Optical amplifiers - Part 9: Semiconductor optical amplifiers (SOAs)

IEC TR 61292-9:2017(E) which is a Technical Report, focuses on semiconductor optical amplifiers (SOAs), especially the specific features and measurement of gain and polarization dependent gain (PDG). In this document, 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. This second edition cancels and replaces the first edition published in 2013. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a) addition of new terms;
b) clarification of noise figure definition.
Keywords: semiconductor optical amplifiers (SOAs), gain and polarization dependent gain (PDG)

General Information

Status
Published
Publication Date
12-Dec-2017
Current Stage
PPUB - Publication issued
Completion Date
13-Dec-2017
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IEC TR 61292-9
Edition 2.0 2017-12
TECHNICAL
REPORT
colour
inside
Optical amplifiers –
Part 9: Semiconductor optical amplifiers (SOAs)
IEC TR 61292-9:2017-12(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TR 61292-9
Edition 2.0 2017-12
TECHNICAL
REPORT
colour
inside
Optical amplifiers –
Part 9: Semiconductor optical amplifiers (SOAs)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.160.10; 33.180.30 ISBN 978-2-8322-5134-8

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 61292-9:2017 © IEC 2017
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms, definitions, abbreviated terms and symbols .......................................................... 7

3.1 Terms and definitions .............................................................................................. 7

3.2 Abbreviated terms ................................................................................................... 8

3.3 Symbols .................................................................................................................. 9

4 Specific features of SOAs ................................................................................................ 9

4.1 SOA chips ............................................................................................................... 9

4.2 Gain ripple ............................................................................................................ 12

4.3 Polarization dependent gain (PDG) ....................................................................... 13

4.3.1 General ......................................................................................................... 13

4.3.2 Polarization insensitive SOAs ........................................................................ 14

4.4 Noise figure (NF) .................................................................................................. 14

4.5 Lifetime of carriers ................................................................................................ 14

4.6 Nonlinear effects ................................................................................................... 14

5 Measurement of SOA output power and PDG ................................................................ 15

5.1 Narrow-band versus broadband light source ......................................................... 15

5.2 Recommended set-up for output power and PDG measurements .......................... 16

5.3 Examples of measurement results obtained by using the recommended set-

up ......................................................................................................................... 17

Annex A (informative) Applications of SOAs ......................................................................... 21

A.1 General ................................................................................................................. 21

A.2 Polarization mode of SOAs ................................................................................... 21

A.3 Reach extender for GPON .................................................................................... 21

A.4 Pre-amplifier in transceivers for 100 Gbit Ethernet ................................................ 21

A.5 Monolithic integration of SOAs .............................................................................. 22

A.6 Reflective SOAs (RSOAs) ..................................................................................... 22

Bibliography .......................................................................................................................... 24

Figure 1 – Schematic diagram of the typical SOA chip .......................................................... 10

Figure 2 – Example of gain dependency on forward current of the SOA chip ......................... 10

Figure 3 – Schematic top view of a typical SOA chip with and without an angled

waveguide structure .............................................................................................................. 11

Figure 4 – Schematic top view of the typical SOA module ..................................................... 12

Figure 5 – Schematic diagram of the optical feedback inside the SOA chip ........................... 12

Figure 6 – Schematic diagram of gain ripple ......................................................................... 13

Figure 7 – Output power and PDG dependence on the wavelength of the SOA chip .............. 15

Figure 8 – Recommended measurement set-up for optical power and PDG of SOA

modules ................................................................................................................................ 16

Figure 9 – Recommended measurement set-up for optical power and PDG of SOA

chips ..................................................................................................................................... 17

Figure 10 – Optical power spectra of three different SOA chips............................................. 18

Figure 11 – Output power and PDG of the SOA chip sample no. 1 as a function of I ............ 18

---------------------- Page: 4 ----------------------
IEC TR 61292-9:2017 © IEC 2017 – 3 –

Figure 12 – Output power and PDG of the SOA chip sample no. 2 as a function of I ........... 19

Figure 13 – Output power and PDG of the SOA chip sample no. 3 as a function of I ............ 19

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

transceivers .......................................................................................................................... 22

Figure A.2 – Schematic diagram of the DFB-LDs-array type wavelength tuneable LD ........... 22

Figure A.3 – Schematic diagram of the seeded WDM-PON system ....................................... 23

---------------------- Page: 5 ----------------------
– 4 – IEC TR 61292-9:2017 © IEC 2017
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

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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.

This second edition cancels and replaces the first edition published in 2013. This edition

constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous

edition:
a) addition of new terms;
b) clarification of noise figure definition.
---------------------- Page: 6 ----------------------
IEC TR 61292-9:2017 © IEC 2017 – 5 –
The text of this technical report is based on the following documents:
Draft TR Report on voting
86C/1465/DTR 86C/1481/RVDTR

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 document has been drafted in accordance with the ISO/IEC Directives, Part 2.

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 document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document 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.
---------------------- Page: 7 ----------------------
– 6 – IEC TR 61292-9:2017 © IEC 2017
INTRODUCTION

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

current optical network systems. IEC TC 86/SC 86C 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 part of IEC 61292 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.
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IEC TR 61292-9:2017 © IEC 2017 – 7 –
OPTICAL AMPLIFIERS –
Part 9: Semiconductor optical amplifiers (SOAs)
1 Scope

This part of IEC 61292, which is a Technical Report, focuses on semiconductor optical

amplifiers (SOAs), especially the specific features and measurement of gain and polarization

dependent gain (PDG).
In this document, 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 Normative references
There are no normative references in this document.
3 Terms, definitions, abbreviated terms and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
SOA

semiconductor optical amplifier that includes the "SOA chip" and the "SOA module"

3.1.2
SOA chip
semiconductor chip that is the active component of the SOA module
3.1.3
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
---------------------- Page: 9 ----------------------
– 8 – IEC TR 61292-9:2017 © IEC 2017
3.1.4
population inversion factor

ratio of the injected carrier density N to the subtraction of the carrier density N where the

stimulated emission is equal to the stimulated absorption from N
n =
N − N

Note 1 to entry: In the semiconductor optical amplifier (SOA) field, the population inversion factor is composed of

not only carrier density parameters but also combination of the confinement factor Γ, the optical gain g, and

internal optical losses α of the optical waveguide of SOA chip. It is defined as:

N Γg
n = ×
N − N Γg − α

Note 2 to entry: The carrier density N where the stimulated emission is equal to the stimulated absorption may

be called "transparent carrier density".
3.2 Abbreviated terms
AR anti-reflection
ASE amplified spontaneous emission
BPF band pass filter
CFP 100G 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
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
---------------------- Page: 10 ----------------------
IEC TR 61292-9:2017 © IEC 2017 – 9 –
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
3.3 Symbols
G optical gain
I forward current
Γ TE mode confinement factor
Γ TM mode confinement factor
L chip length
n effective refractive index
eff
NF noise figure
n population inversion factor
l wavelength
∆l period of gain ripple
ripple
PDCE polarization dependence of coupling efficiency
PDG polarization dependence of active layer gain
active
total polarization dependence of single pass gain
PDG
total
R reflectivity
∆G peak to peak amplitude of gain ripple
ripple
4 Specific features of SOAs
4.1 SOA chips

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

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.
---------------------- Page: 11 ----------------------
– 10 – IEC TR 61292-9:2017 © IEC 2017
Stripe
Optical
Antireflection coatings
output
SOA chip
Active layer
Facet
Length
Electrodes
Facet
Width
Optical
input
IEC
Figure 1 – Schematic diagram of the typical SOA chip

Figure 2 shows the gain dependency on I , which is injected into electrodes at the top and

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

than 150 mA, typical SOA chips can provide optical gain of greater than 20 dB with an input

optical power of around –20 dBm.
0 50 100 150 200
Forward current I (mA)
IEC
Figure 2 – Example of gain dependency on forward current of the SOA chip

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 3 a) and 3 b) show schematic top

views of the conventional SOA chip and the SOA chip with an angled waveguide structure,

respectively. As shown in Figure 3 a), 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

Optical gain G (dB)
Height
---------------------- Page: 12 ----------------------
IEC TR 61292-9:2017 © IEC 2017 – 11 –

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

with the multiple reflections between the facets, since the angle (θ) between the stripe and the

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.
SOA chip
Stripe
Facet
Reflected light
Optical Optical
input output
Facet
AR coating
AR coating Electrode
IEC
3 a) – Schematic top view of the conventional SOA chip
SOA chip
Stripe (angled waveguide structure)
Reflected light
Optical
Facet
output
θ angle
Optical
input
Facet
Electrode
AR coating AR coating
IEC
3 b) – Schematic top view of the SOA chip with angled waveguide structure
Figure 3 – Schematic top view of a typical SOA chip with
and without an angled waveguide structure

One of the other specific features of SOAs is that the gain wavelength band 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

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

and P in InGaAsP. The typical gain wavelength band 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).
---------------------- Page: 13 ----------------------
– 12 – IEC TR 61292-9:2017 © IEC 2017

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

the configuration of the device compared to OFAs.
SOA modules

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 SOA chip 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

gas.
sealed with N
st SOA chip Heatsink
1 lens and holder
Electrode
Isolator
(optional)
Package
Lens holder
Window
Ferrule holder
Fibre
Optical
Ferrule
output
2 lens
Optical
path
Thermistor
Submount
TEC
IEC
Figure 4 – Schematic top view of the typical SOA module
4.2 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.
Input signal
SOA chip
Facet Facet
IEC
Figure 5 – Schematic diagram of the optical feedback inside the SOA chip
---------------------- Page: 14 ----------------------
IEC TR 61292-9:2017 © IEC 2017 – 13 –

The round-trip paths will interfere constructively or destructively depending on the wavelength

of the signal light. This gain dependence on the wavelength is called gain ripple, as shown in

Figure 6.
ripple
Wavelength
IEC
Figure 6 – Schematic diagram of gain ripple

With a chip gain G and facet reflectivity R, the peak-to-peak amplitude of the gain ripple

∆G is shown as Equation (1).
ripple
(1+ G × R)
∆G = (1)
ripple
(1− G × R)

With a signal wavelength l, chip length L, and effective refractive index n , the period of the

eff
gain ripple ∆l is derived from as Equation (2).
ripple
Δl = (2)
ripple
2n L
eff

For example, the SOA chip with n = 3,4, L = 1,2 mm, and l = 1 550 nm has the ∆l of

eff ripple

0,29 nm, approximately. Since the SOA chip has the birefringence between the parallel

transverse electric (TE) and orthogonal transverse magnetic (TM) directions to the chip

substrate, ∆l depends on the polarization mode of the input light.
ripple
4.3 Polarization dependent gain (PDG)
4.3.1 General

The PDG of the SOA chips is mainly caused by the difference of confinement factors between

the TE and TM modes. Generally, the cross-section of the active layer has an anisotropic

structure because the thickness of the active layer (e.g. 0,1 µm) is smaller than its width

(e.g. 1,5 µm). This results in larger TE mode confinement factor (Γ ) than the TM mode

confinement factor (Γ ), which means the TE mode gain will be larger.

PDG is one of the most significant characteristics of SOA modules. The total polarization

dependence of single pass gain (PDG ) in SOA modules is known to be the sum of the

total
) in SOA chips and the polarization
polarization dependence of the active layer gain (PDG
active

dependence of the coupling efficiency (PDCE) between a fibre and a facet at both input and

output ports in SOA modules.
Optical gain
---------------------- Page: 15 ----------------------
– 14 – IEC TR 61292-9:2017 © IEC 2017

In general, SOA chips have an elliptical mode field, so the PDCE will not be zero. Therefore,

unless a specific design is implemented for the PDCE, SOA modules might have a certain

amount of PDG even if the PDG is zero.
total active
4.3.2 Polarization insensitive SOAs
4.3.2.1 General

As described in 4.4.1, the polarization sensitivity of SOA chips is mainly caused by the

difference between Γ and Γ . To achieve the polarization insensitivity, decreasing or

TE TM

compensation of the difference will be needed. It has been reported that the following

techniques lead to a PDG of less than 0,5 dB.
total
4.3.2.2 Bulk active layer with square cross-section waveguide stru
...

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