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ment of reflectivity IS one of the most important factors for
reducing the threshold current for SE lasers [ 1 5 ] . The actual
Fig. 3 Nearfield pattern ofGalnAsP/lnP S E laser
Front half right arc shape IS reflection from holed edge for InP
substrate
reflectivity of this SE laser device can be estimated to be
-97.2%
from the measurement of the gain coefficient
obtained from stripe-contact lasers at 77 K. Further reduction
of the threshold current can be expected by improving the
reflectivity.
Conclusion: This is the first demonstration of a GaInAsP/InP
SE laser grown by CBE. The laser material was grown with a
relatively high growth almost comparable to MOCVD. This
high growth rate is essential for realising SE lasers using
DBRs having a total layer thickness of > 10pm. Furthermore,
good controllability of cavity layer thickness is crucial for SE
lasers to obtain gain matching t o the resonance wavelength.
CBE may fulfill these requirements. It is expected that high
performance SE lasers with DBR and/or quantum wells can
be realised by the CBE technique, and these trials are now
under way.
Acknowledgments: The authors thank Y. Suematsu, President
of Tokyo Institute of Technology, for encouragement of this
work. This work was supported by the Scientific Research
Grant-In-Aid # 6 1 0 6 5 0 0 2 from the Japanese Ministry of Education, Science and Culture.
ZIst January 1992
T. Uchida, N. Yokouchi, T. Miyamoto, Y. Inaba, F. Koyama and K.
Iga (Precision and Intelligence Loboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan)
References
1
KOYAMA, F., KONISHITA, S.,
and
IGA, K . :
‘Room-temperature contin-
uous wave lasing characteristics of a GaAs vertical cavity surfaceemitting laser’, Appl. Phys. Lett., 1989, 55, pp. 221-222
2
3
4
5
6
7
IMAJO, Y., KASUKAWA, A., KASHIWA, S., and OKAMOTO, H.: ‘GaInAsP/
InP semiconductor multilayer reflector grown by metalorganic
chemical vapor deposition and its application to surface emitting
laser diode’, Jpn. J . Appl. Phys., 1990, 29, pp. LI 13O-Ll132
WADA, H., BABIC, D. I., CRAWFORD, D. L., DUDLEY, J. I., BOWERS, J . E,,
Hu, E. L., and MERZ, J. L.: ‘High temperature pulsed operation of
InGaAsP/InP surface emitting lasers’. Proc. 49th Annual Device
Research Conf., 1991, IIIA-8
OSHIKIRI, M., KOYAMA, F., and IGA, K . : ‘Flat surface circular buried
heterostructure surfaceemitting laser with highly reflective SijSiO,
mirrors’, Electron. Lett., 1991, 22, pp. 2038-2039
TSANG, W. T.: ‘From chemical vapor epitaxy to chemical beam
epitaxy’, J . Cryst. Growth, 1989,%, pp. 121-131
UCHIDA, T., UCHIDA, T. K., KOYAMA, F., and IGA, K . : ‘Control Of
GalnAsP/InP layer thickness for surface emitting lasers by chemical beam epitaxy’, to be published in Trans. IEICE Jpn. (in
Japanese)
HEINECKE, H., BAUR. E., EMEIS, N., and SCHIER, M.: ‘Growth of
extremely uniform InP/GaInAs/GaInAsP heterostructures by
MOMBE (CBE) for device integration’. Proc. 3rd Int. Conf.
Chemical Beam Epitaxy and Related Growth Techniques, 1991,
D3
ELECTRONICS LETTERS 12th March 1992 Vol. 28 No. 6
._
~~~
.
~
~~~~~~~
and IGA, K . :
‘GaInAs/InP MQW and DBR growth for surface emitting lasers’,
J . Cryst. Growth, 1991,111, pp. 1062-1065
9 CHOA, F. S., TAI, K., TSANG, W. T., and CHU, S. N. G.: ‘High reflectivity
1.55pm InPflnCaAsP Bragg mirror grown by chemial beam
epitaxy’, Appl. Phys. Lett., 1991, 59, pp. 2820-2822
10 TSANG, w. T., wu, M. C., YANC, L., CHEN, Y. K., and SERGEWT, A. M.:
‘Strained-layer 1.5 pm wavelength InGaAs/InP multiple quantum
well lasers grown by chemical beam epitaxy’, Appl. Phys. Lett.,
1990,26, pp. 2035-2036
11 TSANG, w. T., CHOA, F. s., wu, M. c . , CHEN, Y. K., SERGENT, A. M.,and
SCIORTINO, P. F.. JUN.: ’Very low threshold single quantum well
graded-index separate confinement heterostructure InGaAs/
InGaAsP lasers grown by chemical beam epitaxy’, Appl. Phys.
Lett., 1991,58, pp. 2610-2612
12 SUGIURA, H., NffiUCHI, Y., IGA, R., YAMAUA, T., and YASAKA, H . :
‘InGaAs/InP multiquantum well lasers grown by metalorganic
molecular beam epitaxy (MOMBE)’,Jpn. J . Appl. Phys., 1991, 30,
pp. L286-L288
13 UCHIVA, T., UCHIDA, T . K., YOKOUCHI, N., MIYAMOTO, T., KOYAMA, F.,
and ICA, K . : ‘Beryllium doping to GaInAs/InP quantum wells by
chemical beam epitaxy (CBE)’, Jpn. J . Appl. Phys., 1991, 30, pp.
LI 224-Ll226
14 UCHIDA, T. K., UCHIUA, T., MISE,K., KOYAMA, F., and IGA, K.: ‘Highly
berylhum-doped and lattice-matched GaInAsP/InP growth by
chemical beam epitaxy (CBE)’, Jpn. J. Appl. Phys., 1990, 29, pp.
562-563
15 OSHIKIRI, M., KAWASAKI, H., KOYAMA, F., and IGA, K . : ‘Reflectivity
dependence of threshold current in GaInAsP/InP surface emitting
laser’, IEEE Photonics Techno/.Lett., 1989, 1, pp. 11-13
8
UCHIDA, T. K., UCHIDA, T., MISE, K., KOYAMA, F.,
THEORETICAL THRESHOLD LOWERING
OF COMPRESSIVELY STRAINED
InGaAs/lnGa AsP A N D Ga InAsP/Galn ASP
QUANTUM-WELL LASERS
J. Barrau, B. Brousseau, B. Calvas, J. Y. Emery,
R.J. Simes, C. Starck and L. Goldstein
Indexing terms. Semiconductor lasers. Lasers
The first theoretical evaluation is reported of the effect of
biaxial compression on the quantum-well Ga,In, -.As/
Gao 2olnAso 45p
and
Gao.2oIno.8oAs,P, -J
Ga, ,,In,,,As,.,,P,,,
laser threshold current density. Reference IS made to known experimental results, and a comparison carried out of the potentialities of the two types of
laser.
It has been recently demonstrated that strained Ga,Inl _,As/
GaInAsP/InP and Ga,.,,InAs,P, _y/GaInAsP/InP quantum
well lasers at 1.5 pm have improved characteristics compared
to unstrained lasers, including reduced threshold current
density and high quantum efficiency [l-31. These improvements were predicted because built in axial strain enhances
the heavy hole (HHI-light hole (LH) splitting resulting in a
reduced effective H H mass in the QW plane. However, t o our
knowledge, no quantitative evaluation has been reported.
We have calculated the in-plane dispersion of the valence
sub-bands in GaJn, _,As (x < 0.47) and GaJn, ~ x A s y P , ~ ~ y
( x = 0.2, y > 0.45) compressed wells, embedded in lattice
matched quaternary wells at 1.18 pm (x = 0.2). An example of
the results for each type of well is displayed in Figs. 1 and 2.
The effective mass at the centre of the Brillouin zone is found
to be mhh= 0.1m, in Figs. 1 and 2.
To compare the potential of the different combinations of
well width and strain level that can be adopted for emission at
1.55pm, we define an ‘effective density of states’ poff(~F)
which
represents all the valence sub-hands by a single parabolic
band. With a 2-D isotropic carrier gas, the number of carriers
n is related to the pseudo-Fermi energy by
0
551
where f[eXk) is the Fermi-Dirac distribution, Eik) is the
dispersion relation of the sub-band i; the origin of energies is
We have calculated p.,, for the valence band and report its
value at the transparency threshold (Ecp E; = 0) as a function
of the compressive strain intensity (Fig. 3). NB: the strain level
+
0 - 0
05
0.02
0.04
0.06
K (=/a)
0.08
m
Fig. 1 In-plane dispersion of highest valence sub-bands in
Gao.2,1no.,2As/GalnAsP( 1 , l S p n ) quantum well of width L, = 30A
Correspondingstrain is 128%
Zero of energy coincides with top of valence band of strained bulk
Ga0.28InO.72~
w
E
.
-150
10
15
20
2 5
strain,%
Isr+rsl
Fig. 3 Valence band ‘eflectiw density of states’ ( I O L z c m ~ f m V ~at’ ) ,
threshold of transparency, against strain in GalnAs and GalnAsP wells
Threshold cumnt density (A/cm2)at infinite cavity length, against
strain in InxGal+& well, measured by Thijs et al. [Z])
GaInAs well
GaInAsPwell
0 In,Ga,_,As well [21
determines the composition of the quantum-well layer and the
required well width for laser emission at 1.55 pm. The general
shape of the representative curve resembles the strain dependence of the laser threshold current density measured by Thijs
et al. [Z] on GaJn,-,As/GaInAsP
quantum-well lasers.
These authors point out an unexplained increase of the
threshold at large strain, an effect which is found here theoretically. As the Q W strain is increased from 0%, the hole
effective density of states is reduced as the heavy-hole-like
valence band is separated from the light-hole-like band.
However, there is a minimum after which the effective density
increases as a result of interactions between the lowest energy
valence band and the unconfined states.
The electron binding energy is another useful parameter to
examine to compare the potentialities of the two kinds of well.
We give the result of calculations in Fig. 4 as a function of the
strain intensity. The binding energy is higher for the quaternary Q W case; a more effective electron confinement can thus
be maintained at higher temperature and under higher injection level. Consequently bigher gain and output power, as well
as higher temperature operation, are expected.
I
1
0.02
0.04
0.06
K (Na)
Fig. 2 In-plane
0.08
1(72/21
dispersion of
highest uolence sub-bands in
Gao.,olno.80Aso.82Po.,$GalnAsP
( l . l d p n ) quantum well of width
L, = 60‘4
Corresponding strain is 1.20%
Zero of energy coincides with top of valence band of bulk
~ ~ . 2 0 ~ ~ 0 . 8 0 ~ 0 . 8 2 ~ 0 . , ,
taken at the bottom of the first sub-band. For a single parabolic sub-band, eqn. 1 reduces to
n = pk, T In (1 + drPD3
(2)
where p = m*/& is the density of states of the sub-band. The
‘effectivedensity of states’ is defined as a function ofEp
$
sol
(3)
I
*
I1
a
12
14
I
0
0
13
stratn %
15
16
17
Fa.4 Binding energy of electrons in QW of 1 . 5 5 laser,
~
against
strain intensity
rm
GaInAswell
(4)
552
II 1
+ GaInAsP well
ELECTRONICS LETERS
I
12th March 1992 Vol. 28 No. 6
1
1-
Table 1 presents different possible combinations of well
width/strain for lasers at 1.55 pm. The well width is larger for
quaternary wells than for the ternary wells. This may represent a technological advantage because less dispersion can
be expected [3].
Table 1 POSSIBLE COMBINATIONS
WELL-WIDTH/STRAIN FOR
LASER EMISSION AT 1.55 pm
of identifying the interference fringe order from the output
pattern of an optical fibre interferometer using a broad-hand
optical source. As a result, the unambiguous range of the
output signal is no longer limited to within half a fringe and
absolute phase measurement over a large operating range can
be achieved.
As shown by the upper trace in Fig. I, the output pattern of
an optical fibre white-light interferometer has a distinctive
Well width
Strain
Ternary
1.0
1.2
1.4
3.3
3.0
Quaternary (x
2.1
= 0.2)
10.0
6.0
3.9
In summary, we have presented the first evaluation of the
favourable effect of compressive biaxial strain o n
GaJn, -,As/GaInAsP and Ga,.,,InAs,P, _,/GaInAsP QW
laser threshold via the valence band structure modification.
The theoretical prediction appears to be in good agreement
with measurements of GaJn, _,As/GaInAsP lasers. We have
briefly compared the potentialities of the two solutions and
note several advantages of the quaternary well lasers.
20th January 1992
J. Barrau, B. Brousseau and B. Calvas (Luboratoire de Physique des
Solides, C N R S , INSA, Avenue de Rangueil, 31077 Toulouse Cedex,
France)
J. Y. Emery, R. J. S i e s , C. Starck and L. Goldstein (Alcatel-Alsthom
Recherche, Route de Nozay, 91460 Marcoussis, France)
References
1 n”x,
L. F., THIJS, P. J. A.,DE WAARD,
P. J., BINSMA,
I. I. M.,and
VAN WNGEN,T.: ‘Dependence of polarization, gain, linewidth
enhancement factor, and K-factor on sign of the strain of InGaAs/
InP strained-layer multiquantum well lasers’, Appl. Phys. Lett.,
1991,56, (24), p. 2738
2 ~ J SP. I.
, A., BINSMA,
I. I. M., m”t,
L. F., and VAN WNGEN, T.:
‘Improved performance 1.5 pm wavelength tensile and compres-
sively strained InGaAsfinGaAsP quantum-well laser’. European
Conf. on Optical Communication, Paris, France, September 1991,
pp. 31-38
3 STARCK,
c., EMERY, J. Y., siR. J., MATABO,
M., GOLDSTEIN,
L., and
J.: ‘Strained layer 1 . 5 laser
~
with GalnAsP quantumBARRAU,
wells grown by gas source molecular beam epitaxy‘. ICCBE 3
Conf., Oxford, 1st-5th September 1991
g L---A-----280
80
480
800
680
~82811j
pixel number
Fig. 1 Output signal of experimental electronically-scanned W L I
system and illustration of centralfringe identification procedures
‘near-Gaussian’ visibility profile due to the low coherence property of the broadband source that IS used in the interferometer [2] In theory, there should be a bright (or dark, depending
on the configuration of the interferometer) fringe which has
the maximum amplitude and locates at the centre of the visibility profile where the total optical path difference (OPD)
equals zero This fringe is therefore termed the ‘central fnnge’.
The large-dynamic-range operation of optical fibre WLI
depends on the correct identification of the central fringe.
However, because the visibility profile of the fringe pattern is
very flat near the centre, it is very difficult to distinguish the
central fringe from its adjacent fringes when there is a certain
amount of noise in the output signal A misidentification will
cause a large phase measurement error of more than 2nrad
The upper curve in Fig 2 illustrates the minimum signal-tonoise (S/N) ratio required to identify directly the central fringe
through its maximum visibility (a Gaussian visibility profile IS
assumed) The shadowed bands in the Figure represent the
coherence-length ranges of the typical broadband optical
sources currently available.
80
m
7J
0 60
I
:
al
U1
FRINGE ORDER IDENTIFICATION IN
OPTICAL FIBRE WHITE-LIGHT
INTERFEROMETRY USING CENTROID
ALGORITHM M E T H O D
S. Chen, A. W. Palmer, K. T. V. Grattan and
B. T. Meggitt
?
s
-
40
c
0
m
G 20
0
0
20
40
60
80
100
120
coherence length number o f fringes
Indexing terms: Opticalfibres, Digital signal processing, Algorithms, Interferometers, Optical sensors
A special digital signal processing technique based on a centroid algorithm is presented. It is used to identify the central
fringe from the output fringe pattern produced by an optical
fibre white-light interferometer system to achieve a largedynamic-range and absolute phase measurement. A computer simulation and subsequent experimental verification
have shown that this technqiue is capable of sustaining a
high operating noise level.
Introduction; Optical fibre white-light interferometry (WLI)
has been attracting increasing attention recently in the
research area of optical fihre sensors [I]. It has the potential
ELECTRONICS L E T E R S
12th March 7992 Vol. 28 No. 6
EzEl
Fig. 2 Minimum signal-to-noise ratios required for central fringe identication using maximum visibility or centroid algorithm method for optical
sources with diferent coherence lengths
__ max amp identitication
Q centroid identification
Besides, the S/N ratio of the optical fibre WLI system is
relatively low (40-60dB) because such a system normally
operates at a low optical power and has a high level of noise
at a frequency near the interference signal due to the variation
in the alignment or vibration dunng the scan of the processing
interferometer In particular, the S/N ratio of a system using
the most recently developed ‘electronically-scanned‘ WLI [3]
is even lower because the interference fringes are spatial and
553
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