IEC TR 61292-9:2013

IEC/TR 61292-9 ®
Edition 1.0 2013-11
TECHNICAL
REPORT
colour
inside
Optical amplifiers –
Part 9: Semiconductor optical amplifiers (SOAs)

IEC/TR 61292-9:2013(E)
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IEC/TR 61292-9 ®
Edition 1.0 2013-11
TECHNICAL
REPORT
colour
inside
Optical amplifiers –
Part 9: Semiconductor optical amplifiers (SOAs)

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
S
ICS 33.160.10; 33.180.30 ISBN 978-2-8322-1169-4

– 2 – TR 61292-9 © IEC:2013(E)

CONTENTS
FOREWORD . 4

INTRODUCTION . 6

1 Scope . 7

2 Terms, definitions, abbreviations and symbols . 7

2.1 Terms and definitions . 7

2.2 Abbreviations . 7

2.3 Symbols . 8

3 Specific features of SOAs . 9
3.1 SOA chips . 9
3.2 SOA modules . 11
3.3 Gain ripple . 11
3.4 Polarization dependent gain (PDG) . 13
3.4.1 General . 13
3.4.2 Polarization insensitive SOAs . 13
3.5 Noise figure (NF) . 13
3.6 Lifetime of carriers . 14
3.7 Nonlinear effects . 14
4 Measurement of SOA output power and PDG . 14
4.1 Narrow-band versus broadband light source . 14
4.2 Recommended set-up for output power and PDG measurements . 15
4.3 Examples of measurement results obtained by using the recommended
set-up . 16
Annex A (informative) Applications of SOAs . 19
A.1 General . 19
A.2 Polarization mode of SOAs . 19
A.3 Reach extender for GPON . 19
A.4 Pre-amplifier in transceivers for 100 GE . 19
A.5 Monolithic integration of SOAs . 20
A.6 Reflective SOAs (RSOAs) . 21
Bibliography . 22

Figure 1 – Schematic diagram of the typical SOA chip . 9

Figure 2 – Example of gain dependency on forward current of the SOA chip . 9
Figure 3 – Schematic top view of a typical SOA chip with and without an angled
waveguide structure . 10
Figure 4 – Schematic top view of the typical SOA module . 11
Figure 5 – Schematic diagram of the optical feedback inside the SOA chip . 12
Figure 6 – Schematic diagram of gain ripple . 12
Figure 7 – Output power and PDG dependence on the wavelength of the SOA chip . 14
Figure 8 – Recommended measurement set-up for optical power and PDG of SOA
modules . 15
Figure 9 – Recommended measurement set-up for optical power and PDG of SOA
chips . 16
Figure 10 – Optical power spectra of three different SOA chips. 16
Figure 11 – Output power and PDG of the SOA chip sample no. 1 as a function of I . 17
F
TR 61292-9 © IEC:2013(E) – 3 –

Figure 12 – Output power and PDG of the SOA chip sample no. 2 as a function of I . 17
F
Figure 13 – Output power and PDG of the SOA chip sample no. 3 as a function of I . 18

F
Figure A.1 – Schematic diagram of the receiver section of SOA-incorporated CFP

transceivers . 20

Figure A.2 – Schematic diagram of the DFB-LDs-array type wavelength tuneable LD . 20

Figure A.3 – Schematic diagram of the seeded WDM-PON system . 21

– 4 – TR 61292-9 © IEC:2013(E)

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
OPTICAL AMPLIFIERS –
Part 9: Semiconductor optical amplifiers (SOAs)

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected

data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC/TR 61292-9, which is a technical report, has been prepared by subcommittee 86C: Fibre
optic systems and active devices, of IEC technical committee 86: Fibre optics.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
86C/1148/DTR 86C/1183/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

TR 61292-9 © IEC:2013(E) – 5 –

A list of all parts in the IEC 61292 series, published under the general title Optical amplifiers,

can be found on the IEC website.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be

• reconfirmed,
• withdrawn,
• replaced by a revised edition, or

• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – TR 61292-9 © IEC:2013(E)

INTRODUCTION
Optical amplifiers (OAs) are necessary components as booster, line and pre-amplifiers for

current optical network systems. IEC TC86/SC86C, has published many standards for OAs

and most of them are focused on optical fibre amplifiers (OFAs), which are commonly

deployed in commercial optical network systems. Recently, semiconductor optical amplifiers

(SOAs) have attracted attention for applications in gigabit passive optical network (GPON)

and 100 Gbit Ethernet (GbE) systems. This is because SOA chips are as small as laser

diodes (LDs) and only require an electrical current.

Although SOAs for the 1 310 nm or 1 550 nm bands have been extensively studied since the

1980s, the use of SOAs is still limited to laboratories or field trials. This is due to specific
performance features of SOAs such as gain ripple and polarization dependent gain (PDG).
Thus, there are very few IEC standards addressing SOAs. One example is IEC/TR 61292-3,
which is a technical report for classification, characteristics and applications of OAs including
SOAs. However, it only deals with general information on SOAs and does not contain the
detail information on test methods that are necessary to measure precisely the particular
parameters of SOAs.
This technical report provides a better understanding of specific features of SOAs as well as
information on measuring gain and PDG. It is anticipated that future standards will address
performance and test methodology.

TR 61292-9 © IEC:2013(E) – 7 –

OPTICAL AMPLIFIERS –
Part 9: Semiconductor optical amplifiers (SOAs)

1 Scope
IEC/TR 61292-9, which is a technical report, focuses on SOAs, especially the specific

features and measurement of gain and PDG.
In this report, only the amplifying application of SOAs is described.
Other applications, such as modulation, switching and non-linear functions, are not covered.
Potential applications of SOAs, however, such as reflective SOAs (RSOAs) for the seeded
wavelength division multiplexing passive optical network (WDM-PON), are briefly reviewed in
Annex A.
2 Terms, definitions, abbreviations and symbols
2.1 Terms and definitions
For the purposes of this document, the following terms, definitions, abbreviations and symbols
apply.
2.1.1
SOA
generic term that includes the “SOA chip” and the “SOA module”
2.1.2
SOA chip
semiconductor chip which is the active component of the SOA module
2.1.3
SOA module
fibre-pigtailed optical component that consists of the SOA chip, lenses, optical isolators (if
necessary), a thermoelectric cooler (TEC), a thermistor, a package and fibres

2.2 Abbreviations
AR anti-reflection
ASE amplified spontaneous emission
BPF band pass filter
CFP 100 Gbit form-factor pluggable
CW continuous wave
DEMUX demultiplexer
DFB distributed feedback
EDFA erbium-doped fibre amplifier
FWM four-wave mixing
GbE gigabit Ethernet
GPON gigabit capable passive optical network

– 8 – TR 61292-9 © IEC:2013(E)

LD laser diode
MSA multi-source agreement
MMI multi-mode interference
MQWs multiple quantum wells
NF noise figure
OA optical amplifier
OFA optical fibre amplifier
OLT optical line termination
ONU optical network unit
OPM optical power meter
PC polarization controller
PD photodiode
PDCE polarization dependence of coupling efficiency
PDG polarization dependent gain
PIC photonic integrated circuit
POL polarizer
PON passive optical network
RSOA reflective semiconductor optical amplifier
SLD superluminescent diode
SMF single mode fibre
SOA semiconductor optical amplifier
TE transverse electric
TEC thermoelectric cooler
TIA transimpedance amplifier
TM transverse magnetic
VOA variable optical attenuator
WDM wavelength division multiplexing
XGM cross gain modulation
XPM cross phase modulation
2.3 Symbols
G optical gain
I forward current
F
T TE mode confinement factor
TE
T TM mode confinement factor
TM
L chip length
n effective refractive index
eff
NF noise figure
n spontaneous emission factor
sp
λ wavelength
∆λ period of gain ripple
ripple
PDCE polarization dependence of coupling efficiency
PDG polarization dependence of active layer gain
active
PDG total polarization dependence of single pass gain
total
TR 61292-9 © IEC:2013(E) – 9 –

R reflectivity
Ripple peak to peak amplitude of gain ripple

3 Specific features of SOAs
3.1 SOA chips
IEC  2672/13
Figure 1 – Schematic diagram of the typical SOA chip
Figure 1 shows the schematic diagram of the SOA chip. Similar to LDs, SOA chips are less
than 1,5 mm, 0,5 mm, and 0,2 mm in length, width and height, respectively. Since SOA chips
are made of III-V compound semiconductor materials and developed based on the
technologies used for LDs, the basic physical mechanisms of SOA chips are the same as
those of LDs. Therefore, the population inversion inside the SOA chip is implemented by a
forward current (I ) and the input optical signals are amplified by the stimulated emission of
F
photons in the active layer of the chip. The cross section of the typical active layer is 1,5 µm
and 0,1 µm in width and thickness (height), respectively.

0 50 100 150 200
Forward current I (mA)
F
IEC  2673/13
Figure 2 – Example of gain dependency on forward current of the SOA chip
Optical gain G (dB)
– 10 – TR 61292-9 © IEC:2013(E)

Figure 2 shows the gain dependency on I , which is injected into electrodes at the top and
F
bottom of the SOA chip as shown in Figure 1. The gain of the SOA chip is obtained and

adjusted by simply applying the current. As shown in Figure 2, by increasing the I to greater
F
than 150 mA, typical SOA chips can provide optical gain of greater than 20 dB with an input

optical power of around –20 dBm.

IEC  2674/13
IEC  2675/13
Figure 3 – Schematic top view of a typical SOA chip with
and without an angled waveguide structure
Compared with LDs, the most distinctive feature of SOAs is that the SOA chip has an anti-
reflection (AR) coating on both facets to avoid optical feedback between the facets. Since the
semiconductor materials have a much higher refractive index (>3 is typical) than air, the facet
has a reflectivity of 30 % or above. This feature is suitable for establishing a laser cavity but
not for the SOA chip which requires facet reflectivity of less than 0,1 % over a wavelength
range of greater than 30 nm. To achieve such a low reflectivity, AR coating is employed on
both facets of the SOA chips as shown in Figure 3. Figures 3a and 3b show schematic top

views of the conventional SOA chip and the SOA chip with an angled waveguide structure,
respectively. As shown in Figure 3a, a conventional SOA chip has a straight stripe, which is
normal to both facets where AR coating is applied. The AR coating consists of a multiple-layer
thin film. Each thickness (a quarter wavelength, for example) of the film is controlled within
±4 %. The residual reflectivity will cause intra-cavity interference between the facets, which
leads to gain ripple or laser oscillation. This is because the reflected light is easily coupled
θ) between the stripe and the
with the multiple reflections between the facets, since the angle (
facet is 90°. One of the best ways to suppress intra-cavity feedback is the introduction of an
angled waveguide structure. As shown in Figure 3b, the reflected light cannot be coupled by
multiple reflection when using the angled stripe with θ = 7°. This approach enables the SOA
chip to have a low facet reflectivity of 0,2 %, and less than 0,1 % when combined with AR
coating.
One of the other specific features of SOAs is that the gain-bandwidth of SOA chips can be
varied by only changing the composition of the semiconductor materials using mature LD
technologies (the band engineering technique). For example, long-wavelength (1 300 nm to
1 600 nm) SOA chips have an InGaAsP active layer on an InP substrate, and the peak

TR 61292-9 © IEC:2013(E) – 11 –

wavelength of the gain is adjusted by changing the respective concentrations of In, Ga, As

and P in InGaAsP. The typical gain bandwidth of SOA chips is greater than 40 nm.

Another specific feature of SOA chips is their integration with other semiconductor devices

such as tuneable LDs, electro-absorption modulators and passive waveguides on a single

chip. The integrated SOAs are used, for example, as booster amplifiers in tuneable LDs and

line amplifiers (loss compensators) in photonic integrated circuits (PICs).

In summary, SOAs have completely different physical mechanisms for amplification and for

the configuration of the device compared to OFAs.

3.2 SOA modules
IEC  2676/13
Figure 4 – Schematic top view of the typical SOA module
Figure 4 shows the schematic top view of the SOA module. An SOA chip, a TEC and optical
lenses may be assembled in a butterfly package which has fibre pigtails for the input and
output ports. This is the most common package for SOA modules and its size is almost the
same as that of 14-pin butterfly LD modules. The use of optical isolators (input and/or output)
may depend on the application. For example, optical isolators are not employed in SOA
modules for bidirectional amplification. The TEC is used to stabilize the case (package) since
more than 100 mA of electric current injected into the SOA chip will cause heat inside the chip

to affect polarization characteristics. Similar to LD modules, SOA modules are also
hermetically sealed with N gas.
3.3 Gain ripple
Optical feedback inside the SOA chip, which is the residual reflections from the chip facets,
may lead to round-trip resonances when an input optical signal is launched into the chip, as
shown in Figure 5.
– 12 – T
...