3d qam-dpsk optical transmission employing a single mach

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 38, NO. 22, NOVEMBER 15, 2020 6247

3D QAM-DPSK Optical TransmissionEmploying a Single Mach–Zehnder

Modulator and Optical Direct DetectionHyoung Joon Park , In Ho Ha , Soo-Min Kang , Won-Ho Shin, and Sang-Kook Han , Senior Member, IEEE

Abstract—We propose a novel optical transmission technique,3D QAM-DPSK, that employs a single Mach–Zehnder modulatorbased on a direct detection system. Through 3D QAM-DPSK, theoptical signal is simultaneously transmitted via the intensity andphase of the optical source. The QAM signal is transmitted viathe intensity of the optical source, similar to conventional opticaldirect detection systems. The modified DPSK signal is transmittedvia the phase of the optical source. As a QAM signal has twoaxes, and a DPSK signal has a single axis, the optical modulationtechnique can be treated as 3D QAM-DPSK. In the proposedoptical transmission technique, signal transmission capacity andsignal performance were enhanced in terms of bit error rate, ascompared with ASK-DPSK. The signal transmission performanceof the proposed modulation technique was numerically and exper-imentally demonstrated.

Index Terms—Direct detection, differential phase shiftkeying, ASK-DPSK, mach zehnder modulator, multidimensionalmodulation.

I. INTRODUCTION

W ITH recent developments in 5G-related services andtechnologies, optical infrastructures have been widely

studied. Through the development of network slicing tech-nology, converged wired and wireless optical networks havegarnered extensive research interest [1]–[3]. To improve theconverged wired and wireless optical network, transmissioncapacity enhancement is essential. In order to enhance thetransmission capacity of a passive optical network (PON), sev-eral techniques have been studied. These techniques can bedivided into three categories: enhancing modulation bandwidth[4]–[6], enhancing spectral efficiency [7]–[17], and multiplexing[17]–[25]. Hardware- and software-based bandwidth enhance-ment techniques have been studied [4]–[6]. Because bandwidth

Manuscript received April 3, 2020; revised June 12, 2020 and July 17, 2020;accepted July 18, 2020. Date of publication July 24, 2020; date of current versionNovember 16, 2020. This work was supported by the National Research Foun-dation of Korea under Grant funded by the Korea government (MSIT; Ministryof Science and ICT) under Grant 2019R1A2C3007934. (Corresponding author:Sang-Kook Han.)

The authors are with the Department of Electrical and Electronic Engineering,Yonsei University, Seoul 120-749, South Korea (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this article are available onlineat https://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2020.3011681

enhancement is based on equalization, frequency response hasto be flattened to improve signal performance. However, it isdifficult to significantly improve modulation bandwidth withoutdeveloping an optical device. Techniques developed to enhancespectral efficiency include partial response coding [7], signalshaping [8], faster than Nyquist [9], [10], bit-loading algorithmfor multicarrier transmission [11]–[14], and multidimensionalmodulation [15], [16]. These techniques effectively approach theShannon limit; however, their capability to enhance transmissioncapacity is limited by a single channel. Several multiplexingtechniques have been studied for PONs, such as wavelengthdivision multiplexing (WDM) [17], [18], pulse division mul-tiplexing [19], and polarization division multiplexing (PDM)[20]–[22]. Employing polarization different from PDM, polar-ization shift keying or stokes vector modulation is also pro-posed [23]–[25]. These multiplexing or modulation techniquescould be also employed together to enhance transmission ca-pacity. Employing two or more multiplexing techniques, di-mensions of signal also would be increased. Coherent opticalcommunication can be regarded as phase division multiplex-ing, as signal could be orthogonally transmitted via in-phaseand quadrature-phase [26]–[28]. However, coherent commu-nication is quite sensitive and cost-ineffective as it involvesa complicated IQ modulator and a coherent detector with alocal oscillator. Therefore, as an alternative, Star 16 QAMand ASK-DPSK techniques were suggested to transmit datavia intensity and phase in direct detection (DD) systems [29],[30]. When employing the Star 16 QAM technique, varioushigh order ASK and DPSK could be received simultaneously;however, its transmitter and receiver are difficult to fabricatedue to their numerous device controls and synchronization [30].As opposed to coherent optical transmission and Star 16 QAM,ASK-DPSK transmits signal via intensity and phase with lowcomplexity [29]. As intensity and phase are independent fromeach other, transmission capacity could be effectively improvedwhen compared with general intensity modulation and directdetection (IM/DD) optical transmission. By the orthogonality,ASK-DPSK could be compatible with conventional IM/DDoptical transmissions in terms of the intensity modulation chan-nel. However, the optical modulation index (OMI) in existingASK-DPSK is much less than that of the general IM/DD opti-cal transmission in demodulating DPSK signal. Therefore, the

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mailto:[email protected] -@M ac.kr

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6248 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 38, NO. 22, NOVEMBER 15, 2020

Fig. 1. Intensity and phase modulation using amplitude modulator and phasemodulator.

signal transmitted via intensity could have a smaller signal tonoise ratio (SNR).

In this paper, we propose, for the first time, 3-dimensional(3D) quadrature amplitude modulation based differential phaseshift keying (QAM-DPSK) using a single optical modulator ina DD system. In 3D QAM-DPSK, signal is transmitted simulta-neously via intensity and phase by transmitting QAM signal viaintensity modulation and DPSK signal via phase modulation. Asa QAM signal has two axes and a DPSK has a single axis, wenamed the transmission technique as 3D QAM-DPSK. Signalperformance in terms of bit-error rate (BER) was enhanced. Wetheoretically and experimentally verified the performance of 3DQAM-DPSK and also compared its performance with that ofASK-DPSK.

II. OPERATION PRINCIPLE OF 3D QAM-DPSK

Before introducing 3D QAM-DPSK, the ASK-DPSK tech-nique needs to be discussed. In the ASK-DPSK technique, twomodulators are employed to modulate the ASK signal withintensity and DPSK signal in phase, as shown in Fig. 1. Anon-off keying (OOK) modulation format is used as the ASKsignal, and a differential OOK signal is employed as the DPSKsignal. In output of the laser, the intensity and phase of the opticalsignal are both constant over time. In the output of the amplitudemodulator, intensity varies over time, and phase is constantover time. After the phase modulation of the optical signal atthe phase modulator, intensity is the same as the output of theamplitude modulator, and phase varies over time. This is thesimple and general structure of ASK-DPSK signal generation.As mentioned in [29], the electrical field strength of the opticalsignal at the output of the transmitter could be written as

E(t) =√PA(t) exp{j[w0t+ φ(t)]} (1)

whereP stands for the mean power of the continuous wave (CW)laser,w0 is the optical carrier frequency, A(t) is the modulatedamplitude, and φ(t) is the modulated phase. The receiving op-eration principle is shown in Fig. 2. In order to receive intensityand phase signals, a DD structure and an interferometer structureare employed simultaneously. In Fig. 2, the optical signal is splitinto an intensity detection path and a phase detection path usinga 3 dB coupler. PD1 detects the intensity of the optical signalas a conventional IM/DD system. From [25], electrical current,

Fig. 2. Schematics of intensity and phase receiver.

which is detected by PD1, could be written as

IPD1(t) =1

2kRPA2(t) (2)

where R is the responsivity of the photodiode, and k is theproportionality factor. The demodulation of the intensity signalis also the same as that of a conventional IM/DD system. How-ever, there are some issues in the phase detection path. In phasedetection, a DPSK modulation scheme consisting of a balancedphoto detector (BPD) is employed. The path time difference ofthe interferometer is T, which stands for the symbol duration.To simplify the amplitude and the phase of the electric field, wedenote En(t), An(t), φn(t) as

En(t) = E(nT + t)

An(t) = A(nT + t)

φn(t) = φ(nT + t). (3)

Because the optical field strength of the phase detection pathis 1.5 dB less than E(t), using Equation (3), the electric fieldjust before PD2 is

En(t) =j

2√2[En(t) + En−1(t)]

=j√P

2√2(An(t) exp{j[w0t+ φn(t)]}

+ An−1(t) exp{j[w0(t− T ) + φn−1(t)]}) (4)

Therefore, we could obtain the electrical current after PD2 as

In,PD2(t) =1

8kRP [A2

n(t) +A2n−1(t)

+ 2AnAn−1 cos(φn(t)− φn−1(t))] (5)

which is affected by intensity modulation. By the same method,the final output current, achieved from the current differencebetween PD2 and PD3, could be written as

In(t) =1

2kRPAn(t)An−1(t) cos(φn(t)− φn−1(t)) (6)

From Equation (6), the DPSK signal is demodulated by thecosine term cos(φn(t)− φn−1(t)). Therefore, to demodulatewithout error, the amplitude of the optical field should be largerthan zero. This means that DC offset is required to obtain DPSK,as shown in Fig. 3. Otherwise, the extinction ratio would beworse. To avoid this issue, we proposed 3D QAM-DPSK to


PARK et al.: 3D QAM-DPSK OPTICAL TRANSMISSION EMPLOYING A SINGLE MACH–ZEHNDER MODULATOR AND OPTICAL DIRECT DETECTION 6249

Fig. 3. Optical conversion of electric signal in modulator with DC offset.

Fig. 4. Schematics of 3D QAM-DPSK transmitter.

enhance DPSK accuracy. As DPSK signal, even in our proposedtechnique, is transmitted by phase modulation, the optical phasecould be written simply as

φn = φn(t), 0 < t < T (7)

The proposed QAM-DPSK transmits QAM signal by in-tensity modulation. Conventional QAM modulation modulatesQAM signal by intensity and phase modulation; however, inour proposed technique, QAM signal is modulated on the radiocarrier frequency (fRF ), as shown in Fig. 4. QAM symbols arerepresented by in-phase (I) and quadrature-phase (Q) compo-nents, and I and Q components are modulated at fRF . As theQAM signal is composed of two axes, I and Q, two intensityaxes are employed in the proposed technique. Because the DPSKsignal is transmitted via the phase axis, we named the proposedtechnique 3D QAM-DPSK. To determine the comparability ofthe performance of ASK-DPSK and the proposed 3D QAM-DPSK, their signal properties should be similar. To match themodulation bandwidth and transmission capacity, the symbolduration of the QAM signal should be doubled. However, tomatch the symbol duration, the modulation bandwidth should bedoubled, which increases the transmission capacity. Therefore,we propose two types of 3D QAM-DPSK: one with equaltransmission capacity and another with equal symbol duration,as shown in Fig. 5.

Fig. 5. Signal representation and minimum MRCP of two types of 3D QAM-DPSK.

Fig. 6. Eye diagram of phase axis employing ASK-DPSK.

To detect DPSK from Equation (6), the amplitude of theelectrical field is crucial. Employing ASK-DPSK, the receivedcurrent (6) is divided into six levels, as shown in Fig. 6. In orderto detect phase, the information from all six levels is not needed.The receiver should distinguish whether the level is higher by 3levels or lower by 3 levels. Therefore, the size of the eye openingand bit error rate are determined by the minimum amplitudeof the electric field. Thus, the sum of received current, whichdetermines the BER of DPSK, is written as∫ T

0

I(t)dt

=

∫ T

0

1

8kRP × 4An(t)An−1(t) cos(φn − φn−1)dt. (8)

The minimum received current of phase detection (MRCP)path employing ASK-DPSK is simply calculated as∫ T

0

I(t)dt =

∫ T

0

1

2kRP ×

√F√F cos(φn − φn−1)dt

=1

2kRPFT cos(φn − φn−1) (9)

where F stands for the DC offset of the intensity modulation, asshown in Fig. 3. However, the received current of 3D QAM-DPSK with equal data transmission capacity (QAM-DPSK-EDC) and with equal symbol duration (QAM-DPSK-ESD)would be minimized at some moment, and it is represented inFig. 7. The received current would be minimized in transmitting



Fig. 7. Schematics of modulating intensity and phase employing a singleMZM.

QAM-DPSK-EDC, when

Am =

√W − F

2− W − F

2sin( πTt)+ F

Am−1 =

√W − F

2− W − F

2sin( πTt)+ F (10)

where W is the whole strength of the intensity, as shown in Fig. 3.The MRCP of QAM-DPSK-EDC could be represented as∫ T

0

I(t)dt

=1

2kRP cos(φn(t)− φn−1(t))

∫ T

0

Am(t)Am−1(t)dt (11)

If we denote Ikas

Ik =1

2kRP cos(φn(t)− φn−1(t)) (12)

Equation (11) could be simplified as∫ T

0

I(t)dt =1

2Ik

∫ T

0

W + F

2− W − F

2sin( πTt)dt

=1

2Ik

(W + F

2T − W − F

2T

)

� 1

2IkT (0.182W + 0.818F )

=1

2kRP (0.182W + 0.818F )T cos (φn − φn−1)

(13)

Using Equations (9) and (13), MRCP employing ASK-DPSKand QAM-DPSK-EDC could be compared. When the DC offsetis zero, the MRCP of ASK-DPSK would be∫ T

0

I(F=0)(t)dt = 0 (14)

and MRCP of QAM-DPSK-EDC would be∫ T

0

I(F=0)(t)dt =1

2kRP (0.182W )T cos(φn − φn−1).

(15)

To get the MRCP of QAM-DPSK-ESD, the multiplication ofAm, Am−1 should be minimized, and the moment is shown inFig. 7. Therefore, Am, Am−1 could be represented as

Am =

√W − F

2+

W − F

2sin

(2π

Tt

)+ F

Am−1 =

√W − F

2− W − F

2sin

(2π

Tt

)+ F . (16)

Using Equation (16), the MRCP of QAM-DPSK-ESD couldbe written as,

∫ T

0

I(t)dt =Ik2

∫ T

0

√(W + F )2 − (W − F )2sin2

(2π

Tt

)dt

=Ik2

∫ T

0√W 2 + 6WF + F 2

2+(W − F )2

2cos

(4π

Tt

)dt

= Ik′

∫ T

0

√1 +

(W − F )2

W 2 + 6WF + F 2cos

(4π

Tt

)dt.

(17)

where Ik′ stands for

Ik′ =Ik√

(W 2 + 6WF + F 2)

2√2

. (18)

Equation (17) is too complex to be integrated; hence, a Taylorseries is employed to simplify it. Thus, Equation (17) could bewritten as∫ T

0

I(t)dt � Ik′

∫ T

0

1 +(W − F )2

2(W 2 + 6WF + F 2)cos

(4π

Tt

)dt

− Ik′

∫ T

0

(W − F )4

8(W 2 + 6WF + F 2)2cos2

(4π

Tt

)dt

= Ik′T

(1− (W − F )4

16(W 2 + 6WF + F 2)2

)

=1

2kRPT

√(W 2 + 6WF + F 2)

2√2

×(1− (W − F )4

16(W 2 + 6WF + F 2)2

)cos (φn(t)− φn−1(t))

(19)

Finally, the MRCP of ASK-DPSK, QAM-DPSK-EDC, andQAM-DPSK-ESD could be compared numerically. If we as-sume that F = 0, the MRCP of QAM-DPSK-ESD is written



TABLE IBIT TO SYMBOL MAPPING TABLE

as ∫ T

0

I(F=0)(t)dt � 1

2kRPT

W

2√2

×(15

16

)cos(φn(t)− φn−1(t))

=1

2kRP (0.331W )T cos(φn(t)− φn−1(t)). (20)

Therefore, when F is much smaller than T, considering Equa-tions (14), (15), and (20), the MRCP of QAM-DPSK-EDCbecomes larger than that of ASK-DPSK and smaller than thatof QAM-DPSK-ESD. This means that when F = 0, the DPSKsignal could not be detected using ASK-DPSK, but it can bedetected using QAM-DPSK-EDC and QAM-DPSK-ESD. InEquation (9), the MRCP of QAM-DSPK-EDC when F = 0 isthe same as that of ASK-DPSK when F = 0.182 T. Moreover,the MRCP of QAM-DSPK-ESD when F = 0 is the same as thatof ASK-DPSK when F = 0.331 T. As signal modulation width,as shown in Fig. 3, becomes smaller when F becomes larger, thesignal modulation width of QAM-DPSK-ESD and QAM-DPSKEDC would be larger than the SNR of ASK-DPSK at the sameMRCP. With a larger signal modulation width, the OMI for ananalog signal and the extinction ratio for a digital signal couldbe further increased. To obtain the same MRCP for ASK-DPSKand QAM-DPSK-ESD, 33% of DC offset is necessary, whichdetermines the demodulation accuracy of DPSK signal. Be-cause of the 33% DC offset, the signal modulation width ofASK-DPSK becomes 2/3 of that of QAM-DPSK-ESD, and thismeans that root mean square (RMS) of the ASK-DPSK signalbecomes 4/9 of that of QAM-DPSK-ESD. Therefore, the SNR ofASK-DPSK would be 3 dB less than that of QAM-DPSK-ESD.Since the MRCP is also same employing high order QAM based3D QAM-DPSK, the signal performance of phase dimensionwould be similar to 4QAM based 3D QAM-DPSK. The signalperformance of intensity dimension depends on the order ofQAM, therefore signal performance would be degraded as orderof QAM increases.

Symbol mapping principle of 3D QAM-DPSK-ESD is shownin Table I. QPSK is employed in intensity dimension and DPSK

is employed in phase dimension. Since three bits are modulatedin a symbol, two bits are modulated in intensity dimension andone bit is modulated in phase dimension. The bit is mapped asgray code, which minimizes bit error when symbol error occurs.

As ASK-DPSK transmit signals via the intensity and thephase of light, both an intensity modulator and a phase modu-lator are used in the conventional experiment. However, systemcomplexity would be an issue to employ two modulators. Inour experiment, we employed a single Mach-Zehnder modu-lator (MZM) to transmit the signal modulated in phase andintensity. MZM is known to modulate not only intensity, butalso field. Due to push-pull method, only two states of phasewould be modulated. Simple operation method of modulatingASK-DPSK using single MZM is shown in Fig. 7. If ASK-DPSKis modulated using a single MZM, the MZM voltages V1 andV4 transmits an intensity ‘1’ signal, and MZM voltages V2

and V3 transmits an intensity ‘0’ signal. Thereafter, the opticalphase could be modulated by selecting (V1, V2) or (V3, V4).Thus, using a single MZM, optical intensity and phase could bemodulated to transmit ASK-DPSK and the proposed technique.Similarly, 3D QAM-DPSK could be also modulated using singleMZM. QAM signal is modulated between V1 and V2 or V3 andV4, which is decided by phase dimension. Employing a singleMZM instead of an intensity modulator and a phase modulator,system complexity would be decreased and also synchronizationissue between two modulators would be mitigated.

III. EXPERIMENTAL SETUP

After comparing the performance of the proposed QAM-DSPK-EDC and QAM-DPSK-ESD with ASK-DPSK nu-merically, they will now be compared experimentally. Theexperimental setup for the transmission of optical signal withthe three different techniques is shown in Fig. 8. An externalcavity laser diode was employed as the CW laser source. Thewavelength of optical source was 1550nm. A polarizer controllerwas used to maximize the modulation performance of the MZM.ASK-DPSK, QAM-DPSK-EDC, and QAM-DPSK-ESD signalswere generated using an arbitrary waveform generator (AWG70002A; AWG). The vertical bit resolution of AWG is 10 bitsand sampling rate is set to 10 GHz. converted into optical signalsusing MZM. At the receiver side of Fig. 8, optical signal isdivided into two paths: the intensity detection path and the phasedetection path. At the end of the intensity detection path, a photodetector (PD) receives the intensity of the optical signal. Eventhough only a single MZM is employed in the setup, the intensitysignal could be easily demodulated, as shown in Fig. 7. In thephase detection path, we used 3 dB couplers and optical delaylines to set the optical path difference at one symbol duration in-stead of Mach-Zehnder interferometer. As the optical delay lineis manually controlled, the accuracy of the optical path lengthdifference had some limitations. Thus, the symbol duration ofthe signals was 1 ns. As the symbol duration of DPSK was 1ns, the symbol durations of ASK-DPSK and QAM-DPSK-ESDwere also 1 ns. Hence, the symbol duration of the QAM symbolof QAM-DPSK-EDC was 2 ns. Therefore, total bit rate ofASK-DPSK and QAM-DPSK-EDC was 2 Gbps, and total bit



Fig. 8. Schematics of experimental setup.

rate of QAM-DPSK-ESD was 3 Gbps. Transmission capacitywould be much enhanced with larger modulation bandwidthwith appropriate interferometer. Sampling frequency was set to10 GHz to analyze the experimental results in detail. The DPSKsignal was electrically converted using the BPD and digitallyconverted by the digital phosphor oscilloscope (MSO71604C;DPO).

IV. EXPERIMENTS AND RESULTS

To compare the signal performance of ASK-DPSK, QAM-DPSK-EDC, and QAM-DPSK-ESD, the signals modulated inboth intensity axes and the phase axis have to be considered.First, the intensity axis of the three modulation formats wascompared in Fig. 9. As the intensity axis of ASK-DPSK ismodulated as an OOK signal, an eye diagram was employedto confirm the signal performance. In Fig. 9(a), two levelscould be seen in the eye diagram when F = 0.2 W. However,when F = 0.64 W, the eye becomes more closed, as shownin Fig. 9(b). As expected, the performance of the intensitymodulated signal decreased as F increased. On the other hand,the performance of the phase modulated signal increased as Fincreased. In order to analyze the intensity modulated signalperformance of QAM-DPSK-EDC and QAM-DPSK-ESD, asignal constellation was created, as shown in Fig. 9(c), (d),(e), (f). In QAM-DPSK-EDC, the performance of the intensitymodulated signal was sufficient to transmit without errors whenF = 0.2 W. However, the shape of the signal constellation wasbiased into a specific direction. Therefore, when F = 0.64 W,the signal performance significantly deteriorated. As the symbolduration of QAM is twice of that of DPSK in QAM-DPSK-EDC,phase varied at the middle of the QAM symbol. As the signalwas modulated employing a single MZM, in order to change thephase, amplitude must be also drifted at a short time. This phe-nomenon generated intra-symbol interference and inter-symbolinterference between adjacent QAM symbols, which occurredSNR degradation. Therefore, the performance of intensity inQAM-DPSK-EDC was poorer than expected. However, this isnot the chronic problem of the QAM signal transmission. Whentransmitting QAM-DPSK-ESD, which has the same symbolduration as that of the QAM and DPSK signal, the signalconstellation was less biased into a specific direction, as shown inFig. 9(e). Moreover, in Fig. 9(f), the signal could be demodulated

Fig. 9. Eye diagrams and signal constellations of received signal at intensitydetection path. (a) eye diagram of ASK-DPSK when F = 0.2 W, (b) eye diagramof ASK-DPSK when F = 0.64 W, (c) signal constellation of QAM-DPSK-EDCwhen F = 0.2 W, (d) signal constellation of QAM-DPSK-EDC when F = 0.64W, (e) signal constellation of QAM-DPSK-ESD when F = 0.2 W, and (f) signalconstellation of QAM-DPSK-ESD when F = 0.64 W.

without bit error when F = 0.64 W. As the symbol durations ofQAM and DPSK were the same, there was no voltage drift atthe middle of the symbol unlike in QAM-DPSK-EDC. Onlyinter-symbol interference occurs in QAM-DPSK-ESD. As ex-pected, the signal performance of the intensity modulated signaldecreased as F increased. Signal demodulation received at theintensity detection path had no significant difference with thatof the conventional IM/DD detection.



Fig. 10. Eye diagrams and signal representation of received signal at phase detection path employing ASK-DPSK, (a) eye diagram when F = 0.2 W, (b) eyediagram when F = 0.64 W, and (c) signal representation when F = 0.64 W.

For the next step, the experimental result of signal demodu-lation received at the phase detection path was described. Thereceived current of BPD, when ASK-DPSK was transmitted, isrepresented in Fig. 10. Numerically, six levels have to be seenin the eye diagram of the received current. However, only fourlevels are seen in Fig. 10(a). To numerically analyze the eyediagram, it was expected that some levels would be obscuredby noise, as F = 0.2 W. However, when F = 0.64 W, onlyfive levels are seen in Fig. 10(b). Fig. 10(c) visualizes thedifference between the experimental and the numerical analysis.In Fig. 10(c), the intensity modulated signal of transmitter (Tx)and the received current at the phase detection path are presented.When intensity was continuously large for two frames, that is AL

was modulated for two consecutive frames, the received currentwas at the minimum or maximum level, as expected. However,it was observed that some intermediate steps were overlappedwith each other. As there are some overlaps, the eye diagramwas not vertically symmetric. In detail, when two optical phasesof consecutive frames are different, the levels become near toeach other. Through the experiment, we drew a conclusion aboutnon-symmetric eye diagrams, in that two optical signals modu-lated in different phases at the MZM are not perfectly coherentin terms of phase. The reason is that even the MZM employspush-pull method, optical phase is slightly affected by intensity.However, in demodulating the DPSK signal from the receivedcurrent at the phase detection path, the vertical non-symmetryis not critical. As the eye opening in the middle of eye diagrammostly affects the BER of the DPSK signal, non-symmetry is notcritical. Therefore, we evaluated the performance of the DPSKsignal according to the modulation technique and the value ofthe DC offset F.

The eye diagrams of the DPSK signals using different modu-lation techniques and the corresponding F are shown in Fig. 11.The eye opening of the received current at the end of thephase detection path became larger as F increased, as shownin Fig. 11(b), (c), (d). Moreover, the three highest levels inFig. 11(c) and (d) became closer as F increased, as AL and AS

moved towards each other. In Fig. 11(f), the eye could be hardlyseen when QAM-DPSK-EDC is modulated. As it is difficultto find the difference between the eye diagrams in Fig. 11(b)and 11(f), eye diagrams with few frames were made, as shown

in Fig. 11 (a) and 11(e). Comparing the two eye diagrams,the difference of openings can be clearly seen. Although theeye opening in Fig. 11(a) appears to be large, if we considerthe 6 numerical levels of the eye diagram from ASK-DPSK,the actual eye is actually completely closed. Meanwhile, theeye is open in Fig. 11(e). As it was shown numerically that theDPSK signal is affected by the QAM signal when employingQAM-DPSK-EDC, the eye diagram in Fig. 11(e) is quite dif-ferent from a conventional eye diagram of a digital signal. Asexpected, numerically, the eye openings in Fig. 11(f), (g), (h)also became larger as F increased. Comparing Fig. 11(d) and11(h), the difference of the eye diagrams could be seen even athigh F. The difference is from the An(t)An-1(t) term of Equation(8). Comparing Fig. 11(i) and Fig. 11(e), the eye shapes areclearly different. As the MRCP of QAM-DPSK-ESD was largerthan that of QAM-DPSK-EDC, the eye opening in Fig. 11(j) wasslightly larger than that in Fig. 11(f). The signal performance inFig. 11(g) appears to be better than that of Fig. 11(k); however,the actual signal performance, in terms of the BER, showed thatthe performance of QAM-DPSK-ESD was better.

The signal performance in terms of the BER, according toDC offset and the modulation format, is depicted in Fig. 12 andFig. 13. As expected, numerically, the BER performance of theintensity axis of the three modulation formats decreased as DCoffset F increased. Moreover, as expected, the BER performanceof the phase axis of the three modulation formats were enhancedas DC offset F increased. We expected that at the same F, theBER performance of the intensity axis employing the threemodulation formats would approximate to each other; however,there was quite a large difference. The signal performance interms of BER was lowest when QAM-DPSK-EDC was em-ployed and highest when QAM-DPSK-ESD was employed. Thereason for the poor BER performance of QAM-DPSK-EDCwas the voltage drifting in the middle of the symbol, as ex-plained in Fig. 8 and Fig. 9. The reason for the superior BERperformance of QAM-DPSK-ESD was that the intensity axisof QAM-DPSK-ESD was detected by the matched filter, as itis a QAM signal. This is the case even if its intensity axis wastwice that of ASK-DPSK and QAM-DPSK-EDC. The signalperformance of the phase axis in terms of BER is also comparedin Fig. 12. As numerically analyzed, the BER performance of



Fig. 11. Eye diagrams of received signal at phase detection path employing (a) ASK-DPSK when F = 0.2 W, (b) ASK-DPSK when F = 0.2 W, (c) ASK-DPSKwhen F = 0.5 W, (d) ASK-DPSK when F = 0.64 W, (e) QAM-DPSK-EDC when F = 0.2 W, (f) QAM-DPSK-EDC when F = 0.2 W, (g) QAM-DPSK-EDC whenF = 0.5 W, (h) QAM-DPSK-EDC when F = 0.64 W, (i) QAM-DPSK-ESD when F = 0.2 W, (j) QAM-DPSK-ESD when F = 0.2 W, (k) QAM-DPSK-ESD whenF = 0.5 W, (l) QAM-DPSK-ESD when F = 0.64 W.

Fig. 12. BER performance of intensity and phase dimension according to DCoffset ratio.

the phase axis was the best when employing QAM-DPSK-ESDand the worst when employing ASK-DPSK. As the MRCP of3D QAM-DPSK was larger than that of ASK-DPSK, the twomethods of 3D QAM-DPSK had a better performance thanASK-DPSK. The BER performance of QAM-DPSK-ESD wasbetter than that of QAM-DPSK-EDC, because of the largerMRCP; however, the performance was too effectively enhancedwhen employing 3D QAM-DPSK. The total BER performance

Fig. 13. BER performance of ASK-DPSK and 3D QAM-DPSK according toDC offset ratio.

according to DC offset ratio is represented in Fig. 13. The BERperformance of QAM-DPSK-EDC is better than ASK-DPSK,when DC offset ratio is small. Reversely, BER performance ofASK-DPSK is better than the performance of QAM-DPSK-EDCwhen DC offset ratio is large. However, BER performance ofQAM-DPSK-ESD was the better than two other modulationtechniques. From the facts above, QAM-DPSK-ESD was more



Fig. 14. BER performance of ASK-DPSK and 3D QAM-DPSK according toreceived optical power when DC offset ratio is 25%.

effective than QAM-DPSK-EDC, and the signal performanceof the intensity axes and the phase axis was enhanced whencompared to ASK-DPSK.

To compare the performance of ASK-DPSK and 3D QAM-DPSK in detail, BER performance according to received opticalpower was measured and represented in Fig. 14. When DC offsetratio is 25%, BER performance depends on phase dimension.Thus, performance of QAM-DPSK-ESD was better than QAM-DPSK-EDC and ASK-DPSK. As received optical power getslarger, SNR becomes larger. Therefore, the BER performanceof all three modulation techniques was enhanced by increasingreceived optical power.

V. CONCLUSION

We proposed a novel optical transmission method using 3DQAM-DPSK modulation in an optical DD system. 3D QAM-DPSK transmits QAM via intensity modulation and DPSK viaphase modulation. Unlike conventional modulation techniques,which modulates both intensity and phase, a single MZMis employed to modulate the optical signals. By comparingwith the well-known ASK-DPSK, the signal performance ofthe proposed technique was numerically and experimentallydemonstrated. The BER performance of 3D QAM-DPSK wasat least 100 times lower than that of ASK-DPSK, even thoughthe transmission capacity of the former was 1.5 times higherthan that of ASK-DPSK. Moreover, the signal performance of3D QAM-DPSK would be more enhanced by employing 3Dsymbol mapping and high order modulation. Thus, the proposed3D QAM-DPSK could be helpful to significantly increase trans-mission capacity in non-coherent optical transmission systems.

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Hyoung Joon Park received the B.S. and M.S. degrees in electrical andelectronic engineering from Yonsei University, Seoul, South Korea, in 2014 and2016, respectively, where he is currently working toward the Ph.D. degree inelectrical and electronic engineering. His current research interests include nextgeneration optical access networks, uplink multiple access for OFDMA PON,multidimensional optical transmission, and software defined optical networkphysical layer transmission.

In Ho Ha received the B.S. and M.S. degree in electronic engineering fromYonsei University, Seoul, South Korea, in 2017 and 2019, respectively. He iscurrently working toward the Ph.D. degree in electrical and electronic engi-neering with Yonsei University. His research interests include wireless/wirelineconvergence and next-generation mobile front haul.

Soo-Min Kang received the B.S. and M.S. degrees in electronic engineeringfrom Sogang University, Seoul, Korea, in 2014 and Yonsei University, Seoul,Korea, in 2016, respectively. She is currently working toward the Ph.D. degreein electrical and electronic engineering from Yonsei University. Her currentresearch interests are coherent optical-access networks and digital signal pro-cessing for coherent optical transmission.

Won-Ho Shin received the B.S. and M.S. degrees in electrical and electronicengineering from Yonsei University, Seoul, South Korea, in 2015 and 2017,respectively, where he is currently working toward the Ph.D. degree in electricaland electronic engineering. His current research interests include next generationoptical access networks, optical wireless transmission, and satellite opticalcommunication.

Sang-Kook Han (Senior Member, IEEE) received the B.S. degree in electronicengineering from Yonsei University, Seoul, South Korea, in 1986 and the M.S.and Ph.D. degrees in electrical engineering from the University of Florida,Gainesville, FL, USA, 1988 and 1994, respectively. From 1994 to 1996, he waswith the System IC Laboratory, Hyundai Electronics, where he was involved inthe development of optical devices for telecommunications. He is currently aProfessor with the Department of Electrical and Electronic Engineering, YonseiUniversity. His current research interests include optical devices/systems forcommunications, optical OFDM transmission system, passive optical network,software defined optical network, and visible light communication technologies.


3d qam-dpsk optical transmission employing a single mach

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