28Gbaud 16-QAM modulationwith compact driver modulefor InP MZM

Hitoshi Wakita1a), Munehiko Nagatani1, Shigeru Kanazawa2,Toshihiro Ito2, Eiichi Yamada2, Hiroyuki Ishii1,and Hideyuki Nosaka11 NTT Device Technology Laboratories, NTT Corporation,

3–1 Morinosato Wakamiya, Atsugi City, Kanagawa, Japan2 NTT Device Innovation Center, NTT Corporation,

3–1 Morinosato Wakamiya, Atsugi City, Kanagawa, Japan

a) wakita.hitoshi@lab.ntt.co.jp

Abstract: This paper presents a compact driver module for InP Mach-

Zehnder modulator (MZM). The size of the driver module is 14mm ×8mm × 2.8mm. We reduce the size of this driver module by integrating

two channels for the driver IC, which doesn’t require any external bias tee,

and by making the driver package small. With this driver module and InP

MZM module mounted on evaluation board, we obtained a well separated

16-QAM constellation at the speed of 28Gbaud.

Keywords: InP MZM, driver module, surface-mounted package

Classification: Optoelectronics, Lasers and quantum electronics, Ultrafast

optics, Silicon photonics, Planar lightwave circuits

References

[1] N. Kikuchi and S. Sasaki: J. Lightwave Technol. 28 (2010) 123. DOI:10.1109/JLT.2009.2035827

[2] N. Kikuchi, E. Yamada, Y. Shibata and H. Ishii: Proc. CSICS (2012) 150.DOI:10.1109/CSICS.2012.6340090

[3] H. Wakita, M. Nagatani, S. Ymanaka, H. Tanobe and H. Nosaka: Proc. APMC(2014) FR3A-1.

[4] S. Kanazawa, E. Yamada, T. Goh, N. Kikuchi, Y. Shibata, R. Iga, M. Koutokuand H. Ishii: IEICE (2014) C-4-9.

[5] E. Yamada, S. Kanazawa, A. Ohki, K. Watanabe, Y. Nasu, N. Kikuchi, Y.Shibata, R. Iga and H. Ishii: Proc. OFC (2012) PDP5A.9.

[6] N. Kashio, K. Kurishima, Y. Fukai, M. Ida and S. Yamahata: IEEE Trans.Electron Dev. 57 (2010) 373. DOI:10.1109/TED.2009.2037461

[7] K. Kurishima, M. Ida, N. Kashio and Y. Fukai: IEICE Trans. Electron. E95-C(2012) 1310. DOI:10.1587/transele.E95.C.1310

[8] M. Nagatani, Y. Bouvier, H. Nosaka and K. Murata: Proc. CSICS (2013) 127.DOI:10.1109/CSICS.2013.6659193

[9] H. Wakita, M. Nagatani, M. Mutoh, S. Tsunashima, H. Fukuyama, H. Nosaka,N. Kashio, M. Ida and K. Kurishima: IEICE Tech. Rep. no. 379, MW2013-195(2014) 115.© IEICE 2015

DOI: 10.1587/elex.12.20150656Received July 28, 2015Accepted September 28, 2015Publicized October 8, 2015Copyedited October 25, 2015

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LETTER IEICE Electronics Express, Vol.12, No.20, 1–6

1 Introduction

To deal with the increasing amount of data traffic in optical fiber networks, optical

multilevel transmission is being investigated. One technology for optical multilevel

transmission is 16-QAM transmission [1]. In this QAM system, 100-Gb/s signal is

transmitted at a single wavelength. At higher bit rates, such as 400Gbit/s, parallel-

izing the 100-Gbit/s system can deal with the increasing data traffic. However, this

approach increases the mounting area of components. It is therefore necessary to

reduce the size of the components or channelize them. For multilevel optical

transmission systems, an InP Mach-Zehnder modulator (MZM) is attractive as an

optical IQ modulator because it has a lower driving voltage and is in size smaller

than a lithium-niobate Mach-Zehnder modulator (LN-MZM) [2].

We previously reported the electrical characteristics of the compact quad-

channel driver module [3]. In addition to the effect of the small size itself, the

driver module was designed without any bias tee, which is normally needed in

driver modules for AC coupling between the driver and InP MZM. In this paper, we

present the results of optical 16-QAM modulation with this compact quad-channel

driver module and the InP MZM module, which was also reported [4, 5]. The

obtained 16-QAM constellation exhibits that this driver module can make 100-

Gbit/s or beyond 100-Gbit/s systems smaller.

2 Driver module

The driver IC was fabricated using InP HBT technology [6, 7]. To reduce the

module size, we designed the two-channel driver IC with a fully lumped config-

uration to save active IC area [8]. The driver IC has a fully differential two-stage

amplifier comprising a variable-gain amplifier (VGA) and an output buffer (Obuf )

as shown in Fig. 1.

For the output buffer, we designed a cascode amplifier with 50-Ω termination

resistors. The cascode transistors suppress the Miller effect and thus improve the

output bandwidth. The VGA and the output buffer were arranged in a lumped

configuration. The driver IC does not need any external bias tee for the output

buffer. The purpose of a bias tee is to obtain voltage headroom while keeping the

DC voltage of the output buffer at a low level. However, if we use an external bias

tee, the footprint of the driver module increases because the bias tee consists of

Fig. 1. Block diagram of driver IC per channel.

© IEICE 2015DOI: 10.1587/elex.12.20150656Received July 28, 2015Accepted September 28, 2015Publicized October 8, 2015Copyedited October 25, 2015

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large coils. In this IC, the output buffer absorbs the fall of the output DC operating

point by shifting power supply voltage (Fig. 2).

With this design, we were able to obtain a driver IC that does not require any

external bias tee. The dual channelization and bias-tee-less design can decrease the

footprint of the driver module. The power consumption of this driver IC is 0.95W

per a channel, and is sufficiently suppressed to implement in the compact trans-

ceiver of next generation. This driver module is made of ceramic and surface-

mounted. The target size of the module was 14mm � 8mm for assembly in

compact transceivers. The driver IC in this module was designed without the coils

needed for the bias tee. This reduces the footprint of the module because those coils

require a large mounting area. The driver module has broadband capacitors for DC-

voltage blocking in each channel, so no external DC block is necessary. The

capacitors are 0:4mm � 0:2mm for input and 0:6mm � 0:3mm for output. With

these small capacitors, we were able to reduce the distance between channels in the

module. The RF interface of the package is ground-signal-signal-ground (GSSG).

This also contributes to making the footprint of the driver module smaller because

there is no need for any ground plane between signal lines (Fig. 3(a), Fig. 3(b)).

Fig. 3(c) shows a top view of the quad-channel driver module with two driver ICs.

Fig. 2. Difference in DC operating point of output buffer with andwithout bias tee.

(a) (b) (c)

Fig. 3. Schematic of the driver module and its internal components. (a)1-ch driver module with bias tee [9]. (b) 4-ch driver module. (c)overview of the quad-channel driver module© IEICE 2015

DOI: 10.1587/elex.12.20150656Received July 28, 2015Accepted September 28, 2015Publicized October 8, 2015Copyedited October 25, 2015

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IEICE Electronics Express, Vol.12, No.20, 1–6

In the package, a coplanar waveguide (CPW) is used to ensure good broadband

frequency characteristics. The footprint per channel is 28mm2/ch, 1/8 of that the

221mm/ch2 in our previous report [9]. The external-bias-tee-less design is effective

to reduce mounting area of module.

Fig. 4 shows the differential small signal gain and output return loss of the

driver module. The 3-dB bandwidth is about 22GHz, and output return loss is less

than −10 dB up to 18GHz. It is thought that these characteristics are adequate for

operation at the speed of 28Gbaud.

The four-level output waveform at 28Gbaud is shown in Fig. 5(b). The output

amplitude is about 1.5Vpps (3.0Vppd), and we observed good multilevel eye

opening.

3 Optical 16-QAM modulation with driver module and InP MZM

module

We assembled a transmitter board with the driver and the InP MZM modules

(Fig. 6) and evaluated the performance of the modules with 16-QAM signals at

28Gbaud. The size of InP MZM module is 30mm � 14:6mm � 6:6mm [4, 5], and

is also compact. The InP MZM module has a dual I/Q modulator chip, which

contains four MZMs. The each MZMs have differential input. The RF V� voltage

of the MZMs is 3.0V. Each driving voltage to a modulation electrode in the 16-

Fig. 4. Differential small signal frequency responses of the drivermodule.

(a) (b)

Fig. 5. Measured time-domain response at 28Gbaud. (a) Input wave-form. (b) Output waveform.

© IEICE 2015DOI: 10.1587/elex.12.20150656Received July 28, 2015Accepted September 28, 2015Publicized October 8, 2015Copyedited October 25, 2015

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QAM modulation is a half of the V� voltage, which value is sufficiently small for

being driven by the driver IC. We investigated optical 16-QAM modulation with

these modules. Fig. 6 also shows a measurement system for single polarization (SP)

16-QAM operation. The wavelength of the modulated light was 1550 nm, and the

optical input power was 13 dBm. The operation temperature was 45 °C. The

electrical loss of the driver module and InP MZM module at 20GHz are

−1.5 dB and −1.0 dB, respectively. The electrical loss of transmitter board was

estimated to be 0.08 dB/mm. Therefore, in this transmitter board, we designed the

length of the RF line between driver module and InP MZM module to be less than

5mm to keep the 3-dB bandwidth over 20GHz for 28-Gbaud operation.

Fig. 7 shows the resulting constellation diagram obtained from measurement

system in Fig. 6. A well separated constellation was observed, and the error vector

magnitude (EVM) was 12.8%. This result conclusively demonstrates that this

compact quad-channel driver module is suitable for compact transceivers, such

as CFP2, for optical multilevel transmission.

Fig. 6. Measurement system for 16-QAM constellation and photo-graph of transmitter board.

Fig. 7. Measured 16-QAM constellation diagram at 28Gbaud.

© IEICE 2015DOI: 10.1587/elex.12.20150656Received July 28, 2015Accepted September 28, 2015Publicized October 8, 2015Copyedited October 25, 2015

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4 Conclusion

We described compact quad-channel driver and InP MZM modules and reported

modulation results using an evaluation board. The driver module size of

14mm � 8mm � 2:8mm was accomplished by designing a driver IC that integrates

two channels and requires no external bias tee. In addition to the driver IC design,

adopting small chip capacitors for DC blocking and GSSG via is also effective for

reducing the footprint of the driver module. The bandwidth of the driver module is

22GHz. The driver module provides good multilevel electrical output at 28Gbaud,

and the differential output amplitude is 3.0Vppd. A 16-QAM constellation at

28Gbaud was obtained from the transmitter board equipped with the driver and

InP MZM modules, and EVM was 12.8%. These results show that these modules

are good candidates for compact parallelizing optical 100-Gbit/s transmission

systems.

Acknowledgments

This work was supported in part by “The research and development project for the

ultra-high speed and green photonic networks” of the Ministry of Internal Affairs

and Communications, Japan.

© IEICE 2015DOI: 10.1587/elex.12.20150656Received July 28, 2015Accepted September 28, 2015Publicized October 8, 2015Copyedited October 25, 2015

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