Multiple wavelength amplification in wide
band high power 1550 nm quantum dash
optical amplifier
R. Alizon, D. Hadass, V. Mikhelashvili, G. Eisenstein,
R. Schwertberger, A. Somers, J.P. Reithmaier, A. Forchel,
M. Calligaro, S. Bansropun and M. Krakowski
A wide band, high saturation power InAs=InP 1550 nm quantum dash
optical amplifier is reported. The dependence of crosstalk in multiwavelength amplification on bit rate, power and channel detuning is
studied. Amplification of an eight channel WDM signal covering a
100 nm bandwidth with no crosstalk is demonstrated.
Quantum dot and quantum dash semiconductor optical amplifiers with
wide bandwidths and high saturation powers have been recently
demonstrated in the 1300 nm [1] and 1550 nm [2–3] wavelength
ranges. The wide bandwidth is a direct consequence of the inhomogeneously broadened gain [4] while the large saturation power stems
from a low confinement factor. These two parameters enable distortionless amplification of high bit rate signals [1–2] as well as amplification
of a multiwavelength signal with no crosstalk. Moreover, the specific
dynamics [5] characterising the coupling between the carrier reservoir
(wetting layer) and the quantum dots or dashes reduce the crosstalk
further as the bit rate increases. Namely, multiwavelength amplification
of high bit rate signals is expected to be preferable which is of course an
attractive practical advantage.
In this Letter we report the characteristics of a high performance
InAs=InP quantum dash optical amplifier operating at 1550 nm. We
have used this amplifier to study multiwavelength amplification, in
particular the dependence on signal power, bit rate and channel
detuning. We have demonstrated crosstalk-less amplification of eight
10 Gbit=s channels, covering 100 nm.
The gain region of the quantum dash structure comprised four
quantum dash layers, each five monolayers thick, separated by 25 nm
wide AlInGaAs barriers which were placed within a GRINSCH
structure. The waveguide was defined by a 4.5 mm wide ridge of
length 2.5 mm. The facets were coated with a double layer
Ta2O5=Al2O3 stack yielding a residual reflectivity of 0.01%.
Fig. 1a shows measured small signal gain spectra for several CW
bias levels. The peak small signal gain varies from 21 dB at a bias of
350 mA to 13 dB at 150 mA. The amplifier exhibits a gain larger than
10 dB over a wavelength range of 120 nm at a bias of 350 mA. Fig 1b
shows gain against input power characteristics. The extracted 3 dB
saturation output power ranges from þ16 dBm at 1537 nm to þ18 dBm
at 1585 nm.
9 dBm. The 2.5 Gbit=s rate was chosen since we have previously
demonstrated cross gain modulation at 2.5 Gbit=s over 50 nm [6] with a
different quantum dash amplifier. This rather low bit rate ensures
therefore that our present experiment reaches conservative conclusions.
The interfering channel, modulated at 10 Gbit=s, had a very high input
power (þ5 dBm) and its wavelength was tuned from 1530 to 1550 nm.
Keeping the interfering signal at a shorter wavelength than the signal
ensures maximum saturation [7] once more making this a conservative
experiment.
Fig. 2 Experimental arrangement for multiple wavelength amplification
Fig. 3a shows measured bit error curves for different detunings. We
note that the penalty is detuning dependent as expected in an inhomogeneously broadened amplifier. For detunings of 30 nm and 40 nm,
the penalty is moderate but for 20 nm it is more than 10 dB. We
emphasise the very high, þ5 dBm, input power which actually simulates a large number of channels each carrying the power of a practical
signal to be amplified.
Fig. 3 Evolution of BER at 1570 nm
a Several detunings with interfering channel kept at constant power
b Penalty dependence on bit rate at two different total powers fed into amplifier
The dependence on bit rate which is related to the dynamics of carrier
capture and escape between the quantum dashes and the reservoir
[5, 8, 9] was also tested with two channels, this time of equal power.
The signal wavelength was at 1535 nm and the interfering channel was
at 1565 nm. Fig. 3b shows the penalty against bit rate for a total input
power of 3 dBm and 6 dBm. The advantage of operating at high bit
rate is clearly seen in Fig. 3b where the penalty at 10 Gbit=s was only
3 dB for a power of 3 dBm and 1 dB for 6 dBm. Once more we
point out the high input powers which were used so as to highlight the
limits set by cross talk
Fig. 1 Gain spectra for different bias and gain against input power
characteristics
a Evolution of gain spectrum with bias current from 150 to 350 mA with 50 mA
steps
b Facet to facet gain dependence with input power
Fig. 2 shows the experimental setup for multiwavelength amplification. Two groups of channels are used. One group, the interfering
channels, make use of one to seven wavelengths which are combined in
an array waveguide grating and modulated simultaneously. The eighth
channel (at l1) is modulated by a separate source. All signals were
combined and fed to the quantum dash amplifier, the output signal
of which was filtered so that only the signal at l1 reached the
receiver.
In initial experiments we used only one interfering channel. At
moderate powers per channel we observed no crosstalk induced penalty
at any detuning. To quantify the crosstalk limitations we used a test
signal modulated with 2.5 Gbit=s data at 1570 nm at an input power of
Fig. 4 Input and output signal spectra and BER measurement
a Input signal spectrum
b Output signal spectrum
c BER measurement on signal wavelength tuned across spectrum
ELECTRONICS LETTERS 10th June 2004 Vol. 40 No. 12
Finally, we demonstrate operation of a practical eight channel system.
The seven interfering channels, modulated at 10 Gbit=s, were spectrally
placed in the 1500 to 1600 nm band such that each channel experienced
a gain of at least 10 dB. Each channel had an input power of 20 dBm.
The test signal, also of 20 dBm power, was tuned to various spectral
locations within this band and for each signal wavelength we measured
the bit error curve. Fig. 4a and b show input and output spectra where
the output power of the channels was 10 to 0 dBm. Fig. 4c shows the
bit error rate curves for all the signal wavelengths. The curves overlap
each other and the curve describing the receiver, meaning that the signal
experiences no crosstalk while being amplified.
To conclude, we have described the characteristics of an advanced
InAs=InP quantum dash amplifier operating at 1550 nm. The amplifier
exhibits broad band, large gain and high saturation power. We demonstrate simultaneous amplification of eight channels covering 100 nm
with no crosstalk.
2
Acknowledgment: This research is partially supported by the BigBand
project within the IST program of the EC.
6
# IEE 2004
Electronics Letters online no: 20040531
doi: 10.1049/el:20040531
3
4
5
4 April 2004
7
R. Alizon, D. Hadass, V. Mikhelashvili and G. Eisenstein (Electrical
Engineering Dept., TECHNION, Haifa 32000, Israel )
E-mail: [email protected]
R. Schwertberger, A. Somers, J.P. Reithmaier and A. Forchel (Technische Physik Universitat Wurzburg, Wurzburg 97074, Germany)
8
M. Calligaro, S. Bansropun and M. Krakowski (Thales Research and
Technology, Orsay 91404, France)
9
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