A wavelength converter is able to convert an incoming 1Mdata signal from one wavelength to another.
Figure A.6shows the layout of the Alcatel wavelength converter. This appendix presents the theory of a wavelength converter and describes the Alcatel wavelength converter.
A.1 Theory
The principle of a wavelength converter using SOAs can be explained using a simple SOA model with phase-shift and gain saturation in both amplifiers. The signal is coupled co-propagating with respect to the CWpower into SOA 1. For convenience, both identical SOAs are taken. An incoming CWwave with powerPewand electrical field Eo =
~P;
is equally split in two fields E1 andE2 using an V-tap.The constant (J is not used because we are only interested in the relative behaviour of the two electrical field.
The gain G1 [linear] in the first branch is depending on the input andCWpower as:
(A. 1)
-G1·(O.S*Pcw+PSlgnaf)
Psal3dB (A.2)
WhereGois the unsaturated gain [linear];Pew [W] and Psignal [W]are the CWand signal input powers;
Psat.3dB [W] is the -3dB saturation output power. In the SOA the electrical field E1 is amplified and obtains a phase shift depending on the gain.
Where g1 [NP] and g2 [Np] are the gain of the amplifiers in Nepers.
gj = In(G;) iE[1,2]
The gain G2 [linear] in the second arm is only depending on the CWpower as:
-G2·(O.S"PCW)
(A.3)
(A. 4)
(A.5)
Appendix A Wavelength converter A.2Requirements
(A. 7)
The resulting output field Eout is cross-gain and cross-phase modulated by the signal input power. The output powerPoutis:
(A.8) Using a Matlab script, these equations can be solved, obtaining numerical solutions. From the Matlab script, contour plots can be calculated for variable input powers and extinction ratio. The influence of these parameters on the converted average output power and extinction ratio will be evaluated. The contour plots are made using the parameters in Table A.1. These are realistic SOA values for illustrating the behaviour of the
we.
TableA.1 Parameters of Matlab script to evaluate SOA-WG.
Symbol Parameter Value Unit
Go Unsaturated gain 100
-Psa1,3dB -3 dB saturation power 0 dBm
Pew CWpower -8 dBm
A.2 Requirements
In the STOLAS project, the wavelength converter will operate in the non-inverting range as defined in Figure A.1.
inverting non-inverting
FigureA.1 Transfer function of an MZI-SOA wavelength converter.
In order to define the values of signal input power and extinction ratio for which the incoming signal is converted properly, some boundary conditions should be named. These boundary conditions are:
• The signal input power for a zero, b,o' (Figure A.1), should be larger than bmin,in; this limits the maximum extinction ratio allowed into the
we.
• The extinction ratio of the output should be larger than the input extinction ratio; switching a signal several times through the
we
would otherwise close the signal eye.Appendix A Wavelength converter A.2Requirements
• The output extinction ratio should be larger than the minimal value Emin; the receiver will need a sufficiently large difference between the spaces and marks.
Appendix A.1 presents a set of equations to calculate the transfer function of an MZI-SOA wavelength converter. Figure A.2 shows the transfer function of this
we.
The second maximum at Pin=5dBm is lower than the first one because of the cross-gain modulation in thewe
as described in equation A.7.10r~-~- Transferfun~on wavelengl~converter uSln? Matlab script
----2
'~2'=0----,-.1':-5---'.1'':-0- - - ' . 5 : - - - : - - - = : - - - , , 0 Psignal (dBm)
FigureA.2PoutversusPsignalfor
a we
with identicalSOAs.Figure A.3 shows the contour plot of wavelength converter output power for combinations of extinction ratio and signal input power.
Pout (d8m) versus Psignal and extin
E6
"'
~,
.g~6
~c
.. "
1.1
w , I 0, [.,'I
! I
!
Iv
,'"
/ \\ I
i.
I:>I r.
I 'I I0' I I , ,
·4 -2 0 2 4
Psignal [dBml
~ I
10
FigureA.3 Output power of the
we
for combinations of average power and extinction ratio of the signal input.Figure A.4 shows the contour plots of the output extinction ratio for different combinations of input
AppendixA Wavelength converter
extout(dB)versus Psignal and extin
A.2Requirements
Figure A.4 Output extinction ratio of the
we
for combinations of average power and extinction ratio of the signal input.As an example the working area is plotted in Figure A.5 with the following boundary conditions:
bm1n,in
=
-2dBm-2 dBm, value chosen from Figure A.2. A lower input power results in signal distortion because the signal is on in the crossover range between inverting and non-inverting conversion.
Emln
=
6dB6 dB, expected value of the extinction ratio in the STOLAS system.
Eout >Ein.
Because the signal extinction ratio decreases due to noise in the amplifiers, the
we
should increase the extinction ratio to compensate the noise degradation.WortdncJ area ExtinltlOlI rallo output
E:dlnctlon ratio OlItl'JUt . Elltlncllon ratio Input
10 ':zero' power input (cBl1l)
<NerBge irp.lt power(rBm)
Figure A. 5 The filled area shows the combinations of average power and extinction ratio of the signal input for which the extinction ratio of the converted signal is improved.
Appendix A Wavelength converter A.3 Alcatel wavelength converter
The plot shows the area for which all three conditions are met. The wavelength converter is able to increase the extinction ratio of an input signal for a small range of average power and extinction ratio of the input signal.
It can be concluded that it is possible to use the
we
in the co-propagating, non-inverting setting.Because the area for which the converter works proper1y is very small, an automatic gain control system is necessary in front of the wavelength converter.
A.3 Alcatel wavelength converter
Six independent currents drive the Alcatel wavelength converter. A layout of the Alcatel wavelength converter and the position of currents is presented in Figure A.6. The
we
is highly sensitive to the polatisation of theew
input.FigureA.6 Layout of the Alcatel wavelength converter.
Table A.2 shows four different current settings of the
we
driver unit. It is possible to change thewe
transfer function by tuning the currents and the polarisation of the
ew
input power.TableA.2 Current settings Alcatel Wavelength Converter.
Current Setting 1 Setting 2 Setting 3 Setting 4
11 150 mA 140 mA 148 mA 120 mA
12 40mA 40mA 50 mA 50 mA
13 200 mA 200 mA 349 mA 250 mA
13' 30mA 25mA 276mA 30 mA
14 350 mA 350 mA 240mA 70mA
14' 25mA 25mA OmA 30mA
Appendix A Wavelength converter A.3 Alcatel wavelength converter
4
e-
III~-25
;
---+---ff---,f---+---++~+'rIr___1"'--oa.
:i~ """'III-t---t--...--+....--+----+--~::vf--
o
___ Polarization state 2
Input power (dBm)
Figure A.7The static gain transfer function for co-propagating non-inverting conversion;
current setting4[9].
6
- 0
- - +
1--Polarization state 11-t----1f---;/f1'---.r-"Jl----+-1 ... Polarization slate 2... Polarization slale 3
- - +I-Polarization slale41-t----IHW-f--t----I---'IO-Z-I
Inputpower(dBm)
Figure A.8The static gain transfer function for co-propagating non-inverting conversion;
current setting1[9].
A.3.1 Real-time transfer function we
By applying an AO modulated signal with a large extinction ratio (Figure A9a) and looking at the response of
we
on the rising slope (Figure A9b), it is possible to show the real-time transfer function.The actual transfer can be calculated in Matlab by plotting the
we
output versus the AO input, resulting in a transfer function in mW (Figure A9d) or in dBm (Figure A9c).Appendix A Wavelength converter A.3 Alcatel wavelength converter
AOlnplJ'rrloWC weresponse enAOslope
d.
we transfer'(611:1100,clSTert setting 1
Opticel Power Input [mW] ~calPower'1Jli[dBml
FigureA.9 Transfer function of
we
(a) rising slope AD input signal;(b)