A homodyne coherent receiver for ultra-dense WDM-PON with off the shelf components is presented. It consists of a conventional DFB, phase switched clock signal, an optical coupler instead of a 90º hybrid, balanced photodetectors and digital signal processing. The phase swing for a DBPSK signal was optimized and the performance was experimentally evaluated in terms of the sensitivity for several laser linewidths. The acceptable frequency offset and clock time delay was also assessed. The results exhibit a sensitivity of-48 dBm at a BER of 10-3 and indicate a high tolerance to phase noise.

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We present an optimized carrier recovery architecture based on differential detection for coherent optical receivers that substantially reduces the required DSP hardware resources, aimed to cost-effective transceivers for access networks applications. The proposed architecture shares the 1-symbol complex correlation required for differential phase detection within both the frequency estimation and the phase recovery blocks of the receiver DSP, thus lowering the energy consumption of the digital coherent receiver and increasing the tolerance against fast wavelength drifts of the lasers. We prototyped the proposed carrier recovery in a commercial FPGA for real-time evaluation with DPSK data at 1.25 Gb/s. The optical transmission system implemented direct-phase modulation of commercial DFB lasers, 25 km of single-mode fiber, and a coherent intradyne receiver with low-cost optical front-end based on 3x3 coupler and three photodiodes providing phase-diversity operation. Results show high performance in real-time for DPSK, achieving-55 dBm sensitivity at BER = 10-3 in a 6.25 GHz spaced ultra-dense WDM grid, high tolerance to optical phase noise, and enhanced mitigation of the fast wavelength drifts from lasers enabled by feed-forward DSP correction and feedback LO automatic tuning.

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We experimentally demonstrate a simultaneous amplitude and phase modulation of a monolithically integrated dual electro-absorption modulated laser (DEML). The proposed technique combines a 4-ary direct phase modulation with a 2-level amplitude modulation obtaining an 8-ary amplitude-phase shift keying (8-APSK) external-modulator-free transmitter. Its performance was tested up to 7.5 Gb/s in a 25-km single mode fiber link with intradyne coherent detection. A receiver sensitivity of-42.5 dBm was achieved at FEC limit BER = 4x10-3. The results show that the proposed system can be an efficient flexible transmitter for next generation passive optical networks.