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0 IEE 1997
16 September I997
Electronics Letters Online No: 19971405
A. Vukovic, P. Sewell, T.M. Benson and P.C. Kendall (Department of
Electrical and Electronic Engineering, University of Nottingham,
Nottingham NG7 2RD, United Kingdom)
References
SELTZER, c.P.,
RIVERS, L.J.,
HARLOW, M.J.,
and
‘Low threshold current 1.6 mm InGaAsPiInP tapered
active layer multiquantum well laser with improved coupling to
cleaved singlemode fibre’, Electron. Lett., 1994, 30, pp. 973-974
2 LEALMAN, I.F., SELTZER, c.P., RIVERS, L.J., HARLOW, M.J., and
PERRIN, s.D.: ‘InGdAsP/InP tapered active layer multiquantum well
laser with 1.8 dB coupling loss to cleaved singlemode fibre’,
Electron. Lett., 1994, 30, pp. 1685-1687
1
LEALMAN, I.F.,
PERRIN, s.D.:
3
STERN, M.s.:
‘Semivectorial polarised finite difference method for
optical waveguides with arbitrary index profile’, IEE Proc. J.,
1988, 135, pp. 56&63
4 RAHMAN, B.M.A., and DAVIES, J.B.: ‘Finite-element solution in
integrated optical waveguides’, IEEE J. Lightwave Technol., 1984,
2, pp. 682-688
5 MCILLROY P.W.A : ‘Spectral index method: Single rib waveguide’, in
ROBSON, P.N.,
and
KENDALL, P.C.
(Eds.): ‘Rib waveguide theory by
the spectral index method’ (John Wiley & Sons Inc., 1990), Chap. 5
6
REED, M., BENSON, T.M., SEWELL, P., KENDALL, P.c., BERRY, G.M., and
DEWAR, s.v.: ‘Free space radiation mode analysis of rectangular
dielectric waveguides’, Opt. Qia“mm Electron., 1996, 28, pp. 11751179
M.J.:‘Semiconductor laser facet reflectivity using free
space radiation modes’, ZEE Proc. J., 1993, 140, pp. 49-55
ROBERTSON,
cations. To demonstrate these improvements we report wavelength
conversion of a 40Gbitis signal over a maximum of 24.6nm.
MQ W SOA: The operating conditions of an SOA when used as a
linear amplifier, and when used for nonlinear signal processing
(such as FWM), are very different. In the linear regime, the carrier
density is very high and, due to band filling effects, the wavelength
of highest gain can be as much as 5Onm from the bandedge. However, for good FWM performance, the amplifier is very strongly
saturated i.e. low carrier density. Thus, an MQW SOA with an
active region optimised for linear applications will not be optimised for FWM, and vice versa. We have designed and fabricated
an MQW SOA specifically for FWM.
We use strained ternary MQW material for the active layer as it
has a lower density of states than bulk or quaternary material.
Thus, the inversion factor close to the bandedge will be higher,
which results in a lower noise figure.
For a given carrier density fluctuation, larger index fluctuations
(which are beneficial to FWM efficiency) are produced when the
linewidth enhancement factor (aH)is large. We have positioned
the bandgap wavelength close to the operating wavelength (-1 550
nm) to obtain a large a,. This also ensures that the saturated gain
slope at the operating point with respect to wavelength is either
flat or negative, thus avoiding a strong reduction of the pump-signal power ratio as the beams propagate through the SOA. This could
lead to data pattern dependent effects through gain inodulation.
25
c
20 -15
=/-’
--
mym
i o --
40Gbit/s wavelength conversion over
24.6nm using FWM in a semiconductor
optical amplifier with an optimised MOW
active region
D -15
m
0
5-
<
a
A.E. Kelly, D.D. Marcenac and D. Nesset
Indexing terms: Semiconductor optical amplifiers, Opticalfrequency
Introduction: Semiconductor optical amplifiers (SOAs) are likely
to be key components in future optical fibre networks. This is
because, as well as performing linear amplification, SOAs exhibit
nonlinearities which have proven useful in numerous applications.
An important nonlinear process is four wave mixing (FWM) in
which a signal is mixed in the SOA with a ‘pump’ beam. This
results in the generation of optical sidebands which can be isolated
by filtering and used for demultiplexing [l], wavelength conversion
[2] and dispersion compensation [3].
To avoid waveform distortion caused by gain modulation in the
SOA, the pump power must be high in comparison to the signal.
Furthermore, to reduce ASE noise, the amplifier should be operated in saturation. With these constraints, the FWM efficiency
(and signal-noise ratio) can be too low for high bit rate systems.
Hence, there has been a great deal of interest recently in improving the eficiency of the FWM process in SOAs. Optimisation of
the SOA length has recently led to CW performance improvements [4] and 40 Gbitis transmission over 400km of standard fibre
using mid-span spectral inversion [5]. Since the efficiency of the
FWM process decreases rapidly with increasing pump probe
detunings, the maximum possible wavelength translation is a good
indication of device performance. Recently a record wavelength
conversion range of 18nm has been demonstrated at lOGbit/s [6].
In this Letter, we report the improvements obtained when the
SOA active region has been designed specifically for FWM appli-
ELECTRONICS LETTERS
4th December 1997
Vol. 33
-I?:/
-20 .=
i
i
~
The MQW active region co;sists of seven periods of 40A
unstrained ternary wells and 60A (-1 YOstrain) ternary barriers,
resulting in a photoluminescence wavelength of 1.553pn. SOA
devices were fabricated using buried heterostructure technology. A
2mm long device with multilayer antireflection coated facets was
packaged with lensed fibres. Fig. 1 shows the fibre-to-fibre gain
and the noise figure against current at 1.55pn. The device exhibited 20dB maximum fibre-to-fibre gain and a noise figure (2n,/c,)
of 7.6dB which includes at least a 3dB coupling loss. The increase
in the noise figure at currents 90mA is caused by residual reflections.
4OGbit/s OTDM system performance: To demonstrate the
enhanced performance of the FWM optimised SOA, it was used
to wavelength convert a 4OGbit/s, 27-1 PRBS, optical time division multiplexed (OTDM) data stream. Full details of the OTDM
system can be found in [SI. The power penalty against wavelength
shift at 10-9 bit error ratio (BER) is shown in Fig. 2. The circles
denote wavelength downshifts with an input data signal wavelength of 1557nm and the squares denote wavelength upshifts with
the data signal wavelength at 1543.5nm. Downshifts of up to
24.6nm and upshifts of up to 8.6nm are possible with BERs of <
lVY, indeed a negative penalty is observed for wavelength downshifts of -9nm. The negative penalty is thought to be due to the
No. 25
2123
wavelength dependance of the electroabsorption modulator used
as a demultiplexer. Less than 1dB power penalty is possible for up
to 17.5nm down-conversion and 6.5nm up-conversion.
8
7
m
E
..
0
0
$ 2
=.
0
$,
.... .. .
0
ot
................. -
... ...................
0
0
0
-1 I
-25
-.
0
-15
-10
-5
0
wavelength change, nm
-20
L
5
10
m
Fig. 2 Power penalty for a BER of IP9 against wavelength shij”t
0 signal at 1557nm
References
1
MORIOKA, T.,
TAKARA, H.,
KAWANISHI, S.,
UCHIYAMA, K.,
and
SARUWATARI, M.:
‘Polarisation independent
all
optical
demultiplexing up to 200Gbitis using four wave mixing in a
semiconductor laser amplifier’, Electron. Lett., 1996, 30, pp. 14891491
2 TATHAM, M.c., SHERLOCK, G., and WESTBROOK, L.D.: ‘20-nm optical
wavelength conversion using nondegenerate four-wave mixing’,
Photonics Technol. Lett., 1993, 5 , (ll), pp. 1303-1306
3 TATHAM, M.c.,
GU, x.,
WESTBROOK, L.D.,
SHERLOCK, G.,
and
SPIRIT, D.M.: ‘Transmission of 10Gbit/s directly modulated DFB
signals over 200km standard fibre using mid-span spectral
inversion’, Electron. Lett., 1994, 30, (16), pp. 1335-1336
4 DOTTAVI, A., MARTELLI, F., SPANO, P., MECOZZI, A , SCOTTI, S.,
DALL’ARA, R., ECKNER, .I,
and GUEKOS, G.: ‘Very high efficiency
four-wave mixing in a single semiconductor travelling wave optical
amplifier’, AppI Phys. Lett., 1996, 68, pp. 2186-2188
5
MARCENAC, D.D., NESSET, D., KELLY, A.E., BRIERLY, M., ELLIS, A.D.,
MOODIE, D.G., and FORD, c.w.: ‘40Gbitis transmission over 406km
of NDSF using mid-span spectral inversion in a 2”
long
semiconductor optical amplifier’, Electron. Lett., 1997, 33, (lo), pp.
signal at 1543.5nm
879-881
6 GERAGHTY, D.F., LEE, R.B., VAHALA, K.J., VERDIELL, M., ZIARI, M., and
MATHUR,A.:
BER versus received power for the back-to-back signal and
wavelength down-conversions of 7.4 and 24.6nm are shown in
Fig. 3a. The performance for 4 and 8.6nm wavelength up-conver.
sion are in Fig. 36, again with the back-to-back for comp?nson.
At the extremes of the detunings shown in Fig. 2 (i.e. 24.6nm
down-conversion and 8.6nm up-conversion) there is evidence that
error floors are emerging.
I
\
\
10-
‘Wavelength conversion up to 18nm at 10Gbit/s by
four wave mixing in a semiconductor optical amplifier’, Photonics
Technol. Lett., 1997, 9, (4), pp. 452-454
Compact arrayed waveguide grating multifrequency laser using bulk active material
M.R. Amerfoort, J.B.D. Soole, C. Caneau,
H.P. LeBlanc, A. Rajhel, C. Youtsey and I. Adesida
1o 5
t
r
w
m
10-’
Indexing terms: Semiconductor junction lasers, Waveguide lasers
10.’
10.’
’
-20
-15
-10
-
received power / dBm
a
b
@Bl
Fig. 3 Bit error ratio against receivedpower
a For wavelength down-conversion signal at 1557nm
Introduction: Frequency-selectable lasers at precise operating fre-
0 back-to-back
07.4nm wavelength down-conversion
A 24.6nm wavelength down-conversion
b For wavelength up-conversion signal at 1543.5nm
0 back-to-back
04nm wavelength up-conversion
a 8.6nm wavelength up-conversion
Conclusions: A 2mm long, ternary based, strained MQW SOA
optimised for FWM has been designed and fabricated. The device
was able to convert 40Gbit.k OTDM data over 24.6nm for wavelength downshifts and 8.6nm for wavelength upshifts with BERs
of < lP9.These represent the largest reported wavelength shifts
using FWM in an SOA at 4OGbit/s and demonstrate the excellent
performance of this device. The importance of optimising the
active region characteristics for the particular application has been
highlighted.
Acknowledgment: This work has been partially supported by
ACTS project ‘HIGHWAY’. The authors would like to acknowledge the contributions of D. Moodie, S. Perrin, G. Sherlock, C.
Ford, C. Peed and I. Reid.
0 IEE 1997
13 October 1997
Electronics Letters Online No: 19971380
A.E. Kelly, D.D. Marcenac and D. Nesset (BT
Martlesham Heath, Ipswich IP5 3RE, United Kingdom)
E-mail: [email protected]
2124
An eight-channel arrayed waveguide grating (AWG) intra-cavity
laser employing bulk active material is reported. The use of bulk
material ensures oscillation in one AWG order. Lasing at
200GHz-spaced ITU-defmed frequencies, 192.8-194.2T€€z, is
demonstrated.
Laboratories,
quencies are required for wavelength multiplexed (WDM) telecommunications networks. Such lasers may be obtained using
geometrically-defmed optical cavities. The frst integrated laser of
this type used an etched diffraction grating [l], but this has since
been superseded by lasers employing arrayed waveguide gratings
(AWGs) due to the greater ease of fabrication of the latter [2- 81.
Most of the published work to date on AWG lasers has
employed multi-quantum well (MQW) active material [2 - 41, with
only one report of an AWG laser using bulk active material [5].
The narrow gain band of the latter limits the tendency of the laser
to oscillate in AWG orders other than the one intended, which has
been a problem for MWQ-based AWG lasers. Although this is
now being addressed by carefully tailoring the AWG to suppress
adjacent order transmission 16 81, use of bulk active material
provides a simple alternative that obviates the need for complex
AWG designs.
In this Letter, we report a highly-compact AWG laser employing bulk active material, which is realised by a simple fabrication
scheme. The 8 x 200GHz WDM laser operates with frequencies
on the International Telecommunications Union (ITV designated
grid for WDM transmission systems. Singlemode operation is stable over a wide range of temperature and current injection.
~
Design and fabrication: The goal was to obtain a compact high-
quality WDM laser with the simplest of fabrication techniques. To
this end, a ridge-waveguide amplifier structure was used with a
self-aligned amplifier/guide coupling scheme; only four masking
stages and a single, InP-only, regrowth were required to form the
multi-frequency laser.
The amplifier structure comprised an 0 . 1 5 ~
thick 1 . 5 3 5 ~
InGaAsP active layer, separated from an underlying 0 . 2 thick
~
ELECTRONICS LETTERS
4th December 1997
Vol. 33
No. 25
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