We have generated 4 × 100-Gb/s orthogonal WDM optical signal by employing polarization-division-multiplexed (PDM) return-to-zero (RZ) QPSK modulation format and tight optical filtering technique. The required optical signal-to-noise ratio (OSNR) at bit error ratio (BER) of 2 × 10−3 for the 400Gb/s orthogonal DWDM signal is measured to be ~22.8 dB/0.1nm. After transmission over 1040-km standard single mode fiber (EDFA-only amplification, 80-km amplifier span and fully receiver-side electrical dispersion compensation), the measured BER for all the four orthogonal subchannels are smaller than 2 × 10−3.
©2009 Optical Society of America
400 Gb/s per single channel is expected to be a possible data rate for long-haul (LH) optical transmission after 100 GbE . In recent years, there have been extensive investigations on 100GbE LH optical transmission [1–14] and some early results on 1Tb/s single channel transmission [15,16]), however, there is still no experimental report on 400 Gb/s per channel optical transmission without using optical time division multiplexing technique. To generate single carrier based 400-Gb/s optical signal, even if polarization-division-multiplexed (PDM) 16QAM [10–13] is employed, the baud rate still goes up to 50 Gig baud/s even without considering FEC overhead, coherent receiver which can operate at such high data may not be available in the near future. Alternatively, we can use orthogonal dense wavelength division multiplexing (DWDM) , where multiple spectral peaks or multiple subchannels are used to reduce the baud rate that is required for each sub-wavelength [7,9]. In Ref , we demonstrated a 100-Gb/s transmitter with two spectra peaks which can tolerate large polarization mode dispersion and fiber dispersion. Recently, Masuda  demonstrated 100-Gb/s transmission with a spectral efficiency of 2 bit/Hz/s over 6000 km using a two-subcarrier based all-optical orthogonal frequency division multiplexing (OFDM) format and all-Raman amplification technique. In Ref , we have demonstrated that 100-Gb/s PDM-RZ-QPSK signal can have a relatively good performance even after passing through a 25-GHz interleaver. In , Goldfarb demonstrated that orthogonal DWDM can be used to improve the spectral efficiency. In this paper, we investigate the generation and transmission of 4 × 100-Gb/s orthogonal PDM-RZ-QPSK DWDM signal, where the orthogonal 400-Gb/s DWDM signal is carried by four 25-GHz spaced CW lightwave. After passing through a 100GHz-spaced wavelength-selected switch (WSS), this 400-Gb/s signal is transmitted over 1040-km standard single mode fiber (SMF-28) with an average span loss of 17.3 dB. No Raman amplification or optical dispersion compensation has been used in this experiment.
2. 400-Gb/s orthogonal PDM-RZ-QPSK DWDM signal generation
Figure 1 shows the proposed configuration for 400-Gb/s orthogonal DWDM signal generation and detection. A single-mode CW lightwave is modulated by a phase modulator (PM) driven by a sinusoidal RF source with a repetitive frequency of f. With proper driving voltage, a lightwave with multiple spectral peaks in a fixed frequency spacing and equal amplitude can be generated. Alternatively, we can use an intensity modulator (IM) to generate such an optical signal . For the 4 × 100-Gb/s orthogonal DWDM transmitter, the four peaks is separated into four lightwaves by an wavelength selective device such as array waveguide grating (AWG). Each lightwave is modulated individually with polarization multiplexing scheme to generate a polarization-multiplexed optical signal. In order to improve filtering tolerance, RZ-pulse shaping can be employed (note that the RZ pulse carver can be shared by the four subchannels). QPSK is used in this experiment due to its relatively simpler configuration and higher receiver sensitivity as compared to other higher-level modulation formats. The generated subchannels are combined by an optical coupling component such as a flat top AWG. The four subchannels are combined and then transmitted over fiber to the receiver. At the receiver, the four orthogonal DWDM subchannels are demultplexed before detection.
3. 400-Gb/s orthogonal DWDM signal transmission over 1040-km SMF-28
The experimental setup is shown in Fig. 2 . The 1554.81 nm CW laser (linewidth less than 100 kHz) is modulated by a phase modulator (PM1) with a half-wave voltage smaller than 4 V to generate the required multiple-carrier signal. The driving signal is a 25-GHz sinusoidal RF source with a peak to peak voltage of 7.8 V. The measured optical spectrum after PM1 modulation is shown in Fig. 3(a) . Among many spectra peaks in Fig. 3(a), five peaks are flat with power difference of smaller than 1 dB. The QPSK signal is generated by a Mach-Zehnder intensity modulator (MZM1) cascaded by a phase modulator (PM2). PM2 has the identical performance as PM1. MZM1 is biased at the null point to generate (0, π) phase modulation. MZM1 and PM2 are both driven by a 25-Gb/s data stream with PRBS of 216-2. The 25-Gb/s data is obtained by time-multiplexing two 12.5-Gb/s PRBS signals (each with pattern length of 215-1). The two 25-Gb/s electrical signals are de-correlated by introducing different bit delays with respect to each other. A polarization-maintained EDFA is used to boost the optical power to 16 dBm before the QPSK optical signal is modulated by MZM2, which is driven by a 25-GHz sinusoidal RF signal. After MZM2, a ~40% duty-cycle RZ-shaped signal is generated. The waveform after MZM2 is inserted in Fig. 2. Polarization-multiplexing is achieved by dividing and recombining the signal with 322 symbol delay using a polarization beam combiner (PBC). A 25/50-GHz optical interleaver (IL1) with one input and two output ports is employed to separate the odd and even subchannels. After separation by this interleaver, the signal at one of the two output ports passes through an additional ~5-m fiber delay before the two signal are combined again by another identical 25-GHz interleaver (IL2). This ~5-m fiber is used to de-correlate the odd and even subchannels. Both IL1 and IL2 have a 0.5-dB bandpass bandwidth of 17 GHz and channel isolation of 24 dB. The optical spectra after IL2 are shown in Fig. 3 (b), (c) and (d). Then a 100-GHz WSS is used to select the four orthogonal subchannels to get the required 400-Gbit/s (4 × 100-Gb/s) signal. It should be pointed out that the transmitter in our experiment is different from that in Fig. 1 or our previous one in [3–5]. In this experiment, the baud rate is 25 Gbaud/s and the frequency spacing between the subchannels is equal to the baud rate and therefore theoretically there is no crosstalk between these subchannels if we jointly process all the four subcarriers . Without using joint processing, we observed only about 1dB crosstalk penalty. Hence, we didn’t use another identical transmitter to generate odd or even subchannels in this experiment. However, if the baud rate goes up to 28 Giga baud/s and the frequency spacing remains as 25GHz, the crosstalk between the subchannels can be quite large because these subchannels are no longer be orthogonal. The optical spectrum after the WSS is exhibited in Fig. 3(e). Only four orthogonal subchannels can be seen after the WSS. This WSS has a 0.5, 3 and 20-dB bandpass bandwidth of 79.8, 93.7 and 116.2 GHz, respectively. For transmission, we have added 23 CW loading lightwaves to saturate the EDFAs in the transmission line.
The transmission line consists of 13 spans of 80-km of SMF-28 (the average span loss is 17.3 dB) and EDFA-only amplification. No optical dispersion compensation is used in this experiment. At the sixth span, one bandpass optical filter with bandpass bandwidth of 9 nm is introduced to block the accumulated noise peak occurring in the 1530-1540-nm region (the gain profiles of some of the used EDFAs are not flattened). The total power (after the boost EDFA) launching into the transmission fiber is 12 dBm, corresponding to −2.3 dBm per subchannel at 100 Gb/s.
At the receiver, the measured 100-Gb/s subchannel signal is selected by one tunable optical filter with 3-dB bandwidth of 0.26 nm. Polarization- and phase-diverse coherent detection uses a polarization-diverse 90-degree hybrid, a tunable ECL local oscillator (LO) and four single-ended photodetectors. The sampling and digitization (A/D) function is achieved by using a 4-channel digital storage scope with 50-Gs/s sample rate and 18-GHz electrical bandwidth for the 100-Gb/s data. The captured data is then post-processed using a desktop PC. The signal processing details can be found in . In this experiment, errors were counted over 20 × 60,000 symbols (20 data sets, each data sets consists of 60,000 symbols) so that the average BER for measured signal is based on 4.8 × 106 bits. Figure 5 illustrates the BER curves of each subchannel at 100 Gb/s and 400 Gb/s orthogonal DWDM signal before transmission (back-to-back). At a BER of 2 × 10−3, the required OSNR for subchannel 1, 2, 3 and 4 at 100Gb/s is 17.1, 16.5, 16.5 and 17.1 dB/0.1nm, respectively. For the 400-Gb/s orthogonal DWDM signal, the required OSNR is 22.8dB/0.1nm at a BER of 2 × 10−3. The two side subchannels suffers more from the narrow optical filtering effect of the 100-GHz WSS than the two center subchannels, although they see less WDM crosstalk. We also measured the effect of WDM crosstalk and 25-GHz interleaver. Figure 6 shows the measured results for subchannel 2. It can be seen that the WDM crosstalk introduce about 1dB OSNR penalty while the second 25GHz interleave introduce about 0.4 dB OSNR penalty.
After transmission, all the four subchannels were measured. The OSNR of each subchannel after 1040km transmission is ~21.8 dB/0.1nm (+/−0.5 dB). The BER of subchannel 1, 2, 3 and 4 is given by 1.1 × 10−3, 4 × 10−4, 1 × 10−4 and 1 × 10−3, respectively. The constellations of subchannel 1 before and after transmission are shown in Fig. 4(a) and (b) .
We have experimentally demonstrated successful generation and transmission of an orthogonal 400-Gb/s (4x100-Gb/s) DWDM signal over 1040-km SMF-28 with BER smaller than 2 × 10−3. This 400-Gb/s signal has four orthogonal subchannels and each is modulated by 100-Gb/s PDM-RZ-QPSK. At BER of 2 × 10−3, the required OSNR for this orthogonal 400Gb/s DWDM signal is 22.8dB/0.1nm. If we consider FEC and Ethernet line coding overhead, the bit rate for 400 GbE can go up to 444 Gb/s or 111 Gb/s per subchannel. Because the 0.5-dB bandwidth of the used WSS is a not large enough, the two sideband subchannels will see large penalty for narrow optical filtering effects. Moreover, the spectral overlap between the four subchannels will result in larger interferometric crosstalk due to the loss of orthogonality.
References and links
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