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An all-optical sampling OFDM scheme using PolMux-DQPSK format with single source is proposed and experimentally demonstrated. 5 × 200Gb/s AOS-OFDM signal with spectral efficiency of 3.07bit/s/Hz is successfully transmitted over an 80km SMF link with real-time detection by optical Fourier transform filters. Furthermore, this scheme can coexist with traditional WDM channels and be a promising technique for seamless upgrade of transmission date rate.
©2011 Optical Society of America
1. Introduction
Optical orthogonal frequency division multiplexing (O-OFDM) method has widely been considered as one of the most promising technology for future high-speed optical communication system [1], since its high spectral efficiency, robustness against both chromatic dispersion(CD) and polarization mode dispersion (PMD), and natural compatibility with digital signal processing (DSP)-based implementation [2–4]. Through the use of coherent detection, the line rate of demonstrated OFDM has rapidly advanced to 100Gb/s [5,6] and much lately to 1Tb/s [7]. However these aforementioned systems require higher implementation complexity compared with direct-detection OFDM (DDO-OFDM), such as narrow line-width lasers for receiver-end local oscillators (LO), complex frequency offset and phase noise compensation DSP algorithms. These components restrict the symbol rates of several Gb/s due to the electrical bottleneck of digital-to-analog/analog-to-digital converter and digital signal processor. Furthermore most of these published experiments are off-line [5–9]. At the mean time, a number of real-time transmission systems with G-b/s data rate using electrical implements are demonstrated [10–12]. 2Tbit/s multi-banded coherent WDM is realized in [13] with asymmetric Mach-Zehnder Interferometer (AMZI) and a bandpass filter cascaded for subcarrier selection. To realize Tb/s real-time OFDM system, a useful way is shifting the Fourier transform into the optical domain [14,15]. Recently, a real-time all-optical Fourier transform (OFT) receiver enabled line rate of 10.8Tb/s and 26Tb/s is proposed with high speed optical gates in back-to-back case and 50km SMF transmission [16,17]. We have proposed an all-optical OFDM signal generation with optical cyclic postfixes (OCPs) and real-time detection with optical Fourier transform filters based on fiber Bragg gratings (FBG) in [18]. A simple receiver structure with neither coherent detection nor synchronous optical gates is applied in this system. Moreover, with optical cyclic postfixes inserted, good transmission performance is achieved.
In this paper, a novel all-optical sampling OFDM (AOS-OFDM) scheme employing differential quadrature phase-shift keying (DQPSK) format and polarization multiplexing (PolMux) is demonstrated. A single phase stable ultra-short pulse source is used as optical samples. By an optical OFDM multiplexer, 5 DWDM channels are realized [19]; while in each channel there are 5 all-optical subcarrier channels (SC). 5×200Gb/s PolMux-DQPSK-AOS-OFDM signals are generated and successfully transmitted over 80km single-mode fiber (SMF). The receiver end is highly simplified with real-time detection by using OFT filters.
2. Principles
Nowadays, many optical OFDM systems are based on high speed electronic processing modules such as DAC/ADC and DSPs. At the transmitter, electrical devices perform many dominating functions including serial to parallel conversion (S/P), modulation format, inverse DFT, parallel to serial conversion (P/S) and DAC. The optical devices only carry and detect OFDM signals and provide optical channels. At the receiver end, the process is just inversion of the transmitter which is also done by electronics as shown in Fig. 1(a) . So electrical bottleneck limits the speed and bandwidth of OFDM used in optical communication. In fact, optical technologies can do more complex signal processing functions by taking advantage of high bandwidth and high speed parallel process. Thus, we can shift many electrical functions to optical domain, such as digital sampling, sub-carrier mapping, cyclic postfix or prefix (CP) insertion, etc. Here, we use ultra-short pulses as optical sampling and do sub-carrier mapping by optical OFDM multiplexers as shown in Fig. 1(b). At the receiver, matched optical Fourier transform filters can be employed to extract corresponding sub-carrier channels to do parallel receptions.
Many techniques, including delay interferometers (DI) [17] or arrayed waveguide gratings (AWG) [19], are used to perform all-optical Fourier transform avoiding electrical bottle neck. However, at the receiver, a synchronous optical gate should be used to extract the correct sample [17], which will increase the receiver complexity in a high-speed system. In order to take full advantage of optical devices, we proposed a novel all-optical OFDM technique in ref [18]. The basic principle of all-optical OFDM is similar with that of electrical ones. With optical delay lines and phase shifters, one can use ultrashort optical pulses as samples to do DFT/IDFT process all optically. The structures of optical OFDM MUX-FBG and DMUX-FBG are illustrated in ref [18]. The main advantage to use FBG as AOS-OFDM MUX is to add optical CPs to OFDM signals as in electrical domain and can have more DFT points in a compact structure. As shown in Fig. 2(a) , each sub-grating (SG) is designed for a certain sub-carrier channel, in which there are many sample spots to reflect the incoming optical pulses with proper time delay and phase shifts. In [20], we have built an AOS-OFDM system with 5 separated sub-gratings for 100Gb/s transmission. In order to simplify the system, we fabricate a compact MUX-FBG as shown in Fig. 2(b). Many sub-gratings are cascaded with time delay T, which equals to half of the OFDM symbol period to keep all the channels synchronous and decorrelative. At the receiver, only one stretched DMUX-FBG, whose structure is same with [21], is used for matched optical Fourier transform filter to extract corresponding SC channel.
3. Experiment
The experimental setup is shown in Fig. 3 . A 10GHz ultra-short optical pulse train whose pulse width is 2ps from a mode-locked fiber laser (MLFL) passes through a wavelength converter (WC) to overcome the poor pulse-to-pulse coherence. The WC is based on cross polarization modulation (CPM) effect. Figure 3(A) shows the spectrum of the pump light from the MLFL with central frequency of 193.5THz. By using polarization controllers (PCs), the polarization direction of the pump pulse is maintained along the slow axis of highly nonlinear photonic crystal fiber (HNPCF) and that of the probe light is 45° to the slow axis. The pump light and the 192.6THz probe light couple into the HNPCF to complete CPM effect after being amplified by an erbium-doped fiber amplifier (EDFA) with 25dBm output power. The HNPCF has a high nonlinear coefficient of 12W−1/km. The spectrum after the CPM effect in HNPCF is shown in Fig. 3(B), including the probe and pump light. The output of HNPCF passes through an optical band-pass filter (OBPF) by adjusting its central frequency to avoid disturbance from the pump pulse. The filtered signal is adjusted by a PC and split by a polarization beam splitter (PBS) to extract an optical pulse train whose spectrum is shown in Fig. 3(C). For the CPM effect is dominated by the peak power of the pump pulses, the optical pulse after WC is independent of the pump pulse phase noise and has good coherent characteristics for DQPSK modulation.
Fig. 3 Experimental Setup, MLFL: mode-locked fiber laser; CWL: continuous wave laser; HNPCF: highly nonlinear photonic crystal fiber; OBPF: optical band-pass filter; PPG: pulse pattern generator; MUX: multiplexer; ODL: optical delay line; PC: polarization controller; PBS: polarization beam splitter; PBC: polarization beam coupler; SMF: single mode fiber; DMUX: demultiplexer; AWG: array waveguide grating; BPD: balanced photo detector.
The DQPSK modulator is driven by two synchronous pseudo-random bit sequence (PRBS) from a pulse pattern generator (PPG) at 10Gb/s making the total bit rate 20Gb/s. Then the signal is reflected by the MUX-FBG for generating all-optical OFDM signals. In our fabrication, the grating is written using 30-mW of continuous-wave UV-light at 244-nm from a frequency-doubled Arion laser with hydrogen-loaded Corning SMF28 fiber. The FBG is written using the standard phase mask technology, and the phase shift is achieved by using a piezoelectric nanoautomation stage with 0.1-nm resolution. In this experiment, 5 sub-gratings for different SC channels are made compact in one FBG. The single-way time interval between each SG is set to 50ps which is indicated as T in Fig. 3. T equals to half of the OFDM symbol period (100ps) to keep all the 5 SC channels synchronous and decorrelative. The i-th MUX sub-grating is designed to have 10 reflection sample spots including 2 of them are cyclic postfixes (CPs), the time delay and phase shift of n-th sub-grating are (n-1) ×10ps /2 and (i-3) ×(n-1) π/4(i = 1,2…5) respectively. Detail structure parameters of AOS-OFDM MUX are shown in Table 1 .The MUX-FBG has periodic spectrum and the free spectrum range (FSR) is equal to 100GHz shown in Fig. 3(D), so the multiplexed signals can fit with WDM grids. An optical bandpass filter is used to get 5 channels for transmission with each channel containing 5 OFDM SCs. The spectral of these five channels at D are shown in Fig. 4(a) . Then the signals are tuned by a polarization controller (PC) and fed into a couple of polarization beam splitter (PBS) and polarization beam coupler (PBC) to perform PolMux. An optical delay line (ODL) is used to get different signals asynchronous in two polarizations. The total line rate now is 2(pol.) ×5(WDM channels) ×5(OFDM SCs) ×20Gb/s = 1Tb/s.
Table 1. Parameters for AOS-OFDM MUX
Then the 1Tb/s PolMux-DQPSK-AOS-OFDM signal is boosted into an 80km SMF transmission link with inline dispersion compensation fiber. At the receiver, the signals are separated by an array waveguide grating (AWG) and split by a PBS, and then demutiplexed with a DMUX-FBG which is used as a matched filter by performing optical Fourier transform. The DMUX-FBG is designed to have 8 sample spots, the time delay between each spots is 5ps and phase shift is set to zero, whose spectrum is shown in Fig. 4(a). The central frequency of DMUX FBG can be adjusted by stretching to demultiplex corresponding SCs. After that, a DQPSK demodulator is used to demodulate I and Q signals without coherent detection nor synchronous optical gates. Two balance detectors are used for O/E conversion.
4. Results
The spectrum of the PolMux-DQPSK-AOS-OFDM signal is shown in Fig. 4(a). The bandwidth between first null points of each channel (200Gb/s) is about 65GHz corresponding to the spectral efficiency 3.07bit/s/Hz, which is an advantage for multi-OADM-hop transmission in optical networks. The spectrum of ch3 is zoomed in and the spectra of demultiplexed SC11-SC15 channels for channel 3 in POL-X axis are also shown in Fig. 4(b). Bit error rate (BER) performance of received data for 50 subcarrier channels of two polarizations is shown in Fig. 5 . The BER variation mainly comes from the non-ideal structures of FBGs. After 80km transmission link, the BERs of each SC are still lower than the advanced forward error correction (FEC) limit (2×10−3). The insert (a) and (b) in Fig. 5 are the demodulated constellations of demultiplexed SC12 channels for both B2B and 80km case.
Fig. 5 BER performances of received data for 50 subcarrier channels of two polarizations, demodulated constellation diagram of SC12 channels for (a) B2B and (b) 80km case.
5. Conclusion
A novel high speed real-time all-optical sampling OFDM system has been experimentally demonstrated. The AOS-OFDM system includes 25 SC channels, with the DQPSK format and polarization multiplexing, the total line rate can reach 1Tb/s employing a single optical source. The signal is successfully transmitted over an 80km SMF link with spectral efficiency above 3bit/s/Hz. The receiver is highly simplified by using optical Fourier transform filter with real-time detection. This scheme is a promising technique for seamless upgrade of the transmission date rate fitting with WDM grids.
Acknowledgments
This work was supported by the NSFC under Contract 60932004, 61132004, 61090391.
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