No. 95 · OCTOBER 2022

SUMITOMO ELECTRIC TECHNICAL REVIEWINFOCOMMUNICATIONSHigh-Optical-Output-Power and High-Responsivity
Integrated Coherent Transmitter-Receiver Optical
Sub Assembly for 800 Gbit/s Transmission
Munetaka KUROKAWA*, Masaru TAKECHI, Yasutaka MIZUNO,Taichi MISAWA, Eiichi BANNO, and Hiroshi HARA---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------A digital coherent optical transmission technology has been deployed in not only long-haul and metro networks but also data
center networks. Therefore, there is an increasing demand for a coherent module that can be incorporated in small-sized opticaltransceivers and realize high-speed and high-capacity transmission. Considering the demand, the Optical Internetworking Forumhas standardized an integrated coherent transmit-receive optical sub assembly (IC-TROSA) module in August 2019. This paper
presents an IC-TROSA type-2 module that integrates a tunable laser for 800 Gbit/s applications.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: coherent optical transmission, IC-TROSA, tunable laser, modulator, photodetector

1. Introduction
Internet traffic has been increasing significantly due
to the widespread use of social media, video-based content
delivery, and IoT communications in line with thecommencement of 5G networks. The current challenge is
how to respond to this rapid increase in data communications with fiber-optic communication technology.
Digital coherent optical transmission technology
increases capacity by transmitting not only the state of
optical intensity but also the state of phase as information,
thereby carrying more information in a limited band.
Digital signals are processed on both the transmitting sideand the receiving side to compensate for the degradation of
signal quality that occurs in optical fiber transmission,
thereby achieving long-distance transmission.
Coherent transmission systems with a transmission
speed of 400 Gbit/s have been applied to long-distancenetworks, such as core/metro networks between large
cities. Recently, in order to meet the abovementioned
market demand, standardization of coherent transmission
corresponding to a transmission speed of 800 Gbit/s has
been studied, and deployment of coherent transmission
technology between data centers with shorter transmission
distances has also been promoted.
In applications for data centers, there are demands forhigher transmission speed and smaller coherent modules.
Optical Internetworking Forum (OIF),*1 an industry standardization organization, formulated a standard for integrated coherent transmit-receive optical sub assembly(IC-TROSA).*2 IC-TROSA Type-2(1) is a coherent module
incorporating three main optical devices (i.e., a tunable
laser, a modulator, and a photodetector) and an IC forcontrolling these devices with a size of W 15.1 mm × L
30.0 mm × H 6.5 mm, which can be mounted in a
QSFP-DD*3 optical transceiver.(2)Previously, we developed a high-optical-output-power
tunable laser module, an optical receiver module,(3),(4) andan optical modulator module, and have demonstrated thepotential of InP-based devices capable of 400-Gbit/s transmission.

This paper reports a module design and the measured
characteristics of a coherent module that complies with the
IC-TROSA Type-2 standard and can be used for 800 Gbit/s
applications, by integrating the three functions into a single
package.

2. Configuration of IC-TROSA
Photo 1 shows the appearance of the IC-TROSA wedeveloped. Miniaturization and low profiling have been
attained to achieve a package size of W 14.1 mm × L 27.5mm × H 3.8 mm by using small parts and applying high-integration implementation technology.
A control board, in which ICs are mounted, isarranged at the top of the package and is connected via a
flexible printed circuit (FPC) to attain a total module thickness of 6.2 mm so that the design complies with the
IC-TROSA Type-2 standard. The control board is equipped

Photo 1. Appearance of IC-TROSA and package

High-Optica-Output-Power and High-Responsivity Integrated Coherent Transmitter-Receiver Optical Sub Assembly for 800 Gbit/s Transmission

16（1）SUMITOMO ELECTRIC TECHNICAL REVIEW

No. 95 · OCTOBER 2022with ICs to supply control voltages and currents to thetunable laser, modulator, and photodetector.
Figure 1 shows a model diagram of the IC-TROSA
mounted in the QSFP-DD. The package is sandwiched by atransceiver enclosure at the top and the bottom, and theheat radiation surface of the package is forced to come into
contact with the heat radiation surface of the transceiver. In
the IC-TROSA, the control board is mounted on the
package. Thus, ICs on the control board may become
damaged due to pressure from the enclosure. To cope withthis problem, metal posts are arranged on the four cornersof the package so that the ICs do not come into contact
with the package when pressure is applied and so that they
can withstand vibration and impacts. The design ensureshigh reliability and high heat radiation performance.

Diagonal view

driving the modulator, and a transimpedance amplifier(TIA) for amplifying photodetector output signals.
Thermoelectric coolers are also mounted for maintaining
the temperatures of the tunable laser and modulator atconstant levels.
The arrows in the figure show optical paths among thetunable laser, modulator, and photodetector. The outgoing
beam from the tunable laser is separated by the beam
splitter (BS) into local light for the photodetector andsignal light for the modulator. Part of the light separated to
the local light side is further separated into the wavelength
control function unit that stabilizes the oscillation wavelength.
Regarding the optical configuration on the transmitting side, the signal light is optically coupled with themodulator, the signal light is separated into four channels
in the modulator, and then electrical signals from thedriver-IC are converted into optical modulation signals.
Two pairs of quadrature-modulated signals are output fromthe modulator as X-polarization and Y-polarization opticalmodulation signals. The signals are then combined by thepolarization beam combiner (PBC) outside the modulator
and output from an optical fiber as modulation signals.
Regarding the optical configuration on the receivingside, the modulation signals input from outside via an
optical fiber are separated into X-polarization andY-polarization by the polarization beam splitter (PBS) andoptically coupled with the photodetector. The phase signals
are converted into intensity signals in the 90° hybrid opticalmixer*4 by interference with the local light. The signals arethen converted into electric current by the photodiode (PD)corresponding to the four channels for each signal component. Subsequently, the electric current is converted into
voltage and amplified by the TIA and demodulated as electrical signals.
To realize a size that complies with the standard, wedeveloped compact and multifunctional integrated semiconductor optical devices. Regarding the modulator, aMach-Zehnder (MZ) interference*5 structure is used.
Electrodes are formed in parallel with the optical waveguide, and phase modulation is performed by an electro-optic effect.*6 In general, to attain the desired modula-

DSP*

IC-TROSAQSFP-DD

Cross section

QSFP-DD

Direction ofheat radiation

Package

Height : 6.2 mm

ICs

Control board

Metal post (x4)

DSP

*Digital Signal Processor

Fig. 1. IC-TROSA mounted in the QSFP-DD

3. Module Design
The transmit signals are generated by multiplexing
two optical polarization (X-polarization and Y-polarization)
signals that are phase modulated by the multi-level
encoding modulation method.
The internal configuration of the coherent module isshown in Fig. 2. The coherent module incorporates atunable laser, a modulator, a photodetector, a driver-IC forLC

Receiver side
Optical input

Lens
PBS

Wavelength control
function unit
PD

First lensSecond lens

90ºhybridSigX
SSCs

Modulator

BS

Thermoelectric cooler

PD
PR

PD
PBC
Transmitting side
Optical output

90ºhybridLO

Tunable laser

LC

VOA

SigY

PR

8 PDs

Photodetector

TIA
YI-Ch
YQ-Ch
XQ-Ch
XI-Ch

High frequency wiring

YQ-Ch
Y-port

YI-Ch

X-port

XQ-Ch

FPC

XI-Ch

PD

Thermoelectric cooler

Driver-IC
PR: polarization rotator

Fig. 2. Configuration in IC-TROSA
High-Optica-Output-Power and High-Responsivity Integrated Coherent Transmitter-Receiver Optical Sub Assembly for 800 Gbit/s Transmission

17（2）SUMITOMO ELECTRIC TECHNICAL REVIEW

tion efficiency, there is a problem that the waveguidelengthens and the size of the device becomes longer. We
achieved a compact size of 4.0 mm × 4.4 mm andlow-voltage operation by using a structure wherein the
waveguide is folded 180° inside the device.(5) Regarding
the photodetector integrating 90° hybrid optical mixers, acompact size has been achieved by integrating the functions of each part, which are previously implemented as
separate parts, in the chip. Specifically, two 90° hybrid
optical mixers and eight PDs, four power monitor PDs andtwo variable optical attenuators (VOAs)*7 in each X-port
and Y-port input path, are integrated having a size of 2.6mm × 4.1 mm.(6),(7)For both the modulator and photodetector, a spot-size
converters (SSCs)*8 are integrated into the input/output
ports. The coupling efficiency between the semiconductor
optical devices, and also between the fiber and the modulator/ the photodetector are improved, and the implementation
tolerance during assembly was expanded.

No. 95 · OCTOBER 2022

power was high for both X-polarization and Y-polarization(6 dBm or higher). In the case of transmission based on the
32 QAM*10 modulation format, output of −6.0 dBm or
more is expected with a modulation loss of 13 dB.

Conventional optical system
First lens(alignment lens)Tunable laser

Focusing lensmodulator

Collimated light

Two lens optical system
First lensSecond lens(alignment lens)Collimated light

Tunable laser

4. Optical Characteristic
The tunable laser, modulator, and photodetector are
configured using separate semiconductor optical devices.
Optical parts, such as the beam splitter, must be mounted
between the optical devices. Thus, a spatial optical system
is used for optical connection between the optical devices.
The optical lens system was devised to achieve highoptical coupling efficiency between the tunable laser andthe modulator. A conventional optical system is shown in
the upper part of Fig. 3 (a), wherein the output light fromthe tunable laser is converted into collimated light by onelens and focused on the modulator using a different lens.
However, the tolerance of the lens for the tunable laser is
steep, and a problem exists with a reduction of coupling
efficiency due to misalignment of the fixed lens. We
applied a two-lens optical system as shown in the lower
part of Fig. 3 (a). Specifically, the output light of thetunable laser is turned into collimated light by the two-lensconfiguration. The second lens is used as an alignment lensto flatten the tolerance curve. The results of calculating thetolerance of the alignment lens are shown in Fig. 3 (b). For
example, when a lens whose focal distance is six times thatof the first lens is used as the alignment lens, the tolerance
can also be increased about six times. This effectsuppresses the decrease in coupling efficiency when thelens is fixed, and also effective in reducing fluctuations ofcoupling efficiency against the environment temperaturechange of the package.
In Fig. 4, the optical output power of X-polarization
and Y-polarization from IC-TROSA and the total optical
output power are plotted on the primary axis. The wavelength dependence of the difference in optical output powerof X-polarization and Y-polarization (PDL) is plotted on
the secondary axis. A tunable laser(8) that has a narrow linewidth and a high optical output power was used as the lightsource. The operating condition was 210 mA for the laser
diode (LD) electric current and 250 mA for the SOA*9electric current. Each MZ of the modulator was biased at
the maximum light transmission condition. The maximum
optical output power was 9.2 dBm. The optical output

Focusing lensmodulator

Coupling efficiency [dB](a) Optical configuration between tunable laser
and modulator

0.3 m

1.7 m

Lens offset [ m](b) Comparison of lens tolerance

Package temp. 35 °C

PDL [dB]

Optical output power [dBm]

Fig. 3. Optical system between tunable laser and modulator

Wavelength [nm]Fig. 4. Optical output power on the transmitting side and difference between
the X-polarization and the Y-polarization

High-Optica-Output-Power and High-Responsivity Integrated Coherent Transmitter-Receiver Optical Sub Assembly for 800 Gbit/s Transmission

18（3）SUMITOMO ELECTRIC TECHNICAL REVIEW

high-frequency design.
Figure 7 shows the measurement results of the electrical-optical (E/O) frequency response on the transmittingside of each channel. A 3 dB band of 57 GHz was attained,
which is capable for 800 Gbit/s operation, and flat characteristics were obtained from low to high frequency.
The measurement results of optical-electrical (O/E)
frequency response for a specific channel (XI) as a

Package temp. 35 °C

E/O response [dB]

Signal responsivity [mA/W]

Figure 5 shows the wavelength dependence of responsivities of the signal lights (Sig-X/Y) at a package temperature of 35°C. The voltage applied to the PD was 5 V, andthe TIA power supply voltage was 3.3 V. A responsivity of
62 mA/W or more was attained at a wavelength of 1550
nm. The deviation between the channels was confirmed tobe ±0.3 dB or less.
The package temperature characteristics of the sensitivity of the signal lights and local light (LO) are shown in
Fig. 6. Variations of sensitivity of ±0.3 dB or less wereobtained within a temperature range between −5°C and
75°C.

No. 95 · OCTOBER 2022

Package temp. 35 °C

Frequency [GHz]Fig. 7. Frequency response characteristics on the transmitting side

Wavelength [nm]Variation of responsivity [dB]O/E response [dB]Fig. 5. Wavelength dependence of signal sensitivity

Package temp. 35 °C
XI-Ch

Frequency [GHz]Fig. 8. Frequency response characteristics on the receiving side

Package temperature [°C]
Package temp. 35 °C
XI-Ch

RF CMRR [dB]Fig. 6. Temperature dependence of Signal and LO sensitivity variation

5. Frequency Response Characteristics
The frequency response depends on the performanceof the semiconductor optical device to be mounted, thecharacteristics of the driver-IC and TIA, and the high-frequency design between the FPC and the package, between
the package and the driver-IC, and between the package
and the TIA. We obtained good characteristics over a wide
bandwidth of 50 GHz or more by optimizing the characteristics based on combinations of the optical devices anddriver-IC/TIA and adjusting the overall impedance in the

Frequency [GHz]Fig. 9. CMRR during modulation

High-Optica-Output-Power and High-Responsivity Integrated Coherent Transmitter-Receiver Optical Sub Assembly for 800 Gbit/s Transmission

19（4）SUMITOMO ELECTRIC TECHNICAL REVIEW

function of TIA gain setting (from 0 to 511) are shown in
Fig. 8. A 3 dB bandwidth of 55 GHz was attained at the
optimum value of the TIA setting. In the photodetector,
demodulation signals are affected by the mixing ofcommon-mode signals with differential signals. Thus, ahigh common-mode rejection ratio (CMRR) is required. As
shown in Fig. 9, a CMRR of −15 dB or less wereconfirmed up to 55 GHz by optimizing the high-frequency
design.

6. Conclusion
We have developed a module that complies with the
IC-TROSA Type-2 standard by integrating the tunable
laser, modulator, photodetector, thermoelectric cooler, anddriver-IC for driving the modulator, and TIA for the photodetector into a single package with a size of W 14.1 mm ×L 27.5 mm × H 3.8 mm.(9)A 3 dB bandwidth of more than 55GHz, which
supports a transmission speed of 800 Gbit/s with a 32QAM
modulation format, was confirmed both on the transmittingside and the receiving side, achieving high optical output
power and high sensitivity.
We believe that this module contributes to smaller-sized and higher-speed digital coherent optical transceivers, and realization of an optical communication
system that will underpin the post-5G era.

Technical Terms＊1	Optical Internetworking Forum (OIF): An industry
organization on optical network technology that
promotes standardization.
Integrated Coherent Transmit-Receive Optical＊2	Subassembly (IC-TROSA): A coherent communication
module that integrates both receiving and transmitting
functions. There are two types: Type 1 and Type 2. In
Type 1, the tunable laser is installed outside themodule. In Type 2, the tunable laser is installed inside
the module.
＊3	Quad Small Form-factor Pluggable-Double Density(QSFP-DD): The transceiver size is 72.4 mm × 18.4mm × 8.5 mm.
＊4	90° hybrid optical mixer: A mechanism that outputs
in-phase components and quadrature components byinterfering the signal light with local light in thecoherent receiver.
＊5	Mach-Zehnder (MZ) interference: A waveguide structure
to distribute light by using multi-mode interference,
which occurs in the waveguide.
＊6	Electro-optic effect: A nonlinear optical phenomenon.
This is a technology using changes in the refractive
index when a voltage is applied.
Variable Optical Attenuator (VOA): A function to＊7	adjust the input/output light intensity.
＊8	Spot-Size Converter (SSC): A waveguide structure
whose function is to convert the mode field diameter.
Semiconductor Optical Amplifier (SOA): A＊9	
Semiconductor element that amplifies the input lightintensity by stimulated emission.

No. 95 · OCTOBER 2022＊10	Quadrature Amplitude Modulation (QAM): A method
for modulating two in-phase and quadratureamplitudes that are independent from each other.

References(1)	“Implementation Agreement for Integrated Coherent Transmit-Receive
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M. Takechi, Y. Tateiwa, M. Kurokawa, Y. Fujimura, H. Yagi, andY. Yoneda, “64 GBaud High-bandwidth Micro Intradyne Coherent
Receiver Using High-efficiency and High-speed InP-based Photodetector
Integrated with 90° Hybrid,” OFC2017, paper Th1A.2 (2017)(4)	
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Photodetectors Monolithically Integrated with 90° Hybrid toward Over400 Gb/s Coherent Transmission Systems,” IEICE Transactions onelectronics, Vol.E102-C, No.4, pp. 347-356 (Apr, 2019)(7)	T. Okimoto, H. Yagi, K. Ebihara, K. Yamazaki, S. Okamoto, Y. Ohkura,
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High-Optica-Output-Power and High-Responsivity Integrated Coherent Transmitter-Receiver Optical Sub Assembly for 800 Gbit/s Transmission

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No. 95 · OCTOBER 2022

Contributors The lead author is indicated by an asterisk (*).

M. KUROKAWA*• ‌Assistant Manager, Transmission Devices Laboratory

M. TAKECHI• ‌Senior Assistant General Manager, Transmission
Devices Laboratory

Y. MIZUNO• ‌Transmission Devices Laboratory

T. MISAWA• ‌Transmission Devices Laboratory

E. BANNO• ‌Assistant General Manager, Transmission Devices
Laboratory

H. HARA• ‌Senior Assistant General Manager, Transmission
Devices Laboratory

High-Optica-Output-Power and High-Responsivity Integrated Coherent Transmitter-Receiver Optical Sub Assembly for 800 Gbit/s Transmission

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