CA1076237A - Lasers and photo-detectors - Google Patents

Abstract

ABSTRACT
A semi-conductor light source comprising a single piece of semi-conductor material having formed therein a semi-conductor laser and a semi-conductor optical detector. The semi-conductor laser having a first optical output port, a second optical output port, and a resonant cavity arranged so that in use light is emitted simultaneously through the first and second optical output ports. The semi-conductor laser also includes a pair of reflecting surfaces at right angles to one another which couple an optical output from said first optical output port into said detector.

Description

1~76Z37 !

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The present inve~tion relates to a laser-detector combination7 i.e. an integral unit comprising a laser ~nd a detector arranged to monitor the laser ou~put.

In optical communication s~stems the i~eal light source is a small compact unit producing coherent light-~as an output, A light source which meets these requirements is the GaAs laser. However it is important that the light output from s~ch a laser into an optica~ communications ' ~ system should~e independent o~ the ageing of the laser and i varia~ions in,ambient temperature. For this Eeaison the output I of the laser must be monitored so that the drive curr~nt to the ` laser can be adjus~ed to maintain ~he light output constan~.
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It is has been proposed to use a GaAs diode to monitor the output;of~a GaAs~aser,~and;~to mount the laser and diode in unitary package. Such A device suers from the difficulty that during the fabrication of the package assembly the diode and laser must be aligned so that par~ o .

the laser ou-tput l~S coùpled into the photo diode.
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The presen-t invention a~oids -the more serious alignment problems by forming a laser and a diode detector on the same piece of semi-conductor. This form of construction enables the coupling efficiency between laser and diode to be checked before packaging.

According to the present invention there is provided a semi-conductor light source comprising a single piece of semi-conductor material having a plurality of semi-conductor layers, each layer being chemically dis-tinct fxom adjacent layers, said semi-conductor material ha~ing two physically and electrically distinct regions each of whlch ~ has said layers, one of said regionsconstituting a semi-i conductor laser and the other constituting an optical detector, said semi-conductor laser having a first optical output port, a second optical output port, and a resonant cavity arranged so that in use light is emitted simultaneously through said first and second optical output ports, and means which couple an optical output from said first optical output port into said detector.

Preferably said laser is a GaAs laser and said diode is a GaAs diode.

Embodiments of the in~ention will now be described by way of example with reference to the accompanying drawings in which:-.
Figure 1 shows a section through a GaAs double hetrostructure laser.
~ 3 ; , . . :

I ~ ~ 7 62 3 7 ... .

Figure 2 shows a section along line A-A o~ Figure 1 or a stripe geometry GaAs laser~

Figure 3 shows a section through a ~uried mesa GaAs laser. ~ .

Figure 4.shows diagrammaticall~ the reg;on surrounding the active region of a double double hetros ruc~ure &aAs laser.

~igure S shows a sec~ion thr~ugh the active region of a distributed feedback Ga~s laser. I

Figure 6 shows a laser-detector according to a first :
embodiment of the invention. .

Figur~ 7 shows a plan view of the embodiment illustrated in Figure 6.

Figure 8 shows a section through a laser~detector ~ccording to a second embodiment o~ the invention. .

Figure 9 shows a plan of the laser~detector showm ;n Figure 8. :

, . i Figure 10 shows the use of GaAs bars as a mask, Figure 11: show~ a laser-detector according to a third embodiment of the invention.

, 1~7623'7 It should be realised that the drawings are not to scale and that in fact scales on the drawings have been distorted in places for the sake of claxityO
`' '" ' ' . ':' Before dis~ussing the invention wi h which the present ;i specificatioD is concerned~ a brief outline of a number o different types of Ga~s laser will be given~

Reerrîng to Figure 1 there is illustrated a typical double hetrostructuxe GaAs Laser, it consists basically of five layers o~ material~ The substrate layer 5 consists of GaAs and is approximately 100 microns thick. On this is located a layer 4 o~ GaAlAs approximately 1 micron thick. On top of layer 4 is located layer 3 which consists of GaAlAs ~pproximately 0.3 microns thick. This layer is the active region o~ the laser, i.e. it is the region in which ligh~
I is ge~erated.
., On top of layer 3 is located a layer 2 of GaAlAs approximately , 1 mic~on thick. And on top of layer 2 is loca~ed a layer 1 of GaAs approximately 1 micron thick. De~aîls of typical layer compositions and doping levels etc., are given in Table 1 1, .1, . ~ . ~s 1'~

~L~76~37 .
TAsLE 1 ~Y~ D0DonC ~s~e _Y~Thickness _ I!L~O __~OA
_--1 Ge 2xlO cm P1 micron Ga As .i, 2 (:e 8xlO " P1 microll loO 3GaO . 7 s 3 Si 4x10~7 " . 0.3 micron Al 05Ga 95As . Sn 4Xlol7l~ N1 micron Alo 3GaO 7As Si lx1018 " N100 micron Ga As .. "1 ___ ,.~ ___ ,.

¦ A laser of the form shown in Figure 1 ~ypicaLly has a wide beam divergence~of the order of 40 in a direction transverse to the active layerO This is of course due to diffraction 'l effects caused by the thinness of the active layer~ The ,, .
divergence in the plane of the active layer is much smaller typically of ~he order o 5. The overall width of the active region may be 100 microns. Such a laser,in use) e~hi~its filament formation wherein lasing action i8 confined :
to:long filam~nts within~the active region.~. There is ~o coherency between the light emitted by different ~ilaments.
For many purposes this is an unsatisfactory state of ~~airs.
In order to avoid it the width of the active region is :: 8 .
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7~i237 frequently reduced as shown in Figure 2. Proton or oxygen ion bombardment is employed to limit the extent o~ the area o~ the active region9 so that the ~eion 1 in which light emission can occur is in the form of a comparatively narrow stripe 6 surrounded on either side by a non-active region 7. Lasers having this structure are said to have a stripe geome~ry.

An alternative technique for limiting the widths o~ the active region is to etch away a portion of the region to ~orm a mesa-s~ructure. Such devices suf~er from ~he disadvantage that because o the large diference betwee~

the refractive indices of GaAs and air, a large number of modes are generated by the laser. This disadvan~age can be .
overcome by the use of a buried hetrost~uc~ure. (See GaAs -Ga~ xAlx As buried hetrostructure injection lasers, by To Tsuk~da J. Applo Phys. 45, (1974) P4899). Such a structure is shown in Figure 3 layers 3, 4 and 5 of a double hetros~ructure: laser are formed in a conven~ional manner ~nd then etched away to gi~e a mesa structure. After this has beein done l~yers 2 and 1 are deposited on top of the mesa st~ucture~so that the mesa s~ruct1~re is completely buried~

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I ~ ~ 7 6 2 3 7 ' ¦ In this geometry the active region is surrounded by GaAlAs 9 and the refractive index di~ference between this and the active region is comparatively small~ This means that oIlly a few modes can be supported by ~he activ~
- lay~r.
3, Another stnlcture ~ich is fre~uently used is the double double hetrostructure laser9 (see Reduction of threshold current density în GaAs - Al~ Gal_x As hetrostructure lasers by separa~e optical and carrier confinement ~y M.B. Panish et al Appl. Phys. Lett. 22 (1973) P590) a sectio~ through the active region of this is illustrated in Figure 4. The st~ucture is again similar ~o that employed in the double hetrostructure laser, with the exception that the acti~e layer 3 of the double hetrostructure layer is replaced by a composite layer comprising three separate layers 89 9 and 10~ The central layer 8 is in fact an active region of GaAs in which the l.ight i~ actually generatcd. This is surrou~ded on one slde by a region 9 o .
nGaAlAs ~nd on the o~her side b~ a region 10 of pGaAlAs~
The light produced by ~he laser is generated in region 8 only. However it is free to propagate through regions 9 and 10 so that ~he region in which light can propagate can b~

comparatively thick compared wi~h the double hetrostructure laser. Alternatively the minimum current ~ which laser action - 8- , :

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i can be sustained can be reduced in comparison with ~he double I hetrostructure laser.The optical confi~em~nt occurs at the boundary .
between region 9 and 4 and region 10 and 2.
.

A further type of laser which is so~etimes used is the distributed feedback laser (see GaAs - Gal x AlX - As double hetrostructure distributed feedback lasers by M. Nakamura et al Appln~ P~ys~ Lett. 25 ~1974), P487 D In this lassr a oorrugation 11 i~' formed on the lQyer compri.~ing the active region. This corrugation t~pically has a'depth o~ 800 to 900 angstroms and a period o~ between 1,100 and 3,300 angstroms. Its purpose ~s to act as a diffraction grating.
After the active layer 3 has been grown the corrugations are ~ormed by~

1. coating the active region with a photo resist,

2. exposing the photo res~st to a laser generated ~I .
holographic pattern,

3. removing the exposed photo resist, ~nd :,

4. ion beam machining or back sputtering to remove ; material from the active region where the .
I' ; .
,, photo resist has b,een exposed.
," . After this process has been completed and the remaining photo I resist removed layers 2 and 1 are deposited i~ the : I , tsual manner to compete the laser structure.
Conventional ~ s lasers have a Fabry-Perot cavity, ~hich is ~` tuned to about the 2000th in~erference order. Because the emis~ion spectrum of GaA~ s approximately 500 angs'crom~ wide, 1~'76237 ,1 ' .

this means that the output of a con~entional GaAs laser may include several wavelengths of radiation. By imposing the periodic structure on the active region7 all :, but one interference order in the ~abry-Per~t ~avity is o suppxessed, i~eO only a ~ingle wavelength outpu~ is obtained and only one longitudinal laser mode is permitted.
Furthermore the grating structure ex~ends across ~he entire - width of the active region and it imposes a single phase Il relationship on the wave front pro~agating in the Fabry-Perot ij cavity. This forces the laser to generate a single transverse ¦ mode, i.e. this s~ructure tends to suppress filament formation in the active region.
. . .
The distributed feedback structure ma~ be combined with the double double hetrostructure type of laser. In this case it Ls not the active region itself which has the periodic ~tr~lcture, but the surfaces of one of the regions immediately adJacent to the artive region, î.e. the surface of region 9 say o F;gure 4.

All the devices descri~ed above are known. HoweverJ they ha~e been described because the present invention may employ any of hemy -10- , , :' ."

., ~:: : , ., ... : , -~ 6 2 3 7 , ' ' ' .

.; . . .
.

., Referring now to Figures 6 and 7, ~here is shown a first embodiment of the invent;on~ It co~sists o a single piece of GaAs 127 havin~ the regions 1 to 5 previously .
`, described with reference to Figure 1. The &~As is split . ...
~ by a groove 13 so that there are two electrically distinct ! gr~ups of layers 1 to 4. One half o~ the piece o~ GaAs~ :
j 14, acts as a photo diode, and the other half 15, acts as ., a laser. The laser is optically coupled to ~he photo diode . ,.. :, :
by means of a pair of reflecting surfaces 16 and 17 at 90 to e~ch other.
The~aAs chip is mounted on a piece of type 2A diamond ,. :
~ 18 which is in turn mounted on a copper heat sink. The j , :
~ surface o the type 2A diamond i~ metalised with a layer 19 .. .

; o~ indium deposited on gold d~posite~ on chromium. This metalisation l~yer ha~ a gap i~ it i~ corresponde~ce with the groove 13 in the GaAs~:chip The bottom layer o~ ~he GaAs laser is metalised ,;
with a layer 20 of gold on platinium on tit~nium. T~e bot~om I o~ groove 13 is of course not metalised~ The top sur~ace of th~ GaAs chip is metalised with a layer 2} of germanium gold/

;loy. Contac~ is made to ~he GaAs chip by means of three 1~3i76Z37 electrodes. The ~irst electrode is the laser drive electrode 22, the second electrode is the detector bias electrode 23, and the third electxode is an earth electrode e reflecting surfaces 16 and 17 are formed as a 90 groove 25 in a metal block 26. This is clearly shown in Figure 7.
This form of reflector largely eliminat~s alignment problems.
This is because movement.of the block 26 in the plane o Figure 7 does not affect the optical al~gnment.

The laser portion of the GaAs chip may be made with a stripe geometry as illustrated in Figure 7 where the stripe is i~dicated by reference 27~ Light generated in the laser sectio~ of ~he GaAs ~hip 15 propagates în both directions 28 and 29 in the stripe27. The light output in direc~îon 29 m~y be coupled to a dielectric optical waveguide and used for carrying inormation. The light output in direction 28 is -;
re1ected twice by the metal block 26 and is incident on the photo diode portion o~ the GaAs .hip.

~2 1~7~37 From the above description it should be apparent tha~ a laser-detector formed rom a single GaAs chip can be made by fairly conventional techniques and the optical coupling i of the laser to the detector presents very little problem in the way of aLignmentO ..
' ~Although the com~ined structu~ is intended to opera~e. in : a manner wherein the photo diode output is.used to controL the laser outpuk, the detailed circuit~y whereby this is done does not form a part of the present in~ention. Qui~e conventional . I .
circuitry ma~ be used for this purpose, and so it will not be described.

: .In the embodiment~described above with reference to Figure 6 ~.
- and 7 th~ GaAs chip is mounted p-side down onto a metaLised .~.: type 2A diamond. The metalisatlon is sectioned so that independent electrical contact can be made to the laser and detector parts of the device. It is con~enient ~hat the 'I n-side contact be at earth potential~ hence the use of diamond . as khe intermediate heat sink because of its electrical isolation propertiesO

.1 ' ' . .
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., . . .,, . - ~ . . -~ 7 ~ Z 37, For other designs of laser such as buried mesa and double double hetrostruc~ure lasers where heat sinking would not be necessary because o~ low operating currents în~olved, the laser/~etector could, more con~eniently be bQnded .n-~ide down directly to a copper heat s~k. The groove or channel 13 can be formed ~y chemical etohing or RF
back sputtering using an appropriate mask. Alternatively, ., .
proton or oxygen ion bom~ardment isolation can be used to isolatP the two sections electrically. If this technique is ~ used of course thPre is no physical groove in the GaAs laser, :7 ;. .... ..buk.a..region;.of ele~trically insulating material is formed.
~' The stripe geomet~y employed in the laser may be formed by oxide insulation, proton bom~ardment isolationy localised zinc . j .
diffusio~, or oxygen ion implantation.

A second embodiment of the i~ention is shown with re~e~ence to Figures 8 and 9 of the drawings. ~n this embodiment the .
.~ laser and detector section are longitudinally coupled. In the embodimen~ o~ Figure 6 and 7 the gxoove 13 was ~ormed parallel to the direction of light propaga~ion in the laserO -~ I~ the em~odirnent o~ Figures 8 and 9 a groove 30 is ormed :
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,. , . :. . .. . .. . ... . ~ ~ .... ~
, . . , , . , . ,, , .. ...... . . . .... . , .: ~ ~ . ~ :., ~ 0 7 62 3~ .

per,pPndiculax to the direction of light propagation in the laser. It should thus be apparent that with this structure no coupling mirrors need be used. Apart ~rom the absence ' of mirrors and the use of perpendicular groove 30, the .j :
structure of the em~odiment in Figure 8 and 9 is substa~tially ~i. the same as that shown in Figures 6 and 7 and like components :`1 . -' are given like ree~ence numerals. With ~his ~tructure the ~ .
laser again produces two optical outputs 31 and 32. The ~ optical output 31 may again of course be co~pled into a '1 dielectric optical waveguide. Optical oukput 32 is fed ,! direckly into the detector section of the GaAs chip an~ of ' course with this structure there are no alignme~t problem~
.
since detector and laser are formed simNltaneously on-the same chip. However there is one problem in producing a structure ... .
~: shown in Figures 8 and 9 that is tha end face 33 needs to be ~lat and normal to the junction plane. For this reason : con~ntional chemical etching cannot be used to form the groove 30, However a technique that may be used was reported :
y Sue~atzu e~ al at the semi-conductor laser con~erence in ~ech," ~
A~lan~a U.S.A. in November 1974. In this ~ e the I
: edges of the g~oove are m~sked b~ two GaAs bars 34 and 35,~see Fig,103 ¦ !
~ 15- :~.. . .
1 ~ .

~,. ~ - , . .

.. . . . ..

., .

i , . :

.
. . and the groove is formed by RF back spattering i~
an argon atmospherer The GaAs masks have (110~
cleaves fo~med on the edges of ~he ma~ks so that the .j . , .
crystalographic struc~ure o~ the edge o~ ~he mask is idenkical with the crystalographic struc~ure of the end ace 36 of ~he &aAs laser. I this technique is used it `I . . . .
is ~elieved that ~he end ~ace 33 o~ the laser is l .
:! ' su~ficientl~ flat a~ normal to the junction plane to . ........................................................... ..
pe~mit lasing action.
: ; ' ' ,." ' . .... ~.. An.~lternative structure illustrated in Flgure 11 which ~voids the need ~or a~ accurately ~la~ and normal laser end ~face in ~he dividing section bet~een laser and detec~or : ~mploys a distri~uted feedback structure of the type described with reference to ~igure 5. ~he grating structure is indicated diagrammaticall~ a~ 37. ~n practice the la9er is pumped over a length greater than the length in which he di~raction gra~ing is formed so that the cavi~ becomes .
l.eaky. $he length o~ the grating relative to khe total pump length can be ealcula ed to giVe he required orward power into the system and the requi~d backward power to the . . l - ~16a~ ~ I

~ ~ 7 6~ 37 .

detector. The laser and detector sections of the device can be ;solated in this case either ~y a chemically etched channel? or the device can he isolated by an oxygen ion, or proton bombarded isolation region 380 Because of the passive GaAlAs layers surro~nding the active layer, ~he backward power from ~he laser section will be ef~iciently ~ guided into the detector section. Again with the device ! Fhùwn in Figure 11 in~egers corresponding to those shown I in Figures 8 and 9 are shown by like reerence numerals.
I Apart from the distributed feedba~k st~ucture the device of ~ Figure 11 is identical to that of Figure 8.

``1 ' , .
' As ar ~s straightforward double hetrostructuxe devices are concerned, the integration of a photo detector with a !
laser puts very little limitation on the doping level and thirknesses of the various epitaxial layers. The values g~ven in Ta~le 1 may be used. All that is necessary is that the photo deteckor space charge region ex~end across the active region and that the a~sorption coef~icient o the laser light in the unpumped material is high enough to get absorption o~ the :incoming light. The first criterion is !
easil~ satisfied as the space charge width at a 4 x 1~7 cm-3 , ~
;~ . doping is 0.3 microns at 14 volts. To minimise the . ~7 .' -~ ~-- .

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

~ 37 ., voltage dxop in the n-type GaAIAs, its doping level should be somewhat greater than that of the active layer.
The second criteria rnay also be easily met since it is known from localised "dark line" degradation studies (see;
for ~xample, CW. Degradation at 300 K.o~ Ga As double-he~erostructure jun~tion lasers. II Electronic Gain, by B.W. Hakki and T.L. Paoli, Journal of Applo Phys. 44 (1973) p 4113) that typical absorption coefficients of unpumped regions of double hetrostructure active layers are 100 to 200 , --1 cms . Hence incident radiation wilL be absorbed in a length of sevexal hundred microns which is a very convenient length . ~o employ.
.
,j . , In the longitudinall~ coupled structures, proton or oxygen ion bombardment ~echniques can be employed to limit the : extent of the area of the reverse biased detector element, and hence the influence of leakage currents on the detectability.
Edge leakage e~fects can be similarly eliminat~dO
. ' ' ,, ' ;

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Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A semiconductor light source comprising a single piece of semiconductor material having a plurality of semiconductor layers, each layer being chemically distinct from adjacent layers, said semiconductor material having two physically and electrically distinct regions each of which has said layers, one of said regions constituting a semiconductor laser and the other constituting an optical detector, said semiconductor laser having a first optical output port, a second optical output port, and a resonant cavity arranged so that in use light is emitted simultaneously through said first and second optical output ports, and means which couple an optical output from said first optical output port into said detector.

2. A semiconductor light source as claimed in claim 1, wherein said piece of semiconductor material comprises a substrate of N-type Ga As having located thereon a layer of N-type Ga Al As having located thereon a first layer of P-type Ga Al As having located thereon a second layer of P-type Ga Al As having located thereon a layer of P-type Ga As.

3. A semiconductor light source as claimed in claim 2, wherein said N-type Ga As contains Si as dopant, said N-type Ga Al As comprises Al0.3 Ga0.7 As doped with Sn, said first layer of P-type Ga Al As comprises Al0.05 Ga0.95 As doped with Si, said second layer of P-type Ga Al As comprises Al0.3 Ga0.7 As doped with Ge, and said layer of P-type Ga As contains Ge as a dopant.

4. A semiconductor light source as claimed in claim 1, wherein said optical detector and said laser have parallel optical paths transversely spaced from each other, and said means which couples comprises a pair of reflecting surfaces at substantially 90° to each other.

5. A semiconductor light source as claimed in claim 4, wherein said laser has a stripe geometry.

6. A semiconductor light source as claimed in claim 4, wherein said laser and said optical detector are electrically separated by a groove in said piece of semiconductor material, which groove extends substantially parallel to said optical paths.

7. A semiconductor light source as claimed in claim 4, wherein said laser and said optical detector are electrically separated by a region of electrically insulating material formed within said piece of semiconductor material, said insulating material extending substantially parallel to said optical paths.

8. A semiconductor light source as claimed in claim 1, wherein said optical detector and said laser have substantially colinear optical paths.

9. A semiconductor light source as claimed in claim 89 wherein said laser and said optical detector are electrically separated by a light transmitting groove in said piece of semiconductor material, which groove extends transverse to said optical paths, said groove constituting said means which couple.

10. A semiconductor light source as claimed in claim 8, wherein said laser is a distributed feedback laser having first and second P-type Ga Al As layers and a grating structure superimposed on part of the boundary between the first and second layers.

11. A semiconductor light source as claimed in claim 8, wherein said laser and said optical detector are electrically separated by a region of optically transparent electrically insulating material formed within said piece of semiconductor material, said insulating material extending transverse to said optical path and said optically transparent electrically insulating material constituting said means which couple.

12. A semiconductor light source as claimed in claim 9, wherein said substrate is mounted on a metallic heat sink without an intermediate layer of electrically insulating material.