Generation of optical OFDM signals using 21.4 GS real time digital signal processing
We demonstrate a field programmable gate array (FPGA) based optical orthogonal frequency division multiplexing (OFDM) transmitter implementing real time digital signal processing at a sample rate of 21.4 GS/s. The QPSK-OFDM signal is generated using an 8 bit, 128 point inverse fast Fourier transform (IFFT) core, performing one transform per clock cycle at a clock speed of 167.2 MHz and can be deployed with either a direct-detection or a coherent receiver. The hardware design and the main digital signal processing functions are described, and we show that the main performance limitation is due to the low (4-bit) resolution of the digital-toanalog converter (DAC) and the 8-bit resolution of the IFFT core used. We analyze the back-to-back performance of the transmitter generating an 8.36 Gb/s optical single sideband (SSB) OFDM signal using digital upconversion, suitable for direct-detection. Additionally, we use the device to transmit 8.36 Gb/s SSB OFDM signals over 200 km of uncompensated standard single mode fiber achieving an overall BER
Optical orthogonal frequency-division multiplexing (OFDM) has recently gained substantial interest from both the academic and industrial communities [1] because of its advantages which include high spectral efficiency and simple distortion equalization. Additionally, the OFDM multiband technique proposed in [2] can relax the speed and bandwidth requirement of the signal converters while providing finer switching granularity and more network service flexibility. The suitability of optical OFDM to convey data and services in the next generation of optical networks has been extensively investigated for both direct and coherent detection [2–7]. The two detection schemes have essentially the same transmitter design and only differ in the receiver. Direct-detection OFDM benefits from a simpler and cheaper receiver as it only requires a single photodiode and digital signal processing (DSP), but with lower sensitivity and spectral efficiency. Coherent detection, on the other hand, has a higher sensitivity and spectral efficiency, but comes with additional cost and complexity due to the requirement for a local oscillator laser and other optical components, and also a more complex DSP for phase and polarization tracking. Some of the DSP complexity may be reduced by using a selfcoherent OFDM receiver [8]. In the coherent configuration (CO-OFDM), data rates over 100 Gb/s have been demonstrated over hundreds of kilometers of uncompensated fiber [2,3]. Similarly linear and nonlinear tolerance for direct-detection OFDM has been extensively studied and experimentally verified [5–7]. Recently 108 Gb/s DD-OFDM transmission has been demonstrated over 20 km of standard single-mode fiber (SSMF) using polarization multiplexing [9]. In addition to telecom applications, optical OFDM is also ideally suited to provide 40 and 100 Gb/s Ethernet services for data centers and local area networks deploying multimode fiber
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