Utilizing Hilbert-pair-based digital filtering, intensity modulation and passive optical coupling, optical filterand O-E-O conversion-free ROADMs with excellent flexibility, colorlessness, gridlessness, contentionlessness, adaptability, and transparency to physical-layer network characteristics are proposed and evaluated, for the first time, which offer DSP-enabled dynamic add/drop operations at wavelength, subwavelength, and orthogonal subband levels. Extensive numerical simulations are undertaken to explore the operation characteristics of the proposed ROADMs in IMDD-based optical network nodes. It is shown that the add/drop operation performance is independent of the signal's location in the digital filtering space. In addition, the results also indicate that the drop operation introduces negligible optical power penalties, while for the worst-case scenarios, optical power penalties induced by the add operation can be 3.5 dB. Furthermore, the impacts of key digital filter parameters and intensity modulation-associated drop RF signals on the ROADM add/drop performance are also investigated, based on which optimum ROADM design criteria are identified not only for effectively reducing the digital filter DSP complexity but also simultaneously improving the ROADM performance.Index Terms-Digital filtering, digital signal processing (DSP), intensity modulation, reconfigurable optical add/drop multiplexer (ROADM).

Real-time optical OFDM (OOFDM) transceivers with on-line software-controllable channel reconfigurability and transmission performance adaptability are experimentally demonstrated, for the first time, utilizing Hilbert-pair-based 32-tap digital orthogonal filters implemented in FPGAs. By making use of an 8-bit DAC/ADC operating at 2GS/s, an oversampling factor of 2 and an EML intensity modulator, the demonstrated RF conversion-free transceiver supports end-to-end real-time simultaneous adaptive transmissions, within a 1GHz signal spectrum region, of a 2.03Gb/s in-phase OOFDM channel and a 1.41Gb/s quadrature-phase OOFDM channel over a 25km SSMF IMDD system. In addition, detailed experimental explorations are also undertaken of key physical mechanisms limiting the maximum achievable transmission performance, impacts of transceiver's channel multiplexing/demultiplexing operations on the system BER performance, and the feasibility of utilizing adaptive modulation to combat impairments associated with low-complexity digital filter designs. Furthermore, experimental results indicate that the transceiver incorporating a fixed digital orthogonal filter DSP architecture can be made transparent to various signal modulation formats up to 64-QAM.

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Utilizing Hilbert-pair-based digital filtering, intensity modulation and passive optical coupling, DSP-enabled flexible ROADMs are reported, which are free from both optical filters and O-E-O conversions and also offer excellent flexibility, colorlessness, gridlessness, contentionlessness, adaptability and transparency to physical-layer network characteristics. In this paper, the ROADM performance robustness against variations in numerous network design aspects is, for the first time, extensively explored in IMDD-based optical network nodes. Numerical results show that DSPs not only enable the ROADMs to dynamically and flexibly perform add/drop operations at wavelength, subwavelength and spectrally overlapped orthogonal sub-bands levels, but also considerably improve the ROADM performance robustness against variations in modulation formats, transmission system characteristics/impairments, as well as terminal equipment configurations. Index Terms-Digital filtering, digital signal processing (DSP), intensity modulation, reconfigurable optical add/drop multiplexer (ROADM), performance robustness.

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