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lation fairly well. We found that the first two peaks have
absorption coelficients of a = 1330cm-’ and 950cm-’,
respectively, which are relatively strong for p-type QWs. The
capacity of this system to absorb normally incident radiation
indicates its potential for future application as an infra-red
photodetector.
Acknowledgment: This work is being supported by the US
Olfice of Naval Research and Air Force Olfice of Scientific
Research.
11th March 1992
1.
Katz, Y. Zhang and W. 1. Wang (Department of Efectricaf Engineer-
ing, Columbia University, New York, N Y 10027, U S A )
References
LEVINE, B. F., MALIK, K. 1.. WALKER, I., CHOI, K. K., BETHEA, C. G.,
BLEINMAN, D . A., and VANDENBERY, I. M.: ‘Strong 8.2pm infrared
intersubband absorption in doped GaAs/AlAs quantum well
waveguides’,Appf. Phys. Lett., 1987.50, pp. 273-275
CHOI, K . K., DUTTA, M., NEWMAN, P. G., SAUNDERS, M. L., and IAFRATE,
G. I.: ‘IOpm infrared hot-electron transistors’, Appl. Phys. Lett.,
1990,57, pp. 1348-1350
LEVINE, B. F., GUNAPALA, s. o., KUO, J. M., PEI, s. s., and HUI, s.:
’Normal incidence hole intersubband absorption long wavelength
GaAs/AI,Ga, -.As quantum well infrared photodetectors’, Appl.
Phys. Lett., 1991.59, pp. 1864-1866
XI€, H., KATZ, I., and WANG, w. 1.: ‘Infrared absorption enhancement
in
light-hole
and
heavy-hole
inverted
Ga, _,In,As/Al,.,In,As quantum wells’, Appf. Phys. Lett., 1991,
59, pp. 360-3604
CHANG, Y. C., and JAMES, R. B.: ‘Saturation of intersubband transitions in p-type semiconductor quantum wells’, Phys. Rev. B,
1989.39, pp. 12672-12681
STOLZ, w., MAAN, 1. c . , ALTARELLI, M., TA€”,
L.. and p m , K . :
‘Absorption spectroscopy on Ga, *,Ino 53As/Alo.481no
52A~
multiquantum-well heterostructures’, Phys. Rev. B, 1987, 36, pp.
4310-4315
modulation of an InGaAsP EA modulator. It is also confirmed in experiments that in-line demultiplexing/switchingby
the modulator is effective in reducing amplified spontaneous
emission noise in optical amplifier systems.
A buffer layer loaded InGaAsP (Ag = 144pm) EA modulator [5] with semi-insulating InP buried-heterostructure was
used in the experiments. Small polarisation dependence of the
modulation characteristics of the EA modulator with bulkwaveguide structure is attractive for such in-line applications.
The device length was 210pm. At 1.54pm wavelength, 20dB
extinction ratio was obtained for an applied voltage of - 3 V
for T E mode and - 3.2 V for TM mode, respectively. In contrast with modulators using an optical interferometer, higher
extinction ratio without absorption saturation can be easily
obtained by increasing the bias voltage to the EA modulator,
because the attenuation characteristics are nonperiodic for an
applied voltage. By using these characteristics, we can easily
control the gate width by changing the amplitude of the
applied sinusoidal voltage and bias voltage to the EA modulator.
--
M’
J
a
MASSELINK, W. T., CHANG, Y. C., MORKOC, H., REYNOLDS, D . C.,
LITTON, c . w., BAIAI, K. K., and w, P. w.: ‘Shallow impurity levels
in AIGaAs/GaAs semiconductor quantum wells’, Solid-St. Efeciron., 1986, 29, pp. 205-214
c
Fig. 1 Gate functions obtained by InGaAsP E A modulator driven b y
1.25 G H z sinusoidal modulation with various bias and modulation voltages
10Gbitls OPTICAL DEMULTIPLEXING A N D
SWITCH IN G BY SI N U S0IDALLY DRIVEN
InGaAsP ELECTROABSORPTION
M O D U LATO RS
M. Suzuki, H. Tanaka and Y. Matsushima
Indexing terms
modulation
’
Optical switching, Demultiplexing, Optical
10Gbit/s optical demultiplexing and switching were performed by sinusoidally driven InGaAsP electroabsorption
modulators, for the first time. An optical gate with variable
gate width was obtained just with sinusoidal voltage. In-line
demultiplexingAwitching by the modulator was effective in
reducing the amplified spontaneousemission noise in optical
amplifier systems.
Optical time division multiplexing (OTDM) is expected to be
an elficient way of increasing the transmission capacity of fibre
links. Several schemes have been demonstrated for performing
the optical demultiplexing, e.g. use of LiNbO, modulators [l],
semiconductor laser amplifiers [2], four-wave mixing in fibres
[3], and electroabsorption (EA) modulators [4]. In electrically
controlled demultiplexing, the electronics only have to operate
at the data rate of the base channel. However, in ultrahighspeed and high density OTDM systems, a conventional
driving scheme using square driving signal introduces the significant complexity of ultrabroadband electronics. This Letter
demonstrates that variable gate functions for optical demultiplexing and switching can be obtained just with sinusoidal
934
Horizontal axis is 200ps/division
a V b = -5V,Vm=6Vpeaktopeak
b Vb = -2.1 V, V , = 5.1 V peak to peak
c V, = +0.2V, V, = 2.4Vpeak to peak
d V, = - 1.1V, V, = 5.8V peak to peak
Fig. 1 shows gate functions obtained by 1.25GHz sinusoidal modulation of the EA modulator with various bias voltages V, and modulation voltages V,. The driving conditions
were as follows: (a) V, = - 5 V, V, = 6 V peak to peak, (b)
V,= -2.1V, Vm=5.1Vpeaktopeak,(c)V,= +0.2V, V,=
2.4 V peak to peak, and (d) = - 1.1 V, V, = 5.8 V peak to
peak. As shown in Fig. 2, gate duration was widely changed
from 12.5 to 87.5%, by only changing the amplitude of V, and
V,. Comparing Fig. l a and c , and Fig. l b and d , two gate
durations are complements of each other. A simple optical
demultiplexer can be realised by using the narrow gate function as shown in Fig. la. Furthermore, optical switching can
also be obtained by using an optical 3 dB coupler and two EA
modulators having complementary gate durations.
Fig. 2 shows the experimental results of optical demultiplexing and switching for input optical signals with lOGbit/s repetition rate. A lOGbit/s RZ optical signal amplified by an
Er-doped fibre amplifier as shown in Fig. 2a was coupled into
the modulator with various gating widths. Fig. 2&e show
output optical signals from the modulator with various gate
durations, corresponding to Fig. la-d. A lOGbit/s input
signal was demultiplexed to 8 : n ( n = 1-7), and a complementary output which is necessary for switching operation was
also obtained. 4 : n ( n = 1-3) optical demultiplexing/switching
was also confirmed by 2.5 GHz sinusoidal modulation. This
quite simple driving scheme is attractive for ultrahigh-speed
optical demultiplexing/switching, because the only high-speed
electronics required is for the narrowband R F amplifier.
ELECTRONICS LETTERS 7th May 1992
Vol. 2 8 No. 10
r
t
’
.
Comparing the amplitude of input and output signals as
shown in Fig. 2, demultiplexed signals are apparently large.
This was reflected by the noise reduction effect in the off state
of the modulator. Because the EA modulator can absorb a
wide wavelength of input light for large reverse bias, the
amplified spontaneous emission (ASE) noise of the amplifier
was also absorbed in the off state of the modulator. Fig. 3
shows the spectra for the input signal and signal after the
grating with 25% gating duration obtained by the modulator.
The ASE noise for the output signal was reduced by 6dB over
b
a wide wavelength range from 1.525 to 1.575pm, as expected
from the 25% gating duration. The noise reduction effect of
the modulator will be effective in improving the signal-tonoise ratio.
In conclusion, we have proposed an application of a sinusoidally driven InGaAsP EA modulator to high-speed optical
demultiplexingJswitching.Experiments for 8 : n demultiplexingi
switching for a lOGbit/s signal were successfully demonstrated. The effectiveness of the method for ASE noise
reduction in amplifier systems was also c o n k e d in the temporal and spectral domains. The proposed optical
demultiplexing/switching is advantageous for ultrahigh-speed
OTDM systems, due to the simple driving scheme.
4
i
Acknowledgment: The authors thank N. Edagawa and S.
Yamamoto for useful discussions, and Y. Yoshida for preparing the fibre amplifiers. They also thank K. Ono, T. Yamamoto, and Y. Takahashi, K. Sakai and H. Wakabayashi for
their continued encouragement.
..
I
‘ A
10th March 1992
M. Suzuki, H. Tanaka and Y. Matsushima (KDD R&D Laboratories
2-1-15, Ohara, Kamifukuoka, Saitama 356, Japan)
References
L
C
KOROTKY, s. K., and VESELKA, I. I.: ‘Eficient switching in a 72-Gh/s
Ti : LiNbO, binary multiplexer/demultiplexer’.Conf. Opt. Fiber
Commun., 1990, Paper TuH2, San Francisco
HASEN, P. B., RAYBON, G., WIESENFELD, 1. M., BURRUS, C. A., LOGAN, R.
A., TANBUN-EK, T., and TEMKIN, H.: ‘Optical demultiplexing at
6Gb/s using a semiconductor laser amplifier as an optical gate’,
Photonics Technol. Lett., 1991.3, pp. 1018-1020
ANDREKSON, P. A., OLSSON, N . A., SIMPSON, 1. R., TANBUN-EK,
T.,
and “ER,
M.: ‘16Gbit/s all-optical demultiplexing
using four-wave mixing’, Electron. Lett., 1991,27, pp. 922-924
MOLLENAWR, L. F., LICHTMAN, E., H A R ~ Y ,G . T., NEUBELT, M. I., and
NYMAN, B. M.: ‘Demonstration of error-free soliton transmission
over more than 15,000km at 5Gbit/s, single-channel, and over
11,oOOkm at 10 Gbit/s in a two-cbannel WDM.. Conf. Opt. Fiber
Commun., 1992, PD10, San Jose
SUZUKI, M., TANAKA, H., and AKIBA, s.: ‘Effectof hole pile-up at
heterointerface on modulation voltage in GaInAsP electroabsorption modulator’, Electron. Lett., 1989,25, pp. 88-89
ROGAN, R. A.,
e
Fig. 2 8 n oplicul yariny in rrmporal domain /or IDGhir s rnpur FIYMI,
ohraind h) I 25 GHz sinuwidol modulurion ,$EA modulator
Horizuntal axis i s ?oOp, division
l0Ghit s input signal alter amplification by optical dmplifier
h r Output signdls with demultiplexingrate of 8 I. 8 3. 8 7. and
d 5. re,pectivcly
0
Q U A N T U M WELL LASER WITH SINGLE lnAs
MONOLAYER I N ACTIVE REGION
z
-reference
~~
1
\
525
I550
wavelength,pm
1 575
(I
c
0
c
wavelength, p m
b
Fig. 3 Noise reduction effect in spectral domain b y EA modulator with
25% gating duration
a Spectrum before gating
b Spectrum after gating
ELECTRONICS LE77ERS
7th May 1992 Vol. 28 No. 10
M . L. Dotor, P. Huertas, J. Melkndez, A. Mazuelas,
M . Garriga, D. Golmayo a n d F. Briones
Indexing terms: Semiconductor lasers, Lasers, Semiconductor
urowth. Eoitaxial urowth
A single monolayer thick InAs quantum well laser structure
has been grown at low substrate temperature by atomic layer
molecular beam epitaxy (ALMBE).The laser has an emission
wavelength of -884nm and a threshold current density of
1.97kA/cm2 at room temperature. This value is lower than
values obtained for other monolayer thick quantum well
lasers, and it demonstrates the device quality of epitaxial
layers grown at 350°C by ALMBE.
Recently Sato e t al. [l] observed low temperature photoluminescence (PL) with high intensity and a small linewidth
from GaAs with an InAs monolayer. The peak of the InAs
emission was 200 times higher than the excitonic emissions of
a 400nm-thick undoped GaAs layer. Also, stimulated emission has been observed from photopumped undoped separateconfinement single quantum wells as thin as one monolayer of
InAs [2] and GaAs [3]. These results showed that carrier
collection can be eflicient in these ultrathin quantum well heterostructures because the spatial extent of the wave function is
more important than the well width. In these two cases, the
935
t,
-
t
.
,I
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