the lasant material which is to
`be optically pumped. Moreover, temperature, pressure
`and aging of diode lasers significantly affect, alter and
`change the characteristics of laser diodes, by changing
`the wavelength of the output radiation of laser diode
`pumps. Thermoelectric heaters/coolers and sophisti-
`cated feedback circuits are utilized with a goal toward
`precisely matching the output radiation of laser diodes
`with the absorption band or peak of the lasant material.
`Over the years a number of laser diodes have been
`suggested for matching the output radiation of laser
`pumps with the absorption band of the lasant material,
`however, such laser diodes have resulted in varying
`degrees of success. It is therefore desirable to provide
`an improved optically pumped laser which overcomes
`most, if not all, of the above problems.
`R. B. Allen, GaAlAs Diode Pumped Nd:YAG Laser,
`Technical Report AFAL-TR-72-319, Jan. 1973, pp.
`1-9, describesthe results of a program to develop and
`test a laboratory model of a room-temperature GaAlAs
`diode-pumped Nd:YAG. The Report discloses a laser
`which produced a CW power of more than 80 mW in
`the TEMoo mode, which it was asserted, represented
`the highest level of TEMoo power reported to date for
`a. diode-pumped laser. The GaAlAs light emitting di-
`odes in this -Report were selected for the best room
`temperature spectral match to the Nd:YAG absorption
`band near 805 nm. The CW operating characteristics of
`the 15 best of 1'.-' individually fabricated subarrays are
`given in Table 1 (of the Allen report), which includes
`the peak emission wavelength at 25° C. for 250 mA
`drive current. The distribution of the subarray is found
`in FIG. 4 (of the Allen report), which is a graph of the
`output power versus the peak emission wavelength.
`Twelve of the subarrays have peak emission wave-
`lengths in the range of 805 um plus or minus 5 nm. The
`other three have shorter wavelengths. The CW operat-
`ing characteristics of the 2 worst of the 1'.’ individually
`fabricated subarrays were ignored.
`U.S. Pat. No. 3,946,331 issued to Pollack et a1. de-
`scribes a Nernst lamp for optical pumping of a solid
`state laser. The lamp materials were selected so that the
`light emitted was essentially concentrated in the rela-
`tively narrow pump region of the absorption spectrum
`of the laser crystal.
`W. T. Tsang, Appl. Phys. Letter, Vol. 36, No. 6.
`1930, pp. 441-443 discloses a multiwavelength trans-
`verse-junction-stripe laser, which is capable of emitting
`multiple predominantly single-longitudinal mode emis-
`sions at various wavelengths. In an example, four differ-
`ent outputs at 902.5, 879.3, 853.2 and 827.6 nm were
`obtained simultaneously from a single-wavelength T] S
`laser.
`In contrast, none of the above references disclose or
`suggest an optically pumped laser comprising a laser
`diode array for generating optical pumping radiation,
`such pumping radiation having a bandwidth which is
`about 3 nm to about 15 nm wide and wherein the inten-
`sity of the pumping radiation is substantially uniformly
`distributed over such bandwidth. and a lasant material
`
`

`
`3
`with an absorption band for receiving radiation within
`the bandwidth of the laser diode array.
`SUMMARY OF THE INVENTION
`
`The instant invention discloses an optically pumped
`laser which includes a laser diode array for generating
`optical pumping radiation having a uniform intensity
`distributed over a broad bandwidth, and a Iasant mate-
`rial with an absorption band for receiving radiation
`within such bandwidth.
`An embodiment of the invention includes an optically
`pumped laser comprising: (a) a laser diode array for
`generating optical pumping radiation, said pumping
`radiation having a bandwidth which is about 3 nm to
`about 15 nm wide and wherein the intensity of the
`pumping radiation is substantially uniformly distributed
`over said bandwith. and (la) a lasant material with an
`absorption band for receiving radiation within said
`bandwidth of said laser diode array.
`An object of the invention is to provide an optically
`pumped laser which includes a laser diode array that
`emits optical pumping radiation having a uniform inten-
`sity over a broad bandwidth for providing stable pump-
`ing radiation to a lasant material, thereby providing an
`improved and more stable optically pumped laser
`which provides enhanced stability despite aging and
`temperature, pressure, and current variations.
`Another object of the invention is to provide an opti-
`cally pumped laser that eases and relaxes the require-
`ments of wavelength matching by providing a broad
`bandwidth output radiation from the laser to the ab-
`sorption band of the lasant material.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 of the drawings is a schematic view represen-
`tative of an embodiment of this invention.
`FIG. 2a is a graph of the emission spectrum of a
`conventional diode laser.
`FIG. 2!: is a graph of the emission spectrum of a first
`diode array comprised of two strips suitable for use in
`the practice of this invention.
`FIG. 2c is a graph of the emission spectrum of a sec-
`ond diode array comprised of eleven strips suitable for
`use in the practice of this invention. A conventional
`laser diode array emission spectrum is also shown in
`phantom.
`FIG. 3 of the drawings is a perspective view of a laser
`diode array suitable for use in the practice of this inven-
`tion.
`
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`FIG. 4 of the drawings is a perspective view of an-
`other laser diode array suitable for use in the practice of
`this invention.
`
`FIG. 5 of the drawings is a perspective view of yet
`another laser diode array suitable for use in the practice
`of this invention.
`
`SS
`
`FIG. 6a of the drawings is a perspective view par-
`tially cut away of yet another laser diode array suitable
`for use in the practice of this invention.
`FIG. 6b of the drawings is a graph of the temperature
`versus position of the laser diode array, as set forth in
`FIG. 60.
`
`FIG. 7 of the drawings is a perspective view of still
`another laser diode array suitable for use in the practice
`of this invention.
`
`65
`
`FIG. 8 of the drawings is a perspective view of still
`another laser diode array suitable for use in the practice
`of this invention.
`
`4,901,330
`
`4
`FIG. 9 of the drawings is a perspective view of yet
`another laser diode array suitable for use in the practice
`of this invention.
`
`FIG. 10 of the drawings is a perspective view of
`another laser diode array suitable for use in the practice
`of this invention.
`FIG. 11 of the drawings is a perspective view of
`another laser diode array suitable for use in the practice
`of this invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`While this invention is susceptible of embodiments in
`many forms, there are shown in FIGS. 1-11 several
`specific embodiments suitable for use in the practice of
`this invention, with the understanding that the present
`disclosure is not intended to limit the invention to the
`embodiments illustrated.
`
`Referring to FIG. I, an optical pumping means or
`laser diode array 10 is shown. Laser diode array 10
`consists of elements or heat sinks I2 and 14 with laser
`diodes or strips 13 and 15,
`respectively, attached
`thereto. Light from laser diodes 13 and 15, is guided by
`lens 16 into lasant material 18.
`Such devices are commonly attached to a heat sink,
`and packaged in a metal housing. A highly suitable
`optical laser diode array 10 consists of gallium alumi-
`num arsenide laser diodes. The diode output radiation
`of 13 and 15 should substantially match the absorption
`band of lasant material 18. If lasant materials other than
`Nd:YAG are used.
`then appropriate semiconductor
`materials must be chosen to meet the above wavelength
`criteria.
`In the laser diode array of FIG. 1, diode 13 emits light
`at a wavelength of about 2 nm or less below the absorp-
`tion peak of the lasaut material 13 and laser diode 15
`emits light at a wavelength of about 2 nm or less above
`the absorption peak at the lasant material 18. Laser
`diodes 13 and 15 can be tuned for the appropriate output
`radiation wavelength, by thermoelectric heaters/cool-
`ers, varying the aluminum and/or doping concentration
`in laser diodes 13 and 15. Referring to FIG. 2b, assum-
`ing the absorption peak of the lasant material 18 is at
`about 808 nm, laser diode 13 emits light at a wavelength
`ranging from about 806 nm to about 803 nm, preferably
`about 807.6 nm, and laser diode 15 emits light at a wave-
`length ranging from about 803 nm to about 310 nm,
`preferably about 808.4 nm, under lasing conditions. As
`is known to those skilled in the art, the absorption peak
`of the lasant material can vary from sample to sample.
`Accordingly, the above wavelength values are merely
`exemplary.
`In FIG. 1, the optical pumping means or laser diode
`array 10 with laser diodes 13 and 15, generates an opti-
`cal pumping radiation having a bandwidth which is
`about 3 nm wide and wherein the intensity of the pump-
`ing radiation is substantially uniformly distributed over
`such bandwidth (see phantom (dashed) waveform in
`FIG. 2b). The waveform in FIG. 26 represents a situa-
`tion where the longitudinal waveforms exactly overlap.
`Even if such waveform does not exactly overlap, a
`uniform intensity distributed over a broad bandwidth is
`obtained. The lasant material 18 with a fixed absorption
`band receives radiation within the above bandwidth
`from the laser diode array 10, as is shown in FIG. 1.
`Heat sinks 12 and 14 can be passive in character. Heat
`sinks 12 and 1-t can also include a thermoelectric cooler
`to help maintain laser diodes 13 and 15 at a constant
`
`

`
`5
`temperature and thereby ensure optimal operation of
`laser diodes 13 and 15. During operation the laser diode
`array 10 will be attached to a suitable power supply.
`Electrical leads from laser diodes 13 and 15 which are
`connected to a power supply are not illustrated in FIG.
`1.
`
`Lasant material 18 has a suitable reflective coating on
`input surface 20 and is capable of being pumped by the
`light from laser diode array 10. The lasant material 18
`also has an output surface 22. The reflective coating on
`input surface 20 is highly transparent with respect to
`light produced by the laser diode array 10 but is highly
`reflective with respect to light produced by the lasing of
`lasant material 18.
`Light emitted by the lasing of lasant material 18 is
`passed through a nonlinear optical material 24 to output
`coupler 26 which has a suitable reflective coating on
`surface 28 which is highly reflective with respect to
`light emitted by lasant material 18 but substantially
`transparent to frequency-modified light produced by
`nonlinear optical material 24. Nonlinear optical material
`24 has an output surface 25. Output coupler 26 is config-
`ured in such a manner so that it serves to collirnate the
`output radiation from the laser which passes through it.
`It should be understood, however, that nonlinear opti-
`cal material for 24 is not required for the practice of this
`invention, and merely represents a preferred embodi-
`ment of this invention.
`Laser diode array 10 has an emission spectrum or
`bandwith which is wider than or envelopes the absorp-
`tion band of lasant material 18 which is being pumped.
`The matching of laser light output from laser diode
`array 10 to lasant material 18 is less sensitive to tempera-
`ture and current variations, and aging of laser diodes 13
`and 15, than typical diode pumped lasers because of the
`wide bandwidth of diode laser array 10 as illustrated in
`FIG. 2b.In FIG. 1, only two laser diodes 13 and 15 are
`shown. However, more than ‘two laser diodes may be
`utilized. The bandwidth of laser diode array 10 in FIG.
`2b is at least about twice as wide as a conventional laser
`diode, as illustrated in FIG. 20. Moreover, the laser
`diode array 10 does not require as sophisticated and
`sensitive feedback and thermoelectric heater/cooler
`circuitry to match the absorption band of the lasant
`material 18, because the bandwidth of laser diode array
`10 is wider than conventional narrow bandwidth laser
`diodes. Further, laser diode array 10 eases and relaxes
`the wavelength matching of array 10 to lasant material
`18 by providing a broad bandwidth output radiation
`from array 10 to the absorption band of the lasant mate-
`rial 18, resulting in enhanced stability despite aging and
`temperature, pressure and current variations.
`A stable and steady optically pumped laser such as
`the instant invention has a multitude of uses and is
`highly suitable for situations where environmental vari-
`ations exist, such as but not limited to, metrology, laser
`printers, bar code readers, optical storage, medical ap-
`plications, laser radars, etc. It is particularly advanta-
`geous in situations where severe environmental varia-
`tions exist, such as but not limited to aerospace, automo-
`tive applications,
`industrial sensing, communications,
`gun sights, target designators and many military appli-
`cations, etc.
`It should be noted that an excessively broad emission
`spectrum or bandwidth should not be sought since an
`excessively broad bandwidth would sacrifice pumping
`efficiency and output power. Accordingly,
`the laser
`diode array 10 overcomes the problems associated with
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`4,901,330
`
`6
`narrow bandwidth laser diodes or arrays by sacrificing
`some power output to attain a steady and stable power
`output.
`Lens 16 serves to focus light from laser diodes 13 and
`15 onto lasant material 18. This focusing results in a
`high pumping intensity and an associated high photon
`to photon conversion efficiency in lasant material 18.
`Any conventional optical means for focusing light can
`be used in place of lens 16. For example, a gradient
`index lens, a ball lens, an aspheric lens or a combination
`of lenses can be utilized. Lens 16 is not essential to the
`operation of this invention, and the use of such focusing
`means merely represents a preferred embodiment.
`Any conventional lasant material 18 can be utilized in
`the present invention, provided that it is capable of
`being optically pumped by the laser diode array 10
`selected. Suitable lasant materials include, for example,
`materials consisting of neodymium-doped yttrium vana-
`date (Nd:YV04); neodymium and/or cromium-doped
`gadolinium scandium gallium garnet (Nd, Cr:GSGG);
`thallium, holmium and/or erbium-doped yttrium alumi-
`num garnet (Ttn. Ho, Er:YAG); titanium sapphire (Ti-
`:A1203); glassy and crystalline host materials which are
`doped with an active material. Highly suitable active
`materials include, ions of chromium, titanium and the
`rare earth metals. A neodymium-doped YAG is a
`highly suitable lasant material 18 for use in combination
`with laser diode array 10 producing light having a
`wavelength of about 808 nm. When pumped with light
`of this wavelength,
`the neodymium-doped YAG or
`lasant material 18 can emit light having a wavelength of
`1,064 nm.
`The geometric shape of lasant material 18 can vary
`widely. For example, the lasant material can have lens-
`shaped surfaces or be rhombohedral in shape if desired.
`Although not illustrated in the drawings, an embodi-
`ment can include the use of a fiber of lasant material
`which is end-pumped by the optical pumping means 10.
`Highly suitable fibers for this purpose include, for ex-
`ample, glass optical fibers which are doped with ions of
`a rare earth metal such as neodymium. If a very long
`fiber is required, it can be coiled, on a spool for example,
`in order to minimize the overall length of the laser of
`the instant invention.
`Lasant material 18 has a reflective coating on surface
`20. This coating is conventional in character and is
`selected so as to transmit as much incident pumping
`radiation from laser diodes 13 and 15 as possible, while
`being highly reflective with respect to the radiation or
`light produced by the lasing of lasant material 18.
`For a neodymium-doped YAG rod 18 which is
`pumped with light having a wavelength of 808 nm, the
`coating on input surface 20 should be substantially
`transparent to 808 nm light and highly reflective with
`respect to light having a wavelength of 1,064 nm. In a
`preferred embodiment, this coating will also be highly
`reflective of light having a wavelength of 532 nm, the
`second harmonic of the aforementioned 1,064 run light.
`The wavelength selective mirror which is created by
`the coating on input surface 20 neednot be located on
`the input surface 20 of lasant material 18. If desired, this
`mirror can be located anywhere between laser diode
`array 10 and the lasant material 18, and can consist of a
`coating deposited on any suitable substrate. In addition,
`the mirror can be of any suitable shape.
`Light emitted by the lasing of lasant material 18 is
`passed through nonlinear optical material 24. By proper
`orientation of the crystal structure of the non-linear
`
`

`
`4,901,330
`
`7
`optical material 24 with respect to the incident light
`produced by lasant material 13, the frequency of the
`incident light can be modified, for example, doubled or
`tripled, by passage through nonlinear optical material
`24. For example, light having a wavelength of 1,064 nm
`from a neodymium-doped YAG lasant material 18 can
`be converted to light having a wavelength of 532 mu
`upon passage through nonlinear optical material 24.
`The geometric shape of nonlinear optical material 24
`can vary widely. For example.
`the nonlinear optical
`material can have lens-shaped surfaces or be rhontbohe-
`dral in shape if desired. Further, any such nonlinear
`optical component can comprise heating or cooling
`means to control the temperature of the nonlinear opti-
`cal material 24 and thereby optimize its performance as
`a. hannonic generator. Nonlinear optical material has an
`output surface 25.
`Potassium titanyl phosphate is a preferred nonlinear
`optical material 24. However, any of the many known
`nonlinear optical materials can be utilized in the prac-
`tice of this invention. Such known nonlinear optical
`materials can be a solid or a liquid, and can include, for
`example KH1P04, LiNbO3, KNbO-3,
`I..iIO3, HIO3,
`I(B50g-41-I20, urea and compounds of the formula MTi-
`0(XG4-) where M is selected from the group consisting
`of K, Rb and T1, and X s selected from the group con-
`sisting of P and As. The non-linear optical material 24 is
`not an essential component and its use represents one
`embodiment of this invention.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`suiting mixture of wavelengths is reflected from the
`coating on input surface 20 back through nonlinear
`optical material 24 where some of the residual frequen-
`cy-unmodified light is frequency doubled, and the fre
`quency doubled light is emitted through output coupler
`26. Except for losses which may occur as a result of
`processes such as scattering or absorption, further repe-
`tition of this series of events results in essentially all of
`the light produced by the lasing of lasant material 18
`being frequency doubled and emitted through output
`coupler 26.
`Referring to FIG. 3, there is schematically shown a
`laser diode array 40 which is suitable for use as a source
`of optical pumping radiation in the practice of this in-
`vention. The fabrication of this diode array 40, as well
`as other laser structures hereinafter described. can be
`carried out by liquid-phase epitaxy, molecular beam
`epitaxy and metal-organic chemical vapor deposition,
`which techniques are known in the art. Deposited on
`substrate 42 are layers 44, 4-6, 48, 50, 52, 54, and 56.
`Substrate 42 can be an n+GaAs substrate. Layers 44
`and 52 are confining layers, typically about 1 pm thick,
`which can comprise t1A2x, Gaga, As and pA1,.Ga1.,,As.
`respectively. Layers 4&6 and 50 are waveguide layers
`typically less than or about equal to 0.2 nm thick, which
`can comprise nA1;, G31.y, As and pA1_,.Ga;._,.As. respec-
`tively. Layer 48 is an active region or quantum well
`having a thickness 49 typically less than or about equal
`to 0.2 pm, which can comprise iA1zGa1.zAs. Values for
`X and x’, y and y’. and z typically range from about 0.3
`to 1.0, 0.1 to 0.5, and 0.0 to 0.1. respectively, but are not
`limited to such ranges. Layer 54 is a cap, typically less
`than or about equal to 0.2 pm thick, which can com-
`prise p+GaAs. And layer 56 is a conductive layer typi-
`cally made of a gold/chromium composition or equiva«-
`lent thereof.
`
`As recognized in the art, the conductivity type of the
`above layers can be reversed, which is also true for later
`described embodiments.
`
`A power supply is not included in the figures. As is
`understood by those skilled in the art, the laser diodes,
`light emitting lasers and laser diode arrays illustrated
`herein are properly forward biased by applying a posi-
`tive charge at the appropriate place to energize laser
`diode array 40, and the other diode arrays in FIGS.
`4-11. Layer 56 provides a metalization layer for elec-
`trode connection and current pumping. Also, the bot-
`tom surface 41 of substrate 42 can be metalized to pro-
`vide a contact for the other electrode connection. This
`metalization is conductive and can be made of a gold/-
`tin or gold/’germanium alloy.
`In a conventional laser diode array, the number of
`strips is significant from the point of desired power
`output. Generally, an increase in the number of strips
`will proportionally increase the optical power output at
`a desired wavelength. See FIG. 2c in phantom wherein
`the emission spectrum of a conventional laser diode
`array centered at 308 nm is illustrated. Also, the higher
`the number of emitting cavities, the higher the obtain-
`able peak power output. Typically, laser diode arrays
`include active layers which have the highest index of
`refraction and a low bandwidth to provide a waveguide
`for light wave propagation at a single wavelength under
`lasing conditions. However, such laser diode arrays
`require sensitive and sophisticated feedback circuitry
`and/or
`thermoelectric heaters/coolers
`to carefully
`match the output radiation ofthe array with the absorp-
`tion peak of the lasant material, which is in sharp con-
`
`As a consequence of the fact that nonlinear optical
`material 24 is not 100 percent efficient as a second har-
`monic generator, light passing through this component
`from lasant material 18 will ordinarily consist of a mix-
`ture of frequency doubled or summed light and unmodi-
`lied light. In the case of light having a wavelength of 35
`1,064 nm from neodyrniurn-doped YAG as the lasant
`material 18, the light passed through nonlinear optical
`material 24 will be a mixture of 1,064 run and 532 rim
`wavelengths. This mixture of wavelengths is directed to
`output coupler 26 which has a reflective coating on
`surface 28 which is wavelength selective. This coating
`is conventional in character and is selected in such a
`manner that it is substantially transparent to the 532 nm
`light but highly reflective with respect to the 1,064 nm
`light. Accordingly, essentially only frequency doubled
`light having a wavelength of 532 nm is emitted through
`the output coupler 26.
`The output coupler 26 includes a wavelength selec-
`tive mirror which is created by the coating on surface
`28. It need not be of the precise design illustrated in
`FIG. 1, and can be of any conventional form. For exam-
`ple. the wavelength selective mirror can be created by
`a coating on surface 25 of nonlinear optical material 24.
`In this event, output coupler 26 could be either elimi-
`nated or replaced by optical means whose sole purpose
`is to collimate or otherwise modify the output radiation
`or laser light from the lasant material 20. However, the
`concave shape of the mirror created by the coating on
`surface 28 has the advantage of focusing reflected light,
`which has not been frequency doubled, back onto non-
`linear optical material 24, through lasant material 18 and
`onto the coating on input surface 20. As set forth above.
`in a preferred embodiment. this coating on surface 20 is
`highly reflective of both frequency doubled and un-
`modified light from the lasing of lasant material 18.
`Thus, frequency-unmodified light reflected by the coat-
`ing on surface 28 is partially frequency doubled by
`passage through nonlinear optical material 24, the re-
`
`45
`
`50
`
`55
`
`65
`
`

`
`9
`trast to laser diode array 40. Further, because the band-
`width of the laser diode array 40 is wider than in con-
`ventional narrow bandwidth laser diodes, the radiation
`emitted therefrom is more stable and, less temperature
`and current sensitive, and less sensitive to aging varia-
`tions.
`The current confining channel geometry shown in
`FIG. 3 comprises 11 parallel contact strips which run
`the length of diode array 40. However, any number of
`strips can be utilized in the practice of this invention. A
`conventional proton implant 57 is included which pat-
`terns lateral regions of high and low resistive material in
`a semiconductor device to channel current to a specific
`‘region in a device when properly biased. When laser
`diode array 40 is energized or forwarded biased, current
`is confined to the eleven elongated and narrow strips:
`58, 69. 62, 64, 66, 68, 70, 72, 74, 76 and 78 and channeled
`to the adjacent and corresponding strip areas or emit-
`ting cavities 82, 84, 86, 88, 90, 92, 94, 96, 98, 100 and 102,
`respectively, of active layer 48.
`Each of the above strips has varying widths for con-
`fining current flow to the plurality of strip areas or '
`emitting cavities of the active layer 48. Each strip area
`emits pumping radiation under lasing conditions differ-
`ent from that of each adjacent strip area, thereby pro-
`viding a stable, uniformly distributed wide bandwidth
`laser diode array which is less sensitive to temperature
`and current variations, and less sensitive to aging varia-
`tions.
`The first strip 58 includes a width designated as a in
`FIG. 3, ranging from 0.9 um to 1.1 urn, preferably 1 um,
`the second strip 60 includes a width b ranging from 1.12
`um to 1.37 um, preferably 1.25 um, the third strip 62
`includes a width c ranging from 1.22 um to 1.73 um,
`preferably 1.5? um, the fourth strip 64 includes a width
`d ranging from 1.8 um to 2.2 um, preferably 2 um, the
`fifth strip 66 includes a width e ranging from 2.25 um to
`2.75 um, preferably 2.5 um. the sixth strip 68 includes a
`width f ranging from 2.83 um to 3.47 um, preferably
`3.15 um, the seventh strip 70 includes a width