Photonic vector signal generation employing a novel
optical direct-detection
in-phase/quadrature-phase upconversion
Wen-Jr Jiang,2Chun-Ting Lin,1,* Chung-hung Ho,1Chia-Chien Wei,3Po-Tsung Shih,2Jason (Jyehong) Chen,2and Sien Chi4
1Institute of Photonic Systems, National Chiao Tung University, Tainan 711, Taiwan
2Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan 3Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nautou 545, Taiwan
4Department of Photonics Engineering, Yuan Ze University, Chung Li 320, Taiwan
*Corresponding author: jinting@mail.nctu.edu.tw
Received July 21, 2010; revised October 15, 2010; accepted October 16, 2010; posted October 19, 2010 (Doc. ID 132090); published November 30, 2010
This work demonstrates the feasibility of the generation of an RF direct-detection vector signal using optical in-phase/ quadrature-phase (I/Q) upconversion. The advantage of the proposed transmitter is that no electrical mixer is needed to generate the RF signal. Therefore, I/Q data of RF signals are processed at baseband at the transmitter, which is independent of the carrier frequency of the generated RF signal. A 10 Gb=s 16 quadrature amplitude modulation signal is experimentally demonstrated. Following transmission over a 50 km single-mode fiber, the power penalty is negligible. Moreover, I/Q imbalance of the proposed transmitter is studied and compensated by digital signal processing, which is both numerically and experimentally verified. © 2010 Optical Society of America
OCIS codes: 060.5625, 060.4510.
Radio-over-fiber (RoF) systems have attracted consider-able interest because of their potential use in future broadband wireless communications. The RoF system that distributes RF signals over an optical fiber is a pro-mising approach because of its high bandwidth and low propagation loss. To provide a higher-data-rate trans-mission, high spectral efficiency modulation formats are required, because the bandwidth of a wireless chan-nel is limited. Therefore, high-order quadrature am-plitude modulation (QAM) is a good candidate. The generation of optical RF vector signals using an external Mach–Zehnder modulator (MZM) based on double-sideband (DSB) and single-double-sideband (SSB) modulation schemes have been recently demonstrated [1]. However, both modulation schemes suffer from sensitivity degra-dation because of the limited optical modulation index. Additionally, the DSB signal undergoes performance fad-ing owfad-ing to fiber dispersion. Furthermore, to generate an RF vector signal in a high-frequency band, an electri-cal mixer with a typielectri-cal conversion loss of more than 8 dB is required to upconvert vector signals to an inter-medium or RF, which degrades the performance of the upconverted RF vector signals, especially at the higher frequency. Recently, the generation of optical RF signals by all-optical upconversion has been extensively investi-gated [2–9]. However, the corresponding system requires at least two modulators to upconvert the in-phase (I) and quadrature-phase (Q) signals and occupies much more optical bandwidth than traditional DSB and SSB modula-tion schemes [4–8]. Moreover, [9] needs a local oscillator for homodyne or heterodyne detection, which is not practical for access applications.
This investigation proposes an optical RF vector signal generation approach using optical upconversion and stu-dies its performance both numerically and experimen-tally. The advantage of this architecture is that it requires no electrical mixer. Furthermore, since the
pro-posed system generates only one unmodulated optical subcarrier and one modulated optical subcarrier, i.e., the SSB format, the system does not suffer from RF fad-ing. Moreover, the optimization of their power ratio can be simply realized, because the power of two optical subcarriers can be adjusted independently. Without an electrical mixer to upconvert the RF vector signal, the proposed system can support a high-data-rate signal with excellent performance. However, an important factor that significantly affects system performance is the pre-cision of controlling the amplitude and phase of the input I/Q signals. Therefore, the Gram–Schmidt orthogonali-zation procedure (GSOP) that compensates for the I/Q imbalance is proposed and demonstrated. With I/Q imbalance compensation, both simulation and experi-mental results verify a significant increase in the toler-ance of both amplitude mismatching and conjugate misalignment.
Figure 1presents the proposed optical transmitter to generate a direct-detection wideband optical vector signal. The optical field is modulated by an integrated modulator, which consists of three submodulators: two submodulators for in-phase modulation (MZ-a) and quad-rature-phase modulation (MZ-b), and another submodu-lator (MZ-c) for controlling the phase difference between MZ-a and MZ-b. The optical field at the input of the inte-grated modulator is given byEinðtÞ ¼ EocosðωctÞ, where
Eoandωcare the amplitude and angular frequency of the
optical field, respectively. MZ-a and MZ-b are both biased at the minimum transmission point, and MZ-c maintains a 90° phase shift between the output signals of MZ-a and MZ-b. The optical field at the output of the transmitter is given by
EoutðtÞ ¼ −EosinðπIðtÞ=2VπÞ cosðωctÞ
þ EosinðπQðtÞ=2VπÞ sinðωctÞ; ð1Þ
December 1, 2010 / Vol. 35, No. 23 / OPTICS LETTERS 4069
where IðtÞ and QðtÞ are the in-phase and quadrature-phase data of the vector signal, respectively.
To realize the direct-detection optical vector signals, an unmodulated optical subcarrier is generated atωcþ ωRF,
as shown in Fig.1. Two sinusoidal waves with the same RF frequency but a 90° phase difference are sent to MZ-a and MZ-b after being combined with the electrical I/Q signals. The unmodulated optical subcarrier with carrier suppres-sion is generated at an angular frequency ofωcþ ωRF. The
high-order terms and the interference between the un-modulated and un-modulated subcarriers caused by the nonlinear transfer function of an MZM can be neglected when the modulation depth is small. Accordingly, the optical field at the output of the transmitter can be approximated as
EoutðtÞ ≅ Eof−J0ðmÞ sinðπIðtÞ=2VπÞ cosðωctÞ
þ J0ðmÞ sinðπQðtÞ=2VπÞ sinðωctÞ
− 2J1ðmÞ cosððωcþ ωRFÞtÞg; ð2Þ
wherem is ðV=2VπÞπ and V is the amplitude of the elec-trical sinusoidal driving signal. As determined by square-law photodetection, the beating terms of the modulated and unmodulated signals generate the desired RF vector electrical signal at the frequency ofωRF, and can be
ex-pressed as
Iphoto¼ RJ0ðmÞJ1ðmÞE20½sinðπIðtÞ=2VπÞ cosðωRFtÞ
− sinðπQðtÞ=2VπÞ sinðωRFtÞ; ð3Þ
whereR is the responsivity of the photodiode. Since the modulation depth is small, the equation can be further
simplified as
Iphoto¼ ðπ=2VπÞ
×RJ0ðmÞJ1ðmÞE2
0½IðtÞ cosðωRFtÞ
− QðtÞ sinðωRFtÞ: ð4Þ
Because the generated optical signal has one modu-lated subcarrier and one unmodumodu-lated subcarrier, the proposed system can generate not only on–off-keying sig-nals but also phase-shift keying, QAM, and orthogonal frequency-division multiplexing signals. However, the I/Q data transmit over different paths to the modulator, resulting in amplitude mismatch and conjugate misalign-ment. The possible origins of the amplitude mismatch can be the difference between the powers of the I/Q signals, the differentVπof MZ-a and MZ-b, and the imper-fect splitting ratio between MZ-a and MZ-b. Furthermore, conjugate misalignment arises because MZ-c does not provide an exact 90° phase shift between the output sig-nals of MZ-a and MZ-b. The amplitude mismatch can cause signal distortion, and the conjugate misalignment causes interference between the I/Q signals. These effects can be expressed analytically:
Iphoto¼ ðπ=2VπÞ
×RJ0ðmÞJ1ðmÞE2
0½aIðtÞ cosðωRFtÞ
− QðtÞ sinðωRFt þ θÞ; ð5Þ
wherea and θ are the amplitude mismatch and the con-jugate misalignment parameter, respectively. Figure 2
Fig. 1. (Color online) Experimental setup of proposed optical I/Q upconversion system.
Fig. 2. (Color online) Concept of imbalance effect.
Fig. 3. (Color online) Simulation results of amplitude mis-match and conjugate misalignment.
presents the concept of amplitude mismatch and conjugate misalignment. While Fig.2(a)exhibits the ideal constellation of a square 16-QAM signal, the amplitude mismatch results in unequal powers of the I/Q signals shown in Fig. 2(b). Moreover, the conjugate misalign-ment leads an oblique constellation depicted in Fig.2(c), and the I/Q signals interfere with each other. To solve the issue of I/Q imbalance, the GSOP is proposed [9] to reduce the imbalance effect.
The VPI WDM-TransmissionMaker is used to simulate the effects of I/Q imbalance and the correction using the GSOP. Figures 3(a)and3(b) present the simulation re-sults concerning a 16-QAM signal. A 3 dB amplitude mis-match and a 15° conjugate misalignment result in 8 and 6:4 dB signal-to-noise ration (SNR) degradation, respec-tively. The SNR degradations of∼0:6 and ∼0:9 dB cor-responding to a 3 dB amplitude mismatch and a 15° conjugate misalignment, respectively, are compensated by the GSOP. The results clearly reveal the criticality of the I/Q imbalance for the 16-QAM format and the great performance improvement by the GSOP. The simulation results reveal that the GSOP can remove most penalties caused by the I/Q imbalance for different modulation formats.
Figure 1presents the experimental setup of the pro-posed system. Since the baseband 16-QAM signal is com-plex, the real and imaginary parts are sent from channel one and channel two of a Tektronix AWG7102 arbitrary waveform generator (AWG). The 16-QAM signal data rate is 10 Gb=s, as shown in inset (i) of Fig.1. To realize op-tical direct detection, a new opop-tical subcarrier is gener-ated in one sideband at a frequency that is 8 GHz higher than the original optical carrier, as shown in inset (ii) of Fig. 1. Following square-law detection, an electrical 2:5 GSymbol=s signal at 8 GHz is generated and captured by a Tektronix DPO 71254 with a 50 Gb=s sampling rate, as shown in inset (iii) in Fig.1. The off-line DSP program is used to demodulate the vector signal. The bit error rate (BER) performance is calculated from the measured SNR [2]. Figure4presents the experimental results obtained using GSOP compensation. Since the frequency response of the AWG, amplifier, and modulators is uneven, the feed-forward equalizer is used to reduce the intersymbol interference effect. Figures4(a)and4(b)present experi-mental results concerning the 16-QAM signal, and the GSOP compensation can decrease the SNR degradation from 8.3 to 0:6 dB (from 6.6 to 0:6 dB) due to a 3 dB amplitude mismatch (15° conjugate misalignment). The
results also verify the criticality of the I/Q imbalance for the 16-QAM format. Furthermore, the experimental results agree well with the simulation results. Figure 5
plots the transmission BER curves of the 16-QAM signal with the GSOP compensation. A receiver sensitivity of −11:7 dBm is achieved at a BER of 10−9 in the
back-to-back case. The penalty at the BER of 10−9 is negligible following 50 km single-mode fiber (SMF) transmission. The inserts in Fig.5present the constellation diagrams, and no obvious distortion is observed after fiber trans-mission.
This study proposes the generation of an RF vector signal using all-optical I/Q upconversion for direct detec-tion. 10 Gb=s 16-QAM signals at 8 GHz are demonstrated in both the numerical simulation and the experiment. By applying the GSOP, an SNR degradation of less than 1 dB is achieved for the 16-QAM signals, as a 3 dB amplitude mismatch or 15° conjugate misalignment is applied. Moreover, after transmission over a 50 km SMF, a negli-gible power penalty is observed.
The authors thank the National Science Council of Taiwan (NSCT) for financially supporting this research under contracts NSC99-2221-E-009-047-MY3 and NSC 99-2221-E-009 -046 -MY3.
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Fig. 4. (Color online) Experimental imbalance results.
Fig. 5. (Color online) BER curves of a 10 Gb=s 16-QAM signal. December 1, 2010 / Vol. 35, No. 23 / OPTICS LETTERS 4071
