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200Gs/s Real-time Optical-Sampling-based Orthogonal Frequency Division Multiplexing System Chao Tang, Hongwei Chen, Feifei Yin, Minghua Chen and Shizhong Xie State Key Laboratory on Integrated Optoelectronics, Tsinghua National Laboratory for Information Science and Technology Department of Electronic Engineering, Tsinghua University, Beijing 100084, P.R.China [email protected]

Abstract: An all-optical orthogonal frequency division multiplexing scheme with 200GSample/s optical sampling technique is proposed and experimentally demonstrated. 50Gb/s AO-OFDM data is successfully transmitted over a 20km umcompensated single-mode-fiber link with real-time direct-detection. ©2010 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4230) Multiplexing

1. Introduction Optical orthogonal frequency division multiplexing (O-OFDM) technique has recently attracted growing interests as a promising technology for future high-speed communication systems [1]. It’s considered to have advantages of high tolerance to chromatic dispersion, polarization mode dispersion and optical non-linearity [2]. In conventional optical OFDM system, the symbols are typically generated in electrical domain by multiplexing parallel data into multiple sub-carriers and then converted into optical domain by a complex modulator. This scheme is limited by electronic processing speed in forward/inverse discrete Fourier transform (DFT/IDFT) modules and also the sampling rate of digital-to-analog/analog-to-digital converter (DAC/ADC). Up to now, the highest sampling rate of commercial arbitrary waveform generator (Tektronix AWG7122B) is 24Gs/s. Thus, most existing OFDM systems are off-line, while the highest bit rate of a real-time O-OFDM system is only 12.1Gb/s [3]. Recently all-optical realization methods of OFDM are introduced to overcome the electrical bottle-neck, in which the DFT/IDFT transformation is realized by optical delay lines and phase shifters [4]. An all-optical sampling OFDM (AOS-OFDM) scheme with cyclic postfix (CP) based on fiber Bragg grating (FBG) was proposed in principle [5]. In this paper, a novel all-optical sampling scheme with sub-carrier phase pre-emphasis for optical OFDM multiplexing is presented. Ultra-short optical pulses are applied as optical samples and all-optical OFDM multiplexer/demultiplexer (MUX/DMUX) are based on FBG techniques. The interval betweem each optical sample is only 5ps, so the sampling rate increases to 200Gs/s. With sub-carrier phase pre-emphasis, the demodulated signals have better performance by suppressing the effect of constructive interference between neighbor symbols. In our experiment, 5 optical orthogonal sub-carrier channels (SCs) are multiplexed to generate 50Gb/s NRZ-OFDM signal which is successfully transmitted over a 20km uncompensated single-mode fiber (SMF) link with real-time direct-detection. Spectral efficiency of about 0.8bps/Hz is achieved. 2. Principles

Fig. 1. Pulses overlapping between neighbor symbols

In an AOS-OFDM system, ultra-short optical pulses are applied as sampling pulses. The number of optical sampling pulses in a demodulated OFDM symbol will increase for the linear convolution property of optical OFDM MUX/DMUX [5].Though the decision zone (area (a) in Fig. 1) keep uncontaminated, the sampling pulses between adjacent symbols (area (b) in Fig. 1) with different amplitudes and phases may perform constructive interference which generates very high pulse peaks. This degrades receiving performance due to the optical power limitation of

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the photon detector. In our proposed all-optical sampling scheme, different initial phases are introduced instead of setting all of them the same. With optimum phase design, the constructive interference effect between symbols can be weakened, which will improve performance with the same received optical power. 3. Experiment setup and results

Fig. 2. Experimental setup, (a) spectrum of AOS-OFDM MUX and (b) spectrum of AOS-OFDM DMUX. MLLD: mode-locked laser diode; PPG: pulse pattern generator; EDL: electrical delay line; MZM: Mach-Zehnder modulator; MUX: multiplexer; SMF: single mode fiber; DMUX: demultiplexer; PD: photon detector; BERT: bit error rate tester.

Fig. 2 depicts our all-optical sampling OFDM experimental setup. An ultra-short optical pulse train with pulse width of about 2ps is generated by a mode-locked laser diode (MLLD) with repetition rate 10GHz. The pulse train from pulse pattern generator (PPG) is a 27-1 pseudo-random bit sequence (PRBS) at 10Gb/s. After synchronously modulated with a Mach-Zehnder modulator (MZM), the signal is reflected by the MUX FBG and boosted into 20km SMF. Five MUX sub-FBGs for each SC channel are manufactured in one FBG whose structure is shown in Fig. 2. The single-way time interval between each sub-FBG is set to be 50ps (indicated as T in Fig. 2) which equals to half of OFDM symbol period (100ps) to keep five channels synchronous. The MUX sub-FBG for i-th subcarrier is designed to have 20 reflection sub-gratings including 4 cyclic postfixes (CP) sub-gratings, the single-way time delay and phase shift of n-th sub-grating are (n-1)/2×5ps and (i-3)(n-1) π/8 respectively. Detail structure parameters of OFDM MUX are shown in Table 1. And the DMUX FBG is designed to have 16 reflection sub-gratings, the single-way time delay between each sub-grating is 2.5ps and phase shift is set to zero, which can be tuned by stress to demultiplex corresponding SC channels. The signals after DMUX are amplified and pass through an optical bandpass filter to eliminate the amplified spontaneous emissions (ASE) noise. After a 10Gb/s traditional direct-detection optical receiver, the signals are analysed by a bit error rate tester (BERT). Table 1. Parameters for OFDM MUX MUX Channels M ΔL(μm) ΔΦ Φ0 SC1 20 512.25 - nπ/4 0 SC2 20 512.25 - nπ/8 -3π/8 SC3 20 512.25 0 5π/8 SC4 20 512.25 nπ/8 5π/8 SC5 20 512.25 nπ/4 3π/4 1. M is the number of sub-gratings in one MUX sub-FBG module. 2. ΔL is the distance between each sub-grating, which equals to cT/neffM1, where c is the speed of light in a vacuum, T=50ps is half of the symbol period, neff =1.464 is the effective refractive index of the fiber core. 3. ΔΦ is the phase shift among sub-gratings of each channel. 4. Φ0 is the initiate phase of each MUX sub-FBG. The spectra of AOS-OFDM signal and demultiplexed SC channels are shown in Fig. 3(a). The -20dB bandwidth of the OFDM signal is about 0.5nm corresponding spectral efficiency of 0.8bps/Hz. The OSNR of the AOS-OFDM signal is kept above 20dB. The insertion figures (c) to (g) in Fig. 3 are the demodulated eye-diagrams for channel 1 to channel 5 at receiving power of 0dBm after transmission over a 20km SMF link with a digital sampling oscilloscope (DSO, Tektronix TDS8200, optical bandwidth 65GHz), it’s shown that the middle of the

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eye-diagram can be clearly observed opening and the constructive interference between adjacent eyes is quite weak. The BER curves for both back-to-back (B2B) and post-transmission over 20km SMF are shown in Fig. 3(b). Channel 1 and channel 5 have much better performance than the other 3 channels for less cross-disturbance from neighbor channels. The BERs of all the channels can reach 2e-3, which is the forward error coding (FEC) limit. Only up to 5dBm receiving power penalty can be observed after transmission which is acceptable. There is an optimum receiving power for each channel due to the power saturation of optical receiver. And for each channel, the optimum receiving power get higher after transmission, because the pulse width will be enlarged due to chromatic dispersion of the optical fiber link, which lowers the peak power and weakens the saturation effect.

(a)

(c)

Spectra of demultiplexed signals

Channel 1

(b)

BER vs. Received Power for B2B and post-transmission

(e) Channel 3 (d) Channel 2 (f) Channel 4 Fig. 3 (a) Spectra of demultiplexed signals, (b) BER vs. Received Power for B2B and post-transmission over 20km SMF and (c)--(g) demodulated eye-diagrams

(g)

Channel 5

4. Conclusion In this paper, a novel all-optical sampling scheme with sub-carrier phase pre-emphasis for optical OFDM multiplexing is proposed and experimentally demonstrated. The sampling rate increases to 200Gs/s with 20% CP inserted, and the demodulated signal has good performance by weakening the effect of constructive interference between neighbor symbols. 50Gb/s NRZ-OFDM signals without polarization multiplexing are generated and successfully transmitted over a 20km uncompensated SMF link with real-time direct-detection. The power penalty is less than 5dB and the spectral efficiency reaches 0.8bps/Hz. Higher spectral efficiency can be obtained simply by further applying more complex modulation scheme and polarization multiplexing. 5. Acknowledgement This work was supported in part by NSFC under Contracts No. 60736002, 60807026 and 60932004, the National 863 Program of China under Contract No. 2007AA01Z274. 6. References [1] H. Bao et al, “Transmission simulation of coherent optical OFDM signals in WDM systems,” Opt. Express 15(8), 4410–4418, 2007. [2] J. Armstrong, “OFDM for Optical Communications,” J. Lightw. Technol., 27(3), 189-204, 2009. [3] F. Buchali et al, “Realization of a real-time 12.1 Gb/s optical OFDM transmitter and its application in a 109 Gb/s transmission system with coherent reception,” ECOC 2009, Paper PD2.1. [4] K. Lee, “All optical discrete Fourier transform processor for 100 Gbps OFDM transmission,” Opt. Express,16(6), pp. 4023-4028, 2008. [5] Hongwei Chen et al, “All-optical sampling orthogonal frequency-division multiplexing scheme for high-speed transmission system,” J. Lightw. Technol., 27(21), 4848-4854, 2009.