Note: Descriptions are shown in the official language in which they were submitted.
1~76Z37
!
. ,. ~
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~.
.
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.
.
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.
4-
, 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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' '' '' "' " "' ' " . ' .' " ', " "''' , ' " ~ "; " ' ' '. ', ' " ' .', ' .. . . ' ' ' ' ~ , "
' ' :', ~' ', ' .' : ', ', ' ' ,, ''. ' ' ' , '.. ' '. ' ' '.'' ' . ' . . ''' '', '' ''' ' ,' '."
'' . ' . " " ' ' ' ' I ` ' ' .,, , . ' " ' ' . ~ '" ', " ' ' ', " ' '' . ' ' ' ', ' ' ' ~ " ' " ' ' '.".. ` .. . . " ' . , " ' ' ' ' ' `
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~
.
7~ . :
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- , :
, . ... : .. : : , . .. ~: ~
; .. , . : . , . , . ..... , . , : . :
. , . . ., ~ . - : , ., . :. ,, , . , :
.. . .. .. .. , , . . ,. ,,,.,: .. . . : .,
i.~LO'~623~
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
24
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 ' ' . .
.
~, ~ .
., . . .,, . - ~ . . -
~ 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 :
,,
, . -
.~ ~ .
~14~ ::
' ' ' ' :'
., . . : , ... . -, - , .- .. . ~ ,: :
,. , . :. . .. . .. . ... . ~ ~ .... ~
, . . , , . , . ,, , .. ...... . . . .... . , .: ~ ~ . ~ :.,
~ 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 ~ .
~,. ~ - , . .
.. . . . ..
37
., .
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
. ' ' ,, ' ;
. . .
';1, , ; '
., ;,, ~ .
,: , i
~ 8
,
