Semiconductor Optical Amplifiers (SOAs) as Power

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Semiconductor Optical Amplifiers
(SOAs) as Power Boosters
Applications Note No. 0001
Applications Note No. 0001
SOAs as Power Boosters
Semiconductor Optical Amplifiers (SOAs) as Power Boosters
There is a growing need to manage the increase in loss budgets associated with optical networks
comprising optical nodes which facilitate and promote dynamic wavelength routing. These nodes
are complex at the optical level and in order to provide the necessary functionality, introduce a
loss overhead which has ramifications in respect of system designs (Figure 1a). There is also an
evolutionary move to deploy tuneable laser sources in network architectures for maximum
flexibility and utilisation of the wavelength resource. In general, the output power levels of
tuneable lasers are modest, especially since external modulation is required at data rates up to
and beyond 10Gbit/s introducing additional insertion losses, resulting in the need to boost the
signal prior to transmission. In addition, the ability to perform a limited amount of channel power
equalisation on each wavelength in a WDM multiplex is of benefit (Figure 1b).
SOAs provide a low cost route to providing amplification in such scenarios where it is
advantageous to embed the amplification within the node design or on transmitter line cards.
Longer term they permit higher degrees of integration to be invoked which then translates into
smaller footprint, more cost effective solutions. In this respect SOAs have a clear advantage over
alternative solutions such as EDFAs. See Kamelian Data Sheet on the OPB for typical SOA
parameters for this application.
λ
32 λ
SOA
2x2
2x2
NxN
SOA
SOA
DROP
ADD
2x2
SOA
Figure 1a: SOAs used in optical/add drop to manage the extra losses associated with the
introduction of advanced dynamic re-configurability.
Power
SOA
LD
Mod
SOA
LD
Mod
…
Mod
SOA
MUX
LD
Power
Wavelength
Wavelength
Figure 1b: SOAs used in transmitter modules where in addition to boosting the signal, a limited
amount of channel equalisation is required.
Page 1
Applications Note No. 0001
SOAs as Power Boosters
Semiconductor Optical Amplifiers
Linear operating regime: in amplification, the linear region is the preferred operating regime
since an exact, amplified replica of the input is required. Operating an SOA outside this region
causes distortion since at high output powers, the gain saturates and compresses (Figure 2). The
resulting gain modulation causes patterning in the time domain, because the gain recovery time of
an SOA is typically of the same order as the data modulation speeds. Thus one of the key
operating issues to ensure linear functionality is the management of the input power levels in
order to control the degree into which the device is driven into saturation (see also Kamelian
Application Note No. 0003: “SOAs in Multi-Channel Environments”).
In order to provide some level of channel equalisation, the gain of the SOA can be controlled by
changing the bias current applied. However if the bias current is lowered to lower the gain, the
saturation output power and hence the linear region also reduces which in turn limits the dynamic
range of the variation in gain for a certain output power (Figure 2).
Gain vs Output Power: 1550nm
15
14
13
3dB
12
3dB
3dB
Gain (dB)
11
10
3dB
9
8
7
6
3dB
5
4
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
Output Power(dBm)
180mA
Figure 2:
120mA
90mA
60mA
30mA
3
4
5
5.1dbm
(30mA)
6
7
8
7.9
(60)
9
10
11
12
9.3 10.3 11.2
(90) (120) (180)
Gain vs. output power for a typical SOA showing the linear region of operation and the
3dB output saturation power. Also shown is SOA behaviour in this respect with
variation in the control drive current. Thus for any particular SOA design the output
saturation power reduces with a reduction in gain.
Output saturation power: due to fundamental device physics, for maximum output power,
SOAs are designed such that the gain peak occurs at lower wavelengths with respect to the
desired operating band. The SOA operating regime therefore occurs on the long wavelength tail of
the gain profile (Figure 3, typically offset by around ~50nm from the beginning of the operating
bandwidth).
Page 2
Applications Note No. 0001
SOAs as Power Boosters
21
G ain m ax
O perating Range
19
17
Gain(dB)
15
13
11
9
7
5
3
1400
1465
1450
1528
1500
1550
1563
1600
1650
Wavelength(nm )
Figure 3:
Gain vs. wavelength relationship showing the design strategy that yields a high
saturation output power across, in this case, the c band. The wavelength of the peak
gain is offset in order to function on the longer wavelength segment of the profile.
The wavelength dependence of SOA characteristics translates into trade-offs in parameters with
respect to wavelength (also refer to Kamelian Application Note No. 0002: “SOAs as PreAmplifiers”). This also applies to the output saturation power and this variation must be taken into
consideration in any designs operating over a specified wavelength range (Figure 4). It must be
noted that the all SOA parameters are quoted as a min or max values (as appropriate) across that
specified wavelength band. These parameters can be optimised for any particular application by
accurate movement of the gain peak. In booster applications, the output power is the primary
design parameter of interest.
18
16
14
Gain(dB)& Psat(dBm)
12
Gain
10
Psat
8
6
4
2
0
1525
1530
1535
1540
1545
1550
1555
1560
1565
Wavelength(nm)
Figure 4:
Variation of output power saturation for a typical SOA across the c band (1529nm to
1563nm). This device has a gain peak centred at 1465nm.
Chirp in gain compression: operation of an SOA in gain compression not only results in
patterning but also produces chirp (frequency variations) of the amplified optical signal. The level
of chirp produced is proportional to the amount of gain compression the signal is subject to, the
net effect of additional chirp being to increase the power penalty (and hence attainable
transmission distance) of the link due to resulting increase in dispersion. Unlike directly modulated
lasers the chirp induced is of opposite sign; lasing occurs through current injection to increase the
output power whilst the SOA imparts gain through carrier depletion.
Page 3
Applications Note No. 0001
SOAs as Power Boosters
Figure 5 shows the amount of chirp induced as function of the degree of gain compression for a
typical SOA.
12
0
-0.5
10
Peak Chirp, GHz
-1
-1.5
8
-2
6
-2.5
-3
Chirp Max
-3.5
4
Compression
-4
2
-4.5
-5
0
-30
-25
-20
-15
-10
-5
0
Input Power, dBm
Figure 5:
Chirp as a function of gain compression.
Noise figure (NF): the amplification process is always accompanied by spontaneous emission,
where photons of random phase and polarisation are added to the signal. The noise performance
of an optical amplifier is characterised by the NF, defined as the amount of degradation in the
signal to noise ratio caused by the amplification process. The NF performance of typical SOAs is
defined in Kamelian Application Note No. 0002: “SOAs as Pre-Amplifiers”. In transmitter booster
applications, the NF will play a role but is not as critical as in pre-amplifier applications. In optical
nodes the NF is crucial in defining overall system performance.
Polarisation dependent gain (PDG): in any optical communication system the state of
polarisation at any in-line component is unknown, since installed optical fibre does not preserve
the state of polarisation. Thus, typically, the SOA has to be polarisation insensitive. Through chip
design know-how, very low polarisation dependent gain <0.5dB is available. For the wavelength
dependence of PDG refer to Kamelian Application Note No. 0002: “SOAs as Pre-Amplifiers”. In
transmitter applications, there is a well defined polarisation state emanating from the laser and
PDG is not a critical issue as long as the SOA provides the output power for the required gain. In
mid-span optical node uses, the PDG is important since a random polarisation enters into the
node.
Wide optical bandwidth: SOAs exhibit a ~80nm optical gain bandwidth at the 3dB drop from
the peak gain. Access to a wider bandwidth is possible if the minimum system gain required (at
the extremities) is lower (refer to Kamelian Application Note No. 0004: “SOAs in CWDM Systems).
Centring the gain peak very accurately during the material growth stage means that the SOA can
meet the amplifier needs for all of the low loss transmission window of optical fibres. In DWDM
applications, the SOA provides the required bandwidth easily.
Multi-wavelength operation: the SOA can operate in single and multi-channel environments.
For further details of the performance of the SOA as a power booster in multi-channel scenarios
see “Kamelian Application Note No. 0003: “SOAs in Multi-Channel Environments”.
Data rate transparent: the SOA is able to amplify at data rates ranging from Mbit/s up to and
beyond 40Gbit/s. In this respect it is a future proof technology compatible with any upgrade
scenario since it is also protocol independent.
Page 4
Applications Note No. 0001
SOAs as Power Boosters
Small form factor, amenable to integration: the SOA is housed within a standard 14-pin
butterfly package, the subject of a multi source agreement (MSA) with other leading SOA suppliers
which guarantees system providers with common optical/mechanical specifications. The size of the
package represents a significant improvement on competing optical amplifier solutions. Longer
term, Kamelian’s know-how in on-chip mode expansion technology promotes a manufacturable
solution to the integration of the SOA with other components to yield low cost, highly functional
modules.
Page 5
Applications Note No. 0001
SOAs as Power Boosters
Power Booster Characterisation
Gain compression: Figure 6 and Figure 7 show typical eye diagrams from SOA outputs in the
linear region of operation and in gain compression respectively. Although patterning is clearly
evident in the latter case, the eye diagram remains relatively open nonetheless. Figure 8 reenforces this behaviour highlighting that operation of the SOA in gain compression up to certain
limits does not introduce a significant power penalty. Please note that this measurement does not
consider the effect of the accompanying chirp (see above).
Figure 6:
Typical output eye diagrams for an SOA operating in the linear regime. Also shown are
the probability density functions (pdf) for the ‘1’ and’0’ level. No appreciable patterning
is evident.
Page 6
Applications Note No. 0001
Figure 7:
SOAs as Power Boosters
Typical output eye diagrams for an SOA operating in gain compression. Also shown are
the probability density functions (pdf) for the ‘1’ and’0’ levels. Although patterning is
evident, the eye opening remains clear and acceptable BER performance is maintained
deep into gain compression (see Figure 8).
Page 7
Applications Note No. 0001
SOAs as Power Boosters
3.5
BER Power Penalty, dB
3
2.5
2
1.5
1
0.5
0
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Gain Compression, dB
Figure 8:
BER vs. gain compression for a typical SOA. Operating significantly into gain
compression does not result in an appreciable power penalty.
Power booster in transmission: the performance of SOAs as power boosters can only be
evaluated within a systems context. Figure 9 is a schematic of a generic systems test bed
comprising multi sources (DFBs located on the ITU grid within the c band) and a link length of
60km of standard single mode fibre (SMF28). This enables the characterisation of the power
booster in single and multi channel environments (see Kamelian Application Note No. 0003: “SOAs
in Multi-Channel Environments”), including the combined effect of the factors associated with
operation in gain compression e.g. chirp, patterning.
Tx1
Tx2
Tx3
Tx4
Tx5
Tx6
Tx7
Tx8
60 km Link btb
data
60 km SOA Link
OSA
Rx
BERT
Key:
Figure 9:
SOA,
EDFA
Systems test bed used in the characterisation of the power booster.
Page 8
Applications Note No. 0001
SOAs as Power Boosters
There are two scenarios of interest:
• the back-to-back (btb) measurement which excludes the SOA but includes the effect of the
transmission link. Therefore the effect of dispersion generated within the link provides the
reference measurement. The EDFA is simply used as a means to manage the additional
loss introduced by the optical fibre.
• SOA as a booster followed by the 60km length of single mode fibre. Now the additional
issues with operating in gain compression are included. In the case of single channel
amplification, the characterisation takes into consideration patterning and chirp and relates
this to a power penalty owing to the use of the SOA in this mode.
Figure 10 summarises the power penalty of the btb and SOA boosted cases as a function of input
power. The SOA had a gain of 15dB and an output saturation power of +10dBm, operating at
10Gbit/s. No appreciable increase in the power penalty was evident over the operating range up to
–5dBm input power (representing the 3dB gain compression point for the device under
characterisation. Driving the SOA output further into compression results in an increase in
additional chirp which impinges on the power penalty. This operating range can be extended
through an increase in the output saturation power. More insight into this behaviour is presented
in the Kamelian Application Note No. 0003: “SOAs in Multi-Channel Environments”
4
SOA
SOA+60k fibre
3.5
2.5
2
1.5
Power Penalty, dB
3
1
0.5
0
-20
-18
-16
-14
-12
-10
-8
-6
Input Power to SOA, dBm
Figure 10: Power penalty as a function of input power per channel into an SOA as a power
booster. The btb provides a reference allowing data to be extracted on the impact on
performance of the SOA in this mode of operation.
Page 9
Applications Note No. 0001
4 Stanley Boulevard,
Hamilton Int. Technology Park
High Blantyre, Glasgow, UK
G72 0BN
Tel: +44 (0)1698 722074
Fax: +44 (0)1698 821101
www.kamelian.com
SOAs as Power Boosters
All statements, technical information and recommendations related to Kamelian’s
products are based on information believed to be reliable or accurate. However,
the accuracy or completeness thereof is not guaranteed, and no responsibility is
assumed for any inaccuracies. Before using the product or information, you must
evaluate it and determine if it is suitable for your intended application. The user
assumes all risks and liability whatsoever in connection with the use of a product
or its application.
Copyright © Amphotonix. All rights reserved. March 2012
SAM-DOC-00-0011 v3.0
Page 10

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