ECOC 2010, 19-23 September, 2010, Torino, Italy
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Demonstration of 112-Gbit/s DP-QPSK modulation using InP n-p-i-n Mach-Zehnder modulators Yasuo Shibata, Eiichi Yamada, Takako Yasui, Akira Ohki, Kei Watanabe, Hiroyuki Ishii, Ryuzo Iga, and Hiromi Oohashi NTT Photonics Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198, Japan,
[email protected] Abstract We have successfully demonstrated 112Gbit/s DP-QPSK modulation using a wavelength tunable pulse carver module, in which a wavelength tunable DFB laser array (TLA) and an InP n-p-i-n Mach-Zehnder modulator are co-packaged, and a dual-drive InP Mach-Zehnder IQ modulator module. Introduction Recently there has been a great interest in multilevel modulation formats such as (differential) quadrature phase-shift keying ((D)QPSK) to improve the spectral efficiency of a lightwave communication systems [1]. Among various multi-level modulation formats, dualpolarization quaternary phase-shift keying (DPQPSK) is one of the most promising candidate toward the 100 Gbit/s or higher bit-rate systems [2]. Photonic integration circuit technologies are necessary to realize high performance and reduce the size of the optical components. Integrated modulator has been reported based on the PLC technology [3]. Semiconductor technology is expected to enable further reduction of the modulator / transmitter size. Usually, a (D)QPSK signal is generated by a parallel Mach-Zehnder modulator (P-MZM), in which two sub-MZMs are placed in each arms of a parent MZM. Each sub-MZM is used to generate the in-phase (I) or quadrature-phase (Q) signals, and the parent MZM is used to sum these signals with a phase difference of π/2. Although a P-MZM has been proven to show good performance, it has the drawback that increase in the number of electrodes (or phase modulators), and consequently the number of electric circuits as well as drivers, enlarges the size of modules and transponders. The nested configuration of MZ couplers causes also the excess optical losses. To overcome this, a simple scheme for QPSK signal generation using a single dual-drive Mach-Zehnder modulator (DD-MZM) has been reported [4]. Generation of QPSK signals using a DD-MZM has also been reported [5, 6]. Recently, we have developed an InP MZM with operating point control electrodes, and have demonstrated 40Gbit/s DPSK signal generation in entire C-band region by using multi-chip integration technology [7]. The use of the operating point control technique enables us to use the single MZM as
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IQ modulator in the DD-MZM scheme. In this paper, we demonstrate for the first time the generation of over 100 Gbit/s DP-RZ-QPSK signals using semiconductor MZ modulators. A push-pull type MZM co-packaged with tunable laser diode (TLA) [7] was used to generate the RZ pulse, and another single MZM was used as the DD-MZM or IQ modulator. Device structure Figure 1 shows the schematics of the fabricated n-p-i-n structured InP MZM chip. The MZM chip has a pair of traveling-wave electrodes for highspeed modulation and a separate pair of electrodes for operating point control. The phase modulation electrodes are 2.5 mm long and the operating point control electrodes are 1.1 mm long. The chip is 5.6 mm x 0.8 mm in size. In the push-pull driving configuration, a differential signal was applied to the modulation electrodes around operating points. By introducing the operating point control electrode, the operating points can be set independently from the modulation electrode while the DC bias voltages and consequently the half-wavelength voltages of the two modulation electrodes remain the same. This configuration ensures a high quality modulation without chirping when used in a push-pull configuration with simple symmetric RF-driving condition. When two phase modulators in each MZ arm were driven independently, each phase modulator can be
Modulation electrode Operating Point Control Electrode Fig. 1 Schematic diagram of an InP MZ modulator with operating point control electrode.
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Fig. 3 Optical spectrum of (a) 28-GHz CS-RZ pulses and (b) 28-GHz RZ pulses.
(b) Fig. 2 Photograph of (a) a modulator integrated wavelength tunable light source module (TLA-MZM) used for RZ pulse carver and (b) a dual-drive MZM module.
regard as the I-ch modulator and Q-ch modulator, respectively. To realize IQ modulating operation, the operating point control electrode should be set to give the π/2 phase shift between the two arms of the MZM. Figure 2(a) is a photograph of a fabricated compact InP-based tunable transmitter module (TLA-MZM), which incorporates a TLA and a MZM through hybrid integration. The module size is 41mm (L) x 16 mm (W) x 9 mm (H). The TLA consists of DFB lasers, a multimode interference coupler, and a semiconductor optical amplifier, and thereby serves as a highly stable and highly reliable optical source. The package was equipped with two GPPO connectors to provide a differential RF signal input for push-pull operation and 37 electrical
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lead pins to control the TLA and MZM. We have succeeded in generating 40-Gbit/s DPSK signal in entire C-band region with modulated output power of 5 dBm using this module [7]. Figure 2(b) shows a photograph of the DD-MZM module. The module size is 21 mm (L) x 17 mm (W) x 9 mm (H). The package was equipped with two V-connectors to provide two RF signals independently for dual-drive operation, or provide differential RF signal input for push-pull operation. Experiment First we examined the carver characteristics of the TLA-MZM, in which a TLA and a push-pull MZM are co-packaged. Figure 3(a) shows the observed optical spectrum for 28-GHz CS-RZ pulses using a 14-GHz sinusoidal signal as a driving signal. We can see that the carrier component was suppressed and typical CS-RZ spectrum was observed. Figure 3(b) shows the spectrum for 28-GHz RZ pulses using a 28-GHz sinusoidal driving signal. Though generation of the RZ pulses requires higher bandwidth than CS-RZ pulses, we can obtain both types of pulses using a TLA-MZM. We have confirmed that similar (CS)-RZ pulses could be obtained in entire C-band region using TLA-MZM. Figure 4 shows the experimental setup for DP-
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Fig. 4 Experimental setup for 112Gbit/s DP-QPSK modulation.
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Conclusion We have successfully demonstrated 112 Gbit/s DP-QPSK modulation using InP n-p-i-n MZM modules. A push-pull modulator was used as a pulse carver, and a dual-drive modulator was used as an IQ modulator. We believe that this successful result encourages the development of integrated IQ modulators.
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Fig. 5 Optical waveform of 28-GHz RZ pulses generated by TLA-MZM module.
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Fig. 6 Eye diagrams for demodulated signals of 112-Gbit/s DP-QPSK modulation.
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RZ-QPSK operation. A TLA-MZM module was utilized as the RZ-pulse source. The LD wavelength was set to 1550.2 nm. 28-GHz RZ optical pulses were generated by push-pull driving of the MZM with 28 GHz sinusoidal electrical signals. Figure 5 show the optical waveform of the 28-GHz RZ pulses output by the TLA-MZM module. The pulses were then input into a DD-MZM. A pulse pattern generator generates two sets of 28-Gsymbol/s signals with 3.5 Vpp, which were used to drive each arm of the DD-MZM. The operating point control electrode was set to give the π/2 phase shift between the two arms of the DD-MZM, so that the I-ch signal and Q-ch signal were orthogonally combined. This results in the QPSK operation. The generated 56-Gbit/s QPSK signal was then polarization division multiplexed using a 3-dB coupler, fiber delay line, and a polarization beam combiner, so that 112-Gbit/s DP-QPSK signal was generated. Then, we added amplified spontaneous emission (ASE) noise from an EDFA in order to evaluate the Qfactor of the received signal. On the receiver side, the signals were polarization-demultiplexed and demodulated by a delay line interferometer (DLI) with a FSR of 28 GHz. Two sets of balanced PDs were used to receive the demodulated signals. Figure 6 shows eye diagrams of demodulated signals received by using balanced PD. Clear eye openings were confirmed for both of polarization 1 and 2, and also both I and Q channels. When a DD-MZM is used as an IQ modulator, undesirable intensity transition occurs as well as extra chirp at bit intervals. However, as RZ pulse carver was used to remove the transient region, clear eye opening without amplitude ripple was observed. Owing to the use of the operating point control electrode, we have succeeded in generating 112 Gbit/s DP-QPSK signal using DD-MZM. Figure 7 shows the bit-error-rate characteristics as a function of optical signal-to-noise ratios (OSNRs). Similar characteristics were obtained for all four channels of combination of polarization 1, 2 and IQ channels.
14 13 12 11 10 24 26 28 30 32 34 36 38 40 42 OSNR (dB/0.1nm)
Fig. 7 BER characteristics as a function of optical signal-to-noise ratio. References 1 R. A. Griffin et al., OFC2002, Paper WX6. 2 C. S. R. Fludger et al., OFC2007, Paper PDP22. 3 H. Yamazaki et al., ECOC2008, Paper Mo.3.C.1. 4 K. -P. Ho, et al., J. Lightwave Technol., 23, 764 (2005). 5 D.-J. Krause, et al., Photon. Technol. Lett., 20, 1363 (2008). 6 T. Yoshida et al., OFC2010, Paper OMK4. 7 E. Yamada et al., OFC2010, Paper OWU4.
