Abstract
We describe the operating principle, practical implementation, and experimental
evaluation of a synchronous homodyne coherent optical receiver based on an
optical phase-locking scheme that combines optical injection locking of the
semiconductor laser local oscillator (LO) with low-speed electronic feedback
to give both a large locking bandwidth and a wide tracking range. We demonstrate
phase-error variance as low as 0.002 rad$^{2}$ in 10-GHz bandwidth for locking to a
continuous-wave (CW) signal for a combined signal and LO linewidth of 1.5
MHz (full-width at half-maximum), and robust phase locking to a 10-Gb/s binary
amplitude-shift-keyed (ASK) signal, enabling synchronous back-to-back demodulation
of the signal with low bit error ratio $({\rm BER} < 10^{- 10})$ and improved performance
compared to direct detection at low optical SNR (OSNR). By locking to a low-power
CW pilot carrier in the polarization orthogonal to the data signal, demodulation
of binary phase-shift-keyed (BPSK) data has been achieved, with the required
OSNR at ${\rm BER} = 10^{-3}$ reduced by 3 dB compared to demodulation of ASK data under the
same conditions. The OSNR penalty after transmission of the ASK and BPSK signals
over 40 km of standard single-mode fiber was 1–2 dB at ${\rm BER} = 10^{-3}$, indicating that
the chromatic dispersion sensitivity of the coherent receiver is similar to
that for direct detection, and verifying that the scheme for locking to the
orthogonal pilot is applicable to transmission systems, provided that optical
polarization tracking is employed. In addition, we demonstrate frequency-selective
operation of the coherent receiver, demultiplexing and demodulating one of
a pair of equal power, 10 Gb/s, ASK channels separated by 17.5 GHz, with OSNR
penalty at ${\rm BER} =
10^{- 3}$ of 1.5 dB compared to single-channel operation.
© 2011 IEEE
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