Special Issue

Technologies supporting the achievement of Furukawa Electric Group
Medium-term Management Plan 2022 - 2025
Contribution to the achievement of SDGs
by Furukawa Electric Group

Integrated Coherent Transmit-Receive Optical Sub-Assembly(IC-TROSA) for Data Center Interconnects
Kazuya Nagashima*1, 2, Yozo Ishikawa*1, 2, Atsushi Izawa*1, 2, Masayoshi Nishita*3,
Noritaka Matsubara*2, Hiroyuki Ishii *2, Thanakit Saeyang*4, Masaki Kotoku*1, 2

There is a strong demand for highly integrated optical modules that can beincorporated into a small coherent optical transceiver to provide a high data
rate density in next-generation metro area networks and data center interconnects. This paper
presents a tiny Integrated Coherent Transmit-Receive Optical Sub-Assembly (IC-TROSA) that
integrates a wavelength-tunable laser, an indium phosphide (InP)-based transmitter, and aplanar lightwave circuit (PLC)-based receiver. Electrical-to-optical (E/O) and optical-toelectrical (O/E) bandwidths measure > 40 GHz, which is wide enough to support ≥ 400 Gb/s
operations.

ABSTRACT1.

INTRODUCTION

With the spreading usage of a high-quality video streaming, social media and Smartphone, Internet Traffic has
been rapidly increasing. A digital coherent optical transmission technology that combines a high performance
digital signal processor (DSP) and a coherent detection
has been developed to meet the growing demand forhigh transmission-capacity communication systems. The
digital coherent optical communication systems with atransmission capacity of 100 Gb/s/λ have been introduced into backbone long-distance optical networks
using a wavelength-tunable laser for wavelength divisionmultiplexing (WDM).
Recently, the digital coherent technology has also been
applied to shorter-reach applications, such as metro-area
networks and data center interconnects (DCI). In suchapplications, smaller optical transceivers that can be
operated at higher speed are required to provide a highd a t a - r a t e d e n s i t y. C o n s e q u e n t l y, t h e O p t i c a lInternetworking Forum (OIF), an industry standardization
organization, has issued an implementation agreement (IA)for 400ZR 1), which states a target performance for the
optical link with 400 Gb/s. The IA is meant to be applied tosmall optical transceivers such as Quad Small Form-factor
Pluggable Double Density (QSFP-DD) 2). Furthermore, the
800-Gb/s optical link 3), which requires twice the transmission speed, is also under discussion for future’s optical
Next Generation Photonics Business Innovation Project TeamR&D Division, Telecommunications & Energy Laboratories*3
Fitel Product Division*4
Furukawa FITEL (Thailand)*1*2

links. This organization also issued an IA for an Integrated
Coherent Transmit-Receive Optical Sub-Assembly(IC-TROSA) as a compact optical module to realize sucha small optical transceiver 4). The IA specifies two types,
Type-1 and Type-2, of highly integrated optical modules.
The IC-TROSA Type-1 integrates a dual-polarization
coherent modulator and a receiver in a single ball gridarray (BGA)-type package, whose size is 22.5 (L) × 15 (W)× 3.6 (H) mm. The Type-1 is supposed to be integrated
into an optical transceiver in combination with an external
wavelength-tunable laser. On the other hand, theIC-TROSA Type-2 needs to include a wavelength-tunable
laser in addition to these optical devices in a small goldbox package of 30.0 (L) × 15.1 (W) × 6.5 (H) mm.
Previously, we reported an ultra-compact low-power
Nano-Integrable Tunable Laser Assembly (Nano-ITLA)
using a distributed Bragg reflector/ring resonator monolithically integrated laser chip and an ultra-high-delta
PLC-based wavelength locker. It has a size small enoughthat can be mounted in the QSFP-DD together with an
IC-TROSA Type-1 and it exhibits good transmission characteristics at a 400-Gb/s optical link5). To further minimize
the optical module footprint and reduce the number ofcomponents, the IC-TROSA Type-2 is an attractive optical module. We successfully designed and fabricated an
IC-TROSA Type-2 for 400-Gb/s operation, which integrates the small ITLA, an InP-based transmitter and a
PLC-based receiver in a single package. In this paper, we
describe the design and characteristics of the IC-TROSA
Type-2. Furthermore, we also report the development status for ≥ 800-Gb/s operations that is required in future
optical links.
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Integrated Coherent Transmit-Receive Optical Sub-Assembly (IC-TROSA) for Data Center Interconnects

2.

DESIGN

Figure 1 shows a schematic illustration of the top view of
IC-TROSA Type-2. In our approach, we integrated awavelength-tunable laser, consisting of a monolithic-integrated laser and a PLC-based wavelength locker, a transmitter with an InP-based modulator, and a receiver with a
PLC-based coherent mixer in a hybrid manner to realize
high performance while keeping a simple fabrication process. The wavelength-tunable laser and the modulator
are located on separated thermo electric coolers (TECs)
to control their temperatures individually. The opticalmodule has receptacle-type LC connectors to attach
optical fiber cables easily. A quad modulator driver (DRV)
and a trans-impedance amplifier (TIA) transmit high-speed
signals through a flexible printed circuit (FPC) board
attached to the optical module. In order to achieve a lowcost optical system, a simple 2-lens system with a collimator lens and a focusing lens is implemented. The output light from the wavelength-tunable laser chip is collimated and divided, and then focused to be coupled into
each of the devices. The branching ratio is 75% to themodulator, 22.5% to the coherent mixer and 2.5% to thewavelength locker, respectively.
Figure 2 shows a schematic illustration of the wavelength-tunable laser chip. The laser chip has 2 types ofwavelength-selecting reflectors and a gain section. An
Gold wires

SOA is integrated at the output port to boost and control
the output power. Local micro heating elements areformed on each reflector to enable the lasing wavelength
to be tuned arbitrarily. The laser chip size is 3 (L) ×0.35 (W) mm. A schematic illustration of the PLC-based
wavelength locker is shown in Figure 3. This PLC consists of a SiO2-ZrO2 core that has a high refractive index
difference (Δ) of about 5%5), therefore the minimum bending radius of the waveguides can be small, and weachieved the miniaturized wavelength locker size of 1.7 (L)× 2.5 (W) mm. The wavelength-tunable laser module, which
was previously developed and reported, achieved±0.5 GHz of a wavelength locking accuracy over theoperating case temperature range, and < 100 kHz of anarrow linewidth over the full C-band wavelength range 6).
For the transmitter, we implemented an InP-based optical polarization division multiplexing IQ Mach‐Zehnder(MZ) modulator and a 4-channel modulator DRV. The InPbased modulator has characteristics such as a lower
insertion loss, lower half-wave modulation voltage (Vπ)
higher speed operation compared to those of silicon photonics (SiPh)-based modulators7). Figure 4 is a schematic
illustration of the InP-based modulator. The optical input/
output ports are deployed on the same facet in order tomake a small footprint of the optical module. Spot size
converters (SSC) are integrated in those ports to achievea good coupling efficiency and to broaden the allowable

Pins for DC power supply

4Ch.
TIA

RF PDs…

RF ﬂexible printed circuits

Input port
Coherent mixer

LC
receptacle

BS
Laser chip

IQ modulator

…

4Ch.
DRV

Isolator

Wavelength
lockerTEC
Polarization
beam combiner

IQ modulator

Output port

Pins for DC power supply
Figure 1

Schematic illustration of the IC-TROSA Type-2.

Micro heaters
SOA
DBR (Front mirror)
Gain area
Phase tuningareaPower monitor port
Input port

Output port 1

Output port 2

Ring resonator (Rear mirror)

Figure 2

Schematic illustration of the wavelength-tunable
laser chip.

Figure 3

Schematic illustration of the PLC-based wavelength
locker.

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Integrated Coherent Transmit-Receive Optical Sub-Assembly (IC-TROSA) for Data Center Interconnects

displacement of lens attachment. In addition, the modulator can extend an electrical bandwidth by adopting acapacitively loaded traveling wave electrode (CL-TWE).
These designs contributed to achieving a small modulator
chip size of 9.1 (L) × 3.3 (W) mm, a wide bandwidth of> 40 GHz, and a low driving voltage of 1.5 V 8). The modulator and the modulator driver are connected with gold
wires of 20-μm diameter.

O/E response of the PD with various reverse bias voltages
is shown in Figure 7(a). A 3-dB bandwidth of ≥ 49GHz
can be obtained when the reverse bias voltage ≥ 1V isapplied. To connect the electrodes of the PD and the TIA,
a glass substrate with high-speed transmission lines was
implemented. The transmission lines formed on the top
surface are connected to the TIA with wire bonding. A
common cathode electrode (C) is formed between two
anode electrodes (A) in the differential channels. To mini-

9.1 mm
10.6 mmDiﬀerential RF signals input ports (GSSG)

3.1 mmVT-SSC

Optical signal
input port
Optical signal
output port

CL-TWE
Figure 4

Normalized O/E response / dB

2V
3V

4V
5V

-2
-4

-3 dB

-6
-8
0

10

20

30

40

Frequency / GHz
Figure 750

RotatorLO
input port

Schematic illustration of the PLC-based coherent
mixer.

Cathode (C)Anode (A)Ground (G)8-channel lens
integrated back illuminatedphoto diode array
Glass substrate
Figure 6

Schematic illustration of the PD sub-assembly.

X-Ip
X-In

6

0

Signalinput port

PBS

Anode (A)(b)

290-degOptical Hybrid(Y polarization)

Monitoroutput port

VOAGold wire
Ground (G)Normalized O/E response / dB

0V
1V

4

-10

Y-Ip
Y-In
Y-Qp
Y-Qn

Figure 5

The receiver consists of a PLC-based coherent mixer,
an InGaAs PIN-PD array and a 4-channel TIA. A schematic illustration for the PLC-based coherent mixer is shownin Figure 5. A facet in the right-hand side has a local
oscillator (LO) input port, a signal input port and an output port for a monitor PD. A variable optical attenuator(VOA) and a polarization beam splitter (PBS) are placed in
the signal light path. The coherent mixer interferes withthe signal light and the LO light in a 90-degree opticalhybrid to extract the phase information. After dividing thelight into 8 ports from X-I to Y-Qn according to the phase,
its output is coupled to the PD array on the backside.
Figure 6 shows a schematic illustration of the InGaAs
PIN-PD array mounted on a substrate. We employed abackside-illuminated 8-channel PD array which integrates
a lens array on the backside to effectively enlarge theaperture size while maintaining a small active diameter.
Figure 7 shows optical-to-electrical (O/E) responses. The

6

Output ports
X-Ip
90-deg
X-In
Optical Hybrid
X-Qp(X polarization)
X-Qn

Phase-control electrodes

Schematic illustration of the InP-based modulator.

(a)

4.2 mm

CL-TWE

X-Qp
X-Qn

Y-Ip
Y-In

Y-Qp
Y-Qn

4
2
0
-2
-4

-3 dB

-6
-8
-10

0

10

20

30

40

50

Frequency / GHz

O/E responses (a) PD and (b) PD sub-assembly.

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Integrated Coherent Transmit-Receive Optical Sub-Assembly (IC-TROSA) for Data Center Interconnects

Coherent mixerPDsubassembly

Figure 10 shows a lasing spectrum of the built-in wavelength-tunable laser. We obtained a wavelength tuning
range of 42 nm, which covers the entire C-band range.
Figure 11 (a) shows misalignment tolerance curves of the
focusing lens at the modulator-input port. The verticalaxis is the normalized coupling loss, and the horizontal
axis is the amount of lens misalignment. Measured
horizontal coupling losses are plotted as red-square dots
and measured vertical coupling losses are plotted asblue-diamond dots. Solid lines are the theoreticalcharacteristics. The measured and the theoretical characteristics have a good agreement. Therefore, the lens isproperly aligned as designed. Furthermore, the 1-dB loss
width was ≥ 1 μm owing to integrated SSC, therefore theNormalized output power / dBmize electrical crosstalk, ground electrodes (G) areformed between signal lines. As a result, the electrodearrangement becomes in the order G-A-C-A-G. The
ground electrodes are connected with a gold wire to minimize the wiring length. The 8-channel PD array is mounted on this glass substrate with a flip-chip bonding process. The measured O/E response of the PD sub-assembly is shown in Figure 7(b). The reverse bias was set to
2 V. The O/E responses of all eight channels from X-Ip to
Y-Qn show similar characteristics, and a 3-dB bandwidthof around 40 GHz was obtained for all eight channels. As
shown in Figure 8, this PD assembly is attached to the
PLC-based coherent mixer with UV adhesive to minimize
the receiver size.

1520 1525 1530 1535 1540 1545 1550 1555 1560 1565 1570

Oscillation wavelength / nm

3.

Photograph of the coherent mixer with the PD subassembly.

CHARACTERISTICS

Figure 9 shows a photograph of the prototype IC-TROSA
Type-2 together with a dime. By minimizing the mechanical size of each component and adopting our hybrid-integration techniques, we successfully achieved a mechanical size of 26 (L) × 14 (W) × 4 (H) mm, which is smaller
than the specified size of the OIF’s IC-TROSA Type-2 IA.
In particular, the thickness is 2-mm thinner than the specified height. The smaller size can make wider clearances
between piece parts and accordingly, it helps to integrate
the TROSA into an optical transceiver together with control-ICs.

(a)
Normalized coupling loss / dB

Figure 8

Figure 10 Lasing spectrum of the IC-TROSA Type-2.

6
5
4
3
2

Horizontal

10

Vertical
-3

-2

-1

0

1

2

3

Misalignment of the lens / µm(b)Normalized coupling loss / dB

UV adhesive

6
5
4
3
2

Horizontal

10

Vertical
-4

-3

-2

-1

0

1

2

3

4

Misalignment of the lens / µm
Figure 9

Photograph of the IC-TROSA Type-2.

Figure 11 Coupling loss characteristics as a function of focuslens displacement for (a) a modulator and (b) acoherent mixer.

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Integrated Coherent Transmit-Receive Optical Sub-Assembly (IC-TROSA) for Data Center Interconnects

lens position alignment range is sufficient. Similarly,
Figure 11 (b) shows misalignment tolerance curves of the
focusing lens at the coherent mixer signal input port. The
results match the theoretical values in both horizontal andvertical directions. It indicates that the lens is assembled
as designed.
Figure 12 shows insertion loss characteristics as afunction of wavelength. Red dots plot the loss on thetransmitter side. The insertion loss of the transmitter overthe entire C-band is within 13.6 dB, including the branching loss to the receiver side (approximately 1.25 dB) and
an isolator loss. When the laser chip output-power is controlled to be 17.5 dBm, the output power from the
IC-TROSA is ≥ 3.9 dBm over the entire C-band. Similarly,
the insertion loss of the receiver is plotted as blue dots.
The loss is < 12.4 dB and almost constant regardless of
the wavelength.
15.0

Insertion loss / dB

14.5
14.0
13.5
13.0
12.5
12.0
11.5

Transmitter (TX)

11.0

Receiver (RX)

10.5
10.0
1530

1535

1540

1545

1550

1555

1560

1565

Wavelength / nm
Figure 12 Insertion loss characteristics as a function of
wavelength.

Then, we verified frequency response characteristics.
An electrical-to-optical (E/O) response characteristic of
the particular channel (X-Q) of the transmitter is shown in
Figure 13 (a). We fed an electrical signal to the IC-TROSA
using an RF probe to modulate the lightwave outputtedfrom the tunable laser in a room temperature condition.
The laser was operated at a constant temperature of

15

Transmitter (TX) X-Q channel

12
9
6
3
0
-3
-6
-9
-12
-15
0

10

20

30

40

50

DEVELOPMENT TOWARD A HIGHER
SPEED (≥ 800 GB/S) OPERATION

For future higher-speed optical links, we examined theemployment of higher-speed devices for the next-generation IC-TROSA Type-2 while maintaining its hybrid-integration concept. We implemented a modulator that has a
low loss and a wider bandwidth with an n-i-p-n heterostructure. CL-TWE structure is used for the RF electrodeand termination resistors are integrated on the chip. The
InP-based modulator exhibits higher E/O bandwidth of≥ 67 GHz, and Vπ is ≤ 2 V 9). This modulator and a
4-channel DRV with a bandwidth of ≥ 90 GHz are combined as an ultra-high bandwidth coherent driver modulator. A measured E/O response characteristic is shownin Figure 14 (a) 10). A 3-dB bandwidth of 75 GHz was
obtained even including the loss of the evaluation board.
Therefore, a wide bandwidth can be expected with the
IC-TROSA as well. Similarly, we combined a receiver chipthat integrates a coherent mixer and PDs, and a TIA
whose bandwidth is above 70 GHz. Figure 14 (b) shows ameasured O/E response characteristic of the receiverusing an RF probe. Although the measured data is limited
up to 50 GHz due to the limitation of the measurement
equipment, the bandwidth is obviously over 50 GHz. A
high-speed operation of ≥ 800 Gb/s can be expected.

(b)

18

-18

4.

Normalized O/E response / dB

Normalized E/O response / dB(a)

45°C. As shown in the figure, a 3-dB bandwidth of 45 GHz
was obtained. Figure 13 (b) shows an O/E response characteristic of the receiver's particular channel (Y-Q). Same
as on the transmitter side, an RF probe was used for themeasurement. Thanks to a peaking functionality of the
TIA, the measured 3-dB bandwidth was as high as
46 GHz and it was higher than that of the PD sub-assembly. These E/O and O/E bandwidths are sufficiently higher
than our target value of 40 GHz, which is required for a
400-Gb/s (64-Gbaud) operation.

18
15
12

Receiver (RX) Y-Q channel

9
6
3
0

-3
-6
-9
-12
-15
-18
-21
-24

0

Frequency / GHz

10

20

30

40

50

Frequency / GHz

Figure 13 Frequency response characteristics (a) E/O response and (b) O/E response.

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Integrated Coherent Transmit-Receive Optical Sub-Assembly (IC-TROSA) for Data Center Interconnects

(b)

18
15
12
9
6
3
0
-3
-6
-9
-12
-15
-18

Including evaluation
board loss

0

10

20

30

40

50

60

70

80

90

100

Normalized O/E response / dB

Normalized E/O response / dB

(a)

18
15
12
9
6
3
0
-3
-6
-9
-12
-15
-18
-21
-24

0

10

20

Frequency / GHz
Figure 14 Frequency response characteristics for ≥ 800 Gb/s transmission (a) E/O response

CONCLUSION

We demonstrated an IC-TROSA Type-2 by hybrid integration of following components in a single package, thoseare a wavelength-tunable laser, an InP-based transmitter,and a PLC-based receiver. This IC-TROSA achieves themechanical dimensions of 26 (L) × 14 (W) × 4 (H) mm,
which is smaller than the specified size in the OIF IA for
IC-TROSA Type-2. The built-in laser oscillation covers theentire C-band range. Low insertion losses of ≤ 13.6 dB on
the transmitter side and ≤ 12.4 dB on the receiver side
were achieved. We obtained E/O and O/E bandwidths of≥ 40 GHz, which are sufficiently wide for ≥ 400-Gb/s operations. We will develop the next-generation IC-TROSA
Type-2 capable of ≥ 800-Gb/s operation for supporting
the advanced information society in the future.

ACKNOWLEDGMENT

We would like to thank NTT Device Innovation Center for
their great cooperation in the achievement of theseresults, providing the high-frequency characteristics evaluation data of the 1-Tb/s-class ultra-high bandwidthcoherent driver modulator, which is the result of theirresearch and development.

40

50

Frequency / GHz
10)

5.

30

and (b) O/E response.

REFERENCES
1) Implementation Agreement for 400ZR, IA # OIF-400ZR-01.0,
March 10, 2020.
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3) OIF Launches 800G Coherent and Co-Packaging Framework IA
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		https://www.oiforum.com/oif-launches-800gcoherent-and-copackaging-framework-ia-projects-elects-newboard-memberspositions-officers-and-working-group-chairs/
4) Implementation Agreement for Integrated Coherent TransmitReceive Optical Sub Assembly, IA # OIF-IC-TROSA-01.0, August20, 2019.
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