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Structure and growth We have adopted a somewhat mouied
structure presented in [2] (Fig. 1) Here, the optical resonator is
formed by a 15 5 parr AlAs/Al, 2Ga,As quarter-wavelength stack
Bragg reflector on the substrate side and the natural semconductor-air interface on the top. The Bragg reflector broad reflecbvity
m a m u m is centred at 850 nm The absorber is an undoped
GaAs/&,Gq,As MQW structure with 20 GaAs wells, 9 5nm
thck [3].The thickness of the adjacent Al, ,Ga, ,As layers IS calculated so that the enhancement maximum occurs at 855nm. The
top GaAs cap layer should be kept very thm to avoid s i g ” t
absorption.
sample area yelded a range of samples with varying properties
The area, where conditions discussed m the preceding paragraph
were fulfiied, was selected and test diodes were prepared by mesa
etchmg The contacts were prepared for both the p - and n&,Ga,,As layers, as the Bragg reflector tends to be highly
resistive.
The resulting photocurrent spectra of the tunable RCE photodetector at vanous bias voltages are shown in Fig. 2. Compared to
[4], clearly resolved excitonic peaks enhanced by the resonant cavity have been achieved The shlft of the absorption peak (the tunmg range) is 17nm for 3V reverse bias applied. The crosstalk
rejection ratio deduced from measured data is -3 8dB at 855nm
and -13dB at 872nm The rather low crosstalk rejection ratio at
855nm and relatively wide absorption peaks are caused by the
presence of other transitions, e.g the first light-hole exciton peak
and the exciton linewidth broadening at higher fields.
Conclusion. A tunable resonant-cavlty enhanced photodiode based
on a GaAdAIGaAs material system for the 850nm wavelength
region has been presented. The physical principle of tuning is the
quantum-confiied Stark effect in the GaAs/AlGaAs multiple
quantum well absorber. A tuning range of 17nm at 3V applied
bias has been obtaied
Acknowledgment. Ths work was supported by the COPERNICUS
project No. 12283 ‘DEMACOMINT’.
0 1EE 1991
Electronics Letters Online No 19970037
GaAs substrate
J Waclawek, J KovaC and J Slcriniarova
(Department of
Microelectronics, Slovak Technical University, Ilkovicova 3, SK-812 19
Bratislava, Slovakia)
Fig. 1 Schematic diagram of tunable RCEphotodiode
0.81
I
I
2 October 1996
1
I
B Rheinlander (Faculty of Physics and Geosciences, University of
Leipzig, Linnestrasse 5, 0-04103 Lezpzig, Germany)
V Gottschalch (Faculty of Chemistry and Mineralogy, University of
Leipzig, Lmnestrasse 5,0-04103 Lezpzig, Germany)
References
0
850
860
870
h ,nm
880
890
/87312)
Fig. 2 Photocurrent spectra of tunable RCE photodetector at various
reverse voltages
UNLU, M s , CHYI, J I ,
REED, J , ARSENAIJLT, L , and
‘Resonant cavity-enhanced (RCE) photodetectors’,
IEEE J Quantum Electron, 1991, 27, (2), pp 2025-2034
2 LAI, K , and CAMPBELL, J C ‘Design of a tunable GaAdAlGaAs
multiple-quantum-well resonant-cavity photodetector’, IEEE J
Quantum Electron, 1994, 30, (l), pp 108-114
3 LIN, c H , MEESE, J M , and CHANG, Y c . ‘Optical properties of GaAs/
Al,Ga, .As multiple quantum wells versus electric field including
exciton transition broadening effects in optical modulators’, J
Appl Phys, 1994,7§, (5), pp. 2618-2627
4 LAI, K , HANSING, C , STREETMAN, B G , CAMPBELL J C , LEAVITT R ,
and SIMONIS, G ‘GaAdAlGaAs multiquantum well vertical cavity
tunable photodetector’, J Appl Phys, 1994, 76, (12), pp 82048205
1
KISHINO, K ,
MORKO c, H
(1) ov
(11)
-1
v
(in) -2V
(1v) -3v
The structure has been grown by LP MOCVD using an Aixtron
200 system. Onl57 control samples with the Bragg reflector were
grown under the same growth conditions to verify the position of
the reflection region. More control samples were grown containing
the same structure, except the bottom Bragg reflector, to estimate
the quality of the MQW structure.
Characterisation and results Test diodes have been fabricated
from the control sample and photoresponse spectra have been
measured. They revealed a good quaky of the MQW Excitomc
peaks were present and the QCSE has been demonstrated by
applying a field Zero-bias peak positions were shifted to the red
compared to theoretical expectations [3] and the first heavy-hole
exciton peak occurred at 855nm at zero bias On the sample itself,
reflectivity measurements were taken at dLfferent points As the
exact posihon of the resonance maxmum is very sensitive to the
layers’ thckness, a slight inhomogeneity m growth rate over the
72
K. Ennser, R I . Laming and M.N. Zervas
Indexing terms Optical communication, Optica Jibre
communication, Optical Jibre dispersion, Gratings in fibres
High-bit-rate (lOGbit/s, 1.55pn) phase-encoded duobinary
transmssion ovei nondispersion shlfted fibre lmks usmg grating
dispersion compensators is analysed A reduced sensitivity to
optical nodineanties allows increased transnnssion powers and
thus the &stance, 1700km compared to lO0Okm for the
conventional NRZ format. In addition, for typical links aromd
700km,an mcreased dispersion inargin is observed, equvalent to
f60km compared wth S 5 k m for the NRZ fomiat
ELECTRONICS LETTERS
2nd January 1997
Vol. 33 No. I
Introduction: Dispersion compensating chirped fibre Bragg grat-
ings are attractive to overcome the limitation of long-haul and
high-bit-rate transmission over non-dispersion shifted fibre [1, 21.
In addition, the phase-shifted signalling has been pointed out as a
simple technique to bridge the chromatic dispersion limit [4, 51.
The advantages of this two-level duobinary scheme are that no
decoder is needed and a conventional binary I W D receiver can
be used.
By incorporating phase-shifted duobinary coding, it is possible
to fwther increase the performance of dispersion compensated systems, lOGbit/s transmission up to 700km of standard singlemode
fibre has been experimentally demonstrated with a grating compensator [3].
In this Letter, we compare the impact of self-phase modulation,
amplifier noise and dispersion on the performance of dispersion
compensated lOGbitis systems using either the binary or phaseencoded duobinary transmission format. The duobinary scheme
shows an increased transmission performance and greater dispersion margin than the conventional binary format.
transmitter
+
4
grating
duobinary signal. This ideal duobinary scheme does not show any
advantage [6], however, by overfiltering the data, the energy of '1'
is maintained through propagation, thus reducing both the dispersion effect and sensitivity to fibre nonlinearity. Binary coding is
simulated by removing the duobinary encoder and changing the
filter bandwidth to 10GHz. The inclusion of a narrowband filter
(~10GHz)in the binary encoder shows only a slight improvement
and is not considered here.
The optically amplified link is made up of lOOkm sections of
fibre with 17ps/(nm.km) dispersion and 0.2dBkm loss. The effect
of nonlinearity for different positions of identical gratings has
been studied numerically using the split-step Fourier method. The
best performance was obtained when the dispersion was compensated for by incorporating a grating every 200km with the first
grating at 200km. This is because we are taking advantage of the
interplay between dispersion and nonlinearity. The gratings are
20cm long and designed with a hyperbolic tangent apodisation
profile to give a dispersion of 3400ps/nm, 95% reflectivity and a
3dB bandwidth of O h m . The combined losses of the grating and
circulator (-2dB) are compensated for by increased gain in the
appropriate amplifiers.
System evaluation and discussion: The impact of self-phase modulation for binary and duobinary coding is compared in Fig. 2. To
understand the effect of nonlinearity and amplifier noise, the performance of the binary system with different amplifier noise figures is shown. For low powers (<-2dBm) the maximum distances
are limited by amplifier noise with binary and duobinary being
comparable for the same noise figure. Whereas for high transmission powers, (>-4dBm) the maxi"
distance is limited by nonlinearity and is independent of amplifier noise. Approximately
2dBm higher transmission power can be used with duobinary
transmission, thus allowing an increased maximum link length of
1700km compared to l000km for binary transmission with 6dB
noise figure amplifiers.
1
I
I
liLiilj
Fig. I System configuration of 10GbitIs digital transmission over nondispersion shifted fibre
-L
EDFA erbium-doped fibre amplifier with 6dB noise figure
2.0
t
-2
0
2
L
6
8
10
average fibre -input power, dBm
4
- ,500
a,
I
binary
c
a,
a
/ X
1
?
h
12
0
00
630
665
700
73 5
77 0
fibre length, k m
Fig. 3 Comparison of performance of binary and phase-encoded duobinary 700km dispersion compensated standard fibre link
a Eye-opening penalty against average fibre input power
-L
-2
0
2
4
6
average fibre -input power, dBm
8
b Eye-opening penalty against fibre length for 4dBm average input
power
10
Pig. 2 Maximum fibre length for I dB eye-opening penalty against average fibre input power for IOGbit/s transmission of binary and phaseencoded duobinary coding
1. The phase of the wave is coded generating a two-level intensity
Fig. 3a investigates the eye-opening (EO) penalty against average launch transmission power for a link of 700km. In each case,
the last grating at 600km is optimised. It can be seen that the duobinary system is more robust against noise and nonlinearity and
increases the dynamic range for 1dB EO-penalty from -8 to
-1 1dBm. Fig. 3b shows the sensitivity to link-length. The average
transmission power is 4dBm and the grating is optimised for
700km. For < IdB EO-penalty and binary transmission, the link
must be matched to -+25km, whereas for duobinary transmission
a window of 4 6 0 k m is facilitated. This increased margin may be
very significant in switched networks.
ELECTRONICS LETTERS
No. 'I
NF: Noise figure of the optical amplifier
0 binary (NF = 6dB)
0binary ( N F = 3dB)
A binary ( N F = OdB)
duobinary ( N F = 6dB)
System model: The duobinary transmitter model is shown in Fig.
2nd January 7997
Vol. 33
73
Conclusion: By using a simple phase-shifted duobinary coding
technique the linearly-chirped fibre grating dispersion compensated system has increased noise immunity, bit-rate-length-product
and flexibility in design. For a lOGbit/s transmission system and
grating compensators each 200km, an improvement of 70% of
transmission length of standard fibre has been shown compared
with the conventional NRZ binary signal.
Acknowledgments: K. Ennser acknowledges the Brazilian Council
(CNPq) for financial support. This work was partially supported
by Pirelli Cavi SPA. The ORC is an EPSRC-funded Interdisciplinary Research Centre.
0 IEE 1997
22 October 1996
Electronics Letters Online No: 19970023
I<. Ennser, R.I. Laming and M.N. Zervas (Optoelectronics Research
Centre, University of Southampton, Southampton SO17 IBJ, United
Kingdom)
K. Ennser: On leave from the High-Frequency Institnte, Berlin
Technical University, Germany
E-mail: ke@orc.soton.ac.uk
References
1
OUELLETTE, F.:
2
KASHYAP, R ,
WILLIAMS, D.L.,
3
4
5
6
‘Dispersion cancellation using linearly chirped
Bragg grating filters in optical waveguides’, Opt. Lett., 1987, 12,
(lo), pp. 847-849
CHERNIKOV, S.V.,
CAMPBELL, R.J.,
MCKEE, P.F.,
and TAYLOR, J R : ‘Demonstration of dispersion
compensation in optical fibres using photoinduced chirped
gratings’. Proc. COST Workshop on Optical Fibre, Nice, France,
1994, Paper PD2
LOH, w.H., LAMING, R.I., ELLIS, A.D., and ATKINSON, D.: ‘Dispersion
compensated 10Gbit/s transmission over 700km of standard single
mode fibre with l0cm chirped fibre grating and duobinary
transmitter’. Proc. OFC’96, San Jose, USA, 1996, Paper PD30
PRICE, A J , and LE MERCIER, N : ‘Reduced bandwidth optical digital
intensity inodulation with improved chromatic dispersion
tolerance’, Electron. Lett., 1995, 31, pp. 58-59
YONEGA, K , KUWANO, s , NORIMATSU, s., and SHIBATA, N.: ‘Optical
duobinary transmission system with no receiver sensitivity
degradation’, Electron. Lett., 1995, 31, pp. 302-304
PENNINCKX, D., CHBAT, M , PIERRE, L., and THIERY, J.P : ‘The phaseshaped binary transmission (PSBT): A new technique to transmit
far beyond the chromatic dispersion limit’. Proc. ECOC’96, Oslo,
Norway, 1996, Paper TuD 2.3, pp. 173-176
i0
G.H. Smith, D. Novak and Z . Ahmed
Indexing terms Optical modulation, Radio systems
The authors present a novel method for generating an optical
carrier wth single sideband modulation usmg a dual-electrode
Mach-Zehiider modulator biased at quadrature. It is proposed
and demonstrated expermentally that ths smple techmque can
be used to reduce dispersion power penalties n
.! fibre-ra&o
systems
Introduction Broadband wireless access 1s a promising technology
to provlde future interactive multiniema servlces Radio can promde large bandwdth mobde services, wlule elsewhere in the network it is being considered as the ‘last drop’ to the home In these
systems, the radio frequency (RF) signals can be distributed to
remote antenna sites using optical fibre l d s which promde large
to EM1
bandwidth, low loss, and “unity
The smplest technique for the optical generation and distribution of an R F signal, modulated with data, is a du-ect-detection
scheme via direct or external modulation of a laser To avoid the
effects of laser frequency chirp however, extemally modulated
optical fibre links are the preferred choice. In conventional
74
mtensity modulation, the optical carrier is modulated to generate
an optical field with the carrier and double sidebands (DSB)
When the signal is sent over fibre, chromatic dispersion causes
each spectral component to experience different phase sMts
dependmg on the fibre link distance, modulation frequency, and
the fibre dispersion parameter. If the phase clffeience between the
two optxal sidebands at the photodetector is E, destructive mlxing
wdl cancel all power at the R F frequency [I]. As the RF frequency
mcreases, the effect is even more pronounced and the fibre hnlt
distance becomes severely lmted [2] For example, a 3dB power
degradation m detected R F power occurs for a fibre lmk distance
of 6km in an externally modulated link operating at 2OGHz,
decreasing to 0.7km for radio systems operating at 60GHz [3]
In intensity modulation schemes, dispersion effects can be
reduced by the e h a t i o n of one sideband to produce an optical
single-sideband (SSB). This has been demonstrated prevlously [4]
by usmg an optical fdter to suppress one of the sidebands However, tlus method is l m t e d by the filter characteristics and can be
quite complex to mplement In this Letter, we propose and demonstrate a novel technique for generating an optical carrier with
SSB modulation to overcome the effects of dispersion in fibreradio systems usmg externally modulated lmlts The techmque is
smple as no optical fdtering is required and uses only one dualelectrode Mach-Zehnder modulator (MZM) [5] to produce optical
SSB. We present the theory used to prehct the generation of optical SSB usmg t h s MZM and c o n f m our model with experimental results whch demonstrate the reduced effect of fibre
dispersion
7 9 6 km standard
optical
8 -60
SS B generator
,,553
1553
h,Pm
155L
Fig. 1 Schematic diagram of optical SSB generator and experimental
setup to measure effect of fibre chromatic dispersion
Inset. measured optical spectrum from optical SSB generator with
18GHz applied RF frequency
-electrical
optical
Theory The optical SSB generator using the dual-electrode MZM
is shown in Fig 1 A CW signal from a DFB laser with amplitude
A and frequencyf, is externally modulated by an R F signal wth
amphtude VACand frequency f m using the dual-electrode MZM
The R F signal is applied to both electrodes with 7d2 phase shift
applied to one electrode A DC bias voltage, Vna is also applied
to one electrode whde the other DC terminal is grounded To
model tlus situation, the MZM can be considered as two optical
phase modulators in parallel wth drive signals d 2 out of phase
and with DC voltage apphed to one ann. The output signal Eo(t)
from the MZM is thus represented by
A
Eo(t) = 2 [ cos{w,t
+
+ p. + ancosw,t}
+
cos{w,t
a. sinw,t}]
(1)
where a,= 2@; a, = 271fm; p = (V,dy,), where y, the MZM
switchmg voltage, and a = (VAC
/Q If the MZM is biased so that
p = 1/2 (1 e at quadrature) and is driven such that a < l/n, eqn 1
can be expanded usmg Bessel functions to
A
Eo(t) = -[Jo(an)cosw,t - J o ( a ~ ) s i n d , t
2
-2 4(
a
.
)
cos(w, - U&]
(2)
The Fourier transform of the autocorrelation of Eo(t)will give the
power spectral density &(U)
A2
SE(W) = -$(a!T)Tb(W
+U,}
4
A2
+ -J~(aT).s{w
+
2
ELECTRONICS LETTERS
(W. - U,)}
2nd January 1997
Vol. 33
(3)
No. I
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