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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 3, MARCH 2004 733

A Quaternary Amplitude Shift Keying Modulator forSuppressing Initial Amplitude Distortion

Takuya Nakamura, Jun-Ichi Kani, Member, IEEE, Mitsuhiro Teshima, Member, IEEE, andKatsumi Iwatsuki, Member, IEEE

AbstractThis paper proposes a quaternary amplitude-shift-keying (4ASK) modulation circuit that suppresses amplitudedistortion of the transmitting 4ASK signal. The performance ofthe proposed circuit is quantitatively verified through numericalcalculation in comparison to a conventional alternative. Thefeasibility of the proposed circuit as integrated in a lithium niobatesubstrate is also demonstrated: the suppression of amplitudedistortion is successfully demonstrated and minimum sensitivityof the transmitted 4ASK signal is 1.7 dB better than that offeredby the conventional circuit.

Index TermsAmplitude distortion, eye-opening penalty,MachZehnder modulator (MZM), quaternary amplitude shiftkeying (4ASK).

I. INTRODUCTION

T HE rapid growth predicted for Internet protocol (IP)-basedservices demands that the capacity of wavelength-division-multiplexing (WDM) transmission systems already

installed in core networks and metropolitan networks be greatly

increased. Given the finite gain region of optical amplifiers,

this is possible by increasing the spectral efficiency (bit rate

per channel/wavelength channel spacing, or b/s/Hz), i.e., by

increasing the bit rate per channel and/or by narrowing the

wavelength channel spacing [1].

Table I shows the spectral efficiency of several modulationschemes, as well as effective bandwidths of optical band-path

filters (OBPFs) normalized to wavelength spacing, which in-

fluence the spectral efficiencies. Note that the values of spectral

efficiency achieved with polarization-multiplexing techniques

are not listed. The conventional binary nonreturn-to-zero (NRZ)

modulation scheme offers spectral efficiency of 0.4 b/s/Hz

[2]. The optical duobinary [3] and optical vestigial sideband

(VSB) [4] modulation schemes offer high spectral efficiencies

of 0.6 and 0.64 b/s/Hz, respectively. Optical-code-division

multiplexing (OCDM) can achieve the spectral efficiency of

0.8 b/s/Hz [5], but it demands the use of optical encoders and

decoders, which are relatively complicated.

Another candidate is quaternary amplitude shift keying(4ASK). The 4ASK signal has narrower spectral width than the

binary signal at the same bit rate and can improve the dispersion

tolerance [6] and spectral efficiency in WDM transmission [7].

However, in the conventional 4ASK modulation circuit [6], an

optical 4ASK signal is generated by modulating a continuous

Manuscript received June 30, 2003; revised October 22, 2003.The authors are with the Nippon Telegraph and Telephone Corporation,

NTT Network Innovation Laboratories, Kanagawa 239-0847, Japan (e-mail:[email protected]).

Digital Object Identifier 10.1109/JLT.2004.824465

lightwave with an electrical 4ASK signal that is obtained by

adding one binary signal to another intensity-halved binary

signal. Such configuration translates moderate amplitude dis-

tortion in the original binary signals into significant distortion

of an intermediate level in the optical 4ASK signal.

This paper proposes a novel 4ASK modulation circuit that

suppresses the amplitude distortion; the circuit generates an op-

tical 4ASK signal by optically combining one binary signal and

another intensity-halved binary signal. After describing the con-

figuration of the proposed circuit, its distortion suppression is

quantitatively verified by numerical calculation. It is also shown

that the proposed circuits yield 4ASK signals that achieve thespectral efficiency of 0.8 b/s/Hz. Next, the feasibility of the

proposed circuit as integrated on a lithium niobate (LiNbO )

substrate is confirmed: the suppression of amplitude distortion

is successfully demonstrated and the receiver sensitivity of the

transmitting 4ASK signal is found to be improved compared

with the conventional circuit.

II. CONFIGRATION OF MODULATION CIRCUITS

A. Conventional Modulating Circuit

Fig. 1 shows the configuration of a conventional 4ASK mod-

ulation circuit. Two electrical binary signals are used as orig-

inal signals; one of them is halved in terms of amplitude by

the attenuator. These signals are combined by the power com-

biner to generate an electrical 4ASK signal. The electrical bi-

nary signals are translated into an optical 4ASK signal via a

MachZehnder intensity modulator (MZM). Fig. 2 qualitatively

illustrates the electrical-to-optical (E/O) transfer characteristics.

The original binary signal has moderate amplitude distortion

due to overshoot, undershoot, ringing via electrical circuit, and

so on. Hence, the electrical 4ASK signal in Fig. 2 has amplitude

distortion in all levels, as it is translated from the original binary

signals. In the optical 4ASK output signal, amplitude distortion

at minimum and maximum levels (level 0 and 3) are suppressed

according to characteristics of the MZM, but those at interme-diate levels (level 1 and 2) arenot effectively suppressed because

the transfer characteristic of the MZM is approximately linear at

these levels. These degradations cause significant eye-opening

penalty (EOP) between levels 1 and 2, thus degrading the re-

ceiver sensitivity of the transmitting signal.

B. Proposed Modulating Circuit

Fig. 3 shows the configuration of the proposed modulation

circuit. In this circuit, the input continuous lightwave is divided

by an optical coupler and modulated by electrical binary signals

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734 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 3, MARCH 2004

TABLE ICOMPARISON OF MODULATION SCHEMES

Fig. 1. Configuration of conventional 4ASK modulation circuit.

Fig. 2. Qualitative E/O transfer characteristics of conventional 4ASKmodulation circuit.

via MZMs, thus yielding two optical binary signals. The inten-

sity of one is attenuated with respect to the other in an opticalcontrol section. An optical 4ASK signal is yielded by optically

combining the two binary signals. In this circuit, the amplitude

distortion of each electrical binary signal is suppressed by an

MZM. Accordingly, the yielded optical 4ASK signal has little

amplitude distortion.

The following describes the function of the optical phase con-

trol section in Fig. 3. If the electric fields of two optical binary

signals and are expressed as and , the elec-

tric field power of the output optical 4ASK signal is

expressed as

(1)

Equation (1) indicates that the phase difference between the two

optical binary signals influences the output power of the

4ASK signal. Fig. 4 shows the optical power of each levelversus

the phase difference, where was set as . The ordinate

is normalized to the power of first signal . Levels 0, 1, and2 are constant because either/both or/and is/are 0 and

independent from the phase difference. However, level 3 varies

sinusoidally with the phase difference. If the phase difference

is or , the intervals of each adjoining level are made

equal. The path difference error caused by manufacturing tol-

erances drives the need for the phase difference control section

in Fig. 3. For practical applications, the phase difference must

be precisely controlled, i.e., by employing a feedback control

circuit in which the bias current is dithered. Note that there is a

inherent loss of approximately 3 dB due to the creation of the

equally spaced four-level signal, as can be seen in Fig. 4; one

solution is to increase the input power to the modulator. Also

note that the finite optical extinction ratio of the binary signalscan impact the quality of the 4ASK signal because the circuit is

based on the coherent addition of fields.

III. NUMERICAL COMPARISON OF EOP

A. Degradation of Transmitted 4ASK Signal for Original

Electrical Binary Signal Distortion

We obtained EOPs of the optical 4ASK signal output from

the proposed modulation circuit and compared them with those

of the conventional circuit in the back-to-back situation, when

moderate amplitude distortion was added to the original binary

signals: the distortion was deterministic to simulate the initial

waveform distortion created by overshoot, undershoot, ringingvia electrical circuit, and so on. The bit rate was set to 10 Gb/s,

and the cutoff frequencies of the electrical low-pass filter of the

transmitter/receiver were optimized to 10 and 5 GHz in both

circuits. EOP was defined as the worst of all EOPs between

levels 0 and 1, levels 1 and 2, and levels 2 and 3.

Fig. 5(a) shows the electrical binary signal for which the EOP

was 0.5 dB, and Fig. 5(b) and (c)shows a numerically calculated

eye diagram of the electrical 4ASK signal and optical 4ASK

signal in a conventional circuit, and Fig. 5(d) and (e) shows

the calculated eye diagrams of the optical binary signal and the

yielded optical 4ASK signal in the proposed circuit. In the con-

ventional circuit, the electrical 4ASK signal [see Fig. 5(b)] has

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NAKAMURA et al.: 4ASK MODULATOR FOR SUPPRESSING INITIAL AMPLITUDE DISTORTION 735

Fig. 3. Configuration of proposed 4ASK modulation circuit.

Fig. 4. Relationship of phase difference and optical power of each level.

amplitude distortion in alllevels. Therefore, in the optical 4ASK

output signal in Fig. 5(c), the amplitude distortion at minimum

and maximum levels are suppressed, but those at intermediate

levels are enhanced. In the proposed circuit, on the other hand,

the MZM sections yield two optical binary signals with little

amplitude distortion, as shown in Fig. 5(d). By combining these

optical binary signals, the circuit yields an optical 4ASK signal

that has little amplitude distortion.

Fig. 6 shows the EOP of the optical 4ASK signals of the

proposed and conventional circuits as well as the EOP of

the electrical 4ASK signal of the conventional circuit plotted

against the EOP of the original electrical binary signals. As

shown in the figure, in the conventional circuit, the optical

4ASK signal has worse EOP than the electrical 4ASK signal.

This is because EOPs of intermediate levels become worse due

to the response of the MZM shown in Fig. 2. On the other hand,

the EOP of the proposed circuit is greatly improved. Whenthe EOP of the original binary signal was 0.5 dB, the output

signals of the proposed and conventional circuits had EOP

levels of 0.2 and 1.8 dB, respectively. That is, the proposed

circuit achieves a 1.6-dB improvement in the EOP compared

with the conventional circuit.

B. Degradation in High Spectral Efficiency

We numerically compared the EOP of WDM signals modu-

lated by the 4ASK scheme due to the proposed circuit against

those modulated by NRZ and optical duobinary schemes in the

back-to-back situation. We calculated the EOP of the middle

channel versus the spectral efficiency for the 4ASK scheme,

the NRZ scheme, and the optical duobinary scheme. We exam-

ined only three channels because neighboring channel crosstalk

and coherent beat crosstalk are much stronger than non-neigh-

boring channel crosstalk or power crosstalk [7]. The spectral

efficiency is defined as , where is the bit rate (in bits

per second) and is the channel spacing (in hertz). Parame-

ters used in the numerical calculation are as follows: the cutoff

frequencies of the electrical low-pass filter of the transmitter

(Tx)/receiver (Rx) were set to (Tx: 0.8 B, Rx: 0.7 B), (Tx: 0.3 B,

Rx: 0.8 B), and (Tx: 1.0 B, Rx: 0.5 B) for the NRZ, optical

duobinary, and 4ASK schemes, respectively; they were set at

the minimum values to allow EOPs to be ignored. The simulated

optical multiplexer and demultiplexer were assumed to consist

of a hybrid filter (a MachZehender interferometer) and two ar-

rayed -waveguide gratings with channel spacing of [8] to

suppress more strongly the neighboring channels. As shown inFig. 7, 4ASK held the EOP to less than 0.5 dB at the spectral

efficiency of 0.8 b/s/Hz. This confirms the validity of the pro-

posed circuit in the high-spectral-efficiency WDM condition.

IV. INTEGRATION ON A LiNbO SUBSTRATE

We integrated the proposed circuit shown in Fig. 3 on one

LiNbO substrate and confirmed its feasibility. The two MZM

sections had an insertion loss of 4.09 and 4.30 dB, an extinc-

tion ratio of 28.15 and 28.17 dB, and V of 5.15 and 5.40 V,

respectively.

We measured the receiver sensitivity for an optical 4ASK

signal using the proposed and conventional circuits. Fig. 8shows the experimental setup. A continuous lightwave (CW)

from a laser diode (LD) was modulated by the proposed circuit

(or the conventional circuit for comparison) to generate an

optical 4ASK signal which was received using an optically

preamplified receiver in the back-to-back condition. The bit

rate of electrical signals D1 and D2 was 5 Gb/s while that of the

4ASK signal was set to 10 Gb/s. The frequency of the utilized

light source was 192.850 THz. A pseudorandom bit se-

quence was used as the electrical binary signals. Erbium-doped

fiber amplifiers were used as pre- and postamplifiers. The

noise figure (NF) of the optical preamplifier in the receiver was

6.3 dB, and the input power of the photodetector was around

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Fig. 5. Calculated eye diagram for 4ASK modulating circuit: (a) electrical binary signal, (b) electrical 4ASK signal in conventional circuit, (c) optical 4ASKsignal in conventional circuit, (d) optical binary signal in proposed circuit, and (e) optical 4ASK signal in proposed circuit.

Fig. 6. EOP of 4ASK signals versus EOP of binary electrical signal.

Fig. 7. Calculated spectral efficiency of 4ASK signal in three-channel WDMcondition.

1.0 dBm. After reception in the photodetector, the electrical

4ASK signal was amplified and decoded into two electrical

binary signals. In the decoder shown in figure, the 4ASK signalwas divided into two binary signals and the bit-error rate (BER)

of each signal was measured.

Fig. 9(a) and (b) shows eye diagrams of the output 4ASK sig-

nals in the proposed and the conventional circuits, respectively.

It can be seen that the proposed circuit created an eye diagram

with little amplitude distortion in intermediate levels and clear

eye opening, unlike the conventional one.

Fig. 10 shows the receiver sensitivity characteristics of

each circuit. This figure plots only the BER curves of D2

in Fig. 8. Because the logic of D2 is influenced by that of

D1, the BER of D2 was worse than that of D1. The receiver

sensitivity BER of the proposed and conventional

circuits were 24.4 and 22.7 dBm; namely, the differencein transmitted signal quality between the proposed and the

conventional circuits was found to be 1.7 dB. In addition, the

receiver sensitivity of the transmitting 4ASK signal was 5.6 dB

improved compared with the previously reported value of

19.2 dBm [6]. The decoder circuit has the same configuration

as that in [6], so the difference is thought to be the amount of

circuit noise and thermal noise in the receiver.

The diamond in Fig. 10 shows the ideal receiver sensitivity

( 27.1 dBm) at the BER of , as obtained by numerical

analysis. In the calculations, the spontaneous emission noise of

the optical preamplifier was simulated by generating spectral

components whose real and imaginary parts were independent

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NAKAMURA et al.: 4ASK MODULATOR FOR SUPPRESSING INITIAL AMPLITUDE DISTORTION 737

Fig. 8. Experimental setup.

Fig. 9. Eye diagrams at the output of modulation circuit: (a) proposed circuitand (b) conventional circuit.

Fig. 10. Receiver sensitivity of D2.

Gaussian random variables [9]; the following parameters were

used: the NF of the preamplifier equaled the experimental

one. The full-width at half-maximum (FWHM) of the OBPF

was optimized to minimize the BER. As shown in the figure,

the difference between the ideal value and the experimental

result was 2.7 dB. This difference seems mainly to be due to

the nonlinearity of the electrical amplifier and circuit noise

in the receiver.

V. CONCLUSION

This paper proposed a novel 4ASK modulation circuit that

suppresses amplitude distortion of a transmitted optical 4ASK

signal. The proposed circuit creates an optical 4ASK signal by

optically combining one binary signal and another intensity-

halved binary signal.

After describing the configuration of the proposed circuit, its

suppression of distortion was quantitatively verified by numer-

ical calculations. When the EOP of electrical binary signals was

0.5 dB, the EOP of the optical 4ASK signal was shown to be

0.2 dB. This is 1.6 dB better than that provided by the conven-tional circuit. It was also shown that the proposed circuits yield

4ASK signals that achieve the spectral efficiency of 0.8 b/s/Hz.

The feasibility of the proposed circuit integrated on a LiNbO

substrate was also confirmed: the suppression of amplitude dis-

tortion was successfully demonstrated, and the receiver sensi-

tivity of the transmitted 4ASK signal was 1.7 dB better than that

of the conventional circuit.

ACKNOWLEDGMENT

The authors would like to thank Dr. N. Takachio of NTT Elec-

tronics for his useful comments and Executive Manager N. Fujii

of NTT Network Innovation Laboratories for his continuous

encouragement.

REFERENCES

[1] H.Suzuki et al., 12.5-GHz Spaced 1.28-Tb/s (512-Channel 2 2.5 Gb/s)super-dense WDM transmission over 320-km SMF using multiwave-length generation technique, IEEE Photon. Technol. Lett., vol. 14, pp.405407, Mar. 2002.

[2] A. K. Srivastava et al., Ultradense WDM transmission in L -band,IEEE Photon. Technol. Lett., vol. 11, pp. 15701572, Nov. 2000.

[3] T. Ono et al., Key technologies for terabit/second WDM/systems withhigh spectral efficiency of over 1 bit/s/Hz, IEEE J. Quantum Electron.,vol. 34, pp. 20802088, Nov. 1998.

[4] S. Bigo et al., 10.2Tbit/s (256 2 42.7Gbit/sPDM/WDM) transmission

over 100 km teralight fiber with 1.28 bit/s/Hz spectral efficiency, Proc.OFC 2001, pp. PD25.1PD25.3, 2001.

[5] H. Sotobayashi et al., 1.6 bit/s/Hz, 6.4 Tbit/s OCDM/WDM (4OCDM 2 40 WDM 2 40 Gbit/s) transmission experiment, presentedat the ECOC 2001, Amsterdam, The Netherlands, Sept. 30Oct. 4,2001. Postdeadline Paper.

[6] S. Walklin et al., A 10 Gb/s 4-ary ASK lightwave system, Proc.ECOC97, pp. 255258, 1997.

[7] T. Nakamura et al., 0.8-bps/Hz spectral efficiency super-dense WDMtransmission employing quaternary amplitude shift keying scheme,Proc. OECC 2002, pp. 554555, 2002.

[8] M. Abe et al., MachZehnder interferometer and arrayed-waveguide-grating integrated multi/demultiplexer with photosensitive wavelengthtuning, Electron. Lett., vol. 37, pp. 376377, 2001.

[9] D. Marcuse, Single-channel operation in very long nonlinear fiberswith optical amplifiers at zero dispersion, J. Lightwave Technol., vol.9, pp. 356361, Mar. 1991.

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Takuya Nakamura received the B.E. degree fromNagasaki University, Nagasaki, Japan, in 1996and the M.E. degree from Kumamoto University,Kumamoto, Japan, in 1998.

In 1998, he joined the NTT Optical NetworkSystem Laboratories, Yokosuka, Japan, where hehas been engaged in research on optical accesssystems and optical transmission systems usingdense-wavelength-division-multiplexing (DWDM)

technologies.Mr. Nakamura is a Memberof theInstitute ofElec-trical, Information and Communication Engineers (IEICE) of Japan.

Jun-Ichi Kani (M98) received the B.E. and M.E.degrees in applied physics from Waseda University,Tokyo, Japan, in 1994 and 1996, respectively.

In 1996, he joined the NTT Optical NetworkSystem Laboratories, Yokosuka, Japan, where hehas been engaged in research on ultra-wide-bandwavelength-division-multiplexing technologies andmetropolitan and access area network technologies.

Mr. Kani received the Best Paper Award from theSecond and Third Optoelectronics and Communica-tions Conference (OECC) in 1997 and in 1998, the

Best Paper Award from the European Conference on Networks and OpticalCommunications (NOC) in 1998, the APCC-IEEE ComSoc APB Joint Awardfrom the Asia-Pacific Conference on Communications and IEEE Asia-Pacificboard in 2001, and the Young Scientist Award from IEEE Lasers & Electro-Op-tics Society (LEOS) Japan Chapter in 2003. He is a Member of the Institute ofElectrical, Information and Communication Engineers (IEICE) of Japan.

Mitsuhiro Teshima (M93) received the B.E.and M.E. degrees from the Tokyo Institute ofTechnology, Tokyo, Japan, in 1989 and 1991,respectively.

In 1991, he joined the NTT Transmission SystemLaboratories, Yokosuka, Kanagawa, Japan. Hehas been engaged in research on the frequencycontrol of semiconductor lasers and the opticalcross-connect system. Since 1999, he has been

engaged in research on metropolitan and access areanetwork technologies.Mr. Teshima received the Young Engineers Award from the Institute of Elec-

trical, Information and Communication Engineers (IEICE) of Japan in 1998andthe APCC-IEEE ComSoc APB Joint Award from the Asia-Pacific Conferenceon Communications and IEEE Asia-Pacific board in 2001. He is a Member ofthe IEICE and the Japan Society of Applied Physics (JSAP).

Katsumi Iwatsuki (M02) received the B.E. degreein electronics engineering from the Nagoya Instituteof Technology, Nagoya, Japan, in 1981 and M.E.and Ph.D. degrees in electronics engineering fromthe University of Tokyo, Tokyo, Japan, in 1983 and1986, respectively.

In 1986, he joined the NTT Laboratories, Yoko-suka, Japan, where he has been engaged in researchon optical fiber sensors, optical soliton transmission,and ultra-high-speed nonlinear pulse transmission.Since 1999, he has been engaged in research on

metropolitan and access area network technologies and currently heads thePhotonic Access Systems Research group in the NTT Network InnovationLaboratories.

Dr. Iwatsuki received the Young Engineers Award from the Institute of Elec-trical, Information and Communication Engineers (IEICE) of Japan in 1991andthe APCC-IEEE ComSoc APB Joint Award from the Asia-Pacific Conferenceon Communications and IEEE Asia-Pacific board in 2001. He is a Member ofthe IEICE.

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