for the detection function at data rates up to 900Mbit/s, as
limited by the differential carrier lifetime, and with
a
fibre-to-
fibre amplification of around 10dB. (For the simultaneously
amplified signals, these bit error rates can be achieved at
multi-Gbit/s rates; we experimentally demonstrated 2 Gbit/s.)
The mode of operation of the amplifier device we report here
significantly increases the utility of laser amplifiers in photonic
transmission and switching systems.
We gratefully acknowledge
K.
Bergvall for the AR coating,
and
L.
Atternas for providing the external cavity laser.
M. GUSTAVSSON
A. DJUPSJOBACKA
Ericsson Telecom AB
S-126
25
Stockholm, Sweden
L. THYLEN
References
18th August
1989
1
THYLBN,
L.,
GRANESTRAND,
P., and
DJUPSJOBACKA,
A.:
‘Optical
amplification in switching networks’. Proc. 1st
OSA
topical
meeting on photonic switching, Incline Village, 1987, Paper PDP
8
2 O’MAHONY.
M.
J.:
‘Semiconductor laser optical amplifiers for
use
in
ruture
fiber
systems’,
J.
Lightwave yechnol.,
1988,
LT-6,
pp.
531-544
IKEDA,
M.,
OHGUCHI,
O.,
and YOSHINO, K.: ‘Monolithic LD optical
matrix switches’. Proc. 13th Europ. conf. on optical communica-
tion, 1987
GUSTAVSSON,
M.,
and THYLBN,
L.:
‘Switch matrix with semicon-
ductor laser amplifier gate switches: a performance analysis’. Proc.
2nd OSA topical meeting
on
photonic switching, Salt Lake City,
1989, pp. 18C182
‘Integrated-optic device for high-speed databuses’,
Electron. Lett.,
ALPING, A., BENTLAND,
B.,
and
ENG,
s.
T.:
‘100Mbit/s laser diode
terminal with optical gain for fibre-optic local area networks’,
ibid.,
GUSTAVSSON,
M.,
KARLSSON,
A.,
and
THYLBN,
L.:
‘A
travelling wave
semiconductor laser amplifier for simultaneous amplification and
detection’. Proc. 10th
OSA
topical meeting on integrated and
guided-wave optics, Houston, 1989, pp. 113-1
16
GUSTAVSSON,
M.,
THYLBN,
L.,
DJUPSJOBACKA,
A.,
and KARLSSON,
A.:
‘Travelling wave semiconductor laser amplifiers for simultaneous
amplification and detection: systems experiments’. Proc. 2nd OSA
topical meeting on photonic switching, Salt Lake City, 1989, pp.
159-161
THYLkN,
L., DJUPSIOBACKA, A.,
JANSON,
M.,
and dLDISSEN,
W.:
1985,21, pp. 491-493
1984.20, pp. 794-795
~~
9
GUSTAVSSON,
M.,
KARLSSON,
A.,
and THYLEN, L.: ‘Travelling wave
semiconductor laser amplifier detectors’, submitted
for
publication
10
OLSHANSKY, R., su,
c.
B.,
MANNING,
J., and
POWAZINIK,
w.: ‘Mea-
surement
of
radiative and nonradiative recombination
rates
in
InGaAsP and AlGaAs light sources’,
IEEE
J.
Quantum Electron.,
1984, QE-20, pp. 838-854
ROOM-TEM PERATUR E CONTINUOUS-WAVE
VERTICAL-CAVITY SINGLE-QUANTUM-WELL
M
ICROLASER DIODES
Indexing terms: Semiconductor lasers, Quantum optics, LEDs
Room-temperature continuous and pulsed lasing of vertical-
cavity, single-quantum-well, surface-emitting microlasers
is
achieved
at
-983nm. The active Ga,.,In,.,As single
quantum well
is
lOOA
thick. These microlasers have the
smallest gain medium volumes among
lasers
ever built. The
entire laser structure is grown
by
molecular beam epitaxy
and the microlasers are formed
by
chemically assisted ion-
beam etching. The microlasers are 3-50-pm across. The
minimum threshold currents are
1.1
mA (pulsed) and 1.5 mA
(CY.
Vertical-cavity, surface-emitting lasers’“ have many potential
advantages over conventional edge-emitting lasers owing to
their inherent two-dimensional nature and very short cavity
lengths. The previous results
of
the optically pumped lasers5
and electrically pumped microlasers6 implied very low thresh-
~
ELECTRONICS LETTERS 28th September
1989
Vol.
25
No.
20
1377
old current for a single-quantum-well (SQW) microlaser with
proper current injection into an active gain medium.
We made various sizes (3-100pm) of electrically driven ver-
tical cavity microlasers with
a
100A-thick GaInAs SQW gain
medium. The basic structure is
a
vertical
pin
junction where
electrical current is injected through the bottom and top
mirrors. The sample is grown on an Si-doped
n+
GaAs sub-
strate by molecular beam epitaxy. The bottom (output) mirror
has 23.5 pairs of AlAs/GaAs quarter-wave stack and is Si-
doped
3
x
10’8cm-3. The calculated reflectivity for the
bottom mirror is 99.87% at 980nm. The top mirror consists
of 15 pairs of AlAs/GaAs quarter-wave stack Be-doped
5
x
10’’ cm-3, phase matching superlattices of GaAs/AlAs,
30A of delta-doped (Be,
1
x
10’3cm-2) GaAs for p-type
ohmic contact, and a 15008, thick gold film. The calculated
reflectivity for the top mirror is 99.96% at 980 nm. The spacer
region consists of
a
SQW of
l00A
Ga,.,In,.,As active gain
region surrounded by
a
pair of graded Al,Ga,_,As
(x
=
0.2-
0.5)
and Al,.,Ga,.,As layers
so
that the combined optical
thickness is
a
full wave. The resultant microresonator should
have very high finesse
(>
1OOO) to balance the very small gain
from 100 A-thick Ga,.,In,.,As; otherwise, the lasing would
not be observed. Mesas of diameter 1, 1.5,
2,
3,
4
and 5pm
(Fig.
l),
and
5,
10,
25,
50,
100 and 200pm square are defined
by chemically assisted ion-beam etching. A
-
1500A-thick Ni
layer is patterned on top of the gold film for etch mask. The
mesas are etched about 4-6pm deep through the gold film.
Fig.
1
Very small portion
of
vertical cavity microlasers
of
diameter
1-5
pm
Fig.
2
shows
a
CW laser output characteristic for a 5pm
square microlaser with 1.5 mA threshold at room temperature.
The laser light is linearly polarised above threshold. The
983
nm output passes through the polished GaAs substrate
with only small loss. Room-temperature continuous lasing is
observed for
5,
10
and 25pm microresonators with 1.5, 3.0
and 13.0mA thresholds with no heat sinking applied. Contin-
uous lasing is observed with up to 1.5-2.0 times the threshold
current. For larger CW current, heating seems to red-shift the
gain region out of the Fabry-Perot resonance and the micro-
laser stops lasing. Reduction of the current will again produce
f
lasing at the same power levels as before. If the current is
raised greatly above this point, however, the microlasers are
permanently damaged.
Using 90 ns pulses, the threshold currents are the same
(1.1 mA) for the microlasers of diameter 3-5pm, and 1.1, 2.3,
12 and
45
mA for the microlasers
5,
10, 25 and 50pm square.
The measured single-facet differential quantum efficiency is
about
8-10%.
The threshold currents are marginally lower
than in the previous three-quantum-well microlasers,6 not
three times better. For the larger devices
(>
5
pm), the thresh-
old current density stays almost constant at
1.8
kA/cm’. This
value is still much larger than the theoretical’ minimum
threshold current density
(60
A/cmZ). For the smaller devices
(<
5
pm), the threshold current density increases with the
reduction in size, which we believe is due mainly to surface
recombination of carriers.
LO
I
1
975
)f
?
I
20
1
990
“m
f
0
1
2
3
pig
input
,
mA
Fig.
2
Output against current for 5pm-square microlaser operating
CW
at room temperature without heat-sinking
Inset shows spectrum at 1.8 mA. Spectral line width is 3.5
A,
the
resolution limit of spectrometer
Since the maximum possible gain per pass from a
SQW
is
of the order of any small
loss
of this magnitude can be
critical for the lasing action. Possible sources for the
loss
are
band-tail and/or free carrier absorption by highly doped
impurities, a slight mismatch between the gain maximum and
the peak reflectivity of the mirrors, and scattering. The electri-
cal resistance through the heterostructure mirrors is rather
high (between 565000
R
depending on the
sizes).
This high
resistance and high current density are the main problems
preventing continuous operation at high driving currents.
Reduction of the resistance and better heat sinking should
make more efficient room-temperature continuous micro-
lasers.
In summary, we have demonstrated single-quantum-well
(100
&thick GaInAs), surfaceemitting microlasers with
minimum thresholds of
1.1
mA (pulsed) and 1.5 mA
(CW)
at
room temperature. These microlasers have the smallest gain
medium volumes of any lasers ever built. Even though the
microlasers are not optimised in the present form, the thresh-
old current is low enough to consider driving arrays of micro-
lasers simultaneously. We can expect another order-of-
magnitude reduction in the threshold current (less than
100pA with a
2-3V
source) with proper improvement of
design and processes involved. Arrays of very
low
threshold
microlasers will be critical components for optical computing,
chip-to-chip communication, and photonic switching.
Y.
H. LEE
J.
L.
JEWELL
A. SCHERER*
S.
L. McCALLt
J.
P.
HARBISON*
L. T. FLOREZ*
A TLT Bell Laboratories
Holmdel, NJ 07733, USA
21st July 1989
*Bellcore
Red Bank, NJ 07701, USA
t
AT&T Bell Laboratories
Murray Hill, NJ 07974, USA
References
1
SAKAGUCHI,
T.,
KOYAMA, F.,
and
IGA,
K.:
‘Vertical cavity surface-
emitting laser with an AlGaAs/AIAs Bragg reflector’,
Electron.
Lett.,
1988,24, pp. 928-929
2
IBARAKI, A., KAWASHIMA, K., NRUSAWA, K., ISHIKAWA,
T.,
YAMA-
GUCHI,
T.,
and
NIINA,
T.
:
‘Buried heterostructure GaAs/GaAIAs
distributed Bragg reflector surface emitting laser with very low
threshold (5.2 mA) under room temperature CW conditions’,
Jpn.
BOTEZ,
D.,
ZINKIEWICZ, L.
M.,
ROW,
T.
I.,
MAWST,
L.
L.,
and
PETER-
SON, G.:
‘Low-threshold-current-density vertical-cavity surface
emitting AlGaAs/GaAs diode lasers’. Technical Digest of Con-
ference on Laser and Electro-Optics, Paper FC2, Baltimore, 1989,
p. 380
4
FISHER,
R.
J.,
TAI,
K., HUANG, K. F., DEPPE, D.,
and
CHO,
A.
Y.:
‘Extremely low current threshold in vertical cavity surface emitting
laser diodes by use
of
hybrid reflectors’, to appear in
Appl. Phys.
Lett.
s.
L.,
and
CHO,
A.
Y.:
‘Vertical cavity single quantum well laser’.
Technical digest, CLEO ’89 (Optical Society
of
America, Balti-
more, Maryland), Paper PD 14-1; also to appear in
Appl. Phys.
Lett.,
31st
July 1989
6
JEWELL,
J.
L.,
~CHERER,
A.,
MCCALL,
s.
L., LEE,
Y.
H.,
WALKER,
s.,
HARBISON,
J.
P.,
and
F~REZ,
L.
T.:
‘Low-threshold electrically
pumped vertical-cavity surface-emitting microlasers’,
Electron.
Lett.,
1989, 25, pp. 1123-1 124
7
YARIV, A.:
‘Scaling laws and minimum threshold currents for
J.
Appl. Phys.,
1989,28, pp. L667-L668
3
5
JEWELL,
J.
L.,
HUANG, K.
F.,
TAI, K., LEE, Y. H., FISCHER,
R. I.,
MCCALL,
quantum-confined -semiconductor laser’,
Appl. Phys. Lett.,
1988,
53,
pp. 1033-1035
FABRICATION AND GAIN MEASUREMENTS
FOR BURIED FACET OPTICAL AMPLIFIER
Indexing terms: Optoelectronics, Integrated optics, Optical
communications
The paper reports the fabrication and gain measurements of
buried facet optical amplifiers. Chip gain of 25 dB, gain ripple
of <ldB and gain difference of <ldB for
TE
and TM-
polarised light are observed. The gain is found to decrease
rapidly with increasing temperature. This behaviour is
explained using a model calculation of the radiative and non-
radiative recombination rates in the active region
of
the
amplifier.
Optical amplifiers are currently of interest for applications in
optical communication systems.’.’
Two
important criteria
for
the performance of the optical amplifiers are (i) absence of any
gain ripple, i.e. the modulation of the optical gain at residual
cavity mode wavelengths be negligible, and (ii) the optical gain
must be independent of the polarisation of the input light.
This letter reports the fabrication and optical gain measure-
ments of buried facet
amplifier^^,^
which exhibit very low gain
ripple (<ldB) and
IOW
polarisation dependence
of
gain
(<
1
dB).
The buried facet configuration (Fig. 1) is useful for two
reasons,
(a)
it provides a polarisation-independent reduction
in reflectivity (by a factor of 56100) over a cleaved facet, and
(b)
it allows one to achieve effective reflectivities of
or
less reproducibly simply by putting a conventional anti-
reflection coating
(-
lo-’)
on the facet.4
The fabrication of the device involves the following steps.
The epitaxial layers consisting of the n-InP buffer layer,
undoped GaInAsP
(1
-
1.55 pm) active layer, p-GaInAsP
(2
-
1.3pm) antimeltback layer, p-InP cladding layer and
p-GaInAsP
(2.
-
1.3
pm) contact layer are grown on a (100)-
oriented n-InP substrate by the liquid-phase epitaxy growth
technique. Mesas are then etched on the wafer along
(110)
electrode
~
,
510,
p
-
GaInAsP
ant
I
melt back ant
I
melt back
a
bm
Fig.
1
Schematic diagram
of
buriedfacet amplifer structure
1378
ELECTRONICS LETTERS 28th September
1989
Vol.
25 NO. 20