A 1-Tb/s single-channel coherent optical OFDM (CO-OFDM) signal consisting of continuous 4,104 spectrally-overlapped subcarriers is generated using a novel device of recirculating frequency shifter (RFS). The RFS produces 320.6-GHz wide spectrum using a single laser with superior flatness and tone-to-noise ratio (TNR). The 1-Tb/s CO-OFDM signal is comprised of 36 uncorrelated orthogonal bands achieved by adjusting the delay of the RFS to an integer number of OFDM symbol periods. The 1-Tb/s CO-OFDM signal with a spectral efficiency of 3.3 bit/s/Hz is successfully received after transmission over 600-km SSMF fiber without either Raman amplification or dispersion compensation.
©2009 Optical Society of America
As 100 Gb/s Ethernet (100 GbE) has become increasingly a commercial reality , the next logical pressing issue is the migration path toward 1-Tb/s Ethernet transport. Some industry experts believe that the 1-Tb/s Ethernet standard should be available in the time frame of 2012-2013 . In fact, there had been pioneering Tb/s transmission experiments employing optical time-division multiplexing (OTDM) . But OTDM requires a precise timing alignment for its tributaries and expensive high-order optical dispersion compensation. In the mean time, coherent optical OFDM (CO-OFDM) has recently been demonstrated for its superior performance in spectral efficiency, receiver sensitivity, and polarization-dispersion resilience –, but requires higher implementation complexity compared with direct-detection OFDM (DDO-OFDM) -. CO-OFDM serves as an interesting alternative to the coherent single-carrier system for long-haul transmission –. Coherent optical OFDM (CO-OFDM) has been considered as a promising alternative pathway toward Tb/s transport that possesses high spectral efficiency, resilience to tributary timing alignment and the channel dispersion –. 100-Gb/s CO-OFDM transmission has also been demonstrated using multi-band OFDM to mitigate DAC/ADC electronic bottleneck –. More importantly, for future optical networks, it is anticipated that a pattern of diverse bandwidth demand will emerge: Between major nodes, it is justified to deploy Tb/s transponders in anticipation of large traffic demand; But for some smaller nodes, it is desirable that they can still access a fraction of the multicast 1-Tb/s traffic without resorting to an expensive Tb/s receiver. In this paper, we show that by using multi-band structure of the proposed 1-Tb/s signal, 30-Gb/s coherent receiver can be used to detect 1-Tb/s signal, which provides an option of receiver design in 30-Gb/s granularity, a small fraction of the entire wavelength bandwidth. Additionally, to extend from current 100-Gb/s demonstration – to 1-Tb/s presents significant challenges, requiring 10-fold bandwidth expansion. To optically construct the multi-band CO-OFDM signal using cascaded optical modulators  entails 10 times higher drive voltage, or use of the nonlinear fiber which may introduce unacceptable noise to the Tb/s signal. We here adopt a novel approach of multi-tone generation using a recirculating frequency shifter (RFS) architecture that generates 36 tones spaced at 8.9 GHz with only a single optical IQ modulator without a need for excessively high drive voltage. The highest data rate for coherent detection is achieved at 1.28 Tb/s using pulsed OTDM coherent laser , but no transmission performance is reported. It is expected that both the spectral efficiency and the data rate of the OTDM signal will be greatly affected by the transmission distance due to the introduction of amplifier noise, fiber nonlinearity, and residual dispersion. So it is yet to know the performance of such an OTDM system with some realistic transmission distance. The longest reach for 1 Tb/s per channel is 480 km using dispersion-managed fiber (DMF) with a spectral efficiency of less than 2 bit/s/Hz . In this paper, we extend the report of the first 1-Tb/s CO-OFDM transmission with a record reach of 600 km over SSMF fiber and a spectral efficiency of 3.3 bit/s/Hz without either Raman amplification or optical compensation . Our demonstration signifies that the CO-OFDM may potentially become an attractive candidate for future 1-Tb/s Ethernet transport even with the installed fiber base.
2. Principle of a recirculating frequency shifter (RFS) for bandwidth expansion of multi-band OFDM
One of the main challenges for 1-Tb/s demonstration is to expand OFDM signal bandwidth to the range of 400 GHz. Figure 1(a) shows the architecture of the RFS consisting of a closed fiber loop, an IQ modulator, and two optical amplifiers to compensate the frequency conversion loss. The IQ modulator is driven with two equal but 90 degree phase shifted RF tones through I and Q ports, to induce a frequency shifting to the input optical signal . As shown in Fig. 1(b), in the first round, an OFDM band at the center frequency of f1 (called f1 band) is generated when the original OFDM band at the center frequency of f0 passes through the optical IQ modulator and incurs a frequency shift equal to the drive voltage frequency of f. The f1 band is split into two branches, one coupled out and the other recirculating back to the input of the optical IQ modulator. In the second round, f2 band is generated by shifting f1 band along with a new f1 band which is shifted from original f0 band. Similarly, in the Nth round, we will have fN band shifted from the previous fN-1 band, and fN-1 shifted from previous fN-2, etc. The f N+1 band and beyond will be filtered out by the bandpass filter placed in the loop. With this scheme, the OFDM bands f1 to fN are coming from different rounds and hence contain uncorrelated data pattern. Additionally, such bandwidth expansion does not require excessive drive voltage for the optical modulator. Another major benefit of using the RFS is that we can adjust the delay of the recirculating loop to an integer number (30 in this experiment) of the OFDM symbol periods, and therefore the neighboring bands not only reside at the correct frequency grids, but are also synchronized in OFDM frame at the transmit. To replicate uncorrelated multiple OFDM bands using RFS is thus an extremely useful technique as it does not require duplication of the expensive test equipments including arbitrary waveform generators (AWG) and optical IQ modulators, etc. The RFS has been proposed and demonstrated for a tunable delay, but with only one tone being selected and used . We here extend the application of RFS for multi-tone generation, or more precisely, for bandwidth expansion of uncorrelated multi-band OFDM signal.
In practical systems, the 1-Tb/s multi-band OFDM can be implemented through three-layer mixed-signal optoelectronic architecture laid out in . The center frequencies of individual OFDM bands should originate from the same frequency source, for instance, from the same optical comb generator, and therefore the OFDM subbands are frequency-locked with each other. The phase drift between each OFDM band is not important, and can be easily treated via phase and channel estimation.
3. Experiment setup
Figure 2 shows the experimental setup for the 1-Tb/s CO-OFDM systems. The optical sources for both transmitter and local oscillators are commercially available ECL lasers which have linewidth about 100 kHz. The first OFDM band signal is generated by using a Tektronix Arbitrary Waveform Generator (AWG). The time domain OFDM waveform is generated with a MATLAB program with the parameters as follows: 128 total subcarriers; guard interval 1/8 of the observation period; middle 114 subcarriers filled out of 128, from which 4 pilot subcarriers are used for phase estimation. The real and imaginary parts of the OFDM waveforms are uploaded into the AWG operated at 10 GS/s to generate IQ analog signals, and subsequently fed into I and Q ports of an optical IQ modulator respectively. The net data rate is 15 Gb/s after excluding the overhead of cyclic prefix, pilot tones, and unused middle two subcarriers. The optical output from the optical IQ modulator is fed into the RFS, replicated 36 times in a fashion described in Fig. 1(b), and is subsequently expanded to a 36-band CO-OFDM signal with a data rate of 540 Gb/s. The optical OFDM signal from the RFS is then inserted into a polarization beam splitter, with one branch delayed by one OFDM symbol period (14.4 ns) , and then recombined with a polarization beam combiner to emulate the polarization multiplexing, resulting in a net date rate of 1.08 Tb/s.
Figure 3(a) shows the multi-tone generation if the optical IQ modulation in Fig. 2 is bypassed. It shows a successful 36-tone generation with a tone-to-noise ratio (TNR) of large than 20 dB with a resolution of 0.02 nm. The number of tones is controlled by the bandwidth of the optical bandpass filter in the RFS loop, and the RF tone frequency for the RFS is 8.90625 GHz, phase-locked with the AWG using a 10 MHz reference clock. This is to ensure that all the subcarriers across the entire OFDM spectrum are at the correct uniform frequency grids. The tone frequency of 8.90625 GHz is chosen in a way that no frequency guard band exists between sub-bands. This satisfies the orthogonal-band condition, or in essence, the 1-Tb/s signal is an orthogonal-band-multiplexed OFDM (OBM-OFDM) signal discussed in .
Figure 3(b) shows the optical spectrum of 1.08 Tb/s CO-OFDM signal spanning 320.6 GHz in bandwidth consisting of 4,104 continuous spectrally-overlapped subcarriers, implying a spectral efficiency of 3.3 bit/s/Hz. The signal is then coupled into a recirculation loop comprising 100-km standard single mode fiber (SSMF) and a two-stage EDFA to compensate the loss. The signal is coupled out from the loop and received with a polarization diversity detectors – comprising of a polarization beam splitter, two optical hybrids and four balanced receivers. The performance is detected on a per-band basis by aligning the local laser to the center of each band, and the detected RF signal is anti-alias filtered with a 7-GHz low-pass filter. The four RF signals for the two IQ components are then input into a Tektronix Time Domain-sampling Scope (TDS) and are acquired at 20 GS/s and processed with a MATLAB program using a 2×2 MIMO-OFDM model. The 2×2 MIMO-OFDM signal processing involves – (1) FFT window synchronization using Schmidl format to identify the start of the OFDM frame, (2) software estimation and compensation of the frequency offset, (3) channel estimation in terms of Jones vector and Jones Matrix, (4) phase estimation for each OFDM symbol, and (5) constellation construction for each carrier and BER computation. The channel matrix is estimated by sending 20 OFDM symbols using alternative polarization launch. Much fewer pilot symbols can be used by exploring intra-symbol frequency correlation . We note that the per-band detection of OFDM signal enables the subwavelength bandwidth access with a cost-effective receiver by a node where 1-Tb/s bandwidth is not needed. This fractional bandwidth access is difficult to perform for OTDM-based Tb/s systems. This is because to access each OTDM tributary, meticulous dispersion compensation across broad bandwidth of the Tb/s signal, or a full set of Tb/s receiver is called for.
4. Experiment results
Figure 4 shows the electrical spectrum generated from the arbitrary waveform generator (AWG) with and without 4.4 GHz anti-alias filters. Figure 4(a) indicates strong alias frequency components at both sides of the signal spectrum which will interfere with the adjacent bands and significantly degrade the system performance, if not properly filtered. The roll-off spectral shape is due to the AWG frequency response. After the 4.4 GHz low-pass filter, as shown in Fig. 4(b), a clear signal spectrum without alias components is achieved which can be fed into the optical IQ modulator for up conversion.
Figures 5(a) and 5(b) show the detected electrical spectra for one of the subbands from the 1-Tb/s CO-OFDM signal before and after using a 7.2 GHz electrical anti-alias filter. This is equivalent to placing a 14.4 GHz optical band-pass filter centered around this subband. The anti-alias filter is critical for OBM-OFDM implementation . As is shown in Fig. 5(a), without the electrical anti-alias filter, the electrical spectrum will be broad and covers the entire balanced receiver bandwidth. Such a broad spectrum will have alias effect if sampled at 20 GS/s, indicating that at least 30 GS/s ADC has to be used. However, the filtered spectrum in Fig. 5(b) can be easily sampled with 20 GS/s, or even at a lower speed of 10 GS/s. Additionally, despite the fact that there are some spurious components from neighboring band that is leaked at the edge of the 7.2 GHz filter, since they are orthogonal subcarriers to the interested OFDM subcarriers at the center, they do not contribute to the interference degradation.
Figure 6 shows the BER sensitivity performance for the entire 1.08 Tb/s CO-OFDM signal at the back-to-back. The OSNR required for a BER of 10-3 is 27.0 dB, which is about 11.3 dB higher than 107 Gb/s we measured in . The inset shows the typical constellation diagram for the detected CO-OFDM signal. The additional 1.3 dB OSNR penalty is attributed to the degraded tone-to-noise ratio (TNR) at the right-edge of the CO-OFDM signal spectrum (see Fig. 3(a)). Figure 7 shows the BER performance for all the 36 bands at the reach of 600 km with a launch power of 7.5 dBm, and it can be seen that all the bands can achieve a BER better than 2×10-3, the FEC threshold with 7 % overhead. The inset shows the 1-Tb/s optical signal spectrum at 600-km transmission. It is noted that the reach performance for this first 1-Tb/s CO-OFDM transmission is limited by two factors: (i) the noise accumulation for the edge subcarriers that have gone through most of the frequency shifting, and (ii) the two-stage amplifier exhibits over 9 dB noise figure because of the difficulty of tilt control in the recirculation loop. Both of the two issues can be overcome, and 1000 km and beyond transmission at 1-Tb/s is practically reachable.
A single-channel 1-Tb/s coherent optical OFDM (CO-OFDM) signal consisting of continuous 4,104 spectrally-overlapped subcarriers has been generated using a novel device of recirculating frequency shifter (RFS). The RFS produces 320.6-GHz wide spectrum using a single laser with superior flatness and tone-to-noise ratio (TNR). The 1-Tb/s CO-OFDM signal is comprised of 36 uncorrelated bands achieved by adjusting the delay of the RFS to an integer number of OFDM symbol period. The 1-Tb/s CO-OFDM with a spectral efficiency of 3.3 bit/s/Hz is successfully received after transmission over 600-km SSMF fiber without either Raman amplification or dispersion compensation.
This work was supported by the Australian Research Council (ARC). The authors would like to thank Dr. Tetsuya Kawanishi from National Institute of Information and Communications Technology (NICT), Japan for providing the optical IQ modulator in the experiment.
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