No. 96 · APRIL 2023

SUMITOMO ELECTRIC TECHNICAL REVIEWFEATURED TOPIC

53 GBaud Electro-Absorption Modulator
Integrated Lasers for Intra-Data Center
NetworksMasahiro HONDA*, Akira TAMURA, Kan TAKADA, Kenji SAKURAI,Hironori KANAMORI, and Kazuhiro YAMAJI---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------To cope with the rapid increase in data traffic, 400 Gbit/s optical transceivers have been introduced in data center opticalcommunication systems, requiring high-performance electro-absorption modulator integrated lasers (EMLs). We have developed
four EMLs with center wavelengths of 1271, 1291, 1311, and 1331 nm, which meet the specifications for 400 Gbit/s opticaltransceivers. This paper describes the design and typical characteristics of these new EMLs.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: EML, 53 GBaud, PAM4, data center, 400 Gbit/s optical transceiver

1. Introduction

Table 1. Target Specifications

As smartphones have come into wide use, SNS andvideo subscription services have grown, and teleworking
has become popular due to COVID-19, the volume of data
traffic has steadily increased. To be in line with this trend,
data centers have increasingly moved toward the introduction of 400 Gbit/s optical transceivers to their opticalnetworks. Various standards exist for 400 Gbit/s opticaltransceivers. For standards such as DR4*1 and FR4*2, theelectro-absorption modulator integrated laser (EML) isused for PAM4*3 operation at 53 GBaud modulation (transmission rate = 106 Gbit/s). To achieve 400 Gbit/s transmission, DR4 uses four EMLs with a center wavelength of1311 nm, while FR4 uses one EML each with center wavelengths of 1271, 1291, 1311, and 1331 nm.
Sumitomo Electric Device Innovations, Inc. hasdeveloped and manufactured EMLs for 10 Gbit/s and 25
Gbit/s operations(1)-(3). Drawing on these achievements, wehave recently developed EMLs capable of 53
GBaud-PAM4 operation (hereinafter referred to as “53
GBaud-PAM4 EMLs”) with center wavelengths of 1271,1291, 1311, and 1331 nm.

Specification

Unit

Signaling rate

53.125 ± 100 ppm

GBaud

Modulation format

PAM4-Wavelength

2. Device Design
2-1 Target specifications
Table 1 lists target specifications, for which wereferred to the 400GBASE-FR4 specification defined byIEEE 802.3cu. The 1271, 1291, 1311, and 1331 nm center
wavelengths are termed Lane 0, 1, 2, and 3, respectively.
The relative intensity noise (RIN) is an indicator of thelaser noise level. The transmitter dispersion eye closure
quaternary (TDECQ) is an indicator of the quality of PAM4
waveforms. By reducing the RIN and widening the 3 dB
bandwidth, it is possible to reduce the value of TDECQ.
2-2 Device structure
Photo 1 shows the newly developed 53 GBaud-PAM4
EML. The left side of the photo is a distributed feedback
laser oscillating in a single mode and the right side is amodulator. A direct current is supplied to the laser to emit

Parameter

Lane 01264.5 to 1277.5

Lane 11284.5 to 1297.5

Lane 21304.5 to 1317.5

Lane 31324.5 to 1337.5

nm

Average relative intensity noise≤ -147dB/Hz

3 dB bandwidth≥ 35

GHzTDECQ

≤ 3.4

dB

Optical modulation amplitude

≥ 5.5

dBm

Extinction ratio

≥ 3.5

dB

Photo 1. 53 GBaud-PAM4 EML

light at a certain intensity; a high-frequency electricalsignal is applied to the modulator to produce a high-frequency optical signal.
Figure 1 presents a schematic cross section of theEML. Semiconductor layers were grown on an n-type
indium phosphide (InP) substrate by metal organic chemical vapor deposition. Both the active layer of the laser andthe absorption layer of the modulator comprise multiple
quantum wells (MQWs). With these MQWs having
different structures, both layers were connected by buttjoint growth technology. To reduce the reflection and scattering of light at the butt-joint, the optical intensity distri53 GBaud Electro-Absorption Modulator Integrated Lasers for Intra-Data Center Networks

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distributed-feedback laserp-electrodeNo. 96 · APRIL 2023

opticalmodulator

p-InP
outputn-InPhigh-reﬂection
coating

gratingn-electrode

MQW active layer

anti-reﬂection
coating

MQW absorption layer

Fig. 1. Schematic cross section of EML

butions of the laser and modulator sections overlapped
well. For the material of the facet coatings, highly moisture-resistant dielectrics were selected. Additionally, thefront facet was coated with an anti-reflection coating toreduce the optical feedback to the laser, and the rear facet
was coated with a high-reflection coating to improve
optical output power.
2-3 Laser design
For the creation of a low-noise, high-power laser, it isnecessary to optimize the laser MQWs. Figure 2 shows
schematic band diagrams of laser MQWs. The well depth
is important for both conduction and valence bands. First,for the conduction band, deeper wells are preferable. If thewells are shallow, electrons, being light, escape fromshallow wells, degrading the characteristics [electron overflow, see Fig. 2 (a)]. In particular, care should be taken for
Lane 0, which requires a larger band gap. Second, for thevalence band, shallower wells are preferable. If the wells
are deep, holes, being heavier than electrons, are injected
only in the p-side wells, degrading the characteristics[non-uniform hole injection, see Fig. 2 (a)]. In this development, we optimized the structural parameters of laser
MQWs to achieve deep conduction band wells and shallow
valence band wells, as shown in Fig. 2 (b).

modulator quantum wells and wave functions observed
when the voltage applied to the modulator (VMOD) is 0 V(left) and when a reverse voltage is applied (right). The
reverse voltage application reduces the band gap. This is
known as the quantum-confined Stark effect (QCSE).
Figure 3 (b) presents a schematic diagram of opticalabsorption spectra. When VMOD is 0 V, modulator MQWs
barely absorb laser light because the edge of the absorption
spectrum is away from the lasing wavelength (λLD). On theother hand, reverse voltage application causes the edge ofthe absorption spectrum to shift toward the longer wavelengths due to the QCSE, allowing the modulator MQWs
to absorb laser light, resulting in a lowered optical outputpower. In short, it is possible to control the optical output
power by varying the voltage applied to the modulator.
To create a modulator with a high-bandwidth and ahigh-extinction ratio, important parameters are Δλ [difference between the lasing wavelength and the edge of theabsorption spectrum, see Fig. 3 (b)] and a modulator size.
When the Δλ is small, the laser light is mostly absorbed by
the modulator and the optical output power becomes
extremely low even if VMOD is 0 V; when the Δλ is large,
the modulator absorbs the laser light minimally and theextinction ratio decreases because the edge of the absorption spectrum remains away from the lasing wavelength
even under the application of a reverse voltage. In thisdevelopment, the Δλ was adjusted so that the extinction
ratio would achieve the target. Next, regarding the modulator size, smaller ones are better in terms of bandwidth
because the device capacitance decreases with decreasing
the modulator size. However, for higher extinction ratios, itis necessary to raise the amount of light absorption byusing a larger modulator size. In sum, a tradeoff exists
between the bandwidth and the extinction ratio. In thisdevelopment, we decided the modulator size by fully
taking this tradeoff into account.

(a)

VMOD = 0 V

VMOD < 0 V

conduction band(a)electron overﬂow(b)valence band

conduction
band

Optical absorption(b)valenceband
non-uniform hole injection

LDWavelength

Fig. 2. Schematic band diagrams of laser MQWs
Fig. 3. Operating principle of modulator: (a) schematic diagrams of modulator
quantum wells and wave functions and (b) optical absorption spectra

2-4 Modulator designFirst, this section describes the operating principle of amodulator. Figure 3 (a) shows schematic diagrams of53 GBaud Electro-Absorption Modulator Integrated Lasers for Intra-Data Center Networks

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No. 96 · APRIL 2023

3. Device Characteristics
3-1 Static characteristics
The optical and RIN spectra were evaluated as staticcharacteristics. For these evaluations, the temperature wasset to 60°C, the laser current was set to 100 mA, and themodulator voltage was set to 0 V.
Figure 4 shows the optical spectra. The bars indicate
the target wavelength ranges listed in Table 1. In all lanes,
the main peak is inside the indicated range, and the sidemode suppression ratios (main peak to sub-peak ratios) areapproximately 50 dB.

Lane 010Lane 1

Lane 2

Lane 30Intensity [dB]

-10

3-2 High-frequency characteristics
The frequency response and PAM4 optical waveforms
were evaluated as high-frequency characteristics. For theseevaluations, the temperature was set to 60°C and the laser
current was set to 100 mA.
Figure 6 illustrates the experimental setup used tomeasure frequency response. The electrical signal output
from the lightwave component analyzer traveled throughthe RF probe and entered the EML chip mounted on thesubmount. The optical signal emitted by the EML traveledthrough the optical fiber and entered the lightwave component analyzer. Figure 7 presents the measured frequency
response. In all lanes, the 3 dB bandwidth reached approximately 41 GHz, well above the target of 35 GHz. The pooranti-reflection coating on the front facet is known todegrade frequency response flatness and to adversely affectthe optical waveforms(4). The newly developed EML has analmost flat frequency response up to approximately 30GHz.

-20

DC Voltage

-30
-40

Bias Tee

-50
-60
-70
1260

1280

1300

1320

DC Current

Lightwave
Component
Analyzer

1340

Wavelength [nm]

RF Probe

SubmountFig. 4. Optical spectra

Fig. 6. Experimental setup for frequency response measurement

Figure 5 shows the RIN spectra. The average RIN
values were –152.1, –152.0, –152.1, and –152.0 dB/Hz at
Lane 0, 1, 2, and 3, achieving the target of –147 dB/Hz orless. Moreover, the maximum values observed around 10GHz were –147 dB/Hz or less, with the device certainly
demonstrating superb characteristics.

― Lane 0― Lane 1― Lane 2― Lane 3-140-145

RIN [dB/Hz]

E/O response [dB]

-135

3-150

0-3-9-155― Lane 0― Lane 1― Lane 2― Lane 3-601020304050

Frequency [GHz]-160

Fig. 7. Frequency responses-165-170

051015Frequency [GHz]Fig. 5. RIN spectra

2025Figure 8 illustrates the experimental setup used tomeasure PAM4 optical waveforms. The arbitrary waveformgenerator generated a 53.125 GBaud-PAM4 electricalsignal of the short stress pattern random quaternary(SSPRQ). Using an amplifier, the amplitude was adjusted
to 1.2 Vpp. As performed for the frequency response evaluation, the electrical signal traveled through the RF probe
53 GBaud Electro-Absorption Modulator Integrated Lasers for Intra-Data Center Networks

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No. 96 · APRIL 2023

Technical Terms

and entered the EML chip mounted on the submount. The
optical signal emitted by the EML was captured by theoscilloscope and the signal was processed by an equalizer.
Table 2 shows the measured PAM4 optical waveforms. In
all lanes, clear eye openings were obtained and the TDECQ
was favorable at 2 dB or less. Moreover, the extinction
ratio was approximately 5 dB and the optical modulation
amplitude was approximately 6.8 dB or more. Thus, weconfirmed that our EMLs meet all the target specifications
listed in Table 1.

DR4: The standardized specifications for optical＊1	transceivers with a transmission distance of up to 500m.
FR4: The standardized specifications for optical＊2	transceivers with a transmission distance of up to 2km.
＊3	PAM4: Four-level pulse amplitude modulation. The
amount of information handled is double that ofconventional two-level NRZ modulation.

DC Voltage
Arbitrary Waveform
Generator

Ampliﬁer

Bias Tee

References

RF Probe(1)	
H. Fukasawa et al., “Development of 10Gb/s TOSA/ROSA that
Operates over Wide Temperature Range,” SEI TECHNICAL REVIEW,
No. 71 (October 2010)(2)	H. Fujita et al., “25 Gbit/s Optical Transmitter Modules for OpticalTransceiver,” SEI TECHNICAL REVIEW, No. 80 (April 2015)(3)	R. Teranishi et al., “Integrated TOSA with High-Speed EML Chips for
up to 400 Gbit/s Communication,” SEI TECHNICAL REVIEW, No. 86(April 2018)(4)	C. Sun et al., “Influence of Residual Facet Reflection on the Eye-Diagram
Performance of High-Speed Electroabsorption Modulated Lasers,”
J. Lightw. Technol., vol. 27, no. 15, pp. 2970-2976 (August 2009)

DC CurrentClock

Submount
Oscilloscope
Fig. 8. Experimental setup for 53.125 GBaud PAM4 optical waveform
measurement

4. Conclusion
We have developed 53 GBaud-PAM4 EMLs for 400
Gbit/s optical transceivers with center wavelengths of1271, 1291, 1311, and 1331 nm. For low-noise and highpower operations, electron overflow and non-uniform hole
injection were suppressed by optimizing laser MQWs. In
addition, the Δλ and modulator size were optimized toachieve both high bandwidth and high extinction ratio. The
result was the average RIN of –147 dB/Hz or less and the 3dB bandwidth of 35 GHz or more. In the optical waveform
evaluation, the TDECQ was 2 dB or less, the extinction
ratio was approximately 5 dB, and the optical modulation
amplitude was approximately 6.8 dBm or more. The newly
developed 53 GBaud-PAM4 EMLs for 400 Gbit/s opticaltransceivers have superb performance.
Table 2. 53.125 GBaud-PAM4 optical waveforms
Lane0

Lane1

Lane2

Lane3

Optical waveform(Back to Back)

TDECQ [dB]

2.00

1.88

1.88

1.95

Extinction ratio [dB]

5.06

5.02

5.01

5.06

Optical modulation
amplitude [dBm]

6.87

6.83

7.43

7.42

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No. 96 · APRIL 2023

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

M. HONDA*• ‌Sumitomo Electric Device Innovations, Inc.

A. TAMURA• ‌Sumitomo Electric Device Innovations, Inc.

K. TAKADA• ‌Assistant General Manager, Sumitomo ElectricDevice Innovations, Inc.

K. SAKURAI• ‌Assistant General Manager, Sumitomo ElectricDevice Innovations, Inc.

H. KANAMORI• ‌Assistant General Manager, Sumitomo ElectricDevice Innovations, Inc.

K. YAMAJI• ‌Senior Assistant General Manager, SumitomoElectric Device Innovations, Inc.

53 GBaud Electro-Absorption Modulator Integrated Lasers for Intra-Data Center Networks24（5）