An Endless Optical Phase Delay for Harmonic-Free Phase/Frequency Shifting in Coherent-Lite DCIs and Microwave Photonics

An optical IQ modulator (IQM), when driven by quadrature-phase sinusoidal electrical signals, adds a phase/frequency offset to the optical input. However, harmonics of the applied electrical signals are also generated in the optical domain. In this work, we analytically derive the waveform shapes that achieve a harmonic-free phase/frequency shift with high spectral purity using the IQM. Using these waveforms, an endless optical phase delay (EOPD), wherein the phase delay can be increased or decreased continuously and arbitrarily in a reset-free manner, has been presented. The EOPD can be used in coherent-lite or analog coherent receiver based DCIs, phased array LiDAR, and many other applications. In order to adjust for the bias voltages and control signal gains in the EOPD, some of which may be time varying, a multivariate gradient descent algorithm is presented. Use of the EOPD for phase synchronization in a self-homodyne coherent-lite optical link has been demonstrated, making it a promising solution for low-power high-capacity coherent data center interconnects (DCIs).