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We devised a dual-carrier IQ modulator consisting of a novel complementary frequency shifter (CFS) and two IQ modulators. The CFS generates two optical subcarriers from a single light source and outputs them from separate ports without using any optical demultiplexing filters. We fabricated the modulator in a hybrid configuration of silica planar lightwave circuits and a LiNbO3 phase-modulator array. Frequency-spacing-tunable operation of the CFS was demonstrated. With the modulator, we successfully generated a 100-Gb/s/pol dual-carrier OFDM-QPSK signal.
©2011 Optical Society of America
1. Introduction
Orthogonal frequency-division multiplexing (OFDM) is a promising technology for achieving high spectral efficiency (SE) in long-distance transmission systems [1–3]. It is also expected to enable elastic optical networking [4]. OFDM subcarriers can be generated either in the optical or the digital-electronic domain in the transmitter. The digital-electronic approach is widely used because of the high flexibility and simplicity of the optical setup [5], but the bandwidth of the output optical signal is limited by the bandwidth of the digital electronics. To achieve a large line rate, such as 100-Gb/s, 400-Gb/s, or 1-Tb/s, we have to use the optical approach in place of or in combination with the digital-electronic approach [1,2,6]. A variety of optical multicarrier generators have been reported, such as a Mach-Zehnder modulator (MZM) driven with a sinusoidal signal [1], tandem MZMs [7], a re-circulating frequency shifter (RFS) [2,8], and an MZM with a loop-back waveguide [9]. One of the challenging issues in using these optical subcarrier generators is the implementation of an optical demultiplexing filter, which is required because these generators output all subcarriers from a single output port. We have developed an optical OFDM modulator in which a 2-channel optical demultiplexing filter and two IQ modulators are compactly integrated with a hybrid configuration of silica planar lightwave circuits (PLCs) and LiNbO3 (LN) phase modulators [1,10]. However, optical demultiplexing filters are, in general, inflexible with respect to the frequency spacing, and they are sensitive to both the optical frequency of the input light and temperature.
To overcome these problems, we have devised a novel optical dual-carrier generator, a complementary frequency shifter (CFS), and fabricated an integrated modulator consisting of a CFS and two IQ modulators using the PLC-LN hybrid configuration [11]. The CFS generates two subcarriers and outputs each from a separate output port, eliminating the need for an optical demultiplexing filter. We demonstrated the frequency-spacing tunable operation of the CFS. With the integrated modulator, we successfully generated a 100-Gb/s single-polarization dual-carrier OFDM quadrature-phase-shift-keying (QPSK) signal.
2. Principle of the CFS
As shown in Fig. 1 , the configuration of the CFS is basically identical to that of an IQ modulator with a dual-parallel MZM configuration, except that the output coupler has two output ports, OUT1 and OUT2. This dual-output configuration provides a complementary pair of output signals from the two output ports [12]. By driving the two MZMs with sinusoidal signals with a frequency of fs and a relative delay, τ, of 1/4fs and inputting CW light with a frequency of f0, we can obtain optical outputs with frequencies of f0 + fs and f0-fs from OUT1 and OUT2, respectively. The mechanism of the frequency shift is the same as that of a frequency shifter using a single-sideband modulator (SSB-FS) [13], but a conventional SSB-FS uses only one of the two complementary outputs and discards the other because it has a single-output configuration. The CFS utilizes both of the complementary outputs, serving as a dual-carrier generator that does not require any optical demultiplexing filters. By changing fs and τ ( = 1/4fs), we can flexibly tune the frequency spacing between the two subcarriers.
3. Design and fabrication of the dual-carrier IQ modulator
Figure 2 shows the configuration of the dual-carrier IQ modulator. The optical circuit consists of a CFS and two IQ modulators, IQ1 and IQ2. The two outputs of the CFS are connected to IQ1 and IQ2, respectively, via U-turn waveguides with the same optical-path length. Each modulator has two output ports, which are used as a test and main output, respectively. The main outputs of IQ1 and IQ2 are coupled with a tunable coupler for the fine-tuning of the power balance between the two frequency channels. The input coupler of the CFS is also tunable. The optical circuit is implemented on a hybrid optical chip consisting of two silica-based PLCs (PLC-L and -R) and an X-cut LN chip. The LN chip has an array of six push-pull pairs of straight phase modulators. All other circuit components, including the Y-branches, 3-dB wavelength-insensitive couplers (WINCs), thermo-optic (TO) phase shifters, tunable couplers, and U-turn waveguides, are implemented on the PLCs. We bonded the three chips to each other by using UV-curable adhesive and then mounted them on a SUS package with RF connectors. The PLC-L, LN, and PLC-R chips are 30.0 x 6.4, 64.0 x 6.0, and 14.0 x 6.4 mm, respectively, which gives the total chip length of 108 mm.
4. Characteristics of the CFS
Using the test ports (Test1 and Test2 in Fig. 2), we first tested the CFS. We drove the CFS with sinusoidal clock signals with the amplitude of about 0.8Vπ peak-to-peak while applying only DC biases to IQ1 and IQ2 to maximize the output power from the test ports. Figure 3 shows the optical signal spectra obtained with fs = 12.5 GHz. The horizontal axis is the relative frequency with respect to the input CW light (λ = 1551.2 nm). The peak frequencies for the outputs from Test1 and Test2 are shifted from that of the input light by −12.5 and + 12.5 GHz, respectively. At the relative frequencies of −12.5 and + 12.5 GHz, the intensity ratio between the two outputs is about 40 dB, which is large enough for OFDM applications. The problem is that there are spurious peaks at 0 and ± 25 GHz and their intensities are only about 22 ~30 dB smaller than the main peaks. These spurious peaks will be suppressed by improving the power and modulation-index imbalance in the two arms of each MZM.
Figure 4(a) shows the change in the peak frequency of the two outputs when we changed fs. The peak frequencies change proportionally to fs in opposite directions, and the frequency spacing, which corresponds to the horizontal split between the two lines, is always equal to 2fs. Figure 4(b) shows the intensity ratios of the two outputs measured at each peak frequency. The intensity rations remain at around 40 dB. These results demonstrate the wide-range frequency-spacing tunability, which is difficult to achieve with conventional approaches using an optical demultiplexing filter.
Fig. 4 Driving-frequency, fs, dependence of (a) the peak-frequency shift for each output port and (b) the intensity ratio between the two outputs measured at each peak frequency.
5. Characteristics of the dual-carrier IQ modulator
The static insertion loss of the modulator is 8.3dB at the wavelength of 1550 nm when DC biases for all MZMs and TO phase shifters are adjusted to give the maximum transmittance and the coupling ratios of two tunable couplers are set at 50%. Figure 5 shows the frequency dependence of the electro-optic (E/O) response of the four MZMs. The 3-dB bandwidth of the electro-optic frequency responses are around 25 GHz for all MZMs.
We tested the modulator in the single-polarization back-to-back setup shown in Fig. 6 . We used an external-cavity laser (ECL) with a wavelength of 1551.2 nm and a linewidth of ~30 kHz as a CW light source. The CFS was driven with 12.5-GHz sinusoidal signals, while IQ1 and IQ2 were driven with four 25-Gb/s non-return-to-zero (NRZ) 211-1 pseudo-random bit sequences (PRBSs) with different delays, so that the modulator generated a 100-Gb/s single-polarization dual-carrier OFDM-QPSK signal. On the receiver side, a Mach-Zehnder delay interferometer (MZDI) with a free spectral range of 50 GHz and a MZM gate driven with a 25-GHz sinusoidal signal were used to demultiplex the two frequency channels. The driving signals for the CFS, IQ modulators, and the gate were synchronized to the same clock. The demultiplexed signals were then received with an optical modulation analyzer consisting of a local oscillator (LO) laser, an optical coherent receiver front-end, a 50-GSample/s storage oscilloscope, and offline signal analyzer software with an adaptive digital equalizer. The record length for the analysis was about 2x106 symbols. We received the two frequency channels one by one by adjusting the MZDI and LO to each channel. The optical signal-to-noise ratio (OSNR) was varied by using an amplified spontaneous emission (ASE) source and an optical attenuator and was measured before the MZDI.
Optical signal spectra measured at the output from Test1, Test2, and main output ports are shown in Fig. 7 . The Test1 and Test2 spectra, which correspond to the two frequency channels, have their peaks spaced at 25 GHz, which is equal to the baud rate. The main spectrum agrees with the sum of the Test1 and Test2 spectra, and it has a signal bandwidth of about 75 GHz, which is 3/4 of the bit rate.
Figures 8(a) and (b) show the optical eye diagrams measured before the MZDI and after the gate, respectively, when the IQ1 channel was received. The dual-carrier signal was successfully demultiplexed by the MZDI and the gate. As shown in Fig. 8(b), the pulse intensity of the demultiplexed signal fluctuated with a period of 2 bits. This fluctuation is considered to be the beat between the main signal and the spurious carriers shown in Fig. 3.
The measured bit-error rate (BER) versus OSNR curves and constellations of the two frequency channels, Ch1 and Ch2, are shown in Figs. 9(a) and (b) , respectively. The constellations were measured without adding ASE noise. The required OSNR for BER = 10−3 is 15.9 dB. The results, especially those for BER of ~10−5 or smaller, seem to include some penalty caused by the spurious carriers shown in Fig. 3. No error is observed in the recorded data for Ch1 and Ch2 when the OSNRs are 26.5 and 29.2 dB, respectively.
6. Conclusion
A novel optical dual-carrier generator, the CFS, was devised and demonstrated. The CFS eliminates the need for an optical demultiplexing filter and provides frequency-spacing tunability. We fabricated a dual-carrier IQ modulator, in which a CFS and two IQ modulators are integrated with the PLC-LN hybrid configuration, and successfully generated a 100-Gb/s single-polarization dual-carrier OFDM-QPSK signal. If we use four-level signals to drive the IQ modulators and polarization multiplexing, we will also be able to generate 400-Gb/s dual-carrier dual-polarization 16QAM signal, which may be a good candidate for post-100-Gb/s transmission systems.
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