Optical Measurements in 100 and 400 Gb/s Networks: Will Coherent Receivers Take Over? Fred Heismann Chief Scientist Fiberoptic Test & Measurement
Key Trends in DWDM and Impact on Test & Measurement Complex Modulation Formats with Polarization Multiplexing More sensitive to ASE noise How do I measure in-band OSNR? Coherent Receivers with Phase- and Polarization- Diversity + DSP Large tolerance to CD & PMD Do I still need to measure fiber dispersion? Optical Constellation Analyzer Do I ever need another optical test instrument? Flexible Grid, Superchannels and Nyquist WDM High-Resolution OSA Where are my standards for center frequencies? 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 2
Coherent Receiver with High-Speed Digital Signal Processing 100 Gb/s PM-QPSK LO Laser PBS PBS 0º / 90º Mixer 0º / 90º Mixer PD ADC ADC ADC ADC Digital Signal Processor CD Compensation PMD Compensation Phase Recovery Signal Decoding Data Unprecedented tolerance to fiber dispersion More than 30,000 ps/nm GVD and at least 25 ps mean DGD Do I still need to measure fiber CD and/or PMD? Additional measurement capabilities CD, PMD, PDL and even OSNR Do I still need to measure in-band OSNR? 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 3
Do I Still Need to Measure CD, PMD and / or OSNR? Do I still need to measure fiber link CD and PMD? NO if I want to upgrade my system from 10 Gb/s to 100 Gb/s Dispersion tolerance of coherent receiver is large enough YES if I am going to use new fiber PMD tolerance of coherent receivers is limited Do I still need independent OSNR measurements? YES if my coherent receiver does not measure OSNR Not all coherent receivers have this capability YES for performance verification of newly installed systems Need to establish accurate baseline YES for trouble-shooting of failing systems If receiver does not work no OSNR measurement How else can I measure in-band OSNR of polarizationmultiplexed signals? 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 4
Coherent Optical Constellation / Modulation Analyzer Optical Signal LO Laser PBS PBS 0º / 90º Mixer 0º / 90º Mixer PD ADC ADC ADC ADC Digital Storage Software- Based Digital Signal Processor Results Unprecedented capabilities to analyze optical signals Constellation analysis: EVM, amplitude & phase distortions / offsets, Eye analysis: shape, noise, jitter, Q-factor, crosstalk, OSNR, Polarization analysis: SOP, PMD, PDL, DOP, skew, Bit-pattern analysis: BER, Q-factor, Frequency analysis: Optical spectrum 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 5
Commercial Optical Constellation / Modulation Analyzers High-quality coherent receiver + real-time digitizing oscilloscope + software-based digital signal processing = bulky instrument Not quite field deployable 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 6
Will Measurement Capabilities Migrate to Transponders? Software-based DSP More versatile More capabilities Relatively slow Relatively bulky ASIC-based DSP Less versatile Fewer capabilities Real time processing Small footprint? Coherent Transponder DSP with increased measurement capabilities In-situ analysis of signal quality and fiber properties Why would I ever need another optical test instrument? 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 7
Why Would I Need Other Optical Test Instruments? Fiber loss is still very important Propagation and splice losses, reflections, Especially important for systems with distributed Raman amplification Still need optical power and OTDR measurements Trouble-shooting of failing systems Constellation analyzer requires decoding of received signal If signal cannot be decoded no measurements Need independent instruments for OSNR, PMD, spectrum, Installation and verification of new systems Full fiber link characterization: loss, CD, PMD, OTDR measurements Initial performance evaluation: power levels, center frequencies, OSNR Introduction of optical superchannels and Nyquist WDM Spectral analysis of entire superchannel (>100 GHz BW) High-resolution OSA with wider wavelength range than OMA 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 8
Other Optical Instruments Employing Coherent Detection Coherent optical sampling scope Modulation analyzer with short-pulse LO laser Waveform analysis with much higher bandwidth Coherent OTDR Higher sensitivity longer distances - Submarine cables > 10 000 km Higher resolution (< 10 m) Coherent OFDR Very high-resolution (component testing) High-resolution coherent OSA High spectral resolution ( 5 300 MHz) Leveraging telecommunication components Coherent PMD analyzer In-service PMD measurement (single-polarized signals only) 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 9
Coherent High-Resolution Optical Spectrum Analyzer WDM Input Signals Coherent Receiver with Polarization Diversity LPF PBS 3 db P p (ν) Scanning Local Oscillator Laser 3 db Coherent Mixers with Balanced Detection 3 db LPF RF Power Detectors P s (ν) Laboratory instrument 5 MHz 300 MHz spectral resolution Determined by electrical bandwidth of receiver >70 db dynamic range > 1000 GHz/s tuning speed Sub-GHz accuracy over entire C-band Field instrument 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 10
High-Resolution Spectra of Modulated Optical Signals -10-20 Measured with Coherent OSA (300 MHz Resolution) Optical Power [db] -30-40 -50-60 10 Gb/s NRZ-OOK 43 Gb/s PM-QPSK 2.5 Gb/s NRZ-OOK -70 195.17 195.22 195.27 195.32 Optical Frequency [THz] 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 11
Output Spectrum of Faulty 128 Gb/s PM-QPSK Transmitter -20 128 Gb/s PM-QPSK Optical Power [db] -30-40 -50 Gratingbased OSA High-Resolution OSA -60-80 -40 0 40 80 Relative Frequency [GHz] 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 12
Ultra-Densely-Spaced Nyquist WDM Signals for 400 Gb/s 25 33% higher spectral efficiency than conventional signals Signals are no longer centered on 12.5 GHz ITU grid Relative Optical Power [db] 5 0-5 -10-15 -20 150 GHz 200 GHz 4 x 100 Gb/s or 4 x 200 Gb/s -25-150 -100-50 0 50 100 150 Relative Optical Frequency [GHz] 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 13
High-Resolution Spectral Analysis of Nyquist WDM Signals Densely Spaced Nyquist Signals Signals shall not overlap Precise adjustment of center frequencies is critical 50-GHz Spaced Nyquist Signals How accurately can I measure the center frequencies? Determine from 3 or 10 db bandwidth? What reference frequencies shall I use? Optical subcarriers may not be centered on 12.5 GHz ITU grid Are absolute center frequencies the right metric? Or should it be the relative spacing of center frequencies? 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 14
What Comes Next? In-service OSNR measurements on 100 / 400 Gb/s signals How can we measure in-band noise of polarization-multiplexed signals? - All physical parameters of light are used for data encoding - Polarization extinction method doesn t work Can coherent receivers help us to distinguish signal from noise? - Without decoding of the digital information - When signals are distorted by large CD / PMD and optical filters Compact coherent wavemeters? Hand-held devices for CWDM and DWDM systems When will the technology become cheap enough? Measurement of non-linear noise and crosstalk? Is it possible to distinguish nonlinear effects from ASE noise? - SPM, XPM, XPolM, FWM 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 15
Will Coherent Receivers Take Over? I certainly believe so! Enhanced measurement capabilities in coherent transponders In-situ analysis of signal quality and fiber link properties More T&M instruments using coherent detectors Fault location Trouble shooting Installation and verification tests 2012 JDS Uniphase Corporation JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION 16
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