1 May 2008 Spectrum-efficient 80-Gbit/s differential phase-shift keying transmitter using phase-interleaving technology without optical-time or polarization-division multiplexing
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Abstract
We experimentally demonstrate a spectrum-efficient 80-Gbit/s differential phase-shift keying (DPSK) transmitter for a single wavelength channel using phase-interleaving technology without any optical-time or polarization-division multiplexing. Two cascaded independent 40-Gbit/s modulations were time-interleaved to generate an 80-Gbit/s DPSK signal with a compact spectrum. The proposed 80-Gbit/s DPSK transmitter consists of two independent 40-Gbit/s phase modulators only. No additional high-speed pre- or postcoder is required at the transmitter or receiver side. The proposed scheme is potentially cost-effective.
Lu and Miyazaki: Spectrum-efficient 80-Gbit/s differential phase-shift keying transmitter using phase-interleaving technology without optical-time or polarization-division multiplexing

1.

Introduction

In high-speed optical transmission systems, spectrum efficiency and per-channel capacity should be enhanced in order to reduce the implementation cost per bit as well as transmission impairment. Electrical or optical time-division multiplexing (ETDM, OTDM) has been widely employed to increase the per-channel capacity, and advanced modulation formats and polarization-division multiplexing (PDM) have been used to increase spectrum efficiency. A promising candidate for future optical transmission systems is differential phase-shift keying (DPSK), which offers improved receiver sensitivity and higher tolerance against nonlinear optical effects.1, 2, 3 Recently, a single-channel DPSK system with bit rate of more than 640Gbits has been successfully demonstrated using OTDM and PDM.4 In that system, however, the transmitter employed before multiplexing is operated at only 40Gbits , which indicates that the achievable capacity per wavelength is still limited by the available electronics.

To further increase the transmission capacity with a compact spectrum, it is desirable to explore novel transmitters with high capacity and spectrum efficiency. In this letter, we propose and describe an experimental demonstration of a cost-effective and spectrum-efficient 80-Gbit/s DPSK transmitter using phase-interleaving technology to enhance the spectrum efficiency as well as the per-channel capacity. The proposed high-speed DPSK transmitter offers the following advantages: (1) It is cost-effective, because the implementation is based on two cascaded low-speed phase modulations. (2) For the demonstrated 80-Gbit/s phase-interleaved DPSK signal, a 20-dB bandwidth of only 0.68nm was observed, which promises high spectrum efficiency. (3) It does not require any additional encoders or complicated ETDM, OTDM, or PDM devices at the transmitter and receiver sides.

2.

Operation Principle

Figure 1 illustrates the proposed DPSK transmitter using phase-interleaving technology. Continuous wave (CW) light from a laser diode is independently phase-modulated by two cascaded phase modulators. Here we assume that the bit period of each stage of the phase modulation is T . After the first-stage modulation, the phase of the light is remodulated in the subsequent stage with a relative time offset. The offset can be tuned by using an optical or electrical delay line. As an example, phase patterns before and after phase interleaving are shown in Fig. 2I. The two phase modulations at a bit rate of 1T [Fig. 2a and 2b] are time-shifted with a relative offset T2 , and the resultant final phase pattern [Fig. 2c] after phase interleaving is logically equivalent to the result of an XOR operation between the two independent phase modulation patterns that are applied. At the same time, the modulation speed of the resultant phase modulation is doubled to 2T compared with that of the individual phase modulation, 1T . Thus, a high-speed DPSK signal with bit rate of 2T is generated using components operating at a lower bit rate (1T) .

Fig. 1

The proposed cost-effective DPSK transmitter using phase-interleaving technology. PM: phase modulator; OBPF: optical bandpass filter; MZDI: Mach-Zehnder delay interferometer; EAM: electroabsorption modulator.

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Fig. 2

Operation principle of proposed phase-interleaving technology.

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Interestingly, although the final phase pattern is logically the result of an XOR operation between the two individual phase modulations, the two channels can be simply and independently separated at the receiver side using an optical or electrical demultiplexer. The generated high-speed DPSK signal can also be directly detected by using a high-speed ETDM-based optical receiver.5 At the transmitter side, the precoders for two channels are independently operated at lower bit rate, 1T . As shown in Fig. 2(II), the two phase modulations, ϕ1 and ϕ2 , are introduced by the cascaded phase modulators, respectively. We assume that the phase patterns of ϕ1 and ϕ2 at time slots i and i1 are ϕ1_i , ϕ1_(i1) , and ϕ2_i , ϕ2_(i1) , respectively. As shown in Fig. 2f, the corresponding resultant phase pattern, ϕ3 , at time slots 2i and 2i1 is obtained from the XOR operation between the phase patterns ϕ1 and ϕ2 at time slots i and i1 . The resultant phase ϕ3 at time slots 2i and 2i1 is given by

1

ϕ3_(2i)=ϕ1_iϕ2_(i1)andϕ3_(2i1)=ϕ1_(i1)ϕ2_(i1).
At the receiver side, a Mach-Zehnder delay interferometer (MZDI) with T2 relative delay between the two arms is employed for phase demodulation. Thus, after the phase demodulator, the demodulated data at time slot 2i , k3_(2i) , will be

2

k3_(2i)=ϕ3_(2i)ϕ3_(2i1)=ϕ1_iϕ1_(i1).
This indicates that the demodulated data k3_(2i) are dependent only on the first-stage phase modulation applied in the transmitter. Similarly, the demodulated data at time slot 2i1 , k3_(2i1) , are obtained as

3

k3_(2i+1)=ϕ2_iϕ2_(i1).
It is clear that the demodulated data at time slot 2i+1 result from the second-stage phase modulation at the transmitter side only. Therefore, although the two serially cascaded phase modulations are interleaved in the phase domain, after the phase demodulation, the obtained data are a simple multiplexing of the two channels in the time domain with double the bit rate. Moreover, the differential precoders for the two tributaries are also operated independently at the lower bit rate (1T) rather than the resultant bit rate (2T) . No additional high-speed pre- or postcoder is required at the transmitter or receiver side. This further simplifies the configuration of the transmitter and reduces the implementation cost.

3.

Experiments and Results

An 80-Gbit/s DPSK transmitter using phase interleaving was experimentally demonstrated to verify the proposed scheme. The experiment setup was similar to that illustrated in Fig. 1. The 80-Gbit/s DPSK transmitter consisted of a laser source and two cascaded dual-drive Mach-Zehnder modulators, which were driven by 40-Gbit/s 10311 pseudorandom binary sequence (PRBS) data in push-pull operations with a peak voltage of 2Vπ to introduce phase modulations. The 3-dB bandwidth of the employed modulators’ frequency response was only 23GHz . A tunable electrical delay line was inserted before driving the second phase modulator to ensure a 12.5-ps relative time offset between the two phase modulation tributaries after phase interleaving. At the receiver side, a MZDI with a free spectral range of 164GHz was employed to demodulate the DPSK signal. After demodulation, an electroabsorption modulator (EAM) driven by a 40-GHz clock signal was employed to demultiplex the detected 80-Gbit/s data to 40-Gbit/s data.

The eye diagrams of demodulated 80-Gbit/s data and 40-Gbit/s data after demultiplexing were measured by a high-speed optical sampling oscilloscope and are shown in Fig. 3. The nonuniform eye opening was mainly due to the performance differences between the phase modulators. The measured optical spectrum of the generated 80-Gbit/s phase-interleaved DPSK signal is illustrated in Fig. 4. The optical spectrum of a single-stage 40-Gbit/s DPSK signal is also depicted for comparison. The 20-dB spectral bandwidth of the generated 80-Gbit/s phase-interleaved DPSK was only 0.68nm , whereas that of the conventional 40-Gbit/s DPSK signal was around 0.44nm . This indicates that the generated DPSK signal using phase interleaving offers higher spectrum efficiency than the conventional DPSK signal.

Fig. 3

Eye diagrams of detected (a) 80-Gbit/s DPSK signal after MZDI and (b) 40-Gbit/s data after demultiplexing (10psdiv) .

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Fig. 4

Optical spectra of 40-Gbit/s conventional DPSK signal and 80Gbits phase-interleaved DPSK signal.

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4.

Conclusion

We experimentally demonstrated a cost-effective and spectrum-efficient 80-Gbit/s DPSK transmitter using phase-interleaving technology. The two independent phase modulations were logically combined through an XOR operation with a relative time offset, resulting in the final phase pattern with double the bit rate. Moreover, the proposed DPSK transmitter had a compact spectrum compared with conventional DPSK signals. A 20-dB bandwidth of only 0.68nm was observed for the 80-Gbit/s phase-interleaved DPSK signal.

References

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2.  M. Daikoku, N. Yoshikane, and I. Morita, “Performance comparison of modulation formats for 40Gbits DWDM transmission systems,” in Proc. Optical Fiber Communication Conf. (OFC), paper OFN2 (2005). Google Scholar

3.  W. Shieh, X. Yi, and A. V. Tran, “Label swapping for DPSK encoded labels without wavelength conversion,” in Proc. Optical Fiber Communication Conf. (OFC), paper OTuC1 (2005). Google Scholar

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Guo-Wei Lu, Tetsuya Miyazaki, "Spectrum-efficient 80-Gbit/s differential phase-shift keying transmitter using phase-interleaving technology without optical-time or polarization-division multiplexing," Optical Engineering 47(5), 050501 (1 May 2008). https://doi.org/10.1117/1.2931597
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