Generation of optical millimeter-wave signals and vector formats using an integrated optical I/Q modulator [Invited]

Transcription

1 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 188 Generation of optical millimeter-wave signals and vector formats using an integrated optical I/Q modulator [Invited] Jyehong Chen, 1, * Chun-Ting Lin, 1 Po Tsung Shih, 1 Wen-Jr Jiang, 1 Sheng-Peng Dai, 1 Yu-Min Lin, 2 Peng-Chun Peng, 3 and Sien Chi 1,4 1 Department of Photonics and Institute of Electro-optical Engineering, National Chiao-Tung University, Hsinchu 300, Taiwan 2 Information and Communications Research Labs, Industrial Technology Research Institute, Hsinchu 300, Taiwan 3 Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou County, 545 Taiwan 4 Department of Electrical Engineering, Yuan-Ze University, Chung Li 320, Taiwan *corresponding author: jchen@mail.nctu.edu.tw Received August 20, 2008; revised November 24, 2008; accepted December 4, 2008; published January 23, 2009 Doc. ID This study discusses two key technologies used in radio-over-fiber (RoF) systems, namely, the generation and transmission of millimeter-wave signals and optical modulation schemes capable of carrying vector signal formats and utilizing the continuous performance improvements offered by digital signal processing. A cost-effective frequency-quadrupling technique capable of generating millimeter-wave signals up to 72 GHz is proposed. The generated optical millimeter-wave signals have very high quality, with an optical carrier and harmonic distortion suppression ratio exceeding 36 db. An optical modulation scheme that can support a 64-QAM, 16 Gbits/s orthogonal frequency-division multiplexing RoF system is also demonstrated. Results of this study demonstrate that both methods offer realistic solutions to support future wireless systems Optical Society of America OCIS codes: , , Introduction Following the accelerated development of wireless communications, efficient and costeffective methods of generating and transmitting microwave and millimeter-wave signals are increasingly important. The generation and transmission of microwave and millimeter-wave signals over an optical fiber has been investigated in detail for various applications, including broadband wireless access networks, phase-array antennas, optical sensors, remote antennas, and radars [1 8]. The advantages of using optical fiber as a millimeter-wave signal transmission medium lie in its almost unlimited bandwidth and minimal propagation loss. This study proposes two enabling technologies for future hybrid access networks and discusses them in detail. First, optical millimeter-wave signals are generated via frequency quadrupling. Generation of millimeter-wave signals at frequencies exceeding 40 GHz remains a major challenge because of the frequency response of the LiNbO 3 Mach Zehnder modulator (MZM) or phase modulator (PM) being below 40 GHz. Furthermore, electrical components and equipment capable of operating at frequencies of above 40 GHz, including amplifiers, mixers, and synthesizers, are very expensive. Therefore, developing a cost-effective means of generating optical millimeter-wave signals at frequencies above 40 GHz is of great interest. Several optical millimeter-wave signal generation schemes based on a MZM or PM and designed to achieve frequency multiplication have recently been demonstrated [1 8,10,11]. However, these proposed systems either require multiple optical filters to remove undesired optical sidebands [2 8] or require two cascaded external modulators [8,11], either of which requirement significantly increases system complexity and cost. The required optical filtering severely hinders the implementation of optical upconversion in a wavelength-division-multiplexed (WDM) radio-over-fiber (RoF) sys /09/ /$ Optical Society of America

2 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 189 tem [9]. In addition to RoF techniques, simultaneous transmission of wired baseband (BB) and wireless RF signals for low-cost quadruple-play services (wireless, telephone, television, and Internet) over the constructed passive optical networks (PONs) also attract much attention. This work experimentally demonstrates an optical upconversion system using a frequency-quadrupling approach. Four-channel BB 1.25 Gbits/ s WDM millimeter-wave signals are simultaneously upconverted with optical carrier suppression using only one external modulator without narrowband optical filtering. Since no narrowband optical filter is needed, the proposed system can be implemented in WDM upconversion systems. This study demonstrates the feasibility of a frequency-quadrupling approach that generates optical carrier-suppressed millimeter-wave signals using a single optical in-phase and quadrature (I/Q) modulator and that does not require optical filtering. By setting the bias point of MZMs at the maximum transmission, the obtained millimeter-wave signals have an optical carrier and undesired harmonic distortion suppression ratio above 36 db. Since the optical carrier and harmonic distortion is highly suppressed, the high-purity two-tone millimeter-wave lightwave is not impaired because of fiber dispersion. Second, a remote-heterodyne optical vector modulation scheme based on the same optical I/Q modulator is proposed and shown to be able to support a 64-QAM, 16 Gbits/s orthogonal frequency-division multiplexing (OFDM) RoF system with digital signal processing (DSP) impairment equalization. Coherent communication has long been considered a promising technique to achieve the highest receiver sensitivity with excellent spectral efficiency [12 14]. Because coherent detection, compared with direct detection, retains the optical field phase information, this allows impairments such as dispersion and polarization mode dispersion (PMD) to be easily compensated [13 16]. However, because of the requirements to track the phase and polarization of the incoming signal, an optical coherent receiver is much more complex than a direct detection receiver. With the advance of DSP, the complexity of digital coherent receivers has been dramatically reduced [13]. Although DSP has been widely used in wireless communications to support very high spectral efficiency data formats and correct for various transmission impairments, because of the high data rate in optical communication, DSP has only recently been demonstrated to be capable of supporting a 40 Gbits/s coherent system [17]. Thanks to the dramatic advance of DSP, coherent communication has become much more practical. Nonetheless, there are other difficulties needed to be resolved, e.g., the offset frequency estimation [18] and the spectral linewidth requirement of the local oscillator [19 21]. Ip and Kahn [19] had pointed out, assuming a 1.0 db penalty, a linewidth-to-bit-rate ratio of is required for the 16-QAM format. Yoshida et al. [20] experimentally demonstrated that a light source of 4 khz linewidth is needed for a 64-QAM, 6 Gbits/s system. In this study, a remoteheterodyne optical vector modulation scheme is demonstrated that can support a 64-QAM, 16 Gbits/s OFDM RoF system employing a commercial distributed feedback (DFB) laser with linewidth of the order of several tens of megahertz. The new scheme significantly relaxes the stringent spectral requirement of the light source and at the same time retains optical phase information that allows impairments to be easily compensated. The combination of OFDM and RoF systems (OFDM-RoF) has attracted considerable attention in future broadband wireless access (BWA) for high-speed data and video services with speeds exceeding 10 Gbits/s. RoF technology using the millimeterwave band is a promising solution for providing multigigabits per second service, wide converge, and mobility. However, fiber dispersion continues to limit the transmission length of the optical RF signal above 10 Gbits/s in the millimeter-wave band [10,22]. Recently, OFDM modulation has been demonstrated to exhibit high tolerance to fiber dispersion for long-haul communication, and it is capable of supporting higher-level modulation formats to achieve data rates exceeding 10 Gbits/s [23]. In this study, with an aim of reducing the occupied bandwidth of the RF signal and supporting multigigabit BWA for IPTV, HDTV, or WirelessHD [24] applications, the performance of highly spectral-efficient 16- and 64-QAM OFDM for RoF links is examined. Both 15 Gbits/s 64-QAM and 20 Gbits/s 16-QAM OFDM signals are experimentally demonstrated, and negligible penalties are observed following 25 km of single-

3 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 190 mode fiber (SMF). To the best of the knowledge of the authors, this is the highest data rate being demonstrated in the direct-detection OFDM-RoF system. The remainder of this paper is organized as follows. Section 2 presents a theoretical analysis of optical millimeter-wave signal generation via frequency quadrupling. Section 3 then experimentally demonstrates optical 40, 60, and 72 GHz millimeter-wave signals using electrical 10, 15, and 18 GHz driving signals. Subsequently, Section 4 describes the experimental results of a 15 Gbits/s 64-QAM and a 20 Gbits/s 16-QAM OFDM system, and finally Section 5 presents a summary and conclusions. 2. Theoretical Analysis With MZM biases at maximum transmission, Fig. 1 shows a conceptual diagram of the optical carrier-suppressed millimeter-wave signal generation using a frequencyquadrupling technique. A commercially available optical I/Q modulator is employed in which two sub-mzms (MZ-a and MZ-b) are embedded in each arm of the main modulator (MZ-c). The optical field at the input of the integrated MZM can be written as E in t = E o cos o t, 1 where E o denotes the amplitude of the optical field and 0 represents the angular frequency of the optical carrier. Both MZ-a and MZ-b are biased at maximum transmission. The electrical driving signals sent into MZ-a and MZ-b are V a t =V m cos RF t and V b t =V m cos RF t+ /2, respectively. Moreover, the MZ-c is biased at the minimum transmission to provide an additional phase shift between these two arms. The optical field at the output of the integrated MZM can be expressed as 2 E out t = E o cos o t cos mcos RF t cos o t cos mcos RF t +, 2 where the phase modulation index m is V m /2V. Expanding cos m cos RF t and cos m cos RF t+ /2 using the Bessel function [25], the output optical field can be rewritten as E out t = E o J 4n 2 m cos o + 4n 2 RF t + cos o 4n 2 RF t, n=1 3 where J n denotes the Bessel function of the first kind of order n. When m equals (i.e., V m =2V ) the corresponding values of J 2 m, J 6 m, J 10 m, and J 14 m are , , , and , respectively. The optical sidebands with an order higher than J 6 thus can be neglected without significant errors, and the optical field can be further simplified to Fig. 1. Experimental setup of optical quadruple-frequency millimeter-wave signal generation biased at maximum transmission (PC, polarization controller; OSA, optical spectrum analyzer; ESA, electronic spectrum analyzer).

4 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 191 E out t = E o J 2 m cos o +2 RF t + J 2 m cos o 2 RF t + J 6 m cos o +6 RF t + J 6 m cos o 6 RF t. 4 Following square-law detection using a photodiode (PD) with responsivity R, the photocurrent can be expressed as i t = R E t 2. 5 The cross terms of Eq. (4) produce the desired frequency-quadrupling millimeterwave signal and its harmonic distortion signals. The final millimeter-wave signals are = RE i4 RF 2 o J 2 m J 2 m +2J 2 m J 6 m cos 4 RF t i 8 RF = RE 2 o 2J 2 m J 6 m cos 8 RF t. 6 i 12 RF = RE 2 o J 6 m J 6 m cos 12 RF t Because the values of J 2 m and J 6 m are and , respectively, without causing significant error, the equations can be further simplified if the J 6 m term is dropped in Eq. (6). The final photocurrent is i 4 RF RE o 2 J 2 m J 2 m cos 4 RF t. 7 And this is the desired frequency-quadrupling term. 3. Experimental Results and Discussions To verify the proposed method, experiments are conducted using setups as displayed in Fig. 1. A commercially available optical I/Q MZM is employed to produce the frequency-quadrupling signals. A commercial DFB laser is employed as a cw light source, and a polarization controller (PC) is adopted to adjust the polarization before the light is sent into the MZM. The frequency of the microwave signal is set to 10, 15, and 18 GHz, respectively. The generated optical millimeter-wave signal is amplified using an EDFA and then transmitted over a 50 km single-mode fiber (SMF). The optical power is normalized to 1 dbm before PD detection. Figure 2(a) shows the measured spectrum of the optical 40 GHz millimeter-wave signal using a 10 GHz driving signal before transmission. The optical carrier is effectively suppressed, and the power of the two second-order sidebands that are converted into a 40 GHz electrical millimeter-wave signal after PD detection is 38 db higher than that of the other order sidebands. Sidebands other than the second- and sixthorder sidebands are observed due to the imbalance in the y-junction splitting ratio of the MZM [26]. The suppression of the undesired harmonic sidebands exceeds 38 db and negligibly affects the performance of the optical millimeter-wave signal. Figure 2(b) presents the waveform of the optical millimeter-wave signal with a 50% duty cycle associated with high suppression of the undesired harmonic sidebands. Figure 3(a) presents the electrical spectrum of the generated back-to-back (BTB) millimeter-wave signal with a 40 GHz span and a 30 khz resolution bandwidth. A strong electrical signal with four times the frequency of the driving signal is observed, and the first, second, and third terms of the electrical signal are suppressed to the point where they fall totally below the noise floor. Following 50 km SMF transmission, the spectrum remains very pure and maintains the same suppression ratio as presented in Fig. 3(b). Figure 4(a) reveals that the linewidth of the generated 40 GHz signal is less than 5 Hz and almost equal to that of the 10 GHz driving signal. After transmission over 50 km of SMF, no linewidth broadening of the electrical 40 GHz signal due to fiber dispersion is observed, as shown in Fig. 4(b). Although the bandwidth of the MZM can generate an 80 GHz millimeter-wave signal, limited by the available RF amplifier in the author s laboratory, only 60 and 72 GHz millimeter-wave signals are obtained by using 15 and 18 GHz electrical driving signals, respectively. Figures 5(a) and 5(b) show that the optical powers of the two second-order sidebands of two millimeter-wave signals are at least 36 db higher than those of the other sidebands, which is sufficient for most practical applications. Although the MZM has been used for several decades, bias drift, which affects MZM performance, remains an important issue when the MZM is biased at the maximum or minimum transmission point. In the proposed optical upconversion system, both

5 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 192 Fig. 2. Experimental results of the 40 GHz optical millimeter-wave signal. (a) Optical spectrum. The resolution is 0.01 nm. (b) Optical waveform. MZ-a and MZ-b are biased at the maximum transmission point, and MZ-c is biased at the minimum transmission point. Therefore, the single-channel harmonic distortion suppression ratio (HDSR) and performance degrading due to bias drift are investigated in this work. Since MZ-a is biased at the maximum transmission point, bias drift of MZ-a (MZ-b is similar to MZ-a) decreases the suppression of odd-order optical sidebands. Figure 6(a) illustrates the HDSR of the 20 GHz optical millimeter-wave signal versus MZ-a bias voltage drifts. The voltage drift ratio is defined as V/V 100%, where V is the voltage drift and V is the half-wave voltage of MZ-a (i.e., 4.2 V in this case). The HDSR declines from 39 to 7.5 db when the bias drift is 40%. Figure 6(b) shows the HDSR of the 20 GHz optical millimeter-wave signal MZ-c bias voltage drifts. Differing from the voltage drift effects of MZ-a and MZ-b, the HDSR decreases from 39 to 17 db when bias drift is almost 50%, which corresponds to a voltage deviation of roughly 2.1 V and can be considered an extreme case. Figure 6(c) shows the HDSR of the 20 GHz optical millimeter-wave signal versus all the MZMs bias voltage drifts. When the biases of all sub-mzms drift 30% from the optimal point, the HDSR degrades from 39 to 5 db.

6 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 193 Fig. 3. Electrical spectrum of the generated 40 GHz millimeter-wave signal. (a) BTB, (b) following 50 km of SMF transmission. Figure 7 illustrates the experimental setup of the colorless optical WDM upconversion system. Four DFB lasers, from to nm with 100 GHz channel spacing, are used as the light sources. After the optical coupler, the cw lightwaves are modulated via a single-electrode MZM driven by a 1.25 Gbits/s pseudorandom bit sequence (PRBS) electrical signal with a word length of Each channel of the WDM signals is upconverted simultaneously after the optical I/Q MZM. The upconverted WDM optical millimeter-wave signals are then amplified by an EDFA before they were transmitted over 50 km of SMF. At the receiver, a tunable optical filter (TOF) with a 0.3 nm optical bandwidth is employed to select the desired channel. Figures 8(a) and 8(b) show the optical spectra of a single-channel baseband on off keying (OOK) signal and upconverted optical carrier-suppressed millimeter-wave signal. Only second-order optical sidebands are observed and the optical carrier suppression ratio is higher than 30 db using the proposed frequency-quadrupling technique. Figures 8(c) and 8(d) illustrate the spectra of WDM baseband signals and upconverted WDM optical carrier suppressed millimeter-wave signals. All of the channels are upconverted simultaneously by using only one integrated MZM. Figure 9 plots the bit error rate (BER) curves of the WDM 1.25 Gbits/s RF downconverted OOK signals at 0 km and following 50 km of standard single-mode fiber (SSMF) transmission. The insets show the millimeter-wave eye diagrams of RF backto-back and transmitted signals. The power penalty of the downconverted RF signals at 0 km and following 50 km of transmission for each channel are less than 0.5 db.

7 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 194 Fig. 4. Electrical spectrum of the generated 40 GHz millimeter-wave signal with 100 Hz span. (a) Comparison of generated 40 GHz signal and 10 GHz driving signal. (b) Comparison of generated BTB and following 50 km of SMF transmission 40 GHz signal. The resolution bandwidth is 1Hz. These results successfully demonstrate that the proposed frequency-quadrupling millimeter-wave upconversion technique is a practical solution for future hybrid access networks. 4. Vector Signal Generation of the RoF System 4.A. Experimental Setup Figure 10 depicts the experimental setup of the OFDM RoF system. The principle of the proposed colorless OFDM transmitter is similar to the early OFDM transmitter using a frequency-doubling technique [27]. As shown in insets (i) and (ii) of Fig. 10, the generated optical OFDM signal comprises an upper sideband (USB) and a lower sideband (LSB), where the OFDM data are encoded at the USB and a sinusoidal subcarrier at the LSB. Notably, the optical carrier suppression ratio and the undesired sideband suppression ratio of RF OFDM signals exceed 26 db, which has a negligible influence on the performance of the generated RF signals. Notably, following the PD, the generated electrical signal is the beating term between the LSB and USB. There-

8 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 195 Fig. 5. Optical spectrum of the generated 60 and 72 GHz millimeter-wave signal using 15 and 18 GHz driving signal, respectively. (a) 60 GHz, (b) 72 GHz. fore, a frequency doubling is automatically obtained to reduce the cost of the electronic components, especially for the RF signal in the millimeter-wave range. The OFDM signals are generated by a Tektronix AWG7102 arbitrary waveform generator (AWG) using a MATLAB program. To enable a 20 Gbits/s sampling rate, the AWG is operated in single-channel and interleaved modes. The resolution of the digital-to-analog converter of the AWG is 8 bits. Furthermore, the inverse fast Fourier transform (IFFT) length is 128, resulting in a subcarrier symbol rate of megasymbols/s. The cyclic prefix is set to 1/64 of the symbol time to combat synchronization offsets and intersymbol interference resulting from fiber chromatic dispersion. For the 64-QAM OFDM system, the USB OFDM data occupies channels (i.e., the subcarrier center frequency range from to GHz) with the remaining 112 channels set to 0. A sinusoidal subcarrier with a frequency of GHz is generated at the LSB. Therefore, an optical 15 Gbits/s 64-QAM OFDM signal at 10 GHz that includes 16 subcarriers and occupies a total bandwidth of 2.5 GHz can be generated. For the 16-QAM OFDM system, the OFDM data utilizes channels 1 16 with the remaining 112 channels being set to 0. Following AWG, the 16-QAM OFDM signal is

9 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 196 Fig. 6. HDSR versus MZM bias voltage: (a) MZM-a bias drift, (b) MZM-c bias drift, (c) all MZM bias drift. upconverted to 10 GHz using a mixer and a 10 GHz oscillator. A 10 GHz sinusoidal subcarrier is generated at the LSB. Therefore, an optical 20 Gbits/s 16-QAM OFDM signal incorporating 32 subcarriers and occupying a total bandwidth of 5 GHz can be generated at a center frequency of 20 GHz. At the receiver, the waveform of the generated 64-QAM OFDM signals at 10 GHz is directly captured by a Tektronix DPO with a 50 Gbits/s sampling rate and a

10 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 197 Fig. 7. Experimental setup of the frequency-quadrupled WDM upconversion system. (PC, polarization controller; TOF, tunable optical filter; BPF, bandpass filter; LPF, low-pass filter; PS, phase shifter; BERT, BER tester). 3 db bandwidth of 12.5 GHz. For the generated 16-QAM OFDM signals at 20 GHz, the OFDM data are downconverted to 5 GHz by a 15 GHz oscillator and a mixer. The offline DSP program is employed to demodulate both OFDM signals. The demodulation process includes synchronization, fast Fourier transform (FFT), one-tap equalization, and QAM symbol decoding. Moreover, the BER performance is calculated from the measured error vector magnitude (EVM) [28]. 4.B. Experimental Results and Discussions The relative intensity between the optical subcarrier and optical data-modulated subcarrier strongly influences optical OFDM signal performance. One of the advantages of the proposed OFDM transmitter is that the relative optical intensity between the Fig. 8. Optical spectra of (a) single-channel BB signal, (b) single-channel upconverted signal, (c) WDM BB signals, and (d) WDM upconverted signals.

11 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 198 Fig. 9. BER curves and eye diagram of the RF signals. Fig. 10. Experimental setup of optical RF OFDM generation. Fig. 11. BER versus OPR at receiver powers of 13 dbm.

12 Vol. 8, No. 2 / February 2009 / JOURNAL OF OPTICAL NETWORKING 199 Fig. 12. BER curves of RF OFDM signals. USB and LSB can be easily tuned by adjusting the individual electrical amplitude of the LSB sinusoidal signal and the USB OFDM data signals to optimize the performance of the optical OFDM signals. Figure 11 illustrates the receiver sensitivity of the 64-QAM and 16-QAM OFDM signals versus different optical power ratios (OPRs). The OPR is defined as the total optical power ratio of the LSB subcarrier to the USB OFDM-encoded subcarrier. The optimal OPRs of 15 Gbits/s 64-QAM and 20 Gbits/s 16-QAM OFDM signals are 2.4 and 3.7 db, respectively. Figure 12 shows the BER curves of the 15 Gbits/s 64-QAM and 20 Gbits/s 16-QAM OFDM signals using optimal OPRs after transmission over 25 km of SMF. The sensitivity penalties due to the fiber transmission are negligible. 5. Conclusions This study has demonstrated two key technologies for a RoF system. First, a costeffective frequency-quadrupling technique capable of generating millimeter-wave signals up to 72 GHz was verified. The generated optical millimeter-wave signals are high quality and have optical carrier and harmonic distortion suppression ratios exceeding 36 db. Since no optical filter is needed, the proposed schemes can be utilized in optical upconversion for WDM RoF systems and continuously tunable millimeter-wave signal generation systems. Because the state-of-the-art MZM has an upper-limit frequency response around 40 GHz, the proposed method is capable of generating optical millimeter-wave signals up to 160 GHz. Second, high-spectralefficiency OFDM-RoF signals with 16- and 64-QAM beyond 15 Gbits/s are successfully demonstrated using the proposed colorless OFDM transmitter without optical filtering. The proposed architecture utilizes carrier suppression to achieve frequency doubling and successfully transmits a high-capacity OFDM signal beyond 15 Gbits/s over 25 km of SMF with negligible penalty. Results of this study demonstrate that both methods offer realistic solutions to support future wireless systems Acknowledgments The authors thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contracts NSC E MY2, NSC E , NSC M MY2, and NSC E MY3 References 1. X. Wei, J. Leuthold, and L. Zhang, Delay-interferometer-based optical pulse generator, in Optical Fiber Communication Conference, Technical Digest (CD) (Optical Society of America, 2004), paper WL6. 2. G. Qi, J. Yao, S. Joe, S. Paquet, and C. Belisle, Optical generation and distribution of continuously tunable millimeter-wave signals using an optical phase modulator, J. Lightwave Technol. 23, (2005). 3. G. Qi, J. Yao, J. Seregelyi, S. Paquet, and C. Belisle, Generation and distribution of a

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