1-Tb/s Stokes Vector Direct Detection over 480-km SSMF Transmission Di Che1,2, Qian Hu2, Xi Chen2, An Li2, and William Shieh2 National ICT Australia – Victoria Research Laboratory (NICTA-VRL), VIC 3010, Australia 2 Dept. of Electrical and Electronic Engineering, The University of Melbourne, VIC 3010, Australia 1 Abstract Summary We demonstrate the first 1-Tb/s (10×100Gb/s) optical direct detection over 480-km SSMF transmission using the Stokes vector direct detection (SV-DD) with 7.76bit/s/Hz electrical spectrum efficiency. Introduction With the popularization of personal Internet devices and the development of medium-rich applications, people are now thirsty for more high-performance multimedia experience at their fingertip. As the 100-Gb/s Ethernet has been standardized in 2010 [ 1 ] and increasingly become a commercial reality, the next urgent issue is the migration path toward 1-Tb/s Ethernet transport. Highspeed long-haul transport has achieved the Terabit transmission primarily thanks to the coherent detection [2-3]. For the cost sensitive short-reach networks such as data center interconnects within distance of hundreds of kilometers, the optical direct detection (DD) technology [ 4 - 9 ] has long been regarded as one of the suitable solution, because they can significantly lower the expense compared with coherent counterpart while achieving both high data rate and moderate reach. However, the conventional intensity modulation direct detection (IM-DD) systems cannot undertake the 100 Gb/s to Tb/s transmission due to the two fundamental bottlenecks: (i) chromatic dispersion (CD) induced signal fading due to the lack of phase diversity [4], which limits the transmission distance less than 20 km; and (ii) 2nd order nonlinearity due to photo-detection, which limits the system capacity. The first one can be overcame by using the single sideband modulation (SSB) [5] while the second by leaving frequency gaps in spectrum to separate the signal and 2nd nonlinear component [5-6]. Both approaches reduce the electrical spectrum efficiency (SE) by half, resulting in the total SE only one fourth of single polarization (POL) coherent detection. By applying the double sideband modulation (DSB) and standard balance receiver [2] to the DD system, the signal carrier interleaved DD (SCI-DD) [8] overcomes the above problems and achieves the first 100G single-wavelength single-polarization transmission, with the sacrifice of one third spectral efficiency. To further increase the SE, the Stokes vector DD (SV-DD) was proposed recently [9]. The Stokes vector receiver offers the following advantages: (i) achieving 3dimension detection with phase diversity; (ii) eliminating the 2nd order noise by the balance photo detectors (PD); (iii) realizing polarization independence by conducting the polarization tracking using the digital signal processing (DSP). SV-DD achieves 100% SE with single polarization modulation while reduces the cost by: (i) simpler transmitter design compared with polarization multiplexing (POL-MUX) systems [6]; (ii) no need of local oscillator (LO) at receiver; (iii) simpler DSP due to using less number of FFT operations [7], and no need to track laser frequency offset and phase noise [2]. These merits reveal the potential for SV-DD to be deployed in Terabit short-reach networks. The 160-Gb/s SV-DD transmission of 160-km standard single mode fiber (SSMF) has successfully been demonstrated [9] using single wavelength. In this paper, we will demonstrate the first Terabit wavelength-division multiplexed (WDM) optical direct detection with transmission over 480-km SSMF. This work represents the record reach for singlepolarization modulated 100 Gb/s per wavelength allelectronic direct detection with EDFA-only amplification and without electronic pre-compensation or optical compensation. Principle of Stokes Vector Direct Detection As the name suggests, the Stokes vector (SV) receiver detects the 3 (or 4) components of a Stokes vector. It consists of one 90o optical hybrid and three balance PDs as shown in Fig. 1. Given the 2-D complex signal in Jones space: , the outputs of the receiver correspond to the three components of the SV: 2 2 * *   X Y  S1   X  X  Y  Y     S   S2    X  Y *  X *  Y    2 Re( X  Y * )  (1)  S3   j ( X  Y *  X *  Y )   2 Im( X  Y * )      where Re() and Im() represent for the real part and imaginary part of a complex number. 1(X) PBS 2(X) 90o 3(Y) Optical Hybrid 4(Y) S2 S3 S1 B-PD Fig. 1 Structure of an SV receiver. PBS: Polarization beam splitter; B-PD: Balance photo-detector. In order to recover the signal, the receiver needs to acquire the polarization rotation after the fiber transmission, which is represented by the 3x3 rotation matrix (RM) in the Stokes space. We use the training symbol aided estimation to get the RM. One training period contains 3 orthogonal training symbols with the Jones space representation of (0, 1), (1, 1) and (i, 1), corresponding to the SVs of (-1, 0, 0), (0, 1, 0) and (0, 0, 1). Three columns of the RM are acquired respectively. We multiply the received SV with the inverse of the RM: 2 2 1   X T 2  YT 2    r11 r12 r13   X R  YR    * *  2 Re(XT  YT )    r21 r22 r23    2 Re(X R  YR )  (2)     *  *   2 Im(XT  YT )   r31 r32 r33   2 Im(X R  YR )  where rij is the matrix element of the ith row and jth column of the rotation matrix RM, the subscripts “T” and “R” represent for the transmitter and receiver respectively and the superscript “-1” represents for the inverse. To guarantee the channel is linear, the Y-POL is sent with a constant power, namely carrier (C) while only X-POL is modulated with the signal (S). Therefore, by combining the 2nd and 3rd component of the SV, we have the final output of [9]. Experimental Setup for 1-Tb/s Transmission In this work, we experimentally demonstrate the 1-Tb/s (10 × 100 Gb/s) direct detection with 480-km SSMF transmission using SV-DD. Fig. 2 illustrates the experimental setup. At the transmitter, 10 CW lasers are multiplexed by the coupler with the channel spacing of 50 GHz. The combined light is first split into two branches respectively for the signal and carrier. For the signal branch, the optical signal is fed into a 3-tone generator [9] to achieve wider optical bandwidth, then fed into an I/Q modulator driven by an arbitrary waveform generator (AWG). The RF OFDM signal with a 16-QAM modulation is loaded into the AWG. The FFT size of the OFDM signal is 4096 in which 3420 subcarriers are filled. The cyclic prefix (CP) is 128 points. SV rotation training symbols are added before each OFDM frame with symbol length of 192 points. The AWG operates at a sampling rate of 10 GSa/s, leading to the optical bandwidth of 8.33 GHz for 1-band and total optical bandwidth 25 GHz for 3-band. The raw data rate is 25×4=100 Gb/s for one channel and 1 Tb/s for 10 channels. Counting the OFDM overhead, the data rate is decreased to 969.7 Gb/s before 20 % FEC and 808.1 Gb/s after FEC. This corresponds to pre-FEC electrical spectral efficiency (SE) of 7.76 bits/s/Hz and post-FEC SE of 6.47 bits/s/Hz. The lower branch is a delay line whose fiber length is matched with the upper line to cancel the phase noise between the signal and the carrier. Signal carrier power ratio (CSPR) is maintained to be 0 dB. These two branches are combined with a polarization beam combiner (PBC) and then launched into a recirculation loop which consists of two spans of 80-km SSMF whose loss is compensated by the EDFAs. The inset (i) of Fig. 2 shows the spectrum at the transmitter captured by the optical spectrum analyzer (OSA). Each channel has the baud rate of 25 Gbaud/s, corresponding to 0.2 nm in terms of wavelength; the channel spacing is 0.4 nm (or 50 GHz). Totally 4 nm bandwidth is occupied. At reception, the light is fed into a band pass filter to filter out one channel each time which occupies a 40 GHz bandwidth and carries 100 Gb/s data. The inset (iii) in Fig. 2 shows the spectrum after the filter. The spectrum presents a double sideband modulation (DSB), and the carrier is shown by the power peak at the center. The optical signal is then spilt using a polarization beam splitter (PBS). Polarizations X and Y are equally split into two branches by two 3-dB couplers. Ports 2 and 3 are fed into a standard coherent receiver. Ports 1 and 4 can be fed into a balanced PD, while in experiment we send them to two single-ended PDs and balance them in DSP. The electrical signal is sampled by a real-time oscilloscope at a sampling rate of 50 GSa/s with 15-GHz electrical bandwidth. DSP of the received signal includes: (1) OFDM window synchronization [2]; (2) SV rotation matrix training and polarization recovery; (3) cyclic prefix removal and FFT; (4) channel equalization; (5) constellation reconstruction and BER calculation. Totally 5.24 million bits are collected for BER calculation of one channel. Results and Discussion We first measure the bit error rate (BER) as the function Fig. 2 Experimental setup for 1-Tb/s SV-DD. (Inset (i) spectrum before transmission, (ii) spectrum after 480-km transmission, and (iii) spectrum of channel 6 after wave-shaper.) MUX: multiplexer (10x1 coupler); IM: intensity modulator; I/Q mod. I/Q modulator; AWG: arbitrary waveform generator; PBC/S: polarization beam combiner/splitter; EDFA: erbium doped fiber amplifier; SW: optical switch; BPF: band-pass filter; WSS: wavelength selective switch; PD: photo-detector; B-PD: balance Photo-detector. of the fiber launch power as shown in Fig. 3 to identify the fiber nonlinearity tolerance for this 1-Tb/s system. The optimum launch power is 8 dBm. Then, we measure the BER performance for all the 10 channels at the reach of 480 km with the launch power of 8 dBm. As shown in Fig. 4, all the bands can achieve the BER lower than 0.024, the threshold of 20% FEC [10]. The inset of Fig. 4 is a constellation measured for channel 6 at an OSNR of 34 dB with a BER of 8.8×10-3. Fig. 3 BER performance as a function of fiber launch power for 1 Tb/s SV-DD signal. Fig. 4 BER performance for 100-Gb/s SV-DD tributary channel after 480-km transmission. about 31 dB for 10×25-Gbaud 16-QAM signals to achieve the BER below 20% forward error correction (FEC) threshold. Considering the highest OSNR of 34 dB achieved in the experiment, the system OSNR margin is about 3 dB between 31 dB (20% FEC threshold) and this value. Compared with previous 1Tb/s POL-MUX coherent detection, the required OSNR at 20% FEC threshold is about 25 dB for 1 Tb/s coherent system in [2]. The experimental result indicates a 6 dB OSNR sensitivity penalty for SV-DD, which agrees with the theoretical prediction: since half of the optical power is shared by the carrier at the SV-DD transmitter, there is 3 dB intrinsic OSNR penalty; both carrier and signal suffer from noise degradation in SV-DD, leading to another 3 dB OSNR penalty compared with the coherent system. Nevertheless, this OSNR sensitivity is still much better than the conventional single-end PD based DD [7], because SV-DD does not need a high CSPR to suppress the 2nd order nonlinearity. The performance for this initial 1-Tb/s SV-DD transmission is limited primarily by the multi-tone generator, which has an EDFA inside sacrificing the system OSNR and therefore degrading the OSNR sensitivity. In practice, the 25 Gbaud signal can be generated with a higher sampling rate digital-to-analog converter (DAC). Moreover, by using higher baud rate transmitter with lower modulation format, SV-DD can even support a transmission distance of more than 1000 kilometers, which reveals the flexible capability for SVDD to be deployed in short reach applications. Conclusions We have experimentally demonstrated the first Terabit direct detection reception over 480-km SSMF using the SV-DD with 100% spectrum efficiency with reference to coherent detection. 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