〉〉 Communications Solutions

The Development of a Nano-ITLA for Digital Coherent
Datacenter Interconnects
Masayoshi Nishita*1, Noritaka Matsubara* 1, Junichi Hasegawa* 2, Kazuki Yamaoka* 3,
Maiko Ariga* 3, Yusuke Inaba*1, Masayoshi Kimura* 4, Masaki Wakaba*1, Masahiro Yoshida*1,Kazuomi Maruyama*1, Toshihito Suzuki* 5, Hiroyuki Ishii*1, Yasumasa Kawakita*1
In the compact optical transceivers for datacenter interconnects using digital
coherent technology, a compact and low power consumption wavelength tunable light source is required.
We have developed a Vernier-type monolithic integrated tunable laser chip, that uses aDistributed Bragg Reflector (DBR) and a ring resonator reflector, also have developed extremelyhigh-Δ planar lightwave circuit (extremely-high-Δ PLC) wavelength locker. By mounting them on
the laser module, we have achieved a compact tunable light source Nano-ITLA with a control circuit which meets such requirements. We confirmed that, at a bit rate of 400 Gb/s, this Nano-ITLA
has excellent constellation characteristics, which is an optical output of 17 dBm and a spectral
linewidth of 100 kHz or less in the entire C band.

ABSTRACTINTRODUCTION
1.1. INTRODUCTION
The communication traffic continues to increase rapidly
due to the spread of mobile devices such as
Smartphones and the expansion of services such as thecloud computing, the video distribution, and the Social
Networking Service. Therefore, in order to further
increase the transmission capacity, a large capacitytransmission system of 100 Gb/s per wavelength using a
digital coherent communication that provides the coherent detection using a digital signal processing has been
introduced to the backbone long distance network. Since
the digital coherent technology uses the phase information of the light, the light source is required to have low
phase noise, that is, narrow spectral linewidth.
We have been developing the Micro-ITLA, which is anarrow linewidth wavelength tunable light source with acontrol circuit that meets such requirements1). As a wavelength tunable method, a method that combines aDistributed Feedback (DFB) laser array with an excellent
wavelength stability and a tunable system adjusted by thetemperature has been used. Furthermore, in order toobtain narrow spectral linewidth, linewidth of 100 kHz orless has been achieved with a laser chip that integrates aDistributed Reflector (DR) laser array and an Arrayed
Waveguide Grating (AWG) coupler 2)-4).
In recent years, there has been a direction to introduce
R & D Division, Communication and Energy Laboratory
R & D Division, Planning Department*3
Fitel Products Division*4
R & D Division, Core Technology Fusion Laboratory*5
Furukawa Fitel Optical Device Inc.
*1
*2

a digital coherent communication to the metro / accessareas also to datacenter interconnects, which are shortdistances. There, the compact optical transceivers aremounted at a high density, and therefore, further miniaturization and low power consumption of the light source
are required. However, it has been difficult practically tomeet such a requirement with a laser array type wavelength tunable laser.
Therefore, this time, we have developed a compact
laser module using a newly designed integrated wavelength tunable laser chip and a wavelength locker using
the extremely-high-Δ PLC technology. We will report also
on the miniaturization of the control circuit and theachievement of a compact and low power consumptionNano-ITLA.

DESIGN2.2. DESIGN
2.1 Wavelength Tunable Laser Chip
Figure 1 shows a schematic diagram of the configurationof the developed integrated laser chip. In the laser chip,
the laser section is equipped with two different types of
the wavelength selective type reflectors before and after
the gain region, and in the output end of the laser section,a Semiconductor Optical Amplifier (SOA) that further
amplifies the laser light in order to control the light output
is integrated. The rear side reflector is composed of a ringresonator and a Multi-Mode Interference (MMI) couplerthat splits the light into two. The front side reflector is aDBR. A micro heater is formed on each reflector, and it
makes it possible to control the wavelength spectrum of
the wavelength selective reflector by local heating.
Furukawa Electric Review, No. 52 202144〉〉 Communications Solutions

The Development of a Nano-ITLA for Digital Coherent Datacenter Interconnects

DBR

1

Ring reﬂector

0.8
0.6
0.4
0.2
1575

1570

1565

Wavelength / nm

1560

1555

1550

1545

1540

01535

(a) Calculation example of the reﬂection spectrum ofa DBR and a ring reﬂector.
Overlapping

0.4
0.3
0.20.1

1575

1570

1565

1560

1555

1550

1545

01540

The ring reflector generates periodic reflection peaks
determined by the circumferential length of the ring waveguide, and the reflector itself is compact, so that it is suitable for reducing the power consumption of the heater 5).
On the other hand, the phase of the DBR is cyclicallymodulated and the corresponding periodic reflection
peaks are generated. By optimizing the design, a characteristic with eight reflection peaks having almost the samereflectance is obtained. Since the number of peaks is limited, the wavelength tunable range are limited. Since theperiods of the reflected peaks of the DBR and the ringreflector are designed to be slightly different, it is possible
to select one wavelength in which the reflected peaks
overlap each other and oscillate the laser. As well as thepreviously reported wavelength tunable lasers 6)-8) using
DBRs on both sides, the wideband wavelength tunable
operation is possible by the Vernier scale principle, usingreflectors having two periodic reflection peaks. Figure 2(a) shows a calculation example of the reflection spectrum of the DBR and the ring reflector, and Figure 2 (b)shows the reflection spectrum of their overlap. In theexample of Figure 2, the laser oscillates at 1550 nm,
which has the highest overlap reflectance. By shifting thereflection spectrum of the DBR and the ring reflector to
the wavelength direction with their respective micro heaters, the wavelength with the highest overlap reflectancecan also be shifted, and the laser oscillation can be performed at the desired wavelength. This DBR / ring laser
chip is monolithically integrated on an Indium Phosphide(InP) substrate and is as compact as 3 mm × 0.35 mm. In
addition, an optical circuit is built in between the DBR
and the SOA to split the light and output it backwards.
The output light of this rear monitor port is used as awavelength locker. We have succeeded in developing theworld’s first integrated wavelength tunable laser chip ofthe Vernier type using such a DBR and a ring resonator
type reflector.
2.2 Laser Module with Integrated PLC Wavelength
LockerFigure 3 shows the configuration of the developed laser
module. It consists of a wavelength tunable laser chip, athermoelectric cooler, a beam splitter, an optical isolator,
lenses, photodiodes (PD) and a PLC wavelength locker. In

1535

Figure 1 Structure of the wavelength tunable laser chip.

1530

Ring resonator(Rear mirror)

1530

MMI coupler

1525

Phase
tuningsection1525

DBR (Front mirror)
Gain

Reﬂectance

SOAorder to reduce the module size, the extremely-high-Δ
PLC 9) that we have developed up till now was used forthe wavelength locker, instead of the spatial optical subsystem including the etalon filter, which is a bulk opticalcomponent. The extremely-high-Δ PLC consists of aSiO2-ZrO2 core and has a large specific refractive index
difference (Δ) of about 5%, so that we achieved the miniaturized wavelength locker size of 1.7 mm × 2.5 mm.
Figure 4 is an optical circuit diagram of the PLC. The input
light is split into three paths and then guided to one powermonitor output and two ring resonators. The length of thering resonators are 2565.0 µm and 2565.3 µm, respectively, and their resonance frequencies differ by 20 GHz.

ReﬂectanceMicro heaters

Wavelength / nm(b) Overlapping reﬂection spectrum
Figure 2 Calculation example of the reflection spectrum.
Thermoelectric cooler

FiberLens

PLCWL Locker

Tunable Laser
chipIsolator

Figure 3 Structure of the laser module.
Power monitor
Input

Output1

Output2

Figure 4 Optical circuit diagram of the PLC.

Furukawa Electric Review, No. 52 202145〉〉 Communications Solutions

The Development of a Nano-ITLA for Digital Coherent Datacenter Interconnects

Figure 5 shows the output characteristics of the PLC.
Since the light output intensity depends on the light frequency of the input light, the oscillation wavelength of thelaser can be specified from the light receiving intensity ofthe PD. By selecting two output characteristics and using
an appropriate ring resonator as a wavelength locker, thelaser output power can be controlled at an arbitrary
wavelength with a high accuracy.
Output1/Power monitorr

Output2/Power monitor

1
0.9
0.8

Power ratio

0.7
0.6
0.5
0.4

CHARACTERISTICS
3.3. CHARACTERISTICS
An example of the oscillation spectrum of the manufactured laser module is shown in Figure 7. A wavelength
tunable range of 41 nm was achieved with this device. By
adjusting the SOA current, the fiber output power was
17 dBm in the entire C band. To fully cover the C-band
wavelength, the maximum power consumption of themicro-heater required is 220 mW. Figure 8 shows thespectral linewidth. The phase noise spectrum was measured using a coherent receiver and the linewidth wascalculated from the white noise component of the measured phase noise spectrum. As shown in Figure, 8, thenarrow linewidth of less than 100 kHz were obtained forall the measured wavelength channels. Figure 9 shows
the accuracy of the laser output wavelength when thewavelength is stabilized using a PLC wavelength locker.
The accuracy of the laser output wavelength was within

0.3
0.2

200.1

Figure 5 Output characteristics of the PLC.

0Power / dBm

193.50193.48Frequency / THz

193.46193.44

193.42

193.40193.38193.36193.3410
193.32

193.300-10-20-30-40-50-60
1500 1510 1520 1530 1540 1550 1560 1570 1580 1590
Wavelength / nm

Figure 7 Oscillation spectrum characteristics.
300
Linewidth / kHz2.3 Nano-ITLA
Figure 6 shows a picture of the developed Nano-ITLA. In
Micro-ITLA, the control circuit is configured by using separate general-purpose parts. Here, the control circuit wasdownsized by using a new dedicated IC that puts thesetogether, and by combining it with the compact laser
module, the Nano-ITLA was miniaturized. The size ofNano-ITLA is 25.1 mm × 15.6 mm × 6.4 mm, and the volume
ratio to Micro-ITLA with 37.5 mm × 20.0 mm × 7.5 mm1) isless than half, thus a significant miniaturization wasachieved. It is a size that can be stored in a form factor
called QSFP-DD10) or OSPF 11), which is widely implemented in the compact optical transceivers used for datacenter interconnects.

250200150
1005001520

1530

1540 1550 1560
Wavelength / nm

1570

1580

15.6 mm (W)

6.4 mm (H)

25.1 mm (L)Figure 6 Picture of the developed Nano-ITLA.

Frequency accuracy / GHz

Figure 8 Spectral linewidth characteristics.
32.5
21.5
10.5
0-0.5
-1-1.5-2-2.5-3-10

Setting Frequency
191.44 GHz
193.32 GHz
195.63 GHz

010 20 30 40 50 60Case Temperature / degC

7080Figure 9 Wavelength locking characteristics.

Furukawa Electric Review, No. 52 202146〉〉 Communications Solutions

The Development of a Nano-ITLA for Digital Coherent Datacenter Interconnects±0.5 GHz when the case temperature was in the range of–5°C to 80°C. Therefore, it was confirmed that the wavelength could be controlled sufficiently and stably using
the PLC wavelength locker.
Figure 10 shows the results of a transmission / reception test of the developed Nano-ITLA in the actual digital
coherent communication. The test system for the digital
coherent communication is• Transmitting side: Laser for transmission+ optical modulator• Receiving side: Coherent receiver+ laser for local oscillator+ Digital Signal Processor (DSP)
The characteristics of a multi-level modulation pattern(called a constellation) were evaluated using a DSP thatassumed the transmission at 400 Gb/s. Tests were performed using 16 Quadrature Amplitude Modulation(QAM), 32 QAM, and 64 QAM as the three modulation
formats. First, the developed Nano-ITLA was used as atransmission laser, and for comparison, the laser for alocal oscillator on the receiving side was tested using an
external cavity laser with a spectral linewidth of less than
50 kHz as a reference laser. The results are shown in theupper part of Figure 10. Next, the reference laser and thisNano-ITLA were exchanged, the reference laser was usedas the transmission laser, and the evaluation was performed using this Nano-ITLA as the laser for the localoscillator. The results are shown in the lower part ofFigure 10. In both top and bottom rows, excellent constellations were detected in all three modulation formats,and there was no difference between the reference laser
and this Nano-ITLA. It has been shown that a monolithic
integrated wavelength tunable laser chip consisting of aDBR / ring reflector has narrow linewidth characteristics
comparable to those of a conventional external cavity
laser, and the developed Nano-ITLA can be used for boththe signal and the local oscillating lasers at a bit rate of
400 Gb/s.
DP-16QAM 66.5 Gbaud/s

CONCLUSION
4.4. CONCLUSION
We have developed a compact Nano-ITLA that uses acompact wavelength tunable laser module. In order toreduce the size and the power consumption, we havedeveloped a Vernier-type monolithic integrated tunable
laser chip, that uses a Distributed Bragg Reflector (DBR)
and a ring resonator reflector, also have developed
extremely-high-ΔPLC wavelength locker, and these aremounted on the laser module. We confirmed that, at a bit
rate of 400 Gb/s, this Nano-ITLA has excellent constellation characteristics, which is an optical output of 17 dBm
and a spectral linewidth of 100 kHz or less in the entire Cband. Therefore, this Nano-ITLA is useful as a lightsource for a compact optical transceiver for datacenterinterconnects using digital coherent technology.

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DP-32QAM 53.3 Gbaud/s

DP-64QAM 44.4 Gbaud/s

X

YX

YX

Y

X

Y

X

Y

X

Y

Figure 10 Constellation characteristics.
Top: T he Nano-ITLA is in the transmitting side, and the reference laser is in the receiving side.
Bottom: T he reference laser is in the transmitting side, the Nano-ITLA is in the receiving side.

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The Development of a Nano-ITLA for Digital Coherent Datacenter Interconnects〉〉 Communications Solutions

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