1. Introduction

Multilevel or multi-carrier modulation formats such as quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM) are becoming promising technology to achieve high capacity and high spectral efficiency in future optical transmission systems. Several QAM transmitter schemes have been experimentally demonstrated using commercial modulators [1–3] or integrated optical modules [4,5] with binary [3–5] or multilevel [1,2] driving electronics. Through integration techniques, it is possible to obtain a compact and stable QAM transmitter with binary driving electronics. So far, integrated quad- or hex- parallel Mach-Zehnder modulators (QPMZM [4] or HPMZM [5]), where four or six sub-Mach-Zehnder modulators (sub-MZMs) are embedded on a LiNbO₃ substrate in parallel, have been fabricated and experimentally demonstrated to generate optical 16- or 64- QAM signals, respectively. These schemes are mainly based on a parallel structure, which usually requires hybrid integration technology between LiNbO₃ modulators and silica-based planar lightwave circuits (PLCs) [4,5]. In a QPMZM-based 16-QAM transmitter, four sub-MZMs, forming two in-phase/quadrature (IQ) interferometric structures, were parallel-integrated in one MZ superstructure. Except the DC electrodes for the bias control of each sub-MZM, it requires at least three additional electrodes to adjust the relative phase offsets among embedded sub-MZMs.

In [6], we proposed and fabricated an optical 16-QAM transmitter using a monolithically integrated quad Mach-Zehnder in-phase/quadrature (QMZ-IQ) modulator. Different from the previous parallel integration, four sub-MZMs are integrated and arranged in a single IQ superstructure, where two of them are cascaded in each of the arms (I and Q arms). In principle, only one electrode is required to obtain orthogonal phase offset in the IQ superstructure, which makes the bias-control much easier to handle, and thus provides stable performance. In fact, the QMZ-IQ modulator can be configured to generate different multilevel formats by feeding different binary data streams and operating the modulator at different bias conditions. In this paper, we experimentally demonstrate the generation of both 16-QAM and 8-PSK using the monolithically-integrated QMZ-IQ modulator at 10 Gbaud. We also provide different approaches to explain the operation principle of the 16-QAM synthesis using the integrated modulator, which is helpful to well-understand and optimize the bias-control system of the integrated modulator. By sharing an optical hardware, the re-configurability of multilevel transmitter could offer flexible modulation-format options to network operators to adapt to dynamic network environments.

2. Optical 16 QAM transmitter using QMZ-IQ

As shown in Fig. 1 , an optical 16-QAM signal can be synthesized by the interferometric addition of mutually orthogonal two independent four-level amplitude-shift keying (4-ASK), which is also called as four-level amplitude- and phase-shift keying (4-APSK) in some literatures [7]. The monolithically-integrated QMZ-IQ modulator consists of four sub-MZMs arranged within an IQ superstructure. In each arm of the main superstructure, two sub-MZMs are cascaded in series. One sub-MZM of each arm, MZM-1 or MZM-3, is configured to perform amplitude shift-keying (ASK) modulation with certain extinction ratio (ER), while the other one, MZM-2 or MZM-4, acts as phase-shift keying (PSK) modulator. To ensure that the symbols are equidistant in the resultant QAM constellation, for the ASK part, both MZM-1 and MZM-3 are biased at a point with a 0.6-V _π offset from the transmission null and the positive inflection. The peak-to-peak swing of the driving electronics is 0.8 V _π, which results in ASK modulations with an ER of around 9.5 dB. The sub-MZMs employed for PSK modulations are fully-driven with a driving swing of 2 V _π, and biased at the transmission null. After tandem combination of ER-reduced ASK and PSK modulations, a 4-ASK could be synthesized in each arm. As shown in Fig. 1 (a), two 4-ASK streams are combined through quadrature addition in optical fields, successfully generating a 16-QAM signal. Note that, in this paper, we assume that the un-activated sub-MZMs are set at the peak of transmission.

 

Fig. 1 Operation principle of the proposed multilevel transmitter for 16-QAM generation.

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The operation principle can also been explained by Fig. 1 (b). If only activating the sub-MZMs for ASK modulations (MZM1 and MZM3), an offset 4-QAM is obtained in the first quadrant. While, if only MZM2 and MZM4 are activated for PSK modulations, we can get quadrature phase states in the complex plane, i.e. quadrature phase-shift keying (QPSK). With all of the embedded sub-MZMs activated, the QPSK modulation introduced by the fully-driven sub-MZMs will map the offset 4-QAM constellation to the four different quadratures, thereby obtaining a complete 16-QAM constellation.

In the fabricated QMZ-IQ modulator, four sub-MZMs were integrated on an IQ superstructure using a LiNbO₃ integration platform. Each sub-MZM had a traveling-wave electrode to apply RF and DC for modulation. The measured 3-dB optical bandwidth and V _π of each sub-MZM were 36 GHz and 6 V, respectively. The fiber-to-fiber insertion loss of the modulator was around 6 dB. The monolithic integration offers good long-term stability for QAM generation.

Figure 2 shows a schematic diagram of the experimental setup. CW light from an ECL laser (linewidth: <100 kHz) was fed to the QMZ-IQ modulator to generate a 40-Gb/s 16-QAM. Four de-correlated 10-Gb/s binary data streams were employed to drive the sub-MZMs in QMZ-IQ modulator. RF phase shifters were used to synchronize the modulations in each of the sub-MZMs. The driving swings of sub-MZMs for ASKs were set at around 4.8 V_p-p, while the other sub-MZMs for BPSKs were fully-driven with 12 V_p-p. By tuning the bias in the main MZ superstructure to introduce a π/2 phase difference, finally, a 40-Gb/s QAM was generated using the proposed modulator. At the receiver side, the generated 16-QAM was demodulated by using a digital coherent receiver. After amplification and filtering, the received 16-QAM was mixed with a local oscillator (LO) in an optical 90° hybrid. CW light from another ECL laser was deployed as an LO with a <500 MHz frequency offset from input QAM. The power of LO and input QAM were set at around 9 dBm and −3 dBm, respectively. At the output of the hybrid, optical signals were converted to electrical signals using two balanced detectors, and then sampled for analog-to-digital (A/D) conversion using a real-time oscilloscope (sampling rate: 50 Gsamples/s; analog bandwidth: 12.5 GHz; resolution: 8 bits). The captured data was then processed offline to perform clock recovery, oversampling, retiming, the carrier-phase estimation, FIR filtering and so on. I and Q components were finally recovered for constellation reconstruction and bit-error rate (BER) estimation.

 

Fig. 2 Experimental setup for 16-QAM generation.

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Figure 3 depicts the recovered constellations after demodulation. As we discussed above, 4-ASK signal could be obtained when the sub-MZMs in only one of the arms (I or Q) were activated. Figure 3(a) and (b) show the obtained 4-ASK constellations when the sub-MZMs in I or Q arm were activated individually. As shown in Fig. 3, four obtained phase states in 4-ASK were equispaced each other in optical field, which is critical to ensure the equifield spacing in the resultant 16-QAM constellation. Besides, it clearly shows the phase orthogonality between the obtained two 4-ASK in optical field. As shown in Fig. 3(c), a 16-QAM was finally synthesized by interferometric addition of two orthogonal 4-ASKs. Moreover, the proposed 16-QAM transmitter could be considered as a kind of tandem of offset 4-QAM and QPSK. Figure 4 (a) and (b) show the reconstructed constellations for offset 4-QAM and QPSK, when the sub-MZMs for ASK and PSK were activated separately.

 

Fig. 3 Recovered constellations for (a) (b) 4-ASK and (c) 16-QAM at OSNR of around 24 dB.

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Fig. 4 Recovered constellations for (a) off-set 4-QAM and (b) QPSK at OSNR of around 20 dB.

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By adjusting the OSNR of input 16-QAM signal of the coherent receiver, the BER was evaluated using offline processing as shown in Fig. 5 . Around 17.5-dB OSNR was required to achieve BER of 2 × 10⁻³, corresponding to the forward error correction (FEC) limit. The measured spectrum of 40-Gb/s 16-QAM is shown in the inset of Fig. 5, which illustrates that the bandwidth of the generated 16-QAM is similar to that of binary or quadrature phase-shift keying (PSK or QPSK) at the same baud rate.

 

Fig. 5 Calculated BER of the generated 40-Gb/s 16-QAM. Inset: measured optical spectrum (resolution: 0.01 nm).

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3. Optical 8-PSK transmitter using QMZ-IQ

The proposed QMZ-IQ modulator can also be reconfigured as an 8-PSK transmitter. The configuration and principle is shown in Fig. 6 . Different from the configuration in the 16-QAM transmitter, the two sub-MZMs for ASK, MZM-1 and MZM-3, are driven by complementary data streams (Data-1 and /Data-1) with a peak-to-peak driving swing of around 0.7 V _π. The other sub-MZMs for PSK modulations, MZM-2 and MZM-3, are fully-driven as phase modulators using de-correlated two data stream, Data-2 and Data-3. After superposing an ASK modulation over QPSK, an 8-PSK could be generated by splitting each symbol in QPSK into two symbols with a 45° relative angle as shown in Fig. 6(b). The proposed transmitter is rather simple without including any complicated pre-coder [8]. As shown in Fig. 7 , an 8-PSK constellation was successfully reconstructed after offline processing, which was optimized for 8-PSK. Eight symbols could be clearly distinguished from each other, demonstrating the feasibility of the proposed 8-PSK transmitter scheme based on QMZ-IQ modulator.

 

Fig. 6 (a) The configuration and (b) operation principle for 8-PSK generation using the QMZ-IQ modulator.

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Fig. 7 Recovered 8-PSK constellation at OSNR of around 22 dB.

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4. Conclusion

We have experimentally demonstrated a reconfigurable multilevel transmitter based on a monolithically-integrated QMZ-IQ modulator. By feeding different binary electrical driving signals, QMZ-IQ was configured to generate 16-QAM and 8-PSK. Compared with other parallel integration solutions, the proposed monolithic transmitter using QMZ-IQ features simple bias control and stable performance.