SNR Optimization through Digital Frequency-Domain Pre-Emphasis for Extending Transmission Distances in DML-Based OFDM-PON Systems

Abstract

By optimizing the SNR of OFDM-PON systems through frequency-domain preemphasis to counter the transmission distortion caused by frequency-chirped lasers, we demonstrate 20-Gb/s fixed-16QAM transmission over 40 km with a 20-dB power-budget for a 5.2-GHz signal-bandwidth. OCIS codes: (060.2360) Fiber optics links and subsystems; (060.4080) Modulation IntroductionOrthogonal frequency division multiplexing (OFDM) technology has been widely researched and is one of the more promising approaches to future optical access systems. The higher modulation orders enable bit-rate enhancement through the efficient use of analog front-end bandwidth in optical transceivers. In addition, multi-carrier modulation provides fully flexible services due to its dedicated and shared access modes. These benefits can also be realized in the broadband mobile backhaul, by connecting many radio base stations to aggregation nodes [1]. Using direct modulation and detection, which yields simple optical transceivers, is the most attractive approach for the costeffective implementation of OFDM passive optical networks (PONs) [2,3].In order to increase the reach and the number of splits in PON tree architectures, optical transceivers are being designed to meet the high signal-to-noise ratio (SNR) requirements, to maximize the available optical power budget. As analog electronic and optoelectronic components are further researched and improved, better results continue to be achieved with PON transceivers including low-noise transimpedance amplifiers (TIAs) and high-sensitivity avalanche photodiodes (APDs) [4].However, due to the frequency chirp created by directly modulated laser (DML), OFDM signals suffer waveform distortion in the fiber link, which is influenced by the chirp amount and sign as well as characteristics of the fiber. This chirp-induced dispersion introduces power fading as well as sub-carrier interaction [3,5], leading to SNR degradation on certain sub-carrier frequencies. Therefore, the degraded SNR is insufficient for using high modulation orders, considering that the available SNR at the optical receivers varies throughout a PON tree. This limits the transmission distance as well as the available signal bandwidth of OFDM-PON systems. In this paper, we overcome these limitations through the SNR optimization in a PON system with the frequency-domain pre-emphasis and the OFDM formats of being able to control the power and modulation order for each sub-carrier.

Integrated Gain Switched Comb Source for 100 Gb/s WDM-SSB-DD-OFDM System

“…In the system, amplitude pre-emphasis can be applied to compensate the SNR degradation by studying the EVM of each subcarrier [13]. Fig.…”