Abstract
The Wadley Loop is a method of down-converting RF signals over a wide frequency range using a low-quality widely-tunable oscillator and a high-stability frequency comb reference. Together the widely tunable oscillator and high-stability comb source provide a widely-tunable high-stability receiver. In this paper, we demonstrate an electro-optic version of the Wadley Loop that is able to provide a widely-tunable, high phase stability coherent receiver. This could have applications in Quadrature Amplitude Modulation (QAM) receivers with high constellation sizes, optical OFDM receivers with long symbol durations, and wide-range high spectral resolution optical spectrum analysers.
Highlights
Coherent optical communications requires a local-oscillator laser at the receiver to mix with the incoming signal [1, 2]
The proposed system provides a similar phase stable reference to optically injection locking (OIL) a high-linewidth laser with one comb line of the modelocked laser (MLL) [16]; the Wadley Loop configuration is not limited by the locking bandwidth of the OIL laser, and may be able to compensate for larger frequency offsets
This means that the X-polarization output of the receiver contains the test signal from the transmitter mixed with the widely-tunable oscillator (WTO), the Y-polarization output contains the reference comb from the MLL mixed with the WTO
Summary
Coherent optical communications requires a local-oscillator laser at the receiver to mix with the incoming signal [1, 2]. If the received signal is very sensitive to phase noise, such as in QAM modulation with large constellations [3], or Coherent Optical Orthogonal Frequency Division Multiplexing (COOFDM) [4] with long symbol lengths, the local oscillators must have high phase stability (a narrow linewidth) in the order of 100 kHz. if advanced spectrum shaping techniques, such as Nyquist filters or CO-OFDM are employed, zero frequency offset is preferred to allow accurate operation of the matched filter or FFT. The problem being solved is similar to receiving a narrow-band OFDM subcarrier in a ‘superchannel’ [12], or a signal with low tolerance to phase noise [13] In both cases, a narrow-linewidth, highly tunable, high frequency stability, local oscillator is required. We show that the system leads to reduced rates of phase wander and frequency offset in the constellations of a received optical QPSK signal, and so improves signal quality over a wide range of local oscillator tunings
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