Special Feature

Compact Low-power-consumption
Optical Modulator
Eiichi Yamada†, Ken Tsuzuki, Nobuhiro Kikuchi, andHiroshi Yasaka
Abstract
Optical modulators are indispensable devices for optical fiber communications. They turn light emitted
from a laser diode on and off at ultrahigh speed and generate high-speed optical signals. By employing
the “n-i-n structure” in the core layer, NTT Photonics Laboratories has developed an optical modulator
that is less than one-tenth the size and consumes one-third the power of commercially available devices.
This success is a major step toward broadband technologies that will reduce the load on the environment.

1. Introduction
With the progress of broadband technology, moreand more network equipment is being installed. Inaddition, to handle the rapid increase in traffic, largecapacity large-scale networks are being constructed.
All of that network equipment consumes a lot ofpower, which puts a heavy load on the environment.

The best way to reduce the power consumption is tobuild individual components that consume as littlepower as possible. NTT Photonics Laboratories,
which researches and develops various optical components for optical communications, has recentlysucceeded in developing a compact low-power-consumption semiconductor optical modulator by introducing a new structure [1].

Node interface system···Electrical signal
Electrical signal

Optical input
fiber

Optical signal
receiving &electrical signal
processing circuit

Optical output
fiberElectrical signal
Opticalsignal
Opticalsource

Opticalmodulator

Optical signal generating circuit

Fig. 1. Schematic view of a node interface system using an optical modulator.
† NTT Photonics Laboratories
Atsugi-shi, 243-0198 JapanE-mail: eyamada@aecl.ntt.co.jp

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2. Conversion of electrical signals to optical signals
As shown in Fig. 1, in optical fiber communication
networks, nodes installed in telecommunicationbuildings process optical signals received throughoptical fibers and send them out through other opticalfibers. To enable them to be transmitted via opticalfiber, the electrical signals must be converted into
intensity-modulated optical signals. Since the modulation speed in access systems is relatively low, thisconversion is performed by directly modulating theoptical source, normally a laser diode (LD), withelectrical signals. However, direct modulation of an
LD causes a change in lasing wavelength, which iscalled chirping. Chirping poses a serious problem in
optical fiber transmission in the backbone systems
because it limits the modulation speed and the transmission distance. The solution is external modulation
because it produces very little wavelength chirping.
In external modulation, an optical modulator converts
the fixed-intensity light emitted by the LD into an
intensity-modulated optical signal.
3. Characteristics of various modulators
Most optical modulators use the electro-absorption(EA) effect in a semiconductor or the electrooptic(EO) effect in a dielectric material such as LiNbO3.
Table 1 summarizes the characteristics of commercially available optical modulators and of those produced in this work. Various improvements, such asreduced power consumption, have currently been
made in such modulators.
An EA modulator is compact and fast and can be
driven at voltages below 2 V. An EA modulator
monolithically integrated with an LD provides anoptical source with a modulation speed of 10 Gbit/s.
At present, EA modulator research is focused on
improving performance and reducing cost, and EA
modulators with a low driving voltage of 0.7 V [2]Table 1. Characteristics of commercially available opticalmodulators and of those produced in this work.
Driving
voltage

Modulation
speed

Chipsize

EA modulator

2V

10 Gbit/s< 1 mm

LiNbO3 modulator

3–5V

10–40 Gbit/s~ 60 mmThis work

1V

10–40 Gbit/s~ 5 mm42and 0.79 V [3] have been developed. On the otherhand, optical modulators using the EO effect havebeen used in optical communication networks by
NTT since the mid-1990s. The EO effect is a phenomenon where the refractive index changes when anelectric field is applied to a solid or a liquid. Thechange in the refractive index is proportional to theapplied electric field. This is called the Pockels effect,
and it is commonly used for optical modulators.
The phase modulator and intensity modulator are
optical modulators that use the EO effect. The phasemodulator makes direct use of the phase change
caused by the refractive index change. The intensity
modulator is composed of a Mach Zehnder interferometer, in which the phase change is converted into
an intensity change. Since there is very little wavelength chirping when a Mach-Zehnder modulator isused, it is the most popular optical modulator for
long-distance backbone networks. Recently, there
has been a trend for adopting a modulation format
that includes phase modulation to improve spectral
efficiency. Therefore, there is a growing need for
optical modulators that use the EO effect, from which
the phase modulation is derived.
A Mach-Zehnder modulator made of LiNbO 3offers a large EO effect, small optical propagation
loss, and good coupling efficiency to fibers. However, the driving voltage of commercially available
LiNbO3 Mach-Zehnder modulators is as high as 3 – 5V, and the modules are as large as about 15 × 120mm2. In addition, a complex control circuit is needed
because a driving condition change called “DC drift”
occurs when DC voltage is applied. Attempts havebeen made to reduce the driving voltage of LiNbO3Mach-Zehnder modulators, and 40-Gbit/s modulation with driving voltages of 0.9 V has been demonstrated in a dual-drive operation [4]. Research on
Mach-Zehnder modulators made of semiconductors
such as GaAs and InP is proceeding. However, excess
optical propagation loss and the difficulty of achieving high-speed driving have prevented these modulators from being used in practice.
We have solved these problems by introducing the“n-i-n structure” in the semiconductor layers. This
novel structure allowed us to build a compact highspeed optical modulator with a low driving voltage.
4. Structure of the semiconductor
Mach-Zehnder modulator
A Mach-Zehnder modulator consists of opticalwaveguides. The cross-sectional geometry of a waveNTT Technical Review

Special Feature

BCB

Electrodes

n-InP
Electricfield

Sl-InP
i-MQW

n-InP
SI-InP substrate

BCB: benzocyclobutene

Fig. 2. Cross-sectional geometry of the n-i-n structure optical waveguide.

n-i-n structure waveguides
0, 1, 0, ···Phase modulation
regionOutput light

MMI
couplers

Input lightElectrical signal
termination

Input electrical signal

Fig. 3. Schematic structure of the n-i-n Mach-Zehnder modulator.

guide with the n-i-n structure is shown in Fig. 2. An
n-InP cladding layer, undoped InGaAlAs/InAlAs
intrinsic multi-quantum well (i-MQW), Fe-doped
semi-insulating (SI) InP layer, and n-InP upper
cladding layer were grown on the SI-InP substrate.
The electrical and optical signal losses of the n-type
cladding layer, which is formed on both the signal
and ground electrode sides, are less than 1/20th thoseof the p-type cladding layer normally used in semiconductor optical modulators. The Fe-doped SI-InP
layer between the n-InP upper cladding and undoped
waveguide core provides efficient current blocking.
The SI-InP layer and undoped waveguide core layer
are 1.0 and 0.3 µm thick, respectively. When voltage
is applied to these layers, the intensity of the electricfield increases greatly. Therefore, we can obtain an
efficient index change as a result of the EO effect.
Vol. 3 No. 3 Mar. 2005

Since the refractive index change of InP semiconductor material is 1/7th that of LiNbO3, the key tomaking an optical modulator using the EO effectpracticable is to be able to efficiently apply a highelectric field to the waveguide core layer. The widthof the n-i-n waveguide is 1/4th that of the waveguidein a LiNbO3 modulator, and the gap between electrodes is 1/30th to 1/50th. This makes it possible toapply 100 times the electric field and thereby perform
optical modulation with a low driving voltage. Thewidth of the waveguide and the gap between electrodes are very small in a semiconductor optical modulator. The excellent semiconductor micro-fabrication technologies developed by NTT Photonics Laboratories are what made the n-i-n structure possible.
The structure of an n-i-n Mach-Zehnder modulatoris schematically illustrated in Fig. 3. The modulator
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Special Feature

Vpp = 1.6 V

10 ps/div.

Fig. 5. Eye diagram at 40 Gbit/s. Driving voltage was 1.6 V.
Fig. 4. Photograph of the Mach-Zehnder modulator
chip.The chip size is 0.8 × 4.5 mm2.

consists of two 2 × 2 multimode interference (MMI)
couplers and two parallel phase-shift n-i-n structure
waveguides. First, input light is divided into two andsent into the two optical waveguides by one of thecouplers. Then, when the electric field is applied to
the n-i-n waveguide, the refractive index changes andthe phase of the light transmitted in the waveguidealso changes. The other MMI coupler couples the two
optical waveguides, and the coupled light is introduced to the output port after the two coupled lights
have interfered at the coupler. If the phases of the twolights coincide, the lights are summed and lightappears at the output port. If they are opposite, thelights cancel each other and there is no output light.
The traveling-wave electrodes on the optical waveguides are important. The modulation speed greatlydepends on the high-frequency characteristics of thetraveling-wave electrode. The n-i-n structure waveguide makes it easy to design the traveling-wave electrodes, and the electrodes are much shorter than thosein a LiNbO3 modulator. It is therefore easy to matchthe velocities of the electrical and optical waves, andan operation bandwidth of more than 40 GHz can beobtained. In the n-i-n structure waveguide, although
the optical confinement factor is large and refractive
index modulation is efficient, electrode resistanceaffects the performance because the electrode crosssection is very small. We have decreased the electrode resistance by introducing a 3-µm-thick Au electrode while maintaining the velocity and impedance
matching.
A photograph of the Mach-Zehnder modulator isshown in Fig. 4 in comparison with a matchstick. The
44

chip size is 0.8 × 4.5 mm2. The device length is lessthan 1/10th that of a conventional LiNbO3 modulator.
These modulators can be packaged in a very smallmodule.
5. 40-Gbit/s modulation with low driving voltage
There is strong demand for optical transmission at40 Gbit/s because of the recent increase in transmission capacity. Figure 5 shows an eye diagram
obtained for a 2 31 –1 pseudorandom bit stream(PRBS) at 40 Gbit/s using a device with 4-mm-longelectrodes. The driving voltage was supplied to one
arm of the modulator. We obtained clear eye opening
at a driving voltage of 1.6 V [5], which is about 1/3rdthat of a LiNbO3 modulator.
The Mach-Zehnder modulator has two travelingwave electrodes, and supplying them with differential
signals allows us to halve the driving voltage [6]. Then-i-n modulator can be driven at 1 V at 40 Gbit/s byemploying a push-pull drive configuration.
6. Conclusion
The n-i-n Mach-Zehnder modulator is very attractive as a high-speed modulator because of its low driving voltage, small size, and simple operation. Afterwe confirm its long-term stability and reliability, wewill use it in optical network systems for the next generation.

NTT Technical Review

Special FeatureReferences
Eiichi Yamada[1][2][3][4][5][6]K. Tsuzuki, T. Ishibashi, T. Ito, S. Oku, Y. Shibata, R. Iga, Y. Kondo,
and Y. Tohmori, “40 Gbit/s n-i-n InP Mach-Zehnder modulator witha π voltage of 2.2 V,” Electron. Lett., Vol. 39, No. 20, pp. 1464-1466,2003.
K. Watanabe, N. Kikuchi, H. Arimoto, H. Takano, M. Shirai, M.
Hashimoto, H. Kudo, and R. Takeyari, “Ultra-compact 40-Gbit/s
optical transmitter with a novel hybrid integration technique of electro-absorption modulator on driver-IC,” ECOC-IOOC 2003 Proceedings, Vol. 3, We2.5.1, pp. 448-451, 2003.
H. Fukano, M. Tamura, T. Yamanaka, Y. Kondo, and T. Saitoh,“Very low driving-voltage 40 Gbit/s electroabsorption modulators,”
Proc. 30th European Conference on Optical Communication (ECOC2004), Mo4.4.5, 2004.
M. Sugiyama, M. Doi, S. Taniguchi, T. Nakazawa, and H. Onaka,“Driver-less 40 Gb/s LiNbO3 modulator with sub-1 V drive voltage,”
Proc. Optical Fiber Communication Conference 2002 (OFC 2002),
Postdeadline Papers, FB6, 2002.
K. Tsuzuki, T. Ishibashi, T. Ito, S. Oku, Y. Shibata, T. Ito, R. Iga, Y.
Kondo, and Y. Tohmori, “1.6 V-driven 40-Gbit/s n-i-n Mach-Zehnder modulator based on InP substrate,” Proc. 9th. Optoelectronics andCommunication Conference 2004 (OECC 2004), 15E3-2, pp. 706707, 2004.
K. Tsuzuki, H. Yasaka, T. Ishibashi, T. Ito, S. Oku, R. Iga, Y. Kondo,
and Y. Tohmori, “10-Gbit/s, 100-km SMF transmission using an InPbased n-i-n Mach-Zehnder modulator with a driving voltage of 1.0Vpp,” Proc. Optical Fiber Communication Conference 2004 (OFC2004), Postdeadline Papers, PDP14, 2004.

Senior Research Engineer, Photonic Device
Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees in electrical engineering from Kyoto University, Kyoto
in 1987 and 1989, respectively. In 1989, hejoined NTT where he researched optical soliton
transmission. He is now engaged in R&D ofsemiconductor photonic devices. He received theBest Paper Award from the Institute of Electronics, Information and Communication Engineers(IEICE) of Japan in 1994. He is a member ofIEICE, the Japan Society of Applied Physics(JSAP), and IEEE/LEOS.

Ken Tsuzuki
Research Engineer, Photonic Device Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inapplied physics from Tohoku University, Sendai,
Miyagi in 1993 and 1995, respectively. In 1995,
he joined NTT Opto-electronics Laboratories,
where he engaged in R&D of semiconductor
photonic devices for optical fiber communication
systems. From 2001 to 2002, he was with NTT
Electronics Corporation. Currently, he isengaged in R&D of semiconductor photonicdevices. He is a member of IEICE and JSAP.

Nobuhiro Kikuchi
Photonic Device Laboratory, NTT Photonics
Laboratories.
He received the B.E. and M.E. degrees in electrical and electronics engineering from Kobe
University, Kobe, Hyogo in 1995 and 1998,respectively. In 1998 he joined NTT Optoelectronics Laboratories. Since 1999, he has beenwith NTT Photonics Laboratories, where he isresearching integrated semiconductor opticaldevices for WDM networks. He is a member of
JSAP and IEICE.

Hiroshi Yasaka
Senior Research Engineer, Supervisor, Photonic Device Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees inphysics from Kyushu University, Fukuoka in1983 and 1985 and the Ph.D. degree in electronics engineering from Hokkaido University, Sapporo, Hokkaido in 1993. In 1985, he joined theAtsugi Electrical Communications Laboratories,
NTT where he engaged in R&D of semiconductor photonic devices for optical fiber communication systems. From 1996 to 1998, he was withNTT Optical Network Systems Laboratories,
where he engaged in R&D of photonic transportsystems. From 2000 to 2002, he was with NTT
Electronics Corporation as a technical manager.
He is currently engaged in R&D of semiconductor photonic devices and their monolithicallyintegrated devices. He is a member of IEICE,
JSAP, the Physical Society of Japan, andIEEE/LEOS.

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