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Towards high performance
GalnAsN/GaAsN laser diodes
1.5pm range
ion-etching into ridge waveguide lasers with widths between 1.5 and
5 pm and lengths ranging from 530 to 1200 pm.The thickness, the
indium and the nitrogen content of the QWs were derived from highresolution X-ray diffraction (HR-XRD) on calibration samples.
D. Gollub, M. Fischer and A. Forchel
GaInAsN/GaAsN/AlGaAs double quantum well lasers with emission
at 1.49 pm grown by solid source molecular beam epitaxy is investigated. The devices show the lowest threshold currents (120 mA) and
highest output powers (130 mW pulsed) reported to date for GaAsbased 1.5 pm lasers.
Introduction: Long wavelength lasers emitting at 1.55 and 1.3 pm are
the light sources for optical communication and optical interconnection systems. Lasers at these wavelengths are fabricated almost
exclusively from GaInAsP or AlCaInAs heterostructures based on
InP. Using the GaInNAs material system proposed by Kondow et al.
[ 11 different groups have investigated GaAs-based lasers for 1.3 and
1.5 pm. GaAs-based telecom lasers are attractive, since they would
allow a cost-effective device production on large substrates. In
addition, owing ‘to the large difference in refractive index this
system permits the fabrication of efficient monolithic Bragg reflectors. Furthermore, the high electron confinement of the GaAs-based
material system should allow for temperature-insensitive devices. For
the 1.3 pm system very encouraging results have been reported for
ridge waveguide lasers, DFB lasers and VCSELs by different groups
[2-51. For the 1.5 pm range the first GaInNAs lasers have been
reported by our group earlier [ 6 ] . The threshold current of these
devices has been rather high. Recently GaInNAsSb lasers employing
GaNAsSb barriers with an emission wavelength of 1.49 pm have been
fabricated, yielding improved threshold currents of around 1250 mA
for ridge waveguides (660 x 10 pm’) [7].
In this Letter, we report results on GaInNAs lasers with ternary
quantum well barriers with emission wavelength at 1.49 pm. Ridge
waveguide lasers show strongly improved threshold currents of 120 mA
(530 x 1.8 pm2, one facet HR coated) and 338 mA (1200 x 4 pm2,
facets as cleaved). The maximum output power of the latter device
exceeds 130 mW under pulsed conditions. These are the lowest thresholds and highest output powers for GaAs-based lasers in the 1.5 pm
range reported to date and clearly indicate the potential of GaInNAs for
GaAs-based lasers at 1.5 pm.
1200x4pm2,
as-cleaved
RT, pulsed
807060 50 -
Results: Fig. 1 shows a room temperature (RT, 20°C) light output
against drive current (L(1)) characteristics for a 1200 x 4 pm ridge
waveguide laser operating at 1.491 pm with as-cleaved facets under
pulsed operation. The RT threshold current of the device is about
338 mA. This value is lower by about a factor of three than the best
previously reported threshold for a GaAs-based laser in this wavelength range [7]. The external efficiency from both facets amounts to
about 0.14 W/A. Output powers of more than 130 mW are reached
under pulsed operation. The inset of Fig. 1 shows the RT emission
spectrum at a drive current of 700 mA.
RT, pulsed
Ith=120mA
??
/
/
/
2-1
0
A 530 x 1.8 pm ridge waveguide laser emitting around 1.494 pm has
been HR coated on one facet. The L(1)-characteristics of this device are
shown i n Fig. 2. At RT this device exhibits a record low threshold
current for GaAs-based lasers in the 1.5 pm range of 120 mA under
pulsed conditions. The extemal efficiency on the as-cleaved facet is
0.065 W/A. The maximum output power on the as-cleaved facet
exceeds 15 mW. We point out that the maximum output powers are
thermally limited in the presently unmounted devices. We have also
tested the continuous-wave (CW) capability of the devices. Lasing on
unmounted bars is sustained up to duty cycles of 15%, indicating that
an improved mounting technology could result in CW lasing at 1.5 pm
on GaAs substrates.
1200x4pm2
as-cleaved
pulsed
20 10-
1484 1486 1488 1490 1492 1494 1494
.
0
400
300
Fig. 2 Light output-current characteristic under pulsed operation oj
530 x 1.8 pm2 ridge laser with one facet HR coated
40 30 -
0
200
current, mA
100
1, nm
, . , . , . , . , .
200 400
600 800 1000 1200 1400 1600
Fig. 1 Light output against drive current of as-cleaved I200 x 4 pm’
GaInNAs/GaAsN ridge laser operating close to 1.5 p m under pulsed
operation
Inset: RT emission spectrum at 700 mA
Growth: The SCH laser structure was grown on (001) oriented
n-GaAs substrate by solid source molecular beam epitaxy (MBE).
An EPI-Unibulb radio frequency (RF) plasma source generated
nitrogen radicals from ultra pure N2 gas. The active region of the
lasers - consisting of two 7.6 nm Gao.621no.38Aso.9sNo.os
QWs separated by a 17 nm GaAso.92No,08
barrier - is located at the centre of a
-
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E
60
.
L^
40
Q
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3 40Q
50
20
-
60
70
0
ELECTRONICS LETTERS 26th September 2002 Vol. 38 No. 20
1
’
I
.
I
.
I
1183
Fig. 3 shows a set of L(1)-curves of an as-cleaved 1200 x 4 pm2 ridge
laser measured for different temperatures under pulsed operation.
Lasing with output powers in excess of 20mW is observed up to
temperatures of 60°C. The threshold current increases from 300 to
783 mA as the temperature is increased from 10 to 70°C.
Fig. 4 shows the measured threshold values for the devices in pulsed
mode against temperature. We obtain a characteristic temperature for
the threshold current variationjth(T)=j, exp(T/To) of 72 K for pulsed
operation in the range between 10 and 50°C.
900 1
1200x4pm2 ,as-cleaved
pulsed
1.49pm
2 BORCHERT, B., EGOROV; A.Y., ILLEK, S., KOMAINDA, M., and RIECHERT: H.:
‘ 1.29 pm GalnNAs multiple quantum-well ridge-waveguide laser diodes
with improved performance’, Electron. Lett., 1999, 35, pp. 22062206
3 FISCHER, M., GOLLUB, D., and FORCHEL, A,: ‘1.3 pm GaInAsN laserdiodes
with improved high temperature performance’, Jpn. 1 Appl. Phys., 2002,
41, (1, No.2B), pp. 1161-1163
4 REINHARDT, M., FISCHER, M., KAMP, M., HOFMANN, J., and FORCHEL: A,:
‘ 1.3-pm GaInNAs-AIGaAs distributed feedback lasers’, ZEEE Photonics
Technol. Lett., 2000, 12, pp. 239-241
5 STEINLE, G., RIECHERT, H., and EGOROV; A.Y.: ‘Monolithic VCSEL with
InGaAsN active region emitting at 1.28 pm and CW output power
exceeding 500 pW at room temperature’, Electron. Lett., 2001, 37,
pp. 93-95
6 FISCHER, M., REINHARDT, M., and FORCHEL, A,: ‘GaInAsN/GaAs laser
diodes at 1.52 pm’, Electron. Lett., 2000, 36, pp. 1208-1209
7 HA, W, GAMBIN, V., WISTEY, M., BANK, S., W E N , H., KIM, S., C.ySUen,
Jrand HARRIS, J.S. : ‘Long wavelength GaInNAsSb/GaNAsSb inultiple
quantum well lasers’, Electron. Lett., 2002, 38, pp. 1025-1026
Improving turbo codes by control
of transient chaos in turbo-decoding
algorithms
2004
I
10
’
I
20
.
I
.
30
I
.
40
0
50
’
I
60
‘
I
70
temperature,“C
Fig. 4 Temperature dependence of threshold current under pulsed operution of as-cleaved I200 x 4 p m 2 ridge laser
Compared to the use of GaInNAs/GaAs ridge waveguide lasers with
identical processing and dimensions, the use of a GaInNAs/GaAsN
quantum well results in a significant improvement of the device
properties. We observe a reduction of the threshold current by a
. factor of about 3. In contrast, the efficiency of the devices does not
change systematically. To values for devices with GaInNAs/GaAsN
DQW active layers are similar to those measured for comparable lasers
with GaInNAs/GaAs DQW active layers.
Conclusion: We have realised GaInAsN/GaAsN DQW ridge waveguide laser diodes with emission wavelengths in the 1.5 pm range.
Compared to previous GaInNAs lasers as well as GaInNAsSb lasers
for this wavelength range we observe significantly smaller threshold
current densities, higher output powers and high temperature performance. Our results clearly indicate the potential of GaAs-based
devices for edge and vertical emitters at the main telecom wavelength
window.
Acknowledgments: The authors wish to thank A. Wolf, S. Kuhn and
R. Werner for expert technical assistance. Financial support of the
work at Technische Physik, Wiirzburg University, by the Volkswagen
Foundation and the State of Bavaria is gratefully acknowledged. The
work at nanoplus was supported by the European Commission in the
frame of the IST ‘Gift’ project.
0IEE 2002
Electronics Letters Online No: 20020812
Dol: 10.1049/e1:20020812
16 July 2002
D. Gollub (nanoplus Nanosystems and Technologies GmbH, Oberer
Kirschberg 4, 97218 Gerbrunn, Germany)
E-mail: [email protected]
M. Fischer and A. Forchel (Universitat Wurzburg, Technische Physik,
Am Hubland, 97074 Wurzburg, Germany)
M. Fischer: Now at nanoplus Nanosystems and Technologies GmbH
References
1
s., and
‘GaInNAs: a novel material for long-wavelength range
laser diodes with excellent high-temperature performance’, Jpn. 1
Appl. Phys.,, 1996,35, pp. 1273-1275
KONDOW, M., UOMI, K., NIWA, A,, KITATANI, T., WATAHIKI,
YAZAWA, Y.:
1184
L. Kocarev, Z. Tasev and A. Vardy
A simple technique to control transient chaos in the turbo-decoding
algorithm,which improves the performance of classical turbo codes, is
developed. On average, the turbo-decoding algorithm with control
(when stopped after eight iterations) shows a gain of 0.25-0.3 dB in
the practical signal-to-noiseratios (from 0 to 1 dB), compared to the
classical turbo-decoding algorithm without control.
Introduction: It has been recognised recently that two classes of
codes, namely turbo codes [ l ] and low-density parity-check codes
[2], perform at rates extremely close to the Shannon limit [3]. Both
classes of codes are based on a similar philosophy: constrained
random-code ensembles, described by certain fixed parameters plus
randomness, decoded using iterative decoding algorithms (also known
as message passing decoders). Iterative decoding algorithms may be
viewed as complex nonlinear chaotic dynamical systems. In a
pioneering paper [4], Richardson has presented a geometrical interpretation of the turbo-decoding algorithm, and formalised it as a
discrete-time dynamical system defined on a continuous set. In a
follow-up paper by Agrawal and Vardy [ 5 ] , a bifurcation analysis of
the iterative decoding process as a dynamical system parameterised by
a single parameter, the signal-to-noise ratio (SNR), has been carried
out.
In this Letter, we develop a simple technique to control the transient
chaos in turbo-decoding algorithms and thereby improve the performance of the turbo codes.
Turbo-decoding algorithm as a nonlinear dynamical system: Fig. 1
shows the turbo-coding system. The turbo codeword is obtained as a
concatenation of three component bit sequences: the input information bits i = s o and the parity bits of each constituent convolutional
code, denoted by s1 and s2, respectively. Each of the three sequences
so, s’, and s2 has length n. The input sequence of the second
convolution code has the same Hamming weight as i, but the bits
are rearranged via a permutation n, also known as the ‘interleaver’. In
the simulations presented here, we used a turbo code with n = I024
and identical constituent recursive convolutional codes of rate 1 /2
(CC1 and CC2 in Fig. la) generated by the polynomials
{ D4 D3 D2 + D’
1, D4 + 1} . Overall, this produces a rate- 1/ 3
turbo code. The codewords were transmitted over an additive white
Gaussian noise (AWGN) channel using binary phase-shift keying
(BPSK) modulation.
As shown in [4, 51, the turbo-decoding algorithm (Fig. l b ) can be
written as a dynamical system:
+ +
+
X,(I
+
1) = F,[X,(I),co, c’l
X2(4 = F2[X,(I),
co, c21
ELECTRONICS LE77ERS 26th September 2002
(1)
Vol. 38 No. 20
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