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A novel high speed transmission system using all-optical sampling orthogonal frequency multiplexing (AOS-OFDM) technique is proposed and demonstrated. By utilizing polarization multiplexing (PolMUX) and non-return-to-zero (NRZ) format, the total bit rate is 100Gb/s with high spectral efficiency of 1.6. In addition, optical cyclic postfixes are inserted to help improve the system performance. The 100Gb/s PolMux-NRZ-AOS-OFDM signals can pass through 20km single-mode fiber (SMF) transmission link without any compensation.
©2009 Optical Society of America
1. Introduction
Optical orthogonal frequency division multiplexing (OFDM) method recently arises as a potential technology for future high speed communication systems [1,2]. It is considered to have large tolerance to different fiber transmission impairments, such as chromatic dispersion, polarization mode dispersion and optical fiber non-linearity [3–5]. Many existing optical OFDM systems use electrical circuit to multiplex parallel data into multiple sub-carriers due to OFDM principle and modulate these signals in optical domain by a modulator. Thus, electrical OFDM modulation is limited by electronic processing speed in forward/inverse discrete Fourier transform (DFT/IDFT) module and also the bandwidth of digital-to-analog/analog-to-digital converter (DAC/ADC). If the DFT/IDFT process can be realized by optical method, the OFDM signal process will be very fast and the transmission data rate will also increase greatly. All-optical discrete Fourier transform (DFT) methods combining optical delays and phase shifters are introduced recently. Continuous wave with data modulated is used for transmission and Mach-Zehnder interferometer (MZI) is used as IDFT module [6,7]. Also, coherent WDM signal utilizing OFDM principle is proposed with either coherent comb optical source [8,9] or coherent detection [10]. And a scheme using ultra-short optical pulses as samples for optical DFT/IDFT process is proposed in [11], which has a complex structure of DFT/IDFT modules and needs synchronous pulse carving at the receiver. Recently, we propose a novel optical OFDM scheme with optical cyclic prefix/postfix (OCP) inserted [12]. This scheme can realize simple receiver structure with neither coherent detection nor synchronous optical gate, and also has large chromatic dispersion tolerance. In fact, considering the good performance of fiber Bragg gratings (FBG) in optical code-division multiple-access (OCDMA) systems [13], it’s promising to use FBGs as all optical DFT/IDFT modules. In ref [14], a 20Gb/s double sub-carriers (SC) all-optical OFDM scheme was obtained with FBG based optical DFT/IDFT. The optical cyclic postfixes (CP) can be inserted within the FBG structures.
In this paper, a novel all-optical sampling OFDM (AOS-OFDM) scheme employing traditional non-return-to-zero (NRZ) format and PolMux technique is presented. Five SC channels are utilized to enhance the total bit rate. The multiplexer/demultiplexer (MUX/DMUX) for optical OFDM SCs are made with FBG techniques. Furthermore, this scheme benefits from optical cyclic postfixes inserted. 100Gb/s PolMux-NRZ-AOS-OFDM signals are generated and successfully transmitted over 20km single-mode fiber (SMF) without any compensation.
2. Principle and experiment
2.1. Principle
In discrete all-optical OFDM system, ultra-short optical pulses are used as sampling pulses to perform all-optical OFDM multiplexing and demultiplexing. For the linear convolution property, the demultiplexed OFDM signal has only one orthogonal sample in one bit duration. To solve this problem, optical cyclic prefixes or postfixes samples can be added in one symbol period using FBGs as illustratd in Fig. 1 . The i-th subcarrier (SC) channel signal with optical CPs can be expressed as:
where X is the sample value, N is the sample number in one discrete Fourier transform (DFT) interval, C is the sample number of optical CPs, so in one symbol period T the total sample number is M=N+C. A(t) is the shape of an optical sample pulse, τ and ϕ are the time delay and phase of each sample as described in Fig. 1. Thus, the correct demodulated samples after the AOS-OFDM DMUX module increase to C+1. This method can help to enlarge the decision range in eyediagrams, and also it may give some benefits for dispersion walk-off as the electrical CP’s function in wireless multipath environment [15].2.2.Experiment
In experiment, FBGs are used as AOS-OFDM MUX/DMUX modules for N=8, C=2 case, which means the CP length is 20% of symbol period. The MUX FBG for i-th subcarrier is designed to have 10 reflection sub-gratings, the time delay and phase shift of n-th sub-grating are (n-1)/2×10ps and i(n-1)π/4 respectively. And the DMUX FBG for i-th subcarrier is designed to have 8 reflection sub-gratings, the time delay and phase shift of n-th sub-grating are (n-1)/2×10ps and i(n-1)π/4 respectively. Detail structure parameters of AOS-OFDM MUX/DMUX are shown in Table 1 . The experimental setup is shown in Fig. 2 . An 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 211-1 pseudo-random bit sequence (PRBS) at 10Gb/s. An electrical delay line (EDL) is used to confirm the synchronous NRZ modulation. Then the signal is fed into a coupler and reflected by 5 AOS-OFDM MUX FBGs for different SC channels. An optical delay line (ODL) is used in each path to keep the symbol synchronization. Erbium-doped fiber amplifiers (EDFA) and optical attenuators are used to confirm that 5 subcarrier channel signals have the same power entering into another coupler. Then the combined AOS-OFDM 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 ODL is used for emulating different signals in two polarizations. Then the 100Gb/s PolMux-NRZ-AOS-OFDM signals pass through an optical bandpass filter (OBPF) with 3dB bandwidth of 0.6nm and is amplified by an EDFA. The input optical power of SMF link is 8dBm to avoid nonlinearity in the fiber. After 20km SMF link without any compensation, the signals are firstly split by a PBS and then demutiplexed with corresponding AOS-OFDM DMUX FBGs. At the receiver end, only traditional 10GHz photon detectors (PD) are used without coherent detection.
Table 1. Parameters for AOS-OFDM MUX/DMUX
Fig. 2 Experimental Setup. MLLD: mode-locked laser diode; PPG: pulse pattern generator; EDL: electrical delay line; MZM: Mach-Zehnder modulator; MUX: multiplexer; ODL: optical delay line; PC: polarization controller; PBS: polarization beam splitter; PBC: polarization beam coupler; SMF: single mode fiber; DMUX: demultiplexer; PD: photon detector.
3. Results
Samples in one symbol period of SC5 for bit “1” are shown in Fig. 2 (a). It’s clear that after AOS-OFDM MUX FBG, there are 10 samples in one symbol period of 100ps, which include 8 samples in DFT interval and 2 optical CPs. Figure 2 (b) shows the received demutiplexed SC5 signal with a digital sampling oscilloscope (DSO, Tektronix TDS8200, optical bandwidth 65GHz), and in the middle of the eyediagram there are obviously 3 orthogonal samples which is the same as predicted. The spectrum of this 100Gb/s PolMux-NRZ-AOS-OFDM signal is shown in Fig. 3 . The bandwidth between first null points is 0.496nm corresponding spectral efficiency of 1.61. And the spectra of demultiplexed SC channels in pol-x axis are also shown in Fig. 3, whose SNRs keep above 20dB. Figure 4 shows the eyediagrams of SC5 in B2B and after 20km SMF cases. With the help of optical CPs, the eyediagrams have good opening and tolerance to a certain fiber chromatic dispersion. Bit error rate (BER) performance of 10 channels (with received optical power of −4dBm) is shown in Fig. 5 . The BER variation mainly comes from the difference structures of FBGs. After 20km SMF link, the BERs of each SC are still lower than the advanced forward error correction (FEC) limit (2×10−3) with more uniform performance.
4. Conclusion
A novel high spectral efficiency all-optical sampling OFDM scheme with cyclic postfixes inserted has been proposed for 100Gb/s optical transmission system application. The 100Gb/s PolMux-NRZ-AOS-OFDM system includes 10 sub-carrier channels with 10Gb/s NRZ modulation of each channel, which simplifies the transmitter and receiver design. Experimental results show that the spectral efficiency is higher than 1.6, and this can increase greatly with more complex modulation formats (such as QPSK, M-QAM). Furthermore, with the help of optical CPs, system performance is still good after 20km SMF transmission link without any compensation.
Acknowledgement
This work was supported in part by NSFC under Contract 60632010, 60807026, 60932004,National 863 Program of China under Contract 2007AA01Z264 and the Research Fund for the Doctoral Program of Higher Education of China under Contract 20070003015.
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