CA2078483C - Surface emitting laser - Google Patents

Abstract

Reduction of laser threshold in an electrically pumped vertical cavity laser is the consequence of interpositioning of an electrode layer intermediate the active, photon producing region, and at least one of the two Distributed Bragg Reflectors defining the laser cavity. The advance is a consequence of the lowered pump circuit resistance due to elimination of one or both DBRs - in particular, to elimination of the p-doped DBR - from the pump circuit.

Description

`- -1- 2078483 Surface E-. lI;JI~;~ Laser Back~round of the I~ rltion Technical Eield The invention relates to a category of lasers including vertical cavity 5 lasers, of design known as surface emitting lasers. Most important, such devices appear to satisfy the desire for integratable lasers - lasers to serve in Opto Electronic Integrated_ircuits as well as in all-optic circuits. Contemplated integrated circuits may include electronics - generally semiconductor electronics - serving both foroperation of the lasers and for other purposes.
10 Desc,iplion of the Prior Art Virtually from inception, the emergence of the laser raised expectations of widespread use in integrated circuits - both ancillary to electronic ch~;uilly and in all-photonic ci,cuill y. The development of the elçctric~lly pumped pn junction laser promised to satisfy the desire. Nevertheless, commercially expe-iiçnt integrated15 lasers are not a reality. While there have been a variety of obstacles, I2r loss coupled with high lasing threshold values are central. For specialized purposes, cooled Cil~;uitl~ might suffice; for general use a more economical approach is nçede~
Worldwide effort has addressed the very promising Surface Emitting Laser aka Vertical Cavity Laser, and the consensus is that this approach points the 20 way to commercially feasible OEICs. It is likely prevalent SELs will be based on active regions cont~ining one or more Quantum _ells although active regions based on bulk m~tçri~l are not to be discounted. References tracing introduction and recent development are: Y. Arakawa and A. Yariv, "IEEE J. Quantum Electron.", QE-21, 1666 (1985); Gourley et al, "Applied Physics Letters", 49 (9), 489 (1986) and J. L.
25 Jewell et al, "Optical Fngineçring", 29, 210-214 (1990).
Effort at this time is directed to an SEL structure con.ci~tinp of a p-n junction active region in which photons are generated responsive to pumping current - an active region which in earliest work is based on "bulk", likely homogeneouscomposition, and which, in later work, makes use of Quantum Wells or of 30 superl~ttice structure. The number of quantum wells, more generaUy the thi~1~nçss of the active photon-producing m~tçri~l layer, inescapably dictates lasing threshold.
Desired reduction in I2r heating has led to a decreasing number of QWs, culmin~ting in the 2- or the l-quantum well structure of U.S. Pat. 4,999,842 dated March 12,1991. Most effective cavitation is due to the very high reflectivit,v res~llting from use 35 of Distributed Bragg Reflectors (with reflectivities well over 99%, e.g. for 20+ pair mirror structures on both sides of the active region). See U.S. Pat. 4,999,842 describing a structure having a laser emission threshold at 7 microwatts/llm2 for DBRs of 24-pair, 1/4 wavelength (1/4 ~) layers of GaAs and AlAs, embracing an active region based on an 80 A active layer of In 2 Ga.8As (1/32 wavelength S quantum well) emitting at 980 nm.
While the described work has resulted in acceptable lasing threshold values in the active m~teri~l itself, heating due to high series resistance in the SEL
pump circuit - a circuit including a p-type DBR, the active region, and an n-type DBR - continues to be a problem. Total power efficiency and maximum power 10 obtainable from SELs continues to be low compared to that obtainable from edge-emitting structures (about 5% efficiency and 1 mW power output for SELs vs. 30%
and 100 mW) for edge emitting structures. Origin of the problem is largely the p-type DBR - of the high series resistance due to low hole mobility and the high optical absorption res-llting from increased p-type doping introduced to reduce 15 resi~t~nce. Extensive effort directed to this problem has resulted in optimi7~tiQn of layer-to-layer interf~ces in the mirror (allowing Continuous _ave room temperature operation without heat sinks - but only at the in~icated performance level). Other effort has taken the form of high p-carrier doping levels either throughout the DBR
or at the lowermost level (Y. J. Yang et al, App. Phys. Letters, vol. 58, pp.
20 1780-1782 (April 1991) as well as reduced number of Bragg pairs (by partial, or even complete, substitution of BMgg layers by silver). Both approaches result inassociated optical absorption to lower the differential quantum efficiency of the cavity. While the tMde-off (of lower resi~t~nce for lower quantum efficiency) is a useful design consideration, the overall problem remains unsolved.
25 Summary of the Invention The invention is directed to electrically pumped p-n junction laser uc~u~es, exemplified by SELs in which I2r heating is l~.ccened for given laser output power by removing part or all of one DBR from the electric~l pump circuit.
SLlu~ s of the invention depend upon interpositioning of a layer electrode 30 interm~ te the p-type side of the active region and at least the major part of the DBR on that side of the structure. From the operational standpoint, preferred structures depend upon elimin~tion of part or all of the other DBR - desirably of the e,l~GLy of both mirrors from the electrical circuit, and rely upon positioning of electrode layers on both sides of the active region - the latter, as described in detail, 35 likely including passive layered material ("primary spacers") of such thi~n~c$ as to center active (photon-generating) m~teri~l at the peak of the reslllting st~n~ing wave 3 20784~3 (of the st~nr~ing photonic wave during lasing). In accordance with known practice, reduction of thi~ness of the active m~tçri~l layer as so placed increases the efficiency with which generated photons contribute to the st~n~ing wave while decreasing loss due to absorpdon of the standing wave energy. Active regions, as5 made up of three layers - (1) spacer, (2) active material layer, (3) spacer - in an exemplary structure are of 1/2 ~ thi~ness For most purposes, it is useful to regard such active region together with attendant embracing regions - including at least one electrode layer and, likely, a secondary spacer - as together defining a full ~ cavity.
The preferred structure, inclu~ing another electrode layer and, likely, another 10 associated secondary spacer, is of the same total thickness - likely of ~ thickness.
Material included within such cavity may serve additional function - e.g. the secondary spacer may serve to enh~nce reflectivity, and, accordingly, may be considered part of an adjacent mirror.
A prime operational advantage of SELs of the invention is due to design 15 variations pe. .~ ed by the basic tçaching In addition to structures in which the advantage is gained by elimin~tion of all or part of the n-type DBR (as well of the prime offender - the p-type DBR), separation of electrical and optical function permits further performance advantage. DBRs, or DBR portions no longer included in the pump circuit, may now be optimiæd optically. One such modification entails 20 elimin~tion of photon-absorbing, significant impurity dopant from the excluded portion or the entirety of the DBR on the p-type side of the junction - of the (formerly) p-type DBR layers. This electrode layer is contacted laterally (relative to laser emi.csion, which in accordance with usual terminology is descAbed as vertical).
In a preferred embodiment, a similar electrode underlies the active region, or, 25 ~ltern~tively~ a conven~ional substrate electrode is used, to pump the structure. Such excludedDBRs~pelll&ps of undoped semiconductor, perhaps of dielectric m~tç.ri~l are likely of resistivity of 10- 1 ohm-cm or larger.
A primary value of such electrode layers - at least of that replacing p-type DBR layers in the pump circuit - is lçssel-P,d resistance in the pump circuit.
30 Con.ci(lP~tions dicc!~c~d lead to any of a variety of electrode m~tPri~lc From the perform~nce standpoint, electrodes are desirably met~llic, e.g. gold. This permits least electrode thicknçss (for prescribed conduction) so as to result in least photonic absorption (for invariant electrode pl~çmçnt as centered on a st~n(ling wave node).
Fabrication is the primary consideration that may suggest non-metallic electrodes -35 crystalline lattice matching to allow epitaxial growth, e.g. of secondary spacer andDBR layers, may indicate use of semimetal or heavily doped semiconductor. While advantageous, e.g. in permittinP epitaxial growth, such substit~ltion reduces quantum efficiency due to the increased photonic absorption associated with increased electrode thicknP~s. Greater layer-to-layer variation in refractive index permitted by non-epitaxial growth methods, e.g. by m~nPtron spu~P.. ;ng relieves a restriction on 5 cavity efficiency, Q, to further improve operation. This is a factor in choice of electrode m~teri~l - non-epitaxial DBR growth may even be advantageous with electrodes of semimPtal or semiconductor as well as of metal. At this time relatively small index variation in materials suitable for epitaxy lead to non-epitaxial DBR
growth techniques for longer wavelength values - ~ > 1 ~lm as measured in vacuum10 (e.g.atl.30rl.55llm).
It is proper to consider the inventive te~ching as permitting separation of optical from e~ec~ric~l considerations in the DBR design/fabrication. Accordingly, relaxation of the need for p- or even n-doping of the DBRs adds a degree of freedom.
DBR fabrication, for example, by evaporation techniques, is uncomplicated by the15 need to introduce and control dopant. Now-permitted use of dielectric mirrorsreduces absorption: firstly, due to reduced thicknPss permitted from higher layer-to-layer index v~ri~tion and, secondly, due to minimi7~tion of carrier absorption. Such consideration may, in itself, dictate use of a second electrode layer under the active region to permit use of undoped DBRs, top and bottom.
Improved structures of the invention will have a major effect on a variety of appli~tion~ such as optical computing, optical interconnection, high speed laser printing, and in visible lasers as in displays. RPslllting apparatus, likely based on OEICs cont~ining laser elemPnts described, constitute a $ignifir~nt part of the inventive advance.
25 General Comments It is convenient to describe the invenlive teaching in context of SEL
structures now receiving worldwide attention. The invendon is somewhat broader, in being based on ~llu~;lu~s in which laser cavitation is of such direction as to have a signifi~nt component in the direction of electrical pumping. The obsenation, that 30 absorption for prope-ly placed electrode layers of thick~ess less than approximately 1/4 wavelength (perhaps as large as 0.3 wavelength) may be a minor concern, is of consequence for any such structure in which cavitating energy is, at least in part, tr~n.~mined through the electrode. Structures in which cavitation is designp~lly non-parallel to the electrical pumping direction may benefit.

Devices are described as containing "pn junctions". In fact, a variety of considerations may lead to an active material layer which is nominally of intrinsic conductivity. The r~slllting junction may properly be referred to as "pin".
Such variation is to be considered as included within general reference to "pn" or "junction", etc.
The invention is not primarily concerned with the exact nature of mirrors based on distributed, cooperative in-phase, reflection due to index change between 1/4 A mirror layers. While resulting structures indeed depend upon Braggreflection, they may differ in detail from conventional DBRs. For example, it is not required that alternating layers be made up of periods of identical index pairs.
Pairs may be of differing index to result in graded reflectance, and additional layers, e.g. to result in triplets or higher order periods, may be included.
In accordance with one aspect of the invention there is provided apparatus comprising at least one electrically pumped vertical cavity laser consisting essentially of an active region containing a semiconductor active material layer for generating photons to result in laser emission, said active region comprising p-type and n-type conductivity portions and including a semiconductor junction, said active region being sandwiched between Distributed Bragg Reflectors each comprising a plurality of layers of relatively low and high refractive index for the said laser emission, together with circuit means for electrically pumping said active region to lasing threshold, lasing entailing presence of a standing wave defining a cavity here described as bounded by the said Distributed Bragg Reflectors CHARACTERIZED in that said circuit means includes an electrode layer at least part of which is outside the said active region, whereby at least a major portion of one DBR is excluded from the circuit means, said electrode layer having a conductivity sufficient to result in total conduction of the said circuit greater than that of a circuit including the entire DBR as conductivity-doped to an average carrier concentration of 10l9 carriers/cm3, and in that absorption for laser emission 2 ~ 7 8 ~ 8 3 -5a-is reduced in part due to reduction in conductivity-doping of the excluded portion of the DBR to result in a numerical value of equal to or less than 0.25 for the product r-Ap in which r is circuit resistance in ohms of that portion of the circuit consisting of p-type material and the associated electrode layer and Ap is per-pass photon absorption for that circuit portion expressed as a fraction.
Brief DescriPtion of the Drawing FIG. 1 is a schematic diagram depicting salient features of the relevant portion of an SEL to which reference is made in detailed description of an operating device in accordance with the invention.
FIG. 2, on coordinates of layer thickness and intensity, depicts a standing wave in an operating SEL on the basis of which a variety of design criteria, including standing wave overlap factors, are discussed.
FIG. 3 is a front elevational view of an electrically pumped laser of the invention.
FIG. 4 is a perspective of a portion of an integrated circuit including an array of SELs.
FIG. 5, also in perspective, depicts an integrated circuit containing a laser structure of the invention together with electronic drive circuitry.
FIG. 6 is a diagram relating an exemplary active region design to the standing wave produced in device operation, all in abscissa units of distance and in ordinate units of dopant composition as well as resulting refractive index descriptive of the region itself and in current density with regard to the standing wave.
Detailed Description Nomenclature Description is expedited by definition of terminology used. This is of particular value in view of inconsistent use of many of the terms in the literature.
Active Material Layer - that layer of the SEL primarily responsible for photon generation responsive to pump current.

Active Re~ion - layered portion of the SEL containing the active m~teri~l layer, generally including sandwiching passive layers (pAmary spacer layers), as likely bounded by an electrode layer at least on one surface - likely the upper s~lrfa~e, this electrode thereby replacing the traditional p-type DBR in the 5 electrical pump circuit. From the operating standpoint, where there is no lower electrode layer, the active region may usefully be considered as defined by the mirror - usually an n-type DBR on the underside.
Primary Spacer - layer m~teri~l within the active region embracing the active m~teri~l layer. Usual design dictates that pAmary spacer layers are doped10 with sig~ific~nf impuAty, p-type on one side of the active m~teri~l layer, n-type on the other side of the active mateAal layer, thereby defining or contributing to the pn junction required in contemplated electAcally pumped laser structures of the invention. Layered m~teri~l in addition to the active m~t~ri~l layer and spacer layers within the active region, e.g. serving as part of a DBR, is of conductivity type15 consiQtent with the junction - is genPrally p-type or n-type. For most purposes, ~ddition~l layered m~t~.ri~l within the active region is considered as part of the primary spacer.
Electrode Layer - that layer serving for biasing (for pumping) the active region at least on the p-type side of the SEL. Together with an optional paired 20 electrode layer, or, ~ltern~tively, together with a conventional electrode layer on the m~prsi~e of an n-doped DBR, they define the active region.
Secondary Spacer- any passive material, serving little consequential function in terms of either photon generation or mirror funcdon, outside of the active region - generally in contact with an electrode layer. As di~ sse~l, a secondary25 spacer is optional if the primary spacer is of low refractive index relative to relevant DBR layer - desirable if the primary spacer is of relatively high index so as to assure electrode layer positioning so as to correspond with an energy trough in the st~nding wave.
Conception leading to the disclosure e~t~iled insertion of the p-type 30 electrode into a spacer, so resulting in "secondary" and "primar,v" sp~ers. First commercial devices may be of such design, although variation may be with a view to secondary considerations - for example, the secondary spacer may be of altered index to better complement reflectivity of subsequent DBR layers.

Desi.~n Criteria Requirements of the SEL structure of the invention are ~iSCIlsse~ with ,t;felence to FIGS. 1 and 2. Essential features depicted are: active material layer 10, likely con~tituted of one or more quantum well layers, e.g. as described in U.S. Pat.
5 4,999,842 or, ~ltçrn~tively~ of bulk or supçrl~ttice material. The improvement in power efficiency offered by the inventive approach increases the likelihood of commerci~li7~tion of the Single Quantum Well structure of that patent, and it isuseful to consider layer 10 as cont~ining a single or small number of quantum wells.
In any event, layer 10 is ~i~Cusse~ as sandwiched between primary spacer layers 11 10 and 12. These layers, while shown as con~ti~ltç~ of continuous m~tç~l - whichmay be of uniform or graded composition, may include separately identifi~ble layers, e.g. layers 11 or 12 may include regions serving as DBR layers, or layer 10 may include regions serving as quantum wells in Multi Quantum Well structures.
Compositional grading of layers 11 or 12 may serve to advance secondary design 15 criteria, e.g. reduction of electrical resi~t~nce between an electrode and the active m~teri~l layer. Spacer layers 11 and 12 are doped with .$ignific~nt impurity - e.g. p-type and n-type, respectively. (While structures of the invention may depend upon an upper p-type region for forming the requisite junction, there is no longer a design disadvantage in l~vel~.ing the junction, particularly for preferred structures in which 20 both DBRs are excluded from the pump circuit.) It is convenient to consider the cavity as including: secondary spacer layer (or DBR layer) 16; electrode layer 14;
primary spacer layer 11; active material layer 10; primary spacer layer 12; and if present together with underlying electrode layer 15 and any secondary spacer layer 13. The structure is completed by DBR mirrors 19 and 20. Arrows 21 and 22 25 leplesenl hole and electron flow resulting in operation. Conductive layers may be of metal, may be of semim~t~l, or may be semiconductive. Selection depends upon a variety of factors - generally selection based on tradeoff as between perform~nce and ease/cost of f~.ri~tion. Energy consel vadon is favored by use of metal, e.g. gold, silver, ~ n;l.n~, of resislivily perhaps of the order of 10-6-10-5 ohm-cm (in any 30 event offering lower pump circuit I2r than that of a circuit including the DBR mirror layers now removed from the elect-rical circuit). Expçrimçnt~l work has led to use of such a metal layer of thickn~.ss of perhaps 50-100 A (or of the order of hundredths of a 1/4 wavelength). Non-met~llic electrode layers - sçmimetQl or semiconductor (the latter conl~ining up to 102l carriers/cc) are suitable particularly for shorter 35 wavelength emission - e.g. 0.850 ~lm. Layer thiçkn~cs of - 1/12 ~ and 1/4 ~ for semimet~l and semiconductor electrodes, respectively, yield conductance similar to -8- 2078~83 -that of metal electrodes. Since conductivity is higher for lower bandgap mz~tPri~l, electrode thicknPss may be increaæd less than proportionally with longer wavelength for the same conductivity, thereby decreasing the absorption penalty and increasing advantageous use of semiconductor electrodes.
Regions above electrode layer 14 and below spacer 12 (below electrode layer 15, if present), most importantly consist of DBRs 19 and 20 (although æcondary spacers 16 and 13 may lie within the DBRs.
Other features shown in the figure repreænt preferred characteristics but may be omitted for reasons of economy. Such features include encircling thick 10 conductive layer 17 as well as 18 to lesæn impedance in the pump circuit.
FIG. 1 is de~i~nçd primarily to serve as a basis for the above discussion, and not to exhaustively rep-esenl physical configurations. Discussion is largelycon.~istPnt with an active region 10 supporting a single 1/2 wavelength st~n-~ing wave and with a single quantum well placed accordingly - with such well at the 15 center of region 10, thereby ~sllring pl~cempnt at the crest of the standing wave.
Variations which may ærve economic and/or perform~nre goals may entail bulk or superl~ttice m~teri~l as well as MQW structures. The inventive advance, impo~ ly, contributes toward increased lasing efficiency, and, so, increaæs likelihood of commerci~li7~tion of single quantum well devices (of U.S. Pat.
20 4,999,842). Nevertheless, design flexibility resulting from ability to define layers of tens of A - with wavelengths of thousands of A - permits construction of deviceswith active regions made up of successive quantum wells, or of succes.~ive bulk or supçrl~ttice m~tçri~l layers centered about successive crests. In general, a preference continues to exist for all active layer r~5~teri~1 being positio~ed at a single st~nding 25 wave crest, so that MQW structures are likely to contain no more than the - 6-7 QWs which may fit within ~ 1/4 wavelength about a peak.
As else~.hele in this description, while quantum well structures are certainly ~l~fe.led from the performance standpoint, other considerations largely pel~ining to f~bric~tion cost, may dictate uæ of bulk or superlattice active m~tPri~
FIG. 2, a plot on coordinates of layer thir~nP,ss, ~, on the ordinate and intensity on the ~bscissa, depicts the st~n-ling wave for the center portion of a repreænt~tive structure, 30. The structure shown consists of an active m~t~.ri~llayer 31 sandwiched between primary spacer layers 32 and 33. Electrode layers 34and 35, shown as broken lines, are in contact with the primary spacer layers.
35 Layers 36 and 37 may constitute secondary spacers, or, alternatively, first DBR
mirror layers. As discusæd, uæ of secondary spacers serve, inter alia, to assure g po.~itionql collespondence of electrode layers 34 and 35) with nulls in the st~n~ing wave 37 - likely as required, e.g. on the p-type side of the structure when the primary spacer, e.g. layer 32 is of high refraction index, n, relative to the first DBR mirror layer. For instanres in which the primary spacer is of low index relative to the5 nearest mirror layer, the secondary spacer may be dispensed with. Under such circum~t~nces, layers 36 and/or 37 may be regarded as mirror layers. Regarding the mirror structures as prototypical DBRs, paired layers above secondary spacer 36 are of low, high, low, high index as corresponding with layers 39-42 in that order. If layer 36 serves as a DBR layer, it is of high index. The underside DBR is 10 symmetrical so that layer 41 is of low index, etc.
The active m~teri~l layer 31 is of thickness corresponding with 1/4 ~, or:
~ O/4n (eq. 1) in which:
~0 is the wavelength as measured in vacuum, n = refractive index of the active m~teri~l An electrode layer of gold or other highly conductive metal as used ;"~Pnt~lly, affords useful conductivity in thirknP,ss of the order of 100 A
(pelhaps l/lOOth of a 1/4 wavelength as measured in the metal layer). Semimet~l or heavily doped semiconductor electrode layer material, still affording sufficient20 conductivity for thicknPss below the ~ 1/4 ~, the permitted maximum, may serve in a thi- L lleSS range of pell-aps 1/10-1/12 ~, or 1/2-1/4 ~. (As elsewhere in the description, such measurements are presented as illustrative and not as limitingactual limits depend upon a number of factors, e.g., the lateral dimPn.~ions of the laser.) The absorption of a thin electrode layer (or of other film) centered on a null is:
a~L= 6 ~3L3 ~,3 a (eq. 2) in which:
a = absorption coefficient r = overlap factor in accordance with eq. 3 or eq. 4 L = film thirkne~s The overlap factor ~ of a film centered on a peak of the st~nding wave (centered on an antinode) is:
y=l+ si~_ 2- ~6 (eq.3) lo- 2~78~83 in which:
O = layer ~hicl~n~P.~ in radians from eq. 5.
The overlap factor of a film centered on a null of the st~n-ling wave is:
s~ 6 (eq. 4) 5 in which:
0, layer thickness in radians, = ~, (eq. S) Absorption as calculated above is for the hypothetical condidon of zero reflectivity for the electrode layer. In fact, a metallic electrode layer has significant ~soci~ted reflectivity to result in increased pass length - likely as corresponding 10 with ~ ten reflectdon incident.~ - for radiation which is but single pass for the hypothetical condition. Absorption loss increases linearly with pass length.
Reflectivity for semimetal and semiconductor electrode layers, while finite, is less than that of contemplated metals.
FIG. 3 depicts an SEL structure 50 cont~ining an active region Sl and 15 Bragg mirrors 52 and 53. DBR 52 is con~tituted of alternating low and high refracdve index, 1/4 ~, Bragg layers 54 and 55, and mirror 53 is con~ uled of similar low and high index layers 56 and 57 as discussed in the descriptdon of FIG. 2.
FIG. 3A illusllales an inventive species in accordd"ce with which the SEL provides for a thin, mPt~llic electrode layer 58 separated from active region 59 by primary 20 spacer 60. For the design shown, desired current flow is assured by relatively thick conductive layer 61 to which feed circuitry, not shown, is connected Layer region 61 is in conductive contact with and encircles electrode layer 58 which latter is within the functdonal portion of laser 50. Layer 62 atop electrode layer 58 is the secondary spacer layer, like primary spacer layer 60 of passive m~teri~l - desirably 25 included as di~cussed where primary spacer 60 is of low refractive index relative to that of the lowermost mirror layer of DBR 52. It is succeeded by Bragg layers 54and 55, and may, itself, also serve as a functioning layer of DBR 52. The arr~ngemPnt shown in FIG. 3A depends upon lower electrode 63, which together with electrode layer 58, and by means of electrical circuill ~ not shown, electrically 30 pumps the laser. The relative proportions shown are fairly represerlt~tive of use of a semimetal, a heavily doped semiconductor, or a metal electrode layer 58. The design of FIG. 3A depends upon current blocking region 64 for ch~nnPling pump current through encircled active material layer 59. The structure is completed with ~ltPrn~ting low and high 1/4 wavelength layers 56 and 57 on the undpr~ p~ together 35 co~ u~ g DBR 53, and by substrate 65, the latter n-doped where serving as part of the pump circuit including electrode 63. The requisite pn junction within activem~teri~l layer 59 is the consequence of p-doped spacer 60 and n-doped DBR 53 possibly similar to mirror 52, possibly dicsimilar.
FIG. 3B depicts a version of the inventive structure, similar to FIG. 3A
5 dependent upon a second, relatively thin, likely metallic, electrode layer 58 with its encircling thicker conductive layer 61, overlying (likely relatively thick) primary spacer 60 . The design permits elimin~tion of underlying electrode 63 as well as use of an undoped lower DBR mirror 53 possibly similar to mirror 52, possibly ~i.csimil~r. The structure of FIG. 3B is otherwise as shown in FIG. 3A.
FIG. 4 depicts an array of lasers 70 which may be of the ~et~ design, e.g. as shown in either of FIGS. 3A or 3B. Supporting substrate 71 may, in the instance of the species of FIG. 3A, have served for epitaxial growth of Bragg mirror 53. Other variations may entail removal of lattice-matched substrate following such epitaxial growth to permit replacement with mechanically preferable 15 matPri~l e.g. for more dependable support.
FIG. S is a schPm~tic representation of a portion of an OEIC con~i~ting of a laser 80, together with drive electronics 81 on common substrate 82. Biasing of u~;lure 80 is by means of lead 83 cont~cting an upper electrode layer such as layer 58 of either of FIGS. 3A or 3B. A second current path not shown may contact a 20 lower electrode layer, if present, or, altern~tively, an underlying electrode such as electrode 58 of FIG. 3.
FIG. 6 consists of three figures, 6A, 6B and 6C. FIG. 6A on coordinates of current on the ordinate and ~ictance on the abscissa, depicts standing wave 89 cycling from trough intensity of minimum current value (null) 90 to maximum 25 current value (or crest) 91. Active material layer 92, in a p~pe,ly designed structure, is centered at st~n-ling wave crest 91. The active region includes primary spacers 93 and 94 sandwiching active material layer 92 to, together, define a 1/2 ~ thicknes~s.
Electrode layer 95, centered at null 90 completes the structure as ~pl~sen~e~
FIG. 6B on coordinates of conductivity-composidonal variation (in one 30 in.~t~nce variation of Al content in AlGaAs) on the ordinate and ~istan~e on the abscissa represents a structure of composition serving to produce the standing wave of FIG. 6A. Based on GaAs, in this inst_nce, of Al content ranging from 0% across the active m~teri~l layer 92 to graded compositions of maximum ~lnminllm contentof 40% in the sandwiching primary spacers 93 and 94. Primary spacer 94 achieves a 35 desirable cross-over between loop re~i~t~nce and photonic absorption by means of increasing alnminum content within region 96 (within the range of from 25-40 at.%

2û78~83 for the example shown) and of decreasing alulnin~ content to a 0% or near-0%
~l".~in.. content within region 97, to decrease the conductivity barrier and increase current flow. Mirror layers 98 and 99, in this instance of ~ 15% Al complete that part of the structure diagrammed.
FIG. 6C in ordinate units of refractive index, n, and in the same abscissa units of dist~nce, show the variation in n attend~nt on the compositional changes shown in 6B. Numbering is the same as in FIG. 6B.
The Table reports charactPri.cti~s useful in comparison of modified designs (ex~mplPs numbered 4-9) with the prior art designs (examples 1-3) as treated 10 in the techni~ ul~. It is clear that the inventive advance is usefully incorporated in a large variety of designs, some of which have not yet emerged. It is impractical to describe all such presently contemplated structures, much less toattempt to predict future design~. Full scope of the invention is defined by theappended claims.
Design parameters hPa~ing the columns are those relevant to device design with regard to the thrust of the invention. Column hPa~ings are relevant prim~rily to examples 2 through 9 and are largely inapplicable to the lefe~ ce, prior art device of example 1. Design parameters in the order of the columns are:
"Material" - material relevant to the pump circuit (with reference to 20 examples 1 through 5, designating doping level of semiconductor m~teri~l in the p-side electrode layer of the circuit; with reference to examples 6 through 9, de~ign~ting semimetal or metal composition of the electrode layer). Example 9 provides for a second conducting layer on the n-type side of the SEL as well.
Example 1, included for comparison purposes, pertains to a prior art "standard"
25 structure including a 19 period verdcally biased p-doped DBR within the pump circuit.
"Thi~lfness" is that of the upper electrode layer for examples 2-9. For ex~mple 1, it refers to the penetradon depth of cavitadng radiadon into a GaAs-AlAs DBR. The third column lists values of resistivity of the upper electrode layer in 30 units of micro-ohm-cm.
Absol~,ti~ily, a, is that of the electrode layer as expressed in units of cm~l A is the single pass (or per-pass) absorption for cavitating laser radiadon for the entire resulting SEL - including both mirrors.

The next two columns report cavity quantum efficiency, rl also on this basis - first for mirror reflecdvity of 99.0%; and in the next column, quantum efficiency on the same basis for mirror reflecdvity of 99.8%.
The next column reports values of electrical resistance, r, of the p-type S side of the SEL structure - for examples 2-9 as excluding the upper DBR - i.e. of the p-type m~teri~l and associated electrode layer.
The final column reports the product of r and Ap, the single pass absorpdon of the p-type side of the SEL. This product is a useful figure of merit in ev~ tion of advantages realized by structures of the invendon.
Other relevant design informadon is well-known to those skilled in the art and its inclusion here is unnecess~ry. Such informadon is available from theliterature, see, for example, references cited in "Descripdon of the Prior Art".
ExampleMaterialThiçkness p (,uQcm)a(cm-1) A ~(.990) r~(.998) r(Q) 1. p(S- 10l8)(0.5 llm) 50 .0030 .77 .40 2. p(S 10l8) lllm 10,000 50 .0055 .65 .27 8 3 p(S 10l8) 3~m 10,000 50 .0155 .39 .11 2.7 4. p++(1020) 550 A 1,000 500 .0008 .91 .67 16 5. p++(l02l) 300 A 300 1,500 .0007 .93 .74 8 6. ErAs 200 A 50 10,000 .0008 .93 .71 2 7. W 50 A 11 .0010 .91 .67 3 8. Au 50 A 5 .0006 .94 .77 2 9. Au 50 A 5 .0002 .98 .91 2 F.~mples 2 through 9 involve lessening of pump circuit due to 21 elimin~tion of the upper, or p-doped, DBR, while final example 9 is directed to a 22 structure in which the lower DBR structure is elimin~ted as well. Examples 1 23 through 8 make use of a "convendonal" underside electrode - of an n-type contact 24 electric~lly co~n~cted to the underside of the lower n-doped DBR as used in present 25 SEL de.cigns. Di.~cussion is prim~rily in terms of laser emission at 0.85 llm. Such 26 di~cus.~ion is, of course, illustradve only - l~kely ~i~nific~nt commercial use is 27 expected to soon take the form of emission at longer wavelengths, e.g. at 1.3 llm and 2078~83 1.55 llm. As discussed, the inventive advance is particularly advantageous as practiced at such longer wavelengths.
In all but example 9, the bottom n-type contact, e.g. electrode 63 of FIG. 3, is as used in prior art SEL design.~. Example 9, in~te~(l, utilizes a second S electrode layer on the underside as well. Reported devices as operating at 0.98 llm have utilized upper or p-type Bragg mirrors based on GaAs/AlAs typically conductivity doped to a level of Sx 10 18 cm~3 and, accordingly, resulting in absol~livily, a-SOcm~2. Loss for such a standard p-type mirror is, accordingly, appn~ ately 0.0025 (0.25%). This value is comparable to needed levels for 10 acceptably low threshold current densities, and is :~tt~in~hle for SELs of 1-4 quantum well structure. Reduction in external differential quantum efficiency is expressed in terms of mirror reflectivity, R, and absorption loss, A, in accordance with the relationship:
r~ = (1-R)/(A+l-R) (eq. 6) 15 in which: 1l is laser quantum efficiency (disregarding external pump c~uill~) and other terms are as defined above. The simplified equation pl~senled is valid for R
values of ~ 0.9 and better as qualify the inventive structures.
Examples 2-9 of the Table set forth perform~nre charact~ri.~tics in which biasing on the top side of the SEL - on the p-type side of the SEL - depends upon 20 l~s~e,~e conductivity of an electrode layer, e.g. layer 58 of FIG. 3A, with current introduced via the associated thick conductive layer 61 - to result in near-uniform current distribudon over the p-doped spacer, e.g. layer 60.
The table is described with reference to example numbers: Example 1 -use is made of a "standard" p-type mirror as vertically pumped, in~urring a loss of 25 0.0025, which as increased by the constant n-type per-pass absorption of 0.0005 (the e~pe. ;...~.nt~lly delel-,-ined value ~csumed constant for all of examples 1-8) results in the in~ic~te~ total per-pass absorption, "A" of 0.0030, and in cavity quantum efficiency, ''ll'' of .77 and .40, respectively, for 99% and 99.8% mirror reflectances.
These values of reflectance, experimentally at 0.98 ~m, corresponding with 13 and 30 17 Bragg periods are appl~p,iate for SELs of about four ~luanlulll wells and single quantum well, respectively.
Examples 2-8 are based on use of an electrode layer on the p-type side of the laser while example 9 provides for such layer both on the p- and n-type side.
For cornp~ri~on pul~oses, all exarnples may be regarded as based on electrode layers 35 as sandwiched belween primary and secondary spacers. For some purposes, presence of the optional "secondary spacer", assuring correspondence of electrode 1S 2~78483 ._ layer position with a null on the standing wave, may be treated as the first DBRlayer. Most effective operation is assured by use of such a layer of best optical plope,lies and of lerr~ilive index to most effectively advance both objectives.
The Table is, for most purposes, self-explanatory. It relates 5 conductivity, and consequent permitted thickness of the electrode layer to properties de~l~.linative of SEL operation. It is seen by comparison of examples 2 through 5, all relating to electrode layers of doped semiconductor, that decreasing resistivity, in permitting decreasing thiçkne~s, lessens per-pass absorption to, in turn, result in improved cavity quantum efficiency (ratio of emitted to cavitating radiation).
10 Example 6 is based on an electrode layer of the semimetal, ErAs. Semimetal electrode layers as well as highly doped semiconductor electrode layers, offer arange of crystalline lattice parameters to permit epitaxially growth of subsequent layered m~teri~l - of secondary spacer as well as DBR layers. Otherwise suitableelectrode metals may have lattice parameters to permit epitaxial growth both of the 15 electrode layer and of subsequent material. For example, NiAl as well as CoAl are adequately matched to GaAs. This consideration, primarily concerning f~hriçation~
is of lesser consequence for longer wavelength devices (e.g. 1.3, l.5S ~lm) in which non-epitaxial growth techniques offer thinner, more effective DBR structures in any event.
20 Detailed Desi~n Considerations Extensive effort - both experimentation and study - have produced additional design considerations to signific~ntly advance prospective commercialimp!ement~tion. Results of the effort are reported in this section.
Electrode Layer Optimal design of the electrode represents a cross-over as between the biasinglpump function and device operation. Primary considerations are: electrical, dict~ting minim~l pump circuit recist~nce and, by itself, leading to thick electrode layers; and optical, particularly opdcal absorption, leading to thin electrode layers.
From the m~t~ri~l standpoint, three types of electrode layers are contemplated:
Metal layers are generally desired from the perf )rm~n~e standpoint.
High condu~;~ivilies permit thin layers, which as l,ropelly located at a null in the st~nding wave, interact with least photonic energy and permit low per-pass absorpdon (despite high levels of absorptivity). Gold has been found to be a good choice both for device perform~n~e and fabrication. Ch~o-miç~l stability permits35 considerable freedom in dme in fabricadon. Other consideradons may suggest ~It.orn~tives - higher meldng points of tun&sten and tit~nillm may be an advantage -limits diffusion to permit higher fabrication temperature at some expense to perform~n~e. Silver, with its lesser absorption for shorter wavelength emi~sion,leads to its consideration e.g. in the visible spectrum.
Tabular examples 7-9 are based on 50 A layer thickness. While 5 perform~nce suffers somewhat, use of somewhat greater thicknPss, to 100 A or higher, e.g. to - 400 A may be desirable, e.g. in permifflng less fastidious deposition and/or resulting in higher yield.
SemimPt~l layers, in ongoing work, show improving device perform~n~e - approaching that of metal. Reduction in performance penalty for 10 shorter wavelength emission may lead to its preference due to permitted epitaxial growth. Tabular example 6 is based on use of ErAs, which in layer thickne~s fourtimes that of the metal layers of examples 7-8, results in comparable operalillgchar~t~ri.~ti~s. For such thicknesses, increased absorption dist~nce for increased thic~nPss for semimetal relative to metal is offset by the decrease in abso~ ivily 15 res~llting from reduced number of carriers. Other semimet~l~ offer a range of lattice parameters to satisfy epitaxial growth for contemplated DBR m~teri~ls. See T.
Sands et al, "Stable and Epitaxial Metal/III-V Semiconductor Heterostructures", ~tPri~ Science Reports, vol. 5, no. 3 (Nov. 1990). ModifiratiQn in composition of semimetal electrodes permits adjustment in lattice parameter with little or no effect 20 on conductivity. As an example, sc~n~ium.has been added to ErAs to to more closely match its lattice parameter to that of GaAs to result in Sc,~ Er 1-~ As - e.g. at X = 0.32, m~tching is near-exact. From the electrical standpoint, available resistivities generally in the range of from 3-10-5 to 10-4 ohm-cm suggest layer thiclrnPsses of from 100 A to 400 A for contpmrl~tpd laser structures having 25 symmetrical active regions of applo~imately 30x30 ~lm, or less. Generally, carrier mobilities in s~pmimpt~l~ are greater than those in metals, leading to performance-equivalent thinner electrode layers than earlier expected.
Semiconduct- r materials may be doped sufficiently to serve as a third m~teri~l variant on the electrode layer. Examples 4 and 5 are based on GaAs, p-30 doped to levels of the order of 102-1021 carriers/cm3, e.g. as carbon-doped in accordance with the work of C. R. Abernathy - see, App. Phys. Let., vol. 55, no. 17, pp. 1750-2 (Oct. 23, 1989). Such doping levels have resulted in per-pass absorption again comparable to that of devices using metallic electrode layers (compare ex~mple 5 with eY~mple 8).

The Sll~stratP
For most purposes, substrates are of minim~l th~ npss still ~CSllring high yield of epitaxially grown layers, although some minim~l thi< ~neSs perhaps 250 ~m may be desired from a mech~nical standpoint. This latter, of course, depends on 5 device, IC, or wafer ~imçn.cions, presence or absence of other support, etc. Still other considerations give rise to use of a temporary substrate - e.g. to serve during f~rir~tinn/growth - to be removed/replaced with a view to device operation.
Removal of GaAs after epitaxial DBR growth, followed by replacement with non-m~tching m~teri~l, e.g. diamond or glass, is one example of many, in this instance 10 providing increased transpa~ncy, over the ~ 200-920 nm range, to permit eri~ission through the underside, usually the n-type side, of the laser structure. For wavelength emission within the range of = 920-1000 nm, GaAs generally m~nifçstc adequate transparency. For still longer wavelength - = 1000-1655 nm InP is usefully employed.
Aside from growth requirements, device size and other physical requirements are prime determin~ntc for substrate m~teri~l/design for usual devices providing for top surface emission.
Active Re.~ion Dim~n.sional considerations, primarily of the included active m~tPri~l 20 layer, has been considered (largely in description of FIG. 2). As there in~ic~ted the primary function of photon generation with least absorption loss is optimized by an active m~teri~l layer of a 1/4 wavelength in thiçknPss as measured in the activem~teri~ /4n), sandwiched between primary spacer layers of relatively little absorption. Total thil~nPcc with sandwiching layers is again an integral number of 25 1/2 wavelengths - in devices studied such active region is of one wavelength thi- ~nPss.
Composition choice of the active material is on bases well understood to the skilled artisan. A primary factor concerns emission wavelength. Categories of suitable m~tPri~l.c extensively studied at this time are based on modified GaAs for 30 longer wavelengths within the visible and near-visible spectra, and on modified InP
for longer wavelengths extending further into the infrared spectrum. Exemplary active m~teri~lc, suitably grown by epitaxy on substrates of such binary composilions, of interme~i~t~P ternaries, as well as on ~ltern~tive compositions are known from a variety of references, for example, from above cited U.S. Pat.
35 4,999,842. Such references report a number of active m~teri~ls operated either as buLk or as quantum wells. For convenience, represent~tive compositions are 2078~83 tabulated in terms of associated emission wavelength ("QW" following the active composition identifies m~teri~ls operated as quantum wells rather than buLk):

Active Material ~(nm) InGaAlP QW 630-650 InGaAlP660-670 InGaP QW655-700 InGaP 670 AlGaAs 700-800 GaAs QW800-860 GaAs 880 InGaAs QW880-1100 InGaAsP950-1400 InGaAs QW1400-1600 InGaAs 1655 The active region, generally defined by bridging mirror structures (generally DBRs), or by electrode layer/s as present, also includes the primary spacers. Requirements placed on such spacer m~teri~l are: from the performance standpoint, minim~l absorption for cavitating laser radiation; from the fabrication 20 standpoint, crystalline lattice parameters to permit epitaxial growth both of the spacer and of subsequent m~tP.ri~l (of the active m~teri~l layer and/or secondary spacer or DBR layer). From the dimensional standpoint, thicknpss is such as to result in the required 1/2 ~ or integral number thereof (as allowing for any penell~tion, e.g. in adj~ent DBR layers).
Primary spacers, since within the electrical pump circuit, are doped p- or n-type as part of the functioning pn junction laser. Structures considered have sometimes been based on graded doping/conductivity with conductivity increasing to reach a maximum where in contact with an electrode layer. Such graded composition reduces electrical resi~t~nce at the spacer-electrode interface.
Cavity Mirrors, likely DBRs, pelLaps as including conventional reflectors to result in "hybrid" mirrors, (see U.S. Pat. 4,991,179, issued 2/05/91) are well-known.. ~ltern~ting layers of AlAs and Al % Ga ~ As, resulting in index change of ~ 3.0 - 3.5, have been used for emission wavelengths below about 1 ~lm(as measured in vacuum). As noted, epitaxial growth techniques are appropliate.
35 For longer wavelength emission, e.g. at 1.3, 1.55 llm, reported m~teri~ls are of 2û78~83 substantially lesser index change so that overall advantage makes use of non-epitaxial growth (e.g. of magnetron sputtering deposition) importantly of ~ltern~ting layers of Si and SiO2 with index differences of = 3.5-1.5 - to permit the desired 99+% reflectivity for structures of perhaps 4 mirror periods.
Reflectivity of a DBR interface is proportional to the quantity n2 -n (eq. 7 n2 +n in which:
n 1 = refractive index of DBR layer of lesser index n 2 = refractive index of DBR layer of greater index Study leads to devices which yield cavity reflectivity of > 99.9% for 28 period DBRs of AlAs and 15% AlGaAs (Al.,s Ga 8s As) for emission at 0.85 llm.
Four period DBRs of Si and SiO2 yielded such reflectivity at 1.3 llm.
General Comment - Expected impact of the inventive tP~ching has resulted in e~lensive consideration/experimentation to result in a level of 15 development of considerable sophistication. In a desire to meet the patent laws and to sl1cce~fully survive subsequent court attack, every effort for full disclosure has been made. The very profundity of the teaching has complicated disclosure. Many aspects of the c!~imed invention at its present stage of development - in providing for coh~l--plated problems, in optimizing operation in terms of real conditions -20 represent complications to increase difficulty in description of the general advance.As an example, in-principle discussion of cavitation and related prope. lies of e.g.
is complic~t~d by secondary design considerations reslllting from penetration ofcavitating energy and multiple reflections within an electrode layer to result in an increase in optical length of the cavity. Experimentation and consideration have25 advanced to a level at which it is improper even to provide for cavity dimencions as so co..~-;led. I~ign.~ which may gain in operation from electrode cross-sections of deliberately graded conductivity, deny even this now-simplifying assumption. This consideration is further complicated by difference in behavior based on electrode design as affecting interfacial reflectivity, absorptivity, and thickne~ss.
For the most part, terminology definition and discussion have been in terms of device design appropAate to and likely to be adopted for textbook description. In terms of the above example, the acdve region is initially described as w~co~plicated by the electrode layer, while, in truth, correction must be made for its p.esence. The descAption has been in this general format - with initial ~ cuc~ion in 35 terms of the hypothetical structure as initially disregarding such sophictir~te~

2078~83 sometinles secondary, variations. Only in subsequent description has attention been paid to such m~tters In similar manner, claim language is to some extent n~ces.~rily in terms of such hypothetical structure. The person skilled in the art will, upon reading this description, be fully understanding of the teaching and will be S enabled to carry out the invenlive t~aching without undue e~pe~ i"~ent~tion as required by the patent laws.

Claims (23)

1. Apparatus comprising at least one electrically pumped vertical cavity laser consisting essentially of an active region containing a semiconductor active material layer for generating photons to result in laser emission, said active region comprising p-type and n-type conductivity portions and including a semiconductorjunction, said active region being sandwiched between Distributed Bragg Reflectors each comprising a plurality of layers of relatively low and high refractive index for the said laser emission, together with circuit means for electrically pumping said active region to lasing threshold, lasing entailing presence of a standing wave defining a cavity here described as bounded by the said Distributed Bragg Reflectors CHARACTERIZED in that said circuit means includes an electrode layer at least part of which is outside the said active region, whereby at least a major portion of one DBR is excluded from the circuit means, said electrode layer having a conductivity sufficient to result in total conduction of the said circuit greater than that of a circuit including the entire DBR as conductivity-doped to an average carrier concentration of 1019 carriers/cm3, and in that absorption for laser emission isreduced in part due to reduction in conductivity-doping of the excluded portion of the DBR to result in a numerical value of equal to or less than 0.25 for the product r.Ap in which r is circuit resistance in ohms of that portion of the circuit consisting of p-type material and the associated electrode layer and Ap is per-pass photon absorption for that circuit portion expressed as a fraction.

2. Apparatus of claim 1 in which the thickness of the said electrode layer is less than about 1/4 wavelength for the laser emission as measured within the electrode in the direction defined by the standing wave.

3. Apparatus of claim 2 in which the said active region consists essentially of the said active material layer together with sandwiching primary spacer layers.

4. Apparatus of claim 3 in which the said active material layer is approximately centered on a crest of the standing wave and the thickness of the said active material layer is less than 1/2 wavelength of the standing wave.

5. Apparatus of claim 4 in which the thickness of the said active material layer is of a maximum approximately 1/4 wavelength of the standing wave, in which the said electrode layer is positioned to correspond with a null in the standing wave.

6. Apparatus of claim 5 in which the said primary spacer layers doped with significant impurity resulting in the first such layer being of p-type conductivity and the second such layer being of n-type conductivity, in which the said excluded DBR portion is on the p-type side of the said active region and in which such excluded portion is of resistivity of a minimum of 10-1 ohm-cm.

7. Apparatus of claim 6 in which the active material layer is at least in part of substantially intrinsic conductivity.

8. Apparatus of claim 6 in which substantially the entirety of the said DBR is on the other side of the said electrode layer from the said active region so that substantially the entirety is excluded from the circuit means.

9. Apparatus of claim 8 in which the said electrode layer is metallic and of thickness of a maximum of approximately 400 .ANG..

10. Apparatus of claim 9 in which the said electrode layer comprises at least one material selected from the group consisting of gold, tungsten, molybdenum, copper, titanium, silver, NiAl and CoAl and the electrode layer is of thickness of a maximum of approximately 100 .ANG..

11. Apparatus of claim 10 in which the said electrode layer consists essentially of gold.

12. Apparatus of claim 8 in which the said electrode layer consists essentially of semimetal.

13. Apparatus of claim 12 in which the resistivity of at least a substantial portion of the said electrode layer is within the approximate range of from 3.10-5-10-4 ohm-cm and is of thickness within the approximate range of 100 .ANG.-400 .ANG..

14. Apparatus of claim 13 in which the said electrode layer is comprised of ErAs.

15. Apparatus of claim 14 in which the said electrode layer consists essentially of ScxEr1-xAs.

16. Apparatus of claim 8 in which the said electrode layer consists essentially of semiconductor which is doped with significant impurity to result in an electrical carrier density of at least about 1020/cm3.

17. Apparatus of claim 10, 12 or 16 in which the DRB on the p-type side is epitaxially grown.

18. Apparatus of claim 1 in which the said circuit means provides for pump current which is substantially parallel in direction with the standing wavewithin the said active region and in which laser emission is through the surface of the excluded DBR layer furthest removed from the active region.

19. Apparatus of claim 8 in which the said circuit means includes a second electrode layer intermediate the active region and the second DBR wherebythe second DBR is also excluded from the circuit means.

20. Apparatus of claim 19 in which the second DBR is of resistivity of a minimum of 10 -1 ohm-cm.

21. Apparatus of claim 1 in which the laser is supported on a substrate and in which laser emission is through the substrate.

22. Apparatus of claim 1 comprising an integrated circuit including a plurality of such lasers.

23. Apparatus of claim 22 in which the integrated circuit is an Opto Electronic Integrated Circuit including electronic elements in the circuit means.