High-speed optical DQPSK and FSK modulation using integrated Mach-Zehnder interferometers

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1 High-speed optical DQPSK and FSK modulation using integrated Mach-Zehnder interferometers Tetsuya Kawanishi, Takahide Sakamoto, Tetsuya Miyazaki and Masayuki Izutsu National Institute of Information and Communications Technology, Nukui-kita, Koganei, Tokyo , Japan Phone: , Fax: Takahisa Fujita, Shingo Mori, Kaoru Higuma and Junichiro Ichikawa New Technology Research Laboratories, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi, Funabashi, Chiba , Japan Abstract: We investigated an integrated optical modulator consisting of two Mach-Zehnder interferometers. The modulator can generate optical signals in various types of modulation formats, which have advantages for long-haul transmission, optical labeling, etc. By using a fabricated versatile optical modulator having traveling-wave electrodes designed for high-speed signals, we demonstrated generation of optical 40 Gb/s frequency-shift-keying signals, which can be demodulated by an optical filter. 80 Gb/s optical differential quadrature-shift-keying modulation was also demonstrated, where 40 Gb/s in-phase and quadrature data were, simultaneously, fed to the modulator Optical Society of America OCIS codes: ( ) Modulators; ( ) Modulation References and links 1. Y. Miyamoto, A. Hirano, K. Yonenaga, A. Sano, H. Toba, K. Murata, and O. Mitomi, 320 Gbit/s (8x40 Gbit/s) WDM transmission over 367 km with 120 km repeater spacing using carrier-suppressed return-to-zero format, Electron. Lett. 35 (1999) A. H. Gnauck, G. Raybon, S. Chandrasekhar, J. Leuthold, C. Doerr, L. Stulz, and E. Burrows, 25x 40-Gb/s Copolarized DPSK Transmission Over 12x100-km NZDF With 50-GHz Channel Spacing, Photonics Technol. Lett. 15, (2003) 3. N. Yoshikane and I. Morita, 1.14 b/s/hz Spectrally Efficient 50x85.4 Gb/s Transmission Over 300 km Using Copolarized RZ-DQPSK Signals, J. Lightwave Technol. 23, (2005) 4. A. H. Gnauck, P. J. Winzer, S. Chandrasekher, and C. Dorrer, Spectrally Efficient (0.8 b/s/hz) 1-Tb/s (25x42.7 Gb/s) RZ-DQPSK Transmission Over km SSMF Spans With 7 Optical Add/Drops, ECOC 2004 PD, Th N. Kikuchi, S. Sasaki, K. Sekine, and T. Sugawara, Investigation of Cross Phase Modulation (XPM) Effect on Amplitude- and Phase- Modulated Multi-Level Signals in Dense- WDM Transmission, OFC 2005, OWA4 6. T.Miyazaki, Y.Awaji, Y.kamio, and F.Kubota, Field Demonstration of 160-Gb/s OTDM Signals Using Eight 20-Gb/s 2-bit/symbol Channels over 200Km, OFC 2005 OFF1 7. W. Idler, A. Klekamp, R. Dischler, and B. Wedding, Advantages of Frequency Shift Keying in 10-Gb/s Systems, 2004 IEEE/LEOS Workshop on Advanced Modulation Formats FD3 (2004) 8. T. Sakamoto, T. Kawanishi, T. Miyazaki, and M. Izutsu, Novel Modulation Scheme for Optical Continuous- Phase Frequency-Shift Keying, OFC 2005 OFG2 9. T. Sakamoto, T. Kawanishi, and M. Izutsu, Optical minimum-shift-keying with external modulation scheme, Optics Express. 13, (2005) (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4469

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3 and direct detection [14]. The merit of this FSK labeling is that an FSK transmitter generates the label information on the optical carrier frequency without affecting its intensity. The label information can be extract without affecting the payload signal. FSK signal can bes generated by direct modulation of electric current in a laser light source [14, 20, 21], however the bit rate is limited by the response of the laser. In addition, parasitic intensity modulation should be compensated by using an additional intensity modulator when we need constant amplitude. Recently, we reported an integrated LiNbO 3 versatile optical modulator consisting of two Mach-Zehnder interferometers which can generate optical signals in various modulation formats [22]. The versatile modulator is based on FSK and SSB modulators [11, 23, 24], so that the modulator can shift the output optical frequency [25]. The modulator can also control the in-phase and quadrature components of the output lightwave, and is applicable for quadrature phase-shift-keying (QPSK) and quadrature amplitude modulation (QAM) [16, 17]. In this paper, we investigate optical FSK and QPSK modulation using fabricated versatile modulators, where two sub Mach-Zehnder (MZ) structures were embedded in a main MZ structure. Each MZ structure had a pair of electrodes to obtain push-pull operation, and a traveling wave electrodes designed for 40 Gb/s signals. The modulator provides high-speed and stable FSK modulation, while FSK bit rate of the direct modulation is limited by the response of the laser [10]. We demonstrated 40 Gb/s FSK modulation with 80 GHz frequency deviation, where a pair of 40 GHz sinusoidal signals were fed to the two sub MZ structures, and a 40 Gb/s data signal was applied to the main MZ structure. 80 Gb/s optical DQPSK modulation was also demonstrated, where 40 Gb/s in-phase and quadrature data were, simultaneously, fed to the sub MZ structures in the modulator. 2. Versatile modulator The versatile modulator consists of two sub MZ structures (MZ A and MZ B ) as shown in figure 1. The device structure is almost the same in the FSK modulator [11, 23], but the versatile modulator has six electrodes (A1, A2, B1, B2, C1 and C2) for chirp control and low halfwave voltage [22]. The versatile modulator is composed of six optical phase modulators, where induced phases under the electrodes are denoted by φ A1, φ A2, φ B1, φ B2, φ C1 and φ C2. For simplicity, we assume that the two MZ structures are in balanced push-pull operation, where the amplitude of the signal on the electrode φ A1 (φ B1 ) is equal to that of φ A2 (φ B2 ), but there is 180 phase difference ( φ A1 =- φ A2, φ B1 =- φ B2 ). When the electrodes φ C1 and φ C2 are also in balanced push-pull operation (φ C1 =-φ C2 ), the function would be similar to that of the FSK modulator [23]. When we apply single-tone rf-signals of the same frequency ( f m ) on MZ A and MZ B with 90 phase difference, a frequency shifted lightwave can be generated at the output port of the modulator. The sub MZ structures should be in null-bias point, where the dc-bias can be controlled by the electrodes φ A1, φ A2, φ B1 and φ B2. To eliminate upper sideband (USB) or lower sideband (LSB), 90 optical phase difference should be induced between the optical paths under the electrodes φ C1 and φ C2. The amplitudes of USB and LSB are, respectively, described by [1+iexp( iφ C )]/2 and [1+iexp(iφ C )]/2, where φ C = φ C1 φ C2, and φ C = +90 corresponds to an optimal condition for USB generation. Thus, by feeding a non-return-to-zero (NRZ) signal (source signal, henceforth), whose zero and mark levels respectively correspond to φ C = +90 and 90, to φ C1 or φ C2, we can generate an optical FSK signal, without parasitic intensity modulation. On the other hand, if the single-tone rf-signals are in phase, we can generate a return-to-zero (RZ) OOK signal, where the zero and mark levels of the source signal should be C = +180 and 0, respectively. When MZ A and MZ B are set to be in null-bias points, carrier-suppressed RZ (CSRZ) signals would be generated. The duty cycle of the RZ signals can be controlled by the bias of MZ A and MZ B. When there are rf-signal amplitude differences between φ A1 (φ B1 ) and φ A2 (φ B2 ), the output would be a chirped RZ signal. For (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4471

4 A1 A2 MZ A C1 C2 MZC B1 B2 MZ B Fig. 1. Versatile modulator. Optical response [db] A1 A2 B1 B2 C1 C Frequency [GHz] Fig. 2. Frequency response. BPSK signals, the amplitude induced phase difference of the source signal should be 360. We can also generate QPSK signals by feeding the inphase and quadrature signal components to MZ A and MZ B, respectively, where C should be +90 or 90. In addition, there are some other useful setups for advanced modulation, as shown in Table 1. By changing electric feeding circuit configurations, the versatile modulator can generate various types of modulated signals. We fabricated two types of versatile modulators. One was optimized for FSK (henceforth, FSK modulator), and the other was for QPSK (QPSK modulator). For FSK modulation, a highspeed data signal should be fed to the main MZ structure, to change the output optical frequency according to the data signal. In order to reduce V π for the high-speed data signal, the FSK modulator should have long traveling-wave electrodes for the main MZ structures, so that the length of the sub MZ structure was limited by the wafer size, where V π of the sub MZ structures was higher than 10 V at 40 GHz. Frequency response of the phase modulators in the fabricated FSK (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4472

5 Table 1. Modulation configurations for various formats. OOK BPSK OOK BPSK QPSK MZ A φ A1, φ A2 SS: φ A1 -φ A2 =0 o, 180 o SS: φ A1 = -φ A2 = 0 o, 180 o φ A1 -φ A2 =0 o for NRZ φ A1 -φ A2 =0 o for NRZ SS (In phase) : φ A1 = -φ A2 =0 o, 180 o MZ B φ B1, φ B2 SS: φ B1 -φ B2 = 0 o, 180 o SS: φ B1 = -φ B2 = 0 o, 180 o φ B1 -φ B2 = 0 o for NRZ φ B1 -φ B2 = 0 o for NRZ SS (Quadrature) : φ B1 = -φ B2 = 0 o, 180 o MZ C φ C1, φ C2 φ C1 -φ C2 = 0 o for NRZ φ C1 -φ C2 = 0 o for NRZ SS: φ C1 -φ C2 = 0 o, 180 o SS: φ C1 =-φ C2 = 0 o, 180 o φ C1 -φ C2 = 90 o QPSK FSK SS (MSB) : φ A1 = -φ A2 =0 o, 180 o cos 2πf m t SS (MSB) : φ B1 = -φ B2 = 0 o, 180 o sin 2πf m t SS(LSB) : φ C1 =φ C2 = 0 o, 90 o C1 and C2 are in phase SS: φ C1 -φ C2 = -90 o, 90 o When φa 1 φa2, φb 1 φb2, φc1 φc 2, the output would be chirped. SS: source signal, PS: pulse shape signal for carving, MSB: most significant bit, LSB: least significant bit modulator are shown in Fig. 2. The 3 db bandwidths were about 30 GHz, and the 6 db bandwidths were larger than 40 GHz. The main and sub MZ structures were successfully integrated onto a single-chip using z-cut LiNbO 3 integration platform. The length of the electrodes in the two sub MZ structures (A1,A2,B1 and B2) was 16 mm, while that of the electrodes in the main MZ structure (C1 and C2) was 32 mm. The V π of the sub MZ structures (MZ A and MZ B ) and the main MZ structure (MZ C ) were, respectively, 4.9 V and 2.5 V in push-pull operation at low frequency, where the insertion loss of the modulator was 5.2 db. On the other hand, for QPSK modulation, a pair of data signals are applied to the two sub MZ structures, to achieve control of in-phase and quadrature components. Thus, the QPSK modulator should have long electrodes in the sub MZ structures, to reduce V π of the sub MZ structures. The electrode lengths of the main and sub MZ structures were respectively 16 mm and 32 mm. The V π of the main and sub MZ structures were, respectively, 4.9 V and 2.5 V in push-pull operation at low frequency. Optical 3 db bandwidth of each electrode was larger than 27 GHz. The insertion loss of the QPSK modulator was 5.1 db Gb/s FSK modulation and 40 GHz optical frequency shift Figure 3 shows the experimental setup for FSK modulation. Four sinusoidal electric signals having 90 phase differences were applied to the electrodes A1, A2, B1 and B2, for generation of sideband components. The phase differences were controlled by using tunable delay lines. Figure 4 shows the spectrum of the FSK modulator output, where we applied dc voltage on the main MZ structure. As described in [11], the FSK modulator can suppress the input component (carrier) and one of the sideband (USB or LSB), so that output can be a single mode signal consisting of USB or LSB. This scheme is called single-sideband suppressed-carrier (SSB-SC) (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4473

6 Power divider DC bias for MZA 40GHz A1 A2 Phase shifter 40G NRZ signal C1 FSK modulator Laser diode B1 Oscilloscope B2 Photo Detector C2 DC bias for MZB DC bias for MZc Phase shifter AWG Fig. 3. Experimental setup for FSK modulation. Fig. 4. Optical spectra of frequency shifted signals. modulation. By changing the dc voltage φ C, we can select the output optical frequency (USB or LSB), as shown in Fig. 4. The extinction ratio of undesired components was 17 db. The signal frequency f m was 40 GHz, so that the optical frequency deviation was 80 GHz. By sweeping the frequency of the sinusoidal electric signals as described in [25], we can construct an optical frequency sweeper of ± 40 GHz tunable range. As shown in figure 3, an optical 40 Gb/s FSK signal was generated by feeding a non-returnto-zero (NRZ) pseudo-random-bit-sequence (PRBS) 40 Gb/s data signal to the main MZ structure (electrode C1). Figure 5 shows an optical spectrum of the 40 Gb/s FSK signal. The (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4474

7 Optical power [dbm] Optical frequency [GHz] Fig Gb/s optical FSK signal, where frequency deviation was 80 GHz. Optical power [dbm] USB LSB Optical frequency [GHz] Fig. 6. Spectra of demodulated optical FSK signal. optical FSK signal was demodulated into an OOK signal, by an arrayed-waveguide (AWG). One of the sideband components (USB or LSB) can be taken out from an optical output port of the AWG whose channel separation was 50 GHz, as shown in Figs. 6 and 7. The results show that the eyes are clearly opened both in USB and LSB. The frequency deviation was larger than the bit rate in this experiment, so that the output was a wide-band FSK signal, which can be demodulated incoherently by using a conventional optical filter. However the FSK modulator can be also applied to narrow-band FSK formats, such as continuous-phase FSK, including minimum-shift-keying (MSK), by using synchronization between data for sideband selection and sinusoidal signal for sideband generation [8], and initial phase control technique [9]. (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4475

8 USB LSB 25ps Fig. 7. Eye diagrams of demodulated optical FSK signal Gb/s DQPSK modulation Figure 8 shows the experimental setup for DQPSK modulation. Each of the sub MZ structures was biased for minimum dc transmission, where optical phase difference between the two sub MZ structures was adjusted to π/2 by using the electrode C1 or C2. A pair of NRZ data streams at 40 Gb/s were obtained from a 4 : 1 multiplexer that combines four 10-Gb/s sub channels of PRBS. As shown in figure 8, one of the streams was fed to MZ A for I component modulation, and the other was fed to MZ b for Q component, where the delay between the two streams was adjusted to be 115 bit. The amplitude of I and Q signals at the input ports of the QPSK modulator was 6.5 V (peak-to-peak), corresponding to 2V π at 40 Gb/s, to generate an 80 Gb/s optical DQPSK signal at the output port of the modulator. As shown in Fig. 9, we measured an optical spectrum of a DQPSK signal at the output port of the QPSK modulator, without using any optical filters, where full spectral width measured 20 db down from the maximum of the central wavelength peak was 60 GHz. At the DQPSK demodulator shown in figure 8, the DQPSK signal generated at the modulator was decoded by a one-bit delay interferometer whose constructive and destructive ports were connected to a balanced photodetector. However no precoder was employed for our experiment, and hence there was a deterministic mapping of data from input to output. In order to allow bit-error-ratio (BER) measurements, the error detector was programmed with the expected data sequence. We used a single receiver to decode each 40 Gb/s tributary by adjusting the differential optical phase in the one-bit delay interferometer ( φ) at π/4 or π/4. Figure 10 shows eye diagrams measured at the electric output of the balanced photodetector. In back-to-back transmission, clear eye openings were observed for the two tributaries whose symbol rate was 40 Gsample/s. We measured a back-to-back BER curve of a sub channel extracted from each tributaries by a 1:4 demultiplexer, as shown in Fig. 11, where the receiver sensitivity at the BER of 10 9 was -20 dbm. 5. Conclusion We demonstrated 40 Gb/s FSK and 80 Gb/s DQPSK modulation using integrated versatile modulators. The FSK modulation scheme describe in this paper can be applicable for CPFSK formats, including MSK, which provide enhanced spectral efficiency and receiver sensitivity. To the best of our knowledge, the versatile modulator investigated in this paper is the fastest modulator which can control I and Q components independently, and this is the first time that 80 Gb/s DQPSK modulation has been achieved by an integrated optical modulator. Though any filters were not used for control of the spectral width of the DQPSK signal, the spectrum was very compact, where the full spectral width measured 20 db down from the maximum of the (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4476

9 40Gbps NRZ signal generator DATA Delay DC bias for MZA Error detector DATA Delay I I DQPSK demodulator Q Q A1 A2 C1 QPSK modulator Laser diode B1 B2 C2 DC bias for MZc DC bias for MZB Fig. 8. Experimental setup for DQPSK modulation. Fig Gb/s optical FSK signal. (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4477

10 25ps (a) φ=+π/4 (b) φ= π/4 Fig. 10. Eye diagrams of demodulated optical DQPSK signal. φ=+π/4 φ= π/4 Fig. 11. BER curves of demodulated DQPSK signal. central wavelength peak was 60 GHz. In addition, the versatile modulator can also generate high-speed optical QAM or APSK signals, by applying multi-level data signals to the sub MZ structures. Acknowledgments This study was partially supported by Industrial Technology Research Grant Program in 2004 from New Energy and Industrial Technology Development Organization of Japan. The authors wish to thank Dr. M. Tsuchiya for his fruitful discussion. (C) 2006 OSA 15 May 2006 / Vol. 14, No. 10 / OPTICS EXPRESS 4478