Abstract

Digital signal processing (DSP)-enabled soft reconfigurable optical add/drop multiplexers (Soft-ROADMs) offer flexible add/drop optical switching at wavelength, sub-wavelength and spectrally-overlapped orthogonal (I and Q) sub-band (SB) levels, which makes them highly desirable for enabling flexible reconfigurable optical-wireless converged access networks where both fixed and wireless access services are consolidated in a shared network, enabling network resource efficient and cost-effective connectivity solutions. However, the performance of the targeted sub-band (TSB) extracted by the soft-ROADM drop element is extremely sensitive to drop RF signal phase offset, which is a major limitation impacting the technical feasibility of soft-ROADMs. To overcome this challenge, in this paper, a phase-offset-insensitive soft-ROADM dual-arm IQ drop operation is proposed and experimentally demonstrated. A DSP implemented multi-input multi-output (MIMO)-based I/Q crosstalk mitigation technique is employed to achieve the insensitivity to drop RF signal phase offset. It is shown that, the traditional single-arm I/Q soft-ROADM drop element has a limited drop RF signal phase offset dynamic range of $\sim \pm \,\pi /4$ ( $ < \pm \,0.05\pi $ ) in the presence of I/Q (I&Q) SBs, thus requiring accurate phase control, however, the proposed dual-arm IQ drop element demonstrates the ability to tolerate any arbitrary phase offset, thus completely eliminating the need to dynamically control the drop RF signal's phase. This paper also experimentally demonstrates the effectiveness of the new dual-arm drop element to mitigate any I/Q crosstalk effects caused by, various effects, including the fiber transmission channel-induced frequency response roll-off, symbol timing offset, and propagation delay drifting effects. Importantly, it is shown that the traditional single-arm I/Q drop element offers poor performance stability as the dropped TSB's BER performance degrades within a few minutes, whereas, the new dual-arm drop element achieves a highly stable BER performance. Soft-ROADMs incorporating the newly proposed dual-arm IQ drop element are thus experimentally shown to offer significantly reduced implementation complexity, thus radically increasing their suitability for application in optical-wireless converged access networks.