Formats for coherent optical communications -OPTICAL COMMUNICATIONS
This document discusses formats for coherent optical communications. It explores higher dimensionality modulation formats that apply modulation over more than one parameter such as polarization, time-slot, wavelength, or mode. Both uncoded systems and systems using forward-error correction are studied in terms of spectral efficiency and sensitivity. The document finds that increasing dimensionality at a constant spectral efficiency improves sensitivity substantially for uncoded systems, but provides much smaller gains for coded systems. It also outlines some of the advantages and disadvantages of coherent optical receivers, as well as challenges such as long loop delays and narrow loop bandwidth, and potential solutions through integrated circuits and digital feedback systems.
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Formats for coherent optical communications -OPTICAL COMMUNICATIONS
- 1. FORMATS FOR COHERENT OPTICAL COMMUNICATIONS NITHIN KALLE PALLY OPTICAL COMMUNICATIONS
- 2. Coherent optical communication • Coherent optical communication systems applying modulation formats with a dimensionality of four or higher are investigated and compared to systems using conventional formats. • Higher dimensionality can be achieved by applying modulation over more than one polarization, time-slot, wavelength, mode or core. Both uncoded systems and systems applying forward-error correction (FEC) coding are studied in terms of spectral efficiency and sensitivity. • It is shown that increasing the dimensionality for a constant spectral efficiency improves the sensitivity substantially if no coding is applied, whereas the corresponding gains generally are much smaller in FEC-coded systems.
- 3. Recent Coherent Optical Communication • Coherent detection based on DSP • Local oscillator (LO) laser • Polarization diversity 90° optical hybrid • Balanced detectors • High speed analog to digital convertor (ADC) • High speed digital signal processing (DSP)
- 4. Coherent Optical Receiver – I • Advantages: Dis-advantages: • Electrical circuit complexity Multi-level constellations • Speed limitations High data rate • Cost issues Phase managements • Power consumptions Polarization managements • Polarization managements
- 5. Coherent Optical Receiver – II • Challenges: • Long loop delays (*1ns) • Narrow loop bandwidth (*100MHz) • Transmitting and LO lasers’ linewidth • Sensitive by external variations • Solutions: • Integrated circuits (photonic IC, electrical IC) • Feed-forward loop filter topology • Minimizing Interconnection delays • Digitally operating feedback system
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