The reduction of the LO number for heterodyne coherent detection.

Relative to homodyne coherent detection, heterodyne coherent detection has simple architecture because no 90° hybrid and only half number of photodiodes and analog-to-digital convertor (ADC) chips are required. We experimentally demonstrate that the architecture of heterodyne coherent receivers can be further simplified. When the frequency offset is one half of the channel frequency spacing, one local oscillator (LO) laser can be used for two neighboring wavelength-division-multiplexing (WDM) channels, and therefore the number of LO lasers can be reduced into half compared to homodyne detection. We experimentally demonstrate simplified heterodyne coherent detection of 4 × 196.8-Gb/s polarization-division-multiplexing carrier-suppressed return-to-zero quadrature-phase-shift-keying (PDM-CSRZ-QPSK) modulation after transmission over 1040-km single-mode fiber (SMF)-28 on a 50-GHz grid with bit-error ratio (BER) smaller than pre-forward-error-correction (pre-FEC) limit of 3.8 × 10(-3). To our best knowledge, 196.8 Gb/s is the highest bit rate per channel for heterodyne coherent WDM transmission system. An arrayed waveguide grating (AWG) instead of wavelength selective switch (WSS) is used at the transmitter to spectrally shape and multiplex the WDM signal. We also experimentally demonstrate that heterodyne detection causes 3-dB optical signal-to-noise ratio (OSNR) penalty at the BER of 3.8 × 10(-3) for a certain single channel compared to homodyne detection.